Digital Radiography

1
Digital Radiography Chapter 11Adjuncts to
Radiology Chapter 12

• Brent K. Stewart, PhD, DABMP
• Lois Rutz, M.S. Radiation Safety Engineering,
  Inc.
• a copy of Brent Stewarts unmodified lecture may
  be found at
• http//courses.washington.edu/radxphys/PhysicsCour
  se04-05.html

2
Take Away Five Things You should be able to
Explain after the DR/Adjuncts Lecture

• 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

3
Why Digital/Computed Radiography

• Limitations on Film/Screen radiography
• 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.

4
Why Digital/Computed Radiography cont.

• Screen/film dynamic range 2 to 2.5 orders of
  magnitude.
• Different applications require different
  screen/film combinations.
• Only one original image.
• Films often go missing from ER or ICU and never
  are archived.
• Copies expensive, have inconsistent quality, and
  often are non-diagnostic.
• Archive space expensive, often remote.
• Digitizing film is only way to move images to
  PACS.

5
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.

6
How does Digital/Computed Radiography solve these
problems? Cont.

• Image processing can modify and enhance raw
  (pre-processed) 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.

7
CR vs. DR

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

8
CR

• Detector or Imaging Plate (IP) is essentially a
  type of intensifying screen.
• IP can be used in any bucky or table-top system.
• IP is relatively robust. Requires same care as
  intensifying screens.
• Process is indirect.
• 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.

9
CR and DR Systems
10
Image Production in CR/DR Systems

• 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.
• Processing is related to acquisition system
  characteristics.
• Signal (analog) is converted via ADC to a bit
  value in a digital matrix.
• Digital image is processed.
• Processing is related to desired image
  information.
• Digital matrix is displayed on a video screen or
  printed to paper or film.

11
Signal Processing

• Primarily to accommodate variations in the
  detector/electronics components.
• Involves corrections for dead space,
  non-uniformities, defects.
• 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.

12
Digital Image Correction

• 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
• Gain image G(x,y) G(x,y) - Davg(t)(x,y) Gavg
  (1/N) ? ? G(x,y)
• Make corrections for each detector element (map)
• 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

13
Digital Image Correction
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 310.
14
Detectors

• In order to understand signal processing we need
  to learn about the detectors.
• Photo Stimulable Phosphor Plates
• Photoconductive materials.
• Detector consists of a receptor material (e.g.
  BaF(H)Eu), and a set of signal readout and
  conversion electronics.
• Receptor responsible for the DQE.
• Rest of the system contributes to noise,
  resolution,dynamic range.

15
Detectors in Digital Imaging (1)

• 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 e- produced by
  ionization in the gas (typically high Z gases
  like Xenon Z54, K-edge 35 keV)
• Were used in older CT units

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.32.
16
Detectors in Digital Imaging (2)

• Solid-state materials
• Electrons arranged in bands with conduction band
  usually empty
• Solid-state detectors
• 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.
17
Detectors in Digital Imaging (3)
c.f. Yaffe MJ and Rowlands JA. Phys. Med. Biol.
42 (1997), p. Elements of Digital Radiology, p.
9.
18
Computed Radiography (CR)

• Photostimulable phosphor (PSP)
• Barium fluorohalide 85 BaFBrEu 15 BaFIEu
• e- from Eu2 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 Eu3 ions and emit
  a blue-green (400-500 nm) visible light (VL)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 295.
19
Computed Radiography (CR) System (1)

• 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, 2nd ed., p. 294.
20
Computed Radiography (CR) System (2)

• 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
• 200mm and 100mm pixel size - (14x17 1780x2160
  and 3560x4320, respectively)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 294.
21
Indirect Flat Panel Detectors

• Use an intensifying screen (CsI) 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, 2nd ed., p. 301.
22
Charged-Coupled Devices (CCD)

• Form images from visible light
• Videocams digital cameras
• Each picture element (pixel) a photosensitive
  bucket
• After exposure, the elements electronically
  readout via shift-and-read logic and digitized
• Light focused using lenses or fiber-optics
• Fluoroscopy (II)
• Digital cineradiography (II)
• Digital biopsy system (phosphor screen)
• 1K and 2K CCDs used

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 298-299.
23
Direct Flat Panel Detectors

• Use a layer of photoconductive material (e.g.,
  a-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
• Selenium (Z34)
• CdTe, HgI2 and PbI2

