Medical Staff Conference
Refer to: Current diagnostic uses of computerized tomography in clinical medicine-Medical Staff Conference, University of California, San Francisco. West J Med 126:119-126, Feb 1977
Current Diagnostic Uses of Computerized Tomography in Clinical Medicine These discussions are selected from the weekly staff conferences in the Department of Medicine, University of California, San Francisco. Taken from transcriptions, they are prepared by Drs. David W. Martin, Jr., Associate Professor of Medicine, and H. David Watts, Assistant Professor of Medicine, under the direction of Dr. Lloyd H. Smith, Jr., Professor of Medicine and Chairman of the Department of Medicine. Requests for reprints should be sent to the Department of Medicine, University of California, San Francisco, CA 94143.
DR. SMITH:* There has been a remarkable increase in the use of advanced technology in medical practice during the past decade. This adds to the complexity and expense of diagnosis and therapy, but also lends new assistance to physicians. Perhaps the area where the advances have been most impressive in recent years lies in the use of computerized tomography to image previously inaccessible organs noninvasively. We are pleased to have Dr. Melvyn Korobkin with us today to review the current status of this technology and its application to the diagnosis of diseases of abdominal organs. DR. KOROBKIN: t There are few advances in medicine that can be considered revolutionary, but computerized tomography truly represents such a phenomenon. Many by now are familiar with computed tomography (CT) of the brain, a noninvasive way of imaging known or suspected lesions, whether they be tumors, infarcts, hematomas or abscesses. In the four short years since this technique was introduced into clinical practice, CT brain scanning has met almost unanimous *Lloyd H. Smith, Jr., MD, Professor and Chairman, Department of Medicine. tMelvyn T. Korobkin, MD, Associate Professor in Radiology.
and enthusiastic approval by neurologists, neurosurgeons, radiologists and patients. The presence of brain computerized tomographic scanners in this country has spread rapidly. There is convincing evidence that the number of pneumoencephalograms carried out has dropped drastically, with a similar but lesser decrease in the number of cerebral arteriograms.1 The recent controversy2 regarding the high cost of CT scanners and their relationship to regional health care planning should not obscure the unparalleled advantages of CT scanning for a patient in whom a structural brain lesion is suspected. During the past year prototype models of a CT scanner capable of performing a scan in less than 20 seconds have produced cross section images of the body of remarkable clarity and diagnostic utility. The radiology department in this medical center acquired one of the first of these prototype models, and it is the purpose of my presentation today to show several examples of CT body scanning images, to describe some of the salient clinical and anatomic features of these images, and to speculate where I think CT scanning will be most useful in clinical practice. CT scanning differs from conventional radiology THE WESTERN JOURNAL OF MEDICINE
119
COMPUTERIZED TOMOGRAPHY ABBREVIATIONS USED IN TEXT CT= computed tomography SMA = superior mesenteric artery VMA = vanillylmandelic acid
in several ways. Although x-rays pass through the patient during a CT scan, the detector in the system is a scintillation phototube or a gas-filled ionization chamber, rather than the usual x-ray film. The x-ray tube and the detector system are on opposite sides of the patient, and during an individual scan they rotate about the patient recording information about the internal structure of the thin transverse cross section through which the x-ray beam is passing. The data obtained by the detectors are processed by a high speed minicomputer. Through a complex series of mathematical manipulations the computer "reconstructs" the cross sectional image as an array or grid of picture elements and displays it as an integrated picture on a television monitor. The image seen on the television screen actually represents an analogue display of the digital solutions of thousands of simultaneous linear equations, with each numerical value on the grid representing a close approximation to the x-ray attenuation coefficient of the corresponding tissue within the patient's body. In most cases, the x-ray attenuation coefficient can be conveniently considered to represent the physical density of the tissue. The image on the screen bears a remarkable resemblance to photographs from standard atlases of cross-sectional anatomy. It must be remembered that these images are obtained with the patient lying comfortably on a table, and with the exception of occasional injection of intravenous contrast material, the procedure is free of any pain or significant risk or danger. Table 1 lists the advantages and disadvantages of CT body scanning compared with conventional radiographic procedures. A CT scan is a crosssectional tomogram, displaying an image of a plane approximately one centimeter in thickness. The transaxial orientation of the tomogram is at right angles to the orientation of conventional radiographic tomography, permitting visualization of normal and abnormal tissue in the dorsalventral direction. The most important feature of computed tomography is its ability to differentiate tissues of very similar density (or more precisely, of similar ability to attenuate an x-ray
beam). 120
FEBRUARY 1977 * 126 * 2
One can see different tissue elements on a conventional radiograph only when their densities differ by at least 5 percent. This permits delineation of tissues consisting largely of water, fat, bone or calcium from one another, but abnormalities within the major visceral organs or soft tissues themselves are rarely appreciated without the introduction of contrast media. The density resolution of computed tomography is of the order of one percent. This resolution provides images of tissue contours and internal organ structure previously unavailable in clinical medicine. The disadvantages of CT body scanning are mostly its cost and the length of time required to carry out an examination. At present a CT scanner costs approximately $500,000. Most scanners require three or four minutes for the computer to process the data and reconstruct each image, resulting in a total examination time of at least one hour for a typical patient requiring 8 to 12 images. The cost to the patient, therefore, is higher than for most radiologic procedures (except angiography). The radiation dose to the patient is approximately that received during a barium examination of the gastrointestinal tract. Although the radiation dose is much less than for an abdominal angiogram, the usual precautions regarding unnecessary radiation must be exercised-especially in children and young women. Table 2 lists the three general uses of CT in this hospital and the few other institutions where it is available. CT is a noninvasive way of confirming or excluding the presence of a structural lesion, usually a mass lesion, within the tissues or organs TABLE 1.-Advantages and Disadvantages of Computed Tomography Body Scanning
Advantages Tomographic focusing Transaxial view Remarkable density resolution Minimal discomfort or danger Disadvantages Cost Length of procedure Moderate radiation dose TABLE 2.-Uses of Computed Tomographic Body
Scanning Determine the presence or absence of a structural lesion. Determine the size and extent of the lesion. Suggest the specific diagnosis of the lesion. Monitor the effect of therapy on the size and extent of the lesion.
