ANNUAL REVIEWS
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Ann. Rev. Med. 1979. 30:189-98 Copyright © 1979 by Annual Reviews Inc. All rights reserved
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
COMPUTED TOMOGRAPHY
.7315
IN NEUROLOGIC DIAGNOSIS Anne G. Osborn, MD. Department of Radiology, University of Utah College of Medicine, Salt Lake City, Utah 84132
INTRODUCTION The instant, universal acceptance of computed tomography (CT) in the evaluation of a wide spectrum of intracranial lesions attests to its great value in neurologic diagnosis. The reliability of CT in demonstrating a broad range of intracerebral disorders has been well established (1). CT has been said to provide more diagnostic information on the general spectrum of intracranial disease than any other single neuroradiologic examination (2) and for the first time permits direct visualization of important but formerly obscure areas such as the orbit and posterior fossa. This report reviews the basic physical principles of CT scanning, then briefly discusses the utility, economics and efficacy, limitations, and new developments in cranial CT as an adjunct to neurologic diagnosis.
BASIC PRINCIPLES OF CT SCANNING The ability to distinguish small differences in the x-ray attenuation of various intracranial tissues-ordinarily not distinguishable with conven tional neurodiagnostic procedures-in large part accounts for the unparal leled success of CT (3). A series of narrowly collimated x-ray exposures is made of an object as viewed from many different angles, and the data thus acquired is then used to reconstruct sectional images that display the object in cross section. The first commercially available CT scanners employed a translate-rotate data-gathering format; i.e. after one complete transverse scan, the x-ray tube and its highly sensitive detector system were rotated 1° around the object to be imaged and the scan repeated, usually through an arc of 180°. 189
0066-4219/79/0401-0189$01.00
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
190
OSBORN
Thus 180 different data profiles were obtained of the imaged object and the entire section was then reconstructed by a computer using a convolution algorithm or "filtered back projection" method (3, 4). More recent developments in CT technology include fan-beam systems (where the x-ray source and detectors are rotated synchronously), fixed or stationary detector systems with a fan-beam radiation source, and (still in the future) fixed-source, fixed-detector scanners. These innovations have both improved resolution and reduced the requisite scan time from several minutes to only a few seconds. UTILITY, EFFICACY, AND ECONOMICS
Virtually all neurodiagnostic techniques, with the possible exception of EEG, have been used less often as the -capabilities of CT have become known to clinicians (5). CT scans in many instances obviate the need for other, more invasive diagnostic procedures such as cerebral angiography. In other cases, CT has added important complementary information that facilitates patient management (2). Some authors estimate that CT is approximately 9�98% accurate in imaging most intracranial lesions and is thus one of the most effective medical detection systems available (6). The utility of CT in the assessment of intracranial trauma and/or hemorrhage, degenerative diseases, hy drocephalus, mass lesions, and congenital malformations has been thor oughly documented (7-13). CT has also proved particularly helpful for follow-up studies on patients with cerebral tumors undergoing surgery and/or radiation therapy (14), ventricular shunting for hydrocephalus (15), in evaluating the treatment course of patients with cerebral abscess (16), and for delineating lesions in the orbit, sella, and posterior fossa. CT has profoundly altered the management of posterior fossa tumors in children, many of whom have been explored surgically after investigation by CT only (17). On the other hand, CT has resulted in little demonstrable improve ment either in the cost of care or management of patients with cerebrovas cular disease (18). Abrams & McNeil (19-21) pointed out that the remoteness of health outcome from the point of a diagnostic test such as CT leads to interim reliance on more proximate measures of that test's efficacy, such as (a) the degree to which CT furnishes new or additional diagnostic information not otherwise available; (b) its accuracy; (c) its effect on the morbidity and mortality of diagnostic and therapeutic procedures; (d) its impact on treat ment planning; and (e) changes in cost and/or savings incident to its use. While the lack of hard, controlled data makes evaluation of many efficacy studies hazardous, the efficacy of CT in difficult management problems
