ABSTRACT

Magnetic resonance imaging (MRI) is one of the newest techniques available for imaging in hospitals. It is a so-called tomographic technique (i.e. it takes ‘slices’ through the body) based on the phenomenon of nuclear magnetic resonance (NMR). Technically it is a complex technique, but some understanding can be gained using qualitative arguments.

Nuclear spin

X-rays are not used in MRI scanning, but instead the patient is placed in a relatively strong magnetic field (around 30,000 times as strong as the Earth’s magnetic field). The nuclei of hydrogen atoms (protons) like many other nuclei have a special property called nuclear spin. This means that in some respects they behave like tiny bar magnets, and in a static magnetic field they have just two possible orientations; either aligned with, or against, the magnetic field (see Figures 1 and 8 below). More protons align with the magnetic field, as this requires less energy. So, within the patient the net magnetization within the tissues is aligned parallel to the applied magnetic field.

Figure 1. Randomly oriented protons in a magnetic field

Figure 2. Protons in the presence of a magnetic field

Transitions away from this parallel state can be brought about by the application of a radiofrequency (rf) field, typically in the region of 20 to 100 MHz for many MRI scanners. The frequency required, which is referred to as the resonant or Larmor frequency, depends linearly on the strength of the static magnetic field. The rf field is applied in the form of a pulse of short (microseconds) duration. The signal detected by the scanner is the component of the net magnetization vector perpendicular to the applied magnetic field.

The signals are detected using specially designed and shaped radiofrequency coils (or antenna), for example for heads, knees, etc. The fact that these coils can be placed immediately adjacent to an area of interest makes an important contribution to the quality of the final image.

Creating body slices

MRI can be used to ‘slice’ the body in any direction. As well as producing ‘axial’ images, it can also produce images of the sagittal plane (i.e. a vertical plane that divides the body into right and left sections), coronal plane (a vertical plane that divides the body into front and back sections) or any plane in between. The contrast can be changed in MRI images producing what are called T1, T2, or proton density weighted images.

Proton density weighting

Once the net magnetization has been excited away from its position parallel with the magnetic field by the rf pulse, it moves or ‘relaxes’ back to its original state. The times taken for this relaxation are governed by T1 and T2 time constants. These T1 and T2 values vary for different types of tissue.

MR images are formed using a series of rf pulses in a carefully timed sequence. By varying the timings of these pulse sequences the final images can highlight the differences in T1 or T2. (It is also possible to set the timing within a pulse sequence so that the contrast is independent of both T1 and T2, and so depends on just proton density.)

T1-weighted images show tissues with a large value of T1 (e.g. water) as dark. In T2-weighted images tissues with a large value of T2 are bright. Water has a high T2 so shows up bright in a T2 image. This feature can be used to highlight disease. Figures 3 and 4, below, show T2- and T1-weighted MR images.

Figure 3. T2 axial of normal brain

Figure 4. Sagittal T1 MRI image of the pediatric head

Advantages and disadvantages of MRI

The advantages of MRI are:

• excellent spatial resolution
• excellent soft tissue discrimination (for example, white and grey matter in the brain can be distinguished)
• no ionizing radiation
• slices can be taken in any direction
• the strength of the magnetic fields and the power levels for the radiofrequency irradiation are thought to be well below potentially damaging levels. [However, as a precaution, MRI is not normally carried out during pregnancy.]

The disadvantages are:

• high cost of equipment and maintenance
• relatively slow
• not suitable for all patients (e.g. MRI scanners, magnetic fields and rf pulses can upset the operations of pacemakers; heating can occur in some metallic implants)
• the ‘projectile effect’: ferromagnetic materials experience strong forces in the static magnetic field and objects such as a bunch of keys or an oxygen cylinder can become lethal weapons

Vocal tract imaging

Together with radiography (use of X-rays) and videofluoroscopy, MRI scanning is a popular technique for making images of the vocal tract. It is a useful tool for the instrumental measurement of voice.

Acknowledgement

[Information last accessed: 29 July 2017]

This article is adapted from ‘Imaging in medicine’. An OpenLearn (http://www.open.edu/openlearn/) chunk reworked by permission of The Open University copyright © 2016 – made available under the terms of the Creative Commons Licence v4.0 http://creativecommons.org/licenses/by-nc-sa/4.0/deed.en_GB. As such, it is also made available under the same licence agreement.

Header image: Liz West in Flickr made available under Creative Commons Attribution 2.0 Licence.