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MRI Scanners

Magnetic resonance imaging (MRI) uses the body's natural magnetic properties to produce high resolution images of the inside of the body. It does this using the properties of the hydrogen ion.

The hydrogen proton can be likened to the planet earth, spinning on its axis, with a north-south pole. Because it is charged, it generates a magnetic field, like any moving charge. The field generated is that of a tiny bar magnet. In the absence of an external field these tiny bar magnets are randomly oriented

When the body is placed in a strong magnetic field, such as in an MRI scanner, the protons' axes all line up. This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner.

When additional energy (in the form of a radio wave) is added to the magnetic field, the magnetic vector is deflected. The radio wave frequency (RF) that causes the hydrogen nuclei to resonate is dependent on the strength of the applied magnetic field.

The strength of the magnetic field can be altered electronically from head to toe using a series of electric coils, and, by altering the local magnetic field by small increments, different slices of the body will resonate with different frequencies.

When the radiofrequency source is switched off the magnetic vector returns to its resting state, and this causes a signal to be emitted. It is this signal which is used to create the MR images. Receiver coils are used around the body part in question to act as aerials to improve the detection of the emitted signal. The intensity of the received signal is then plotted on a grey scale and cross sectional images are built up.

Multiple transmitted radiofrequency pulses can be used in sequence to emphasise particular tissues or abnormalities. Different tissues relax at different rates when the transmitted radiofrequency pulse is switched off so different tissues can be distinguished.

An MRI examination is thus made up of a series of pulses of different frequencies.. Different tissues (such as fat and water) have different relaxation times and can be identified separately. By using a “fat suppression” pulse sequence, for example, the signal from fat will be removed, leaving only the signal from any abnormalities lying within it.

Most diseases manifest themselves by an increase in water content, so MRI is a sensitive test for the detection of disease. The exact nature of the pathology can be more difficult to ascertain: for example, infection and tumour can in some cases look similar. A careful analysis of the images by a radiologist will often yield the correct answer.

There are no known biological hazards of MRI because, unlike x ray and computed tomography, MRI uses radiation in the radiofrequency range which is found all around us and does not damage tissue as it passes through.

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