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

Abstract

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.

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

“…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. Rapid B 1 ϩ mapping has recently been demonstrated using a SE segmented-EPI sequence (18). Similar to conventional SE techniques, signal is acquired at excitation angles of 60 and 120°for maximum sensitivity to small variations in B 1 ϩ .…”

Caffeine-induced uncoupling of cerebral blood flow and oxygen metabolism: A calibrated BOLD fMRI study

“…CBF time series were computed by taking the surround subtraction of the tag/control image series from the first echo data (Liu and Wong, 2005). These time series were then corrected for inhomogeneities in the coil sensitivity profiles using the smoothed minimum contrast images (Wang et al, 2005), and converted to physiological units (ml/100ml/min) using the CSF image as a reference signal (Chalela et al, 2000). Mean baseline pre and post-dose CBF values were calculated by averaging over all data points and voxels within each of the two ROIs.…”

On the steady‐state properties of actual flip angle imaging (AFI)

“…Therefore, accurate B 1 mapping is required for a variety of applications such as quantitative image processing (1) and multielement transmit applications (2,3) like RF-shimming (4), supported by appropriate high-field MRI systems. Most existing B 1 mapping approaches employ multiple acquisitions with different flip angles (5)(6)(7)(8)(9) or stimulated echoes in multipulse sequences (10 -13). Despite recent advances (1,8,9,13), many of these techniques are still too complicated and time-consuming for routine use.…”