Medical Imaging II PDF

Medical Imaging II Vicky Varghese Nuclear Medicine Imaging Projection radiography and computed tomography, two imaging modalities that rely on the transmission of photons through the body to form images Nuclear medicine, an imaging modality that relies on the emission of photons...

Medical Imaging II Vicky Varghese Nuclear Medicine Imaging Projection radiography and computed tomography, two imaging modalities that rely on the transmission of photons through the body to form images Nuclear medicine, an imaging modality that relies on the emission of photons from within the body The ionizing radiation emitted when the radioactive atom undergoes radioactive decay is used to determine the location of the molecule within the body  A radiotracer is injected into a peripheral arm vein of the patient, or the patient inhales or ingests the radiotracer Specialized instrumentation produces images of the internal distribution of radioactivity, which is assumed to mirror the distribution of the compound of interest These images are compared with known distributions in different disease states In order to most accurately depict the biodistribution of radiotracers, instrumentation that emphasizes high image quality and quantitative accuracy has been developed  Nuclear medicine is used whenever a physician needs information on physiologic or biochemical function The two most common nuclear medicine procedures are bone scanning and myocardial perfusion imaging A projection radiograph can show a fracture; a bone scan can show active metabolism during the healing process Introduction Nuclear medicine relies on radiopharmaceuticals introduced into the body to trace the spatial and temporal distribution of the underlying physiological and biochemical processes that govern the radiopharmaceutical’s distribution Radiopharmaceuticals consist of both a chemical compound (the ‘‘pharmaceutical’’ portion of the word) and a radioactive atom (the ‘‘radio’’ portion of the word) The radioactive atom undergoes radioactive decay, radiation is emitted that leaves the body. External imaging devices, such as scintillation cameras, record these radiation emissions coming from the patient and produce either a planar, two-dimensional image or cross-sectional, tomographic images. Depending on the specific radiotracer, different physiological or biochemical functions are imaged Differences from the projection radiography In nuclear medicine, however, each different radiotracer produces a depiction of a completely different physiological or biochemical function Radiation that forms the basis of the image is not transmitted through the body, but arises from the radioactive atoms within the body hence referred to as emission imaging Nomenclature An atom consists of a nucleus of protons and neutrons, which together are called nucleon The atomic number Z is equal to the number of protons in the nucleus and defines the element The mass number A is equal to the number of nucleons in the nucleus The term nuclide refers to any unique combination of protons and neutrons that forms a nucleus. If a particular nuclide is radioactive (i.e., it can undergo radioactive decay), it is termed a radionuclide  Atoms with the same atomic number but different mass number (i.e., different numbers of neutrons) are called isotopes. For example, carbon-11 Atoms with the same mass number but different atomic numbers are called isobars. For example, carbon-11 decays to boron-11; the two are isobars Atoms with the same number of neutrons are called isotones Atoms with the same atomic and mass number (i.e., the same nuclide) but with different energy levels are called isomers  The sum of the masses of the constituents of an atom (i.e., the protons, neutrons, and electrons) is greater than the atom’s actual mass. The difference between the sum of the masses of the atom’s constituents and the actual mass is called the mass defect The energy required to remove an electron completely from an atom is the electron binding energy, which is greater for electrons in the orbits closer to the nucleus because of the greater attractive force of the nucleus for inner electrons Radioactive decay is the process by which an atom rearranges its constituent protons and neutrons to end up with lower inherent energy  Radioactivity describes how many radioactive atoms are undergoing radioactive decay every second The half-life t1/2 is the time it takes for the radioactivity (or the number of radioactive atoms) to decrease by a factor of 2. Line of Stability If the different unique combinations of protons and neutrons as a nucleus that are found in nature are cataloged, they can be separated into two groups: nonradioactive nuclides, radioactive nuclides the total number of nucleons, and the ratio of neutrons to protons, determine whether a nuclide is stable or radioactive. Modes of Decay There are four main modes of decay: 1. alpha decay, which results in emission of an alpha particle; 2. beta decay, which results in emission of a beta particle 