Free-breathing R2* Characterization of the Placenta During Normal Early Gestation Using a Multiecho 3D Stack-of-Radial Technique
Tess Armstrong1,2, Dapeng Liu1, Thomas Martin1,2, Alto Stemmer3, Yutaka Natsuaki4, Sherin U. Devaskar5, Carla Janzen6, Teresa Chanlaw5, Rinat Masamed1, Daniel Margolis7, Kyunghyun Sung1,2, and Holden H. Wu1,2

1Radiological Sciences, University of California Los Angeles, Los Angeles, CA, United States, 2Physics and Biology in Medicine, University of California Los Angeles, Los Angeles, CA, United States, 3Siemens Healthcare GmbH, Erlangen, Germany, 4Siemens Healthcare, Los Angeles, CA, United States, 5Pediatrics, David Geffen School of Medicine at UCLA, Los Angeles, CA, United States, 6Obstetrics and Gynecology, David Geffen School of Medicine at UCLA, Los Angeles, CA, United States, 7Radiology, Weill Cornell Medical College, New York, NY, United States


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.


Placental ischemic-hypoxia, a condition caused by abnormal placental vascular development, leads to negative outcomes such as fetal growth restriction, preterm labor, and spontaneous abortion1,2. Detection of placental hypoxia during early gestation can allow earlier reversal by interventions. Measurement of placental hypoxia can be performed by chorionic villous sampling, however, this is an invasive procedure with potential complications3. Alternatively, multiecho gradient echo MRI techniques can non-invasively characterize hypoxia by quantifying transverse relaxation rate (R2* = 1/T2*), which is related to tissue oxygen content4–6. However, conventional MRI techniques using Cartesian sampling are susceptible to maternal and fetal motion-induced coherent aliasing artifacts and are often performed with a breath-hold (BH)4,6. In addition, there is very limited understanding of placental R2* in both normal and abnormal early gestation. In this work, we propose and evaluate a novel free-breathing (FB) multiecho 3D stack-of-radial (FB Radial) technique for quantifying R2* in the placenta at 3 T and report results for normal early gestation.


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.


Linear correlation analysis in the R2* phantom showed a significant correlation for Protocol a (ρ = 0.9989, P << 0.05) and Protocol b (ρ = 0.9984, P << 0.05) in mean R2* between the proposed Radial and the Cartesian techniques for the range of expected placenta R2* values (Fig. 3). This supports the use of the Radial technique for placental R2* quantification. In vivo placenta R2* maps were successfully obtained (Fig. 2). Placenta R2* values are reported in Table 2. The range of placental R2* values in early normal gestation was 5–30 s-1 at 3T. A trend towards positive correlation was observed between placental R2* and GA (Fig. 3).


Similar to our observed trend, a positive correlation between R2* and GA has also been observed in a previous study in the placenta at 1.5T for later GA of 25-30 weeks (4). With further research, we may find significant correlation between placental R2* and GA throughout gestation and characterize differences between the normal and hypoxic placenta using our FB Radial technique.


We have demonstrated successful R2* mapping with a range of 5–30s-1 at 3T using the new FB Radial technique in the placenta during early normal gestation. Our new technique and the measured normative range of R2* may be an important foundation to further study and improve management of abnormal pregnancies with placental ischemic-hypoxia.


This work acknowledges the use of the ISMRM Fat-Water Toolbox (http://ismrm.org/workshops/FatWater12/data.htm). This work was funded by the grant NICHD U01-HD087221. The authors would like to thank Irish del Rosario, Shams Rashid, and Xinran Zhong for their help with this project.


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Figure 1: (a-c) 3D stack-of-radial technique. (a) Radial readouts are acquired along kz before rotating. (b) Radial readouts are rotated continually by the golden angle (θG). (c) Multiple echoes are acquired every TR using a bipolar readout gradient. (d) Reconstruction pipeline for the 3D stack-of-radial data.

Table 1: Representative sequence parameters for the in vivo placenta scans.

Figure 2: In vivo placenta FB Radial source images at representative echo times (TEs), the axial T2 HASTE, PDFF and R2* maps with representative ROIs.

Table 2: R2* values and gestational age (GA) for the in vivo placenta scans for each volunteer and protocol. aFB Radial: Protocol a. bFB Radial: Protocol b.

Figure 3: (a) Linear correlation plots for the mean Radial R2* versus Cartesian R2* in a R2* phantom using Protocol a (ρ = 0.9989, P << 0.05) and Protocol b (ρ = 0.9984, P << 0.05). (b) The linear correlation for placental R2* versus gestational age (GA) in weeks using the FB Radial technique (ρ = 0.2339, P = 0.2309).

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)