Review CR & DIGITAL IMAGING (1)

2012 – RT 244 wk 15

References: Bushong Vol 9www.sprawls.org/resources/DIGRAD/classroom.htm

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ObjectivesObjectivesDigital imaging review

Review CR fundamentals

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Digital ImagingDigital ImagingImage acquisition that produces

an electronic image that can be viewed and manipulated on a computer.

Examples?

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Methods to Digitize an Image• 1. Film Digitizer

• 2. Video Camera (vidicon or plumbicon)

• 3. Computed Radiography

• 4. Direct Radiography– PACS– DICOM

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Digital

Radiography

DirectCapture

IndirectCapture

Direct-to-DigitalRadiography

(DDR)-Selenium

ComputedRadiography

(CR) - PSL

LaserScanningDigitizers

Direct-to-DigitalRadiographySilicon Scint.

DDR CR

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Computer Language

• Computers operate on the Binary Number System

• It has only two digits, 0 and 1

• Computers function by converting all data into binary values.

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ByteByteRepresents one character, digit, or

value. A bit describes the smallest unit of

measure 0 or 1 – computers ultimately understand only 0 or 1

Byte are 8 bitsA kilobyte represents 1024 bytes,

megabyte is 1 million bytes, gigabyte is approximately 1 billion bytes

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Alphabet in Binary

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What is a Pixel? 11

Basics of Digital Images

• digital images are a (matrix) of pixel (picture element) values

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Pixel

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Computed Radiography

Fundamentals of

Computerized Radiography

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CR SYSTEM CR SYSTEM COMPONENTSCOMPONENTS

What are the CR system components?

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CR SYSTEM CR SYSTEM COMPONENTSCOMPONENTS

CASSETTES (phosphor plates)

ID STATIONIMAGE PREVIEW (QC) STATION

DIGITIZERVIEWING STATION

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Imaging Plate (IP)Imaging Plate (IP)Contained in a cassette

Handled the same as S/F cassettes

Processed more like daylight processor with no chemicals

IP has lead backing to reduce scatter

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CR – PSP plate• photostimulable phosphor (PSP) plate

• Exit photons energizes the PSP plate

• The energy is stored in traps on plate (latent image)

• PLATE scanned in CR READER

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Imaging Plate Imaging Plate ConstructionConstructionA thin sheet of plastic

IP’s have several layers◦A protective layer. This is a very

thin, tough, clear plastic that protects the phosphor layer

◦A phosphor or active layer. This is a layer of photostimulable phosphor that “traps” electrons during exposure

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Active Layer - Crystals

• The materials that make up the PSP plate are from the barium fluorohalide family.

• Barium fluorohalide, chlorohalide, or bromohalide crystals. The most common crystal uses is barium fluorohalide with europium

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Acquiring the Image What is the correct order?

• Violet light is captured by PMT – is amplified and converted into a digital signal

• cassette is put into the reader, the imaging plate is extracted

• light is sent to the analog to digital converter (ADC). To convert light to binary.

• e- return to ground state, visible light is emitted

• remnant beam interacts with electrons in the barium fluorohalide crystals

• imaging plate is scanned with a helium laser beam or solid-state laser diodes 22

Acquiring the Image• The remnant beam interacts with electrons in

the barium fluorohalide crystals. This interaction stimulates, or gives energy to, electrons in the crystals, allowing them to enter the conductive layer, where they are trapped in an area of the crystal known as the color or phosphor center.

• This trapped signal will remain for hours, even days, although deterioration begins almost immediately. IR should be processed as soon as possible.

• The trapped signal is never completely lost.

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Imaging Plate Imaging Plate ConstructionConstructionA reflective layer. This is a layer

that sends light in a forward direction when released in the cassette reader. This layer may be black to reduce the spread of stimulating light and the escape of emitted light. Some detail is lost in this process.

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IP Construction

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Cross section of a PSP Cross section of a PSP screenscreen

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Needle PSP increase the absorption of x-rays and limit the spread of light emission

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IP DesignIP DesignDesigned to optimize the

intensity of light release. (CE)

Enhance the absorption of x-rays (DQE)

Limit the spread of light emission for more detail.

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Photostimulable Luminescence• When the cassette is put into the reader,

the imaging plate is extracted and scanned with a helium laser beam or, in more recent systems, solid-state laser diodes. This beam, about 100μm wide with a wavelength of 633 nm (or 670 to 690 nm for solid state), scans the plate with red light in a raster pattern and gives energy to the trapped electrons.

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X-ray interaction with a PSP screen

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X-ray interactions with the screen phosphors causes an e- to excited

When e- return to groundstate visible light is emitted

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CR Phosphor PlatesCR Phosphor Plates

ABSORPTION EMISSION

X-RAY

LIGHT

LASER STIMULATION

ELECTRONTRAP

ELECTRONTRAP

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CR Reader – PSP plateCR Reader – PSP plateStimulates the matrix of trapped E-

by a RED OR ULTRAVIOLET laser light

Trapped E- energy is released in a form of VIOLET/BLUE light

Violet light is captured by PMT – is amplified and converted into a digital signal

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Producing a PSL signalProducing a PSL signal50% of the excited e- return to

ground state immediately, resulting in light (VIOLET/BLUE) emission.

Slow scan = plate Fast scan = laser

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How CR worksHow CR worksReleased light is captured by a

PMT (photo multiplier tube). An ultrasensitive photomultiplier tube or CCD (charged couple device)

PSP light is amplified by the PMT or CCD

This light is sent to the analog to digital converter (ADC). To convert light to binary.

