Magnetic Resonance Imaging MRI Explained

An MRI (Magnetic Resonance Imager) is a Medical Imaging tool designed to image soft tissue in the human body. Essentially the machine uses a Radio Frequency Transmitter and Receiver combined with a strong magnet and a high speed computer to create images in the form of electronic ‘slices’ of the human body. Most modern systems use a magnet that is around 1.5 Tesla in field strength, that’s around 10,000 times stronger than the earth’s magnetic field!

Because of the intense field, which is invisible but incredibly strong, there is a very important safety concern while in a MRI room. No magnetic material in the form of metal containing iron is allowed into the room. If this was to happen the metal accelerates straight into the centre of the magnet like a missile! this can injure you. Every person is given a questionnaire to fill out this is to check if they are safe to enter the room. i.e. they have no magnetic material on them which could hurt them.

Once a person is checked for any magnetic (containing iron) implants, rods, screws, pacemakers, aneurysm clips in the brain, etc. they may be admitted into the magnet room.

The scanner uses a strong ‘static’ (non-moving) magnetic field, and a varying or ‘Gradient’ magnetic field to scan a patient. It is this switching on and off of the Gradients that creates a loud banging and machine-gun sounding noise-the patient is given ear-plugs during the scan for this reason.

The machine is tuned to protons inside Hydrogen atoms within the human body. Hydrogen is abundant inside the human body in the form of water and fat molecules. Hydrogen protons, while under the influence of the magnetic field of the MRI scanner, spin and flip around in a fashion quite similar to many spinning clocks. An MRI image is a kind of map of pixels showing distribution of water and fat nuclei within the region of interest or ‘slice’. How the Hydrogen protons behave determines the degree of white or black pixels within the picture. Depending on the type of scan, water can appear bright white or black, with fat, muscle, brain and other tissue falling between these two extremes. Bone quite often appears dark-grey to black because of the way in which hydrogen is bound-up within the structure of the bone. So MRI is not generally used to image bone.