Table 3.

Differential diagnosis table for Primary Aortoenteric Fistula (AEF) and Inflammatory Abdominal Aortic Aneurysm

DIAGNOSIS	CT	MR	ULTRASOUND	NUCLEAR IMAGING
Primary AEF	Extravasation of intravenous contrast into the bowel is the most specific sign, though rare Leakage of enteric contrast into the periaortic space Ectopic gas adjacent to or within the aorta Hematoma in the retroperitoneum or within the bowel wall Obliteration of the fat plane that separates the aorta from the affected segment of bowel Focal thickening and tethering of a bowel loop immediately adjacent to the aorta, which usually is diseased	Not typically used in the emergency setting Also, longer acquisition times and need for greater technical expertise limits use	Hypoechoic projection from the aorta communicating with an adjacent segment of bowel Loss of fat plane between aorta and bowel Presence of periaortic gas, demonstrated as hyperechoic foci with “dirty-shadowing” Color doppler may demonstrate the pulsatile jet from the aorta into the bowel	Not typically used in the emergency setting. Tagged-RBC scanning can be used in slow or intermittent bleeding.
Inflammatory abdominal aortic aneurysm	May possess morphologic features of both mycotic aneurysms and retroperitoneal fibrosis, as detailed below However, they should not demonstrate abscesses, periaortic gas or infective complications of adjacent structures which would suggest mycotic aneurysm instead	May possess morphologic features of both mycotic aneurysms and retroperitoneal fibrosis, as detailed below However, they should not demonstrate abscesses, periaortic gas or infective complications of adjacent structures which would suggest mycotic aneurysm instead.	May possess morphologic features of both mycotic aneurysms and retroperitoneal fibrosis, as detailed below However, they should not demonstrate abscesses, periaortic gas or infective complications of adjacent structures which would suggest mycotic aneurysm instead.	Aortic wall thickening and perianeurysmal fibrotic tissue typically shows increased uptake on 18F-FDG PET/CT
Retroperitoneal fibrosis	Rind-like layer of soft-tissue enveloping aorta anywhere from the level of the renal arteries to iliac vessels, inferior vena cava and ureters. Usually lies anterior and lateral to the aorta, spares the posterior aspect, and does not typically displace the aorta off the spine. Early stage: avid enhancement Late, inactive stages: little or absent enhancement Hydroureteronephrosis if ureters involved.	Low signal intensity on T1 and variable signal intensity on T2 depending on the degree of active inflammation Early, active: high T2 signal intensity and early contrast enhancement Late, inactive: usually exhibiting low T2 signal and little or no contrast enhancement.	Well demarcated hypoechoic or isoechoic retroperitoneal mass with irregular contours anterior to the lower lumbar spine or sacral promontory.	Increased uptake with Ga-67 and 18F-FDG PET/CT may be seen during the active inflammatory stage.
Mycotic aneurysm	May have similar imaging features to inflammatory aneurysms. Aneurysm can be saccular or complex, with lobular contour Concentric or eccentric periaortic inflammatory soft tissue, fat stranding, fluid or abscess Periaortic gas and abscess may be present Periaortic inflammatory soft tissue may appear as: ▪ Hypoattenuating concentric rim with homogeneous contrast enhancement ▪ Heterogeneous in attenuation with poor contrast enhancement ▪ Rim enhancement upon developing necrosis Other features: adjacent vertebral body destruction, psoas abscess, osteomyelitis or discitis Rapid aneurysmal expansion of the aorta over a short period of time on serial CTs Can rupture with adjacent hematoma.	Similar morphologic features as on CT Wall of aneurysm can show intense enhancement which may be most apparent on black blood sequences Periaortic soft tissue mass demonstrates low signal intensity on T1 and high signal intensity on T2 with variable enhancement	Hypoechoic structure with color or spectral flow on doppler imaging, with associated hypoechoic or heterogenous perilesional inflammatory tissue or hematoma. Aneurysm may demonstrate turbulent flow and “yin-yang” pattern on color doppler imaging. May demonstrate turbulent flow	Radiotracers Ga 67 citrate and 111 In–labeled WBC may show increased tracer uptake within the soft tissues surrounding the aneurysm. Tend to show FDG-avidity on PET imaging of 4.5 SUV-max or more
Infectious aortitis	Similar findings to mycotic aneurysm but with normal caliber	Similar findings to mycotic aneurysm but with normal caliber	Similar findings to mycotic aneurysm but with normal caliber	Similar findings to mycotic aneurysm but with normal caliber