PET/CT

What is PET?
Positron emission tomography (PET) is a name given to the technique of imaging the spatial and temporal distribution in the body of radioactive atoms which emit positive electron (positrons) in their nuclear decay.

PET is a non-invasive, diagnostic imaging technique used for measuring the metabolic activity of cells in the human body. All cells need energy, and this energy is obtained from sugar. Abnormal cells often require more energy therefore consume more sugar than surrounding healthy tissue.

PET is a revolutionary technology that provides diagnostic information no other imaging test can provide. PET works by imaging the biological functions of the human body to reveal the presence and state of disease.

To create images, compounds like simple sugar (glucose for ex.) are labeled with low level radioactive tracers and are injected into the patient. A scanning machine records the radioactive signals, these tracers emit as they journey through the human body and collect in the various organs and tissues targeted for examination.

A computer reassembles the emissions into images, resulting in whole body, 3D pictures. These images show normal functioning organs and tissues, as well as portions of the body which are acute cancer sites.

PET is unique because it produces images of the body’s basic biochemistry or function. Traditional diagnostic techniques, such as x-rays, CT scans or MRI, produce images of the body’s anatomy or structures. The premise of these techniques is that the change in structure or anatomy caused by disease can be seen when compared to previous or normal images.

Biochemical processes are also altered with disease and may occur before there is a change in anatomy. PET can determine how an organ is functioning, rather than just seeing how it looks, as it provides additional information and enhances the way a physician traditionally diagnose disease. Scientific study has found that PET imaging is quite effective at diagnosing, staging, restaging and monitoring certain cancers.

How Does PET Work?

1. Biological principles:
   PET evaluates basic biological processes-perfusion, metabolism, and biosynthesis, through taking advantage of cellular needs for glucose. By studying the accumulation patterns of glucose, PET enables a radiologist of assessing cell viability, abnormalities in metabolism and nutrient use, tissues and organs that show elevated accumulations of glucose demonstrate inappropriately high rates of cell metabolism. Therefor unlike other medical imaging techniques, PET is a dynamic rather than a static assessment of tissue health and integrity.
2. Biochemical principles:
   Glucose is the starting molecule for glycolysis, a metabolic step common to almost every life form on earth. The body receives FDG (Fluorodeoxyglucose), a PET isotope tagged with glucose as if it were ordinary glucose molecules.
   However, once the FDG enters the cell, glycolysis, “the breaking of glucose” cannot proceed and FDG accumulates in the cell.
   In other words, FDG is not metabolized after it enters the cells.
   Cells with abnormally high metabolic rates take up large amounts of FDG, however Malignant tumors which typically demonstrate high rates of cell division, require higher than normal amounts of glucose to meet their metabolic needs. This makes it possible to both identify and stage malignancy in situ without having to resort to invasive biopsy or surgical methods.
3. Physical principles:
   When FDG decays, it emits a positron - hence the positron emission - this positron almost immediately comes in contact with an electron later on, the combined particles undergo an annihilation reaction turning the mass of the two particles into two 511-kev gamma photons, the photons which are emitted at 180° to each other are easily detected externally as the location to be detected is the site of the 180° emission of two gamma rays, this site is also called a coincidence line. A coincidence line provides detection for forming tomographic images with PET.

What Advantages Does PET Offer to Physicians?
PET enables the physicians to better determine the extent and spread of the disease. When provided with better information about early presence or spread of cancer, physicians can select the treatment protocol most effective and appropriate for their patient.
What’s startling about PET is that the scanner can detect tissue abnormalities even in the absence of structural changes.

What are the Clinical Applications of PET?
The most common applications of PET are in the field of oncology (cancer), considering PET provides vital diagnostic information that can alter the course of cancer treatment and sometimes helps in avoiding unwarranted surgery.

