• The kidneys are high flow demand end-organs, which receive blood from one or more renal arteries.

• Doppler waveform demonstrates a rapid upstroke, sharp peak, and a low resistive monophasic waveform consistent with continuous diastolic forward.

• Normal aortic and renal artery velocity is 60-100 cm/sec

• RI = PSV-EDV/PSV

• Normal 0.5-0.7 (dependent on age & area sampled)

  • RI is normally higher in elderly w/o renal insufficiency and neonates.

  • Measurement of peripheral resistance; independent of angle correction.

  • Measured in segmental arteries of the upper, mid and lower poles.

  • Useful and sensitive noninvasive marker of renal function.

• 0.7 - 0.8 indeterminate

• >0.8 indicates pathologic impedance to flow

  • Nonspecific but may indicate renal artery stenosis, acute urinary obstruction, or medical renal disease.

  • Increased intrarenal RI is considered a marker of intrarenal arterial stiffness and worsening tubulointerstitial damage.

• Measure interlobar renal arteries (adjacent to medullary pyramids) in 3 locations of each kidney (upper pole, interpolar region, lower pole) and reported as average.

ELEVATED RI

Typically implanted in the R > L iliac fossa; usually single but en-bloc transplant of both kidneys into recipient can be occasionally performed (pediatric donor to adult recipient)

• Goal is to determine whether there is treatable surgical or vascular complication. US cannot reliably differentiate b/n various causes of parenchymal rejection and requires bx for definite diagnosis.

• Elevated RI (>0.7) suggests renal dysfunction

5 categories to look for when evaluating: artery, vein, parenchyma, collecting system, area around tx (collections)

• US Cannot distinguish b/n parenchymal causes: rejection, ATN, C-I toxicity -> get bx

• US can distinguish morphological from parenchymal dz

• Renal artery

  • RA stenosis:

    • Usually at anastomosis

    • Elevated velocities at site of narrowing

    • Tardus-parvus waveforms distal to stenosis

    • Usually takes several weeks to months to develop.

  • RA kink

    • Folding or twisting of vascular pedicle 2/2 malposition or lack of space (compartment syndrome)

    • Immediately post surgery; same sono signs as RAS. easily corrected at surgery.

  • RA thrombosis

• Renal vein thrombosis: 

  • RA may show reversal of diastolic flow

• AV fistulae / Pseudoaneurysm 2/2 renal bx

RAS findings via MRA 

• PSV >200 cm/sec 

• Renal artery: aortic velocity ratio > 3.5 

• Reduced/absent EDV suggests stenosis distal to area of interest.

• Focal areas of aliasing

• Like carotid artery, tardus et parvus waveform suggests stenosis proximal to the transducer, known as inflow lesion.

RAS findings via interlobular artery

• Early systolic acceleration <300 cm/sec 

• Limitations:

  • Decreased sensitivity w/ borderline stenosis

  • Cant distinguish high grade from complete occlusion w collateral flow

  • Unable to localize stenosis

  • Variation in cursor location for measurement of systolic acceleration

  • Dependent on compliant vessels 

Intrarenal doppler cannot solely be used to dx RAS

MEDICAL COMPLICATIONS

Generally cannot be differentiated on US. Bx is needed.

Hyperacute rejection: w/n first few hrs of tx (very rare, ABO blood type incompatibility.

Acute tubular necrosis: immediate few postoperative days; usually sequela of preimplantation ischemia.

Acute rejection: w/n 3 months of tx

Chronic rejection: after 3 months of tx

Drug toxicity: cyclosporine is nephrotoxic

REJECTION

ATN

TX RELATED MALIGNANCY