MR physics

The basic process

The way MR images are generated is complicated and is much harder to understand than plain radiography, CT and ultrasound. It has strong underpinnings in physics which must be understood before any real sense of 'how it works' is gained. 

What follows is a very abbreviated, 'broad strokes' description of the process. Essentially, the process can be broken down into four parts: 1) Preparation, 2) Excitation, 3) Spatial Encoding and 4) Signal Acquisition. For a more detailed description of each part of the process, please refer to the links scattered throughout this introduction and at the bottom of the page. 

Preparation

The patient is placed in a static magnetic field. Hydrogen protons within the patient's body align to the magnetic field.  Additional preparation sequences can also be performed to manipulate the contrast in the image, e.g., inversion prep.

Excitation

An RF pulse(Radio Frequency pulse) is emitted from the scanner, tuned to a specific range of frequencies at which Hydrogen protons precess. This results in some of the Hydrogen protons being "knocked" 180o out of alignment with the static magnetic field and being forced into phase with other Hydrogen protons.  As the energy from the RF pulse is dissipated, the Hydrogen protons will return to alignment with the Static magnetic field. The MRI signal is derived from the Hydrogen protons as they move back into alignment with the magnetic field, and fall out of "phase" with each other.  (The actual process is much more complicated broken down into T1 relaxation, and T2 decay).

Spatial Encoding

Spatial encoding of the MRI signal is accomplished through the use of Gradients (smaller magnetic fields) which perturb the main magnetic field, and cause Hydrogen protons in different locations to precess at slightly different rates. The portion of the gradient coils and the associated current that is perpendicular to the main magnetic field cause a force (Lorentz force) on the coils. The Gradients are turned on and off very quickly in this process causing them to vibrate causing the majority of the noise associated with the MRI environment. This occurs even though they are embedded in epoxy!

Signal Acquisition

As the protons undergo relaxation, the change in the local magnetic fields create currents in the receive coils. These currents can be detected as a change in voltage. The signal must then be sampled, converting the analog signal to digital, and then is stored for processing. The MRI signal is then broken down and spatially located to produce images.  

Standard exam

Multiple image sets are obtained in the standard exam (which varies from facility to facility). Exam times vary according to part of anatomy being studied, pathology expected, and Radiologist preferences. Occasionally, a contrast material may be used to enhance images. Typically, exams are ordered without and with contrast for comparison purposes. Very rarely, and only in certain circumstances are exams ordered with contrast only. After the exam the patient is removed from the scanner and given post procedure instructions (information about contrast material if used, sedation if used, and time when to expect a report from the examination). 

MRI physics
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rID: 22558
Section: Physics
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