Interlobular vasculature in renal transplants: a power Doppler US study with MR correlation

B-Flow Sonography vs. Color Doppler Sonography for the Assessment of Vascularity in Pediatric Kidney Transplantation

OBJECTIVE

 To compare B-flow sonography (BFS) with color Doppler sonography (CDS) for imaging of kidney transplant vascularization in children.

PATIENTS AND METHODS

 All children receiving a kidney transplantation who underwent a protocol-based ultrasound examination (Loqiq 9, GE Medical Systems, Milwaukee, WI, USA) using the BFS and CDS technique with equal settings and probe position between January 2013 and January 2016 were retrospectively assessed (n = 40). The obtained datasets were visually graded according to the following criteria: (I) delineation of the renal vascular tree (Grade 1 - clear demarcation of interlobar, together with arcuate and interlobular vessels; Grade 2 - clear demarcation of interlobar and cortical vessels, but no distinction of interlobular from arcuate vessels; Grade 3 - only clear demarcation of interlobar vessels, Grade 4 - insufficient demarcation) (II) delineation of cortical vessel density in ventral, lateral, and dorsal part of the transplant, (III) smallest vessel-capsule distance, and (IV) maximum cortical vessel count. Comparison between methods was performed using Fisher's exact and paired sample t-tests.

RESULTS

 Applying a curved transducer (C1-6), BFS showed superior delineation of the renal vascular tree (p < 0.001), a lower vessel-capsule distance (p < 0.001), a higher cortical vessel count (p < 0.001), and a higher cortical vessel density in the superficial cortex (p = 0.01) than CDS. In the dorsal and lateral aspects of the transplant, cortical vessel density was lower with BFS (both p < 0.001). Using a linear high-resolution transducer (ML 6-15), no significant differences between the methods were found.

CONCLUSION

 Improved imaging of kidney transplant vascularization can be achieved in children by adding BFS to a standard protocol. The BFS technique is especially beneficial for overall assessment of the renal vascular tree together with the extent of cortical vascularization on curved array images.

KEY POINTS

  · Depiction of vascular tree and ventral cortical vessels is improved by BFS.. · The dorso-lateral cortex was better represented with CDS because of higher penetration.. · Additional monitoring with BFS improves the monitoring of transplant viability..

CITATION FORMAT

· Dammann E, Groth M, Schild R et al. B-Flow Sonography vs. Color Doppler Sonography for the Assessment of Vascularity in Pediatric Kidney Transplantation. Fortschr Röntgenstr 2021; 193: 49 - 60.

RADIOLOGICAL IMAGING IN RENAL TRANSPLANTATION

SUMMARY – Radiological diagnostic methods have a significant role in the preoperative and postoperative care of patients after kidney transplantation. Improvement and innovations in technology, but also the growing experience of the radiologists who deal with kidney transplant patients as part of the transplant team lead to earlier detection of complications in the postoperative period, which are the leading cause of transplant failure. In this article, we describe, through diagnostic imaging examples, detailed evaluation of all possible complications that can occur after kidney transplantation, with evaluation of different possible diagnostic methods that can be used in the preoperative assessment and postoperative follow up and care of the transplanted patient. The goal of this article is to demonstrate and summarize in detail the possible complications of renal transplantation and how to best diagnostically approach them, with special reference to ultrasound which is the main imaging method for this group of conditions.

Diffusion-weighted imaging of the kidneys in haemolytic uraemic syndrome

OBJECTIVES

To evaluate the kidneys of patients with haemolytic uraemic syndrome (HUS) using diffusion-weighted imaging (DWI) and Doppler ultrasound (US) compared with healthy controls.

MATERIALS AND METHODS

Fifteen patients (mean age 33.3 years; three male; 12 female) with diarrhoea-positive HUS and 15 healthy volunteers were prospectively evaluated with DWI and Doppler US. A total apparent diffusion coefficient (ADC_TOT), and ADCs predominantly reflecting microperfusion (ADC_LOW) and diffusion (ADC_HIGH) were calculated. Doppler US evaluated renal vascularity and flow.

