Mapping of the Radiofrequency Field

102

111

Phased array coils are finding widespread applications in both the research and the clinical setting. However, intensity nonuniformities with such coils can reduce the potential benefits of these coils, particularly for applications such as tissue segmentation. In this work, a method is described for correcting the nonuniform signal response based on in vivo measures of both the transmission field of body coil and the reception sensitivity of phased array coils, separately. For a uniform phantom, the reception sensitivity can be calculated using both Bloch equations and transmission field maps. For a heterogeneous object such as a brain, a minimal contrast acquisition must be obtained to map the receiver nonuniformities. This transmit field/ receiver sensitivity (TFRS) approach is compared with the standard methods of using the body coil to obtain a reference scan and low-pass filtering. The quantitative comparison results shows that the TFRS approach provides superior results in correcting intensity nonuniformities for a uniform phantom. This approach reduces the ratio between signal intensity SD of an image and its mean intensity from approximately 21% before correction to 13% after correction. Results are also shown demonstrating the utility of this approach in vivo with human brain images. The method is general and can be applied with most pulse sequences, any coil combination for transmission and reception, and in any anatomic region. Magn Reson Med 53:666 -674, 2005.

Section: Resultsmentioning

confidence: 99%

In vivo method for correcting transmit/receive nonuniformities with phased array coils

Wang

Qiu

Constable

2005

102

111

“…Inaccurate angles can arise from two sources: slice profile errors, which are minimized using 3D sequences, and B 1 ϩ inhomogeneity. The latter problem is exacerbated at higher field strengths and needs to be corrected (16). Even some coil configurations at 1.5 T require correction, as 25% deviations have been reported in certain regions of a breast coil (19).…”

Section: Discussionmentioning

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%

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

Cheng

Wright

2006

307

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...

“…7a. The measured B 1 map for the same slice, acquired using the double-angle method (13,14), can be seen in Fig. 7b.…”

Section: Phantom Resultsmentioning

confidence: 99%

Fat suppression for ¹H MRSI at 7T using spectrally selective adiabatic inversion recovery

Balchandani

Spielman

2008

Proton magnetic resonance spectroscopic imaging ( 1 H MRSI) at 7T offers many advantages, including increased SNR and spectral resolution. However, technical difficulties associated with operating at high fields, such as increased B 1 and B 0 inhomogeneity, severe chemical shift localization error, and converging T 1 values, make the suppression of the broad lipid peaks which can obscure targeted metabolite signals, particularly challenging. Conventional short tau inversion recovery can successfully suppress fat without restricting the selected volume, but only with significant metabolite signal loss. In this work, we have designed two new pulses for frequency-selective inversion recovery that achieve B 1 -insensitive fat suppression without degrading the signal from the major metabolites of interest. The first is a spectrally selective adiabatic pulse to be used in a volumetric 1 H MRSI sequence and the second is a spatial-spectral adiabatic pulse geared toward multi-slice 1 H MRSI. Partial interior volume selection may be used in addition to the pulses, to exclude areas with severe B 0 inhomogeneity. Some differences in the spectral profile as well as degree of suppression make each pulse valuable for different applications. 7T phantom and in vivo data show that both pulses significantly suppress fat, while leaving most of the metabolite signal intact. Magn Reson Med 59:980-988, 2008.