Radioisotopes and Radiopharmeceuticals
Magnetic Resonance Imaging
Nuclear science is widely applied in medicine. The
versatility of nuclear techniques allows for a range of applications
in medical diagnosis and treatment of bodily conditions. In the
United States alone, over 3500 hospitals perform 10 million nuclear
medicine procedures each year.
Radioisotopes and Radiopharmeceuticals
In medicine, substances containing radioisotopes are given the specialised name
radiopharmeceuticals. Many nuclear medicine procedures rely
upon radiopharmeceuticals to work.
act as tracers. (See Science Applications
for more about this.)
A huge array of different compounds are found
in the human body, and doctors can follow these through the body
using radiopharmeceuticals. Radiopharmeceuticals are simply the
ordinary compounds found within the body - except they are
synthesised to contain an atom which is radioactive. Thus, a
radiopharmeceutical is just a normal compound that has been
radioactively 'tagged', and can thus be tracked with radiation measuring devices.
Consider a molecule of glucose
(a type of sugar found in the body). When it is
artificially synthesised as a tracer, scientists replace a stable carbon atom
with a radioactive one, consequently tagging it.
This resultant tagged molecule is a
radiopharmeceutical. The trail of radiation that it emits while it
passes through the body can be monitored using Geiger meters or
scintillation counters. This trail can then be mapped into a computer
to show doctors how the compound is processed by the body. The
tracing technique is particularly useful for monitoring digestion of
food. It has also been used in the study of how amino acids, critical
substances in our bodies, are formed.
Radioisotopes can also
be used to calculate the permeability of cell walls - that is, how
easily cell walls can be penetrated. Radioactively tagged molecules
are injected into the fluid surrounding the cell. The presence of
radioactivity inside or outside of the cell is monitored
over time, allowing the permeability to be determined.
For example, nerve impulses in our bodies involve
sodium and potassium ions passing through 'sodium-potassium pumps'.
By placing radioactive sodium ions outside cell walls of nerve cells and
monitoring the level of radioactivity, the activity of the pump can be determined.
Radiopharmeceuticals can also be used to determine activity of
compounds concentrate in certain parts of the body. Therefore in the
analysis of a certain part of the body, the patient is given the
corresponding radiopharmeceutical that will accumulate most there.
Once in the body, the
distribution of the radioactively labelled compounds is monitored by
a network of "gamma cameras" around the patient, which work
by detecting the radiation from the compounds. The data collected by
the cameras is processed by a computer, allowing doctors to analyse
the patient's body. High concentrations of the
radioactively tagged compound in or around an organ can indicate
overactivity; low concentrations can indicate underactivity.
One example of this is the common test for thyroid gland functionality.
Patients are given a capsule of often iodine-123, and
a scan takes place usually on the next day to assess the condition of
Magnetic Resonance Imaging
Magnetic resonance imaging is based on the vibrations that nuclei
experience when bombarded by radio waves in a magnetic field.
See Nuclear Magnetic Resonance in science.
The alternative name, magnetic resonance imaging (MRI) arises
from a modification of nuclear magnetic resonance (NMR).
The word "nuclear" is omitted so as to avoid
the negative connotations. In fact,
NMR has nothing to do with radiation or radioactivity and is relatively
safe compared to other medical imaging techniques.
"Nuclear" is relevant because of the process' relation with nuclei.
Magnetic resonance imaging is used as an analytical tool in scanning
various parts of the body. It is especially used in brain scans
and is one of the best techniques known for such procedures.
The technique is also used in obtaining highly accurate images of
body tissues - much more precise than computer aided tomography (CAT) scans
or ultrasound. The resolution of magnetic resonance images typically
is of the order of 0.5 to 1 mm, one of the most precise techniques available.
resolution of various medical imaging techniques. Even though some
techniques are more precise than others, not all can be used for a given need.
|positron emission tomography||80-100|
In a hospital, magnetic resonance imaging machines consist of a large
hollow cylinder where the patient is placed. In this cylinder, there are several
kilometres of wire wrapped around in a coil. When a current is passed through
this wire, a very strong magnetic field is generated, and is especially concentrated
in the centre of the cylinder. This provides the magnetic field for the
resonance of nuclei.
The patient to be scanned is placed in the cylinder, and then the machine generates
a magnetic resonance image, which is then used by doctors
to determine the patient's condition.
sample magnetic resonance image of a brain scan
Radiation can cause cancer. Normally, cells that are exposed to radiation die.
However, if the radiation does not kill the cell, they cell may be mutated by the
radiation, and can start to reproduce. These multiplying mutant cells constitute
However, although radiation can
cause cancer, it can also be used to treat it.
Rapidly multiplying cancer
cells are particularly prone to radiation. Doctors can administer
radiation to kill these cancerous cells. However, some surrounding
normal cells are unavoidably killed as well, due to the large doses
of radiation required. Thus patients who undergo radiation therapy
also experience side effects such as radiation sickness. To try to
minimise these side-effects, narrow beams of radiation are used, and
the beam continuously rotates around the body, minimising the
radiation to normal areas but concentrating it around the cancerous
Cross-section view of how a radiation beam rotates around a point so that
radiation is concentrated there, but minimised everywhere else.
Radiation is also used
to sterilise surgical equipment and bandages. By bombarding the
object to be sterilised with radiation, bacteria and viruses can be
killed. This procedure is very common in hospitals for cleaning
reusable surgical equipment between operations and sterilising
blood for transfusions.
It is also used in the manufacture of disposable medical products
such as bandages and syringes where sterilisation is required before sale or use.
Radiation-based sterilisation is especially useful in sterilising heat-sensitive
or steam-sensitive materials.
The technique of single photon emission tomography, also known as SPET,
involves administering the patient with a radiopharmeceutical, and
then rotating a gamma camera or other radiation detector around the patient to
detect the emitted radiation from many different angles. These different angles are put
together by a computer to produce an image useful for the doctors.
The general concept of moving a detector around a patient to gain
data from different angles is known as computer-aided tomography, or CAT.
Thus SPET is a type of CAT.
Another technique called positron emission tomography, or PET, makes use
of positron emitting atoms such as carbon-11, nitrogen-13 and oxygen-15.
These atoms are again incorporated into radiopharmeceuticals that concentrate
themselves in the part of the body to be studied once injected or consumed by the patient.
When these atoms beta-decay, the emitted positron will move and eventually collide
with an electron elsewhere in the body. Once this happens, the positron and electron
are both destroyed, producing two gamma rays. These resultant gamma rays
radiate out in exactly opposite directions.
A ring of detectors is placed around the patient, and when the gamma rays are detected,
a computer can calculate the time difference and therefore work out where
the annihation event occurred. However, with today's technology,
these time differences cannot be measured accurately enough to generate
useful data. (The best possible resolution is about 8cm). Nevertheless,
if this could be refined, the biggest advantage of PET would be the lower
scanning time and that
a lower dose of radiopharmeceutical could be used. This is because
the detection in PET is done capturing two gamma rays at once with one large
circular detector, whereas with other CAT techniques one small detector that
rotates around the patient is used.