Digital Radiography Chapter 11 Adjuncts to Radiology Chapter

Digital Radiography – Chapter 11 Adjuncts to Radiology – Chapter 12 Brent K. Stewart, Ph. D, DABMP Lois Rutz, M. S. Radiation Safety Engineering, Inc. a copy of Brent Stewart’s unmodified lecture may be found at: http: //courses. washington. edu/radxphys/Physics. Course 04 -05. html Brent K. Stewart, Ph. D, DABMP 1

Take Away: Five Things You should be able to Explain after the DR/Adjuncts Lecture v v v The various types of detectors used in digital imaging (e. g. , scintillators, photoconductors, etc. ) The differences between the various technologies used for digital radiography (e. g. , CR, indirect and direct DR) Benefits of each type (e. g. , resolution, dose efficiency) Why digital image correction and processing are necessary or useful and how they are executed The various types of adjuncts to radiology (e. g. , DSA or dual-energy imaging), what issue they are trying to resolve, mechanism exploited and end result Brent K. Stewart, Ph. D, DABMP 2

Why Digital/Computed Radiography v Limitations on Film/Screen radiography v v v Screen/Film system is image receptor and display Image characteristics depend on Screen/Film and Film processing. Modification of image difficult to control (e. g. development temperature). Image appearance depends on technique settings. Image quality cannot be repaired after development. Retake only solution to poor I. Q. Brent K. Stewart, Ph. D, DABMP 3

Why Digital/Computed Radiography cont. v Screen/film dynamic range 2 to 2. 5 orders of magnitude. v v Different applications require different screen/film combinations. Only one “original” image. v v Films often “go missing” from ER or ICU and never are archived. Copies expensive, have inconsistent quality, and often are nondiagnostic. Archive space expensive, often remote. Digitizing film is only way to move images to PACS. Brent K. Stewart, Ph. D, DABMP 4

How does Digital/Computed Radiography solve these problems? • Decouples imaging chain components. • Detector, image processing, display all “independent” entities. • Independent in design but not in application. • Detector can make use of extended dynamic range. • Solid state detectors have improved DQE. • Electronics can apply corrections to input signals. • In particular, over/under exposure can be corrected, reducing retakes. Brent K. Stewart, Ph. D, DABMP 5

How does Digital/Computed Radiography solve these problems? Cont. • Image processing can modify and enhance raw (preprocessed) data. • Images can be displayed on workstations which permit interactive display processing. • Image data is stored digitally. “Original image” is available everywhere and at any time. Brent K. Stewart, Ph. D, DABMP 6

CR vs. DR v CR also known as a Photostimulable Phosphor system. v v CR uses an imaging plate similar to an intensifying screen as the receptor. CR systems are indirect digital systems. v v Indirect systems convert x-radiation to the final digital image through one or more stages. DR digital radiography v v Uses a fixed detector such as amorphous selenium plate as the receptor. Can be a direct or an indirect digital system. v When direct it is sometimes called DDR for direct digital radiography Brent K. Stewart, Ph. D, DABMP 7

CR v Detector or Imaging Plate (IP) is essentially a type of intensifying screen. v v v IP can be used in any bucky or table-top system. IP is relatively robust. Requires same care as intensifying screens. Process is indirect. v v X-ray creates excitation center. Plate reader uses red light to stimulate centers to release blue light. Blue light is directed to a photo-electric transducer (pmt or other). Electric signal digitized to make raw image. Brent K. Stewart, Ph. D, DABMP 8

CR and DR Systems Brent K. Stewart, Ph. D, DABMP 9

Image Production in CR/DR Systems v v v Radiation through the patient creates a latent image on the receptor. Receptor is “read” by some process and latent image is converted to an electronic signal. Signal is processed. v v v Signal (analog) is converted via ADC to a bit value in a digital matrix. Digital image is processed. v v Processing is related to acquisition system characteristics. Processing is related to desired image information. Digital matrix is displayed on a video screen or printed to paper or film. Brent K. Stewart, Ph. D, DABMP 10

