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Objective:To characterize the MRI features of GIST and correlate the MRl features with histopathology. To analyze the value of MR imaging in the diagnosis and evaluation of aggressive risk in gastrointestinal stromal tumors.Materials and Methods:The clinical and MR imaging findings of 37 patients with histological and immunohistochemical diagnosis of GIST were reviewed. The pathologic diagnosis was made according to the tumor size and mitotic rate. MR imaging was performed on 1.5 Tesla scanner (Signa Infinity, GE). MR examinations included Tl-Weighted and fat-suppressed T2-Weighted imaging, diffusion-weighted imaging (DWI) with the b value of 0,500s/mm2 by single-shot echo-planar imaging (EPI) sequence, and 3D dynamic contrast enhancement MRI (DCE-MRI) by using 3D-LAVA sequence. According to the lesion’size and mitosis rate,37 cases were classified to low-risk, mid-risk and high-risk groups. MRI evaluations were made regarding the lesions" site, growth pattern, size, shape, margin, signal intensity on T1 WI and T2WI before enhancement, uniformity, enhancement pattern, internal structure after enhancement, kinetic curve pattern on dynamic study, the ADC values on DWI, direct invasion to adjacent organ and distant metastasis. The MRI findings were compared with the aggressive risk groups.Results:Among the 37 cases(11 stomach,9 duodenum,7 small bowel,2 colon,3 rectum,3 mesentery,1 omentum, and 1 retroperitoneum),15 low-risk GIST,10 mid-risk GIST,12 high-risk GIST. Primary tumors were typically exophytic (91.9%). GIST with higher aggressive risk were more likely to have irregular shape, ill-defined margin, heterogeneous signal intensity on MRI images, direct invasion to adjacent organ and and distant metastasis. All lesions were more enhanced on venous phase of post-contrasted MR than arterial phase, The time-signal intensity curve (TIC) of dynamic contrast enhancement of 37 tumors on MRI appeared as persistent enhancing pattern in 29 cases. Rapid enhanced and slow wash-out on delayed images pattern in 8 cases. The TIC patterns between the three groups showed no statistically significant difference (P＞0.05). The ADC values of high/mid/low-risk GIST were 1.127±0.205 (×10-3mm2/s),1.236±0.254 (×10-3mm2/s) and 1.478±0.344 (×10-3mm2/s), respectively. The ADC values of high-risk and mid-risk GIST were lower than that of low-risk, and there was statiscally significant difference between high/mid-risk and low-risk groups (P<0.05).Conclusion:GIST demonstrate characteristic MR imaging features, which correspond well with histopathologic findings. Usually, arise from the gastrointestinal tract, solitary, predominantly exophytic, well-defined margins, heterogeneous signal intensity, moderate or severe enhancement and characteristic ADC values are helpful in the diagnosis of GIST on MR imaging. The presence of irregular shape, ill-defined margin, heterogeneous signal intensity, lower ADC values are suggestive of higher aggressive risk. It is difficult for the degree of enhancement and the time-signal intensity curve pattern to differentiate aggressive risk of GIST. MR imaging and ADC values on DWI have a potential role in diagnosing and evaluating the aggressive risk of GIST pre-operative, but the final diagnosis still depends on the histopathologic and immunohistochemical examination.
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