MRI is performed using a number of different sequences which highlight different types of tissue in the human body. One sequence, called “T2 weighted images,” highlights tissues which contain various amounts of water. Tissues with a high fluid (water) component are bright on T2-weighted images or “high signal on T2 weighted images.” Desmoid tumors contain active fibroblasts which have a component of fluid in their cells and extracellular spaces, and therefore, are high in T2 signal.
Desmoid tumors also contain collagen (which is produced by the fibroblasts). Collagen alone has very little water content, and therefore, is low in T2 signal. When desmoid tumors contain active fibroblasts in their early, growing stages they are called “cellular” desmoid tumors because they have a high proportion of cells and relatively less collagen. These cells and their large extracellular spaces are bright in T2 signal. However, when the fibroblasts regress over time, leaving mostly collagen behind, desmoid tumors become low in T2 signal and will no longer grow. Reducing the cellularity of desmoid tumors, and therefore, their growth potential, is one of the goals of systemic therapy, and can be monitored using MRI.