50mT Ultra-Low Field MRI System Study And Its Applications In Brain Stroke Diagnosis

The mortality and disability of stroke are among the highest in the china and around the word.Early prehospital diagnosis is an effective way to reduce the mortality and disability of stroke.Bedside monitoring is a direct demand during the patients with severe stroke was being cured at the ICU.Currently,the main approaches to early diagnosis and bedside monitoring are mobile CT and monitoring the intracranial pressure to report the change by implanting sensor after craniotomy.Mobile CT has ionizing radiation,and its diagnosis for ischemic stroke is to exclude the hemorrhage stroke.Inplanting pressure sensor in brain will hurt patient and bring the risk of infection.Therefore,there are an urgently demand for non-invansive and non-radiative imaging equipment for stroke imaging.Medical personnel can use this equipment to diagnosis and monitor the stroke disease of patients,to reduce damage of life and healthy from stork.As a non-invansive and non-radiative imaging technology,MRI was widely used in medical imaging,especially in stroke imaging that it is more accurate than CT.Generally in hospital,highfield superconductor MRI and middle/low field permanent MRI can provide higher quality and fast imaging speed.But it can't be applied in general department as a imaging equipment,because of its ten tons of weight,large footprint,high demanding for electromagnetic shielding and static magntic shielding.When MRI come into stroke imaging in clinical department,we need to miniature and lightweight the MRI system.It's meaning the magntic field will rapidly decrease.This dissertation mainly starts from the design of electromagnetic components in magnetic resonance imaging system based on electromagnetic field theory.Focusing on ultra-low field MRI for stroke desease imaging,a H-type magnet,gradient coil,radio-frequency transmitter and receiver was optimized and designed,and manufacture,measuere corresponding prototypes.Finally,the applied experimental study of stroke imaging was carried out.The specific research work includes the following aspects:1)50mT ultra-low field MRI magnet for brain imaging:The H-typed magnet structure was chosen,after comparing the research status of lightweight MRI for brian imaging.Aiming at the serious comsuption of computing resource and time for FEM simulations of magnet's optimization,uniform design was bringing in H-typed MRI magnet optimization.The uniformity design reduced the simulation models from 15~4 to15.After 5 iterations,a H-typed magnet within 2000ppm homogeneity was acquired,and its homogeneity of actual measurement exceeded 8000ppm.In order to improve the homogeneity of the magnet,the influence value of shimming pieces to magnetic field distributions was study wih different size and different location.It provides a guidance for the location and size of shimming piece during passive shimming.The homogeneity of magnet was improved from 8000+ppm to 120ppm after passive shimiming.The magnetic field strength and of this magnet is 50.9m T,and it weights 350kg.2)the electromagnetic inverse problems research of biplanar gradient and shimming coils:For the application of electromagnetic inverse problem in circular biplanar gradient design based equivalent magnetic diploe method,the element subdivision of equal area was being proposed.The simulated non-linearity of designed X/Y/Z gradient coils were3.71%,3.74%,1.09%,correspondingly.The effeciences were 12.5 m T/A/100A,13.5m T/A/100A,36 m T/A/100A.And the non-linearity influence of manufacture and assembling error was simulated,the results show the max error was generated by the deviation of gradient field direction.Based on magntic diopole and stream function,second order active shimming coils(z~2-(x~2+y~2)/2?xz)was designed and manufacture.3)RF coils design and optimization.Aiming to the H-typed MRI magnet for stroke imaging,a metamorphic solenoid RF transmitting coil was proposed.And particle swarm otpimzation method was used to optimize the distance between the adjacent coils.It has the RF magnetic field inhomogeneity belew 5%in region of interest.A semi-shperical RF receiving coil and a solenoid RF receiving coil which has an elliptic cross-section was designed and manufactured for brain imaging.4)the performance testing and clinical experiments of ultra-low field MRI.Based on manufactured MRI components,a 50mT ultra-low field MRI system for stroke imaging was set up supplemented others electronic components.Firstly,the image performance is tested,and the resolution of the water mold result coil can reach 1mm.Secondly,the difference between the brain structure and the image is explained.The signal-to-noise ratio of brain tissue in T1-weighted images and T2-weighted images was tested,the SNR value was 9.7 and 4.0 corresponding.Finally,the system was deployed in the hospital.And,clinical experiments were being conducted,mainly targeted to image patients with hemorrhagic stroke,ischemic stroke and brain cysts.The images showed that the lesions were specific.In this dissertation,50mT ultra-low field MRI system was all-around researched,based on the works of electromagnetic components design theory,performance testing,clinical experiments for brain stroke.It will provide a novel possible to earlier prehospital diagnosis and bedside imaging for stroke.