Computed Tomography PDF

Computed Tomography  COMPUTED  TOMOGRAPHY  Physical Principles : BY AHMED JASEM ABASS  MSC of Medical Imaging You don’t have to know how to be build a car to be a good driver.  CT scanners were first introduced in 1971 with a single detector for brain study under the...

Computed Tomography  COMPUTED  TOMOGRAPHY  Physical Principles : BY AHMED JASEM ABASS  MSC of Medical Imaging You don’t have to know how to be build a car to be a good driver.  CT scanners were first introduced in 1971 with a single detector for brain study under the leadership of Sir Godfrey Hounsfield, an electrical engineer at EMI (Electric and Musical Industries Ltd). After that, it has undergone multiple improvements, with an increase in the number of detectors and a decrease in the scan time. What is a CT scanner?  an X-ray device capable of cross-sectional imaging  creates images of slices through the patient Why CT?  conventional radiography suffers from collapsing of 3D structures onto a 2D image  although the resolution of CT is lower, it has extremely good low contrast resolution enabling the detection of very small changes in tissue type  CT gives accurate diagnostic information about the distribution of structures inside the body  In conventional X-ray imaging, the entire thickness of the body is projected on a film : structures overlap and are difficult to distinguish). One of the problems is the loss of information about depth.  Tomography It is a non-invasive medical imaging modality that combines the use of X-rays and computer processing to generate tomographic (‘slices’) of the area scanned.  Tomos = slice; Graphein = to write  Definition - imaging of an object by analyzing its slices.  A CT Scanner is  CAT: Computerized Axial Tomography  Spiral CT.  Multi-Slice CT or Multi-Detector. Matrix  The image is represented as a MATRIX of numbers.  Matrix: A two dimensional array of numbers arranged in rows and columns.  Each individual element or number in the image matrix represents a three dimensional volume element in the object, called a VOXEL.  The VOXEL is represented in the image as a two-dimensional element called PIXEL - (picture element).  Field of view (FOV) is the diameter of the body region area being imaged (e.g., 25 cm for a head or 40 cm for an abdomen).  CT pixel size is determined by dividing the FOV by the matrix size, which is generally 512 x 512 in CT. Pixel vr Voxel  PIXEL Size Dependencies:  MATRIX SIZE  FOV  VOXEL Size Dependencies  FOV  MATRIX SIZE  SLICE THICKNESS Attenuation  Attenuation is the reduction of the intensity of an x-ray beam as it traverses matter. The reduction may be caused by absorption or by deflection (scatter) of photons from the beam and can be affected by different factors such as beam energy and atomic number of the absorber. CT numbers  The numbers in the image matrix are called CT numbers.  Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object.  To obtain a visual image, the CT numbers are assigned different shades of gray on a gray scale.  Each shade of gray represents the x-ray attenuation within the corresponding voxel.  This number is compared to the attenuation value of water and displayed on a scale of units named Hounsfield units (HU) after Sir Godfrey Hounsfield.  This scale assigns water as an attenuation value (HU) of zero. The range of CT numbers is 2000 HU wide although some modern scanners have a greater range of HU up to 4000. Each number represents a shade of grey with +1000 (white) and –1000 (black) at either end of the spectrum. Hounsfield values (HU) CT numbers Windowing  Windowing is the process of using the calculated Hounsfield units to make an image.  The various radio density amplitudes are mapped to 256 shades of gray. These shades of gray can be distributed over a wide range of HU values to get an overview of structures.  Alternatively, these shades of gray can be distributed over a narrow range of HU values (called a "narrow window") centered over the average HU value of a particular structure to be evaluated. In this way, variations in the internal makeup of the structure can be discerned. This is a commonly used image processing technique known as contrast compression. Windowing (Cont..)  For example, to evaluate the abdomen in order to find Small masses in the liver, one might use liver windows. Choosing 70 HU as an average HU value for liver, the shades of gray can be distributed over a narrow window or range. One could use 170 HU as the narrow window, with 85 HU above and 85 HU below it, with 70 HU average value; Therefore the liver window would extend from -15 HU to +155 HU.  All the shades of gray for the image would be distributed in this range of Hounsfield values.  Any HU value below -15 would be pure black, and any HU value above 155 HU would be pure white in this example.  Using this same logic, bone windows would use a "wide window" (to evaluate everything from fat-containing medullary bone that contains the marrow, to the dense cortical bone). Windowing  Window level is CT number selected for centre of the range of numbers displayed on the image.  Window width is total range of values selected.  Width determines contrast.  A narrow window enhances inherent contrast.  Window level determines the brightness. Window level (WL) and window width (WW) These two images are of the same section, viewed at different window settings. (A) A window level of +40 with a window width (B) of 350 reveals structures within the mediastinum but no lung parenchyma can be seen. (B) The window level is –600 with a window width of 1500 Hounsfield units. This enables details of the lung parenchyma to be seen, at the expense of the (A) mediastinum. Imaging System  Scanner:  Gantry.  Patient Couch.  Gantry Houses:  X-ray Tube.  Generator.  Filter.  Collimators.  Detectors.  Patient Couch :  450 Pounds (204 Kg) Distributed Weight Limit.  Scannable Range: Coverage From Head To Thigh (162 Cm). Gantry Characteristics  Tilting Range, (Tilting Range Of Most Scanners +30 To - 30 Degrees).  Aperture, (Most Of The Scanners Have 70 cm Aperture). Thank You