Color Flow Ultrasound Spatial Sampling Beam Density for Partial Volume-Corrected Three-Dimensional Volume Flow (3DVF): Theory, Simulation, and Experiment.

OBJECTIVE: The purpose of this study was to quantify the accuracy of partial volume-corrected three-dimensional volume flow (3DVF) measurements as a function of spatial sampling beam density using carefully-designed parametric analyses in order to inform the target applications of 3DVF. METHODS: Experimental investigations employed a mechanically-swept curvilinear ultrasound array to acquire 3D color flow (6.3 MHz) images in flow phantoms consisting of four lumen diameters (6.35, 4.88, 3.18 and 1.65 mm) with volume flow rates of 440, 260, 110 and 30 mL/min, respectively. Partial volume-corrected three-dimensional volume flow (3DVF) measurements, based on the Gaussian surface integration principle, were computed at five regions of interest positioned between depths of 2 and 6 cm in 1 cm increments. At each depth, the color flow beam point spread function (PSF) was also determined, using in-phase/quadrature data, such that 3DVF bias could then be related to spatial sampling beam density. Corresponding simulations were performed for a laminar parabolic flow profile that was sampled using the experimentally-measured PSFs. Volume flow was computed for all combinations of lumen diameters and the PSFs at each depth. RESULTS: Accurate 3DVF measurements, i.e., bias less than ±20%, were achieved for spatial sampling beam densities where at least 6 elevational color flow beams could be positioned across the lumen. In these cases, greater than 8 lateral color flow beams were present. PSF measurements showed an average lateral-to-elevational beam width asymmetry of 1:2. Volume flow measurement bias increased as the color flow beam spatial sampling density within the lumen decreased. CONCLUSION: Applications of 3DVF, particularly those in the clinical domain, should focus on areas where a spatial sampling density of 6 × 6 (lateral x elevational) beams can be realized in order to minimize measurement bias. Matrix-based ultrasound arrays that possess symmetric PSFs may be advantageous to achieve adequate beam densities in smaller vessels.

Measurement of Cerebral Metabolism Under Non-Chronic Hemodynamic Conditions.

BACKGROUND: Blood flow to the brain is a critical physiological function and is useful to monitor in critical care settings. Despite that, a surrogate is most likely measured instead of actual blood flow. Such surrogates include velocity measurements in the carotid artery and systemic blood pressure, even though true blood flow can actually be obtained using MRI and other modalities. Ultrasound is regularly used to measure blood flow and is, under certain conditions, able to provide quantitative volumetric blood flow in milliliters per minute. Unfortunately, most times the resulting flow data is not valid due to unmet assumptions (such as flow profile and angle correction). Color flow, acquired in three dimensions, has been shown to yield quantitative blood flow without any assumptions (3DVF). METHODS: Here we are testing whether color flow can perform during physiological conditions common to severe injury. Specifically, we are simulating severe traumatic brain injury (epidural hematoma) as well as hemorrhagic shock with 50% blood loss. Blood flow was measured in the carotid artery of a cohort of 7 Yorkshire mix pigs (40-60 kg) using 3DVF (4D16L, LOGIQ 9, GE HealthCare, Milwaukee, WI, USA) and compared to an invasive flow meter (TS420, Transonic Systems Inc., Ithaca, NY, USA). RESULTS: Six distinct physiological conditions were achieved: baseline, hematoma, baseline 2, hemorrhagic shock, hemorrhagic shock plus hematoma, and post-hemorrhage resuscitation. Mean cerebral oxygen extraction ratio varied from 40.6% ± 13.0% of baseline to a peak of 68.4% ± 15.6% during hemorrhagic shock. On average 3DVF estimated blood flow with a bias of -9.6% (-14.3% root mean squared error) relative to the invasive flow meter. No significant flow estimation error was detected during phases of flow reversal, that was seen in the carotid artery during traumatic conditions. The invasive flow meter showed a median error of -11.5% to 39.7%. CONCLUSIONS: Results suggest that absolute volumetric carotid blood flow to the brain can be obtained and potentially become a more specific biomarker related to cerebral hemodynamics than current surrogate markers.

