Contributions of chemical and diffusive exchange to T1ρ dispersion

Variations in local magnetic susceptibility may induce magnetic field gradients that affect the signals acquired for MR imaging. Under appropriate diffusion conditions, such fields produce effects similar to slow chemical exchange. These effects may also be found in combination with other chemical exchange processes at multiple time scales. We investigate these effects with simulations and measurements to determine their contributions to rotating frame (R_1ρ) relaxation in model systems. Simulations of diffusive and chemical exchange effects on R_1ρ dispersion were performed using the Bloch equations. Additionally, R_1ρ dispersion was measured in suspensions of Sephadex and latex beads with varying spin locking fields at 9.4 T. A novel analysis method was used to iteratively fit for apparent chemical and diffusive exchange rates with a model by Chopra et al. Single‐ and double‐inflection points in R_1ρ dispersion profiles were observed, respectively, in simulations of slow diffusive exchange alone and when combined with rapid chemical exchange. These simulations were consistent with measurements of R_1ρ in latex bead suspensions and small‐diameter Sephadex beads that showed single‐ and double‐inflection points, respectively. These observations, along with measurements following changes in temperature and pH, are consistent with the combined effects of slow diffusion and rapid ^?OH exchange processes. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Ultrashort TE T1ρ magic angle imaging

An ultrashort TE T₁ρ sequence was used to measure T₁ρ of the goat posterior cruciate ligament (n = 1) and human Achilles tendon specimens (n = 6) at a series of angles relative to the B₀ field and spin‐lock field strengths to investigate the contribution of dipole–dipole interaction to T₁ρ relaxation. Preliminary results showed a significant magic angle effect. T₁ρ of the posterior cruciate ligament increased from 6.9 ± 1.3 ms at 0° to 36 ± 5 ms at 55° and then gradually reduced to 12 ± 3 ms at 90°. Mean T₁ρ of the Achilles tendon increased from 5.5 ± 2.2 ms at 0° to 40 ± 5 ms at 55°. T₁ρ dispersion study showed a significant T₁ρ increase from 2.3 ± 0.9 ms to 11 ± 3 ms at 0° as the spin‐lock field strength increased from 150 Hz to 1 kHz, and from 30 ± 3 ms to 42 ± 4 ms at 55° as the spin‐lock field strength increased from 100 to 500 Hz. These results suggest that dipolar interaction is the dominant T₁ρ relaxation mechanism in tendons and ligaments. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

T1ρ of protein solutions at very low fields: Dependence on molecular weight,concentration, and structure

The effect of molecular weight, concentration, and structure on 1/T₁ρ, the rotating frame relaxation rate, was investigated for several proteins using the on-resonance spin-lock technique, for locking fields B₁ < 200 μT. The measured values of 1/T₁ρ, were fitted to a simple theoretical model to obtain the dispersion curves 1/T₁ρ(ω₁) and the relaxation rate at zero B₁ field, 1/T₁ρ,(O). 1/T₁ρ, was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra- and intermolecular hydrogen and disulfide bonds especially contributed to 1/T₁ρ. In all samples, 1/T₁ρ(O) was equal to 1/T₂ρ measured at the main magnetic field B_o = 0.1 T, but at higher locking fields the dispersion curves mono-tonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for 1/T₁ρ relaxation in protein solutions is not valid at very low B₁ fields. The underlying mechanism for the relaxation rate 1/T₁ρ at B₁ fields below 200 μT is discussed.     

In vivo 3.0‐tesla magnetic resonance T1ρ and T2 relaxation mapping in subjects with intervertebral disc degeneration and clinical symptoms

The purpose of this study is (1) to determine the correlation between T_1ρ and T₂ and degenerative grade in intervertebral discs using in vivo 3.0‐T MRI, and (2) to determine the association between T_1ρ and T₂ and clinical findings as quantified by the SF‐36 Questionnaire and Oswestry Disability Index. Sixteen subjects participated in this study, and each completed SF‐36 and Oswestry Disability Index questionnaires. MRI T_1ρ and T₂ mapping was performed to determine T_1ρ (77 discs) and T₂ (44 discs) in the nucleus of the intervertebral disc, and T₂‐weighted images were acquired for Pfirrmann grading of disc degeneration. Pfirrmann grade was correlated with both T_1ρ (r = ?0.84; P < 0.01) and T₂ (r = ?0.61; P < 0.01). Mixed‐effects models demonstrate that only T_1ρ was associated with clinical questionnaires (R²_SF‐36 = 0.55, R²_O.D.I. = 0.56; P < 0.05). Although the averaged values of T_1ρ and T₂ were significantly correlated, they presented differences in spatial distribution and dynamic range, thus suggesting different sensitivities to tissue composition. This study suggests that T_1ρ may be sensitive to early degenerative changes (corroborating previous studies) and clinical symptoms in intervertebral disc degeneration. Magn Reson Med 63:1193–1200, 2010. © 2010 Wiley‐Liss, Inc.     

