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.

Figure 7: Randomly orientated protons in a magnetic field

Figure 8: Protons in the presence of a magnetic field

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Figure 8

Figure 8: 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 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]

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Transcript: Magnetic Resonance Imaging (MRI)

Thankfully, for this patient, a CT scan has already shown that the top of the spine hasn’t been fractured, but damage to the brain has yet to be ruled out. Magnetic resonance imaging, or MRI, with its excellent soft tissue discrimination, is the best way to check for this.

To obtain images the patient is placed in a large static magnetic field, produced by a super-conducting magnet. This powerful field, typically 1.5 T in strength, causes the nuclei of the hydrogen atoms in the body to line up. But they can be flipped into another direction by a radio frequency pulse. The way these nuclei then relax back to their original position depends on the environment of the hydrogen atoms, in other words on tissue type.

When imaging small parts of the body, specialized coils are used to detect the radio frequency signal given off by the relaxing nuclei. Here a head coil is being used.

Laser alignment is used so that the patient can be moved to the correct position in the scanner.

Because the powerful magnetic fields can disrupt pacemakers and cause heating of metal implants, every patient must be carefully checked before entering the scanner room.

With MRI it is possible to choose any direction for the image slices. In this case the radiographer is taking an axial pilot scan to give a series of coronal images.

There are many different imaging sequences that can be used, but most of them produce an image where the intensity depends mainly on one of three characteristics – T1, T2 and proton density.

The intensity in these images depends largely on T2, the spin-spin relaxation. Substances with a long T2, such as water, appear bright. This is obvious if you look at the eyes in this image.

Thankfully for this patient the images show no abnormality.

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Magnetic Resonance Imaging (MRI)

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.

Figure 9: T₂ axial of normal brain

Figure 10: Sagittal T₁ MRI image of the paediatric head

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.