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RADT 2403 Chapter 16
Digital Radiography
| Question | Answer |
|---|---|
| What does SPR stand for? | Scanned Projection Radiography |
| What generation of CT is scanned projection used? | 3rd |
| What is scanned projection used for in CT? | positioning of a FOV |
| What is the capture element? | What captures the x-ray photon |
| What is the capture element in CR? | A photostimulable phosphor |
| What are the capture elements used in DR? | Cesium iodide, gadolinium oxysulfide, amorphouse selenium |
| What is the coupling element? | What transfers the x-ray generated signal for collection. |
| What are types of coupling elements? | Lens, fiber optics, contact layer, amorphous selenium |
| What is the collection element? | What collects the signal |
| What kind of devices can collect the signal? | CCD, Photodiode and TFT |
| How do CDD and Photodiode collect the signal? | Light sensitive device that collects photons |
| How does TFT collect the signal? | Charge sensitive device that collects electrons. |
| What does CDD stand for? | Charge-couple device |
| What does TFT stand for? | Thin-film Transistor |
| What are the two types of Indirect DR? | Scintillator and a charge coupled device (CCD) ; Scintillator and amorphouse silicon and thin-film transistor. |
| What is a scintillator? | Phosphor that converts x-rays to light. |
| What is a commonly used scintillator? | Cesium iodide |
| What is a charge-coupled device? | Converts light signal to digital-based signal |
| What are charge-coupled devices used in place of? | Photodiodes |
| How big are charge-coupled devices? | 1-2 cm in size |
| what material are charge-couple devices based off of? | silicon |
| What is the process for Indirect DR using a scintillator and CCD? | X-ray > Phosphor (scintillator > visible light > CCD > Digital signal |
| What is the process for Indirect DR using a scintillator, and TFT? | X-ray > Phosphor (scintillator) > visible light > Photodetector > TFT > digital signal |
| What is the thin film transistor TFT? | Electronic component that includes the readout, charge collector, "Thin film" of an active semiconductor |
| TFTS act as _______ that allow the pixels to change state rapidly? | individual switches |
| Because TFTs act as individual switches that allow pixels to change state rapidly, what does the allow the pixel to do? | Turn on or off much quicker. |
| What is Direct Radiography (DR)? | "Cassette less" type of digital imaging that uses a detector array to record remnant radiation |
| What are the two types of DR? | Indirect Capture and Direct Capture |
| What is the process of Indirect capture? | Converts x-rays > light > electronic/digital signal |
| What is the process of Direct capture? | Converts x-rays directly > electronic/digital signal |
| What are the 5 types of image receptor materials? | Charge coupled device (linear response curve); Cesium iodide/ charge-coupled device ; cesium iodide and amorphous silicone; Amorphous selenium; Barium Fluorhalide with Europium phosphor CR. |
| What kind of device is a charge coupled device? | A device for the movement of an electrical charge |
| 3 Qualities of a charge-coupled device? | High sensitivity; Wide dynamic range; Radiation response is linear. |
| With a charge-coupled device, image contrast is ________ to image receptor x-ray exposure. | Unrelated |
| Charge coupled device can be used in both ____ and _____ capture | indirect or direct. |
| Cesium iodide/Charge coupled device is used in what kind of capture? | Indirect |
| The CsI phosphor absorbs the x-ray photons and transmits scintillation light to the CCD by way of ______? | fiber optic bundles |
| Cesium Iodide/Amorphous Silicone is used in what kind of capture? | Indirect |
| What is amorphous silicone? | a fluid which is sprayed on |
| Cesium Iodide/amorphouse silicone has high ______? | photoelectric capture |
| For CsI/Amoruphous silicone, _______ and ________ are imbedded in each pixel | Capacitor and TFT |
| With CsI/amorphouse silicone, what captures the x-ray photons and transmits scintillation light to the collection element (TFT)? | CsI phosphor |
| What is scintillation? | flash of light produced in certain materials when they absorb ionizing radiation |
| TFT is a type of ______ | semiconductor |
| TFT is a _____ collection electrode | charge |
| TFT is an electronic switch that is _____ during the x-ray exposure allowing a _____ to be stored in a capacitor | closed; charge |
| The captured charge from TFT is _______ to the x-ray exposure received to the "Dexel". | proportional |
| What happens to the switch for TFT once the exposure is complete? | It opens and electric charge flows to the charge amplifier. |
| How is DEXEL arranged? | In rows and columns |
| DEXEL includes what components? | the TFT and storage capacitor |
| Only ____% of the Dexel contributes to the image | 80 |
| ______% of DEXEL is for the components. Called what? | 20 ; The Fill factor |
| DEXEL is comparable to what? | A farm plot. Have to have fields and the barn. |
| Amorphous selenium is used in what capture? | Direct |
| Amorphous selenium uses no_____ | scintillation phosphor. |
| The a-Se is what both elements? | The capture element and the coupling element. |
| X-ray photons hits the a-Se and is what? | directly converted into a charge. |
| What are some CCD qualities for Indirect DR? | high sensitivity and wide dynamic range; responds to a large variety of light energies; allows lower techniques to be used. |
| What are some imaging challenges in Indirect DR? How is this overcome? | Several CCD's must be tiled together to cover the detector array...leaving seams where light cannot be detected...overcome using a software interpolation termed "flat-field correction" |
| For Indirect DR using a scintillator/a-Si/ and TFT, a-Si is what element? | The X-ray collection element? |
| the a-Si is painted on to supporting material (glass) to create what? | "flat panel detector" |
