Imaging characterization helps presurgical triage and planning

Multislice CT improves liver lesion detection

By: Dushyant Sahani, M.D., Anil Shetty, M.B.B.S., And Sanjay Saini, M.D.

Major advances in CT technology have helped elucidate the imaging features of primary and metastatic liver tumors. Detection and characterization of both focal and diffuse pathologic conditions in the liver require an understanding of the principles of hepatic perfusion. In the fasting state, normal liver parenchyma receives about 70% of its blood from the portal vein and 30% from the hepatic artery.1,2 Most primary and metastatic liver tumors, however, receive their blood from the hepatic artery.

Prior to surgical treatment of liver tumors, it is important to detect, characterize, and accurately localize them. Improved detection and characterization can help determine which hepatic tumors may be amenable to aggressive surgical techniques and which indicate palliative treatment.3,4 The characterization of liver lesions as benign or malignant is important for the correct triage of patients to surgical versus nonsurgical therapies. Spiral CT is the imaging modality used most frequently for the preoperative depiction of focal liver lesions. The use of a dual-phase CT technique, with imaging during both the arterial and portal venous phases of enhancement, is particularly important for the depiction of hypervascular liver lesions.5,6

It is possible with spiral CT to image the entire liver during each of two somewhat distinct phases of contrast media enhancement: the earlier, fleeting hepatic arterial-dominant phase, and the later and more prolonged portal venous-dominant phase.6 Detection of hepatic lesions often requires a tailored protocol to optimize the phase of enhancement to improve lesion conspicuity. This is particularly important for hypervascular lesions, in which scan timing and contrast media flow rates may be crucial to optimize detection in the often-fleeting hepatic arterial phase.6

Spiral CT scanners with multiple detector rows dramatically increase acquisition speeds.7 Since as many as four sections are obtained during each gantry rotation, the entire liver can be examined in as little as five to seven seconds, depending on the gantry rotation and table speed. This increase in speed not only allows optimization of dual-phase imaging but also provides some unique opportunities. Digital subtraction of CT images has been possible in the past, but the technique was limited by respiratory misregistration from multiple breath-holds. With multidetector-row spiral CT, it is now possible to acquire two phases of hepatic contrast enhancement during the same breath-hold, potentially negating the effects of respiratory misregistration. This facilitates digital subtraction of the precontrast CT scan from the hepatic arterial CT scan to emphasize the various features of early contrast enhancement.8

It is important to note that the protocols vary with multislice spiral CT when compared with single-slice spiral CT. The rate of flow of contrast is increased to 4 to 5 mL/sec, and the time delay should be adjusted for the phase. In general, an early arterial phase should be imaged 20 to 25 seconds after the start of the contrast administration, and the late arterial phase 30 to 35 seconds after. The thinner slices during the study also help reconstruct the vascular tree of the liver and determine the volume of the tumor or the segment to be resected for a transplant donor.

APPLICATIONS

The imaging techniques and protocols detailed below are for the Lightspeed Qx/i multislice spiral scanner from GE Medical Systems. The table lists the advantages of multislice CT.

-Lesion detection and characterization. Technical advances in multiphasic spiral CT have improved the detection and characterization of hepatic neoplasms, particularly hypervascular varieties such as hepatocellular carcinoma (HCC).10,11 The temporal window for the arterial phase imaging is only 20 to 30 seconds in duration. This small window makes it important to coordinate the speed and timing of contrast injection and scan acquisition. Thinner slices decrease partial volume averaging and are helpful in lesion detection. Metastases are best detected when hepatic arterial and portal venous phases are combined, but hypervascular metastases are more easily identified during the hepatic arterial phase of enhancement. As small benign hepatic masses are common even in a patient with known malignant disease, the chance of a lesion being malignant is only about 50%.10 Therefore, liver lesion characterization is an equally important clinical objective, and the enhancement patterns on these phases help this characterization.11

-Benign hepatic tumors. Hemangioma is the most common benign liver tumor. These tumors have a distinctive enhancement pattern characterized by sequential contrast opacification beginning at the periphery as one or more nodular or globular areas of enhancement (Figure 1) and proceeding toward the center.10

On unenhanced CT, focal nodular hyperplasia (FNH) usually appears as a homogeneous hypodense lesion with a well-defined low attenuation scar in one third of cases. Due to the prominent arterial vascular supply, FNH undergoes marked homogeneous enhancement during the arterial phase of contrast-enhanced CT, except for the scar that often enhances in delayed scans (Figure 2).

Hepatocellular adenoma may be hypodense on unenhanced CT images, due to the presence of fat, old necrosis, or old hemorrhage, or it may be hyperdense owing to recent hemorrhage or large amounts of glycogen. Substantial enhancement during the arterial and early portal venous phases of contrast enhancement may be noted.

Simple hepatic cysts are common liver lesions appearing on CT as well-circumscribed, homogeneous masses of near-water attenuation value (<20 HU) that show no enhancement after IV contrast material administration. Small lesions may appear to have higher density because of partial volume averaging.