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
191
common to general neurologic practice has nevertheless been studied. For example, because of the low yield of therapeutically significant lesions, it has rarely been justifiable to subject those chronic childhood seizure patients without progressive neurological defect to the morbidity of pneumoenceph alography or arteriography. As a noninvasive screening procedure, CT has disclosed structural abnormalities in 30% of such cases. While the most common abnormality detected is either focal or generalized atrophy (15%), only about 2% of the abnormalities discovered by CT are potentially of therapeutic significance (22). However, the demonstration of either a static process or findings of a normal CT may be helpful in some cases to both family and physician. The utility of CT in the assessment of childhood macrocrania has also been documented (23). Only 18% of these sometimes puzzling patients had a normal CT. The remainder had significant hydrocephalus, neoplasms, subdural hematomas or effusions, or a variety of abnormalities involving the
cerebral parenchyma. Patients presenting with the chief comp"laint of chronic headache or a presumptive diagnosis of temporal lobe epilepsy form a significant portion of outpatients scheduled for CT as a primary or "screening" procedure. In patients with headache and no objective neurologic findings, CT scans are usually normal. In contrast, a recent study found 34% with headache and associated objective neurologic findings had abnormal CT scan (55% were regarded as potentially treatable; the majority of the others had demonstra ble cortical atrophy). Assessing the cost at $240 for each examination, the calculated lower bounds for costs of case findings in these groups were $4,363 and $500 per patient respectively (24). At one university hospital, 35% of all CT scans requested resulted in a positive, clinically important diagnosis for an overall case-finding cost of $800. Case-finding costs here varied from a low (and therefore high positive yield) of $411 for patients in coma to $3,500 for patients with headaches as their only indication for CT scanning (24). Since CT became available, one study reports a progressive decrease in the proportion of cerebral angiograms that are normal (from 36 to 16%) (25). Hence CT is also making those other neurodiagnostic evaluations that are still performed more efficient for patients with suspected lesions. Prelim inary data also show that in many cases, CT can replace or reduce more "conventional" diagnostic studies, resulting in significant reduction in hos pital stay and cost (26), particularly in certain patient groups such as those with extracerebral collections or intracerebral neoplasms. Problems that still need to be resolved include proper utilization, further demonstra tion of cost-effectiveness, and evaluation of future technological develop ments.
192
OSBORN
COMPLEMENTARY ROLE OF CT AND OTHER NEURODIAGNOSTIC PROCEDURES While it has replaced more invasive procedures in some instances, the complementary role of
CT and additional neuroradiologic studies has been (2). The accuracy of CT varies according to the
well documented in others
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
organ system examined as well as the locus and specific nature of the lesion
(27). CT
may detect blood in the subarachnoid space, intracerebral or in
traventricular hematomas, the thrombosed portion of a giant aneurysm, etc. However, angiography in the patient with aneurysm and subarach noid hemorrhage is still necessary to visualize the base of the aneu rysm, its relationship to adjacent vessels, and the presence of vascular spasm.
CT can demonstrate areas of cerebral ischemia or infarction, if they are of sufficient size, and can easily distinguish between hemorrhagic and is chemic infarction. Here again, angiography is necessary to delineate precise
vascular details such as vessel occlusions, stenosis, ulcerating plaques, and patterns of collateral blood flow. CT alone is usually sufficient as a diagnostic procedure in patients with acute head injury and can accurately distinguish traumatic edema from frank intercerebral hematoma. Angiography may be necessary to delineate vessel lacerations, traumatic fistulae, and isodense (resolving) balanced sub dural hemotomas. While
CT is more accurate than angiography or radionuclide scans in
detecting both primary and metastatic neoplasms, angiography is necessary for the delineation of feeding vessels, occluded dural sinuses, etc, and is often helpful in distinguishing between the many various entities (such as infarction) that may resemble a neoplasm (Figure
1).
