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CT in the diagnosis of supratentorial tumors
CT in the Diagnosis of Supratentorial
Tumors
David 0. Davis, M.D.
T
HE computed tomogram (CT) is approximately 98% accurate in detecting intracranial lesions,” and thus is one of the most effective detection systems utilized in medicine. In comparison, radionuclide scans detect about 75%80%” of intracranial masses.While they are especially effective in detecting higher grade astrocytomas, meningiomas, and abscesses,radionuclide scans are ineffective in low grade astrocytomas and cystic abnormalities. CT scanning is also highly useful in the evaluation or analysis of a lesion, not only permitting localization, but often yielding definitive information about its character. At the time of clinical presentation, many intracranial lesions have produced a definite mass. The mass may be solid or cystic, may be surrounded by edema, and may have variable inherent neovascularity. Furthermore, tumor may cause obstruction of the cerebrospinal fluid pathway, leading to hydrocephalus, which is clearly shown by CT. The tumor itself may calcify to a variable degree. It is well known that a high percentage of oligodendrogliomas calcify and that astrocytomas, especially those of lower grade, may also have associated calcification. Calcification is shown on plain radiographs in lo%-20% of meningiomas. We have been severely limited in our ability to discern this calcification in the past. CT has a greater sensitivity to differences in contrast and therefore allows detection of lesser concentrations of calcification than may be seen on plain skull films or even on conventional tomograms. CT detection of intracranial massesis based on changes related to the above-mentioned characteristics of intracranial lesions. Normal brain tissue has an EMI number ranging from approximately 8 to 20. Intracranial massesmay or may not have David 0. Davis, M.D.: Professor of Radiology, The George Washington University Medical Center, Washington, D. C. 0 1977 by Grune & Stratton, Inc.
Seminars
in Roenfgenology,
Vol.
XII,
No. 2 (April),
1977
attenuation values different from normal brain. Experimental work by Phelps et a1.i5 with a monoenergetic x-ray beam has shown that the attenuation coefficient of various tumors may not significantly differ from that of normal tissue. If a massis isodense with the surrounding brain, it may be invisible on the scan. However, decreased density because of edema, or increased density caused by calcification or hemorrhage, may still allow its detection. Extensive edema in the tissue reduces the absorption value by virtue of the increased amount of water in the tissue. Utilizing the standard display system, this results in a lucent, or less dense, zone. Calcification in a mass leads to increased absorption of the x-rays and therefore increased density. Lesions containing fluid or fatty substance are readily identified by virtue of their lucent content combined with mass effect. The presence of a mass may also be inferred by ventricular deformity, as in air encephalography. Displacement or deformity implies a mass. At the present time, the axial projection usually utilized for CT scanning does not allow detailed evaluation of the type of displacement. But this is less important, since we usually are able to identify the site of the lesion directly and are not as dependent upon indirect signs. However, while a lesion may be well shown, its specific localization to a given lobe of the brain may be difficult. It is fairly easy to place a mid- and high convexity lesion in the appropriate lobe as long as you realize that, because of angulation, the present scanners tend to show less of the frontal region as the sections progress toward the vertex. It may thus be difficult to place a lesion above or below the sylvian fissure, especially with a temporal lobe mass that is markedly elevating the fissure. It is important to be able to accurately localize a lesion and to transfer this information from the CT scan to external landmarks, so that surgical approach can be planned.’ Distension of the ventricular system, especially in the absence of evidence of surface brain shrinkage, suggests hydrocephalus, which implies an ob97
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Ethier4 has shown that the intravenous injection of a large volume of iodinated contrast medium prior to CT scanning, which he dubbed computed angiotomography, is very effective in increasing the inherent density of some brain lesions (Fig. 1). Gado et a1.6V7showed that this increased density was in large measure secondary to diffusion or leakage of the iodine-containing contrast medium through the damaged blood-brain barrier into the
Fig. 1. Metastatic carcinoma of the breast showing contrast enhancement. (A) Radiolucency of left parasagittal parietal region, without an obvious mass. (BI Scan at a slightly higher level after injection of contrast material shows a discrete mass adjacent to the falx, representing metastatic tumor superficially located. The lucency on the precontrast scan represents edema secondary from the tumor.
strutting lesion. Sometimes ventricular distension may be due to an intraventricular lesion. Some of this distension is secondary to the mass itself or to obstruction caused by it. Rarely, it may be due to increased production of cerebrospinal fluid by a choroid plexus papilloma. Usually, ventricular lesions are well shown because of the large difference in contrast between the surrounding cerebrospinal fluid and the tumor itself.
Fig. 2. Astrocytoma grade 2. (A) There is attenuaof both tion and obliteration of the anterior portion lateral ventricles secondary to tumor infiltration of the corpus callosum and medial frontal lobes. Other sections showed almost complete obliteration of the frontal horn tips. The tumor itself is not visible. (B) The strip of faintly increased density in the midline is the result of contrast enhancement of a portion of the tumor.
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adjacent tissues, although in some instances the enhancement is in part attributable to increased vascularity within the neoplasm. BRAIN
TUMORS
Zimmerman,lg in his series of 5199 patients, found that 31% had gliomas and 20% had meta-
static tumor. Obviously, some of the metastatic lesions were below the tentorium, but it is safe to say that about 50% of diagnosable brain tumors are supratentorial. Zimmerman also showed that about 70% of the gliomas were astrocytomas of all grades. Thus, if we discuss astrocytomas and metastatic tumor, we will cover a great portion of the malignant brain tumors commonly seen. Astrocytomas
Low grade astrocytomas often run a benign course. They tend to cause little mass effect or surrounding reaction and may be difficult to detect with CT. If they lie close to the ventricular system, they may cause deformity of the ventricles. Their inherent neovascularity or minor damage to the blood-brain barrier may allow their identification by contrast enhancement (Figs. 2 and 9). Grades 3 and 4 astrocytomas are often bulky lesions. They usually show great contrast enhancement, which makes their identification easy. On nonenhanced CT scans, mottled lucency of edema surrounding the central nodular massis often seen, with or without distinct displacement or distortion of the ventricular system. Extensive irregular con-
Fig. 3. Astrocytoma. (Al Scan after contrast medium injection showing mottled radiolucencies surrounding a central nodule that is minimally enhanced in the left temporal region. No clear-cut mass effect was present. Angiography was normal. It was elected to follow the patient without treatment for a few months. (BI Six months later. Note the more prominent edema surrounding the tumor and the increased enhancement of the nodule, which now has a central lucency.
