Transcript Digital Imaging - radprofkopso.com

Digital Imaging
STACY KOPSO, M.ED., RT(R)(M)
REFERENCES
ODIA DIGITAL ACADEMY, ARRT
HTTP://WWW.ONLINELEARNINGCENTER.COM
Advantages
 Large Dynamic Range
 Post Processing
 Independence of adjustments in brightness and contrast
through window width and level
 Image enhancement and analysis (CAD)
 Accurately labeled data
 Storage (PACS)
 Ability to transmit data to remote sites
Dynamic Range
 Large dynamic range
 Image receptor's ability to respond to different exposure levels
 The number of signal values that the receptor is capable of
capturing.
 The greater the number of signal values that a receptor is
capable of capturing, the greater the receptor's dynamic range.
Exposure Latitude
 The range of exposure values to the receptor that
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produce an acceptable range of densities for diagnostic
purposes
Automatic rescaling is the reason the digital system can
produce an image even when significant exposure errors
occur
Underexposure of 50% or greater results in a mottled
image or an image with the appearance of noise
Overexposure greater than 100% to 200% results in a
loss of image contrast, depending on the exam that is
performed
Digital imaging has greater latitude than film screen
Exposure Latitude
Exposure Indicator
 Image brightness and contrast no longer are linked
to exposure factors
 Dose Creep
 Technologists must use the exposure indicator to
determine if an image is properly exposed or within
the acceptable range of under- or overexposure.
 S# Scheiner's system index for exposure index
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German astronomer Julius Scheiner
Exposure Indicators for Cassette-Based
Fuji / Philips / Konica
S Number, Sensitivity Number
Increase in exposure =decrease in S#
Carestream (Kodak)
EI, Exposure Index
Increase in exposure=increase in E
Agfa
lgM, Logarithm of a Histogram Median
Increase in exposure=increase in IgM
Digital Receptors
 Cassette-Based Systems
 Computed Radiography (CR)
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Photostimulable Phosphor
 Cassetteless Systems
 Digital Radiography (DR)
Photostimulable Phosphor
 Flat panel
 Charge-coupled device (CCD)
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Cassette-Based Systems
 Integrated with existing radiographic equipment
 AEC must be recalibrated and techniques must be adjusted
 Parts of a cassette-based system
 Image receptor- Photostimulable phosphor plate (PSP)
 Plate reader
 Computer workstation
Photostimulable Plate
 Protective layer
 Thin layer of plastic to protect the phosphor layer
 Phosphor layer
 Barium fluorohalide
 Europium (activator)
 Turbid phosphor layer
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Random distribution of phosphor crystals within the active layer
Structured phosphor layer
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Columnar phosphor crystals within the active layer
PSP Phosphor Layers
Turbid phosphor
Columnar phosphor
Photostimulable Plate (cont’d)
 Reflective layer
 Reflects light released during the reading phase toward the
photodetector
 Conductive layer
 Reduces and conducts away static electricity
 Color layer
 Absorbs stimulating light and reflect emitted light
 Support layer
 Sturdy material to give rigidity to the plate
 Backing layer
 Soft layer that protects the back of the plate
Photostimulable Phosphor Plate
Phosphor side
Back side/barcode
Characteristics of Photostimulable Phosphors
 PSP Conversion Efficiency
 The ability of the storage phosphor to convert the signal
exiting the patient into trapped electrons.
 Absorption efficiency
 A measure of how effective the phosphor is at absorbing the xray photons.
Photostimulable Phosphor
Plate Response
 PSP is exposed to x-rays
 Phosphor atoms are ionized
 Half of the removed electrons are “trapped” in the
conduction band (energy level of an atom)
 These trapped electrons represent the latent image
 PSP plate is exposed to the laser of the reader
Plate Scanning
 The laser beam scans across the plate
 which causes the electrons (elevated into F traps by
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the x-ray beam) to drop back down into their normal
orbit.
When those electrons drop back into their normal
orbit, they emit light.
