Sarma V.S. Akella

Proteoglycan‐induced changes in T1ρ‐relaxation of articular cartilage at 4T

Proteoglycan (PG) depletion‐induced changes in T1ρ (spin‐lattice relaxation in rotating frame) relaxation and dispersion in articular cartilage were studied at 4T. Using a spin‐lock cluster pre‐encoded fast spin echo sequence, T1ρ maps of healthy bovine specimens and specimens that were subjected to PG depletion were computed at varying spin‐lock frequencies. Sequential PG depletion was induced by trypsinization of cartilage for varying amounts of time. Results demonstrated that over 50% depletion of PG from bovine articular cartilage resulted in average T1ρ increases from 110–170 ms. Regression analysis of the data showed a strong correlation (R2 = 0.987) between changes in PG and T1ρ. T1ρ values were highest at the superficial zone and decreased gradually in the middle zone and again showed an increasing trend in the region near the subchondral bone. The potentials of this method in detecting early degenerative changes of cartilage are discussed. Also, T1ρ‐dispersion changes as a function of PG depletion are described. Magn Reson Med 46:419–423, 2001.

T1ρ relaxation mapping in human osteoarthritis (OA) cartilage: Comparison of T1ρ with T2

To quantify the spin‐lattice relaxation time in the rotating frame (T1ρ) in various clinical grades of human osteoarthritis (OA) cartilage specimens obtained from total knee replacement surgery, and to correlate the T1ρ with OA disease progression and compare it with the transverse relaxation time (T2).

Proteoglycan Depletion–Induced Changes in Transverse Relaxation Maps of Cartilage

RATIONALE AND OBJECTIVES The authors performed this study to (a) measure changes in T2 relaxation rates, signal-to-noise ratio (SNR), and contrast with sequential depletion of proteoglycan in cartilage; (b) determine whether there is a relationship between the T2 relaxation rate and proteoglycan in cartilage; and (c) compare the T2 mapping method with the spin-lattice relaxation time in the rotating frame (T1rho) mapping method in the quantification of proteoglycan-induced changes. MATERIALS AND METHODS T2- and T1rho-weighted magnetic resonance (MR) images were obtained in five bovine patellae. All images were obtained with a 4-T whole-body MR unit and a 10-cm-diameter transmit-receive quadrature birdcage coil tuned to 170 MHz. T2 and T1rho maps were computed. RESULTS The SNR and contrast on the T2-weighted images were, on average, about 43% lower than those on the corresponding T1rho-weighted images. The T2 relaxation rates varied randomly without any particular trend, which yielded a poor correlation with sequential depletion of proteoglycan (R2 = 0.008, P < .70). There was excellent linear correlation between the percentage of proteoglycan in the tissue and the T1rho relaxation rate (R2 = 0.85, P < .0001). CONCLUSION T2-weighted imaging neither yields quantitative information about the changes in proteoglycan distribution in cartilage nor can be used for longitudinal studies to quantify proteoglycan-induced changes. T1rho-weighted imaging, however, is sensitive to sequential depletion of proteoglycan in bovine cartilage and can be used to quantify proteoglycan-induced changes.

Reduction of residual dipolar interaction in cartilage by spin-lock technique

The influence of radiofrequency (RF) spin‐lock pulse on the laminar appearance of articular cartilage in MR images was investigated. Spin‐lock MRI experiments were performed on bovine cartilage plugs on a 4.7 Tesla small‐bore MRI scanner, and on human knee cartilage in vivo on a 1.5 Tesla clinical scanner. When the normal to the surface of cartilage was parallel to B0, a typical laminar appearence was exhibited in T2‐weighted images of cartilage plugs, but was absent in T1ρ‐weighted images of the same plugs. At the “magic angle” orientation (when the normal to the surface of cartilage was 54.7° with respect to B0), neither the T2 nor the T1ρ images demonstrated laminae. At the same time, T1ρ values were greater than T2 at both orientations throughout the cartilage. T1ρ dispersion (i.e., the dependence of the relaxation rate on the spin‐lock frequency ω1) was observed, which reached a steady‐state value of close to 2 kHz in both parallel and magic‐angle orientations. These results suggest that residual dipolar interaction from motionally‐restricted water and relaxation processes, such as chemical exchange, contribute to T1ρ dispersion in cartilage. Further, one can reduce the laminar appearance in human articular cartilage by applying spin‐lock RF pulses, which may lead to a more accurate diagnosis of degenerative changes in cartilage. Magn Reson Med 52:1103–1109, 2004.