Placental assessment using spectral analysis of the envelope of umbilical venous waveforms in sheep.

INTRODUCTION: This study was designed to test the efficacy of an ultrasound flow measurement method to evaluate placental function in a hyperandrogenic sheep model that produces placental morphologic changes and an intrauterine growth restriction (IUGR) phenotype. MATERIALS AND METHODS: Pregnant ewes were assigned randomly between control (n = 12) and testosterone-treatment (T-treated, n = 22) groups. The T-treated group was injected twice weekly intramuscularly (IM) with 100 mg testosterone propionate. Control sheep were injected with corn oil vehicle. Lambs were delivered at 119.5 ± 0.48 days gestation. At the time of delivery of each lamb, flow spectra were generated from one fetal artery and two fetal veins, and the spectral envelopes examined using fast Fourier transform analysis. Base 10 logarithms of the ratio of the amplitudes of the maternal and fetal spectral peaks (LRSP) in the venous power spectrum were compared in the T-treated and control populations. In addition, we calculated the resistive index (RI) for the artery defined as ((peak systole - min diastole)/peak systole). Two-tailed T-tests were used for comparisons. RESULTS: LRSPs, after removal of significant outliers, were -0.158 ± 0.238 for T-treated and 0.057 ± 0.213 for control (p = 0.015) animals. RIs for the T-treated sheep fetuses were 0.506 ± 0.137 and 0.497 ± 0.086 for controls (p = 0.792) DISCUSSION: LRSP analysis distinguishes between T-treated and control sheep, whereas RIs do not. LRSP has the potential to identify compromised pregnancies.

Bedside Cerebral Blood Flow Quantification in Neonates.

Measurement of blood flow to the brain in neonates would be a very valuable addition to the medical diagnostic armamentarium. Such conditions such as assessment of closure of a patent ductus arteriosus (PDA) would greatly benefit from such an evaluation. However, measurement of cerebral blood flow in a clinical setting has proven very difficult and, as such, is rarely employed. Present techniques are often cumbersome, difficult to perform and potentially dangerous for very low birth weight (VLBW) infants. We have been developing an ultrasound blood volume flow technique that could be routinely used to assess blood flow to the brain in neonates. By scanning through the anterior fontanelles of 10 normal, full-term newborn infants, we were able to estimate total brain blood flows that closely match those published in the literature using much more invasive and technically demanding methods. Our method is safe, easy to do, does not require contrast agents and can be performed in the baby's incubator. The method has the potential for monitoring and assessing blood flows to the brain and could be used to routinely assess cerebral blood flow in many different clinical conditions.

Umbilical Vein Pulse Wave Spectral Analysis: A Possible Method for Placental Assessment Through Evaluation of Maternal and Fetal Flow Components.

OBJECTIVES: Placental blood flow analysis is complicated by having both maternal and fetal flow components. Using the Fast Fourier Transform (FFT) of the umbilical venous pulse wave spectra (PW) envelope, we could simultaneously assess maternal/fetal blood flow in the placenta and investigate if normal and intrauterine growth restriction (IUGR)/pre-eclamptic pregnancies could be distinguished. METHODS: This retrospective study included normal gestations (N = 11) and gestations with IUGR, pre-eclampsia, or both (N = 13). Umbilical vein PW were acquired and spectral envelopes were identified as a function of time and analyzed by FFT. Base-10 logarithms of the ratios of the maternal/fetal spectral peaks (LRSP) were compared in normal and IUGR/pre-eclamptic populations (two-tailed t-test). Body mass index (BMI), gestational age at scan time, placental position, and weight-normalized umbilical vein blood volume flow (two-tailed t-test, analysis of variance [ANOVA] analysis) were tested. P < .05 was considered significant. RESULTS: The LRSP for normal and IUGR/pre-eclamptic pregnancies were 0.141 ± 0.180 and -0.072 ± 0.262 (mean ± standard deviation), respectively (P = .033). We detected differences between normal gestations and combinations of LRSP and weight-normalized umbilical venous blood flows. Placental effects based on LRSPs and blood flow may act synergistically in cases with both pre-eclampsia and IUGR (P = .014). No other significant associations were seen. CONCLUSIONS: In this preliminary study, we showed that umbilical venous flow contains markers related to placental maternal/fetal blood flow, which can be used to assess IUGR and pre-eclampsia. When coupled with umbilical cord blood flow, this new marker may potentially identify the primary causes of the two conditions.