Fig. 4. Glioblastoma with extensive contrast enhancement. The large right frontal tumor shows extensive surrounding edema. There is a central lucency, suggesting necrosis, or at least decreased leakage. The right frontal horn is compressed and displaced posteriorly, and there is slight midline displacement to the left.
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Fig. 7. Metastatic melanoma. Precontrast scan shows extensive edema scattered throughout the cortex and white matter of the brain, with two dense melanomatous nodules clearly evident. Fig. 5. Astrocytoma grade 3 with an elliptical lesion in the right occipitoparietal area. The rim cyst is discretely enhanced. There is compression atrium end anterior displacement of the ventricle right. The thickness of the rim tends to exclude a cyst. Rarely meningioma may mimic this appearance.
cystic of the of the on the benign
trast enhancement may occur from marked leakage of the contrast medium into the tissues as well as from neovasculature within the lesion (Fig. 3). Frequently, the central portion of the lesion does not enhance with contrast medium, while the periphery does (Figs. 4 and 5). While some of these may be cystic and others may have a necrotic
Fig. 6. Metastatic leiomvosarcoma. (A) Postcontrast scan shows a large mass in the right caudate-capsular zone, indenting and perhaps growing into the body of the lateral ventricle. There was no significant enhancement. (6) After a month of radiation therapy, the scan shows marked reduction in the size of the mass.
center, some solid nonnecrotic tumors do not enhance centrally, possibly due to the failure of the iodine to perfuse throughout the lesion.14 In other words, there may be a multicompartmental flow pattern, with a more active flow zone surrounding a more slowly perfused central zone. Extensive displacement of the ventricular system may be associated with hippocampal herniation, with obliteration of the perimesencephalic cisterns
Fig. 6. Metastatic lung carcinoma. Postcontrest scan shows two sharply defined lesions, indicating metastases. Little edema is present (compare with Fig. 7).
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Fig. 10. Metastatic lung carcinoma. Large right frontal lobe mass with ring-like contrast enhancement, extensive edema, and mild mass effect, reminiscent of the astrocytoma in Fig. 4. It could also be mistaken for an abscess.
CT not only lends itself to the diagnosis of these lesions but a!so to their follow-up after treatment. Shrinkage of the tumor after radiation or chemotherapy may be well shown (Fig. 6). Metastatic Tumors
Fig. 9. Multiple glioblastomas. (AI Precontrast scan shows ventricular deformity and left posterior frontal edema with areas of faintly increased density immediately anterior to the deformities and edema. (B) Postcontrast scan shows a large irregular lobulated central lesion, a second lesion in the left posterior frontal region, and two additional areas of faintly increased density adjacent to the posterior portion of the body of the right ventricle (arrows).
and contiguity of the temporal lobe with the brain stem. Some gliomas may arise within or grow into the ventricular system, presenting as a mass sharply outlined by ventricular fluid, enabling their detection even without contrast enhancement.
Some metastases cause a great deal of edema and, while the metastatic nodule itself may not be clearly shown, the edema and mass effect are obvious (Fig. 7). Other metastases are isodense with no edema, and are consequently invisible on a plain CT scan.13 All those familiar with CT scans have seen patients in whom there is no evidence of a lesion on the plain scan, but multiple lesions are shown on the postcontrast scan. Multiple lesions are generally metastatic deposits (Fig. 8) but rarely multifocal gliomas (Fig. 9) and multiple abscessesoccur. Some metastatic lesions may cause generalized edema of the brain, which is easily shown by CT. However, treatment with steroids for 2-3 days may reduce the edema and render the lesions invisible. Postcontrast scans show enhancement of the nodules, however. It may be difficult or impossible to distinguish a malignant astrocytoma from a solitary metastatic lesion. Both may have considerable edema, a large mass effect, and complete or ring-like contrast enhancement (Fig. 10).
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Fig. 11. Huge intraventricular ependymoma on a postcontrast scan. The left lateral ventricle is enlarged and displaced to the right. We have seen similar findings in intraventricular granuloma.
Ependymomas Ependymomas show essentially the same CT pattern as malignant astrocytomas, i.e., a mass of variable size, density, and degree of contrast enhancement. Since most supratentorial ependymomas are juxtaventricular in origin, they may be indistinguishable from centrally placed glioblastomas or metastases. Intraventricular ependymomas are usually well shown (Fig. 11) and may be mimicked by intraventricular granulomas or abscesses. The presence of calcification within a lesion tends to exclude metastatic disease. However, calcification does not appreciably help in the identification of the cell type, since any tumor of the glioma series may show intrinsic calcification. Meningiomus Meningiomas frequently show calcification on CT scan (Fig. 12). The amount of calcium is variable, so that the EMI value may range from 20 to more than 100. It is important to understand the concept of “volume averaging” in this respect. With thick volume of the slice, a small amount of fairly dense calcification may average out to a much lower number than with a thin, slice. The clinical presentation usually allows differentiation
Fig. 12. Meningioma. (A) Precontrast scan shows a homogeneous increased density in the right frontal lobe with displacement and deformity of the ventricular n/stem and minimal surrounding edema. The nature of the density suggests faint or psammomatous calcification. (6) Postcontrast scan shows enhancement of the falx meningioma.
of a lower density calcification from hemorrhage, which may present with EMI numbers in the same range. The presence of a diffusely radiodense lesion on the surface of the brain, be it lateral or medial convexity or near the base of the skull, suggestsa meningioma. Some meningiomas are isodense and may not be shown precontrast. Others present a lucent zone within the brain, either from edema or from extensive encephalomalacia of the surrounding brain. The true character and size of the tumor nodule
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Fig. 14. Recurrent cystic astrocytoma. Postcontrast scan reveals a sharply defined central lucency, surrounded by a ring of enhanced cyst wall, in turn surrounded by irregular mottled edema. There is probably a discrete nodule on the periphery of the lesion (arrow].
is generally present throughout the tumor. Occasionally though, a meningioma may be enhanced in a ring-like fashion, similar to a glioma or abscess. The highly calcified meningiomas may show no contrast enhancement. This may indicate that there is minimal neovascularity or that the enhancement does not appreciably affect the high values created by the calcification.
