Wednesday April 27, 2011
THE DOCTOR SAYS
By Dr MILTON LUM
No, a PET scan doesn’t scan your pets. It’s a variation of a CT scan that produces three dimensional colour images.
THERE have been exponential advances in medicine in the last five decades, both in diagnostics and therapeutics. This has been made possible through technological discoveries or improvements in other fields of human activity. The CT or CAT scan is one such example.
The CT scan is a large ring-shaped medical device which produces a series of x-ray beams as it rotates around the body in small movements. Images are built up of the various parts of the body scanned during this process.
These images, which are called tomograms, are of greater detail than that of ordinary x-rays, which uses a single x-ray beam only. The CT scan can produce images of various body parts like the organs in the chest and abdomen, blood vessels, and bones.
A positron emission tomography (PET) scan is a variation of a CT scan that produces three dimensional colour images. It detects the presence of a radioactive substance, called a tracer, in the body, and produces images that show where the tracer is concentrated.
A PET scan is used in the diagnosis and evaluation of the development of a condition, as well as monitoring of the progress of treatment.
A radioactive substance is produced in a device called a cyclotron. It is attached to a natural compound like glucose, water, or ammonia to produce a compound called a radiotracer, a small amount of which is then injected into a vein, or inhaled during breathing.
The radiotracer travels to the body parts that utilise the natural compound, eg radioactive fluorodeoxyglucose (FDG) is attached to glucose to produce a radiotracer which travels to body parts that utilise glucose. As cancer cells utilise glucose in a different manner than normal cells, FDG is used to demonstrate cancer tissue.
As the body breaks down the radiotracer, energy is produced by the positively charged particles, called positrons. The latter is detected and transmitted to a computer, where it produces three dimensional images. The rate of breakdown of the radiotracer in different organs and tissues vary, thereby producing images of different colours and brightness.
There is a standard operating procedure whenever a PET scan is ordered, with instructions given to the patient by the doctor prior to having the scan.
It is usual practice to advise a patient not to eat anything for four to six hours prior to a PET scan, and to drink plenty of water before that.
The patient will be asked about his or her medical condition, consumption of medicines, history of allergies, and the possibility of pregnancy in women in the reproductive age group. In general, PET scans are not done in pregnant women unless there are medical or surgical conditions that require evaluation and/or treatment. This is because of the possibility that x-rays may harm the developing foetus.
Children are more susceptible to radiation risks than adults. As such, PET scans are only done if the child has a serious condition, which is of greater risk than the PET scan itself.
Any jewellery or metal on the body has to be removed because they interfere with the scanning process. Hair clips, dentures, contact lenses, and hearing aids will also have to be removed, particularly if the head is to be scanned.
The patient will be asked to change into a hospital gown.
Some people have a fear of being closed in without means of escape (claustrophobia, which is derived from the Latin word claustrum, which means “shut in place”, and the Greek word phobos, which means “fear”). Having a PET scan can be claustrophobic. If one has this problem or an anxiety about the scan, one should inform the staff when making the appointment.
This is usually addressed by advice and information or the prescription of a sedative to be taken prior to the scan. It is advisable to arrange for a relative or friend to drive if one is going to take a sedative.
A small amount of radiotracer will be injected into a vein in the arm, or inhaled during breathing. It takes about 30 to 90 minutes for the radiotracer to travel round the body. One needs to be quiescent and refrain from talking or moving about.
At the appropriate time, the patient will be taken to the room where the PET scanner is located and asked to lie down on an examination table, which automatically moves in and out of the hollow part of the scanner. Images will only be taken of the body part that is inside the ring.
After the position of the examination table is adjusted to ensure that the exact part of the body is in the ring of the scanner, the radiographer or radiologist will leave the examination room.
He or she will carry out the scanning from a control room, which has a window with a full view of the patient. The reason is to reduce the radiation exposure to staff carrying out x-rays daily. Communication with the staff in the control room during the scan will be through an intercom.
The patient will be requested to be still and breathe normally to prevent any blurring of the images taken. One may be asked to breathe in, breathe out, or to hold the breath at certain stage(s) of the procedure.
The x-ray in the ring of the CT scan rotates around the patient, taking images as it does so. With the completion of each rotation, the bed on which the patient is lying is moved forward a little bit, and the cycle continues until the examination of the body part required is completed
The PET scan is painless. However, if one feels unwell or needs assistance, one can press a buzzer or use the intercom to contact the staff.
As the radiation exposure is small, there are no side effects. The drinking of large amount of fluids will help in getting rid of the radiotracer from the body. The radiotracer would usually be totally excreted from the body in a few hours.
The many x-ray images taken will be stored in a computer system. The radiologist will study and analyse the images taken on a monitor screen from various angles and then write a report for the doctor who ordered the scan.
With the completion of the scan, one can leave the hospital, unless one is hospitalised. It is advisable to ask the staff when to expect the results and to confirm one’s appointment with the doctor who ordered the scan.
A PET scan is a complementary investigation, ie it is used in conjunction with other imaging modalities like x-rays, CT scan, or magnetic resonance imaging (MRI). Its advantage over other imaging investigations is that it provides information about the functioning of a body part at the cellular level instead of providing information about what it looks like.
CT and MRI scans detect changes later than PET scans, after the disease has caused changes in the structure of organs or tissues.
PET scans are used in the diagnosis and evaluation of various medical conditions as well as in the monitoring of treatment.
The common uses of PET scans are:
> Cancer – showing up a cancer, determining its stage, showing if it has spread to other body parts, decision making on the best treatment, and showing how well the treatment is working.
> Heart disease – identification of diseased parts and assessment of function.
> Epilepsy – detection of the affected part of the brain and providing information about the suitability of certain medications.
> Alzheimer’s disease – diagnosis.
> Brain tumours – diagnosis.
> Lungs – diagnosis.
> Breasts – diagnosis, biopsy.
PETS scans are also used in research, eg brain function and ageing.
The exposure to radiation during a PET scan is generally at levels that are safe and insufficient to cause harm to a patient. The amount of radiation is about the same as in most CT scans. Moreover, the radiation does not last for very long in the body.
Pregnant or breastfeeding women are advised to inform their doctor before having a PET scan. This is because foetuses and infants are more susceptible to the effects of radiation because their organs are still growing.
An allergic reaction to the radiotracer may occur, although this is very rare, in which case there is pain, redness, or swelling at the site of the injection.
As with all things in medicine, the benefits and the risks of any intervention have to be considered.
If a PET scan is used in the diagnosis of a medical or surgical condition, or to monitor treatment, the benefits will have to outweigh the potential risks.
■ Dr Milton Lum is a member of the board of Medical Defence Malaysia. This article is not intended to replace, dictate or define evaluation by a qualified doctor. The views expressed do not represent that of any organisation the writer is associated with.