Indmedica Home | About Indmedica | Medical Jobs | Advertise On Indmedica
Search Indmedica Web
Indmedica - India's premier medical portal

Journal of the Academy of Hospital Administration

Role of Positron Emission Tomography (PET) in Clinical Set Up

Author(s): N.R. Jaganathan*

Vol. 14, No. 2 (2002-07 - 2002-12)

Positron emission tomography, PET is the new non-invasive molecular imaging technique in clinical medicine that allows the physician to examine the heart, brain and other organs. PET gives images based on the detection of subatomic particles that are emitted from a radioactive substance (radiopharmaceutical) given to the patient. PET imaging is unique in that it shows the functioning of organs and tissues, while other imaging techniques - such as X-ray and CT - show structure. PET is useful modality for detection of cancer, coronary artery disease and several brain diseases and disorders. It provides information about the metabolic activities of tumors and reveals metabolic changes that may occur with the treatment.

Basic Aspects of PET

It is well known that unstable nuclides with an excess of protons in the nucleus can undergo radioactive decay via positron emission. A positron is an antimatter electron; it has the same rest mass as an electron, but has a +1 charge. Nuclides undergoing positron decay increase their neutron-to-proton ratio. Once emitted, the positron travels several millimeters in tissue, depositing its kinetic energy in the process of ionization of atoms in the tissue. When the positron expends all its kinetic energy and is at rest, it meets up with a free unbound electron in the tissue. Since the positron is an antimatter electron, mutual annihilation occurs. In order to obey the law of conservation of energy, two 511 Kev gamma ray photons appear in the place of the position of the positron and electron. These two photons are emitted exactly 180 degrees back-to-back which are detected in PET. The information is then fed into a computer to be converted into a complex picture.

Thus, the physics behind PET images is that the radioactive substance decay leads to the ejection of positive electron, which collides with an electron in the tissue. This collision result in a conversion from mass to energy, resulting in the emission of two gamma rays heading off in exact opposite directions. Special crystals, called photomultiplier-scintillator detectors, within the PET scanner detect the gamma rays. The camera records the millions of gamma rays being emitted, and a computer uses the information to generate an image of the area where radioactive substance has accumulated. The medical importance of PET lies in the existence of isotopes like 11C, 13N, 15O and 18F, which are essential elements of all living systems, and their physiological processes. Hence, tissue-specific and chemistry-specific tracers can be synthesized and injected into human/animals to study the physiological functions of normal and pathological tissues in vivo.

PET scanner and Procedure for Scanning:

PET instrument looks like a large, square shape doughnut. Within this machine are multiple rings of detectors that record the emission of energy from the radioactive substance. PET is usually carried out on an out patient basis. The patient need to fast (except water) at least four to six hours before the scan. The patient would be lying on a cushioned table and will be moved into the hole of the scanner. Before the examination, a radioactive substance (produced in a machine called cyclotron) is attached, or tagged, to a natural body compound, most commonly glucose, but sometimes water and ammonia. This process is called radiolabelling. Normally the radioactive substance is given to the patient through intravenous injection and it will take approximately 30 - 60 minutes for the radioactive substance to travel through the body and be absorbed by the tissue. There after the scanning begins and this may take another 30 - 45 minutes. Patients with heart disease may undergo a stress test in which PET scans are obtained while they are at rest, then after the administration of a drug to alter the blood flow to the heart. The administration of the radioactive substance will feel like a slight pinprick if given by intravenous injection. The patient has to remain still during the investigation. No special preparation or change in the daily routine is required.

Once this substance is administered to the patient, the radioactivity begins to breakdown in the body, resulting in the release of energy that is detected by the PET scanner. Different colors or degree of brightness on a PET image represents different levels of body function. For example, because healthy tissue uses glucose for energy, it accumulates some of the radiolabelled glucose, which will show up as background areas on the PET images. Cancerous tissue, which uses more glucose than normal tissue, will absorb more of the radiolabelled substance and appear brighter on the PET images. The following are some of the tracers used for studies of brain: fluorodeoxyglucose (FDG), 15O or 15O water and 11C or 11O carbon monoxide. FDG is used to carry out physiological studies of memory, cognition, etc., as well as to detect and diagnose several kinds of tumors and other diseases of the human brain. 15O water is a molecule of common water with radioactive oxygen in place of the non-radioactive isotope. Thus it distributes mainly in the blood and can be used to measure blood flow and in functional brain studies. 11C or 15O carbon monoxide is used to quantify and make changes of cerebral blood volume.