COMPUTERIZED TOMOGRAPHY
of the abdomen. In addition to detecting the presence of an abnormality, certain lesions have a characteristic CT density that permits a specific pathologic diagnosis to be made. Even when a diagnosis is made or can be made by other modalities, CT body scanning permits a convenient and unique way to plan appropriate therapyespecially if radiation therapy is indicated-and to follow the effect of specific therapy on the extent of the lesion. As I just indicated, most of the early experience with CT body scanning has centered on studies of the abdomen. It has been especially useful in studying the pancreas and retroperitoneum where conventional imaging procedures, including diagnostic ultrasound, have major limitations. Although there are certain anatomic sites outside the abdomen where the use of CT may be valuable, especially the pleura, mediastinum, and soft tissues of the extremities and axial skeleton, I will confine my discussion today to computed tomography of the abdomen.
attenuation numbers (and therefore different shades of gray on the television screen), the perivisceral fat exerts the major influence on the useful visualization of abdominal structures (Figure 1). Obese patients generally have a large amount of abdominal perivisceral fat, but a surprising number of nonobese patients may also harbor this radiologically useful characteristic of body constitution. Obviously emaciated patients usually have very little perivisceral fat, often leading to a nondiagnostic examination3 (Figure 2). Intraperitoneal fluid or ascites can be easily identified by a separation of the liver and spleen from the adjacent rib cage by tissue with the approximate density of water, a density lower than that of the parenchymal organs. If this separation is limited to the liver or spleen alone, hemorrhage
General Studies of the Abdomen Cross sectional images of the abdomen vary in their quality and utility in proportion to the amount of retroperitoneal and perivisceral fat present. This fatty material, producing a low density background to the reconstructed images transmitted to the television monitor, is responsible for the clarity and definition of the borders of the individual tissues and organs. Although some of these organs differ from each other in CT
Figure 2.-Virtual absence of perivisceral fat in this patient makes it difficult to delineate the abdominal organs.
Figure 1.-The view is looking up from the supine patient's feet. A generous amount of abdominal fat helps outline the liver (L), kidneys (K), aorta (A), inferior vena cava (V), left renal vein (RV), and body and head of the pancreas (P). Notice the superior mesenteric artery between the aorta and pancreas.
Figure 3.-The tail and body of the pancreas (P) appear normal in size, shape and texture. Also seen in cross-section are the liver, spleen (S), jejunum (J) and colon.
THE WESTERN JOURNAL OF MEDICINE
121
COMPUTERIZED TOMOGRAPHY
from that specific organ should be considered. Diffuse or localized high density medium within the lower density intraperitoneal fluid has been observed in patients seeded with metastases3 or with presumed clotted blood following a biopsy study of the liver.
Pancreas There has been widespread enthusiasm surrounding the use of CT in the evaluation of known or suspected pancreatic disease. Both the normal and abnormal pancreas can be routinely imaged in most patients. Only when there is a pronounced paucity of perivisceral fat in the abdomen is it
Figure 4.-A normal head of the pancreas (P) is seen between the duodenum (D), inferior vena cava (V) and superior mesenteric artery (S).
difficult to visualize the size and contour of the pancreas. Certain anatomic features help define the normal organ. Although the pancreas may occasionally lie horizontally in the abdomen and be seen on a single transverse section, most often it assumes an oblique orientation, only specific portions appearing on two or three successive levels.
The tail usually appears on the highest section, adjacent to the splenic hilum and the upper pole of the left kidney (Figure 3). The body is identified as the curving structure just anterior (ventral) to the superior mesenteric artery (SMA) and its surrounding plane of fat. This particular relationship to the SMA iS important because the third
Figure 6.-A pseudocyst in the head of the pancreas (Ps) is seen as a well-circumscribed, low density mass.
t
a
Figure 5.-The body and tail of the pancreas are notably enlarged in this patient with hemorrhagic pancreatitis. The pancreas has a lower density (more dark) than normal, presumably due to edema. The gallbladder (G) is also enlarged.
122
FEBRUARY 1977 * 126 * 2
Figure 7.-A carcinoma produced this focal enlarge-
ment of the head (H) of the pancreas. Note the body and tail are also in this cross-section, and appear normal.