Fig. 13. Meningioma. (A) The precontrast study shows extensive encephalomalacia anterior to a poorly visible nodule on the surface of the brain. (B) Postcontrast scan shows marked enhancement of the nodule.
itself may not be shown until the postcontrast scan (Fig. 13). Contrast enhancement of meningiomas is usually striking, with an increase in the EM1 value of the tumor itself ranging from 10 to 30 units. Enhancement usually is distributed evenly throughout the tumor, probably because the fine neovasculature
Fig. 15. Epidermoid cyst. Nonenhanced scan shows a sharply marginated cystic lucency in the left frontotemporal area. The absorption values were et or slightly below zero. The ventricular system was displaced slightly to the right on other sections. There was no contrast enhancement.
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Fig. 16. Epidermoid cyst in the left temporal lobe with a sharply defined lucent zone with absorption values at or slightly below the values for cerebrospinal fluid. The postcontrast study showed a suggestion of a rim about the cystic lesion. The cyst fluid contained cholesterol crystals.
cysts Cysts in the brain are generally characterized by low absorption values and sharply circumscribed margins. 3 The cyst may accompany a solid neoplasm or may be an independent lesion. In cystic neoplasms, contrast enhancement of the rim of the cyst may occur if it is surrounded by tumor tissue (Fig. 14) but others may not show enhancement. Most cysts are benign and are either arachnoid cysts or epidermoid cysts (Figs. 15 and 16). While the latter may have an intrinsic absorption value well below that of water, the values of most seem to range approximately at or slightly below the O-6 level that is ascribed to the cerebrospinal fluid. In epidermoid cyst, faint contrast enhancement of a rim of tissue adjacent to the cyst may occur, presumably not in the cyst wall but rather the slightly damaged adjacent brain tissue. Since they are usually slow growing, cysts seldom cause significant edema. Cysts may not cause as much deformity of the central structures as expected from their size, since the brain has had the opportunity to accommodate to their growth.
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Fig. 17. Colloid cyst. Sharply defined, slightly mottled nodular density in the foramen of Monro with accompanying hydrocephalus, as is evidenced by enlargement of the frontal and temporal horns. This is the classic appearance of colloid cyst.
Colloid cysts represent a distinct entity. Usually arising at the foramen of Monro, they often present with hydrocephalus due to partial obstruction of the ventricular pathway (Fig. 17). All of our colloid cysts have been radiodense, with values
Fig. 18. Classic appearance of choroid plexus papilloma with intraventricular mass with irregular margins and central fibrous stroma, which is slightly less dense than the main portion of the tumor. The peripheral portion enhanced markedly after contrast injection.
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Fig. 19. Pineal teratoma. (A) Precontrast study shows moderate calcification in a posterior third ventricular mass. The patient has had a ventricular shunt. (Bj Postcontrast study shows marked enhancement of the mass. This is characteristic of pineal neoplasm.
ranging from 25 to 30; some are reported to show values in the 15-20 range. This radiodensity and the central or slightly eccentric localization at the foramen of Monro specifically identifies the lesion.17 So far, no contrast enhancement has been shown with these tumors. Another example of colloid cyst is shown on page 132.
Fig. 20. Ectopic pinealoma fsuprasellar or infundibular dysgerminomaj. (A) Postcontrast injection scan shows a nodular mass (arrow) in the suprasellar region. (B) Higher cut reveals enlargement of the ventricles with marked encasement of the ventricular system by intraventricular spread of the tumor. Identical findings have been seen in medulloblastoma.
Miscellarzeous Tumors Choroid plexus papilloma. This is a highly vascular lesion presenting in the ventricular system, either in the atrium of a lateral ventricle or in the fourth ventricle. Hydrocephalus may be a signifi-
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reduction of the ventricular size, but the local deformity may remain. Pineal neoplasm. These are so strategically placed that they usually cause hydrocephalus. ‘* Their localization, frequent intrinsic calcification, and marked contrast enhancement permits the diagnosis (Fig. 19). Other lesions in the posterior portion of the third ventricle, such as a metastasis, may mimic pinealoma. So-called ectopic pinealoma is actually a teratoma. It usually arises in the re-
Fig. 21. Abscess. (A) Precontrast study shows an irregular lucent zone in the left frontal lobe with ventricular deformity and midline displacement. (B) Portcontrast scan shows a characteristic ring enhancement of the abscess wall.
cant component, either because of obstruction or because of overproduction of CSF. Focal enlargement of the ventricle may be present at the site of the tumor. Irregular, frond-like margins are quite well shown on the base-line scan and extensive enhancement is present after the injection of iodinated contrast.’ Central lucencies representing the fibrous stroma of the tumor may (Fig. 19) incorrectly imply the presence of a central cyst (Fig. 18). Removal of the lesion usually results in
Fig.22. ,Abscess. (A) The patient had trauma with fracture of the cribriform plate, cerebrospinal fluid leakage, and pneumocephalus. He later developed confusion. This scan shows an extensive lucent zone involving most of the right frontal lobe. There is faint visualization of a rim lying between a zone of edema and the central cavity. [B) Postcontrast injection shows intense opacification of the thick capsule.
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gion of the infundibulum and presents as a suprasellar mass. It may grow upward into the hypothalamus, obstructing the foramen of Monro and causing hydrocephalus. These lesions share with medulloblastoma and ependymoma the potential for intraventricular and subarachnoid spread. Total encasement of the ventricular system may occur. The injection of contrast material may markedly enhance the mass, leading to its identification asaneoplasm (Fig. 20). Another example is shown on page 130.
Abscesses Abscesses often present merely as lucent zones in the brain on nonenhanced scans. The lucency is secondary to the edema and cerebritis around the abscess. Mass effect may not be discernible if the lesion is on the brain surface, as they often are. Contrast injection classically shows a thin-walled cyst with a sharply defined capsular ring of enhancement (Figs. 21 and 22). In some cases, the central necrotic zone is small or absent, indicating minimal central necrosis (Fig. 23). The clinical picture may allow a reasonably accurate diagnosis, but often it is difficult to distinguish a sharply defined tumor from an abscess.
