High spectral and spatial resolution (HiSS)¹ MRI is a well-established technique for examining the spectral characteristics of proton signals at voxel locations in high-resolution magnetic resonance images. The method can be used to robustly separate contributions from water and fat signals² and to probe tissue composition by analysing the spectral components of the water and fat resonances³. Data acquisition is performed using EPSI^4,5 to allow rapid spatial and spectral encoding with a scan duration that is suitable for clinical application. However, EPSI is limited by the requirement that the echo spacing is sufficiently long to perform the required spatial encoding in the readout direction, resulting in a corresponding limit on the spectral bandwidth that is available. In turn, this leads to aliasing of fat or water signals, which complicates the data analysis. The problem scales with field strength due to the increasing separation of fat and water resonances. This paper introduces a readout-segmented version of EPSI, which makes it possible to perform HiSS with significantly increased spectral bandwidth, which is independent of the spatial resolution or available gradient strength; it is demonstrated how this modified technique avoids the aliasing problems that are encountered using standard EPSI at a field strength of 3T. A drawback is the increased acquisition time due to the additional scans required to acquire the multiple readout segments

Fig. 1 shows the pulse diagram for the sequence used in the study. A train of gradient echoes is sampled using a sinusoidal readout gradient, which is preceded by stepped encoding gradients in both readout and phase-encoding directions. Data were acquired from the knee of a healthy volunteer using a Siemens 3T Skyra system and a wrap-around flex coil. Imaging parameters were as follows: FOV 200mm; matrix 256 x 256; slice thickness 2mm; TR 150ms; 192 echoes with spacing 360μs, corresponding to a spectral bandwidth of 2.8kHz; scan time 8 mins. 19 secs. After acquisition and standard reconstruction of images for each echo time, Fourier transformation was applied in the echo dimension to provide a proton spectrum for each voxel location. Fat and water images were generated by integrating the respective regions of the spectra..

Fig. 2 shows four images from the multi-echo data set, showing a high level of anatomical detail and no discernable artefacts relating to the readout-segmented encoding applied in the head-feet direction. As seen in fig. 3, the spectroscopic analysis of the data made it possible to cleanly separate the signal contributions from water and fat resonances and to generate the corresponding spectrally selective images. Fig. 4 shows single-voxel spectra from four anatomical regions of the knee, each showing a dominant spectral component according to the respective tissue type.Readout segmentation is used routinely with echo-planar imaging (EPI) to reduce the echo spacing and associated susceptibility artefacts⁶, but has not been used previously in conjunction with EPSI. As demonstrated by the images and spectra shown in this study, the technique decouples the spatial resolution in the readout direction from the echo spacing, thereby making it possible to simultaneously achieve a high spatial resolution and a high spectral bandwidth. The compromise for this improvement is an increase in the overall scan time due to the acquisition of multiple readout segments. This could be mitigated by using partial Fourier in the readout direction to reduce the number of acquired readout segments⁷ and by matching the echo spacing carefully to the bandwidth requirements of the application. The results presented in this study could be improved by introducing time-domain filtering before generating the proton spectra; this would be particularly beneficial in the case of the the fat image of fig. 3, whose SNR is affected by the rapid T₂ decay of fat during the multi-echo readout.This preliminary study has demonstrated how the application of readout segmentation to multiple-gradient-echo sequences substantially increases the spectral bandwidth that can be achieved when imaging at high spatial resolution. This approach promises to have a high clinical impact by improving the reliability of data analysis in HiSS studies in general and by facilitating the transfer of the technique to higher field strengths of 3T and above. Furthermore, the propsed sequence will also be a significant advantage for metabolite imaging using EPSI, in particiular for studies at ultra-high field strengths or with nuclei other than ¹H, when a higher spectral bandwidth is required.