Review of Quantitative Knee Articular Cartilage MR Imaging

Abstract

Osteoarthritis (OA) is one of the most prevalent disorders in today's society, resulting in significant socio-economic costs and morbidity. MRI is widely used as a non-invasive imaging tool for OA of the knee. However, conventional knee MRI has limitations to detect subtle early cartilage degeneration before morphological changes are visually apparent. Novel MRI pulse sequences for cartilage assessment have recently received increased attention due to newly developed compositional MRI techniques, including: T2 mapping, T1rho mapping, delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), sodium MRI, diffusion-weighted imaging (DWI)/ diffusion tensor imaging (DTI), ultrashort TE (uTE), and glycosaminoglycan specific chemical exchange saturation transfer (gagCEST) imaging. In this article, we will first review these quantitative assessments. Then, we will discuss the variations of quantitative values of knee articular cartilage with cartilage layer (depth)- and angle (regional)-dependent approaches. Multiple MRI sequence techniques can discern qualitative differences in knee cartilage. Normal articular hyaline cartilage has a zonal variation in T2 relaxation times with increasing T2 values from the subchondral bone to the articular surface. T1rho values were also higher in the superficial layer than in the deep layer in most locations in the medial and lateral femoral condyles, including the weight-bearing portion. Magic angle effect on T2 mapping is clearly observed in the both medial and lateral femoral condyles, especially within the deep layers. One of the limitations for clinical use of these compositional assessments is a long scan time. Recent new approaches with compressed sensing (CS) and MR fingerprinting (MRF) have potential to provide accurate and fast quantitative cartilage assessments.

Keywords: knee cartilage; magnetic resonance imaging; quantitative.

Fig. 1

T2 profiles on 2D surface map of (a) the whole layer, (b) the deep layer, and (c) the superficial layer of the entire femoral cartilage. Arrows indicate ± 54.7° (the magic angle). (Reprinted with permission from #6).

Fig. 2

Articular segmentation with angle- or layer-dependent approach. (a) After manual cartilage extraction, the central point of the cartilage (red dot) was automatically approximated. (b) Static magnetic field (B0) was defined as 0 degrees, with negative and positive angles located anterior and posterior to the central point. (c) Radial lines from a central point divided cartilage into 4-degree segments. (d) Segmentation of cartilage into deep (0%–50%) and superficial layers (51%–100%) of relative thickness. (e) T2 profiles were generated for whole thickness, deep, and superficial layers of cartilage. (Reprinted with permission from #6).

Fig. 3

Difference in average T1rho values between the superficial and deep layers at the medial condyle, lateral condyle, trochlea, and the entire femoral cartilage. Average T1rho values in the superficial layer of the femoral articular cartilage are higher than in the deep layer over the entire femur, medial condyle, and lateral condyle, with a statistically significant difference (P < 0.05). (Reprinted with permission from #67).

Fig. 4

2D and 3D knee cartilage T2 mapping using SENSE or CS. (a) 2D T2 mapping with SENSE (factor = 2, 7 min 5 sec), (b) 2D T2 mapping with CS (factor = 3, 4 min 44 sec), (c) 3D T2 mapping with SENSE (factor = 4, 11 min 28 sec), and (d) 3D T2 mapping with CS (factor = 6, 8 min 2 sec). (Courtesy of Dr. Atsuya Watanabe and Mr. Takayuki Sakai at Eastern Chiba Medical Center, Japan). CS, compressed singing; SENSE, sensitivity encoding.