Signal Processing v v Primarily to accommodate variations in the detector/electronics components. Involves corrections for dead space, non-uniformities, defects. v v v Could be developed to compensate for MTF losses. All systems, PSP or Direct, do some sort of processing and scaling. Ultimate goal is to present the image processing module with “true” image pixels. Brent K. Stewart, Ph. D, DABMP 11

Digital Image Correction v v v Interpolation to fill in dead pixel and row/column defects Subtracting out average dark noise image Davg(t)(x, y) Differences in detector element digital values for flat field v v Make corrections for each detector element (map) v v v Gain image: G(x, y) =G’(x, y) - Davg(t)(x, y); Gavg =(1/N) ∙ G(x, y) I(x, y) = Gavg ∙ [Iraw(x, y) - Davg(t)(x, y)] / G(x, y) Done for DR and in a similar manner for CT (later) Not performed for CR on a pixel by pixel basis, although there are corrections on a column basis for differences in light conduction efficiency in the light guide to the PMT Brent K. Stewart, Ph. D, DABMP 12

Digital Image Correction c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 310. Brent K. Stewart, Ph. D, DABMP 13

Detectors v In order to understand signal processing we need to learn about the detectors. v v Photo Stimulable Phosphor Plates Photoconductive materials. Detector consists of a receptor material (e. g. Ba. F(H)Eu), and a set of signal readout and conversion electronics. Receptor responsible for the DQE. v Rest of the system contributes to noise, resolution, dynamic range. Brent K. Stewart, Ph. D, DABMP 14

Detectors in Digital Imaging (1) v v Gas and solid-state detectors Energy deposited to e- through Compton and photoelectric interactions Gas detectors – apply high voltage across a chamber and measuring the flow of eproduced by ionization in the gas (typically high Z gases like Xenon: Z=54, K-edge = 35 ke. V) Were used in older CT units c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 32. Brent K. Stewart, Ph. D, DABMP 15

Detectors in Digital Imaging (2) v Solid-state materials v v Electrons arranged in bands with conduction band usually empty Solid-state detectors v v v Scintillators – some deposited energy converted to visible light Photoconductors – charge collected and measured directly Photostimulable phosphors – energy stored in electron traps c. f. Yaffe MJ and Rowlands JA. Phys. Med. Biol. 42 (1997), p. Elements of Digital Radiology, p. 10. Brent K. Stewart, Ph. D, DABMP 16

Detectors in Digital Imaging (3) c. f. Yaffe MJ and Rowlands JA. Phys. Med. Biol. 42 (1997), p. Elements of Digital Radiology, p. 9. Brent K. Stewart, Ph. D, DABMP 17

Computed Radiography (CR) v v v Photostimulable phosphor (PSP) Barium fluorohalide: 85% Ba. FBr: Eu + 15% Ba. FI: Eu e- from Eu 2+ liberated through absorption of x-rays by PSP Liberated e- fall from the conduction band into ‘trapping sites’ near F-centers By low energy laser light (700 nm) stimulation the e- are re-promoted into the conduction band where some recombine with the Eu 3+ ions and emit a blue-green (400500 nm) visible light (VL) c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 295. Brent K. Stewart, Ph. D, DABMP 18

Computed Radiography (CR) System (1) v v v Imaging plate (IP) made of PSP is exposed identically to SF radiography in Bucky IP in CR cassette taken to CR reader where the IP is separated from cassette IP is transferred across a stage with stepping motors and scanned by a laser beam (~700 nm) swept across the IP by a rotating polygonal mirror Light emitted from the IP is collected by a fiber-optic bundle and funneled into a photomultiplier tube (PMT) PMT converts VL into e- current c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 294. Brent K. Stewart, Ph. D, DABMP 19