Large sellar and suprasellar masses are easily detected by CT. But angiog raphy and/or pneumonencephalography may be necessary both for differ ential diagnosis (Figure
2) and to delineate the exact relationship of a given
mass to adjacent structures such as the optic chiasm and anterior recesses of the third ventricle. Pituitary microadenomas are also delineated best by pneumoencephalography combined with thin-section hypocycloidal tomog raphy. While
CT has proved exceptionally helpful in the detection of posterior
fossa lesions, cerebellopontine-angle tumors smaller than one centimeter are best delineated by contrast cisternography. Rarely, air studies with tomog raphy may be necessary to distinguish between lesions such as cystically dilated fourth ventricle or cystic fourth ventricular neoplasm, which can have identical appearances on
CT scan.
193
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
Figure 1
A.
Contrast-enhanced cr scan showing a large left temporal mass lesion (arrows)
with some peripheral edema and left to right subfalcine shift of the lateral ventricles. Does the lesion represent a tumor?
B.
Left common carotid angiogram, lateral view, midarterial
phase. Same patient as A. A partially thrombosed giant middle cerebral artery aneurysm
(arrows) is present, accounting for the unusual cr scan.
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
194
Figure 2
OSBORN
cr scans demonstrating a variety of contrast-enhancing lesions (arrows) in the
suprasellar area. A. partiaJIy calcified; D.
Giant aneurysm; B. Meningioma; E.
Hypothalamic glioma; C.
Craniopharyngioma,
Pituitary adenoma with suprasellar extension; F.
Third ventricular colloid cyst.
LIMITATIONS OF CT SCANS While highly efficacious in many instances, various limitations are present in CT scanning (21). Some of these limitations are imposed by the system itself. Lesions below the inherent limits of resolution of the system itself either in size or attenuation difference from surrounding normal tissue may not be detected at all. Some examples include small infarcts or metastases and early infiltrating neoplasms. Lesions may be obscured by adjacent structures, particularly at the skull base. Artifacts introduced by voluntary or biological motion can either obscure abnormalities actually present or create the appearance of non existent lesions. Even with the use of contrast enhancement, vascular details
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
195
such as small vessel occlusions or stenosis, small to medium size aneurysms, and the vascular supply to vascular malformations or neoplasms may not be delineated. Other limitations on
CT scanning are observer imposed. Normal struc
tures (such as a prominent jugular tubercle) may be erroneously interpreted as a lesion (acoustic neurinoma). Failure to recognize such factors as par
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
tial-volume effect (inclusion of only part of a lesion or structure in a section of finite thickness) may also lead to misdiagnosis. Failure to obtain a com plete scan series, including a vertex pair, may result in failure to visualize as much as
10-15%
of the total intracranial volume
(28).
As the general experience with CT has increased, that virtually "anything can look like anything else" has become a caveat. Infarcts and resolving hematomas can be indistinguishable from neoplasm, neoplasms can resem ble abscesses, abscesses can be identical to cavitating metastases, and metas tases can mimic primary tumors (Figure 1). A variety of completely different pathologic entities in a similar location may closely resemble one another and therefore require additional diagnostic procedures (such as angiography) to assist in the radiographic differential diagnosis (Figure
2).
CONCLUSION: NEW DIRECTIONS IN CRANIAL CT SCANNING The impending development of ultra rapid scanning systems will make both dynamic studies and gated scans of rapidly moving objects technically feasible. Currently available rapid scan units render heavy sedation or anesthesia unnecessary in virtually all uncooperative or pediatric age pa tients. While computer reconstructions of frontal and lateral views of the brain from axial scan data have been performed experimentally since the advent of
CT scanning, regenerated coronal and sagittal slices are now a standard (29). With the advent of large
feature of some commercially available units
aperture air-gap body scanners, true or nonregenerated scans with signifi cantly imprOVed resolution (Figure
3)
can be obtai.ned
Techniques for stereotaxic CT-guided surgery are being developed
(31).