Extrinsic Tumors Tumors that arise outside the dura and grow through it into the brain may be identified by CT
Fig. 23. Abscess. The postcontrast scan shows a nodular density in the left occipitoparietal region without a central cavity. Some abscesses may not have a central necrotic zone or it may fail to visualize by scan.
scan and their origin indicated. If the brain is affected, considerable edema may be present. Care must be taken to distinguish an artifact from a tumor. Rescanning with slight adjustments in the section level may aid in this distinction. These lesions usually begin in or grow through the bony structures of the skull. The bone lesions can be identified provided that appropriate windowing is utilized. Generally, the use of a wide window width and high window levels will allow these bony lesions to be shown. The tumor will often show considerable contrast enhancement, which aids in determining the extent of involvement. Direct spread may occur from a metastatic lesion in the skull, or from a primary lesion of the paranasal structures. The latter may grow directly through the floor of the anterior fossa into the frontal lobes. Lymphoma or sarcoma may also arise extracerebrally and grow into the brain, causing a pattern of edema, mass effect, and contrast enhancement identical to that described above. DIFFERENTIAL
DIAGNOSIS
The brain has a limited response to pathologic lesions. The presence of edema and a mass may indicate brain tumor or cerebral infarction. Utilizing CT, it may be difficult to separate the two. On many occasions, the clinical presentation aids greatly in this differentiation, but in some cerebrovascular insults, especially those involving the penetrating arteries feeding the central structures of the brain, the clinical presentation may be misleading. The injection of contrast material is usually very helpful. Central cerebrovascular infarction is seldom enhanced by the contrast material. In this situation, it is best to delay 2-4 weeks and perform a follow-up CT scan. Encephalomalacia secondary to infarction causes sharply defined lucencies with lower EM1 values than the usual tumorous edema.’ Additionally, the brain shrinkage may lead to dilation of the ventricular system toward the site of the lesion, strongly suggesting a diagnosis of cerebral infarction. Surface infarction may enhance with contrast injection,‘8 the pattern of enhancement usually being relatively sharply defined and regular be-
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cause of projection of the cortex and subcortex into the white matter. In addition, the enhancement of surface infarcts is often peripheral. Obviously, the use of angiography in these patients may also be helpful, since demonstration of blocked vessels, collateral channels, or retrograde flow favors stroke over tumor.16 Radionuclide scans also may be helpful5 Central abscess may be extremely difficult to distinguish from malignant glioma and, rarely, metastasis or meningioma. The abscess tends to cause a great deal of edema, and postcontrast scans tend to show a ring of density surrounding a relatively lucent central zone, thus closely mimicking the central necrotic zone seen with glioblastomas. Huckman has shown that the average EMI value of the central lucency of an abscess is often higher
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than that of a glioblastoma,gY’O but the evaluation of a given patient may be extremely difficult. In general, abscess tends to have a thinner, more sharply defined contrast-enhanced wall than neoplasm, but surgical confirmation is often necessary. It is important to maintain a high degree of suspicion about ring densities, remembering always that such a lesion may be an abscess,thus requiring further evaluation. About a third of cerebral abscessesshow a solid density after enhancement without a ring sign. Angiography may aid in the diagnosis, but if the angiogram shows an avascular mass, the problem still remains. In our own institution, we tend to tap these lesions in order to be sure of the diagnosis before rupture of the abscess or surface thrombophlebitis occurs.
REFERENCES 1. Cronqvist S, Brismar J, Kjellin K, et al: Computer assisted axial tomography in cerebrovascular lesions. Acta Radio1 (Diagn) 16:135-145,1975 2. Davis DO: Neuroradiologic diagnosis of temporal lobe masses. Presented at the Radiological Society of North America Meeting, Chicago, 1976 3. Davis KR, Roberson GH, Taveras JM, et al: Diagnosis of epidermoid tumor by computed tomography. Analysis and evaluation of findings. Radiology 119:347353,1976 4. Ethier R: The use of large volume contrast media injections in computed tomography. Presented at the First International Symposium on Computed Tomography, Montreal, 1974 5. Gado MH, Coleman RE, Merlis AL, et al: Comparison of computerized tomography and radionuclide imaging in “stroke.” Stroke 7: 109-113, 1976 6. Gado MH, Phelps ME, Coleman RE: An extravascular component of contrast enhancement in cranial computed tomography. Part I. The tissue-blood ratio of contrast enhancement. Radiology 117:589-593,197s 7. Gado MH, Phelps ME, Coleman RE: An extravascular component of contrast enhancement in cranial computed tomography. Part II. Contrast enhancement and the blood-tissue barrier. Radiology 117:595-597, 1975 8. Harwood-Nash DC: Computed cranial tomography in children with intracranial neoplasms, in Salamon G (ed): Advances in Cerebral Angiography. Berlin, Springer, 1975, pp 349-352 9. Huckman MS: Clinical experience with the intravenous infusion of iodinated contrast material as an adjunct to computed tomography. Surg Neurol 4:297318, 1975
10. Huckman MS: Analysis of CT numbers in tumors and abscesses. Presented at the American Society of Neuroradiology Annual Meeting, Atlanta, 1976 11. Knaus WA, Davis DO: Computerized tomography versus radionuclide brain scanning in the diagnosis of brain tumor. Presented at the American Federation for Clinical Research, Atlantic City, May, 1976 12. Messina AV, Potts G, Sigel RM, et al: Computed tomography: Evaluation of the posterior third ventricle. Radiology 119:581-592, 1976 13. New PF, Scott WR, Schnur JA, et al: Computed tomography with the EM1 scanner in the diagnosis of primary and metastatic intracranial neoplasms. Radiology 114:75-87,1975 14. Norman D: Analysis of use of iodinated contrast in brain tumor diagnosis with CT. Presented at the Radiological Society of North America Meeting, Chicago, 1975 15. Phelps ME, Hoffman EJ, Ter-Pogossian MM: Attenuation coefficients of various body tissues, fluids and lesions at photon energies of 18 to 136 KeV. Radiology 117:573-583, 1975 16. Report of the Joint Committee for Stroke Facilities. XII. Computed tomography in the management of cerebrovascular disease. Stroke 6:103-107, 1975 17. Sackett JF, Messina AV, Petit0 CK: Computed tomography and magnification vertebral angiotomography in the diagnosis of colloid cysts of the third ventricle. Radiology 116:95-100, 1975 18. Wing SD, Norman D, Pollock JA, et al: Contrast enhancement of cerebral infarcts in computed tomography. Radiology 121:89-92, 1976 19. Zimmerman HM: The ten most common types of brain tumor. Semin Roentgen01 6:48-58, 1971
Tumors
David 0. Davis, M.D.
