The imaging evaluation of oncology patients requires accurate depiction and characterization of all hepatic and extrahepatic tumors. While helical computed tomography (CT) has been the workhorse of most radiology departments, recent advances in abdominal magnetic resonance imaging (MRI) have moved it to the forefront of oncologic imaging at our institution, the Sharp Memorial Hospital.[1,2]
Compared to helical CT, MRI of the abdomen offers the potential advantages of superior soft-tissue contrast and multiplanar imaging. The extent to which we are able to distinguish tumor from normal abdominal soft tissues is central to our ability to accurately depict the dimensions of a tumor. Magnetic resonance images display a much wider range of soft-tissue contrast, making tumor masses easier to distinguish from adjacent soft tissues (Figure 1). The addition of contrast agents provides differential enhancement of tumor and normal soft tissues, thus further improving the delineation of hepatic and extrahepatic tumor (Figure 2).
Among the recent advances in MRI are faster pulse sequences, breath-hold imaging, and use of intravenous contrast agents and surface coils, all of which have improved image quality and shortened examination times. The versatility of abdominal MRI is unmatched by any other imaging examination. For these reasons, at our institution, MRI has evolved from a problem-solving study to become the primary imaging examination in many patients with malignancy. In this review, we will discuss the spectrum of oncologic applications for abdominal MRI and will highlight areas in which newer MRI techniques offer significant advantages over helical CT.
The depiction of a focal liver lesion requires a difference in signal intensity between the lesion and the adjacent liver parenchyma. The lesion may be either more or less intense than the surrounding liver. Most liver masses are easily depicted on unenhanced T1-weighted or T2-weighted MRIs (Figure 3). However, some tumors produce minimal changes in T1- and T2-relaxation and, therefore, show limited contrast with the surrounding liver.
Contrast agents are used to accentuate the inherent differences in liver-lesion signal intensity. These contrast agents facilitate differential enhancement of liver parenchyma and masses, as well as promote lesion depiction and, to some degree, lesion characterization (Figure 4).[3,4]
Gadolinium chelates were the first intravenous contrast agents to be approved. These nonspecific extracellular agents rapidly equilibrate from the intravascular space into the extracellular space after injection. The use of gadolinium chelates has become an integral part of MRI of the liver and extrahepatic abdomen. Although liver-specific contrast agents are now available, gadolinium chelates continue to offer significant advantages in abdominal MRI.
Gadolinium chelates uniquely provide important information about tumor perfusion, which is key in the assessment of liver masses. They assist with the detection and characterization of liver lesions and in the establishment of the volume of viable perfused tumor.
Gadolinium chelates are equally important for MRI of the extrahepatic abdomen. The interstitial accumulation of these agents within peritoneal, omental, and gastrointestinal tumor produces marked enhancement and is crucial to accurate tumor staging. Depiction of lesions within solid visceral organs, such as the pancreas, kidneys, and spleen, also improves following gadolinium injection.
For effective hepatic MRI, we combine unenhanced axial T1-weighted spoiled gradient recalled echo (SPGR), and fat suppressed T2-weighted MRI with rapid, serial dynamic gadolinium-enhanced SPGR MRI. Three sets of dynamic gadolinium-enhanced liver images are obtained during the arterial, portal-venous, and equilibrium phases of liver enhancement. On high-field strength systems, rapid SPGR T1-weighted sequences are obtained after a 0.1-mmol/kg bolus injection of gadolinium chelate. By imaging the entire liver volume during a short period of suspended respiration, motion artifact is eliminated, while the intravascular gadolinium provides important information on tumor perfusion.
Compared to CT with iodinated contrast material, MRI has proven to be superior in evaluating metastatic liver disease.[5-9] Semelka et al compared single-phase helical CT and MRI with fat suppressed T2-weighted and gadolinium-enhanced SPGR in 89 patients with focal liver masses. In 49% of patients, MRI detected more lesions than did helical CT, with MRI depicting 519 true-positive liver masses vs 295 lesions depicted with helical CT.