Comparison of Variations Between Spectral Doppler and Gaussian Surface Integration Methods for Umbilical Vein Blood Volume Flow.

OBJECTIVES: We are studying a new method for estimating blood volume flow that uses 3-dimensional ultrasound to measure the total integrated flux through an ultrasound-generated Gaussian surface that intersects the umbilical cord. This method makes none of the assumptions typically required with standard 1-dimensional spectral Doppler volume flow estimates. We compared the variations in volume flow estimates between techniques in the umbilical vein. METHODS: The study was Institutional Review Board approved, and all 12 patients gave informed consent. Because we had no reference standard for the true umbilical vein volume flow, we compared the variations of the measurements for the flow measurement techniques. At least 3 separate spectral Doppler and 3 separate Gaussian surface measurements were made along the umbilical vein. Means, standard deviations, and coefficients of variation (standard deviation/mean) for the flow estimation techniques were calculated for each patient. P < .05 was considered significant. RESULTS: The ranges of the mean volume flow estimates were 174 to 577 mL/min for the spectral Doppler method and 100 to 341 mL/min for the Gaussian surface integration (GSI) method. The mean standard deviations (mean ± SD) were 161 ± 95 and 45 ± 48 mL/min for the spectral Doppler and GSI methods, respectively (P < .003). The mean coefficients of variation were 0.46 ± 0.17 and 0.18 ± 0.14 for the spectral Doppler and GSI methods respectively (P < 0.002). CONCLUSIONS: The new volume flow estimation method using 3-dimensional ultrasound appears to have significantly less variation in estimates than the standard 1-dimensional spectral Doppler method.

Three-dimensional US for Quantification of Volumetric Blood Flow: Multisite Multisystem Results from within the Quantitative Imaging Biomarkers Alliance.

Background Quantitative blood flow (QBF) measurements that use pulsed-wave US rely on difficult-to-meet conditions. Imaging biomarkers need to be quantitative and user and machine independent. Surrogate markers (eg, resistive index) fail to quantify actual volumetric flow. Standardization is possible, but relies on collaboration between users, manufacturers, and the U.S. Food and Drug Administration. Purpose To evaluate a Quantitative Imaging Biomarkers Alliance-supported, user- and machine-independent US method for quantitatively measuring QBF. Materials and Methods In this prospective study (March 2017 to March 2019), three different clinical US scanners were used to benchmark QBF in a calibrated flow phantom at three different laboratories each. Testing conditions involved changes in flow rate (1-12 mL/sec), imaging depth (2.5-7 cm), color flow gain (0%-100%), and flow past a stenosis. Each condition was performed under constant and pulsatile flow at 60 beats per minute, thus yielding eight distinct testing conditions. QBF was computed from three-dimensional color flow velocity, power, and scan geometry by using Gauss theorem. Statistical analysis was performed between systems and between laboratories. Systems and laboratories were anonymized when reporting results. Results For systems 1, 2, and 3, flow rate for constant and pulsatile flow was measured, respectively, with biases of 3.5% and 24.9%, 3.0% and 2.1%, and -22.1% and -10.9%. Coefficients of variation were 6.9% and 7.7%, 3.3% and 8.2%, and 9.6% and 17.3%, respectively. For changes in imaging depth, biases were 3.7% and 27.2%, -2.0% and -0.9%, and -22.8% and -5.9%, respectively. Respective coefficients of variation were 10.0% and 9.2%, 4.6% and 6.9%, and 10.1% and 11.6%. For changes in color flow gain, biases after filling the lumen with color pixels were 6.3% and 18.5%, 8.5% and 9.0%, and 16.6% and 6.2%, respectively. Respective coefficients of variation were 10.8% and 4.3%, 7.3% and 6.7%, and 6.7% and 5.3%. Poststenotic flow biases were 1.8% and 31.2%, 5.7% and -3.1%, and -18.3% and -18.2%, respectively. Conclusion Interlaboratory bias and variation of US-derived quantitative blood flow indicated its potential to become a clinical biomarker for the blood supply to end organs. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Forsberg in this issue.

