Proton T1rho-dispersion imaging of rodent brain at 1.9 T

Magnetic resonance myocardial T1ρ mapping : Technical overview, challenges, emerging developments, and clinical applications

The potential of cardiac magnetic resonance to improve cardiovascular care and patient management is considerable. Myocardial T1-rho (T1ρ) mapping, in particular, has emerged as a promising biomarker for quantifying myocardial injuries without exogenous contrast agents. Its potential as a contrast-agent-free ("needle-free") and cost-effective diagnostic marker promises high impact both in terms of clinical outcomes and patient comfort. However, myocardial T1ρ mapping is still at a nascent stage of development and the evidence supporting its diagnostic performance and clinical effectiveness is scant, though likely to change with technological improvements. The present review aims at providing a primer on the essentials of myocardial T1ρ mapping, and to describe the current range of clinical applications of the technique to detect and quantify myocardial injuries. We also delineate the important limitations and challenges for clinical deployment, including the urgent need for standardization, the evaluation of bias, and the critical importance of clinical testing. We conclude by outlining technical developments to be expected in the future. If needle-free myocardial T1ρ mapping is shown to improve patient diagnosis and prognosis, and can be effectively integrated in cardiovascular practice, it will fulfill its potential as an essential component of a cardiac magnetic resonance examination.

New insights into rotating frame relaxation at high field

Measurements of spin-lock relaxation rates in the rotating frame (R1ρ ) at high magnetic fields afford the ability to probe not only relatively slow molecular motions, but also other dynamic processes, such as chemical exchange and diffusion. In particular, measurements of the variation (or dispersion) of R1ρ with locking field allow the derivation of quantitative parameters that describe these processes. Measurements in deuterated solutions demonstrate the manner and degree to which exchange dominates relaxation at high fields (4.7 T, 7 T) in simple solutions, whereas temperature and pH are shown to be very influential factors affecting the rates of proton exchange. Simulations and experiments show that multiple exchanging pools of protons in realistic tissues can be assumed to behave independently of each other. R1ρ measurements can be combined to derive an exchange rate contrast (ERC) that produces images whose intensities emphasize protons with specific exchange rates rather than chemical shifts. In addition, water diffusion in the presence of intrinsic susceptibility gradients may produce significant effects on R1ρ dispersions at high fields. The exchange and diffusion effects act independently of each other, as confirmed by simulation and experimentally in studies of red blood cells at different levels of oxygenation. Collectively, R1ρ measurements provide an ability to quantify exchange processes, to provide images that depict protons with specific exchange rates and to describe the microstructure of tissues containing magnetic inhomogeneities. As such, they complement traditional T1 or T2 measurements and provide additional insights from measurements of R1ρ at a single locking field. Copyright © 2016 John Wiley & Sons, Ltd.

Estimation of the regional cerebral metabolic rate of oxygen consumption with proton detected 17O MRI during precision 17O2 inhalation in swine

Despite the importance of metabolic disturbances in many diseases, there are currently no clinically used methods for the detection of oxidative metabolism in vivo. To address this deficiency, (17)O MRI techniques are scaled from small animals to swine as a large animal model of human inhalation and circulation. The hemispheric cerebral metabolic rate of oxygen consumption (CMRO(2)) is estimated in swine by detection of metabolically produced H(2)(17)O by rapid T(1rho)-weighted proton magnetic resonance imaging on a 1.5T clinical scanner. The (17)O is delivered as oxygen gas by a custom, minimal-loss, precision delivery breathing circuit and converted to H(2)(17)O by oxidative metabolism. A model for gas arterial input is presented for the deeply breathing large animal. The arterial input function for recirculation of metabolic water is measured by arterial blood sampling and high field (17)O spectroscopy. It is found that minimal metabolic water "wash-in" occurs before 60s. A high temporal resolution pulse sequence is employed to measure CMRO(2) during those 60s after delivery begins. Only about one tidal volume of (17)O enriched oxygen gas is used per measurement. Proton measurements of signal change due to metabolically produced water are correlated with (17)O in vivo spectroscopy. Using these techniques, the hemispheric CMRO(2) in swine is estimated to be 1.23+/-.26 micromol/g/min, consistent with existing literature values. All of the technology used to perform these CMRO(2) estimates can easily be adapted to clinical MR scanners, and it is hoped that this work will lead to future studies of human disease.

