3 NUCLEAR MEDICINE Gamma Camera
Nuclear medicine is a branch or specialty of medicine and medical imaging that uses radionuclides in the diagnosis and treatment of disease. In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals.
These radiopharmaceuticals, once administered to the patient, can localize to specific organs or cellular receptors. This property of radiopharmaceuticals allows nuclear medicine the ability to image the extent of a disease-process in the body, based on the cellular function and physiology, rather than relying on physical changes in the tissue anatomy. In some diseases nuclear medicine studies can identify medical problems at an earlier stage than other diagnostic tests.
Nuclear medicine tests differ from most other imaging modalities in that diagnostic tests primarily show the physiological function of the system being investigated as opposed to traditional anatomical imaging such as CT or MRI. Nuclear medicine imaging studies are generally more organ or tissue specific (e.g.: lungs scan, heart scan, bone scan, brain scan, etc.) than those in conventional radiology imaging, which focus on a particular section of the body (e.g.: chest X-ray, abdomen/pelvis CT scan, head CT scan, etc.). In addition, there are nuclear medicine studies that allow imaging of the whole body based on certain cellular functions.
Technetium-99m is used in 20 million diagnostic nuclear medical procedures every year. Approximately 85 percent of diagnostic imaging procedures in nuclear medicine use this isotope. Depending on the type of nuclear medicine procedure, the Tc-99m is tagged (labelled or bound) to a pharmaceutical that transports the Tc-99m to its required location.
In nuclear medicine imaging, radiopharmaceuticals are taken internally, for example intravenously or orally. Then, external detectors (gamma cameras) capture and form images from the radiation emitted by the radiopharmaceuticals. This process is unlike a diagnostic X-ray where external radiation is passed through the body to form an image.
Gamma camera A gamma camera, also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy. The applications of scintigraphy include nuclear medical imaging to view and analyse images of the human body or the distribution of medically injected, inhaled, or ingested radionuclides emitting gamma rays.
Once a radiopharmaceutical has been administered, it is necessary to detect the gamma ray emissions in order to attain the functional information. The instrument used in Nuclear Medicine for the detection of gamma rays is known as the Gamma camera. The components making up the gamma camera are: 1-collimator 2-detector crystal 3-photomultiplier tubes 4-position logic circuits 5-data analysis computer.
1. Camera Collimator The first object that an emitted gamma photon encounters after exiting the body is the collimator. The collimator is a pattern of holes through gamma ray absorbing material, usually lead or tungsten, that allows the projection of the gamma ray image onto the detector crystal. The collimator achieves this by only allowing those gamma rays traveling along certain directions to reach the detector. The mostly commonly used collimators are the parallel hole collimators , either high sensitivity or high resolution.
A- In the absence of collimation there is no relationship between the position at which a gamma ray hits the detector B- The parallel-hole collimator forms an image by excluding all gamma rays except those travelling parallel to the hole’s axis.
2. Scintillation Detector In order to detect the gamma photon we use scintillation detectors. A Thallium-activated Sodium Iodide [NaI(Tl)] detector crystal is generally used in Gamma cameras. This is due to this crystal's optimal detection efficiency for the gamma ray. A detector crystal may be circular or rectangular. A gamma ray photon interacts with the detector by means of the photoelectric effect with the iodide ions of the crystal of the Sodium Iodide , this interaction causes the release light.
3. Photomultiplier Tubes Only a very small amount of light is given off from the scintillation detector. Therefore, photomultiplier tubes are attached to the back of the crystal. At the face of a photomultipler tube (PMT) is a photocathode which, when stimulated by light, ejects electrons. The PMT is an instrument that detects and amplifies the electrons that are produced by the photocathode. This electron from the cathode is focused on a dynode which absorbs this electron and re-emits many more electrons (usually 6 to 10). These new electrons are focused on the next dynode and the process is repeated over and over in an array of dynodes. At the base of the photomultiplier tube is an anode which attracts the final large cluster of electrons and converts them into an electrical pulse.
4. Position Circuit The position logic circuits immediately follow the photomultiplier tube array and they receive the electrical impulses from the tubes in the summing matrix circuit. 5. Data Analysis Computer Finally, in order to deal with the incoming projection data and to process it into a readable image, a processing computer is used. The computer may use various different methods to reconstruct an image.
SPECT/CT • Scintigraphy ("scint") is the use of gamma cameras to capture emitted radiation from internal radioisotopes to create two-dimensionalimages. • SPECT (single photon emission computed tomography) imaging, as used in nuclear cardiac stress testing, is performed using gamma cameras, usually one, two or three detectors or heads, are slowly rotated around the patient's torso. • SPECT/CT The 3D images of SPECT gamma camera can be mounted with CT to obtain anatomical together with physiological images.
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