T
HE computed tomogram (CT) is approximately 98% accurate in detecting intracranial lesions,” and thus is one of the most effective detection systems utilized in medicine. In comparison, radionuclide scans detect about 75%80%” of intracranial masses.While they are especially effective in detecting higher grade astrocytomas, meningiomas, and abscesses,radionuclide scans are ineffective in low grade astrocytomas and cystic abnormalities. CT scanning is also highly useful in the evaluation or analysis of a lesion, not only permitting localization, but often yielding definitive information about its character. At the time of clinical presentation, many intracranial lesions have produced a definite mass. The mass may be solid or cystic, may be surrounded by edema, and may have variable inherent neovascularity. Furthermore, tumor may cause obstruction of the cerebrospinal fluid pathway, leading to hydrocephalus, which is clearly shown by CT. The tumor itself may calcify to a variable degree. It is well known that a high percentage of oligodendrogliomas calcify and that astrocytomas, especially those of lower grade, may also have associated calcification. Calcification is shown on plain radiographs in lo%-20% of meningiomas. We have been severely limited in our ability to discern this calcification in the past. CT has a greater sensitivity to differences in contrast and therefore allows detection of lesser concentrations of calcification than may be seen on plain skull films or even on conventional tomograms. CT detection of intracranial massesis based on changes related to the above-mentioned characteristics of intracranial lesions. Normal brain tissue has an EMI number ranging from approximately 8 to 20. Intracranial massesmay or may not have David 0. Davis, M.D.: Professor of Radiology, The George Washington University Medical Center, Washington, D. C. 0 1977 by Grune & Stratton, Inc.
Seminars
in Roenfgenology,
Vol.
XII,
No. 2 (April),
1977
attenuation values different from normal brain. Experimental work by Phelps et a1.i5 with a monoenergetic x-ray beam has shown that the attenuation coefficient of various tumors may not significantly differ from that of normal tissue. If a massis isodense with the surrounding brain, it may be invisible on the scan. However, decreased density because of edema, or increased density caused by calcification or hemorrhage, may still allow its detection. Extensive edema in the tissue reduces the absorption value by virtue of the increased amount of water in the tissue. Utilizing the standard display system, this results in a lucent, or less dense, zone. Calcification in a mass leads to increased absorption of the x-rays and therefore increased density. Lesions containing fluid or fatty substance are readily identified by virtue of their lucent content combined with mass effect. The presence of a mass may also be inferred by ventricular deformity, as in air encephalography. Displacement or deformity implies a mass. At the present time, the axial projection usually utilized for CT scanning does not allow detailed evaluation of the type of displacement. But this is less important, since we usually are able to identify the site of the lesion directly and are not as dependent upon indirect signs. However, while a lesion may be well shown, its specific localization to a given lobe of the brain may be difficult. It is fairly easy to place a mid- and high convexity lesion in the appropriate lobe as long as you realize that, because of angulation, the present scanners tend to show less of the frontal region as the sections progress toward the vertex. It may thus be difficult to place a lesion above or below the sylvian fissure, especially with a temporal lobe mass that is markedly elevating the fissure. It is important to be able to accurately localize a lesion and to transfer this information from the CT scan to external landmarks, so that surgical approach can be planned.’ Distension of the ventricular system, especially in the absence of evidence of surface brain shrinkage, suggests hydrocephalus, which implies an ob97
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Ethier4 has shown that the intravenous injection of a large volume of iodinated contrast medium prior to CT scanning, which he dubbed computed angiotomography, is very effective in increasing the inherent density of some brain lesions (Fig. 1). Gado et a1.6V7showed that this increased density was in large measure secondary to diffusion or leakage of the iodine-containing contrast medium through the damaged blood-brain barrier into the
Fig. 1. Metastatic carcinoma of the breast showing contrast enhancement. (A) Radiolucency of left parasagittal parietal region, without an obvious mass. (BI Scan at a slightly higher level after injection of contrast material shows a discrete mass adjacent to the falx, representing metastatic tumor superficially located. The lucency on the precontrast scan represents edema secondary from the tumor.
strutting lesion. Sometimes ventricular distension may be due to an intraventricular lesion. Some of this distension is secondary to the mass itself or to obstruction caused by it. Rarely, it may be due to increased production of cerebrospinal fluid by a choroid plexus papilloma. Usually, ventricular lesions are well shown because of the large difference in contrast between the surrounding cerebrospinal fluid and the tumor itself.
Fig. 2. Astrocytoma grade 2. (A) There is attenuaof both tion and obliteration of the anterior portion lateral ventricles secondary to tumor infiltration of the corpus callosum and medial frontal lobes. Other sections showed almost complete obliteration of the frontal horn tips. The tumor itself is not visible. (B) The strip of faintly increased density in the midline is the result of contrast enhancement of a portion of the tumor.
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adjacent tissues, although in some instances the enhancement is in part attributable to increased vascularity within the neoplasm. BRAIN
TUMORS
Zimmerman,lg in his series of 5199 patients, found that 31% had gliomas and 20% had meta-
static tumor. Obviously, some of the metastatic lesions were below the tentorium, but it is safe to say that about 50% of diagnosable brain tumors are supratentorial. Zimmerman also showed that about 70% of the gliomas were astrocytomas of all grades. Thus, if we discuss astrocytomas and metastatic tumor, we will cover a great portion of the malignant brain tumors commonly seen. Astrocytomas
Low grade astrocytomas often run a benign course. They tend to cause little mass effect or surrounding reaction and may be difficult to detect with CT. If they lie close to the ventricular system, they may cause deformity of the ventricles. Their inherent neovascularity or minor damage to the blood-brain barrier may allow their identification by contrast enhancement (Figs. 2 and 9). Grades 3 and 4 astrocytomas are often bulky lesions. They usually show great contrast enhancement, which makes their identification easy. On nonenhanced CT scans, mottled lucency of edema surrounding the central nodular massis often seen, with or without distinct displacement or distortion of the ventricular system. Extensive irregular con-
Fig. 3. Astrocytoma. (Al Scan after contrast medium injection showing mottled radiolucencies surrounding a central nodule that is minimally enhanced in the left temporal region. No clear-cut mass effect was present. Angiography was normal. It was elected to follow the patient without treatment for a few months. (BI Six months later. Note the more prominent edema surrounding the tumor and the increased enhancement of the nodule, which now has a central lucency.
