In a recent study, we presented our initial results concerning FSE imaging at 4.7 T ( 6). However, the extension to high field is not necessarily straightforward, due to the aforementioned issues of B 1 inhomogeneity and SAR power restrictions. FSE is used clinically as an efficient method for obtaining high-quality T 2-weighted images. The pulse sequence we implemented is fast spin echo (FSE) ( 4, 5). 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. This means, for some RF-intensive pulse sequences, that the power deposition may exceed permissible specific absorption rate (SAR) safety limits. In addition, the RF power required for spin excitation increases with the magnetic field strength. 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. 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). 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.Īlong with the advantages of high-field MR systems, there are also associated problems, ranging from technical challenges to safety concerns. Over recent years, the use of high-field MR scanners for imaging the human body has become increasingly wide-spread, due to the improvements they offer in signal-to-noise ratio (SNR) and susceptibility contrast ( 1, 2). This study demonstrates that high-field FSE produces images of the human brain with high spatial resolution, SNR, and tissue contrast, within currently prescribed power deposition guidelines. Thirty-four slice data sets (slice thickness = 2 mm in-plane resolution = 0.469 mm acquisition time = 11 min 20 s) from normal volunteers are presented, which allow visualization of brain anatomy in fine detail. 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. 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. 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). It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. 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.
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