MR imaging

MRI examination:
     
Major technical advances in MR imaging have led to its wider use in the evaluation of cancer patients. MR imaging is a powerful tool in the evaluation of primary liver neoplasms. Determination of tumor extent and tissue characterization is provided with standard spin-echo T1- and T2- weighted imaging and is enhanced by the application of advanced sequences such as gradient-echo, fast spin-echo, and fat suppression techniques. Intravenously administered contrast agents, such as gadopentenate dimeglumine provides additional opportunities for lesion characterization. Fat-suppressed T1- weighted imaging with dynamic gadolinium enhancement has also yielded results comparable with those of contrast-enhanced CT. Dynamic hepatic arterial-phase contrast material-enhanced imaging is essential with both CT and MR imaging. Biphasic contrast material-enhanced dynamic MR imaging is an important technique for evaluating liver and bone disease. However, in the liver several potential diagnostic pitfalls may be encountered, including lobar, segmental, subsegmental, and subcapsular hyperperfusion abnormalities; early-enhancing pseudolesions, particularly in the medial segment of the left hepatic lobe; heterogeneous hyperperfusion abnormalities throughout the liver; and hypointense pseudolesions due to vascular artifacts, unenhanced hepatic vessels, partial volume artifacts, magnetic susceptibility artifacts, and regenerative nodules in cirrhosis. These abnormalities sometimes have appearances similar to those of true lesions or tumor spread to the surrounding liver parenchyma on arterial-dominant phase dynamic MR images. In most cases, however, no corresponding abnormalities are seen with other pulse sequences or on delayed-phase MR images. Recurrence of hypervascular tumor tissue may sometimes be seen only during the arterial phase of a multiphasic protocol. This fact emphasizes the importance of scanning technique. Arterial-phase images are most useful for evaluating hypervascular hepatomas, and hypervascular bone metastases (thyroid cancer, renal cell carcinoma) whereas differentiation of coagulated areas from hypoattenuating tumor tissue is usually easiest with images obtained during the (portal) venous and equilibrium phases in patients who undergo treatment for hepatic and bone metastases. MR imaging was used to determine whether the ablation is complete and to screen for early recurrences that may benefit from reablation. Complete ablation creates an area of necrosis that, at MR imaging, is of low signal intensity compared with the surrounding tissue, is often homogeneous, and has smooth margins. Precontrast T1-weighted spoiled GRE images were acquired in the transverse and coronal planes, and T2-weighted fat-saturated turbo spin-echo or spin-echo images were acquired in the transverse plane. Gadolinium-based contrast material, administered intravenously, was used in conjunction with a breath-hold spoiled GRE sequence in all patients. Gadolinium chelates were injected at a dose of 0.1 mmol per kilogram of body weight, and images were acquired immediately, at 45 seconds, at 90 seconds, and at 5 minutes after administration. Image acquisition immediately after the administration of contrast material was performed in the hepatic arterial-dominant phase, which was defined as the enhancement phase in which contrast material is present in portal veins and hepatic arteries and not present in hepatic veins. Recent studies have also shown that current magnetic resonance (MR) imaging-with hepatic arterial-dominant phase, spoiled gradient-echo (GRE) techniques and enhancement with a gadolinium-based contrast agent or with T2-weighted fat-saturated spin-echo techniques-may be equivalent or superior to CTAP (CT during arterial portography) for the detection of liver metastases. RC Smelka et al. reported a mean sensitivity and specificity for lesion detection (liver metastases) were, respectively, 0.884 and 0.444 for CTAP (CT during arterial portography) and 0.968 and 0.857 for MR imaging. CTAP and MR were not different with respect to sensitivity (P = .50) but were marginally different with respect to specificity (P = .063), which did not, however, achieve statistical significance. No false-positive or false-negative lesions due to perfusion abnormalities were detected at MR imaging. This study suggested that MR imaging was as sensitive as, and more accurate than, CTAP. Although a trend for greater specificity was observed, a significant difference was not reported, which presumably reflected, in part, the small number of patients. MR imaging also demonstrated lesions that were missed or misclassified at CTAP. The multiphase technique is also very useful in bone metastases evaluation after radio frequency ablation. This technique allows to detect residual tumor in all types of bone metastases (different vascularization). 

Successful treatment of 3 metastases from colon cancer. MR imaging obtained 3 months after radio-frequency ablation shows ablation defects without contrast enhancement, suggestive of a complete response.

Successful treatment of 3 metastases from colon cancer. MR imaging obtained 3 months after radio-frequency ablation shows ablation defects without contrast enhancement, suggestive of a complete response.

Large metastases from colon cancer. Incomplete ablation. MR imaging obtained 10 days after the procedure shows irregular shape contrast enhancement with nodular pattern.

Incomplete ablation of a large HCC lesion. MR imaging obtained 3 months after the radio-frequency ablation shows a nodular contrast enhancement in the border of the tumor.