Abnormal placental vascular development leads to ischemic-hypoxia thereby causing fetal growth restriction, preterm labor, and spontaneous abortion. Multiecho Cartesian MRI can characterize placental hypoxia by quantifying R2*, but is susceptible to motion artifacts. We have developed a new free-breathing (FB) multiecho R2* quantification technique using 3D stack-of-radial imaging (Radial). In n=16 subjects as part of an IRB-approved study, we observed an R2* range of 5 – 30s-1 at 3 T using the new FB Radial technique in the placenta during early normal gestation. Our new technique and the measured normative range of R2* may improve management of pregnancies with placental ischemic-hypoxia.
Phantom Experiments: A bipolar multi-echo RF-spoiled GRE sequence using the golden-angle-ordered(7) 3D stack-of-radial trajectory was developed (Fig. 1a-c). A phantom with test tubes containing concentrations of 30mg/mL, 10mg/mL, 6mg/mL, 3.75mg/mL, 1.875mg/mL, and 0.9375mg/mL of a superparamagnetic iron oxide agent was scanned with Cartesian (8) and Radial sequences to assess R2* with two protocols (Protocols a and b in Table 1).
In Vivo Experiments: IRB approval and informed consent was obtained for this study. In vivo placenta scans were acquired feet-first supine in n = 16 healthy subjects during time frames of 14-18 weeks and 19-23 weeks of gestation on 3T MRI scanners (Skyra or Prisma, Siemens, Germany) using FB Radial and T2 HASTE(9). Of the 16 subjects, n=12 were scanned during both time frames, and n=4 were scanned once either during 14-18 weeks or 19-23 weeks (28 total scans) (Table 2). Protocols a and b were used in different scans to acquire images (Table 1). Initially subjects were scanned with Protocol a to get a baseline for expected R2* values. After determining that the R2* range was relatively low (5 – 30s-1), more echoes were added to improve fitting for this range. A body array coil and spine coil were used for all acquisitions.
Reconstruction: Radial reconstruction and post-processing was performed offline with MATLAB R2013b (MathWorks, Natwick, MA, USA) using gradient correction (10, 11), 3D gridding, a linear density compensation function, and adaptive coil combination(12). A 7-peak fat signal model (13) with single effective R2* per voxel was used (14–16). The Cartesian multiecho and T2 HASTE sequences were reconstructed on the scanner.
Analysis: T2 HASTE and FB Radial placental images were viewed in OsiriX 6.0 (Pixmeo, Switzerland). ROIs were drawn on the slice containing the majority of the placenta using the T2 HASTE images. The ROIs were applied to the R2* maps in anatomically corresponding regions (Fig. 2) to measure R2*. Linear correlation analysis was performed for R2* as a function of gestational age (GA) in weeks.
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