Effects of B1 inhomogeneity correction for three-dimensional variable flip angle T1 measurements in hip dGEMRIC at 3 T and 1.5 T

Delayed gadolinium-enhanced MRI of cartilage is a technique for studying the development of osteoarthritis using quantitative T(1) measurements. Three-dimensional variable flip angle is a promising method for performing such measurements rapidly, by using two successive spoiled gradient echo sequences with different excitation pulse flip angles. However, the three-dimensional variable flip angle method is very sensitive to inhomogeneities in the transmitted B(1) field in vivo. In this study, a method for correcting for such inhomogeneities, using an additional B(1) mapping spin-echo sequence, was evaluated. Phantom studies concluded that three-dimensional variable flip angle with B(1) correction calculates accurate T(1) values also in areas with high B(1) deviation. Retrospective analysis of in vivo hip delayed gadolinium-enhanced MRI of cartilage data from 40 subjects showed the difference between three-dimensional variable flip angle with and without B(1) correction to be generally two to three times higher at 3 T than at 1.5 T. In conclusion, the B(1) variations should always be taken into account, both at 1.5 T and at 3 T.     

Gd-EOB-DTPA增强MRI T1 mapping技术在肝脏疾病的应用进展

T_1 mapping成像是定量分析组织T_1值的方法,主要包括Look-Locker(LL)序列、改良的Look-Locker反转恢复序列(MOLLI)及可变多翻转角成像技术。近年来,基于Gd-EOB-DTPA的T_1 mapping成像广泛应用于肝脏。根据注射对比剂前后肝实质的T_1变化,不仅能用于对肝脏损伤程度、肝纤维化分期、肝硬化病人肝储备功能的评估,而且对肝脏局灶性病变的鉴别、肝细胞肝癌分化程度的预测也有一定的价值。就Gd-EOB-DTPA增强MRI T_1 mapping技术在肝脏疾病中的应用现状及研究进展予以综述。     

Novel method for rapid, simultaneous T1, T*2, and proton density quantification.

An imaging method called "quantification of relaxation times and proton density by twin-echo saturation-recovery turbo-field echo" (QRAPTEST) is presented as a means of quickly determining the longitudinal T(1) and transverse T(2) (*) relaxation time and proton density (PD) within a single sequence. The method also includes an estimation of the B(1) field inhomogeneity. High-resolution images covering large volumes can be achieved within clinically acceptable times of 5-10 min. The range of accuracy for determining T(1), T(2) (*), and PD values is flexible and can be optimized relative to any anticipated values. We validated the experimental results against existing methods, and provide a clinical example in which quantification of the whole brain using 1.5 mm(3) voxels was achieved in less than 8 min.     

Simultaneous variable flip angle-actual flip angle imaging method for improved accuracy and precision of three-dimensional T1 and B1 measurements

A new time-efficient and accurate technique for simultaneous mapping of T(1) and B(1) is proposed based on a combination of the actual flip angle (FA) imaging and variable FA methods. Variable FA-actual FA imaging utilizes a single actual FA imaging and one or more spoiled gradient-echo acquisitions with a simultaneous nonlinear fitting procedure to yield accurate T(1)/B(1) maps. The advantage of variable FA-actual FA imaging is high accuracy at either short T(1) times or long repetition times in the actual FA imaging sequence. Simulations show this method is accurate to 0.03% in FA and 0.07% in T(1) for ratios of repetition time to T1 time over the range of 0.01-0.45. We show for the case of brain imaging that it is sufficient to use only one small FA spoiled gradient-echo acquisition, which results in reduced spoiling requirements and a significant scan time reduction compared to the original variable FA method. In vivo validation yielded high-quality 3D T(1) maps and T(1) measurements within 10% of previously published values and within a clinically acceptable scan time. The variable FA-actual FA imaging method will increase the accuracy and clinical feasibility of many quantitative MRI methods requiring T(1)/B(1) mapping such as dynamic contrast enhanced perfusion and quantitative magnetization transfer imaging.     