COMPUTERIZED TOMOGRAPHY
portion of the duodenum can otherwise simulate the body of the pancreas, but the transverse duodenum lies posterior (dorsal) to the SMA, between it and the aorta. The head of the pancreas is not so conveniently identified. Its margins often blend with the second portion of the duodenum and the inferior vena cava. When a doubt exists about the precise size and contour of the head of the pancreas, a repeat scan following the ingestion of a dilute solution of iodinated oral contrast material usually resolves the question by outlining the lumen of the second portion of the duodenum lying just lateral to the pancreatic head (Figure 4). On rare occasion intravenous injection of iodinated urographic contrast material has been necessary to identify the lumen of the inferior vena cava lying dorsal to the head of the pancreas. Experience with CT scans of the pancreas in many normal patients is necessary to learn the normal variations in size and contour of this organ. In general, the organ tends to gradually increase in size from tail to head, and any impression of enlargement of a specific portion must take into account the appearance of the other portions. In a supine scan any portion of the pancreas with an anterior-posterior dimension greater than 2/3 the transverse diameter of the adjacent vertebral body should be suspected of enlargement.4 Acute pancreatitis results in a diffusely enlarged gland with an attenuation coefficient or CT number slightly less than normal (Figure 5). Pseudocysts are readily seen as round or oval, well-circumscribed zones of a notably reduced
CT number (Figure 6). A calcified rim or diffuse pancreatic calcification of chronic pancreatitis is often associated. Intravenous injection of iodinated contrast material usually helps enhance the visualization of the low density center of a pancreatic pseudocyst. In those cases of pancreatic carcinoma diagnosed by CT usually there has been shown a localized area of enlargement (Figures 7 and 8), although in a minority a more diffuse increase in size has been shown.3-5 Some have had poorly marginated low density zones within them, representing areas of necrosis, and many have obliterated the normal fat plane posterior to the gland. It is possible that a focal area of enlargement secondary to inflammation could simulate a neoplasm on the CT scan, but clinical features will
Figure 9.-The branching, low density structures in the liver represent dilated intrahepatic bile ducts (B).
M.. * -VW 4w i.
Figure 8.-Another carcinoma in the head of the pancreas. Oral contrast material fills some of the bowel, including the duodenum (D) lateral to the pancreatic mass.
Figure 10.-Liver metastases. Both the right and left lobes of the liver contain multiple small and large areas of decreased density.
THE WESTERN JOURNAL OF MEDICINE
123
COMPUTERIZED TOMOGRAPHY
often suggest the correct differential diagnosis. Clearly pancreatic tumors too small to alter the expected size and contour of the pancreas may remain undetected. Unfortunately there is no reliable difference in CT attenuation coefficient between pancreatic carcinoma and normal pancreatic parenchyma.
Liver CT of the liver has been used mostly to evaluate unexplained jaundice and to search for single or multiple mass lesions. Etiologic diagnosis of obstructive jaundice can be made when dilated intraheptic biliary radicles are visualized (Figure 9). They are characterized by low density zones within the liver-curvilinear near the hilum, horizontal in the left lobe and spherical in the right lobe (where they are seen end-on).36 They are visualized only because the attenuation coefficient of bile is lower than that of normal liver
parenchyma. Unfortunately the normal portal venous tree, coursing through the liver adjacent to the biliary radicles, occasionally produces a CT image identical to that observed with dilated biliary ducts. Injection of intravenous urographic contrast material enhances visualization of biliary ducts by perfusing the extraductal tissues, while it obscures the portal venous structures by perfusing them and elevating their density to a level near that of the surrounding parenchyma. Among the questions yet to be answered are: (1) the degree and duration of obstruction necessary before dilatation of the intrahepatic ducts occurs, (2) the degree of dilatation necessary before it can be imaged by CT and (3) the temporal relationship between dilatation of the common
Figure 12.-The normal retroperitoneal structures are usually well outlined by fat. Note the symmetrical psoas muscles just lateral to the vertebral body.
Figure 11.-Upper, a cyst (C) in the lateral aspect of the right kidney had a density reading similar to water. Lower, intravenous contrast material increases the density of the renal parenchyma, but not that of the cyst.
124
FEBRUARY 1977 * 126 * 2
Figure 13.-The right psoas muscle is much larger than the left, and contains a low density core within it (A). A pyogenic abscess was subsequently drained at surgical operation.
COMPUTERIZED TOMOGRAPHY
bile duct and of the intraheptic bile ducts. I have seen one case of an obstructed, dilated common bile duct with intrahepatic ducts of normal caliber in a patient with liver cirrhosis, and one case of dilated intrahepatic ducts not seen on a 21/2 minute scanner. Accurate delineation of dilated bile ducts has also been reported with recent advances in gray-scale ultrasonography. It will be important to determine the sensitivity and specificity of this finding using these two noninvasive modalities. CT evaluation of the liver should provide a rational first step in the medical evaluation of uncxplained jaundice. If dilated ducts are seen on the CT scan, a percutaneous transhepatic cholangiogram using the small caliber needle technique should follow in order to confirm the diagnosis and locate the precise site of obstruction. Frequently the site of obstruction can be deduced, especially if a mass is shown in the head of the pancreas. If dilated ducts are not shown by CT, conservative medical therapy for presumed nonobstructive disease can be elected, or an endoscopic retrograde cholangiopancreatogram can be done in some cases. CT of the liver shows mass lesions which possess attenuation coefficients sufficiently different from normal liver (Figure 10). When such differences occur, lesions that are smaller and deeper than those seen on conventional radionuclide liver scans can be identified. Sometimes, however, very large lesions easily seen on isotopic scans are not visualized by CT because their density is only
Figure 14.-A mantle of tissue (N) completely traverses the prevertebral space, obscuring the normal borders of the aorta and inferior vena cava. This pattem is characteristic of diffuse retroperitoneal lymphade-
nopathy.