Indirect Flat Panel Detector (for comparison)
Direct Flat Panel Detector
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 304.
24
Thin-Film Transistors (TFT)

• 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, 2nd ed., p. 301.
25
Resolution and Fill Factor

• Dimension of detector element largely determines
  spatial resolution
• 200mm and 100mm pixel size typical
• For dimension of a mm - Nyquist frequency FN
  1/2a
• If a 100mm ? 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, 2nd ed., p. 303.
26
Image Digitization and Processing

• After acquisition and correction of raw data, the
  image is ready for display processing.
• The image data consists of a matrix of numbers.
  Each pixel is one matrix point. Each gray scale
  is a digital value.
• 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.

27
Digital Storage of Images

• Usually stored as a 2D 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
• CT 512x512x12 bits/pixel
• CR 1760x2140x10 bits/pixel
• DR 2048x2560x16 bits/pixel

c.f. Huang, HK. Elements of Digital Radiology, p.
8.
28
Image Processing

• 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.
• Process involves mathematical filters.
• Image data is display processed to give desired
  contrast and density.
• Process involves re-mapping along a chosen
  display (HD) curve

29
Generic Display Processing

• Different manufacturers may use different
  versions of generic image processing methods.
• E.g. Musica, Ptone
• All describe means of scaling and modifying image
  appearance.
• Different manufacturers use different exposure
  indicators.
• E.g. EI, S, IgM
• All describe the relationship between the
  exposure to the detector and the pixel value.

30
Generic Elements of Display Processing

• Exposure Recognition.
• Adjust for high/low average exposure
• Signal Equalization
• Adjust regions of low/high signal value
• Grayscale Rendition
• Convert signal values to display values
• Edge Enhancement
• Sharpen edges
• M. Flynn, RSNA 1999

31
Image Processing
32
Computed Radiography (CR) System (3)

• IP dynamic range 104, about 100x that of S-F
  (102)
• Very wide latitude ? flat contrast
• Image processing required
• Enhance contrast
• Spatial-frequency filtering
• CRs 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, 2nd ed., p. 296.
33
Unsharpmasked Spatial Frequency Processing
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 313.
34
Global Processing

• Most common global image processing window/level
• Global processing algorithm
• I(x,y) c I(x,y) a essentially y mx
  b
• Level (brightness) set by a
• Window (contrast) set by c
• I 2N/wwI-wl-(ww/2), where ww window
  width and wl window level
• Need threshold limits when max/min 2N-1, 0
  digital values encountered
• If I(x,y) gt Tmax?I(x,y) Tmax
• If I(x,y) lt Tmin?I(x,y) Tmin

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 92 and 311.
35
Image Processing Based on Convolution

• 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., 2D 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, 2nd ed., p. 312.
36
Image Processing Based on Convolution

• Delta function kernel
• Blurring kernel (normalization) also known as
  low-pass filter
• Edge sharpening kernel

0 0 0
0 1 0
0 0 0
1/9 1/9 1/9
1/9 1/9 1/9
1/9 1/9 1/9
-1 -1 -1
-1 9 -1
-1 -1 -1
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 313.
37
Image Processing Based on Convolution

• 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 19F MRI

38
Median and Sigma Filtering

• Convolution of an image with a kernel where all
  the values are the same, e.g. (1/NxM),
  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 gt m s
  or lt m s and then take the average and place in
  output image

39
Multiresolution/Multiscale Processing and
Adaptive Histogram Equalization (AHE)

• 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 2N-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

40
Histogram Equalization
Properly Exposed Image
Over-exposed Image
Under-exposed Image
Histogram Equalized Image
c.f. http//www.wavemetrics.com/products/igorpro/i
mageprocessing/imagetransforms/histmodification.ht
m
41
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.
c.f. http//www.wavemetrics.com/products/igorpro/i
mageprocessing/imagetransforms/histmodification.ht
m
42
Contrast vs. Spatial Resolution in Digital Imaging

• S-F mammography can produce images w/ gt 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, 2nd ed., p. 315.
43
Digital Imaging Systems and DQE

