Standardization of power Doppler quantification of blood flow in the human fetus using the aorta and inferior vena cava

Improving coronary ultrafast Doppler angiography using fractional moving blood volume and motion-adaptive ensemble length

Coronary microperfusion assessment is a key parameter for understanding cardiac function. Currently, coronary ultrafast Doppler angiography is the only non-invasive clinical imaging technique able to assess coronary microcirculation quantitatively in humans. In this study, we propose to use fractional moving blood volume (FMBV), proportional to the red blood cell concentration, as a metric for perfusion. FMBV compares the power Doppler in a region of interest (ROI) inside the myocardium to the power Doppler of a reference area in the heart chamber, fully filled with blood. This normalization gives then relative values of the ROI blood filling. However, due to the impact of ultrasound attenuation and elevation focus on power Doppler values, the reference area and the ROI need to be at the same depth to allow this normalization. This condition is rarely satisfied in vivo due to the cardiac anatomy. Hereby, we propose to locally compensate the attenuation between the ROI and the reference, by measuring the attenuation law on a phantom. We quantified the efficiency of this approach by comparing FMBV with and without compensation on a flow phantom. Compensated FMBV was able to estimate the ground-truth FMBV with less than 5% variation. This method was then adapted to the in vivo case of myocardial perfusion imaging during heart surgery on human neonates. The translation from in vitro to in vivo required an additional clutter filtering step to ensure that blood signals could be correctly identified in the fast-moving myocardium. We applied the singular value decomposition filter on temporal sliding windows whose lengths were a function of myocardium motion. This motion-adaptive temporal sliding window approach was able to improve blood and tissue separation in terms of contrast-to-noise ratio, as compared to well-established constant-length sliding window approaches. Therefore, compensated FMBV and singular value decomposition assisted with motion-adaptive temporal sliding windows improves the quantification of blood volume in coronary ultrafast Doppler angiography.

Automated Visualization and Quantification of Spiral Artery Blood Flow Entering the First-Trimester Placenta, Using 3-D Power Doppler Ultrasound

The goal of our research was to quantify the placental vascularity in 3-D at 11-13 + 6 wk of pregnancy at precise distances from the utero-placental interface (UPI) using 3-D power Doppler ultrasound. With this automated image analysis technique, differences in vascularity between normal and pathologic pregnancies may be observed. The algorithm was validated using a computer-generated image phantom and applied retrospectively in 143 patients. The following features from the PD data were recorded: The number of spiral artery jets into the inter-villous space, total geometric and PD area. These were automatically measured at discrete millimeter distances from the UPI. Differences in features were compared with pregnancy outcomes: Pre-eclamptic versus normal, all small-for-gestational age (SGA) to appropriate-for-gestational age (AGA) patients and AGA versus SGA in normotensives (Mann-Whitney). The Benjamini-Hochberg procedure was used (false discovery rate 10%) for multiple comparison testing. Features decreased with increasing distance from the UPI (Kruskal-Wallis test; p <0.001). At 2- 3 mm from the UPI, all features were smaller in pre-eclamptic compared with normal patients and for some in SGA compared with AGA patients (p <0.05). For AGA versus SGA in normotensive patients, no significant differences were found. Number of jets measured at 2-5 mm from the UPI did not vary because of the position of the placenta in the uterus (ANOVA; p > 0.05). This method provides a new in-vivo imaging tool for examining spiral artery development through pregnancy. Size and number of entrances of blood flow into the UPI could potentially be used to identify high-risk pregnancies and may provide a new imaging biomarker for placental insufficiency.

A technique for the estimation of fractional moving blood volume by using three-dimensional power Doppler US

PURPOSE

To (a) demonstrate an image-processing method that can automatically measure the power Doppler signal in a three-dimensional ( 3D three-dimensional ) ultrasonographic (US) volume by using the location of organs within the image and (b) compare 3D three-dimensional fractional moving blood volume ( FMBV fractional moving blood volume ) results with commonly used, unstandardized measures of 3D three-dimensional power Doppler by using the human placenta as the organ of interest.