RESULTS

When compared with controls, kidneys affected by HUS showed reduced cortical ADC values (ADC_TOT 1.79±0.22 vs. 2.04±0.1x10^-3 mm²/s, P 0.001), resulting in either low corticomedullary differences (11/15 patients) or an inverted corticomedullary pattern (4/15 patients). Reduction of cortical ADC values was associated with a decrease of cortical vascularity on Doppler US (ADC_TOT, P<0.001; ADC_LOW, P 0.047). Kidneys with complete absence of the cortical vasculature on Doppler US (four patients) also demonstrated limited diffusion (ADC_HIGH, P 0.002). Low glomerular filtration rate, requirement for haemodialysis during hospitalization, and longer duration of haemodialysis were associated with decreased cortical diffusivity (ADC_TOT: P 0.04, 0.007, and <0.001, respectively).

CONCLUSION

DWI shows qualitative and quantitative abnormalities in kidneys affected by HUS, thereby extending the non-invasive assessment of renal parenchymal damage.

KEY POINTS

• In HUS, DWI is feasible for functional characterization of kidney involvement. • Kidneys affected by HUS showed reduced cortical diffusivity. • Decreased cortical diffusivity was associated with lower kidney function. • Requirement and duration of haemodialysis was linked to degree of cortical alterations.

Correlation between Doppler parameters and renal cortical fibrosis in lupus nephritis: a preliminary observation

To assess the relationship between renal Doppler parameters and renal cortical fibrosis in lupus nephritis (LN), we retrospectively reviewed 24 patients with LN underwent both renal color Doppler sonography and renal biopsy. The angle-corrected Doppler parameters, including peak systolic velocity (PSV), end diastolic velocity (EDV) and resistive index (RI) at the main and interlobar renal arteries were measured. The Doppler parameters and PSV and EDV ratios of the interlobar artery to main renal artery were compared with histopathologic analysis of the kidney biopsy specimen. On the basis of renal cortical fibrosis, the 24 cases of LN were divided into two groups: mild (6%-25%) renal cortex fibrosis (n = 13) and moderate (26%-50%) renal cortex fibrosis (n = 11). An independent-samples two tailed t test was used to statistically analyze the differences in PSV, EDV and RI between the two groups. Receiver operating characteristic was analyzed for assessing the accuracy of interlobar artery PSV and EDV in predicting moderate renal cortical fibrosis. In our result, both PSV and EDV in moderate renal cortex fibrosis were lower than that in mild renal cortex fibrosis. There were statistically significant differences in PSV and EDV at the interlobar artery, EDV and RI at the main renal artery, and PSV and EDV ratios of the interlobar artery to main renal artery between the two groups (all p < 0.05). The area under receiver operating characteristic curves of PSV and EDV for predicting >26% renal cortical fibrosis was 0.96 and 0.90, respectively. The optimal cutoff values for differentiating >26% renal cortical fibrosis from those <25% were PSV 30 cm/s (sensitivity = 0.92; specificity = 1) and EDV 13 cm/s (sensitivity = 0.77; specificity = 1). Therefore, the values of PSV and EDV at the interlobar artery can potentially be used as hemodynamic indicators of renal cortical fibrosis, which may non-invasively assist in monitoring the progression of renal cortical fibrosis in LN, especially in patients with contraindications to renal biopsy.

Doppler parameters in renal transplant dysfunction: correlations with histopathologic changes

OBJECTIVES

The aim of this study was to assess the relationship between intrarenal Doppler parameters and histopathologic changes shown on kidney biopsy in renal transplant dysfunction.

METHODS

We retrospectively reviewed the records of 113 patients (61 men and 52 women; age range, 22-76 years; mean age ± SD, 50.9 ± 12.7 years) who underwent both transplanted kidney sonography and biopsy from May 1, 2007, to May 31, 2009. Doppler parameters of the interlobar arteries, including the peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistive index (RI), were compared with kidney biopsy findings. According to histopathologic findings, the 113 patients were divided into two groups: 1, interstitial fibrosis/tubular atrophy and vascular/glomerular sclerosis (n = 79); and 2, edematous changes in glomeruli without fibrosis (n = 34). The correlations between Doppler parameters and histopathologic findings were statistically analyzed.

RESULTS

There were statistically significant differences in the PSV and EDV of the interlobar arteries between groups 1 and 2. Both the PSV and EDV in group 1 were significantly lower than those in group 2 (P < .001). There was no significant difference in the RI of the interlobar arteries between the two groups (P > .05). There were no significant differences in the PSV, EDV, and RI of the main renal artery between the two groups (all P > .05).

