Large GISTs
As the tumor grows it may project outside the bowel (exophytic growth) and/or inside the bowel (intraluminal growth), but they most commonly grow exophytically such that the bulk of the tumor projects into the abdominal cavity. If the tumor outstrips its blood supply, it can necrose internally, creating a central fluid-filled cavity with bleeding and cavitations that can eventually ulcerate and communicate into the lumen of the bowel. In that case, barium swallow may show an air, air-fluid levels or oral contrast media accumulation within these areas. Mucosal ulcerations may also be present. In contrast-enhanced CT images, large GISTs appear as heterogeneous masses due to areas of living tumor cells surrounding bleeding, necrosis or cysts, which is radiographically seen as a peripheral enhancement pattern with a low attenuation center. In MRI studies, the degree of necrosis and bleeding affects the signal intensity pattern. Areas of bleeding within the tumor will vary its signal intensity depending on how long ago the bleeding occurred. The solid portions of the tumor are typically low signal intensity on T1-weighted images, are high signal intensity on T2-weighted images and enhanced after administration of gadolinium. Signal-intensity voids are present if there is gas within areas of necrotic tumor. Features of malignancy
Malignancy is characterized by local invasion and metastases, usually to the liver, omentum and peritoneum. However, cases of metastases to bone, pleura, lungs and retroperitoneum have been seen.
In a CT scan, abnormalities may be seen in 87% of patients and it should be made with both oral and intravenous contrast. Among imaging studies, MRI has the best tissue contrast, which aids in the identification of masses within the GI tract (intramural masses). Intravenous contrast material is needed to evaluate lesion vascularity. Preferred imaging modalities in the evaluation of GISTs are CT and MRI,: 20–21  and, in selected situations, endoscopic ultrasound. CT advantages include its ability to demonstrate evidence of nearby organ invasion, ascites, and metastases. The ability of an MRI to produce images in multiple planes is helpful in determining the bowel as the organ of origin (which is difficult when the tumor is very large), facilitating diagnosis. Small GISTs
Since GISTs arise from the bowel layer called muscularis propria (which is deeper to the mucosa and submucosa from a luminal perspective), small GIST imaging usually suggest a submucosal process or a mass within the bowel wall. In barium swallow studies, these GISTs most commonly present with smooth borders forming right or obtuse angles with the nearby bowel wall, as seen with any other intramural mass. The mucosal surface is usually intact except for areas of ulceration, which are generally present in 50% of GISTs. Ulcerations fill with barium causing a bulls eye or target lesion appearance. In contrast-enhanced CT, small GISTs are seen as smooth, sharply defined intramural masses with homogeneous attenuation.