A plug-and-play gradient insert for fast and silent MRI

The aim of this thesis was to present a novel plug-and-play gradient insert for enhanced brain imaging and silent or ultrasonic encoding, which was designed to operate with a 7T MR-scanner. In particular, we showcased the benefits of using this gradient insert for echo-planar imaging readouts for fMRI and introduced a novel silent imaging method leveraging the fast switching capabilities of this gradient insert. Furthermore, we explored the potential use of this novel silent imaging method for acceleration and investigated its application to T1-weighted anatomical imaging and spectroscopic imaging.

Chapter 2 focused on the design and characterization of the plug-and-play gradient insert. The novel design feature of this gradient insert is its ease-of-use, with only 45 kg it can be (de)-installed within 15 minutes, and its high slew rate of 1300 T/m/s. We characterized the gradient insert in terms of its field distribution, geometric distortion, minimal-echo spacing and peripheral nerve stimulation threshold. We showed that compared to a conventional setup, at least a 2-fold reduction in echo-spacing was possible while not being limited by peripheral nerve stimulation.

In Chapter 3, we introduced a novel silent gradient-axis and method for sound reduction. The silent gradient-axis consisted of the gradient insert from Chapter 2, modified to be resonant at the inaudible frequency of 20 kHz. We characterized this setup in terms of PNS, SAR and sound level to showcase its safety. Furthermore, we presented an imaging sequence that uses the silent gradient-axis to yield fast and quiet imaging.

Chapter 4 explored the acceleration capabilities of the silent gradient-axis. The acceleration capabilities were assessed based on g-factor maps for different combinations of scan parameters like the silent gradient amplitude and readout bandwidth. Here, we showed that the silent gradient can provide acceleration similar to other WAVE-imaging methods but without an additional sound burden.

Chapter 5 showcased the application of the silent gradient to a T1-weighted anatomical imaging sequence. We compared a quiet T1-weighted acquisition using the silent gradient with a conventional T1 weighted acquisition in terms of quantitative and qualitative image quality, sound level and subject experience. We showed that the quiet T1-weighted acquisition produced acceptable image quality and provided a more comfortable experience while significantly reducing sound.

In Chapter 6, we applied the silent gradient-axis to spectroscopic imaging by introducing a silent EPSI readout. Phantom measurements were used to compare spectra from conventional phase-encoded spectroscopic imaging, the silent EPSI readout and a conventional EPSI readout and compare their SNR. We showed that the silent EPSI readout was faster than conventional phase-encoded spectroscopic imaging without any additional sound while featuring a higher SNR efficiency.

All in all, this thesis showed that even a single-axis gradient insert can offer a substantial increase in scan efficiency and spatiotemporal resolution for EPI-readouts, which was achieved without affecting the day-to-day operation of the MR-scanner. In addition, this thesis introduced a novel silent gradient and silent imaging method that leveraged the fast-switching capabilities of this gradient insert and could yield sound reduction without compromising scan time.