CONCLUSIONS

The PSV and EDV of the interlobar artery have statistical correlations with histopathologic types in renal transplant dysfunction. Both the PSV and EDV in interstitial fibrosis/tubular atrophy and vascular/glomerular sclerosis seem lower than those in glomerulopathy without fibrosis. Hence, the PSV and EDV of the interlobar artery may potentially be used as hemodynamic indicators for monitoring the progress of renal transplants.

Renal vascular perfusion index in a canine model

Decreased renal perfusion plays an important role in the progression toward renal failure. In this study, a novel measure was proposed to quantify renal perfusion using canine model. Serial renal vascular images at different vascular areas including the whole vascular tree, interlobar, arcuate and interlobular vessels were captured. Image processing software was designed to analyze the changes of power Doppler intensity of colored pixels within regions-of-interest (ROI). For a given ROI, the power Doppler vascular index (PDVI) was found to fluctuate with the cardiac cycle. It was also noted that the power Doppler signals generated by arterial vessels have different fluctuating waveforms and different phase compared with the signal derived from venous vessels. A power Doppler correlation-map was developed to differentiate the arteries and veins in the ROI. Using the serial power Doppler images and the derived flow direction information, the interlobular perfusion can be strongly quantified. The renal vascular perfusion index (RVPI) defined as the ratio of PDVI(max) versus PDVI(min) was significantly higher in the interlobular vessel areas than three other areas for seven healthy dogs. The RVPI resembles the systolic/diastolic (S/D) ratio that commonly reflects arterial hemodynamics. RVPI and power Doppler correlation-map reveal more "dynamic" sense of vascular perfusion and provide a novel approach for the examination of renal function in clinical practice.

Value of contrast-enhanced ultrasonography for detecting renal infarcts proven by contrast enhanced CT. A feasibility study

The effectiveness of contrast-enhanced ultrasonography (CEUS) in the evaluation of patients with acute renal infarcts was investigated, using contrast-enhanced helical computed tomography (CT) as the reference imaging procedure. Twenty-seven consecutive patients with acute renal infarcts detected with contrast-enhanced helical CT underwent CEUS. Digital cine-clips of CEUS were evaluated by two independent readers blinded to CT findings. Image quality was rated subjectively on a four-point scale. Then, readers were asked to assign a confidence level in diagnosis of renal infarct at the upper pole, medium portion, and lower pole of each kidney according to a five-degree scale, ranging from definitely absent to definitely present. ROC curve analysis was employed to assess the overall confidence of diagnosis of infarct, and weighted kappa values were calculated to assess inter-reader agreement. The subjective image quality of CEUS was lower than the image quality of CT at the upper poles. However, the diagnostic performance of CEUS was excellent (area under receiver-operator characteristic curve 0.992 +/- 0.006 for reader 1; 0.991 +/- 0.007 for reader 2), with very good inter-reader agreement (weighted kappa value = 0.83). CEUS is a reproducible tool to detect acute renal infarcts in men, with a diagnostic performance approaching that of CT.

Evaluation of Renal Cortical Perfusion by Noninvasive Power Doppler Ultrasound During Vascular Occlusion and Reperfusion

BACKGROUND

Urine output, a frequently used resuscitation end point, is presumed to represent renal cortical perfusion. However, no noninvasive method for direct measurement of renal perfusion exists. Power Doppler ultrasound (PDUS) is a method that reportedly is sensitive to low-velocity and microvascular blood flow and can depict it. This study aimed to develop a quantitative technique for PDUS image analysis, and to evaluate the ability of PDUS to quantify cortical perfusion during renal ischemia induced by vascular occlusion.

METHODS

A method was developed to determine the mean gray-scale intensity of PDUS images from within the renal cortex (PDUS image intensity). This index was hypothesized to represent renal cortical microvascular blood flow. Renal cortical blood flow was determined using fluorescent microspheres in five swine. Renal artery flow was measured with an ultrasonic flow probe. Power Doppler ultrasound was performed at baseline; at 75%, 50%, and 25% of baseline renal artery flow; and during reperfusion.

RESULTS

Subjectively, PDUS images showed decreases in image intensity corresponding to renal artery occlusion and increases after reperfusion. Cortical blood flow correlated well with renal artery flow (n = 25; r2 = 0.868) and with PDUS image intensity (n = 25; r2 = 0.844).

CONCLUSION

Noninvasive power Doppler ultrasound image intensity correlated well with invasively measured renal cortical blood flow, and may be useful during resuscitation of injured and critically ill patients.

