Gastrointestinal
Gastrointest Radiol 1, 201-208 (1976)
Radiology ~ by Springer-Verlag 1976
Computed Tomography of the Abdomen: Initial Experience J e r r y P. P e t a s n i c k a n d J o h n W . C l a r k Department of Radiology, Rush-Presbyterian St. Luke's Medical Center, Chicago, illinois, U.S.A.
Abstract. C o m p u t e d t o m o g r a p h y
has provided a new d i m e n s i o n in t h e r o e n t g e n o l o g i c e v a l u a t i o n o f t h e a b domen. Normal structures not visible on conventional examinations are clearly identified. Abnormalities are recognized by their alterations in anatomic form or by their effect on tissue absorption values. Our early e x p e r i e n c e s u g g e s t s t h a t in t h e a b d o m e n c o m p u t e d t o m o g r a p h y will b e m o s t v a l u a b l e in d e t e c t i n g l e s i o n s in t h o s e sites l e a s t a c c e s s i b l e t o c o n v e n t i o n a l r o e n t g e n o g r a p h i c m e t h o d s s u c h as t h e liver, s p l e e n , p a n c r e a s and retroperitoneum.
Key words: A b d o m e n , tomography,
technique
abnormalities - Neoplasms,
- Computed diagnosis.
Computed tomography (CT) has had a dramatic imp a c t o n t h e p r a c t i c e o f n e u r o r a d i o l o g y s i n c e its i n i t i a l d e s c r i p t i o n b y H o u n s f i e l d [1] a n d A m b r o s e [2]. A s a result, m a j o r c h a n g e s h a v e o c c u r r e d in t h e p a t t e r n of utilization of other diagnostic neuroradiologic e x a m i n a t i o n s [3]. R e c e n t l y , a d v a n c e s in d e s i g n h a v e a l l o w e d t h i s t e c h n i q u e t o b e u s e d in t h e e v a l u a t i o n of other regions of the body. Several whole body s c a n n e r s h a v e b e e n in u s e f o r o n e o r m o r e y e a r s a n d p r e l i m i n a r y r e p o r t s h a v e d e m o n s t r a t e d t h e i r usef u l n e s s p a r t i c u l a r l y in e x a m i n i n g t h e a b d o m e n [4-9]. A n E M I C T 5000 b o d y s c a n n e r h a s b e e n in u s e a t the Rush-Presbyterian St. L u k e ' s M e d i c a l C e n t e r s i n c e A p r i l 1976 a n d e x a m p l e s o f its a p p l i c a t i o n will be demonstrated.
Materials and Methods The EMI body scanner permits the abdomen to be examined as a series of reconstructed transaxial tomographic sections 13 mm Address reprint requests to." Jerry P. Petasnick, M.D., Department
of Radiology, Rush-Presbyterian St. Luke's Medical Center, 1753 W. Congress Parkway, Chicago, IL 60612, U.S.A.
in thickness. The scanning gantry contains a scanning frame on which an X-ray tube is mounted diametrically opposite an array of 30 highly senitive sodium iodide crystal detectors. The patient lies on a movable table that extends through the scanning gantry and allows any portion of the body to be positioned between the X-ray source and the detector array. A tightly collimated fanshaped X-ray beam is used for examination. The scanning frame traverses linearly across the patient and the photons in the beam emerging from the patient are measured by the detectors. The linear traverse takes just over 1 s during which time more than 18,000 readings of the intensity of the X-ray photons in the emergent beam are recorded by the detectors. At the end of the linear traverse the scanning frame with the X-ray tube and detectors rotates around the patient by 10~ and another linear traverse is made. The process of traversing and 10~ rotation continues until 18 linear traverses spanning 180~ have been completed. This sequence requiring 20 s produces one tomographic section. The readings taken during the scanning process are digitized and continuously introduced into a computer during the scanning process. The completed computed tomogram of the section under examination represents a cross-sectional image of the X-ray absorption values. The reconstructed image representing the anatomic structures of the section scanned is displayed on a television monitor in various shades of gray corresponding to the calculated absorption values. The image can be manipulated on the display console to permit a selective display of any particular absorption value from the wide spectrum of absorption values obtained during the scan. The images are oriented as though viewed from the patient's feet. This places the supine patient's right side to the left of the viewing console. Preliminary absorbed dose measurements on our unit indicate that the integrated surface dose during a study of the abdomen (140 kV, 28 mA, 20 s) is 2.6 rad/scan [10]. The surface dose for a series of eight sequential sections is increase to about 5 rads because of the scattered radiation. The integrated surface dose is about the same as that from conventional roentgenographic exams such as intravenous pyelography with nephrotomography but less than that from selective abdominal arteriography.
Results Computed tomography permits anatomic structures and pathologic alterations to be viewed in a manner not possible by conventional roentgenographic techniques. Anatomic structures can be identified by computed tomography when only negligible differences
202
J.P. Petasnick and J.W. Clark: Computed Tomography of the Abdomen
in attenuation coefficients exist provided these structures are separated by interfaces of different attenuation properties. Abnormalities can be recognized as alterations in anatomic form or variations in tissue absorption coefficient or both. Examples demonstrating the use of computed tomography will be considered in the following sections.
Peritoneal Cavity Ascitic fluid can be readily identified because its density is less than that of most intra-abdominal organs (Fig. 1). The liver and often the spleen are displaced away from the lateral abdominal wall and their margins clearly visualized. Parietal peritoneal tumor implants can be identified.
