Radiology CME Basics of computed tomography (CT) Vikas ... - IJCCI!

Radiology CME Basics of computed tomography (CT) Vikas Chaudhary, Specialist and Head, Department of Radiodiagnosis, Employees’ State Insurance Corporation (ESIC) Model Hospital, Gurgaon-122001, Haryana, India. E-mail: [email protected] International Journal of Clinical Cases and Investigations 2013. Volume 5 (Issue 4), 4:5, 1st January 2014 Keywords : Computed tomography, Hounsfield unit, window level, window width, rays

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Principles of CT scanning: CT scanning uses a computer to reconstruct cross-sectional images of the body from measurement of x-rays transmitted through thin slices of the patient tissue. As a narrow, thin- collimated beam of x-ray on one side passes through the patient, it is attenuated by absorption. The sensitive detectors on the opposite side of the patient measure x-ray transmission through the slice. Multiple pencil or fan x-ray beams are passed from different directions through a desired volume, while the x-ray tube rotates 360° around the patient. The three dimensional volume or transverse slice of the patient tissue is considered to be made of multiple ‘voxels’ that have a width & height (in x-y plane), and depth (along z axis; equals slice thickness). Each beam passes through a number of voxels as it traverses the volume. Using this data (of transmitted x-rays from different directions), high computational power computers then calculate (by simple mathematical equations) the amount of x-ray attenuation done by each voxel. Once the attenuation for each voxel is determined, the computer assigns a Hounsfield unit (HU) to each voxel. Thereby, each voxel is assigned a shade of gray by the computer based on the degree of x-ray attenuation done by that voxel. Although different tissues within a voxel attenuate at different rates, a voxel is assigned only one shade of gray/HU based on the average attenuation done by all the tissues within that voxel. In Hounsfeld scale, tissue density is expressed in different shades of gray in relation to its x-ray absorption. The scale ranges from -1000 (air), to 0 (water), to +1000 (cortical bone). ‘Pixel’, a picture element, is 2D projection of a voxel on the computer screen that has only width and height. An anatomic image is made up of a matrix of picture elements (pixels), each of which represents a volume element (voxel) of patient tissue. The resultant image represents shades of gray in a range of pixel values described as window-width. Window-width influences the contrast of the image. Pixel with values greater than the upper limit of the window-width are displayed white, and pixel with values less than the lower limit of the

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window-width are displayed black. The center of the gray scale is described as windowlevel; it should be set as close as possible to the HU of the tissue to be examined. To analyze optimally all of the anatomic information of any particular slice, the image is viewed at different window-width and window- level settings. CT images are acquired usually in axial plane; however, images may be reformatted in sagittal, coronal, or oblique planes or as 3D image.1 In 1991, the introduction of volumetric CT scanning, using spiral or helical scanners, has revolutionized the diagnostic imaging. Helical CT (volume data acquisition in single breath hold) has several advantages over conventional CT (one slice per breath hold). Helical/spiral CT has faster gantry rotation, more powerful x-ray tubes, improved interpolation algorithms, dramatically improved speed of image acquisition, decreased incidence of misregistration between consecutive axial slices, improved spatial resolution, reduced patient dose & volume of intravenous (IV) contrast and enhanced multiplanar and 3D reconstruction. Multidetector helical CT (MDCT) is a recent advancement in CT imaging; it utilizes the principles of the helical scanner but incorporates multiple rows of detectors. Available system has moved from two slices to 64 slices. 256 slice scanners have also been developed and are now available. The key advantage of MDCT is the speed (it is 5-8 times faster than single-slice helical CT); however, radiation dose is 3-5 times higher with MDCT than with single-slice CT. Retrospective creation of thinner or thicker sections from the same raw data and improved 3D rendering with diminished helical artifacts are additional advantages. Moreover, large coverage area and high resolution allows high-detail CT angiography, cardiac imaging, virtual CT colonoscopy, bronchoscopy and temporal bone imaging.2 CT artifacts degrade the image and may cause errors in diagnosis. Volume averaging is present in every CT image. Slices above and below the image that is being interpreted must be examined for sources of volume averaging that may be misinterpreted as pathology. A beam-hardening artifact is seen as low density streaks extending from structures of high attenuation like bone. A motion artifact is demonstrated as blurred or duplicated image, as a result of voluntary or involuntary patient movement, breathing, cardiac motion, vessel pulsation or peristalsis. Streak artifacts arise from high-density sharp-edged objects, such as shotgun pellets, vascular clips or artificial denture.3 Advantages & Disadvantages of CT over MRI: The advantages of CT compared with MR include rapid scan acquisition, superior cortical bone details and demonstration of acute hemorrhage/calcifications. Main disadvantages of CT are relative high amount of ionizing radiation and contrast allergy which may occasionally be fatal.1-3 References: 1. Miraldi F, Sims MS, Wiesen EJ. Imaging principles in computed tomography. In: Haaga JR, Lanzieri, CF, Gilkeson RC, eds. CT and MR imaging of the whole body, 4th ed. St. Louis, MO: Mosby, 2003:2-36. 2. Rydberg J, Buckwalter KA, Caldemeyer KS, Philips MD, Conces DJ Jr, Aisen AM, et al. Multisection CT: scanning techniques and clinical applications. RadioGraphics 2000;20:1787-806. 3. Barrett JF, Keat N. Artifacts in CT: recognition and avoidance. RadioGraphics 2004;24:1679-91.

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