Simultaneous multi-segment (SMSeg) EPI over multiple focal regions

Abstract

Objective.This study aimed at developing a simultaneous multi-segment (SMSeg) imaging technique using a two-dimensional (2D) RF pulse in conjunction with echo planar imaging (EPI) to image multiple focal regions.Approach.The SMSeg technique leveraged periodic replicates of the excitation profile of a 2D RF pulse to simultaneously excite multiple focal regions at different locations. These locations were controlled by rotating and scaling transmit k-space trajectories. The resulting multiple isolated focal regions were projected into a composite 'slice' for display. GRAPPA-based parallel imaging was incorporated into SMSeg by taking advantage of coil sensitivity variations in both the phase-encoded and slice-selection directions. The SMSeg technique was implemented at 3 T in a single-shot gradient-echo EPI sequence and demonstrated in a phantom and human brains for both anatomic imaging and functional imaging.Main results.In both the phantom and the human brain, SMSeg images from three focal regions were simultaneously acquired. SMSeg imaging enabled up to a six-fold acceleration in parallel imaging without causing appreciable residual aliasing artifacts when compared with a conventional gradient-echo EPI sequence with the same acceleration factor. In the functional imaging experiment, BOLD activations associated with a visuomotor task were simultaneously detected in two non-coplanar segments (each with a size of 240 × 30 mm²), corresponding to visual and motor cortices, respectively.Significance.Our study has demonstrated that SMSeg imaging can be a viable method for studying multiple focal regions simultaneously.

Keywords: 2D RF pulse; EPI; parallel imaging; reduced FOV; simultaneous multi-segment imaging.

Figure 1:

(A): A 2D RF pulse that produces a central spatial response and multiple replicates in the plane defined by the slice-selection (i.e., z-direction) and phase-encoding (i.e., y-direction) axes. (B): The excitation k-space trajectory was first designed as a square in the plane defined by the phase-encoded (k_y) and slice-selection (k_z) directions, and then rotated by α degree in the k_y-k_z plane, followed by scaling the relative amplitude of the phase-encoding and slice-selection gradients to achieve a desired excitation k-space coverage. After scaling, the rotation angle became β, and the perpendicular distance between two neighboring excitation k-space lines changed from ΔK_orig to ΔK. (C): The corresponding spatial profiles simulated using the Bloch equations. According to the properties of Fourier transform, k-space rotation by an α angle results in the excitation profile to rotate by the same angle (from the image on the left to the image in the middle). A subsequent gradient scaling produces a final tilt angle β. The center-to-center distance between two adjacent segments is determined by 1/ΔK.

Figure 2:

The left panel (A) illustrates the principles of SMSeg imaging and the formation of a composite “slice”. Three obliquely distributed segments were simultaneously excited and projected into a composite slice as shown at the bottom of (A). The right panel shows one representative axial composite slice of the phantom from the SMSeg EPI sequence (top row) without acceleration (B) and with acceleration factor (AF) of 2 (C), 4 (D), and 6 (E), together with the conventional 2D GRE-EPI images (bottom row) at the location corresponding to the central segment with different AFs (F, G, H, and I). The three segments in (B), (C), (D), and (E) were from the three different locations as shown in (A). The images from conventional 2D GRE-EPI with 4- and 6-fold accelerations [(H) and (I)] exhibited residual aliasing artifacts (indicated by the red arrows). These artifacts were substantially reduced in the corresponding composite images in [(D) and (E)] using SMSeg EPI due to the deployment of additional sensitivity variation in the slice-selection direction. The SMSeg allowed for a greater AF in parallel imaging.

Figure 3:

(A): Illustration of the multiple composite slices, each with a distinctive color and each containing three segments. Only three consecutive slices are shown for simplicity, although more were acquired. (B): Two representative slices of SMSeg echo-planar images (selected from a total of 10 slices) from a healthy human brain (24-year-old male) with different acceleration factors (AF). The three segments were positioned at the frontal, temporal, and occipital lobes, respectively, as shown in (A). A relatively large field of view (FOV, 240 × 240 mm²) was used to cover the three prescribed segments in different axial planes. As the AF increased, geometric distortion decreased as expected, especially at the frontal lobe.

Figure 4:

(A): The three segments at different locations (yellow boxes) were simultaneously excited by a 2D RF pulse with a titled transmit k-space design shown in Figure 1. The lower and upper segments covered the visual and motor cortices, respectively. Because the projections of the three segments spanned a limited FOV, a small FOV along the phase-encoded direction (240 × 120 mm²) was used as shown in (A). (B): A composite SMSeg image of (A) obtained with GRAPPA-based parallel imaging (AF = 2) in an fMRI study (10 averages of images obtained during the baseline) from a representative healthy subject (32-year-old male). (C): A composite activation map illustrating simultaneous activations in the visual (the lower segment) and motor cortices (primary motor cortex and supplementary motor area in the upper segment). In contrast, no activations were detected in the middle segment, consistent with the absence of visual or motor functional regions in that segment. The color bar shows the SPM t-values.