• Remember the equation for DQE(f)?
• DQE(f)
• 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 a-Si systems using CsI and Gd2O2S
  rather than a-Se (mean x-ray E Z)
• a-Si DQE falling off more rapidly than a-Se
  (geometry)

a-Si DR
a-Se DR
44
Digital versus Analog Processes Implementation

• 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
• Same exam process and equipment as screen-film
  radiography
• Many exam rooms serviced by one reader
• Lower initial cost
• Benefits of DR
• Throughput ? radiographs available immediately
  for QC read

45
Patient Dose Considerations

• 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
• Bone, spine and extremities 200
• Chest 300
• General imaging including abdomen and pelvis
  300/400
• Flat panel detectors can reduce radiation dose by
  2-3x as compared with CR for the same image
  quality due to ? quantum absorption efficiency
  conversion efficiency

46
Using the CR Sensitivity Number to Track Dose
47
Huda Ch6 Digital X-ray Imaging Question

• 12. Photostimulable phosphor systems do NOT
  include
• A. Analog-to-digital converters
• B. Barium fluorohalide
• C. Light detectors (blue)
• D. Red light lasers
• E. Video cameras

48
Huda Ch6 Digital X-ray Imaging Question

• 11. Which of the following x-ray detector
  materials emits visible light
• A. Xenon
• B. Mercuric iodide
• C. Lead iodide
• D. Selenium
• E. Cesium iodide

49
Raphex 2002 Question Digital Radiography

• D47. 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.

50
Huda Ch6 Digital X-ray Imaging Question

• 13. Photoconductors convert x-ray energy
  directly into
• A. Light
• B. Current
• C. Heat
• D. Charge
• E. RF energy

51
Huda Ch6 Digital X-ray Imaging Question

• 15. Which of the following does NOT involve image
  processing
• A. Background subtraction
• B. Energy subtraction
• C. Histogram equalization
• D. K-edge filtering
• E. Low-pass filtering

52
Huda Ch6 Digital X-ray Imaging Question

• 14. Processing a digital x-ray image by
  unsharpmask enhancement would increase
  the
• A. Bit depth per pixel
• B. Matrix size
• C. Patient dose
• D. Visibility of edges
• E. Limiting spatial resolution

53
Adjuncts and other interesting stuff
54
Geometric (Linear) Tomography

• 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, 2nd ed., p. 318.
55
Digital Tomosynthesis

• 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, 2nd ed., p. 320.
56
Temporal Subtraction

• Digital Subtraction Angiography (DSA) usually
  1K resolution
• Mask (background) subtracted from images
  during/post contrast injection ? lt 1 trans.
  visualized
• Motion can cause misregistration artifacts
• Digital value proportional to contrast
  concentration and vessel thickness
• 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, 2nd ed., p. 322.
57
Dual-Energy Subtraction

• Exploits differences between the Z of bone (Zeff
  13) and soft tissue (Zeff 7.6)
• Images taken either at two different kVp
  (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 _at_ SCCA

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 324.
58
Dual-Energy Subtraction
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 325.
59
Huda Ch6 Digital X-ray Imaging Question

• 22. The matrix size in a DSA image is
  typically
• A. 128 x 128
• B. 256 x 256
• C. 512 x 512
• D. 1024 x 1024
• E. 2048 x 2048

60
Huda Ch6 Digital X-ray Imaging Question

• 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

61
Raphex 2003 Question Digital Radiography

• D51. 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

62
Huda Ch6 Digital X-ray Imaging Question

• 17. The Nyquist frequency for a 1K digital
  photospot image (25 cm image intensifier
  diameter) is
• A. 1 lp/mm
• B. 2 lp/mm
• C. 4 lp/mm
• D. 8 lp/mm
• E. 10 lp/mm
• FN (lp/mm) 1/2a 1/2(1024 lines/250 mm)
  2.048 2

63
Digital Representation of Data (1)

• Bits, Bytes and Words
• 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
• kilobytes (kB) - 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

64
Digital Representation of Data (2)

• Digital Representation of Different Types of Data
• Alphanumeric text, integers, and non-integer data
• Storage of Positive Integers
• In general, n bits have 2n possible permutations
  and can represent integers from 0 to 2n-1 (the
  range usually denoted with square brackets)
• n bits represents 2n values with range 0, 2n-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

65
Conversion of Analog Data to Digital Form

• 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
• 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, 2nd ed., p. 69.
66
Periodic Table of the Elements
c.f. http//www.ktf-split.hr/periodni/en/