This light is directed to the photodetector
Photodetector amplifies the light energy and
converts it to an electrical signal
Signal is passes through an ADC where it is digitized
Reader
 Optical System
 Consists of a laser light, filters and beam-shaping devices
 Drive mechanism
 Moves the PSP plate through the reader
 Photodetector
 Senses the light released from the PSP plate during scanning
 This light is then sent to an analog-to-digital converter
(ADC)
 The ADC converts it to an electronic signal for the display
computer
Light Guide Assembly
 Directs the light that a PSP emits to a
photomultiplier tube
 The light guide collects the light and directs it back
into a device called a photomultiplier tube. The tube
converts the light into an electronic signal, which is
digitized by the analog-to-digital converter, or ADC.
Light Guide Assemble
Erasure Lamp
 White light
 Removes the rest of the trapped electrons
 Ghost image
 Insufficient erasure of an image
Sampling
 Time based event of the signal that is being sent from the
photodetector to the ADC
 Sampling frequency
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The number of pixels sampled per millimeter as the laser scans each
line of the imaging plate
 Sampling pitch
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How digital detectors sample the x-ray exposure
Cassette-based
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Distance between laser beam positions during processing of the plate
Cassetteless
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Distance between adjacent DELs
Sampling Frequency
 Sampling frequency of PSP plates
 Range from 5 pixels/mm up to 20 pixels/mm
 The greater the sampling frequency equates to a smaller pixel
size and increased spatial resolution.
Cassette less Systems
INDIRECT AND DIRECT
Cassette less Systems (DR)
 A detector array replaces the bucky assembly
 Instant viewing
 Indirect and direct capture
 Indirect uses 2 forms of capture
Charge Coupled Device (CCD)
 Scintillator
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Indirect Capture
 Indirect
 Two forms of capture
 Each form uses a scintillator
 Charge Coupled device
 Thin-Film transistor
Scintillator
 Scintillator
 Absorbs xray energy and emits visible light in response
 Material- Cesium iodide
 Converts the x-ray beam to light and then that light is
converted into electrons to create an image.
 Uses amorphous silicon as the photodetector and a thin film
transistor array (TFT)
 Indirect capture
Charge Coupled Device (CCD)
 Very light sensitive
 Cesium iodide phosphor plate connected to CCD by
fiberoptic (pg 159)
 Xrays are absorbed by the scintillator and converted
to light
 Light is then transmitted to CCD where it is
converted to an electronic signal for viewing
 Tiling- joins the CCD’s together
Charge Coupled Device
 The light from scintillator material strikes the silicon
in the CCD silicon chip. The electrons then are
collected by the CCD chip elements we refer to as
pixels.
Charge Coupled Device
Thin Film Transistor (TFT)
 Uses cesium iodide or gadolinum oxysulfide as the
phosphor
 Photodetector
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amorphous silicon
 TFT array
 Contains the readout, charge collector and light sensitive
elements
 Configured into a network of pixels/DEL’s covered by the
scintillator plate
 Each pixel contains a photodetector and transistor
Thin Film Transistors (TFT)
 The transistor operates as a gate
 During an x-ray exposure, the gates are turned off, the
electric charge cannot flow
 The image is built up in the dels in the form of an electric
charge
 The amount of charge in each del is proportional to the
number of x-rays absorbed in that region of the detector
 Following the exposure, the “gates” are turned on one
row at a time, and the amount of charge stored in each
del is transferred for digitization and storage in the
output digital matrix.
Thin Film Transistor (TFT)
 X-ray energy is absorbed by the photodetectors and
converted to electric charges
 These charges are captured and transmitted by the
TFT array to the workstation
Direct Capture
Direct Capture
 Direct capture
 v (amorphous selenium) and a TFT array
 Converts the x-ray beam into electrons to create an image
 Use amorphous selenium as the photoconductor to convert the
x-ray beam to electrons that thin film transistors (TFT) then
collect
 Better resolution than indirect
Flat Panel Image Receptors
 Scintillator- uses amorphous silicon and a thin film
transistor (TFT) array
 Non-Scintillator-uses amorphous selenium and a
TFT array
Detective Quantum Efficiency (DQE)
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Efficiency of the detector
An expression of the exposure level required to produce an image.
