Hybrid Technology in Nuclear Medicine - Assignment Example

Hybrid Technology in Nuclear Medicine Essay Introduction Medical imaging is the mainstay of clinical medicine. It is now unquestionable that in the management of all diseases accurate investigation and diagnosis play the role of cornerstones. In certain specialties, physical imaging techniques play central role, not only in diagnosis but also in planning and monitoring of the treatment. The diagnostic medical imaging began with ionizing x-radiation. It is still widely used as the conventional method of radiologic imaging. With the advent of new technologies and better understanding of the physics of imaging technologies, there had been new methods of diagnosis that manipulated cellular metabolism and nuclear components of the cells at different stages of pathologic alterations to that opened new vistas of nuclear medical imaging. However, this progress was not easy since it called for specialized skills of physicists, engineers, and chemists, whose collective knowledge created a hybrid system in collaboration with medical expertise, primarily to complement medical management of diseases, and since then diagnostic medical imaging became a team activity (Beyer et al., 2000). In this assignment, the introduction of hybrid technologies such as PET and SPECT will be reviewed from current literature with the background physical principles applicable in these diagnostic imaging modalities, and it will be investigated how the basic principles of computerized tomography has advanced based on these platforms. Innovative technologies have their benefits, but implementing them in the clinical areas used to convention is always a challenge. This assignment will explore this trade-off based on current advances so an idea of future developments and implications of these technologies are evident to the reader. X-ray Imaging applies the concept that human body serves as a complex object. Human body demonstrates the characteristics of any other physical object. These include transmissivity, opacity, emissivity, reflectivity, conductivity, and magnetizability. A specially designed imaging system that can capture the changes in these characteristics with disease would serve as an imaging device. The variations in these characteristics can be analyzed to yield information which can be clinically significant to lead to a pathologic diagnosis. In the X-ray that serves as the prototype of the initial investigative modality used basically cathode rays to create images of the human body parts on the fluorescent screens (Rappoport et al., 2004). This was effected by emission of x-rays from the cathode ray tube. X-ray can be attenuated by different materials of different densities and can be transmitted through and can be captured by a photographic plate. Later, it was established that X-ray is electromagnetic in nature. These comprise of photons with specific wavelengths, frequency, and energy. The electromagnetic spectrum of X-ray can be divided into several bands. Those with long wavelengths were used in magnetic resonance imaging. The next significant wave length is X-ray which is used in conventional radiography. Some ultra-short waves, which are high-energy gamma rays, are used in nuclear imaging (Valk et al., 2003). Physical Principles of X-ray If we consider the physical principles, X-rays are generated in an X-ray tube which comprises of a vacuum tube with a cathode and an anode in an environment of high potential difference. When electricity is passed, the cathode current releases electrons at the cathode through thermal excitation. The voltage difference between the cathode and the anode then drives these electrons in an accelerated fashion which then hit the anode plate to release the energy, which in part is X-ray. It is important to be aware that in order to produce an image from the attenuated X-ray beam, these beams need to be captured and converted to an image. Recent advents of technologies have enabled digital capture of images. Traditionally, however, screen-film detectors and the image intensifiers are still in use. X-rays images are taken on photographic films (Ball and Moore, 1998). However, photographic films are very inefficient in capturing the X-rays. The output image in a film is just 2% of the incident X-ray photons. This percentage of the photons indicates the absorption efficiency which is determined by the probability of an X-ray photon quantum absorbed by the detector. This raises a potential disadvantage of prohibitively large doses of radiation due mainly to the low sensitivity of the film. To circumvent this problem, an intensifying screen is used in front of the film which is designed to absorb most of the X-ray photons by the heavy chemical element in the screen. The X-ray film contains an emulsion of silver halide crystals which when exposed to light would absorb optical energy and undergo physical changes. Absorption of sufficient amount of phonons lead to darker spots called development centers. When the film is developed, in a given area of the film, the more the grains are involved, the darker is the image. This would lead to formation of a negative. In radiography, the final image is a negative image (Burger et al., 2002). Computerized Tomography Conventional radiography leads to planar images, where certain clinical situations will demand three-dimensional images of the pathologic process in an organ system. To solve this problem, X-ray computed tomography was devised as a modality, which produces cross-sectional images representing the X-ray attenuation properties of the body. The basic principles involved in this modality are X-rays are produced by an X-ray tube, attenuated by the patient's body, and detected by an X-ray detector (Hsieh, 2003). Using ultrathin X-ray beams, the entire field of view is scanned in a set of lines in basically two geometric fashions. These are parallel beam geometry and fan beam geometry. This process is repeated in a large number of angles which enables measurements of line attenuation at all possible angles of the field of view for all possible distances. These