Computed Radiography (CR) System (2) v v v Electronic signal output from PMT input to an ADC Digital output from ADC stored Raster swept out by rotating polygonal mirror and stage stepping motors produces I(t) into PMT which eventually translates into the stored DV(x, y): PMT→ADC→RAM IP exposed to bright light to erase any remaining trapped e- (~50%) IP mechanically reinserted into cassette ready for use 200 mm and 100 mm pixel size (14”x 17”: 1780 x 2160 and 3560 x 4320, respectively) c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 294. Brent K. Stewart, Ph. D, DABMP 20

Indirect Flat Panel Detectors v v Use an intensifying screen (Cs. I) to generate VL photons from an x-ray exposure Light photons absorbed by individual array photodetectors Each element of the array (pixel) consists of transistor (readout) electronics and a photodetector area The manufacture of these arrays is similar to that used in laptop screens: thin-film transistors (TFT) c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 301. Brent K. Stewart, Ph. D, DABMP 21

Charged-Coupled Devices (CCD) v v v Form images from visible light Videocams & digital cameras Each picture element (pixel) a photosensitive ‘bucket’ After exposure, the elements electronically readout via ‘shiftand-read’ logic and digitized Light focused using lenses or fiber-optics v v Fluoroscopy (II) Digital cineradiography (II) Digital biopsy system (phosphor screen) 1 K and 2 K CCDs used c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , pp. 298 -299. Brent K. Stewart, Ph. D, DABMP 22

Direct Flat Panel Detectors v v v Use a layer of photoconductive material (e. g. , α-Se) atop a TFT array e- released in the detector layer from x-ray interactions used to form the image directly X-ray→e-→TFT → ADC→RAM High degree of e- directionality through application of E field Photoconductive material can be made thick w/o degradation of spatial resolution Photoconductive materials v v Selenium (Z=34) Cd. Te, Hg. I 2 and Pb. I 2 Indirect Flat Panel Detector (for comparison) Direct Flat Panel Detector c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 304. Brent K. Stewart, Ph. D, DABMP 23

Thin-Film Transistors (TFT) v v After the exposure is complete and the e- have been stored in the photodetection area (capacitor), rows in the TFT are scanned, activating the transistor gates Transistor source (connected to photodetector capacitors is shunted through the drain to associated charge amplifiers Amplified signal from each pixel then digitized and stored X-ray→VL→e-→ADC→RAM c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 301. Brent K. Stewart, Ph. D, DABMP 24

Resolution and Fill Factor v v v Dimension of detector element largely determines spatial resolution 200 mm and 100 mm pixel size typical For dimension of ‘a’ mm - Nyquist frequency: FN = 1/2 a If a = 100 mm → FN = 5 cycle/mm Fill factor = (light sensitive area)/(detector element area) Trade-off between spatial resolution and contrast resolution c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 303. Brent K. Stewart, Ph. D, DABMP 25

Image Digitization and Processing v After acquisition and correction of raw data, the image is ready for display processing. v The image data consists of a matrix of numbers. Each pixel is one matrix point. Each gray scale is a digital value. v For example: a matrix can have 1024 x 1024 pixels and each pixel will have a value from 0 to 1024. Each value is related to the radiation exposure which created that pixel. Brent K. Stewart, Ph. D, DABMP 26

Digital Storage of Images v v v Usually stored as a 2 D array (matrix) of data, I(x, y): I(1, 1), I(2, 1), … I(n, m-1), I(n, m) Each minute region of the image is called a pixel (picture element) represented by one value (e. g. , digital value, gray level or Hounsfield unit) Typical matrices: v v v CT: 512 x 12 bits/pixel CR: 1760 x 2140 x 10 bits/pixel DR: 2048 x 2560 x 16 bits/pixel c. f. Huang, HK. Elements of Digital Radiology, p. 8. Brent K. Stewart, Ph. D, DABMP 27