Relatively simple computer graphics systems for digitization of two-dimen sional information and its display in three dimensions have been used experimentally (R. Brown and W. R. Glenn, personal communication) and should be available for general use in the near future. Body CT is being utilized with increasing frequency in the evaluation of traumatic spinal injuries (32), the narrow canal syndrome, and in a variety of congenital lesions such as craniovertebral anomalies and spinal dysraph-
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
196
OSBORN
ism (33). The availability of metrizamide will also make CT-assisted mye lography and cisternography a widely available procedure (34). Finally, positron or emission CT scanning (ECAT) systems capable of providing high contrast, high resolution quantitative images of metabolic or physiologic parameters in the brain are being developed (35). This new technique promises to be particularly helpful in those neurologic disorders such as acute demyelinating diseases or small infarcts where routine trans mission CT is often negative.
Figure 3
Direct sagittal cr scan of the face and paranasal sinuses. The scanner window
width was set to optimize visualization of structures such as the ethmoid sinuses (arrows) .
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
197
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
Literature Cited 1. Evens, R. G. 1976. New frontier for radiology: Computed tomography. Am. J. Roentgenol. 126:1117-29 2. Davis, K. R., Poletti, C. E., Roberson, G. H., Tadmor, R., Kjellberg, R. N. 1978. Complementary role of computed tomography and other neuroradiologic procedures. Surg. Neural 8:437-46 3. Ter-Pogossian, M. M. 1977. Computer ized cranial tomography: Equipment and physics. Semin. Roentgenol 12: 13-25 4. Brooks, R. A., DiChiro, G. 1976. Prin ciples of computer assisted tomography (CAT) in radiographic and radioiso topic imaging. Phys. Med. BioL 21:689732 5. Baker, H. L. Jr. 1975. The impact of computed tomography on neuroradi ologic practice. Radiology 116:637-40 6. Davis, D. 0. 1977. CT in the diagnosis of su ratentOrial tumors. Semin. Roent gena. 12:97-108 7. Baker, H. L. Jr., Campbell, J. K., Houser, O. W., Reese, D. F. 1975. Early experience with the EMI scanner for study of the brain. Radiology 116: 327-33 8. Harwood-Nash, D. C. 1977. Congenital craniocerebral abnormalities and com puted tomography. Semin. Roentgenol. 12:39-51 9. Huckman, M. S., Fox, J. H., Ramsey, R. G. 1977. Computed tomography in the dia�osis of degenerative diseases of the br81n. Semin. Roentgenol. 12:63-75 10. Davis, K. R., Taveras, J. M., Roberson, G. H., Ackerman, R. H., Dreeslath, J. N. 1977. Computed tomography in head trauma. Semin. Roentgenol 12: 53-62 11. EI Gammal, T., Allen, M. B. Jr., Lott, T. 1977. Computer assisted tomogra phy and pneumoencephalography in nontumorous hydrocephalus in infants and children. J. Comput. Assist. To mogr. 1:204-10 12. Zimmerman, R. A., Bilaniuk, L. A., GenneraIIi, T., Bruce, D., Dolinskas, C., Uzzell, B. 1978. Cranial computed tomography in diagnosis and manage ment of acute head trauma. Am. J. Roentgenol. 131:27-34 13. Larson, E. B., Omenn, G. S., Magno, J. 1978. Impact of computed tomography in the care of patients with suspected hydrocephalus. Am. J. Roentgenol 131:41-44 14. Kretzchmar, K., Aulich, A., Schindler, E., Lange, S., Grumme, T., Muse, W. 1978. The diagnostic value of CT for
y
radiotherapy
of
cerebral
Neuraradiology 14:245-50
tumors.
15. Palmieri, A., Menichelli, F., Pasquini, U., Salvatini, U. 1978. Role of com puted tomography in the postoperative evaluation of Infantile hydrocephalus. Neuraradiology 14:257-62 16. Whelan, M. A., Hilal, S. K. 1978. The
computed tomography of brain ab scesse s. Presented.at 16th Ann. Meet.
