In a 43‑year‑old pregnant woman with a right‑sided renal bruit and a normal soft abdomen, what is the first‑line imaging study to evaluate a suspected renal vascular abnormality?

Imaging for Suspected Renal Vascular Abnormality in Pregnancy

In a 43-year-old pregnant woman with a right renal bruit, ultrasound with color Doppler of the kidneys and retroperitoneum is the definitive first-line imaging study. 1

Primary Recommendation

Perform color Doppler ultrasound of the kidneys and bladder retroperitoneal as your initial and often sole imaging modality. This study allows you to:

• Assess renal anatomy and identify structural abnormalities 1
• Measure renal resistive indices (RI) to detect vascular dysfunction 1
• Evaluate renal perfusion patterns without radiation exposure 2
• Distinguish physiologic from pathologic processes 3

The American College of Radiology explicitly designates ultrasound as the first-line study for diagnosis of maternal renal anatomic abnormalities in pregnancy. 1

Key Diagnostic Parameters to Obtain

When performing the color Doppler study, ensure measurement of:

• Renal resistive indices (RI): Values >0.70 suggest underlying kidney dysfunction 1, 3
• RI difference between kidneys: A difference of 0.04 or greater indicates pathologic obstruction or vascular compromise 1, 3
• Ureteral jet evaluation: Helps distinguish obstructive from non-obstructive pathology 1, 3
• Renal pelvis anteroposterior diameter: To detect hydronephrosis 2

When to Advance to Second-Line Imaging

If ultrasound findings are equivocal or non-diagnostic, proceed to MRI/MRU without contrast rather than CT. 1, 3

Specific indications for MRI include:

• Ultrasound shows hydronephrosis >2 cm, calculi, or masses 2
• Persistent symptoms despite conservative management 2
• Need for detailed vascular anatomy that ultrasound cannot provide 4
• Suspicion of renal artery stenosis requiring anatomic confirmation 5

MRU without IV contrast is preferred over CT in pregnant patients and can be combined with anatomical imaging for comprehensive evaluation. 1

Critical Pitfalls to Avoid

Do not order CT as your initial imaging study. CT should be reserved exclusively for problematic situations where diagnosis cannot be made on ultrasound or MRI. 1, 6 When CT is absolutely required, use low-dose protocols with fetal exposure <20 mGy. 2

Avoid plain radiography (KUB) and intravenous urography (IVP). These modalities are considered obsolete and provide no diagnostic benefit in this clinical scenario while exposing the fetus to unnecessary radiation. 1

Do not assume a renal bruit always indicates hemodynamically significant stenosis. A case report demonstrates that an audible bruit may arise from an artery not contributing to hypertension, while a significant stenosis causing hypertension can be silent. 7 The ultrasound findings will guide whether further vascular imaging is warranted.

Clinical Context

While a renal bruit raises suspicion for renovascular disease (particularly fibromuscular dysplasia in a woman of childbearing age), physiologic hydronephrosis occurs in 70-90% of pregnant women and is typically more pronounced on the right side. 3 The color Doppler ultrasound will help distinguish between:

• Physiologic pregnancy-related changes
• Renal artery stenosis (atherosclerotic or fibromuscular dysplasia)
• Pathologic obstruction requiring intervention
• Incidental renal anatomic variants 2

The ultrasound is sufficient to confirm the diagnosis in 96% of cases of renal pathology in pregnancy. 8 Only 4% of cases require MRI or low-dose CT for definitive diagnosis. 8