Myocardial Perfusion Imaging by using Different Radionuclides
Medicine has developed over the years and is still a yet fast growing subject area, especially the area of molecular medicine. Molecular medicine, also known as nuclear medicine incorporates the use of special probes for the examination, visualisation and characterisation of different physiological processes at different levels, particularly cellular and subcellular levels. The history of nuclear medicine dates back to about the early 1950s when familiarity developed around the use of radionuclides for the visualisation of internal anatomic structures and also to trace chemical processes taking place in the body. Since then, several radionuclides have been discovered.
Radionuclides are atoms which possess unstable nuclei and are radioactive (Ramamoorthy, Padmanabhan & Venkatesh 2008). Usually, when an atom becomes unstable, its nucleus tries to become stable by emitting energy in the form of either alpha or beta particles. In some other situations, the nucleus might give out energy in the form of gamma rays instead of alpha or beta particles. Radionuclides can be found naturally although the ones used in medicine and other scientific fields are produced artificially.
Radionuclides, also known as radioisotopes (radioactive isotopes) can be divided into three classes: the cosmogenic radionuclides, primordial radionuclides, and the secondary radionuclides (Ramamoorthy 2009). All these groups of radionuclides occur naturally. However, artificial radionuclides can be produced by using nuclear reactors, cyclotrons, or linear accelerators. The common radionuclides in use in nuclear medicine include Technetium (99m-Tc), Iodine (131-I), Thallium (201-Tl), Gallium (67-Ga), Yttrium (90-Y), Chromium (51-Cr), Phosphorus (32-P), Strontium (89-Sr), and Indium (111-In) (Schuster et al. 2008). The radionuclides used in diagnostic medicine are the ones that emit gamma rays while the ones that emit alpha and beta particles are used therapeutically in nuclear medicine. In the field of diagnostic medicine, the most commonly used radionuclide is Technetium. Technetium-99m can be produced by cyclotrons and nuclear reactors through molybdenum. It can therefore be said to be the product of decay of Molybdenum. It has almost ideal chemical properties which are needed for use in medical diagnosis. It has a very short half-life of about 6 hours (6.02 hours) and the low energy emitted plus its ease of chelation makes it possible to fit into a large range of radiodiagnostic materials. Also, it can be easily and rapidly bound to several materials and therefore can be used as a marker or a label for different substances of interest for many diagnostic purposes. However, in about ten percent of other medical examinations, other radionuclides are preferred because of their specific properties corresponding to the organ of interest, for instance, using Iodine (131-I) for the thyroid or using Thallium (201-Tl) for cardiac studies.
Apart from other general uses of radionuclides, myocardial perfusion imaging specifically requires the use of radionuclides as tracers (for example, thallium-201 of technetium-99m) which are taken up and held on to by the cardiac muscles. The end result of the uptake of radioactive tracers is a three dimensional objective image which is quantifiable as it shows the intensity of tracer uptake within the myocardium (atrial or ventricular) (Bengal 2009). The intensity of the tracer at any point on the image directly implies either blood flow sufficiency (perfusion) to that portion of the myocardium; of the ratio of live myocardium to fibrosed regions; or both (Henzlova et al. 2009). On this image, regions of ischemia or infarction appear as “cold spots” (Dilsizian et al. 2009). In actual practice, the tracer intensity or concentration on the image is normalised to a normal myocardial region, that is, the region that shows the most radiotracer uptake. Therefore, the myocardial perfusion image can be said to be an image of relative perfusion of the myocardium (El Fakhri et al. 2009).
In addition, myocardial perfusion imaging can also show the motions of the region of the myocardium in question and calculate precisely the ejection fraction of the muscle, especially, the left ventricular ejection fraction (Strauss et al. 2008). This is much more evident when the individual under the test is subjected to an exercise and the procedure is combined with an exercise electrocardiography.
The use of radionuclides in medicine can be enhanced with the use of specialised machines which can accentuate the kind of images produced by the procedure. Radionuclide imaging can be divided into positron imaging and single-photon imaging, and the techniques which make use of these imaging modalities are Positron Emission Tomography (PET) and Single-Photon Emission Computed Tomography (SPECT) respectively. In both techniques, the radionuclide tracer is introduced into a molecule which has a predilection for the region or part of the body of interest, and in this case, the heart or myocardium. The uptake of the tracer over a particular period of time is measured and used to get some information about the anatomical, physiological or biochemical process of interest.
