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: Iterative reconstruction Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Stu...

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: Iterative reconstruction Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Students of second class 79 Iterative reconstruction Another approach to image reconstruction is based on iterative techniques. Iteration is defined as a procedure in which repetition of a sequence of operations results in values successively closer to a desired result. Said another way, iteration is a computational, mathematical procedure in which a cycle of operations is repeated, often, to approximate the desired result more closel. An iterative reconstrution starts with an assumption (for example, that all points in the matrix have the same value) and compares this assumption with measured values, makes corrections to bring the two into agreement, and then repeats this process over and over until the assumed and measured values are the same or within accept- able limits Consider the following numeric illustration: 1. Initial estimate: Compute the average of four elements and assign it to each pixel, that is, 1 + 2 + 3 + 4 = 10; 10/4 = 2.5 80 2. First correction for error (original horizontal ray sums minus the new horizontal ray sums divided by 2) = (3 5)/2 and (7 5)/2 = 2/2 and 2/2 = and 1.0 3. Second estimate: 4. The second correction for error (original vertical ray sums minus new vertical ray sums divided by 2) = (4 5)/2 and (6 5)/2 = 1.0/2 and +1.0/2 = 0.5 and +0.5: The final matrix solution is thus In this technique, repeated estimations of the x ray photon counts that would be acquired in each projection are calculated , and compares them with actual measured counts (forward the projection) acquired by the detector array. At each step, the ratio of estimated to actual x ray counts is used to formulate a correction factor that is used to create the next estimate (back projecting the ratio). 81 This process is repeated over and over again resulting in movement of the estimated x ray photon count distribution ever closer to the actual, measured photon count distribution, Fig. 1. Fig. 1: Iterative reconstruction techniques used in CT. Today, iterative reconstruction algorithms have resurfaced because of the availability of high-speed computing. The primary advantages of iterative image reconstruction algorithms are to reduce image noise and minimize the higher radiation dose inherent in the filtered back-projection algorithm. 82