Radiation Protection in Digital Radiology

IAEAInternational Atomic Energy Agency

Radiation Protection in Digital Radiology

Optimising DR DisplaysL08

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Educational Objectives

• List three major differences between DR displays and transilluminated films

• Explain how CRTs and LCDs differ with respect to the display of medical images

• Appreciate the differences between medical and commercial grade flat panels

• Give an example of how differences between a technologist’s display and a radiologist’s display can contribute to unnecessary radiation exposure.

Radiation Protection in Digital Radiology L08 Optimising DR Displays

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The Cathode Ray Tube (CRT) is a fifty-year old device for displaying electronic

images• Electrons produced in a

vacuum tube strike a luminescent screen

• The path of the electron beam is deflected by a coil

• The amount of light produced in any position is related to the intensity of the electron beam at that time

• Color can be produced by means of a shadow mask or aperture grill

Bushberg et al. 2002 The Essential Physics of Medical Imaging 2nd EdRadiation Protection in Digital Radiology L08 Optimising DR Displays

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The CRT provides a dynamic display of even stationary images

• The electron beam scans across the face of the display in a raster fashion

• The standard video frame rate is 30 fps (SMPTE)• Historical lowest rate to avoid

perception of flicker• Convenient: ½ of 60 Hz• Alternate frame rates, such as

24 fps for motion pictures• Interleaved display would use 2

frames for one image – higher spatial resolution

• A picture element (pixel) is an arbitrary segment of a scan line

Bushberg et al. 2002 The Essential Physics of Medical Imaging 2nd Ed


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Active Matrix Liquid Crystal Displays (AMLCD) present electronic images by a

different method• The LCD controls the transmission

of a uniform backlight

• The transmission of light through a given LC cell can be considered binary (on/off)• Actually much more complex

• A pixel is composed of six components arranged in a chevron pattern• Two domains

• Three colors

• “Active” refers to control of each pixel independently via TFT array

“Flat Panel”

Bushberg et al. 2002 The Essential Physics of Medical Imaging 2nd EdRadiation Protection in Digital Radiology L08 Optimising DR Displays

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Light transmitted by a flat panel is a composite of pixel component

states

• Un-calibrated response is irregular

• Display controller needs 10-12 bits for medical applications


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Ability to produce color, limits performance of commercial flat

panels

• Color filter allows only 3-5% transmittance of backlight vs. 8-15% for monochrome

• Combination of sub-pixel intensities to yield true white is additional complication


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Off-axis viewing is problematic with flat panels

• CRT emissive luminance is Lambertian, the intensity appears the same from all viewing angles

• Flat panel transmissive luminance is non-Lambertian, the intensity appears different from any viewing angle other than normal (rounds?)

• This is not a problem for single viewer, unless the viewer must move (interventional?, surgery?)

• Even if radiographer has same display as radiologist, off-axis viewing differences can cause discrepancy in rendering the image


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“How do you know that it’s okay for the physician to view images on that

display?”

• Capabilities of the display technology• Characteristics of the images to be viewed• Idiosyncrasies of human visual system• Configuration of the display device• Calibration of the display device• Local viewing environment• Workstation software and controls• Viewing task to be performed• Active maintenance of display quality• Ambient lighting condition


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Digital imaging is often“display-limited”

• The information contained in the image cannot be presented by the display in a single rendering• Spatial resolution• Contrast resolution• Dynamic range

• Workstations address this problem by software tools to display a portion of the image at full resolution• Zoom and Pan• Window-width and window-level


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The smallest feature that can be displayed is limited by the number of

pixels

Pixels Array size Spot size (mm)

1MP 900 X 1100"1K X 1K”

0.35

2 MP 1200 X 1600“1K X 1.5K”

0.25

3 MP 1500 X 2000“1.5K X 2K”

0.20?

