Digital Image Quality:

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Digital Image Quality

• Is there a sacrifice?

Terri L. Fauber, R.T. (R)(M) Virginia
Commonwealth University School of Allied Health
Professions Department of Radiation Sciences
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Topics

• Radiation Risk
• Image Receptors
• Patient Exposure
• Research Experiment
• Reducing Patient Exposure

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Radiation Exposure

• High-dose radiation exposure is harmful.
• Low-dose radiation exposure can be harmful.
• Practice ALARA
• As Low as Reasonably Achievable.

4
Low Dose Exposure

• Important societal issues
• Cancer screening tests
• Nuclear power
• Occupational exposure
• Frequent-flyer
• Manned space exploration
• Radiologic terrorism
• Brenner et al, 2003

5
Radiation Risk

• Most of our knowledge about risk to low doses of
  radiation is estimated using extrapolation
  techniques.
• Estimating a value beyond the range of known
  values.

Some believe low dose risks have been
underestimated and others believe it may be
overestimated.
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Radiation Risk

• The debate continues on the health risks due to
  low-dose radiation exposures.
• Linear dose-response relationship.
• Threshold vs. nonthreshold.
• Radiation hormesis.

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Linear Nonthreshold Effect

• The response is directly proportional to the
  radiation dose.
• Even a small dose will have an effect.

Response
Dose
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Linear Threshold

• A certain level of radiation dose must be
  reached before an effect occurs.

Response
Dose
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Radiation Risk

• The U.S. Department of Health and Human Services
  identified x-rays as a known human carcinogen.
• National Academies support the belief that low
  levels of radiation may cause harm.

(Supports linear nonthreshold risk model.)
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Radiation Risk

• Exposure to x-rays (lt 0.2 Gy or 20 rads)
  is strongly associated with leukemia and cancer
  of the thyroid, breast and lung.
• (11th Report on Carcinogens, 2005)
• Direct epidemiological evidence demonstrates
  that an organ dose of 0.01 Gy or 1 RAD of
  diagnostic x-rays is associated with an
  increase in cancer risk.
• (Brenner et al, 2003)

Risks may vary for acute vs. protracted low dose
exposure.
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USRT National Study

• In 1982 a national longitudinal study of R.T.s
  certified between 1926 and 1982 was initiated to
  evaluate health outcomes associated with
  long-term occupational exposure to radiation.
• The third survey was completed in 2004.

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USRT National Study

• Radiographers who began working before 1950 had
  an increased risk of developing breast cancer.
• Compared with similar people in the general U.S.
  population, radiologic technologists had lower
  mortality risks for all causes of death and for
  all cancers combined.

Mohan et al, 2003
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Radiation Hormesis

• Proposes small doses of radiation may actually
  be healthful by stimulating the immune system.
• Some scientific evidence indicates that low
  radiation doses increase life span.

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Radiation Hormesis

• Proponents say benefits of radiation have been
  known for the past century.
• It is suggested that this evidence has been
  suppressed by special interest groups.

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Biological Variability

• Mutagens
• Carcinogens
• Tumor promoters
• Cancer-protective substances

• Environment
• Diet
• Lifestyle
• Age

Prasad at el, 2003
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The Continuing Debate

• Until undisputed research confirms or refutes the
  harmful effects of low radiation dose, we will
  not truly know its effect.
• Regardless, we must continue to protect our
  patients from unnecessary radiation exposure.

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Image Receptor Technology

• Film-screen
• Exposure errors apparent on image.
• Repeats typically due to exposure errors.
• Digital
• Wide dynamic range.
• Reduction in repeats due to exposure errors.
• Provides information on radiation exposure to
  receptor.