Uses of PET in Clinical Medicine:

Today PET imaging is having a major impact in patient care and its clinical applications are increasing, particularly since the introduction of whole-body imaging. It is used to detect cancer and to examine the effects of cancer therapy by characterizing biochemical changes with the cancer. PET of the heart can be used to determine blood flow to the heart muscle and help evaluate signs of coronary artery disease. Together with myocardial metabolism study, PET can differentiate non-functioning heart muscle from normal heart muscle that will benefit in coronary bypass surgery cases. PET scans of the brain are used to evaluate patients who have memory disorders, brain tumors, seizures and degenerative diseases like Alzheimer's, Huntington's, and Parkinson's. Within the first few hours of stroke, PET may be useful in determining treatment therapies. In general PET imaging is very accurate in differentiating malignant from benign growths, as well as showing the spread of malignant tumors. It can help detect recurrent brain tumors and tumors of the lung, colon, breast, lymph nodes, skin, and other organs. In addition, PET imaging can be used to determine what combination of treatment is most likely to be successful in managing a patient's tumor.

Other Considerations:

Because of the complexity of the PET scanner equipment lot of planning is needed before one decides to purchase. It needs a medical cyclotron, six lead-shielded hotcells with associated radiochemistry facilities, radiopharmacy and a whole body PET Scanner. Because the radioactivity is short lived, the radiation exposure is extremely low. Moreover, the amount of radioactive material injected is also very low it does not affect the normal processes of the body. The radioactive material may expose the fetus of patients who are pregnant or the infants of women who are breast-feeding to the radiation. The risk to the fetus and the infant should be considered related to the information gain from the potential result of the PET examination. PET can give false results if a patient's chemical balances are not normal. Specifically, the results of diabetic patients can be adversely affected because of blood sugar or blood insulin levels.

Reference Materials

  1. Atlas of Clinical PET. Michael M. Maisey, Richards L. Wall and Salley F. Barrington, Edward Arnold, 1999.
  2. PET in Clinical Oncology. Helmut J. Wieler and R. Edward Coleman, Springer, Berlin, 2000.
  3. Clinical PE. Gustav K von Schulthess. (Ed), Lippincot Williams & Wilkins, Philadelphia, 1999.
  4. Role of PET with F - 18 - fluorodeoxyglucose in the diagnosis, staging and chemotherapy evaluation in particular with non-small cell lung center. Sigrid Stroobaruti, Leuven University Press, 2002
  5. Cardiac PET. Markens Schwaiger, Kluwer Academic, 1995.
  6. Clinical PET. Correlation With Morphological Cross- sectional Imaging. Gustav K. von Schulthess, Lippincot, 2000.
  7. Molecular Imaging in Oncology. E. Edmund Kim and Edward F. Jackson, Springer, 1999.
  8. Atlas of Clinical PET. M. Maisey, Oxford University Press, 1999.
  9. Wahl RL, Zasadny K, Helvie M, Hutchins GD, Weber B, Cody R, (1993) Metabolic monitoring of breast cancer chemohormotheraphy using positron emission tomography: initial evaluation. J. Clin Oncol 11: 2101 - 2111.
  10. Tyler JL, Diksic M, Villemure J-J, Evans AC, Meyer E, Yamoto YL, Feindel W (1987) Metabolic and hemodynamic evaluation of gliomas using positron emission tomography. J. Nucl Med 28: 1123 - 1133.
  11. Hoh CK, Glaspy J, Rosen P, Dahlborn M, Lee SJ, Kunkel L, Howkin RA, Maddahi J, Phelps ME (1997) Whole-body FDG-PET imaging for staging of Hodgkin's disease and lymphoma. J. Nucl Med 38: 343 - 348.

* Department of NMR, All India Institute of Medical Sciences New Delhi - 110029, India.

Access free medical resources from Wiley-Blackwell now!

About Indmedica - Conditions of Usage - Advertise On Indmedica - Contact Us

Copyright © 2005 Indmedica