PET provides the following critical information:

• Whether or not the tumor is malignant.
• The extent of cancer.
• Whether or not the cancer has spread to other organs.
• Monitoring cancer recurrence.
• Monitoring the effectiveness of chemotherapy and radiotherapy.
• Beyond Oncology, most applications are in the field of infection/inflammatory conditions, neurology and cardiology.

Is PET Safe?
Safety issues have been well documented with PET imaging and biological damage is minimal. Because positrons collide with electrons, the patient is exposed to a minute amount of radiation, very similar to that of a single x-ray procedure. Also the radiopharmaceutical used in the scan, decays completely a short time after the injection.

How Can I Prepare For a Scan?
It is important to arrive at our PET center on time.
For PET scan, we have to order the FDG, as this custom made radiopharmaceutical is calibrated to your weight and time of procedure. Because of the very short half-life of FDG the injection must be timely, or the PET scan results may be compromised.

Loose/leisure clothing is recommended, we also suggest a sweater because the room temperature may be low. Generally, if these recommendations are followed it is not necessary for PET patients to change into examination gown.

PET procedures should be coordinated with other aspects of the patient’s cancer treatments, because the radiopharmaceutical uptake may be compromised by other drugs and treatments, special consideration and coordination is necessary for patients receiving chemotherapy and radiation therapy. It is important to tell us if you are receiving chemotherapy, radiation therapy or if they just had surgery.

What Will The Patient Experience?
A PET scan is a simple procedure, one hour before the PET scan is acquired, the patient is injected with the radiopharmaceutical Fluorodeoxyglucose (FDG). FDG is made up of two components -Fluorine-18 a radioisotope with a half-life of 109 minutes, and glucose (sugar). Glucose is tagged to metabolic marker in normal and abnormal tissues, considering a cancerous tissue is more metabolically active, it will have a higher uptake of this sugar allowing the PET images to show the location and extent of the disease.

It takes about sixty minutes for the glucose to be distributed throughout the body, during this uptake time the patient is comfortable in a quiet room and is encouraged to drink water. After a sufficient uptake time the patient is scanned with a PET scanner, sensitive to the energy emitted by FDG, later on the camera detects the radioactive emissions and reconstructs the activity into 3D images.

In appearance and size, the PET scanner is similar to CT scanner, A PET/CT study requires ten to fifteen minutes.

How is PET different from CT and MRI?
CT and MRI are focused on structural damage and anatomical change to tissues, PET images evaluate the physiological change to tissues related to metabolism. Since metabolic change occurs before structural change, PET detects disease before CT or MRI, which is important as early detection and appropriate selection of treatment can have a major impact on the cost, as well as the outcome, of treatment.

Also, while CT and MRI are focused on imaging a particular area of the body, a single PET scan can give information about the entire body. In oncology, this can be a crucial advantage when additional tumors and the spread of the disease are sometimes discovered in a single PET image. In addition to reducing the cost, the replacement of multiple tests is also a great convenience for patients and physicians, as the extent of the disease is established more quickly and with greater confidence.

Is PET Cost Effective?
According to researchers and clinicians, the benefit and cost-saving potential of PET lies in its ability to:

• Detecting a disease before anatomical deterioration.
• Stage and trace disease progression.
• Limit patient exposure to invasive diagnostic and treatment techniques.
• Eliminate the need for redundant diagnostic and surgical tests and surgical procedures.
• Influence the use of the most appropriate and effective treatment protocols.
• Track treatment efficacy.
• Guide surgical procedures.

In Summary, the key benefits of PET are:

• Safe, non-invasive diagnostic procedure.
• Detailed diagnostic information, not available from other tests.
• Shorter time for definitive diagnosis.
• Earlier detection of disease with fewer invasive diagnostic procedures.
• Precise staging of the disease and better monitoring of cancer recurrences.
• More effective tracking of the results of chemotherapy and radiotherapy.
• Can result in less invasive surgery and/or avoidance of some surgeries.
• Can contribute to lowering the overall cost of care.