Three-dimensional US Fractional Moving Blood Volume: Validation of Renal Perfusion Quantification.

Background Three-dimensional (3D) fractional moving blood volume (FMBV) derived from 3D power Doppler US has been proposed for noninvasive approximation of perfusion. However, 3D FMBV has never been applied in animals against a ground truth. Purpose To determine the correlation between 3D FMBV and the reference standard of fluorescent microspheres (FMS) for measurement of renal perfusion in a porcine model. Materials and Methods From February 2017 to September 2017, adult pigs were administered FMS before and after measurement of renal 3D FMBV at baseline (100%) and approximately 75%, 50%, and 25% flow levels by using US machines from two different vendors. The 3D power Doppler US volumes were converted and segmented, and correlations between FMS and 3D FMBV were made with simple linear regression (r2). Similarity and reproducibility of manual segmentation were determined with the Dice similarity coefficient and 3D FMBV reproducibility (intraclass correlation coefficient [ICC]). Results Thirteen pigs were studied with 33 flow measurements. Kidney volume (mean Dice similarity coefficient ± standard deviation, 0.89 ± 0.01) and renal segmentation (coefficient of variation = 12.6%; ICC = 0.86) were consistent. The 3D FMBV calculations had high reproducibility (ICC = 0.97; 95% confidence interval: 0.96, 0.98). The 3D FMBV per-pig correlation showed excellent correlation for US machines from both vendors (mean r2 = 0.96 [range, 0.92-1.0] and 0.93 [range, 0.78-1.0], respectively). The correlation between 3D FMBV and perfusion measured with microspheres was high for both US machines (r2 = 0.80 [P < .001] and 0.70 [P < .001], respectively). Conclusion The strong correlation between three-dimensional (3D) fractional moving blood volume (FMBV) and fluorescent microspheres indicates that 3D FMBV shows excellent correlation to perfusion and good reproducibility. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Morrell et al in this issue.

Partial Volume Effect and Correction for 3-D Color Flow Acquisition of Volumetric Blood Flow.

Blood volume flow (VF) estimation is becoming an integral part of quantitative medical imaging. Three-dimensional color flow can be used to measure volumetric flow, but partial volume correction (PVC) is essential due to finite beamwidths and lumen diameters. Color flow power was previously assumed to be directly proportional to the perfused fractional color flow beam area (voxel). We investigate the relationship between color flow power and fractionally perfused voxels. We simulate 3-D color flow imaging using Field II based on a 3.75-MHz mechanically swept linear array. A 16-mm-diameter tube with laminar flow was embedded into soft tissue. We investigated two study scenarios where soft tissue backscatter is 1) 40 dB higher and 2) 40 dB lower, relative to blood. Velocity and power were computed from color flow packets ( n = 16 ) using autocorrelation. Study 1 employed a convolution-based wall filter. Study 2 did not employ a wall filter. VF was computed from the resulting color flow data, as published previously. Partial volume voxels in Study 1 show lesser power than those in Study 2, likely due to wall filter effects. An "S"-shaped relationship was found between color flow power and fractionally perfused voxel area in Study 2, which could be due to an asymmetric lateral-elevational point spread function. Flow computation is biased low by 7.3% and 7.9% in Study 1 and Study 2, respectively. Uncorrected simulation estimates are biased high by 41.5% and 12.5% in Study 1 and Study 2, respectively. Our findings show that PVC improves 3-D VF estimation and that wall filter processing alters the proportionality between color flow power and fractionally perfused voxel area.