Fig. 4. Glioblastoma with extensive contrast enhancement. The large right frontal tumor shows extensive surrounding edema. There is a central lucency, suggesting necrosis, or at least decreased leakage. The right frontal horn is compressed and displaced posteriorly, and there is slight midline displacement to the left.
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Fig. 7. Metastatic melanoma. Precontrast scan shows extensive edema scattered throughout the cortex and white matter of the brain, with two dense melanomatous nodules clearly evident. Fig. 5. Astrocytoma grade 3 with an elliptical lesion in the right occipitoparietal area. The rim cyst is discretely enhanced. There is compression atrium end anterior displacement of the ventricle right. The thickness of the rim tends to exclude a cyst. Rarely meningioma may mimic this appearance.
cystic of the of the on the benign
trast enhancement may occur from marked leakage of the contrast medium into the tissues as well as from neovasculature within the lesion (Fig. 3). Frequently, the central portion of the lesion does not enhance with contrast medium, while the periphery does (Figs. 4 and 5). While some of these may be cystic and others may have a necrotic
Fig. 6. Metastatic leiomvosarcoma. (A) Postcontrast scan shows a large mass in the right caudate-capsular zone, indenting and perhaps growing into the body of the lateral ventricle. There was no significant enhancement. (6) After a month of radiation therapy, the scan shows marked reduction in the size of the mass.
center, some solid nonnecrotic tumors do not enhance centrally, possibly due to the failure of the iodine to perfuse throughout the lesion.14 In other words, there may be a multicompartmental flow pattern, with a more active flow zone surrounding a more slowly perfused central zone. Extensive displacement of the ventricular system may be associated with hippocampal herniation, with obliteration of the perimesencephalic cisterns
Fig. 6. Metastatic lung carcinoma. Postcontrest scan shows two sharply defined lesions, indicating metastases. Little edema is present (compare with Fig. 7).
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Fig. 10. Metastatic lung carcinoma. Large right frontal lobe mass with ring-like contrast enhancement, extensive edema, and mild mass effect, reminiscent of the astrocytoma in Fig. 4. It could also be mistaken for an abscess.
CT not only lends itself to the diagnosis of these lesions but a!so to their follow-up after treatment. Shrinkage of the tumor after radiation or chemotherapy may be well shown (Fig. 6). Metastatic Tumors
Fig. 9. Multiple glioblastomas. (AI Precontrast scan shows ventricular deformity and left posterior frontal edema with areas of faintly increased density immediately anterior to the deformities and edema. (B) Postcontrast scan shows a large irregular lobulated central lesion, a second lesion in the left posterior frontal region, and two additional areas of faintly increased density adjacent to the posterior portion of the body of the right ventricle (arrows).
and contiguity of the temporal lobe with the brain stem. Some gliomas may arise within or grow into the ventricular system, presenting as a mass sharply outlined by ventricular fluid, enabling their detection even without contrast enhancement.
Some metastases cause a great deal of edema and, while the metastatic nodule itself may not be clearly shown, the edema and mass effect are obvious (Fig. 7). Other metastases are isodense with no edema, and are consequently invisible on a plain CT scan.13 All those familiar with CT scans have seen patients in whom there is no evidence of a lesion on the plain scan, but multiple lesions are shown on the postcontrast scan. Multiple lesions are generally metastatic deposits (Fig. 8) but rarely multifocal gliomas (Fig. 9) and multiple abscessesoccur. Some metastatic lesions may cause generalized edema of the brain, which is easily shown by CT. However, treatment with steroids for 2-3 days may reduce the edema and render the lesions invisible. Postcontrast scans show enhancement of the nodules, however. It may be difficult or impossible to distinguish a malignant astrocytoma from a solitary metastatic lesion. Both may have considerable edema, a large mass effect, and complete or ring-like contrast enhancement (Fig. 10).
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Fig. 11. Huge intraventricular ependymoma on a postcontrast scan. The left lateral ventricle is enlarged and displaced to the right. We have seen similar findings in intraventricular granuloma.
Ependymomas Ependymomas show essentially the same CT pattern as malignant astrocytomas, i.e., a mass of variable size, density, and degree of contrast enhancement. Since most supratentorial ependymomas are juxtaventricular in origin, they may be indistinguishable from centrally placed glioblastomas or metastases. Intraventricular ependymomas are usually well shown (Fig. 11) and may be mimicked by intraventricular granulomas or abscesses. The presence of calcification within a lesion tends to exclude metastatic disease. However, calcification does not appreciably help in the identification of the cell type, since any tumor of the glioma series may show intrinsic calcification. Meningiomus Meningiomas frequently show calcification on CT scan (Fig. 12). The amount of calcium is variable, so that the EMI value may range from 20 to more than 100. It is important to understand the concept of “volume averaging” in this respect. With thick volume of the slice, a small amount of fairly dense calcification may average out to a much lower number than with a thin, slice. The clinical presentation usually allows differentiation
Fig. 12. Meningioma. (A) Precontrast scan shows a homogeneous increased density in the right frontal lobe with displacement and deformity of the ventricular n/stem and minimal surrounding edema. The nature of the density suggests faint or psammomatous calcification. (6) Postcontrast scan shows enhancement of the falx meningioma.
of a lower density calcification from hemorrhage, which may present with EMI numbers in the same range. The presence of a diffusely radiodense lesion on the surface of the brain, be it lateral or medial convexity or near the base of the skull, suggestsa meningioma. Some meningiomas are isodense and may not be shown precontrast. Others present a lucent zone within the brain, either from edema or from extensive encephalomalacia of the surrounding brain. The true character and size of the tumor nodule
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Fig. 14. Recurrent cystic astrocytoma. Postcontrast scan reveals a sharply defined central lucency, surrounded by a ring of enhanced cyst wall, in turn surrounded by irregular mottled edema. There is probably a discrete nodule on the periphery of the lesion (arrow].
is generally present throughout the tumor. Occasionally though, a meningioma may be enhanced in a ring-like fashion, similar to a glioma or abscess. The highly calcified meningiomas may show no contrast enhancement. This may indicate that there is minimal neovascularity or that the enhancement does not appreciably affect the high values created by the calcification.
