Pet-ct Scanning

[Henrietta Naughton]

Positronemission tomography (PET) is a type of nuclear medicine procedure that measures metabolic activity of the cells of body tissues. PET is actually a combination of nuclear medicine and biochemical analysis. Used mostly in patients with brain or heart conditions and cancer, PET helps to visualize the biochemical changes taking place in the body, such as the metabolism (the process by which cells change food into energy after food is digested and absorbed into the blood) of the heart muscle.

PET differs from other nuclear medicine examinations in that PET detects metabolism within body tissues, whereas other types of nuclear medicine examinations detect the amount of a radioactive substance collected in body tissue in a certain location to examine the tissue's function.

Since PET is a type of nuclear medicine procedure, this means that a tiny amount of a radioactive substance, called a radiopharmaceutical (radionuclide or radioactive tracer), is used during the procedure to assist in the examination of the tissue under study. Specifically, PET studies evaluate the metabolism of a particular organ or tissue, so that information about the physiology (functionality) and anatomy (structure) of the organ or tissue is evaluated, as well as its biochemical properties. Thus, PET may detect biochemical changes in an organ or tissue that can identify the onset of a disease process before anatomical changes related to the disease can be seen with other imaging processes such as computed tomography (CT) or magnetic resonance imaging (MRI).

PET is most often used by oncologists (doctors specializing in cancer treatment), neurologists and neurosurgeons (doctors specializing in treatment and surgery of the brain and nervous system), and cardiologists (doctors specializing in the treatment of the heart). However, as advances in PET technologies continue, this procedure is beginning to be used more widely in other areas.

PET may also be used in conjunction with other diagnostic tests, such as computed tomography (CT) or magnetic resonance imaging (MRI) to provide more definitive information about malignant (cancerous) tumors and other lesions. Newer technology combines PET and CT into one scanner, known as PET/CT. PET/CT shows particular promise in the diagnosis and treatment of lung cancer, evaluating epilepsy, Alzheimer's disease and coronary artery disease.

Originally, PET procedures were performed in dedicated PET centers, because the equipment to make the radiopharmaceuticals, including a cyclotron and a radiochemistry lab, had to be available, in addition to the PET scanner. Now, the radiopharmaceuticals are produced in many areas and are sent to PET centers, so that only the scanner is required to perform a PET scan.

Further increasing the availability of PET imaging is a technology called gamma camera systems (devices used to scan patients who have been injected with small amounts of radionuclides and currently in use with other nuclear medicine procedures). These systems have been adapted for use in PET scan procedures. The gamma camera system can complete a scan more quickly, and at less cost, than a traditional PET scan.

The radionuclides used in PET scans are made by attaching a radioactive atom to chemical substances that are used naturally by the particular organ or tissue during its metabolic process. For example, in PET scans of the brain, a radioactive atom is applied to glucose (blood sugar) to create a radionuclide called fluorodeoxyglucose (FDG), because the brain uses glucose for its metabolism. FDG is widely used in PET scanning.

Other substances may be used for PET scanning, depending on the purpose of the scan. If blood flow and perfusion of an organ or tissue is of interest, the radionuclide may be a type of radioactive oxygen, carbon, nitrogen, or gallium.

The radionuclide is administered into a vein through an intravenous (IV) line. Next, the PET scanner slowly moves over the part of the body being examined. Positrons are emitted by the breakdown of the radionuclide. Gamma rays called annihilation photons are created when positrons collide with electrons near the decay event. The scanner then detects the annihilation photons, which arrive at the detectors in coincidence at 180 degrees apart from one another. A computer analyzes those gamma rays and uses the information to create an image map of the organ or tissue being studied. The amount of the radionuclide collected in the tissue affects how brightly the tissue appears on the image, and indicates the level of organ or tissue function.

In general, PET scans may be used to evaluate organs and/or tissues for the presence of disease or other conditions. PET may also be used to evaluate the function of organs, such as the heart or brain. The most common use of PET is in the detection of cancer and the evaluation of cancer treatment.

In some cases, an initial scan may be performed prior to the injection of the radionuclide, depending on the type of study being done. The patient will be positioned on a padded table inside the scanner.

The radionuclide will be injected into the IV. The radionuclide will be allowed to concentrate in the organ or tissue for about 30 to 60 minutes. The patient will remain in the facility during this time. The patient will not be hazardous to other people, as the radionuclide emits less radiation than a standard X-ray.