A measure of a receptor's ability to create an output signal that
accurately represents the input signal (x-ray beam).
Expressed as the percentage of the x-ray energy that strikes the
receptor that is successfully converted to an output signal used in
creating the image.
A measure of how well the signal-to-noise (SNR) ratio is preserved
A receptor with a high DQE will require less dose to create
an optimal image when compared to a receptor with a low
QE
Referred to as potential speed
Detector Element
 Each square in the matrix is a detector element
(DEL). DELs collect the electrons given off by the
amorphous selenium or the amorphous silicon
Detector Element
 DELs collect electrons that are extracted from the
detector assembly and converted into a digital value
by an ADC.
 That process creates the image that displays on our
monitor.
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controls the recorded detail, or spatial resolution, for the flatpanel device.
fixed and set by manufacturer
Digital Image Characteristics
Digital Image Characteristics
 The digital image is a matrix of numbers, known as
pixels, that corresponds to the intensity of the x-ray
beam that strikes a particular area.
Pixels
Pixel
 Combination of rows and columns of pixels inside a
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matrix
Each matrix is a picture element known as a pixel
Each pixel is recorded as a single numerical value,
which is represented as a single brightness level on
the monitor
The numerical value is determined by the
attenuation of xrays passing through the tissue
Bone, high attenuation=low value (increase
brightness/decrease density)
Pixel
 Each pixel has a bit depth (number of bits) that
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determines the exit radiation recorded
Controls the exact pixel brightness/shades of gray
Determined by the analog to digital converter
Large bit depth= greater number of shades of gray
displayed
Greater shades of gray= better contrast resolution
Pixel pitch- distance between center of one pixel and
the center of an adjacent pixel (microns)
Pixel
 Pixel Size is measured from side to side of the pixel.
 Pixel Pitch is measured from the center of one pixel
to the center of an adjacent one. Determines the
maximum spatial resolution
Pixel
 Pixel density measures the number of pixels
contained within a unit area.
Matrix
 As the analog signal is digitized it is divided into a
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matrix
The detector size or field of view is the useful
imaging area of the digital receptor
The size of the matrix determines the resolution
The larger the matrix, the greater the number of
smaller pixels = increase in resolution
Disadvantage of larger matrix
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Computer processing time, transmission and digital storage
space increases as matrix increases
Matrix
 Matrix Size or Field of View
 Rows and columns of pixels
 Denser pixels within a fixed receptor size require a larger
image matrix to accommodate more squares within the image
receptor.
 If the receptor size increases with a fixed pixel size, more pixels
of the same size must be added for a larger image matrix.
Spatial Resolution
 Spatial resolution is the ability of the system to
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record adjacent small structures.
Measured by the size of the pixel used to create an
image.
The micron (µm) measures pixel size.
1 millimeter = 1 thousand micrometers
CR uses sampling frequency
DR uses detector element size (DEL)
Spatial Resolution
 Measured in line pairs per mm (lp/mm)
 Determined by pixel size
 Decrease in pixel size=increase resolution
 Cassette based
 Sampling frequency (↑sampling=↑resolution)
 Direct Radiography
 Detector element size (DEL) with flat panel detector
 As DEL increases, spatial resolution increases
Spatial Resolution
 The ability of an imaging system to allow two adjacent structures
to be visualized as being separate, or the distinctness of an edge
in the image (sharpness)
 Limiting spatial resolution
 The ability of a detector to resolve small structures and is measures
using a bar pattern
 Modulation Transfer Function
 Measure of the ability of the system to preserve signal contrast
 Ideal expression of digital detector image resolution
Flat Panel Spatial Resolution
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Determined by detector element size
Fixed
Increase the DEL size, you increase spatial resolution.
Flat Panel Spatial Resolution
Sampling Frequency & Spatial Resolution
 Nyquist Frequency
 Determines the maximum spatial resolution for a given sampling
frequency.
