Investigating the effect of exchange and multicomponent T(1) relaxation on the short repetition time spoiled steady-state signal and the DESPOT1 T(1) quantification method

PURPOSE: To examine the spoiled steady-state (spoiled gradient-recalled echo sequence [SPGR]) signal arising from two-compartment systems and the role of experimental parameters, in particular TR for resolving signal from each compartment. MATERIALS AND METHODS: Using Bloch-McConnell simulations, we examined the SPGR signal from two-component systems in which T(1) is much greater than the mean residence time (tau(m)) of proton spins in each component. Specifically, we examined the role of TR on the ability to resolve each components signal, as well as the influence of experimental parameters on derived DESPOT1 T(1) values. RESULTS: Results revealed that when TR < or = 0.01 tau(m), the measured SPGR signal may be modeled as a summation of signal from each species using a no-exchange approximation. Additionally, under this short TR condition, the driven equilibrium single pulse observation of T(1) (DESPOT1) mapping approach provides T(1) values preferentially biased toward the short or long T(1) species, depending on the choice of flip angles. CONCLUSION: The ability to model the SPGR signal using a no-exchange approximation may permit the quantification multicomponent T(1) relaxation in vivo. Additionally, the ability to preferentially weight the DESPOT1 T(1) value toward the short or long T(1) may provide a useful window into these components.     

Change in the proton T1 of fat and water in mixture

This work describes observed changes in the proton T₁ relaxation time of both water and lipid when they are in relatively homogeneous mixtures. Results obtained from vegetable oil–water emulsions, pork kidney and lard mixtures, and excised samples of white and brown adipose tissues are presented to demonstrate this change in T₁ as a function of mixture fat fraction. As an initial proof of concept, a simpler acetone‐water experiment was performed to take advantage of complete miscibility between acetone and water and both components' single chemical shift peaks. Single‐voxel MR spectroscopy was used to measure the T₁ of predominant methylene spins in fat and the T₁ of water spins in each setup. In the vegetable oil–water emulsions, the T₁ of fat varied by as much as 3‐fold when water was the dominant mixture component. The T₁ of pure lard increased by 170 msec (+37%) when it was blended with lean kidney tissue in a 16% fatty mixture. The fat T₁ of lipid‐rich white adipose tissue was 312 msec. In contrast, the fat T₁ of leaner brown adipose tissue (fat fraction 53%) was 460 msec. A change in the water T₁ from that of pure water was also observed in the experiments. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

T1rho-prepared balanced gradient echo for rapid 3D T1rho MRI

PURPOSE: To develop a T1rho-prepared, balanced gradient echo (b-GRE) pulse sequence for rapid three-dimensional (3D) T1rho relaxation mapping within the time constraints of a clinical exam (<10 minutes), examine the effect of acquisition on the measured T1rho relaxation time and optimize 3D T1rho pulse sequences for the knee joint and spine. MATERIALS AND METHODS: A pulse sequence consisting of inversion recovery-prepared, fat saturation, T1rho-preparation, and b-GRE image acquisition was used to obtain 3D volume coverage of the patellofemoral and tibiofemoral cartilage and lower lumbar spine. Multiple T1rho-weighted images at various contrast times (spin-lock pulse duration [TSL]) were used to construct a T1rho relaxation map in both phantoms and in the knee joint and spine in vivo. The transient signal decay during b-GRE image acquisition was corrected using a k-space filter. The T1rho-prepared b-GRE sequence was compared to a standard T1rho-prepared spin echo (SE) sequence and pulse sequence parameters were optimized numerically using the Bloch equations. RESULTS: The b-GRE transient signal decay was found to depend on the initial T1rho-preparation and the corresponding T1rho map was altered by variations in the point spread function with TSL. In a two compartment phantom, the steady state response was found to elevate T1rho from 91.4+/-6.5 to 293.8+/-31 and 66.9+/-3.5 to 661+/-207 with no change in the goodness-of-fit parameter R2. Phase encoding along the longest cartilage dimension and a transient signal decay k-space filter retained T1rho contrast. Measurement of T1rho using the T1rho-prepared b-GRE sequence matches standard T1rho-prepared SE in the medial patellar and lateral patellar cartilage compartments. T1rho-preparedb-GRE T1rho was found to have low interscan variability between four separate scans. Mean patellar cartilage T1rho was elevated compared to femoral and tibial cartilage T1rho. CONCLUSION: The T1rho-prepared b-GRE acquisition rapidly and reliably accelerates T1rho quantification of tissues offset partially by a TSL-dependent point spread function.     

Spatial distribution and relationship of T1ρ and T2 relaxation times in knee cartilage with osteoarthritis

T_1ρ and T₂ relaxation time constants have been proposed to probe biochemical changes in osteoarthritic cartilage. This study aimed to evaluate the spatial correlation and distribution of T_1ρ and T₂ values in osteoarthritic cartilage. Ten patients with osteoarthritis (OA) and 10 controls were studied at 3T. The spatial correlation of T_1ρ and T₂ values was investigated using Z‐scores. The spatial variation of T_1ρ and T₂ values in patellar cartilage was studied in different cartilage layers. The distribution of these relaxation time constants was measured using texture analysis parameters based on gray‐level co‐occurrence matrices (GLCM). The mean Z‐scores for T_1ρ and T₂ values were significantly higher in OA patients vs. controls (P < 0.05). Regional correlation coefficients of T_1ρ and T₂ Z‐scores showed a large range in both controls and OA patients (0.2–0.7). OA patients had significantly greater GLCM contrast and entropy of T_1ρ values than controls (P < 0.05). In summary, T_1ρ and T₂ values are not only increased but are also more heterogeneous in osteoarthritic cartilage. T_1ρ and T₂ values show different spatial distributions and may provide complementary information regarding cartilage degeneration in OA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, 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.     