Spin-lock MRI with amplitude- and phase-modulated adiabatic waveforms: an MR simulation study

INTRODUCTION

Image contrast between tissue types can be generated based on their T1/T2 ratio using spin-lock MRI techniques. An interesting application of such a concept would be to generate contrast in tissue with tissue relaxation times modified using exogenous contrast agents. An amplitude-modulated adiabatic waveform has been shown in the past to perform spin-lock MRI. However, implementation of this waveform may not prove to be efficient and practical in research or a clinical setup due to high radiofrequency power deposition. Recent advancement in software and hardware MR technology allows implementation of amplitude- and phase-modulated adiabatic waveforms on MR systems. The aim of this work was to explore role of adiabatic waveforms in performing rho imaging and demonstrate that amplitude- and phase-modulated waveforms [e.g., hyperbolic secant, B1 independent rotation-4 (BIR-4) waveforms] can be used to distinguish materials that differ in T1/T2 ratio.

METHODS AND RESULTS

MR simulation was performed using computer routines implemented in MATLAB environment (Mathworks, Natick, MA). Modified Bloch equations with trapezoidal, hyperbolic secant and BIR-4 waveforms were used to perform MR simulation. Trapezoidal waveforms were only used for comparison to other waveforms. Gadolinium DTPA (Gad-DTPA) (T1/T2 approximately 1) and manganese chloride (MnCl(2)) (T1/T2 approximately 10) were used as examples of contrast agents due to their routine use in clinical and research setups and more importantly because they provide good examples of materials differing in T1/T2 ratios. Results of spin locking using trapezoidal waveform agree very well with the previously published results, thereby validating the computer routines used in this MR simulation. Plots of M(rho) (magnetization vector in rho domain) vs. offset frequency show distinct curves for these materials differing in T1/T2 for the three waveforms. BIR-4 waveform demonstrated a 40% difference in M(rho) ( approximately 150 Hz) for the materials. Rate of spin lock with hyperbolic secant waveform was rapid compared to other waveforms.

DISCUSSION

MR simulation using contrast agents Gad-DTPA and MnCl(2) provided a useful way to demonstrate that amplitude- and phase-modulated adiabatic waveforms can be used to perform spin-lock imaging. Future work involves implementation of these waveforms on MR scanners and performing in vivo imaging to generate tissue contrast based on relaxation times ratio.

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

PURPOSE

To quantify the spin-lattice relaxation time in the rotating frame (T1rho) in various clinical grades of human osteoarthritis (OA) cartilage specimens obtained from total knee replacement surgery, and to correlate the T1rho with OA disease progression and compare it with the transverse relaxation time (T2).

MATERIALS AND METHODS

Human cartilage specimens were obtained from consenting patients (N = 8) who underwent total replacement of the knee joint at the Pennsylvania Hospital, Philadelphia, PA, USA. T2- and T1rho-weighted images were obtained on a 4.0 Tesla whole-body GE Signa scanner (GEMS, Milwaukee, WI, USA). A 7-cm diameter transmit/receive quadrature birdcage coil tuned to 170 MHz was employed.

RESULTS

All of the surgical knee replacement OA cartilage specimens showed elevated relaxation times (T2 and T1rho) compared to healthy cartilage tissue. In various grades of OA specimens, the T1rho relaxation times varied from 62 +/- 5 msec to 100 +/- 8 msec (mean +/- SEM) depending on the degree of cartilage degeneration. However, T2 relaxation times varied only from 32 +/- 2 msec to 45 +/- 4 msec (mean +/- SEM) on the same cartilage specimens. The increase in T2 and T1rho in various clinical grades of OA specimens were approximately 5-50% and 30-120%, respectively, compared to healthy specimens. The degenerative status of the cartilage specimens was also confirmed by histological evaluation.

