Desai, S. Desai, Smruti A. Desal, S. Desiraju, T. Dhand, R. Dhawan, R. Dhawan, I. Dhirawani, M. Dhot, P. Dickson, Rumona ; Awasthi, Shally ; Williamson, Paula ; Demellweek, Colin ; Garner, Paul Effects of treatment for intestinal helminth infection on growth and cognitive performance in children: systematic review of randomised trials British Medical Journal, Dikshit, M. Dikshit, Madhu ; Kumari, Ranjana Modulation of platelet aggregation response by factors released from polymorphonuclear leukocytes Hematology, 2 1. Dikshit, Madhu ; Sharma, Prashant Nitric oxide mediated modulation of free radical generation response in the rat polymorphonuclear leukocytes: a flowcytometric study Methods in Cell Science, 24 Dixit, Chetna ; Dikshit, Madhu A flowcytometric method for evaluation of acid secretion from isolated rat gastric mucosal cells Journal of Pharmacological and Toxicological Methods, 45 1.
Dixit, Chetna ; Rastogi, Leena ; Dikshit, Madhu Effect of nitric oxide modulators on pylorus-ligation-induced ulcers in the rat Pharmacological Research, 39 1. Doval, D. Drake, N. Du, Z. Dubey, S. Duggal, K. Dutta, D. Dutta, G. Dutta, P. Dutta, S. Dutta, N. Edison, E. Edison, Eunice S. Ellwood, P. Ellwood, p. Enas, Enas A. Fakiola, Michaela ; Nancy Miller, E.
Farooqui, A. Faruque, S. Abdul ; Roy, S. Faruque, Shah M. Farzadfar, Farshad ; Finucane, Mariel M. Fatima, Anees ; Vemuganti, Geeta K. Fernandes, Merle ; Sridhar, Mittanamalli S. Fernandes, Merle ; Vemuganti, Geeta K. Fernandes, M. Fernandez, T. Fida, S. Finucane, Mariel M. Frank, Mark B. Fries, P. Froilano De Mello, I. Gagee, P. Gaitonde, B. Ganatra, R. Indian Journal of Medical and Paediatric Oncology, 28 3. Ganessan, K. Gangal, S. Gangal, Sharad V. Ganguly, Anasua ; Sharma, R.
Ganguly, N. Garg, P. Garg, Pallavi ; Nandy, Ranjan K. Garg, Prashant ; Mahesh, S. Garg, Prashant ; Vemuganti, Geeta K. Garg, S. Garg, Surabhi ; Ramamurthy, T. Garg, Surabhi ; Saha, Pradip K. Garg, U. Gawade, S. Geevarghese, G. George, Korula ; George, Susan S. George, S. Purification, characterization and substrate specificity Biochemical Journal, 2. George, T. George, B. Gerstein, H. Ghosh, A. Ghosh, Amit ; Pal, S. Ghosh, Amit ; Poddar, Ramendra K.
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Gopinath, Gomathy ; Sailaja, K. Gopinathan, R. Gopinathan, U. Gorowara, Sushumma ; Ganguly, Nirmal K. Gorowara, Sushumna ; Ganguly, Nirmal K. Goswami, Deepti ; Goswami, Ravinder ; Banerjee, Uma ; Dadhwal, Vatsla ; Miglani, Sunita ; Lattif, Ali Abdul ; Kochupillai, Narayana Pattern of Candida species isolated from patients with diabetes mellitus and vulvovaginal candidiasis and their response to single dose oral fluconazole therapy Journal of Infection, 52 2.
Goswami, R. Goswami, Ravinder ; Goswami, Deepti ; Kabra, Madhulika ; Gupta, Nandita ; Dubey, Sudhisha ; Dadhwal, Vatsala Prevalence of the triple X syndrome in phenotypically normal women with premature ovarian failure and its association with autoimmune thyroid disorders Fertility and Sterility, 80 4. Goswami, Ravinder ; Jaleel, Abdul ; Kochupillai, Narayana Paniker Insulin antibody response to bovine insulin therapy: functional significance among insulin requiring young diabetics in India Diabetes Research and Clinical Practice, 49 1. A clinical-pathological conference International Journal of Cardiology, 49 3.
Goswami, Ravinder ; Tandon, Rakesh K. Goswami, Ravinder ; Gupta, Nandita ; Ray, Debarti ; Singh, Namrata ; Tomar, Neeraj Pattern of hydroxy vitamin D response at short 2 month and long 1 year interval after 8 weeks of oral supplementation with cholecalciferol in Asian Indians with chronic hypovitaminosis D British Journal of Nutrition, Gott, V. Gott, Vincent L. Govind, Chhabi K. Govindarajan, Vaidehi ; Agarwal, V. Govindaswamy, M. Goyal, J. Goyal, Jyotsna ; Ganguly, Nirmal K. Gray, Lincoln C. Grewal, R. Grover, R. Guarino, Linda A. Guerin, Philippe J.
Guha, A. Guha, S. Guhathakurta, B. Gulati, L. Guleria, R. Gupta, A. Gupta, Aarti ; Kumar, Lalit Long-term outcome of patients with AL amyloidosis treated with high-dose melphalan and stem cell transplantation Indian Journal of Medical and Paediatric Oncology, 29 1. Gupta, D. Gupta, Deepak ; Kumar, Lalit Newly diagnosed multiple myeloma: allograft or autograft? The National Medical Journal of India, 20 6. Gupta, Jayashree Das ; Satishchandra, P. Gupta, K. Gupta, Naveen ; Reddy, Vanga Shiva ; Maiti, Sankar ; Ghosh, Amit Cloning, expression, and sequence analysis of the gene encoding the alkali-stable, thermostable endoxylanase from alkalophilic, mesophilic bacillus sp.
Gupta, Nupur ; Sharma, J. Gupta, P. Sen ; Nair, G. Gupta, Piyush ; Indrayan, Abhaya Effect of vitamin A supplementation on childhood morbidity and mortality: critical review of Indian studies Indian Pediatrics, Gupta, R. Gupta, Rashmi ; Iyer, Venkateswaran K. Gupta, S. Gupta, Satish K. Gupta, U. Gupta, V. Gupta, Y. Gupta, Yogendra K. Gupta, Anu ; Sharma, V. Gurav, Yogesh K. Hakim, S. Haley, Bradd J. Handa, R. Hanjan, S.
Harhay, Michael O. Harinarayan, C. The Israel Medical Association Journal, Haroon, Nigil ; Srivastava, Rajni ; Misra, Ramnath ; Aggarwal, Amita A novel predictor of clinical response to methotrexate in patients with rheumatoid arthritis: a pilot study of in vitro T cell cytokine suppression The Journal of Rheumatology, 35 6. Harrison, Paul W. Hasan, J. Hasan, Nur A. Hasker, E. Hawrami, K. Hawrami, Khidir ; Hitman, Graham A. Hayaran, A. Hegde, L. Heller, Richard F. Henry, C. Herring, F. Hinenoya, A. Hitman, G.
Hitman, Graham A. Holden, Brien A. Honavar, Santosh G. Moorthy ; Vedantam, Rajshekhar A computed tomography-based localizer to determine the entry site of the ventricular end of a parietal ventriculoperitoneal shunt Neurosurgery, Hull, Russell D. Huq, Anwar ; Bradley Sack, R. Husain, M. Hussain, Amjad ; Das, Suman R. Hussain, T. Huxley, R. Ibrahim, S. Idris, I. Idris, M. Indira Nath, Forward to the past The Lancet, 2. Indira Nath, ; Dubey, S. Indira Nath, ; Laal, Suman Nucleotide sequence and deduced amino acid sequence of Mycobacterium leprae gene showing homology to bacterial atp operon Nucleic Acids Research, 18 Indira Nath, ; Sood, S.