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Digital Radiography

• An advantage of digital radiography is its wider
  dynamic range.
• Produces diagnostic densities within a wider
  range of exposures
• Computer can display a wider range of optical
  densities

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Digital Image Processing

• Adjustments made for low or high exposures
• Exposure Indicator a number displayed to
  indicate the level of x-ray exposure received to
  the imaging plate.
• Kodak Exposure index (EI) uses a logarithmic
  scale and every 300 EI change in exposure x 2.
• Agfa Log median value (LgM) uses logarithmic
  scale and every 0.3 change in exposure x 2
• Fuji Sensitivity number (S)
• S value is inversely proportional to the mR
  (exposure) reaching the image receptor, an S
  value of 200 indicates proper exposure, 400 S
  value ½ as much exposure and 100 S value 2 x
  exposure

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Digital Radiography

• Computed Radiography (Cassette-based)

Memory storage
Digital data
Reader (laser)
Display monitor
X-ray photons exiting pt.
Laser printer
Photostimulable phosphor

• Direct Digital Radiography (Cassetteless)

Memory storage
Digital data
Display monitor
X-ray photons exiting pt.
Laser printer
Fixed imaging plate
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Computed Radiography
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Direct Digital Radiography
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Digital Image Receptors

• Computed radiography (CR)
• Photostimulable phosphor (IP).
• CR reader extracts latent image.
• Light energies converted to digital data.

• Direct Digital Radiography (DR)
• Electronic detectors.
• Direct capture of latent image.
• Indirect or direct conversion of latent image
  to digital image.

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Digital Image Characteristics

• A digital image is displayed as a combination
  of rows and columns known as matrix
• The smallest component of the matrix is the
  pixel (picture element)
• The location of the pixel within the image
  matrix corresponds to an area within the
  patient or volume of tissue referred to as
  voxel

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Matrix Size
For a given field of view, a larger matrix size
includes a greater number of smaller pixels.
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Image Characteristics

• Each pixel is assigned a numeric value that
  represents a shade of gray based on the
  attenuation characteristics of the volume of
  tissue imaged

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Pixel Depth

• Each pixel has a bit depth and the number of
  bits determines the number of shades of gray the
  system is capable of displaying on the digital
  images.
• 10- and 12- bit pixel can display 1024 and 4096
  shades of gray, respectively.
• Increasing pixel bit depth improves image quality

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Digital Image Visibility

• The center or midpoint of the window level and
  the width of the window will demonstrate density
  and contrast of the displayed image.

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Image Receptors

• Digital
• Wide exposure latitude.
• Acquisition of data separate from processing
  and display.
• Post-processing capabilities.

• Film-screen
• Narrow exposure latitude.
• Film acquires,
• processes and displays image.
• No changes after processing.

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Current Literature

• Digital technologies compensate for under- and
  overexposures.
• Low radiation exposures increase the amount of
  quantum mottle (noise).
• High radiation exposures produce a more visually
  appealing image
• (less noisy).

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Current Literature

• New relationships exist between radiation
  exposure and digital image receptors and
  radiographers might not be aware of increased
  radiation exposure to patients.
• Radiographers might not be as precise in
  selecting optimal exposure techniques.

Exposure Factor Creep
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Patient Radiation Exposure

• Compagnone et al, 2006 study compared patient
  radiation doses for six standard exams using
  film-screen, CR, and DR.
• Entrance skin dose (ESD) and Effective dose (E)
  were calculated.
• Image quality was assessed by radiologists.

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Study Findings

• CR generally results in higher ESDs than
  film-screen and DR.
• Effective doses for DR were lower than CR and
  Film-screen
• Radiologists preferred the appearance of the DR
  images.

(Compagnone et al, 2006.)
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Patient Radiation Exposure

• Warren-Forward et al, 2007 conducted a
  retrospective analysis of exposure indices for
  chest and lumbar exams within two hospitals.
• Also, phantoms were exposed to a range of
  kilovoltages to evaluate the relationship
  between exposure indices and ESD.

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Study Findings

• A high percentage of exposure indices were
  outside of the recommended range for both
  hospitals, indicating that over- and
  underexposures had occurred.
• The experimental phantom exposures found that a
  small increase in exposure indices produced a
  large increase in entrance-surface dose.

(Warren-Forward et al, 2007.)
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Patient Radiation Exposure

• Concerns raised regarding patient CR exposures,
  especially for pediatric patients.
• Optimum image quality not necessary for
  follow-up CR images.
• Manufacturer-recommended exposure ranges may be
  set too high.