Evaluation of Umbilical Vein Blood Volume Flow in Preeclampsia by Angle-Independent 3D Sonography.

OBJECTIVES: To investigate the association between umbilical vein blood volume flow and the condition of preeclampsia in an at-risk maternal patient cohort. Umbilical vein volume flow was quantified by a 3-dimensional (3D) sonographic technique that overcomes several limitations of standard sonographic flow measurement methods. METHODS: A total of 35 patients, each with a singleton pregnancy, were recruited to provide 5 patients with preeclampsia, derived as a subset from a 26-patient at-risk group, and 9 patients with normal pregnancies. An ultrasound system equipped with a 2.0-8.0-MHz transducer was used to acquire multivolume 3D color flow and power mode data sets to compute the mean umbilical vein volume flow in patients with normal pregnancies and preeclampsia. RESULTS: The gestational ages of the pregnancies ranged from 29.7 to 34.3 weeks in the patients with preeclampsia and from 25.9 to 34.7 weeks in the patients with normal pregnancies. Comparisons between patients with normal pregnancies and those with preeclampsia showed weight-normalized flow with a moderately high separation between groups (P = .11) and depth-corrected, weight-normalized flow with a statistically significant difference between groups (P = .035). Umbilical vein volume flow measurements were highly reproducible in the mean estimate, with an intrapatient relative SE of 12.1% ± 5.9% and an intrameasurement relative SE of 5.6% ± 1.9 %. In patients who developed pregnancy-induced hypertension or severe pregnancy-induced hypertension, umbilical vein volume flow suggested gestational hypertensive disorder before clinical diagnosis. CONCLUSIONS: Results indicate that mean depth-corrected, weight-normalized umbilical vein volume flow is reduced in pregnancies complicated by preeclampsia and that volume flow may indicate hypertensive disorder earlier in gestation. Volume flow measurements are highly reproducible, and further study in a larger clinical population is encouraged to determine whether 3D volume flow can complement the management of preeclampsia and, in general, at-risk pregnancy.

Volumetric blood flow in transjugular intrahepatic portosystemic shunt revision using 3-dimensional Doppler sonography.

OBJECTIVES: Three-dimensional (3D)/4-dimensional (4D) sonographic measurement of blood volume flow in transjugular intrahepatic porto systemic shunt revision with the intention of objective assessment of shunt patency. METHODS: A total of 17 patients were recruited (12 male and 5 female; mean age, 55 years; range, 30-69 years). An ultrasound system equipped with a 2.0-5.0-MHz probe was used to acquire multivolume 3D/4D color Doppler data sets to assess prerevision and postrevision shunt volume flow. Volume flow was computed offline based on the principle of surface integration of Doppler-measured velocity vectors in a lateral-elevational c-surface positioned at the color flow focal depth (range, 8.0-11.5 cm). Volume flow was compared to routine measurements of the prerevision and postrevision portosystemic pressure gradient. Prerevision volume flow was compared with the outcome to determine whether a flow threshold for revision could be defined. RESULTS: Linear regression of data from revised transjugular intrahepatic portosystemic shunt cases showed an inverse correlation between the mean-normalized change in prerevision and postrevision shunt volume flow and the mean-normalized change in the prerevision and postrevision portosystemic pressure gradient (r(2) = 0.51; P = .020). Increased shunt blood flow corresponded to a decreased pressure gradient. Comparison of prerevision flows showed preliminary threshold development at 1534 mL/min, below which a shunt revision may be recommended (P = .21; area under the receiver operating characteristic curve = 0.78). CONCLUSIONS: Shunt volume flow measurement with 3D/4D Doppler sonography provides a potential alternative to standard pulsed wave Doppler metrics as an indicator of shunt function and predictor of revision.

A method to validate quantitative high-frequency power doppler ultrasound with fluorescence in vivo video microscopy.

