What is MRI?
Magnetic resonance imaging (MRI) sends pulses of magnetism through the body, with each pulse making all hydrogen atoms point the same way, as if these atoms are magnets. As these atoms change direction, they emit signals, which are measured by the scanner. And when the magnetism stops, the hydrogen atoms revert to other positions, emitting more signals. These signals are seen as relaxation times. 

Melanin has a short T1 relaxation times compared to surrounding liver tissue, so that metastases are visible on MRI. Most but not all melanoma metastases also have short T2 relaxation times and such lesions are more likely to be detected using contrast.

How sensitive is this test?
MRI can detect lesions as small as 5 mm. 

How can sensitivity be enhanced?
Sensitivity can be enhanced by injecting gadolinium (e.g., Primovist) intravenously and/or by diffusion weighting, which can be done without contrast. 

Diffusion weighting shows how much the water molecules have drifted between pulses and provides information not possible with standard MRI. After treatment of liver metastases from uveal melanoma, diffusion MRI shows response to therapy before standard MRI, according to a study published by M Bujis et al in AJR 2008. This is because when cells die, the membranes disintegrate so that the water molecules can diffuse more.  

What about false positive results?
A study by Maeda T published in the Japanese Journal of Clinical Oncology in 2007 reported finding hepatic metastases from uveal melanoma in 15 patients and benign lesions in another five (i.e., vascular lesions and cysts with hemorrhage). Diffusion weighting does not distinguish malignant tumors from other lesions. 

What other advantages does MRI have?
MRI does not expose patients to dangerous radiation. It can be performed repeatedly.

Are there any risks?
In patients with pre-existing disease, gadolinium can rarely cause nephrogenic systemic fibrosis, which can be fatal.