Stanford is the first health care institution in Northern California to offer patients a powerful new diagnostic imaging system known as Positron Emission Tomography/Computerized Tomography (PET/CT) scanning.

Today, most PET scans are performed on instruments that are combined PET and CT scanners. The combined PET/CT scans provide images that pinpoint the location of abnormal metabolic activity within the body, like malignant tumor cells. The combined scans have been shown to provide more accurate diagnoses than the two scans performed separately.

Every PET/CT scan at Stanford is reviewed and correlated by both a board certified nuclear medicine doctor and a board certified radiologist at a daily joint review session. Separate full reports are generated from each division for each patient.

The PET and CT scans are done at the same time on the same machine. The physician is able to precisely overlay the metabolic data of the PET scan and the detailed anatomic data of the CT scan to make a more detailed image than either test would make by itself. A small amount of radioactive glucose (sugar) is injected into a vein. The PET scanner rotates around the body and makes a picture of where glucose is being used in the body. Malignant tumor cells show up brighter in the picture because they are more active and take up more glucose than normal cells do. Some people are sensitive to the radioactive glucose and may have nausea, headache, or vomiting.

PET, or positron emission tomography, monitors the biochemical functioning of cells by detecting how they process certain compounds, such as glucose (sugar). Cancer cells metabolize glucose at a much higher level than normal tissues.

CT, or computed tomography, yields a detailed picture of the body's anatomical structures by taking cross-sectional images or X-ray slices of the body. While CT does an excellent job of depicting structures and anatomy, it may miss small or early stage tumors.

Currently, doctors can overlay the results of PET and CT scans performed separately to identify and locate tumors. However, because a patient may not be positioned identically for both scans, the two images can be difficult to line up exactly, degrading the accuracy of the diagnostic information.

The combined PET/CT machine allows doctors to rapidly perform both scans in one session without having to move the patient. This means doctors can precisely overlay the metabolic data of the PET scan and the detailed anatomic data of the CT scan to pinpoint the location and stage of tumors.

Positron emission tomography (PET) scanning is an imaging modality primarily used in oncology. It utilizes radiotracers to measure various metabolic processes in the body. Various changes in metabolism, blood flow, and regional chemical composition can be analyzed by it. Radio-tracers can be injected, swallowed, or inhaled depending upon the site of the body being examined, and the tracer gets trapped in various tissues of the body depending upon the affinity. Areas of higher activity show higher uptake and brighter spots on images. Unstable nuclei of radioactive tracers emit positrons that produce gamma rays when combined with neighboring electrons. The gamma rays are detected by a ring of detectors in the scanner. A computer then uses this data to create a 3D image of the tracer in the body. Various tracers are utilized depending on the targets.

The tracer may be administered intravenously, orally, or via inhalation. It takes some time to distribute throughout the body. If a PET-CT is to be done, a contrast may be administered intravenously or orally. Positioning depends upon the site to be scanned. The PET machine has a central hole through which the patient slides. First, images are generally scout images to assess correct positioning. Sometimes, breath-holding may be required. The scan takes anything from 30 minutes to 1 hour.

FDG-PET is used for diagnosis, staging, and monitoring cancers, particularly in Hodgkin's lymphoma,[2] non-Hodgkin lymphoma,[3] and lung cancer.[4][5][6] In a study, the likelihood ratio for malignancy in a solitary pulmonary nodule with an abnormal FDG-PET scan was 7.11. This study suggested that the FDG-PET scan is more accurate than the standard criteria for diagnosis. FDG-PET can be used as an adjunct test in solitary pulmonary nodule evaluation.[7] In assessing FDG-PET in staging patients with non-small cell carcinoma, FDG-PET had a higher sensitivity (71% vs 43%), positive predictive value (44% vs 31%), negative predictive value (91% vs 84%) & accuracy (76% vs 68%) than computed tomography (CT) scan for N2 lymph nodes. Meanwhile, FDG-PET had a higher sensitivity (67% vs. 41%) but lower specificity (78% vs. 88%) than a CT scan for N1 lymph nodes. It accurately upstaged 28 patients (7%) with unsuspected metastasis & down-staged 23 patients (6%). Hence, the FDG-PET scan allows for improved patient selection & accurately stages the mediastinum. However, there were many false positives in lymph nodes, and it may miss N2 disease in the #5, #6, and #7 stations.

3a8082e126