(Radiographics. 2002;22:701-719.)
© RSNA, 2002
Special Focus Session
Multidetector CT: Abdominal Visceral Imaging1
W. Dennis Foley, MD
1 From the Department of Radiology, Medical College of Wisconsin, Froedtert Hospital East, 9200 W Wisconsin Ave, Milwaukee, WI 53226. From the Plenary Session, Special Focus Session: Multisection CT (Multidetector Row CT): Applications in the Chest and Abdomen, presented at the 2000 RSNA scientific assembly. Received August 6, 2001; revision requested January 11, 2001 and received March 4; accepted March 6. Address correspondence to the author (e-mail: dfoley@mcw.edu).
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Abstract
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Multidetector computed tomography (CT) has diffused rapidly into clinical imaging practice in a short time. Major improvements in relation to single-detector CT are faster image acquisition and improved z-axis resolution. Abdominal visceral imaging with multidetector CT uses both the vascular and parenchymal phases of contrast material enhancement to provide a combined angiographic and multipass organ-directed study. All examinations are performed with standard contrast material loads.
© RSNA, 2002
Index Terms: Computed tomography (CT), multi–detector row • Bile ducts, CT, 76.12114 • Kidney, CT, 81.12114 • Liver, CT, 761.12114 • Pancreas, CT, 770.12114
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Introduction
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Multidetector computed tomography (CT), initially introduced in 1998, has had a substantial impact on the performance parameters and clinical applications of helical CT. Rapid volume coverage speed is combined with thin image thicknesses to create a volume data set best suited for workstation analysis, either in two-dimensional axial display, multiplanar reformations, or three-dimensional imaging (1). Precise timing and rapid sequence acquisition also allow multiple abdominal imaging passes to be performed at precisely defined circulation phases.
Major applications of multidetector CT are CT angiographic studies, pulmonary embolism detection and related indirect CT venography, trauma, and abdominal visceral imaging. Abdominal visceral imaging with multidetector CT uses both the vascular and parenchymal phases of contrast material enhancement to provide a combined angiographic and organ-directed study. In this article, the utility of multidetector CT in hepatic, pancreatic, renal, and abdominal trauma applications is reviewed, with emphasis on acquisition techniques tailored to contrast material pharmacokinetics to improve lesion detection and characterization.
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Advantages of Multidetector CT Technology
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The generic advantages of multidetector CT (ie, faster acquisition and thinner sections, compared with those used in single-detector CT) derive from the use of a multiple-row detector with narrow detector collimation, either symmetric or asymmetric in design, in conjunction with a relatively wide x-ray beam and rapid table translation. In symmetric detector design, the central and peripheral detector rows have equal detector collimation (eg, 1.25 mm in the LightSpeed by GE Medical Systems, Milwaukee, Wis). With asymmetric detector design, the collimation of the central detector row is narrower than that of the more peripheral detector rows. For example, two different systems have detector row collimations of 1 mm, 1.25 mm, 2.25 mm, and 5 mm from center to periphery (Volume Zoom; Siemens Medical Solutions, Iselin, NJ, and MX8000; Marconi Medical Systems, Cleveland, Ohio), whereas another system has four central 0.5-mm detector rows and 16 peripheral (eight on each side) 1-mm detector rows (Aquillion; Toshiba America Medical Systems, Tustin, Calif).
With multidetector CT technology, there is relatively less loading on the x-ray tube, and tube overloading and tube cooling are not practical concerns. Multidetector CT technology has the same advantages as single-detector helical CT (ie, registered volume data set with overlapping reconstructions to reduce partial volume effect and effective use of intravenously injected contrast material). In addition, multidetector CT technology allows the acquisition of different image thicknesses from the same data set by using helical reconstruction weighting algorithms and interpolation of adjacent helical data sets.
Because the two systems have comparable photon statistics incident on the detector and equal image thickness, the signal-to-noise ratios of multidetector CT and single-detector CT systems are relatively equivalent. Because the x-ray beam of the multidetector CT system is wide compared with that of the single-detector CT system, less tube output is required for the same longitudinal or z-axis coverage. When thinner image thicknesses are chosen for a multidetector CT data acquisition and the same signal-to-noise ratio is desired, increased tube current and radiation dose result. Choice of image thickness, and thus radiation dose, should be determined by the clinical objectives of the study. Thin image thickness is used for the detection of small focal lesions and for CT angiography.
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Clinical Applications
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Hepatic Imaging
Contrast Material Pharmacokinetics.—
The liver has complex circulatory dynamics. Approximately 80% of the hepatic blood supply is derived from the portal venous system and 20% from the hepatic artery. The injected bolus of contrast material initially enhances the liver via inflow through the hepatic arteries. The isolated pure arterial phase of enhancement is usually 8–10 seconds in duration (2) and is followed by a second "late arterial/portal venous inflow phase" of enhancement. In this phase, hepatic contrast enhancement predominantly reflects arterial inflow with a substantial, though minor, component secondary to splanchnic venous inflow of contrast material into the portal vein and hepatic parenchyma.
With single-detector CT systems, a "double-pass" hepatic helical technique was instituted, in which the first pass was an arterial dominant phase (3). This first imaging pass was performed 20–50 seconds after the beginning of an intravenous injection of the contrast material bolus, and it encompassed both the pure arterial phase and the late arterial/portal venous inflow phase of hepatic enhancement. Subsequently, beginning 60 seconds after the start of the contrast material injection, a "portal venous phase" imaging pass was performed. The term portal venous phase was used to indicate that the majority of hepatic enhancement in this phase was secondary to portal venous inflow (4).
With multidetector CT systems, the initial admixed arterial dominant phase used with a single-detector CT system can be subdivided into a pure early arterial phase and a late arterial/portal venous inflow phase. Each of these phases has a temporal window of approximately 8–10 seconds (5). The phase of maximum hepatic parenchymal enhancement and hepatic venous opacification occurs about 45 seconds after the beginning of the pure early arterial phase. An imaging pass performed during this circulatory phase has been entitled the "hepatic phase," and corresponds in timing to the portal venous phase imaging pass previously described (Fig 1).
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Figure 1. Schematic of injection and image data acquisition for multiphase hepatic CT. Injection (white bar) is performed at a rate of 5 mL/sec for 30 seconds for a total volume of 150 mL of 60% iodinated contrast material (42-g iodine load). There are three separate acquisition intervals shown sequentially: the early arterial phase, the late arterial/portal venous inflow phase, and the hepatic phase. Each acquisition technique uses a pitch value of 6 (table speed per rotation divided by detector collimation—modified definition). In this schematic, the injection to scan delay before the first acquisition is 20 seconds. Injection to scan delay is determined for each individual with a preliminary mini-bolus injection and can vary from 12 to 25 seconds.
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Accurate acquisition timing for multipass imaging depends on the assessment of circulation time in individual patients. This assessment is made by using either a preliminary injection of a small bolus of contrast material ("mini-bolus injection") or online bolus tracking software.
