Reduced field of view imaging using a static second-order gradient for functional MRI applications

• Published in: Magnetic resonance in medicine Vol. 75; no. 2; pp. 817 - 822
• Main Authors: Islam, Haisam, Glover, Gary H.
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01.02.2016; Wiley Subscription Services, Inc
• Subjects: Autocorrelation functions; Brain - anatomy & histology; Excitation; Field of view; Functional magnetic resonance imaging; Humans; Image Enhancement - methods; Magnetic resonance imaging; Magnetic Resonance Imaging - methods; Phantoms, Imaging; Radio frequency; Radio Waves; reduced FOV; selective excitation; Signal-To-Noise Ratio; Time series; Visual field; Visual stimuli; Z2 gradient; Z2 gradient; reduced FOV; selective excitation
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.25650

Purpose Imaging using reduced FOV excitation allows higher resolution or signal‐to‐noise ratio (SNR) per scan time but often requires long radiofrequency pulses. The goal of this study was to improve a recent reduced field of view (FOV) method that uses a second‐order shim gradient to decrease pulse length and evaluate its use in functional MRI (fMRI) applications. Theory and Methods The method, which was initially limited to excite thin disc‐shaped regions at the isocenter, was extended to excite thicker regions off the isocenter and produced accurate excitation profiles on a grid phantom. Visual stimulation fMRI scans were performed with full and reduced FOV. The resolution of the time series images and functional activation maps were assessed using the full‐width half‐maxima of the autocorrelation functions (FACFs) of the noise images and the activation map values, respectively. Results The resolution was higher in the reduced FOV time series images (4.1% ± 3.7% FACF reduction, P < 0.02) and functional activation maps (3.1% ± 3.4% FACF reduction, P < 0.01), but the SNR was lower (by 26.5% ± 16.9%). However, for a few subjects, the targeted region could not be localized to the reduced FOV due to the low Z2 gradient strength. Conclusion The results of this study suggest that the proposed method is feasible, though it would benefit from a stronger gradient coil. Magn Reson Med 75:817–822, 2016. © 2015 Wiley Periodicals, Inc.