T1- and T2-weighted images covering the liver and upper abdomen may be acquired prior to contrast injection to search for other pathologies that might account for the patient's abdominal symptoms. The amount of abdominal intraperitoneal and subcutaneous fat can be assessed on these images. Most patients with mesenteric ischemia will have a scaphoid abdomen and less than 15 mm of subcutaneous fat over the rectus abdominis muscles.
3D CE-MRA permits a morphologic analysis of the mesenteric arteries. The high incidence of visceral artery stenosis in an asymptomatic population makes it difficult to determine the clinical significance of a morphologic finding. For patients with mesenteric artery stenosis or an equivocal history of mesenteric ischemia it may be difficult to predict whether correcting the mesenteric artery stenosis will alleviate symptoms. Functional MR imaging may complement 3D CE-MRA in this regard. A cine phase contrast sequence can be employed to assess blood flow in the superior mesenteric vein (SMV) following caloric stimulation. In patients with mesenteric ischemia the increased post-prandial blood-flow within the SMV is out of proportion to that in the SMA. This effect is due to recruitment of collateral flow. By exploiting the known paramagnetic effect of deoxyhemoglobin, Li et al showed close correlation between T2 measurements of blood in the SMV and oxygen saturation in an in vivo animal model . Identifying low oxygen saturation in the SMV compared to the IVC suggests ischemia.
When the CA and SMA are patent, the possibility of branch vessel stenosis and regional ischemia can be assessed by looking at bowel wall enhancement (see below). Bowel mucosa normally enhances greatly immediately following gadolinium administration. Regional areas of diminished or delayed bowel enhancement are suggestive of regional ischemia. The combination of morphologic 3D CE MRA of the splanchnic vasculature with functional assessment of mesenteric flow holds considerable promise as the emerging modality of choice for evaluation of patients with suspected mesenteric ischemia.
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