Doppler ultrasound has proven useful in distinguishing between active and inactive disease, though no conclusions can be drawn from it about the severity of active disease.¹¹ The role of CT in Crohn's disease has been firmly established, though exposure to ionizing radiation limits its widespread use. A correlation between disease activity and the degree of mural enhancement seen on CT has been demonstrated.¹²

Crohn's disease will last throughout a patient's lifetime. The disease manifests at a relatively young age in approximately 50% of cases. Optimal diagnostic workup requires that patients be monitored frequently. Monitoring techniques should consequently be noninvasive or minimally invasive and, above all, patient-friendly.

Although a perfect tool that meets all these requirements is not presently available, MRI has been advocated as one of the most promising techniques for small bowel imaging. The modality can provide an accurate assessment of disease activity, producing multiplanar images with high soft-tissue contrast. No ionizing radiation is involved, making long-term follow-up feasible, and the discomfort and risks are lower than with other methods.^5,13 Disadvantages include the higher costs associated with MRI and the longer examination time, especially when compared with CT.⁴

Ultrafast MR sequences with breath-hold or respiratory triggering can provide high-quality panoramic images of the gastrointestinal tract and extraintestinal, intra-abdominal structures. The introduction of cine-MR mode has added valuable functional information on the motility of affected loops. This is despite the lower temporal resolution compared with conventional enteroclysis. ^5,14 The impact of MRI on evaluations of Crohn's disease is consequently increasing, and imaging protocols, including small bowel distension techniques (enteroclysis versus followthrough) and MR sequences, have been standardized.³

MRI assessment of active disease has been shown to influence patient management significantly, regardless of the number of segments involved,¹⁵ as therapy is tailored toward individual patients rather than individual segments. MRI features of activity are consequently analyzed in most studies on a per-patient basis.

KEY MURAL MRI FINDINGS

Visualizing changes in the appearance of the bowel wall helps confirm the presence and phase of disease.

• Bowel wall thickness. Bowel wall thickness has been widely tested as a way of discriminating between active and inactive Crohn's disease (Figure 1).^15-19 One evaluation of therapy response in patients with active Crohn's disease found a substantial decrease of bowel wall thickness. Median values of wall thickness in the small bowel before and after treatment were 4.9 mm and 4.4 mm, respectively (p = 0.03).¹⁸ Researchers in a separate study concluded that on a per-patient basis, a bowel wall thickness of 4 mm or more has a sensitivity of 88% and a specificity of 75% for active Crohn's disease.¹⁵