Quantitative assessment of cartilage morphology and trabecular bone structure with magnetic resonance imaging at 7 tesla: B0 field homogeneity, SNR, CNR, and reproducibility

Jin Zuo, Radu I. Bolbos, Suchandrima Banerjee, Xiaojuan Li, Sharmila Majumdar

The high signal-to-noise ratio (SNR) of ultra-high field magnetic resonance imaging (MRI) has great potential for improved soft tissue contrast in musculoskeletal imaging. Despite the superior SNR, ultra-high field MRI faces several challenges such as increased chemical shift differences, radiofrequency (RF) power deposition and main (B0) field and RF (B1) field inhomogeneities. So far, not many studies of quantitative musculoskeletal imaging have been performed to utilize this SNR advantage and to address these challenges. This study aims to examine the precision and the feasibility of quantitative measurements of cartilage morphology and trabecular bone structure of the knee at 7 Tesla as well as evaluate the B0 field homogeneity, SNR and contrast to-noise ratio (CNR) of the quantification.

High resolution sagittal spoiled gradient recalled (SPGR) images (for cartilage morphology measurement) and high resolution axial Fiesta-C images (for trabecular bone structure measurement) were acquired on healthy volunteers using GE 7 Tesla scanners. Volunteers were repositioned between the repeated scans to test the precision of the measurements. The homogeneity of the B0 field was evaluated with both axial and sagittal images (Figure 1). Comparable knee images were acquired at both 3 Tesla and 7 Tesla for SNR and CNR evaluation (Figure 2). The acquired SPGR images were segmented into five compartments for cartilage morphological measurements: later/medial femur condyle (LFC/MFC), later/medial tibia (LT/MT) and patella (PAT). The acquired Fiesta-C images were segmented into three compartments for trabecular bone structure analysis: femur, LFC and MFC. Cartilage volume and cartilage thickness, trabecular structure parameters (App. BV/TV, App. Tb.N., App. Tb.Sp. and App. Tb.Th.) were computed for each individual knee compartment and the whole knee joint. The coefficients of variation (CV) were less than 2% for cartilage thickness and volume. For trabecular bone structure measurement, the CVs were less than 3.5%. This excellent reproducibility indicated that quantitative assessment of cartilage morphology and trabecular bone structure can be achieved at ultra high field such as 7 Tesla accurately. Combined with superior signal-to-noise ratio, musculoskeletal imaging will have wide applications at ultra high field in the near future.