MAGNETIC RESONANCE IMAGING OF MOVING PATIENTS AT ULTRA-HIGH FIELD: REAL-TIME MOTION CORRECTED PARALLEL-TRANSMIT PULSE DESIGN
Lead Research Organisation:
Cardiff University
Department Name: Sch of Psychology
Abstract
Ultra-high field (UHF, 7T and above) magnetic resonance imaging (MRI) scanners present a unique opportunity to study the brain at much higher resolution than previously possible. Many brain MRI acquisitions last upwards of several minutes. Any small deviation in subject position during this time, due to involuntary movement or breathing, can change the imaged volume, leading to destructive artefacts in the images, and necessitating a rescan of the patient. Additionally, questions arise regarding patient safety, as effects of motion on the small amounts of tissue heating which occur during MRI are not well understood. Motion is specifically problematic with uncooperative patients such as in paediatric imaging, or for patients with Parkinson's or dementia. Sedation, which is common practice in such cases, is invasive, affects results obtained for non-structural MRI techniques (e.g. functional MRI), and has been reported to cause adverse side effects and even admittance to emergency care.
To overcome electromagnetic interference at UHF, parallel transmission (pTx) hardware can be used, significantly improving image quality and control over tissue heating. pTx allows radiofrequency (RF) pulses -used to generate the MRI signal- to be delivered by multiple, independently controlled RF channels. However, the benefits of pTx come at a cost of exacerbating the motion-related problems described above, as the channels' RF interacts in complex ways. The extra degrees of freedom offered by pTx (i.e. because the channels are independently controlled) mean that the problem could be overcome with a new approach to pTx RF pulse design, given a more comprehensive understanding of the interactions between channels under conditions of motion, and application of that to the RF pulse design process.
My research approaches this problem in two ways. Firstly, I am developing a motion-robust approach to pTx pulse design. Using computer simulations, I initially investigated the effects of motion in different pTx contexts to better understand the dynamics involved. Following this, I devised a new approach which has reduced the sensitivity of pTx pulses to motion in simulations, meaning that image quality remains high even in the case of patient motion. My immediate next steps are to continue simulations to ensure that the safety (tissue heating) issue also benefits from my approach, and validate the approach using scanner experiments.
Beyond reducing sensitivity by introducing motion-robustness, a second focus of my research contributes towards a real-time pTx pulse design method. This would allow real-time updates to be applied to the scanner, compensating for patient motion as it occurs throughout the scan, and therefore more accurately and effectively negating all motion-induced effects. I am using a machine learning approach to train a neural network with simulated training data for development of the approach, before testing on experiments.
Implications of my work are widespread. By removing motion-related concerns over pTx, the benefits of UHF MRI (e.g. the higher image resolution) can be applied clinically. Areas such as epilepsy and MS diagnosis and prognosis have already demonstrated benefits of this in research contexts, but the technical challenges mentioned here currently prevent its widespread or clinical use
To overcome electromagnetic interference at UHF, parallel transmission (pTx) hardware can be used, significantly improving image quality and control over tissue heating. pTx allows radiofrequency (RF) pulses -used to generate the MRI signal- to be delivered by multiple, independently controlled RF channels. However, the benefits of pTx come at a cost of exacerbating the motion-related problems described above, as the channels' RF interacts in complex ways. The extra degrees of freedom offered by pTx (i.e. because the channels are independently controlled) mean that the problem could be overcome with a new approach to pTx RF pulse design, given a more comprehensive understanding of the interactions between channels under conditions of motion, and application of that to the RF pulse design process.
My research approaches this problem in two ways. Firstly, I am developing a motion-robust approach to pTx pulse design. Using computer simulations, I initially investigated the effects of motion in different pTx contexts to better understand the dynamics involved. Following this, I devised a new approach which has reduced the sensitivity of pTx pulses to motion in simulations, meaning that image quality remains high even in the case of patient motion. My immediate next steps are to continue simulations to ensure that the safety (tissue heating) issue also benefits from my approach, and validate the approach using scanner experiments.
Beyond reducing sensitivity by introducing motion-robustness, a second focus of my research contributes towards a real-time pTx pulse design method. This would allow real-time updates to be applied to the scanner, compensating for patient motion as it occurs throughout the scan, and therefore more accurately and effectively negating all motion-induced effects. I am using a machine learning approach to train a neural network with simulated training data for development of the approach, before testing on experiments.
Implications of my work are widespread. By removing motion-related concerns over pTx, the benefits of UHF MRI (e.g. the higher image resolution) can be applied clinically. Areas such as epilepsy and MS diagnosis and prognosis have already demonstrated benefits of this in research contexts, but the technical challenges mentioned here currently prevent its widespread or clinical use
