Direct comparison of BOLD measurements acquired using functional spectroscopy versus EPI

Introduction: Functional spectroscopy (FS) measures changes in T2* associated with changes in blood oxygenation within a single volume of interest (VOI) [1]. FS uses a double spin-echo PRESS pulse sequence that gives a water-dominated free induction decay (FID) reflecting changes in VOI neural activation through a blood oxygenation level-dependent (BOLD) signal. While several groups have demonstrated that BOLD can be measured using FS, no study has directly compared FS BOLD measurements with echo-planar imaging (EPI), which is by far the most widely used pulse sequence for functional neuroimaging. To address this question we developed a pulse sequence that allows the simultaneous acquisition of FS and EPI data. We used this pulse sequence to test whether FS is more sensitive to BOLD changes than EPI by measuring the same BOLD signal with both sequences. Methods: A combined FS/EPI pulse sequence was developed to measure BOLD using both techniques simultaneously (Figure 1). Within a single TR, first a standard EPI slice acquisition is performed, followed by the FS sequence for single VOI without water suppression. Although only one EPI slice and FS VOI are shown here, many of either can be collected each TR. The sequence was implemented using the IDEA system provided by Siemens for pulse sequence development. To compare FS and EPI BOLD measurements, four subjects were scanned using a 1.5 T Avanto system and a 32-channel head coil (Siemens Healthcare, Erlangen, Germany). During all functional scans subjects viewed flashing checkerboard stimuli designed to produce strong visual cortex activation in a block design (16s blocks with an initial baseline of 46s). A VOI in visual cortex was identified in each subject by performing an initial functional localizer scan, during which whole-brain EPI volumes were collected while the subject viewed the visual stimulus (TR=2s, TE=40ms, BW=2298Hz/px, 64x64 matrix size, FOV=208mm, 32 slices, 3mm slice thickness, 106 measurements). Just after imaging, the resulting volumes were analyzed using FSL to reveal voxels that responded strongly to the visual stimulus. This activation map was used to place the center of a VOI in primary visual cortex (Figure 2). After locating a VOI, each subject performed 3 scans to measure VOI BOLD: 1) a simultaneous FS/EPI scan, 2) an individual FS scan, and 3) an individual EPI scan. Imaging parameters for FS were: TR=2s, TE=40ms, VOI size=9x9x9mm, vector size=1024, BW=2300Hz. The EPI parameters were identical to the functional localizer scan except only a single 9mm thick slice was acquired. The position and orientation of the FS VOI and the EPI slice were matched so that the 3x3 square of voxels in the center of the EPI slice exactly matched the FS VOI. The simultaneous FS/EPI run was always performed second, and the order of the individual FS and individual EPI scans were alternated (first or third) between subjects.