Fig. 13. Meningioma. (A) The precontrast study shows extensive encephalomalacia anterior to a poorly visible nodule on the surface of the brain. (B) Postcontrast scan shows marked enhancement of the nodule.
itself may not be shown until the postcontrast scan (Fig. 13). Contrast enhancement of meningiomas is usually striking, with an increase in the EM1 value of the tumor itself ranging from 10 to 30 units. Enhancement usually is distributed evenly throughout the tumor, probably because the fine neovasculature
Fig. 15. Epidermoid cyst. Nonenhanced scan shows a sharply marginated cystic lucency in the left frontotemporal area. The absorption values were et or slightly below zero. The ventricular system was displaced slightly to the right on other sections. There was no contrast enhancement.
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Fig. 16. Epidermoid cyst in the left temporal lobe with a sharply defined lucent zone with absorption values at or slightly below the values for cerebrospinal fluid. The postcontrast study showed a suggestion of a rim about the cystic lesion. The cyst fluid contained cholesterol crystals.
cysts Cysts in the brain are generally characterized by low absorption values and sharply circumscribed margins. 3 The cyst may accompany a solid neoplasm or may be an independent lesion. In cystic neoplasms, contrast enhancement of the rim of the cyst may occur if it is surrounded by tumor tissue (Fig. 14) but others may not show enhancement. Most cysts are benign and are either arachnoid cysts or epidermoid cysts (Figs. 15 and 16). While the latter may have an intrinsic absorption value well below that of water, the values of most seem to range approximately at or slightly below the O-6 level that is ascribed to the cerebrospinal fluid. In epidermoid cyst, faint contrast enhancement of a rim of tissue adjacent to the cyst may occur, presumably not in the cyst wall but rather the slightly damaged adjacent brain tissue. Since they are usually slow growing, cysts seldom cause significant edema. Cysts may not cause as much deformity of the central structures as expected from their size, since the brain has had the opportunity to accommodate to their growth.
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Fig. 17. Colloid cyst. Sharply defined, slightly mottled nodular density in the foramen of Monro with accompanying hydrocephalus, as is evidenced by enlargement of the frontal and temporal horns. This is the classic appearance of colloid cyst.
Colloid cysts represent a distinct entity. Usually arising at the foramen of Monro, they often present with hydrocephalus due to partial obstruction of the ventricular pathway (Fig. 17). All of our colloid cysts have been radiodense, with values
Fig. 18. Classic appearance of choroid plexus papilloma with intraventricular mass with irregular margins and central fibrous stroma, which is slightly less dense than the main portion of the tumor. The peripheral portion enhanced markedly after contrast injection.
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Fig. 19. Pineal teratoma. (A) Precontrast study shows moderate calcification in a posterior third ventricular mass. The patient has had a ventricular shunt. (Bj Postcontrast study shows marked enhancement of the mass. This is characteristic of pineal neoplasm.
ranging from 25 to 30; some are reported to show values in the 15-20 range. This radiodensity and the central or slightly eccentric localization at the foramen of Monro specifically identifies the lesion.17 So far, no contrast enhancement has been shown with these tumors. Another example of colloid cyst is shown on page 132.
Fig. 20. Ectopic pinealoma fsuprasellar or infundibular dysgerminomaj. (A) Postcontrast injection scan shows a nodular mass (arrow) in the suprasellar region. (B) Higher cut reveals enlargement of the ventricles with marked encasement of the ventricular system by intraventricular spread of the tumor. Identical findings have been seen in medulloblastoma.
Miscellarzeous Tumors Choroid plexus papilloma. This is a highly vascular lesion presenting in the ventricular system, either in the atrium of a lateral ventricle or in the fourth ventricle. Hydrocephalus may be a signifi-
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reduction of the ventricular size, but the local deformity may remain. Pineal neoplasm. These are so strategically placed that they usually cause hydrocephalus. ‘* Their localization, frequent intrinsic calcification, and marked contrast enhancement permits the diagnosis (Fig. 19). Other lesions in the posterior portion of the third ventricle, such as a metastasis, may mimic pinealoma. So-called ectopic pinealoma is actually a teratoma. It usually arises in the re-
Fig. 21. Abscess. (A) Precontrast study shows an irregular lucent zone in the left frontal lobe with ventricular deformity and midline displacement. (B) Portcontrast scan shows a characteristic ring enhancement of the abscess wall.
cant component, either because of obstruction or because of overproduction of CSF. Focal enlargement of the ventricle may be present at the site of the tumor. Irregular, frond-like margins are quite well shown on the base-line scan and extensive enhancement is present after the injection of iodinated contrast.’ Central lucencies representing the fibrous stroma of the tumor may (Fig. 19) incorrectly imply the presence of a central cyst (Fig. 18). Removal of the lesion usually results in
Fig.22. ,Abscess. (A) The patient had trauma with fracture of the cribriform plate, cerebrospinal fluid leakage, and pneumocephalus. He later developed confusion. This scan shows an extensive lucent zone involving most of the right frontal lobe. There is faint visualization of a rim lying between a zone of edema and the central cavity. [B) Postcontrast injection shows intense opacification of the thick capsule.
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gion of the infundibulum and presents as a suprasellar mass. It may grow upward into the hypothalamus, obstructing the foramen of Monro and causing hydrocephalus. These lesions share with medulloblastoma and ependymoma the potential for intraventricular and subarachnoid spread. Total encasement of the ventricular system may occur. The injection of contrast material may markedly enhance the mass, leading to its identification asaneoplasm (Fig. 20). Another example is shown on page 130.
Abscesses Abscesses often present merely as lucent zones in the brain on nonenhanced scans. The lucency is secondary to the edema and cerebritis around the abscess. Mass effect may not be discernible if the lesion is on the brain surface, as they often are. Contrast injection classically shows a thin-walled cyst with a sharply defined capsular ring of enhancement (Figs. 21 and 22). In some cases, the central necrotic zone is small or absent, indicating minimal central necrosis (Fig. 23). The clinical picture may allow a reasonably accurate diagnosis, but often it is difficult to distinguish a sharply defined tumor from an abscess.
