Quantification of power Doppler and the index 'fractional moving blood volume' (FMBV)

Abstract

Complete fetal assessment should ideally encompass both structural and functional components. Accurate assessment of regional blood flow, in particular impaired or redistributed flow, is necessary for functional fetal assessment. Various methods of evaluating perfusion in the adult have been well summarized by Jansson et al.1, but the mainstay in the fetus has been Doppler ultrasound2. Doppler ultrasound allows the evaluation of moving particles within the fetal circulation but has not yet been proved useful for direct measurement of regional perfusion. It would appear simple to use quantified Doppler to evaluate flow volumes in single large vessels, by combining accurate gray-scale measurement of vessel diameter with spectral Doppler calculation of the speed of movement of the blood cells within that vessel3-5. However, even for isolated large vessels, inaccuracies in both the gray-scale and the Doppler component have so far precluded accurate volume flow measurements, with the pulsatile expansion of vessels such as the fetal aorta being in the range of 10–15% of the minimum diameter3. This becomes significantly more complex when extrapolated to tissue perfusion, where blood vessels are tortuous and multidirectional, prohibiting accurate vascular diameter measurement or angle of insonation correction for the Doppler frequency shift. For this reason fetal vascular assessment has so far relied upon the measurement of indices of resistance to blood flow in isolated blood vessels, including the systolic/diastolic ratio, the resistance index and the pulsatility index. The advantage of these measurements has been that an alteration in the angle of insonation will result in an equal increment in both the numerator and the denominator, and thus the ratio remains consistent. Based upon these ratios, comments on tissue perfusion and vascular compliance may be made, as an extrapolation from the perceived resistance to flow; the most studied cases are the progressive changes in the indices of the umbilical and middle cerebral arteries and ductus venosus in fetal growth restriction6. A newer option in the search for a tool to evaluate regional perfusion is power Doppler. All machines that produce color Doppler through autocorrelation inherently have power Doppler capability, as both power and spectral modes are derived from the same information7. Whereas spectral (velocity-based) Doppler examines and displays frequency shifts, power Doppler instead displays details of the intensity or amplitude of these frequency shifts above a certain threshold value (the noise filter setting). While the frequency of the Doppler shift from a given particle (such as a red blood cell) is related to the cosine of the angle of insonation, the amplitude of the scatter is not. This means that power Doppler imaging is (essentially) angle-independent8. Background noise is more easily filtered out, as it is always of low amplitude9, allowing the display dynamic range to be significantly increased10, with higher gain levels9. These combined attributes make power Doppler more suited to evaluation of multidirectional flow and hence perfusion. It is for this reason that various research groups are investigating power Doppler as a tool for quantification rather than observation. The development of these techniques have been well documented11-18. Power Doppler is ideally suited to measurement of values that approximate to perfusion, though are not, by definition, perfusion measurements themselves19. Instead it gives a measurement of the number of scatterers (predominantly red blood cells) and their amplitude within tissue. In-vitro experiments have shown power Doppler intensity to correlate with fluid flow8, 20, 21. Regional vascularity as measured with power Doppler may be assessed by counting the number of color pixels and the intensity of color within these pixels in a given region on power flow mapping. This may be performed by software within the ultrasound machine itself, or clips or images may be exported from the ultrasound machine for offline digital analysis. The most commonly used techniques are to compare the density of colored pixels (percent of total area), the number of visible vessels (per unit area) in a region of interest, or the summed pixel values and compare with normal tissue1. One of the concerns with quantification of power Doppler images is that while numerical information can be ascribed to a given image or clip in this fashion, outside of the examination in question measurements of power Doppler may have little meaning, as they are based on purely arbitrary indices. Any factor that alters the color pixel count could affect vascularity assessment, including effects of attenuation (maternal habitus, fetal depth, intervening tissue type) and machine settings (aperture size, gain, scale, filter and persistence). Though there is time-gain-compensation in power mode Doppler, it is unlikely to be effective over the whole field of