What is the best imaging modality to rule out an abscess near the ischium?

Best Imaging Modality to Rule Out Abscess Near the Ischium

MRI is the recommended imaging modality for ruling out an abscess near the ischium due to its superior ability to detect and characterize fluid collections, abscesses, and fistulous tracts. 1

Imaging Options and Their Effectiveness

MRI

• Provides excellent spatial and contrast resolution for detecting both superficial and deep soft tissue infections 1
• Superior for detecting fluid and inflammation in soft tissues, making it ideal for abscess identification 1
• Can accurately define anatomical location, size, and relationship to surrounding structures 1
• Contrast-enhanced MRI further increases diagnostic sensitivity for abscesses and fistulas 1
• Particularly useful for occult supralevator abscesses, complex anal fistula, or perianal Crohn's disease 1

CT Scan

• Useful in acute settings due to widespread availability and short acquisition time 1
• Intravenous contrast is preferred to help visualize and characterize fluid collections and abscesses 1
• Reported sensitivity of 77% for detecting perirectal abscesses, which is lower than MRI 1
• Limited by poor spatial resolution in the pelvis and difficulty differentiating between fistula tracts and inflammation 1
• Particularly helpful for detecting soft tissue gas, which may indicate infection 1

Ultrasound

• Useful for detecting fluid collections, including abscesses 1
• High sensitivity (96.7%) and specificity (85.7%) for diagnosing superficial abscesses 1
• Can evaluate internal characteristics of abscess cavity such as necrosis or debris 1
• Limited for deeper soft tissue structures compared to MRI and CT 1
• Point-of-care transperineal ultrasound is emerging as a valid tool for diagnosing perineal and perirectal abscesses 1

Clinical Considerations

When to Use Imaging

• Imaging is recommended in cases of atypical presentation or when physical examination is inconclusive 1
• Particularly important when suspecting occult supralevator abscesses, complex anal fistula, or perianal Crohn's disease 1
• For abscesses near the ischium (ischioanal/ischiorectal), imaging helps define the extent and relationship to surrounding structures 1

Abscess Classification Relevant to Imaging

• Ischioanal abscesses penetrate through the external anal sphincter into the ischioanal space 1
• Proper identification of anatomical location is crucial for surgical planning 1
• Imaging should report abscess size at its largest diameter in two perpendicular planes 1
• Any relationship with fistulae should be identified and reported 1

Special Considerations

• For patients with suspected Crohn's disease, combining bowel and pelvic assessment may be beneficial 1
• An interval of at least 4 weeks is recommended for examinations performed after acute perianal surgery 1
• Consider potential complications like horseshoe abscess formation, which requires specific identification 1

Pitfalls and Caveats

• CT has limited sensitivity (77%) for perirectal abscesses and may miss small collections 1
• Ultrasound is operator-dependent and may not adequately visualize deep structures near the ischium 1
• Physical examination alone may miss deep abscesses, particularly in immunocompromised patients 1
• Fistulography, once commonly used, has low accuracy (as low as 16%) and has been replaced by more accurate imaging techniques 1

In conclusion, while CT and ultrasound have roles in specific clinical scenarios, MRI remains the gold standard for ruling out an abscess near the ischium due to its superior soft tissue contrast and ability to accurately characterize the extent of disease.