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Chap 5 Flat-panel DR
Imaging 2 - digital
| Question | Answer |
|---|---|
| Physical basis for the CR image is the IP detector, which is made of a photostimulable phosphor (PSP) (3 steps): | After X-ray exposure, latent image is formed, Laser beam extracts image information signal, Signal is digitized and sent to a computer for preprocessing. |
| Two categories of flat panel DR detectors: | Indirect TFT and direct TFT. |
| Indirect and direct are based on the type of x-ray absorber used, what do each use? | indirect uses phosphor, direct uses a photoconductor. |
| Systems that use phosphor (indirect detectors) and systems that use photoconductors (direct) differ by converting x-rays to electrical how? | phosphor (indirect)- convert x-ray energy into electrical through an intermediate stage of light photons, photoconductors (direct) convert energy energy directly into electrical charge without the intermediate stage. |
| 2 types of indirect digital detectors? | charged coupling device (CCD) digital detector, and the flat-panel thin film transistor (TFT) digital detector. (difference between them is technical factors on how to convert light to electrical) |
| CCD digital detector is based on an _____ conversion process and uses a _______ to convert light to electrical signal. | indirect, CCD chip |
| Which detector is not classified as a flat-panel digital detector but is available commercially? | CCD |
| 4 main components of a CCD detector? | x-ray absorber, scintillator, light optics, the CCD. (which is a sensor chip.. or many...) |
| What does the CCD (sensor chips) do? | captures the light, and is also an electrical charge readout device. |
| Scintillator screen does what in CCD detectors? **** | detects x-rays and converts them into light |
| An indirect flat-panel (TFT) digital detector uses several components to convert x-rays into light that is subsequently converted into electrical charges, these components are: | x-ray scintillator (conversion layer usually cesium iodide or gradolinium), an amorphous silicon (a-Si) photodiode flat panel layer, with a thin-film transistor (TFT) (for reader of the electrical charges by the photodiode) |
| Scintillator layer used in indirect flat-panel (TFT) is usually either ___________ or ________. | cesium iodide (Csl) or gradolinium oxysulfide (Gd2O2S) |
| (indirect flat panel (TFT) detectors) ______ crystals are deposited in a needle like fashion (_______ phosphor) and run in the direction of the x-ray beam. | cesium iodide (Csi) crystals are deposited in needle like fashion, (structured phosphors) |
| (indirect flat panel (TFT) detectors) ______ crystals are deposited powdered particles (_______ phosphor). | gradolinium oxysulfide crystals, (turbid phosphors) |
| Csi crystals (______ phosphor). Gradolinium oxysulfide (_____ phosphor). | Csi- structured, Gradolinium- turbid |
| 4 limitations of CR: | X-ray detection of CR is inefficient and this affects image quality and dose, Spatial resolution of CR is less than FS radiography. CR IPs can easily be damaged. CR IPs must be transported to a separate image processor for image data extraction |
| (flat panel DR) Selenium drum technology developed specifically for: | imaging the chest |
| CCD indirect digital detector order: | x-ray, scintillator- light, lens or fiber optic, CCD - electrical charge |
| CCD. Silicon chip is made of millions of pixels forming a matrix array of pixels. Light from the _____ falls upon each pixel to produce __________ (charges) in direct proportion to the amount of light falling upon them. | scintillator, produce electron-hole pairs. |
| CCD. Bucket brigade? | During readout, system electronics provide systematic collection of changes on each chip in a manner referred to as a “bucket brigade”. (the charge pattern from the readout of pixels is sent to the ADC for digitization) |
| CCD. What happens after the bucket brigade? | The charge pattern from the readout of pixels (from the light of the scintillator falling on each pixel that produced electron-hole pairs...) is sent to the ADC for digitization. Then sent to computer for processing. |
| (indirect flat panel (TFT) detectors) ______ phosphors produce lateral spreading of light, which destroys the spatial resolution of an image. | powdered. (this is why structured phosphor like Csi crystals are better) |
| TFT stands for? | thin-film transistor |
| Indirect flat panel TFT detectors: following the x-ray scintillator (x-ray detection medium) is what? | an a-Si photodiode flat-panel layer |
| Indirect flat panel TFT detectors: a-Si photodiode layer is for what? | to convert the light from the x-ray detection scintillator into electrical charges |
| Indirect flat panel TFT detectors: purpose of the capacitor? | collect and store the electrical charge produced in the a-Si photodiode array. |
| *** indirect detectors use a light sensitive photodiode to capture light from the ___________ phosphor such as _____ to produce electrical charges, the direct detectors use an ____ photoconductor to convert x-rays _____ into electrical charges. | scintillator phosphor such as CsI, a-Se photoconductor to convert x-rays directly |
| Indirect TFT: the purpose of the a-Si photodiode layer is ? | To convert th elight from the x-ray detection scintillator into electrical charges. |
| Indirect TFT: the purpose of the capacitator is ? | To collect and store the electrical charge produced in the a-Si photodiode array. |
| Direct TFT: the photoconductor is usually what substance? | amorphous selenium (a-Se) |
