Radiological Imaging Modalities

Preoperative radiological imaging is the primary diagnostic and staging modality for hepatobiliary tumors, and still plays an essential part in the decisionmaking process regarding surgical resection. However, overall false negative rates lie between 40% and 70% depending upon the technique used and the type of tumor examined.13,14 There is a wealth of information concerning the efficiency of the many radiological imaging methods, but substantial differences in resolution, costs and availability of the various techniques have led to conflicting statistics when comparing alternative modalities. It is not the object of this chapter to provide a complete description of all the available modern imaging techniques, but rather to put the standard modalities at our disposal in perspective of the current practice of hepatobiliary surgery.

Conventional Imaging Techniques

Ultrasonography (US)

By virtue of its simplicity and availability, transabdominal US still plays a crucial role in detecting and evaluating liver malignancies. Variations in contrast within abnormal liver tissues and between solid or cystic tumors are ideally suited to detection with US. It provides multiplanar imaging with excellent spatial resolution, while hepatic vascular anatomy and patency can be accurately displayed using color Doppler flow techniques.15 US is, however, highly operator dependent and easily restricted by patient habitus or interference by bowel gas and the concealing rib cage. Although sensitivity as high as 94% has been reported for the detection of small hepatomas, results are generally much more variable with sensitivities ranging from 20-76% in the detection of colorectal liver metastases.16,17 Nevertheless, US is suitable for the screening of liver metastases during the follow-up of colorectal cancer, and the development of HCC in patients with known cirrhosis or chronic hepatitis.

Computed Tomography (CT)

The principal imaging methods used in the preoperative assessment of liver tumors are based on CT scanning.18 Various enhancement techniques exist including dynamic contrast bolus, delayed scanning and arterial portography (CTAP). Conventional CT will detect hepatic involvement in 90% of cases, but only approximately 70% of actual individual lesions are reliably documented. CT is unable to characterize smaller lesions due to the partial volume effect, while nodules under 1 cm in size remain undetected. Another problem is the limited accuracy in demonstrating small peripheral surface lesions, especially in the left liver lobe where they may be obscured by artifacts caused by cardiac motion or contrast in the stomach. Rapid spiral CT scanning optimizes contrast dynamics and will increase sensitivity to well over the present 85%. CTAP appears to be even more sensitive,

but it is an invasive and cumbersome technique. Furthermore, laminar flow perfusion defects cause pseudo-lesions, resulting in false positive rates as high as 30-40%.16'19,20 CT with arterial iodinated oil emulsions may offer advantages in detecting HCC in cirrhotic livers.

Because of its ability to rule out residual primary disease, local recurrence or secondary tumor deposits at remote sites (such as lung and mediastinum) while simultaneously evaluating intrahepatic tumor, CT remains the primary imaging modality for the staging of patients with hepatobiliary malignancies.

Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) is increasingly available and is particularly useful for characterizing certain benign tumors (e.g., hemangiomas) and for detecting and staging HCC.13,14,21 Faster, dynamic MRI with gadolinium contrast and enhancement with super paramagnetic iron oxide (SPIO) are promising new techniques, but detection accuracy rates have yet to be defined.

Problems in Liver Imaging

Lesion Threshold

Inherent to the resolution of the imaging technique used, there is a threshold size for the ability to both detect and discriminate the nature of liver mass lesions. Both US and CT are unable to delineate tumors under 1 cm. Although MRI can detect smaller lesions, these subcentimeter "nuisance nodules"21 are frequently too small to exhibit distinctive morphologic features or allow guided biopsy.

The Hepatic Substrate

Coexistent abnormalities in the substrate of the liver parenchyma complicate the radiologic assessment of suspect liver tumors. The high incidence of silent, incidental benign tumors in the adult population is the most frequent cause of diagnostic dilemmas. Small, often multiple, cavernous hemangiomas are especially difficult to differentiate but also cysts, focal nodular hyperplasia and liver adenomas may be a cause for concern. Their significance lies in confusion with metastatic disease, particularly when the lesions are under 15 mm in size.

Parenchymal disease, especially cirrhosis, reduces the sensitivity and accuracy of radiologic imaging. Initial recognition of tumor and differentiation from regeneration nodules are seriously affected. Fatty infiltration of the liver is another common occurrence and lowers the attenuation of hepatic parenchyma. Although usually diffuse, focal areas may be spared of fat and retain their normal density, mimicking a tumor. Alternatively, the fatty deposit itself may appear as a focal lesion indistinguishable from metastatic disease.

Requirements for Surgical Decision Making1

Deficiencies in current radiologic imaging mean that only 30-50% of candidates for curative surgery will be found to be resectable at laparotomy. The main reason for unresectability is unsuspected extrahepatic disease (i.e., small volume peritoneal seeding or hepatic lymph node involvement) found in two thirds of cases. In the remaining one third, undetected local intrahepatic conditions (i.e., extra deposits, proximity to major vascular structures or degree of cirrhosis) are the reason for unfruitful laparotomy.

Existing standard imaging techniques (US, CT, MR) have overall sensitivities and specificities of between 60-85%. A comprehensive preoperative assessment will therefore require a combination of different imaging modalities, with considerable time and costs involved.

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