A comparison of Power Doppler with conventional sonographic imaging for the evaluation of renal artery stenosis

Duplex/CD sonography is used as a routine method for the assessment of RAs [1, 2]. This technique is a reliable diagnostic tool in order to display vascular structures as well as velocity and flow direction of red blood cells by mapping the Doppler frequency shift information. However, the detection of RAs through the anterior abdominal wall with this conventional technique is hampered in a substantial group of patients either by anatomical origin of RAs from a plane that may be almost perpendicular to the ultrasonic beam [3–5], or by several technical issues like bowel gas, breathing movements and obesity. In such cases RAs exploration through modified windows, like the margino-renal, lateral window, is crucial [2]. Furthermore, it is intriguing to complete the sonographic evaluation with PD. PD generates an intravascular color map reflecting the integrated power in the Doppler signal, which essentially depends on the density of red blood cells within the sample volume. Since increased sensitivity to visualize the continuity of blood flow in arterial stenosis is claimed for PD, we evaluated this new technique in a prospective comparison with conventional imaging for the diagnosis of RA stenosis.

RA stenosis is a common cause of secondary hypertension and angiography is considered to be the gold standard for the morphologic visualization of RA stenosis [7, 8]. Yet, angiography is costly, invasive, and associated with inherent morbidity. Therefore, it cannot be used as a screening method and it must be reserved for the confirmation of a diagnosis suggested by other techniques and for interventional procedures. Either computed tomographic or magnetic resonance angiography and captopril renography can evaluate the renal perfusion and the haemodynamic significance of a stenosis [1, 9, 10]. All these techniques are safer alternatives to angiography; nevertheless, scintigraphy does not provide any anatomical information and, moreover, high cost and not widespread availability of all of them may somehow preclude a routine use in clinical practice [11].

Sonography, as a combination of grey-scale B-mode, CD and PW spectral Doppler, is a totally non-invasive technique that is widely used as a first-line examination to advance the diagnosis of RA stenosis [1, 2, 6]. Yet, conventional sonography often requires a high degree of technical expertise in order to provide accurate results. In other words, it may be technically very demanding and time consuming to clearly identify and interrogate RAs because of their angle of origin from the aorta and their sinuous course; further issues arise from anatomical variants, that can result in false negatives, and from technically inadequate test conditions such as body habitus, inability to hold the breath, bowel gas.

As an alternative and indirect method of identifying RA stenosis, many groups use to analyze intrarenal waveforms, as pointed out by the original observations by Handa [12] and Stavros [13], looking for the tardus-parvus effect. Good results can be obtained only with some quantitative parameters [12–14], whose value in clinical practice has been reviewed elsewhere [2]. Although such "indirect" parameters can overcome technical demands inherent to main RAs interrogation, many groups -as we do- consider them less reliable than "direct" ones [15–18]. Indeed, intrarenal PW examination may be relatively easy in patients without abnormalities, as our control group. To obtain diagnostic-quality spectral signals when the clinical suspicion is high, however, is almost as challenging and time consuming as to obtain satisfactory images of the main RAs with CD and PW interrogation. Moreover, the tardus-parvus technique is limited by a low sensitivity due to the intrinsic variability of Doppler waveforms in patients without abnormalities [16–18] and by the fact that the degree of stenosis necessary to consistently cause downstream flow changes is still unknown: the simple look at the waveform shape and the observation of the presence or absence of an end-systolic peak may only allow to differentiate normal from generically abnormal flow [1, 2].

The development of PD has been permitting further diagnostic possibilities for renal sonography [19, 20]. CD and PD have distinct applications: the knowledge of advantages and limitations of each is essential for their proper application. Both CD and PD elaborate the Doppler frequency shift. The main difference consists in the parameters of the signal that are processed: CD mode processes the mean frequency shift to provide color intensity representing velocity, and the phase shift to indicate flow direction. PD mode generates an intravascular color map reflecting the integrated power in the Doppler signal [21]. Such a parameter depends on the amount of red blood cells producing the Doppler frequency shift, regardless of their flow velocity and direction.

