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3 X-RAY COMPUTED TOMOGRAPHY
Pages 23-36

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From page 23...
... The technological improvements in the first 20 years of CT can best be characterized by a comparison of typical scan parameters from 1972 and 1994, as shown in Table 3.1. The greatest progress has been made in reducing scan times and in improving spatial resolution.
From page 24...
... In CT scanners the x-ray attenuation according to equation 3.2 is measured along a variety of lines within a plane perpendicular to the long axis of the patient with the goal of reconstructing a map of the attenuation coefficients a for this plane. The resulting attenuation coefficients are usually expressed with reference to water: apatient-awater a fin HU)
From page 25...
... The image then can be reconstructed either after rearranging the central projections into paraHel projections with parallel beam reconstruction techniques, or more directly by using the central projections with a modified fan beam reconstruction algorithm. A variant of the fan beam system employs a number of x-ray tubes and detector arrays rotating around the patient with the x-ray focal spots shining through the gaps be ~ Beam hardening is the effect of different attenuations of the different parts of the x-ray spectrum when the x-ray passes through an object.
From page 26...
... This configuration allows for a shorter exposure time since the system has to be rotated only through a small angle rather than 1800 or more in order to obtain a complete data set for image reconstruction. However, because of the very high cost involved and because of the improved speed of slip ring or electron beam based systems, this variant of the conventional fan beam system has not gained wide distribution.
From page 27...
... Compared to the rare-gas ionization detector, the scintillation detector usuaBy has a Tower geometric efficiency (ratio of active versus ineffective frontal area) , which is, however, generally more than compensated for by the latter's higher quantum efficiency.2 In fan beam systems a collimator is usually employed in front of the scintilIator elements to reduce the effects of scat 2The quantum efficiency is a measure of detector (material)
From page 28...
... A reduction of the noise level requires an increase in x-ray intensity or data acquisition time, along with an increase in patient dose. The signal dependency of the quantum noise leads to typical radial noise structures in the reconstructed images.
From page 29...
... For example, cletector-related problems in fan beam systems usually manifest themselves as ring-shaped artifacts, whereas in systems based on stationary ring detectors they will have the character of streaks or stripes. 3.2.4 Quantitative CT Although in the overwhelming majority of all examinations the resulting images are inspected only visually and qualitatively by the radiologist providing the diagnosis, a few special applications require absolute evaluation of the attenuation coefficients.
From page 30...
... The requirement for short acquisition times comes both from the necessity to effectively freeze the motion of organs like the heart and from the desire to cover large areas of the patient in the shortest possible time to improve patient comfort and patient throughput. To avoid the relatively greater effects of quantum noise associated with shorter scan times, the most critical and technically most demanding requirement is improvement to both instantaneous and sustained x-ray power from the tube or electron beam system.
From page 31...
... Such imaging is achieved by moving the patient slowly through the gantry while continuously acquiring data. The resulting helical movement of the x-ray tube focal spot relative to the patient allows rapid volumetric data acquisition over large areas of the body.
From page 32...
... 3.4 Electron Beam Techniques With conventional techniques, scan times cannot be shortened arbitrarily, because considerable masses have to be accelerated and moved, and because higher continuous x-ray power is difficult to achieve with the current rotating anode tubes. A technical alternative to overcome these problems and facilitate subsecond scanning is the use of electron beam techniques to replace conventional x-ray tubes.
From page 33...
... Further research into faster filtered backprojection implementations or the increased use of Fourier domain-based reconstruction methods, along with an increase in fairly inexpensive processor speed, should help to close this obvious gap in the near future. Similarly, as with conventional CT scanners, the development of advanced detector array technologies will improve the spatial sampling of electron beam type systems.
From page 34...
... Applications of CT angiography established to date range from the diagnosis of renal artery stenosis to examinations of complex vascular abnormalities and aortic aneurysms. 3.6 Research Opportunities To summarize, the following areas of scientific or technological development appear to have the greatest potential for enhancing the applications an clinical usefulness of x-ray CT: · increased instantaneous and, even more importantly, sustained x-ray power capabilities in both conventional x-ray tubes and electron beam systems; New and innovative technologies for the required high-intensity x-ray source, beyond those currently available; · Two-dimensional detector arrays encompassing a larger solid angle and allowing improved spatial resolution along the scanner axis, inclu(ling the associated high-throughput data acquisition electronics; · Decrease in image reconstruction times, for example, through higherperformance Tow-cost processors and the increased use of Fourier domain based reconstruction algorithms; · Mathematical means for utilizing the information contained in scatteret1 photons; · Effective means for the correction of patient motion-relate(1 artifacts; · Better and easier-to-use three-dimensional data reduction and visualization techniques; .
From page 35...
... (ells.) , Ultrafast Computed Tomography in Cardiac Imaging: Principles and Practice, Futura Publishing Co., Tnc., Mount Kisco, N.Y., 1992.


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