Radiographic Techniques for Upper Limbs - Lecture 2 PDF

 contrast  Contrast is the difference in density between structures of interest within the image.  A high contrast image has a greater difference between the grey shades displayed  but a smaller range of greys.  A low contrast image has a smaller difference (i.e. it’s more difficult...

 contrast  Contrast is the difference in density between structures of interest within the image.  A high contrast image has a greater difference between the grey shades displayed  but a smaller range of greys.  A low contrast image has a smaller difference (i.e. it’s more difficult to make out different areas)  but a larger range of greys.  contrast contrast The contrast seen on the radiographic image ( film) or digital display can be described in three ways. 1- subject contrast Is features of object ( subject) under examination , the difference in radiation intensities emerging from the subject. 2- subjective contrast In the personal appreciation of the differences in the optical density or computer screen brightness when the image is reviewed 3- radiographic contrast the differences in the optical density on different parts of the processed film or in computer brightness recorded resolution Resolution is the measure of how far apart two objects must be before they can be seen as separate details in the image. noise Its is the variation in the level of grey in the image that are distributed over the area but unrelated to the structures being imaged. noise Reducing the proportion of noise in an image will improve the quality. The main way to achieve this is to increase the number of photons detected  noise  Increasing the dose (mA): higher number of photons and smaller proportion of noise  Using an image receptor with a greater attenuation coefficient: more photons are absorbed and converted into a signal  Make the image receptor thicker: again, more photons will be absorbed and converted into a signal  Using larger detector elements: more area to absorb photons per pixel. exposure factors  The radiation output from the x ray tube is product  1- current applied to the x ray tube ( measured in milliamps )  2- the duration of exposure ( measured in seconds)  3- the voltage applied to the x ray tube (measured in kilovoltage ) exposure factors  The radiation output is normally known as the intensity of the x ray beam and need to be varied to enable different body parts to be imaged. Factors affect the intensity of radiation beam.  1- the medium through which the beam passes  2- the distance between the x ray source and patinet  3- if the grid of bucky is used  4- filtration is applied to the x ray unshapness  The boundaries between a dark and a light area may be ill-defined, resulting in a blurred edge.  There are four causes of unsharpness:  1- Geometric unsharpness  2- Image receptor unsharpness  3- Movement unsharpness  4- Edge unsharpness  distortion  distorted image will be produced if not all parts of the image are magnified by the same amount.  artifact  There are a variety of patient and system factors that can create artefacts:  1- Motion artefact  2- Double exposure  3- Grid cut off  4- Radio-opaque objects on or external to the patient