5 MP 2000 X 2500“2K X 2K”

0.15

CRT Beam spot size for 300 X 400 mm Field

Flynn MJ 2004Radiation Protection in Digital Radiology L08 Optimising DR Displays

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• Interference pattern between fixed grid lines and down-sampling rate for display

• Disappeared on zoom

• Bad choices• Display default

magnification factor

• Line rate of grid

“Corduroy” artefact


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This is one place where size doesn’t matter

• The physical size of the display affects only viewing distance• The pixel matrix is what matters

• Large displays are useful for interventional, surgical, and multiple simultaneous observers• Projection Displays• Plasma Displays• DLP displays

• Small displays may be useful for reference or image navigation• PDA• Cell Phone• LEP (light emitting polymers)


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Spatial resolution of the display is limited by blur

• Blur is a major factor in CRT displays because of the dynamic way the pixel is produced

• Blur is much improved in flat panels because of the stationary structure of the pixel• a 3MP flat panel performs as well as a 5MP CRT display


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Electronic displays are limited with respect to maximum luminance

• Typical light box luminance is >500 fL (1700 cd/m2)• Typical medical CRT is 70-90 fL (240-300 cd/m2 )

• minimum ACR is 50 fL (171 cd/m2 ) for primary interpretation

• Typical general purpose CRT is 30 fL (100 cd/m2 )• Medical monochrome LCD is 200 fL (700 cd/m2 )• Typical consumer electronics color LCD display is

60 fL (200 cd/m2 )• Paper SSA20-06 Visser M et al. RSNA 2005

describes prototype backlight up to 2000 cd/m2


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Because of their limited luminance, the viewing environment for electronic

displays is critical

• Ambient illumination limits the contrast that can be appreciated from an electronic display

• The higher the ambient illumination, the higher the maximum luminance the display will need.

• The more reflective the display, the lower the allowable level of ambient illumination.• Big problem with CRT

• Changes in ambient illumination strongly affect contrast in the dark areas of the display, so one strategy is to raise the minimum luminance.• Convenient for flat panels with poor black levels


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The luminance function of electronic displays is not appropriate for viewing

digital images

• The luminance function is modified by a software look-up-table (LUT) in an attempt to elicit equal human visual response for equal changes in grayscale value.

• The mathematics of this transformation are defined in DICOM Part 14 Grayscale Standard Display Function (GSDF).

• The result is that a graph of luminance expressed in units of “just noticeable differences” (JND) is linear with respect to grayscale value.


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Luminance (Cd/m^2) versus PV

1

10

100

1000

0 2000 4000 6000 8000 10000 12000 14000 16000 18000

Pixel ValueL

um

ina

nc

e (

Cd

/m^

2)

Measured (Nominal Lmax=600)

GSDF

(serial num ber A2I-05975)

JND Index versus Input Pixel Value

0

100

200

300

400

500

600

0 2000 4000 6000 8000 10000 12000 14000 16000 18000

Pixel Value

JN

D In

de

x

Series1

Least Squares Fit

(serial number A2I-05975)

R^2 = 0.99984

Luminance of properly calibrated display is curved function of greyscale value

Luminance translated into JND is linear function of greyscale value


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Dell Model E771P Color Monitor

y = 4.888x + 36.444

R2 = 0.9856

y = 4.3844x + 60.704

R2 = 0.9706

0

100

200

300

400

500

600

0 20 40 60 80 100

SMPTE %

JND

In

dex

Stentor

Webb1000

Linear (Stentor)

Linear (Webb1000)

Viewer software can affect display function


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“The best electronic image, improperly displayed is terrible.”• CRT monitors degrade over time. LCDs last longer.

• The wrong display Look-up-table (LUT) can spoil a great electronic image (DICOM Part 14 GSDF)

• Test patterns, notably the SMPTE, can make display problems obvious.


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Just because you ran the calibration routine does not mean the display is DICOM Part

14 compliantDome Flatpanel post cal

y = 5.8183x + 202.51

R2 = 0.9721

0100200300400500600700800900

0 20 40 60 80 100 120

SMPTE%

JND

in

dex


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Dome Flatpanel post cal

y = 4.286x + 150.2

R2 = 0.9717

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120

SMPTE%

JND

in

dex

HP Color

For GSDF conformance, consumer color flat panels require control of downloadable color

ramps


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Display problems affect the radiologist’s ability to practice digital radiology

• Potential errors (hard or soft copy)• Incorrect GSDF calibration• Inadequate matrix

• Moire’ (interference) patterns• Inadequate spatial resolution

• Incorrect or missing demographics or annotations

• Inadequate viewing conditions• QC => Radiologist “Film” critique


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Quantifiable Consequences of Degraded Performance

