Synopsis

Keywords: Body: Digestive, Body: Body, Cardiovascular: Angiography

Patients with ischemia or vasculitis involving the bowel usually present with acute abdominal pain and are imaged with CT. However, there are several scenarios where MR may be utilized or preferred. This presentation will discuss MR protocol optimization to efficiently diagnose these conditions, explain the pathophysiology and imaging findings of these entities, and review differential diagnoses.

Patients with ischemia or vasculitis involving the bowel typically present with acute abdominal pain. Clinically the differential diagnosis for acute abdominal pain is broad and historically CT has been the examination of choice for evaluating these patients. CT is widely available and provides a rapid diagnosis. However, CT may not be appropriate for all patients and there are several scenarios where MR may be utilized or preferred. Patients may have an allergy to iodinated contrast preventing the performance of CT. Radiation exposure concerns may exist in younger patients, pregnancy, or patients presenting with recurrent or chronic symptoms requiring multiple evaluations. Additionally, the diagnosis of bowel ischemia may be diagnosed when evaluating patients for other conditions on focused MR exams.
Protocol Optimization
Optimized and efficient MR protocols are needed to allow rapid imaging and diagnosis given the acuity of these patients [1-5]. Patients may have difficulty with breath holding or remaining stationary because of the abdominal pain, leading to motion artifacts. Depending on the acuity of the patient and suspected diagnosis, several different MR protocols can be performed. In the acute setting, a rapid screening exam consisting of multiplanar single-shot T2-weighted sequences with and without fat-suppression provides an excellent overview of the bowel. Balanced steady-state free precession (bSSFP) sequences are also useful as they are less sensitive to intraluminal flow artifacts in the bowel and provide an excellent overview of the vasculature if intravenous contrast is not administered. 3D T1-weighted GRE sequences are helpful in demonstrating associated hemorrhage. Detection of bowel ischemia may be improved with the administration of intravenous contrast. Rapid dynamic contrast-enhanced 3D GRE sequences have the potential to acquire multiple 3D datasets during bowel enhancement and motion compensation techniques can reduce respiratory motion artifacts or allow acquisition during free breathing. If there is concern for bowel pathology in a patient with more mild or chronic symptoms, an optimized bowel exam using MR enterography provides more information. If a primary vascular abnormality is of concern, MRA can be performed 6. A hybrid protocol combining early MRA sequences with MREN sequences could also be considered for those patients where optimized vascular and bowel imaging is desired.
Mesenteric Ischemia
Mesenteric ischemia can be classified into acute, chronic, or acute-on-chronic. Acute mesenteric ischemia can be further divided into occlusive (embolic, thrombotic, or venous) or nonocclusive. Bowel findings and location vary on the type of ischemia and can include bowel wall thinning, decreased mural enhancement, mesenteric edema, pneumatosis, mesenteric and portal venous air, and pneumoperitoneum. Wall thickening may be seen with reperfusion and is greatest with venous ischemia [7].
Vasculitis
Vasculitides are classified based on vessel size involved however frequently there can be overlap [8]. The most common vasculitides involving the gastrointestinal tract include polyarteritis nodosa (PAN), ANCA-associated vasculitis, IgA vasculitis, Bechet disease, and vasculitis associated with systemic diseases such as systemic lupus erythematosus, however, can occur with any of the vasculitides. Patients may present with abdominal pain, GI bleeding, nausea, vomiting, and diarrhea. The bowel findings in the vasculitides are often nonspecific and related to the associated ischemia or hemorrhage with segmental areas of wall thickening and edema. If perivascular soft tissue thickening or vascular stenoses are visualized the diagnosis of vasculitis can be suggested however this finding is frequently absent. Other suggestive findings include a history of vasculitis, ischemic changes in other end-organs, recurrent transient areas of wall thickening which occur in different locations and ischemic changes in young patients. Complications include bowel infarction, perforation, and stricture formation. There are numerous conditions which can produce wall thickening, mural edema, and mural inflammation including angioedema which can be recurrent and occur in different locations [9]. Correlation with prior imaging is important to evaluate the temporal and spatial changes. Review of imaging of other body parts to identify vascular changes and other end-organ damage also can be helpful. Given that the imaging findings are frequently nonspecific, the clinical diagnosis requires correlation with history, physical exam, and laboratory testing. Endoscopic biopsies are usually not helpful given their superficial nature.
Emerging Techniques
Ferumoxytol-enhanced MRA provides prolonged intravascular contrast enhancement allowing a longer time window for imaging and the ability to obtain higher resolution imaging without concern for accurate timing to the contrast bolus. Ferumoxytol-enhanced MRA has been used to assess the vasculature in patients with advanced kidney disease undergoing evaluation for renal transplant and may be useful in other scenarios such as suboptimal evaluation of the mesenteric vasculature by other techniques [6,10,11].
4D flow techniques allow more physiologic information regarding the severity of chronic mesenteric ischemia than anatomic images alone and may be helpful in determining the hemodynamic significance of stenoses in chronic mesenteric ischemia [6,12].

Acknowledgements

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