CONCLUSION

Preliminary results from a limited number of knee specimens (N = 8) suggest that T1rho relaxation mapping is a sensitive noninvasive marker for quantitatively predicting and monitoring the status of macromolecules in early OA. Furthermore, T1rho has a higher dynamic range (>100%) for detecting early pathology compared to T2. This higher dynamic range can be exploited to measure even small macromolecular changes with greater accuracy compared to T2. Because of these advantages, T1rho relaxation mapping may be useful for evaluating early OA therapy.

In vivo 17O NMR approaches for brain study at high field

17O is the only stable oxygen isotope that can be detected by NMR. The quadrupolar moment of 17O spin (I = 5/2) can interact with local electric field gradients, resulting in extremely short T1 and T2 relaxation times which are in the range of several milliseconds. One unique NMR property of 17O spin is the independence of 17O relaxation times on the magnetic field strength, and this makes it possible to achieve a large sensitivity gain for in vivo 17O NMR applications at high fields. In vivo 17O NMR has two major applications for studying brain function and cerebral bioenergetics. The first application is to measure the cerebral blood flow (CBF) through monitoring the washout of inert H2 17O tracer in the brain tissue following an intravascular bolus injection of the 17O-labeled water. The second application, perhaps the most important one, is to determine the cerebral metabolic rate of oxygen utilization (CMRO2) through monitoring the dynamic changes of metabolically generated H2 17O from inhaled 17O-labeled oxygen gas in the brain tissue. One great merit of in vivo 17O NMR for the determination of CMRO2 is that only the metabolic H2 17O is detectable. This merit dramatically simplifies both CMRO2 measurement and quantification compared to other established methods. There are two major NMR approaches for monitoring H2 17O in vivo, namely direct approach by using 17O NMR detection (referred as direct in vivo 17O NMR approach) and indirect approach by using 1H NMR detection for measuring the changes in T2- or T1rho-weighted proton NMR signals caused by the 17O-1H scalar coupling and proton chemical exchange (referred as indirect in vivo 17O NMR approach). Both approaches are suitable for CBF measurements. However, recent studies indicated that the direct in vivo 17O NMR approach at high/ultrahigh fields appears to offer significant advantages for quantifying and imaging CMRO2. New developments have further demonstrated the feasibility for establishing a completely noninvasive in vivo 17O NMR approach for imaging CMRO2 in a rat brain during a brief 17O2 inhalation. This approach should be promising for studying the central role of oxidative metabolism in brain function and neurological diseases. Finally, the similar approach could potentially be applied to image CMRO2 noninvasively in human brain.

3D-T1rho-relaxation mapping of articular cartilage: in vivo assessment of early degenerative changes in symptomatic osteoarthritic subjects

RATIONALE AND OBJECTIVES

To determine the in vivo feasibility of quantifying early degenerative changes in patellofemoral joint of symptomatic human knee using spin-lattice relaxation time in the rotating frame (T(1rho)) magnetic resonance imaging (MRI).

MATERIALS AND METHODS

All the MRI experiments were performed on a 1.5 T whole-body GE Signa clinical scanner using a custom built 15-cm diameter transmit-receive quadrature birdcage radiofrequency coil. The T(1rho)-prepared magnetization was imaged with a three-dimensional gradient-echo pulse sequence pre-encoded with a three-pulse cluster consisting of two hard 90 degrees pulses and a low power spin-lock pulse. Quantitative T(1rho) relaxation maps of asymptomatic (n = 8 males), and six symptomatic human volunteers (four men, two women) were computed using a appropriate signal expression.

RESULTS

All six symptomatic volunteers showed elevation in T(1rho) relaxation times when compared with asymptomatic subjects. In symptomatic population, the T(1rho) relaxation times varied from 63 +/- 4 ms to 95 +/- 12 ms (mean +/- standard deviation) depending on the degree of cartilage degeneration. The increase in T(1rho) of symptomatic population was statistically significant (n = 6, P <.002) when compared with corresponding asymptomatic population. However, in asymptomatic population the relaxation times varied only from approximately 45 to 55 ms (n = 8, age range 22-45 years).

