10/26/2015. Study Harder

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1 This presentation is a professional collaboration of development time prepared by: Rex Christensen Terri Jurkiewicz and Diane Kawamura Study Harder CR detection is inefficient, inferior to film screen radiography. Spatial resolution in film screen is superior to CR lp/mm film screen, 3-5 lp/mm CR CR imaging plates are easily damaged CR imaging plates must be processed for image extraction. Digital radiography systems which are commonly referred to as DR systems will be called flat panel digital or cassetteless imaging systems. 1

2 A flat panel detector is a single unit with components allowing the digitization of the x-ray image with out moving an imaging plate to be scanned or read. The computer acts as the interface between the x-ray equipment and the Flat panel detector as well As the software abilities Such as post processing for optimal image display. Image display, storage, and communication are all output processes. The monitor is used for viewing, post-processing, and interpretation by the radiologist. Storage of the images is possible through a PACS system, and a network allows the images to be viewed or sent worldwide. There are two types of flat panel radiography: 1. Indirect detection 2. Direct detection 2

3 Indirect detectors use a phosphor which convert x-rays into electrical charge thru an intermediate stage of light photons. Direct detectors use a photoconductor which converts x-ray directly into an electrical charge without the intermediate stage. The Scintillator layer in indirect detectors is usually : 1. Cesium Iodide (CsI) or 2. Gadolinium oxysulfide(gd 2 O 2 S) A scintillator is a bulk crystalline material. Scintillation the property of luminescence when excited by ionizing radiation. These have been used in the past by image intensifiers in fluoroscopy equipment, and rare earth film cassettes. 3

4 Light is emitted when x-rays are absorbed by the phosphor, and scatters multiple times off of the powdered particles before it escapes the screen. The powdered phosphors produce lateral light spread which destroys spatial resolution. Cesium Iodide scintillator has a needle structure which minimize lateral light spread and thus improves resolution. The needle diameter is approximately 6 micrometers. The x-ray beam runs the direction of the needles. This helps reduce the spread of lateral light improving spatial resolution CCD Digital detectors use a charge couple device chip which converts that light that is produced by the phosphor into an electrical charge. Although it is not a true flat panel detector, it is widely used, and so is included in the indirect flat panel radiography. 4

5 The CCD detector uses a silicon chip which is made of millions of pixels to form a matrix array. The CCD collects light from the scintillator and each pixel produces an electron hole pair in proportion to the amount of light which has hit the pixel. Developed in the 1970s Highly light-sensitive device 3 principal advantageous imaging characteristics: 1. Sensitivity 2. Dynamic range 3. Size RADT 3463 Computerized Imaging 18 The charge-coupled device has higher sensitivity for radiation and a much wider dynamic range than screen-film image receptors SENSITIVITY Ability of the charge-coupled device to detect and respond to very low levels of visible light Important for low patient radiation dose in digital imaging RADT 3463 Computerized Imaging 19 RADT 3463 Computerized Imaging 20 5

6 Ability to respond to a wide range of light intensity Very dim to very bright Very small making it highly adaptable to various forms of digital radiography 1 to 2 cm Pixel size is 100 x 100 m Radiation response of a charge-coupled device compared with a 400 speed screen-film image receptor RADT 3463 Computerized Imaging 21 RADT 3463 Computerized Imaging 22 The Scintillator layer in indirect detectors is usually : 1. Cesium Iodide (CsI) or 2. Gadolinium oxysulfide(gd 2 O 2 S) A scintillator is a bulk crystalline material. Scintillation the property of luminescence when excited by ionizing radiation. These have been used in the past by image intensifiers in fluoroscopy equipment, and rare earth film cassettes. 6

7 Light is emitted when x-rays are absorbed by the phosphor, and scatters multiple times off of the powdered particles before it escapes the screen. The powdered phosphors produce lateral light spread which destroys spatial resolution. Cesium Iodide scintillator has a needle structure which minimize lateral light spread and thus improves resolution. The needle diameter is approximately 6 micrometers. The x-ray beam runs the direction of the needles. This helps reduce the spread of lateral light improving spatial resolution After the x-rays have gone through the scintillator layer, it hits the amorphous silicon layer. This layer converts the light into an electrical charge. Adjacent to the photodiode is the thin film transistor(tft) and storage capacitors which collect the electrical charge This is an indirect conversion process which requires an: X-ray Scintillator (conversion layer) Amorphous silicon photodiode with a TFT array (it reads the electrical charges) 7