Delineation of lesion borders is often easier on MRI, allowing for more consistent measurement of hepatic metastasis (Figure 4).
Hypervascular Hepatic Metastases
Hypervascular metastases from renal cell carcinoma, pancreatic islet cell tumors, breast carcinoma, thyroid carcinoma, sarcomas, and carcinoid tumors are supplied by the hepatic artery and enhance rapidly following the injection of gadolinium chelates. Since 75% to 80% of the blood supply to the liver derives from the portal vein, there is only minimal liver enhancement on these early images. Thus, on arterial phase images, hypervascular metastases will show marked enhancement against a background of minimally enhancing liver parenchyma (Figure 5).
Hypovascular Hepatic Metastases
Hypovascular metastases arise from colon carcinoma, pancreatic carcinoma, transitional cell carcinoma, and lung cancer and are best depicted on portal-venous phase gadolinium-enhanced images. These metastases receive a minimal supply of blood from the hepatic artery or portal vein. During the portal-venous phase, the liver parenchyma demonstrates marked enhancement while the hypovascular metastases show minimal enhancement, producing the greatest difference in liver-lesion signal intensity (Figure 6).[6,7,9] Gradual peripheral enhancement and heterogenous central enhancement of these lesions occurs on later images.
On arterial phase images, hypovascular metastases are poorly visualized, appearing only vaguely distinct from the adjacent liver. Since neither the tumor nor the liver is enhancing at this point, these hypovascular lesions may not be depicted. However, due to the high-contrast resolution of MRI, it is not uncommon to see thin peripheral enhancement of hypovascular metastases on gadolinium-enhanced capillary phase images. On equilibrium phase gadolinium-enhanced images, hypovascular metastases will become indistinct due to nonspecific interstitial accumulation of the contrast material. The pattern of peripheral washout of the contrast may also be noted.
Hepatocellular carcinomas (HCC) manifest a highly variable appearance on unenhanced T1-weighted and T2-weighted images. On T1-weighted imaging, HCC are most often hypointense compared with normal liver, although hyperintense lesions or areas of hyperintensity within a hypointense HCC can be seen. These hyperintense regions within the HCC reflect the presence of fat, copper, or protein. On T2-weighted imaging, HCC are generally hyperintense, although well-differentiated HCC may be isointense compared with liver parenchyma, thus limiting their detection.
Due to the variable appearance of HCC on T1-weighted and T2-weighted MRIs, dynamic gadolinium-enhanced imaging can play an important role in the diagnosis of this primary liver tumor.[10,11] Compared to helical CT with iodinated contrast material, the superior contrast resolution of MRI facilitates detection of small enhancing HCC on arterial phase gadolinium-enhanced SPGR images. In the setting of cirrhosis, any abnormal foci on arterial phase images should be considered highly suggestive of a developing HCC. In a recent study, Yamashita et al found that arterial phase gadolinium-enhanced MRI is superior to helical CT for detecting hepatocellular carcinoma.
Following IV injection of gadolinium, the pattern and degree of enhancement of HCC is related to tumor differentiation and histologic subtype. Well-differentiated tumors often show minimal arterial phase enhancement that washes out on portal-venous phase images, leaving the tumor hypointense compared with the adjacent liver. The presence of a capsule will be indicated by a hypointense rim on early images that enhances on delayed images.
Moderately or poorly differentiated HCC are characterized by dilated sinusoidal spaces, which produce moderate or marked enhancement with gadolinium on arterial phase images. Enhancing tumor nodules are often best depicted on these arterial phase dynamic images. Portal-venous invasion by an HCC is most accurately identified on gadolinium-enhanced breath-hold SPGR images. Expansion of the portal vein and replacement of the intravascular gadolinium by tumor thrombus is evidence of portal-venous tumor extension.