Driven-equilibrium single-pulse observation of T1 relaxation. A reevaluation of a rapid “new” method for determining NMR spin-lattice relaxation times

312

396

Rapid 3D mapping of T 1 relaxation times is valuable in diverse clinical applications. Recently, the variable flip angle (VFA) spoiled gradient recalled echo approach was shown to be a practical alternative to conventional methods, providing better precision and speed. However, the method is known to be sensitive to transmit field (B 1 ؉ ) inhomogeneity and can result in significant systematic errors in T 1 estimates, especially at high field strengths. The main challenge is to improve the accuracy of the VFA approach without sacrificing speed. In this article, the VFA method was optimized for both accuracy and precision by considering the influence of imperfect transmit fields, noise bias, and selection of flip angles. An analytic solution was developed for systematic B 1 ؉ -induced T 1 errors and allows simple correction of T 1 measurements acquired with any imaging parameters. A noise threshold was also identified and provided a guideline for avoiding T 1 biases. Finally, it was shown that three flip angles were the most efficient for maintaining accuracy and high precision over large ranges of Rapid and accurate measurement of the longitudinal, or T 1 , relaxation time has long challenged MRI scientists but remains an important goal because of its clinical relevance across a diverse range of applications. Many of these applications, however, including dynamic contrast-enhanced studies of cancer, perfusion studies of muscle, diagnosis of neurologic disorders such as multiple sclerosis and epilepsy, and enhanced tissue discrimination for image-guided procedures, require low-noise, high-resolution mapping over a large volume. These requirements cannot be achieved in a clinically acceptable time frame (Ͻ30 min) using conventional methods of inversion-or saturation-recovery (1-3). An alternative method (4) involves determining T 1 from a set of two or more spoiled gradient recalled-echo (SPGR) images acquired with varied flip angles. Advantages of this approach include low power deposition compared to spin echo techniques and low spatial distortion compared to rapid echo planar imaging. Most importantly, the accuracy has been shown to be similar to that achieved with conventional and accelerated techniques but with a significant reduction in imaging time (5,6).A dominant source of error in the variable flip angle (VFA) approach is inaccurate knowledge of the flip angles due to transmit field B 1 ϩ inhomogeneity. Although the impact on T 1 accuracy is known (7,8), correction of measured T 1 has been reported in only a few studies (9,10), mainly due to the complexity and long scan time requirements of mapping the B 1 ϩ field. A second source of systematic error is noise-induced bias. The effect on T 1 measurements is generally subtle and may be the reason it has not, to our knowledge, been previously reported. However, these errors can be appreciable below a certain signal-tonoise (SNR) threshold, particularly with multiple angles, and one must identify and image at a suitable SNR level to ensure accurate measureme...

Section: Discussionmentioning

confidence: 99%

Section: Choice Of Flip Anglesmentioning

confidence: 99%

Section: Choice Of Flip Anglesmentioning

confidence: 99%

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Rapid high‐resolution T₁ mapping by variable flip angles: Accurate and precise measurements in the presence of radiofrequency field inhomogeneity

Cheng

Wright

2006

312

396

“…The variable nutation angle method originally introduced in 1974 (10) and investigated by a number of authors (11)(12)(13)(14) calculates T 1 with an accuracy similar to that achieved by the IR and SR techniques, but with a significant decrease in acquisition time. The sequence involves establishing a spoiled steady state followed by the collection of spoiled gradient echo (SPGR) images over a range of flip angles.…”

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confidence: 99%

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

Deoni

Rutt

Peters

2003

661

815

A novel, fully 3D, high-resolution T 1 and T 2 relaxation time mapping method is presented. The method is based on steadystate 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 ؋ 25 ؋ 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. A fast and accurate method of determining the longitudinal, T 1 , and transverse, T 2 , relaxation constants on a voxelby-voxel basis has long been a goal of MRI scientists. Rigorous characterization of T 1 and T 2 may allow for greater tissue discrimination, segmentation, and classification, thereby improving disease detection and monitoring, as well as enhancing the images used for image-guided surgical procedures. Absolute determination of T 1 and T 2 is clinically useful in areas such as in-flow perfusion studies (1) and dynamic contrast agent studies (2), as well as in the diagnosis of epilepsy (3) and in determining the severity of Parkinson's disease (4). Therefore, a method that permits simultaneous T 1 and T 2 determination in a rapid manner would be useful in a wide range of imaging applications.In order to be clinically useful for neuroimaging applications, T 1 and T 2 maps should be of high resolution, with a voxel volume less than 1 mm 3 , and have low noise. Imaging time should be less than 30 min for a large volume (25 ϫ 25 ϫ 10 cm) with minimal postprocessing time. Ideally, postprocessing would be performed at the scanner console.Despite the long acquisition times, the principal methods for T 1 and T 2 mapping remain inversion-recovery (IR) and saturation-recovery (SR) for T 1 , and spin echo (SE) and multiple or fast spin echo (mSE, FSE) for T 2 . Although alternative methods (5-9) have been developed to rapidly and accurately determine T 1 or T 2 , the low signal-to-noise ratio (SNR), lengthy reconstruction time, or special hardware requirements associated with these newer methods reduce their appeal.The variable nutation angle method original...

“…Furthermore, these sequences show high sensitivity to respiratory and organ motion (11). The aim of this study was to optimize a fast T 1 mapping technique for accurate T 1 quantification in abdominal CE-MRI using the variable FA (VFA) approach (12,13). Optimization methods were developed to maximize the signalto-noise ratio (SNR) and ensure effective spoiling and steady state for a defined T 1 range and a limited TR and acquisition time.…”

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confidence: 99%

Optimized and combined T₁ and B₁ mapping technique for fast and accurate T₁ quantification in contrast‐enhanced abdominal MRI

Treier

Steingoetter

Fried

et al. 2007

112

140

Fast T 1 mapping techniques are a valuable means of quantitatively assessing the distribution and dynamics of intravenously or orally applied paramagnetic contrast agents (CAs) by noninvasive imaging. In this study a fast T 1 mapping technique based on the variable flip angle (VFA) approach was optimized for accurate T 1 quantification in abdominal contrast-enhanced (CE) MRI. Optimization methods were developed to maximize the signal-to-noise ratio (SNR) and ensure effective RF and gradient spoiling, as well as a steady state, for a defined T 1 range of 100 -800 ms and a limited acquisition time. We corrected B 1 field inhomogeneities by performing an additional measurement using an optimized fast B 1 mapping technique. Highprecision in vitro and abdominal in vivo T 1 maps were successfully generated at a voxel size of 2.8 ؋ 2.8 ؋ 15 mm 3 and a temporal resolution of 2.