Extrinsic Tumors Tumors that arise outside the dura and grow through it into the brain may be identified by CT
Fig. 23. Abscess. The postcontrast scan shows a nodular density in the left occipitoparietal region without a central cavity. Some abscesses may not have a central necrotic zone or it may fail to visualize by scan.
scan and their origin indicated. If the brain is affected, considerable edema may be present. Care must be taken to distinguish an artifact from a tumor. Rescanning with slight adjustments in the section level may aid in this distinction. These lesions usually begin in or grow through the bony structures of the skull. The bone lesions can be identified provided that appropriate windowing is utilized. Generally, the use of a wide window width and high window levels will allow these bony lesions to be shown. The tumor will often show considerable contrast enhancement, which aids in determining the extent of involvement. Direct spread may occur from a metastatic lesion in the skull, or from a primary lesion of the paranasal structures. The latter may grow directly through the floor of the anterior fossa into the frontal lobes. Lymphoma or sarcoma may also arise extracerebrally and grow into the brain, causing a pattern of edema, mass effect, and contrast enhancement identical to that described above. DIFFERENTIAL
DIAGNOSIS
The brain has a limited response to pathologic lesions. The presence of edema and a mass may indicate brain tumor or cerebral infarction. Utilizing CT, it may be difficult to separate the two. On many occasions, the clinical presentation aids greatly in this differentiation, but in some cerebrovascular insults, especially those involving the penetrating arteries feeding the central structures of the brain, the clinical presentation may be misleading. The injection of contrast material is usually very helpful. Central cerebrovascular infarction is seldom enhanced by the contrast material. In this situation, it is best to delay 2-4 weeks and perform a follow-up CT scan. Encephalomalacia secondary to infarction causes sharply defined lucencies with lower EM1 values than the usual tumorous edema.’ Additionally, the brain shrinkage may lead to dilation of the ventricular system toward the site of the lesion, strongly suggesting a diagnosis of cerebral infarction. Surface infarction may enhance with contrast injection,‘8 the pattern of enhancement usually being relatively sharply defined and regular be-
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cause of projection of the cortex and subcortex into the white matter. In addition, the enhancement of surface infarcts is often peripheral. Obviously, the use of angiography in these patients may also be helpful, since demonstration of blocked vessels, collateral channels, or retrograde flow favors stroke over tumor.16 Radionuclide scans also may be helpful5 Central abscess may be extremely difficult to distinguish from malignant glioma and, rarely, metastasis or meningioma. The abscess tends to cause a great deal of edema, and postcontrast scans tend to show a ring of density surrounding a relatively lucent central zone, thus closely mimicking the central necrotic zone seen with glioblastomas. Huckman has shown that the average EMI value of the central lucency of an abscess is often higher
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than that of a glioblastoma,gY’O but the evaluation of a given patient may be extremely difficult. In general, abscess tends to have a thinner, more sharply defined contrast-enhanced wall than neoplasm, but surgical confirmation is often necessary. It is important to maintain a high degree of suspicion about ring densities, remembering always that such a lesion may be an abscess,thus requiring further evaluation. About a third of cerebral abscessesshow a solid density after enhancement without a ring sign. Angiography may aid in the diagnosis, but if the angiogram shows an avascular mass, the problem still remains. In our own institution, we tend to tap these lesions in order to be sure of the diagnosis before rupture of the abscess or surface thrombophlebitis occurs.
REFERENCES 1. Cronqvist S, Brismar J, Kjellin K, et al: Computer assisted axial tomography in cerebrovascular lesions. Acta Radio1 (Diagn) 16:135-145,1975 2. Davis DO: Neuroradiologic diagnosis of temporal lobe masses. Presented at the Radiological Society of North America Meeting, Chicago, 1976 3. Davis KR, Roberson GH, Taveras JM, et al: Diagnosis of epidermoid tumor by computed tomography. Analysis and evaluation of findings. Radiology 119:347353,1976 4. Ethier R: The use of large volume contrast media injections in computed tomography. Presented at the First International Symposium on Computed Tomography, Montreal, 1974 5. Gado MH, Coleman RE, Merlis AL, et al: Comparison of computerized tomography and radionuclide imaging in “stroke.” Stroke 7: 109-113, 1976 6. Gado MH, Phelps ME, Coleman RE: An extravascular component of contrast enhancement in cranial computed tomography. Part I. The tissue-blood ratio of contrast enhancement. Radiology 117:589-593,197s 7. Gado MH, Phelps ME, Coleman RE: An extravascular component of contrast enhancement in cranial computed tomography. Part II. Contrast enhancement and the blood-tissue barrier. Radiology 117:595-597, 1975 8. Harwood-Nash DC: Computed cranial tomography in children with intracranial neoplasms, in Salamon G (ed): Advances in Cerebral Angiography. Berlin, Springer, 1975, pp 349-352 9. Huckman MS: Clinical experience with the intravenous infusion of iodinated contrast material as an adjunct to computed tomography. Surg Neurol 4:297318, 1975
10. Huckman MS: Analysis of CT numbers in tumors and abscesses. Presented at the American Society of Neuroradiology Annual Meeting, Atlanta, 1976 11. Knaus WA, Davis DO: Computerized tomography versus radionuclide brain scanning in the diagnosis of brain tumor. Presented at the American Federation for Clinical Research, Atlantic City, May, 1976 12. Messina AV, Potts G, Sigel RM, et al: Computed tomography: Evaluation of the posterior third ventricle. Radiology 119:581-592, 1976 13. New PF, Scott WR, Schnur JA, et al: Computed tomography with the EM1 scanner in the diagnosis of primary and metastatic intracranial neoplasms. Radiology 114:75-87,1975 14. Norman D: Analysis of use of iodinated contrast in brain tumor diagnosis with CT. Presented at the Radiological Society of North America Meeting, Chicago, 1975 15. Phelps ME, Hoffman EJ, Ter-Pogossian MM: Attenuation coefficients of various body tissues, fluids and lesions at photon energies of 18 to 136 KeV. Radiology 117:573-583, 1975 16. Report of the Joint Committee for Stroke Facilities. XII. Computed tomography in the management of cerebrovascular disease. Stroke 6:103-107, 1975 17. Sackett JF, Messina AV, Petit0 CK: Computed tomography and magnification vertebral angiotomography in the diagnosis of colloid cysts of the third ventricle. Radiology 116:95-100, 1975 18. Wing SD, Norman D, Pollock JA, et al: Contrast enhancement of cerebral infarcts in computed tomography. Radiology 121:89-92, 1976 19. Zimmerman HM: The ten most common types of brain tumor. Semin Roentgen01 6:48-58, 1971