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Helical CT

Diagnostic Pitfalls of Arterial Phase Imaging of the Upper Abdomen

¹ All authors: The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, Baltimore, MD 21287.

Received May 11, 1999; accepted after revision July 2, 1999.

 
Address correspondence to B. A. Urban, Department of Radiology, Johns Hopkins Hospital, 600 N. Wolfe St., Baltimore, MD 21287.

Presented at the annual meeting of the American Roentgen Ray Society, New Orleans, May 1999.

Introduction

Top Introduction Arterial Phase Helical CT:... Liver Pancreas Spleen Conclusion References

 
Helical CT is routinely used in evaluation of the abdomen and pelvis. Advantages of helical CT scanning, including rapid volumetric data acquisition and optimal contrast enhancement, enable accurate diagnosis of a wide spectrum of disease processes [1]. However, early arterial phase scanning can result in various phenomena that are potential diagnostic pitfalls if not appropriately recognized [2, 3, 4, 5, 6]. Many of these pitfalls result from early parenchymal enhancement or partial vascular opacification—especially in the parenchymal organs of the upper abdomen, most notably in the liver. In addition, the appearance of true disease during the arterial phase can at times be misleading or nonspecific. This report provides an overview of potential pitfalls of arterial phase imaging of the liver, pancreas, and spleen.

Arterial Phase Helical CT: Indications and Technique

Top Introduction Arterial Phase Helical CT:... Liver Pancreas Spleen Conclusion References

 
Arterial phase helical CT refers to any acquisition obtained during peak arterial contrast opacification. Arterial phase CT images are often obtained when hypervascular tumors or metastases are suspected or when vascular anatomy is of particular interest, particularly in evaluation of the liver or kidneys [1]. Acquisitions obtained during the arterial phase are often combined with images obtained during other phases of enhancement in the evaluation of most solid abdominal organs [1]. Arterial phase images of the upper abdomen produce marked enhancement of the aorta and major arteries, the spleen, pancreas, and renal cortex.

The specific protocol for obtaining arterial phase helical CT images varies depending on the clinical indication and the region of interest [1]. Typically, arterial phase images are obtained 20-30 sec after the onset of peripheral IV injection of 120-150 ml of iodinated contrast material. The optimal rate of injection is usually 3-4 ml/sec. The degree of enhancement is dependant on the rate and timing of the injection and the patient's cardiac output and body habitus. Thin-collimation (2- to 3-mm) images are ideal for evaluation of vascular anatomy; a slightly thicker collimation (5 mm) is used for evaluation of most solid organs. Depending on the organ of interest and the clinical indication, arterial phase scans are often supplemented with additional helical acquisitions (dual-phase helical CT).

Liver

Top Introduction Arterial Phase Helical CT:... Liver Pancreas Spleen Conclusion References

 
Arterial phase imaging is particularly important for evaluation of the liver [1]. Some lesions are markedly vascular and are best seen on the arterial phase acquisition. In addition, arterial phase imaging is used for evaluation of the anatomy of the hepatic artery. Arterial phase images are usually obtained in conjunction with a portal venous phase acquisition, typically obtained 70-90 sec after IV injection of contrast material.

Arterial phase imaging can reveal a variety of pseudolesions, most of which result from normal vascular variations or changes in the normal dual blood supply to a portion of liver [2, 3, 4]. Common pseudolesions are seen in the gallbladder fossa, the porta hepatis, and the medial segment of the left lobe of the liver near the falciform ligament [4] (Fig. 1). A transient hepatic attenuation difference of increased enhancement can mimic true disease but is often the result of vascular compromise or thrombosis [4]. Hyperattenuation in the region of thrombosis results from compensatory increased hepatic arterial flow and the lack of dilution of contrast material during the arterial phase because of the diminished, nonopacified early portal venous return. Transient attenuation differences from peripheral portal vein occlusion have a wedge-shaped distribution [4] (Fig. 2A, 2B). Complete portal vein thrombosis can result in diffuse peripheral hyperenhancement if central cavernous transformation has developed (Fig. 3). Extrahepatic vascular causes, notably occlusion of the superior vena cava, can produce a striking hyperenhancement of the medial segment of the left lobe via collateral inferior epigastric pathways (Fig. 4).