Ultrasound score is more predictive than serum creatinine in assessment of cellular rejection in cynomolgus monkey renal allografts

RATIONALE AND OBJECTIVES

To investigate whether ultrasound (US), in particular the use of an ultrasound scoring system, can provide more diagnostic information than clinical parameters, such as serum creatinine, for the diagnosis and determination of the degree of cellular rejection in renal allografts in the cynomolgus monkey (Macaca fascicularis).

METHODS

Sixty-eight cynomolgus monkeys with life-supporting renal allografts were examined with a 7.5MHz linear ultrasound transducer. One-hundred fifty two-dimensional, spectral, and power Doppler examinations were performed and four ultrasound parameters, percentage increase in graft volume, cortical thickness, resistive index (RI) of the renal arcuate artery, and power Doppler (PD) scores were recorded from serial examinations. An ultrasound score was assigned to each graft based on the number of those parameters that were abnormal; a score of 1 indicated that all four were normal, and a score of 5 that all four were abnormal. Each parameter and the combined score were compared with serum creatinine values and histology and evaluated statistically using Spearman rank correlation.

RESULTS

In animals with dysfunctioning allografts (serum creatinine elevations >200 micromol/L), Spearman rank correlation showed a significant correlation between the US score and the histology score: between 200 and 500 micromol/L, r = 0.309, P = 0.046, n = 31 and if > 500 micromol/L, r = 0.486, P = 0.005, n = 30. In those same animals, no correlation could be shown between serum creatinine values and the US score or between the serum creatinine values and the histologic diagnosis. In contrast to the US score, single ultrasound parameters were not found to correlate to histologic findings.

CONCLUSION

The application of ultrasound imaging in nonhuman primate renal transplant models provides valuable information concerning the presence and severity of cellular rejection in cases of graft dysfunction and the US score has a better predictive value of histology than serum creatinine values alone.

Ultrasonography in renal transplantation

Sonography is a simple, inexpensive, and readily available imaging modality that has become an essential component of the management of renal transplantation. It is indicated in almost all patients with acute renal failure and also is useful in the evaluation of pain, infection, and hematuria and the performance of percutaneous biopsy. Although many aspects of sonography are similar in native and transplanted kidneys, there are important differences and problems unique to the renal allograft, which form the basis for this review. The anatomy of renal transplantation and changes that accompany parenchymal disorders are discussed, but particular attention focuses on problems related to the urinary tract, fluid collections, and vascular disorders. By becoming more familiar with transplant sonography, nephrologists will be better able to incorporate this indispensable tool into the care of their patients.

Technique of color Doppler quantification of vascularity in transplanted kidneys

A new technique for the quantification of vascularity based on the analysis of color Doppler images is described. We utilized velocity information obtained directly from cineloops transferred to a computer for off-line analysis. This methodology was used in transplanted kidneys to assess parenchymal vascularity on the basis of percentage color pixel density and mean flow velocity in mid-kidney cross-sectional regions of interest and the distance from the most peripheral color pixels to the capsule of the kidney. Other color Doppler quantitative methods have lacked reproducibility, and therefore before evaluation of the clinical usefulness of this technique, intraobserver reproducibility and interobserver reliability were assessed in 42 patients; no statistically significant variation was demonstrated. In 13 patients with normally functioning transplants, the mean maximum color pixel density was 34.7+/-13.4%, the mean flow velocity was 5.2+/-0.9 cm/second, and the mean distance to the capsule was 3.3+/-1.1 mm.

Power Doppler imaging and resistance index measurement in the evaluation of acute renal transplant rejection

PURPOSE

This study was designed to test the hypotheses that power Doppler imaging has a predictive value in the assessment of acute renal allograft rejection and that the information garnered from the combination of resistance index (RI) determination and power Doppler imaging has an even greater predictive value in this assessment.

METHODS

Power Doppler images obtained prior to 96 sequential renal allograft biopsies in 92 patients with suspected renal transplant rejection were retrospectively graded for parenchymal vascularity on a scale of 1 to 4: 1, normal, uniform cortical flow; 2, mild peripheral cortical hypoperfusion; 3, vascular pruning in cortex and medulla; and 4, no visible parenchymal flow, with flow seen only in central vessels. Power Doppler grades and RIs for these cases were compared to graft biopsy results.