Liver The normal liver (Fig. 2) is homogeneous in density and varies considerably in size and configuration. The liver is usually denser than the other intra-abdominal organs and increases in density following the intravenous administration of iodinated contrast media. In the normal density range, normal peripheral bile ducts and arteriovenous structures are usually not visualized. In some normal patients, non-dilated central bile ducts can be seen and arteriovenous structures may be identified if the fat content of the liver is increased (Fig. 3). The right lobe of the liver occupies most of the right half of the abdomen in sections near the diaphragm (Fig. 2). In the more caudally located sections the right renal fossa can be identified posteriorly and the inferior tip of the right lobe assumes a more anterior position (Fig. 4). The left lobe extends across the midline to varying degrees and is much smaller in volume than the right lobe. In most patients there is no clear-cut delineation between the right and left lobes. The porta hepatis can be identified as a transverse cleft on the posterior surface of the liver; however, the structures within the porta hepatis are usually not seen. Tumors in the liver, whether benign or malignant are generally less dense than the normal liver parenchyma (Figs. 5 and 7). This difference is usually accentuated by the intravenous administration of iodinated contrast media. Malignant tumors, primary or metastatic, usually present as clearly delineated masses less dense than normal liver parenchyma, but higher in density than hepatic cysts or dilated bile ducts. At times, however, the initial density of the tumor may be almost the same as that of normal liver parenchyma making identification difficult
(Fig. 6A). However, these lesions are usually seen following the intravenous administration of iodinated contrast media as the density of the normal liver increases more than that of the tumor (Fig. 6B). Tumors which appear vascular on angiographic examination are usually less dense than normal liver parenchyma even after the intravenous administration of contrast media; however, on occasion, tumor masses clearly seen on the pre-infusion scan disappear following the administration of contrast media. For this reason, pre-infusion scans should always be obtained when contrast media is used. Cystic lesions can be readily detected by their low density {Fig. 7). Reliance on configuration alone may produce diagnostic problems as some metastatic lesions have smooth margins (Fig. 6). Hepatic abscesses are less dense than liver and may have irregular margins [6, 8, 11]. They may mimic primary or metastatic neoplasms. History is usually helpful in establishing a diagnosis.
Biliarv Tract Normal bile ducts are usually not visualized; however, following dilatation the bile ducts are readily seen because their density is less than that of normal liver parenchyma (Fig. 8). In the region of the porta hepatis a branching pattern is seen (Fig. 8A), while the more peripherally located ducts present as smooth round defects (Fig. 8 B). They can usually be differentiated from metastatic lesions because they are more sharply circumscribed and are less dense than metastatic tumor; however, care must be taken in their analysis since liver metastases may be present in patients with obstructive jaundice. Stanley et al. [9] have demonstrated that computed tomography is highly accurate in differentiating obstructive from n o n - o b structive jaundice. The gallbladder can usually be identified as a welldefined oval area of watea" or slightly higher density in the region of the porta hepatis (Fig. 12). Dilatation is easily recognized (Figs. 8 and 10) and may help to localize the site of obstruction in patients with jaundice. Calcified gallstones are readily detected and oftentimes gallstones not seen on plain films of the abdomen may be identified (Fig. 12). Primary evaluation of the gallbladder by computed tomography is of limited use except in the above conditions or occasionally in some patients with a non-visualizing gallbladder.
Spleen Splenic size, configuration and density can be readily evaluated. Splenic enlargement can be identified and
Fig. 1. A 55-year-old female with ascites. Scan of the upper abdomen demonstrates a zone of decreased density along the lateral margin of the right lobe of the liver (L). Spleen (S) and fundus of stomach (F) also seen on this section Fig. 2. Normal scan through the upper abdomen demonstrating liver (L), spleen (5), fundus of stomach (F). The aorta (A) can be seen anterior to the vertebral centrum
Fig. 3. A 45-year-old male with fatty infiltration of the liver. Note the striking difference in the density of the liver (L) and spleen (5). The linear structures of increased density within the liver are normal vascular structures. K, upper pole of left kidney, V, inferior vena cava Fig. 4. Normal scan through the mid-abdomen at the level of the left renal artery (lower arrow) and vein (upper arrow). The left renal vcin runs anterior to the aorta to the inferior vena cava (V). The head of the pancreas (P) and duodenum are just anterior to the inferior vena cava Fig. 5. A 48-year-old female with carcinoma of the colon metastatic to liver. There is a large irregular low density mass in the posterolateral portion of the right lobe of the liver and a second low density mass anteriorly near the midline
Fig. 6 A and B. A 56-year-old female with m e l a n o m a metastatic to liver A Pre-infusion scan through upper a b d o m e n raises question of area of decreased density anteriorly in medial portion of right lobe of the liver B Post-infusion scan following injection of 100 cc of 60% renografin clearly delineates a large low density mass in the right lobe of liver
Fig. 7. A 47-year-old female with polycystic disease of the liver and kidneys. There are multiple low density masses (water density) throughout the liver