Providing a representation of a different property of blood flow, PD has several advantages over CD; it has greater sensitivity to detect the blood flow itself and, with the same angle of insonation, PD images permit better definition of the intravascular surfaces and visualization of continuity of flow than CD ones. In other words, PD images are independent from the angle of insonation [21]. This is particularly useful in arterial stenosis, since PD imaging results in an angiography-like visualization of the whole vascular lumen [22, 23]. Indeed, PD permits to clearly represent in color the low-velocity post-stenotic flow. This could be even possible with broad band CD flow, keeping PRF as low as possible; with such a modification of PRF, yet, the aliasing phenomenon would lose its significance and the frame rate would be extremely low. Moreover, PD improves the visibility of vessels that are sinuous or kinked or lying on a plane perpendicular to the ultrasonic beam, due to its independence from the angle of insonation. Yet, PD disadvantages mainly consist in long scanning time that makes PD images susceptible to soft-tissue flash artifacts and movement artifacts; moreover, PD does not yield directional or velocity information and, consequently, does not permit to detect aliasing, although it gives the operator the possibility to place the PW sample volume at the most stenotic tract of the vessel. Finally, the frame rate is much lower with PD than with CD, so that arteries are not distinguished from veins on the basis of their color pulsatility. Accordingly, PD is a useful tool, but it is complementary to Duplex/CD in vascular sonography: they should be used together for optimal diagnostic results, CD and aliasing being the guides to PD activation.

Although there are no universally accepted criteria to make a diagnosis of RA stenosis with PD, displayed perfusion characteristics are similar to those obtained from angiography (Figure 2 and 3). According to our observations, therefore, if an intriguing, angiography-like, interpretation of PD images is given, even of a single frame with a color "minus", this technique is likely to permit an estimation, though not as exact as with angiography [20, 24], of the degree of stenosis, thus of the haemodynamic significance of a plaque. Furthermore, with the appropriate learning curve, PD could shorten the overall imaging time when the angiography-like diagnosis of stenosis is obtained, mainly because it would be not necessary to perform long scans in deep inspiration in order to achieve a stable PW flow trace.

In our experience, the evaluation through the lateral window seems particularly useful in the case of left RA stenosis: this is due, in part, to the absence of the good sonographic window created by the liver for right RA. Radiological studies have showed that the left RA has in most cases a lateral origin from the aorta [3–5]: thus the lateral approach, with the patient recumbent on the right side, allows the operator to avoid usual abdominal obstacles and makes the ultrasonic beam aligned as much as possible to the axis of the vessel: the result is the highest intensity of the left RA Doppler signal.

The main limitation of this study is the small number of patients the we could enroll, due to the low prevalence of RA stenosis in the whole population of patients with hypertension. Moreover, the overall high specificity (93%) of sonography is not surprising and is consistent with the literature [1]. In our hypertensive patients there was a relevant clinical suspicion of RA stenosis, quite apart from the subsequent angiographic demonstration: this circumstance represents a relevant bias in the selection of patients to be investigated, as to let us obtain high sensitivity and negative predictive values, especially with PD (table 1). Therefore, a larger prospective study is needed, in order to compare PD-guided sonography with conventional sonography for the diagnosis of RA stenosis in an unselected cohort of patients with hypertension.

This study demonstrates that PD is superior to conventional imaging, in terms of sensitivity and specificity, for the diagnosis of RA stenosis, because it allows a clear visualization of the whole stenotic vascular lumen. Due to its intrinsic limitations, PD cannot replace conventional sonographic techniques and especially CD, whose aliasing phenomenon is the most useful guide to activate PD during the examination. When PD is used as an adjunctive tool in vascular sonography, it enables a more accurate imaging of renovascular disease with results that seem comparable to selective angiography.