CONCLUSION

Preliminary results demonstrated the in vivo feasibility of quantifying early biochemical changes in symptomatic osteoarthritis subjects employing T(1rho)-weighted MRI on a 1.5 T clinical scanner. This study on limited number of symptomatic population shows that T(1rho)-weighted MRI provides a noninvasive marker for quantitation of early degenerative changes of cartilage in vivo. However, further studies are needed to correlate early osteoarthritis determined from arthroscopy with T(1rho) in a large symptomatic population.

Off-resonance binomial preparatory pulse technique for high sensitivity MRI of H2O17

H2O17 is a freely diffusible tracer and naturally occurring isotope of O16 detectable by MRI and has been shown to be useful for assessing cerebral perfusion in animal studies using direct and indirect MR detection techniques. However, earlier MR techniques are either not sensitive to changes in small concentrations of tissue H2O17 or are not practical for use on clinical scanners. In this work, a novel method for detecting H2O17 with high sensitivity has been proposed that uses a short, intense, binomial preparatory spin lock pulse with resonance offsets for high sensitive rapid, multislice imaging of tissue H2O17. Two sets of phantom experiments were performed on the 1.5 T and repeated on 3.0 T to assess the feasibility of the proposed technique. The phantom was constructed using ovalbumin and doped with 0.4 atom% and 1.0 atom% H2O17. After obtaining initial parameters, the proposed technique was validated in an anesthetized primate model that was injected with 1.8 cc of 40 atom% H2O17. Phantom experiments showed that the proposed technique was able to detect H2O17 with relatively high sensitivity and high B1 amplitude (and small offsets) preparatory pulses produced similar results as low B1 amplitude (and larger offsets). Primate brain study showed a 42.97% difference in mean signal intensity between pre- and post-H2O17 injection. The proposed technique was successfully implemented on a clinical scanner and was able to detect H2O17 with relatively high sensitivity. Primate study has shown that such a technique can be successfully used for human imaging applications to investigate and assess cerebral perfusion.

T1rho-weighted MRI using a surface coil to transmit spin-lock pulses

T1rho-weighted MRI is a novel basis for generating tissue contrast. However, it suffers from sensitivity to B1 inhomogeneity. First, excitation with a spatially varying B1 causes flip-angle artifacts and second, spin locking with an inhomogeneous B1 results in non-uniform T1rho contrast. In this study, we overcome the former complication with a specially designed spin-locking pulse sequence and we successfully obtain T1rho-weighted images with a surface coil. In this pulse sequence, the spin-lock pulse was divided into segments of equal duration and alternating phase. This "self-compensating" T1rho-preparatory pulse sequence was analyzed and the effect of an inhomogeneous B1 field was simulated using the Bloch equations. T1rho-weighted MR images of a phantom and a human knee joint in vivo were obtained on a clinical scanner with a surface coil to demonstrate the utility of the pulse sequence. The self-compensating T1rho-prepared pulses sequence resulted in substantially reduced image artifacts compared to the conventional, single-phase spin-lock pulse.

In vivo quantification ofT1? 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 T1rho contrast with an additional factor of intrinsic T2rho weighting, which hinders direct measurement of T1rho. A method is presented to compensate the MS-SL-generated data with regard to T2rho in an effort to accurately calculate multislice T1rho maps in a feasible experimental time. The T2rho-compensated multislice T1rho maps produced errors in the measurement of T1rho in healthy patellar cartilage of approximately 5% compared to the gold standard measurement of T1rho acquired with single-slice spin-lock pulse sequence. The MS-SL sequence has potential as an important clinical tool for the acquisition of multislice T1rho-weighted images and/or quantitative multislice T1rho maps.

Pulse sequence for multisliceT1?-weighted MRI

A 2D multislice spin-lock (MS-SL) MR pulse sequence is presented for rapid volumetric T1rho-weighted imaging. Image quality is compared with T1rho-weighted data collected using a single-slice (SS) SL sequence and T2-weighted data from a standard MS spin-echo (SE) sequence. Saturation of longitudinal magnetization by the application of nonselective SL pulses is experimentally measured and theoretically modeled as T2rho decay. The saturation data is used to correct the image data as a function of the SL pulse duration to make quantitative measurements of T1rho. Measurements of T1rho using the saturation-corrected MS-SL data are nearly identical to those measured using an SS-SL sequence. The MS-SL sequence produces quantitative T1rho maps of an entire sample volume with the high-SNR advantages conferred by SE-based sequences.