slightly different from normal parenchyma. Intravenous injection of iodinated contrast material can be used to increase the CT number of normal liver and thus enhance visualization of low density lesions. However, a precontrast examination of the liver must never be omitted because occasionally a low density vascular lesion will be obscured by contrast injection. Even if a radionuclide liver scan shows a filling defect, CT scanning can determine which defects are due to cysts and which to simply an anatomic thinning of the liver. CT scans showing pronounced diminution of liver density in the presence of fatty infiltration, or a notable increase in liver density in hemochromatosis, have also been
described.3-6
Kidney Computed tomography is useful in evaluation of three types of renal abnormalities seen at urography: (1) renal mass, (2) nonfunctioning kidney and (3) abnormal renal axis. Renal cysts present a CT image of a mass having a CT number near that of water, a number that does not change following intravenous injection of urographic contrast material (Figure 11 ) .3 The CT number of a renal carcinoma is close to that of the normal renal parenchyma, and it increases in density with contrast injection, though not to the same degree as the adjacent normal tissue. Solid renal masses other than cancer presumably will show the similar CT characteristics; further experience is necessary to ascertain whether or not any differences will reliably distinguish renal cancer from other noncystic lesions. As of the time that this report is being presented, the accuracy of CT in distinguishing cysts from solid lesions has not been established. I have seen one case of multiple benign, hypervascular angiomyolipomas in which the correct diagnosis was made by CT because of the readily apparent fatty nature of the tumors. The role of CT in routine evaluation of renal masses will probably be a limited one in places where high quality ultrasonography is present, because the diagnostic accuracy of the latter can be as high as 95 percent. CT will probably be reserved for those renal masses in which the ultrasound pattern is indeterminate in type, or where the sonogram is technically suboptimal. Because the density of any mass depends on whether all or a part is included in the volume of a given picture element, it is possible that very small inTHE WESTERN JOURNAL OF MEDICINE
125
COMPUTERIZED TOMOGRAPHY
trarenal lesions will not be confidently differentiated by CT evaluation. A kidney nonfunctioning at urography can be easily evaluated by CT. Hydronephrosis presents a characteristic image of an oval, water density mass in the renal sinus. Even if the diagnosis of obstructive hydronephrosis is apparent at urography, CT can be used to determine whether a mass is present at the site of obstruction. We recently examined a hypertensive child with elevated vanillylmandelic acid (VMA) levels whose urogram showed an obstructed ureter near its entrance into the bladder. A 2 cm oval mass was shown on the CT section taken at the site of obstruction. It proved to be an extra-adrenal pheochromocytoma. One of the most perplexing problems encountered during urography is that of an abnormal renal axis, with the kidney lying in a vertical orientation or with the upper pole more lateral in position than the lower pole. Such a finding usually raises the possibility of a paraspinal retroperitoneal mass, but previously this could lead to an often unrewarding series of diagnostic maneuvers. A CT scan centered on the site of altered renal axis safely leads to a rapid diagnosis. Frequently a soft tissue mass is easily identified, at other times just a large amount of retroperitoneal fat lies between the spine and the kidney.
Retroperitoneal Space More than any other area of the body, the retroperitoneal space is uniquely and spectacularly imaged by CT scanning (Figure 12). The pancreas, of course, lies at the cephalic end of the retroperitoneum, and we have already described the salient features of CT pancreatic scanning. The remaining portion of the retroperitoneum is frequently the site of neoplasms, abscesses and hematomas (Figure 13), all of which are notoriously difficult to diagnose by conventional imag-
126
FEBRUARY 1977 * 126 * 2
ing techniques. The aorta, inferior vena cava and both psoas tnuscles are usually well outlined by surrounding fat, and any significant amount of abnormal tissue is usually readily apparent on the CT scan. Even the CT demonstration of a normal retroperitoneum can be very useful in a patient with unexplained abdominal or back pain. Lymph nodes are normally seen as very small structures of variable number surrounding the aorta and inferior vena cava, many lying between the spine and the great vessels. When these nodes become grossly enlarged, a CT diagnosis of retroperitoneal adenopathy is easily made, particularly when the nodes occur in the normally clear space between the aorta and left psoas muscle (Figure 14). But current resolution of CT scanners does not allow diagnosis of infiltrative disease of normal sized nodes. Although negative findings on CT scan do not exclude significant disease in normal sized nodes, enlarged retroperitoneal nodes shown in patients with known or suspected lymphomas may in selected cases obviate lymphangiography. Scans following lymphangiography in patients have shown that the lateral extent of dispersion of the lymphangiographic contrast material in lymphomatous nodes often underestimates the true lateral extent of the abnormal tissue. Clearly our general approach to the diagnosis and evaluation of space occupying lesions is in the process of being revolutionized by the noninvasive computerized tomography. REFERENCES 1. Baker HL: The impact of computed tomography on neuroradiologic practice. Radiology 116:637-640, 1975 2. Shapiro SH: CAT fever. N Engl J Med 294:954-956, 1976 3. Sagel S: Early clinical experience with motionless whole body computed tomography. Radiology 119:321-330, 1976 4. Haaga JR: Precise biopsy localization by computed tomography. Radiology 118:603-607, 1976 5. Sheedy PF: Computed tomography of the body; initial clinical trial with the EMI prototype. Am J Roentgenol Radium Ther Nucl Med 127:23-51, 1976 6. Stanley RJ: Computed tomography of the body; early trends in application and accuracy of the method. Am J Roentgenol Radium Ther Nucl Med 127:53-67, 1976
Refer to: Current diagnostic uses of computerized tomography in clinical medicine-Medical Staff Conference, University of California, San Francisco. West J Med 126:119-126, Feb 1977
Current Diagnostic Uses of Computerized Tomography in Clinical Medicine These discussions are selected from the weekly staff conferences in the Department of Medicine, University of California, San Francisco. Taken from transcriptions, they are prepared by Drs. David W. Martin, Jr., Associate Professor of Medicine, and H. David Watts, Assistant Professor of Medicine, under the direction of Dr. Lloyd H. Smith, Jr., Professor of Medicine and Chairman of the Department of Medicine. Requests for reprints should be sent to the Department of Medicine, University of California, San Francisco, CA 94143.