Image Processing v v Image data is scaled to present image with appropriate gray scale (O. D. ) values regardless of the actual radiation used to produce the image. Image data is frequency enhanced around structures of importance. v v Process involves mathematical filters. Image data is display processed to give desired contrast and density. v Process involves re-mapping along a chosen display (“H&D”) curve Brent K. Stewart, Ph. D, DABMP 28

Generic Display Processing v Different manufacturers may use different versions of generic image processing methods. v v v E. g. Musica, Ptone All describe means of scaling and modifying image appearance. Different manufacturers use different exposure indicators. v v E. g. EI, S, Ig. M All describe the relationship between the exposure to the detector and the pixel value. Brent K. Stewart, Ph. D, DABMP 29

Generic Elements of Display Processing v Exposure Recognition. v v Signal Equalization: v v Adjust regions of low/high signal value Grayscale Rendition v v Adjust for high/low average exposure Convert signal values to display values Edge Enhancement: v Sharpen edges v M. Flynn, RSNA 1999 Brent K. Stewart, Ph. D, DABMP 30

Image Processing Brent K. Stewart, Ph. D, DABMP 31

Computed Radiography (CR) System (3) v v v IP dynamic range = 104, about 100 x that of S-F (102) Very wide latitude → flat contrast Image processing required: v v v Enhance contrast Spatial-frequency filtering CR’s wide latitude and image processing capabilities produce reasonable OD or DV for either under or overexposed exams Helps in portable radiography: where the tight exposure limits of S-F are hard to achieve Underexposed → ↑ quantum mottle and overexposed → unnecessary patient dose c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 296. Brent K. Stewart, Ph. D, DABMP 32

Unsharpmasked Spatial Frequency Processing c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 313. Brent K. Stewart, Ph. D, DABMP 33

Global Processing v v Most common global image processing: window/level Global processing algorithm v v v v I’(x, y) = c ∙ [I(x, y) – a]: essentially y = mx + b Level (brightness) set by a Window (contrast) set by c I’ = [2 N/ww]∙[I-{wl-(ww/2)}], where ww = window width and wl = window level Need threshold limits when max/min [2 N-1, 0] digital values encountered If I’(x, y) > Tmax→I’(x, y) = Tmax If I’(x, y) < Tmin→I’(x, y) = Tmin c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , pp. 92 and 311. Brent K. Stewart, Ph. D, DABMP 34

Image Processing Based on Convolution v v Convolution: Ch. 10 - Image Quality and Ch. 13 - CT Defined mathematically as passing a N-dimensional convolution kernel over an N-dimensional numeric array (e. g. , 2 D image or CT transmission profile) At each location (x, y, z, t, . . . ) in the number array multiply the convolution kernel values by the associated values in the numeric array and sum Place the sum into a new numeric array at the same location c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 312. Brent K. Stewart, Ph. D, DABMP 35

Image Processing Based on Convolution v v v Delta function kernel 0 0 1 0 0 Blurring kernel (normalization) also known as low-pass filter 1/9 1/9 1/9 Edge sharpening kernel -1 -1 9 -1 -1 c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 313. Brent K. Stewart, Ph. D, DABMP 36

Image Processing Based on Convolution v v Convolution kernels can be much larger than 3 x 3, but usually N x M with N and M odd Can also perform edge sharpening by subtracting blurred image from original → high-frequency detail (harmonization) The edge sharpened image can then be added back to the original image to make up for some blurring in the original image: CR unsharpmasking - freq. processing The effects of convolution cannot in general be undone by a ‘de-convolution’ process due to the presence of noise, but a deconvolution kernel can be applied to produce an approximation: 19 F MRI Brent K. Stewart, Ph. D, DABMP 37

