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Osteochondral Junction (OCJ) Imaging Using a Fast T1-weighted 3D Ultrashort Echo Time Cones Sequence at 3T
Zhenyu Cai1,2, Zhao Wei2, Mingxin Chen2, Saeed Jerban2, Hyungseok Jang2, Eric Chang2,3, Jiang Du2, and Yajun Ma2
1Radiology, Fuwai Hospital Chinese Academy of Medical Sciences, Shenzhen, shenzhen, China, 2Radiology, University of California San Diego, San Diego, CA, United States, 3VA San Diego Healthcare System, San Diego, CA, United States

Synopsis

The osteochondral junction (OCJ) is the region where calcified cartilage meets subchondral bone (SCB), and is likely to be highly related to osteoarthritis (OA). However, it is difficult to image OCJ tissues due to their relatively short transverse relaxation times, which cause little or no signal to appear with conventional imaging sequences. In this study, we developed a 3D T1-weighed fast ultrashort echo time cones sequence with fat saturation (FS-UTE-Cones) to generate a high OCJ contrast image of the human knee on a clinical 3T MRI scanner.

Introduction

Several studies have shown that the region of the osteochondral junction (OCJ), which encompasses the tissues between the uncalcified layers of cartilage and the marrow spaces of the trabecular bone, is highly related to osteoarthritis (OA) (1-4). However, magnetic resonance imaging (MRI) of the OCJ region is difficult due to the tissues’ short transverse relaxation times (i.e. short T2), which result in little or no signal with conventional sequences. Ultrashort echo time (UTE) sequences have been employed to overcome this limitation (5-11). However, most techniques are based on 2D UTE sequences, which are unable to cover the whole knee joint due to strong out-of-slice signal excitation, especially for off-center slices (12). In this study, we developed a fast 3D T1-weighed UTE Cones sequence with fat saturation (FS-UTE-Cones) to highlight OCJ tissues in human knee on a clinical 3T MRI scanner. This study examined an ex vivo patellar cartilage sample, cadaveric whole knee joint, and an in vivo knee joint.

Methods

The features of 3D FS-UTE-Cones sequence can be found in Figure 1 (13). A product FatSat module was used to suppress fat. To improve the acquisition efficiency, a series of spokes (Nsp) was acquired after each FatSat pulse (Figure 1A). For each spoke, a short rectangular pulse (e.g., 100µs) was used for non-selective signal excitation (Figure 1B), followed by a Cones data acquisition (Figure 1C). To investigate the T1 values for the different layers of cartilage (i.e. superficial, middle, deep, and calcified layers), a patellar cartilage sample was scanned in a homemade birdcage coil with a 3D UTE-Cones actual flip angle imaging and variable flip angle (3D UTE-Cones AFI-VFA) sequence (14): 1) UTE-Cones AFI: TR=20/100ms, flip angle=45°; 2) UTE-Cones VFA: TR=20ms; flip angle=4°, 7°, 10°, 15°, 20°, 25° and 30°. Other sequence parameters were: FOV=7×7×6.4cm3, acquisition matrix=192×192×64, TE=0.032 ms, receiver bandwidth=100 kHz, and a total scan time of 45.5 min. In vivo knee imaging was performed on five healthy volunteers (29-45 years of age, 3 males and 2 females) and five patients with OA (48-70 years of age, 4 males and 1female). Informed consent was obtained from all subjects in accordance with guidelines of the institutional review board. A healthy knee joint from a 36-year-old volunteer was scanned with the FS-UTE-Cones sequence with different flip angles (i.e. flip angle=5°, 10°, 15°, 20°, 25°, and 30°) to investigate the contrast change of the OCJ tissues. A transmit and receive knee coil was used for RF excitation and signal reception. Other sequence parameters were: FOV=13×13×8.4cm3, acquisition matrix=256×256×42, TR/TE=80/0.032 ms, Nsp=5, τ=6ms, receiver bandwidth=166 kHz, and a total scan time of 3 min. The clinical T2- and PD-weighted sequences were used for comparison. Then, a high contrast image of the OCJ was obtained by scanning a healthy knee joint from a 37-year-old volunteer with the optimized T1-weighted FS-UTE-Cones sequence (flip angle=30°). Six abnormal cadaveric whole knee joints and five OA patients were also scanned with the T1-weighted FS-UTE-Cones sequence and compared with the clinical T2-weighted sequence.