RESULTS

No statistically significant association was observed between parenchymal vascularity as graded by power Doppler imaging, RI, and the presence of acute transplant rejection.

CONCLUSIONS

Neither grading of vascularity on power Doppler images, RI measurement, nor the combination of these methods is an accurate means of detecting renal allograft rejection.

Contribution of power Doppler sonography to the detection of renal allograft rejection in the cynomolgus monkey

RATIONALE AND OBJECTIVES

To evaluate whether a change in the power Doppler (PD) flow signals produced by the renal cortical interlobular vasculature of allografts in cynomolgus monkey transplant models is useful for the detection of cellular rejection and vasculopathies.

METHODS

Seventy-three monkeys with life-supporting allografts (bilateral native kidney nephrectomy) and 20 monkeys with allografts implanted with only unilateral native kidney nephrectomy were examined with ultrasound that included an examination with PD. Each graft received a PD score of 3 (normal cortical blush), 2 (reduced flow, no blush), or 1 (absence of cortical flow), and the results were compared with histology either from ultrasound-guided biopsy or at necropsy.

RESULTS

One hundred seventy-one allograft examinations (histological and PD) were compared. Histologically normal grafts were statistically more likely to have normal PD findings than were those with reduced flow or absent flow. Allografts with reduced flow had statistically more severe cellular rejection than those with normal flow. Also, vasculopathies were present in all three PD groups.

CONCLUSIONS

Reduced renal cortical flow in the cynomolgus monkey renal allograft indicates that more severe degrees of cellular rejection are present compared with allografts with normal flow. Overlap in the histological diagnoses of allografts with normal and reduced flow exists, and the finding of reduced flow with PD may be prognostically important and indicates the need for tissue sampling.

Power Doppler imaging: initial evaluation as a screening examination for carotid artery stenosis

PURPOSE

To evaluate power Doppler imaging as a possible screening examination for carotid artery stenosis.

MATERIALS AND METHODS

In the principal pilot study, a prospective, blinded comparison of power Doppler imaging with duplex Doppler imaging, the reference-standard method, was conducted in 100 consecutive patients routinely referred for carotid artery imaging at a large, private multispecialty clinic. In the validation pilot study, a prospective, blinded comparison of power Doppler imaging with digital subtraction angiography, the reference-standard method, was conducted in 20 consecutive patients routinely referred at a teaching hospital. Using conservative assumptions, the authors performed cost-effectiveness analysis.

RESULTS

Power Doppler imaging produced diagnostic-quality images in 89% of patients. When the images of the patients with nondiagnostic examinations were regarded as positive, power Doppler imaging had an area under the receiver operating characteristic curve, A(z), of 0.87, sensitivity of 70%, and specificity of 91%. The validation study results were very similar. The cost-effectiveness of screening and, as indicated, duplex Doppler imaging as the definitive diagnostic examination and endarterectomy was $47,000 per quality-adjusted life-year.

CONCLUSION

The A(z) value for power Doppler imaging compares well with that for mammography, a generally accepted screening examination, and with most other imaging examinations. Power Doppler imaging is likely to be a reasonably accurate and cost-effective screening examination for carotid artery stenosis in asymptomatic populations.

Power Doppler imaging of acute renal transplant rejection

PURPOSE

We evaluated the usefulness of power Doppler imaging (PDI) in diagnosing acute renal-transplant rejection.

METHODS

Twenty-eight patients underwent 33 renal-transplant biopsies for suspected acute rejection. Patterns of renal parenchymal vascularity revealed by PDI in patients with abnormal biopsy results were compared with patterns in a group who had normal biopsy results. PDI examinations were reviewed retrospectively by 2 independent radiologists who had no knowledge of the biopsy results. A PDI diagnosis of acute rejection required marked vascular pruning in both the cortex and medulla. PDI results then were compared with transplant-biopsy results.

RESULTS

The sensitivity and specificity of PDI for diagnosing acute renal-transplant rejection were 40% and 100%, respectively. None of the patients with negative biopsy results had PDI abnormalities. The negative predictive value of PDI was 33%, and the positive predictive value was 100%.

CONCLUSIONS

In our study, an abnormal sonogram was highly predictive of acute transplant rejection. However, a normal sonogram did not exclude the possibility of rejection.

