What Is A Pet Scan?

Importance of medical imaging

Medical imaging techniques have been a crucial diagnostic tool for confirming diseases, outlining the treatment approach and monitoring the progress and effectiveness of the treatment, thereby allowing for better patient care. For example, ultrasound imaging in pregnant women helps doctors monitor the baby’s development or ascertain any abnormalities early on. Medical resonance imaging (MRI) and computed tomography (CT) scans help determine the effectiveness of the treatment and make changes to the treatment protocol if required. Medical imaging is useful in preventive care; mammograms are used to detect early signs of breast cancer, allowing early treatment initiation to reduce breast cancer-related fatalities.

Positron emission tomography (PET)

Unlike X-rays, MRI and CT scans, which show the structural changes in organs, PET is an imaging technique that utilises radioactive tracers to identify metabolic changes in the tissues, indicating damage or dysfunction. Changes in cellular function can be visible before changes in the anatomy of the organ; thus, it is beneficial in the early diagnosis of diseases. This article focuses on the principle of PET scan, its applications, advantages and limitations.

Basic principle of operation

A radioactive tracer is intravenously injected into the bloodstream. The tracer compounds are naturally found in our body, such as glucose, ammonia, or water, and are tagged with a radioactive material. For example, fluorine-18 (18F) fluoro-deoxyglucose (FDG), known as [18F] FDG PET, is a glucose analogue that is taken up by the cells instead of regular glucose for cellular activities. The positron emissions produced by the radioactive tracer are detected by the camera. The computer attached to the camera can reconstruct a two-dimensional or three-dimensional image of the organ being examined. A PET scan usually takes about 30 min.

The areas where the radioactive tracer uptake is more suggest increased metabolic activity, for example, cancer cells, and appear brighter (known as ‘hot spots’) compared with those of the normal tissues. PET and CT are commonly performed together as information regarding the cellular function and anatomy can be simultaneously obtained providing a more accurate diagnosis.^1,2,3

Preparation for PET scan

Before the test

• Make a list of all the medications that you are currently taking and treatments that you have previously undergone, such as radiotherapy, chemotherapy or surgery.
• Mention any allergic reactions encountered with radiotracers and contrast dyes (for CT).
• Inform the staff if you have diabetes
• Inform them if you feel uncomfortable in closed spaces.
• Inform them if you are pregnant or breastfeeding.
• Avoid eating at least 8 hours before the test. You can have water and medications as usual. Avoid your diabetic medications before the test.

On the day of the test

• On the day of the exam, you must remove all metal objects on your body as it interferes with imaging.
• A blood sample is collected to measure glucose levels and the radiotracer is intravenously administered. You will be asked to wait for approximately 60 min for the radiotracer to distribute in the body, after which, you will be examined via the PET scanner.
• The PET scanner is a huge doughnut-shaped machine, with detectors located inside the machine. In the scanning room, you will be asked to lie on the table. You may be instructed to lie still or hold your breath. Once you are in the appropriate position, the technician will leave the room. They will be able to see you on the TV screen and you can communicate with the technician via intercom regarding any discomfort you experience. The table will move back and forth in the scanner to capture the images.

After the test

• Following the exam, you should drink enough water to flush out the radiotracer from your body. 
• If you are a nursing mother, you must wait for 24 hrs before resuming breastfeeding.^3,4

Applications

Oncology

A PET scan can differentiate between cancerous and benign tumours. It is also used to determine the stage of cancer. The term staging refers to determining the phase, progression or severity of the cancer. Once the cancer stage is determined, the doctor can plan the treatment approach. PET scans can also determine if the cancer has metastasised, which means whether the cancer has spread to other parts of the body.

If a patient is undergoing cancer treatment, a PET scan can determine if the patient is responding to the treatment, thereby determining its effectiveness. If the treatment is not working, the doctor can modify the treatment.

