User:GyroMagician/MRI hardware
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Contents |
[edit] Introduction
[edit] Main Magnet
[edit] Shims
[edit] Gradients
During an imaging sequence the static B0 field is deliberately altered by the addition of linear field gradients used to localize the NMR signal. An MRI system typically has three orthogonal gradient coils. The magnetic field generated by each coil is aligned to the main static field, but varies in the three orthogonal directions x, y and z (picture).
Gradient coils are large resistive coils, set inside the bore of the main magnet. Due to the high current carried by the coils during an imaging sequence (~600A), the coils are often water cooled to prevent over heating. Gradient coils typically produce a field gradient of 30mT/m, with the highest absolute field of approximately 6mT produced at the edge of the field of view.
Early imaging experiments used shim coils used to generate field gradients [13], but it was quickly found that the imaging speed was limited by the time taken to switch the shim coils. Separate, low-inductance higher-power gradient coils are now used.
Magnetic gradients are generated by three coils, producing orthogonal gradients in the x, y and z directions of the scanner. These are usually resistive electromagnets powered by sophisticated amplifiers which permit rapid and precise adjustments to their field strength and direction.
Typical gradient systems are capable of producing gradients from 20 mT/m to 100 mT/m (i.e. in a 1.5 T magnet, when a maximal z-axis gradient is applied the field strength may be 1.45 T at one end of a 1 m long bore, and 1.55 T at the other).
It is the magnetic gradients that determine the plane of imaging - because the orthogonal gradients can be combined freely, any plane can be selected for imaging.
Scan speed is dependent on the strength and switching speed (slew rate) of the gradient system.
Switching speed is determined by the inductance of the gradient coils. Gradient strength is determined by the power of the gradient amplifiers, and the efficiency and cooling of the gradient coils.
Scan speed is dependent on performance of the gradient system. Stronger gradients allow for faster imaging, or for higher resolution; similarly gradients systems capable of faster switching can also permit faster scanning. However, gradient performance is limited by safety concerns over nerve stimulation.
