UTE-based adiabatic T1ρ is sensitive to enzymatic proteoglygan degradation in human articular cartilage
Lidi Wan1,2, Adam C Searleman1, Yajun Ma1, Jonathan H Wong3, Mark E Murphy4, Jiang Du1, Guangyu Tang2, and Eric Y Chang1,3

1Department of Radiology, UC San Diego, San Diego, CA, United States, 2Department of Radiology, Shanghai Tenth People's Hospital of Tongji University, Shanghai, China, 3Radiology Service, VA San Diego Healthcare System, San Diego, CA, United States, 4Orthopaedic Surgery Service, VA San Diego Healthcare System, San Diego, CA, United States


A series of quantitative UTE techniques have been developed to assess articular cartilage. The early stage of osteoarthritis is characterized by proteoglycan (PG) loss in cartilage. This study aimed to determine if quantitative UTE-based biomarkers are sensitive to PG loss induced by chondroitinase ABC in cadaveric human cartilage. Pure cartilage wafers were exposed to sequential enzymatic digestion. MR imaging was performed before and after sequential digestion. PG loss was observed after digestion, with a corresponding increase in UTE adiabatic T1ρ values as compared to controls.


The onset of osteoarthritis (OA) is primarily associated with biochemical alterations, and proteoglycan (PG) loss in articular cartilage has been hypothesized as an initiating factor (1,2). Noninvasive detection of PG depletion by MR imaging would help early stage diagnosis (3,4), making timely treatment of the disease while it is still reversible a clinical possibility. Articular cartilage, especially the deep radial and calcified layers, contains a significant amount of short T2 components (< 10 milliseconds), where the signal decays so fast that little or no signal can be detected with conventional MR sequences (5). Previous studies have shown that conventional MRI techniques, including T1-, T2-, and magnetization transfer (MT)-weighted imaging, are inconclusive in detecting early stage changes in OA (6,7). 3D ultrashort echo time (UTE) Cones MRI sequences, with TEs on the order of microseconds, allow for direct volumetric imaging and quantitative assessment of cartilage, including the deep radial and calcified layers. This study aimed to determine if quantitative 3D UTE-Cones-based biomarkers are sensitive to PG loss induced by enzyme in human cartilage and also aimed to discuss the implications of detecting OA at early stages.


Sample preparation: 16 osteochondral cores were harvested from one cadaveric specimen (31-year-old male donor) and the osseous components were removed using a scalpel. The cartilage wafers were soaked in buffer solution (50 mM Tris, 60 mM sodium acetate, and 0.02% bovine serum albumin, pH 8.0) for one hour before baseline MR scan.

MR sequences: All imaging was performed on a 3T clinical MRI scanner (MR750, GE Healthcare Technologies, Milwaukee, WI, USA) using a homemade 30 ml birdcage coil. The following four imaging protocols were performed: A) 3D UTE-Cones magnetization transfer (3D UTE-Cones-MT) with three saturation pulse powers (q = 400°, 600°, and 800°) and five frequency offsets (Df = 2, 5, 10, 20, and 50 kHz) (8); B) 3D UTE-cones with actual flip angle imaging and variable flip angles (3D UTE-Cones AFI-VFA) with flip angles (FA) of 5°, 10°, 20°, and 30°, and a TR of 20 ms (9); C) 3D UTE-Cones with adiabatic T1ρ preparation (3D UTE-Cones-AdiabT1ρ) with spin-locking time (TSL) of 0, 12, 24, 36, 48, 72, and 96 ms (10); D) 3D UTE-T2* with TEs of 0.032, 4.1, 8.1, 12.1, 16.1, and 32 ms. Other imaging parameters included: FOV = 5 cm, matrix=160×160, slice thickness=0.5mm, 60 slices. The total scan time was 78 min.

Enzymatic digestion and histology: Samples (n=8) were incubated at 37 ℃ for 44 hours in a 2 ml 0.1U/ml solution of chondroitinase ABC (C3667, Sigma-Aldrich, St. Louis, MO) and buffer solution, in order to induce proteoglycan loss. The controls (n=8) were immersed at 37 ℃ for 44 hours in just buffer solution. Buffer and enzyme solution were changed after 22 hours. At the end of the digestion, samples were rinsed in buffer solution for 30 minutes. After MR imaging, cartilage wafers were fixed in 10% zinc formalin, paraffin-embedded, and sectioned. Safrain-O and fast green stains were used for PG detection.

Data analysis: Three consecutive slices at the center of each wafer were used for global region of interest (ROI) analysis. T1, AdiabT1ρ, MT modeling of macromolecular fraction, and T2* values were calculated for all cartilage samples, before and after enzymatic digestion using previously reported methods (11). Pixel maps were also obtained. Two-sided paired t test was used for statistical analysis.


Figures 1 and 2 show the estimated T1, adiabT1ρ, macromolecular fraction, and T2* values before and after incubation. Although increases in T1, macromolecular fraction, and T2* were observed, the differences were non-significant (p>0.05). However, adiabT1ρ significantly increased in enzyme-treated samples after digestion (p=0.01).

Figure 3 shows representative pixel maps and corresponding histology. The enzyme-treated sample demonstrated increased adiabT1ρ values after digestion, and PG loss was confirmed by decreased Safrain-O staining.


In our study, we found that adiabT1ρ relaxation, as measured with the 3D UTE-Cones-AdiabT1ρ sequence, was most sensitive for assessing enzyme-induced PG loss. Increasing trends were seen with the other measures, but did not reach statistical significance.

Using a FSE readout at 9.4T, Nissi et al. did not observe significant changes in adiabT1ρ after chondroitinase ABC treatment (11). Our results suggest that the 3D UTE-Cones-AdiabT1ρ sequence, with its advantage of detecting short T2 components, may be used to detect PG loss on a clinical scanner.


PG loss in cartilage was observed after digestion, with a corresponding increase in UTE adiabaticT1ρ values. T1ρ relaxation as measured with the 3D UTE-Cones-AdiabT1ρ sequence has potential for detecting PG loss in articular cartilage, thus facilitating the diagnosis of OA at early stages.


The authors acknowledge grant support from VA Clinical Science and Rehabilitation R&D Awards (I01CX001388 and I01RX002604) and NIH (R01AR062581).


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Figure 1. Quantitative UTE MRI biomarkers, including T1, AdiabT1ρ, macromolecular fraction, T2* for both enzyme-treated and control groups before and after incubation.

Figure 2. Value changes of quantitative UTE biomarkers (T1, AdiabT1ρ, macromolecular fraction, and T2*) before and after incubation for enzyme-treated and control groups are shown in A, B, C, and D, respectively, with dots/squares representing the mean values and with error bars representing the standard deviation of measurements. AdiabT1ρ values significantly increased in samples after enzymatic digestion. UTE adiabatic T1ρ was sensitive to PG loss induced by chondrotinase ABC and can distiguish between enzyme-treated and control samples.

Figure 3. UTE adiabatic T1ρ pixel mapping of representative enzyme-treated (A) and control (B) samples before and after incubation, respectively. The enzyme-treated sample has obvious increased T1ρ value after exposure to chondroitinase ABC as compared to the control. The corresponding Safrain-O and fast green stained photomicrographs in (C) and (D), respectively, confirmed PG loss in the enzyme-treated group.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)