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Hepatobiliary FDG PET/CT


Liver

The segmental anatomy of the liver is based on the hepatic veins, thus the segmental anatomy of the liver is more difficult to appreciate on contrast computed tomography (CT). The liver is divided into right and left lobes by the middle hepatic vein. The right lobe is divided into anterior and posterior segments by the right hepatic vein, and the left lobe is divided into medial and lateral segments by the left hepatic vein ( Fig. 12.1 ). On a noncontrast CT, this may lead to confusion with defining the left and right hepatic lobes ( Fig. 12.2 ). The falciform ligament may be confused as the structure that separates the left and right hepatic lobes, but this is actually the plane of the left hepatic vein, separating the medial and lateral segments of the left lobe. On a noncontrast CT scan, the intralobar fissure, separating the left and right hepatic lobes, is better approximated by the location of the gallbladder fossa.

FIG. 12.1, Hepatic Segmental Anatomy.

FIG. 12.2, (A) Axial noncontrast CT through the liver demonstrating the falciform ligament (arrow) in the gallbladder fossa (arrow) . (B) The same axial noncontrast CT image with the course of the left hepatic vein marked using the falciform ligament (red line) in the course of the middle hepatic vein approximated using the gallbladder fossa (yellow line) . The portion of the liver between the red and yellow lines is sometimes mistaken for a portion of the right hepatic lobe but is actually the medial segment of the left hepatic lobe.

When confronted with a focal liver lesion, knowledge of the differential diagnosis for solid and cystic liver masses is valuable. In the differential diagnosis, consider malignancies, including primary malignancies, metastases, and lymphoma, as well as benign etiologies including benign neoplasms and infections.

Let's start with solid liver lesions ( Fig. 12.3 ). Solid liver lesions may be malignant or benign. Malignant solid liver lesions include metastases, lymphoma, and primary malignancies such as hepatocellular carcinoma (HCC) and cholangiocarcinoma (CC). Benign solid liver lesions are most commonly hemangiomas, focal nodular hyperplasia (FNH), or adenomas. With this differential diagnosis for solid liver lesions, clinical history and imaging characteristics can then be used to narrow the differential or determine diagnosis. Of course, biopsy may sometimes be required for the definitive diagnosis.

FIG. 12.3, Differential Diagnosis for Solid Liver Lesions.

Now let's consider cystic liver lesions ( Fig. 12.4 ). Cystic lesions may also be malignant or benign. For malignancies, metastases may be cystic, HCC and CC may be cystic, and we would add biliary cystadenocarcinomas. Lymphoma is less likely to be cystic. For benign cystic liver lesions, we would consider cysts, infections (pyogenic, amoebic, echinococcal), biliary hamartomas (von Meyenburg complexes), and biliary cystadenomas. This produces a differential diagnosis that is different from that of solid liver lesions.

FIG. 12.4, Differential Diagnosis for Cyst Liver Lesions.

With the knowledge of common differentials for solid and cystic liver lesions, we can now address specific scenarios of liver lesions on fluorodeoxyglucose positron emission tomography (FDG PET)/CT.

Hepatic Metastases

The liver is one of the most common organs for metastatic involvement. In a patient with a known malignancy with a propensity to metastasize to the liver, FDG-avid liver lesions are highly suspicious for liver metastases ( Fig. 12.5 ). FDG-avid liver metastases may be visualized with corresponding low-attenuation lesions on CT; however, the lack of low-attenuation lesions on the corresponding CT does not prevent the diagnosis of metastasis. Several possible reasons may contribute to the lack of an apparent lesion on CT. First and foremost, most FDG PET/CT scans are performed without intravenous contrast, and scans without intravenous contrast will greatly limit the visualization of liver lesions on CT, particularly on soft tissue CT windows. Adjusting the CT window to that of the liver window may help to visualize a corresponding low-attenuation lesion on CT. Second, even with intravenous contrast, FDG PET may demonstrate FDG-avid liver metastases without a corresponding lesion on CT ( Fig. 12.6 ). This is particularly apparent when the patient has hepatic steatosis. In the setting of hepatic steatosis, the low-attenuation liver parenchyma may mask low-attenuation liver lesions which are more apparent on FDG PET.

FIG. 12.5, FDG-avid Liver Metastases from Gallbladder Cancer.

FIG. 12.6, Liver Metastases Seen on FDG PET but Not Contrast-enhanced CT due to Hepatic Steatosis.

Although much less common, it is also possible to visualize the development of liver metastases on the CT component of the PET/CT before the lesion becomes appreciably FDG avid ( Fig. 12.7 ). The background FDG avidity of the liver may mask small or only mildly FDG-avid liver metastases. To optimize the detection of liver metastases on FDG PET/CT, view the CT images on a liver window, even if the CT was performed without intravenous contrast. A narrow liver window will help to visualize low-attenuation liver metastases from non- or mildly FDG-avid malignancy.