8 A direct flat-panel detector uses a TFT matrix array very similar to that used for the other detector type, thus the common flat-panel designation. However, the capture medium, instead of a phosphor (or scintillator), is a photoconductor. Current detectors typically employ amorphous selenium for that purpose. The x-ray photons can be captured by the photo conductor layer and their energy is directly converted to charge with no intermediary light conversion stage. Is the most common photoconductor because it has excellent x-ray detection ability and high spatial resolution which is paramount. The photoconductor converts the x-ray photons directly into an electrical charge. The TFT and electronics collects and stores the electrical charges for the digital image. 8

9 The flat panel is a matrix of detectors, each can be regarded as a pixel. It is also called an Active Matrix Array. Each pixel has 3 mains components. 1. TFT 2. Capacitor 3. Sensing area The sensing area is where the received data from the above layer is captured. The fill factor is a ratio of the sensing area to the area of the pixel. Pixel Geometry The pixel face is occupied by conductors, capacitors, and the thin-film transistor (20%) Fill Factor: portion of the pixel element that is occupied by the sensitive image receptor (80%) Fill Factor= Sensing area of pixel Area of Pixel A fill factor of 80% means that 20 % of the area is filled with other components such as the TFT and storage capacitor, and 80% is the sensing area. The higher the fill factor the better the spatial and contrast resolution. RADT 3463 Computerized Imaging 35 9

10 The rows and columns receive a signal from each pixel. Each pixel contains a TFT (or switch), storage capacitor, and sensing area. The sensing area picks up the light from the scintillator in indirect flat panel, and with direct flat panel the TFT picks up the x-ray photons once they pass through the patient. Differing body parts require different size of detectors and therefore different matrices. Pixel size and spacing also known as pixel pitch determines the spatial resolution. The larger the pixel pitch the lower the spatial resolution. More pixels = Better resolution Latitude refers to the amount of grays visualized in an image or the width of high to low densities seen in an image. In CR and flat panel imaging the latitude is much greater than film screen imaging. Advantage is a wide range of exposure levels that can be seen and visualized as acceptable to the observer. Exposure indicator (EI) or (S number) or (lgm log of median values) aids in prevention of dose creep. Each manufacturer has a range of numbers in which the technologist can keep apprised of what entails a good radiograph exposure. 10

11 Collimation Patient position Centering Image processing Body habitus/disease processes Characteristics that affect the image quality: A. Spatial Resolution B. MTF C. Dynamic Range D. DQE E. Image Lag G. Artifacts It is the ability of a system to visualize small or fine details in an image. This is highly dependent on pixel size in the digital imaging system. Modulation Transfer Function measures the ability of the detector to transfer the spatial resolution characteristics to the image. OR - a measure of the transfer of modulation (or contrast) from the subject to the image. In other words, it measures how faithfully the detector reproduces (or transfers) detail from the object to the image. A perfect MTF is 1 or perfect detection. As spatial frequency increases the MTF decreases 11

12 It is the detected range of levels of radiation during an exposure. There is an increased range of shades of gray in digital imaging over film screen imaging. Increase in spatial frequency = better resolution Decrease in spatial frequency = better contrast Detective Quantum Efficiency is the efficiency of the detector to convert the x-ray signal into a useful image signal. A perfect DQE is 1 DQE provides information about the signal to noise ratio (SNR) In imaging you want good contrast in your image. This is the signal but there is also noise in the signal due to photons striking the detector (quantum or electronic noise) Image lag is the persistence of the image, or the production of an image after the radiation beam has turned off. Trapped in the metastable state. Image lag times vary and are shortest for flat panel digital imaging detectors. You want a high Signal to Noise ratio (high signal low noise) 12

13 Various flaws in components can lead to artifacts. Dust Scratches Chemical Reactions Defective pixels Poor electronics performance Scattered radiation grids Portable exams Surgical exams Fluoroscopic exams 13