Fast high-resolution T1 mapping of the human brain.

A sequence for the acquisition of high-resolution T1 maps, based on magnetization-prepared multislice fast low-angle shot (FLASH) imaging, is presented. In contrast to similar methods, no saturation pulses are used, resulting in an increased dynamic range of the relaxation process. Furthermore, it is possible to acquire data during all relaxation delays because only slice-selective radiofrequency (RF) pulses are used for inversion and excitation. This allows for a reduction of the total acquisition time, or scanning with a reduced bandwidth, which improves the signal-to-noise ratio (SNR). The method generates quantitative T1 maps with an in-plane resolution of 1 mm, slice thickness of 4 mm, and whole-brain coverage in a clinically acceptable imaging time of about 19 s per slice. It is shown that the use of off-center RF pulses does not result in imperfect inversion or magnetization transfer (MT) effects. In addition, an improved fitting algorithm based on smoothed flip angle maps is presented and tested successfully.     

Gd-DTPA bolus tracking in the myocardium using T1 fast acquisition relaxation mapping (T1 FARM).

MRI methods currently used for bolus tracking in the myocardium, such as saturation recovery turbo-fast low-angle shot (FLASH) (srTFL), are limited by signal intensity (SI) saturation at high contrast agent (CA) concentrations. By using T1 fast acquisition relaxation mapping (T1 FARM), a Gd-DTPA bolus (0.075 vs. 0.025 mmol/kg) may be injected without causing saturation. This study tested the feasibility of in vivo T1 FARM bolus tracking under rest/stress conditions in seven beagles with multiple permanently occluded branches of the left anterior descending (LAD) coronary artery. Although it underestimated the myocardial perfusion reserve (MPR) measured ex vivo using radioactive microspheres (mean +/- SEM; 3.60 +/- 0.26), the MPR determined upon application of the modified Kety model (1.86 +/- 0.10) enabled distinction between normal and infarcted tissue. The partition coefficient (lambda) estimated at rest and stress using the modified Kety model underestimated ex vivo radioactive measurements in infarcted tissue (0.25 +/- 0.01 vs. 0.26 +/- 0.01 vs. 0.79 +/- 0.08 ml/g, P < 0.0001) yet was accurate in normal tissue (0.28 +/- 0.01 vs. 0.30 +/- 0.01 vs. 0.33 +/- 0.01 ml/g, P = NS). Thus, although unsuitable for myocardial viability assessment, T1 FARM bolus tracking shows potential for assessment of myocardial perfusion.     

High magnetic field water and metabolite proton T1 and T2 relaxation in rat brain in vivo.

Comprehensive and quantitative measurements of T1 and T2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T1) and decrease (T2) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T2 relaxation and a strong increase of T1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T1/T2-based anatomical MRI quickly levels off beyond approximately 7 T and may actually decrease at higher magnetic fields.     

Suitability of T(1Gd) as the dGEMRIC index at 1.5T and 3.0T.

Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) is based on the theory that Gd-DTPA(2-) will distribute in inverse relation to cartilage glycosaminoglycan (GAG). T(1Gd) (T(1) after penetration of a 0.2 mmol/kg dose of Gd-DTPA(2-)) has been used as the dGEMRIC index, although (1/T(1Gd)-1/T(1o)) should be more representative of Gd-DTPA(2-) concentration (where T(1o) = T(1) before contrast). T(1o) and T(1Gd) were measured in 20 volunteers at both 1.5T and 3T and the correlation between the metrics of T(1Gd) and (1/T(1Gd)-1/T(1o)) was calculated. There was a high correlation coefficient between the two metrics at both field strengths, with R = 0.94, 0.93, and 0.90 for central medial femur, posterior medial femur, and medial tibia, respectively, at 1.5T and 0.87, 0.94, 0.96 at 3T. In all cases P < 0.0001. Therefore, these data suggest that, for native cartilage, the current practice of measuring T(1Gd) (but not also T(1o)) is adequate at both 1.5T and 3T.     

T1, T2 and proton density measurements in the grading of cerebral gliomas

The possibility that cerebral tumours may be graded by measuring T1 or T2 with magnetic resonance (MR) imaging was studied. A consecutive series of patients with subsequently verified gliomas was enrolled, and studied with MR. Patients who had prior surgical, chemotherapeutic or steroid treatment were excluded. Single slice multiple saturation recovery and multiple spin echo techniques were used to measure T1, T2 and proton density in the tumour. In 33 patients with cerebral gliomas there were 5 grade I, 12 grade II, 7 grade III and 9 grade IV. T1 and T2 values tended to be smaller in grade I gliomas than in grades II, III and IV gliomas. Relaxation parameters overlapped considerably in tumours with different grades. Proton density values did not show much change between different grades of gliomas. Relaxation parameters cannot be used to determine tumour grade reliably. Correspondence to: S. Newman