Intravenous access is obtained with a 20-gauge plastic venous cannula in an antecubital vein. For patients with poor venous access, a high-frequency linear array ultrasound (US) probe is used to guide needle placement into a suitable antecubital vein. The initial mini-bolus injection is performed with 20 mL of 60% iodinated contrast material injected at a rate of 5 mL/sec. A scan of a single level is performed in the upper abdominal aorta beginning 10 seconds after the start of the injection, with one scan performed every 2 seconds during a 20-second acquisition. With a region of interest placed in the abdominal aorta (Fig 2), a time attenuation curve is obtained to map the arrival time and washout of the contrast material bolus in the abdominal aorta. The injection to scan delay corresponds to the time interval between the beginning of the contrast material injection and the peak level of contrast enhancement in the upper abdominal aorta as registered on the time attenuation curve. In our experience, the arrival time of an intravenously injected contrast material bolus in the aorta can vary between 12 and 25 seconds. Because the temporal window for the initial early arterial phase is only 8 seconds, both accurate circulation timing and rapid image acquisition are mandatory if a defined early arterial phase image is to be acquired in individual patients.
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Figure 2. Time attenuation curve of the abdominal aorta obtained after mini-bolus injection (5 mL/sec for 4 seconds; volume, 20 mL of 60% iodinated contrast material) into an antecubital vein. Time from the beginning of injection until the first image and region of interest reading is 10 seconds. This first data point (10 seconds after the beginning of injection) is registered as zero on the time axis. Each data point (1) is obtained from the single level scan sequence obtained at 2-second interscan delays. In this example, peak aortic enhancement is obtained at 14 seconds after the beginning of injection.
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Water is used as the enteric contrast agent in the upper gastrointestinal tract to allow a three-dimensional volume data set to be used for multiplanar CT angiography. The patient should receive 8 oz of water 15 minutes before the study and an additional 8 oz immediately before the mini-bolus injection.
The patient receives a bolus injection of 150 mL of 60% iodinated contrast material administered at the same rate as the mini-bolus injection (ie, 5 mL/sec). Acquisition parameters— specifically, table speed per rotation and scan rotation speed— are set to allow full coverage of the liver in less than 8 seconds.
The first two imaging passes are timed to the early arterial phase and the late arterial/portal venous inflow phase. Because first pass data may be used for three-dimensional CT angiography, the image thickness should be 2.5 mm or less. During the same patient breath hold and 15 seconds after the contrast material bolus reaches the aorta, the late arterial/portal venous inflow phase occurs. Acquisition duration for this second pass is also 8 seconds or less. The total duration of imaging in a single breath hold is maximum at 23 seconds (8 seconds for early arterial phase, 7-second intergroup delay, and 8 seconds for late arterial/portal venous inflow phase).
Hypervascular Hepatic Neoplasms.—
A clinical imaging trial has demonstrated that hypervascular hepatic neoplasms, either primary or metastatic, are best demonstrated during the late arterial/portal venous inflow phase rather than during the early arterial phase (5). This timing presumably reflects the time interval for distribution of contrast-enhanced hepatic arterial blood into the tumor neovasculature and diffusion into the interstices of the tumor (Fig 3). Early arterial phase imaging of the hepatic and mesenteric arterial circulation is reserved for patients in whom CT angiography would be of benefit, such as potential transplant recipients, candidates for hepatic resection or cryoablation, or candidates for chemoembolization. In patients who are not candidates for these surgical or angiographic procedures, the first early arterial phase imaging pass is not performed and imaging passes during the second late arterial/portal venous inflow phase and subsequent hepatic phase are used instead.
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Figure 3a. Multidetector CT images from the early arterial phase (a), late arterial/portal venous inflow phase (b), and hepatic phase (c) in a patient with hepatic cirrhosis and two small focal hypervascular hepatocellular carcinomas in the right lobe (arrows in b). Maximum tumor-liver contrast is obtained in the late arterial/portal venous inflow phase (b).
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Figure 3b. Multidetector CT images from the early arterial phase (a), late arterial/portal venous inflow phase (b), and hepatic phase (c) in a patient with hepatic cirrhosis and two small focal hypervascular hepatocellular carcinomas in the right lobe (arrows in b). Maximum tumor-liver contrast is obtained in the late arterial/portal venous inflow phase (b).
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Figure 3c. Multidetector CT images from the early arterial phase (a), late arterial/portal venous inflow phase (b), and hepatic phase (c) in a patient with hepatic cirrhosis and two small focal hypervascular hepatocellular carcinomas in the right lobe (arrows in b). Maximum tumor-liver contrast is obtained in the late arterial/portal venous inflow phase (b).
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The aim of CT angiography of the hepatic and mesenteric circulation is to provide a tool for therapy planning by defining arterial stenosis and anomalies, particularly aberrant or replaced right and left hepatic arteries arising from the superior mesenteric and left gastric arteries, respectively (Fig 4). In addition, in patients who are candidates for surgically implanted chemoinfusion catheters, the anatomy of the gastroduodenal artery, length of the more distant proper hepatic artery (if present), and the anatomic origin of both the right and left hepatic arteries are important (Fig 5). A combined CT angiogram/CT portogram can be obtained from the late arterial/portal venous inflow imaging phase (Fig 6). This study is valuable for mapping the extrahepatic portal venous system in patients with suspected pancreatic or bile duct malignancy and in patients who are potential transplant recipients. Venouscompression, venous stenosis or thrombosis, and portal systemic venous collateral vessels can be displayed.
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Figure 4. Hepatic/mesenteric CT angiogram of the same patient as in Figure 3, produced with image data obtained from the first pass of a multiphase hepatic CT examination performed with 2.5-mm image thickness and table speed of 15 mm per rotation at a pitch value of 6.0 (see Fig 1). This is a volume-rendered display. CHA = common hepatic artery, GDA = gastroduodenal artery, LHA = left hepatic artery, PHA = proper hepatic artery, RHA = right hepatic artery, SA = splenic artery, SMA = superior mesenteric artery.
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Figure 5. More posterior coronal plane (top) and more anterior coronal plane (bottom) reformated images demonstrate the position of a hepatic arterial infusion chemotherapy catheter (black arrows) surgically implanted in the gastroduodenal artery (bottom) and the short length of the proper hepatic artery before division into right and left hepatic artery branches (white arrows) (top). Normal anatomic arterial branching had been demonstrated at preoperative CT angiography, enabling the surgeon to place the infusion catheter with knowledge that the chemotherapy agent would be delivered to both the right and left hepatic lobes. This is a planar maximum intensity projection (MIP) display obtained from the first pass data of a triple pass hepatic CT examination performed with a 2.5-mm image thickness with a table speed of 15 mm per rotation at a pitch value of 6.0 (see Fig 1).
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Figure 6. Hepatic hilar cholangiocarcinoma involving the common hepatic duct and occluding the right intrahepatic branch of the portal vein in a 70-year-old woman. Slab coronal plane MIP image reformatted from admixed portal venous inflow/hepatic phase image data (2.5-mm image thickness and table speed of 15 mm per rotation at a pitch value of 6.0 [see Fig 1]) demonstrates right intrahepatic portal vein occlusion (arrow). The superior mesenteric, extrahepatic portal, and left intrahepatic portal veins are patent. An endoscopically placed biliary stent is located in the common duct distal to the tumor. Transhepatic biliary stent placement was performed subsequent to the CT examination.
