Oscillating steady-state imaging (OSSI): A novel method for functional MRI

Abstract

Purpose: Signal-to-noise ratio (SNR) is crucial for high-resolution fMRI; however, current methods for SNR improvement are limited. A new approach, called oscillating steady-state imaging (OSSI), produces a signal that is large and $T_2^{\ast }$ -weighted, and is demonstrated to produce improved SNR compared to gradient echo (GRE) imaging with matched effective TE and spatial-temporal acquisition characteristics for high-resolution fMRI.

Methods: Quadratic phase sequences were combined with balanced gradients to produce a large, oscillating steady-state signal. The quadratic phase progression was periodic over short intervals such as 10 TRs, inducing a frequency-dependent phase dispersal. Images over one period were combined to produce a single image with effectively $T_2^{\ast }$ -weighting. The OSSI parameters were explored through simulation and phantom data, and 2D and 3D human fMRI data were collected using OSSI and GRE imaging.

Results: Phantom and human OSSI data showed highly reproducible signal oscillations with greater signal strength than GRE. Compared to single slice GRE with matched effective TE and spatial-temporal resolution, OSSI yielded more activation in the visual cortex by a factor of 1.84 and an improvement in temporal SNR by a factor of 1.83. Voxelwise percentage change comparisons between OSSI and GRE demonstrate a similar $T_2^{\ast }$ -weighted contrast mechanism with additional $T_2^{\text{'}}$ -weighting of about 15 ms immediately after the RF pulse.

Conclusions: OSSI is a new acquisition method that exploits a large, oscillating signal that is $T_2^{\ast }$ -weighted and suitable for fMRI. The steady-state signal from balanced gradients creates higher signal strength than single slice GRE at varying TEs, enabling greater volumes of functional activity and higher SNR for high-resolution fMRI.

Keywords: $T_2^{\ast }$ -weighting; BOLD contrast; Oscillating steady-state imaging (OSSI); functional MRI; high SNR; high-resolution.