Digital, PACS, Artifacts. Flashcards
DR utilizes an array of electronic x-ray detectors that take the remnant beam &
convert it to electronic signals to make available to the computer
In DR detectors, the materials used for detecting the x-ray signal and the sensors are
permanently enclosed inside a rigid protective housing
The permanent location of the detectors of a DR system is usually:
usually just below the radiographic tabletop, where the Bucky tray is
The image receptor can vary in:
size. 14 x 14, 14 x 17, or 17 x 17 inches
Electronic direct readout detectors are what differentiate
DR from CR.
Examples of types of collection elements:
Photodiodes
Charge-coupled devices (CCD)
Complementary Metal Oxide Semiconductors (CMOS) or
Thin-film transistors (TFT)
The CCD and photodiode are light sensitive devices that
collect light photons.
The photoconductor utilizes
specialized pixels that convert x-rays into light
The TFT is a charge-sensitive device that
collects electrons and creates the signal.
The DR flat panel image receptor consists of
an array of detectors and thin film transistors (TFT)
Exit radiation interacts with the detector and an electrical charge is created
This charge is stored temporarily in the
transistor (TFT) until ready to be read out.
The pattern of electrical charges is what makes up the
latent image.
There are two types of flat panel detectors utilized:
Direct and Indirect. They differ in how the x-ray exposure is converted to an electric charge
Thin-film transistor (TFT) detector arrays may be used in
both direct and indirect-conversion detectors
X-ray photons are absorbed by a scintillator, which is
a radiation-conversion material
In direct conversion, a scintillator absorbs x-rays and converts them to:
electrons which are stored in the TFT detectors
The TFT absorbs the electrons and does what with them?
generates electrical charges
The scintillator used in direct conversion is:
amorphous selenium
The thin film transistor is an array of:
small pixels which absorb electrons and generates electrical charges.
The thin film transistor reacts like ______ to send the electrical charges to the image processor
a switch
How many pixels can be read and converted to a composite digital image in less than 1 second?
Over 1 million.
Indirect also uses TFT technology
The difference is:
that indirect has a two-step process:
1) x-ray photons are converted to light
2) light photons are converted to an electrical signal
The scintillator in indirect conversion converts X-rays into:
visible light.
The scintillator in indirect conversion is made up of:
Cesium iodide (CsI) Gadolinium - Rare-earth element used in intensifying screens
After the indirect conversion scintillator converts X-rays into light, the light is then converted into an electric charge by
photodetectors such as:
Amorphous silicon photodiode
Charge-coupled devices (CCDs)
Cesium Iodide Crystals are crystalline needles that act as:
light directing tubes (much like fiber optics).
What is the benefit of using cesium iodide crystals as a scintillator?
This process allows greater detection of x-rays because there is almost no light spread therefore creating better resolution
CsI has a high capture rate because of
its high atomic number
Since x-ray interaction with CsI is high it results in
low patient dose
CsI needles absorb the x-ray photons, convert their energy into light & channels it to the
amorphous silicon photodiode array.
Indirect conversion by CsI crystalline needles process X-ray photons with _______ and very _____.
low noise, quickly. Approximately 30 million pixels/sec)
What is is the oldest indirect-conversion DR system?
Charge Coupled Device
Within charged couple devices, the light from the phosphor is linked by _______ to the CCD array
fiber optic bundles
After the image is transferred to the charge coupled device it
converts the light into an electrical charge
Where is the charge created within a charge coupled device sent to?
an analog-digital converter
Even though these CCD-based detectors require special equipment they are:
widely available and relatively low cost
The advantages of CCD are:
Sensitivity
Dynamic Range
Size
Sensitivity of a charge coupled device:
the ability of the CCD to detect and respond to very low levels of visible light
Dynamic range of a charge coupled device:
the ability of the CCD to respond to a wide range of light intensity, from very dim to very bright
Size of a charge coupled device:
CCD is very small making it highly adaptable to DR (1-2cm)
Complementary Metal Oxide Semiconductor (CMOS) Indirect conversion
A system that utilizes special pixel sensors that convert x-rays into light photons
Each sensor in CMOS has an amplifier which:
converts the light photons into electrical charges.
