What’s Inside the Magnet and Why?

Abstract

Three types of magnetic field are used to generate images for magnetic resonance imaging, (MRI):

A strong, constant magnetic field, a gradient magnetic field that can be rapidly switched on and off and a radiofrequency (RF) magnetic field. A typical MRI system therefore consists of three main components that generate these three fields; the main magnet, the gradient coil assembly and the integral rf body transmitter coil. The most common main magnet configurations are superconducting, with a horizontal patient bore. The magnet coils generate a highly uniform magnetic field at the centre of the patient bore which is brought to within a specified limit of homogeneity by a process known as shimming. Three cylindrical copper windings (gradient coils) positioned inside the inner bore of the cryostat each generates a magnetic field gradient which varies the resonant frequency, enabling the MR signal to be encoded in three dimensions. This provides the unique ability to directly acquire cross sectional images in any orthogonal or oblique plane. The integral RF body transmitter coil generates a much smaller magnetic field that oscillates at the resonant (Larmor) frequency, causing the hydrogen nuclei in the patient’s tissue to resonate and then to re-emit the energy, also in the form of an oscillating rf magnetic field. This MR Signal is detected by a RF receiver coil. For Cardiac MRI this is usually a dedicated receiver coil or coil array.