Fractional moving blood volume ( FMBV ) concept can be applied to 3D power Doppler quantification

We read with great interest a recent study in UOG that concluded that fractional moving blood volume (FMBV) cannot be applied to three-dimensional power Doppler (3DPD)1. In a region known to have 100% moving blood, the authors observed no differences in vascularization index (VI), flow index (FI) or vascularization flow index (VFI) when gain level was changed, which is the basis on which FMBV is applied to standardize vascularity measurements on two-dimensional imaging1. However, we believe that this study had an important weakness: the authors evaluated FMBV using only a low pulse repetition frequency (PRF): 0.9 KHz. Low PRFs increase the sensitivity to detect low blood velocity, but they also reduce the capacity of distinguishing between higher blood velocities, because any blood velocity over a certain limit will be quantified as 100%, regardless of the true velocity. The average value of blood flow velocity in the umbilical artery is probably higher than the limit established by a PRF of 0.9 KHz. Therefore, one might expect that all color values inside the brightest region of the umbilical artery would be 100%, regardless of gain, because the velocity would be far higher than the maximum threshold. Hence, we believe that the authors' conclusions about the inapplicability of the FMBV technique to 3DPD could be premature. We performed a small study to investigate this hypothesis. Pregnant women aged > 18 years, with singleton pregnancy, placental insertion at the anterior uterine wall and no known diseases, who were undergoing a regular second-trimester ultrasound examination were invited to participate in this study. The research protocol was approved by the local ethics committee, and all women signed informed consent. 3DPD datasets were acquired at the insertion of the umbilical cord in the placenta, using fixed settings, except for PRF and gain: power, 100%; wall motion filter, low1; freq, mid; flow res, mid1; smooth, 5/6; ensemble, 12; line den, 7; power Doppler map, 5 (gently color, on); balance, G ≥ 200; artifact, on; L. filter, 2. We acquired four 3DPD datasets, at gains of −3, –6, –9 and −12, using PRFs of 0.9 KHz and 1.8 KHz, without moving the transducer between acquisitions. We assessed 3DPD indices (VI, FI and VFI) from small spherical samples inside the umbilical artery, close to its insertion into the placenta, evaluating a region of 100% moving blood. We evaluated and analyzed 10 pregnant women aged between 19 and 28 years. The VI from the smallest spherical samples of all acquired 3DPD datasets was 100%; therefore, VFI was always equal to FI. VFI and FI increased linearly with increasing gain using each PRF, but more so using PRF = 1.8 KHz than with PRF = 0.9 KHz (R2 = 0.64 vs 0.46, Figure 1, Table 1). Although 3DPD quantification has been investigated in hundreds of studies2, its reproducibility and clinical utility are limited3, 4. The FMBV technique applied to 3DPD emerged as an attempt to standardize this method, to compensate for the effect of attenuation1. In our study, we observed that, when using the higher of the two PRF levels, we obtained well-distributed graphic curves. The clear correlation of FI and VFI with gain show that FMBV can be applied to 3DPD. Therefore, we conclude that, using a suitable PRF, the FMBV concept can be applied as an internal standardization tool for 3DPD. One should bear in mind that other promising techniques to standardize 3DPD quantification (e.g. volumetric pulsatility index) are currently being studied5-8. C. A. Soares†‡, A. H. Miyague†‡, F. M. Filho†‡, N. Raine-Fenning§ and W. P. Martins*†‡ †Department of Obstetrics and Gynecology, Medical School of Ribeirao Preto, University of Sao Paulo (DGO-FMRP-USP), Av. Bandeirantes, 3900, 8 andar, Ribeirao Preto, Brazil; ‡Ultrasonography and Retraining Medical School of Ribeirao Preto (EURP), Ribeirao Preto, Brazil CEP: 14049–900; §Division of Obstetrics & Gynaecology, School of Human Development, University of Nottingham, Nottingham, UK