4 Magnetic resonance imaging
Magnetic resonance imaging (MRI) is one of the newest techniques available for imaging in hospitals. It is a 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.
X-rays are not used, but instead the patient is placed in a ‘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 7 and 8 below). More protons align with the magnetic field, as this requires less energy. So, within the patient the net magnetisation within the tissues is aligned parallel to the applied 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 magnetisation vector perpendicular to the applied magnetic field.
The signals are detected using specially designed and shaped radiofrequency coils (or antena), 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.
In the following video clip a head coil is used. As you are watching the video, pay particular attention to the main differences in image appearance between MRI and CT.
Click to view the clip about magnetic resonance imaging [3 minutes 24 seconds]
Transcript: Magnetic Resonance Imaging (MRI)
Take a moment to consider what you think are the main difference between MRI and CT images.
MRI can be used to ‘slice’ the body in any direction. As well as producing ‘axial’ images, it can also produce sagittal or coronal images (or any plane in between.) The contrast can be changed in MRI images producing what were called T1, T2, or proton density weighted images. The contrast in CT images is fixed, depending on the attenuation coefficient.
We will now look at T1, T2 and proton density weighting in a bit more detail. Once the net magnetisation 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 9 and 10, below, show T2- and T1-weighted MR images.
You may think that since MRI uses strong magnetic fields and rf waves, rather than X-rays, it is completely safe. Actually, two safety hazards were mentioned in the video clip. Can you remember what they were?
MRI scanners, magnetic fields and rf pulses can upset the operations of pacemakers. In addition, heating can occur in some metallic implants.
Another major hazard of MRI scanners not mentioned in the video is 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.
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.
Now that you understand more about MRI, take a little time to consider what you think the advantages and disadvantages of this technique would be.
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 disadvantages are:
high cost of equipment and maintenance;
not suitable for all patients (e.g. those with a heart pacemaker).