Proton spin‐lock ratio imaging for quantitation of glycosaminoglycans in articular cartilage

To quantify glycosaminoglycans (GAG) in intact bovine patellar cartilage using the proton spin‐lock ratio imaging method. This approach exploits spin‐lattice relaxation time in the rotating frame (T1ρ) imaging and T1ρ relaxivity (R1ρ).

In vivo measurement of T1ρ dispersion in the human brain at 1.5 tesla

To measure T1ρ relaxation times and T1ρ dispersion in the human brain in vivo.

In vivo quantification of T1ρ using a multislice spin‐lock pulse sequence

A multislice spin‐lock (MS‐SL) pulse sequence is implemented on a clinical scanner to acquire multiple images with spin‐lock‐generated contrast of the knee joints of six healthy human subjects. The MS‐SL sequence produces images with T1ρ contrast with an additional factor of intrinsic T2ρ weighting, which hinders direct measurement of T1ρ. A method is presented to compensate the MS‐SL‐generated data with regard to T2ρ in an effort to accurately calculate multislice T1ρ maps in a feasible experimental time. The T2ρ‐compensated multislice T1ρ maps produced errors in the measurement of T1ρ in healthy patellar cartilage of ∼5% compared to the gold standard measurement of T1ρ acquired with single‐slice spin‐lock pulse sequence. The MS‐SL sequence has potential as an important clinical tool for the acquisition of multislice T1ρ‐weighted images and/or quantitative multislice T1ρ maps. Magn Reson Med 52:1453–1458, 2004.

T1ρ-relaxation mapping of human femoral-tibial cartilage in vivo

To demonstrate the in vivo feasibility of measuring spin‐lattice relaxation time in the rotating frame (T1ρ); and T1ρ‐dispersion in human femoral cartilage. Furthermore, we aimed to compute the baseline T1ρ‐relaxation times and spin‐lock contrast (SLC) maps on healthy volunteers, and compare relaxation times and signal‐to‐noise ratio (SNR) with corresponding T2‐weighted images.

Depth-dependent proton magnetization transfer in articular cartilage

To measure the proton magnetization transfer ratio (MTR) maps in control and collagen‐depleted bovine patellar cartilage specimens as a function of cartilage depth during mechanical compression.

T1ρ MR imaging of the human wrist in vivo

Abstract Rationale and Objectives The purpose of this study was ( a ) to demonstrate the feasibility of computing T1ρ maps of, and T1ρ dispersion in, human wrist cartilage at MR imaging in vivo and ( b ) to compare T1ρ and T2 weighting in terms of magnitude of relaxation times and signal intensity contrast. Materials and Methods T2 and T1ρ magnetic resonance images of wrist joints in healthy volunteers ( n = 5) were obtained with a spin-echo sequence and a fast spin-echo sequence pre-encoded with a spin-lock pulse cluster. A 1.5-T clinical imager was used (Signa; GE Medical Systems, Milwaukee, Wis) with a 9.5-cm-diameter transmit-receive quadrature birdcage coil tuned to 63.75 MHz. Results T1ρ relaxation times at a spin-lock frequency of 500 Hz vary from 40.5 msec ± 0.85 to 56.6 msec ± 4.83, and T2 relaxation times vary from 28.1 msec ± 1.88 to 34.5 msec ± 2.63 (mean ± standard error of the mean, n = 5, P Conclusion It was possible to perform T2- and T1ρ-weighted MR imaging of human wrist cartilage in vivo with standard clinical imagers. The higher signal-to-noise ratio and improved contrast between cartilage and surrounding fat achieved with T1ρ imaging may provide better definition of lesions and accurate quantitation of small changes in cartilage degeneration.