Gradient heating and cooling
Why do the scanner walls get so warm?
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The walls along the inner bore of an MR scanner frequently become noticeably warm during the course of an imaging study. This heating is primarily caused by eddy currents and resistive heating as current is passed through the imaging gradients.
Gradient coils are driven by powerful pulse-width modulated (PWM) amplifiers located in an equipment room that is usually directly adjacent to the magnet (scanning) room. For intense imaging applications is it not unusual for peak coil driving voltages to reach 2500 V and currents to exceed 1000 A. Intense gradient heat is created, with maximum internal coil temperatures reaching 55-60 °C. The power required to operate a gradient coil scales with the fifth power of its radius. Thus gradient coil design and cooling is even more challenging for today's wide-bore (70 cm) scanners.
All modern scanners use some sort of fluidic cooling to minimize heating. This is generally accomplished by circulating de-ionized water or an ethylene glycol solution from a heat exchange pump ("chiller") located in the adjacent magnet equipment room. The water flows through tubes, jackets, or channels in close relation to the gradient coils as shown in the illustration above. Some scanners also include forced air cooling around the gradients.
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Paired ('in'-'out') gray hoses (arrows) circulate water/ethylene glycol around the gradient coils for cooling
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Chiller (inside components)
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Chiller (outside components)
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More efficient cooling and reduction in "hot spots" have also resulted from improved design of the cylindrical formers (shells) on which the gradient coils are wound. These formers are being increasingly constructed from alumina-ceramic materials with markedly superior thermal conductivity properties than older plastic-fiberglass composites.
In addition to heating of the gradients themselves, warmth also comes from eddy current heating of the radiofrequency (RF)-shield. The RF-shield lies between the gradient and RF-coils and serves to decouple the two during RF transmission. The RF-shield is closely apposed to the thin RF-coil and is thus located very near the inner plastic wall of the scanner bore. Care must be taken to keep large patients from touching the inner walls of the scanner as rarely burns have occurred when the patient's body becomes part of an eddy current loop.
In addition to heating of the gradients themselves, warmth also comes from eddy current heating of the radiofrequency (RF)-shield. The RF-shield lies between the gradient and RF-coils and serves to decouple the two during RF transmission. The RF-shield is closely apposed to the thin RF-coil and is thus located very near the inner plastic wall of the scanner bore. Care must be taken to keep large patients from touching the inner walls of the scanner as rarely burns have occurred when the patient's body becomes part of an eddy current loop.
References
Doty FD. MRI gradient coil optimization. In: Blumler P, Blumich B, Botto R, Fukushima E (eds), Spatially Resolved Magnetic Resonance. Wiley; 1998, pp 647-674.
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Doty FD. MRI gradient coil optimization. In: Blumler P, Blumich B, Botto R, Fukushima E (eds), Spatially Resolved Magnetic Resonance. Wiley; 1998, pp 647-674.
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.