MATERIALS AND METHODS

This is a retrospective study of scans obtained as part of a prospective study of imaging placental biomarkers with US, performed with ethical approval and written informed consent. One hundred forty-three consecutive female patients were examined by using an image-processing technique. Three-dimensional FMBV fractional moving blood volume was measured on the vasculature from the uteroplacental interface to a depth 5 mm into the placenta by using a normalization volume 10 mm outside the uteroplacental interface and compared against the Virtual Organ Computer-aided AnaLysis ( VOCAL Virtual Organ Computer-aided AnaLysis ; GE Healthcare, Milwaukee, Wis) vascularization flow index ( VFI vascularization flow index ). Intra- and interobserver variability was assessed in a subset of 18 volumes. Wilcoxon signed rank test and intraclass correlation coefficients were used to assess measurement repeatability.

RESULTS

The mean 3D three-dimensional FMBV fractional moving blood volume value ± standard deviation was 11.78% ± 9.30 (range, 0.012%-44.16%). Mean VFI vascularization flow index was 2.26 ± 0.96 (range, 0.15-6.06). Linear regression of VFI vascularization flow index versus FMBV fractional moving blood volume produced an R(2) value of 0.211 and was significantly different in distribution (P < .001). Intraclass correlation coefficient analysis showed higher FMBV fractional moving blood volume values than VFI vascularization flow index for intra- and interobserver variability; intraobserver values were 0.95 for FMBV fractional moving blood volume (95% confidence interval [ CI confidence interval ]: 0.90, 0.98) versus 0.899 for VFI vascularization flow index (95% CI confidence interval : 0.78, 0.96), and interobserver values were 0.93 for FMBV fractional moving blood volume (95% CI confidence interval : 0.82, 0.97) versus 0.67 for VFI vascularization flow index (95% CI confidence interval : 0.32, 0.86).

CONCLUSION

The extension of an existing two-dimensional standardized power Doppler measurement into 3D three-dimensional by using an image-processing technique was shown in an in utero placental study. Three-dimensional FMBV fractional moving blood volume and VFI vascularization flow index produced significantly different results. FMBV fractional moving blood volume performed better than VFI vascularization flow index in repeatability studies. Further studies are needed to assess accuracy against a reference standard.

Inapplicability of fractional moving blood volume technique to standardize Virtual Organ Computer-aided AnaLysis indices for quantified three-dimensional power Doppler

OBJECTIVE

To determine whether the technique of fractional moving blood volume (FMBV) is applicable to Virtual Organ Computer-aided AnaLysis II (VOCAL II™)-based indices to quantify three-dimensional power Doppler ultrasound (3D-PDU) by investigating the effect of gain level on the indices measured at a possible reference point for standardization.

METHODS

Ten women with singleton pregnancy between 33+3 and 37+5 weeks' gestation were recruited. The optimal position for 3D acquisition of cord insertion into the placenta was identified and static 3D-PDU volumes were acquired using consistent machine configurations. Without moving the probe or the participant changing position, successive 3D volumes were stored at -3, -5, -7 and -9 dB and at the individualized sub-noise gain (SNG) level. Volumes were excluded if flash artifact was present, in which case all five volumes were reacquired. Using 4D View software, the cord insertion was magnified and the smallest sphere possible was used to measure vascularization index (VI), flow index (FI) and vascularization flow index (VFI). The associations between VOCAL indices and gain level were assessed using Pearson's correlation coefficient.

RESULTS

VOCAL indices for cord insertion correlated poorly with gain level, whether fundamental or relative to SNG level (R(2) = 0.07 and 0.04, respectively). VI was consistently 100% and mean FI and VFI were 99.5 (SD, 0.57), with all values > 97 irrespective of gain level.

CONCLUSIONS

Whilst previous work has shown that gain correlates well with placental tissue VOCAL indices, the correlation between gain level and VOCAL indices in an area of 100% vascularity at the cord insertion is poor. Regions of 100% vascularity appear to be artificially assigned a value approaching 100% for all VOCAL indices irrespective of gain level. This precludes using the technique of VOCAL indices from large vessels to standardize power Doppler measurements and the FMBV index is therefore not applicable to image analysis using VOCAL.

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.

Influence of power Doppler gain setting on Virtual Organ Computer-aided AnaLysis indices in vivo: can use of the individual sub-noise gain level optimize information?

OBJECTIVES

To demonstrate the influence of gain setting on the calculated Virtual Organ Computer-aided AnaLysis (VOCAL(™)) three-dimensional (3D) indices and define a point, the sub-noise gain (SNG), at which maximum information is available without noise artifact.