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Measures of morbidity and mortality in children Indian Pediatrics, Basic philosophy of statistical tests, confidence intervals and sample size determination Indian Pediatrics, Statistical inference from qualitative data: proportions, relative risks and odds ratios Indian Pediatrics, Methods of sampling and data collection Indian Pediatrics, 36 9. Indrayan, Abhaya ; Rustagi, Jagdish S. Indrayan , Abhaya Statistical fallacies in orthopedic research Indian Journal of Orthopaedics, 41 1. Innes Asher, M. Isaac, Poonnoose Santosh ; Vedantam, Rajshekhar Rate of resolution of histologically verified intracranial tuberculomas Neurosurgery, 53 4.
Ishida, Nobuo ; Rao, Gullapalli N. Islam, M. Iyengar, N. Jacewicz, M. Jacewicz, Mary ; Feldman, Henry A. Jacewicz, Mary S. Jacob, A. Jadaun, G. Jaganathan, N. Jagannathan, N. Influence of molecular geometry on the carboxyl carbon tensors in alkanedicarboxylic acids and related compounds Magnetic Resonance in Chemistry, 27 Jagannathan, Naranamangalam R.
Jagannathan, S. Jagarlamudi, R. Jaggi, Kuldeep S. Jaggi, Manu ; Khar, R. Jain, A. Jain, Amita ; Chaturvedi, Umesh C. Jain, Amita ; Kumar, Pradeep ; Awasthi, Shally High ampicillin resistance in different biotypes and serotypes of Haemophilus influenzae colonizing the nasopharynx of healthy school-going Indian children Journal of Mediacal Mictobiology, 55 2. Jain, Anil ; Kar, P. Jain, Naresh C. Jain, S. Jain, Sanjay K. Jain, Vishal ; Saini, Deepti ; Goswami, Pooja ; Sinha, Subrata A phage antibody to the active site of human placental alkaline phosphatase with higher affinity to the enzyme-substrate complex Molecular Immunology, 44 4.
Jain, Suman ; Visser, Leo H. Jain, Sumana ; Sharma, Ratna ; Wadhwa, Shashi Effect of prenatal species-specific and music stimulation on the postnatal auditory preference of domestic chick Indian Journal of Physiology and Pharmacology, 48 2. Jameel, S. Jamgaonkar, A. A retrospective study Acta Virologica, 47 3. Jarvis, James N. Jayakar, Selwyn S. Jayandharan, G. Jayanthi, L. Jayshree, R. Jeejeebhoy, K. Jeemon, P. Jeevanram, R. Part B. Nuclear Medicine and Biology, 13 3. Jha, Ashish ; Sharma, Surendra K.
Jha, Tara K. Jhondhale, D. Joglekar, S. John, Albert M. John, Joshi ; Kumar, Velayudhan Mohan ; Gopinath, Gomathy Recovery of sleep after fetal preoptic transplantation in medial preoptic area-lesioned rats Sleep, Jonathan, Ashish ; Rajshekhar, Vedantam Endoscopic third ventriculostomy for chronic hydrocephalus after tuberculous meningitis Surgical Neurology, 63 1. Joseph, L. Nuclear Medicine and Biology, 14 2. Nuclear Medicine and Biology, 14 5.
Joseph, Arjun ; Kang, Gagandeep Making can-do into must-do: the way forward to health and wealth? Indian Journal of Medical Ethics, 7 4. Joseph Joy, M. Joshi, M. Joshi, S. Joshi, Shashank R. Joshi, Shripad P. Joshy, L. Jothikumar, N. Jotwani, Geeta ; Itoh, Kazuo ; Wadhwa, Shashi Immunohistochemical localization of tyrosine hydroxylase, substance P, neuropeptide-Y and leucine-enkephalin in developing human retinal amacrine cells Developmental Brain Research, 77 2.
Jotwani, G. Julka, P. Kabra, S. Kadhiravan, T. Kadhiravan, Tamilarasu ; Sharma, Surendra K. Kailas, S. Kailash, Uma ; Hedau, S. Kala, Mrinalini ; Bajaj, Kiran ; Sinha, Subrata Direct antigen capture by soluble scFv antibodies - A method for detection, characterization, and determination of affinity Applied Biochemistry and Biotechnology, 90 1. Kala, Mrinalini ; Misra, Anjan ; Saini, Deepti ; Bajaj, Kiran ; Sinha, Subrata Phage displayed antibodies to heat stable alkaline phosphatase: framework region as a determinant of specificity Journal of Biochemistry, 4.
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Kalimi, Ghulam H. Kalimi, H. Ghulam ; Sirsat, M. Satyavati Phorbol ester tumor promoter affects the mouse epidermal gap junctions Cancer Letters, 22 3. Tropical Medicine and International Health, 11 1. Kalra, Anju ; Dubey, M. Kalyanaraman, K. Kalyanaraman, S. Journal of Clinical Microbiology, 46 8. Kamat, Anupa ; Ravi, V. Kamble, R. Kambo, P. Kameshwar Rao, A. Kamruzzaman, M. Kandarkar, S. Kang, G. Kang, Gagandeep Rotavirus genotypes and severity of diarrheal disease Clinical Infectious Diseases, 43 3.
Kang, Gagandeep ; Pulimood, Anna B. Kanungo, S. Kanwar, Rupinder Kaur ; Ganguly, N. Kanwar, S. Kanwara, Rupinder Kaur ; Ganguly, N. Kaplan, G. Kapoor, A. Kapoor, Ashish ; Ratnapriya, R. Kapoor, P. Kapur, Kavita ; Kapur, A. Kar, P. American Journal of Gastroenterology, Kar, Sudeshna ; Ghosh, Ranajit K.
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Karmali, M. Karmarkar, M. Kartha, C. Kataria, R. Katoch, K. Katoch, V. Katoch, Vishwa M. Kaul, C. Kaul, Pankaj ; Malla, N. Kaur, G. Kaur, Gurvinder ; Singh, P. Kaur, H. Kaur, Jaspreet ; Sundar, Shyam ; Singh, Neeloo Molecular docking, structure-activity relationship and biological evaluation of the anticancer drug monastrol as a pteridine reductase inhibitor in a clinical isolate of Leishmania donovani Journal of Antimicrobial Chemotherapy, 65 8.
Kaur, M. Kaur, Rupinder ; Ganguly, N. Kaur, S. Kauri, S. Kauser, H. Kenney, Richard T. Kesavulu, M. Prakasha ; Ramya, T. Keshari, Ravi S. Khaitan, A. Khan, A. Khan, Asis ; Das, S. Khanduja, V. Khanna, M. Khanna, Madhu ; Chaturvedi, U. Khanna, Neelam ; Chandramuki, A. Khanna, Sumant ; Ravi, V. Khanna, N. Khardori, Romesh ; Bajaj, Jasbir S. Khare, M. Khare, P. Khilnani, G. Khilnani, Gopi C. Khokhani, R. Khubchandani, M. Khurana, Seema ; Ganguly, Nirmal K. Khuroo, M.
