Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

When film-screen image receptors are used, an inadvertent overexposure of thepatient will result in a dark film, which provides immediate feedback to the technologistregarding the technique factors (and relative dose) used. However, whendigital image receptors are used, overexposure of the patient can produce excellentimages because the electronic systems compensate for (and essentially mask) largefluctuations in exposure. Consequently, high-exposure conditions may go unnoticedunless appropriate measures for periodic monitoring are taken. For this reason,a quality control program should be in place to ensure proper exposure levels.Most digital detector systems provide an estimate of the incident exposure that canassist in the evaluation of exposure trends. The exposures necessary to produce goodimages are directly related to the detective quantum efficiency (DQE) of the detector:Detectors with high DQEs make more efficient use of the x-rays and thereforerequire less exposure for adequate signal-to-noise ratios. X-ray exposure levelsshould be tailored to the needs of the specific clinical examination, in considerationof the digital detector and its DQE. For example, it is generally accepted that CRimaging systems require about twice the exposure of a corresponding 400-speedscreen-film detector for comparable image quality. The CR system is thereforeequivalent to a 200-speed screen-film system for most general radiographic examinations.Screen-film cassettes for extremity radiography are slower (i.e., requiremore radiation) than those used for general radiography, and higher exposure levelsare required for extremity imaging with CR as well (equivalent to 75- tolOO-speedscreen-film systems).Flat panel detectors can reduce radiation dose by about twofold to threefoldfor adult imaging, compared with CR for the same image quality, owing to thebetter quantum absorption and conversion efficiency associated with that technology.Hard copy display refers to displaying images on film, and soft copy display refersusing video monitors. Digital radiographic images produced by flat panel TFTarrays or by CR systems are significantly larger in matrix size than any other imagesproduced in the radiology department. For example, typical SPECT and positronemission tomography (PET) images are 128 X 128 pixels, MRl images are typically256 X 256 pixels, and CT images are 512 X 512 pixels. Digital subtraction angiographyimages are often 1,024 X 1,024 pixels. Digital chest radiographs using 100-~m pixels result in an image matrix of 3,500 X 4,300 pixels. A typical personalcomputer display screen is slightly larger than 1 X 1 k, and therefore could simultaneouslydisplay 64 full-resolution SPECT images, 16 full-resolution MRl images,or 4 full-resolution CT images. However, the 3.5- X 4.3 k-pixel digital chest imageis much too large for a 1 X 1 k monitor; only one-fifteenth of the image data couldbe displayed at anyone time. Therefore, when digital radiographs are included inthe image mix, the display resolution of the monitors used for soft copy readingmust be increased if entire images are to be displayed at full or nearly full spatial resolution.This increases the cost of soft copy display workstations. A more detaileddiscussion of soft copy display is given in Chapter 17.