Computed Tomography Protocol and Image Processing Technique

Many cross-sectional imaging modalities are available for the evaluation of patients with GISTs, including ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET). CT is the mainstay of abdominal imaging. It is widely available and highly accurate, particularly in assessing liver metastases.32,33 If properly performed, CT will adequately address most clinical concerns in patients with GISTs. CT features provide information that may help in differentiating GISTs from lymphoma and epithelial gastrointestinal tumors. With the introduction of new targeted medical therapy for GISTs, CT is increasingly utilized to assess tumor response and to evaluate for disease recurrence. In addition, CT is currently the first imaging modality requested in patients with suspected bowel obstruction, which is reported in up to 30% of GISTs.34-36 Occasionally a small bowel neoplasm may be detected in these cases. CT may also be utilized to guide tissue biopsy.

The recent introduction of multidetector row CT, combined with multiplanar reconstructions and high-fidelity volume rendering, can provide comprehensive evaluation of the abdomen, particularly the vascular anatomy37-39; this is particularly useful in determining the origin of large exo-phytic GISTs, which may be difficult to determine in an axial plane. Multiplanar volume rendering and maximum intensity projection techniques are also used to accurately delineate small mucosal lesions, to better characterize the morphology of the lesion, and to detect hepatic and peritoneal metastases.

Although CT may detect an incidental GIST, patients are more commonly referred for accurate tumor localization, characterization, staging, and surveillance for metastases or recurrence after surgery. For routine scanning of the abdomen and pelvis, a detector configuration of 4 x 2.5 mm, table speed of 15 mm, and pitch of 6:1 allow for adequate coverage in a single breath-hold of 20-25 seconds. Image reconstruction of 5 mm, with optional 2.5-mm overlap, can be performed in such cases. Newer 16-slice multidetector CT scanners allow the use of 1.5-mm detectors with 2-mm-thick slices at 1-mm intervals. Contrast enhancement is typically achieved using 120mL (2mL/kg) nonionic contrast media injected intravenously, with a power injector, at a rate of 3ml/s. Depend ing on the clinical indication, arterial and/or portal venous imaging is performed using a 25-second and 60- to 65-second scan delay, respectively. In patients undergoing dual-phase CT of the liver, in addition to CT of the chest, abdomen, and pelvis, arterial-phase images of the liver should be obtained in the first breath-hold at 25 seconds, followed by scans through the chest, abdomen, and pelvis in the second breath-hold at 65 seconds. For the evaluation of the abdominal vascular anatomy before possible resection, a detector configuration of 16 x 0.75-mm collimation and 0.5-mm intervals will result in superior three-dimensional image reconstruction and volume rendering techniques.

Positive oral contrast is not administered when imaging GIST because it may degrade image reconstruction and obscure small mucosal lesions. Positive oral contrast agents may also mix unevenly with gastric and intestinal fluid, resulting in pseudotumor.40,41 When imaging the stomach and small bowel, 750 mL water is recommended as a negative contrast agent, given to the patient approximately 15 minutes before imaging. Patients also receive an additional 250 mL immediately before the study to ensure adequate distention of the stomach. Water is well tolerated and results in good gastric and proximal small bowel distension as well as excellent visualization of the enhancing gastric wall.42 One disadvantage of using water as oral contrast is the suboptimal distension of the distal small bowel. Some authors have advocated using positive contrast initially, followed by water to allow adequate distension of the stomach and small bowel.43 Alternatively, oral metoclopramide can be administered to improve ileal distension and reduce bowel peristalsis.44 GISTs that are detected incidentally may be seen on a routine CT scan of the abdomen, which is usually performed with high-density oral contrast.

At our institution, CT scanning is performed using a mul-tislice variable detector array (Sensation 16; Siemens Medical Solutions, Malvern, PA). Multidetector row CT images are acquired as a volume data set during a single breath-hold. All CT imaging data, in the original resolution of 512 x 512, are sent from the scanner to a freestanding workstation for postprocessing (Leonardo with In Space software; Siemens). Volume rendering allows the best approach to visualize the stomach and small bowel compared to other rendering algo-rithms.42,45 Volume rendering utilizes all the attenuation information in any given slab of tissue, and real-time adjustments can be performed to accentuate the stomach and small bowel. Histograms of the relative density values are manipulated through trapezoid control of variables, such as width, level, opacity, and brightness. This function assigns opacity and color to each voxel and can be instantaneously adjusted to alter the final display. It is often helpful to start with two-dimensional multiplanar reconstructions and then proceed to the three-dimensional volume rendering. Initial two-dimensional multiplanar reconstructions allow for quick assessment of the abdomen in the axial, coronal, and sagittal planes. The main advantage of three-dimensional volume rendering is the enhanced depth perception, which improves visualization of a complex mass or tortuous vessels. Interactive application of different orientations and cut planes enhance the visualization of the bowel and the display of tumor in any plane that is necessary for the surgeon or referring physician.

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