Influence of instrument settings on flow signal and background noise in power Doppler US. An experimental study using a flow phantom with hyperechoic background

How to use power Doppler ultrasound in transvaginal assessment of uterine fibroids

Measuring vascularization in uterine fibroids is important for their diagnosis, treatment and prognosis. Vascularization can be measured by power Doppler ultrasound. The power Doppler signal depends on fibroid characteristics and on a variety of ultrasound-machine settings. Literature describing which machine settings influence the power Doppler signal is limited. Each manufacturer names settings and presets at their own discretion, with little information available publicly. Consistency of machine settings is important for correct interpretation of images in daily practice and is essential in yielding reproducible data for research. The aims of this paper, drawing from both a literature search and semistructured interviews with ultrasound-machine engineers and clinical experts in gynecological ultrasound, were: (1) to provide comprehensive background information on ultrasound physics and fibroid characteristics; (2) to present an overview of machine settings relevant to both two- and three-dimensional power Doppler, including power Doppler frequency, pulse repetition frequency, gain, wall-motion filter, acoustic power, persistence and signal rise; and (3) to provide a step-by-step tutorial on the optimal settings for vascular evaluation of uterine fibroids using power Doppler. The step-by-step tutorial comprises six steps to optimize the power Doppler signal, create a preset and acquire a reliable three-dimensional volume. This step-by-step tutorial should help research groups and clinicians to use power Doppler correctly and reproducibly in the evaluation of uterine fibroids. © 2022 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

3D power Doppler in uterine fibroids; influence of gain, cardiac cycle and off-line measurement techniques

This study answers the question of whether ultrasound machine settings and the cardiac cycle can influence 3D power Doppler (3D PD) indices in the evaluation of uterine fibroid vascularisation. These parameters were reported to affect the vascular indices and cause undesired variation. 3D PD ultrasound was performed using three different gain settings: a fixed predetermined gain (50 dB), a higher gain (65 dB) and an individualised subjectively most optimal gain. Two consecutive 3D PD sweeps were taken to evaluate the effect of the cardiac cycle. A predetermined most optimal fixed gain setting was not different from the individually most optimal chosen gain in vascular assessment of fibroids. A higher gain corresponded with a significantly higher vascular index (VI). Potential variation during the cardiac cycle does not disturb the VI in fibroids.

Determining the relationship between three-dimensional power Doppler data and true blood flow characteristics: an in-vitro flow phantom experiment

OBJECTIVES

Three-dimensional (3D) ultrasound can be used to acquire power Doppler data which can be quantified to give an objective impression about blood flow within a tissue or organ. Proprietary software can be used to calculate three indices of vascularity: vascularization index (VI), flow index (FI) and vascularization flow index (VFI). Although these indices appear to have a predictive value in the clinical setting and can be shown to vary between different patient populations and over time within the same population, their relationship with true in-vivo blood flow characteristics has not been established. The objective was to examine the effect of flow rate, vessel number, attenuation and erythrocyte density on these indices.

METHODS

A computer-driven flow phantom was used to continuously pump a nylon particle-based blood mimic (Orgasol(trade mark)) around a closed system through three different ultrasound test tanks. These tanks were designed specifically for these experiments and contained C-Flex(trade mark) tubing, in a variety of arrangements, encased in an agar-based tissue mimic. The test tanks were insonated with a modified 3D transvaginal 4-8-MHz ultrasound transducer and 3D power Doppler data were then acquired over a graduated series of flow rates, depths and blood mimic concentrations. Regression analysis was used to determine the resulting relationships.

RESULTS

The VI increased linearly with an increase in flow rate (P < 0.05), whereas the FI increased in a cubic manner with a more rapid initial increase (P < 0.05). The VI demonstrated a similar linear increase with an increase in the erythrocyte mimic density (P < 0.05), whereas the FI increased markedly with a small change in erythrocyte mimic density and then plateaued (P < 0.01). There was a significant reduction in each index as the distance between the transducer and vessel increased (P < 0.05). Patterns similar to those seen in relation to the change in flow rate were evident, with a more linear relationship between depth and the VI and VFI than between depth and the FI, although the FI remained relatively constant and was not significantly affected by distance from the transducer until a depth of 55 mm was reached. Although a positive linear relationship was seen between vessel number and VI and VFI (P < 0.05) the FI demonstrated a very different and complex, cubic relationship (P < 0.001), increasing linearly until a maximum of three vessels were present when it decreased, and no overall correlation was seen (P > 0.05).

