Positron Emission Tomography (PET) scans, combined with Computed Tomography (CT), known as Pet/ct scans, represent a significant advancement in medical imaging. This powerful diagnostic tool utilizes small amounts of radioactive materials, called radiotracers, along with specialized cameras and computer processing to evaluate the function of organs and tissues within the body. PET/CT excels at identifying changes at the cellular level, often detecting diseases in their earliest stages, sometimes before other imaging techniques are capable of revealing abnormalities.
Prior to undergoing a PET/CT scan, it’s crucial to inform your doctor if you suspect you might be pregnant or if you are currently breastfeeding. Your healthcare provider will give you specific preparation instructions tailored to the type of PET/CT exam you are scheduled for. It’s also important to discuss any recent illnesses, pre-existing medical conditions, all medications you are currently taking, and any allergies you have – particularly allergies to contrast materials. Typically, your doctor will advise you to abstain from eating and only drink water for several hours leading up to your scan. It’s recommended to leave jewelry at home and wear loose, comfortable clothing on the day of your appointment. You may be asked to change into a gown for the duration of the exam.
Understanding PET/CT Scanning
PET/CT scanning is a sophisticated form of nuclear medicine imaging. Nuclear medicine leverages radiotracers to diagnose, assess, and treat a wide spectrum of diseases. These encompass cancer, cardiovascular disease, gastrointestinal disorders, endocrine imbalances, neurological conditions, and various other medical issues. The unique strength of nuclear medicine exams lies in their ability to pinpoint molecular activity within the body. This capability allows for the detection of disease in its nascent stages and provides valuable insights into a patient’s response to ongoing treatments.
Nuclear medicine procedures are generally non-invasive and typically painless, with the exception of the minor discomfort associated with intravenous injections. These diagnostic tests employ radiopharmaceuticals or radiotracers to aid in the diagnosis and evaluation of medical conditions.
Radiotracers are essentially molecules that are linked to, or “labeled” with, a minute quantity of radioactive material. These radiotracers are designed to accumulate in areas of the body where there is tumor growth or inflammation. They can also be engineered to bind to specific proteins within the body. Fluorine-18 fluorodeoxyglucose (FDG) is the most frequently used radiotracer. FDG is a molecule that closely resembles glucose, the body’s primary source of energy. Cancer cells, characterized by their heightened metabolic activity, tend to absorb glucose at an accelerated rate. This increased glucose uptake is readily visualized on PET scans. This characteristic allows physicians to detect cancerous activity often before it becomes apparent on other types of imaging scans. FDG is just one example, with numerous other radiotracers currently in use or under development to target different biological processes.
The radiotracer is usually administered via injection, although depending on the specific exam, it may also be swallowed or inhaled as a gas. Once administered, the radiotracer travels through the body and accumulates in the area of interest. A specialized PET camera detects the gamma rays emitted by the radiotracer. The camera then works in conjunction with a computer to generate images and provide quantitative molecular information about the targeted tissues or organs.
Modern medical imaging often integrates nuclear medicine images with Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) to produce what are known as fused or co-registered images. This image fusion technique allows doctors to correlate and interpret information from two distinct imaging modalities within a single image. The result is more precise diagnostic information and a more accurate diagnosis. Integrated Single Photon Emission CT/CT (SPECT/CT) and Positron Emission Tomography/CT (PET/CT) systems can perform both types of scans concurrently. PET/MRI is an emerging technology that combines PET and MRI, though its availability is not yet widespread.
A PET scan excels at measuring vital bodily functions, particularly metabolic activity. This capability is invaluable in helping doctors assess the functional status of organs and tissues.
CT imaging utilizes specialized X-ray equipment, and sometimes contrast material, to capture multiple cross-sectional images of the body’s internal structures. A radiologist then reviews and interprets these images on a computer monitor. CT scans are particularly adept at providing detailed anatomical information.
In contemporary medical practice, combined PET/CT scanners are utilized for the vast majority of PET scans. The integration of PET and CT in a single scan offers the advantage of precisely localizing abnormal metabolic activity within the corresponding anatomical context provided by the CT scan. This synergy often leads to more accurate diagnoses compared to performing PET and CT scans separately.
Common Applications of PET/CT Procedures
Doctors utilize PET and PET/CT scans for a wide range of clinical applications, including:
- Cancer Detection and Diagnosis: PET/CT is highly effective in detecting cancerous tumors and aiding in the diagnostic process.
- Cancer Staging: Determining if and how far cancer has spread (metastasized) within the body is crucial for treatment planning. PET/CT plays a vital role in cancer staging.
