Computed Tomography Equipment Techniques PDF

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: Interpolation Algorithms Pitch Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target populatio...

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: Interpolation Algorithms Pitch Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Students of second class 23 Scientific Content: Interpolation Algorithms Reconstruction of an image at any z-axis position is possible because of a mathematical process called interpolation. Figure (1) presents a graphic representation of interpolation and extrapolation. If one wishes to estimate a value between known values, that is an interpolation; if one wishes to estimate a the value beyond range of known values, that is an extrapolation. Fig(1): Interpolation estimates a value between two known values. Extrapolation estimates a value beyond known values. During helical CT, image data are received continuously, as shown by the data points in Figure 2A. When an image is reconstructed, as in Figure 2,B, the plane of the image does not contain enough data for reconstruction. The data in that plane must be estimated by interpolation. Data interpolation is performed by a special computer program called an interpolation algorithm. 24 Figs.(2): A, During multislice helical computed tomography, image data are continuously sampled. B, Interpolation of data is performed to reconstruct the image in any transverse plane. Image interpolation creates a number of new slices between known slices in order to obtain an isotropic volume image. The problem with continuous tube and table motion was that projections precessed in a helical motion around the patient and did not lie in a single plane. This meant that conventional reconstruction algorithms could not work. Helical CT scanning produces a data set in which the x-ray source has travelled in helical trajectory around the patient, (the data are acquired in a helical path around the patient). Present day CT reconstruction algorithms assume that the x-ray source has negotiated a circular not a helical path around the patient. To compensate for these differences in the acquisition geometry, before the actual CT reconstruction the helical data set is interpolated into a series of planar image data sets (the reconstruction plane of interest). Interpolation is essentially a weighted average of the data from either side of the reconstruction plane, with slightly different weighting factors used for each projection angle. 25 Figs.(3): Data interpolation In summary: Interpolation Algorithms are the mathematical process required to reconstruct axial images from the spiral volume data set. 26 Pitch During helical scans, the table motion causes displacement of the fan beam projections along the z axis; the relative displacement is a function of the table speed and the beam width. The ratio of table displacement per 360° rotation to section thickness is termed pitch. Pitch is the table movement per rotation divided by beam width. pitch = table travel / beam width pitch = 1 - coils of the helix are in contact pitch < 1 - coils of the helix overlap pitch > 1 - coils of the helix are separated For example If beam width is 10cm, the table moves 10cm during one tube rotation, then pitch is 1, so, x-ray beam associated with consecutive helical loops are contiguous. If beam width is 10cm and table moves 15cm per tube rotation, then pitch is 1.5 So, a gap exists between the x-ray beam edge of consecutive loop. If beam width is 10cm and table moves 7.5cm then pitch is 0.75, so, beams and consecutive loops overlap by 2.5 (doubly irradiating the underlying tissues). Fig (4): Illustration of pitch concepts 27 The relationship between the volume of tissue imaged and pitch is given as follows: Advantages of helical CT scanner Fast scan times and large volume of data collected. Minimizes motion artifacts. Less mis-registration between consecutive slices. Reduced patient dose. Improved spatial resolution. Enhanced multiplaner or 3D renderings. Improved temporal resolution 28