Evaluation of a motion-robust 2D chemical shift-encoded technique for R2* and field map quantification in ferumoxytol-enhanced MRI of the placenta in pregnant rhesus macaques

Abstract

Background: 3D chemical shift-encoded (CSE)-MRI techniques enable assessment of ferumoxytol concentration but are unreliable in the presence of motion.

Purpose: To evaluate a motion-robust 2D-sequential CSE-MRI for R2* and B0 mapping in ferumoxytol-enhanced MRI of the placenta.

Study type: Prospective.

Animal model: Pregnant rhesus macaques.

Field strength/sequence: 3.0T/CSE-MRI.

Assessment: 2D-sequential CSE-MRI was compared with 3D respiratory-gated CSE-MRI in placental imaging of 11 anesthetized animals at multiple timepoints before and after ferumoxytol administration, and in ferumoxytol phantoms (0 μg/mL-440 μg/mL). Motion artifacts of CSE-MRI in 10 pregnant women without ferumoxytol administration were assessed retrospectively by three blinded readers (4-point Likert scale). The repeatability of CSE-MRI in seven pregnant women was also prospectively studied.

Statistical tests: Placental R2* and boundary B0 field measurements (ΔB0) were compared between 2D-sequential and 3D respiratory-gated CSE-MRI using linear regression and Bland-Altman analysis.

Results: In phantoms, a slope of 0.94 (r² = 0.99, concordance correlation coefficient ρ = 0.99), and bias of -4.8 s^-1 (limit of agreement [LOA], -41.4 s^-1 , +31.8 s^-1 ) in R2*, and a slope of 1.07 (r² = 1.00, ρ = 0.99) and bias of 11.4 Hz (LOA -12.0 Hz, +34.8 Hz) in ΔB0 were obtained in 2D CSE-MRI compared with 3D CSE-MRI for reference R2* ≤390 s^-1 . In animals, a slope of 0.92 (r² = 0.97, ρ = 0.98) and bias of -2.2 s^-1 (LOA -55.6 s^-1 , +51.3 s^-1 ) in R2*, and a slope of 1.05 (r² = 0.95, ρ = 0.97) and bias of 0.4 Hz (LOA -9.0 Hz, +9.7 Hz) in ΔB0 were obtained. In humans, motion-impaired R2* maps in 3D CSE-MRI (Reader 1: 1.8 ± 0.6, Reader 2: 1.3 ± 0.7, Reader 3: 1.9 ± 0.6), while 2D CSE-MRI was motion-free (Reader 1: 2.9 ± 0.3, Reader 2: 3.0 ± 0, Reader 3: 3.0 ± 0). A mean difference of 0.66 s^-1 and coefficient of repeatability of 9.48 s^-1 for placental R2* were observed in the repeated 2D CSE-MRI.

Data conclusion: 2D-sequential CSE-MRI provides accurate R2* and B0 measurements in ferumoxytol-enhanced placental MRI of animals in the presence of respiratory motion, and motion-robustness in human placental imaging.

Level of evidence: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:580-592.

Keywords: B0 field; R2*; chemical shift-encoded; ferumoxytol; motion-robust; placenta.

Figure 1.

T1-weighted Structural reference image (a), R2* map (b) and B0 field map (c) of a single slice in 3D CSE-MRI acquisition of four vials in one batch of the phantom scans, R2* (e) and ΔB0 (f) of all eight vials with varying ferumoxytol concentrations measured in the four CSE-MRI acquisition. An example of boundary B0 field measurement is indicated by two yellow dashed oval ROIs in the zoomed B0 field map (d).

Figure 2.

Results from phantom scans: linear regression analysis (a1, f1) and Bland-Altman analysis (a2-f2) of R2* and ΔB0 measured in 2D CSE-MRI (a, d), 2D CSE-MRI high SNR (b, e), and 3D CSE-MRI (c, f) in comparison with the reference measured in 3D CSE-MRI high SNR. Small bias on both R2* and ΔB0 measurements at relatively low R2* and ΔB0 values indicates good correlation of measurements in the three CSE-MRI acquisitions and the reference CSE-MRI acquisition, while the large bias at high R2* and ΔB0 values indicated limited measurable range of each CSE-MRI acquisition. Particularly in 2D CSE-MRI, high linear correlation and small bias on both R2* and ΔB0 were observed for reference R2* values below 390.0 s⁻¹ (bold markers, fonts, and lines in a1, a2, d1, d2).

Figure 3.

Representative anatomic images, R2* maps, and B0 field maps from 2D and 3D CSE-MRI acquisitions in a pregnant rhesus macaque (Rhesus #4) at scans before (Pre) and immediately after (Post) ferumoxytol administration. Two placental discs and the uterus cavity are delineated by blue lines and orange lines, respectively. The amniotic fluid is indicated with a green arrow on the anatomic image.

Figure 4.

Linear regression analysis (a, c, e) and Bland-Altman analysis (b, d, f) of R2*, log(R2*) and ΔB0 measured in 2D CSE-MRI in comparison with the measurements in 3D CSE-MRI of anterior placental disc, posterior placental disc and both together in animals. R2* and ΔB0 measurements are consistent in 2D and 3D CSE-MRI acquisitions.

Figure 5.

Anatomic images, R2* maps, and B0 field maps of 2D and 3D CSE-MRI in two pregnant women. Motion artifacts are observed in the R2* map and B0 field map in 3D CSE-MRI of Subject #1, while no obvious artifacts are observed by the radiologist in 2D CSE-MRI. The placental disc of this subject (delineated by blue lines) was located in a different location of the maternal pelvic for 2D vs 3D CSE-MRI (see yellow arrows), suggesting the presence of fetal motion during this exam. No obvious artifacts or obvious movement of the placenta are observed by the radiologist in 3D or 2D CSE-MRI in Subject #10.