This charge is converted by an analog-to-digital converter.
Takes up less space/highly efficient.
CCD vs. CMOS
- CMOS more susceptible to noise
- Light sensitivity, resolution and quality slightly less than CCD
- Uses very little power compared to CCD
- CMOS costs less
- Less pixel fill factor with CMOS
Regardless of type of digital imaging system the varying electrical signals are sent to
the ADC for conversion into digital data
Digitized x-ray intensities or pixels are patterned in computer to form
the image matrix
Image matrix is
digital composite of varying x-ray intensities exiting the pt
DQE is
a measurement of x-rays that are absorbed when they hit the detector
DQE shows
how efficiently a system converts the x-ray input signal into a useful output image
Systems with higher quantum efficiency can produce
higher quality images at lower dose
Both indirect & direct DR has increased DQE over CR
In addition to this, direct conversion has:
increased DQE because there is no light conversion step, no light spread and therefore no light to blur the recorded signal output
Detector size is critical
They must be
large enough to cover the entire area to be imaged and small enough to be practical
In DR the ________ size determines resolution
Detector ELement (DEL)
The major cause of noise in the digital system is
electronic noise. This is the main factor limiting quality.
If not enough time is allowed for signal conversion the image will suffer.
electronic memory artifact
If exposures are taken in too rapid sequences, there may not be enough time for each previous exposure to transfer the entire signal
Preprocessing
takes place in the computer where the algorithms determine the image histogram
Postprocessing
is done by the technologist through various user functions. This is when exposure errors are corrected
Postprocessing Image Enhancement
Once the image is converted into digital (numerical) data the computer can perform post processing image enhancement.
The data within the collimated area produce a graphic representation of the optimal densities called
a histogram
image sampling
the plate is scanned and the image’s location and orientation are determined.
Size of signal is determined and a value is placed on each pixel.
A histogram is generated from the image data.
sampling:
the intensity of light or radiation from each pixel is measured by the detectors
Each histogram varies for:
each body part (anatomy specific)
What are the values of the x axis and y axis on a histogram?
x-axis is the amount of exposure read
y-axis is the number of pixels for each exposure
(how often pixels occurred and how many pixels were recorded)
Automatic rescaling:
means that the images are produced with uniform density and contrast, regardless of the amount of exposure
Automatic rescaling can result in:
Quantum mottle (too little exposure) Loss of contrast and loss of distinct edges (too much exposure)
Equalization
another software in which you are able to brighten a specific area
The LUT (Look Up Table) is used as
a reference to evaluate the raw information and correct the light values
LUT are data stored in the computer that is used to
substitute new values for each pixel during the processing
There is a LUT for every anatomic part
The appropriate LUT is automatically selected to give desired contrast characteristics to
match the type procedure
The LUT can be graphed by plotting the
original values on the horizontal axis and the new values on the vertical axis
Using the Look Up Table, Contrast can be increased or decreased by
changing the slope of this graph.
Brightness (density) can be increased or decreased by moving the line up or down the y-axis
Latitude refers to
the amount of error that can be made and still result in the capture of a quality image
The exposure latitude is much ______ for digital than that of screen/film
greater
If exposure is more than 50% below the ideal exposure then
quantum mottle results
If the exposure is more than 200% above the ideal exposure then __________ results
contrast loss
The biggest difference between digital and film/screen radiography lies in the ability to
manipulate the digitized pixel values that leads to greater exposure latitude
there is less _______ latitude with digital as compared to film/screen.
positioning
When positioning in DR, Beam, part and receptor alignment must be
right on in order to get a properly exposed image
In digital detectors where x-ray photon energy stimulates a phosphor there is always a spreading of light. The more light spread:
the less the image looks like the object and the lower the Modulation Transfer Function (MTF)
Modulation transfer function is
a numeric value that is used to measure spatial resolution. It is obtained from a graph utilizing numbers from 0 to 1 (0% to 100%)
MTF (Modulation transfer function) is a ratio of the image to the object so if a system is perfect it would have an MTF of
100% or if measured on a scale of 0 to 1 the MTF would be 1
“Formula” for MTF (Modulation transfer function):
MTF= information recorded in an image
information available in the part
When any component of the system is compromised the overall quality of the system is affected
MTF is showing us
if each system is “doing its part “ to contribute to the efficiency of the entire system
Types of Postprocessing Techniques:
Subtraction
Contrast Enhancement
Edge Enhancement
Black/white reversal
Subtraction
Removes superimposed structures to make anatomic area of interest more visible
Contrast Enhancement
Alters the pixel values to display different brightness levels
Edge Enhancement
Improves visibility & accentuates lines between difference adjacent tissues.