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The third imaging pass, the hepatic phase, corresponds in timing to what is conventionally labeled as the portal venous phase. In this imaging pass, the hepatic parenchyma achieves the maximum level of hepatic enhancement. Patients with vascular primary or metastatic neoplasms (the latter including metastatic islet cell and carcinoid tumors) may have tumor foci that are relatively necrotic or hypovascular, particularly after therapy. In these patients, the hepatic tumor burden is estimated from the extent of enhancing tumor identified on the late arterial phase images as well as the hypoattenuating tumor best seen on the hepatic phase images.
Primary hepatocellular carcinoma may be complicated by venous tumor thrombus and arterioportal fistula (6). In patients with venous tumor thrombus, a solid tumor cast grows from the hepatic parenchyma into the venous lumen and is supplied by arterial neovasculature. The typical appearance has been described in the angiographic literature as "threads and streaks." The same appearance can be identified on early arterial phase multidetector CT images, both on the two- and subsequently three-dimensional CT angiographic display (Fig 7).
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Figure 7. Multifocal hepatocellular carcinoma and expansile tumor thrombus in the portal vein in a middle-aged man. Coronal plane MIP image reformatted from late arterial/portal venous inflow phase image data (2.5-mm image thickness and table speed of 15 mm per rotation at a pitch value of 6.0 [see Fig 1]) shows the head of the expansile tumor thrombus in the extrahepatic portal vein (black arrow). Note prominent neovasculature within the tumor thrombus (white arrow). Multifocal tumor appears as low-attenuation lesions in the periphery of the right hepatic lobe.
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Arterioportal fistula (Fig 8) may occur in conjunction with tumor thrombus in the portal venous system, as well as with arterial pseudoaneurysms that occur after blunt or penetrating trauma. Opacification of the intrahepatic portal vein during the early arterial phase is definitive evidence of an arterioportal fistula.
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Figure 8a. Hepatic cirrhosis, hepatocellular carcinoma, tumor thrombus, and arterioportal fistula in a middle-aged man. (a, b) Axial scans from the arterial phase sequence demonstrate the hypertrophied left hepatic artery and tumor thrombus in the left portal vein (arrows in a) and the head of the thrombus in the main portal vein (curved arrow in b), with retrograde fill of the right intrahepatic posterior portal vein branch (straight arrow in b). (c) Coronal plane MIP image reformatted from early arterial phase image data (2.5-mm image thickness and table speed of 15 mm per rotation at a pitch value of 6.0 [see Fig 1]) demonstrates tumor thrombus in the left portal vein (curved arrow) with opacification of the intra- and extrahepatic portal vein (arrowheads). The portal vein is opacified retrograde to the portal vein confluence. Note the opacified superior mesenteric artery (straight arrow) with no opacification of the superior mesenteric vein. Lack of mesenteric venous opacification is expected for a pure early arterial phase image. Opacification of the portal vein in this phase is indicative of an arterioportal fistula.
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Figure 8b. Hepatic cirrhosis, hepatocellular carcinoma, tumor thrombus, and arterioportal fistula in a middle-aged man. (a, b) Axial scans from the arterial phase sequence demonstrate the hypertrophied left hepatic artery and tumor thrombus in the left portal vein (arrows in a) and the head of the thrombus in the main portal vein (curved arrow in b), with retrograde fill of the right intrahepatic posterior portal vein branch (straight arrow in b). (c) Coronal plane MIP image reformatted from early arterial phase image data (2.5-mm image thickness and table speed of 15 mm per rotation at a pitch value of 6.0 [see Fig 1]) demonstrates tumor thrombus in the left portal vein (curved arrow) with opacification of the intra- and extrahepatic portal vein (arrowheads). The portal vein is opacified retrograde to the portal vein confluence. Note the opacified superior mesenteric artery (straight arrow) with no opacification of the superior mesenteric vein. Lack of mesenteric venous opacification is expected for a pure early arterial phase image. Opacification of the portal vein in this phase is indicative of an arterioportal fistula.
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Figure 8c. Hepatic cirrhosis, hepatocellular carcinoma, tumor thrombus, and arterioportal fistula in a middle-aged man. (a, b) Axial scans from the arterial phase sequence demonstrate the hypertrophied left hepatic artery and tumor thrombus in the left portal vein (arrows in a) and the head of the thrombus in the main portal vein (curved arrow in b), with retrograde fill of the right intrahepatic posterior portal vein branch (straight arrow in b). (c) Coronal plane MIP image reformatted from early arterial phase image data (2.5-mm image thickness and table speed of 15 mm per rotation at a pitch value of 6.0 [see Fig 1]) demonstrates tumor thrombus in the left portal vein (curved arrow) with opacification of the intra- and extrahepatic portal vein (arrowheads). The portal vein is opacified retrograde to the portal vein confluence. Note the opacified superior mesenteric artery (straight arrow) with no opacification of the superior mesenteric vein. Lack of mesenteric venous opacification is expected for a pure early arterial phase image. Opacification of the portal vein in this phase is indicative of an arterioportal fistula.
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Hypovascular Hepatic Tumors.—
Hypovascular metastases are best evaluated by using conventional hepatic phase imaging with an injection to scan delay of 60 seconds following an intravenous bolus injection of a 42-g iodine load of contrast material (3 mL/sec for 150 mL of 60% iodinated contrast material) (7). With multidetector CT technology, a range of image thicknesses may be used. In general, image thicknesses greater than 5 mm are not employed with multidetector scan-ners. In patients undergoing initial evaluation for suspected metastases from the common hypovascular tumor types (colon, breast, lung, pancreas, endometrium), an image thickness of 3.75 mm (LightSpeed; GE Medical Systems) is used (Fig 9). The liver is scanned in 8–10 seconds, with continued more caudal scanning of the abdomen and, if clinically indicated, the pelvis. An abdominal or abdominopelvic scan can be obtained within one patient breath hold.
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Figure 9a. Metastatic pancreatic carcinoma in a 64-year-old man. Scan sequence was obtained 60 seconds after the beginning of the intravenous contrast material injection (42-g iodine load, 3 mL/sec for 150 mL of 60% iodinated contrast material); 3.75-mm image thickness was used. (a) First scan shows three small metastases in segments 7 and 8. Note the clear display of a posterior subcapsular lesion (arrow). (b) Subsequent scan shows multiple small metastases in the right and left lobes and the caudate lobe. Note the clear distinction between adjacent small solid metastases (curved arrow) and the cyst (straight arrow) in the anterior left hepatic lobe. Air is present in the left intrahepatic bile ducts. (c) Subsequent scan shows a hypovascular solid neoplasm in the pancreatic head and body that partly encases the portal vein confluence (black arrow). Note the dilated distal pancreatic duct (white arrows).
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Figure 9b. Metastatic pancreatic carcinoma in a 64-year-old man. Scan sequence was obtained 60 seconds after the beginning of the intravenous contrast material injection (42-g iodine load, 3 mL/sec for 150 mL of 60% iodinated contrast material); 3.75-mm image thickness was used. (a) First scan shows three small metastases in segments 7 and 8. Note the clear display of a posterior subcapsular lesion (arrow). (b) Subsequent scan shows multiple small metastases in the right and left lobes and the caudate lobe. Note the clear distinction between adjacent small solid metastases (curved arrow) and the cyst (straight arrow) in the anterior left hepatic lobe. Air is present in the left intrahepatic bile ducts. (c) Subsequent scan shows a hypovascular solid neoplasm in the pancreatic head and body that partly encases the portal vein confluence (black arrow). Note the dilated distal pancreatic duct (white arrows).