CONCLUSIONS

The VI, FI and VFI are all significantly affected by volume flow, attenuation, vessel number and erythrocyte density, but in different ways. The VI and VFI seem to have a more predictable relationship, whereas the FI often demonstrates a more complex cubic relationship that is not always logical. Further work is required to establish the effect of other confounding parameters before valid conclusions may be made and a better understanding of 3D power Doppler ultrasound imaging achieved.

Evaluation of the effect of machine settings on quantitative three-dimensional power Doppler angiography: an in-vitro flow phantom experiment

OBJECTIVES

Three-dimensional (3D) ultrasound is being used increasingly to acquire and subsequently quantify power Doppler data within the clinical setting. One proprietary software package calculates three 3D vascular indices: the vascularization index (VI), the flow index (FI), and the vascularization flow index (VFI). Our aim was to evaluate how different settings affect the Doppler signal in terms of its quantification by these three indices within a 3D dataset.

METHODS

A computer-driven 'flow phantom' was used to continuously pump a nylon particle-based blood mimic (Orgasol(trade mark)) around a closed system through a C-flex(trade mark) tube embedded in an agar-based tissue mimic. The test tanks were insonated with a modified 3D transvaginal 4-8-MHz ultrasound transducer (V530D) and power Doppler data were acquired over a series of different settings. Each experiment involved the manipulation of just one Doppler setting in order to study it in isolation.

RESULTS

As expected, all of the power Doppler settings, when altered, were found to effect significant changes (P < 0.05) in the VI, FI and VFI. The gain and signal power had the greatest effect, producing no Doppler signals at the lowest settings and the highest recordable indices at the maximum settings. The pulse repetition frequency (PRF) was the next most influential setting but a Doppler signal was seen and measurable at all of the different settings. The other Doppler settings had a much less profound effect on the vascular indices, with subtle but significantly different measures across the full range of settings. The speed of data acquisition was also found to affect the vascular indices, all of which were reduced when the fast mode was used although the only significant effect was on the VFI.

CONCLUSIONS

The VI, FI and VFI are all affected significantly by variations in power Doppler settings and by the speed of acquisition. The gain and signal power have the greatest effect on the power Doppler signal, followed closely by the PRF. The other settings and speed of acquisition also influence the signal, but to a much lesser degree. It is essential to maintain Doppler settings if any meaningful comparisons are to be made within and between subjects.

Signal losses with real-time three-dimensional power Doppler imaging

Power Doppler imaging (PDI) has been shown to be influenced by the wall filter when assessing arterial stenoses. Real-time 3-D Doppler imaging may likely become a widespread practice in the near future, but how the wall filter could affect PDI during the cardiac cycle has not been investigated. The objective of the study was to demonstrate that the wall filter may produce unexpected major signal losses in real-time 3-D PDI. To test our hypothesis, we first validated binary images obtained from analytical simulations with in vitro PDI acquisitions performed in a tube under pulsatile flow conditions. We then simulated PDI images in the presence of a severe stenosis, considering physiological conditions by finite element modeling. Power Doppler imaging simulations revealed important signal losses within the lumen area at different instants of the flow cycle, and there was a very good concordance between measured and predicted PDI binary images in the tube. Our results show that the wall filter may induce severe PDI signal losses that could negatively influence the assessment of vascular stenosis. Clinicians should therefore be aware of this cause of signal loss to properly interpret power Doppler angiographic images.

A straightforward algorithm for the quantification of power Doppler signals

RATIONALE AND OBJECTIVES

To document a simple algorithm that quantifies power Doppler signals by computer-based statistical image analysis.

METHODS

Personal computer-based software was developed. Color pixels representing power Doppler signals were separated from the background gray pixels in power Doppler images converted in BMP format. To avoid misregistration during this segmentation, a threshold value is required to adequately distinguish between gray and color pixels. Equating the intensity of each color pixel to a Power Value (PV), a histogram of Power Values in power Doppler images was obtained.

RESULTS

The software was designed to allow users to adjust two thresholds (Difference Threshold and Intensity Threshold) by observing the segmentation results in real time. The software calculates the following indexes within any region of interest as defined by the operator: mean PV, peak PV, standard deviation of PV, percentage power Doppler area, and integrated PV per unit area.

CONCLUSIONS

Power Doppler signals can be objectively quantified using this straightforward algorithm. This simple and practical method can be applied to comparative or longitudinal studies upon vascularity or blood flow.