MRI Machines

An MRI or Magnetic Resonance Imaging machine uses magnetism to look inside the human body. How an MRI works is a fascinating concept. An extremely powerful magnetic field of 1.5 to 3.0 Tesla surrounds the patient. A Tesla is a unit of measurement of the magnets strength. To give an idea of the strength of an MRI magnet, the Earth’s core is pulling on objects with an intensity of about 0.5 Tesla. OK so the MRI is 3 times stronger right? Not quite, there are a lot of Physics calculations to consider but in reality the MRI is about 10,000 to 20,000 times stronger and this is based mostly on the proximity and mass of the object to the magnetic field.
This magnetic field is created by taking several thousand turns of wire around a large open core and running current through it. These magnet wires are turned into superconductors by bathing them in liquid helium. The liquid helium drops the temperature of the wire to -450 Fahrenheit and effectively removes all resistance from the wire. This is the primary magnet and is responsible for aligning the hydrogen atoms in the human body.

MRI’s also use three other types of magnets called gradient magnets that have approximately 1/1000th of the strength of the primary magnet. The purpose of the gradient magnet is to focus on a specific part of the body. Each gradient magnet is responsible for focusing one plane of the 3-D image created by the primary magnet also sometimes just referred to as the coil. Those planes are coronal, sagittal, and axial.

The coronal plane separates the front of the body from the back. Imagine laying the human body on a table and cutting through it so the front half could be lifted away and the back would still be on the table. The Sagittal Plane would cut the body in half separating the left side from the right side. The Axial Plane separates the top half of the body from the lower half. For a better understanding see the image above.

The images produced all show up as a gray scale image of cross-sectional slices of the body. The varying shades of gray indicate the thickness, type and consistency of the tissue. After these slices are put in order a 3-dimensional image can be displayed. The MRI uses different types of coils to surround the body part that is intended to be scanned. These coils further fine tune the image by picking up magnetic induction being produced by the primary magnet and gradient magnets.

Some areas of the MRI in my hospital that have been problematic are the SSM, (System Support Module), the RF Gradient Amplifier, the Cold Head and various mechanical issues. We have an older model 1.5 Tesla magnet that requires periodic refilling of the liquid helium. I monitor the boil off rate as does the company that refills it. The boil off rate is the rate at which the liquid helium warms up enough to turn into gas and escape into the atmosphere. The normal refill rate of my magnet is around once every three months and has been for years. We recently noticed that the refills were becoming more frequent and checked the pressure of the magnet to find it had risen. Normally this indicates that the cold head is getting weak and sure enough it was. The cold head is the part of the magnet that keeps the liquid helium chilled at -450 Fahrenheit. The normal lifespan of a cold head is about 3 or 4 years. Ours was 9 years old! We replaced it and the boil off rate dropped.

Occasionally we get complaints that the image is hard to focus or one plane is out of whack. Normally a quick calibration of the gradient takes care of it. One particular instance the images had no resolution at all. The RF Gradient Amplifier circuit had become defective. This circuit is what boosts or amplifies the Micro or Pico voltages that are being picked up by the body coil, head coil, knee coil, or any one of a dozen different coils that are placed around the patient to help improve the signal. The gradient magnets themselves are pretty sturdy and usually respond well to the calibration procedure and start acting right again.