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Figure 9c. Metastatic pancreatic carcinoma in a 64-year-old man. Scan sequence was obtained 60 seconds after the beginning of the intravenous contrast material injection (42-g iodine load, 3 mL/sec for 150 mL of 60% iodinated contrast material); 3.75-mm image thickness was used. (a) First scan shows three small metastases in segments 7 and 8. Note the clear display of a posterior subcapsular lesion (arrow). (b) Subsequent scan shows multiple small metastases in the right and left lobes and the caudate lobe. Note the clear distinction between adjacent small solid metastases (curved arrow) and the cyst (straight arrow) in the anterior left hepatic lobe. Air is present in the left intrahepatic bile ducts. (c) Subsequent scan shows a hypovascular solid neoplasm in the pancreatic head and body that partly encases the portal vein confluence (black arrow). Note the dilated distal pancreatic duct (white arrows).
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Arterial Perfusion Anomalies.—
Hepatic perfusion anomalies occur in patients with focal interruptions to the liver blood supply and venous drainage and in patients with vascular hepatic neoplasms, both before and after ablative therapy. Perfusion anomalies are most pronounced during arterial phase imaging.
In patients with lobar or segmental portal vein stenosis or thrombosis or in patients with hepatic vein stenosis or thrombosis, there may be compensatory increased hepatic arterial inflow to the affected segments or lobes (Fig 10).
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Figure 10. Hepatic hilar cholangiocarcinoma in a 70-year-old woman (same patient as in Fig 6). Image obtained from the late arterial/portal venous inflow phase shows a central tumor mass involving the hepatic duct confluence and the medial segment of the left hepatic lobe (white arrow). The right portal vein is occluded. Increased enhancement of the right hepatic lobe reflects compensatorily increased arterial flow. Note the hypertrophied right hepatic artery branch (black arrow). The left hepatic lobe, particularly the lateral segment, is relatively hypertrophied, reflecting the subacute nature of the right portal vein occlusion.
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In patients with vascular hepatic neoplasms, the surrounding normal hepatic parenchyma may be relatively hyperenhanced during the late arterial phase of imaging. This hyperenhancement is secondary to a "sump" effect, in that the liver tumor produces increased hepatic arterial inflow in a segmental or lobar distribution to supply both the tumor and adjacent normal parenchyma (Fig 11); thus, the increased hepatic arterial inflow will result in hyperenhancement of both. Distinction of tumor margin from the adjacent hyperenhanced normal parenchyma is important for accurate tumor sizing.
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Figure 11a. Metastatic pancreatic islet cell carcinoma in a 40-year-old man. Early arterial phase (a) and late arterial/portal venous inflow phase (b) images both show multifocal hepatic tumor, but the tumor conspicuity is greater in the late arterial/portal venous inflow phase (b). In addition, there is focal geographic enhancement of adjacent normal hepatic parenchyma secondary to a "sump" effect. Tumor margins are more distinct, although less enhanced, on the early arterial phase image (a) when the transient hepatic attenuation difference (THAD) effect is not evident.
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Figure 11b. Metastatic pancreatic islet cell carcinoma in a 40-year-old man. Early arterial phase (a) and late arterial/portal venous inflow phase (b) images both show multifocal hepatic tumor, but the tumor conspicuity is greater in the late arterial/portal venous inflow phase (b). In addition, there is focal geographic enhancement of adjacent normal hepatic parenchyma secondary to a "sump" effect. Tumor margins are more distinct, although less enhanced, on the early arterial phase image (a) when the transient hepatic attenuation difference (THAD) effect is not evident.
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Arterioportal fistulas occur in vascularized hepatic tumors with neocirculation and after penetrating trauma such as percutaneous biopsy. The increased arterial flow demands of the focal arterioportal fistula result in hyperenhancement of adjacent distal hepatic parenchyma (Fig 12). Arterial perfusion anomalies associated with arterioportal fistula and venous interruption should be distinguished from hepatocellular carcinoma. Hepatocellular carcinoma is characterized by neovasculature in an inhomogeneously enhancing infiltrating or expanding tumor.
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Figure 12a. Focal hepatic arterioportal fistula after biopsy in a patient with hepatic cirrhosis and hypertrophy of the left hepatic and caudate lobes. Axial early arterial phase images demonstrate a pseudoaneurysm (straight arrow in a) and early filling of the portal vein (arrowheads in b). Note the geographic triangular region of increased hepatic enhancement due to the transient hepatic attenuation difference effect between the pseudoaneurysm/arterioportal fistula and the liver capsule. Retraction of the liver capsule is presumed secondary to focal hepatic scarring.
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Figure 12b. Focal hepatic arterioportal fistula after biopsy in a patient with hepatic cirrhosis and hypertrophy of the left hepatic and caudate lobes. Axial early arterial phase images demonstrate a pseudoaneurysm (straight arrow in a) and early filling of the portal vein (arrowheads in b). Note the geographic triangular region of increased hepatic enhancement due to the transient hepatic attenuation difference effect between the pseudoaneurysm/arterioportal fistula and the liver capsule. Retraction of the liver capsule is presumed secondary to focal hepatic scarring.
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Another cause of hepatic perfusion anomalies is tumor ablation in which the ablation process (either cryoablation, ethanol injection, or radio-frequency ablation) results in peripheral portal venous thrombosis outside the margin of the tumor. Compensatorily increased hepatic arterial flow to the normal parenchyma adjacent to the tumor results in a rim, either partial or complete, of hyperenhancing normal hepatic parenchyma (Fig 13). This geographically enhanced normal parenchyma borders the outer margin of the ablated tumor and is distinguishable from incompletely ablated tumor, in which enhancing tissue would be seen inside the tumor margin.
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Figure 13a. Multifocal hepatocellular carcinoma in a 45-year-old man successfully treated with percutaneous ethanol ablation therapy. Both ablated lesions in the anterior inferior right lobe (a) and the medial segment left lobe (b) are characterized by uniform hypoattenuation and sharp margination. There is geographic hyperenhancement in a small zone of hepatic parenchyma peripheral to both ablated lesions (arrow in a) and (arrowhead in b) secondary to perfusion anomaly. Note the biliary ductal dilatation in the lateral segment of the left hepatic lobe. The dilatation is secondary to a biliary stricture resulting from intravasation of alcohol deep to the treated lesion during one of the treatment sessions.
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Figure 13b. Multifocal hepatocellular carcinoma in a 45-year-old man successfully treated with percutaneous ethanol ablation therapy. Both ablated lesions in the anterior inferior right lobe (a) and the medial segment left lobe (b) are characterized by uniform hypoattenuation and sharp margination. There is geographic hyperenhancement in a small zone of hepatic parenchyma peripheral to both ablated lesions (arrow in a) and (arrowhead in b) secondary to perfusion anomaly. Note the biliary ductal dilatation in the lateral segment of the left hepatic lobe. The dilatation is secondary to a biliary stricture resulting from intravasation of alcohol deep to the treated lesion during one of the treatment sessions.