Fig. 8 A and B. 65-year-old male with obstructive jaundice as a result of metastases to the region of the porta hepatis from carcinoma of the bladder A Scan through upper a b d o m e n demonstrates low density dilated bile ducts radiating from the porta hepatis. Areas of increased density in the vertebral centrum are metastases. S, fluid filled stomach B Scan through mid-abdomen. The dilated peripheral bile ducts are seen in the periphery of the liver as smoothly circumscribed low density defects. The gallbladder (G) is dilated and there is a mass (arrows) anterior to the inferior vena cava (V) extending into the region of the porta hepatis
Fig. 9. Scan through upper a b d o m e n demonstrating normal pancreas (arrows) anterior to the aorta (A) and superior mesenteric artery. The head of the pancreas lies medial to the second portion of the d u o d e n u m and anterior to the inferior vena cava (V). L, liver, K, left kidney Fig. 10A and B. A 62-year-old male with carcinoma of the head of the pancreas A Upper tomographic section demonstrates dilatation of the gallbladder
(G) B Lower tomographic section demonstrates enlargement of the head of the pancreas (arrows) with posterior extension to obscure inferior vena cava Fig. 11. A 37-year-old female with pseudocyst of the pancreas. There is a large low density mass occupying most of the anterior portion of the abdomen. The mass extends posteriorly to obliterate the inferior vena cava and it displaces the right kidney posteriorly and to the right Fig. 12. A 37-year-old male with a mass in the right kidney on intravenous pyelography. A scan through the upper a b d o m e n following injection of 100 cc of 60% renografin demonstrates a cyst in the right kidney. The gallstones (arrow) could not be identified on the plain film of the a b d o m e n
Fig. 13A and B. A 43-year-old male with large hypernephroma metastatic to the liver. A There is a large mass arising from the medial portion of the right kidney (K). A mass is present in the medial aspect of the right lobe of the liver (arrows). Its density is only slightly less than that of normal liver making identification difficult. B The renal mass extends medially to involve the psoas muscle. Note lateral displacement of the lower pole of the right kidney. Calcification can be seen in the wall of the aorta
Fig. 14. A 70-year-old male with a large aneurysm of the abdominal aorta (arrows)
Fig. 15. A 60-year-old female with cystadeno-carcinoma of the ovary. Scan through the pelvis demonstrates a multilobulated mass of low density occupying most of the pelvis
Fig. 16. A 48-year-old male with a neurofibroma involving the spinal canal. There is destruction of the right side of the neural arch and the mass extends laterally to displace the crura of the right diaphragm
J.P. Petasnick and J.W. Clark: Computed Tomography of the Abdomen mass lesions differentiated. Normally, the splenic density is less than that of the liver; however, if the spleen is involved with a diffuse infiltrative process or the liver parenchyma is replaced by fat, this relationship can change.
Pancreas The pancreas is recognized by its characteristic size, shape and location anterior to the proximal portion of the superior mesenteric artery (Fig. 9). The pancreas can be identified in most patients and is surrounded by a thin rim of retroperitoneal fat which is continuous with the fat surrounding the aorta and superior mesenteric artery. The head of the pancreas lies just anterior to the inferior vena cava and within the loop formed by the second and third portions of the duodenum. At times it may be difficult to separate the head of the pancreas from the duodenum which creates problems in the diagnosis of enlargement of this portion of the pancreas. Administration of dilute water-soluble oral contrast media may help to delineate the duodenum. The body of the pancreas lies anterior to the spine, aorta and superior mesenteric artery. The tail of the pancreas lies anterior to the upper pole of the left kidney and near the hilum of the spleen. Knowledge of the anatomic position of the pancreas will help differentiate it from the third portion of the duodenum. This portion of the duodenum may have a shape similar to the pancreas but its position between the aorta and superior mesenteric artery allows the differentiation between duodenum and pancreas to be made. The recognition of the relationship of the pancreas to other organs is essential because there is often no difference in the density of the pancreas and adjacent organs. In thin or debililated patients the absence of retroperitoneal fat may make it impossible to identify the pancreas. The pancreas may be oriented horizontally or obliquely within the abdomen. If it is oriented horizontally the complete organ can be seen on one or more of the 13-mm-thick sections (Fig. 9) ; however, if it lies obliquely only a portion will be identified on any one section and examination of several contiguous sections is necessary to visualize the entire gland. The size of the normal pancreas varies considerably. Haaga et al. [12] have related pancreatic size to the transverse diameter of the vertebral body in the same section. They state that the width of the normal body and tail of the pancreas is one-third to two-thirds the tranverse diameter of the vertebral body and the width of the head of the pancreas should
207
be no greater than the full transverse diameter of the vertebral body. The most important finding in carcinoma of the pancreas is enlargement of the pancreas (Fig. 10). Although enlargement of the pancreas can be readily detected, this does not allow a specific diagnosis of carcinoma to be made since the organ may also be enlarged in pancreatitis. If a pseudocyst is present, it can be identified by its low density (Fig. 1 1). The absorption values of pancreatic carcinoma, normal pancreas and pancreatitis are so similar that these conditions cannot be differentiated. Moreover, because of this similarity in absorption values, small lesions that have not enlarged the gland cannot be detected. The fat planes surrounding the pancreas may disappear as a result of tumor infiltration or pancreatitis. In thin or debilitated patients this sign is unreliable. Other retroperitoneal tumors may produce similar changes in the fat planes surrounding the pancreas.