Should be viewed with & without edge enhancement so as not to miss any details that may be obscured by enhancement process.
Black/white reversal
Reverses the gray scale from the original radiograph
Smoothing
Filtering process that suppresses image noise by averaging pixel frequency with surrounding pixel values
Disadvantages to smoothing:
Reduction in noise comes with a reduction in contrast
Some spatial resolution is sacrificed
Edge Enhancement is useful for:
larger structures such as organs & soft tissues
Smoothing is useful for:
viewing smaller structures
Digital images can be altered while being viewed on a CRT and then printed onto film by
a laser camera.
Laser printers - describe wet & dry
Wet – use liquid chemicals (developer & fixer)
Dry – chemicals are part of the film (image created by use of heat)
Digital vs. film-screen – computer has the ability to adjust density to
compensate for technique errors
Transformation from analog to digital results in
loss of info
Flat panel direct capture detectors improved spatial resolution compared to
CR
Pixel size – major factor in level of
spatial resolution
High resolution monitors (1000 lines) have
improved image display.
2000-line resolution have been recommended
Because the range of stored densities in the computer is so much wider than what we can see, the digital image is only a small part of total data obtained by the computer
Therefore each image is only a ______ on the total range of data
“window”
As window level increases image density
increases.
As window level increases the overall brightness ______ and when window level decreases the overall brightness _______.
increases, decreases
By adjusting the window level on monitor we change
the image brightness
Window width
control that adjusts contrast
Wide window width =
lower contrast (more shades of gray)
Noise contributes
no useful information and detracts from quality of the image
What is primary source of noise in digital?
Quantum mottle.
Noise related to CCD technology
Statistical noise
Dark current noise
Amplification noise
Statistical Noise
Created by lack of light photons from the scintillator or not enough x-rays striking the scintillator
This causes the output signal to be noisy resulting in Quantum Mottle
Dark Current Noise
- Occurs when the CCD chip operates without radiation stimulation
- As a result of temperature rising
- Adds unnecessary info to the image
- Some manufacturers add thermostat cooling device
Amplification Noise
- Defects inherent in the manufacturing of the pixel elements
- Some pixels may not work at all
- The more bad pixels, the worse the detector efficiency
- The lower the cost of the CCD, the higher the number of defects
Noise is measured as
a signal-to-noise ratio (S/N)
The Signal (in signal to noise ratio) is:
the information required from the image
The Noise (in signal to noise ratio) is:
the unwanted information on an image
A high S/N indicates
little noise in the image 1000:1
The noisiest component of most digital systems is
the television camera (fluoro)
Image Stitching
Algorithms rely on overlapping exposures to verify registration marks & image contrast edges to align multiple exposures
Depending on system, annotation may not
transfer to PACS
PACS
Picture Archival and Communication System is a networked group of computers, servers, and archives that can be used to manage digital images
PACS can accept any image that is in _________________ format
Digital Imaging and COmmunications in Medicine (DICOM)
DICOM is
a network that allows imaging of modalities and PACS to communicate in the same “language”
DICOM is a universally accepted standard for:
exchanging medical images among the modality, viewing stations, and the archive
PACS serves as a
file room, reading room, duplicator and courier
PACS is custom designed for
each facility
facilities pushed to have RIS, HIS and EMR interfaced with PACS. What is RIS, HIS, and EMR?
