Magnetic resonance imaging, or MRI, is a way of obtaining very detailed images of organs and tissues throughout the body without the need for x-rays. Instead, it uses a strong magnetic field (typically 1 to 2 T) , radio waves, a rapidly changing magnetic field, and a computer to demonstrate whether or not there is an injury or some disease process present. For this procedure, the patient is placed within the MR scanner—typically a large, tunnel or doughnut-shaped magnet that is open at both ends. The magnetic field aligns atomic nuclei that are present in the body's tissues. Radio waves then cause these particles to produce signals that are picked up by a receiver within the scanner. The signals are specially characterized using the changing magnetic field, and computer-processed to create very sharp images of tissues as "slices" that can be viewed in any plane or from any direction.
The powerful magnetic field of the MR system will attract
ferromagnetic objects and may cause them to move suddenly and with
great force. This can pose a possible risk to the patient or anyone in
an object's flight path. Great care is taken to be certain that objects
such as screwdrivers and oxygen tanks are not brought into the MR
system area. It is vital that patients remove any metallic belongings
in advance of an MRI exam, including watches, jewelry, and items of
clothing that have metallic threads or fasteners.
All MR imaging systems are equipped with a set of resistive wire
windings known as gradient coils, which produce the gradient magnetic
field. The gradient magnetic field is a time varying magnetic field,
and this property requires that different safety precautions be taken
as opposed to the static magnetic field. Gradients provide
position-dependent variation in magnetic field strength and are pulsed
on and off during and between RF excitation pulses. The purpose of
these gradients is to spatially encode information contained in the
emitted RF signal. The amplitudes of these gradient magnetic fields
(typically 20 mT/m) may seem small in comparison with the main static
magnetic field, but the salient feature is not the strength of the
field but the rate of change of field
generated when the field is switched on
or off. A quantity called the slew
rate, which is the rate of change of gradient amplitude is
usually quoted in evaluating gradient performance and potential
physiological effects. Typical slew rates range from 20 to 80 T/m/s.
Faster rise times of the gradient produces shorter echo spacing and
better image resolution.