Slice selection in TSE

[Alexandra van Dongen]

Dear JEMRIS community,

I am working on a slice selection gradient in a TSE sequence in JEMRIS. The problem I have, is that the slice selection gradients cause misalignments in the k-space and introduce artifacts in the image.

The easiest way to illustrate is as follows. I have attached an example sequence, which is a standard 2D TSE with a linear readout. The slice selection gradients (GSS) work on the GX axis, with an area of:

(BW * DUR * 2 * Pi) / SliSel

Here, BW is the bandwidth of the RF pulse, DUR is the duration of the RF pulse and SliSel is the thickness of my selected dimension in mm.

When there is no GSS, the k-space aligns behaves as expected (see image 1). When I increase the strength of the GSS, every other k-space line will be misaligned (see image 2). Increasing the GSS by e.g. larger BW or smaller SliSel will increase the misalignment.

The resultant image artifacts seem like ghosts. I have attached in image from the standard 2D sphere as well (radius 16 mm, {dx,dy} 0.5 mm) (image 3). It was created with the above sequence, and the following adjustments:

-          FOVx = FOVy = 64

-          Nx = Ny = 64

Has anyone encountered this before? And any ideas on how to solve it?