Median and Sigma Filtering v v v Convolution of an image with a kernel where all the values are the same, e. g. (1/Nx. M), essentially performs an average over the kernel footprint Smoothing or noise reduction This can make the resulting output value susceptible to outliers (high or low) Median filter: rank order values in kernel footprint and take the median (middle) value Sigma filter: set sigma (s) value (e. g. , 1) and throw out all values in kernel footprint > m + s or < m – s and then take the average and place in output image Brent K. Stewart, Ph. D, DABMP 38

Multiresolution/Multiscale Processing and Adaptive Histogram Equalization (AHE) v v v Some CR systems (Agfa/Fuji) make use of multiresolution image processing (AKA unsharpmasking) to enhance spatial resolution Wavelet or pyramidal processing on multiple frequency scales Histogram equalization re-distributes image digital values to uniformly span the entire digital value range [2 N-1, 0] to maximize contrast AHE does this on a spatial sub-region basis in an image rather than the entire image Fuji ‘Dynamic Range Control’ (DRC) a version of AHE that operates on sub-regions of digital values Brent K. Stewart, Ph. D, DABMP 39

Histogram Equalization Properly Exposed Image Over-exposed Image Under-exposed Image Histogram Equalized Image c. f. http: //www. wavemetrics. com/products/igorpro//imageprocessing/ http: //www. wavemetrics. com/products/ imageprocessing/imagetransforms/histmodification. htm Brent K. Stewart, Ph. D, DABMP 40

Global and Adaptive Histogram Equalization The following images illustrate the differences between global and adaptive histogram equalization. MR image with the corresponding gray-scale histogram. The histogram has a peak at minimum intensity consistent with the relatively dark nature of the image. Global histogram equalization and the final gray -scale histogram. Comparing the results with the figure above we can see that the distribution was shifted towards higher values while the peak at minimum intensity remains. Adaptive histogram equalization shows better contrast over different parts of the image. The corresponding gray-scale histogram lacks the mid -levels present in the global histogram equalization as a result of setting a high contrast level. Brent K. Stewart, Ph. D, DABMP c. f. http: //www. wavemetrics. com/products/igorpro//imageprocessing/ http: //www. wavemetrics. com/products/ imageprocessing/imagetransforms/histmodification. htm 41

Contrast vs. Spatial Resolution in Digital Imaging v v S-F mammography can produce images w/ > 20 lp/mm According to Nyquist criterion would require 25 mm/pixel resulting in a 7, 200 x 9, 600 image (132 Mbytes/image) Digital systems have inferior spatial resolution However, due to wide dynamic range of digital detectors and image processing capabilities, digital systems have superior contrast resolution c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 315. Brent K. Stewart, Ph. D, DABMP 42

Digital Imaging Systems and DQE v v v v Remember the equation for DQE(f)? DQE(f) = α-Si DR α-Se DR How can we account for this? Both CR and the screens in film/screens made thin Film higher spatial resolution than CR DQE higher for α-Si systems using Cs. I and Gd 2 O 2 S rather than α-Se (mean x-ray E & Z) α-Si DQE falling off more rapidly than α-Se (geometry) Brent K. Stewart, Ph. D, DABMP 43

Digital versus Analog Processes & Implementation v v Although some of the previous image reception systems were labeled ‘digital’, the initial stage of those devices produce an analog signal that is later digitized CR: x-rays→VL→PMT→current→voltage→ADC CCD, direct & indirect digital detectors: stored e- → ADC Benefits of CR v v Same exam process and equipment as screen-film radiography Many exam rooms serviced by one reader Lower initial cost Benefits of DR v Throughput ↑: radiographs available immediately for QC & read Brent K. Stewart, Ph. D, DABMP 44

Patient Dose Considerations v v Over and underexposed digital receptors produce images with reasonable OD or gray scale values As overexposure can occur, need monitoring program CR IP acts like a 200 speed S-F system wrt. QDE Use the CR sensitivity (‘S’) number to track dose v v Bone, spine and extremities: 200 Chest: 300 General imaging including abdomen and pelvis: 300/400 Flat panel detectors can reduce radiation dose by 2 -3 x as compared with CR for the same image quality due to ↑ quantum absorption efficiency & conversion efficiency Brent K. Stewart, Ph. D, DABMP 45

