Estimation of fractional moving blood volume in fetal lung using Power Doppler ultrasound, methodological aspects

Perfusion estimation of the fetal lung is an important predictor of its maturity and function after birth. Ultrasound (US) power Doppler has previously been used to assess the perfusion of the fetal lung, based on the mean pixel intensity (MPI) over a region-of-interest (ROI). The drawback is that the MPI is not only dependent on the amount of flowing blood, but also depth, gain and attenuation in overlying tissue layers. In this study, power Doppler images have been analysed according to a previously published method that attempts to compensate for such unwanted variations, resulting in a measure termed fractional moving blood volume (FMBV). A total of 29 singleton pregnancies with normally grown fetuses were evaluated after 35 weeks of gestation. For reliable interpatient comparisons, we found it always necessary to use a well-defined section of the fetal lung. Therefore, all scans were performed in a transverse plane of the fetal thorax with a four-chamber view of the heart, through the intercostal space. ROIs in 12 left and 17 right fetal lungs were defined (the fetal lung closest to the transducer was always examined). No differences in estimated FMBV or MPI were found between left and right lungs. Similarly, there was no significant difference between images acquired during systole and diastole. FMBV is compensated for depth and, also, for other sources of power variation, as reflected in the coefficient of variation: 0.14 for FMBV and 0.24 for MPI. A scan on a test phantom reveals that the power in dB is linearly related to velocity over a limited range, suggesting that FMBV might be useful for discriminating between normal and decreased fetal lung perfusion.