Functional MRI of the motor cortex using a conventional gradient system: Comparison of FLASH and EPI techniques

Gradient echo (GE) and echo planar imaging (EPI) techniques are two differe nt approaches to functional MRI (fMRI). In contrast to GE sequences, the ul tra short EPI technique facilitates fMRI experiments with high spatial and temporal resolution or mapping of the whole brain. Although it has become t he method of choice for fMRI, EPI is generally restricted to modern scanner s with a strong gradient system. The aim of our study was to evaluate the a pplicability of EPI for fMRI of the motor cortex using a 1.5 T scanner with a conventional gradient system of 10 mT/m (rise time: 1 ms). Therefore, EP I was compared with a well-established high resolution fast low angle shot (FLASH) technique (matrix size 128(2)). The PLASH technique was applied add itionally with a 64(2) matrix size to exclude influences caused by differen t spatial resolution, because the EPI sequence was restricted to a 64(2) ma trix size. A total of 35 healthy volunteers were included in this study. Th e task consisted of clenching and spreading of the right hand. FLASH and EP I techniques were compared regarding geometric distortions as well as quali tative and quantitative fMRI criteria: Mean signal increase between activat ion and rest and the area of activation were measured within the contralate ral, ipsilateral, and supplementary motor cortex. The quality of subtractio n images between activation and rest, as well as the quality of z-maps and time course within activated regions of interest, was evaluated visually. E PI revealed significant distortions of the anterior and postior brain margi ns; lateral distortions (relevant for the motor cortex) could be neglected in most cases. The mean signal increase was significantly higher using FLAS H 128(2) compared to FLASH 64(2) and EPI 64(2), whereas the activated areas proved to be smaller in FLASH 128(2) functional images. Both results can b e explained by well-documented partial volume effects, caused by different voxel size. Similar quality of the subtraction images and of the time cours es in. different regions of interest were found for all techniques under in vestigation, but slightly reduced quality of z-map in FLASH 128(2). Within the limits of reproducibility and measurement accuracy, the location of con tralateral activation was similar using FLASH and EPI sequences. In conclus ion, EPI proved to be a reliable technique for fMRI of the motor cortex, ev en on an MR scanner with a conventional gradient system. (C) 1998 Elsevier Science Inc.