Using the CR Sensitivity Number to Track Dose Brent K. Stewart, Ph. D, DABMP 46

Huda Ch 6: Digital X-ray Imaging Question v 12. Photostimulable phosphor systems do NOT include: v A. Analog-to-digital converters B. Barium fluorohalide C. Light detectors (blue) D. Red light lasers E. Video cameras v v Brent K. Stewart, Ph. D, DABMP 47

Huda Ch 6: Digital X-ray Imaging Question v 11. Which of the following x-ray detector materials emits visible light: v A. Xenon B. Mercuric iodide C. Lead iodide D. Selenium E. Cesium iodide v v Brent K. Stewart, Ph. D, DABMP 48

Raphex 2002 Question: Digital Radiography v v v D 47. Concerning computed radiography (CR), which of the following is true? A. Numerous, small solid-state detectors are used to capture the x-ray exposure patterns. B. It has better spatial resolution than film. C. It is ideal for portable x-ray examinations, when phototiming cannot be used. D. It is associated with high reject/repeat rates. E. The image capture, storage, and display are performed by the receiver. Brent K. Stewart, Ph. D, DABMP 49

Huda Ch 6: Digital X-ray Imaging Question v 13. Photoconductors convert x-ray energy directly into: v A. Light B. Current C. Heat D. Charge E. RF energy v v Brent K. Stewart, Ph. D, DABMP 50

Huda Ch 6: Digital X-ray Imaging Question v 15. Which of the following does NOT involve image processing: v A. Background subtraction B. Energy subtraction C. Histogram equalization D. K-edge filtering E. Low-pass filtering v v Brent K. Stewart, Ph. D, DABMP 51

Huda Ch 6: Digital X-ray Imaging Question v 14. Processing a digital x-ray image by unsharpmask enhancement would increase the: v A. Bit depth per pixel B. Matrix size C. Patient dose D. Visibility of edges E. Limiting spatial resolution v v Brent K. Stewart, Ph. D, DABMP 52

Adjuncts and other interesting stuff Brent K. Stewart, Ph. D, DABMP 53

Geometric (Linear) Tomography v v v With the advent of CT, geometric tomography has only limited clinical utility where only one or a few planes of objects with high contrast are desired, e. g. , IVP Desired slice through patient set at pivot point (focal plane) The tomographic process blurs out regions outside the focal plane, but still contributes to overall loss of contrast Larger tomographic angles result in a lessening of out of plane contributions High dose, comparable to CT for many tomographic slices c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 318. Brent K. Stewart, Ph. D, DABMP 54

Digital Tomosynthesis v v Improved version of geometric tomography where a digital detector saves an image at each of several tube angles This allows reconstruction of multiple planes through the object through shifting the various images through a certain distance before summing them Much more dose efficient, but still suffers from out of plane blurring effects Either CR or DR used c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 320. Brent K. Stewart, Ph. D, DABMP 55

Temporal Subtraction v v Digital Subtraction Angiography (DSA) – usually 1 K resolution Mask (background) subtracted from images during/post contrast injection: Δ < 1% trans. visualized Motion cause misregistration artifacts Digital value proportional to contrast concentration and vessel thickness v v v Is = ln(Im) – ln(Ic) = mvessel ∙ tvessel Temporal subtraction works best when time differences between images is short Possible to spatially warp images taken over a longer period of time c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 322. Brent K. Stewart, Ph. D, DABMP 56

Dual-Energy Subtraction v v v Exploits differences between the Z of bone (Zeff ≈ 13) and soft tissue (Zeff ≈ 7. 6) Images taken either at two different k. Vp (two-shot) One image (one-shot) taken with energy separation provided by a filter (sandwich) Iout = loge(Ilow) – R ∙ loge(Ihigh), where R is altered to produce soft-tissue predominant or bone predominant images GE Chest DR @ SCCA c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 324. Brent K. Stewart, Ph. D, DABMP 57

