A PET scan, which stands for Positron Emission Tomography, is a type of nuclear medicine imaging test. It uses a radioactive substance called a tracer to look at organs and tissues in the body. Before getting a PET scan, patients are often told to sit in a dark room for up to an hour. There are a few important reasons for this requirement:
To allow the tracer to distribute in the body
The tracer used in PET scans gives off radiation that can be detected by the PET scanner. For the scan to be useful, the tracer needs time to travel through the bloodstream and collect in the organs or tissues being examined. Sitting in a dark room allows the tracer to distribute evenly. Rushing immediately to the scanner won’t allow this needed time for circulation. Tracers often take 30-60 minutes to reach an ideal distribution.
To avoid exposure to external light and radiation
The tracer gives off a type of light called gamma rays. Exposure to external light and radiation could interfere with detection of these gamma rays during the scan. Sitting in a dark, isolated room reduces potential interference. It allows the camera to capture a clear picture of the tracer distribution. Ambient light and radiation in the environment can muddle the scan images.
To promote relaxation and comfort
PET scans require the patient to lie still for up to 2 hours in the scanning machine. Sitting quietly in a darkened room helps patients relax and get comfortable before this long scan. Being relaxed assists patients in lying perfectly still, which leads to clearer scan images. The darkness promotes a comfortable, restful environment.
How Long Do You Have to Sit in the Dark Room?
The time patients sit in the dark before a PET scan can vary. The most common times are:
- 30-60 minutes
- 45-90 minutes
- 1 hour
The radiology department will provide specific instructions on how long to sit quietly avoiding light. 1 hour in a dark room is typical. However, the timing depends on:
The tracer used
Different PET scan tracers require different circulation times. For example, FDG, the most common tracer, requires 30-60 minutes before scanning. Other tracers may need longer or shorter dark adaptation periods. The properties of the tracer determine the required distribution time.
The organ or tissue being imaged
Certain organs need longer dark adaptation times due to their anatomy and physiology. For example, cardiac PET scans often require 60-90 minutes of tracer circulation compared to 30-60 minutes for most other scan types. This allows the heart time to absorb sufficient tracer amounts.
Patient’s health status
Some medical conditions can slow down the tracer circulation time. Patients with poor circulation may need the full 60-90 minutes for the tracer to adequately distribute before the PET scan. Health providers evaluate patients individually to determine appropriate dark room duration.
Can You Do Anything During the Waiting Period?
The dark room period before a PET scan requires minimal activity and exposure to external stimuli. However, patients do not have to sit completely still and silent. The following activities are okay during the waiting time:
Listening to music
Listening to music with headphones is permitted as long as the music device does not give off light. Music can help relax the patient and make the hour pass by quicker.
Talking quietly
Patients may bring a friend or relative to sit with them in the dark room. Quiet conversation passes the time. However, boisterous talking and loud voices are not allowed.
Using the restroom
Brief bathroom breaks are permitted during the waiting period. However, frequent lengthy trips can disrupt the tracer circulation. Patients should try to use the restroom before starting the waiting period.
Drinking water
Patients may drink small sips of water to take medications or stay hydrated. But excessive eating and drinking should be avoided.Remaining still and relaxing is the goal.
Breathing exercises
Deep breathing, meditation, and other quiet relaxation techniques are fine in the dark room. They can help reduce patient anxiety prior to the scan.
Reading
Reading print material is generally not allowed because it exposes the eyes to light. However, some facilities permit reading on light-free e-readers.
Activities that should be avoided include: bright lights, cell phone use, excessive talking, large meals, and moving around. The key is staying relaxed and limiting light exposure.
Purpose of the Tracer
The radioactive tracer injected prior to the scan may also be called a radiopharmaceutical. This tracer gives off subatomic particles called positrons as it decays. The PET scanner detects these particles. When positrons collide with electrons in the body, gamma rays are generated. The PET scanner constructs 3D images by mapping the gamma ray signals.
Different tracers are used to image different aspects of body function. Some common uses include:
FDG tracer
FDG stands for 18F-fluorodeoxyglucose. This tracer contains radioactive fluorine. It is similar to glucose, so it accumulates in areas where glucose metabolism is high, like the brain and heart. FDG PET scans are often used to:
- Evaluate brain disorders
- Assess heart health
- Detect and stage cancers
- Monitor cancer treatment response
Amyloid tracer
Amyloid tracers bind to amyloid plaques in the brain. This allows PET detection of Alzheimer’s disease. Examples include florbetapir F18 and flutemetamol F18. Amyloid PET helps differentiate Alzheimer’s from other types of dementia.
Dopamine transporter tracers
Examples are FDOPA F18, raclopride C11, and fallypride C11. These tracers evaluate the dopamine system in the brain. They help diagnose Parkinson’s disease and movement disorders. They may also be used in psychiatric disorders like schizophrenia.
Glucose metabolism tracers
In addition to FDG, other radioactive glucose analogs like fluorothymidine F18 are used to measure metabolic activity in the body. This can help identify tumors and infections.
Tau protein tracers
Tau PET tracers bind to tau proteins in the brain. Build up of tau proteins is linked to brain diseases like Alzheimer’s. Examples include flortaucipir F18 and THK tracers.
How PET Scans Work
PET scans produce 3D images of functional processes in the body. Here is an overview of how they work:
- The radioactive tracer is injected into the patient intravenously.
- Over 30-90 minutes, the tracer circulates and accumulates in the target organ/tissue.
- The patient lies still on a table that slides into the PET scanner.
- As the tracer decays, positrons are emitted. The positrons collide with electrons producing gamma rays.
- The PET scanner contains rings of detectors that pick up the gamma rays.
- A computer uses the detector information to reconstruct 3D images. Higher gamma ray concentrations show as brighter spots on the images.
- A radiologist interprets the PET images to evaluate organ/tissue function.
PET scans are highly sensitive and can detect tiny metabolic changes long before anatomical changes are visible on CT or MRI scans. The amount of radiation exposure from a PET scan is similar to 8-10 x-rays.
PET vs CT vs MRI
While PET scans look at body function, CT (computed tomography) and MRI (magnetic resonance imaging) scans provide anatomical images.
| Scan Type | Details | Uses |
|---|---|---|
| PET scan | Uses radioactive tracers to image functional processes in the body like metabolism, blood flow, and neuron activity. | Cancer, heart disease, brain disorders |
| CT scan | Combines multiple x-ray images to create cross-sectional anatomical images. | Cancers, injuries, stones, organ anatomy |
| MRI scan | Uses magnetic fields and radio waves to construct anatomical images. | Brain disorders, muscle/joint injuries, cancers |
PET scans are often combined with CT or MRI to overlay anatomy and function. For example, a PET/CT scan fuses functional information from PET with anatomical structures seen on CT.
Conclusion
Sitting in a dark, quiet room for up to an hour before a PET scan allows the radioactive tracer time to distribute evenly throughout the body. This leads to the best quality images. Avoiding light also reduces interference with detection of the tracer by the PET scanner. While waiting, patients can listen to music, talk quietly, or do relaxation exercises as long as they avoid exposure to bright light. After this tracer circulation period, the PET scanner can effectively map the tracer concentration and construct diagnostic images of organ and tissue function.