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Kochupillai, V. Kohli, N. Koley, H. Komoroski, R. Kompella, Viswanadh B. Kono, R. Kooner, Jaspal S. Koshi, R. Kotekar, A. Kothari, N. Kothari, Nikhil ; Keshari, Ravi S. Kovai, Vilas ; Rao, Gullapalli N. Krishnaiah, S. Krishnaiah, Sannapaneni ; Nirmalan, Praveen K. Krishnan, G. Krishnan, K. Rai Aureomycin in the treatment of typhoid, typhus, cystitis and pertussis Indian Medical Gazette, 85 5. Krishnan, R. Venkata ; Pal, G. Krishnan, Savithri ; Krishnan, A.
Krishnaswamy, Kamala ; Gopalan, C. Kulkarni, A. Kulkarni, H. Kulkarni, Vaijayantee ; Rajshekhar, Vedantam ; Raghuram, Lakshminarayan Accelerated spondylotic changes adjacent to the fused segment following central cervical corpectomy: magnetic resonance imaging study evidence Journal of Neurosurgery: Spine, 1. Kulkarni, Vyjayanti ; Rajshekhar, Vedantam Chronic non-healing ulcers of the scalp after trauma Postgraduate Medical Journal, 76 Kulshreshtha, R.
Kumar, A. Kumar, Amit ; Malaviya, A. Kumar, D. Kumar, G. Samson Sujit ; Rajshekhar, Vedantam Acute graft extrusion following central corpectomy in patients with cervical spondylotic myelopathy and ossified posterior longitudinal ligament Journal of Clinical Neuroscience, 16 3. Srinivasa Intraoperative mapping of sacral nervous system S2 - 4 British Journal of Neurosurgery, 20 6. Kumar, K. Kumar, L. Experience from a developing country Bone Marrow Transplantation, Study of febrile episodespi Journal of the Association of Physicians of India, 40 1.
A study of cases Journal of the Association of Physicians of India, 38 Kumar, Lalit ; Gulati, Subhash C. Kumar, Mahesh ; Jagannathan, N. Kumar, P. Kumar, Pratik ; Rehani, M. Kumar, R. Kumar, Rajeev ; Indrayan, Abhaya A nomogram for single-stage cluster-sample surveys in a community for estimation of a prevalence rate International Journal of Epidemiology, 31 2.
Kumar, S. Kumar, Sachin ; Barthwal, Manoj Kumar ; Dikshit, Madhu Nitrite-mediated modulation of HL cell cycle and proliferation: involvement of cyclin-dependent kinase 2 activation Journal of Pharmacology and Experimental Therapeutics, 3. Kumar, Sen Sushanta ; Premalatha, G. Kumar, Virendra ; Jagannathan, N. Kumar, Virendra ; Jagannathan, Naranamangalam R. Kumari, R. Kumta, U. Kundapur, A.
Kunimoto, Derek Y. Kurazono, H. LaRocque, Regina C. Laal, S. Infection and Immunity, 50 3. Lakshmi, R. Lakshmi, S. Purification and phosphoprotein nature Journal of Biosciences, 6 1.
Lakshmy, Ramakrishnan ; Mohan, Viswanathan ; et. Lal, Anil P. Lam, J. Lanchbury, J. Lara, Ruben J. Lee, C. Lee, V. Lewis, R. Lewis, V. Lindsay, Brianna ; Ramamurthy, T. Lockwood, Diana N. Loganathan, G. Lohman, Lawrence E. Lonn, Eva M. Loos, U. Wochnschr, Luthra, S. Madan, K. Madan, Z. Madegowda, R. Madesh, M. Madhavi, Y. Mahadev, P. Mahadevan, Anita ; Vaidya, Sunil R. Mahajan, R. Mahajan, Supriya Dinkar ; Aalinkeel, Ravikumar ; Singh, Shailini ; Shah, Pankaj ; Kochupillai, Narayana Pre-pregnancy weight and weight gain during pregnancy are important determinants in the endocrine modulation of fetal growth restriction Journal of the Turkish-German Gynecological Association, 8 1.
Mahakur, A. Mahalanabis, D. Mahapatra, A. Mahendra, Ankit ; Misra, Ramnath ; Aggarwal, Amita Th1 and Th17 predominance in the enthesitis-related arthritis form of juvenile idiopathic arthritis The Journal of Rheumatology, 36 8. Majumdar, R. Majumdar, Ramanath ; Balasubramanian, A. Makharia, G. Malaviya, A. Malaviya, B. Malaviya, P. Malhotra, Rajeev Kumar ; Indrayan, A. Malik, P. Malik, Pushpa ; Singh, A. Malvi, S.
Mangalam, Ashutosh K. Mangalik, A. Maniar, Hina S. Manickasundari, M. Manikandan, K. Wirtschafter, MD, Eric L. Berman, MD, and Carolyn S. McDonald, MD] 7. Ford, MD, and Carol L. Karp, MD 8. Stewart, MD 9. Jordan, MD, and Richard R. Anderson, MD Pulido, MD Fletcher, MD Netland, MD, PhD Flynn, Jr. Smiddy, MD Cunningham, Jr. Wang, MD, PhD Plager, MD; written by Edward G. Buckley, MD, David A. Plager, MD, Michael X. Repka, MD, and M. Parks, MD and Gunter K. Traboulsi, MD www. Policeni, MD Andrew G. Lee, MD Wendy R. Johnson, Bruno A. Policeni, Andrew G. Lee, Wendy R. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Oxford University Press.
Wirtschafter, Eric L. Berman, Carolyn S. Includes bibliographical references. ISBN 1. Eye—Magnetic resonance imaging—Atlases. Visual pathways—Magnetic resonance imaging—Atlases. Visual pathways—Tomography—Atlases. Johnson, Michael C. Michael Curtis , — II. Wirtschafter, Jonathan Dine, — Magnetic resonance imaging and computed tomography.
Series: Ophthalmology monographs ; 6. Eye Diseases—diagnosis—Atlases. Magnetic Resonance Imaging—methods—Atlases. Tomography, X-Ray Computed—methods—Atlases. W1 OPL v. W55 The material represents the approach, ideas, statements, or opinion of the authors, not necessarily the only or best method or procedure in every case, nor the position of the Academy.
The Academy does not endorse any of the products or companies, if any, mentioned in this monograph. Some material on recent developments may include information on drug or device applications that are not considered community standard, that reflect indications not included in approved FDA labeling, or that are approved for use only in restricted research settings.
This information is provided as education only so physicians may be aware of alternative methods of the practice of medicine, and should not be considered endorsement, promotion, or in any way encouragement to use such applications. The FDA has stated that it is the responsibility of the physician to determine the FDA status of each drug or device he or she wishes to use in clinical practice, and to use these products with appropriate patient consent and in compliance with applicable law.
The Academy and OUP are not liable to anyone for any errors, inaccuracies, or omissions obtained here. The Academy specifically disclaims any and all liability for injury or other damages of any kind v vi Legal Notice for any and all claims that may arise out of the use of any practice, technique, or drug described in any material by any author, whether such claims are asserted by a physician or any other person. Preface Neuroimaging and orbital imaging using computed tomography CT and magnetic resonance imaging MRI techniques have revolutionized the evaluation, management, and treatment of orbital and neuro-ophthalmic disorders.
It is our hope that this revision of the original AAO neuroimaging monograph will assist clinicians in understanding the mechanics of the imaging techniques i. The authors of the current edition wish to acknowledge and thank the authors of the fi rst edition Dr. Eric Berman, Dr. Carolyn Johnson for their work, which served as the basis for this revision. We would also like to acknowledge Dr. Daniel Thedens, PhD, for his work in reviewing the physics section of this monograph.