3. positron decay, which results in emission of a positron; 4. isomeric transition, which results in emission of a gamma ray. These ionizing radiations fall into two classes: particulate radiation and electromagnetic radiation. Radiotracers There are about 1,500 known radionuclides, of which 200 or so can be purchased. Out of these, however, only a dozen or so are suitable for nuclear medicine for several reasons We desire ‘‘clean’’ gamma ray emitters—that is, ones that do not also emit alpha and beta particles (because these particles would only contribute to patient radiation dose without usefully contributing to image formation) In nuclear medicine we would prefer that there would be no attenuation of the radiation. This is because in nuclear medicine we are trying to determine the location of the emitters Another important property of a radiotracer is its half-life. We need to be able to form images in a matter of minutes, not seconds or hours. it must be useful and safe to ‘‘trace’’ within the body, either by itself or attached to a compound  Iodine, for example is a naturally occurring substance in the body, accumulating in the thyroid gland. Iodine-123 or I-131 in a sodium salt can be administered orally and measured in the thyroid to assess thyroid function. Technetium-99m can be used to label diethylene triamine pentaacetic acid (DTPA), which is filtered by the kidneys, and serial images of the kidneys can be used to assess , renal function. Gaseous O2 in which one oxygen atom has been replaced by oxygen-15 is used to measure blood flow and to assess oxygen metabolism with positron emission tomography. Fluorodeoxyglucose (FDG) is used by the body like glucose, except that the (labeled) fluorine- 18 atoms remain where the molecule is first used. Imaging the uptake of FDG in the brain, for example, is thought to reveal aspects of the mental processes involved in motor, perceptual, and cognitive tasks Planar Scintigraphy As in diagnostic x-ray imaging, nuclear medicine imaging evolved from projection imaging to tomographic imaging Projection studies in nuclear medicine, called planar scintigraphy, have always used the Anger scintillation camera, a type of electronic detection instrumentation. The corresponding tomographic imaging method, called single photon emission computed tomography (SPECT), uses one or more rotating Anger cameras to obtain projection data from multiple angles Another imaging method, called positron emission tomography (PET), is based on radiotracers labeled with radioactive atoms whose decay produces a positron that is subsequently annihilated, producing two gamma photons Together, SPECT and PET are referred to as emission computed tomography Instrumentation The three basic imaging modalities in nuclear medicine—planar imaging, SPECT, and PET Planar imaging and SPECT use radiotracers that are gamma emitters, while PET uses radiotracers that emit positrons SPECT and PET require tomographic reconstruction techniques (like CT), while planar imaging forms images by projection  Collimators The collimator is a 1- to 2-inch thick slab of lead of the same dimensions as that of the scintillation crystal, with a geometric array of holes in it The collimator provides an interface between the patient and the scintillation crystal by allowing only those photons traveling in an appropriate direction to interact with the crystal There are several types of collimators used with Anger cameras: parallelhole, converging, diverging, and pinhole. The most commonly used collimator is the parallel- hole collimator, which consists of an array of parallel holes perpendicular to the crystal face Scintillation Crystal This type of detector is based on the property of certain crystals to emit light photons (scintillate) after deposition of energy in the crystal by ionizing radiation. The most commonly used scintillation crystal in nuclear instrumentation is sodium iodide with ‘‘thallium doping,’’ which is written as NaI[Tl] The scintillation crystals in a gamma camera are typically 10–25 inches in diameter and 1/4–1 inch thick. The thicker crystals are used for high-energy gamma rays, while the thinner crystals are used for low-energy gamma rays Photomultiplier Tubes Each gamma photon that interacts in the scintillation crystal (by a photoelectric or Compton scattering process) produces a burst of light in the crystal, comprising thousands of light (scintillation) photons. This light is reflected and channeled out of the back of the crystal, through a glass plate and is incident upon an array of photomultiplier tubes. Positioning Logic When a gamma photon interacts with the crystal, thousands of scintillation photons are produced, and every PMT produces an output pulse. The goal of the Anger camera’s positioning logic circuitry is to determine both where the event occurred on the face of the crystal and the combined output of all the tubes, which represents the light output of the crystal These output signals are denoted as X and Y for the estimated two-dimensional position of the event