Spectroscopic SWIFT

INTRODUCTION Often the ability to obtain chemical-shift information is needed and advantageous in MRI. In addition to providing chemical information, the acquired spectral dimension can also be used to reduce image blur resulting from frequency shifts of all types. This work describes a new 3D and 4D spectroscopic technique involving an intrinsic frequency dimension. The method is based on an old idea to reveal both the spatial distributions and intrinsic frequency spectra of NMR signals from a set of projections obtained in the presence of fixed spatial gradients of varied strength and orientation [1]. Instead of collecting the FID after applying a hard RF pulse, as done in the original method, the new technique uses SWIFT (SWeep Imaging with Fourier Transform) [2], which is less demanding of RF field amplitude, and therefore, is more applicable to human study [3]. Due to the absence of an “echo time” in the SWIFT sequence, the presented method can be applied for spectroscopy of objects having transverse relaxation times, T2, in microsecond time scale. Consequently the main application of this method is expected to be in ultrashort T2 mapping, which is in the focus of this work. Other applications such as fat-water separation and imaging near metallic implants will be illustrated and discussed. THE DESCRIPTION OF THE METHOD In the presence of a field gradient of strength G the angle, θ, between the direction of a projection and the frequency axis in the “pseudo space” is given by ( ) atan / GD θ γ ≈ Ω