- Treatment Monitoring: Assessing the effectiveness of cancer treatments, such as chemotherapy or radiation therapy, is essential. PET/CT scans can help determine if a treatment is working and if a tumor is responding.
- Cancer Recurrence Detection: After cancer treatment, PET/CT can be used to monitor for signs of cancer recurrence.
- Prognosis Evaluation: PET/CT findings can contribute to a more accurate prognosis, or prediction of the likely course of a disease.
- Tissue Metabolism and Viability Assessment: PET/CT can evaluate the metabolic activity and viability of tissues, which is important in various conditions beyond cancer.
- Heart Attack Impact Assessment: In patients who have experienced a heart attack (myocardial infarction), PET/CT can help determine the extent of damage to the heart muscle.
- Identification of Heart Muscle Viability for Revascularization: PET/CT, often in conjunction with myocardial perfusion scans, can identify areas of the heart muscle that would benefit from procedures like angioplasty or coronary artery bypass surgery to restore blood flow.
- Brain Abnormality Evaluation: PET/CT is used to investigate brain abnormalities such as tumors, memory disorders, seizures, and other central nervous system disorders.
- Brain and Heart Function Mapping: PET/CT can be used in research settings to map normal brain and heart function, enhancing our understanding of these vital organs.
Preparing for Your PET/CT Scan
For your comfort during the PET/CT scan, you may be provided with a gown to wear, or you may be allowed to remain in your own clothing, provided it is free of metal.
It is imperative for women to inform their physician and the radiology technologist if they are pregnant or breastfeeding. Nuclear medicine procedures involve small amounts of radiation, and precautions are necessary for pregnant and breastfeeding women.
Prior to the scan, inform your doctor and the technologist about all medications you are taking, including over-the-counter drugs, vitamins, and herbal supplements. It is also important to disclose any allergies you have, any recent illnesses, and any other medical conditions you may have.
You will receive specific preparation instructions depending on the type of PET/CT scan you are scheduled for. Patients with diabetes will receive tailored instructions to ensure the accuracy of the scan.
If you are breastfeeding, consult with your radiologist or doctor for guidance on how to proceed. It may be advisable to pump breast milk beforehand and store it for use until the radiotracer and CT contrast material (if used) are no longer present in your body.
To avoid interference with the CT images, it is essential to leave all metal objects at home. This includes jewelry, eyeglasses, dentures, and hairpins. You may also be asked to remove hearing aids and removable dental work before the scan.
Generally, you will be instructed not to eat anything for several hours before a whole-body PET/CT scan. Food intake can alter the distribution of the PET radiotracer in your body, potentially leading to a suboptimal scan. This could necessitate repeating the scan on another day, so adhering to fasting instructions is crucial. Avoid drinking sugary or caloric liquids for several hours before the scan, but drinking water is encouraged to stay hydrated. If you have diabetes, your doctor will provide you with specific dietary guidelines. Be sure to inform your doctor about all medications you are taking and any allergies, especially to contrast materials or iodine, as contrast is sometimes used in conjunction with the CT portion of the PET/CT scan.
Your doctor will also assess for any pre-existing conditions that might increase the risk associated with receiving intravenous contrast material, if contrast is planned for your scan.
PET/CT Equipment Overview
A PET scanner is a sizable machine characterized by a large, circular opening in the center, resembling a donut shape. It shares a similar appearance with CT and MRI units. Within the machine, multiple rings of detectors are positioned to record the energy emissions emanating from the radiotracer within your body.
The CT scanner component is also typically a large, donut-shaped apparatus with a short tunnel at its center. During the CT scan, you will lie on a narrow table that slides into and out of this tunnel. An X-ray tube and electronic X-ray detectors are positioned opposite each other in a ring, termed a gantry, and rotate around you to acquire the CT images. The computer workstation responsible for processing the imaging data is located in a separate control room. From this control room, a technologist operates the scanner and closely monitors you throughout the exam, maintaining direct visual and verbal contact via a speaker and microphone system.
Combined PET/CT scanners integrate the features of both PET and CT scanners into a single unit.
The images generated by the PET/CT scan are created by a computer system using the data acquired from the gamma camera detectors.
How PET/CT Procedure Works
Conventional X-ray exams generate images by transmitting X-rays through the body. In contrast, nuclear medicine, including PET/CT, employs radioactive materials known as radiopharmaceuticals or radiotracers. These radiotracers are typically injected into the bloodstream, although they can also be swallowed or inhaled. The radiotracer accumulates in the targeted area of the body, where it emits gamma rays. Specialized cameras detect this energy, and with the aid of sophisticated computer processing, generate detailed pictures that depict both the structure and function of organs and tissues.