METHODS

Pregnant women were recruited at the time of their pregnancy-dating scan. Five identical static 3D power Doppler volumes of the placenta were acquired using identical machine settings apart from altering the power Doppler gain setting. The gain settings included the individualized SNG setting (determined by increasing gain until noise artifact was visible, then reducing it until the artifact just disappeared). The data were analyzed using VOCAL II. Vascularization index (VI), flow index (FI) and vascularization flow index (VFI) were calculated for the same sample at five different power Doppler gain levels. The relationship between the values calculated for the VOCAL indices and the gain value was explored using linear regression analysis.

RESULTS

Results from 50 women were analyzed. The percentage difference in VI and VFI from that observed at the SNG level in each woman was significantly linearly related to the gain setting relative to that at the SNG point (VI: r(2) = 0.68, P < 0.0001; VFI: r(2) = 0.72, P < 0.0001), with the values produced for VI and VFI decreasing as the gain was turned down. There was a distinct 'turning point' at the SNG level with linear relationships above and below, but with significantly different gradients (P ≤ 0.001). This relationship was not demonstrated for FI.

CONCLUSION

The SNG setting appears to represent each individual's optimum gain level. Using this may improve meaningful comparisons of VI and VFI between patients.

Evaluation of neonatal regional cerebral perfusion using power Doppler and the index fractional moving blood volume

BACKGROUND

The high cerebral morbidity of premature neonates is thought to be related to changes in tissue perfusion in vulnerable areas of the brain. Quantification of power Doppler (PD) images using the index fractional moving blood volume (FMBV) may allow measurement of regional cerebral perfusion.

OBJECTIVE

To evaluate the reproducibility of calculating FMBV using PD ultrasound images to estimate cerebral perfusion.

METHODS

Two experienced clinicians performed head ultrasounds on 24 normally-grown neonates at less than 33 weeks' gestation. Both clinicians independently acquired and stored three PD images in two different coronal planes. FMBV was calculated offline after selecting two predefined regions of interest within these planes (basal ganglia and subependymal regions). Reproducibility was evaluated by calculating the intraclass correlation coefficient (intraCC) and the interclass correlation coefficient (interCC).

RESULTS

FMBV was successfully evaluated in 24/24 neonates by both clinicians. The intraCC for repeatability for observer A was 1.00 (95% CI 1.00-1.00) for the basal ganglia and 0.99 (95% CI 0.99-1.00) for the subependymal region, and for observer B was 0.99 (95% CI 0.99-1.00) for the basal ganglia and 0.96 (95% CI 0.92-0.98) for the subependymal region. The interCC was 0.86 (95% CI 0.68-0.94) for the basal ganglia and 0.93 (95% CI 0.86-0.97) for the subependymal region.

CONCLUSION

Using standardised settings and a well-defined region of interest, the calculation of FMBV using PD images is a reproducible method of estimating neonatal regional cerebral perfusion.

Normal reference ranges of fetal regional cerebral blood perfusion as measured by fractional moving blood volume

OBJECTIVES

To establish normal reference intervals of fetal regional brain blood perfusion using power Doppler ultrasound as measured by fractional moving blood volume (FMBV).

METHODS

A cohort of consecutive singleton normally grown fetuses was selected including at least 12 fetuses for each completed week of gestation between 24 and 41 weeks. Cerebral blood perfusion was estimated using conventional power Doppler ultrasound in the following brain regions: frontal area, basal ganglia and posterior brain. Five consecutive good-quality images were recorded in each area and the region of interest was delineated offline. The FMBV was quantified as the average of all images and expressed as a percentage. Normal reference ranges were constructed by means of the LMS (lambda-mu-sigma) method.

RESULTS

A total of 230 fetuses were included. The median gestational age at evaluation and at delivery was 33.1 (range, 24.0-41.0) and 39.7 (range, 34.9-42.3) weeks, respectively. From 24 to 41 weeks' gestation, the mean FMBV increased from 13.21 to 14.97% in the frontal area, 11.17 to 14.86% in the basal ganglia and 4.83 to 6.70% in the posterior brain.

CONCLUSIONS

Normal data of fetal cerebral blood perfusion in the frontal area, basal ganglia and posterior brain are provided, which could be of clinical use in the assessment of fetal brain circulation.

