Magnetic Resonance Imaging and Endoanal Ultrasonography

These are two excellent imaging modalities that provide anatomic detail of the anorectal region. Each of these modalities has distinct advantages and disadvantages. Endoanal ultrasonography is rapid, less costly, portable, and accurate. However, there is a learning curve for image interpretation, interobserver interpretation is fair, there is a high false-positive rate for sphincter defects, and images are difficult to interpret when there is a great deal of fibrosis. This last point is particularly important for image interpretation in recurrent fistulas and incontinence after multiple prior sphincter surgeries. Magnetic resonance imaging offers clear images of the entire pelvis, and provides clear images of the anal sphincter musculature. Moreover, MRI has the capabilities for multiplanar images and computer manipulation of the images (T1- and T2-weighted images), which facilitate the interpretation of fibrosis from other tissue densities. However, MRI is expensive, requires specialized endoanal coils for optimal imaging of the anorectal region, and with current technology is time-consuming.

Nonetheless, these two different modalities should be regarded as complementary and not competitive to one another. The endoanal ultrasound is an office-based examination performed by the surgeon, and may be regarded as an extension of the physical examination. Some have labeled the ultrasound as the "surgeon's stethoscope."Many patients will be accurately diagnosed by ultrasonography and no further imaging modalities will be needed. However,for the patient in whom the ultrasound may not be accurate, as in the case of complex fistulas, additional information may be gained with an endoanal MRI. Further studies are currently underway to better delineate the role of each of these modalities in our diagnostic armamentarium.

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