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Focal Benign Hepatic Tumors.—
The characteristic imaging features of benign hepatic lesions such as cyst, hemangioma, focal nodular hyperplasia, or hepatic adenoma observed with multidetector CT are the same as those seen with single-detector CT, and there are no additional findings or changes in diagnostic criteria resulting from the use of the rapid thin-section imaging technique. However, the "imaging window" that optimizes detection of these transiently enhancing hypervascular neoplasms is the late arterial/portal venous inflow phase, the same phase used to evaluate malignant primary and secondary vascular hepatic neoplasms (5).
Biliary Disease.—
Use of thin-section hepatic CT techniques with multidetector CT results in better demonstration of the intrahepatic and extrahepatic biliary tree than can be achieved with the thicker section thicknesses used with single-detector CT. Thin-section imaging performed before the administration of contrast material (precontrast) may be useful in detecting partly calcified ductal stones (8). Postcontrast imaging is useful in defining the site and extent of biliary tract obstruction and, in cases of malignant duct obstruction, the location and size of lymphadenopathy and hepatic metastases. Enhanced thin-section late arterial/portal venous inflow phase or hepatic phase CT may be combined with overlapped reconstructions to provide a data set for three-dimensional CT cholangiography. Curved planar reformation is a useful display technique for defining the site of biliary obstruction and demonstrating tumor encasement of adjacent periductal vessels (9).
Diffuse Liver Disease.—
The most common diffuse liver diseases identified in clinical practice include fatty infiltration, cirrhosis, hemosiderosis, and Budd-Chiari syndrome. The utility of multidetector CT is most apparent in the evaluation of patients with underlying hepatic cirrhosis who are at risk for hepatocellular carcinoma. The appropriate CT technique is a combination of late arterial/portal venous inflow phase and hepatic phase imaging. This approach maximizes the detection of hepatocellular carcinoma, a lesion that occurs with a 20% prevalence in patients with chronic cirrhosis (10). Lesions that simulate hepatocellular carcinoma include coincidental hemangiomas, focal nodular hyperplasia, adenomas, and localized vascularized fibrous scars. For a patient awaiting transplantation surgery, the addition of early arterial phase imaging allows for production of a CT arteriogram and CT portal venograms, which can help define important anatomic vascular variants and stenosis or thrombosis of the extrahepatic portal venous system, all of which are significant findings for surgical planning.
Focal Benign Liver Disease.—
Benign focal lesions include cyst, abscess, hematoma, and infarct. These entities have no distinctive features on multidetector CT scans beyond those already well seen with single-detector CT.
Pancreatic Imaging
Accurate detection and staging of pancreatic adenocarcinoma are major challenges. Patients may present either with painless jaundice, in which carcinoma of the pancreatic head is likely, or with abdominal pain and weight loss but no jaundice, in which tumors are likely to be more advanced and to involve the body and tail. Coexistent pancreatitis may be present.
Contrast Material Pharmacokinetics.—
A triple pass imaging technique that includes an early arterial phase, pancreatic phase (11) (corresponding in timing to the late arterial/portal venous inflow phase of the liver) (Figs 14, 15), and subsequent hepatic phase can be employed. Thin-section technique (1.25 mm in the early arterial phase and 2.5 mm in the pancreatic phase) and rapid coverage are used in conjunction with rapid intravenous bolus injection of the standard 42-g iodine load delivered at 5 mL/sec. As with hepatic imaging, preliminary mini-bolus injection is used to determine arrival time of contrast material in the abdominal aorta and thus the start time for the early arterial phase. The early arterial phase is used in patients with suspected pancreatic carcinoma who are potential candidates for surgery, and it can be used to produce a CT angiogram for a vascular road map. Focal tumor, focal pancreatitis, and pseudocyst are best delineated in the pancreatic phase. In addition, this phase provides data for a reformatted CT angioportogram.
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Figure 14. Schematic of multiphase pancreatic CT with a multidetector CT scanner outlines the first two acquisition passes: the early arterial phase and the subsequent "pancreatic" phase. Injection (white bar) of 60% iodinated contrast material (42-g iodine load) is performed at a rate of 5 mL/sec for 30 seconds. The first imaging pass uses a pitch value of 6 with 1.25-mm detector collimation and 2.5-mm image thickness; 1.25-mm image thickness can be generated from this data set for CT angiography. The pancreatic phase uses a pitch value of 3 (table speed per rotation divided by detector collimation—modified definition) and 3.75-mm image thickness. A conventional hepatic phase, beginning 60 seconds after the start of contrast material injection, is performed in patients with suspected pancreatic carcinoma. Injection to scan delay is set from the preliminary mini-bolus injection. In this schematic, the injection to scan delay is 20 seconds, but it can vary between 12 and 25 seconds.
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Figure 15a. Hilar cholangiocarcinoma. Pancreatic phase (a) and hepatic phase (b) images both demonstrate intrahepatic biliary dilatation. On the pancreatic phase image (a), note the increased enhancement of the pancreatic head and body (arrowheads) compared with the hepatic parenchyma. A more uniform splenic enhancement is present on the hepatic phase image (b) compared with the pancreatic phase image (a). Pancreatic phase imaging is used to better delineate small hypoattenuating intrapancreatic tumors. Peripancreatic vessel encasement, lymphadenopathy, hepatic metastases, and peritoneal tumor are equally well demonstrated with both phases.
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Figure 15b. Hilar cholangiocarcinoma. Pancreatic phase (a) and hepatic phase (b) images both demonstrate intrahepatic biliary dilatation. On the pancreatic phase image (a), note the increased enhancement of the pancreatic head and body (arrowheads) compared with the hepatic parenchyma. A more uniform splenic enhancement is present on the hepatic phase image (b) compared with the pancreatic phase image (a). Pancreatic phase imaging is used to better delineate small hypoattenuating intrapancreatic tumors. Peripancreatic vessel encasement, lymphadenopathy, hepatic metastases, and peritoneal tumor are equally well demonstrated with both phases.
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As with triple pass hepatic CT, water is used as the enteric contrast material for triple pass pancreatic CT in patients with suspected pancreatic carcinoma in order to provide a suitable imaging template for CT arteriography and venography.
Pancreatic Adenocarcinoma.—
The major imaging findings better demonstrated by multidetector CT compared with single-detector CT are a hypoattenuating pancreatic mass associated with pancreatic ductal dilatation proximal to the tumor mass. In tumors located in the pancreatic head, biliary ductal dilatation is expected (Fig 16). An important characteristic for determining potential resectability is perivascular tumor invasion, particularly in relation to the celiac and mesenteric arteries, the splenic and superior mesenteric veins, and the portal vein confluence (Figs 17, 18). More pronounced peripancreatic tumor extension may involve the left renal vein.
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Figure 16a. Carcinoma of the pancreatic head and body, biliary and pancreatic ductal dilatation, and celiac and hepatic artery encasement in a 40-year-old woman. (a) A more cephalic pancreatic phase image shows celiac artery encasement (white arrow) and a biliary stent (curved arrow) in a dilated suprapancreatic common duct. The portal vein is encased (straight black arrow). (b) A more caudal pancreatic phase image demonstrates pancreatic ductal dilatation in the more proximal atrophic pancreas (curved arrow), anterior cystic components of the tumor (arrowhead), and encasement of the origin of a replaced right hepatic artery arising from the right proximal superior mesenteric artery (straight arrow).
