Chapter 10 Digital Imaging Receptors
Disadvantages of CCD Systems
-CCD is limited in size called Tiles -Tiles must be pieced together like a puzzle or mosaic, so there is missing data at the seams -Computer system must interpolate pixel values to correct for these seams
Indirect Capture W/CCD Scintillator converts remnant beam into light
-CESIUM IODIDE (Csl) Phosphor plate -Coupled to CCD via fiber-optic bundle or optical lens system; Light emitted from scintillator is transmitted to CCD, then converted into electronic signal and processed/displayed by workstation.
Primary parts of the CR system:
-Cassette -Photostimulable Phosphor (PSP) Plate [inside the cassette] -Plate reader -Computer workstation
Radiology Information System (RIS)
-Combines reports with images in pt folders -Allows fast comparison with previous studies -Appointment scheduler
ARRT Content Specs divide digital imaging into two parts:
-Computed Radiography (CR) -Digital Radiography (DR)
How PSP Plates work When a PSP Plate is exposed to X-rays
-Emits light immediately after exposure -Approximately 50% of electrons are removed from the phosphor atoms and are trapped in the conductive band. -Quantity and distribution of captured electrons is proportional to exposure and represents the latent image (can be stored for hours, but starts to deteriorate immediately)
Digital Imaging and Communication in Medicine (DICOM) connects
-HIS: Hospital Information System -RIS: Radiology Information System -EMR: Electronic Medical Records
After the X-ray exposure, there are three adjustments made to the histogram before the image is displayed:
-Histogram Analysis IDs the useful pixels ("Values of Interest"), used for processing -Rescaling adjust the pixel values to fit the average histogram of the body part and adjust for small exposure errors. -A lookup table (LUT) is used to give the image just the right amount of brightness and contrast.
VOI is used to determine what section of the histogram data set should be included in the displayed image:
-Identifies exposure field -Identifies edges of the image -Excludes data outside the desired field
CR READER
-Laser beam sweeps back and forth to release phosphor electrons healed in conduction band -Laser doesn't actually move; it is reflected by a rotating or oscillating mirror -Liberated electrons release excess energy as light -Light is directed to photodetector, which amplifies light energy and converts into analog signal -Analog signal processed by Analog-to-Digital Converter (ADC)
Advantages of CMOS
-Less expensive to manufacture -Requires a fraction of the power to run
Charged coupled Device (CCD) developed by the military
-Light sensitive (can respond to a wide range of light intensities) -Wide dynamic range
Cassette
-Made of lightweight plastic, is simply a container for the Photostimulable Phosphor (PSP) Plate. -Inside is lined with felt material to prevent static buildup and dust collection. -Backing contains a sheet of aluminum to absorb X-rays that penetrate the place; reduces the amount of backscatter radiation that strikes the plate. *-A sturdy, light-proof container for film. Also a sturdy protective container for the Photostimulable Phosphor Plate in Computed Radiography.
Modulation Transfer Function (MTF)
-Measure of the ability of the system to preserve signal contrast -Value will be between 0 (min. Difference) and 1 (max. Difference)
A Laser beam is projected onto a phosphor layer and
-Releases trapped electrons from conductive layer and returns them to respective valence shells -excess energy is emitted as light energy directed at photodetector -signal is analog and must be converted via ADC digital format
Conversion to CR What kind of changes/challenges were there?
-Required little modifications for technologists who were used to selecting receptor based on body part to be imaged. -Large learning curve for computer technology
Complementary Metal Oxide Semiconductor (CMOS)
-Scintillator device made up of a crystalline silicon matrix -each detector element has it's own amplifier, photodiode, and storage capacitor, and is surrounded by transistors.
Direct Capture
-Skips the scintillator process of indirect capture, thereby increasing resolution -Uses photoconductor of Amourphous SELENIUM (a-Se) and TFT array -Top layer is electrode: needed to provide electric field prior to exposure(prep the device)
Direct Radiography (DR)
-Systems are those that have detectors that directly capture and read out an electronic image signal. *ARRT Standard Definition
Values of Interest (VOI)
-stored histogram models and determine what section of the histogram data set should be included in the displayed image. Digital rad works best when at least two, but preferably four, collimated edges can be measured by computer.
Computed Radiography (CR)
-systems are those that use storage phosphors to temporarily store energy representing the image signal. The phosphor then undergoes a process to extract the latent image. *ARRT Standard Definition. -A digital imaging system that uses a cassette, a Photostimulable Phosphor plate, a plate reader, and a computer workstation to acquire and display a digital image. *Books Definition
Human eye can see how many shades of gray?
30 at a particular level -approximately 500 shades of gray total -digital radiography uses 14-bit dynamic range (16,384)
What is the average size of digital rad study?
38 MB
DICOM (Digital Imaging and Communications in Medicine)
A common computer language that allows different systems of a Picture Archiving and Communication system(PACs) to communicate with each other
Photoconductor
A device that absorbs X-rays and creates electrical charges in proportion to the x-ray exposure received.
Direct Radiography (DR) systems
A digital imaging system that uses a detector array in place of the Bucky assembly; The imaging-forming radiation is captured and transferred to a computer from the detector array for almost instant viewing at the control panel.
Charged-Coupled Device (CCD)
A light-sensitive semiconducting device that generates an electrical charge when stimulated by light and stores this charge in a capacitor.
Signal-to-Noise Ratio (SNR)
A method of describing the strength of the radiation exposure compared with the amount of noise apparent in a digital image.
Structured Phosphor
A phosphor layer that has columnar phosphor crystals within the active layer resembling needles standing on end and packed together.
Turbid Phosphor
A phosphor layer with a random distribution of phosphor crystals within the active layer and can be used with both CR and DR systems.
Region of interest
A quantitative function of digital imaging that allows for the pixel value of a selected area of interest to be calculated. This value can help characterize disease.
Picture Archiving and Communication Systems (PACs)
A secure network for transmitting and exchange of patient images and data, display (viewing and workstations), and storage (archiving)
Smoothing
A software function to suppress noise. Image noise is considered a high-frequency variation in the histogram, and post processing adjustment of these high frequencies can reduce noise.
Look up Tables (LUT)
Adjustments ensure the digital image displays with the ideal contrast and brightness. A raw digital image is very gray, with minimal contrast. The radiography system processes this raw image using an LUT to give the image just the right amount of brightness and contrast. Different body parts use different LUTs since different body parts need different levels of brightness and contrast.
Image inversion
Allows for the change of the image from negative (bone is white) to positive (bone is dark). Some pathological conditions are better identified in this way.
Image flip
Allows for the flipping of the image so that it is oriented properly for interpretation
Annotation
Allows test to be added to the image to identify areas of interest or add information important for diagnosis. Should not be used in place of anatomic side markers
Magnification
An electronic magnifying glass is available for use with digital images. With very high-resolution monitors, such as those used by the radiologist, magnification to see fine detail is possible.
Thin Film Transistors (TFT)
Are small electronic components layers into glass substrate
What had to be recalibrated for CR?
Automatic Exposure Control (AEC)
Conversion to CR Cassette (Image Receptors) were
Basically the same size and appearance as traditional film/screen.
Luminance
Brightness level of the workstation monitor
Digital Imaging has been used by other modalities for years:
CT, Sonography, Nuc. Med., and MRI.
A Detector Array can
Capture and transfer a Remnant beam to a computer with: -Almost instant viewing at a control panel -Indirect capture VS Direct capture
DR READER
Cassette-less Digital Radiography Systems
Alternative to CCD
Complementary Metal Oxide Semiconductor (CMOS)
CR systems allow(ed)
Conventional film/screen radiography rooms to be converted to digital with few modifications to the x-ray machine.
Indirect Capture DR
Currently there are two forms of Indirect Capture. -Charge-Coupled Device (CCD) + X-ray Scintillator + Optics -Thin-Film Transistor (TFT) + X-ray Scintillator + Photodetectors
Sampling pitch
Describes how digital detectors sample the X-ray exposure. They do so discretely- that is, at specific locations separated by specific intervals. different in CR and DR.
Size of the matrix
Determines the resolution. - the larger the matrix, the greater the number of pixels and the greater the resolution.
Pixel pitch
Distance between the center of one pixel and the center of an adjacent pixel, measured in microns.
Digital Imaging and Communication in Medicine (DICOM)
First formulated in 1983 to allow different computer systems (workstations, laser printers, storage, etc...) to interact and share information.
What is a Histogram?
Graphical representation of recorded exposure values
Histogram
Graphics representation of a data set, it represents the numbers of digital pixel values verses the relative prevalence of those values in the image.
What step in the histogram identifies values of interest?
Histogram Analysis
Anticipated histogram
In digital imaging, each anatomical part has an__?
Edge enhancement
Increases the contrast along the edge of a structure through a sophisticated software function. The part must have been sufficiently exposed and have a low signal to noise ratio, because noise is also enhanced.
Two general categories of DR are
Indirect Capture and Direct Capture
Interpolate
Is a type of estimation, a method of constructing new data points within the range of a discrete set of known date. Estimation, hypothesis
Photostimulable Phosphor (PSP) Plate
Is made of several layers: -Protective layer is a thin layer of plastic to protect the phosphor layer -Phosphor Layer (most important layer, the heart of the plate) contains Bromide Phosphor: Barium Fluorohalide (85%) and Europium Iodine (15%) -May be turbid or structured -Reflective Layer, reflects light released during the reading phase toward the photodetector -Conductive Layer, reduces and conducts away statice electricity -Support layer, provides rigidity to the plate -Color layer (light shield) New technology, absorbs stimulating light and reflect emitted light. -Soft backing protects the plate. ***Color layer and Support layer and switched in the book?
Sampling Pitch in DR
Is the distance between adjacent detector elements (DELs)
Sampling Pitch in CR
Is the distance between laser beam positions during processing of the plate.
Sampling Frequency
Is the frequency at which a data sample is acquired from the detector, it is expressed as sampling pitch.
Photodetector
Is used to sense the light released from the PSP plate during scanning. It amplifies this light, but it is in an analog electronic signal form. To make it digital, this amplified signal is sent to the analog-to-digital converter (ADC) that converts it to a digital electronic signal for the display computer.
Digital imaging requires
LOTS of storage space
Radiography was___ to make the transition to Digital Imaging.
Last
Dose-area product (DAP)
Measurement of exposure in air, followed by computation to estimate absorbed dose to the patient (cGY-m^2)
Detective quantum efficiency (DQE)
Measurement of the efficiency of an image receptor in converting X-ray exposure it receives into quality radiographic image
Disadvantages of CMOS
Not yet readily available for radiography, used in mammography and dental
Exposure Indicators (EI)
Numerical value indicating level of radiation exposure to image receptor
PSP Plate layers
PRO, PHO, REF, CON, SUP, COL
How PSP Plates work PSP Plate is processed by a CR reader
PSP Plate is exposed to laser from the reader and releases stored energy (Photostimulable luminescence), converting to a digital signal and becoming a manifest image.
*Book summary of CR Process
PSP plate is exposed to X-ray energy, and many electrons are trapped in the conduction bands as a latent image. When the cassette is inserted into the READER, the plate is removed and fed into the scanning area at a controlled and precise speed. As the plate moves forward, a laser beam is projected onto the phosphor layer, the laser energy releases the trapped electrons from the conduction band. These electrons return to their respective valence shells, the excess energy is given off as spontaneous light energy. This light energy is directed to the Photodetector via a Fiber-Optic bundle or a solid, light conducting material. Photodetector sends this light to the ADC, where it is converted to a digital electronic signal that is sent to the computer for processing and display.
Pixel
Picture element -two dimensional square that creates matrix
Rescaling
Process that adjusts the original histogram from the exposure to match the expected histogram. PPT*-Computer will correct for under/over-exposure by rescaling actual histogram (X-ray taken) to the anticipated histogram (X-ray expected) -Selecting appropriate algorithm in computer is important so that rescaling is correct for body part imaged. -adjusting the achieved histogram to match the desired(expected) histogram
Dynamic range
Range of EI that an image receptor can respond to and acquire data
exposure latitude
Range of Exposure values that will produce acceptable range for diagnosis
Which of the following adjust for under/over exposure?
Rescaling *PPT says LUT...but it also says that Under/Over exposure is adjusted by rescaling
Matrix
Rows and columns of pixels -Higher the matrix, the more pixels, the higher the spatial resolution. -Higher the matrix, smaller the pixels
Indirect Capture (w/TFT)
Scintillator is made of CESIUM IODIDE (Csl) or GADOLINIUM OXYSULFIDE (GOS)
Equalization
Software-weighted processing function whereby underexposed areas (light areas) are made darker and overexposed areas (dark areas) are made lighter. The effect is a image that appears to have lower contrast so that dense and lucent structures are better seen within the same image.
What had to be adjusted/updated for CR?
Technique charts and anatomic program systems
Bit Depth
The image is digitized by both location(spatial resolution) and intensity(grayscale) -the number of bits, is the available grayscale and refers to the number of shades of gray that can be displayed with in a pixel. -CR systems have bit depth of 2^14 or 16,384 shades of gray.
Detector array
The physical component consisting of multiple detector that efficiently absorb the transmitted radiation and accurately convert it to an electrical signal for display on a computer workstation.
Diffusion
The process by which washing works; exposes the film to water that contains less thiosulfate than the film.
Detector size/Field of View
The useful imaging area of the digital receptor
Drive mechanism
Within the reader removes the plate from the cassette and moves the plate into the scanning area at a controlled and precise speed
Histogram values
X-Axis=Range of exposure/pixel values Y-Axis=Frequency or the number of pixels in the radiograph with that specific value
ID (w/TFT) Photodetector is generally an Amourphous SILICON (a-Si) photodiode
a-Si is a liquid that can be painted onto a foundation, makes flat panel detectors possible.
Window leveling
brightness of image
Window width
contrast of the image
