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Show Journal of Clilliml Neuro-ophthalmology 7(2): 96-97, 1987. Editorial Comments Imaging in Neuro-ophthalmological Disorders © 1987 Raven Press, New York The article by Zermeno et a1. in this issue of the Journal requires a few comments concerning imaging in neuro-ophthalmological disorders. Widespread experience with magnetic resonance imaging (MRI) points out the increased sensitivity of this technique over computed tomography (CT) when abnormalities of brain parenchyma are suspected. This is particularly obvious in imaging of the posterior fossa and brainstem. With CT, the combination of streak artifacts from the petrous bones and the relatively low contrast resolution of CT makes MRI a preferred imaging procedure for brainstem abnormalities. Only when acute bleeding is suspected does CT have an advantage over MRI. This advantage is related to the fact that acute bleeding can be unequivocally diagnosed when an area of increased density is seen on CT. With MRI the situation is more complex, since the signal is related to the duration of the bleeding, the breakdown products of the blood, the pulse sequence used, and the field strength of the magnet. Since most infarcts are nonhemorrhagic, MRI is preferred in detecting the presence of vascular disease and for demyelinating processes, early neoplastic and inflammatory changes, and congenital abnormalities. With those points in mind, one should look carefully at the article by Zermeno et a1. The authors believed that their patient was unusual because in those instances where a midbrain infarct is unilateral, there is insufficient brain damage to cause somnolent mutism. This latter situation requires a larger lesion, i.e., one that is bilateral, in order to affect the level of consciousness. The author postulates that in order for just a unilateral infarct to have caused this combination of somnolent mutism and unilateral third nerve palsy, there must have been sufficient edema elicited by the infarct to cause compression of the contralateral reticular activating system. The lack of definite edema on CT (note the normal density of the contralateral midbrain and the lack of compression of the ambient cisterns) and the fact that this patient continued to deTJoonstrate somnolent mutism 4 weeksafta the acute event (long after the effects 96 of the edema should have diminished) suggests that the CT did not completely display the pathological process that occurred. Since the reticular activating system has a wide anatomic distribution in the brainstem, a more likely explanation is that the CT simply did not show the total area of stem involvement. Given the clinical setting, MRI would probably have shown bilateral signal abnormalities, one in the region where the CT is positive as shown in Fig. 1 and the other in the contralateral portion of the stem, at a distance, removed from the right third nerve nucleus. When MRI is not available, thin CT sections (~5 mm) are appropriate under the circumstances described in this case and should be done as part of the initial study. Coronal CT is frequently a helpful complimentary examination, particularly when the findings on routine axial CT are uncertain, when the neurological findings do not match the CT findings, or when artifacts are present on the initial study. Post i.v. contrast CT is helpful to rule out an enhancing mass that may have clinically mimicked an infarct and to grossly delineate the posterior fossa arterial circulation. CT performed in this manner is recommended as the first study only when there is no means of obtaining an MRI. While MR is superior to CT in detecting brain abnormalities, CT remains the procedure of choice in nearly all suspected orbital abnormalities. The bony details of the orbit are better defined with CT, and the thinner cuts available with high-resolution CT units allows sections down to 1.5 mm to be obtained, thus achieving greater spatial resolution. The chemical shift phenomenon at the boundary of the orbital fat and the optic nerve causes bothersome artifacts on MR!. Even with the use of orbital surface coils, the time of scanning, particularly on T2 weighted images (-15-20 min) introduces the problem of globe movement, which distorts the image. In the future, we can look forward to thinner MRI sections on high field strength systems and to decreased scanning time as a consequence of the many newly introduced EDITORIAL COMMENTS 97 J. Lawton Smith, M.D. Robert M. Quencer, M.D. TABLE 1. Choice of imaging procedure in cranial and orbital abnormalities (a neuroophthalmo/ ogical-neuroradio/ogical consensus) two heads together, and the following is presented to help the clinician who at times now faces a dilemma as whether to order a computed tomographic scan or a magnetic resonance scan. It is hoped that the table will help the clinician. Grading: + + highly favored, + favored, = equal. • Based on plain magnetic resonance, Le., no paramagnetic contrast agent. b Magnetic resonance becomes an even more favored procedure when there is disease in the posterior fossa, especially the brainstem. The favored position of magnetic resonance relates mainly to its sensitivity in detecting the presence of these lesions, not in determining their causes. c Procedure of choice based on high-resolution computed t~mography, done pre- and post-enhancement, and high field strength magnetic resonance technology available in January 1987. Future advances in magnetic resonance will in all likelihood, enhance the position of magnetic reso: nance relative to computed tomography (A.M.a.). + ++ + + + ++ ++ ++ ++ + ++ + Computed tomography (pre and post Magnetic enhancement) resonance" ++ ++ + + + ++ + + ++ Location/disease Orbit Orbital mass Optic nerve abnormality Optic canal Intrasellar abnormality (microadenomas) Suprasellar mass with chiasm compression Intrinsic chiasmal lesion Bony lesions of skull vault or skull base Extra-axial lesions Meningioma Neurinoma Hematoma Acute Chronic Cavernous sinus Intra-axial lesionsb Tumor Inflammatory Multiple sclerosis Metastases Infarction Hematoma Acute Chronic Leukodystrophies Atrophy Hydrocephalus Vascular disease Infarction AV malformation Aneurysm Sinus thrombosis Dural fistula Congenital brain malformation Neuroimaging