Am. Soc. Neuroradiol., New Orleans 17. Boltshauser, E., Hamalatha, H., Grant, D. N., Tiel, K. 1977. Impact of comput erized axial tomography on the man agement of posterior fossa tumors in childhood. J. Neural Neurasurg. Psy chiatry 40:209-13 18. Larson, E. B., Omenn, G. S., Loop, 1. W. 1978. Computed tomography in pa tients with cerebrovascular disease: im pact of a new technology in patient care. Am. J. Roentgenol 131:35-40 19. Abrams, H. L., McNeil, B. J. 1978. Medical implications of computed tomography I. N. Engl. J. Med. 298: 255-61 20. Abrams, H. L., McNeil, B. J. 1978. Medical implications of computed tomography II. N. Engl. J. Med. 298: 310-18 21. Abrams, H. L., McNeil, B. J. 1978. Computed tomography: cost and effi cacy implications. Am. J. Roentgenol 131:81-87 22. Backman, D. S., Hodges, F. J. III, Fre man, J. M. 1976. Computed axial tomography in chronic seizure disor ders of childhood. Pediatrics 58:828-32 23. Kingsley, D., Kendall, B. E. 1978. The value of computed tomography in the evaluation of the enlarged head. Neuraradiology 15:59-71 24. Knaus, W. A., Davis, D. O. 1978. Utili zation and cost-effectiveness of cranial computed tomography at a university hospital. J. Comput. Assist. Tomogr. 2:209-14 25. Larson, E. B., Omenn, G. S., Margulis, M. T., Loop, J. W. 1977. Impact of computed tomography on utilization of cerebral angiograms. Am. J. Roent genol 129:1-3 26. Bahr, A. L., Hodges, F. J. III. 1978. Efficacy of computerized tomography of the head in changing patient care and health cost: A retrospective study. Am. J. Roentgenol 131:45-49 27. Davis, K. R., Taveras, J. M., Roberson,. G. H., Ackerman, R. H. 1976. Some limitations of computed tomography in
198
OSBORN the diagnosis of neurological diseases.
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
Am. J. Roentgenol 127:111-23
28. Wing, S. D., Osborn, A. G., Wing, R. W. 1978. The vertex scan: An impor tant component of cranial computed tomography. Am. J. Roentgenol 130: 765-67 29. Rosenbaum, H. E. 1977. Computerized tomographic scans of brain: Three dimensional. Arch. Neurol 34:386-87 30. Osborn, A. G., Anderson, R. E. 1978. Direct sagittal cr scans of the face and paranasal sinuses. Radiology. 129: 81-87 31. Bergstrom, M., Greitz, T. 1976. Stereo taxic computed tomography. Am. J. Roentgenol 127:167-70 32. Tadmor, R., Davis, K. R., Roberson,
G. H., New, P. F. J., Taveras, J. M. 1978. Computed tomographic evalu ation of traumatic spinal injuries. Radi ology 127:825-27 33. Wolpert, S. M., Scott, R. M., Carter, B. L. 1977. Computed tomography in spi nal dysraphism. Surg. Neurol. 8:199206 34. Drayer, B. P., Rosenbaum, A. E., Hyman, H. B. 1977. Cerebrospinal fluid imaging using serial metrizamide cr cisternography. Neuroradiology 13: 7-17 35. Phelps, M. E., Hoffman, E. J., Huang, S. C., Kuhl, D. E. 1978. A new comput erized tomographic imaging system for positron-emItting radiopharmaceuti cals. J. Nucl Med. 19:635-47
Further
Quick links to online content
Ann. Rev. Med. 1979. 30:189-98 Copyright © 1979 by Annual Reviews Inc. All rights reserved
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
COMPUTED TOMOGRAPHY
.7315
IN NEUROLOGIC DIAGNOSIS Anne G. Osborn, MD. Department of Radiology, University of Utah College of Medicine, Salt Lake City, Utah 84132
INTRODUCTION The instant, universal acceptance of computed tomography (CT) in the evaluation of a wide spectrum of intracranial lesions attests to its great value in neurologic diagnosis. The reliability of CT in demonstrating a broad range of intracerebral disorders has been well established (1). CT has been said to provide more diagnostic information on the general spectrum of intracranial disease than any other single neuroradiologic examination (2) and for the first time permits direct visualization of important but formerly obscure areas such as the orbit and posterior fossa. This report reviews the basic physical principles of CT scanning, then briefly discusses the utility, economics and efficacy, limitations, and new developments in cranial CT as an adjunct to neurologic diagnosis.
