MRI Physics- What is MRI? – Deconstructing MRI Physics Test Review.

What is MRI?


What is MRI?

MRI or Magnetic Resonance Imaging is a rapidly growing imaging modality that uses an external magnetic field, radio frequencies, gradients, computer hardware/software, and numerous other components to obtain diagnostic images. Unlike CT, MRI does not use harmful ionizing radiation which is used in X-ray but instead utilizes non-ionizing radiation to obtain highly detailed images of soft tissue, the chemical metabolism of tissue or tumors, the patency of vasculature and many other structures and functions within the body. This guide is intended for future or current MRI Technologists to either review for their registry exam, be it ARRT or ARMRIT, or further their knowledge within the field of MRI Physics. While this first segment may seem technical, even to those familiar with the concepts each area of interest and importance to the registry exam will be covered in easily understandable language and analogies as the guide progresses.

In MRI, a patient who has been screened for unsafe metals and implanted devices is introduced to an external magnetic field, and the patient’s anatomical area of interest is placed as close to the center (or middle) of the bore of the magnet as possible, known as isocenter. The hydrogen nuclei within the body, which consists of a single proton are spinning around throughout much of the bodies tissue with random orientation. This spin is referred to in MRI as a Magnetic Moment.

In the presence of this external magnetic field, however, hydrogen nuclei lose their random orientation and align themselves in excess toward, or parallel to this external magnetic field. This is to say that while some nuclei align themselves against, or anti-parallel to the external magnetic field, more nuclei align themselves in the direction of the external magnetic field. This collection of excess magnetic moments (spinning nuclei) spinning in the parallel direction is referred to as a Net Magnetization Vector.

These hydrogen nuclei spin, or precess, around the external magnetic field at a certain frequency that is dependent on how powerful the external magnetic field is, which is measured in Gauss or Tesla.

A radiofrequency (RF) pulse of a similar frequency to that of the precessional frequency of the hydrogen nuclei is sent in and disrupts the excess nuclei parallel to the external magnetic field. This RF pulse flips these nuclei away from the external magnetic field and all the nuclei are temporarily in phase or precessing at the same rate in the same direction with the same orientation. As soon as this RF pulse dissipates, the hydrogen nuclei begin to lose phase and relax back to their natural state, back in the direction of the external magnetic field,.

This relaxation and loss of phase produces an analog signal within the receiver coils and is measured in several ways depending on the desired contrast weighting of the image. This signal is spatially encoded or simply honed into the area of interest (brain, foot, abdomen etc.), by gradient coils. These coils create small variations in the precessional frequency of the hydrogen nuclei, and in doing so are capable of only sampling hydrogen nuclei in the anatomical region of interest. A one-dimensional linear gradient can be thought of as follows.

Imagine that in lying on the table within the bore of the magnet the linear gradient can cause the hydrogen nuclei at the base of your feet to revolve or precess once per second. These nuclei would spin faster and faster as they progress further up your body (superior) until the nuclei at the top of your head revolve or precess 10 times per second. If you wanted images of your brain, you could selectively tell the machine to only collect data points from hydrogen atoms precessing between 9 and 10 times per second. Anything below 9, or the mid-neck down in this scenario, would not be sampled and data from that area would not be included, and only information from that anatomical area of interest would be sampled and collected to create the image.

Gradients are also the primary source of noise during an MRI due to rapidly changing electrical currents causing vibrations within the coils themselves. These rapidly changing electrical currents are what allow the gradient coils to create a gradient magnetic field, and the noise (while improvements are being made) is currently a necessity of this process.

In reviewing the fundamentals of Magnetic Resonance Imagine it is obvious that there are three major components that are necessary for obtaining and collecting a signal.

  1. The External Magnetic Field – To create a Net Magnetization Vector.
  2. A Radiofrequency Transmitter and Receiver- To disrupt the nuclei and collect data.
  3. A Magnetic Gradient – for spacial encoding.


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