By Dr Geeta Kadayaparth & Dr Abhishek Gupta
in the field of Oncology, nuclear imaging tests have proven to be a game changer for the treatment of cancer. With their precision and accuracy, PET scans have been advantageous for oncologists in deciding the due course of treatment.
Positron emission tomography (PET) scans can show up a cancer, reveal the stage of the cancer, show whether the cancer has spread, help doctors decide on the most appropriate cancer treatment, and give them an indication on the effectiveness of ongoing chemotherapy. A PET scan several weeks after starting radiation treatment for lung cancer can indicate whether the tumor will respond to the treatment.
The machine detects pairs of gamma rays that are emitted indirectly by a tracer (positron-emitting radionuclide), which is placed in the body on a biologically active molecule. The images are reconstructed by computer analysis. Modern machines often use a CT X-ray scan which is performed on a patient at the same time in the same machine.
A PET scan uses radiation, or nuclear medicine imaging, to produce 3-dimensional, color images of the functional processes within the human body. It can also be used to diagnose a health condition, as well as for finding out how an existing condition is developing and to see how effective an ongoing treatment is.
Patients are told to not consume any food for at least four to six hours prior to a scan, but to drink plenty of water instead. PET scans are not painful and once the scan is complete, patients can go home. Patients should drinks lots of liquid afterward to flush the radioactive drugs from system.
Before carrying out a PET scan, a radioactive medicine is produced in a cyclotron (a type of machine). The radioactive medicine is then tagged to a natural chemical. This natural chemical could be glucose, water, or ammonia. The tagged natural chemical is known as a radiotracer. The radiotracer is then inserted into the human body. When it is inside, the radiotracer will go to areas inside the body that use the natural chemical. For example, FDG (fluorodeoxyglucose – a radioactive drug) is tagged to glucose to make a radiotracer. The glucose goes into those parts of the body that use glucose for energy. Cancers use glucose differently from normal tissue – so, FDG can show up cancers. It detects the energy emitted by positively-charge particles (positrons). As the radiotracer is broken down inside the patient’s body, positrons are made. This energy appears as a 3-dimensional image on a computer monitor. The image reveals how parts of the patients body function by the way they break down the radiotracer. A PET image will display different levels of positrons according to brightness and color.
PET Vs X-rays and MRI
PET scans are used alongside X-rays or MRI (magnetic resonance imaging) scans. They are used to make a diagnosis or to get more data about a health condition. They are also useful in finding out how effective current treatment is. The biggest advantage of a PET scan, compared to an MRI scan or X-ray, is that it can reveal how a part of the patient’s body is functioning, rather than just how it looks. Medical researchers find this aspect of PET scans particularly useful. A CT or MRI scan can assess the size and shape of body organs and tissue, but cannot assess its function, which a PET scan does. In other words, MRI or CT scans tell you what an organ looks like, while a PET scan can tell you how it is working.
How does the PET work?
A small quantity of radiotracer is either injected into the patient’s arm or breathed in as a gas. The radiotracer may take anything from 30 minutes to 90 minutes to reach the targeted part of the body during which patient is asked to stay still and not talk. When a patient is ready they will be taken to the room where the PET scan is and will lie down on a cushioned examination table and images of the body are taken.
While the scan is in process, it is vital that the patient keep as still as possible. Depending on which part of the body is being scanned, the whole process takes from about 30 to 60 minutes. A buzzer is given to the patients to alert the staff if felt unwell.
Most patients can go home as soon as the scan has been done. Doctors advise people to consume lots of liquids to flush the radioactive drugs out of their system more quickly. Experts say the radiotracers should have left the body completely within three to four hours after entering the body.
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