Method for reduced SART1?-weighted MRI

A reduced specific absorption rate (SAR) version of the T(1rho)-weighted MR pulse sequence was designed and implemented. The reduced SAR method employs a partial k-space acquisition approach in which a full power spin-lock pulse is applied to only the central phase-encode lines of k-space, while the remainder of k-space receives a low-power spin-lock pulse. Acquisition of high- and low-power phase-encode lines are interspersed chronologically to minimize average power deposition. In this way, the majority of signal energy in the central portion of k-space receives full T(1rho)-weighting, while the average SAR of the overall acquisition can be reduced, thereby lowering the minimum safely allowable TR. The pulse sequence was used to create T(1rho) maps of a phantom, an in vivo mouse brain, and the brain of a human volunteer. In the images of the human brain, SAR was reduced by 40% while the measurements of T(1rho) differed by only 2%. The reduced SAR sequence enables T(1rho)-weighted MRI in a clinical setting, even at high field strengths.

In vivo proton MR three-dimensional T1rho mapping of human articular cartilage: initial experience

The purpose of this study was to demonstrate the feasibility of computing three-dimensional relaxation maps of spin-lattice relaxation time in the rotating frame (T1rho) from in vivo magnetic resonance (MR) images of the human patellofemoral joint. T1rho was measured by applying a three-dimensional gradient-echo pulse sequence in six healthy subjects and one symptomatic subject by using a 1.5-T MR imager and a 15-cm-diameter transmit-receive quadrature birdcage radiofrequency coil. Average T1rho measured in healthy patellar cartilage was 49.7 msec +/- 3.2 (mean +/- SD). Two-dimensional T1rho-weighted images were obtained with a fast spin-echo pulse sequence for comparison. There was good correlation between two-dimensional and three-dimensional T1rho values for the six healthy subjects (R2 = 0.88, slope = 1.16).

T 1 rho-relaxation mapping of human femoral-tibial cartilage in vivo

PURPOSE

To demonstrate the in vivo feasibility of measuring spin-lattice relaxation time in the rotating frame (T(1rho)); and T(1rho)-dispersion in human femoral cartilage. Furthermore, we aimed to compute the baseline T(1rho)-relaxation times and spin-lock contrast (SLC) maps on healthy volunteers, and compare relaxation times and signal-to-noise ratio (SNR) with corresponding T(2)-weighted images.

MATERIALS AND METHODS

All MR imaging experiments were performed on a 1.5 T GE Signa scanner (GEMS, Milwaukee, WI) using a custom built 15-cm transmit-receive quadrature birdcage radio-frequency (RF) coil. The T(1rho)-prepared magnetization was imaged with a single-slice two-dimensional fast spin-echo (FSE) pulse sequence preencoded with a three-pulse cluster consisting of two hard 90 degrees pulses and a low power spin-lock pulse. T(1rho)-dispersion imaging was performed by varying the spin-lock frequency from 100 to 500 Hz in five steps in addition to varying the length of the spin-lock pulse.

RESULTS

The average T(1rho)-relaxation times in the weight-bearing (WB) and nonweight-bearing (NWB) regions of the femoral condyle were 42.2 +/- 3.6 msec and 55.7 +/- 2.3 msec (mean +/- SD, N = 5, P < 0.0001), respectively. In the same regions, the corresponding T(2)-relaxation times were 31.8 +/- 1.5 msec and 37.6 +/- 3.6 msec (mean +/- SD, N = 5, P < 0.0099). T(1rho)-weighted images have approximately 20%-30% higher SNR than the corresponding T(2)-weighted images for similar echo time. The average SLC in the WB region of femoral cartilage was 30 +/-4.0%. Furthermore, SLC maps provide better contrast between fluid and articular surface of femoral-tibial joint than T(1rho)-maps. The T(1rho)-relaxation times varied from 32 msec to 42 msec ( approximately 31%) in the WB and 37 msec to 56 msec ( approximately 51%) in NWB regions of femoral condyle, respectively, in the frequency range 0-500 Hz (T(1rho)-dispersion).

