Digital Radiography Basic Concepts  Image Quality Concepts – Spatial Resolution (limiting resolution) – Noise: Quantum Mottle  Nature of the Digital Image – Spatial Digitization – Analog-to-Digital Conversion  Digital Radiography Factors – – – Spatial Digitization and Resolution ADC and Noise ADC and Dynamic Range Basic Concepts: Limiting Resolution  Limiting Resolution (simplest form): refers to the smallest, closely spaced objects for which separate images can be seen  Measurement and Units: Bar pattern Basic Concepts: Limiting Resolution Limiting Resolution (simplest form):  Measurement and Units: Bar pattern Measured using bar pattern  (lead strips separated by spaces) and expressed as smallest visible bar size or highest spatial frequency (line-pairs/mm)  Sources Blurring in radiography: – – – Focal spot (all types of radiography) Motion (all types of radiography) Receptor blur - depends on receptor Radiography Image Receptor Blur Dual Receptors (Screens) Noise and Image Quality Image Noise: Quantum mottle  Quantum mottle (QM) refers to the “graininess” of x-ray images  QM is caused by using a limited number of x-ray photons to make an image  QM interferes with ability to details  Using more photons (more mAs) reduces noise but increases radiation exposure The Nature of the Digital Image Basic Concepts: Resolution and Noise  The Digitization Process  – Spatial Digitization – Analog-to-Digital Conversion (ADC)  Radiation Dose, Noise and Resolution – Resolution versus Dose: receptor thickness – Dose versus Image Noise (Quantum mottle)  Dynamic Range The Digitization Process  Every “image” starts out in analog form: – – “light” image emitted by screen “light” image from intensifier output phosphor – TV camera voltages – Stimulated light from computed radiography  Analog “image” must be converted (digitized) to matrix of pixels stored as binary numbers Conventional CR Scanning Flying Spot CR Scan In a conventional flying spot CR reader, stimulated output exposure (scan level) from the IP is proportional to the laser intensity I and dwell time Td Absorption Efficiency CR Blur Dynamic Range (Latitude)  Dynamic Range, or latitude refers the range of exposures which provide useful diagnostic information For film, is the the range of exposures that provide acceptable optical densities (ie, not too dark and not too light) Dynamic Range LGM and OD vs Exposure LG M or O D 3.5 2.5 Agfa CR vs Lanex Reg Lanex Reg CR-200 1.5 0.5 0 0.30.60.91.21.51.82.12.42.7 3.33.63.94.24.54.8 Log Relative Exposure (Log (Ei/Eo) Dynamic Range (Latitude) CR vs Film Dynamic Range (latitude): range of exposures providing useful diagnostic information  Regular F/S: 16:1 (between 0.5 and 2.5 OD) (exposure yielding 2.5 OD is 16x exposure yielding 0.5 OD)  CR: >10,000:1 (between minimum and maximum measurable scan levels)  CR and “Film Density” 1/8 x E 4xE Radiation Dose with DR  How much is enough ? – –  Image Noise (Quantum mottle) required image quality How much is too much? – – Patient radiation exposure concerns possible saturation of parts of image (all black) Noise and Proper Exposure Levels LgM=1.10 LgM=2.1 0.1 Exposure Correct Exposure FUTURE CR TECHNOLOGY New phosphors and scan head technology  Dual Energy  “Flat Panel” CR Receptor Devices Dual Energy Imaging