Radiation Protection in Digital Radiology Optimising DR Displays L08

IAEA International Atomic Energy Agency

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.

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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

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Bushberg et al. 2002 The Essential Physics of Medical Imaging 2 nd 3 Ed Radiation Protection in Digital Radiology

L08 Optimising DR Displays

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

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Bushberg et al. 2002 The Essential Physics of Medical Imaging 2 nd Ed 4 Radiation Protection in Digital Radiology

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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”

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Bushberg et al. 2002 The Essential Physics of Medical Imaging 2 nd Ed 5 Radiation Protection in Digital Radiology

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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

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CRT Beam spot size for 300 X 400 mm Field Pixels Array size Spot size (mm) 1MP 2 MP 3 MP 5 MP 900 X 1100 "1K X 1K” 1200 X 1600 “1K X 1.5K” 1500 X 2000 “1.5K X 2K” 2000 X 2500 “2K X 2K” 0.35

0.25

0.20?

0.15

Flynn MJ 2004 Radiation Protection in Digital Radiology

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“Corduroy” artefact • • • 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

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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/m 2 ) Typical medical CRT is 70-90 fL (240-300 cd/m 2 ) •

minimum ACR is 50 fL (171

cd/m 2 )

for primary interpretation

Typical general purpose CRT is 30 fL (100 cd/m 2 ) Medical monochrome LCD is

200 fL

(700 cd/m 2 ) Typical consumer electronics color LCD display is 60 fL (

200 cd/m 2

) Paper SSA20-06 Visser M et al. RSNA 2005 describes prototype backlight up to 2000 cd/m 2

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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

(serial number A2I-05975) 1000 100 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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10 Measured (Nominal Lmax=600) GSDF 1 0 2000 4000 6000 8000 10000

Pixel Value

12000 14000 16000 18000

JND Index versus Input Pixel Value

(serial number A2I-05975) 600 500 400 300 200 R^2 = 0.99984

100 0 0 2000 4000 6000 8000 10000

Pixel Value

12000 Series1 Least Squares Fit 14000 Radiation Protection in Digital Radiology

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16000 18000 18

Viewer software can affect display function

600 500 400 300 200 100 0 0

Dell Model E771P Color Monitor

y = 4.3844x + 60.704

R 2 = 0.9706

20 40 y = 4.888x + 36.444

R 2 = 0.9856

SMPTE %

60 80 100 Stentor Webb1000 Linear (Stentor) Linear (Webb1000)

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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 compliant

Dome Flatpanel post cal

900 800 700 600 500 400 300 200 100 0 0 20 y = 5.8183x + 202.51

R 2 = 0.9721

40 60

SMPTE%

80 100 120

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For GSDF conformance, consumer color flat panels require control of downloadable color ramps 700 600 500 400 300 200 100 0 0

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y = 4.286x + 150.2

R 2 = 0.9717

20 40 60

SMPTE%

80 100 Radiation Protection in Digital Radiology

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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

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http://aapm.org/pubs/reports/OR_03.pdf

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Some aspects of display performance can change over time

 GSDF  Luminance Uniformity  Noise (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

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Ref: Raimond Pohlman and Jeff Shepard, UT MDACC

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Difference in appearance on two GSDF calibrated displays

Acquisition Station

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PACS Display Even with proper calibration, viewer interpretation of greyscales can differ … Radiation Protection in Digital Radiology

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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

IAEA WW=2747, WL=4897

16384 12288 8192 4096 0 0 4096 8192

Input Grayscale

12288 16384 Radiation Protection in Digital Radiology

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DX VOI LUT PACS Linear LUT 31

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 2 nd 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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