High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

Abstract

In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162°) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B 1 ) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B 1 inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness ‫؍‬ 2 Over recent years, the use of high-field MR scanners for imaging the human body has become increasingly widespread, due to the improvements they offer in signal-tonoise ratio (SNR) and susceptibility contrast (1,2). These benefits can be exploited to allow 1) the acquisition of MR images with higher spatial resolution and thus greater anatomical detail, 2) more robust detection of susceptibility-related changes in signal intensity (e.g., BOLD contrast in functional MRI (fMRI)), and 3) easier discrimination of metabolite signals in MR spectra, due to increased dispersion as well as higher SNR.Along with the advantages of high-field MR systems, there are also associated problems, ranging from technical challenges to safety concerns. At higher fields, the wavelength of the RF excitation pulses approaches the dimensions of the object being imaged, causing inhomogeneities in the RF (B 1 ) field via RF focusing effects (3). As a result, the RF flip angle varies over the imaging field of view (FOV), potentially leading to variations in both signal intensity and contrast across images. In addition, the RF power required for spin excitation increases with the magnetic field strength. This means, for some RF-intensive pulse sequences, that the power deposition may exceed permissible specific absorption rate (SAR) safety limits.In this work, we describe the implementation and application of a standard clinical MRI pulse sequence on a high-field 4.7-T whole-body system. The pulse sequence we implemented is fast spin echo (FSE) (4,5). FSE is used clinically as an efficient method for obtaining high-quality T 2 -weighted images. However, the extension to high field is not necessarily straightforward, due to the aforementioned issues of B 1 inhomogeneity and SAR power restrictions. In a recent study, we presented our initial results concerning FSE imaging at 4.7 T (6). Here we investigate in more detail the factors involved in using FSE as a technique for imaging the human brain at 4.7 T. To do this, we assess the extent of B 1 variation within the brain and its effect on image signal in...

Time‐efficient fast spin echo imaging at 4.7 T with low refocusing angles

“…A recent work at 4.7 T demonstrated signal differences of 20 -25% in edge slices when using refocusing angles of 162°, and attributed these differences to MT signal attenuation (6). While the experimental methodologies differ, our results (Fig.…”

Potential role of high‐field MRI for studies in Parkinson's disease

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Selective effect of Apo e4 on CA3 and dentate in normal aging and Alzheimer's disease using high resolution MRI at 4 T

“…The following sequences, which were part of a larger research imaging and spectroscopy protocol, were acquired: 1. For the measurement of hippocampal subfields, a high resolution T2 weighted fast spin echo sequence (TR/TE: 3500/19 ms, echo train length 15, 18.6 ms echo spacing, 160° flip angle, 100% oversampling in ky direction, 0.4 × 0.4 mm in plane resolution, 2 mm slice thickness, 24 interleaved slices without gap, acquisition time 5:30 min (adapted from DeVita et al 2003;Thomas et al 2004), angulated perpendicular to the long axis of the hippocampal formation, 2. For the measurement of total hippocampal volume a volumetric T1-weighted gradient echo MRI (MPRAGE) TR/TE/TI = 2300/3/950 ms, 7° flip angle, 1.0 × 1.0 × 1.0 mm3 resolution, acquisition time 5.17 min and 3.…”