Flow quantification with high-frequency (>20 MHz) power Doppler ultrasound can be performed objectively using the wall-filter selection curve (WFSC) method to select the cutoff velocity that yields a best-estimate color pixel density (CPD). An in vivo video microscopy system (IVVM) is combined with high-frequency power Doppler ultrasound to provide a method for validation of CPD measurements based on WFSCs in mouse testicular vessels. The ultrasound and IVVM systems are instrumented so that the mouse remains on the same imaging platform when switching between the two modalities. In vivo video microscopy provides gold-standard measurements of vascular diameter to validate power Doppler CPD estimates. Measurements in four image planes from three mice exhibit wide variation in the optimal cutoff velocity and indicate that a predetermined cutoff velocity setting can introduce significant errors in studies intended to quantify vascularity. Consistent with previously published flow-phantom data, in vivo WFSCs exhibited three characteristic regions and detectable plateaus. Selection of a cutoff velocity at the right end of the plateau yielded a CPD close to the gold-standard vascular volume fraction estimated using IVVM. An investigator can implement the WFSC method to help adapt cutoff velocity to current blood flow conditions and thereby improve the accuracy of power Doppler for quantitative microvascular imaging.

Three-dimensional sonographic measurement of blood volume flow in the umbilical cord.

OBJECTIVES: Three-dimensional (3D) umbilical cord blood volume flow measurement with the intention of providing a straightforward, consistent, and accurate method that overcomes the limitations associated with traditional pulsed wave Doppler flow measurement and provides a means by which to recognize and manage at-risk pregnancies. METHODS: The first study involved 3D sonographic volume flow measurements in 7 healthy ewes whose pregnancies ranged from 18 to 19 weeks' gestation (7 singletons). Sonographic umbilical arterial and venous flow measurements from each fetus were compared to the corresponding average measured arterial/venous flow to assess the feasibility of measurement in a static vessel. A second complementary study involved 3D sonographic volume flow measurements in 7 healthy women whose pregnancies ranged from 17.9 to 36.3 weeks' gestation (6 singletons and 1 twin). Umbilical venous flow measurements were compared to similar flow measurements reported in the literature. Pregnancy outcomes were abstracted from the medical records of the recruited patients. RESULTS: In the fetal sheep model, arterial/venous flow comparisons yielded errors of 10% or less for 8 of the 9 measurements. In the clinical study, venous flow measurements showed agreement with the literature over a range of gestational ages. Two of the 7 patients in the clinical study had lower flow than anticipated for gestational age; one had a subsequent diagnosis of intrauterine growth restriction, and the other had preeclampsia. CONCLUSIONS: Accurate measurement of umbilical blood volume flow can be performed with relative ease in both the sheep model and in humans using the proposed 3D sonographic flow measurement technique. Results encourage further development of the method as a means for diagnosis and identification of at-risk pregnancies.

Improved objective selection of power Doppler wall-filter cut-off velocity for accurate vascular quantification.

The wall-filter selection curve method is proposed to objectively identify a cut-off velocity that minimizes artifacts in power Doppler images. A selection curve, which is constructed by plotting the color pixel density (CPD) as a function of the cut-off velocity, exhibits characteristic intervals hypothesized to include the optimum cut-off velocity. This article presents an improved implementation of the method that automatically detects characteristic intervals in a selection curve and selects an operating point cut-off velocity along a characteristic interval. The method is applied to subregions within the Doppler image to adapt the cut-off velocity to local variations in vascularity. The method's performance is evaluated in 30-MHz power Doppler images of a four-vessel flow phantom. At high (>5 mm/s) flow velocities, qualitative improvements in vessel delineation are achieved and the CPD in the resulting images is accurate to within 3% of the vascular volume fraction of the phantom.

Objective selection of high-frequency power Doppler wall filter cutoff velocity for regions of interest containing multiple small vessels.

