COMPARATIVE STUDY: QDESS VERSUS RAFO-4 PERFORMANCE IN 5-MINUTE, SIMULTANEOUS, RELIABLE 3D T2 MAPPING AND MORPHOLOGICAL MR IMAGING

INTRODUCTION
Cartilage T2 is a non-invasive MRI biomarker for KOA as it is sensitive to the underlying collagen hydration/organization. Cartilage microstructural changes seen in early KOA result in elevated T2. Cartilage T2 maps could be used in DMOAD clinical trials.
Quantitative DESS (qDESS) simultaneously acquires 3D, morphological whole knee images and quantitative T2 maps in ∼5 minutes. Recently, we developed RaFo-4 balanced Steady State Free Precession (RaFo-4 bSSFP) that also has the potential to simultaneously acquire 3D, morphological whole knee images with high SNR efficiency and quantitative cartilage T2 maps in ∼5 minutes. RaFo-4 uses machine learning (Random Forest) to estimate voxel-level cartilage T2 from bSSFP images. In this preliminary study, we compared qDESS and RaFo-4 bSSFP in morphological imaging and cartilage T2 mapping.
OBJECTIVE
1) Which technique (qDESS or RaFo-4 bSSFP) has better test-retest repeatability of cartilage T2 maps? 2) Which technique gives higher quality morphological images, as quantified using SNR of femoral, patellar, and tibial cartilage and CNR of cartilage-muscle, cartilage-synovial fluid, and synovial fluid-muscle?
METHODS
10 healthy volunteers (HVs: 7F, 3M, 20-40 age range) were scanned on a 3T Siemens Verio (Erlangen, Germany) using an 8-channel knee coil with ethics approval. Test-retest 3D (80 slices) sagittal knee images were acquired using qDESS (water excitation, 20° flip angle, 21.77 ms TR, 6 ms TE, 364 Hz/Px receiver bandwidth, 0 dummy scans per volume) and bSSFP (water excitation, 22° flip angle, 8.6 ms TR, 4.3 ms TE, 364 Hz/Px receiver bandwidth, 0 dummy scans per volume) for both knees of each HV with knee repositioning. qDESS and bSSFP were resolution- (0.4 × 0.4 × 1.5 mm3 voxel volume, 150 × 150 × 120 mm3 field of view) and scan time-matched (5:05 min. for qDESS and 5:04 min for bSSFP). 4 separate phase-cycled bSSFP images were acquired with phase cycling increments [0°, 90°, 180°, 270°]. Parallel imaging was used (GRAPPA R=2 for bSSFP and qDESS with 24 reference lines; 6/8th phase/slice partial Fourier for bSSFP). Cartilage in qDESS images was segmented using DOSMA and those segmentation masks were used on the bSSFP images. Test-retest repeatability was calculated using the ICC and coefficient of variation (CoV) after removing outlier T2 estimates (T2 < 20 ms, T2 > 90 ms). The percentage of outlier estimates was also calculated. For quantitatively evaluating morphological image quality, SNR and CNR were calculated from the Root Sum of Squares (RSOS) of the two qDESS echos and four phase-cycled bSSFP images.
RESULTS
1) In Fig1, RaFo-4 preserves cartilage T2 spatial variations seen in qDESS T2 maps (red and pink arrows indicate regions of low and high T2 values, respectively) and it produces visually smoother maps. While 10-20% of qDESS estimates are outliers, RaFo-4 estimates no outliers, a unique feature of the algorithm. RaFo-4 bSSFP shows good to excellent test-retest repeatability without estimating any outliers (ICC = 0.74-0.91, CoV = 2.01-3.58%) whereas qDESS shows excellent test-retest repeatability after removing outliers (ICC = 0.87-0.97, CoV = 1.48-1.61%). 2) RaFo-4 bSSFP has consistently higher SNRs and higher/comparable CNRs compared to qDESS (Tab1).
CONCLUSION
RaFo-4 bSSFP is a promising 5-minute alternative to qDESS as it provides more reliable cartilage T2 maps and better morphological image quality. Future work includes testing both techniques on a larger cohort of HVs and early KOA patients and comparing performance.