What is the preferred initial imaging modality for evaluating an abdominal/pelvic mass?

Preferred Initial Imaging Modality for Evaluating an Abdominal/Pelvic Mass

Ultrasound is the preferred initial imaging modality for evaluating an abdominal/pelvic mass, with transvaginal and transabdominal approaches combined with color Doppler being the most appropriate first-line investigation. 1

Initial Imaging Selection Algorithm

Step 1: Ultrasound Evaluation

• Transvaginal ultrasound combined with transabdominal ultrasound provides high sensitivity (>90%) for initial assessment of pelvic pathology and can often definitively characterize the origin of pelvic masses 1
• Color Doppler evaluation should be included as a standard component to evaluate vascularity and help determine the origin of masses, with a "bridging vessel sign" helping to identify masses of uterine origin such as pedunculated fibroids 1
• Ultrasound is superior to CT for distinguishing cystic from solid masses, though it may be limited in defining tissue planes 2
• For suspected adnexal masses specifically, the American College of Radiology recommends US duplex Doppler pelvis, US pelvis transvaginal, and US pelvis transabdominal as complementary procedures 3

Step 2: If Ultrasound is Inconclusive or Limited

For Pelvic Masses:

• MRI pelvis with IV contrast is the preferred second-line imaging modality when ultrasound is inconclusive, providing superior tissue characterization 1
• MRI has high soft-tissue contrast resolution, allowing for direct visualization of pelvic organs, muscles, and fascia 1, 4
• MRI is excellent for characterizing uterine masses, including size, number, location, and degree of degeneration 1

For Abdominal Masses:

• CT abdomen and pelvis with IV contrast is recommended when ultrasound is inconclusive, particularly for suspected aortic aneurysms 3
• CT provides better definition of tissue planes and demonstrates contiguous anatomical structures such as the rectum, bony pelvis, vertebral bodies, abdominal aorta, and inferior vena cava 2
• CT is superior for demonstrating calcifications and is rarely affected by overlying bowel gas 2

Special Considerations

For Suspected Aortic Pathology:

• CT angiography (CTA) with IV contrast is considered the reference standard for diagnosis and management decision-making of abdominal aortic aneurysms 3
• CTA provides rapid image acquisition with high spatial resolution of the aorta and branch vessels 3
• For patients who cannot undergo CT with contrast, MRA can be considered as an alternative with sensitivity approaching 90% 5

For Reproductive Age Women:

• Ultrasound should always be the initial imaging modality 3, 1
• If pregnancy is suspected or confirmed, MRI without contrast is preferred over CT when ultrasound is inconclusive 3
• MRI abdomen and pelvis may be appropriate if index of suspicion is high for appendicitis or bowel abnormalities, especially late in pregnancy 3

For Masses Highly Suspicious for Malignancy:

• CT abdomen and pelvis with IV contrast and MRI pelvis without and with IV contrast are recommended as complementary procedures 3
• MRI provides better tissue characterization while CT is better for evaluating for distant metastatic disease 3, 1

Common Pitfalls to Avoid

• Failure to use both transabdominal and transvaginal approaches when performing ultrasound, leading to incomplete evaluation 1
• Relying solely on transabdominal ultrasound for large pelvic masses, which may have limited visualization due to size constraints 1
• Using CT as first-line imaging when MRI would provide superior tissue characterization for indeterminate masses 1
• Not using color Doppler to identify "bridging vessel sign" that helps differentiate pedunculated fibroids from adnexal masses 1

Contrast Considerations

• For CT, IV contrast generally improves diagnostic accuracy, though unenhanced CT has been found to be more sensitive than ultrasound in identifying abdominal aortic aneurysms 3
• For urgent/emergent abdominal/pelvic CT in hospitalized patients, studies suggest no significant difference in diagnostic accuracy between enhanced and unenhanced imaging 6
• For MRI, contrast-enhanced images are useful in defining intratumoral architecture and tumor borders of adnexal masses, resulting in better determination of malignancy (accuracy 95%) compared to T2-weighted images (85%) 7