Kidney The kidneys are well demonstrated on computed tomography because of the abundant perinephric fat surrounding the kidneys and separating them from adjacent structures (Figs. 4 and 12). The renal sinus with its structures is easily seen and delineates the inner parenchymal margin. The normal renal parenchyma is slightly less dense than the liver, spleen and pancreas but increases in density following the administration of urographic contrast media. Although there is abundant fat surrounding the kidney, the perirenal and pararenal spaces cannot be delineated. Renal masses are readily identified because they alter the contour of the kidney; however, computed tomography has been of limited value in their evaluation because of the high diagnostic yield with present uroradiologic methods. Solid renal masses are differentiated from cystic lesions by their difference in density. Benign simple cysts present as smooth, sharply demarcated, round or oval masses with low absorption coefficients (slightly higher than water). They are clearly seen without contrast enhancement and usually do not change in density after contrast administration (Fig. 12). Solid tumors are recognized by their change in renal contour. These masses have a density slightly less than normal renal parenchyma (Fig. 13) and demonstrate a variable change in density following the injection of contrast media. Computed tomography is of greatest value in determining the extent of tumor involvement and the presence of metastatic disease, particularly to the liver (Fig. 13). Renal parenchymal and calyceal calculi are clearly
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J.P. Petasnick and J.W. Clark: Computed Tomography of the Abdomen
seen and readily localized. Computed tomography has also been helpful in evaluating non-visualizing kidneys. A dilated renal pelvis can be recognized and evaluation of renal size and parenchymal thickness is possible.
Retroperitoneum The retroperitoneal region is difficult to evaluate with conventional roentgenographic techniques; however, if there is sufficient retroperitoneal fat, the structures in the retroperitoneal space can be well visualized by computed tomography (Fig. 2). The aorta and inferior vena cava are sharply delineated in the normal patient and can serve as indicators of disease. The inferior vena cava or aorta may disappear because of enlargement of surrounding nodes or envelopment by a mass or there may be displacement or compression (Figs. 10 and 13). Aneurysms of the aorta are readily detected (Fig. 14) and can be precisely localized if the renal vessels are visualized.
Pelvis The anatomy of the pelvis is clearly defined and usually reliably constant when compared to the upper abdomen. Delineation of the size, extent and type of tumor mass is the most c o m m o n indication for computed tomography of the pelvis. Evaluation of the absorption value allows the differentiation of solid and cystic masses; however, some malignant neoplasms, particularly in the ovary, present as cystic lesions. Therefore, it may be difficult to separate benign and malignant pelvic masses (Fig. 15). Distortion of the bladder can be readily identified and enlargement of the prostate and uterus detected.
Musculoskeletal The osseous structures are readily identified because of their relatively high absorption coefficient. Destructive lesions can be identified and extension into adjacent soft tissues evaluated (Fig. 16).
Discussion
Computed tomography has provided a new dimension in the roentgenographic evaluation of the abdomen. The ability to recognize abnormalities as alter-
ations in anatomic form or variation in tissue absorption coefficient has made it possible to evaluate the parenchyma of intra-abdominal organs. Pathology can be recognized by its direct effect on the parenchyma rather than by its effect on intraparenchymal structures. Computed tomography has been used in the initial detection of lesions, in the determination of the extent of known disease and in the differentiation of benign and malignant diseases. Our early experience and that of others [4~9, 11, 12] suggests that computed tomography is most valuable in detecting lesions in those sites least accessible to conventional roentgenographic techniques: liver, spleen, pancreas and retroperitoneum. Studies are in order to compare the diagnostic accuracy of computed tomography with conventional abdominal examinations as well as with abdominal ultrasound, radionuclide scanning and lymphography.
References 1. Hounsfield GN: Computed transverse axial scanning (tomography). Part 1: Description of system. Brit J Radiol 46: 1016- 1022, 1973 2. Ambrose J : Computed transverse axial scanning (tomography). Part 2: Clinical application. Brit J Radiol 46." 1023 1047. 1973 3. Baker HL Jr: The impact of computed tomography on neuroradiologic practice. Radiology 116.637 640, 1975 4. Alfidi RJ, Haaga J, Meaney TF, MacIntyre WJ, Gonzalez L, Tarar R, Zelch MG, Boller M, Cook SA, Jelden G: Computed tomography of the thorax and abdomen; A preliminary report. Radiology 117.'257 264, 1975 5. Twigg HL, Axelbaum SP, Schellinger D: Computerized body tomography with the ACTA scanner. J A M A 234.'314 317, 1975 6. Sagel S, Stanley R J, Evens RG: Early clinical experience with motionless whole body computed tomography. Radiology 119.'321 330, 1976 7. Stephens DH, Hanery RR, Sheedy PFII: Computed tomography of the abdomen. Radiology 119.'331 335, 1976 8. Sheedy PFII, Stephens DH, Hattery RR, Robert R, Muhm JR, Gartman GW: Compt~ted tomography of the body: Initial clinical trial with the EMI prototype. Am J Roentgenol 127:23-51, 1976 9. Stanley RJ, Sagel SS, Levitt RG: Computed tomography of the body: Early trends in application and accuracy of the method. An+ J Roentgenol 127.'53-67, 1976 10. Liu SS: Personal Communication 11. Alfedi RJ, Haaga JR, Havrilla TR, Pepe RG, Cook SA : Computed tomography of the liver. Am J Roentgenol 127.'69 74. 1976 12. Haaga JR, Alfidi RJ, Zelch MG, Meaney TF, Boller M, Gonzalez L, Jelden GL: Computed tomography of the pancreas. Radiolog)" 120:589 595, 1976
Received: October 1, 1976; accepted: October 25, 1976
Gastrointest Radiol 1, 201-208 (1976)
Radiology ~ by Springer-Verlag 1976
Computed Tomography of the Abdomen: Initial Experience J e r r y P. P e t a s n i c k a n d J o h n W . C l a r k Department of Radiology, Rush-Presbyterian St. Luke's Medical Center, Chicago, illinois, U.S.A.
