Discriminative imaging of maternal and fetal blood flow within the placenta using ultrafast ultrasound

Abstract

Being able to map accurately placental blood flow in clinics could have major implications in the diagnosis and follow-up of pregnancy complications such as intrauterine growth restriction (IUGR). Moreover, the impact of such an imaging modality for a better diagnosis of placental dysfunction would require to solve the unsolved problem of discriminating the strongly intricated maternal and fetal vascular networks. However, no current imaging modality allows both to achieve sufficient sensitivity and selectivity to tell these entangled flows apart. Although ultrasound imaging would be the clinical modality of choice for such a problem, conventional Doppler echography both lacks of sensibility to detect and map the placenta microvascularization and a concept to discriminate both entangled flows. In this work, we propose to use an ultrafast Doppler imaging approach both to map with an enhanced sensitivity the small vessels of the placenta (~100 μm) and to assess the variation of the Doppler frequency simultaneously in all pixels of the image within a cardiac cycle. This approach is evaluated in vivo in the placenta of pregnant rabbits: By studying the local flow pulsatility pixel per pixel, it becomes possible to separate maternal and fetal blood in 2D from their pulsatile behavior. Significance Statement: The in vivo ability to image and discriminate maternal and fetal blood flow within the placenta is an unsolved problem which could improve the diagnosis of pregnancy complications such as intrauterine growth restriction or preeclampsia. To date, no imaging modality has both sufficient sensitivity and selectivity to discriminate these intimately entangled flows. We demonstrate that Ultrafast Doppler ultrasound method with a frame rate 100x faster than conventional imaging solves this issue. It permits the mapping of small vessels of the placenta (~100 μm) in 2D with an enhanced sensitivity. By assessing pixel-per-pixel pulsatility within single cardiac cycles, it achieves maternal and fetal blood flow discrimination.

Figure 1. Placement of the ultrasound probe on the rabbit placenta.

Modified with permission from Fischer, B. et al. (2012). Rabbit as a reproductive model for human health. Reproduction 144, 1-10.

Figure 2. Ultrafast Doppler Sequence.

(a) Each ultrafast ultrasound image is composed of three tilted plane wave images. (b) The placenta is insonified with ultrafast imaging during 6 s. (c) From each spatial pixel of the 2D field of view, we can extract a 3000 points temporal signal with tissue and blood signal. (d) After wall filtering the blood signal can be extracted. (e) At each time T and for each spatial pixel, UFD acquisition permits to compute the PW Doppler spectrum.

Figure 3. Ultrafast imaging of the placenta.

(a,b) Respectively UDF and conventional Power Doppler image of the placenta containing maternal and fetal blood. (c–e) UFD PW Doppler of respectively a non pulsatile vessel, a pulsatile vessel and noise within one spatial pixel. (f–h) Averaged UFD PW Doppler of respectively a non pulsatile vessel, a pulsatile vessel and noise within one spatial pixel.

Figure 4. TVCF of pulsatile and non pulsatile flows.

The green dotted line represents the TVCF of the noise. The black point (+/− standard deviation) represents the distribution of the TVCF of non pulsatile flows. The blue point (+/− standard deviation) represents the distribution of the TVCF of pulsatile flows.

Figure 5. Discrimination of maternal and fetal flows.

(a) Results of the discrimination of fetal blood (blue) and maternal blood (red). (b) Maternal blood computed from the difference between the UFD image before and after the maternal aortic clamping. (c) Maternal blood computed from the pulsatility discrimination algorithm. (d) Fetal blood computed from the pulsatility discrimination algorithm. (e) UFD Power Doppler image of fetal blood acquired after the aortic clamping. (f) UFD PW Doppler extracted from the vessel circled with a blue line on (c).