measurements then can be computed to reconstruct the actual attenuation of at each given point of the scanned slice. Other imaging modalities, such as, MR, PET, and SPECT use this principle of image reconstruction, the term CT is used for X-ray computerized axial tomography (Hendee and Ritenour, 2002). The utility of the CT techniques actually is comprehensible from its clinical use. Actually, the main virtue of CT is its ability to produce a series of cross-sectional images of any region of the human body. Projection images are also taken in radiography, but in CT, true 3D images can be reconstructed with a better and enhanced contrast. This is mainly used to acquire anatomic images of different parts of the body. The physical principles of CT are based on X-ray attenuation, and therefore, the same contrast agents used in conventional radiographic imaging can be used in CT (Kalender, 2006). The most important benefit of the CT is to gain highly sensitive images of the minor intensity differences in contrast to radiographic images. Therefore, enhancement of contrast is feasible with small concentrations contrast agents. Thus, this technique would be able to detect even subtle pathological changes, small anatomical differences, changes in diffusion volume, alterations in perfusions, or even changes in functional diffusion. However, this display of anatomical changes comes at the cost of high radiation doses, which are in order of 10 to 100 times higher than the radiation dose required for radiographic images for a given anatomic area. Although a very important diagnostic modality, the potential harm from high radiation dosage cannot be ignored. While the image quality is a very important determinant of this imaging modality, correct use of equipment, optimal maintenance of the equipment, use of dose limiting low integrated tube current, a limited scan range, and use of modulated tube current may limit the dose (Guy and Ffytche, 2005). Nuclear Imaging The evolution of radiologic imaging is essentially a journey from methods of imaging human anatomy with X-rays generated external to the body and passing through it to the modalities of imaging physiological functions using radiation that emanates or caused to emanate from inside the human body. The initial modalities used tracer doses of radioisotopes, and the imaging technologies involved a range of techniques that led to production of images of the distribution of these radionuclide-labeled agents in the area of view. Evidently, these techniques are designed to demonstrate physiological processes and functions of the individual organs and would be determined by blood flow, blood volume, and other metabolic processes (Townsend et al., 2003). This array of techniques not only demonstrates the anatomy, these have predilections for physiologic and metabolic processes at the cellular level. The early development of these imaging equipment included developed of rectilinear scanners and the scintillation cameras. Analyzing the principles of most commonly used imaging of this category, the planar static imaging, we can consider that these comprise of single-view images consisting of two-dimensional distributions of projections of three-dimensional distribution of activity in the detector field of view (Townsend and Cherry, 2001). Principles Since the 1950s, technological improvements in the field led to development of gamma camera which has over the time evolved into Anger scintillation camera and the 2D planar detector to produce a 2D projection image without scanning. Applying the principles and methods of X-ray computed tomography; the Anger camera can use the projection images to compute the original spatial distribution of the radionuclide within a slice or a volume. The preceding tomographic system is known as SPECT that stands for single photon emission computed tomography (Carney and Townsend, 2003). It was subsequently demonstrated that two such scintillation cameras may be combined to detect a pair of photons that originate following emission of positron. This phenomenon was used to develop Positron Emission Tomography or PET which detects photon pairs. Since the 1970s, PET systems were developed for human studies and since the last decade it has been in regular clinical use (Townsend et al., 2003). PET In positron emission tomography or PET, the decay of the radionuclide tracer which essentially is an unstable molecular isotope following its involvement in the metabolic process would decay and would produce gamma rays. This would allow the remaining concentration of the tracer as a function of position and time. Two approximately antiparallel gamma rays each of 511 keV in vivo can then be measured by surrounding array of scintillation detectors provides each detector remains in electronic coincidence with those placed on the opposite side of the subject. Although, there are apparent benefits with PET imaging, the very principle of planar scintigraphy involves production of a 2D projection of a 3D object distribution. This can seriously jeopardize the image quality since there is considerable superimposition of non-target activity. This would restrict the measurement of the organ function and would prohibit accurate quantification of the function. SPECT, on the contrary utilizes multi-cross-sectional images of tissue function, thereby eliminating the effects of overlying and underlying radioactivities causing superposition artifacts (Townsend, 2001). SPECT SPECT involves the use of 99 Pertechnetate from which a single gamma ray is emitted per disintegration, whereas in PET radioisotope such as 68 Gallium leads to emission of two gamma rays when the positron arises from nuclear disintegration. In SPECT, two general types of imaging devices are used, limited angle or transaxial. In the limited angle, photons are detected within a limited angular range simultaneously from several sections of the body leading to images that are constructed parallel to the face of the detector. In contrast, in the transaxial SPECT, the detectors surround the body and hence move around to accomplish a complete 360 degree angular sampling of