-Malignant tumors. Due to their predominant arterial supply, HCCs are seen as transiently hyperdense masses in the arterial phase of hepatic enhancement. They become isodense with hepatic parenchyma or hypodense in the portal venous phase of enhancement. On delayed images, the capsule and septa demonstrate prolonged enhancement. In the detection of hypervascular HCC, most investigators agree that arterial phase images are superior to portal and delayed phase images.11,12 Therefore, the arterial phase imaging is mandatory in multiphasic dynamic CT (Figure 3). Other reports recommend triple-phase helical CT for the detection and characterization of HCC.13 Contrast-enhanced CT is also capable of showing vascular invasion and arterioportal shunting associated with HCC. Diffusely infiltrating and small HCCs in cirrhosis may be difficult to detect with CT: The rate of HCC detection ranges from 38% to 84% with conventional CT and is 87% with biphasic helical CT.11 The delayed phase of helical CT has been reported to show higher liver-HCC contrast than does the portal venous phase, thus improving the rate of detection of well-differentiated hypovascular HCCs.14

Fibrolamellar HCC appears on CT as a large, well-defined vascular mass with a lobulated surface and, often, a central scar. Intrahepatic cholangiocarcinoma arises from the epithelium of small intrahepatic bile ducts. CT usually reveals a hypodense mass with irregular margins that shows mild peripheral enhancement and pooling of contrast material on delayed images (Figure 2).12 Likewise, metastases are best detected when hepatic arterial and portal venous phases are combined (Figure 4). Braga et al demonstrated improved liver lesion detection with dual-phase helical CT at 77% to 90% in comparison with Mangafodipir-enhanced MRI at 69% to 72%.15

PROTOCOLS

-Preoperative imaging before liver resection. It is important to detect, characterize, and accurately localize liver tumors prior to surgical treatment. The selection of tumors that may be amenable to aggressive surgical techniques and palliative treatments is crucial.3,4 Faster imaging permits acquisition of multiple thin, high-resolution slices through the liver without motion artifacts.7 Significant improvement in image processing and reconstruction algorithms accompanies this advance in image acquisition techniques. It is possible to create 3D reconstructions of the entire liver and view its relationship to the focal hepatic lesions. The dual hepatic arterial, portal venous, and hepatic venous vascular map can be generated, and this serves as a vascular road map for planning liver resection. Multislice helical CT is also able to demonstrate any aberrant arterial supply to the liver.16,17 Any nondominant accessory or replaced hepatic arteries present require ligation before the placement of an intra-arterial chemotherapy pump just distal to the origin of the gastroduodenal artery for correct target delivery of the chemotherapeutic agent (Figure 3).17

- Imaging evaluation of liver transplant donors. Imaging evaluation is critical to successful liver transplant from living donors. The role of imaging is to identify conditions in the donor that contraindicate liver transplantation as well as to detect variant anatomy of the hepatic angioarchitecture and the biliary tree that may influence the surgical technique.18 MSCT can perform comprehensive evaluation of the liver donor, including assessment of liver parenchyma for diffuse abnormalities such as fatty infiltration or incidental focal lesions, as well as calculations of total and graft liver volume. Evaluation of the liver vascular anatomy and variants (arterial, portal, and hepatic venous) is also needed.

Multidetector CT provides clearer, more accurate anatomic relationships because it allows the lengthwise display of vessels (Figures 4A and 4B). The 3D liver volumetric analysis aids in the "virtual liver resection" for presurgical planning and educational purposes for residents and fellows on appropriate dissection technique for major hepatic resections (Figure 6). Likewise, 3D CT angiograms can provide information similar to conventional angiography.19 This imaging is of increasing importance, especially given advances in hepatic resection techniques that rely fundamentally on knowledge of hepatic vascular variants. Technical advantages include a standard protocol that allows optimum visualization of all hepatic vasculature for all subjects.

-Liver perfusion imaging. Functional imaging in the abdomen is a topic of research, but, although this approach has evolved for the brain, for the abdomen it is only beginning. Recent understanding of the process of angiogenesis shows how tumors derive a vascular supply from adjacent tissue, an essential step in growth and metastatic spread.20,21 This breakthrough has generated interest among researchers in using imaging for the assessment of tumor angiogenesis, an investigation critical to our basic understanding of disease behavior in the abdomen, and promises to lead directly to improved treatment plans.

Dynamic CT and compartmental modeling can be used as noninvasive tools to quantify liver perfusion.20-22 To obtain the perfusion data, a rapid series of images is acquired without table movement following a bolus of contrast medium. Blood flow, blood volume, and mean transit times can be calculated by mathematically modeling the temporal changes in contrast enhancement in the tissue and vascular system. It has been shown that perfusion CT offers the ability to positively identify patients with chronic liver disease and metastasis.22 This principle can be applied to assess the therapeutic response of liver tumors to novel anticancer therapies. Although most of the work on CT perfusion imaging has been done with single-slice helical CT, multislice CT capability allows increased z-axis coverage with thin-slice acquisition without any tube heating constraints.

Dr. Sahani is an assistant radiologist at Massachusetts General Hospital in Boston and a clinical instructor at Harvard Medical School. Dr. Shetty is an intern in radiology at MGH, and Dr. Saini is a professor of radiology at Harvard and director of CT at MGH.

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