DR. SMITH:* There has been a remarkable increase in the use of advanced technology in medical practice during the past decade. This adds to the complexity and expense of diagnosis and therapy, but also lends new assistance to physicians. Perhaps the area where the advances have been most impressive in recent years lies in the use of computerized tomography to image previously inaccessible organs noninvasively. We are pleased to have Dr. Melvyn Korobkin with us today to review the current status of this technology and its application to the diagnosis of diseases of abdominal organs. DR. KOROBKIN: t There are few advances in medicine that can be considered revolutionary, but computerized tomography truly represents such a phenomenon. Many by now are familiar with computed tomography (CT) of the brain, a noninvasive way of imaging known or suspected lesions, whether they be tumors, infarcts, hematomas or abscesses. In the four short years since this technique was introduced into clinical practice, CT brain scanning has met almost unanimous *Lloyd H. Smith, Jr., MD, Professor and Chairman, Department of Medicine. tMelvyn T. Korobkin, MD, Associate Professor in Radiology.
and enthusiastic approval by neurologists, neurosurgeons, radiologists and patients. The presence of brain computerized tomographic scanners in this country has spread rapidly. There is convincing evidence that the number of pneumoencephalograms carried out has dropped drastically, with a similar but lesser decrease in the number of cerebral arteriograms.1 The recent controversy2 regarding the high cost of CT scanners and their relationship to regional health care planning should not obscure the unparalleled advantages of CT scanning for a patient in whom a structural brain lesion is suspected. During the past year prototype models of a CT scanner capable of performing a scan in less than 20 seconds have produced cross section images of the body of remarkable clarity and diagnostic utility. The radiology department in this medical center acquired one of the first of these prototype models, and it is the purpose of my presentation today to show several examples of CT body scanning images, to describe some of the salient clinical and anatomic features of these images, and to speculate where I think CT scanning will be most useful in clinical practice. CT scanning differs from conventional radiology THE WESTERN JOURNAL OF MEDICINE
119
COMPUTERIZED TOMOGRAPHY ABBREVIATIONS USED IN TEXT CT= computed tomography SMA = superior mesenteric artery VMA = vanillylmandelic acid
in several ways. Although x-rays pass through the patient during a CT scan, the detector in the system is a scintillation phototube or a gas-filled ionization chamber, rather than the usual x-ray film. The x-ray tube and the detector system are on opposite sides of the patient, and during an individual scan they rotate about the patient recording information about the internal structure of the thin transverse cross section through which the x-ray beam is passing. The data obtained by the detectors are processed by a high speed minicomputer. Through a complex series of mathematical manipulations the computer "reconstructs" the cross sectional image as an array or grid of picture elements and displays it as an integrated picture on a television monitor. The image seen on the television screen actually represents an analogue display of the digital solutions of thousands of simultaneous linear equations, with each numerical value on the grid representing a close approximation to the x-ray attenuation coefficient of the corresponding tissue within the patient's body. In most cases, the x-ray attenuation coefficient can be conveniently considered to represent the physical density of the tissue. The image on the screen bears a remarkable resemblance to photographs from standard atlases of cross-sectional anatomy. It must be remembered that these images are obtained with the patient lying comfortably on a table, and with the exception of occasional injection of intravenous contrast material, the procedure is free of any pain or significant risk or danger. Table 1 lists the advantages and disadvantages of CT body scanning compared with conventional radiographic procedures. A CT scan is a crosssectional tomogram, displaying an image of a plane approximately one centimeter in thickness. The transaxial orientation of the tomogram is at right angles to the orientation of conventional radiographic tomography, permitting visualization of normal and abnormal tissue in the dorsalventral direction. The most important feature of computed tomography is its ability to differentiate tissues of very similar density (or more precisely, of similar ability to attenuate an x-ray
beam). 120
FEBRUARY 1977 * 126 * 2
One can see different tissue elements on a conventional radiograph only when their densities differ by at least 5 percent. This permits delineation of tissues consisting largely of water, fat, bone or calcium from one another, but abnormalities within the major visceral organs or soft tissues themselves are rarely appreciated without the introduction of contrast media. The density resolution of computed tomography is of the order of one percent. This resolution provides images of tissue contours and internal organ structure previously unavailable in clinical medicine. The disadvantages of CT body scanning are mostly its cost and the length of time required to carry out an examination. At present a CT scanner costs approximately $500,000. Most scanners require three or four minutes for the computer to process the data and reconstruct each image, resulting in a total examination time of at least one hour for a typical patient requiring 8 to 12 images. The cost to the patient, therefore, is higher than for most radiologic procedures (except angiography). The radiation dose to the patient is approximately that received during a barium examination of the gastrointestinal tract. Although the radiation dose is much less than for an abdominal angiogram, the usual precautions regarding unnecessary radiation must be exercised-especially in children and young women. Table 2 lists the three general uses of CT in this hospital and the few other institutions where it is available. CT is a noninvasive way of confirming or excluding the presence of a structural lesion, usually a mass lesion, within the tissues or organs TABLE 1.-Advantages and Disadvantages of Computed Tomography Body Scanning
Advantages Tomographic focusing Transaxial view Remarkable density resolution Minimal discomfort or danger Disadvantages Cost Length of procedure Moderate radiation dose TABLE 2.-Uses of Computed Tomographic Body
Scanning Determine the presence or absence of a structural lesion. Determine the size and extent of the lesion. Suggest the specific diagnosis of the lesion. Monitor the effect of therapy on the size and extent of the lesion.