View larger version (168K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —76-year-old woman with hepatic pseudolesion near falciform ligament. Arterial phase helical CT scan shows characteristic flow-related pseudolesion (arrow) in medial segment of left lobe of liver. This pseudolesion is the most common one seen during arterial phase and is related to separate venous drainage into left gastric veins. It is often mistaken for tumor; characteristic appearance and anatomic localization are keys to accurate diagnosis. Focal fat can have similar appearance.

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —52-year-old woman with transient hepatic attenuation difference from peripheral portal vein thrombosis. Arterial phase helical CT scan reveals peripheral hypervascular transient hepatic attenuation difference (arrowheads). Hyperattenuation in liver distal to thrombosis results from combination of compensatory hepatic arterial flow and diminished early portal venous return.

View larger version (124K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —52-year-old woman with transient hepatic attenuation difference from peripheral portal vein thrombosis. Portal venous phase helical CT scan shows homogeneously enhanced liver. Small peripheral portal vein thrombus is now appreciated (arrow).

View larger version (165K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3. —44-year-old man with hepatic pseudolesions from portal vein thrombosis. Arterial phase helical CT scan reveals circumferential hyperperfusion (arrowheads) in patient with complete portal vein thrombosis (arrow).

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —27-year-old woman with lymphoma and superior vena cava obstruction. Arterial phase helical CT scan reveals marked early enhancement in medial segment of left lobe of liver (arrow). Collaterals from superior vena cava obstruction involve inferior epigastric veins and course through liver, resulting in characteristic enhancing pseudolesion. Large lymphoma filled anterior mediastinum (not shown).

Other pitfalls of arterial phase imaging include lesion detection and characterization. Unopacified hepatic veins can be mistaken for true lesions and decrease lesion conspicuity [2, 3] (Fig. 5A, 5B). Poor hepatic parenchymal enhancement markedly limits detection of hypovascular tumors, which are best seen on portal venous phase images [5]. Large tumors can be entirely missed (Fig. 6A, 6B). In the trauma patient, lack of early enhancement can also make detection of liver lacerations problematic. Vascular hepatic tumors can often mimic the appearance of aneurysms or vascular malformations (Figs. 7 and 8). In addition, enhancement of vascular tumors can be brief and transient, making localization difficult at biopsy. Peripheral enhancement of hemangiomas can be difficult to appreciate during the arterial phase (Fig. 9A, 9B). Nodular enhancement of hemangiomas is more typical.

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —31-year-old woman with metastatic colonic cancer. Arterial phase helical CT scan shows many hypodense lesions, of which only one is true metastasis. Unfortunately, lesion detection can be difficult and lesion conspicuity can be markedly compromised during arterial phase imaging.

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —31-year-old woman with metastatic colonic cancer. Portal venous phase helical CT scan easily reveals metastatic lesion (arrow). Normal vessels have now opacified.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A. —54-year-old woman with multiple hepatic adenomas. Arterial phase helical CT scan reveals subtle inhomogeneous hepatic enhancement. Because most contrast material reaches liver via portal vein, arterial phase images typically show poor parenchymal enhancement, which can severely limit lesion detection as shown in this case.

View larger version (130K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B. —54-year-old woman with multiple hepatic adenomas. Portal venous phase helical CT scan reveals many lesions (arrowheads), which can now be seen easily because lesions are contrasted against enhanced liver.

View larger version (162K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7. —41-year-old woman with metastatic islet cell tumor. Arterial phase helical CT scan reveals homogeneously enhancing hypervascular metastasis in liver (arrow). Hypervascular tumors can be difficult to differentiate from vascular lesions and aneurysms.

View larger version (157K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8. —66-year-old man with hepatic artery pseudoaneurysm. Arterial phase helical CT scan shows enhancing hepatic artery pseudoaneurysm (arrow). Note similar appearance of hepatic artery pseudoaneurysm and enhancing metastatic islet cell tumor shown in Figure 7.

