Implementation And Improvement Of Retrospective Respiratory Navigator Gating Technique At Low-Field MRI

Abdominal imaging is one of the important clinical application areas of magnetic resonance imaging (MRI), but image artifacts caused by respiratory motion remains a major problem. Due to the limitation of hardware performance, for the same scanning technique, low-filed MRI system is more prone to suffer from respiratory motion artifacts because of longer scan time. If not compensated for or corrected, respiratory motion can produce image blurring, ghosting, intensity loss, and misregistration. These image artifacts can obscure important anatomic structures, making diagnosis difficult or even impossible.As a simple and reliable method, retrospective navigator gating technique can effectively reduce the motion artifacts caused by respiratory motion. This technique has been successfully applied at high-field super-conduct MRI system, but it’s application at low-field MRI system has not yet been reported. In this thesis, the design and implementation of retrospective respiratory gating technique at low-field MRI system were first investigated and solved.On the other hand, however, long scan time seriously restricts the clinical application of the retrospective respiratory navigator gating technique. In addition, low signal-to-noise ratio (SNR) efficiency is also a very big drawback of retrospective respiratory navigator gating technique. To solve these problems, in this thesis, we developed and evaluated a modified method to improve the performance of retrospective respiratory navigator gating technique.The main contents of this thesis include:1. The design and implementation of navigator sequenceThe first and most important step for retrospective respiratory navigator gating is to design and implement pencil beam excitation pulse. In this thesis, RF pulses for pencil beam excitation were designed by solving the Bloch equations using a small flip angle approximation. Specifically, a spiral excitation k-space trajectory were designed according to the gradients amplitude and maximal slew rate constraints; A non-unitary sampling density was used with shortened pulse duration and improved excitation profile; The evaluation of the RF design was simulated by Shinnar-Le Roux algorithm in which the excitation profile was numerically computed; The actual k-space trajectory was measured using Duyn calibration technique (DCT), and the k-space design was adjusted in the light of the measured results; An intensity based edge detection algorithm was used to detect the position of diaphragm.2. The implementation and improvement of retrospective respiratory navigator gating technique.2D GRE sequence with retrospective respiratory gating was realized, and then a modified retrospective navigator gating technique with weighted data acquisition and post-processing method that provides improved image quality was presented. Experiments in phantom and seven volunteers demonstrate that the proposed method provides an enhanced SNR and reduced ghost-to-image ratio compared to the conventional method. The proposed method can also be used to reduce imaging time while maintaining comparable image quality.