T1ρ MRI of human musculoskeletal system

Magnetic resonance imaging (MRI) offers the direct visualization of the human musculoskeletal (MSK) system, especially all diarthrodial tissues including cartilage, bone, menisci, ligaments, tendon, hip, synovium, etc. Conventional MRI techniques based on T₁‐ and T₂‐weighted, proton density (PD) contrast are inconclusive in quantifying early biochemically degenerative changes in MSK system in general and articular cartilage in particular. In recent years, quantitative MR parameter mapping techniques have been used to quantify the biochemical changes in articular cartilage, with a special emphasis on evaluating joint injury, cartilage degeneration, and soft tissue repair. In this article we focus on cartilage biochemical composition, basic principles of T_1ρ MRI, implementation of T_1ρ pulse sequences, biochemical validation, and summarize the potential applications of the T_1ρ MRI technique in MSK diseases including osteoarthritis (OA), anterior cruciate ligament (ACL) injury, and knee joint repair. Finally, we also review the potential advantages, challenges, and future prospects of T_1ρ MRI for widespread clinical translation. J. Magn. Reson. Imaging 2015;41:586–600. © 2014 Wiley Periodicals, Inc.     

T1ρ-relaxation in articular cartilage: Effects of enzymatic degradation

Spin-lattice relaxation in the rotating frame (T_1ρ) dispersion spectroscopy and imaging were used to study normal and enzymatically degraded bovine articular cartilage. Normal specimens demonstrate significant T_1ρ “dispersion” (～60 to ～130 ms) in the 100 Hz to 9 kHz frequency range. Proteoglycan-degraded specimens have 33% greater T_1ρ values than collagen-degraded or normal samples. T_1ρ-weighted images reveal structure not found in conventional T₁-or T₂-weighted images. Our results suggest that T_1ρ measurements are selectively sensitive to proteoglycan content. The potential of this method in distinguishing the early degenerative changes in cartilage associated with osteoarthritis is discussed.     

Measurement of intervertebral disc pressure with T1ρ MRI

The aim of this study is to demonstrate T_1ρ MRI's capability for measuring intervertebral disc osmotic pressure. Self‐coregistered sodium and T_1ρ‐weighted MR images were acquired on ex vivo bovine intervertebral discs (N = 12) on a 3 T clinical MRI scanner. The sodium MR images were used to calculate effective nucleus pulposus fixed‐charge‐density (mean = 138.2 ± 27.6 mM) and subsequently osmotic pressure (mean = 0.53 ± 0.18 atm), whereas the T_1ρ‐weighted images were used to compute T_1ρ relaxation maps. A significant linear correlation (R = 0.56, P < 0.01) between nucleus pulposus fixed‐charge‐density and T_1ρ relaxation time constant was observed. More importantly, a significant power correlation (R = 0.72, P < 0.01) between nucleus pulposus osmotic pressure as predicted by sodium MRI and T_1ρ relaxation time constant was also observed. The current clinical method for assessing disc pressure is discography, which is an invasive procedure that has been shown to have negative effects on disc biomechanical and biochemical properties. In contrast, T_1ρ MRI is noninvasive and can be easily implemented in a clinical setting due to its superior signal‐to‐noise ratio compared with sodium MRI. Therefore, T_1ρ MRI may serve as a noninvasive clinical tool for the longitudinal evaluation of disc osmotic pressure. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

T1ρ MRI quantification of arthroscopically confirmed cartilage degeneration

Nine asymptomatic subjects and six patients underwent T₁ρ MRI to determine whether Outerbridge grade 1 or 2 cartilage degeneration observed during arthroscopy could be detected noninvasively. MRI was performed 2‐3 months postarthroscopy, using sagittal T₁‐weighted and axial and coronal T₁ρ MRI, from which spatial T₁ρ relaxation maps were calculated from segmented T₁‐weighted images. Median T₁ρ relaxation times of patients with arthroscopically documented cartilage degeneration and asymptomatic subjects were significantly different (P < 0.001), and median T₁ρ exceeded asymptomatic articular cartilage median T₁ρ by 2.5 to 9.2 ms. In eight observations of mild cartilage degeneration at arthroscopy (Outerbridge grades 1 and 2), mean compartment T₁ρ was elevated in five, but in all observations, large foci of increased T₁ρ were observed. It was determined that T₁ρ could detect some, but not all, Outerbridge grade 1 and 2 cartilage degeneration but that a larger patient population is needed to determine the sensitivity to these changes. Magn Reson Med 63:1376–1382, 2010. © 2010 Wiley‐Liss, Inc.