What is the best initial imaging study, CT scan or X-ray, for pelvic pain radiating to the leg?

Imaging for Pelvic Pain Radiating to the Leg

Ultrasound should be used as the initial imaging modality for evaluating pelvic pain radiating to the leg, with CT reserved for cases where ultrasound is inconclusive or when non-gynecological causes are strongly suspected. 1

Initial Imaging Approach

Ultrasound as First-Line

• Transvaginal ultrasound combined with transabdominal ultrasound is the most appropriate initial imaging study for pelvic pain radiating to the leg, especially when gynecological causes are suspected 1
• Ultrasound provides excellent visualization of pelvic structures with no radiation exposure and is widely regarded as the initial imaging study of choice 1
• Color and spectral Doppler should be included as standard components of the examination to evaluate internal vascularity of pelvic structures 1

When to Use CT Instead of Ultrasound

• CT with IV contrast should be considered as the initial imaging modality when:
  • Non-gynecological causes are strongly suspected (e.g., diverticulitis, appendicitis) 1
  • The pain is poorly localized or involves both abdomen and pelvis 1
  • There is suspicion of urolithiasis (in which case non-contrast CT "stone protocol" would be appropriate) 1
  • The patient has a broad differential diagnosis including both gynecological and non-gynecological etiologies 1

Evidence-Based Considerations

Limitations of X-ray

• There is currently no evidence to support the use of radiography to evaluate pelvic pain radiating to the leg 1
• Radiography cannot visualize soft tissue abnormalities that are common causes of pelvic pain 1
• Plain radiographs have limited utility except in cases of suspected pelvic fractures or foreign bodies 2

Advantages of CT

• CT with IV contrast has higher sensitivity than ultrasound (89% versus 70%) for urgent diagnoses in adults with abdominopelvic pain 1
• CT provides approximately 88% overall accuracy compared with surgical diagnosis 1
• CT is superior for detecting non-gynecological causes such as diverticulitis, with sensitivity of 81% versus 61% for ultrasound 1

Role of MRI

• MRI should be considered as a problem-solving examination after initial imaging with ultrasound or CT 1
• MRI provides excellent soft tissue contrast and is particularly valuable when endometriosis or fistulizing disease is suspected 1
• MRI is preferred over CT in pregnant patients for assessing non-gynecological pathologies 1

Clinical Decision Algorithm

1. Start with ultrasound (transvaginal + transabdominal with Doppler) if:

   • Gynecological etiology is suspected
   • Patient is of reproductive age
   • Pregnancy is possible or confirmed
   • Pain is well-localized to the pelvis 1, 3

2. Start with CT abdomen and pelvis with IV contrast if:

   • Non-gynecological cause is strongly suspected
   • Pain involves both abdomen and pelvis
   • Patient has risk factors for diverticulitis or other gastrointestinal pathology
   • Ultrasound is likely to be limited (e.g., obesity, extensive bowel gas) 1, 4

3. Consider non-contrast CT if:

   • Urolithiasis is suspected 1

4. Consider MRI if:

   • Initial imaging is inconclusive
   • Endometriosis is suspected
   • Patient is pregnant and ultrasound is inconclusive 1

Common Pitfalls to Avoid

• Relying solely on plain radiographs, which have very limited utility in evaluating pelvic pain 1
• Automatically ordering CT without considering ultrasound first, especially in younger patients or when gynecological causes are suspected 3
• Performing unnecessary follow-up ultrasound after CT when the CT findings are definitive for benign conditions 5, 6
• Failing to consider vascular causes of pelvic pain radiating to the leg, which may require specific imaging protocols 1
• Not using IV contrast for CT when evaluating most non-urolithiasis causes of pelvic pain, which significantly limits diagnostic capability 1, 4

Remember that the imaging approach should be guided by the suspected etiology, patient demographics, and clinical presentation. The American College of Radiology recommends ultrasound as the first-line imaging modality for most cases of pelvic pain, with CT reserved for specific indications or when ultrasound is inconclusive 7.