PET scans specifically rely on radiotracer injections.
Unlike other imaging modalities, nuclear medicine and PET/CT focus on physiological processes occurring within the body, such as metabolic rates and levels of various biochemical activities. Areas exhibiting higher concentrations of radiotracer are referred to as “hot spots.” These hot spots may indicate regions of elevated chemical or metabolic activity. Conversely, areas with lower radiotracer concentrations, termed “cold spots,” suggest reduced activity.
Step-by-Step PET/CT Procedure
Nuclear medicine exams, including PET/CT scans, are conducted on both outpatients and hospitalized patients.
You will be positioned lying down on a comfortable exam table. If required, a nurse or technologist will insert an intravenous (IV) catheter into a vein, typically in your hand or arm. This IV line is used for radiotracer injection in PET scans.
Following the radiotracer injection, there is typically a waiting period of approximately 30 to 60 minutes. This allows sufficient time for the radiotracer to circulate throughout your body and be absorbed by the tissues or organs being examined. During this uptake period, you will be asked to rest quietly and minimize movement and talking to ensure optimal image quality.
You may be asked to drink a contrast agent that will localize in the intestines. This helps radiologists to better interpret the images and differentiate structures within the abdomen and pelvis.
Once the uptake phase is complete, you will be moved into the PET/CT scanner to commence the imaging process. Remaining still throughout the imaging is crucial to prevent blurring of the images. The CT scan portion is typically performed first, immediately followed by the PET scan. In some instances, a second CT scan, this time with intravenous contrast, may be performed after the PET scan to enhance anatomical detail. The CT scan itself is very rapid, usually taking less than two minutes. The PET scan acquisition takes longer, typically ranging from 20 to 30 minutes.
The total scanning time for a PET/CT procedure is generally around 30 minutes, although this can vary depending on the specific clinical indication and the area of the body being scanned.
In certain situations, depending on the area under investigation, additional tests involving different radiotracers or pharmacological agents may be employed. This can extend the overall procedure time to as long as three hours. For example, in cardiac PET studies, you may undergo scans both before and after exercise or before and after the administration of an IV medication that increases blood flow to the heart, to assess heart muscle perfusion and viability under stress.
After the initial scan acquisition is complete, you may be asked to wait briefly while the technologist reviews the images to ensure they are of adequate quality and diagnostic for interpretation. Occasionally, additional images may be acquired to clarify or better visualize specific areas or structures. The need for additional images is not necessarily indicative of a problem with the exam or the presence of an abnormality; it is a routine part of quality assurance in medical imaging.
If you have an IV line in place for the procedure, the technologist will typically remove it after the exam is finished, unless you are scheduled for another procedure that same day that requires continued IV access.
What to Expect During and After Your PET/CT Scan
With the exception of the minor discomfort associated with the intravenous injection, most nuclear medicine procedures, including PET/CT scans, are painless. Reports of significant discomfort or adverse side effects are uncommon.
You may experience a slight pinprick sensation when the technologist inserts the needle for the IV line. During the radiotracer injection, some patients report a cool sensation moving up their arm. Generally, there are no other immediate side effects related to the radiotracer administration.
PET scans specifically utilize radiotracer injections and do not typically involve other invasive procedures.
In some nuclear medicine procedures (though not typically PET/CT), a catheter may be placed into the bladder. This can cause temporary discomfort.
Maintaining stillness during the PET/CT scan is important for image quality, and while the imaging itself is painless, remaining in one position for an extended period can cause some individuals to experience mild discomfort.
If you have a fear of confined spaces (claustrophobia), you may feel anxious during the scan as you are positioned within the PET/CT scanner. If you are known to be claustrophobic, inform your doctor beforehand, as strategies can be employed to minimize anxiety.
Unless your doctor provides you with specific instructions to the contrary, you can typically resume your normal activities immediately after your PET/CT scan. A technologist, nurse, or doctor will provide you with any necessary specific instructions before you are discharged from the imaging facility.
The small amount of radiotracer administered will naturally lose its radioactivity over time through radioactive decay. It will also be eliminated from your body through urine and stool in the hours and days following the exam. Drinking plenty of fluids, especially water, after the scan will help to expedite the elimination of the radiotracer from your system.