View larger version (141K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A. —72-year-old man with small hemangioma. Arterial phase helical CT scan shows focal lesion (arrowhead) with subtle peripheral nodularity. Early phase arterial acquisitions often show hemangiomas before peripheral enhancement and, as a result, such hemangiomas are often classified as indeterminate or misdiagnosed as tumors.

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B. —72-year-old man with small hemangioma. Portal venous phase helical CT scan reveals increasing peripheral enhancement of hemangioma (arrowhead).

Arterial phase imaging in patients with primary or secondary parenchymal liver disease can produce problematic enhancement patterns (Fig. 10). Portal hypertension can delay hepatic enhancement and underestimate parenchymal disease or produce portal vein pseudothrombosis (Figs. 11A, 11B and 12A, 12B, 12C, 12D). Peripheral arterioportal venous shunts are sometimes seen on arterial phase images and can mimic small tumors [6] (Fig. 13).

View larger version (148K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10. —40-year-old woman with hepatic congestion after heart transplantation. Arterial phase helical CT scan shows markedly inhomogeneous hepatic enhancement. Heart failure and hepatic congestion can produce mosaic pattern of hepatic enhancement. Arterial phase scanning can markedly exaggerate this appearance, which should not be mistaken for infiltrative tumor. Mild ascites is present.

View larger version (146K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11A. —47-year-old woman with autoimmune hepatitis, cirrhosis, and hepatic necrosis. Arterial phase helical CT scan reveals cirrhotic liver with relatively homogeneous enhancement.

View larger version (143K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11B. —47-year-old woman with autoimmune hepatitis, cirrhosis, and hepatic necrosis. Portal venous helical CT scan better reveals more extensive heterogeneous hepatic enhancement from necrosis, which was proven at biopsy. Evaluation of arterial phase images alone can lead to underestimation of liver disease because images are obtained before peak parenchymal enhancement. This limitation can be especially problematic in imaging of the cirrhotic liver.

View larger version (150K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12A. —41-year-old woman with cirrhosis and pseudothrombosis of portal vein and superior mesenteric vein. Arterial phase helical CT scan of liver shows apparent portal vein thrombosis (arrow). Pseudothrombosis is most commonly encountered in patients with portal hypertension and delayed portal venous return. Note hepatic cirrhosis and ascites.

View larger version (158K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12B. —41-year-old woman with cirrhosis and pseudothrombosis of portal vein and superior mesenteric vein. Arterial phase helical CT scan near pancreas shows apparent superior mesenteric vein thrombosis (arrow).

View larger version (151K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12C. —41-year-old woman with cirrhosis and pseudothrombosis of portal vein and superior mesenteric vein. Portal venous phase helical CT scan reveals patent portal vein (arrowheads).

View larger version (177K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12D. —41-year-old woman with cirrhosis and pseudothrombosis of portal vein and superior mesenteric vein. Portal venous phase helical CT scan reveals patent superior mesenteric vein (arrow) with no evidence of mass.

View larger version (112K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13. —47-year-old man with cirrhosis and peripheral hypervascular lesions. Arterial phase helical CT scan reveals several small hypervascular lesions (arrows). Detection and characterization of very small lesions using arterial phase CT scans is difficult. Many of these lesions may be considered indeterminate and may simply represent small arterioportal venous shunts. Workup and follow-up appropriate for these small lesions remain ambiguous.

Pancreas

Top Introduction Arterial Phase Helical CT:... Liver Pancreas Spleen Conclusion References

 
Arterial phase helical CT is useful in staging pancreatic cancer and in detecting hypervascular pancreatic tumors, particularly islet cell tumors. A pitfall of arterial phase imaging can result when a portal vein confluence is unopacified, thus simulating the appearance of a thrombus or tumor (Fig. 12A, 12B, 12C, 12D). Venous encasement by tumor can be underestimated [7] (Fig. 14A, 14B). In addition, enhancement of the normal pancreas and peripancreatic vasculature can be pronounced, making hypovascular solid tumors mistakenly appear cystic (Fig. 15A, 15B).

View larger version (140K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14A. —59-year-old man with unresectable pancreatic tumor. Arterial phase helical CT scan shows unopacified portal vein and superior mesenteric vein, thus visualization of vascular encasement is markedly limited.

