Chapter 20 Digital Display Device (Image production)

radiologist use

QC monitors

extra light that's not coming from the source

Ambient lighting

a computer terminal allowing access to an manipulation of radiologic images and permanent savings of changes into PAC system

-most are flat panel LCD screens that are designed to reduce the effect of ambient lighting on the image

-monitors control the brightness of the image, defined as the intensity of light representing the various pixels comprising the image

Work station monitors

PAC

picture archiving computer system

a display monitor that uses a layer of liquid crystals between two polarized sheets of glass to control the blocking or emission of light from behind the screen

ex; laptop or tv screen

-the display monitors is the weakest link in the modern medical imaging chain and misdiagnosis can result from poor display quality

-the luminance of lcd should be checked and logged once a year

LCD(liquid crystal display)

normally lower resolution, used to review acquired images for exposure and positioning before they are sent to the radiologist

-are now viewed electronically, quality assurance for the displace monitors has become extremely important

Monitors radiograph use

high resolution for maximum detail and better contrast resolution

Monitors the radiologist use

-consistency between all display monitors within a particular workstation is essential

-all monitors should have the same level of luminance and be set at the same contrast

-cleaned monthly

3 things monitors need to work properly

AAPM

American Association of Physicists in Medicine

-patterns and outline related producers that can be downloaded from their website for display on a particular monitor to check each specific display

-measure and observation should be routinely

SMPTE

Society of Motion Picture and Television Engineers

-generic test pattern which includes elements for most of the image qualities all in one comprehensive pattern

-observation of 5% and 95% luminance patches

-helps to point out any gross deviations( stir away what it supposed be) in luminance adjustment

-when a new monitor is installed its test by smpte for brightness, contrast, and resolution

ACR

American College of Radiology

-requires a minimum brightness of 250 lumens

SMPTE, AAPM,ACR

all included test for maximum luminance, luminance response, luminance uniformity, contrast, resolution, noise, reflectance, and others

Performance assessment standards

The amount of light emitted, passing through or reflected from a object

ex; lamp or candle in a dark room

Luminance

The ability to clearly distinguish the individual parts of an object, resolve detail

Resolution

radiologist use

-used for diagnosis and more stringent guidelines for quality control

- to pass inspection for defective pixels fifteen or fewer bad pixel across the entire screen, three or fewer bad pixels within any circle of 1cm diameter, and three or fewer bad adjacent pixels

Class 1 monitors

radiographic use

-may still be classified as workstations but are not used for diagnosis

-to pass inspection for defective pixels 2 bad pixel out of 1 million

Class 2 monitors

brightness from screen

-measured in candela per square meters(cd/m^2)

-one candela generates one lumen per steradian

-typically display has a luminance of 50 to 300 cd/m^2, the max is 800

-the brightness for radiologist screen range from 500-600 lm

Luminance meter

the measurement of luminance of an area of a radiographic image displayed on a monitor

Brightness

the unit of luminous flux, or the intensity of light s perceived by the human eye

-how we measure something like a cup of sugar

Lumen

unit used to measure the volume of space within the dome shaped hemisphere

Steradian

the measure of intensity of light incident on a surface

-measured in lumens per square(1/m^2) or lux

-describe how well objects in our field of view illuminated

-the maximum ambient lightening radiologic reading room should be less than one fourth of normal office lighting

Illuminance

device use to measure light intensity

-photometric evaluations of digital device and ambient light levels is essential to image QC

-read out in units of lux, lumens, or candelas/m^2 which is equivalent to the same

Photometer

near range photometer- used in close proximity to display device

telescopic photometer-used to test from distance of lm

2 types of photometer

arises from problems that cause the displayed image to be geometric different from the original image

-affect size, shape of image features

-tested by TG 18 QC & TG LPV/LPH

-by testing the whole screen you can find pincushions and barre like distortions

-all lines should appear straight

Geometric (shape) distortion

cathode ray (CRT) based display device but rare in digital display device

Distortions are common in

ex; sew in pins with holes

Pincushion distortions

ex; jail bars

Barrelike distortions

an ideal digital display device has a luminance that is based on the light generated only by the device itself

-specular reflection; mirror image

-diffuse reflection; light randomly scattered

Reflection

image acquired by a digital image modality is stored as an array of pixel values

-pixel values are also called grayscale values and they are sent to a digital display device as presentation values or p values

-displayed luminance consist of light produced by the light digital display device (ddl)

-we see p values, then transform into ddl then we view them as luminance levels

luminance response

the quantitative measure of the ability of the digital display device to produce separable images of different points of an object with high fidelity

-test using AAPM TG 18 CX & QC

Display spatial resolution

an image along with image contrast and size is an important factor in determine the visibility of an object

-any high frequency fluctuations or pattern that interfere with detection of the true signal are classified as noise

-a blur on a image

Display Noise

lcd electric charge is applied to turn a pixel off so that it is dark against a light background, so truly dead hardware pixels are identified by bringing up a solid bock background and examining it for white spots

-when a pixel become "stuck" in a "on" state we look for black spots on a solid white field

Dead pixel

used for evaluating diffuse reflection

-one has to observe the threshold of visibility for low contrast pattern under both ambient lighting conditions and in total darkness

AAPM TG 18 AG

used to perform a qualitative evaluation of the luminance response of a digital display device

-pattern with half 16 area of half moon targets to evaluate the luminance response

AAPM TG 18 CT

brightness uniform across the entire screen

-verified by measuring brightness in 5 areas of the screen

-reading should be within 20% of each other

AAPM TG 18 UN

the maximum variation of luminance across the display area when a uniform pattern is display

AAPM TG 18 UNL

the DICOM standard include a grayscale standard display function

-was developed to bring values assigned to the dynamic range into linear alignment with human perception, with increments called JNDS or just noticeable differences

-10% of luminance response

Grayscale Standard Display Function (GSDF)

Digital Imaging and Communications in Medicine guidelines which standardize the behavior of all the various digital device used in PAC system

DICOM

used to evaluate display spatial resolution

- 4 or less is a good spatial resolution

AAPM TG 18 CX

used to evaluate spatial resolution uniformity

AAPM TG 18 QC

4 squares divided

AAPM TG 18 AFC

chest x-ray

AAPM TG 18 CH

2 types of photometer