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Equals ½ of the sampling frequency. If 10 pixels/mm are scanned,
Nyquist frequency is a maximum of 5lp/mm
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Increasing the sampling frequency increases the time it takes to display a
visual image.
Detective Quantum Efficiency (DQE)
 Efficiency of the detector
 The exposure level required to produce an image
 Potential Speed class (similar to film)
 Higher DQE = lower pt dose (balance dose/noise)
 A measure of how well the signal-to-noise ratio
(SNR) is preserved in an image
 SNR is how clearly a very faint object appears in an
image.
 Signal (meaningful information) Noise (background
information)
Modulation Transfer Function
 A measure of the ability of the system to preserve
signal contrast as a function of spatial resolution
 Ideal expression of digital detector image resolution
Dose Area Product
 Measures the entrance skin exposure delivered to the
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patient
Measured by detector inside or on collimator
Commonly employed with cassette-less systems
Depends on exposure factors and field size
Reflects dose to patient and total volume of tissue
being irradiated
Pre-Processing
Histogram
 Histograms graphically represent a collection of
exposure values extracted from the receptor
 Quantization-The number of bars that make up each
histogram represents different sampling frequencies
Histogram
 The computer analyzes the histogram using
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processing algorithms
Compares it to a preestablished histogram specific to
the anatomic part being imaged
These stored histogram models have values of
interest
Computer identifies the exposure field and the edges
of the image.
All exposure data outside this field are excluded from
histogram
Histogram Error
 Inadequate collimation results in a wider histogram.
Exposure date outside of the area of interest is
included.
Post processing
 Window Width
 The shade of gray displayed (contrast of the image)
 Window Leveling
 Controls brightness/density
 Shuttering
 Removing distracting light that surrounds the image (fake collimation)
 Masking
 Suppressing frequencies of lesser importance
 Causes small detail loss
 Edge Enhancement
 Increases contrast along the edge. Can only be done with a low signal to
noise ratio
 Smoothing
 Reducing noise
Brightness
The brightness of the digital image is equivalent to the
term density that was applied to the analog image.
Adjusted through window level
Controlled by mAs setting
Contrast
 Contrast is determined by the difference in adjacent
densities contained within the image.
 Adjusted through window width
 Controlled by kVp
LUT
 Look up table
 Primary factor influencing contrast
 Histograms of luminance values used as a reference
to evaluate the intensities and predetermined
grayscale values
 Rescaling for every anatomic part
Image Blur
 Receptor Blur
 Geometric Blur
 Motion Blur
Receptor Blur
 Receptor blur (PSP)
 The sampling frequency of a PSP plate controls image blur
with the photostimulable phosphor digital receptor.
Flat-Panel Detector Blur
 Detector Element Size
 DEL size contributes to the image blur present in a flat panel
detector receptor. The larger DELs in a flat panel detector
cause more image blur.
Geometric Blur
 Focal Spot Size
 Source-to-image receptor distance (SID)
 Object-to-image receptor distance (OID)
Motion Blur
Image Noise
 Undesirable fluctuations in the brightness of an
image
 Due to quantum(mottle) and electronic noise
 Low mAs, fast IR, high kVp contribute to QM
Spatial Resolution
 The receptor size is made to fill the field of view of
the display monitor. The receptor size and matrix
size are directly proportional.
Picture Archiving and
Communication Systems
PACS AND DICOM
PACS AND DICOM
 PACS
 An electronic network for communication between
the image acquisition modalities, display stations
and storage (file room and reading room)
 For these different systems to communicate with
each other, a common language is necessary
 Dicom
 Digital imaging and communications in medicine
PACS Storage
 Long term
 Optical disk, tape, magnetic disks
 Short term
 RAID Redundant array of independent disks
 Hard drives
 Magnetic disks
 HIS- Hospital information system
 Contains full patient information
 RIS – Radiology information system
 Contains radiology reports and specific radiology information
about the patient
 HL-7
 Demographics, orders and claims