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Figure 16b. Carcinoma of the pancreatic head and body, biliary and pancreatic ductal dilatation, and celiac and hepatic artery encasement in a 40-year-old woman. (a) A more cephalic pancreatic phase image shows celiac artery encasement (white arrow) and a biliary stent (curved arrow) in a dilated suprapancreatic common duct. The portal vein is encased (straight black arrow). (b) A more caudal pancreatic phase image demonstrates pancreatic ductal dilatation in the more proximal atrophic pancreas (curved arrow), anterior cystic components of the tumor (arrowhead), and encasement of the origin of a replaced right hepatic artery arising from the right proximal superior mesenteric artery (straight arrow).
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Figure 17. Three-dimensional volume-rendered hepatic/mesenteric CT arteriogram of a patient with unresectable carcinoma of the pancreatic head and body (same patient as in Fig 16) shows biliary (straight solid arrow) and pancreatic duct (open arrow) stents. Irregular narrowing of the proximal replaced right hepatic artery (wavy arrow) and proximal common hepatic artery (arrowhead) is noted. A retroaortic left renal vein is seen. Image data were obtained from the first pass of a multiphase pancreatic CT study with 1.25-mm detector collimation, table speed of 7.5 mm per rotation, and a pitch value of 6.0 (see Fig 14).
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Figure 18a. Carcinoma of the pancreatic head and uncinate process involving the portal vein confluence in a 45-year-old woman. (a) Axial image demonstrates a diffuse hypoattenuating tumor mass in the pancreatic head contiguous to the junction of the more posterior superior mesenteric vein (straight arrow) and the more anterior splenic vein (curved arrow). A biliary stent is present in the common duct (arrowhead). (b) Multiplanar MIP image from the late arterial/portal venous inflow phase demonstrates focal encasement at the portal vein confluence (arrows). An endoscopically placed biliary duct stent extends from the hepatic hilum to the duodenum. Data were obtained from the pancreatic phase with 3.75-mm detector collimation, table speed of 11.25 mm per rotation, and a pitch value of 3.0 (see Fig 14).
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Figure 18b. Carcinoma of the pancreatic head and uncinate process involving the portal vein confluence in a 45-year-old woman. (a) Axial image demonstrates a diffuse hypoattenuating tumor mass in the pancreatic head contiguous to the junction of the more posterior superior mesenteric vein (straight arrow) and the more anterior splenic vein (curved arrow). A biliary stent is present in the common duct (arrowhead). (b) Multiplanar MIP image from the late arterial/portal venous inflow phase demonstrates focal encasement at the portal vein confluence (arrows). An endoscopically placed biliary duct stent extends from the hepatic hilum to the duodenum. Data were obtained from the pancreatic phase with 3.75-mm detector collimation, table speed of 11.25 mm per rotation, and a pitch value of 3.0 (see Fig 14).
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Regional peripancreatic lymphadenopathy, including extension to nodes in the hepatoduodenal ligament and at the porta hepatis, can be detected. Use of thin-section technique during the hepatic phase optimizes detection of hepatic metastases.
Invasion of the root of the mesentery and transverse mesocolon associated with ascites are ominous findings that suggest peritoneal carcinomatosis.
Multiplanar reformations including curved planar reformations optimize display of pancreatic tumors and local metastases by providing a high-resolution image of the whole organ in planes other than the axial plane, with a better demonstration of peripancreatic tumor extension, vascular invasion, and lymphadenopathy.
Intraductal Pancreatic Mucinous Tumors.—
Intraductal pancreatic mucinous tumors include tumors otherwise referred to as ductectatic mucinous tumors and localized cystic mucinous tumors of the macrocystic type. The major advantage of thin-section multidetector CT technique (1.25 or 2.5 mm) is duct visualization to detect the extent of ductectatic tumor. In patients with localized cystic neoplasms, relationships of the neoplasms to surrounding vessels and adjacent organs can be well displayed (12) (Fig 19). Intraductal pancreatic mucinous tumors may be found as small cystic pancreatic lesions and can be incidental findings on studies performed for other reasons. Intraductal pancreatic mucinous tumors are potentially malignant. Patients with ductectatic tumors beyond branch duct involvement should undergo endoscopic retrograde cholangiopancreatography to confirm the diagnosis before exploratory surgery and removal. Most small side branch ductectatic tumors are benign, but the imaging appearance does not allow differentiation of benign from malignant disease (13). Focal chronic pancreatitis with associated pseudocysts is the most common and important differential diagnosis.
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Figure 19a. Intraductal papillary mucinous tumor with peripancreatic extension in an 80-year-old woman. (a) Most cephalic pancreatic phase image demonstrates a multiloculated cystic tumor cephalad to the pancreatic body, compressing the lesser curvature of the stomach and contiguous to the caudate lobe. The left gastric artery (curved arrow) is on the anterior surface of the cystic tumor mass. Intrahepatic extension of cystic tumor is seen adjacent to the portal vein at the hepatic hilum (straight arrows). (b) Pancreatic phase image at the level of the pancreatic body and tail demonstrates diffuse ectatic dilatation of the main pancreatic duct with large intraductal calcifications in the proximal duct (arrows). (c) More caudal pancreatic phase image through the inferior pancreatic head demonstrates irregular dilatation of the main pancreatic duct (arrow) associated with atrophy of the gland parenchyma. Note the relative atrophy of the right hepatic lobe and hypertrophy of the left hepatic lobe seen on all three images.
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Figure 19b. Intraductal papillary mucinous tumor with peripancreatic extension in an 80-year-old woman. (a) Most cephalic pancreatic phase image demonstrates a multiloculated cystic tumor cephalad to the pancreatic body, compressing the lesser curvature of the stomach and contiguous to the caudate lobe. The left gastric artery (curved arrow) is on the anterior surface of the cystic tumor mass. Intrahepatic extension of cystic tumor is seen adjacent to the portal vein at the hepatic hilum (straight arrows). (b) Pancreatic phase image at the level of the pancreatic body and tail demonstrates diffuse ectatic dilatation of the main pancreatic duct with large intraductal calcifications in the proximal duct (arrows). (c) More caudal pancreatic phase image through the inferior pancreatic head demonstrates irregular dilatation of the main pancreatic duct (arrow) associated with atrophy of the gland parenchyma. Note the relative atrophy of the right hepatic lobe and hypertrophy of the left hepatic lobe seen on all three images.
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Figure 19c. Intraductal papillary mucinous tumor with peripancreatic extension in an 80-year-old woman. (a) Most cephalic pancreatic phase image demonstrates a multiloculated cystic tumor cephalad to the pancreatic body, compressing the lesser curvature of the stomach and contiguous to the caudate lobe. The left gastric artery (curved arrow) is on the anterior surface of the cystic tumor mass. Intrahepatic extension of cystic tumor is seen adjacent to the portal vein at the hepatic hilum (straight arrows). (b) Pancreatic phase image at the level of the pancreatic body and tail demonstrates diffuse ectatic dilatation of the main pancreatic duct with large intraductal calcifications in the proximal duct (arrows). (c) More caudal pancreatic phase image through the inferior pancreatic head demonstrates irregular dilatation of the main pancreatic duct (arrow) associated with atrophy of the gland parenchyma. Note the relative atrophy of the right hepatic lobe and hypertrophy of the left hepatic lobe seen on all three images.