BASIC PRINCIPLES OF CT SCANNING The ability to distinguish small differences in the x-ray attenuation of various intracranial tissues-ordinarily not distinguishable with conven tional neurodiagnostic procedures-in large part accounts for the unparal leled success of CT (3). A series of narrowly collimated x-ray exposures is made of an object as viewed from many different angles, and the data thus acquired is then used to reconstruct sectional images that display the object in cross section. The first commercially available CT scanners employed a translate-rotate data-gathering format; i.e. after one complete transverse scan, the x-ray tube and its highly sensitive detector system were rotated 1° around the object to be imaged and the scan repeated, usually through an arc of 180°. 189
0066-4219/79/0401-0189$01.00
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
190
OSBORN
Thus 180 different data profiles were obtained of the imaged object and the entire section was then reconstructed by a computer using a convolution algorithm or "filtered back projection" method (3, 4). More recent developments in CT technology include fan-beam systems (where the x-ray source and detectors are rotated synchronously), fixed or stationary detector systems with a fan-beam radiation source, and (still in the future) fixed-source, fixed-detector scanners. These innovations have both improved resolution and reduced the requisite scan time from several minutes to only a few seconds. UTILITY, EFFICACY, AND ECONOMICS
Virtually all neurodiagnostic techniques, with the possible exception of EEG, have been used less often as the -capabilities of CT have become known to clinicians (5). CT scans in many instances obviate the need for other, more invasive diagnostic procedures such as cerebral angiography. In other cases, CT has added important complementary information that facilitates patient management (2). Some authors estimate that CT is approximately 9�98% accurate in imaging most intracranial lesions and is thus one of the most effective medical detection systems available (6). The utility of CT in the assessment of intracranial trauma and/or hemorrhage, degenerative diseases, hy drocephalus, mass lesions, and congenital malformations has been thor oughly documented (7-13). CT has also proved particularly helpful for follow-up studies on patients with cerebral tumors undergoing surgery and/or radiation therapy (14), ventricular shunting for hydrocephalus (15), in evaluating the treatment course of patients with cerebral abscess (16), and for delineating lesions in the orbit, sella, and posterior fossa. CT has profoundly altered the management of posterior fossa tumors in children, many of whom have been explored surgically after investigation by CT only (17). On the other hand, CT has resulted in little demonstrable improve ment either in the cost of care or management of patients with cerebrovas cular disease (18). Abrams & McNeil (19-21) pointed out that the remoteness of health outcome from the point of a diagnostic test such as CT leads to interim reliance on more proximate measures of that test's efficacy, such as (a) the degree to which CT furnishes new or additional diagnostic information not otherwise available; (b) its accuracy; (c) its effect on the morbidity and mortality of diagnostic and therapeutic procedures; (d) its impact on treat ment planning; and (e) changes in cost and/or savings incident to its use. While the lack of hard, controlled data makes evaluation of many efficacy studies hazardous, the efficacy of CT in difficult management problems
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
191
common to general neurologic practice has nevertheless been studied. For example, because of the low yield of therapeutically significant lesions, it has rarely been justifiable to subject those chronic childhood seizure patients without progressive neurological defect to the morbidity of pneumoenceph alography or arteriography. As a noninvasive screening procedure, CT has disclosed structural abnormalities in 30% of such cases. While the most common abnormality detected is either focal or generalized atrophy (15%), only about 2% of the abnormalities discovered by CT are potentially of therapeutic significance (22). However, the demonstration of either a static process or findings of a normal CT may be helpful in some cases to both family and physician. The utility of CT in the assessment of childhood macrocrania has also been documented (23). Only 18% of these sometimes puzzling patients had a normal CT. The remainder had significant hydrocephalus, neoplasms, subdural hematomas or effusions, or a variety of abnormalities involving the