COMPUTERIZED TOMOGRAPHY
of the abdomen. In addition to detecting the presence of an abnormality, certain lesions have a characteristic CT density that permits a specific pathologic diagnosis to be made. Even when a diagnosis is made or can be made by other modalities, CT body scanning permits a convenient and unique way to plan appropriate therapyespecially if radiation therapy is indicated-and to follow the effect of specific therapy on the extent of the lesion. As I just indicated, most of the early experience with CT body scanning has centered on studies of the abdomen. It has been especially useful in studying the pancreas and retroperitoneum where conventional imaging procedures, including diagnostic ultrasound, have major limitations. Although there are certain anatomic sites outside the abdomen where the use of CT may be valuable, especially the pleura, mediastinum, and soft tissues of the extremities and axial skeleton, I will confine my discussion today to computed tomography of the abdomen.
attenuation numbers (and therefore different shades of gray on the television screen), the perivisceral fat exerts the major influence on the useful visualization of abdominal structures (Figure 1). Obese patients generally have a large amount of abdominal perivisceral fat, but a surprising number of nonobese patients may also harbor this radiologically useful characteristic of body constitution. Obviously emaciated patients usually have very little perivisceral fat, often leading to a nondiagnostic examination3 (Figure 2). Intraperitoneal fluid or ascites can be easily identified by a separation of the liver and spleen from the adjacent rib cage by tissue with the approximate density of water, a density lower than that of the parenchymal organs. If this separation is limited to the liver or spleen alone, hemorrhage
General Studies of the Abdomen Cross sectional images of the abdomen vary in their quality and utility in proportion to the amount of retroperitoneal and perivisceral fat present. This fatty material, producing a low density background to the reconstructed images transmitted to the television monitor, is responsible for the clarity and definition of the borders of the individual tissues and organs. Although some of these organs differ from each other in CT
Figure 2.-Virtual absence of perivisceral fat in this patient makes it difficult to delineate the abdominal organs.
Figure 1.-The view is looking up from the supine patient's feet. A generous amount of abdominal fat helps outline the liver (L), kidneys (K), aorta (A), inferior vena cava (V), left renal vein (RV), and body and head of the pancreas (P). Notice the superior mesenteric artery between the aorta and pancreas.
Figure 3.-The tail and body of the pancreas (P) appear normal in size, shape and texture. Also seen in cross-section are the liver, spleen (S), jejunum (J) and colon.
THE WESTERN JOURNAL OF MEDICINE
121
COMPUTERIZED TOMOGRAPHY
from that specific organ should be considered. Diffuse or localized high density medium within the lower density intraperitoneal fluid has been observed in patients seeded with metastases3 or with presumed clotted blood following a biopsy study of the liver.
Pancreas There has been widespread enthusiasm surrounding the use of CT in the evaluation of known or suspected pancreatic disease. Both the normal and abnormal pancreas can be routinely imaged in most patients. Only when there is a pronounced paucity of perivisceral fat in the abdomen is it
Figure 4.-A normal head of the pancreas (P) is seen between the duodenum (D), inferior vena cava (V) and superior mesenteric artery (S).
difficult to visualize the size and contour of the pancreas. Certain anatomic features help define the normal organ. Although the pancreas may occasionally lie horizontally in the abdomen and be seen on a single transverse section, most often it assumes an oblique orientation, only specific portions appearing on two or three successive levels.
The tail usually appears on the highest section, adjacent to the splenic hilum and the upper pole of the left kidney (Figure 3). The body is identified as the curving structure just anterior (ventral) to the superior mesenteric artery (SMA) and its surrounding plane of fat. This particular relationship to the SMA iS important because the third
Figure 6.-A pseudocyst in the head of the pancreas (Ps) is seen as a well-circumscribed, low density mass.
t
a
Figure 5.-The body and tail of the pancreas are notably enlarged in this patient with hemorrhagic pancreatitis. The pancreas has a lower density (more dark) than normal, presumably due to edema. The gallbladder (G) is also enlarged.
122
FEBRUARY 1977 * 126 * 2
Figure 7.-A carcinoma produced this focal enlarge-
ment of the head (H) of the pancreas. Note the body and tail are also in this cross-section, and appear normal.
