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Measurement of Fetal Volume Blood Flow Accurate measurement of volumes of flow in peripheral vessels is of clinical interest because it reflects differential perfusion of organs in the fetus, both during normal growth and in response to adverse circumstances In animal models, flow of blood can be measured with a high degree of accuracy with electromagnetic167 or transit-time flowmeters.168 Because these methods require an invasive approach, they cannot currently be applied in studies of human fetuses Noninvasive methods to measure the volume of flow in the aorta are confounded by methodologic difficulties, mostly in relation to accurate measurements of aortic diameter, an issue that has still not been resolved, despite improvements in the resolution of ultrasound.124,169 Human fetal blood flow has been measured with a high degree of accuracy using cardiac magnetic resonance In this chapter we will concentrate on the much more extensively reported echo-Doppler techniques The method traditionally used to estimate volume of flow in a vessel is from the product of the velocity time integral of the Doppler signal in the vessel, the heart rate, and a separate estimation of the area of the lumen from static measurements of its diameter The inherent error in this method is estimated to be as large as one-fifth when measuring fetal arteries.124 This error mostly results from limits of resolution of the diameter, but failure to account for changes in pulsatile flow may account for almost one-tenth of the difference in volume of flow compared with static assessment.170 Estimations of venous flow have been reported,171 but the potential error is further compounded by the assumption that the vessel under examination is circular Simultaneous measurement of the diameter of the pulsatile vessel and the mean velocity of flow allows an estimate of flow volume in the descending aorta of the healthy fetus.169 The volume of aortic flow, measured in this way, has shown a linear increase with increasing gestation, being estimated at 225 mL/min per kg body weight in fetuses of 250 days Estimation of Perfusion Using Power Doppler Methods using newer Doppler modalities have been evaluated as surrogate estimates of perfusion Power Doppler was initially investigated as a method to estimate perfusion in the adult, but an important methodologic problem was the formation of rouleaux that artificially elevated the maximal value for power Doppler amplitude Fortunately, studies in the fetus confirm that fetal blood does not form rouleaux to any significant degree, permitting a comparison of perfusion of different organs Studies using quantification of power Doppler have suggested an increase in power Doppler signals from the placenta, lungs, spleen, liver, and kidney up to 34 weeks of gestation, followed by a decrease in all but the spleen, which remains constant Abnormalities in the ratio of volumes in the brain and lungs were seen in high-risk pregnancies, but methodologic problems using this technique may limit the conclusions drawn by such studies Mean pixel intensity was the original method developed to assess perfusion over a region of interest but was dependent not only on the volume of flow of blood but also depth, gain, and attenuation in overlying layers of tissue This was superseded by an alternative method, the fractional moving volume, that attempted to compensate for these confounding variables by assigning a value of 100% amplitude to a large vessel within the region of interest and comparing the amplitude in smaller vessels, such as those supplying the fetal kidney.172–174 Pitfalls limiting reliability of this method include the depth of the imaged organ from the transducer Phantom studies describe that the power is linearly related to velocity over a limited range, suggesting it may be useful to discriminate between normal and decreased fetal perfusion in organs such as the lung Validation of measurements with power Doppler ultrasound has been performed in sheep using radioactive microspheres, showing good correlations in the adrenal gland.175 It was proposed that power Doppler methods may be improved by combining them with 3D standardized techniques to identify a reproducible anatomic plane for measurement,176 and this technology is now available using virtual organ computer-aided analysis (VOCAL) It is now possible to acquire a 3D power Doppler image of the organ of interest, and VOCAL software in the ultrasound machine can calculate a vascularization index representing the percentage of colored over total voxels in the organ to assess its vascularity (Fig 6.18).177 FIG 6.18 Power Doppler and three-dimensional (3D) standardized techniques have enabled a method of volume flow assessment using virtual organ computer-aided analysis (VOCAL) This figure shows a 3D power Doppler image of a sacrococcygeal teratoma and VOCAL software in the ultrasound machine can calculate a vascularization index (VI%) representing the percentage of colored over total voxels in the organ to assess its vascularity 3D and 4D Echocardiography and Quantification of Volume and Ejection Fraction All imaging techniques depend on excellent quality of imaging, so 3D volume estimation is only as good as the cross-sectional image, and quality may be compromised by fetal movements and maternal respiration Newer technology with shorter acquisition times permit live real-time 3D imaging and the rapid acquisition of a 3D volume set that can be manipulated later offline for expert analysis.178 More recently, physiologic information on stroke volume, cardiac output, and ejection fraction has been reported using 3D inversion mode sonography and 4D spatiotemporal image correlation using the VOCAL modalities.179,180