Abstract. C o m p u t e d t o m o g r a p h y
has provided a new d i m e n s i o n in t h e r o e n t g e n o l o g i c e v a l u a t i o n o f t h e a b domen. Normal structures not visible on conventional examinations are clearly identified. Abnormalities are recognized by their alterations in anatomic form or by their effect on tissue absorption values. Our early e x p e r i e n c e s u g g e s t s t h a t in t h e a b d o m e n c o m p u t e d t o m o g r a p h y will b e m o s t v a l u a b l e in d e t e c t i n g l e s i o n s in t h o s e sites l e a s t a c c e s s i b l e t o c o n v e n t i o n a l r o e n t g e n o g r a p h i c m e t h o d s s u c h as t h e liver, s p l e e n , p a n c r e a s and retroperitoneum.
Key words: A b d o m e n , tomography,
technique
abnormalities - Neoplasms,
- Computed diagnosis.
Computed tomography (CT) has had a dramatic imp a c t o n t h e p r a c t i c e o f n e u r o r a d i o l o g y s i n c e its i n i t i a l d e s c r i p t i o n b y H o u n s f i e l d [1] a n d A m b r o s e [2]. A s a result, m a j o r c h a n g e s h a v e o c c u r r e d in t h e p a t t e r n of utilization of other diagnostic neuroradiologic e x a m i n a t i o n s [3]. R e c e n t l y , a d v a n c e s in d e s i g n h a v e a l l o w e d t h i s t e c h n i q u e t o b e u s e d in t h e e v a l u a t i o n of other regions of the body. Several whole body s c a n n e r s h a v e b e e n in u s e f o r o n e o r m o r e y e a r s a n d p r e l i m i n a r y r e p o r t s h a v e d e m o n s t r a t e d t h e i r usef u l n e s s p a r t i c u l a r l y in e x a m i n i n g t h e a b d o m e n [4-9]. A n E M I C T 5000 b o d y s c a n n e r h a s b e e n in u s e a t the Rush-Presbyterian St. L u k e ' s M e d i c a l C e n t e r s i n c e A p r i l 1976 a n d e x a m p l e s o f its a p p l i c a t i o n will be demonstrated.
Materials and Methods The EMI body scanner permits the abdomen to be examined as a series of reconstructed transaxial tomographic sections 13 mm Address reprint requests to." Jerry P. Petasnick, M.D., Department
of Radiology, Rush-Presbyterian St. Luke's Medical Center, 1753 W. Congress Parkway, Chicago, IL 60612, U.S.A.
in thickness. The scanning gantry contains a scanning frame on which an X-ray tube is mounted diametrically opposite an array of 30 highly senitive sodium iodide crystal detectors. The patient lies on a movable table that extends through the scanning gantry and allows any portion of the body to be positioned between the X-ray source and the detector array. A tightly collimated fanshaped X-ray beam is used for examination. The scanning frame traverses linearly across the patient and the photons in the beam emerging from the patient are measured by the detectors. The linear traverse takes just over 1 s during which time more than 18,000 readings of the intensity of the X-ray photons in the emergent beam are recorded by the detectors. At the end of the linear traverse the scanning frame with the X-ray tube and detectors rotates around the patient by 10~ and another linear traverse is made. The process of traversing and 10~ rotation continues until 18 linear traverses spanning 180~ have been completed. This sequence requiring 20 s produces one tomographic section. The readings taken during the scanning process are digitized and continuously introduced into a computer during the scanning process. The completed computed tomogram of the section under examination represents a cross-sectional image of the X-ray absorption values. The reconstructed image representing the anatomic structures of the section scanned is displayed on a television monitor in various shades of gray corresponding to the calculated absorption values. The image can be manipulated on the display console to permit a selective display of any particular absorption value from the wide spectrum of absorption values obtained during the scan. The images are oriented as though viewed from the patient's feet. This places the supine patient's right side to the left of the viewing console. Preliminary absorbed dose measurements on our unit indicate that the integrated surface dose during a study of the abdomen (140 kV, 28 mA, 20 s) is 2.6 rad/scan [10]. The surface dose for a series of eight sequential sections is increase to about 5 rads because of the scattered radiation. The integrated surface dose is about the same as that from conventional roentgenographic exams such as intravenous pyelography with nephrotomography but less than that from selective abdominal arteriography.
Results Computed tomography permits anatomic structures and pathologic alterations to be viewed in a manner not possible by conventional roentgenographic techniques. Anatomic structures can be identified by computed tomography when only negligible differences
202
J.P. Petasnick and J.W. Clark: Computed Tomography of the Abdomen
in attenuation coefficients exist provided these structures are separated by interfaces of different attenuation properties. Abnormalities can be recognized as alterations in anatomic form or variations in tissue absorption coefficient or both. Examples demonstrating the use of computed tomography will be considered in the following sections.
Peritoneal Cavity Ascitic fluid can be readily identified because its density is less than that of most intra-abdominal organs (Fig. 1). The liver and often the spleen are displaced away from the lateral abdominal wall and their margins clearly visualized. Parietal peritoneal tumor implants can be identified.
