Introduction:
Positron emission tomography is a type of nuclear medicine imaging that uses a special dye called radioactive tracers or radiopharmaceuticals to detect normal and abnormal metabolic activity in your body.
What Are the Uses of Positron Emission Tomography?
Positron emission tomography is commonly used to detect:
1. Cancer:
The metabolic rate of cancer cells is higher than normal cells. As cancer cells have high chemical activity levels, it appears as a bright spot on PET scans. PET scan can be used for:
-
Detecting cancers such as brain cancer, cervical cancer, lung cancer, thyroid cancer, esophageal cancer, pancreatic cancer, and head and neck cancers.
-
To check whether cancer has spread.
-
To check whether cancer treatment like chemotherapy is working.
-
Checking for cancer recurrence.
2. Heart Diseases:
PET scan is used to detect the areas of decreased blood flow in the heart. Healthy heart tissues can take more radioactive tracer than unhealthy heart tissue with decreased blood flow. It will help the doctors to choose your treatment options, such as angioplasty or coronary artery bypass surgery. It can also predict a heart attack or stroke by detecting the hardening of arteries (atherosclerosis).
3. Brain Disorders:
During PET scans, radioactive tracers are attached to glucose compounds in the brain. Detecting the radioactive tracers helps in the interpretation of the areas of the brain which use a higher rate of glucose. PET scan is useful in finding the following central nervous system disorders:
-
Epilepsy (a nerve disorder that affects the brain causing uncontrolled movements in a patient).
-
Parkinson’s disease (a central nervous system disorder affecting the movements of the patient).
-
Alzheimer’s disease (a brain disease affecting the memory, thinking skills, and functional skills of the patients).
-
Depression (a feeling of sadness or loss of interest).
-
Trauma in the head.
4. Infection:
PET scan can be useful for detecting bacterial infections, specifically enterobacterial types related to endocarditis, septic arthritis, osteomyelitis, and infections of the central nervous system.
How Does a PET Scan Work?
In a positron emission tomography (PET) scan, your doctor may inject a small amount of radioactive tracer or sugar called fluorodeoxyglucose-18 (FGD-18) into the vein in your arm, or you swallow or inhale the tracers as gas. The radioactive tracers enter your blood and are absorbed by the cells in your body. Cells that need more energy may absorb more sugar. Cancer cells absorb more sugar than healthy cells. The patient will be asked to wait for one hour for the radioactive tracers to be absorbed by the tissues. You are asked to rest quietly and not allowed to move, talk or do any other activities to minimize the brain stimulation before the scan.
What are the instructions given before your PET scan?
Your doctor may give a few instructions prior to your positron emission tomography (PET) scan. Inform your doctor about the medications you are taking.
-
48 Hours Before the Scan:
You have to avoid vigorous exercises and deep tissue massages 48 hours before the scan.
-
24 Hours Before the Scan:
You will be instructed to take a low sugar, low carbohydrate diet.
There are certain foods you should avoid:
- Cereal.
- Pasta.
- Starchy vegetables.
- Bread.
- Fruit and fruit juices.
- Alcohol.
- Caffeinated drinks.
- Candies, including chewing gums.
- Milk and milk products.
- Rice.
- Six Hours Before the Scan:
- You should not eat anything.
- You are allowed to drink water.
- You can take your regular medication.
- Foods which you are allowed to take are meat, non-starchy vegetables like carrot, broccoli, nuts, eggs, greens, and diet soda.
How Does a Pet Scan Perform?
The technician will explain the procedure and give the instructions you have to follow during the scan. You need to remove any clothing or jewelry, or other objects that interfere with the scan and empty your bladder before the scan. You are asked to lie down on a sliding table that is moved into the large tunnel-shaped scanner. During the scan, you should not move as it may blur the image. The scanner does not produce any radiation; it just detects and records the distribution of the radioactive tracer in your body. This procedure may take about 30 to 45 minutes. If multiple scans are required, it may take about three hours. You may be asked to hold your breath for a few seconds. You may hear clicking sounds and buzzing noise during the scan. Do not panic with the sounds. The images are sent to a radiologist specializing in PET scans for review and interpretation of the scan.
What Are the Factors to Be Considered While Taking a PET Scan?
-
Pregnancy - This test is not safe for pregnant women as babies in the womb are more sensitive to radioactive tracers.
-
Breastfeeding - Pump the breast milk before the test as you are not advised to pump 24 hours after the scan.
-
Diabetic Patients - Fasting may affect your blood sugar level, so take your regular dose of medicine and have a light meal four hours before the procedure.
-
Allergies - Allergic to the radioactive dye.
-
Claustrophobia - Afraid of closed spaces. If you are claustrophobic, you will be given medicine to make you feel sleepy and reduce your anxiety.
What are the instructions given before your PET scan?
After your PET scan, the radioactive materials injected into your body may remain for about a few hours to days.
-
Limit your contact with pregnant women and infants.
-
Drink plenty of water which helps to flush out the tracers from your body.
What Are the Limitations of PET Scans?
-
PET scans may be less accurate in certain conditions, such as slow-growing, less active tumor cells, as they may not absorb much tracer. Small tumors less than seven millimeters may not be detectable.
-
Diabetic patients undergoing PET scans make sure that their blood sugar levels are less than 200 g/dl before the scan. If the sugar level is high, radiotracer will not be effectively absorbed by the cells. If your insulin level is high, it will absorb more radiotracer, which affects the result.
-
Obese (overweight) people cannot fit into the scanning chamber. It might affect the image quality.
What Are the Benefits of PET Combination Scanning?
When a PET scan is combined with computed tomography (CT) scan or magnetic resonance imaging (MRI), we can get an accurate diagnosis. Modern PET scanners are now available with integrated CT scanners (PET-CT) or MRI (PET-MRI) scanners. It is convenient for the patient to avoid two scans taken at two different times.
What Are the Interpretations of Positron Emission Tomography?
Final images showing “hot spots” are the places where there are an increased number of radioactive tracers absorbed by the cells. Areas of high cellular metabolism may be suggestive of cancer.
Images showing “cold spots” are the places where there are fewer radioactive traces. It may indicate less metabolic active areas, often due to reduced blood flow or tissue death.
Conclusion:
Positron emission tomography (PET) scan is a diagnostic tool that can detect diseases before it shows up in other imaging techniques such as computerized tomography and magnetic resonance imaging. Combining a PET scan with CT or MRI helps in precise diagnosis. Detecting the changes in the body at the cellular level identifies the early stage of the disease and can save many lives. PET has now become an important tool in preclinical studies.
