Mei Wu1,2, Yanping Xue1, Yajun Ma1, Claire Tang1, Meghan Shen1, Saeed Jerban1, Eric Y Chang1,3, and Jiang Du1
1Department of Radiology, University of California, San Diego, San Diego, CA, United States, 2Department of Radiology, Guangzhou First People’s Hospital, School of Medicine, South China University of Technology, Guangzhou, China, 3Radiology Service, VA San Diego Healthcare System, San Diego, CA, United States
Synopsis
The study protocol included three-dimensional
Ultrashort Echo Time Cones actual flip angle imaging (3D UTE-Cones-AFI) for T1
measurement and UTE-Cones with adiabatic T1ρ (AdiabT1ρ) preparation
for AdiabT1ρ measurement. We applied the 3D UTE-Cones
AdiabT1ρ sequence to healthy volunteers and patients
with different degrees of OA for a systematic evaluation of its clinical
performance. Results showed that
the 3D UTE-Cones AdiabT1ρ sequence could be used for high resolution
imaging and quantitative assessment of the knee cartilage, and that the AdiabT1ρ biomarker
showed a significant positive relationship with WORMS.
Introduction
Knee osteoarthritis (OA) affects
millions of people worldwide. The loss of proteoglycans (PGs) in
articular cartilage is one of the most significant early changes in OA. Spin
lattice relaxation in the rotating frame (T1ρ)
has been proposed to probe this and other biochemical changes (1-3). However, a
major confounding factor in regular continuous wave T1ρ (CW-T1ρ) imaging of
articular cartilage is the magic angle effect (4-5). To address this challenge, researchers have
proposed trains of adiabatic full passage (AFP) pulses to generate adiabatic T1ρ (AdiabT1ρ)
relaxation (6-12). Previous studies have shown that AdiabT1ρ is less sensitive
to the magic angle effect compared with CW-T1ρ relaxation in
bovine cartilage (10, 11). However, the AdiabT1ρ sequence is based
on conventional Cartesian data acquisition with an echo time (TE) of several
milliseconds or longer, which is too long to image many knee joint tissues with
short T2*s, such as the deep layers of articular cartilage, menisci, ligaments,
and tendons (13-16). More recently, the combination of 3D ultrashort echo time
(UTE) Cones data acquisition and adiabatic T1ρ preparation (3D UTE
Cones-AdiabT1ρ)
has been proposed for potentially magic angle-insensitive imaging of both short
and long T2 tissues or tissue components in the knee joint. Preliminary studies
of patellar cartilage show a much reduced magic angle effect as compared to
CW-T1ρ and T2 (17). In this study, we aimed to apply this sequence to healthy
volunteers and patients with different degrees of OA for a systematic
evaluation of its clinical performance. Method
In this study a total of 65 human subjects (aged 20-88
years, mean age 54.8±16.9 years; 32males, 33 females; 28 left knees, 37 right
knees) were recruited following approval from the Institutional Review Board.
Written informed consent was obtained from each participant before 3D
UTE Cones-AdiabT1ρ imaging (Figure 1) on a 3T
clinical MR system (MR750, GE Healthcare, Milwaukee, WI). The imaging
protocol included 3D UTE-Cones actual flip angle (AFI) imaging for B1 mapping, a variable
flip angle (VFA) approach for T1 mapping, and AdiabT1ρ imaging (18,19).
Typical imaging parameters included a field-of-view (FOV) of 15×15×10.8 cm3
and receiver bandwidth of 166 kHz. Other sequence parameters were: 1) 3D UTE-Cones AFI:
TR1/TR2 = 20/100 ms, flip angle = 45°; 2) 3D VFA UTE-Cones:
TR = 20
ms; flip angle = 5°, 10°, 20°, and 30°; 3) 3D AdiabT1ρ UTE-Cones with
FatSat: TR = 500
ms; flip angle = 10°; number of spokes per Adiabatic preparation (Nsp) = 25; and
pairs of adiabatic IR pulse (NIR) = 0, 2, 4, 6, 8, 12 and 16;
corresponding to spin-locking time (TSL) of 0, 12, 24, 36, 48, 72, and
96 ms, respectively; with each sequence scan time = 2 min 34 sec (for a total scan time of 17
minutes for all seven TSLs). Radiography and clinical T2- and
PD-weighted images were also obtained for morphological evaluation of cartilage
degeneration. The elastix motion
registration was applied before quantification. Articular cartilage was divided
into 13 sub-regions (Figure 2). AdiabT1ρ values
were compared with Modified
Whole-Organ Magnetic Resonance Imaging Scores (MWORMS) (20) for all 65 human subjects. Results and Discussion
Figure 3 shows representative 3D UTE Cones-AdiabT1ρ images of the
femoral articular cartilage of a normal volunteer and an OA patient,
respectively. Excellent AdiabT1ρ fitting of the 3D AdiabT1ρ UTE-Cones images with different TSLs
demonstrates a T1ρ of 34.9 ± 3.7 ms for the normal
femoral condyle and 47.9 ± 6.6 ms for the abnormal
femoral condyle, consistent with PG depletion in the latter case.
All subjects were
further subdivided into two groups (the extent groups include: WORMS 0 =
controls, WORMS 1, 2, 2.5 = regional lesions, WORMS 3, 4, 5 = diffuse lesions;
the depth groups include: WORMS 0 = controls, WORMS 1, 2, 3, 4 = partial
thickness lesions, WORMS 2.5, 5 = full-thickness lesions) according to the
extent and the depth of the cartilage lesions. The Spearman correlation
coefficient showed a significant positive relationship between the T1ρ values,
T1 values, and the WORMS (P < 0.001), as shown in Table 1. Additionally, T1ρ has a significantly higher
correlation with WORMS than T1 does (Table 1). Table 2 further
demonstrates the strong correlation between AdiabT1ρ values of some femur and patella sub-regions and WORMS by using a linear regression analysis (P
< 0.0038).
The preliminary results from 65 human subjects
demonstrate the potential of the 3D UTE-Cones AdiabT1ρ imaging sequence
for quantitative evaluation of cartilage degeneration. This sequence can also
be used for systematic evaluation of other knee joint tissues such as the
menisci, ligaments, and tendons. A systematic evaluation of all the knee joint
tissues will likely provide improved evaluation of OA. Limitations of this
study are that the number of patients is relatively small and that the protocol
is relatively long. However, the total scan time can be significantly reduced
by using small number of TSLs, as well as more advanced image reconstruction
and data processing techniques, such as parallel imaging, compressed sensing,
and deep learning techniques. Conclusion
The
3D UTE-Cones-AdiabT1ρ sequence can be used for high resolution imaging and quantitative assessment of the knee
cartilage, and the AdiabT1ρ biomarker shows a significant positive
relationship with WORMS.Acknowledgements
The authors are thankful for
support from R01AR075825,
2R01AR062581, and 1R01AR068987.References
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