Color Doppler sonography in early renal transplantation follow-up: resistive index measurements versus power Doppler sonography

OBJECTIVE

This study was designed to compare power Doppler images of perfusion with interlobar resistive index measurements obtained during the early monitoring of renal graft transplant to diagnose cortical perfusion abnormalities and assess prognosis.

SUBJECTS AND METHODS

Thirty-one patients underwent color Doppler sonography (4-7 MHz and 7-10 MHz) on day 6+/-2 after renal transplantation. Cortical vessel density was assessed visually and classified as either normal or decreased. Twelve months after transplantation, the results of power Doppler imaging and the levels of resistive index were reviewed in light of clinical and laboratory findings, graft biopsy results, and functional outcome.

RESULTS

No significant relationship was observed among power Doppler grades, levels of resistive index, and renal function. Power Doppler grades and resistive index levels failed to allow us to distinguish between tubulopathy and rejection. However, we found a statistically significant relationship between renal function at 12 months after transplantation and power Doppler grade (p = .04).

CONCLUSION

This study suggests that color Doppler sonography is insensitive in revealing and in allowing radiologists to differentiate the causes of graft dysfunction. However, power Doppler sonography allows a prediction of the functional recovery of the graft at 12 months after transplantation not provided by resistive index levels.

Power Doppler sonography: clinical applications

OBJECTIVE

Color Doppler imaging (CD) has had a great impact on ultrasonography (US). This technique depicts local flow by encoding an estimate of the mean Doppler frequency shift at a particular position in color. However, the choice of the mean frequency shift as the parameter for representing flow in color Doppler is somewhat arbitrary. Power Doppler ultrasound is a technique that encodes the power in the Doppler signal in color. This parameter is fundamentally different from the mean frequency shift. The frequency is determined by the velocity of the red blood cells, while the power depends on the amount of blood present. Providing an image of a different property of blood flow, power Doppler has shown several key advantages over colour Doppler, including higher sensitivity to flow, better edge definition and depiction of continuity of flow. In this paper we review the results of power Doppler clinical studies.

MATERIALS AND METHODS

All relevant information available in the literature on the potential clinical applications of this technique was revised to give a detailed survey.

RESULTS

The increased flow sensitivity and better vascular detailing of power Doppler have been used to detect flow presence and characteristics in vessels that are poorly imaged with conventional color Doppler. The improved depiction of tissue vasculature has shown potential advantages, especially in some areas, such as the cortex of native kidneys and renal allografts, the prepuberal testis, the infant hip and the bowel wall, in which color Doppler is not sensitive enough to detect clinically important, slow and poor flow in small vessels. In inflammatory conditions, power Doppler was valuable in depicting increased flow in vessels that are dilated because of inflammatory response. In this field, advantages have been reported in acute cholecystitis and in inflammatory states of musculoskeletal tissues. The higher sensitivity to slow flow and the improved detailing of the course of tortuous and irregular vessels made power Doppler a promising technique to image intratumoral vessels and, thereby, to ameliorate the accuracy of color Doppler in predicting the likelihood of benign versus malignant nature of nodules. Specific flow patterns, missed at color Doppler studies, have been indicated with power Doppler in some tumors of the liver and breast. In different settings, power Doppler also permitted to monitor serial blood flow changes after therapy and to display them as color intensity, allowing the observer to distinguish flow changes.

CONCLUSION

Although the actual role of power Doppler in changing patient management has not been assessed yet, this technique can depict flow which was previously undetectable, and thus permits an easier and more confident diagnosis in body regions where the ultrasound signal is weak because blood vessels are small.

Power Doppler: it's a good thing

Power Doppler is a new method of ultrasound flow imaging the utility of which is currently under investigation. This technique creates a flow map based on the integrated power of the Doppler spectrum, rather than mean Doppler frequency. Power Doppler imaging is inherently more sensitive in terms of flow detection than standard color Doppler imaging; therefore, power Doppler can display flow for situations in which color Doppler is ineffective and can even display tissue perfusion in highly vascular organs such as the kidneys. Furthermore, power Doppler is not effected by aliasing, nor is it effected by blooming in the same way as color Doppler, which has deleterious effects on color Doppler flow images. The control of blooming with power Doppler may be of great importance in the clinical application of echo-enhancing agents. This review article discusses in detail the technological advantages and disadvantages of power Doppler flow imaging. In addition, it provides a synopsis of the preliminary research studies that have been conducted to date with respect to the clinical applications of power Doppler.