Reproducibility of regional placental vascularity/perfusion measurement using 3D power Doppler

OBJECTIVES

To assess reproducibility and regional variability of placental perfusion measurement using three-dimensional (3D) power Doppler VOCAL() (Virtual Organ Computer-aided AnaLysis).

METHODS

Twenty pregnant women at 26-34 weeks' gestation with normally grown, biophysically normal, singleton pregnancies with anterior placentae had placental power Doppler mapping data stored digitally from each of the four placental quadrants. Each was imaged by two investigators, with two datasets stored per investigator per quadrant. 5760 data values from the 320 datasets were evaluated by the same two investigators. Power Doppler imaging of the placental cord insertion was performed to generate a value for standardization as 'fractional moving blood volume' if appropriate. The vascularization index (VI), flow index (FI) and vascularization flow index (VFI) were calculated from spherical regions-of-interest to assess reproducibility within and between quadrants and between investigators for both acquisition and analysis.

RESULTS

We found extensive variability for all readings. For repeated measurements within the same dataset the intra-analyzer intraclass correlation coefficient (ICC) range was: 0.24-0.57 for VI, 0.33-0.78 for FI and 0.12-0.48 for VFI. The corresponding interanalyzer ICC range was: 0.38-0.92 for VI, 0.40-0.85 for FI and 0.10-0.92 for VFI. The intra-acquirer variability (paired t-test) mean differences range was: - 3.91 to 4.71 for VI, - 2.68 to 3.31 for FI and - 2.23 to 2.78 for VFI; the corresponding interacquirer variability (paired t-test) range was: - 1.92 to 5.18 for VI, - 3.06 to 2.04 for FI and - 1.69 to 2.60 for VFI. The regional variability range (coefficient of variation) was: 6.28-126.34% for VI, 2.26-49.01% for FI and 6.09-151.55% for VFI. For all analyzed data, FI showed least variability and cord values for VI were consistently 100% (mean VFI, 98.4 and 98.8 between observers).

CONCLUSIONS

There is insufficient evidence to support the meaning, reliability or reproducibility of VOCAL (VI, FI or VFI) as a tool to quantify placental perfusion, despite its use in multiple publications and journal submissions. There is poor reproducibility at the most fundamental level. Further investigation into the reproducibility of placental perfusion and quantification using VOCAL is required before development and application as a clinically useful tool.

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.

Evaluation of fetal regional cerebral blood perfusion using power Doppler ultrasound and the estimation of fractional moving blood volume

OBJECTIVE

To standardize the evaluation of regional fetal brain blood perfusion, using power Doppler ultrasound (PDU) to estimate the fractional moving blood volume (FMBV) and to evaluate the reproducibility of this estimation.

METHODS

Brain blood perfusion was evaluated in 35 normally grown fetuses at 28-30 weeks of gestation, using PDU. The following cerebral regions were included in the PDU color box: anterior sagittal, complete sagittal, basal ganglia, and cerebellar. Ten consecutive good-quality images of each anatomical plane were recorded and the delimitation of the region of interest (ROI) was performed off-line. FMBV was quantified in the ROI of all images and the mean considered as the final value. Differences between regions, variability, reproducibility and agreement between observers were assessed.

RESULTS

Power Doppler images of the described anatomical planes were obtained in all cases, regardless of fetal position. The median time for the acquisition of the images was 7 (range 4-12) min. Mean (range) FMBV values were: anterior sagittal, 16.5 (10.7-22.8)%, inter-patient coefficient of variation (CV) 0.22; complete sagittal, 13.5 (8.8-16.1)%, CV 0.27; basal ganglia, 18.3 (10.7-27.6)%, CV 0.27; and cerebellar, 6.6 (3.0-11.0)%, CV 0.38. There were statistically significant differences in FMBV between cerebellar and complete sagittal ROIs with the frontal and basal ganglia regions. Reproducibility analyses showed an intraclass correlation coefficient of 0.91 (95% CI 0.67-0.97) and an interclass correlation coefficient of 0.87 (95% CI 0.70-0.94). Interobserver agreement showed a mean difference between observers of -0.2 (SD 2.7) with 95% limits of agreement -5.6 to 5.2.

CONCLUSIONS

When the regions of interest are well defined, the FMBV estimate offers a method to quantify blood flow perfusion in different fetal cerebral areas. There appear to be regional differences in FMBV within the fetal brain.