Liver The normal liver (Fig. 2) is homogeneous in density and varies considerably in size and configuration. The liver is usually denser than the other intra-abdominal organs and increases in density following the intravenous administration of iodinated contrast media. In the normal density range, normal peripheral bile ducts and arteriovenous structures are usually not visualized. In some normal patients, non-dilated central bile ducts can be seen and arteriovenous structures may be identified if the fat content of the liver is increased (Fig. 3). The right lobe of the liver occupies most of the right half of the abdomen in sections near the diaphragm (Fig. 2). In the more caudally located sections the right renal fossa can be identified posteriorly and the inferior tip of the right lobe assumes a more anterior position (Fig. 4). The left lobe extends across the midline to varying degrees and is much smaller in volume than the right lobe. In most patients there is no clear-cut delineation between the right and left lobes. The porta hepatis can be identified as a transverse cleft on the posterior surface of the liver; however, the structures within the porta hepatis are usually not seen. Tumors in the liver, whether benign or malignant are generally less dense than the normal liver parenchyma (Figs. 5 and 7). This difference is usually accentuated by the intravenous administration of iodinated contrast media. Malignant tumors, primary or metastatic, usually present as clearly delineated masses less dense than normal liver parenchyma, but higher in density than hepatic cysts or dilated bile ducts. At times, however, the initial density of the tumor may be almost the same as that of normal liver parenchyma making identification difficult
(Fig. 6A). However, these lesions are usually seen following the intravenous administration of iodinated contrast media as the density of the normal liver increases more than that of the tumor (Fig. 6B). Tumors which appear vascular on angiographic examination are usually less dense than normal liver parenchyma even after the intravenous administration of contrast media; however, on occasion, tumor masses clearly seen on the pre-infusion scan disappear following the administration of contrast media. For this reason, pre-infusion scans should always be obtained when contrast media is used. Cystic lesions can be readily detected by their low density {Fig. 7). Reliance on configuration alone may produce diagnostic problems as some metastatic lesions have smooth margins (Fig. 6). Hepatic abscesses are less dense than liver and may have irregular margins [6, 8, 11]. They may mimic primary or metastatic neoplasms. History is usually helpful in establishing a diagnosis.
Biliarv Tract Normal bile ducts are usually not visualized; however, following dilatation the bile ducts are readily seen because their density is less than that of normal liver parenchyma (Fig. 8). In the region of the porta hepatis a branching pattern is seen (Fig. 8A), while the more peripherally located ducts present as smooth round defects (Fig. 8 B). They can usually be differentiated from metastatic lesions because they are more sharply circumscribed and are less dense than metastatic tumor; however, care must be taken in their analysis since liver metastases may be present in patients with obstructive jaundice. Stanley et al. [9] have demonstrated that computed tomography is highly accurate in differentiating obstructive from n o n - o b structive jaundice. The gallbladder can usually be identified as a welldefined oval area of watea" or slightly higher density in the region of the porta hepatis (Fig. 12). Dilatation is easily recognized (Figs. 8 and 10) and may help to localize the site of obstruction in patients with jaundice. Calcified gallstones are readily detected and oftentimes gallstones not seen on plain films of the abdomen may be identified (Fig. 12). Primary evaluation of the gallbladder by computed tomography is of limited use except in the above conditions or occasionally in some patients with a non-visualizing gallbladder.
Spleen Splenic size, configuration and density can be readily evaluated. Splenic enlargement can be identified and
Fig. 1. A 55-year-old female with ascites. Scan of the upper abdomen demonstrates a zone of decreased density along the lateral margin of the right lobe of the liver (L). Spleen (S) and fundus of stomach (F) also seen on this section Fig. 2. Normal scan through the upper abdomen demonstrating liver (L), spleen (5), fundus of stomach (F). The aorta (A) can be seen anterior to the vertebral centrum
Fig. 3. A 45-year-old male with fatty infiltration of the liver. Note the striking difference in the density of the liver (L) and spleen (5). The linear structures of increased density within the liver are normal vascular structures. K, upper pole of left kidney, V, inferior vena cava Fig. 4. Normal scan through the mid-abdomen at the level of the left renal artery (lower arrow) and vein (upper arrow). The left renal vcin runs anterior to the aorta to the inferior vena cava (V). The head of the pancreas (P) and duodenum are just anterior to the inferior vena cava Fig. 5. A 48-year-old female with carcinoma of the colon metastatic to liver. There is a large irregular low density mass in the posterolateral portion of the right lobe of the liver and a second low density mass anteriorly near the midline
Fig. 6 A and B. A 56-year-old female with m e l a n o m a metastatic to liver A Pre-infusion scan through upper a b d o m e n raises question of area of decreased density anteriorly in medial portion of right lobe of the liver B Post-infusion scan following injection of 100 cc of 60% renografin clearly delineates a large low density mass in the right lobe of liver
Fig. 7. A 47-year-old female with polycystic disease of the liver and kidneys. There are multiple low density masses (water density) throughout the liver