Dual-Energy Subtraction c. f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed. , p. 325. Brent K. Stewart, Ph. D, DABMP 58

Huda Ch 6: Digital X-ray Imaging Question v 22. The matrix size in a DSA image is typically: v A. 128 x 128 B. 256 x 256 C. 512 x 512 D. 1024 x 1024 E. 2048 x 2048 v v Brent K. Stewart, Ph. D, DABMP 59

Huda Ch 6: Digital X-ray Imaging Question v v v 25. Changing the DSA matrix from 10242 to 20482 would NOT increase the: A. Data digitization rate B. Data storage requirement C. Image processing time D. Spatial resolution E. Pixel size Brent K. Stewart, Ph. D, DABMP 60

Raphex 2003 Question: Digital Radiography v v v D 51. A flat panel digital radiographic detector has a square 20 x 20 cm image receptor field. The full field of the detector is coupled to a nominal 2048 x 2048 CCD array. The relative spatial resolution (lp/mm) when going from a 20 x 20 cm to a 10 x 10 cm field of view is: A. Four times better B. Twice as good C. The same D. Half as good E. One fourth as good Brent K. Stewart, Ph. D, DABMP 61

Huda Ch 6: Digital X-ray Imaging Question v 17. The Nyquist frequency for a 1 K digital photospot image (25 cm image intensifier diameter) is: v v A. 1 lp/mm B. 2 lp/mm C. 4 lp/mm D. 8 lp/mm E. 10 lp/mm v FN (lp/mm) = 1/2 a = 1/2(1024 lines/250 mm) = 2. 048 ≈ 2 v v v Brent K. Stewart, Ph. D, DABMP 62

Digital Representation of Data (1) v Bits, Bytes and Words v v Smallest unit of storage capacity = 1 bit (binary digit: 1 or 0) Bits grouped into bytes: 8 bits = byte Word = 16, 32 or 64 bits, depending on the computer system addressing architecture Computer storage capacity is measured in: v v kilobytes (k. B) - 210 bytes = 1024 bytes a thousand bytes megabytes (MB) - 220 bytes = 1024 kilobytes a million bytes gigabytes (GB) - 230 bytes = 1024 megabytes a billion bytes terabytes (TB) - 240 bytes = 1024 gigabytes a trillion bytes Brent K. Stewart, Ph. D, DABMP 63

Digital Representation of Data (2) v Digital Representation of Different Types of Data v v Alphanumeric text, integers, and non-integer data Storage of Positive Integers v v v In general, n bits have 2 n possible permutations and can represent integers from 0 to 2 n-1 (the range usually denoted with square brackets): n bits represents 2 n values with range [0, 2 n-1] 8 bits represents 28 = 256 values with range [0, 255] 10 bits represents 210 = 1024 values with range [0, 1023] 12 bits represents 212 = 4096 values with range [0, 4095] 16 bits represents 216 = 65, 536 values with range [0, 65535] Brent K. Stewart, Ph. D, DABMP 64

Conversion of Analog Data to Digital Form v v v The electronic measuring devices of medical scanners (e. g. , transducers and detectors) produce analog signals Analog to digital conversion (analog to digital converter – ADC) ADCs characterized by v v sampling rate or frequency (e. g. , samples/sec – 1 MHz) number of bits output per sample (e. g. , 12 bits/sample = 12 -bit ADC) c. f. Bushberg, et al. , The Essential Physics of Medical Imaging, 2 nd ed. , p. 69. Brent K. Stewart, Ph. D, DABMP 65

Periodic Table of the Elements c. f. http: //www. ktf -split. hr/periodni/en/ http: //www. ktf-split. hr/ Brent K. Stewart, Ph. D, DABMP 66