Sonography (a combination of gray-scale B-mode, color Doppler/CDUS and power spectral Doppler/PW) represents a totally non-invasive, easily available, low-cost and well tolerated technique that is able to detect RAS and renovascular diseases. The resistive index (RI), pulsatility index, systolic/diastolic ratio, peak systolic velocity (PSV), acceleration time, acceleration index, and other parameters are used to quantitatively analyze the characteristics of Doppler waveforms of renal blood flow. Of these parameters, the RI and PSV are most commonly used clinically. Since consistent RI values are obtained regardless of the Doppler incident angle, the RI is suitable for examining the renal artery with an irregular direction of blood flow.
Indications for RDU include (not limited to):
1. Evaluation of patients with hypertension, particularly when there is a moderate to high suspicion of RVHT (uncontrolled hypertension despite optimal therapy, hypertension with a progressive decline in renal function and abrupt onset of hypertension);
2. Follow-up of patients with known renovascular disease who have undergone renal artery stent placement or other renal artery interventions or have known unilateral stenosis with concern for stenosis in the contralateral kidney;
3. Evaluation of an abdominal or flank bruit;
4. Evaluation of a suspected vascular abnormality such as an aneurysm, pseudoaneurysm, arteriovenous malformation or fistula;
5. Evaluation of renal insufficiency in a patient at risk for renovascular disease;
6. Evaluation of renal artery blood flow in patients with known aortic dissection, trauma, or other abnormalities that may compromise blood flow to the kidneys; and
7. Evaluation of discrepant renal size.
There are no absolute contraindications to performing this examination.
Specifications of the Examination
The study is generally performed for both kidneys (previous 8-12 hours fasting; patient in the supine position; anterior abdominal wall or the flank approach; low-frequency scanhead 2.5-5.0 MHz); if not, the report should state the reason for a unilateral study. The study consists of grayscale imaging of the kidneys with spectral and color Doppler imaging of the extrarenal and intrarenal vessels.
A. Grayscale Imaging
The longest renal length should be measured and reported. In patients who have not had recent cross-sectional imaging of the kidneys, a complete renal ultrasound examination may be considered.
B. Spectral and Color Doppler Evaluation
Analysis of main renal artery and intrarenal arterial waveforms should be used to evaluate for RAS. Careful attention to technique is important to ensure accurate examination results, including selecting a transducer that is appropriate for the patient’s body habitus, optimizing color Doppler parameters, using an appropriate sample volume, optimizing the velocity scale for the size of the waveform to avoid aliasing (this may require adjusting the scale, baseline, or frequency or selecting a lower-frequency transducer), and using the lowest feasible angle of insonation. Angle correction is essential for determining blood flow velocity. The angle between the direction of flowing blood and the applied RDU signal should not exceed 60° (optimal 30-60°).
1. Main Renal Artery Evaluation
The entire main renal artery should be scanned along its long axis; inability to visualize the entire or part (especially the origin) of the main renal artery should be reported. Spectral Doppler imaging should be performed along the vessel’s length from the origin to the hilum at the lowest feasible angle of insonation. The greatest PSV should be recorded at the origin/proximal portion, at the mid aspect, and near the hilum. A PSV should also be recorded at any site of color aliasing or suspected stenosis. If there is significant stenosis, a Doppler waveform should be recorded within the stenosis and distal to the stenosis. An effort should also be made to search for accessory renal arteries. When visualized, PSV should be recorded as described above. An appropriate angle-corrected spectral waveform from the abdominal aorta at the level of the renal arteries should be recorded. The aortic PSV is used to calculate the ratio of the PSV in the renal artery to the aorta.
2. Intrarenal Evaluation
Spectral waveforms should be recorded from segmental arteries (first pathologically altered) in the upper and lower poles and the interpolar region (midportion) of each kidney. Intrarenal analysis consists of quantitative and/or qualitative evaluation of the Doppler waveforms. Quantitative evaluation may include acceleration times, acceleration indices or RI. For qualitative analysis, the morphology of the waveform should be assessed for a normal systolic upstroke or tardus parvus changes.
Renal vascular diseases and renovascular hypertension (RVHT)
Renal vascular diseases include: RVHT, nephroangiosclerosis, vasculitis (poliarteritis nodosa, Wegener disease, etc), thrombotic microangiopathies (haemolytic-uraemic syndrome, thrombotic thrombocytopenic purpura), renal infarcts, cortical necrosis, scleroderma, toxaemia of pregnancy. RVHT is a consequence of RAS; it is potentially curable if arterial patency is reconstituted. In 90–95% of cases RVHT is caused by atherosclerotic RAS (proximal 2cm) and is primarily seen in older male patients, over 50 years of age. Fibromuscular dysplasia is more common in women and younger patients (average 35 years). A very important finding is asymmetry of kidney dimensions on RDU. Presence of RAS does not necessarily imply that the patient has RVTH; more than 50% of normotensive people have a certain degree of RAS. It is considered that RAS that reduces the diameter of artery more than 60–70% causes RVHT. The main goal of imaging in diagnosis of RAS is to detect a potentially curable lesion in a normal-sized kidney.
RDU criteria for the diagnosis of significant RAS (>50%) are as follows: 1) increase in renal peak systolic velocity/PSV (PSV>180 cm/s) – the most important findng 2) reno-aortic ratio/RAS calculated as ratio between renal and aortic PSV >3,5 3) turbulent flow at the post-stenotic region, 4) visualization of the renal artery without detectable Doppler signal (occlusion) 5) reduction in flow velocity (parvus-tardus waveform, slow systolic acceleration < 3m/sec) and 6) resistance index/RI in distal arterial segments and intrarenally (RI>0,80) with Δ RI (right-left) > 0.05.
Renal vein thrombosis (RVT)
RDU is very accurate in the diagnosis of RVT - directly visualise a distended vein, filled with the thrombus, without flow. In adults RVT is most commonly seen in patients with kidney cancer that commonly infiltrates the renal vein and inferior vena cava. In these patients, malignant venous thrombus may demonstrate vascularisation on RDU.
Indirect signs of RVT are an alteration of intrarenal arterial spectrum, with extreme elevation of RI (intrarenal oedema) caused by impaired venous drainage. In adults RVT may be also caused by nephritic syndrome, hypercoagulable conditions, oral contraceptive intake, retroperitoneal fibrosis and thrombophlebitis migrans. In children more common causes are dehydration and trauma. Early diagnosis is important to ensure adequate therapy.