Morphologic MRI can detect only macroscopic changes associated with advanced cartilage lesions. While quantitative MR parameters can enable early diagnosis, scan time limitations, inconsistency between measurements and unsettled diagnostic interpretation have prevented extensive clinical validation, restricting their use to research settings. Magnetic resonance fingerprinting provides robust and reliable reconstruction of multiple parameters from a single, fast MR acquisition, which could enable to translate quantitative imaging into clinical practice.
Purpose
MRI is a powerful tool for noninvasive evaluation of joints [1-4]. MRI, for example, has emerged as the preferred diagnostic modality to determine the presence and extent of lesions in the hip articular cartilage and acetabular labrum [2], due to its multiplanar image acquisition capability and its high soft tissue contrast. However, routine MRI can diagnose cartilage defects only if there are morphologic changes, whereas cartilage may already be irreversibly compromised at a biochemical level despite appearing as normal. Continued development in the field of quantitative MRI in recent years has seen the emergence of techniques able to probe the earliest biochemical changes in cartilage [5-11]. However, scan time limitations, inconsistency between measurements, and time‐consuming cartilage segmentation for diagnostic interpretation have prevented extensive clinical validation of quantitative MR techniques, restricting their common use to research settings. The introduction of magnetic resonance fingerprinting (MRF) [12], a novel approach to data acquisition, postprocessing, and visualization, might enable robust and reliable reconstruction of multiple parameters from a single, fast MR acquisition.Methods
We have recently developed an MRF technique for potentially reproducible T1 and T2 mapping in the hip with clinically acceptable resolution (0.6 × 0.6 mm2) and total scan time of approximately 7 minutes [13]. Since transmit field (B1+) nonuniformity can bias quantitative hip imaging at 3 T [14], our MRF implementation was designed to simultaneously measure B1+, in addition to proton density (PD), T1, and T2. To simplify workflow and enhance reproducibility, an automatic slice positioning system was implemented. We employed a multimodal multilabel classification strategy to segment the hip cartilage, exploiting the different contrasts provided by our MRF framework (T1, T2, PD). To assess the intrinsic reproducibility of our technique, we scanned a multicompartment phantom on three different clinical 3 T MR scanners. To assess the in vivo reproducibility of our quantitative hip cartilage evaluation method, we scanned three healthy volunteers six times on the same day, twice on each of the three clinical scanners used for phantom validation.Results
The accuracy of the phantom T1 and T2 values was estimated using gold standard measurements as the reference. The estimation error was less than 3% for the range of relaxation times expected for cartilage. The average inter-scanner coefficient of variation (CV) among all phantom compartments was 1.5% and 0.9% for T1 and T2. The median of T1 and T2 within the segmented hip cartilage was consistent among repeated acquisitions with different scanners for all three volunteers. The intrascanner variability was ~1% and 3% for T1 and T2, respectively. Inter-scanner variability was 2% for T1 and 5.5% for T2.Discussion/Conclusion
We introduced a new technique for clinically feasible quantitative imaging (PD, T1, and T2) of the hip articular cartilage. By combining model‐based parameter estimation together with automatic slice positioning and semiautomatic segmentation, we showed that quantitative cartilage evaluation could be highly reproducible. We have recently begun a longitudinal study on a large cohort of patients with hip pathologies, which is aimed at assessing the usefulness of quantitative cartilage evaluation with the long-term goal of translating quantitative cartilage assessment into routine hip imaging protocols.