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Islet Cell Pancreatic Tumor and Carcinoid Tumor.—
Islet cell tumors are focal hypervascular pancreatic lesions. They may be either functional or nonfunctional, and they secrete a variety of hormones including gastrin, insulin, glucagon, and vasoactive intestinal polypeptide.
Patients with suspected islet cell tumors are evaluated for tumor location and possible metastases (14) by using a triple pass pancreatic/hepatic technique. Findings from preoperative imaging are correlated with those from intraoperative US, the technique that has the greatest sensitivity for defining small pancreatic lesions.
A major advantage of multipass abdominal imaging in patients with islet cell or carcinoid tumors is detection and localization of vascular hepatic metastases (Fig 20). As with other vascular hepatic lesions, the optimal phase for detecting these hepatic metastases is the late arterial/portal venous inflow phase. If tumor ablation therapy is being considered, multidetector CT is important in the preoperative planning for combined resection and ablation.
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Figure 20a. Multifocal hepatic metastases from intestinal carcinoid tumor in a 59-year-old man. (a) Late arterial/portal venous inflow phase image shows multifocal enhancing tumor masses varying in size from several millimeters to several centimeters in all hepatic lobes and segments. (b) Hepatic phase image at the same level shows that the carcinoid metastases have become predominantly isoattenuating. Several focal lesions in the right lobe (arrowheads) are now hypoattenuating relative to the surrounding normal hepatic parenchyma.
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Figure 20b. Multifocal hepatic metastases from intestinal carcinoid tumor in a 59-year-old man. (a) Late arterial/portal venous inflow phase image shows multifocal enhancing tumor masses varying in size from several millimeters to several centimeters in all hepatic lobes and segments. (b) Hepatic phase image at the same level shows that the carcinoid metastases have become predominantly isoattenuating. Several focal lesions in the right lobe (arrowheads) are now hypoattenuating relative to the surrounding normal hepatic parenchyma.
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Pancreatitis.—
The extent and severity of pancreatitis as determined with an imaging study is assessed according to the degree of pancreatic enlargement, number and size of pancreatic pseudocysts, peripancreatic effusion, and pancreatic necrosis (15). Associated complications include peripancreatic vascular thrombosis and gastrointestinal fistulization. In patients with acute pancreatitis, a double pass helical imaging technique that includes pancreatic and hepatic phase acquisitions is potentially beneficial for outlining focal areas of pancreatic necrosis. In contradistinction to the imaging techniques used for patients with suspected pancreatic carcinoma, positive enteric contrast material is used in the CT imaging of patients with pancreatitis to better delineate adjacent pathologic conditions of the bowel including fistulas.
Renal Imaging
Renal CT studies may be performed without contrast material enhancement, with both precontrast and dynamic postcontrast imaging, and with only dynamic postcontrast imaging, which is timed to occur in specific pharmacokinetic phases (ie, dedicated triple pass postcontrast imaging).
Contrast Material Pharmacokinetics.—
The transit time of contrast material through the renal circulation is very rapid, with a 6-second temporal window between initial arterial opacification and opacification of the renal vein; 25% of the circulating contrast material bolus passes through the renal circulation. The enhancement phases of the kidney can be subdivided into four phases: (a) a vascular phase, which consists of a brief arterial phase that corresponds in timing to the arterial phase used for hepatic and pancreatic imaging; (b) a combined venous/angionephrogram phase, which corresponds in timing to the late arterial/portal venous inflow phase or pancreatic phase; (c) a nephrogram phase, which occurs 2 minutes after the beginning of the contrast material injection; and finally (d) an excretory phase, which occurs 5–10 minutes after the beginning of the contrast material injection. The classic angionephrogram represents a combination of preferential arterial flow to the renal cortex (90% of renal blood flow) and rapid glomerular filtration. The temporal window for the angionephrogram phase is approximately 60 seconds. During this time, the medullary pyramids progressively enhance because of tubular filtration and slow medullary circulation. By 2 minutes, the nephrogram becomes uniform. The nephrogram phase is preferred for imaging solid renal neoplasms (Fig 21). Caliceal contrast enhancement appears at 3–5 minutes.
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Figure 21. Schematic of multiphase aortorenal CT angiography with a multidetector CT scanner outlines injection and image data acquisition. Injection (white bar) is 5 mL/sec for 30 seconds of 60% iodinated contrast material (42-g iodine load). An early arterial phase, late arterial/renal venous/portal venous phase, and nephrogram phase are shown sequentially. Each imaging pass uses a pitch value of 6 (table speed per rotation divided by detector collimation—modified definition). In patients with suspected renal cell carcinoma, preliminary baseline non-contrast-enhanced images of the kidneys are obtained.
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Nonenhanced Renal CT.—
Nonenhanced CT of the kidneys and urinary tract is a standard method for evaluating suspected urinary tract lithiasis. A 5-mm image thickness is relatively standard for both single-detector CT and multidetector CT (16).
Pre- and Postcontrast Renal CT.—
This approach is used to evaluate a suspected complex cystic or solid mass. The postcontrast imaging phase is subdivided into an initial dynamic postcontrast sequence and a subsequent delayed (5–10 minutes) postcontrast sequence. Lesions are evaluated for imaging characteristics (eg, attenuation, wall definition, homogeneity, septation, nodularity, calcification) as well as for sequential enhancement changes (17). An image thickness of 5 mm is used (Fig 22). The initial dynamic postcontrast imaging pass extends from the diaphragm inferiorly to the aortic bifurcation and is used to evaluate possible hepatic or abdominal metastases from a solid renal neoplasm. The delayed postcontrast images are obtained through the kidneys only.
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Figure 22. Right upper pole renal cell carcinoma in a 55-year-old man. Angionephrogram phase image demonstrates a solid enhancing tumor mass in the right upper pole (arrow). Postcontrast attenuation was 50 HU and precontrast attenuation (not shown) was 20 HU. A postcontrast attenuation value of greater than 20-25 HU and enhancement of greater than 10-15 HU are consistent with a solid renal mass.
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Regions of interest are placed on identical locations in the renal mass for each of the three phases. This evaluation is best performed at a workstation and with the split screen technique, in which identical levels are displayed simultaneously.
Recent publications have questioned the accuracy of the CT number criteria used to distinguish renal cysts from solid renal lesions. The accepted range of Hounsfield units for renal cyst fluid is 0–20 HU. However, a phenomenon described as "pseudoenhancement" has been observed in intrarenal cysts examined during the nephrogram phase of enhancement. The explanation for an attenuation value of greater than 20 HU in a simple fluid cyst is overcorrection for beam hardening in the reconstruction algorithm. Thus, a renal cystic lesion that measures in the range of 0–20 HU on a precontrast study may have Hounsfield unit values of up to 35 HU if the lesion is intrarenal and examined during the nephrogram phase.
