Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Direct Digital Radiography or Direct Capture Radiography Yu zixi Radiological department Taishan Medical College Late 1990’s A new approach to imaging appeared DR or DDR or Direct Capture imaging Too early to tell which system will prevail Directed Digital Radiography (DDR) Directed digital radiography, a term used to describe total electronic imaging capturing. Eliminates the need for an image plate altogether. DDR Systems IMAGE CAPTURE CR PSP – photostimulable phosphor plate REPLACES FILM IN THE CASSETTE DR – NO CASSETTE – PHOTONS CAPTURED DIRECTLY ONTO A TRANSISTOR SENT DIRECTLY TO A MONITOR DIRECT RADIOGRAPHY uses a transistor receiver (like bucky) that captures and converts x-ray energy directly into digital signal seen immediately on monitor then sent to PACS/ printer/ other workstations FOR VIEWING CR vs DR CR imaging plate processed in a Digital Reader Signal sent to computer Viewed on a monitor DR transistor receiver (like bucky) directly into digital signal seen immediately on monitor DDR Digital CR Radiography Direct Capture Indirect Capture Direct-to-Digital Radiography (DDR)-Selenium Computed Radiography (CR) - PSL Direct-to-Digital Radiography Silicon Scint. Laser Scanning Digitizers Two types of DDR systems Both are based on the thin-film transistor as an active matrix array (AMA) Built the size of a conventional S/F receptor Active Matrix Array (AMA) Pixels are read sequentially, one at a time Each TFT and detector represents a pixel DEL = charge collecting detector element DEL Digital Value Digital Value depends on: Charge collected by DEL. Bit depth 10 bit =1 - 1024 12 bit =1 - 4096 DEL collects e- Unlike CR plates, only the exposed pixels contribute to the image data base. One exposure = Detector Readout DDR using cesium iodide scintillation phosphors CsI is coated over an active matrix array (AMA) of amorphus silicon (a-Si) photodiodes Amorphus means without shape Photodiodes are used to detect light or measure its intensity also called a charge coupled device (CCD) DDR steps using cesium iodide Exit x-rays interact with CsI scintillation phosphor to produce light The light interact with the a-Si to produce a signal The TFT stores the signal until readout, one pixel at a time CsI phosphor light detected by the AMA of silicon photodiodes DDR only using amorphous selenium (a-Se) The exit x-ray photon interact with the a-Si (detector element/DEL). Photon energy is trapped on detector (signal) The TFT stores the signal until readout, one pixel at a time Active matrix array of silicon photodiodes Advantages/Disadvantages CsI phosphors have high detective quantum efficiency (DQE) = lower patient dose DQE = % of x-rays absorbed by the phosphors a-Se only: there is no spreading of light in the phosphor = better spatial resolution F/S & DDR imaging systems F/S & DDR imaging systems Image Resolution – (how sharply is the image seen) CR & DR 4000 x 4000 image only as good a monitor* 525 vs 1000 line more pixels = more memory needed to store resolution dependent on pixel size CR 2-5 lp/mm RAD 3-6 lp/mm DR 3-5 lp/mm IMAGE APPEARS SHARPER BECAUSE CONTRAST CAN BE ADJUSTED BY THE COMPUTER – (DIFFERENCES IN DENSITY) Image Resolution Pixel Pitch Spatial resolution determined by pixel pitch. Detector element (DEL) size 140 μm = ~3.7 lp/mm 100 μm = ~ 5.0 lp/mm Signal Sampling Frequency Good sampling under sampling DR Initial expense ---high very low dose to patient image quality of 100s using a 400s technique Therfore,¼ the dose needed to make the image Flat Panel TFT Detectors Have to be very careful with terminology One vendor claims: “Detector has sharpness of 100 speed screen” May be true: TFT detectors can have very sharp edges due to DEL alignment But , Spatial resolution is not as good as 100 speed screen. TFT detector = 3.4 lp/mm 100 speed screen = 8 – 10 lp/mm TFT Array Detectors Detector is refreshed after exposure If no exposures are produced. . . detector refreshed every 30 – 45 sec Built in AEC, An ion chamber between grid and detector Patient Dose Important factors that affect patient dose DQE: when using CsI systems Both systems “fill factor” The percentage of the pixel face that contains the x-ray detector. Fill factor is approximately 80% Fill Factor DDR has all the advantages of CR imaging techniques Post processing & PACS Advantages of DDR Fast speed, large throughput High spatial resolution Low patient radiation dose Can be updated Less noise Powerful post-processing THANK YOU!