cerebral parenchyma. Patients presenting with the chief comp"laint of chronic headache or a presumptive diagnosis of temporal lobe epilepsy form a significant portion of outpatients scheduled for CT as a primary or "screening" procedure. In patients with headache and no objective neurologic findings, CT scans are usually normal. In contrast, a recent study found 34% with headache and associated objective neurologic findings had abnormal CT scan (55% were regarded as potentially treatable; the majority of the others had demonstra ble cortical atrophy). Assessing the cost at $240 for each examination, the calculated lower bounds for costs of case findings in these groups were $4,363 and $500 per patient respectively (24). At one university hospital, 35% of all CT scans requested resulted in a positive, clinically important diagnosis for an overall case-finding cost of $800. Case-finding costs here varied from a low (and therefore high positive yield) of $411 for patients in coma to $3,500 for patients with headaches as their only indication for CT scanning (24). Since CT became available, one study reports a progressive decrease in the proportion of cerebral angiograms that are normal (from 36 to 16%) (25). Hence CT is also making those other neurodiagnostic evaluations that are still performed more efficient for patients with suspected lesions. Prelim inary data also show that in many cases, CT can replace or reduce more "conventional" diagnostic studies, resulting in significant reduction in hos pital stay and cost (26), particularly in certain patient groups such as those with extracerebral collections or intracerebral neoplasms. Problems that still need to be resolved include proper utilization, further demonstra tion of cost-effectiveness, and evaluation of future technological develop ments.
192
OSBORN
COMPLEMENTARY ROLE OF CT AND OTHER NEURODIAGNOSTIC PROCEDURES While it has replaced more invasive procedures in some instances, the complementary role of
CT and additional neuroradiologic studies has been (2). The accuracy of CT varies according to the
well documented in others
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
organ system examined as well as the locus and specific nature of the lesion
(27). CT
may detect blood in the subarachnoid space, intracerebral or in
traventricular hematomas, the thrombosed portion of a giant aneurysm, etc. However, angiography in the patient with aneurysm and subarach noid hemorrhage is still necessary to visualize the base of the aneu rysm, its relationship to adjacent vessels, and the presence of vascular spasm.
CT can demonstrate areas of cerebral ischemia or infarction, if they are of sufficient size, and can easily distinguish between hemorrhagic and is chemic infarction. Here again, angiography is necessary to delineate precise
vascular details such as vessel occlusions, stenosis, ulcerating plaques, and patterns of collateral blood flow. CT alone is usually sufficient as a diagnostic procedure in patients with acute head injury and can accurately distinguish traumatic edema from frank intercerebral hematoma. Angiography may be necessary to delineate vessel lacerations, traumatic fistulae, and isodense (resolving) balanced sub dural hemotomas. While
CT is more accurate than angiography or radionuclide scans in
detecting both primary and metastatic neoplasms, angiography is necessary for the delineation of feeding vessels, occluded dural sinuses, etc, and is often helpful in distinguishing between the many various entities (such as infarction) that may resemble a neoplasm (Figure
1).
Large sellar and suprasellar masses are easily detected by CT. But angiog raphy and/or pneumonencephalography may be necessary both for differ ential diagnosis (Figure
2) and to delineate the exact relationship of a given
mass to adjacent structures such as the optic chiasm and anterior recesses of the third ventricle. Pituitary microadenomas are also delineated best by pneumoencephalography combined with thin-section hypocycloidal tomog raphy. While
CT has proved exceptionally helpful in the detection of posterior
fossa lesions, cerebellopontine-angle tumors smaller than one centimeter are best delineated by contrast cisternography. Rarely, air studies with tomog raphy may be necessary to distinguish between lesions such as cystically dilated fourth ventricle or cystic fourth ventricular neoplasm, which can have identical appearances on
CT scan.
193
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
Figure 1
A.
Contrast-enhanced cr scan showing a large left temporal mass lesion (arrows)
with some peripheral edema and left to right subfalcine shift of the lateral ventricles. Does the lesion represent a tumor?
B.
Left common carotid angiogram, lateral view, midarterial
phase. Same patient as A. A partially thrombosed giant middle cerebral artery aneurysm
(arrows) is present, accounting for the unusual cr scan.