Image Interpretation and Results
A radiologist, or another physician with specialized training in nuclear medicine, will meticulously interpret the PET/CT images and generate a comprehensive report. This report will then be sent to your referring physician, who ordered the scan.
If a diagnostic CT scan was part of your PET/CT exam, a radiologist with specialized expertise in interpreting CT exams will also contribute to the report, focusing on the CT findings.
Your referring physician will then discuss the results of your PET/CT scan with you, explain the findings, and outline any necessary next steps in your care.
Benefits and Risks of PET/CT
Benefits of PET/CT
- Unique Functional and Anatomical Information: PET/CT provides unique information about both the function (metabolic activity) and anatomy of body structures, often unattainable through other imaging procedures alone.
- Superior Diagnostic and Treatment Information: For many diseases, PET/CT provides the most valuable diagnostic and treatment-related information available.
- Less Invasive than Exploratory Surgery: A PET/CT scan is significantly less invasive than exploratory surgery and can often provide more precise diagnostic information, potentially avoiding the need for surgical procedures for diagnostic purposes.
- Early Disease Detection: By detecting changes at the cellular level, PET/CT imaging has the remarkable ability to detect the early onset of disease, often before structural changes become apparent on other imaging tests like CT or MRI.
- Enhanced Accuracy and Detail: Combined PET/CT scans offer greater detail and a higher level of diagnostic accuracy because both scans are performed sequentially in the same session, without requiring the patient to change position between scans. This minimizes the potential for misregistration and improves anatomical correlation.
- Patient Convenience: Undergoing both CT and PET scans in a single appointment is more convenient for patients compared to scheduling and undergoing two separate exams at different times.
Risks of PET/CT
- Low Radiation Exposure: Nuclear medicine exams, including PET/CT, utilize only small doses of radiotracers, resulting in relatively low radiation exposure. For diagnostic exams, the potential benefits of the information gained from the scan significantly outweigh the very low radiation risk.
- Long History of Safe Use: Nuclear medicine diagnostic procedures have been safely used in clinical practice for over six decades, and there are no known long-term adverse effects associated with such low-dose radiation exposure.
- Benefit-Risk Assessment: Your doctor will always carefully weigh the potential benefits of a PET/CT scan against any potential risks before recommending the procedure. Significant risks are rare, and your doctor will discuss any relevant risks with you prior to the scan and provide an opportunity for you to ask questions.
- Rare Allergic Reactions: Allergic reactions to radiotracers are exceedingly rare and are typically mild if they do occur. It is essential to inform the nuclear medicine personnel of any known allergies you have and to describe any prior experiences with nuclear medicine exams.
- Minor Injection Site Discomfort: The radiotracer injection may cause slight, transient pain and redness at the injection site, which typically resolves rapidly.
- Pregnancy and Breastfeeding Considerations: Women who are pregnant or breastfeeding must inform their doctor and radiology technologist. Radiation safety precautions are in place to minimize radiation exposure to the fetus or infant.
Limitations of PET/CT
- Procedure Time: Nuclear medicine procedures, including PET/CT, can be time-consuming. The radiotracer uptake period can take from several hours to days in some specialized exams, and the imaging acquisition itself can also take up to several hours in certain cases. However, advancements in PET/CT technology, particularly with newer equipment, are continually working to shorten procedure times.
- Image Resolution: The image resolution of nuclear medicine images, including PET images, may not be as high as that of CT or MRI in terms of fine anatomical detail. However, PET scans are exceptionally sensitive for detecting a wide range of disease processes at the molecular level. The functional information they provide is often unique and unobtainable using other imaging techniques that focus primarily on anatomical structure.
- Blood Sugar Level Effects: Altered blood sugar or blood insulin levels can adversely affect the test results, particularly in diabetic patients or individuals who have eaten shortly before the exam. Strict adherence to fasting instructions is crucial for optimal image quality and accurate interpretation, especially for FDG-PET scans.
- Radiotracer Decay and Timing: Radiotracers decay relatively quickly and are effective for only a limited time window. Therefore, it is essential to arrive on time for your scheduled PET/CT appointment and receive the radioactive material at the designated time. Late arrival may necessitate rescheduling the procedure to ensure optimal radiotracer activity and image quality.
- Patient Size Limitations: Individuals with significant obesity may not fit comfortably within the bore (opening) of some conventional PET/CT units. However, modern PET/CT scanners are designed with wider bores to accommodate a broader range of patient sizes.
This information provides a comprehensive overview of PET/CT scans. For more specific information related to your individual medical situation and planned PET/CT procedure, always consult with your doctor or healthcare provider.
