What imaging study is most useful in diagnosing a Spigelian hernia?

A CT scan is particularly useful in diagnosing a Spigelian hernia due to its ability to provide detailed cross-sectional images of the abdominal wall and the surrounding structures. This imaging modality can effectively reveal the presence of the hernia, demonstrating the defect in the Spigelian fascia and any associated herniated contents, such as intestines or omentum.

While ultrasound can also be useful in certain hernia evaluations, it may not always provide the comprehensive anatomical detail necessary for confident diagnosis, especially in cases where there is uncertainty about the hernia's extent or relationship to other structures. X-rays are limited in their use for diagnosing hernias since they do not show soft tissue structures well, and MRI is generally not the first-line imaging for this condition due to cost, availability, and the fact that CT offers sufficient detail for diagnosis.

Thus, the CT scan stands out as the preferred method for its accuracy and ability to visualize the necessary anatomy involved in Spigelian hernias.