Technetium
99m
Tc-99 m is
an ideal marker radionuclide for imaging as it is:-
- a pure gamma-emitter,
- has a physical half-life of only 6 hours
and
- emits g-rays
of energy that it ideal for detection with a g-camera
(140 keV).
Technetium 99m is a very useful radionuclide
in gamma imaging because it only emits gamma rays and these are of an
energy that are easily detected by a gamma camera (140keV). It has an
ideal half life (six hours) which is long enough for diagnostic procedures
to take place but short enough for the patient not to be inconvenienced
unduly by remaining ‘radioactive’ for too long a period after
the investigation. It is suitable not only for use alone but also for
attachment to a wide range of compounds for tracer experiments. It can
easily be produced ‘in situ’ using a ‘cow’ as it is
the daughter nucleus of the decay of Molybdenum 99 and is easily separated
from the parent (more hazardous b-emitter)
by a saline flush. A fresh supply is brought to the hospital fortnightly
and then the ‘cow’ is ‘milked’ as and when required.
The effective half-life of a radionuclide
is the time taken for the activity of the sample to reduce to one half
of its value in the body of a patient. Effective half life has two components:
the physical half life (related to the probability of spontaneous decay
of the atom – this is fixed and reliably known) and the biological
half life (related to the time taken for half of the radionuclide being
expelled from the body by natural biological processes eg. excretion,
sweating, respiration etc.). Biological half life varies with the individual
and the target organ for the radionuclide as it is dependant on metabolic
processes. This makes it less reliably predictable and poses problems
for dosage calculations.
lE
= lP
+ lB
Where:-
lE
= effective decay constant = ln 2/TE(
effective half life)
lp
= physical decay constant = ln 2/TP(
physical half life)
lB
= biological decay constant = ln 2/TB(
biological half life)
1/TE = 1/TP
+ 1/TB This means that when Technetium 99m is used as a radioactive
tracer and is studied with the gamma camera the operator has to take into
account not only its 6 hour half life but also the rate at which it will
be flushed biologically out of the body. This will depend upon the chemical
compound to which it is attached and the way an individual’s metabolism
copes with that compound. Use of Technetium 99m A
Technetium-99m - antimony sulphide colloid can be used as a
radioactive marker to examine the function of the lymph nodes. It is injected
into the drainage areas to be visualised. Cancerous cells divide more
frequently than non-cancerous ones and therefore a 'hot-spot' of high
activity results from cancerous growth.
A 'butterfly' can be used
to ensure that an injection of radioactive material is inserted into the
correct site.
The radionuclide in the
syringe is shielded to protect the person giving the injection.
The gamma ray is electromagnetic radiation of very high penetration
power. Therefore more rays exit the body and are available for detection
than interact with the patient's tissue. These can be detected by a gamma
camera and the concentration of radioactive tracer in various parts of the
body can be ascertained.
Gamma rays cannot be focused by refraction
therefore a lead collimator is used to direct rays from a point on the
patient towards a single point on a sodium iodide crystal. This absorbs
g-rays
emanating from other parts of the body before they activate the crystal.
A crystal of sodium iodide fluoresces
when a gamma ray interacts with one of its orbital electrons, promoting
it to a higher energy level. On its return to ground state a photon is
emitted. If this energy is in the visible region a flash of light is seen.
This light is detected using a photomultiplier
(see diagram) which is a device in which incident photons create
measurable electrical pulses. The device is based on the photoelectric
effect (see diagram). It uses large electric fields to accelerate electrons
and, through a cascade sequence, amplify the signal.
Gamma Camera employing parallel
hole collimation
- uses more radiation than pin-hole type
without losing resolution
- is ten times more sensitive when scanning
than a pin-hole type
The gamma camera is made to scan the area
of interest approximately four hours after the patient has been injected
with the radioactive tracer. This gives it time to circulate and accumulate
in 'hot-spots' of rapid cell division. The second scan taken within the
24-hour period can then be compared to the first to diagnose suspicion
of overactive lymph node activity.
The camera display can be made on a monitor
and videoed. Hard copies of the scans can then be taken and compared.
Gamma camera scan
of kidneys
The electrical output can be translated into
colour coded graphical display using electronic circuitry.
Heart Gamma Camera Scan - note use
of colour to give instant visual recognition of variations in concentration
Scanning is programmed electronically
so that the area of interest is looked at. The chemical is chosen because
it takes part in a normal biological function of the lymphatic system
any slight abnormalities in function can be identified by comparison of
the two scans. Current experimentation with a tiny probe containing a
miniaturised gamma camera, hopes to combine keyhole surgery with nuclear
medicine and hence reduce the risk due to exposure dose of the patient,
making more frequent scans a possibility.