fast scanning techniques. Time will tell whether these improvements will cause MRI to replace CT as the preferred means of imaging the orbit. For now, orbital MRI does have a few limited advantages. It can image the orbital directly in any plane, including oblique imaging. MRI uses no radiation and, as presently used, has no known adverse affects. This is an advantage when a patient may have to be imaged many times over the course of several years for an intraorbital abnormality. In this instance, the total radiation dose to the lens is a clinical consideration, and MRI would be a better way of following up the orbital lesion. In addition, any intraorbital process that can spread through the optic canal, such as an optic nerve sheath meningioma or an optic glioma, should be studied with MRI. We have found changes in the intracranial portion of the optic nerve that CT could not detect. Fluid accumulation within the optic sheath is better seen with MRI than Cf, although A-mode echography remains the best and least expensive way of detecting such fluid. Recent work has attempted to differentiate between various types of orbital masses by their MRI signal characteristics-Le., comparing masses that have long versus short T2 relaxation times-but not enough data have been accumulated to show the efficacy of this approach. In summary, the clinician must not assume that CT will show the full extent of a brain abnormality. MRI is far more sensitive and in most cases should replace Cf as the preferred initial study of the brain. Only in the orbit does Cf hold an advantage, but this may be short-lived, given the rapid technological advances that have recently occurred in MRI. MRI should be given first priority when studies of the brain are ordered. Robert M. Quencer, M.D. Departments of Radiology and Neurological Surgery University of Miami School of Medicine Miami, Florida There is so much emphasis on the advantages of magnetic resonance imaging in the literature these days that high-resolution computed tomography often appears to suffer deprecation by comparative neglect. This is the personal opinion of one of us a.L.S.). It therefore seemed appropriate to put JClin Neuro-ophthalmol, Vol. 7, No.2, 1987 EDITORIAL COMMENTS 97 J. Lawton Smith, M.D. Robert M. Quencer, M.D. TABLE 1. Choice of imaging procedure in cranial and orbital abnormalities (a neuroophthalmo/ ogical-neuroradio/ogical consensus) two heads together, and the following is presented to help the clinician who at times now faces a dilemma as whether to order a computed tomographic scan or a magnetic resonance scan. It is hoped that the table will help the clinician. Grading: + + highly favored, + favored, = equal. • Based on plain magnetic resonance, Le., no paramagnetic contrast agent. b Magnetic resonance becomes an even more favored procedure when there is disease in the posterior fossa, especially the brainstem. The favored position of magnetic resonance relates mainly to its sensitivity in detecting the presence of these lesions, not in determining their causes. c Procedure of choice based on high-resolution computed t~mography, done pre- and post-enhancement, and high field strength magnetic resonance technology available in January 1987. Future advances in magnetic resonance will in all likelihood, enhance the position of magnetic reso: nance relative to computed tomography (A.M.a.). + ++ + + + ++ ++ ++ ++ + ++ + Computed tomography (pre and post Magnetic enhancement) resonance" ++ ++ + + + ++ + + ++ Location/disease Orbit Orbital mass Optic nerve abnormality Optic canal Intrasellar abnormality (microadenomas) Suprasellar mass with chiasm compression Intrinsic chiasmal lesion Bony lesions of skull vault or skull base Extra-axial lesions Meningioma Neurinoma Hematoma Acute Chronic Cavernous sinus Intra-axial lesionsb Tumor Inflammatory Multiple sclerosis Metastases Infarction Hematoma Acute Chronic Leukodystrophies Atrophy Hydrocephalus Vascular disease Infarction AV malformation Aneurysm Sinus thrombosis Dural fistula Congenital brain malformation Neuroimaging fast scanning techniques. Time will tell whether these improvements will cause MRI to replace CT as the preferred means of imaging the orbit. For now, orbital MRI does have a few limited advantages. It can image the orbital directly in any plane, including oblique imaging. MRI uses no radiation and, as presently used, has no known adverse affects. This is an advantage when a patient may have to be imaged many times over the course of several years for an intraorbital abnormality. In this instance, the total radiation dose to the lens is a clinical consideration, and MRI would be a better way of following up the orbital lesion. In addition, any intraorbital process that can spread through the optic canal, such as an optic nerve sheath meningioma or an optic glioma, should be studied with MRI. We have found changes in the intracranial portion of the optic nerve that CT could not detect. Fluid accumulation within the optic sheath is better seen with MRI than Cf, although A-mode echography remains the best and least expensive way of detecting such fluid. Recent work has attempted to differentiate between various types of orbital masses by their MRI signal characteristics-Le., comparing masses that have long versus short T2 relaxation times-but not enough data have been accumulated to show the efficacy of this approach. In summary, the clinician must not assume that CT will show the full extent of a brain abnormality. MRI is far more sensitive and in most cases should replace Cf as the preferred initial study of the brain. Only in the orbit does Cf hold an advantage, but this may be short-lived, given the rapid technological advances that have recently occurred in MRI. MRI should be given first priority when studies of the brain are ordered. Robert M. Quencer, M.D. Departments of Radiology and Neurological Surgery University of Miami School of Medicine Miami, Florida There is so much emphasis on the advantages of magnetic resonance imaging in the literature these days that high-resolution computed tomography often appears to suffer deprecation by comparative neglect. This is the personal opinion of one of us a.L.S.). It therefore seemed appropriate to put JClin Neuro-ophthalmol, Vol. 7, No.2, 1987 |