Study On Structure Of Ultra-Low Field Head MRI Magnet

Since the discovery of Nuclear Magnetic Resonance(NMR)technology in 1946,it has attracted much attention due to its advantages of non-invasive and high precision.The main magnet is the core structure of the magnetic resonance imaging system.The stable static magnetic field provided by the main magnet directly determines the imaging quality of the system.Although the high-field-strength magnetic resonance imaging system with superconducting magnets as the core has high imaging accuracy,it is bulky and costly.The clinical need for a lightweight and portable magnet structure,ultra-low field imaging technology came into being.In the ultra-low field magnetic resonance imaging system,in order to meet the requirements of lightweight and movable,a head main magnet structure that is sufficiently lightweight and can provide the uniform static magnetic field required for imaging is required.In this paper,under the background of ultra-low field magnetic resonance imaging,the main magnet structure of the head is studied,and the design is made from two directions of electromagnetic and permanent magnets.The design process combines simulation software and algorithms to optimize the structure and design methods of traditional magnets.The main work of this article is as follows:(1)The design optimization of the saddle type permanently conductive magnet.This paper compares and discusses the single-circuit saddle magnet and the double-circuit saddle magnet,and analyzes the compensation effect of the double-circuit structure on the magnetic field.By discretizing the saddle-shaped coil,the effect of each turn of the saddle-shaped coil at different spatial positions on the magnetic field of the target field is calculated,the linear programming method is used to search and optimize,and the current distribution is combined with the non-linear programming method to regularize the way to successfully obtain A double-loop saddle-type electromagnet structure that meets the design requirements.The working current of the electromagnet structure is21 A,the outer loop magnet is 2.2m high,the radius is 0.556 m,and the radian is147.25 °;the inner loop magnet is 1.89 m high,the radius is 0.476 m,and the radian is58.7 °.The magnet can provide a static magnetic field of 47 m T in a spherical area with a diameter of 240 mm,and its simulated uniformity is 1881 ppm.(2)H-type permanent magnet design optimization.In this paper,by building a Matlab and Comsol joint simulation platform,the Matsol software is used to control the Comsol software for modeling and simulation,and the joint simulation platform and genetic algorithm are combined to greatly improve the optimization efficiency.By adopting a sectorized structure,the influence of the iron yoke on the target field is reduced,and the uniformity of the magnetic field in the target area is improved.The final design completes the H-type permanent magnet structure that meets the requirements.The permanent magnet is 0.85 m long,0.74 m wide,and 0.44 m high.It can provide a 59 m T static magnetic field in a spherical area of 240 mm in diameter,and its simulation uniformity is 1498 ppm.(3)Research on weight loss of H-type permanent magnets.By optimizing the shape of the iron yoke,it is possible to ensure weight loss without affecting the magnetic field distribution of the target area as much as possible,and the actual weight loss is 18.5kg.The ring magnets,which can theoretically be further reduced in weight,were preliminarily explored,and the single ring,double ring,and triple ring structures were optimized,and the preliminary optimized structure that met the design requirements was obtained.Can lose weight by 0.35%,2.43%,and 3.19% respectively.(4)Experimental research.In this paper,the magnetic field measurement of the designed and processed H-type magnet was carried out,and the measured uniformity was 7486 ppm without shimming.Passive shimming improves the uniformity to 59 ppm.An ultra-low field magnetic resonance imaging platform was built with the H-shaped main magnet as the core,and head imaging experiments were performed on normal humans and patients with cerebral hemorrhage.The image effect is good,and the lesion area can be clearly distinguished.