Accuracy of T1 measurement with 3-D Look-Locker technique for dGEMRIC

PURPOSE: To validate the accuracy of T1 measurement by three-dimensional Look-Locker method (3D LL) for delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) of human subjects with and without osteoarthritis (OA), as compared with two-dimensional inversion recovery fast spin-echo (2D IR-FSE) technique. MATERIALS AND METHODS: MR sagittal images of the knees were acquired for T1 mapping in 29 subjects with standard 2D IR-FSE and 3D LL sequences 90-135 min following administration of 0.2 mmol/kg Gd-DTPA(2-). T1 maps of femoral and tibial cartilage were generated using custom software. Comparisons in T1 values between the two techniques were performed using regression analysis. RESULTS: Good agreement in T1 values between 2D IR-FSE and 3D LL was observed (R values of 0.90, and 0.85, and 0.86 for all, OA, and control subjects, respectively) when acquired within 15 min. CONCLUSION: The 3D LL sequence provides accurate T1 estimates of articular cartilage with advantages of entire joint coverage, shorter acquisition time, and a wide range of inversion times sampled within a single acquisition.     

Comparison of delayed gadolinium enhanced MRI of cartilage (dGEMRIC) using inversion recovery and fast T1 mapping sequences

The delayed Gadolinium Enhanced MRI of Cartilage (dGEMRIC) technique has shown promising results in pilot clinical studies of early osteoarthritis. Currently, its broader acceptance is limited by the long scan time and the need for postprocessing to calculate the T1 maps. A fast T1 mapping imaging technique based on two spoiled gradient echo images was implemented. In phantom studies, an appropriate flip angle combination optimized for center T1 of 756 to 955 ms yielded excellent agreement with T1 measured using the inversion recovery technique in the range of 200 to 900 ms, of interest in normal and diseased cartilage. In vivo validation was performed by serially imaging 26 hips using the inversion recovery and the Fast 2 angle T1 mapping techniques (center T1 756 ms). Excellent correlation with Pearson correlation coefficient R2 of 0.74 was seen and Bland‐Altman plots demonstrated no systematic bias. Magn Reson Med 60:768–773, 2008. © 2008 Wiley‐Liss, Inc.     

T1 relaxation time at 0.2 Tesla for monitoring regional hyperthermia: feasibility study in muscle and adipose tissue.

The purpose of this study was to characterize T(1), particularly in the hyperthermia temperature range (ca. 37-44 degrees C), in order to control regional hyperthermia with MR monitoring using 0.2 Tesla, and to improve T(1) mapping. A single-slice and a new multislice "T One by Multiple Read-Out Pulses" (TOMROP) pulse sequence were used for fast T(1) mapping in a clinical MRI hyperthermia hybrid system. Temporal stability, temperature sensitivity, and reversibility of T(1) were investigated in a polyamidacryl gel phantom and in samples of muscle and adipose tissues from turkey and pig, and verified in patients. In the gel phantom a high linear correlation between T(1) and temperature (R(2) = 0.97) was observed. In muscle and adipose tissue, T(1) and temperature had a linear relationship below a breakpoint of 43 degrees C. Above this breakpoint muscle tissue showed irreversible tissue changes; these effects were not visible in adipose tissue. The ex vivo results were confirmed in vivo under clinical conditions. T(1) mapping allows the characterization of hyperthermia-related tissue response in healthy tissue. T(1), in combination with fast mapping, is suitable for controlling regional hyperthermia at 0.2 T within the hybrid system.     

Rapid combined T1 and T2 mapping using gradient recalled acquisition in the steady state.

A novel, fully 3D, high-resolution T(1) and T(2) relaxation time mapping method is presented. The method is based on steady-state imaging with T(1) and T(2) information derived from either spoiling or fully refocusing the transverse magnetization following each excitation pulse. T(1) is extracted from a pair of spoiled gradient recalled echo (SPGR) images acquired at optimized flip angles. This T(1) information is combined with two refocused steady-state free precession (SSFP) images to determine T(2). T(1) and T(2) accuracy was evaluated against inversion recovery (IR) and spin-echo (SE) results, respectively. Error within the T(1) and T(2) maps, determined from both phantom and in vivo measurements, is approximately 7% for T(1) between 300 and 2000 ms and 7% for T(2) between 30 and 150 ms. The efficiency of the method, defined as the signal-to-noise ratio (SNR) of the final map per voxel volume per square root scan time, was evaluated against alternative mapping methods. With an efficiency of three times that of multipoint IR and three times that of multiecho SE, our combined approach represents the most efficient of those examined. Acquisition time for a whole brain T(1) map (25 x 25 x 10 cm) is less than 8 min with 1 mm(3) isotropic voxels. An additional 7 min is required for an identically sized T(2) map and postprocessing time is less than 1 min on a 1 GHz PIII PC. The method therefore permits real-time clinical acquisition and display of whole brain T(1) and T(2) maps for the first time.     

