Posted by: Thixia | April 12, 2008

MRI 6 of 14

More MRI Magnets



The magnets make MRI systems heavy, but they get lighter with each new generation. For example, at the institution where I work, we are getting ready to replace an eight-year-old scanner that weighs about 17,000 lbs (7,711 kg) with a new one that weighs about 9,700 lbs (4,400 kg). The new magnet will also be about 4 feet shorter (about 6 feet / 1.8 m long) than our current one. This is very important to claustrophobic patients. Our current system cannot handle anyone who weighs more than 295 pounds (134 kg). The new one will be able to accommodate patients over 400 pounds (181 kg). The systems are getting more and more patient friendly.  

Photo courtesy NASA
This MRI image shows some of the internal organs in someone’s upper torso.

A very uniform, or homogeneous, magnetic field of incredible strength and stability is critical for high-quality imaging. It forms the main magnetic field. Magnets like those described above make this field possible.


Another type of magnet found in every MRI system is called a gradient magnet. There are three gradient magnets inside the MRI machine. These magnets are very, very low strength compared to the main magnetic field; they may range in strength from 180 gauss to 270 gauss, or 18 to 27 millitesla (thousandths of a tesla). The function of the gradient magnets will become clear later in this article.


The main magnet immerses the patient in a stable and very intense magnetic field, and the gradient magnets create a variable field. The rest of an MRI system consists of a very powerful computer system, some equipment that allows us to transmit RF (radio frequency) pulses into the patient’s body while they are in the scanner, and many other secondary components.

Let’s find out about some of the basics involved in creating an image.


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