For a patient who has concluded the cancer treatment, a PET scan can be used to confirm the absence of cancer and to ensure no chance of recurrence. PET scans have been used to detect cancers of the lung, breast, oesophagus, thyroid; lymphoma; melanoma and colorectal cancer.^5,6

Neurology

Brain PET scans can assist doctors in the following ways:

• It can aid in the early diagnosis, progression and severity of neurodegenerative diseases, such as Alzheimer’s disease. When an amyloid PET scan is negative, the possibility of Alzheimer’s disease is excluded.
• Patients with mild cognitive impairment can be classified into the possibility of developing dementia using a combination of FDG, tau and amyloid PET scans. This aids in impeding disease progression and making appropriate decisions regarding treatment.
• PET scans can determine if memory loss is due to depression or vascular dementia, which is caused by ischaemic strokes that reduce the flow of blood to the brain. Dementia can occur when a stroke damages the part of the brain that controls the memory function.
• It can diagnose Parkinson’s disease and distinguish it from other movement disorders. It can map the areas of the brain involved in speech and movement. 18F-Fluorodopa (FDOPA) PET scans can differentiate between parkinsonian and non-parkinsonian syndromes as in the former condition the uptake of the radiotracer FDOPA will be reduced, whereas in the latter condition, the uptake will be preserved.
• It can help determine the site of onset of an epileptic seizure in the pre-surgical workup. In approximately one-third of the patients with epilepsy, the seizures cannot be controlled using anti-epileptic drugs. FDG PET will show reduced metabolism of glucose at the site of onset of the seizure and sometimes along the epilepsy network. The accurate location of the onset of seizure allows the surgeon to resect the part that will provide either freedom from seizures or improved control of seizures.
• It can determine if the brain tumour is cancerous or not. It aids in staging the tumour, creating an appropriate treatment plan and assessing the response to the treatment. It can also distinguish if the changes observed in the scan are due to a recurrent tumour or scar tissue.^7,8

Cardiology

A cardiac PET scan can diagnose coronary artery disease by accurately determining the flow of blood between the heart muscle and the coronary arteries. Myocardial viability is also determined using a cardiac PET scan to evaluate the extent of tissue damage caused by a heart attack.

A viability test is conducted in two parts. First, a radiotracer, such as rubidium-82 or ammonia-13, is intravenously injected to determine the resting blood flow in the heart. Second, FDG is injected. The images collected will show the areas of tissue damage with no uptake of glucose, whereas healthy cells or those recovering from tissue damage will show glucose uptake. Cardiac PET scans can help cardiologists decide whether the patient will benefit from an angioplasty, coronary artery bypass surgery or a heart transplant.^9,10,11

Detection of infection and inflammation

FDG PET is useful for the assessment of pathologic conditions, such as sarcoidosis, spinal osteomyelitis, vasculitis and fever of unknown origin.¹²

Advantages

• PET scans exhibit high sensitivity and specificity in the diagnosis of various medical conditions in the early stages, enabling rapid intervention and improving patient outcomes.
• It is capable of detecting changes in metabolic activity that can aid in early diagnosis before the structural changes appear using other imaging techniques.
• It provides insight into the stage and progress of the disease, thereby allowing a personalised treatment approach, which drastically improves treatment outcomes.
• As it can assess the response to treatment, doctors can assess the effectiveness of the treatment and make modifications accordingly.^13,14

Limitations

• The types of equipment used in PET, such as the cyclotron and cameras, are expensive, and thus, are not readily available in many hospitals.
• The administration of the radiotracer may pose a risk in the development of cancer in the future.
• If there is a chemical imbalance in the body, a PET scan can reveal false-positive results. This is true in the case of diabetic patients where test results may be affected due to altered blood sugar or insulin levels.
• As the radioactive tracer rapidly decays, it is important to be on time for your appointment and get the radiotracer administered at the scheduled time.
• People who are obese may not fit into the conventional PET scanner unit.
• The procedure is time-consuming as it takes time for the radiotracer to accumulate in the region of interest (hours to days) in addition to the time required for imaging.
• The resolution obtained from the organs of the body is lower compared with CT or MRI.¹⁵

Summary

PET is an advanced diagnostic tool that can provide precise information on changes in metabolic and chemical activities and blood flow. It has been immensely useful in the diagnosis of cancer, cardiovascular diseases and neurological diseases. It provides a functional evaluation of the body, allowing doctors to spot abnormalities early, provide individualised treatment plans and track the progress of the disease for positive patient outcomes. Integrating PET with other imaging modalities improves diagnostic accuracy and benefits patient care.