MR enterography remains the modality of choice for imaging children with known or suspected inflammatory bowel disease. In the pediatric population, enterography presents unique challenges related to motion artifact. New sequences have been developed that minimize motion degradation and improve the diagnostic quality of images. In this educational exhibit we will: 1) Review the role of MR enterography in pediatric inflammatory bowel disease (IBD) and discuss key MR findings in the diagnosis of IBD 2) Discuss current methods and protocols in MR enterography and 3) Describe the variable refocusing flip angle single shot fast spin echo T2-weighted sequence and its application in pediatric MR enterography.

1. MR Enterography of Inflammatory Bowel Disease in Pediatric Patients

IBD is a broad category of disorders mainly comprised of Crohn disease and ulcerative colitis, and usually presents in the pediatric population, with a peak incidence between 15 – 30 years of age.¹ Although the diagnosis of IBD is made through biopsy and histologic assessment of bowel inflammation, characterization of disease course and complications is made through follow-up imaging.

MR enterography has become the modality of choice for imaging pediatric bowel disorders. MR enterography allows for excellent characterization of the bowel wall and lumen, as well as the adjacent mesentery and organs, without use of ionizing radiation.² Key MR findings of inflammatory bowel disease include any combination of the following: bowel wall thickening, hyperenhancement, edema, reduced diffusion, and/or luminal narrowing/stricturing. Additional findings include mesenteric edema or fat stranding, fibrofatty proliferation, lymphadenopathy, and sinus tracts or fistulas.²

2. Current Enterography Methods

Current MR enterography protocols can be performed with or without the administration of general anesthesia. Studies last approximately 45 minutes, and patients who are scanned without general anesthesia are instructed to remain still and follow breath hold commands. Glucagon is usually administered to slow bowel peristalsis. The MR enterography protocol at our institution includes axial and coronal SSFSE, axial DWI, coronal T2-weighted cine, and optionally, a coronal T1-weighted pre and post-gadolinium. All sequences serve different purposes, and of these sequences, SSFSE is particularly important for the anatomic visualization of the bowel wall.

SSFSE has a fast acquisition time, which reduces image degradation from patient motion. In the pediatric population, variable patient cooperation with breath-holds and positioning causes motion-related artifact, and fast acquisition sequences are integral to producing diagnostic quality images. Patient respiration, peristalsis, intraluminal flow in bowel, and the existence of gas-tissue interfaces are all technical factors that complicate the acquisition of quality images. SSFSE requires long echo train lengths, and is often limited by blurring artifact from T2 signal decay. SSFSE with variable refocusing flip angle (vrfSSFSE) offers several advantages over the conventional sequence in terms of imaging speed and improved image sharpness.³

3. vrfSSFSE and its application in pediatric MR enterography

Instead of the static flip angle used with SSFSE, vrfSSFSE modulates the refocusing flip angle in order to reduce SAR and blurring due to T2 decay (Table 1).³ A reduction in SAR allows for shortening of the minimum TR, which allows shorter breath hold sequences and greater slice coverage. Recent studies have demonstrated that vrfSSFSE allows for an approximately two-fold increase in acquisition speed and allows for full-Fourier k-space acquisition, which creates a significant improvement in subjective image quality metrics.^4,5

At our institution, we scanned five patients (mean age 15 ± 1.8 years) using the vrfSSFSE sequence, and obtained subjectively sharper images, with better contrast resolution and sharper visualization of the bowel wall (Figure 1). Other studies have also demonstrated a quantitatively higher rating of subjective image quality with the vrfSSFSE sequence as compared to the conventional SSFSE.⁴

MR enterography remains the primary imaging modality for children with known or suspected inflammatory bowel disease. MR enterography is particularly difficult to perform in children, as even the SSFSE sequence is susceptible to motion artifact. vrfSSFSE is an improvement on the conventional SSFSE sequence, with reduced scan times that also allow for improvements in image quality.