Placental perfusion imaging using velocity-selective arterial spin labeling

Abstract

Purpose: The placenta remains the least understood human organ in large part because of the lack of non-invasive tools currently available to examine placental function in vivo. This study investigates the feasibility of using velocity-selective arterial spin labeling (VSASL) to assess placental perfusion.

Methods: In placental perfusion imaging, VSASL was compared with pseudocontinuous ASL (PCASL), which is currently the standard technique in brain ASL. Reproducibility of placental VSASL was evaluated using two repeated scans within the same imaging session. Inflow-dependence of placental VSASL was investigated by modulating VSASL signal using maternal inhalation of 100% oxygen and variation of cutoff velocity. All experiments were performed in healthy pregnant volunteers at 1.5 T.

Results: Apparent placental perfusion measured using PCASL with two different labeling locations was only 16% and 9% of that of VSASL (n=7, p<0.01 for both). Placental VSASL was highly reproducible based on within-subject coefficient of variation of 3.5%, repeatability of 19.7 ml/100 g/min, and intraclass correlation coefficient of 0.97 (n=14). Placental VSASL was also found to be dependent on blood inflow given that the absolute change in apparent placental perfusion with maternal hyperoxia was significantly larger than that of two repeated scans under normoxia (n=7, p<0.01) and there was a significant difference in apparent placental perfusion between different cutoff velocities (n=6, p<0.01).

Conclusion: This study demonstrates the feasibility of non-invasive placental perfusion imaging using VSASL and lays the groundwork for acquiring placental perfusion images in pregnancies at high risk where placental function is impaired.

Keywords: ASL; VSASL; arterial spin labeling; placental perfusion; velocity-selective arterial spin labeling.

Figure 1

Apparent placental perfusion measured using VSASL and PCASL with labeling plane above and below the imaging slab. VSASL showed dramatically higher perfusion measurement than either of PCASL methods in all subjects. With PCASL, upper labeling showed higher placental perfusion signal in the placenta that was located relatively lower in the uterus (Subject 1–4) and lower labeling showed higher placental perfusion signal in the placenta that was located relatively higher in the uterus (Subject 5–7).

Figure 2

Comparison of placental ASL images acquired using different labeling methods as well as localizer and anatomical images in Subject 4 and 6 from Figure 1. As shown in the localizer, the placenta was located relatively lower in Subject 4 and higher in Subject 6 inside the uterus. The common imaging slab of VSASL and PCASL and the two different locations of the labeling plane of PCASL are shown in the localizer images. In the other images, the placenta is delineated with the dotted line. PCASL signal in the placenta was higher with upper labeling in Subject 4 and with lower labeling in Subject 6, compared to the other location of the labeling plane. For both subjects, the average VSASL signal in the placenta was substantially higher than either PCASL scheme.

Figure 3

Correlation (a) and Bland-Altman (b) plots of two repeated measurements of placental perfusion measured using VSASL back to back in the same scan session. In b, solid and dotted lines represent the mean difference and 95% limits of agreement respectively.

Figure 4

Apparent placental perfusion in response to maternal hyperoxia induced by 100% oxygen inhalation, compared to the repeated measurements of placental perfusion before oxygen inhalation. During hyperoxia, apparent placental perfusion decreased in Subject 1, 2, and 5, and increased in the other subjects.

Figure 5

Placental VSASL images of the reproducibility test (baseline, repeat) and hyperoxia scan acquired in Subject 4 and 7 from Figure 4. The placenta is delineated with the dotted line. While VSASL signal in the placenta was similar between the two scans before oxygen inhalation for both subjects, VSASL under hyperoxia demonstrated reduced signal in Subject 2 and increased signal in Subject 7 compared to their VSASL signal under normoxia.

Figure 6

Apparent placental perfusion measured using VSASL with cutoff velocity (Vcut) of 2 cm/s, 4 cm/s, and infinity (i.e.no labeling). Apparent placental perfusion was reduced significantly as cutoff velocity increased.

Figure 7

Temporal standard deviation (SD) of 2D VSASL labeled signal time series in an ROI drawn within the placenta, with cutoff velocity of 2 cm/s, 4cm/s and infinity. No significant difference was present between the three cutoff velocities, confirming that contribution of subject motion to VSASL signal was not significant.