Radial Hadamard-encoded 19F-MRI

Abstract

Objectives: Developing a ¹⁹F imaging method to acquire images of the molecular inflammation tracer perfluorooctyl bromide (PFOB) without chemical shift artifacts.

Materials and methods: PFOB is a molecular tracer that can be used to track the response of myeloid cells. However, imaging of PFOB with ¹⁹F-MRI is challenging due to its complex spectrum which leads to unwanted chemical shift artifacts. Spectral HE allows for separate reconstructions of each peak of the PFOB spectrum, which was combined into a single image after resonance shift correction. In this work, a Hadamard-encoded (HE) radial 3D UTE sequence was tested in phantoms and in vivo in a pig, measuring the ¹⁹F signal in the spleen at different times after injection.

Results: Chemical shift artifacts were effectively suppressed with HE, and an SNR > 100 was observed for the ¹⁹F signal in the spleen 2 days after injection. The signal decreased over time, and 7 days after injection it was reduced by 30%.

Discussion: Chemical shift artifact correction using HE allowed for in vivo ¹⁹F PFOB imaging of labeled monocytes with a high SNR. Compared to spectrally selective excitation, HE increased the PFOB ¹⁹F-MRI signal by 10%, and the simple HE-algorithm could be directly integrated into the image reconstruction of the MRI system.

Keywords: Fluorine-19 magnetic resonance imaging; Magnetic resonance imaging; Perfluorocarbons.

Fig. 1

Chemical structure and chemical shift spectrum of PFOB at 3 T

Fig. 2

Different steps during the reconstruction process shown with the same windowing. Left: Chemical shift artifacts in a radial sequence, Middle: Reconstructed signals for each peak after Hadamard-decoding. Due to chemical shifts within the CF₂ multi resonance, this image appears blurred. Right: Combination of all signals via sum of squares

Fig. 3

Phyllotaxis pattern of consecutively acquired spokes intersecting the unit sphere. The end-points of consecutive k-space spokes follow a spiral on the surface of a sphere with radius k_Max. A spiral where the k_z-component decreases with every spoke (red) is followed by a spiral with increasing k_z-component (orange)

Fig. 4

a Spectrum of perfluorooctyl bromide (PFOB), b measured FLASH signal curves of different resonances at TR = 10 ms

Fig. 5

Comparison between excitation with Gaussian and sinc-shaped RF pulses for all PFOB resonances

Fig. 6

¹⁹F Phantom images acquired with the HE UTE sequence. Top: Comparison of individual reconstructions of each resonance with the combined image (mean of 10 averages). Bottom: Comparison of noise between each resonance and the combined image (Standard deviation of 10 averages)

Fig. 7

Image reconstructed with Hadamard-decoding (left) and iterative deconvolution (right)

Fig. 8

a Combined ¹H (gray) and ¹⁹F (green) image of a porcine abdomen in vivo 2 days after PFOB-NE injection. b Outlines of a segmentation of the spleen together with the fluorine signal. The three green spots outside of the body of the pig are ¹⁹F marker vials. Due to blurring from the CF₂ off-resonances, the edges appear less sharp in the ¹⁹F-signal

Fig. 9

¹⁹F signal in the spleen at different times after injection on a logarithmic scale