Medical tracers

​​In a nutshell

Medical tracers are radioactive isotopes bound to substances which are consumed and processed by the body. Gamma cameras are used to observe radiation from within a patient. PET scans use $$\beta^+$$ emission to detect and build images of organ and tissue function within the body.

Medical tracers

Medical tracers are radioactive isotopes which are bound to a substance which is then used by the body. The medical tracers are then ingested, injected or inhaled to enter the required part of the body. Medical tracers are used as a diagnostic technique to show the function and the structure of tissues and organs. 

The two requirements for a medical tracer is that they have short half-lives and they emit gamma radiation. They need to have a short half-life as patients need to not be radioactive for long amounts of time after the procedure, but long enough for the instruments to detect the radiation. They need to be gamma emitters as it is the least ionising and most penetrative, so can be detected outside the body.

The two most common medical tracers are fluorine$$-18$$​ and technetium$$-99m$$​. Both of these have relatively small half-lives. Fluorine$$-18$$​ has a half-life of $$110\, minutes$$ and technetium$$-99m$$​ has a half-life of $$6 \, hours$$. Both decay into more stable isotopes.

Example

When the radioactive substance is bound to a substance such as glucose, the body will send the glucose to the muscles or organs which are using the most amount of energy. Cancers divide uncontrollably and use up lots of the bodies energy, so they need lots of fuel. Radioactive glucose would collect in and around the cancers which would then highlight on image produced.

Another example is iodine. Radioactive iodine is taken in by the thyroid gland, if the thyroid gland wasn't working properly then the radioactive iodine wouldn't collect in the thyroid gland which could help with a diagnosis of a faulty thyroid. 

Tracers can also show blood flow in the brain. This is especially helpful as x-rays and CT scans can't image the brain as it is protected by a thick layer of bone, your skull! 

Gamma camera

Gamma emitted from a patients body is observed by a piece of equipment called a gamma camera. When enough time has lapsed for the medical tracer to reach the area of interest the patient is then observed using the gamma camera.

The gamma camera consists of several different parts:

1 To computer 2 Electronic circuit 3 Photomultiplier tubes 4 Lead collimator 5 Lead shield 6 Sodium iodide crystal 7 A gamma camera

The gamma camera is encased in a lead shield. This is to prevent any background radiation or any radiation not parallel to the collimating slits to pass into the sensitive equipment.

The lead collimators is to ensure that only gamma radiation parallel to the collimator can enter the gamma camera. They are a series of narrow lead lined columns which absorb any radiation which isn't perpendicular to the surface of the gamma camera.

After the lead collimator is a sodium iodine crystal which emit a flash of light when gamma radiation reaches it. This part of the gamma camera is called the scintillator.

Next is the photomultiplier tubes which detect the intensity and location of the flashes of light from the crystal. 

Finally is the electronic circuit which collect the signals from the photomultiplier tubes and convert them into electrical signals which are then sent to a computer to process and build an image. 

PET scans

Positive emission tomography scans are large devices which detect gamma radiation from a patient. 

1 PET scan 2 Monitor 3 PET machine 4 Rotating scanner 5 Motorised exam table 6 Brain scan

The patient is given a medical tracer containing a positron emitting isotope with a short half life, like nitrogen$$-12$$​, oxygen$$-15$$​ or fluorine$$-18$$​. 

The tracer undergoes $$\beta ^+$$ decay and releases a positron. The positron then gets annihilated by a nearby electron and emits two gamma photons travelling in opposite directions. 

Gamma cameras all around the body in the PET scan then detect the location and delay of the gamma photons and send the information to a computer which then builds up a composite image of the radioactivity in the body. 

Depending on the tracer and the substance it is bound to, the tracer will gather in different parts of the body and will help build a picture of how the body is processing the substance being used. 

Example

If the radioactive tracer is bound to glucose then the tracer will go to the place with the highest metabolic rate in the body. Cancers have a high metabolic rate due to their constant growth and division. 

A PET scan allows for a much more detailed and accurate image to be compiled by the computer which will help the doctors in their diagnosis of the patient.

However the disadvantage of a PET scan is that they are incredibly costly to build and maintain and the half-lives used are quite short which means that there is only a narrow window in which the patient can be imaged. As the tracer is radioactive the patient can't just be given another dose to be imaged again. 

The benefits and drawbacks of using ionising radiation

There is always a risk associated with using ionising radiation when dealing with patients. However there must always be a risk vs reward procedure to be followed. 

Example

If a patient has a suspected cancer or illness which could have a devastating consequence on their life expectancy if ionising radiation isn't used to diagnose their condition, then the risk of exposure to the ionising radiation is dwarfed by the potential reward of getting a diagnosis and then a cure. 

Ionising radiation is a non-invasive technique and will almost always carry significantly less risk than invasive techniques such as operations and other invasive techniques. 

Risks of ionising radiation

X-rays, $$\alpha$$, $$\beta$$ and $$\gamma$$ radiation are all ionising radiation. They interact with atoms or molecules to form ions which can then damage cells and living tissues. They can do this in several ways depending on the exposure and activity of the radiation:

The large scale affects of ionising radiation can include skin burns, tumours, hair loss, radiation sickness and ultimately death.  

This is why it is incredibly important to weigh up the benefits and drawbacks for each diagnosis on an individual and per patient basis.