| Direct TFT: the photoconductor (a-Se) does what? | detects x-ray photons from the patient and converts them directly into electrical charges. |
| Flat panel TFT: What is the matrix or active matrix array-whats it consist of? | consists of rows and columns that play a role in addressing and readout of the signal from each pixel. |
| Flat panel TFT: each pixel contains what 3 things? | a TFT (switch), a storage capacitor, and a sensing area. |
| The sensing area (in each pixel of a TFT) will detect the light from the CsI scintillator in the indirect TFT detector, or in the case of a direct TFT, ______________. | x-ray photons passing through the patient. (since there isnt a CsI scintillator?) |
| The sensing/storage element (of a pixel...) of an indirect TFT detector is the _________, and it is the ____________ in the direct TFT that uses the a-Se photoconductor. | photodiode, storage capacitor. |
| The ______________ of the pixel receives the data from the layer above it that captures x-rays that are converted to light (indirect) or electrical charges (direct). | sensing area |
| Fill factor is what? | defined as the ratio of sensing area of the pixel to the area of the pixel itself |
| Fill factor is expressed how? | as a percentage. (Fill factor of 80% means that 20% of the pixel area is occupied by the detector electronics and 80% is sensing area) |
| Fill factor affects what 2 things? | spatial resolution and contrast resolution (detail and signal-to-noise) |
| Detectors with ____ fill factors (____ sensing areas) will provide better spatial and contrast resolution. | high, large |
| Indirect TFT detectors use a light sensitive ___________ to capture light from the scintillator phophor (such as CsI) to produce _________. Direct TFT detectors use an a-Se ___________ to convert the x-rays _________. | photodiode, to produce electrical charges. Photoconductor, directly into electrical charges. |
| x-ray attenuation depends on what 3 factors? | atomic number (Z#), density, and thickness of attenuating material. |
| X-ray scintillators are what for indirect flat panel TFTs? | CsI and Gd2O2S (gradolinium) |
| General operating principles/steps of indirect TFT? | utilize a scintillator layer such as CsI to convert photons into light, these light photons strike the a-Si photodiode, which converts them to electrical charges. |
| General operating principles/steps of direct TFT? | photons fall upon the a-Se photoconductor layered on top of a matrix of a a-Si TFT array. As xrays strike the a-Se, electral charges are created and the electrical field causes them to move towards the TFT elements where they are collected and stored. |
| Before the flat panel detector can be used, it must be prepared. This is referred to as __________. | initialization. |
| What is exposure latitude? | examines the response of the image receptor to the radiation falling upon it. |
| EI does or does not represent an equivalent for patient dose entrance exposure? | does not |
| Raw digital data is first subject to pre-processing and presented as _______________ image. | "for processing" image. |
| After the image is presented as "for processing" image, it then undergoes digital image _____________, and the resultant image is labeled a _________________ image. | post processing, "for presentation" image. |
| ____ processing improves contrast/appearance for the radiologist. | post |
| Image obtained initially from the detector is referred to as what? | flat-field image. |
| You base scaling the histogram on what? | the exposure falling on the detector. |
| Third step in image processing is ___________, where the adjusted or scaled raw data values are mapped to the "for presentation" values to display an optimum image. | contrast enhancement. |
| "processing optimization rules" are what | avoid manipulating processing on a case-by-case basis- creates inconsistency in image appearance. Ensure modified processing parameters work on a variety of cases/people sizes. |
| the ability of the imaging system to resolve fine details present in an object is called | spatial resolution |
| 3 tests to measure spatial resolution? | bar test pattern, sharp-edged object, narrow slit. |
| What is the modulation transfer function? *** | math function that measures the ability of the detector to transfer its spatial resolution characteristics to the image. |
| An MTF of ____ represents a perfect detector. | 1 |
| High frequencies contain _____, low frequencies contain ______. | fine details, contrast. |
| A system that has a higher spatial frequency at an MTF of .1 will show _____ spatial resolution than a system with below MTF of .1. | better |
| High MTF value at lower spatial frequencies means that the detector provides better _____. | contrast resolution |
| Dynamic range of a digital detector is what | the response of the detector to different levels of radiation exosure |
| What is DQE? | Detective quantum efficiency. "the efficiency of a detector to convert the x-radiation signal at its entrance window into useful image signal. |
| It is important to have low or high SNR? | high |
| DQE can be expressed as : | DQE= SNR 2out / SNR 2in |
| A perfect digital detector would have a DQE of __. | 1 |
| As the spatial frequencies increase, the DQE ______ | decreases rapidly |
| Image lag is aka | memory effect |
| Image lag refers to: | the persistance of the image, that is, charge is still being produced after the radiation beam from the tube has been turned off. |
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Zoest35
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