We have tried to update, rather than completely replace, the content of the original version, especially in the areas of neuroimaging that have not changed significantly over time. To update the monograph, a literature review was performed by the current authors using a systematic English-language MEDLINE search — limited to articles with relevance to neuro-ophthalmic and orbital imaging.
In the original version of this monograph, Chapter 1 provided a nonmathematical introduction to the physics of imaging. We have tried to simplify this portion of the original text and provide a more concise and clinically meaningful context for the basics of the imaging techniques for the targeted audience of this monograph. The basic physics for the imaging studies might help the reader better understand CT and MRI such that studies are ordered and interpreted more precisely and efficiently. Chapters 1 through 3 are intended to provide a basic framework for the normal and pathologic radiologic fi ndings seen in various disease entities of interest to the ophthalmologist.
Chapter 4, the last section of the monograph, provides the reader with guidelines for ordering the proper imaging study and provides specific pathologic examples of interest to the ophthalmologist. Michael C. Wirtschafter is deceased. The prescribing i. In addition, the prescribing physician must communicate the imaging question and provide relevant clinical information to the interpreting physician i.
Since the publication of the original edition of this monograph, newer techniques and special sequences have improved the ability to detect pathology in the orbit and brain of interest to the ophthalmologist. Table A-1 lists the clinical scenarios for which an imaging study might be ordered by an ophthalmologist. In this second version of the monograph, we update the original content and summarize the recent neuroradiologic literature on the various modalities applicable to CT and MRI for ophthalmology.
The mainstays for orbital and neuroimaging are conventional radiography e. The x-ray has been known since the time of Roentgen, for which he received a Nobel Prize. The role of the traditional skull or orbital radiograph has been supplanted in the modern era by current CT and MR techniques, and we will not discuss in detail the radiograph beyond its historical uses. At our institutions, we still use the skull radiograph in a limited role to prescreen patients with a history or suspicion for metallic foreign body prior to MR studies.
As a general rule, patients who xi xii Introduction Table A Unilateral or bilateral visual loss e.
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Efferent neurogenic pupillary defects e. Afferent pupillary defects e. Proptosis e. Diplopia or external ophthalmoplegia 6. Eye lid abnormalities e. Oscillopsia e. Ophthalmoscopic abnormalities suggesting an orbital or intracranial lesion e. Ocular or orbital trauma e. We have chosen to emphasize vascular imaging advances e. The goal of the monograph is to reinforce the critical importance of accurate, complete, and timely communication of the clinical fi ndings, the differential diagnosis, and the presumptive topographical location of the suspected lesion from the prescribing ophthalmologist to the interpreting radiologist in order to perform the optimal imaging study and to ultimately receive the best interpretation.
We provide tables to summarize the indications and best imaging recommendations for specific ophthalmic entities, and we have collected a number of examples of specific radiographic pathology to illustrate the relevant entities. About the Authors Michael C. Bruno Policeni, MD, is a clinical assistant professor of diagnostic radiology— neuroradiology, University of Iowa. Andrew G. Lee, MD is chair of the department of ophthalmology at The Methodist Hospital in Houston, Texas, and is professor of ophthalmology in neurology and neurological surgery at Weill Cornell Medical College.
Lee serves on the Editorial Board of 12 journals including the American Journal of Ophthalmology, the Canadian Journal of Ophthalmology, and Eye and he has published over peerreviewed articles, 40 book chapters, and 3 full textbooks in ophthalmology. Wendy K.
Smoker has served on the Editorial Board of multiple radiology journals, previously a deputy editor of Radiology. This accomplishment rests on a broadly based body of knowledge and techniques that encompasses much of modern physics. The scientific foundation involves notions of atomic and molecular structure and the concept of nuclear MR NMR. First observed in the late s, the actual phenomenon of NMR could be produced in uniform, bulk materials contained in test tubes or similar chambers.
In , Paul C. Lauterbur suggested that magnetic field gradients could encode position-dependent imaging information. Hounsfield Nobel Prize in Physiology and Medicine had before them. Some phenomena of light can best be described by classic wave theory, while other phenomena of light are best illustrated by quantum particle theory, even though the two approaches may have apparent contradictions. Similarly, the physics 3 4 Neuroimaging in Ophthalmology of MRI can sometimes best be understood with a quantum-mechanical model, in which each proton can be in either one of two states parallel or antiparallel , or with a classic mechanical model, in which the net magnetic moment vector of all protons can be located with regard to a three-dimensional frame of reference.
This three-dimensional frame of reference may be considered either as stationary, with its x and y axes fi xed and perpendicular to the main static magnetic field the laboratory frame of reference , or as rotating, with its x and y axes rotating in synchrony with the precessing protons the rotating frame of reference. MRI is based on the principle that the nuclei of certain atoms become polarized or aligned display magnetic moments when placed in a static magnetic field Figure In particular, it is the odd number of protons in the nuclei of hydrogen 1H , sodium 23Na , and phosphorus 31P that is responsible for the magnetic moments in human tissues.
Comparison of the magnetic properties of a compass with the MR behavior of protons in several states. A No magnet: in the absence of a magnetic field, the protons are randomly aligned. B Compass needle in magnetic field: the compass needle aligns with the magnet but can point in only one direction. More protons align with the field than against it. The possibility of two orientations for the protons differs from the one orientation allowed for the compass needle.
Such a pulse is said to tilt the protons and is used in various imaging sequences. The nuclear magnetization vector M rotates at the Larmor frequency around the main magnetic field vector B0, which is defi ned as oriented in the z axis. M rotates in the clockwise direction from y to x as viewed from above at the Larmor frequency. The MRI signal obtained from the protons in each voxel must be detected and processed to obtain useful information.
When the magnetic field designated as B 0 is activated, all of the atomic nuclei will be affected. However, a slightly smaller fraction a few parts per million of the nuclei will align against B 0 than with it, creating a net magnetic vector in the longitudinal direction of the magnetic field, designated the z axis Figure The behavior of hydrogen nuclei protons in a magnetic field can be compared and contrasted to that of a compass.
The compass needle always points in one direction, but protons can align either parallel or antiparallel to the magnetic field. The protons aligned in the parallel direction are at a lower energy level than those aligned in the antiparallel direction. A quantum of energy must be absorbed by a proton oriented in the parallel direction for it to be transformed into a proton oriented in the antiparallel direction.
Conversely, a proton oriented in the antiparallel direction must lose a quantum of energy to be transformed to a parallel orientation. Transformations between parallel and antiparallel orientations can occur if the proton gains or emits energy by gaining or emitting one photon as a radiofrequency [RF] wave of the correct energy.
Moreover, a proton in an antiparallel orientation can be struck by one photon and emit two photons, resulting in an energy loss sufficient to transform the proton to a parallel orientation. Low-strength magnets cause nearly equal numbers of protons to align in each direction, but as the magnetic field strength is increased, the energy difference between the two states also increases and more of the magnetic dipoles of the individual protons tend to align in the lowest energy state parallel to the main magnetic field.
This creates a net magnetic vector parallel to the direction of the magnet. For each isotope that possesses a nuclear magnetic moment e. The Larmor frequency is related to intrinsic properties of that element 6 Neuroimaging in Ophthalmology and is directly proportional to the strength of the static magnetic field measured in Tesla, abbreviated T. Thus, application of an RF wave of resonant frequency equal to the Larmor frequency provides the energy that will produce a realignment of the magnetic vector. This application of energy is called excitation.
