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: Reconstruction methods: Backprojection reconstruction Filtered Backprojection Name of the instructor: Lec. D...

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: Reconstruction methods: Backprojection reconstruction Filtered Backprojection Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Students of second class 72 Image Reconstruction The image obtained in CT is different from that obtained in conventional radiography, Fig.1, in which rays form an image directly on the image receptor. While with CT imaging systems, it is created from data received and represent a depictions of relative attenuation of x rays as they pass through the body. The x-rays from a stored electronic image that is displayed as a matrix of intensities. Fig. (1): The most conspicuous difference between conventional radiographic imaging and CT imaging. attenuating ability is related to its density and represents the likelihood that an x ray photon will pass through the tissue to be recorded by the detectors rather than the photon from -ray attenuating ability is expressed by its attenuation coefficient, tissue density, the hi 73 (explained earlier). The higher the However, the attenuation coefficient of a tissue is not constant and may be altered by the tissue thickness and the energy of the x ray photon (KeV). Image Reconstruction Techniques Image reconstruction is a mathematical process that generates tomographic images from x-ray projection data acquired at many different angles around the patient. The reconstruction process is based on the use of an algorithm that uses the attenuation data measured by detectors to systematically build up the image for viewing and interpretation. Image reconstruction involves several algorithms to calculate all the from a set of projection data. The algorithms applicable to CT include back-projection, iterative methods, and analytic methods. Currently, there are currently two forms of image reconstruction: Filtered back-projection (FBP) and Iterative reconstruction (IR). Back-Projection Back-projection is a simple procedure that does not require much understanding of mathematics. Back-projection, also called the summation method or near superposition method. Back- projection can be best explained with a graphical or numerical approach. Consider four beams of x rays that pass through an unknown object to produce four projection profiles P1, P2, P3, and P4 (Fig. 2). The problem involves the use of these profiles to reconstruct an image of the unknown object (black dot) in the box. 74 The projected datasets are back-projected to form the corresponding images BP1, BP2, BP3, and BP4. The reconstruction involves summing these back-projected images to form an image of the object. Fig.2: Graphic representation of the back-projection reconstruction technique. BP involves summing the data from hundreds of projection angles to reconstruct the image. Since the data from a projection angle of 0° is identical to the data from a projection angle of 180°, only the data from a 180° gantry rotation is necessary to reconstruct the full CT image. The displayed CT image is composed of the CT number data (Hounsfield unit data) from the summed projection information. 75 Back-projection can also be explained with the following 2 × 2 matrix: Four separate equations can be generated for the four unknowns, 1, 2, 3, and 4: A computer can solve these equations very quickly. BP advantages 40 slices per second). The problem with the back-projection technique is that it does not produce a sharp image of the object and therefore is not used in clinical CT. The most striking artifact of back-projection is the typical star pattern that occurs because points outside a high- density object receive some of the back-projected intensity of that object. 76 Filtered Back-Projection Filtered back-projection is also referred to as the convolution method (Fig. 3). The projection profile is filtered or convolved to remove the typical starlike blurring that is characteristic of the simple back-projection technique. The steps in the filtered back-projection method (Fig. 3, B) are as follows: 1. All projection profiles are obtained. 2. The logarithm of the data is obtained. 3. The logarithmic values are multiplied by a digital filter, or convolution filter, to generate a set of filtered profiles. 4. The filtered profiles are then back-projected. 5. The filtered projections are summed and the negative and positive components are therefore canceled, which produces an image free of blurring. Fig. 3: Back- and filtered back techniques used in CT. A, Back-projection results in an unsharp image.B, Filtered back-projection uses a digital filter to remove blurring, which produces a sharp image. projection 77 The image quality is acceptable, but not optimal and thus, its major disadvantage is its limitations in image quality due to the necessary filtering used with this technique. These filtering techniques accentuate noise and mandate the need for higher radiation doses to permit adequate image quality. The excess image noise using FBP results from the inaccuracy of several assumptions used in this technique that limit spatial resolution and lead to increased streak artifact and relatively poor low contrast detectability. FBP tends to falter in larger patients due to increased tissue attenuation and in intentional low radiation dose scanning, which is becoming more important as understanding and awareness of the effects of cumulative radiation dose are realized. However, the advantages and acceptability of FBP have traditionally limited the incentive to change reconstruction methods. However, with the increased numbers of CT scans and the advanced applications such as cardiac CT angiography, the importance of more radiation efficient reconstruction methods has been emphasized, mandating the onset of IR. 78