Quantitative magnetization transfer imaging of human brain at 7 T

Abstract

Quantitative magnetization transfer (qMT) imaging yields indices describing the interactions between free water protons and immobile macromolecular protons. These indices include the macromolecular to free pool size ratio (PSR), which has been shown to be correlated with myelin content in white matter. Because of the long scan times required for whole-brain imaging (≈20–30 min), qMT studies of the human brain have not found widespread application. Herein, we investigated whether the increased signal-to-noise ratio available at 7.0 T could be used to reduce qMT scan times. More specifically, we developed a selective inversion recovery (SIR) qMT imaging protocol with a i) novel transmit radiofrequency (B1+) and static field (B0) insensitive inversion pulse, ii) turbo field-echo readout, and iii) reduced TR. In vivo qMT data were obtained in the brains of healthy volunteers at 7.0 T using the resulting protocol (scan time≈40 s/slice, resolution=2×2×3 mm3). Reliability was also assessed in repeated acquisitions. The results of this study demonstrate that SIR qMT imaging can be reliably performed within the radiofrequency power restrictions present at 7.0 T, even in the presence of large B1+ and B0 inhomogeneities. Consistent with qMT studies at lower field strengths, the observed PSR values were higher in white matter (mean±SD=17.6±1.3%) relative to gray matter (10.3±1.6%) at 7.0 T. In addition, regional variations in PSR were observed in white matter. Together, these results suggest that qMT measurements are feasible at 7.0 T and may eventually allow for the high-resolution assessment of changes in composition throughout the normal and diseased human brain in vivo.

Effects of magnetization transfer on T 1 contrast in human brain white matter

“…Values of between 2.4 and 2.0 s −1 were found in the corpus callosum ROI at 3 T and 7 T respectively, which is on the low end of the 2.5–4 s −1 range reported in literature (Sled, Levesque et al 2004, Yarnykh and Yuan 2004, Samsonov, Alexander et al 2012, Dortch, Moore et al 2013). In part, this may be related to methodological differences.…”

“…The distinctly different T 1 relaxation between grey and white matter has been attributed on their different myelin content: myelin rich white matter contains an up to 30% fraction of proteins and lipids (Randall 1938), whose largely invisible hydrogen ( 1 H) protons exhibit rapid T 1 relaxation (Deese, Dratz et al 1982, Ellena, Hutton et al 1985, Du, Sheth et al 2014) and, through magnetization transfer, accelerate T 1 relaxation of MRI visible water 1 H protons (WP’s). Thus, study of T 1 relaxation with inversion recovery may allow quantification of this magnetization transfer (MT) (Edzes and Samulski 1977, Gochberg and Gore 2003, Dortch, Moore et al 2013) and aid in determining brain myelin content (Stuber, Morawski et al 2014), which has important neuro-scientific and clinical applications (Dinse, Waehnert et al 2013). MRI techniques such as MP2RAGE (Van Gelderen, Koretsky et al 2006, Marques, Kober et al 2010) and DESPOT1 (Deoni, Rutt et al 2008) have recently been proposed and are increasingly being used for this purpose (Dinse, Hartwich et al 2015).…”

A rapid approach for quantitative magnetization transfer imaging in thigh muscles using the pulsed saturation method

“…B 0 inhomogeneities were mapped using a dual-echo gradient echo sequence (25), with TR/TE 1 /TE 2 = 25/2.3/4.6 ms, excitation flip angle of 25°, SENSE = 1.3, and N EX = 1. T 1 data were acquired using an inversion recovery pulse with turbo-field-echo (TFE) readout (26), followed by a train of saturation pulses and a reduced pre-delay (TD) of 1.5 s (27), where TD is the time from the last saturation pulse to the inversion pulse. Seven inversion-recovery time (TI) points were collected, with TI = 50, 100, 200, 500, 1000, 2000, and 6000 ms.…”

Variable density magnetization transfer (vdMT) imaging for 7 T MR imaging

“…In order to improve the efficiency covering k-space, the readout of the sequence was changed to reduce the scan time for anatomical MT imaging of the human brain at 7T (Dortch et al, 2013;Mougin et al, 2010). In these studies, turbo field-echo readout (TFE; or MPRAGE) was used to more efficiently cover the k-space than conventional MT imaging method.…”

“…(R2.14) Recently, Dortch et al demonstrated whole brain covered qMT imaging at 7T. Although they improved the efficiency covering kspace by using TFE readout, the acquisition time was relatively long for routine patient scan (Dortch et al, 2013). Using a fast image acquisition technique such as Simultaneous MultiSlice (Feinberg and Setsompop, 2013) or an EPI readout sequence may allow qMT imaging to be performed in a reasonable scan time; this subject should be assessed in future studies.…”