This energy is re-emitted from the protons over time through a process known as relaxation. During relaxation, the torque of the static magnetic field exerted on the magnetic moment of the protons causes them to exhibit a type of movement called precession: the protons behave like small tops spinning around the axis of the magnetic field vector see Figure We are familiar with the force of gravity that causes the precession of a spinning top or gyroscope; when the device falls over, relaxation is completed.
In the case of MRI, the magnetic force causes complete relaxation when all of the protons are realigned with the main magnetic field. Relaxation is the process whereby the absorbed energy is redistributed among the aligned protons there is also some loss to the neighboring nuclei. Relaxation releases energy at the Larmor frequency and this energy can be detected with an antenna, often the same antenna that was used to excite the protons.
The imposition of the main magnetic field is somewhat analogous to installing the strings on a musical instrument and then applying tension to them tuning. The tuning is accomplished by the gradient magnetic field. This RF pulse provides energy that is absorbed by the protons and brings them to a more excited state, which changes the direction of their magnetic vectors.
The length and amplitude of the pulse can be varied to control the degree of change in the magnetic vector. The resultant movements of the protons can be called fl ips or tilts see Figure In current clinical practice, only protons are used to produce MRI scans, because protons are abundant in the human body as water and the signal from hydrogen protons is easily detected.
Most of the energy re-emitted by the protons in the tissue is absorbed within the tissue, but a small fraction of the energy is absorbed by the antenna receiver coil. The detected signal is proportional to the spin density— the number of nuclear magnetic moments per unit volume. The signal is received by an antenna arranged to detect precession of the magnetic dipole of protons only while they remain aligned in the transverse plane.
When a signal is recorded as described earlier, it is described as a measurement of free induction decay FID. The longitudinal decay of each proton within the slice is somewhat analogous to that of each atom within a volume of unstable isotopes of one element, in that it behaves independently with its own timing. FID signals are not used clinically 7 Magnetic Resonance Imaging because their half-lives are not sufficiently long to allow the application of gradients necessary to localize the signals and generate an image in human patients.
The pulse sequences that are used to produce clinically useful signals are discussed in Sections to The emitted signals are transformed by the computer into an image by a mathematical process called two- or three-dimensional Fourier transformation. This produces the familiar pictures seen on the monitors. NMR was initially a technique applied to small volumes of homogeneous chemicals in a vessel within homogeneous magnetic fields. To provide useful clinical information about the inhomogeneous environments of protons within a tissue space such as the skull or orbit, it is necessary to arrange for the selective excitation of a slice of tissue with an MRI scanning device.
To accomplish this, a weak inhomogeneous gradient static magnetic field is superimposed on the strong homogeneous static magnetic field supplied by the main magnet Figure Gradient magnets can alter the local magnetic field strength represented by arrow length from the uniform strength imposed by the main magnet A. When the gradient magnet field is superimposed B , unequal field strength results. Although the progression is continuous throughout the magnetic field, the strength in only three regions is shown for simplicity. The extent of the magnetic gradient is called its bandwidth.
The gradient magnet tunes the system for the extraction of spatial information. MR scanning device, demonstrating the main magnet, gradient magnet coils, and RF surface coils. The use of surface coils is optional. B0, the magnetic vector of the main magnet, is in the longitudinal plane z axis and is shown above the device.
The transverse plane x, y axes is perpendicular to the longitudinal axis. The inhomogeneous field varies in a predictable way with the location in the bore of the magnet. Three gradient coils are required to set up a threedimensional imaging system. A spatial coordinate system can be established that permits imaging of the axial, coronal, and sagittal orthogonal planes as well as oblique planes. Manipulating the gradient fields through orthogonal planes provides data for Fourier transformation and spatial reconstruction.
This can be done by periodically adding y and z gradient fields to the static x gradient field. The ability to directly obtain information in the midsagittal plane of the skull is one of the principal advantages of MRI over computed tomography CT ; however, with the new multislice CT, thin-section images with isotropic voxels volume elements are available and generate excellent orthogonal plane reconstructions.
Once the x, y, and z gradients have been established, a change of the exciting RF is the only requirement for changing the parallel plane that is being imaged. The thickness of the parallel planes slice thickness is controlled by the bandwidth of the RF pulse used in the excitation Figure Although RF receiver coils are built into the scanner, they can be temporarily disconnected and replaced by smaller coils applied directly to parts of the body surface such as the orbit.
Surface coils such as head and orbit coils have specific uses for evaluating the orbit and visual system.
Professor of Ophthalmology at the Stanford University Medical Center
Acquisition of a single-slice, two-dimensional Fourier transform image. A The gradients of the magnetic field strengths within the scanned volume produced by the main and gradient magnets. The direction of the B0 arrow indicates the main magnetic field. The direction of increasing phase difference and increasing frequency within the slice is indicated by the arrows. B The dark volume is that excited by the RF pulses within the selected frequency range: the wider the range, the greater is the slice thickness.
This parameter is also designated as Gs, the slice- selection gradient. C The frequency-encoding gradient Gf is shown arbitrarily along the y axis of the transverse plane. D The phase- encoding gradient Gp is shown arbitrarily along the x axis of the transverse plane. Spatial localization within three-dimensional volumes voxels can be defi ned by the application of these gradients.
The application of the x gradient while detecting the MRI signal assigns a unique frequency to each voxel according to its x coordinates. The application of a y gradient for a short period before the detection of the MRI signal assigns a unique phase to each voxel according to its y coordinates. Selective excitation of protons within a single slice will result when the precession frequency of the protons within the slice is identical to that of the transmitted exciting RF see Figure Because the Larmor frequency is determined by the strength of the static magnetic field, it follows that the Larmor frequency will vary in a predictable manner within tissue placed in an inhomogeneous magnetic field.
This inhomogeneity results when a gradient coil is turned on at the same time as the RF coil that emits the Larmor frequency. This can be used to create the selective excitation of tissue within a single slice. Useful clinical information depends on receiving, analyzing, and displaying the analog RF signals from the excited protons.
MRI scanning device. A general-purpose computer operating in a digital domain is used to control various operations in the analog domain, including the magnetic gradient coils and the RF antenna for its transmission T and receiver R functions. After the RF signal is received, it is converted from analog to digital for processing within the computer and displayed on the image display graphic device as well as the printing device. Redrawn with permission from Atlas SW. Magnetic Resonance Imaging of the Brain and Spine.
There are two methods in general clinical use for spatially encoding MRI data: frequency encoding and phase encoding. Frequency encoding uses an antenna to record the FID frequency spectrum produced while the magnetic field gradient is switched on. The protons excited at different positions within the plane will resonate at different frequencies based on their position in the plane Figure The combined signal from such protons can be subjected to Fourier analysis, and projection of their relative location and intensity can be made.
Thousands of repeated determinations at many angles within the plane can be made as the gradient is rotated. Phase encoding also uses a magnetic field gradient to encode spatial information but in a slightly different way. Phase encoding uses the gradient to set up a difference in the amount of rotation or phase in the proton signal as a function of location Magnetic Resonance Imaging Excited tissue FID signal 11 Fourier transformation A B Amplitude z x y C Frequency distance Figure Extraction of spatial information.