Fast and optimized T1 mapping technique for the noninvasive quantification of gastric secretion

PURPOSE: To evaluate the noninvasive quantification of gastric secretion volume after administration of a labeled viscous glucose solution by fast T(1) mapping. MATERIALS AND METHODS: T(1) values of a series of labeled and diluted glucose solutions were measured in vitro to characterize the interrelationship between T(1) and contrast agent concentration (C(Gd)) as well as the dependency of relaxivity and reference T(1) (T(10)) on the macromolecular concentration. Abdominal T(1) mapping in five healthy volunteers of different body mass index was performed after filling an intragastric balloon with a labeled and diluted glucose solution. In additional ex vivo experiments, T(1) values of gastric (GJ) and duodenal juice (DJ) and 0.1 N HCl solution were determined. RESULTS: A linear relationship between relaxivity and macromolecular concentration and between T(10) and macromolecular concentration was found. The in vitro T(1)-C(Gd) calibration curve was successfully validated in all volunteers. T(1) values of GJ, DJ, and HCl (2939 msec vs. 2858 msec vs. 2760 msec) were close to the T(1) of water ( approximately 3000 msec). CONCLUSION: The presented method allows one to noninvasively quantify the spatial distribution of gastric secretory products in the human stomach and provides a valuable tool for evaluating the efficacy of drugs to stimulate/inhibit gastric secretion.     

Improved encoding strategy for CPMG-based Bloch-Siegert B(1)(+) mapping

Bloch-Siegert (BS) based B(1)(+) mapping methods use off-resonant pulses to encode quantitative B(1)(+) information into the signal phase. It was recently shown that the principle behind BS-based B(1)(+) mapping can be expanded from spin echo (BS-SE) and gradient-echo (BS-FLASH) based BS B(1)(+) mapping to methods such as Carr, Purcell, Meiboom, Gill (CPMG)-based turbo-spin echo (BS-CPMG-TSE) and multi-spin echo (BS-CPMG-MSE) imaging. If CPMG conditions are preserved, BS-CPMG-TSE allows fast acquisition of the B(1)(+) information and BS-CPMG-MSE enables simultaneous mapping of B(1)(+), M(0), and T(2). To date, however, two separate MRI experiments must be performed to enable the calculation of B(1)(+) maps. This study investigated a modified encoding strategy for CPMG BS-based methods to overcome this limitation. By applying a "bipolar" off-resonant BS pulse before the refocusing pulse train, the needed phase information was able to be encoded into different echo images of one echo train. Thus, this technique allowed simultaneous B(1)(+) and T(2) mapping in a single BS-CPMG-MSE experiment. To allow single-shot B(1)(+) mapping, this method was also applied to turbo-spin echo imaging. Furthermore, the presented modification intrinsically minimizes phase-based image artifacts in BS-CPMG-TSE experiments.     

Full-brain T1 mapping through inversion recovery fast spin echo imaging with time-efficient slice ordering.

Brain T1 mapping has important clinical applications in detecting brain disorders. Conventional T1 mapping techniques are usually based on inversion recovery spin echo (IRSE) imaging or its more time-efficient counterpart inversion recovery fast spin echo (IRFSE) imaging because they can deliver good image quality. Multiple inversion times are required to accurately estimate T1 over a wide range of values. Without acquisition optimization, both the IRSE and the IRFSE T1 mapping techniques require long scan times to image the whole brain. To reduce the scan time and maintain the quality of the T1 maps, we propose a new full-brain T1 mapping pulse sequence based on a multislice inversion recovery fast spin echo imaging using a time-efficient slice ordering technique.     

Combined T1-T2 mapping of human femoro-tibial cartilage with turbo-mixed imaging at 1.5T

PURPOSE: To evaluate the influence of Gd-DTPA on cartilage T2 mapping using turbo-mixed (tMIX) imaging, and to show the possible usefulness of the tMIX technique for simultaneously acquiring T1 and T2 information in cartilage. MATERIALS AND METHODS: Twenty volunteers underwent MRI of the knee using the tMIX sequence before and after gadolinium administration. T1 and T2 maps were calculated. The mean T1 was determined on the pre- and postcontrast T1 maps. T2 relaxation values before and after gadolinium administration were statistically analyzed. RESULTS: The obtained relaxation values are in correspondence with previously published data. The mean T1 before gadolinium administration was 449 msec +/- 34.2 msec (SD), and after gadolinium administration it was 357 msec +/- 55.8 msec (SD). The postcontrast T1 relaxation range was 221.5-572.8 msec. The mean T2 of the precontrast T2 maps was 34.2 msec +/- 3.1 msec (SD), and the mean T2 of the postcontrast T2 maps was 32.5 msec +/- 3.1 msec (SD). These are statistically significant different values. A correction for the postcontrast T2 values, using a back-calculation algorithm, yielded a 98% correlation with the precontrast T2 values. CONCLUSION: The absolute difference of pre- and postcontrast T2 is very small and is ruled out using the back-calculation algorithm. Combined T1-T2 tMIX cartilage mapping is a valuable alternative for separate T1 and T2 cartilage mapping.     

