Radiology Call Resources for Trainees

RI is a nonspecific marker of intrarenal vascular resistance and is influenced by age, heart rate, vascular compliance, and intrinsic renal disease.

• RI >0.8: markedly elevated resistance; may be associated with renal parenchymal disease and poorer renal prognosis.
• RI around 0.7 is commonly used as a practical upper reference point in adults, but should not be interpreted in isolation.

• RAR ≥3.5 supports hemodynamically significant renal artery stenosis when aortic velocity is suitable for ratio assessment.
• RAR becomes unreliable when the reference aortic velocity is markedly abnormal.

• Main renal artery PSV >200 cm/s is a commonly used direct criterion for renal artery stenosis.
• PSV should be interpreted together with RAR and post-stenotic/tardus-parvus findings rather than as a stand-alone test.

• Prolonged intrarenal acceleration time (approximately ≥0.08 s in commonly used criteria) can support significant proximal stenosis.
• Tardus-parvus morphology, delayed systolic upstroke, and reduced acceleration are indirect findings and have lower diagnostic accuracy than direct velocity measurements.

RI is nonspecific. Elevation may occur with rejection, acute tubular injury, drug toxicity, infection, obstruction, venous outflow abnormality, or compressive perinephric collections. Trends and the full Doppler/gray-scale examination are more useful than an isolated RI value.

Suspicious findings include focal marked PSV elevation, a high renal-to-iliac velocity ratio, post-stenotic spectral broadening/turbulence, and downstream tardus-parvus waveforms. Exact thresholds vary across institutions and studies; an isolated elevated PSV in a tortuous transplant artery is not diagnostic.

• Absent arterial perfusion is the key Doppler finding.
• Evaluate venous flow as well, because severe global perfusion abnormalities may have more than one vascular cause.

• Reduced or absent main renal vein flow.
• Reversed diastolic arterial flow is an important associated finding, but is not specific by itself.
• Gray-scale findings may include graft enlargement and edema.

• Hematoma
• Seroma
• Lymphocele
• Abscess
• Urinoma

Assess collecting-system dilation in conjunction with bladder distention, ureteral course, peritransplant collections, renal function, and the postoperative interval. Mild collecting-system prominence can be physiologic in some transplant kidneys.