From the basic laws of physics, unstable nuclides may take either of two pathways in order to become stable. The first pathway involves conversion of a proton to a neutron resulting in the release of a positron particle, usually represented as e+ or β+. And when a positron hits an electron, there is a kind of obliteration (annihilation effect) of both particles but with the release of an equivalent energy. The energy is usually in the form of two gamma rays or photons which possess the same energy of 511 keV and in opposite directions (Liu et al. 2009). This is probably a reason why positron emission tomography is referred to as dual emission tomography. To achieve this, only positron-emitting radionuclides are used or indicated in positron emission tomography such as Nitrogen-13, Fluorine-18, Oxygen-15, and Carbon-11.
Based on the production of positrons, PET machines are installed with two detectors on opposite sides so that the direction from where the gamma ray is being developed can be detected with very high precision (Robertson et al. 2009). Unlike a typical camera which makes use of photographic films, PET scanners and even SPECT scanners make use of highly sensitive scintillation detectors. These detectors are made up of materials which have dense crystalline properties. Some of the materials include cesium fluoride and bismuth germanium oxide. The main role of these materials is to sense the invisible gamma rays and convert them to rays within the visible light spectrum. The visible light ray is then converted to a pulse of electric signal by a photomultiplier tube (PMT). Therefore, the detector can be said to be made up of the dense crystalline material and the photomultiplier tube. As mentioned earlier, annihilation events are better picked up when more than one detector is used. All the information gotten from the procedure is sent to computer which can later be used to produce clear and real images using basic computed tomography techniques.
Single photon emission computed tomography (SPECT) is very much similar to positron emission tomography (PET), however, while PET involves gamma rays, SPECT involves the use of beta emitters and the use of more specialised equipment. It also makes use of a gamma camera which rotates around the body of the person undergoing the procedure. The rotating gamma camera can be compared to the detector in a computerised tomography machine which obtains images at different axial levels. SPECT is very valuable in medical diagnosis because of its special ability to select, with pin-point accuracy, the exact point where there is an anatomic or physiologic abnormality by making a series of bi-dimensional slices which are then converted and reconstructed by a computer to form three-dimensional images (Minarik et al. 2008). The SPECT machine can be said to be an advancement over the PET scanner and the computerised tomography machine, as it combines both principles.
Unlike PET which makes use of radionuclides that produce positrons, SPECT makes use of proton-rich radionuclides that produce a single gamma photon (ray) (Iagaru et al. 2009). Radioactive isotopes that achieve ground state configuration or decay by means of electron capture with or without gamma emission are used more often in SPECT, and examples include majorly Technetium-99m and Iodine-123. The spatial resolution of the detectors used in SPECT does not possess a comparable theoretical limit because the gamma photons which are produced are emitted directly from the location of decay. This is another reason why the detection instruments used in SPECT differs from the ones in PET. SPECT employs the use of a collimator which is placed between the radiation detector and the subject of interest (Okarvi & Jammaz 2009). The collimator has several minute holes which selectively allow the passage of photons travelling on a parallel trajectory to reach the detecting surface from the subject.
The resolution of quality of the images produced by either of the two methods is determined by the physical and chemical properties of the radionuclides used. The energy emitted by the radionuclide determines the image quality. Also, the interactions of the radionuclides are also important. Some of the physical characteristics that influence image quality include attenuation, partial volume effects, scatter, spatial resolution, and radioactive decay time.
The basic of the effects caused by attenuation is determined by the interactions of the photons with the detectors. Not all the emissions from the radionuclides get to the PET or SPECT detectors. This occurs as a result of the interactions (which may be photoelectric absorption or Compton scattering) between the emissions and surrounding tissues. Only a very small fraction of the emitted radiation gets to the detectors. Also, due to the difference in density between the various body matters, attenuation through bone, soft tissue, air and fluid differs. Tissue thickness also influences the image resolution. Cardiac muscles have varied thickness, as the ventricles are thicker and bulkier than the atria. Therefore, the intensity of radiation emitted from a thicker portion would be different from the radiation emitted by a radionuclide absorbed by a less thick portion.
Since all radionuclides have different half-lives, inconsistencies in images produced by the diagnostic imaging modalities may arise. However, in terms of patient safety, radionuclides with very short half lives are preferred to those with longer radioactive decay time (Buscombe, Hirji, & Witney-Smith 2008). Those with short radiodecay time give out a reduced total dose to the patient undergoing the procedure, compared with those with longer half-lives.
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- Dr P. Ronald (Author), 2010, Myocardial Perfusion Imaging Using Different Radionuclides, Munich, GRIN Verlag, https://www.grin.com/document/368310