SAR-reduced spin-echo-based Bloch-Siegert B(1)(+) mapping: BS-SE-BURST

Fast and accurate B(1)(+) mapping is possible using phase-based Bloch-Siegert (BS) methods. Importantly, the off-resonant pulses needed for BS B(1)(+) mapping methods can easily be implemented in multiple MR sequences. BS-based B(1)(+) mapping has thus been introduced for gradient echo (BS-FLASH), spin-echo (BS-SE), and Carr, Purcell, Meiboom, Gill (CPMG)-based multi-SE and turbo-SE sequences. When using SE and multi-SE/turbo-SE-based BS sequences, however, the high intrinsic specific absorption rates must be considered in clinical situations. This study introduces a fast BS B(1)(+) mapping method based on a SE-BURST sequence (BS-SE-BURST). With SE-BURST sequences, multiple low-magnitude excitation pulses are applied prior to the refocusing pulse. Thus, multiple and different phase-encoded echoes can be acquired per excitation cycle. Compared with a SE sequence, this excitation strategy results in a similar signal-to-noise ratio (SNR) per unit time but with reduced specific absorption rate. The proposed BS-SE-BURST sequence was implemented on a conventional 3 T whole body MRI scanner and applied successfully.     

Error reduction and parameter optimization of the TAPIR method for fast T1 mapping.

A methodology is presented for the reduction of both systematic and random errors in T(1) determination using TAPIR, a Look-Locker-based fast T(1) mapping technique. The relations between various sequence parameters were carefully investigated in order to develop recipes for choosing optimal sequence parameters. Theoretical predictions for the optimal flip angle were verified experimentally. Inversion pulse imperfections were identified as the main source of systematic errors in T(1) determination with TAPIR. An effective remedy is demonstrated which includes extension of the measurement protocol to include a special sequence for mapping the inversion efficiency itself.     

Fast cardiac T(1) mapping in mice using a model-based compressed sensing method

Direct measurement of the longitudinal relaxation time T₁ provides objective and quantitative diagnostic information. However, current T₁ mapping methods are generally time consuming without the aid of fast imaging. This study used a model‐based compressed sensing method for fast cardiac T₁ mapping in small animals. Based on the physics of magnetization recovery, the aliasing artifact associated with under‐sampling was removed by exploiting the sparsity of the signals in the T₁ recovery direction. Simulation study was performed to evaluate the reconstruction accuracy under various experimental conditions. Several approaches that accounted for phase variations were compared for optimized reconstruction in the phantom study. In vivo validation was performed on a cardiac manganese‐enhanced MRI study using mice. Accurate reconstruction of the under‐sampled images and the resulting T₁ maps were achieved in both simulation and MRI studies on phantom and in vivo mice. These results suggest that the current compressed sensing method allows fast (<80 s) T₁ mapping of the mouse heart at high spatial resolution (234 × 469 μm²). Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

T1 corrected B1 mapping using multi‐TR gradient echo sequences

This work presents a new approach toward a fast, simultaneous amplitude of radiofrequency field (B₁) and T₁ mapping technique. The new method is based on the “actual flip angle imaging” (AFI) sequence. However, the single pulse repetition time (TR) pair used in the standard AFI sequence is replaced by multiple pulse repetition time sets. The resulting method was called “multiple TR B₁/T₁ mapping” (MTM). In this study, MTM was investigated and compared to standard AFI in simulations and experiments. Feasibility and reliability of MTM were proven in phantom and in vivo experiments. Error propagation theory was applied to identify optimal sequence parameters and to facilitate a systematic noise comparison to standard AFI. In terms of accuracy and signal‐to‐noise ratio, the presented method outperforms standard AFI B₁ mapping over a wide range of T₁. Finally, the capability of MTM to determine T₁ was analyzed qualitatively and quantitatively, yielding good agreement with reference measurements. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

A B1‐insensitive high resolution 2D T1 mapping pulse sequence for dGEMRIC of the HIP at 3 Tesla

