Info

Figure 11.2 • Time-activity curve of a molybdenum-99/technetium-99m generator system demonstrating the in-growth of technetium-99m and subsequent elution. (Reprinted with permission from Bushberg, J. T., et al.: The Essential Physics of Medical Imaging, 2nd ed. Philadelphia: Lippincott Williams and Wilkins, 2002.)

The major difference between electromagnetic radiation (x-rays or y-rays) and particulate radiation (fi+, ¡3~, and a particles) lies in the ability of electromagnetic rays to penetrate matter. Whereas particles travel only a few millimeters before expending all their energy, x-rays and y-rays distribute their energy more diffusely and can travel through several centimeters of tissue.

Particle emitters deliver highly localized radiation doses to biological molecules. Damage results from the direct absorption of this radiation energy. This is known as the direct effect of radiation damage. The radiation dose of particle emitters is generally clinically useful for therapeutic use, with limited benefits for diagnostic use.

X-rays and y-rays deliver more uniform doses in a less concentrated way throughout the irradiated volume of tissue. Damage is indirectly produced by formation of free radicals (atoms or molecules with an unpaired electron). The indirect effect of radiation damage generally involves aqueous free radicals as intermediaries in the transfer of radiation energy to the biological molecules.

All biological systems contain water as the most abundant molecule (~80%). Radiolysis of water is the most likely event in the initiation of biological damage from diagnostic x-rays and y-rays. The absorption of energy by a water molecule results in the ejection of an electron with the formation of a free radical ion (H2O'+). The free radical ion dissociates to yield a hydrogen ion (H+) and a hydroxyl free radical (HO). The hydroxyl free radicals combine to form hydrogen peroxide (H2O2), which is an oxidizing agent. In addition, hydrogen free radicals (H) can form, which can combine with oxygen (O2) and form a hydroperoxy-free radical (HO2). These free radical intermediates are very reactive chemically and can attack and alter chemical bonds.

The only significant "target" molecule for biological damage is DnA. Types of DNA damage include single- and double-chain breakage as well as intermolecular or intramolecular cross-linking of double-stranded DNA. With the direct effect of radiation, the damage makes cell replication impossible. Cell death occurs. For the indirect effect of radiation, if the damage is not lethal but changes the genetic sequence or structure, mutations occur that may lead to cancer or birth of genetically damaged offspring. Some effects of radiation may develop within a few hours; others may take years to become apparent. Consequently, the effects of ionizing radiation on human beings may be classified as somatic (affecting the irradiated person) or genetic (affecting progeny).

Radiation dose can only be estimated, and its "measurement" is called radiation dosimetry. In the case of x-ray exposure, most radiation "doses" in the literature are described as the entrance exposure (in roentgens per minute) to the patient. In diagnostic nuclear medicine procedures, patients are irradiated by radiopharmaceuticals localized in certain organs or distributed throughout their bodies. Because the radionuclides are taken internally, there are many variables, and the radiation absorbed dose (rad) to individual patients cannot be measured. It can only be estimated by calculation. Under normal circumstances, no radiation worker or patient undergoing diagnostic investigation by radiopharmaceutical or radiographic procedures should ever suffer from any acute or long-term injury. Typical radiation doses to patients from radiopharmaceuticals are similar to, or less than, those from radiographic procedures.

Radiopharmaceuticals

Medical science provides a framework or paradigm in which to understand disease and to maintain health. Nuclear medicine is the branch of medical science that contributes to medicine by the use of the radiotracer method for diagnosis, and the use of in vivo radioactivity for therapy.

Nuclear medicine generally involves the administration of radioactively labeled compounds to trace a biological process. This process may be mechanical (gastric emptying, blood flow, cardiac wall motion) or a variety of other physiological functions. Within the concept of a "radiotracer" is the implication that the agent administered will not disturb the functional aspects of the process you wish to examine. In nuclear medicine, this concept is used to trace physiological processes in vivo and then compare them with known normal images or levels. These images are interpreted within the context of relevant pathophysiology to allow diagnosis of disease. The information obtained from diagnostic images can also be used to follow the patient for improvement following treatment. In clinical practice, nuclear medicine also makes use of in vitro diagnostic methods (radioimmunoassay) as well as in vivo radiopharmaceutical therapy.

The most common application of nuclear medicine is to image the distribution of radiopharmaceuticals in specific tissues or organ systems with a scintillation (Anger) camera for diagnostic purposes. Fundamentally, these instruments or cameras allow in vivo detection and localization of radiotracers. The purpose of the gamma camera is to record the location and intensity of the radiation within the imaging field (Fig. 11.3).