CONCLUSION

The feasibility of performing in vivo T(1rho) relaxation mapping in femoral cartilage at 1.5T clinical scanner without exceeding Food and Drug Administration (FDA) limits on specific absorption rate (SAR) of RF energy was demonstrated.

Three-dimensional T1rho-weighted MRI at 1.5 Tesla

PURPOSE

To design and implement a magnetic resonance imaging (MRI) pulse sequence capable of performing three-dimensional T(1rho)-weighted MRI on a 1.5-T clinical scanner, and determine the optimal sequence parameters, both theoretically and experimentally, so that the energy deposition by the radiofrequency pulses in the sequence, measured as the specific absorption rate (SAR), does not exceed safety guidelines for imaging human subjects.

MATERIALS AND METHODS

A three-pulse cluster was pre-encoded to a three-dimensional gradient-echo imaging sequence to create a three-dimensional, T(1rho)-weighted MRI pulse sequence. Imaging experiments were performed on a GE clinical scanner with a custom-built knee-coil. We validated the performance of this sequence by imaging articular cartilage of a bovine patella and comparing T(1rho) values measured by this sequence to those obtained with a previously tested two-dimensional imaging sequence. Using a previously developed model for SAR calculation, the imaging parameters were adjusted such that the energy deposition by the radiofrequency pulses in the sequence did not exceed safety guidelines for imaging human subjects. The actual temperature increase due to the sequence was measured in a phantom by a MRI-based temperature mapping technique. Following these experiments, the performance of this sequence was demonstrated in vivo by obtaining T(1rho)-weighted images of the knee joint of a healthy individual.

RESULTS

Calculated T(1rho) of articular cartilage in the specimen was similar for both and three-dimensional and two-dimensional methods (84 +/- 2 msec and 80 +/- 3 msec, respectively). The temperature increase in the phantom resulting from the sequence was 0.015 degrees C, which is well below the established safety guidelines. Images of the human knee joint in vivo demonstrate a clear delineation of cartilage from surrounding tissues.

CONCLUSION

We developed and implemented a three-dimensional T(1rho)-weighted pulse sequence on a 1.5-T clinical scanner.

Artifacts in T(1rho)-weighted imaging: correction with a self-compensating spin-locking pulse

Significant artifacts arise in T(1rho)-weighted imaging when nutation angles suffer small deviations from their expected values. These artifacts vary with spin-locking time and amplitude, severely limiting attempts to perform quantitative imaging or measurement of T(1rho) relaxation times. A theoretical model explaining the origin of these artifacts is presented in the context of a T(1rho)-prepared fast spin-echo imaging sequence. Experimentally obtained artifacts are compared to those predicted by theory and related to B(1) inhomogeneity. Finally, a "self-compensating" spin-locking preparatory pulse cluster is presented, in which the second half of the spin-locking pulse is phase-shifted by 180 degrees. Use of this pulse sequence maintains relatively uniform signal intensity despite large variations in flip angle, greatly reducing artifacts in T(1rho)-weighted imaging.

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

PURPOSE

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 (T(1rho)) imaging and T(1rho) relaxivity (R(1rho)).

MATERIALS AND METHODS

All the magnetic resonance imaging (MRI) experiments were performed on a 4-T whole-body GE Signa scanner (GEMS, Milwaukee, WI), and spectroscopy experiments of chondroitin sulfate (CS) phantoms were done on a 2-T custom-built spectrometer. A custom-built 11-cm-diameter transmit-receive birdcage coil, which was tuned to a proton frequency of 170 MHz, was employed for the imaging experiments. T(1rho) measurements were made using a fast spin echo (FSE) sequence pre-encoded with a three-pulse cluster consisting of two 90 degrees hard pulses separated by a low-power rectangle pulse for spin-locking.

RESULTS

The methodology is first validated on CS phantoms and then used to quantify GAG content in intact bovine cartilage (N = 5). There is a good agreement between the GAG map calculated from the T(1rho) ratio imaging method (71 +/- 4%) and GAG measured from spectrophotometric assay (75 +/- 5%) in intact bovine tissue.

CONCLUSION

We have demonstrated a proton spin-lock ratio imaging method to quantify absolute GAG distribution in the cartilage in a noninvasive and nondestructive manner.

Water magnetic relaxation dispersion in biological systems: the contribution of proton exchange and implications for the noninvasive detection of cartilage degradation

Magnetic relaxation has been used extensively to study and characterize biological tissues. In particular, spin-lattice relaxation in the rotating frame (T(1rho)) of water in protein solutions has been demonstrated to be sensitive to macromolecular weight and composition. However, the nature of the contribution from low frequency processes to water relaxation remains unclear. We have examined this problem by studying the water T(1rho) dispersion in peptide solutions ((14)N- and (15)N-labeled), glycosaminoglycan solutions, and samples of bovine articular cartilage before and after proteoglycan degradation. We find in model systems and tissue that hydrogen exchange from NH and OH groups to water dominates the low frequency water T(1rho) dispersion, in the context of the model used to interpret the relaxation data. Further, low frequency dispersion changes are correlated with loss of proteoglycan from the extra-cellular matrix of articular cartilage. This finding has significance for the noninvasive detection of matrix degradation.

Human knee: in vivo T1(rho)-weighted MR imaging at 1.5 T--preliminary experience

A fast spin-echo sequence weighted with a time constant that defines the magnetic relaxation of spins under the influence of a radio-frequency field (T1(rho)) was used in six subjects to measure magnetic resonance (MR) relaxation times in the knee joint with a 1.5-T MR imager. A quantitative comparison of T2- and T1(rho)-weighted MR images was also performed. Substantial T1(rho) dispersion was demonstrated in human articular cartilage, but muscle did not demonstrate much dispersion. T1(rho)-weighted images depicted a chondral lesion with 25% better signal-difference-to-noise ratios than comparable T2-weighted images. This technique may depict cartilage and muscular abnormalities.

T1rho imaging of murine brain tumors at 4 T

RATIONALE AND OBJECTIVES

The goal of this study was to evaluate the utility of T1rho weighting in magnetic resonance imaging of murine brain tumors.

MATERIALS AND METHODS

S91 Cloudman melanoma was implanted in mouse brains (n = 4). A T2-weighted spin-echo (SE) and a T1rho-weighted fast SE-based sequence were performed on a 4-T clinical imager. T2 and T1rho maps were computed. The tumor-to-normal-tissue contrast was compared between T2-weighted, T1rho-weighted, proton-density-weighted, and pre- and postcontrast T1-weighted SE images.

RESULTS

The tumor-tissue contrast of the T1rho-weighted images was similar to that of the T2-weighted images but less than that of the postcontrast T1-weighted images. The T1rho-weighted images provided better definition of tumor boundaries than T2-weighted images. At spin-locking powers of 0.5 and 1.5 kHz, the T1rho of the tumor was 64.0 msec +/- 0.46 and 68.65 msec +/- 0.59, respectively. There was no significant inter- or intra-animal variation in T1rho for tumor or normal brain cortex.

CONCLUSION

T1rho-weighted imaging performed at low spin-lock strengths qualitatively depicted tumor borders better than proton-density or T2-weighted imaging and could be useful in treatment planning when combined with other imaging sequences.

T1rho imaging using magnetization-prepared projection encoding (MaPPE)

T1rho contrast weighting using a magnetization-prepared projection encoding (MaPPE) pulse sequence was investigated. Fast radial imaging was implemented by applying magnetization preparation pulses, each followed by multiple RF alpha pulses encoding radial trajectories of k-space. Acquiring multiple views per preparatory pulse imposes view-to-view variation; the resultant distortion of the point-spread function is examined. The issue of maximizing signal while preserving the intended contrast weighting is addressed. Under modification of repetition time and flip angle (alpha), three distinct behavior regimes of the sequence are identified. The utility of the pulse sequence as a quantitative relaxation measurement tool is also examined by comparing imaging and spectroscopy experiments. A mouse was imaged in vitro to demonstrate the viability of application to MR histology. These images exhibit the utility of spinlocking and projection encoding as an aftemative contrast source to both T2-weighted MaPPE images and conventional T2-weighted spin-echo images.