COMPUTERIZED TOMOGRAPHY
portion of the duodenum can otherwise simulate the body of the pancreas, but the transverse duodenum lies posterior (dorsal) to the SMA, between it and the aorta. The head of the pancreas is not so conveniently identified. Its margins often blend with the second portion of the duodenum and the inferior vena cava. When a doubt exists about the precise size and contour of the head of the pancreas, a repeat scan following the ingestion of a dilute solution of iodinated oral contrast material usually resolves the question by outlining the lumen of the second portion of the duodenum lying just lateral to the pancreatic head (Figure 4). On rare occasion intravenous injection of iodinated urographic contrast material has been necessary to identify the lumen of the inferior vena cava lying dorsal to the head of the pancreas. Experience with CT scans of the pancreas in many normal patients is necessary to learn the normal variations in size and contour of this organ. In general, the organ tends to gradually increase in size from tail to head, and any impression of enlargement of a specific portion must take into account the appearance of the other portions. In a supine scan any portion of the pancreas with an anterior-posterior dimension greater than 2/3 the transverse diameter of the adjacent vertebral body should be suspected of enlargement.4 Acute pancreatitis results in a diffusely enlarged gland with an attenuation coefficient or CT number slightly less than normal (Figure 5). Pseudocysts are readily seen as round or oval, well-circumscribed zones of a notably reduced
CT number (Figure 6). A calcified rim or diffuse pancreatic calcification of chronic pancreatitis is often associated. Intravenous injection of iodinated contrast material usually helps enhance the visualization of the low density center of a pancreatic pseudocyst. In those cases of pancreatic carcinoma diagnosed by CT usually there has been shown a localized area of enlargement (Figures 7 and 8), although in a minority a more diffuse increase in size has been shown.3-5 Some have had poorly marginated low density zones within them, representing areas of necrosis, and many have obliterated the normal fat plane posterior to the gland. It is possible that a focal area of enlargement secondary to inflammation could simulate a neoplasm on the CT scan, but clinical features will
Figure 9.-The branching, low density structures in the liver represent dilated intrahepatic bile ducts (B).
M.. * -VW 4w i.
Figure 8.-Another carcinoma in the head of the pancreas. Oral contrast material fills some of the bowel, including the duodenum (D) lateral to the pancreatic mass.
Figure 10.-Liver metastases. Both the right and left lobes of the liver contain multiple small and large areas of decreased density.
THE WESTERN JOURNAL OF MEDICINE
123
COMPUTERIZED TOMOGRAPHY
often suggest the correct differential diagnosis. Clearly pancreatic tumors too small to alter the expected size and contour of the pancreas may remain undetected. Unfortunately there is no reliable difference in CT attenuation coefficient between pancreatic carcinoma and normal pancreatic parenchyma.
Liver CT of the liver has been used mostly to evaluate unexplained jaundice and to search for single or multiple mass lesions. Etiologic diagnosis of obstructive jaundice can be made when dilated intraheptic biliary radicles are visualized (Figure 9). They are characterized by low density zones within the liver-curvilinear near the hilum, horizontal in the left lobe and spherical in the right lobe (where they are seen end-on).36 They are visualized only because the attenuation coefficient of bile is lower than that of normal liver
parenchyma. Unfortunately the normal portal venous tree, coursing through the liver adjacent to the biliary radicles, occasionally produces a CT image identical to that observed with dilated biliary ducts. Injection of intravenous urographic contrast material enhances visualization of biliary ducts by perfusing the extraductal tissues, while it obscures the portal venous structures by perfusing them and elevating their density to a level near that of the surrounding parenchyma. Among the questions yet to be answered are: (1) the degree and duration of obstruction necessary before dilatation of the intrahepatic ducts occurs, (2) the degree of dilatation necessary before it can be imaged by CT and (3) the temporal relationship between dilatation of the common
Figure 12.-The normal retroperitoneal structures are usually well outlined by fat. Note the symmetrical psoas muscles just lateral to the vertebral body.
Figure 11.-Upper, a cyst (C) in the lateral aspect of the right kidney had a density reading similar to water. Lower, intravenous contrast material increases the density of the renal parenchyma, but not that of the cyst.
124
FEBRUARY 1977 * 126 * 2
Figure 13.-The right psoas muscle is much larger than the left, and contains a low density core within it (A). A pyogenic abscess was subsequently drained at surgical operation.
COMPUTERIZED TOMOGRAPHY
bile duct and of the intraheptic bile ducts. I have seen one case of an obstructed, dilated common bile duct with intrahepatic ducts of normal caliber in a patient with liver cirrhosis, and one case of dilated intrahepatic ducts not seen on a 21/2 minute scanner. Accurate delineation of dilated bile ducts has also been reported with recent advances in gray-scale ultrasonography. It will be important to determine the sensitivity and specificity of this finding using these two noninvasive modalities. CT evaluation of the liver should provide a rational first step in the medical evaluation of uncxplained jaundice. If dilated ducts are seen on the CT scan, a percutaneous transhepatic cholangiogram using the small caliber needle technique should follow in order to confirm the diagnosis and locate the precise site of obstruction. Frequently the site of obstruction can be deduced, especially if a mass is shown in the head of the pancreas. If dilated ducts are not shown by CT, conservative medical therapy for presumed nonobstructive disease can be elected, or an endoscopic retrograde cholangiopancreatogram can be done in some cases. CT of the liver shows mass lesions which possess attenuation coefficients sufficiently different from normal liver (Figure 10). When such differences occur, lesions that are smaller and deeper than those seen on conventional radionuclide liver scans can be identified. Sometimes, however, very large lesions easily seen on isotopic scans are not visualized by CT because their density is only
Figure 14.-A mantle of tissue (N) completely traverses the prevertebral space, obscuring the normal borders of the aorta and inferior vena cava. This pattem is characteristic of diffuse retroperitoneal lymphade-
nopathy.