Radiation in the form of gamma photons (occasionally x-rays) initially enters the camera through the collimator, which usually is a sheet of lead with multiple small, precisely made holes. The collimator covers the detector's crystal. The purpose of the collimator is to decrease scattered radiation and to increase the overall resolution of the system. Photons that are not blocked by the collimator then enter a large sodium iodide (with a small amount of thallium) crystal that absorbs y-rays. The absorbed energy in the crystal is emitted as a flash of light (called a scintillation), which is proportional to the

Figure 11.3 • Patient being imaged with a scintillation (Anger) camera. (Reprinted with permission from Bushberg, J. T., et al.: The Essential Physics of Medical Imaging, 2nd ed. Philadelphia: Lippincott Williams and Wilkins, 2002.)

energy of the 7-ray. Coupled to the back of the NaI(Tl) crystal are photomultiplier (PM) tubes that convert the light flashes to electrical pulses proportional to the amount of light. To localize the original source of the photon (and create an image), a computer assigns x-y spatial coordinates to the various 7-rays coming from the patient and stores this information in a matrix. After data collection, the image is converted into a signal for display on a video monitor. The images obtained with the scintillation camera are called scintigrams, scintigraphs, or scans.

Nuclear medicine imaging studies involve the generation of images that represent the functional status of various organs in the body. When interfaced with computer systems, information regarding dynamic physiological parameters such as organ perfusion, metabolism, excretion, and the presence or absence of obstruction can be obtained. Images can be focused on a portion of the body, or an image of the whole body can be acquired by moving the camera from head to toe.

Cross-sectional images of organs can be obtained by rotating a position-sensitive scintillation camera detector about the patient. This type of procedure is called single photon emission computed tomography (SPECT), which is the counterpart of CT or computed axial tomography (CAT) scans in diagnostic radiology. Most SPECT systems (Fig. 11.4) use one to three scintillation detectors that rotate about the patient.

TV Monitor

TV Monitor -

TV Monitor

Figure 11.4 • Schematic diagram of a rotating triple-detector scintillation camera system for single photon emission computed tomography (SPECT).

Independent Radiation Detectors

Annihilation Photons

TV Monitor -

Independent Radiation Detectors

Annihilation Photons

Figure 11.5 • Schematic diagram of a PET imaging system with multiple scintillation detectors that localize positron decay by coincident detection of annihilation photons.

Figure 11.4 • Schematic diagram of a rotating triple-detector scintillation camera system for single photon emission computed tomography (SPECT).

Figure 11.5 • Schematic diagram of a PET imaging system with multiple scintillation detectors that localize positron decay by coincident detection of annihilation photons.

SPECT is routinely used when imaging the brain or heart to demonstrate three-dimensional distribution of radioactivity in these organs.

A newer modality for imaging uses multiple detector heads to image positron-emitting radiopharmaceuticals by PET (Fig. 11.5). The physics associated with annihilation radiation allow PET cameras to utilize coincident circuitry to facilitate the acquisition of high resolution images for the two photons emitted approximately 180 degrees from each during positron decay. When the photon pair is detected by two detectors, it is known that a decay event took place along a straight line between those detectors. Computer models will calculate a three-dimensional image of the subsequent data set to prepare PET images of the area of interest. Although various positron emitting nuclides can be covalently attached to numerous biologically important molecules for development of PET radiopharmaceuticals, 18F-fluorodeoxyglucose (18F-FDG) is by far, the major clinical PET agent.

The incidence of adverse reactions to radiopharmaceuti-cals is very low. Most adverse events are allergic reactions and occur within minutes of intravenous injection. In the case of radiolabeled murine antibodies, an anaphylactic reaction may occur on rare occasions.

Fluorine Radiochemistry

The clinically useful radioisotope of fluorine for organ imaging is fluorine-18. Fluorine-18 is produced in a cyclotron according to the 18O(p,n)18F nuclear reaction. Fluorine-18 (ty2 = 110 min) decays predominantly by positron emission to oxygen-18 with 7-ray emissions of 511 keV (194%). Fluorine-18 can be attached to several physiologically active molecules and, with the great strength of the C-F bond, appears to be a very useful label for radiopharmaceuticals.2 Radiotracer production involves relatively complicated synthetic pathways, and the preparation of high-specific-activity compounds presents many problems. The short half-life of fluorine-18 makes it necessary to complete the synthetic and purification procedure quickly.

Was this article helpful?

0 0
Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment