Stacy kopso, M.Ed., rt(r)(m) ODIA digital Academy, arrt

Stacy kopso, M.Ed., rt(r)(m) ODIA digital Academy, arrt
Digital Imaging Stacy kopso, M.Ed., rt(r)(m) References ODIA digital Academy, arrt

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. On the other hand, a digital receptor can produce an image that’s acceptable at 50 percent underexposure. Only clue is quantum noise It would look the same in many respects as an ideal image and would not require a repeat. Digital imaging allows up to a 100 percent overexposure, above the ideal, which provides no visual cue to a technologist that the patient was overexposed. In this respect, we may be using more radiation than necessary to create images with a digital receptor, or not enough.

Exposure Latitude The range of exposure values to the receptor that 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 latitude for digital is linear vs films exposure latitude Straight line region aka body

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 German astronomer Julius Scheiner image brightness and contrast no longer are linked to exposure factors, technologists must use the exposure indicator to determine if an image is properly exposed or within the acceptable range of under- or overexposure. Dose creep- gradually increase exposure

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 Based on the histogram analysis

Exposure Indicators for Flat Panel
Philips EI Measures the speed class of operation Increasing the exposure decreases the EI value Siemens EXI Directly related to the exposure level Increasing the exposure increases the EXI value Canon REX Increasing the exposure increases the REX value

Dose Area Product Measures the entrance skin exposure delivered to the 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 reducing the exposure field size affects the entrance skin exposure to the patient as measured by a DAP meter.

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) Too high of a DQE = noise on image (technique too low)

Digital Receptors Cassette-Based Systems Cassetteless Systems
Computed Radiography (CR) Photostimulable Phosphor Cassetteless Systems Digital Radiography (DR) Flat panel Charge-coupled device (CCD)

Cassette holds the PSP plate
Cassette less PSP plate is placed inside a large piece of equipment (bucky) Same type of PSP for all exams except for mammography Plates constructed for higher resolution use are either manufactured differently at the phosphor layer or designed with dual-sided reading technology that allows the phosphor layer to be read from both sides of the imaging plate.

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 Cassette is simply a container for the PSP plate/ made of plastic Inside is lined with felt material to prevent static buildup and dust collection Backing aluminum to absorb x-rays that penetrate the plate and reduces the amount of backscatter radiation that strikes the plate

Photostimulable Phosphor Plate
The front, white, side contains the photostimulable phosphors and holds the latent image until processing occurs in the image reader. The back side is black and only contains two barcode stickers.

Photostimulable Phosphor Plate
Imprinting the pts ID /information This is also where we tell the computer software what type of processing codes to use for a specific image based on the particular anatomical structure. For example, if a plate is scanned for a PA chest but is used for a lateral chest radiograph, the image won’t display properly due to the incorrect processing codes.

Photostimulable Plate
Protective layer Thin layer of plastic to protect the phosphor layer Phosphor layer Barium fluorohalide Europium (activator) Turbid phosphor layer Random distribution of phosphor crystals within the active layer Structured phosphor layer Columnar phosphor crystals within the active layer Columnar resembles needles standing on end and packed together Turbid or structured/ depends on the manufacturer

PSP Phosphor Layers Turbid phosphor Columnar phosphor
Depends on the manufacturer

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

Characteristics of Photostimulable Phosphors Characteristics of Photostimulable Phosphors 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 x-ray 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 Energy is released and converted to a digital signal Becomes a manifest image Quantity and distribution are proportional to exposure and represent the latent image Latent image- several sensitivity specks with many silver ions attracted to them. Appear as radiographic density on the manifest image after processing

PSP plate response The energy photons elevate from the resting energy level at the valance band –to a higher energy level at the conduction band A portion of the electrons are trapped in the F-center (higher than resting energy level) These trapped electrons are the ones that create the latent image The electrons that are not trapped, migrate back to the valance band and release light Following exposure, the red laser beam scans crystals to release almost all of the electrons in the F trap. These electrons emit light as they migrate back to the lower energy valance band 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. Following the red laser scanning, the plate is exposed to an intense form of white light which forces the rest of the trapped electrons to release. The white light exposure erases the plate.

Erasure Lamp Bright light that removes the rest of the trapped electrons When the bulbs are defective or dirty, the device can’t completely erase plates and subsequent images are poor quality

Removes the rest of the trapped electrons Ghost image
Insufficient erasure of an image

PSP Reader

Reader Optical System Drive mechanism Photodetector
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. Collects light given off during PSP image extraction The light guide collects the light and directs it toward the analog-digital converter, which converts the light photons into digital information. Works much like fiber optic technology to move information from the PSP to the photomultiplier tube the light the PSP emits during the image extraction process is not exceptionally bright. This is the reason a majority of the photons must be collected and then converted in the photomultiplier tube. This is also why routinely cleaning the light guide assembly is important to efficiently operating a PSP plate reader.

Light Guide Assemble

Plate Scanning The laser beam scans across the plate
which causes the electrons (elevated into F traps by 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 The scanning process and extraction of image data is the same whether a system is cassette-based or cassette-less

PSP exposed to laser of the reader

Sampling Time based event of the signal that is being sent from the photodetector to the ADC Sampling frequency The number of pixels sampled per millimeter as the laser scans each line of the imaging plate Sampling pitch How digital detectors sample the x-ray exposure Cassette-based Distance between laser beam positions during processing of the plate Cassetteless Distance between adjacent DELs

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

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. Spatial resolution losses occur because of blurring caused by geometric factors, detector element effective aperture size, and motion of the patient

This is a flying spot scanner
a higher sampling frequency the pixel size is smaller and we increase the spatial resolution. The lower sampling frequency equates to a larger pixel size and lower spatial resolution. How frequently the signal extracted from the PSP is sampled determines the recorded detail.

Matrix As the analog signal is digitized it is divided into a 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 Computer processing time, transmission and digital storage space increases as matrix increases

Pixel Combination of rows and columns of pixels inside a 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)

Pixels Each pixel has a bit depth (number of bits) that 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)

PSP Plate Scanning Line Scan Flying Spot Scan
Used with cassette-less PSP The plate is either pulled underneath the linear scanner or the plate remains stationary and the laser scanner moves across the plate. Flying Spot Scan Cassette based A technologist inserts the plate, the plate moves through the reader, and then during its movement, the laser beam moves across the plate.

Line Scan Cassette less scanning
The plate is either pulled underneath the linear scanner or the plate remains stationary and the laser scanner moves across the plate.

Detective Quantum Efficiency (DQE)
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 The receptor with the highest DQE requires the least exposure but may result in a greater amount of image noise.

Pre-Processing

Histogram The computer analyzes the histogram using 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 Computer software Automatic rescaling takes place in the end

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 Part of preprocessing ADC must quantize, or turn, that continuous stream of electrons into unique values. The histogram provides a tally of each unique digital value the signal extracts from the receptor. Histograms with a higher sampling frequency have more bars packed close together. X-axis is the amount of exposure (sampling frequency) Y-axis is the incidence of pixels for each exposure level

Exposure Field Borders
Image on lt- recognized exposure fields Image on rt- If at least three edges are not identified, then all data including scatter outside the filed may be involved in the histogram, resulting in a rescaling error

Cassetteless Systems INDIRECT AND DIRECT

Digital Radiography A detector array replaces the bucky assembly
Instant viewing Indirect and direct capture Indirect uses 2 forms of capture Charge Coupled Device (CCD) Scintillator

Indirect Capture Two forms of capture Each form uses a scintillator
Charge Coupled device Thin-Film transistor

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. Indirect form of image capture

X-rays strike the scintillator, which converts the energy to light photons.
These light photons reflect off a mirror onto a focusing lens that focuses them onto the CCD.

Charge Coupled Device

Thin Film Transistor (TFT)
Uses cesium iodide or gadolinum oxysulfide as the phosphor Photodetector 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

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 Scintillator has an extra step than the Non-Scintillator does before it converts the x-ray beam into electrons

A scintillator converts x-ray photons into light, which are emitted from the scintillator and interact with a photoconductive material typically made of amorphous silicon to convert the light photons into electrons. The electrons created in the amorphous silicon then migrate to thin film transistors and produce an electric signal. The illustration on this page shows the three-step process associated with the image capture that occurs in a scintillator flat-panel receptor.

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 Does not use a scintillator Photoconductor absorbs x-rays and creates electric charges in proportion to the x-ray exposure received

Indirect capture on left (uses a scintillator)
Scintillator is a 3 step process before it reaches the TFT’s Non-scintillator is only a 2 step

Flat Panel Image Receptors
Scintillator- uses amorphous silicon and a thin film transistor (TFT) array Non-Scintillator-uses amorphous selenium and a TFT array Detector array seen here, replaces the bucky assembly. Requires a complete x-ray unit replacement Non-scintillator is direct capture

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 Scintillator based flat-panel device Doesn’t matter if it’s a scintillator or non scintillator, each matrix still have a DEL

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. controls the recorded detail, or spatial resolution, for the flat-panel device. fixed and set by manufacturer

Flat Panel Spatial Resolution
Determined by detector element size Fixed Increase the DEL size, you decrease spatial resolution.

Flat Panel Spatial Resolution
If we want a higher resolution image, we must use a flat panel detector with smaller DELs

PSP with 100 micron pixels has a lower resolution than a 100 micron DEL
Difference is in how they create an image The PSP loses some resolution when a laser scans the plate The flat panel does not loose any resolution

The receptor size processed(sampled) at 100 microns will contain 4x the amount of information but will take longer to process and will increase the storage space

Sampling Frequency & Spatial Resolution
Nyquist Frequency Determines the maximum spatial resolution for a given sampling frequency. Equals ½ of the sampling frequency. If 10 pixels/mm are scanned, Nyquist frequency is a maximum of 5lp/mm Increasing the sampling frequency increases the time it takes to display a visual image. This frequency determines the level of spatial resolution for an image receptor. For example: 2.5 lp/mm requires a sampling frequency of 5 pixels/mm 5.0 lp/mm requires a sampling frequency of 10 pixels/mm 10.0 lp/mm requires a sampling frequency of 20 pixels/mm

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 size used to produce an image decreases, recorded detail increases. The number of pixels sampled per millimeter off the plate controls spatial resolution.

Digital Image Spatial Resolution
Measured by the size of the pixel used to create an image. The micron (µm) measures pixel size. 1 millimeter = 1 thousand micrometers

Digital Spatial Resolution
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. Going to effect how long it takes an image to display on a screen and requires more file space to store the image.

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.

Pixel density measures the number of pixels contained within a unit area. Pixels are smaller in a 10 pixels per millimeter image, that pixel density has a greater resolution than a 5 pixels per millimeter pixel density.

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

Image Characteristics
Brightness Contrast Image Blur Image Noise Spatial Resolution

Post processing Window Width Window Leveling Shuttering Masking
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 Excessive density/ insufficient density in analog Image on left lower brightness Image on rt higher brightness

Contrast Contrast is determined by the difference in adjacent densities contained within the image. Adjusted through window width Controlled by kVp Muscle organs fatty pads Image on left Bone scale contrast Image on rt Soft tissue gray scale Digital imaging is more sensitive to scatter radiation than film because they capture and record the low energy photons which causes histogram analysis errors

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. A low sampling frequency suffices for imaging large parts, such as a pelvis A higher sampling frequency, on the other hand, provides greater spatial resolution for examining extremities when you need to look at fine structures If the sampling is low frequency, you’ll see more blur than when sampling at a higher frequency.

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) How do each affect blur? Same is true with digital as it is with film. A large FSS will give you less detail= increase in penumbra(blur) than a small FSS SID and blur has an inverse relationship (increasing SID=decrease in blur) increases the SID, which decreases beam divergence and object magnification Watch video on SID OID minimize the object-to-image distance to reduce blur by decreasing magnification

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 Quantum- too few photons reaching the image receptor to form the image Electronic components of digital imaging system

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

Receptor Size The receptor size is made to fill the field of view of the display monitor. The receptor size and matrix size are directly proportional Image on left is taken on a 8x10 film Image on rt is taken on a 14x17 film

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 2 megapixel for radiologists 1 megapixel for other staff 5 megapixel for mammograms

PACS Storage Long term Short term Optical disk, tape, magnetic disks
RAID Redundant array of independent disks Hard drives Magnetic disks

HIS- Hospital information system RIS – Radiology 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

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