Questions 1-11 are based on the following
Magnetic resonance imaging (MRI) uses the body's
natural magnetic properties to produce detailed images from
any part of the body. For imaging purposes the hydrogen
nucleus (a single proton) is used because of its abundance in
water and fat.
The hydrogen proton can be likened to the planet earth,
spinning on its axis, with a north-south pole. In this respect it
behaves like a small bar magnet. Under normal
circumstances, these hydrogen proton "bar magnets" spin in
the body with their axes randomly aligned. When the body is
placed in a strong magnetic field, such as an MRI scanner,
the protons' axes all line up. This uniform alignment creates a
magnetic vector oriented along the axis of the MRI scanner. MRI scanners
come in different field strengths, usually
between 0.5 and 1.5 tesla.
The strength of the magnetic field can be altered
electronically from head to toe using a series of gradient
electric coils, and, by altering the local magnetic field by
these small increments, different slices of the body will
resonate as different frequencies are applied.
When the radiofrequency source is switched off the
magnetic vector returns to its resting state, and this causes a
signal (also a radio wave) 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. A
different emphasis occurs because different tissues relax at
different rates when the transmitted radiofrequency pulse is
switched off. The time taken for the protons to fully relax is
measured in two ways. The first is the time taken for the
magnetic vector to return to its resting state and the second is
the time needed for the axial spin to return to its resting state.
The first is called T1 relaxation, the second is called T2
An MR examination is thus made up of a series of pulse
sequences. 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.