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Research Experiment

• Investigated the effect of varying radiation
  exposures on CR image quality.
• Optical density.
• Density differences (contrast).
• Low and high.
• Resolution ( of line pairs visible)

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Research Question

• What effect will extreme variation in the
  quantity of radiation incident on the CR imaging
  plate have on optical density, contrast and
  resolution?

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Research Design

• True Experimental
• Independent Variable
• Quantity of radiation incident on the CR
  imaging plate (IP).
• Dependent Variables
• Optical densities.
• Density differences.
• of line pairs visible.

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Equipment

• GE MVP 60 3-phase radiographic unit.
• Fuji FCR 1 Shot QC phantom.
• Fuji FCR XG-1 Smart CR Reader.
• 5 14 x 17 Fuji Smart CR IPs, Type C.
• FujiFilm FM DPL laser printer.
• X-rite Densitometer Model 301.

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Baseline Phantom Image

• Contrast Patches
• High-density differences.
• Low-density differences.
• Resolution Test Pattern
• Line pairs/mm.
• Density Circle

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Data Collection

• Baseline image created according to Fuji QC
  specifications.
• Exposure groups determined by dividing baseline
  mAs by a factor of 2 and multiplying baseline
  mAs by a factor of 2.
• Exposures ranged from 1 to 125 mAs, yielding 8
  groups.
• Five images were exposed, processed and printed
  for each exposure group.

• Optical densities were measured with a
  densitometer.
• High- and low-contrast density patches were
  measured and the difference between the
  circle densities calculated.
• of Lp/mm was determined visually using a
  magnifying glass.

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Results

• S number
• 1 mAs 1847
• 2 mAs 818
• 4 mAs 396
• 8 mAs 204
• 16 mAs 102
• 32 mAs 52
• 64 mAs 27
• 125 mAs 13

• Resolution in Line pairs/mm
• 1 mAs 2.500
• 2 mAs 2.625
• 4 mAs 2.875
• 8 mAs 2.875
• 16 mAs 2.700
• 32 mAs 2.750
• 64 mAs 2.750
• 125 mAs 2.875

Inverse proportional change in S
Less resolution at extremely low mAs
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Optical Density
8
125
1
Increasing mAs
Range 1.40 1.432
Difference .032 lt.05 O.D.)
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High-density Differences
8
4
2
1
125
64
32
16
mAs
Difference 0.09 gt 0.05 O.D.
Range 0.868 - 0.776
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Research Findings

• Exposure indicator reflected proportional change
  in mAs.
• Optical density stable for extreme exposure
  variation.
• Low-density differences stable for extreme
  exposure variation.
• High-density differences decreased for high
  exposures (decreases contrast).
• Resolution decreased at low exposures.

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Conclusions

• Computed radiography can compensate for extreme
  radiation exposure variability.
• Resolution decreased at low exposures.
• High-density differences (contrast) decreased at
  high exposures.

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Implications

• Exposure errors will not reduce image quality.
• The radiographer is responsible for selecting
  exposure techniques that will produce quality
  digital images with the least amount of
  exposure to the patient.

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Additional Research

• Replicate study on other types of digital
  equipment.
• Investigate extreme exposure variation on the
  image quality of patient- equivalent phantoms.
• Investigate radiologists perception of digital
  quality for images created with extreme exposure
  variation.

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• Recognize and comprehend the different
  relationship between radiation exposure and
  digital imaging systems.

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Reducing Patient Exposure

• Select exposure techniques that reduce patient
  exposure while maintaining diagnostic image
  quality.
• Use higher kVp and lower mAs whenever possible.
• Monitor exposure indicator for feedback on
  patient exposure.

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Strategies

• Use automatic exposure control (AEC) devices
  routinely and correctly.
• Develop and use technique charts.
• Use less exposure for acceptable quality
  whenever possible.

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Continuing Education

• Become more knowledgeable about digital
  technologies.
• Dosimetric monitoring systems may be a reality,
  including
• Reference values.
• Calculated entrance skin dose and dose-area
  product (DAP).
• Warning message for exposure errors.
• Patient exposure dose audits

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This research project was supported by a grant
from the ASRT Education and Research Foundation.