FIG. 12.7, Liver Metastasis Seen on Noncontrast CT but Not FDG PET.

Following therapy, decreases in liver metastases may be apparent by either decreasing size on CT or FDG avidity on FDG PET ( Fig. 12.8 ). In some scenarios, decreases in FDG avidity will better represent treatment response and decreases in size on following treatment of initially FDG-avid pancreatic lymphoma, on CT. This is particularly apparent in the setting of hepatic pseudocirrhosis ( Fig. 12.9 ). Pseudocirrhosis is a result of successful treatment of hepatic metastases, with retraction of the liver capsule in regions of decreasing metastases. This produces a liver contour and appearance similar to that of a patient with cirrhosis but without secondary signs of cirrhosis such as splenomegaly or varices. Pseudocirrhosis is most commonly seen after treatment of patients with breast cancer. The altered morphology of a pseudocirrhotic liver may make it very difficult to determine the extent of residual malignancy on CT.

FIG. 12.8, Decreased Liver Metastases Following Therapy.

FIG. 12.9, Extent of Liver Metastasis Response to Therapy Better Visualized by FDG PET than by Contrast-enhanced CT due to Hepatic Pseudocirrhosis.

In addition, some systemic therapies may not cause liver metastases to decrease in size, but FDG avidity may still accurately reflect metabolic activity or quiescence in the lesion ( Fig. 12.10 ). This is apparent with some immune-modulating therapies such as ipilimumab. Ipilimumab is a monoclonal antibody that helps to incite a stronger immune reaction against malignancy. These immune therapies have demonstrated substantial benefit in the treatment of melanoma, lung cancer, and other malignancies, but treatment response may not be reflected by changes in tumor size. FDG avidity may help to evaluate treatment response in patients on these immune therapies.

FIG. 12.10, Treatment Response in a Patient With Metastatic Melanoma Following Ipilimumab and Nivolumab Therapy Demonstrated by FDG PET.

Interventional radiology (IR) oblations of liver metastases are another area in which FDG PET may serve to better visualize residual or recurrent malignancy than does anatomic imaging. Following ablation of liver metastases, the resulting anatomic defect is usually larger than the original metastasis and makes further evaluation of that region of liver parenchyma difficult. FDG PET is often more sensitive for the detection of residual or recurrent malignancy at the site of IR oblations ( Fig. 12.11 ).

FIG. 12.11, Recurrence of Malignancy Following Interventional Radiology Ablation of a Liver Metastasis, Better Visualized by FDG PET than by Contrast-enhanced CT.

Lymphoma

FDG PET often provides greater sensitivity for detection of active hepatic lymphoma than does anatomic imaging. After diagnosis, FDG PET provides valuable information about response to therapy in hepatic lymphoma, similar to other sites of lymphomatous involvement ( Fig. 12.12 ). The Lugano Criteria state that (for initially FDG-avid lymphoma) posttreatment reduction of FDG avidity to less than liver background represents a complete response to treatment, whereas residual FDG avidity greater than liver background is suspicious for residual active lymphoma.

FIG. 12.12, Response to Therapy in Hepatic Lymphoma, Better Visualized by FDG PET than by CT.

Hepatocellular Carcinoma

Individual HCC tumors vary in tumor grade. Well-differentiated HCCs are more histologically similar to normal hepatocytes, whereas poorly differentiated HCCs are more aggressive. FDG uptake in HCC correlates with tumor differentiation, with more poorly differentiated tumors tending to be more FDG avid. Because of the variability of FDG avidity in HCC, FDG PET is neither sensitive nor specific for detection of primary HCC. Contrast-enhanced CT or magnetic resonance (MR) with multiple phases of image acquisition is far more accurate for initial detection and local staging of HCC. The value of FDG PET/CT for HCC is the detection of nodal and distant metastases, in particular for subcentimeter nodal metastases and osseous metastases. Thus FDG PET/CT may assist with distant staging of HCC.

Cholangiocarcinoma

CC is a primary malignancy of the biliary ducts, which may occur within the liver or involve the extrahepatic ducts. CC most commonly occurs in the common bile duct. As is the case for HCC, FDG PET/CT is not used for detection or local staging, where contrast-enhanced CT and MR are superior. FDG avidity of CCs will vary by histology. Nodular forms of CC tend to be more FDG avid, whereas infiltrating forms are less apparent on FDG PET. This may be due to lower cellular density in infiltrating forms of CC, so there are fewer cells per unit of volume to accumulate FDG. FDG avidity of CCs has also been shown to vary by location of the tumor. Peripherally located CCs are more apparent on FDG PET than are central/hilar lesions. As seen with other malignancies, the role of FDG PET/CT in CC is limited to the detection of unsuspected nodal and distant metastases. The role of FDG PET/CT for this has not been rigorously evaluated.

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