RIS – Radiology Information system
HIS – Hospital Information System
EMR – Electronic Medical Record
Parts of PACS
Reading stations Physician review stations Web-access Technologist quality control stations Administrative stations Archive systems Interfaces to various hospital & radiology systems
Fundamental Parts of PACS
Image Acquisition
Display Workstations
Network
Archive Servers
Display Workstations
any computer that health care worker uses to view images
Workstations inside & outside of Radiology
Software enables user to manipulate image
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File Room/Image Management
- May be used to look up exams for a physician or to print copies for pt
- More popular is burning CDs
- File room also used for correcting pt demographics
Network
The manner in which many computers can be connected to interact with one another
Various types of devices that allow storage, retrieval and viewing of images
PACS workstations
Remote PACS workstations
Departmental mainframe
Hospital mainframe
Teleradiology is
the process of remote transmission and viewing of images
Who produced DICOM?
The ACR in conjunction with National Electrical Manufacturers Assoc. (NEMA)
Archive Servers
– the file room of PACS. The central hub that receives all images before being released to radiologists
Archives are chosen for:
System need
System cost
System compatibility
Images are stored in archives on:
on magnetic tape or optical disc
PACS archive controls
the receipt, storage and distribution of new and historic images
Workflow is
the amount of work or exams completed over a period of time
Workflow Manager in Radiology is
how the examination is completed from order entry to transcribed report
Navigation functions are used to
move through images, series studies and patients
The work list is used to
navigate through patients.
Access orders through work list
Image Manipulation & Enhancement Function
Any tool used to change the appearance of the image
Image Manipulation & Enhancement tools include:
Window/level Annotation Flipped image Pan, zoom and magnify Measurements
Pan, zoom, and magnify
used primarily by the radiologist to increase the size of an area on the image
Measurements
most commonly used is the distance measurement
Query/retrieve Function
used to retrieve studies from the archive
CD burning option
Print films for outside use
Advanced Workstation Function
Usually placed on specialty workstations for the radiologist or tech QC station to further enhance images
Reading station advanced functions:
Multiplanar reconstruction (MPR)
Maximum intensity projection & minimum intensity projection (MIP and MinIp)
Volume rendering technique (VRT)
Shaded surface display (SSD)
Multiplanar Reconstruction (MPR)
One of the most commonly used 3-D rendering techniques
Maximum intensity projection (MIP)Minimum intensity projection (MinIp)
Used to visualize vessels and air-filled structures. Performed after injection of contrast on CT & MRI.
Volume rendering technique (VRT)
Allows user to assign colors based on intensity of tissue so that bone, contrast agent, and organs can be seen in different colors
Shaded surface display
Uses a threshold of pixel intensity values: everything below is removed, everything above is assigned a color and shown as
3-D
Application Service Provider (ASP)
A company that provides outsourcing of archiving and management functions for a pay-per-use –per-month charge. Used by hospitals that do not have the capital funds or the personnel to implement and operate the complex archive needed for a PACS
ASPs give smaller institutions access to
the level of hardware and software they could not otherwise afford
ASP also used as
a disaster recovery mechanism
Disaster Recovery
Involves making copies of each tape or disk and sending to another building or off-site location
What is the most integral part of a PACS?
Archive.
Artifacts are
any undesirable densities on the processed image other than those caused by scatter radiation or fog.
The four types of artifacts seen in digital are:
Imaging plate artifacts
Plate reader artifacts
Image processing artifacts
Printer artifacts
Imaging Plate Artifacts
As plate ages becomes prone to cracks from action of removing and replacing plate within reader
What does an imaging plate artifact appear as?
Cracks appear as areas of lucency on the image. Cracks usually first become visible on the IP edges. As deterioration progresses cracks appear closer to the clinically used areas of the IP
Adhesive tape used to secure markers can
leave residue on the IR
When humidity is low static can cause hair to
cling to IP creating artifact
Backscatter can cause
dark line artifacts. Areas of the lead coating on the cassette that are worn or cracked allow scatter to image these weak areas
Plate Reader Artifacts
Intermittent appearance of extraneous line patterns can be caused by problems in the plate reader’s electronics
Reader electronics may have to be replaced to remedy this problem
Horizontal white lines caused by dirt on the light guide in the plate reader are examples of what type of artifact?
plate reader artifact.
In the event where a plate reader artifact is discovered:
Service personnel will need to clean the light guide
