DK1212_C004

DK1212_C004 - 4 X-Ray Computed Tomography X-ray computed...

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155 4 X-Ray Computed Tomography X-ray computed tomography (CT) became the historically first tomo- graphic modality entirely based on digital reconstruction of images. It has qualitatively changed the field of medical imaging. Contemporarily, it is probably the most common computerized tomographic modality, with excellent spatial resolution, fast image acquisition enabling even real-time imaging, and rather generic application. For a deeper study see, besides other rich literature, [4], [6], [7], [9], [10], [21], [26], [27], [30], the main sources used in this chapter. 4.1 IMAGING PRINCIPLE AND GEOMETRY 4.1.1 Principle of a Slice Projection Measurement Classical x-ray radiography provides attenuation images that are combinations of projections of all layers forming the thickness of the object (see Section 3.1.1). There is no way to separate the infor- mation from different layers, except by subjective evaluation using a priori anatomical knowledge. It is possible to enhance the image of an individual layer by blurring images of other layers on special x-ray equipment, where the radiation source and detection plane © 2006 by Taylor & Francis Group, LLC
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156 Jan are moved simultaneously with respect to the patient during expo- sition in a proper way (classical x-ray tomography). Nevertheless, besides other drawbacks, this does not enable the determination of the spatial attenuation distribution, which would allow the recon- structing of images for any desired plane exactly. Computed tomog- raphy (CT) solves the problem of determining this inner distribution by measuring individual object-influenced intensity values for many differently oriented rays, and subsequently reconstructing the data on internal attenuation distribution computationally. The basic principle of CT measurement can be seen in Figure 4.1. The x-ray source, together with primary collimators, provides a fine beam of radiation (ideally an infinitesimally narrow ray) that passes Figure 4.1 Principle of measurement of projections — basic rectangular arrangement. I m ( τ ) Θ a projection X-ray source primary collimator X-ray beam X-ray detector secondary collimator a slice of object rotating frame r © 2006 by Taylor & Francis Group, LLC
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X-Ray Computed Tomography 157 the object, the intensity of the beam is then measured by a detector. Even with a good primary collimation, the beam is always slightly divergent, thus increasing the diameter of the measured volume of the object when approaching the detector side. This is compensated for by the secondary (detector) collimator, which excludes all the rays outside of the desired beam, in this way also suppressing the radi- ation scattered along the way through the object. The effective diam- eter of the resulting beam may be in the range of about 0.5 to 10 mm.
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This note was uploaded on 02/27/2008 for the course BME 525 taught by Professor Singh during the Fall '07 term at USC.

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DK1212_C004 - 4 X-Ray Computed Tomography X-ray computed...

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