3D-EPI with parallel imaging acceleration along two Axis: Evaluated with Phantom Study and BOLD fMRI

Abstract

The function of the human brain has for many years been an intriguing field of research, and has led to the discovery of functional MRI (fMRI). fMRI is dependent on high spatial and temporal resolution to acquire detailed activation maps of the brain. The required high temporal resolution is especially hard to achieve in MRI, as the data acquisition is time consuming. In this work, an accelerated three-dimensional echo planar imaging (3D-EPI) sequence is used to increase the temporal resolution in fMRI. The increased temporal resolution is achieved by accelerating the sequence in two phase encoding directions. As a result it is possible to acquire a full 3D-volume of the brain in only 0.5 seconds. A parallel imaging technique named GRAPPA has been used to accelerate the sequence. The present work include a comparison study between a 16 channel and a 32 channel head coil available at the scanner, and a fMRI study where the sensitivity of the accelerated 3D-EPI sequence to BOLD activation has been compared with a standard 2D-EPI sequence. The results from the coil comparison show a significant improvement in image quality for the 32 channel coil, especially when high acceleration factors is applied. Ghosting artifacts were reduced and the SNR in the images increased, compared to the results from the 16 channel coil. In the fMRI study the results show significantly higher z-values for the 3D-EPI sequence, which indicates an increased sensitivity to the BOLD signal compared to the 2D-EPI sequence. The observed increase in sensitivity provides a clear indication that 3D-EPI sequence has a promising future within fMRI.