In this article, I want to use an MRI (magnetic resonance imaging) medical scanner design as a familiar example for what needs to be done in a power supply design architecture to allow it to function properly in a high magnetic field environment.

An MRI room is a very harsh electromagnetic (EM) environment for electronics. Shielding may be used to enclose electronic devices to attenuate the strength of magnetic fields down to a level at which the devices can function properly and not be destroyed. Without proper shielding, the magnetic fields produced by the MRI scans will damage electronics and may also pull ferromagnetic objects into the bore of the MRI.

In the other direction, interference from electronics can cause false images to be obtained from the MRI, so that aspect of the power supply design must be addressed.

Until recently, power supplies designed into the MRI environment have had limited functionality depending upon how close they are placed near the MRI unit. The magnetic field that is present typically causes power supply failure. An object made from ferromagnetic material will be pulled into the MRI machine, and become attached to the MRI magnet--not a good thing. A power supply with iron-core based transformers and inductors are candidates for this disastrous situation.

In the past, power supplies were prevented from being drawn into the MRI machine via Velcro strips to keep them attached to the floor or other anchors in the room. In this case, a long, shielded cable is used to connect the power supply to the patient monitor being powered. This method goes against two key features of an MR patient monitor. Cost needs to be kept down, but the shielded cable is expensive. Mobility is necessary, and the range of the patient monitor is limited by the shielded cable it is attached to. The best design is to have the power supply attached to the patient monitoring unit. This would lower the cost of the shielded cable, because it can be much shorter. The mobility of the patient monitor is also increased, because the power supply is not attached to a fixed anchor.

Creating an optimum power supply

Designing a power supply board takes many resources, and a great deal of testing needs to be performed. A custom power supply would need to be produced pretty quickly for the completion of the MRI scanner project, and may not be at the same quality of an off-the-shelf one, which had years of resources deployed in its development. In addition, the power supply would also have to comply with safety regulations regarding patient monitoring systems, such as IEC60601. Because such a power supply would have already gone through the regulation process, it would just need to be qualified for the purposes of the particular project architecture.

MRI equipment and its surrounding environment

MRI uses a magnetic field and pulses of radio wave energy in order to create images of organs and structures inside the body. The magnetic field generated by the coil is typically in the range of 1 to 4 Tesla, which is a huge magnetic field that will have a detrimental effect on some of the electrical equipment such as power supplies that may get their transformers saturated and will not be able to function in such an environment. For safety and patient comfort during the MRI session, some equipment requires the power supply to be as close as possible to the load, meaning the power-unit must operate safely while exposed to the high magnetic field generated by the coil.

MRI uses a large magnet and radio waves to look at organs and structures inside the human body. There are numerous challenging design requirements when designing a power supply for an MRI architecture. Because of the sensitivity of the measurements made by an MRI machine, the oscillator frequency of the power supplies need to be precisely placed at a frequency that will not corrupt the MRI image.

[Continue reading on EDN US: Power designs for MRI environments]

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