Fig. 8 A and B. 65-year-old male with obstructive jaundice as a result of metastases to the region of the porta hepatis from carcinoma of the bladder A Scan through upper a b d o m e n demonstrates low density dilated bile ducts radiating from the porta hepatis. Areas of increased density in the vertebral centrum are metastases. S, fluid filled stomach B Scan through mid-abdomen. The dilated peripheral bile ducts are seen in the periphery of the liver as smoothly circumscribed low density defects. The gallbladder (G) is dilated and there is a mass (arrows) anterior to the inferior vena cava (V) extending into the region of the porta hepatis
Fig. 9. Scan through upper a b d o m e n demonstrating normal pancreas (arrows) anterior to the aorta (A) and superior mesenteric artery. The head of the pancreas lies medial to the second portion of the d u o d e n u m and anterior to the inferior vena cava (V). L, liver, K, left kidney Fig. 10A and B. A 62-year-old male with carcinoma of the head of the pancreas A Upper tomographic section demonstrates dilatation of the gallbladder
(G) B Lower tomographic section demonstrates enlargement of the head of the pancreas (arrows) with posterior extension to obscure inferior vena cava Fig. 11. A 37-year-old female with pseudocyst of the pancreas. There is a large low density mass occupying most of the anterior portion of the abdomen. The mass extends posteriorly to obliterate the inferior vena cava and it displaces the right kidney posteriorly and to the right Fig. 12. A 37-year-old male with a mass in the right kidney on intravenous pyelography. A scan through the upper a b d o m e n following injection of 100 cc of 60% renografin demonstrates a cyst in the right kidney. The gallstones (arrow) could not be identified on the plain film of the a b d o m e n
Fig. 13A and B. A 43-year-old male with large hypernephroma metastatic to the liver. A There is a large mass arising from the medial portion of the right kidney (K). A mass is present in the medial aspect of the right lobe of the liver (arrows). Its density is only slightly less than that of normal liver making identification difficult. B The renal mass extends medially to involve the psoas muscle. Note lateral displacement of the lower pole of the right kidney. Calcification can be seen in the wall of the aorta
Fig. 14. A 70-year-old male with a large aneurysm of the abdominal aorta (arrows)
Fig. 15. A 60-year-old female with cystadeno-carcinoma of the ovary. Scan through the pelvis demonstrates a multilobulated mass of low density occupying most of the pelvis
Fig. 16. A 48-year-old male with a neurofibroma involving the spinal canal. There is destruction of the right side of the neural arch and the mass extends laterally to displace the crura of the right diaphragm
J.P. Petasnick and J.W. Clark: Computed Tomography of the Abdomen mass lesions differentiated. Normally, the splenic density is less than that of the liver; however, if the spleen is involved with a diffuse infiltrative process or the liver parenchyma is replaced by fat, this relationship can change.
Pancreas The pancreas is recognized by its characteristic size, shape and location anterior to the proximal portion of the superior mesenteric artery (Fig. 9). The pancreas can be identified in most patients and is surrounded by a thin rim of retroperitoneal fat which is continuous with the fat surrounding the aorta and superior mesenteric artery. The head of the pancreas lies just anterior to the inferior vena cava and within the loop formed by the second and third portions of the duodenum. At times it may be difficult to separate the head of the pancreas from the duodenum which creates problems in the diagnosis of enlargement of this portion of the pancreas. Administration of dilute water-soluble oral contrast media may help to delineate the duodenum. The body of the pancreas lies anterior to the spine, aorta and superior mesenteric artery. The tail of the pancreas lies anterior to the upper pole of the left kidney and near the hilum of the spleen. Knowledge of the anatomic position of the pancreas will help differentiate it from the third portion of the duodenum. This portion of the duodenum may have a shape similar to the pancreas but its position between the aorta and superior mesenteric artery allows the differentiation between duodenum and pancreas to be made. The recognition of the relationship of the pancreas to other organs is essential because there is often no difference in the density of the pancreas and adjacent organs. In thin or debililated patients the absence of retroperitoneal fat may make it impossible to identify the pancreas. The pancreas may be oriented horizontally or obliquely within the abdomen. If it is oriented horizontally the complete organ can be seen on one or more of the 13-mm-thick sections (Fig. 9) ; however, if it lies obliquely only a portion will be identified on any one section and examination of several contiguous sections is necessary to visualize the entire gland. The size of the normal pancreas varies considerably. Haaga et al. [12] have related pancreatic size to the transverse diameter of the vertebral body in the same section. They state that the width of the normal body and tail of the pancreas is one-third to two-thirds the tranverse diameter of the vertebral body and the width of the head of the pancreas should
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be no greater than the full transverse diameter of the vertebral body. The most important finding in carcinoma of the pancreas is enlargement of the pancreas (Fig. 10). Although enlargement of the pancreas can be readily detected, this does not allow a specific diagnosis of carcinoma to be made since the organ may also be enlarged in pancreatitis. If a pseudocyst is present, it can be identified by its low density (Fig. 1 1). The absorption values of pancreatic carcinoma, normal pancreas and pancreatitis are so similar that these conditions cannot be differentiated. Moreover, because of this similarity in absorption values, small lesions that have not enlarged the gland cannot be detected. The fat planes surrounding the pancreas may disappear as a result of tumor infiltration or pancreatitis. In thin or debilitated patients this sign is unreliable. Other retroperitoneal tumors may produce similar changes in the fat planes surrounding the pancreas.