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Sequence of CR imaging

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Processing of digital images can be used to

change most image characteristics. • Three possibilities include

processing methods to:• Adjust and optimize the

image contrast characteristics

• LUT & Processing Algorithms

• Reduce image noise • Increase visibility of detail • Some type of digital

image processing is used with most of the medical imaging modalities.

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Brightness & Contrast

• Optimum kVp & mAs has changed for digital

• kVp changes for SUBJECT contrast – not image contrast

• mAs does not influence DENSITY the same as it did with F/S

• The image is POSTPROCESSED – with changing PROCESSING ALGORITHMS

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• digital processing methods that can be used to adjust the contrast characteristics of an image. 

•Look Up Table (LUT) processing •Windowing •Are used in digital radiography as well as with many of the other imaging modalities.

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DIGITAL “DENSITY”

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Dynamic Range

• The range of exposure values to which the

• image receptor will respond.

• The greater the range of values that a

• receptor will respond to the greater the

• dynamic range.

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Characteristic curveof radiographic film

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Widow level & width

Same photons at the image receptor

Image is post processed – changing

Brightness and contrast of image appearance

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• The ability to window is a valuable feature of all digital images.

• Windowing is the process of selecting some segment of the total pixel value range

• and then displaying the pixel values within that segment over the full brightness (shades of gray) range from white to black.

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windowing

• Important point...Contrast will be visible only for the pixel values that are within the selected window. 

• All pixel values that are either below or above the window will be all white or all black and display no contrast.

• The person controlling the display can adjust both the center and the width of the window.  The combination of these two parameters determine the range of pixel values that will be displayed with contrast in the image.

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advantages of windowing?51

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Detective Quantum EfficiencyDQE

• An indicator of the potential “speed class” or dose level required to acquire an optimal image.

• The DQE performance is obtained by

comparing the image noise of a detector

with that expected for an “ideal” detector

having the same signal-response characteristics.

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Exposure Latitude

• It is the optimal exposure range

• relative to the “ideal” exposure

• that produces a quality image at

• an appropriate patient dose.

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Why do digital systems havesignificantly greater latitude?

• Linear response give the imaging plates greater latitude

• Area receiving little radiation can be enhanced by the computer

• Higher densities can be separated and brought down to the visible density ranges

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Exposure Latitude

• The analog receptor exposure latitude ranges from approximately

• 30% underexposed

• to 50% overexposed relative to

• the “ideal” exposure level.

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Exposure Latitude

The digital image receptor

• exposure latitude ranges from

• approximately

• 50% underexposed

• to 100% over exposure

• relative to the “ideal” exposure level.

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Exposure Indicators

• Imaging plates get a signal from the exposure they receive

• The value of the signal is calculated from the region identified as the anatomy of interest

• The signal for the plate is an average of all signals given to the plate

NoteIt is important to note that just because a• digital imaging system has the capacity to• produce an image from gross underexposure• or gross overexposure it does not equate to• greater exposure latitude. • The reason the system is capable of producing

an image when significant exposure errors occur is through a process called automatic rescaling.

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• In a digital system, underexposure of

• 50% or greater will result in a mottled

• image.

• In a digital system, overexposure

• greater than 200% of the ideal will result

• in loss of image contrast.

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Histogram showing pixel values in an image. The pixel values in gray are on the horizontal with the total number for each on the vertical.

DarkerLighter

Screen / Film Imaging = self regulating

2 mAsUnder exposed

6 mAsCorrect exposure

24 mAsOver exposed

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CR imaging results in 10,000 shades of gray

Fixed kVp exposures

mAs = 0.5S = 357

mAs = 1.0 S = 175

mAs = 2.0S = 86

mAs = 5.0S = 35 71

LUT

• Look Up Table (LUT)

• Each anatomic area has a LUT

• Used to adjust contrast and density

• Other terms that may be used for this– Contrast rescaling– Contrast processing– Gradation processing– Tone scaling

LUT

• The image data from the histogram is rescaled for application of the LUT

• The LUT maps the adjusted data through a “S” curve that is similar to an H & D curve

• The result is an image that has the correct contrast and brightness (density)

Characteristic curve & Characteristic curve & histogramhistogram

UnderexposedUnderexposed

OverexposedOverexposed

Just right!Just right!

LOOK UP TABLE (LUT)

Linear LUT

Black Shirt

Facial Tones

Black Saturation

White Saturation

* No Detail in Black Areas* High Contrast* Only Detail in White Areas can be seen

* No Detail in White Areas* Low Contrast* Only Detail in Black Areas can be seen

Digital Images – Bit Depth

• Pixel values can be any bit depth (values from 0 to 1023)

• Bit depth = # or gray shades available for image display

• Image contrast can be manipulated to stretched or contracted to alter the displayed contrast.

• Typically use “window width” and “window level” to alter displayed contrast and brightness

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Display Bit Depth1 bit 6 bit 8 bit

2 shades 64 shades 256 shades

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BIT Depth

• The number of gray shades available for image display.

Number of gray shades is 2n .

• Where n is the number of bits available for each pixel

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Digital - GrayscaleBit depth.

Number of gray shades available for display

• 8 bit 256

• 10 bit 1024

• 12 bit 4096

• 14 bit 16384

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CR Image QualityCR Image QualityPixels, Field of View, Image receptorSampling frequency, Quantization,

Nyquest frequencyNoiseMagnificationImage compression (lossless vs

lossee)

Questions?

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