Early detection of cartilage degeneration in the hip may help prevent onset and progression of osteoarthritis in young patients with femoroacetabular impingement. Delayed gadolinium‐enhanced MRI of cartilage is sensitive to cartilage glycosaminoglycan loss and could serve as a diagnostic tool for early cartilage degeneration. We propose a new high resolution 2D T₁ mapping saturation–recovery pulse sequence with fast spin echo readout for delayed gadolinium‐enhanced magnetic resonance imaging of cartilage of the hip at 3 T. The proposed sequence was validated in a phantom and in 10 hips, using radial imaging planes, against a rigorous multipoint saturation–recovery pulse sequence with fast spin echo readout. T₁ measurements by the two pulse sequences were strongly correlated (R² > 0.95) and in excellent agreement (mean difference = ?8.7 ms; upper and lower 95% limits of agreement = 64.5 and ?81.9 ms, respectively). T₁ measurements were insensitive to B₁₊ variation as large as 20%, making the proposed T₁ mapping technique suitable for 3 T. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Interleaved T1 and T2 relaxation time mapping for cardiac applications

Purpose

To diagnose acute myocardial infarction (MI) with MRI, T₁‐weighted and T₂‐weighted images are required to detect necrosis and edema. The calculation of both T₁ and T₂ maps can be relevant for quantitative diagnosis. In this work, we present a simultaneous quantification of T₁‐T₂ relaxation times of a short‐axis view of the heart in a single scan.

Materials and Methods

An electrocardiograph (ECG)‐triggered, navigator‐gated, interleaved T₁ and T₂ mapping sequence was implemented for the quantification of the T₁ and T₂ values of phantoms, healthy volunteers, and three patients with acute MI. The proposed acquisition scheme consisted of an interleaved two‐dimensional (2D) steady‐state free precession (SSFP) sequence with three different modules: an inversion‐recovery (IR) sequence with multiple time delays, followed by a delay of one cardiac cycle for magnetization recovery and a T₂‐preparation pulse with multiple echo‐times for T₂ quantification.

Results

Measurements of in vivo relaxation times were in good agreement with literature values. The interleaved sequence was able to measure T₁ and T₂ relaxation times of the myocardium.

Conclusion

The interleaved sequence acquires data for the calculation of T₁ and T₂ maps in only one scan without the need for registration. This technique has the potential to differentiate between acute and chronic MI by estimating the concentration of gadolinium diethylenetriamine pentaacetic acid (Gd‐DTPA) in the necrotic tissue and to assess the extent of edema from T₂ maps. J. Magn. Reson. Imaging 2009;29:480–487. © 2009 Wiley‐Liss, Inc.     

Mapping of the B1 field distribution of a surface coil resonator using EPR imaging.

Surface coil resonators have been widely used to perform topical EPR spectroscopy. They are usually positioned adjacent to or implanted within the body. For EPR applications these resonators have a number of important advantages over other resonator designs due to their ease of sample accessibility, mechanical fabrication, implementation of electronic tuning and coupling functions, and low susceptibility to sample motions. However, a disadvantage is their B(1) field inhomogeneity, which limits their usefulness for 3D imaging applications. We show that this problem can be addressed by mapping and correcting the B(1) field distribution. We report the use of EPR imaging (EPRI) to map the B(1) distribution of a surface coil resonator. We show that EPRI provides a fast, accurate, and reliable technique to evaluate the B(1) distribution. 3D EPRI was performed on phantoms, prepared using three different saline concentrations, to obtain the B(1) distribution. The information obtained from the phantoms was used to correct the images of living animals. With the use of this B(1) correction technique, surface coil resonators can be applied to perform 3D mapping of the distribution of free radicals in biological samples and living systems.     

Indirect 17(O)-magnetic resonance imaging of cerebral blood flow in the rat.

Proton T(1rho)-dispersion MRI is demonstrated for indirect, in vivo detection of (17)O in the brain. This technique, which may be readily implemented on any clinical MRI scanner, is applied towards high-resolution, quantitative mapping of cerebral blood flow (CBF) in the rat by monitoring the clearance of (17)O-enriched water. Strategies are derived and employed for 1) quantitation of absolute H(2) (17)O tracer concentration from a ratio of high- and low-frequency spin-locked T(1rho) images, and 2) mapping CBF without having to transform the T(1rho) signal to H(2) (17)O tracer concentration. Absolute regional blood flow was mapped in a single 3-mm brain slice at an in-plane resolution of 0.4 x 0.8 mm within a 5-min tracer washout time; these data are consistent with the less localized CBF measurements reported in the literature. T(1rho)-weighted imaging yields excellent signal-to-noise ratios, spatiotemporal resolution, and anatomical contrast for mapping CBF.