Dedicated Triple Pass Postcontrast Technique.—
The usual indication for this technique is preoperative staging of a lesion that is highly suspected of being renal cell carcinoma. The examination includes arterial and venous phases for vascular mapping and a nephrogram phase to define tumor size and location (eg, polar or perihilar) and tumor margination in relation to the renal sinus (18) (Figs 23, 24). This information is critical for planning potential nephron-sparing surgery and cryoablation procedures.
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Figure 23a. Central right renal cell carcinoma in a 35-year-old woman. (a) Aortorenal CT angiogram from planar MIP reformatted data shows immediate prehilar branching of the single right renal artery (RRA, arrow) with segmental arteries supplying branches to the central tumor (arrowhead). No tumor thrombus is evident in the two separate right renal veins. Data were obtained from the first pass of an aortorenal CT angiographic study with a 1.25-mm image thickness, table speed of 7.5 mm per rotation, and a pitch value of 6.0 (see Fig 21). (b) Axial nephrogram phase image shows the tumor, which abuts and compresses the renal pelvis (arrows). Based on the central tumor location, supply by multiple segmental renal arteries, and invasion of the renal pelvis, total nephrectomy was necessary.
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Figure 23b. Central right renal cell carcinoma in a 35-year-old woman. (a) Aortorenal CT angiogram from planar MIP reformatted data shows immediate prehilar branching of the single right renal artery (RRA, arrow) with segmental arteries supplying branches to the central tumor (arrowhead). No tumor thrombus is evident in the two separate right renal veins. Data were obtained from the first pass of an aortorenal CT angiographic study with a 1.25-mm image thickness, table speed of 7.5 mm per rotation, and a pitch value of 6.0 (see Fig 21). (b) Axial nephrogram phase image shows the tumor, which abuts and compresses the renal pelvis (arrows). Based on the central tumor location, supply by multiple segmental renal arteries, and invasion of the renal pelvis, total nephrectomy was necessary.
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Figure 24. Right upper pole renal cell carcinoma in a 55-year-old man (same patient as in Fig 22). CT angiogram from MIP reformatted data shows a right upper pole solid renal mass (curved arrow) that is supplied by a single upper pole segmental renal artery branch (straight arrow). There are two separate patent right renal veins (arrowheads). Segmental nephrectomy was performed. Data were obtained from the first pass of an aortorenal CT angiogram with a 1.25-mm image thickness, table speed of 7.5 mm per rotation, and a pitch value of 6.0.
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The arterial phase imaging is preceded by a preliminary mini-bolus injection to determine the time that contrast material reaches the aorta. The injection to scan delay is set from the preliminary mini-bolus injection. Thin image thickness (1.25 mm) is used with rapid table speed to enable CT angiography of the aortorenal circulation. Coverage is limited to the region between the upper pole of the kidney and the aortic bifurcation. Within the same patient breath hold, a repeat imaging pass is performed in the venous phase to delineate renal vein anatomy and potential venous tumor thrombi. This scan sequence extends to the diaphragm to detect possible vascular hepatic metastases, since this timing corresponds to the late arterial/portal venous inflow phase. The third imaging pass is a nephrogram phase study of the kidneys that begins 180 seconds after the injection of contrast material. Scan coverage includes the kidneys only.
Focal Benign Renal Disease.—
Focal benign conditions affecting the kidneys include focal nephronia, abscess, infarct, hematoma, and urinoma. Imaging features seen at multidetector CT correspond to those already well demonstrated by single-detector CT. Focal inflammatory disease and infarct are usually imaged in a postcontrast mode only. In the case of focal nephronia and abscess, the diagnosis is strongly suspected clinically.
Imaging of Blunt Abdominal Trauma
Patients in whom blunt abdominal and pelvic trauma is suspected require an accurate rapid assessment before treatment planning. Important diagnostic findings include solid organ injuries such as laceration and infarction and associated active bleeding (Figs 25–27), mesenteric and bowel injuries, bladder rupture, major vessel trauma, diaphragmatic rupture, and musculoskeletal injury to the lumbar spine and pelvis (19).
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Figure 25. Blunt abdominal trauma with splenic laceration, perisplenic hematoma, and vascular contrast material extravasation. Axial multidetector CT scan shows the vascular contrast material extravasation as focal areas of enhancement in the intrasplenic/perisplenic hematoma (arrows), which are higher in attenuation than adjacent normal splenic parenchyma.
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Figure 26. Multifocal right hepatic lobe lacerations with a focus of active hemorrhage. Axial multidetector CT scan shows focal contrast material extravasation (arrow), which has higher attenuation than that of intrahepatic vascular structures and the enhanced hepatic parenchyma. The finding represents active hemorrhage. Hepatic lacerations extend to the capsule, and there is perihepatic hematoma.
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Figure 27. Active bleeding from the superior mesenteric vein. Axial multidetector CT scan shows the site of venous contrast material extravasation (curved arrow), which is adjacent to a small-caliber mesenteric arterial arcade branch (straight arrow). Extravasated contrast material also outlines bowel loops subadjacent to the right lateral abdominal wall.
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Trauma imaging studies are performed with contrast material enhancement. Positive enteric contrast material may be used in imaging the upper gastrointestinal tract. Image acquisition starts 60 seconds after the beginning of an intravenous contrast material injection of a 42-g iodine load (150 mL of 60% iodinated contrast material injected at 3 mL/sec). Scan coverage is from the diaphragm to the inferior pelvis. With a half-second scan rotation speed and table speed of 15 mm per rotation, the total acquisition time is approximately 10 seconds.
The major advantage of using multidetector CT for trauma cases is the ability to combine an arterial vascular phase study of the thorax (to look for traumatic aortic disruption) with a parenchymal phase examination of the abdomen. In these combined thoracoabdominal studies, contrast material is injected at 5 mL/sec. It has been our empiric experience that additional cases of hepatic, splenic, mesenteric, and pelvic bleeding have been better demonstrated with multidetector CT technique, perhaps reflecting the higher intraarterial iodine concentration achieved with the more rapid bolus injection.
If skeletal trauma is detected, images can be reconstructed at 2.5-mm thicknesses by using a bone algorithm with a limited field of view and 50% overlapped reconstructions. This approach allows the generation of a three-dimensional volume data set for assessment of pelvic and thoracolumbar spinal fractures.
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Conclusions
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Radiologists should be well versed in imaging technology, vascular physiology, and contrast material pharmacokinetics and evolving clinical needs such as tumor detection and staging in abdominal visceral imaging. With this background information, multidetector CT technology can be applied effectively to the evaluation of suspected hepatic, pancreatic, and renal pathologic conditions.
Multidetector CT is a powerful imaging tool that is still in a relatively early stage of development. Improved capability with more than four-channel multidetector systems has been implemented. Eight-channel multidetector CT scanners are now available, and 16-channel multidetector CT systems should be introduced into clinical practice in mid-2002. Increasingly, multidetector CT data will be reviewed at a workstation as a volume data set with isotopic resolution, which allows the radiologist to interrogate the data set, either in the vascular or parenchymal phase, in the most appropriate imaging plane to derive clinically relevant information. In departments without picture archiving and communica-tion systems in CT, limited film recording of the large volume data sets is usually performed and the complete study, including overlapping reconstructions, is interrogated at a workstation.
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Footnotes
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Abbreviation: MIP = maximum intensity projection
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References
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Abstract
Introduction