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14B. —59-year-old man with unresectable pancreatic tumor. Portal venous phase helical CT scan produces adequate venous opacification and reveals superior mesenteric vein occlusion (arrow).

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 15A. —40-year-old woman with pancreatic tumor mimicking pseudocyst. Arterial phase helical CT scan shows lesion that was initially diagnosed as pseudocyst (arrow). In fact, lesion represents low-density solid adenocarcinoma. As shown in this case, arterial phase helical CT can produce contrast enhancement of normal vessels and parenchyma that is so marked that hypovascular solid masses appear cystic. Similar difficulties can occur when evaluating renal masses. Note tiny hepatic metastases are present (arrowheads).

View larger version (161K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 15B. —40-year-old woman with pancreatic tumor mimicking pseudocyst. Arterial phase helical CT scan obtained at 3-month follow-up reveals marked increase in size of liver metastases (arrowheads). To avoid this potential pitfall, obtain density values when evaluating any potentially cystic mass.

Spleen

Top Introduction Arterial Phase Helical CT:... Liver Pancreas Spleen Conclusion References

 
Variable contrast enhancement of the normal spleen during arterial phase imaging results from differential flow through the vascular sinusoids of the red pulp [2, 3, 8]. Patterns of normal inhomogeneous enhancement are typically sinusoidal or cordlike and can be easily differentiated from true splenic disease in most patients (Fig. 16). Rarely, the appearance of the normal spleen during the arterial phase can mimic disease. Obtaining delayed images may be helpful in confusing cases.

View larger version (172K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 16. —50-year-old man with splenic pseudolesions. Arterial phase helical CT scan reveals normal inhomogeneous enhancement of spleen. Typically, this finding is only seen on arterial phase acquisition and is exaggerated in patients with heart failure, splenic vein occlusion, or portal hypertension. Inhomogeneous enhancement results from variable flow rates through red pulp and should not be confused with splenic lacerations.

Conclusion

Top Introduction Arterial Phase Helical CT:... Liver Pancreas Spleen Conclusion References

 
Familiarity with the common pitfalls of arterial phase imaging is essential for accurate detection and characterization of disease in the liver, pancreas, and spleen. Correlation with an additional portal venous acquisition (dual-phase helical CT) is often necessary, especially in the evaluation of hepatic disease.

References

Top Introduction Arterial Phase Helical CT:... Liver Pancreas Spleen Conclusion References

 

1. Zeman RK, Baron RL, Jeffrey RB Jr, Klein J, Siegel MJ, Silverman PM. Helical body CT: evolution of scanning protocols. AJR 1998;170:1427-1438[Free Full Text]
2. Silverman PM, Cooper CJ, Weltman DI, Zeman RK. Helical CT: practical considerations and potential pitfalls. RadioGraphics 1995;15:25-36[Abstract]
3. Herts BR, Einstein DM, Paushter DM. Spiral CT of the abdomen: artifacts and potential pitfalls. AJR 1993;161:1185-1190[Abstract/Free Full Text]
4. Chen WP, Chen JH, Hwang JI, et al. Spectrum of transient hepatic attenuation differences in biphasic helical CT. AJR 1999;172:419-424[Free Full Text]
5. Miller FH, Butler RS, Hoff FL, Fitzgerald SW, Nemcek AA Jr, Gore RM. Using triphasic helical CT to detect focal hepatic lesions in patients with neoplasms. AJR 1998;171:643-649[Abstract/Free Full Text]
6. Kim TK, Choi BI, Han JK, Chung JW, Park JH, Han MC. Nontumorous arterioportal shunt mimicking hypervascular tumor in cirrhotic liver: two-phase spiral CT findings. Radiology 1998;208:597-603[Abstract/Free Full Text]
7. Graf O, Boland GW, Warshaw AL, Fernandez-del-Castillo C, Hahn PF, Mueller PR. Arterial versus portal venous helical CT for revealing pancreatic adenocarcinoma: conspicuity of tumor and critical vascular anatomy. AJR 1997;169:119-123[Abstract/Free Full Text]
8. Urban BA, Fishman EK. Helical CT of the spleen. AJR 1998;170:997-1003[Free Full Text]

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