Results and Discussion

Figure 2 shows a 3D FS-UTE-Cones image of the patellar cartilage sample and corresponding fitting curves and T1 values of different regions of cartilage. The T1 values of uncalcified cartilage decrease from the superficial layer (890±11 ms) to the middle layer (847±4 ms), then the deep layer (729±7 ms). The calcified cartilage has a much lower T1 of 465±8 ms. The significantly lower T1 values of the calcified cartilage make it possible to highlight the OCJ region with a highly T1-weighted UTE sequence. The OCJ tissues appear dark in the clinical T2- and PD-FSE images (Figure 3A and 3B) due to the short T2 relaxation. However, the signals in the OCJ region can be detected with the PD-weighted FS-UTE-Cones sequence with low flip angles (Figure 3C and 3D), but without OCJ contrast. In comparison, the OCJ region was highlighted with the T1-weighed FS-UTE-Cones sequence with high flip angles (e.g. 25° and 30°) as a bright band. Figure 4 demonstrated a high OCJ contrast in whole knee joint imaging with the proposed T1-weighted FS-UTE-Cones sequence. Continuous bright bands can be found in the OCJ region of patellar, femoral, and tibial cartilage. Both the abnormal knee joint specimen and knee joints of the OA patients show morphological changes in the OCJ region (Figure 5), including regions with ill-defined or focal absence of bright band adjacent and parallel to the SCB (arrows).

Conclusion

The proposed 3D T1-weighted FS-UTE-Cones sequence is a novel imaging method that can highlight the OCJ region of human knee joint. This fast technique could be used to detect signal changes in OCJ tissues which may be related to OA.

Acknowledgements

The authors acknowledge grant support from NIH (R01AR075825, 2R01AR062581, 1R01 AR068987), and the VA Clinical Science and Rehabilitation R&DAwards (I01RX002604)

References

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Figures

Figure 1 The 3D T1-weighted FS-UTE-Cones sequence. This sequence employs a FatSat module for fat suppression, followed by multiple spoke UTE-Cones data acquisition (A). A short rectangular pulse is used for signal excitation followed by 3D spiral sampling with a minimal nominal TE of 32 µs (B). The spiral trajectories are arranged with conical view ordering (C).

Figure 2 T1 measurement for a patellar cartilage sample on superficial (A), middle (B), deep (C), and calcified layers (D) layers. The corresponding fitting curves and T1 values are shown in E-H. T1 value decreases gradually from the superficial layer to the calcified layer.

Figure 3 In vivo knee imaging from a 36-year-old volunteer with the clinical T2- and PD-weighted sequences (A and B), together with the FS-UTE-Cones with different flip angles (C-H). OCJ regions are dark in T2- and PD-weighted FSE images. In contrast, FS-UTE-Cones sequence can detect signals from OCJ tissues. With a higher flip angle in FS-UTE-Cones imaging (i.e. higher T1-weighting), the OCJ tissues show a higher contrast.

Figure 4 Whole knee joint OCJ imaging from a 37-year-old healthy volunteer. Continuous bright bands can be seen in the OCJ region of patellar, femoral, and tibial cartilage.

Figure 5 Imaging of abnormal cadaveric knee joints (first two rows) and knee joints from OA patients (last two rows). The clinical T2-weighted images are used for comparison with the proposed T1-weighted FS-UTE-Cones images. The arrows show abnormal regions with reduction or loss of high intensity band in OCJ regions of T1-weighted FS-UTE-Cones images.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)
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