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
194
Figure 2
OSBORN
cr scans demonstrating a variety of contrast-enhancing lesions (arrows) in the
suprasellar area. A. partiaJIy calcified; D.
Giant aneurysm; B. Meningioma; E.
Hypothalamic glioma; C.
Craniopharyngioma,
Pituitary adenoma with suprasellar extension; F.
Third ventricular colloid cyst.
LIMITATIONS OF CT SCANS While highly efficacious in many instances, various limitations are present in CT scanning (21). Some of these limitations are imposed by the system itself. Lesions below the inherent limits of resolution of the system itself either in size or attenuation difference from surrounding normal tissue may not be detected at all. Some examples include small infarcts or metastases and early infiltrating neoplasms. Lesions may be obscured by adjacent structures, particularly at the skull base. Artifacts introduced by voluntary or biological motion can either obscure abnormalities actually present or create the appearance of non existent lesions. Even with the use of contrast enhancement, vascular details
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
195
such as small vessel occlusions or stenosis, small to medium size aneurysms, and the vascular supply to vascular malformations or neoplasms may not be delineated. Other limitations on
CT scanning are observer imposed. Normal struc
tures (such as a prominent jugular tubercle) may be erroneously interpreted as a lesion (acoustic neurinoma). Failure to recognize such factors as par
Annu. Rev. Med. 1979.30:189-198. Downloaded from www.annualreviews.org Access provided by Technische Universiteit Eindhoven on 01/24/15. For personal use only.
tial-volume effect (inclusion of only part of a lesion or structure in a section of finite thickness) may also lead to misdiagnosis. Failure to obtain a com plete scan series, including a vertex pair, may result in failure to visualize as much as
10-15%
of the total intracranial volume
(28).
As the general experience with CT has increased, that virtually "anything can look like anything else" has become a caveat. Infarcts and resolving hematomas can be indistinguishable from neoplasm, neoplasms can resem ble abscesses, abscesses can be identical to cavitating metastases, and metas tases can mimic primary tumors (Figure 1). A variety of completely different pathologic entities in a similar location may closely resemble one another and therefore require additional diagnostic procedures (such as angiography) to assist in the radiographic differential diagnosis (Figure
2).
CONCLUSION: NEW DIRECTIONS IN CRANIAL CT SCANNING The impending development of ultra rapid scanning systems will make both dynamic studies and gated scans of rapidly moving objects technically feasible. Currently available rapid scan units render heavy sedation or anesthesia unnecessary in virtually all uncooperative or pediatric age pa tients. While computer reconstructions of frontal and lateral views of the brain from axial scan data have been performed experimentally since the advent of
CT scanning, regenerated coronal and sagittal slices are now a standard (29). With the advent of large
feature of some commercially available units
aperture air-gap body scanners, true or nonregenerated scans with signifi cantly imprOVed resolution (Figure
3)
can be obtai.ned
Techniques for stereotaxic CT-guided surgery are being developed
(31).
Relatively simple computer graphics systems for digitization of two-dimen sional information and its display in three dimensions have been used experimentally (R. Brown and W. R. Glenn, personal communication) and should be available for general use in the near future. Body CT is being utilized with increasing frequency in the evaluation of traumatic spinal injuries (32), the narrow canal syndrome, and in a variety of congenital lesions such as craniovertebral anomalies and spinal dysraph-
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196
OSBORN
ism (33). The availability of metrizamide will also make CT-assisted mye lography and cisternography a widely available procedure (34). Finally, positron or emission CT scanning (ECAT) systems capable of providing high contrast, high resolution quantitative images of metabolic or physiologic parameters in the brain are being developed (35). This new technique promises to be particularly helpful in those neurologic disorders such as acute demyelinating diseases or small infarcts where routine trans mission CT is often negative.
Figure 3
Direct sagittal cr scan of the face and paranasal sinuses. The scanner window
width was set to optimize visualization of structures such as the ethmoid sinuses (arrows) .
COMPUTED TOMOGRAPHY IN NEUROLOGIC DIAGNOSIS
197
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