| What is a TFT: | electronic switch that allows a capacitor to collect charge (from freed electrons) until the exposure terminated. |
| Indirect DR using a scintillator/a-Si and TFT is arranged in what? | in a network of dexels (detector elements) |
| Each dexel contains what? | a detector face of a-Si, a charge collection electrode, a capacitor, and a TFT. |
| How does TFT function in image acquisition? | Closes during exposure, opens when exposure ends, sends charge collected by capacitor to charge amplifier. |
| What does the charge amplifier in TFT do? | Located outside the detecter; Converts charge to a digital signal. |
| What is the DEXEL Fill Factor? | the sensitivity and ability of dexels to convert x-ray photons to a digital signal. |
| How efficient is the fill factor? | 80% |
| What is the effect of high fill factor on image quality? | Increased contrast and spatial resolution; high signal-to noise ratios. |
| Components of DR? | TFT and a photoconductor of a-Se |
| Where is amorphous selenium (a-Se) sandwiched between? | charged electrodes. |
| How is DR Arranged? | in rows and columns termed an Active Matrix Array (AMA) |
| What is Active Matrix Array and what does it do? | Network of gatelines and datalines; Allows dexels to be read in sequence. |
| What is the Image acquistion process for DR? | x-rays free electrons in a-Se, creates positively charged holes; free electrons attracted to positively charged electrode; electron holes attracted to negatively charged electrode, builiding on SE layer at bottom of detector; stored capacitor for exposure |
| X-rays incident upon a layer of a-Se generate electron-hole pair vie what? | the internal photoelectric effect. |
| A bias voltage applied to the depth of the selenium layer does what? | Draws the electrons and holes to corresponding electrodes. |
| The generated current from bias voltage is proportional to what? | the intensity of the irradiation |
| What are more steps to the image acquisition process for DR? | 5-10 Volts applied to Se layer; Charge sent to ADC and computer for processing. |
| What does applying voltage to Se layer do? | Opens TFT gates sequentially and dumps the stored charge. |
| How many dexels can be converted to a visible image in less than 1 second? | 1 million |
| What are the three main steps to digitizing an analog image? | Scanning, sampling, and quantization. |
| What does the scanning step do in digitizing an analog image? | process of dividing image up into a matrix...each cell becomes a pixel. |
| What does the sampling step do in digitizing an analog image? | process of detecting and measuring exposure received by the detector. |
| What does the quantization step of digitizing an analog image do? | Assigns a value to each pixel relative to the energy it received. |
| Digital radiography is more efficient in what? | tme, space, and personnel than screen-film radiography. |
| What are the three elements described in digital imaging? | capture element, coupling element, and collection element. |
| What is the capture element? | that in which the x-ray is captured. |
| What is the capture element for CR? | The photostimulable phosphor. |
| What are the capture elements for DR? | sodium iodide (NaI), cesium iodide (CsI), gadolinium oxysulfide (GdOS), or amorphous selenium (a-Se). |
| What is the coupling element? | That which transfers the x-ray generated signal to the collection element. |
| What are examples of coupling elements? | lens, fiber-optic assembly, contact layer or a-Se. |
| What is the collection element? | photodiode, charge-coupled device (CCD) or a thin-film transistor (TFT). |
| The CCD and photodiode collect what? | light photons. |
| The TFT collects what? | electrons |
| What element is the CCD? | light sensing element. |
| What does sensitivity of the CCD refer to? | ability of CCD to detect and respond to low levels of visible light. |
| What does dynamic range refer to with CCD? | ability of the CCD to respond to a wide range of light intensity, from dim to bright. |
| How is CCD different from screen film? | CCD has higher sensitivity for radiation and a much wider dynamic range. |
| CCD radiation response is _______ | linear. |
| CsI/CCD is an indirect DR process by which x-rays are first converted to _____ and then to an electronic signal. | light. |
| CsI/a-Si is an indrect DR process by which x-rays are first converted first to _____ and then to electronic signal. | light. |
| Spatial resolution in DR is ________ | pixel limited. |
| a-Se is a direct DR process by which x-rays are _______ | converted directly into an electronic signal. |
| X-rays incident on a-Se create what? | electron hole pairs through direct ionization of selenium. |
| Created charge on a-Se is collected by what? | a storage capacitor. |
| What is DM | Digital mammography. |
| What do line spread functions show? | |
| no DR system can match screen-film radiagraphy for... | spatial resolution. |
| ___________ is more important than spatial resolution for mammography or soft tissue radiography. | contrast resolution. |
| What is DMT? | Digital mammographic tomosynthesis |
| SPR has better _________ but limited spatial resolution. | contrast resolution. |
| Which is a principal disadvantage of the use of an area beam versus scanned projection radiography? | Scatter radiation |
| In scanned projection digital radiography, which of the following moves to produce an image? | The patient |
| A scanned projection radiograph is... | virtually scatter-free |
| What is part of SPR? | detector array |
| Which of the following has the largest line spread function? | screen-film |
| Advantage of area beam over SPR? | short exposure time and reduced motion blur. |
| What is unique to direct radiography? | no cassette is needed. |
| An x-ray system used for direct radiography has what? | a linear H& D curve. |
| Principal limitation of SPR is what? | exam time |
| What could be used as a radiation detector in direct radiography? | BaFlCl |
Created by:
Cjenki10