A, B Fourier transformation. C The sounds emitted by two tuning forks originating from the same volume merge to produce a complex waveform. Fourier transformation right identifies the source and amplitude of the two signals. RF signals emitted from relaxing protons at slightly different positions within a signal slice of tissue undergoing MRI scanning emit signals that are measured as free induction decay FID. The protons at slightly different distances along the y axis emit slightly different resonant frequencies, indicated by large and small tuning forks.
Frequency-encoding analysis is performed while the magnetic field gradient is switched on. Thus, phase encoding generates a distance-dependent effect in a direction perpendicular to the frequency-encoding direction. The phaseencoding gradient is turned on after the RF pulse but before the antenna records data. Hundreds, or even thousands, of repeated determinations are obtained at different gradient strengths on each repetition.
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The gradient strength can be controlled by varying the current strength and direction in the coil. The combination of all of these different phase-encoding steps can be used to localize the protons in a second, or even third, direction within the magnet. The only difference is that the signal changes arising from the frequency-encoding gradient are generated during the recording of the FID signal, whereas the changes due to the phase-encoding gradient are generated before the FID signal is recorded. The signal intensity, represented on a two-dimensional projection as a pixel picture element , is proportional to the number of protons precessing within a three-dimensional voxel volume element within the acquisition matrix.
Repeated measurements at all projections in the transverse x,y plane are used to calculate the signal intensity at each pixel within the plane. The digital methods of calculation and image construction are related to those used in computed tomography. T1 and T2 are time constants resulting from inherent tissue characteristics that correspond to the behavior of protons whose nuclei precess in response to applied magnetic and RFstimuli Table TR repetition time , TE time to echo , and TI time for inversion are time intervals selected by the personnel performing the scan and are independent of any inherent tissue characteristics.
These time intervals are discussed in Section In MRI scanning, RF pulses of a selected energy and duration are applied to reorient the net magnetic vector from the z axis that was imposed by the static magnetic field. The frequency required of the RF pulse is directly related to the magnetic field strength. With a field strength of 1. These pulses are used, either singly or in combination, Table T1 and T2 in Various Mammalian Tissues at 1. A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from 1— MHz: dependence on tissue type, NMR frequency, temperature, species, excision, and age.
Med Phys. Magnetic Resonance Imaging 13 Longitudinal magnetization to produce different pulse sequences. The term longitudinal plane is used for the z axis, the vector of the magnetic field prior to the onset of the RF pulse. After the pulse is terminated, there will be zero magnetization from the protons in the longitudinal axis. Subsequently, the protons will slowly realign with the longitudinal magnetic field. This exponential process is called longitudinal relaxation. This type of relaxation occurs as the stimulated protons lose their kinetic energy due to the retarding forces of neighboring nuclei in the lattice.
Spin-lattice relaxation is essentially a thermal reaction, with the transfer of energy as heat from the protons to the surrounding molecular environment lattice. Due to slight imperfections in the static magnetic field and local variations in the magnetic moments of neighboring protons and unpaired electrons, some protons are exposed to a stronger field than others and precess at a faster rate than adjacent protons, because the rate of precession always depends on the local magnetic field experienced.
The interaction of the magnetic moments of the faster-precessing with the slower-precessing protons causes loss of energy and entropy. The protons exchange their spins with their neighbors—thus, the term spin-spin relaxation. In a quantum-mechanical model, parallel and antiparallel protons are converted to the opposite state. T1 relaxation. M0 is the maximal value and is not achieved until several multiples of T1 have passed.
If the repetition time TR is equal to T1, longitudinal remagnetization will not have time to be fully completed. Spin-echo and T2 relaxation. Thereafter, the faster-precessing and the slower-processing protons interact magnetically and their spins begin to dephase rapidly within the transverse plane so that their vectors spread to occupy an ever-enlarging sector and ultimately a disc. What is illustrated is a pure T2 effect; the actual process involves a combination of T2 and T1 relaxation so that the disc illustrated becomes conical, as shown in Figure Some residual dephasing is present at the echo owing to T2 effects, which cannot be reversed by the refocusing pulse.
F, proton spins with faster precession; S, proton spins with slower precession. Clinical Magnetic Resonance Imaging. The magnetic vectors thus spread out like the opening of a fan within the transverse plane from their initial location in one direction along the y axis. This process of loss of phase coherence among spins is called transverse relaxation. This reflects the time-dependent interaction of proton spins, causing nuclei to precess at different rates and deviate from the uniform motion created on initial excitation.
T1 and T2 relaxation compared. Note that T2 relaxation is completed much more rapidly than T1 relaxation. The signal emitted represents both T1 and T2 relaxation according to the characteristics of the protons within the tissue. Note that the amplitude of the signal falls rapidly by the time the protons have dephased or are close to the transverse plane during T2 decay. Philadelphia: WB Saunders Co; Longitudinal magnetization increases after the termination of the RF pulse with a time constant of T1 , while transverse magnetization decreases after the termination of the RF pulse with a time constant of T2.
T1 relaxation and T2 relaxation occur simultaneously, but T2 relaxation is completed much more rapidly than T1 relaxation Figure At the end of transverse relaxation, when the transverse magnetization has nearly reached zero, the magnetic vectors can be represented as located along the edge of a disc in the transverse plane.
They are then brought into the shape of a cone of decreasing diameter during longitudinal relaxation. The tissue characteristics associated with T1 and T2 are discussed in Section The scanner operator performing the procedure sets the time between pulses, which is called the repetition time TR.
This represents the interval of time between cycles of excitation and relaxation. However, repeating the RF pulse with TR less than the average T1 leads to reduced signal strength and a noisier image Figure These repeated patterns of RF bursts are called pulse sequences. Varying only TR affects the appearance of tissues. Equilibrium magnetization and magnetic saturation.
A Greater equilibrium longitudinal magnetization results with a long TR between series of several RF pulses. B With a short TR, the equilibrium magnetization is less and the saturation of the protons is greater. Decreased equilibrium magnetization is associated with decreased signal strength on T1-weighted images. When the protons align with equal numbers in the parallel and antiparallel directions, they are said to be saturated; this phenomenon forms the basis for saturation-recovery scanning.
The solid lines represent the longitudinal relaxation following each pulse. Top curve in color connects the maximum relaxation at the end of each repetition. Signal produced by a train of RF pulses separated by a repetition time of TR. The free induction decay FID signal is measured immediately after each pulse. The signal intensity that is related to T1 relaxation time is illustrated in Figure , which has been calculated from data similar to those in Table The first pattern is T1-weighted and the last is described as proton-density-weighted.
MRI signal calculated as a function of the pulse repetition time TR for gray matter GM , white matter WM , and cerebrospinal fluid CSF , using the parameters of Table and assuming typical relaxation times in proton densities. Gray matter and white matter are isointense when TR is approximately 2 sec and also below 0. Note that CSF is essentially isointense with gray and white matter when TR is in the vicinity of 5 sec. If TR is allowed to become greater than 5 sec, CSF will have the highest signal intensity while the proton density of the gray and white matter will provide contrast for the brain.
Compare Figure Saturation is defi ned in MRI as an equilibrium condition in which an equal number of protons are aligned with and against a magnetic field such that no further absorption of energy of the RF pulse will take place. Thus, the vector sum of all the proton magnetizations is zero and there is no net magnetic vector. Saturation can be produced by repeated pulses having interpulse intervals much shorter than T1. The effect of decreasing TR on the equilibrium magnetization series of pulses is shown in Figure Repeating pulses more frequently than shown i.