the emanating photons from single or multiple sections of the body. Consequently, the constructed image planes are perpendicular to the face of the radiation detectors. The imaging parameters improve considerably with these variations against PET, and there are discernible contrasts between regions of differing functions. Moreover, with SPECT, the observer can have better spatial localization, improved detection of abnormal function, and greatly improved quantification, although spatial resolution is compromised (Antoch et al., 2002). Challenges It is to be considered that this type of imaging has its own challenges. In comparison to X-ray, the number of detected photons is much smaller. Noise would play very important roles in quality of the images. The imaging process is considered to be stochastic in these methods. Collimation is very important in both PET and SPECT. In tomography, the position of the point source is known, and every photon that is detected may provide information about the projection line, which is a line that connects the source with the detection point. In PET and SPECT, the source has unknown spatial distribution. Therefore, some collimation is needed to be applied in order to detect the information about this distribution. In PET mechanical collimation is not needed; however, in SPECT collimation is done with a mechanical collimator which is a thick lead plate with holes designed to absorb all the photons not propagating parallel to the axes of the holes. This leads to absorption of most of the photons, and the sensitivity of the investigation suffers, In PET both photons are detected through an electronic coincidence circuit (Valk et al., 2003). Hybrid Technology PET and SPECT systems can be combined with a CT or MR system, and this new generation of imaging systems are known as hybrid systems. Several combinations are possible, and between the PET/CT and SPECT/CT, PET/CT system is currently popular and has been being increasingly used in clinical practice. The problem of attenuation correction has been solved with the use of CT image. The basic problem is long duration of examination, where motion artifacts may arise (Hawkes et al., 2003). Therefore, the immediate challenge that this modality faces is registration mismatch between the transmission and emission images. Although not straightforward, nonrigid registration may solve this issue. The next most important technical challenge is problems due to energy dependence of the linear attenuation coefficient. In PET, the photon energy is 511 keV. In the CT component of this hybrid system, the X-ray source transmits an energy spectrum with a maximum energy defined by the voltage in the tube. This problem has been attempted to be solved by approximation of single average effective energy from the X-ray energy spectrum, and the relationship between the attenuation coefficients at these effective energies is assumed to be piecewise linear (Hawkes et al., 2003). Clinical Examples The study of bone metabolism has been facilitated by combination of SPECT and CT scanners. This system utilizes the combination of metabolic information from the SPECT and anatomic information from the CT. This considerably improves the diagnostic accuracy of the bone disorders. PET is considered to be the gold standard to evaluate myocardial viability. A perfusion CT scan is the imaging of choice to detect pulmonary embolism (Coleman et al., 2005). Future There is possibility of continuous improvements and advancements in these imaging modalities. Research is underway to develop improved systems by developing TOF and hybrid systems, new detectors, and provision of elimination of motion artifacts. The research indicates that progression in this area of imaging will mainly take place by development of new generation of tracers. As is apparent, more clinical indications will be created by labeling new compounds with PET tracers. There will also be a shift to more specific biomarkers that target to specific receptors at the cellular level (Beyer et al., 2000). Conclusion The successes of PET, SPECT, and other hybrid technology indicate the future of imaging. There has been immense progress in this area in the last two decades, and it is clearly indicated that clinical imaging will go beyond these developments with the goal being evolution towards visualization of the biologic processes at the cell level. This would open the avenues to study cellular function and molecular pathways in vivo. Certain techniques that are being researched are imaging of gene regulation, protein-protein interactions, and stem cell tracking (Coleman et al., 2005). This would lead to a new discipline called molecular imaging with an entirely new shift of focus from studies of anatomy to studies of function at the level of the molecules. Reference List Antoch, G., Freudenberg, LS., Egelhof, T., et al., (2002). Focal tracer uptake; a potential artifact in contrast-enhanced dual-modality PET/CT scans. J Nucl Med;43:1339-1342. Ball, J. and Moore, JD., (1998). Essential Physics for Radiographers. Oxford: Blackwell Science, 1998. 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An Introduction to the Principles of Medical Imaging. London: Imperial College Press, revised edition, 2005. Hawkes, DJ., Hill, DL., Hallpike, L., and Bailey, DL., (2003). Coregistration of structural and functional images. In: Valk P, Bailey DL, Townsend DW,Maisey MN, editors. Positron Emission Tomography: Basic Science and Clinical Practice. London: Springer:181-197. Hendee, WR. and Ritenour, ER., (2002). Medical Imaging Physics. New York: Wiley-Liss, fourth edition, 2002. Hsieh, J., (2003). Computed Tomography: Principles, Design, Artifacts, and Recent Advances. Cambridge: Cambridge University Press, SPIE Publications, SPIE Press Monograph, Volume PM114, 2003. Kalender, WA., (2006). Computed Tomography: Fundamentals, System Technology, Image Quality, Applications. Cambridge: Cambridge University Press, Wiley-VCH, SPIE Press Monograph Volume PM114, second edition, 2006. Rappoport, V., Carney, J., and Townsend, DW., (2004). 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