Having a PET Scan...
Your doctor has referred you for a PET scan and there are several guidelines you will be asked to follow. You will be asked to not eat or drink anything expect water and medications for six hours prior your scan. Other instructions include:• Please plan to be on time for your scan appointment.
• Please allow 3 hours for the PET examination.
• It must be 3 ~ 6 weeks since your radiation and/or surgery.
• Women who are pregnant must not be scanned, and nursing mothers should ask for specific instructions.
• Diabetics should ask for specific instructions.
• You should wear comfortable clothing.
• You should drink at least 8 glasses of water the day before the scan.Before the scan, a small IV will be placed in a vein that will be used to check your blood sugar level and to inject the radioactive substance (FDG) for the test. This line remains in place until the scan is completed.
After the radioactive dose (FDG) injection, up to an hour will elapse before the scan begins. It is important to lie quietly without talking or moving during this period. You will also be asked to lie still during the scan, which usually lasts about 60 minutes. If you are having more than one scan to evaluate different areas of your body, the procedure may take up to two hours to complete. You should plan to be at the Center for about 3 hours overall for the entire process.
Most of the FDG collects in your bladder, so it is important that you urinate shortly after the scan is completed. Drink plenty of extra fluids for the rest of the day and urinate frequently to help eliminate the substance from your system as quickly as possible. The radioactive substance used in the study has a very short half life, and most of it is gone within the first four to eight hours after the injection.
Following the scan, the information is processed and filmed by a technologist and then interpreted by a trained, board certified nuclear medicine physician. A report is then prepared and sent directly to the physician who ordered the test, who will discuss the results with you.
What is a PET Scan?
P.E.T., which stands for positron emission tomography, is an imaging technique that relies on changes in tissue biochemistry. Most well known conventional medical imaging techniques, such as X ray, ultrasound, CT and MRI, depend on changes in the anatomy or structure of organs. P.E.T. is able to image changes in cells and tissues often before there are changes in anatomy. Biologically active natural compounds such as oxygen, carbon and glucose labeled with radioactive isotopes are given intravenously and react in the body identically to their non radioactive counterparts. The normal and abnormal activity (distribution) can be imaged with a P.E.T. scanner. The tracer amounts used do not have a pharmacological effect and there are no known biological or radiation hazards associated with P.E.T. imaging.
The most commonly used radiopharmaceutical in P.E.T. imaging is 18F-fluorodeoxy-glucose, or simply FDG. This compound is radioactive sugar. Glucose is metabolized in cancer tissues to a greater extent than in normal tissues; consequently cancerous tissues can be seen on the FDG-P.E.T. scan. This unique information can have a profound impact on treatment decisions (e.g., Is that new mass cancerous and has it spread?).Most FDG-P.E.T. imaging today is done to evaluate cancer patients. Through the efforts of a number of nuclear medicine societies, the FDA has approved FDG-P.E.T imaging for several cancer related problems. A partial list of these cancer-related problems are presented below:
The evaluation of solitary lung nodules for the presence of cancer.
The preoperative staging and re-staging of some lung cancers.
The staging and re-staging of lymphomas.
Staging and re-staging of malignant melanomas (skin cancers) prior to surgery.
The staging and re-staging of colon cancer.
The staging and re-staging of breast cancer.Other cancers, such as liver, head and neck tumors, pancreas, thyroid cancer, and soft tissue sarcomas, can be detected and followed with FDG-P.E.T.
A number of cost effectiveness analyses have shown that thousands of dollars can be saved without loss of life expectancy when P.E.T. is used in the evaluation of cancer patients. P.E.T. may result in the earlier diagnosis of cancer. Unnecessary procedures and surgeries may be avoided if the patient is found to have more extensive disease than expected. Treatment that is more appropriate or even life saving may be offered to the patient shown to have disease that is less advanced than expected. Costly batteries of tests ordinarily required for staging might be reduced to a single P.E.T. scan.
How is a PET Scan performed?
PET scans can be done on an outpatient basis. It is also possible that some hospital inpatients may undergo a PET examination for certain conditions. Although each PET center may have specific protocols in place, LIFESCAN Chicago uses the following procedure:
• A small amount of radioactive material is injected into the vein 40 ~ 60 minutes before scanning
• The patient lies on a table that slides into the middle of the PET scanner for 40 ~ 50 minutes
• The scanner detects gamma rays and maps an image of the area, allowing the physician to see the location of the metabolic process. For example, glucose (or sugar, which the body uses to produce energy), combined with a radioisotope, will show where glucose is being used in the brain, the heart muscle, or in a growing tumor.
• The entire procedure is expected to last from 2 ~ 3 hours, depending on the
type of PET examination the patient is undergoing.
In what conditions are PET scans helpful?
Oncology (Tumors): The largest growth area of PET is in the area of tumor evaluation. The information PET provides on the active status of tumors has proven extremely helpful for determining methods of therapy or surgery. Scans performed after treatment will demonstrate the effectiveness of that treatment on the tumor.Neurology: Many patients with epilepsy do not respond well to drug therapy. When surgical intervention is required, PET can be helpful in locating the exact area of the brain causing the seizures. Imaging the brain with radioactive glucose has also been found useful in diagnosing Alzheimer's Disease and other conditions that may cause memory loss or mental deterioration.
Cardiology: PET is very helpful in determining whether coronary bypass surgery will be beneficial. The information is important in selecting those patients who are considered good candidates for this type of surgery.
How PET differs from CT/MRI?
P.E.T. is extremely sensitive and effective in detecting recurrent cancer, especially in patients with positive and rising tumor markers who are otherwise asymptomatic. This information is very practical and enables the physician and patient to make cost effective treatment decisions, such as avoiding unnecessary surgery, directing more complete surgery, or allowing surgery when it had seemed contraindicated.
In patients who have received chemotherapy, it is difficult to demonstrate early shrinkage of the cancer when using CT or MRI to monitor tumor response, even when the treatment is effective in controlling or stopping tumor growth. In contrast, P.E.T. can often show decreased or even absent tumor metabolism after only one or two cycles of chemotherapy. P.E.T. can determine ineffective therapy early on, saving money on expensive treatments and morbidity, but can also save precious time otherwise wasted while waiting for the desired response. With this information, early modification of treatment and selection of a new therapeutic regimen can occur. A P.E.T. scan’s ability to detect treatment response cannot be matched by CT or MRI.CT, MRI and P.E.T. imaging all provide a non invasive way to “see” the inner workings of the human body. P.E.T. however, does not have to wait for changes in the anatomical structure of the patient to determine staging, recurrence, therapy response and differentiation of scar and residual disease.
P.E.T. can determine the nature of solitary pulmonary nodules that are smaller than 4 cm in size and this is very useful for preoperative staging. CT will show a lesion but will not determine whether it is benign or malignant. CT is also less accurate in finding metastases, and cannot differentiate malignancy from scar or other tissue in checking recurrence of colon cancer (P.E.T. sensitivity is 96% vs. CT sensitivity of 53%.)
In determining the extent of malignant melanoma (staging), P.E.T. is more sensitive and accurate than CT (P.E.T. 96% vs. CT 55%). P.E.T. is also more sensitive and specific than CT for staging lymphoma. CT cannot always differentiate benign from malignant tissue in re-staging and surveillance of lymphoma patients. In initial staging of breast cancer, P.E.T. evaluation of the axilla has a sensitivity and specificity of 81% and 100% in an area where CT is not very useful in differentiating between benign and malignant tissue. In re-staging ovarian cancer after chemotherapy treatment, P.E.T. sensitivity is 94% vs. CT 73%, and the specificity of P.E.T. is 100% vs. CT 38%.