High-frequency (> 20 MHz) power Doppler ultrasound is frequently used to quantify vascularity in preclinical studies of small animal angiogenic models, but quantitative images can be difficult to obtain in the presence of flow artifacts. To improve flow quantification, color pixel density (CPD) can be plotted as a function of wall filter cutoff velocity to produce a wall-filter selection curve that can be used to estimate actual vascular volume fraction. A mathematical model based on receiver operating characteristic statistics is developed to study the behavior of wall-filter selection curves. The model is compared to experimental data acquired with a 30-MHz transducer and a custom-designed multiple-vessel flow phantom capable of mimicking a range of blood vessel sizes (200-300 microm), blood flow velocities (1-10 mm/s), and blood vessel orientations. At high flow rates, wall-filter selection curves for multiple-vessel regions include a plateau whose CPD corresponds with the total vascular volume fraction. Conversely, the vascular volume fraction of a subset of vessels is obtained at low flow rates. Detection of the volume fraction of all vessels is ensured when a plateau is > 0.5 mm/s in length and begins at a wall filter cutoff < 2 mm/s.

Estimating vascular volume fraction in a network of small blood vessels with high-frequency power Doppler ultrasound.

Quantitative images of high-frequency (> 20 MHz) power Doppler ultrasound can be difficult to obtain in the presence of flow artifacts due to power Doppler's sensitivity to operator-dependent acquisition settings. To improve flow quantification, color pixel density (CPD) can be plotted as a function of wall filter cut-off velocity to produce a wall-filter selection curve that can be used to estimate vascular volume fraction by locating the plateau along the curve. The behavior of the wall-filter selection curve in a multiple-vessel region of interest is studied using a custom-designed multiple-vessel flow phantom. The flow phantom is capable of mimicking a range of blood vessel sizes (200-300 microm), blood flow velocities (1-10 mm/s), and blood vessel orientations (long-axis and transverse). At high flow rates, single-vessel wall-filter selection curves superimpose to produce a multiple-vessel curve where the CPD at the left-most plateau corresponds with the actual vascular volume fraction. However, interpretation of the multiple-vessel wall-filter selection curve is not straightforward when the flow rate in the vascular network is low.

Detectability of small blood vessels with high-frequency power Doppler and selection of wall filter cut-off velocity for microvascular imaging.

Power Doppler imaging of physiologic and pathologic angiogenesis is widely used in preclinical studies to track normal development, disease progression and treatment efficacy but can be challenging given the presence of small blood vessels and slow flow velocities. Power Doppler images can be plagued with false-positive color pixels or undetected vessels, thereby complicating the interpretation of vascularity metrics such as color pixel density (CPD). As an initial step toward improved microvascular quantification, flow-phantom experiments were performed to establish relationships between vessel detection and various combinations of vessel size (160, 200, 250, 300 and 360 microm), flow velocity (4, 3, 2, 1 and 0.5 mm/s) and transducer frequency (30 and 40 MHz) while varying the wall filter cut-off velocity. Receiver operating characteristic (ROC) curves and areas under ROC curves indicate that good vessel detection performance can be achieved with a 40-MHz transducer for flow velocities > or =2 mm/s and with a 30-MHz transducer for flow velocities > or =1 mm/s. In the second part of the analysis, CPD was plotted as a function of wall filter cut-off velocity for each flow-phantom data set. Three distinct regions were observed: overestimation of CPD at low cut-offs, underestimation of CPD at high cut-offs and a plateau at intermediate cut-offs. The CPD at the plateau closely matched the phantom's vascular volume fraction and the length of the plateau corresponded with the flow-detection performance of the Doppler system assessed using ROC analysis. Color pixel density vs. wall filter cut-off curves from analogous in vivo experiments exhibited the same shape, including a distinct CPD plateau. The similar shape of the flow-phantom and in vivo curves suggests that the presence of a plateau in vivo can be used to identify the best-estimate CPD value that can be treated as a quantitative vascularity metric. The ability to identify the best CPD estimate is expected to improve quantification of angiogenesis and anti-vascular treatment responses with power Doppler.