Many patients with cancer end up having whole-body imaging studies, either positron emission tomography and computed tomography (PET/CT) or magnetic resonance imaging (MRI) scans. Some of these patients are already exposed to radiation as part of their therapy, so MRI provides a valuable radiation-free study for oncologists to monitor their patients and any possible metastasis.
Whole-body MRI is commonly used to image these cancer patients to evaluate the presence of secondary (or metastatic) lesions, which can change the course of their treatment. Metastatic lesions occur when the cancer has spread beyond the origin point. Metastases can occur in both near and far areas of the body compared to the origin point.
Diffusion weighted magnetic resonance imaging
Diffusion weighted imaging (DWI) is an MR technique that uses the Brownian motion of water molecules. Because of this, DWI can provide information about lesions and soft tissue structures. This imaging method is used for a number of things, including imaging cancer patients.
One study compared whole-body MRI including DW sequences, as well as a number of other sequences, to PET/CT to assess advanced melanoma.1 The study detected 120 lesions on the whole-body MRI scans of 35 patients. Of these lesions, 70 were malignant. The team states that the sensitivity and specificity were both higher on MRI compared to PET/CT. The sensitivity was 82% for MRI and 72.8% for PET/CT, while the specificity was 97% on MRI and 92.7% on PET/CT.1 Adding DWI, MRI detected 14 more malignant lesions. DWI was also found to be more accurate for detecting metastases in bone, liver, subcutaneous and intra-peritoneal sites compared to PET/CT.1
Optimizing your scan
In some cases, non-optimal DWI may produce scintigraphy-like images, which may lead to false positive results. One team from Oran, Algeria has noted that there may be an efficient way to avoid these types of images.2 They suggest conducting DW-MRI with an inversion time technique. Adjusting their scan in this way allowed them to have better fat suppression, which, in turn, helps to suppress the background signals from the body.
Magnet homogeneity describes how uniform a magnetic field is, and for MRI increased homogeneity may mean increased quality.2 If the signal is not uniform, like when an item is causing a distortion in the magnetic field, the images may have distortions (called artifacts). Using an MR system with strong magnet homogeneity allows for acquisition of large field of view images. It also helps reduce distortions in off-center imaging, like that of the limbs.
Magnetic resonance whole-body imaging may present one way to evaluate metastases in all types of cancers. In melanoma imaging, DW-MRI was found to be more sensitive and specific than PET/CT. If an inversion time technique is added to the DW-MRI, the images may have more uniformity. All of this can affect the treatment course for patients with cancer, as their physicians work to prevent the cancer's spread.
- Comparative study of two whole-body imaging techniques in the case of melanoma metastases: advantages of multi-contrast MRI examination including a diffusion-weighted sequence in comparison with PET-CT. https://www.ncbi.nlm.nih.gov/pubmed/19497694. Eur J Radiol. Last accessed September 19, 2019.
- Whole-body diffusion for evaluation of metastatic lesions. SIGNA Pulse of MR. http://www.gesignapulse.com/signapulse/spring_2019/MobilePagedArticle.action?articleId=1488812&app=false. Last accessed September 19, 2019.