Relaxation after the fi nal pulse of such a series occurs from an initial condition of zero net magnetization, rather than from an initial condition of partial magnetization as would occur after a single RF pulse. Saturation-recovery techniques are used in T1-weighted scans of the sella to remove the effects of unsaturated protons in blood moving through the imaging volume.
Saturationrecovery techniques also are useful in removing swallowing and respiratory motion artifacts. Table Typical signal intensities for various tissues are compared. Bottom, The proton densities of various tissues are shown and expressed as a percentage of MRI-visible protons in tissue compared with pure water. Note that the differences between tissues are small; this limits the usefulness of proton-density-weighted images. Saturation-recovery pulse sequence. Relaxation is not complete between pulses.
The tissue represented by the curve printed in white has a shorter T1 relaxation time than the tissue characterized by the curve printed in gray. Gray matter has a shorter T1 relaxation time than white matter. Because the FID signal will be related to the transverse magnetization, contrast on the printed image will result because the gray matter will produce a greater signal intensity than white matter for tissues of equal proton density. The spin-echo technique minimizes the effects of inhomogeneities in the static magnetic field.
The cycle can be thought of as fl ipping the protons into phase at the end of the pulse. The resultant signal is measured again. The T2 image is obtained in a later echo, on the order of 60 to msec after the excitation see Figure First echo images also called proton-density-weighted images have also been commonly obtained in the past. However, these have mainly been replaced by FLAIR fluid-attenuated inversion recovery images except for specific indications that are outside the scope of this monograph. Use of a spin-echo technique to produce a T2-weighted signal due to signal decay T2 contrast.
The first and second echoes, TE1 and TE2 , are asymmetric in this example. Inversion-recovery pulse sequence. During TI, the proton spins relax partially, some of them returning to the parallel orientation. Note the multiple crossover points at which gray matter, white matter, and CSF are predicted to be isointense.
The latter cannot emit a signal there is an absence of transverse magnetization due to the absence of longitudinal magnetization. The inversion-recovery technique thus allows the signal of a given tissue to be suppressed by selecting a TI adapted to the T1 of this tissue. At an optimum TI about 0. One of many fat-suppression techniques useful in imaging the orbit, STIR is especially valuable when combined with contrast agents, so that a contrast-enhanced tumor will produce an intense signal while normal bright orbital fat is suppressed on a T1-weighted image.
Even without flow produced by external forces, free water molecules surrounding a hydrogen ion move rapidly and their rotational and translational Brownian movement produces rapid fluctuation in the magnetic fields adjacent to the protons. T2 relaxation times are fundamentally faster than T1 relaxation times. These molecular water—proton interactions accelerate T1 and T2 relaxation. Water molecules attached bound to proteins or to 22 Neuroimaging in Ophthalmology cell membranes this is especially prominent in fat cells move less rapidly than unattached free water molecules within tissue Figure Water molecules in flowing blood move rapidly.
In adipose tissue and proteinaceous solutions, limited movement of water molecules occurs at a fluctuation rate equal to that of the resonant frequencies of the protons. Such solutions have rapid T1 and intermediate T2 relaxation times. However, in pure water, the molecules fluctuate more rapidly than the protons and do not affect them as much, giving pure water long T1 and T2 relaxation times, measured in seconds as opposed to milliseconds.
When water molecules are very tightly bound to collagen in tendons and scar tissue, their slow fluctuation promotes rapid T2 relaxation but not T1 relaxation. Thus, such tissues have T2 relaxation times of less than 50 msec, while the T1 relaxation times may be in the vicinity of 1 sec. Slower-moving molecules, such as proteins, produce less rapid fluctuations in the magnetic field experienced by their protons and do not affect T1 relaxation as much as the faster-moving free water molecules.
In biologic tissues, water is often bound to proteins, causing a change in the T1 relaxation of that protein. Efficient shortening of T1 relaxation occurs in the region of the curve representing the movement of water molecules bound to proteins and cell membranes, which restricts molecular motion compared to free water molecules. Perturbations in the bound-water and free-water content of tissues contribute to the contrast of abnormal from normal tissues. The proton density of structures contributes the majority of information concerning the structures.
Tissues with higher proton density produce the most intense signals. Flow is an important consideration in fluids that do not remain stationary. If protons are stimulated by the RF pulse and leave the slice being scanned before the image is sampled, then no signal will be obtained. Similarly, the protons that have come into the slice after the RF pulse have not been exposed to the pulse, and such protons will not contribute to the image.
As a result, a flow void is seen where rapid flow exists. Thus, large vessels appear dark on typical T1- and T2-weighted images using spin-echo sequences unless special techniques are used to display them i. The electron clouds around atomic nuclei shield them from the applied magnetic field during the RF pulse, creating alterations in the local field that the nuclei encounter. For example, protons associated with water have a different Larmor frequency than protons associated with lipids because of differences in the electron cloud configuration. Relaxation time of protons versus frequency Hz of molecular motion of water molecules.
Free water has rapid diffusion and long T1 and T2 times free-water region. Bound water associated with restricted motion has shorter T1 and T2 times bound-water region. Note that the T1 relaxation time of bound water is efficiently shortened as a function of its frequency of molecular motion. The clinical differentiation of biological fluids cerebrospinal fluid and vitreous from tissues occurs in the region represented by the right half of the bound-water region. Here T1 becomes much longer than T2. High signal intensity is represented by the light regions of the T1 and T2 curves.
On T1-weighted images, high signal intensity is associated with short T1 times so that fatty tissue will be characteristically bright. On T2-weighted images, the high signal intensity is associated with long T2 times so that free water and edema in tissue will be characteristically bright. Note that free water molecules diffuse randomly in all directions. Bound water is associated with macromolecules and cell membranes and has T1 and T2 relaxation times that are shorter than those of free water. Collagen has tightly bound water with long T1 relaxation times and very short T2 relaxation times.
Image contrast as a function of TR and TE. Bottom, The to msec time corresponds to the fi rst-echo times used clinically. Top, Corresponds to the second-echo times. Right, The TR shown corresponds to the TR used in a spin-echo technique, thus producing a proton-density-weighted scan on the first echo and a T2-weighted scan on the second echo.
The T1-weighted signal is produced when TR is in the range shown on the right. Table provides characteristics useful in differentiating the different types of MRI protocols. Note that gray matter and white matter have essentially equal proton densities, thus limiting the usefulness of this technique for differentiating normal anatomy. Look at the vitreous cavity and the CSF to make an initial evaluation of the weighting of a scan. If normal vitreous and CSF are hypointense, the scan is T1-weighted. If the CSF and vitreous are bright, then the scan is likely a T2-weighted study.
There is insufficient appreciation of the ability of MRI to adequately image bone and detect fractures. It is true that water molecules in cortical bone are tightly bound and few in number, producing no detectable signal. The black image of cortical bone can be appreciated as having contrast with regard to adjacent medullary bone, muscle, and fat. Moreover, hyperintense signals due to the intrusion of hemorrhage and edema into the normal low signal of cortical bone should alert to the presence of a fracture or tumor. Tissues that inherently have a shorter T1 will have greater signal intensity at a given TR than those with a longer T1 Figure Fat has a short T1, while gray matter, white matter, muscle, and CSF demonstrate increasingly long T1 relaxation times.