Kidney The kidneys are well demonstrated on computed tomography because of the abundant perinephric fat surrounding the kidneys and separating them from adjacent structures (Figs. 4 and 12). The renal sinus with its structures is easily seen and delineates the inner parenchymal margin. The normal renal parenchyma is slightly less dense than the liver, spleen and pancreas but increases in density following the administration of urographic contrast media. Although there is abundant fat surrounding the kidney, the perirenal and pararenal spaces cannot be delineated. Renal masses are readily identified because they alter the contour of the kidney; however, computed tomography has been of limited value in their evaluation because of the high diagnostic yield with present uroradiologic methods. Solid renal masses are differentiated from cystic lesions by their difference in density. Benign simple cysts present as smooth, sharply demarcated, round or oval masses with low absorption coefficients (slightly higher than water). They are clearly seen without contrast enhancement and usually do not change in density after contrast administration (Fig. 12). Solid tumors are recognized by their change in renal contour. These masses have a density slightly less than normal renal parenchyma (Fig. 13) and demonstrate a variable change in density following the injection of contrast media. Computed tomography is of greatest value in determining the extent of tumor involvement and the presence of metastatic disease, particularly to the liver (Fig. 13). Renal parenchymal and calyceal calculi are clearly
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J.P. Petasnick and J.W. Clark: Computed Tomography of the Abdomen
seen and readily localized. Computed tomography has also been helpful in evaluating non-visualizing kidneys. A dilated renal pelvis can be recognized and evaluation of renal size and parenchymal thickness is possible.
Retroperitoneum The retroperitoneal region is difficult to evaluate with conventional roentgenographic techniques; however, if there is sufficient retroperitoneal fat, the structures in the retroperitoneal space can be well visualized by computed tomography (Fig. 2). The aorta and inferior vena cava are sharply delineated in the normal patient and can serve as indicators of disease. The inferior vena cava or aorta may disappear because of enlargement of surrounding nodes or envelopment by a mass or there may be displacement or compression (Figs. 10 and 13). Aneurysms of the aorta are readily detected (Fig. 14) and can be precisely localized if the renal vessels are visualized.
Pelvis The anatomy of the pelvis is clearly defined and usually reliably constant when compared to the upper abdomen. Delineation of the size, extent and type of tumor mass is the most c o m m o n indication for computed tomography of the pelvis. Evaluation of the absorption value allows the differentiation of solid and cystic masses; however, some malignant neoplasms, particularly in the ovary, present as cystic lesions. Therefore, it may be difficult to separate benign and malignant pelvic masses (Fig. 15). Distortion of the bladder can be readily identified and enlargement of the prostate and uterus detected.
Musculoskeletal The osseous structures are readily identified because of their relatively high absorption coefficient. Destructive lesions can be identified and extension into adjacent soft tissues evaluated (Fig. 16).
Discussion
Computed tomography has provided a new dimension in the roentgenographic evaluation of the abdomen. The ability to recognize abnormalities as alter-
ations in anatomic form or variation in tissue absorption coefficient has made it possible to evaluate the parenchyma of intra-abdominal organs. Pathology can be recognized by its direct effect on the parenchyma rather than by its effect on intraparenchymal structures. Computed tomography has been used in the initial detection of lesions, in the determination of the extent of known disease and in the differentiation of benign and malignant diseases. Our early experience and that of others [4~9, 11, 12] suggests that computed tomography is most valuable in detecting lesions in those sites least accessible to conventional roentgenographic techniques: liver, spleen, pancreas and retroperitoneum. Studies are in order to compare the diagnostic accuracy of computed tomography with conventional abdominal examinations as well as with abdominal ultrasound, radionuclide scanning and lymphography.
References 1. Hounsfield GN: Computed transverse axial scanning (tomography). Part 1: Description of system. Brit J Radiol 46: 1016- 1022, 1973 2. Ambrose J : Computed transverse axial scanning (tomography). Part 2: Clinical application. Brit J Radiol 46." 1023 1047. 1973 3. Baker HL Jr: The impact of computed tomography on neuroradiologic practice. Radiology 116.637 640, 1975 4. Alfidi RJ, Haaga J, Meaney TF, MacIntyre WJ, Gonzalez L, Tarar R, Zelch MG, Boller M, Cook SA, Jelden G: Computed tomography of the thorax and abdomen; A preliminary report. Radiology 117.'257 264, 1975 5. Twigg HL, Axelbaum SP, Schellinger D: Computerized body tomography with the ACTA scanner. J A M A 234.'314 317, 1975 6. Sagel S, Stanley R J, Evens RG: Early clinical experience with motionless whole body computed tomography. Radiology 119.'321 330, 1976 7. Stephens DH, Hanery RR, Sheedy PFII: Computed tomography of the abdomen. Radiology 119.'331 335, 1976 8. Sheedy PFII, Stephens DH, Hattery RR, Robert R, Muhm JR, Gartman GW: Compt~ted tomography of the body: Initial clinical trial with the EMI prototype. Am J Roentgenol 127:23-51, 1976 9. Stanley RJ, Sagel SS, Levitt RG: Computed tomography of the body: Early trends in application and accuracy of the method. An+ J Roentgenol 127.'53-67, 1976 10. Liu SS: Personal Communication 11. Alfedi RJ, Haaga JR, Havrilla TR, Pepe RG, Cook SA : Computed tomography of the liver. Am J Roentgenol 127.'69 74. 1976 12. Haaga JR, Alfidi RJ, Zelch MG, Meaney TF, Boller M, Gonzalez L, Jelden GL: Computed tomography of the pancreas. Radiolog)" 120:589 595, 1976
Received: October 1, 1976; accepted: October 25, 1976