• Loss of Contrast Sensitivity

• Loss of Sharpness/Spatial Resolution

• Loss of Dynamic Range

• Increase in Noise

• Decrease in System Speed

• Geometric Distortion

• Artefacts


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AAPM Task Group 18 has developed procedures for assessing display

quality• GSDF

• Luminance Uniformity

• Noise (Low contrast performance)

• Resolution and resolution uniformity (CRT only)

• Veiling Glare (CRT only)

• Geometric Distortion (CRT only)

• Bandwidth Artifacts (CRT only)

• Dead Pixel Count (LCD only)


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AAPM Task Group 18 report on assessment of display performance for medical imaging

systems

• Recommended tests and frequency

• Useful test patterns

http://aapm.org/pubs/reports/OR_03.pdfRadiation Protection in Digital Radiology L08 Optimising DR Displays

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Some aspects of display performance can change over time

GSDFLuminance UniformityNoise (Low contrast performance)Resolution and resolution uniformity (CRT only)

X Veiling Glare (CRT only)Geometric Distortion (CRT only)Bandwidth Artifacts (CRT only)Dead Pixel Count (LCD only)


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To assure display quality, you are going to have to measure it

• Will need photometer• May need a chromaticity attachment

• Will need test patterns• MANY available from TG18

• Will need to measure more stuff, more frequently (monthly) with CRT• Useful lifetime of CRTs is limited compared to

flat panels


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Good news is that remote QC technology is available

• Automatic GSDF calibration

• Automatic monitoring and compensation for changes in ambient lighting and maximum luminance

• Remote monitoring, reporting, and adjustment via SNMP client.• Luminance level, drive level, system temperature,

etc

Ref: Raimond Pohlman and Jeff Shepard, UT MDACC Radiation Protection in Digital Radiology L08 Optimising DR Displays

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Difference in appearance on two GSDF calibrated displays

Acquisition Station PACS DisplayEven with proper calibration, viewer interpretation of greyscales can differ …


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Discrepancy between the DX VOI LUT and the PACS Linear LUT

• PACS viewer applied linear LUT to greyscales intended to have sigmoidal LUT

• Consequence: clipped light and dark regions

WW=2747, WL=4897

0

4096

8192

12288

16384

0 4096 8192 12288 16384

Input Grayscale

Ou

tpu

t G

rays

cale

DX VOI LUT

PACS Linear LUT


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Teleradiology – the forgotten display

• With remote viewing, one can no longer control what display is going to be used to display the image.

• ACR Standard calls for transmission and assessment of SMPTE test pattern weekly.

• Only workable approach is to provide the physician with an assessment tool at session log-in where he must affirm that he can see features.


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Field Effect Display (FED) may challenge AMLCD

• Can be built as thin as LCD

• Emissive display: no backlight


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Conclusions:

• Active Matrix Liquid Crystal Displays will continue to displace Cathode Ray Tube Displays for medical imaging

• Displays for medical imaging require special calibration according to DICOM GSDF

• Increasing use of pseudo-color in digital imaging imposes special demands on displays

• Novel display technologies are likely to find use in specific limited applications, except possibly FEDS


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Answer True or False

• The spatial resolution in flat panel monitors are better than CRT

• There can be artefacts arising from display screens

• The display systems can be used in any kind of environment


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Answer True or False

• True. Blur in flat panel monitors is less than the CRT monitors because of the stationary structure of the pixel.

• True. Corduroy artefact. It is the interference pattern between fixed grid lines and down-sampling rate for display.

• False. Ambient illumination limits the contrast that can be appreciated from an electronic display


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References:

• Bushberg JT, Seibert JA, Leidholdt EM Jr, and Boone JM. The Essential Physics of Medical Imaging 2nd Ed. Lippincott Williams and Wilkins. Philadelphia. (2002) 933).

• Flynn MJ. Softcopy Display: Technology, Performance, and Quality. In Specifications, Performance Evaluations and Quality Assurance of Radiographic and Fluoroscopic Systems in the Digital Era. Goldman LW and Yester MV eds. AAPM Monograph No. 30.Medical Physics Publishing. Madison. (2004) 335-351.

• Baldano A. Principles of Cathode-Ray Tube and Liquid Crystal Display Devices. In Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics. (2003) 91-102.

• Samei E, Badano A, Chakraborty D et al. Assessment of display performance for medical imaging systems: Executive

summary of AAPM TG18 report. Medical Physics 32(4)(2005)1205-1225.


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