T1-weighted images may provide better anatomic detail than T2-weighted images T2WI because the shorter time required to acquire images means less artifact induced by movement, including vascular pulsations of the brain. T1 weighting is particularly good for imaging normal anatomic detail in the brain and orbit Figures to Excellent contrast is observed between the high signal intensity of fat and the less intense signals of muscle and vessels.
T1-weighted MRI can be useful for evaluating choroidal melanoma because the stable free radicals within melanin have paramagnetic properties that decrease both T1 and T2 relaxation times. The differential diagnosis for substances that are bright on the precontrast T1-weighted MR images includes proteinaceous fluid, fat, blood depending on the stage , calcification, and melanin. The posterior pituitary gland can also demonstrate high signal on T1 Figure The ophthalmologist cannot always rely on fat being bright on T1WI because special suppression sequences can suppress the normal hyperintense signal of fat on T1WI i.
The ophthalmologist must be familiar with these special scanning protocols to avoid misdiagnosis. The GM is relatively darker than the WM because it contains more water. Also compare Figure Reprinted with permission from Latchaw RE. Louis, Mo: Mosby—Year Book; Figure Axial T1-weighted left and T2-weighted right MRI of the cervical spinal cord just below the cervicomedullary junction demonstrating normal anatomy. Note the vertebral artery VA flow voids surrounded by bright cerebrospinal fluid signal on the T2-weighted study.
One common lesion in this location might be a lateral medullary infarct producing a Wallenberg syndrome clinically. LPM, lateral pterygoid muscle; MM, masseter muscle. Note this is at the level of the sixth cranial nerve nucleus at the facial colliculus FaC visible bilaterally formed by the seventh cranial nerve wrapping around the sixth cranial nerve nucleus. Note how the vitreous and cerebrospinal fluid are dark on the T1-weighted image and bright on the T2-weighted image. A dorsal midbrain lesion might produce light near dissociation of the pupils, an upgaze paresis, and convergence-retraction nystagmus on attempted upgaze.
The optic nerves ON can be seen approaching the optic chiasm OC. The optic tracts OT are better visualized on the T2-weighted image when surrounded by bright cerebrospinal fluid CSF signal. The oculomotor nerve nucleus is located in the dorsal midbrain at the level of the superior colliculus. Axial T1-weighted left and T2-weighted right MRI demonstrating normal anatomy at the level of third ventricle V3 between the two thalami T. A thalamic lesion might produce a supranuclear vertical gaze palsy.
The cistern of velum interpositum VICi is an anterosuperior extension of the quadrigeminal plate cistern. Note the dark cerebrospinal fluid signal in the lateral ventricles on T1-weighted image and bright CSF signal on T2-weighted image. The genu anterior and splenium posterior of the corpus callosum CC are represented. The internal cerebral veins ICV are paired midline structures located in the roof of the third ventricle. Note the septum pellucidum SP , which may be absent in entities such as septo-optic dysplasia.
White matter of the centrum semiovale CeS ovals can be affected by demyelinating lesions such as multiple sclerosis. The central sulcus CS separates the precentral motor cortex single dot from the postcentral sensory cortex double dots. FL, frontal lobe; PL, parietal lobe. The inferior oblique muscle IO can be seen at this level.
It arises from the medial aspect of the inferior orbital rim. Also seen is the infraorbital nerve IoN within the infraorbital canal in the floor of the orbit right. Coronal T1-weighted left and T2-weighted right MRI demonstrating normal anatomy also in the mid-globe G region slightly posterior to Figure Note the lacrimal gland LG evident in the superotemporal orbit. Coronal T1-weighted left and T2-weighted right MRI demonstrating normal anatomy at the level of the posterior globe.
Note the various sizes of the extraocular muscles seen in cross section. Note the dark cerebrospinal fluid signal on the T1-weighted image immediately surrounding the normal optic nerve ON within the bright fat signal of the orbit. Note the extraocular muscles form the annulus of Zinn AZ at this level. Coronal T1-weighted left and T2-weighted right MRI demonstrating normal posterior orbital apex anatomy.
Also note the anterior clinoid process AC , which is bright on T1-weighted imaging due to bone marrow fat within the process. Note the optic chiasm in the suprasellar cistern SCi. Note the supraclinoid internal carotid artery ICA just prior to its bifurcation. The pituitary stalk PS is also seen. The pituitary gland PiG is seen within the sella turcica and lies approximately 10 mm below the optic chiasm. Before visual field defects become apparent, pituitary lesions must be large enough to break through the diaphragma sellae and extend into the suprasellar cistern, where they compress the visual pathway commonly producing bitemporal hemianopic field defects.
Coronal T1-weighted left and T2-weighted right MRI at the level of the third ventricle V3 demonstrating normal anatomy. Cerebrospinal fluid within the Meckel caves MC is visualized bilaterally dark on T1- and bright on T2-weighted imaging. Note several structures of the inner ear at this level, the semicircular canal ScC and vestibule Ve , seen best on the T2-weighted image.
The tentorium cerebelli TC is a crescentic, arched, duplicated dural membrane that covers the cerebellum and supports the occipital lobe. Fat suppression sequences allow better visualization of underlying pathologic lesions on T1WI of the orbit, particularly after contrast has been administered. Without fat suppression, the overlying bright fat T1 signal might obscure underlying contrast enhancement e.
Parasagittal T1weighted MRI demonstrating normal anatomy. Parasagittal T1weighted MRI demonstrating normal orbital anatomy. Fat suppression techniques can also confi rm the content of fat-containing lesions, such as dermoid cysts Figure and lipomas. Although the ophthalmologist does not have to order T1 fat-suppressed images, they should strongly consider fat suppression for all postcontrast orbital MR scans e. At most centers, postcontrast imaging with fat suppression is an established protocol. Ophthalmologists should be aware that inadequate or incomplete fat suppression can be caused by metallic artifact e.
Parasagittal T1-weighted MRI demonstrating normal anatomy. Note the S-shaped course of the internal carotid artery ICA , which marks the location of the cavernous sinus. Also note the basilar artery BA flow void black. Increased T2 signal is demonstrated within the pineal gland consistent with a pineal cyst. It may be difficult to differentiate the inferior orbit from the superior orbit without identifying adjacent structures such as the paranasal sinuses.
Axial T1-weighted left and T2-weighted right MRI of the normal orbit at a level just inferior to the optic nerve. Note the course of the optic nerve ON through the optic canal. The fourth cranial nerve CN4 decussates and exits the brainstem dorsally in the region of the superior medullary velum SMV , beneath the inferior colliculus. Traumatic fourth cranial nerve palsies have been attributed to injury in the area of the posterior decussation from contact with the edge of the tentorium cerebelli. Note the optic chiasm OC and adjacent structures. Careful review of other sequences might confi rm that the abnormal signal is indeed artifact.
Note the melanoma is bright on unenhanced T1-weighted imaging and dark on T2-weighted imaging consistent with its melanin content. Sagittal T1-weighted MRI demonstrating the normal bright signal of the posterior pituitary gland arrow. Axial postcontrast T1-weighted MRI with fat suppression demonstrating normal anatomy of the inferior orbit. Axial postcontrast T1-weighted MRI with fat suppression demonstrating normal anatomy of the mid-orbit. Note the loss of the usual bright fat signal in the orbit and subcutaneous tissue with this fat suppression sequence compare Figure , left.
Also note enhancement of the highly vascular extraocular muscles and choroid Ch. Axial postcontrast T1-weighted MRI with fat suppression demonstrating normal orbital anatomy at the level of the optic chiasm OC.
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