slightly different from normal parenchyma. Intravenous injection of iodinated contrast material can be used to increase the CT number of normal liver and thus enhance visualization of low density lesions. However, a precontrast examination of the liver must never be omitted because occasionally a low density vascular lesion will be obscured by contrast injection. Even if a radionuclide liver scan shows a filling defect, CT scanning can determine which defects are due to cysts and which to simply an anatomic thinning of the liver. CT scans showing pronounced diminution of liver density in the presence of fatty infiltration, or a notable increase in liver density in hemochromatosis, have also been
described.3-6
Kidney Computed tomography is useful in evaluation of three types of renal abnormalities seen at urography: (1) renal mass, (2) nonfunctioning kidney and (3) abnormal renal axis. Renal cysts present a CT image of a mass having a CT number near that of water, a number that does not change following intravenous injection of urographic contrast material (Figure 11 ) .3 The CT number of a renal carcinoma is close to that of the normal renal parenchyma, and it increases in density with contrast injection, though not to the same degree as the adjacent normal tissue. Solid renal masses other than cancer presumably will show the similar CT characteristics; further experience is necessary to ascertain whether or not any differences will reliably distinguish renal cancer from other noncystic lesions. As of the time that this report is being presented, the accuracy of CT in distinguishing cysts from solid lesions has not been established. I have seen one case of multiple benign, hypervascular angiomyolipomas in which the correct diagnosis was made by CT because of the readily apparent fatty nature of the tumors. The role of CT in routine evaluation of renal masses will probably be a limited one in places where high quality ultrasonography is present, because the diagnostic accuracy of the latter can be as high as 95 percent. CT will probably be reserved for those renal masses in which the ultrasound pattern is indeterminate in type, or where the sonogram is technically suboptimal. Because the density of any mass depends on whether all or a part is included in the volume of a given picture element, it is possible that very small inTHE WESTERN JOURNAL OF MEDICINE
125
COMPUTERIZED TOMOGRAPHY
trarenal lesions will not be confidently differentiated by CT evaluation. A kidney nonfunctioning at urography can be easily evaluated by CT. Hydronephrosis presents a characteristic image of an oval, water density mass in the renal sinus. Even if the diagnosis of obstructive hydronephrosis is apparent at urography, CT can be used to determine whether a mass is present at the site of obstruction. We recently examined a hypertensive child with elevated vanillylmandelic acid (VMA) levels whose urogram showed an obstructed ureter near its entrance into the bladder. A 2 cm oval mass was shown on the CT section taken at the site of obstruction. It proved to be an extra-adrenal pheochromocytoma. One of the most perplexing problems encountered during urography is that of an abnormal renal axis, with the kidney lying in a vertical orientation or with the upper pole more lateral in position than the lower pole. Such a finding usually raises the possibility of a paraspinal retroperitoneal mass, but previously this could lead to an often unrewarding series of diagnostic maneuvers. A CT scan centered on the site of altered renal axis safely leads to a rapid diagnosis. Frequently a soft tissue mass is easily identified, at other times just a large amount of retroperitoneal fat lies between the spine and the kidney.
Retroperitoneal Space More than any other area of the body, the retroperitoneal space is uniquely and spectacularly imaged by CT scanning (Figure 12). The pancreas, of course, lies at the cephalic end of the retroperitoneum, and we have already described the salient features of CT pancreatic scanning. The remaining portion of the retroperitoneum is frequently the site of neoplasms, abscesses and hematomas (Figure 13), all of which are notoriously difficult to diagnose by conventional imag-
126
FEBRUARY 1977 * 126 * 2
ing techniques. The aorta, inferior vena cava and both psoas tnuscles are usually well outlined by surrounding fat, and any significant amount of abnormal tissue is usually readily apparent on the CT scan. Even the CT demonstration of a normal retroperitoneum can be very useful in a patient with unexplained abdominal or back pain. Lymph nodes are normally seen as very small structures of variable number surrounding the aorta and inferior vena cava, many lying between the spine and the great vessels. When these nodes become grossly enlarged, a CT diagnosis of retroperitoneal adenopathy is easily made, particularly when the nodes occur in the normally clear space between the aorta and left psoas muscle (Figure 14). But current resolution of CT scanners does not allow diagnosis of infiltrative disease of normal sized nodes. Although negative findings on CT scan do not exclude significant disease in normal sized nodes, enlarged retroperitoneal nodes shown in patients with known or suspected lymphomas may in selected cases obviate lymphangiography. Scans following lymphangiography in patients have shown that the lateral extent of dispersion of the lymphangiographic contrast material in lymphomatous nodes often underestimates the true lateral extent of the abnormal tissue. Clearly our general approach to the diagnosis and evaluation of space occupying lesions is in the process of being revolutionized by the noninvasive computerized tomography. REFERENCES 1. Baker HL: The impact of computed tomography on neuroradiologic practice. Radiology 116:637-640, 1975 2. Shapiro SH: CAT fever. N Engl J Med 294:954-956, 1976 3. Sagel S: Early clinical experience with motionless whole body computed tomography. Radiology 119:321-330, 1976 4. Haaga JR: Precise biopsy localization by computed tomography. Radiology 118:603-607, 1976 5. Sheedy PF: Computed tomography of the body; initial clinical trial with the EMI prototype. Am J Roentgenol Radium Ther Nucl Med 127:23-51, 1976 6. Stanley RJ: Computed tomography of the body; early trends in application and accuracy of the method. Am J Roentgenol Radium Ther Nucl Med 127:53-67, 1976
