Abstract
Image blurring that results from off-resonance and fast T2* signal decay is a common issue in radial ultrashort echo time magnetic resonance imaging (MRI) sequences. One solution is to use a higher readout bandwidth, but this may be impractical for some techniques such as pointwise-encoding time reduction with radial acquisition (PETRA), which is a hybrid method of zero echo time and single-point imaging techniques. Specifically, PETRA has severe specific absorption rate (SAR) and radiofrequency (RF) pulse peak power limitations when using higher bandwidths in human measurements. In this study, we introduce gradient modulation (GM) to PETRA to reduce image-blurring artifacts while keeping SAR and RF peak power low. GM-PETRA tolerance to image blurring was evaluated in simulations and experiments by comparison with the conventional PETRA technique. We performed inner ear imaging of a healthy subject at 7 T. GM-PETRA showed significantly less image blurring as a result of off-resonance and fast T2* signal decay compared to PETRA. In in vivo imaging, GM-PETRA nicely captured complex structures of the inner ear such as the cochlea and semicircular canals. GM can improve PETRA image quality and mitigate SAR and RF peak power limitations without special hardware modification in clinical scanners.
Highlights
Image blurring that results from off-resonance and fast signal decay of extremely short T2* spins is a common problem for radial ultrashort echo time magnetic resonance imaging (MRI) sequences such as zero echo time (ZTE) [1, 2] and sweep imaging with Fourier transformation [3] techniques
We introduce a novel pointwise-encoding time reduction with radial acquisition (PETRA) technique with gradient modulation (GM) that enables high readout bandwidths while keeping a relatively low excitation bandwidth
By increasing the GM-PETRA amplitude from 60 to 125 kHz (GM-PETRA 60-125 kHz), off-resonance artifacts became comparable to PETRA with a 120-kHz excitation
Summary
Image blurring that results from off-resonance and fast signal decay of extremely short T2* spins is a common problem for radial ultrashort echo time magnetic resonance imaging (MRI) sequences such as zero echo time (ZTE) [1, 2] and sweep imaging with Fourier transformation [3] techniques. A high excitation bandwidth (ie, short pulse length) leads to an increase in the specific absorption rate (SAR) and RF pulse peak power, which are the major limitations of ZTE in human applications [4]. Another severe limitation of using higher bandwidths in ZTE is an increasing demand for fast switching between transmit and receive (T/R) modes. The slow T/R switching of most standard clinical MRI scanners (Ͼ20 s) results in missing critical data points in a large region around the center of k-space [5]
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