Imaging of the active B1 field in vivo

Stollberger, Rudolf; Wach, Paul

doi:10.1002/mrm.1910350217

Cited by 327 publications

(343 citation statements)

References 14 publications

Supporting

Mentioning

339

Contrasting

Order By: Relevance

“…For example, a three-angle set can be derived from four angles, as the smaller two angles are often within 1 point of each other and only one need to be retained. Ten angles optimized for TR ϭ 5 ms from a genetic algorithm (11) were also evaluated, consisting of the set 10␣ ' [2,3,4,5,7,9,11,14,17,22]. Although our straightforward strategy of combining appropriate ideal angles was not as sophisticated as these other approaches for multiple angle selection, we showed that comparable precision profiles could be achieved.…”

Section: Choice Of Flip Anglesmentioning

confidence: 99%

“…However, substantial inhomogeneity may arise when using separate transmit/receive coil systems or when imaging at field strengths above 1.5 T. An accurate approach to account for these variations is to map the B 1 ϩ field distribution using a double-angle method (16,17). Standard spin echo (SE) acquisitions are the most robust, as they are insensitive to B 0 variations, but are impractical in the clinic due to long scan times.…”

Section: Simulation Studiesmentioning

confidence: 99%

See 1 more Smart Citation

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

Cheng

Wright

2006

Magnetic Resonance in Med

308

396

View full text Add to dashboard Cite

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...

show abstract

Section: Choice Of Flip Anglesmentioning

confidence: 99%

Section: Simulation Studiesmentioning

confidence: 99%

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

Cheng

Wright

2006

Magnetic Resonance in Med

308

396

View full text Add to dashboard Cite

show abstract

“…For further quantitative assessment of the coil efficiency, the amplitude B 1xy was investigated by taking advantage of the principle of reciprocity (1,21). The B 1xy distribution was measured with a doubleangle approach utilizing a simple spin-echo sequence (30). Briefly, effects from longitudinal relaxation can be ignored if the repetition time is sufficiently long (T R Ն 5 T 1 ).…”

Section: In Situ Experiments On Phantomsmentioning

confidence: 99%

Reengineered helmet coil for human brain studies at 3 Tesla

Driesel

Merkle

Hetzer

et al. 2005

Concepts Magn. Reson.

View full text Add to dashboard Cite

ABSTRACT:We describe the further development of a circularly polarized helmet coil for magnetic resonance imaging (MRI) of the human brain at 3 T. The coil is used in transmit and receive (transceive) mode, a useful alternative over commercially available quadrature headcoils with scanners where phased arrays are unavailable. The coil is simple to build. Its structure is based on an assembly of two coplanar dual-loop coils of the split-circle design, which are arranged in crossed fashion. Both coils circumscribe dome-like the human head. Because of the symmetry of the assembly, a high degree of circularly polarized radiofrequency (RF) is obtained within the brain. Bench and in situ measurements of the RF magnetic field, B 1 , indicated good axial homogeneity and a moderate gradient along the symmetry axis. Compared to a commercial birdcage coil, the signal-to-noise ratio was increased up to a factor of 1.7 as a result of its superior filling factor. Initial applications in healthy volunteers included anatomical MRI and functional imaging with a cognitive paradigm.

show abstract

“…A high‐resolution GRE image was acquired to provide a reference for muscle segmentation (TR/TE = 100/3.45 ms, flip angle =10°, matrix 512 × 240, FOV 44 × 20.6 cm 2 ). B 1 maps were acquired with the double angle method11 (TE/TR = 11/7000 ms, FOV 44 × 20.6 cm 2 , 40 × 10 mm slices, nominal flip angles 60° and 120°), with B 1 error expressed as the percentage deviation of the actual flip angle from that nominally prescribed.…”

Section: Methodsmentioning

confidence: 99%

Stability and sensitivity of waterT₂obtained with IDEAL‐CPMG in healthy and fat‐infiltrated skeletal muscle

et al. 2016

View full text Add to dashboard Cite

Quantifying muscle water T 2 (T 2‐water) independently of intramuscular fat content is essential in establishing T 2‐water as an outcome measure for imminent new therapy trials in neuromuscular diseases. IDEAL‐CPMG combines chemical shift fat–water separation with T 2 relaxometry to obtain such a measure. Here we evaluate the reproducibility and B 1 sensitivity of IDEAL‐CPMG T 2‐water and fat fraction (f.f.) values in healthy subjects, and demonstrate the potential of the method to quantify T 2‐water variation in diseased muscle displaying varying degrees of fatty infiltration.The calf muscles of 11 healthy individuals (40.5 ± 10.2 years) were scanned twice at 3 T with an inter‐scan interval of 4 weeks using IDEAL‐CPMG, and 12 patients with hypokalemic periodic paralysis (HypoPP) (42.3 ± 11.5 years) were also imaged. An exponential was fitted to the signal decay of the separated water and fat components to determine T 2‐water and the fat signal amplitude muscle regions manually segmented.Overall mean calf‐level muscle T 2‐water in healthy subjects was 31.2 ± 2.0 ms, without significant inter‐muscle differences (p = 0.37). Inter‐subject and inter‐scan coefficients of variation were 5.7% and 3.2% respectively for T 2‐water and 41.1% and 15.4% for f.f. Bland–Altman mean bias and ±95% coefficients of repeatability were for T 2‐water (0.15, −2.65, 2.95) ms and f.f. (−0.02, −1.99, 2.03)%. There was no relationship between T 2‐water (ρ = 0.16, p = 0.07) or f.f. (ρ = 0.03, p = 0.7761) and B 1 error or any correlation between T 2‐water and f.f. in the healthy subjects (ρ = 0.07, p = 0.40). In HypoPP there was a measurable relationship between T 2‐water and f.f. (ρ = 0.59, p < 0.001).IDEAL‐CPMG provides a feasible way to quantify T 2‐water in muscle that is reproducible and sensitive to meaningful physiological changes without post hoc modeling of the fat contribution. In patients, IDEAL‐CPMG measured elevations in T 2‐water and f.f. while showing a weak relationship between these parameters, thus showing promise as a practical means of quantifying muscle water in patient populations.

show abstract

Imaging of the active B₁ field in vivo

Cited by 327 publications

References 14 publications

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

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

Reengineered helmet coil for human brain studies at 3 Tesla

Stability and sensitivity of waterT₂obtained with IDEAL‐CPMG in healthy and fat‐infiltrated skeletal muscle

Contact Info

Product

Resources

About

Imaging of the active B1 field in vivo

Cited by 327 publications

References 14 publications

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

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

Reengineered helmet coil for human brain studies at 3 Tesla

Stability and sensitivity of waterT2obtained with IDEAL‐CPMG in healthy and fat‐infiltrated skeletal muscle

Contact Info

Product

Resources

About

Imaging of the active B₁ field in vivo

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

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

Stability and sensitivity of waterT₂obtained with IDEAL‐CPMG in healthy and fat‐infiltrated skeletal muscle