4. Flat-Panel Radiography Flashcards
Limitations of CR
- X-ray detection of CR is inefficient & this affects
image quality & dose. - The spatial resolution of CR is less than Film-Screen radiography. CR 3-5 lp/mm, FS 10-15
lp/mm - CR Imaging plate can easily be damaged,
susceptible to scratches & cracking. - CR Imaging plate must be transported to a
separate image processor (reader) for image data
extraction
Introduced as
early as ____ for
use in radiographic
imaging
1995, Flat-panel digital
radiography
A-Si
Amorphous Silicon
A-Se
Amorphous Selenium
Digital detectors used before 1995 were \_\_\_\_\_ on FPD technology
not based
(CCD chip) Slot-scan digital detector
1990
Selenium drum digital detector
1994
- Chest imaging
- Thoravision (Philipps Medical Systems)
- Insert figure 5-2
conducts electrons when struck by
light or X-ray photons
Photoconductor
- Selenium 34
- Silicon 14
Flat-panel
Digital
Radiography:
S Y S T E M
C O M P O N E N T S
- Pre-amplifiers
- Switching Control
- Central Logic Circuits
- ADC’s
- Internal Memory
It is important to note that X-ray detection &
digitization of the X-ray signal take place
within the _____
flat-panel detector
Types of FPD Detectors
- Indirect Detectors
- Direct Detectors
-uses phosphors
-phosphors convert X-ray energy into electrical charge
through an intermediate stage of light photons.
Indirect Detectors
-use a photoconductor
-photoconductors convert x-ray energy into electrical
charge without the intermediate stage.
Direct Detectors
2 TYPES OF INDIRECT DETECTORS:
technical
components
- Charged-Coupled Device (CCD) Digital Detector
2. Flat-Panel Thin Film Transistor (TFT) Digital Detector
The most prominent difference between these
two types of detectors is the technical
component used to ____
convert light into electrical
signals
based on an indirect conversion process
CCD (Charged-coupled Device) digital detectors
uses a CCD chip to convert light to
electrical charge
CCD (Charged-coupled Device) digital detectors
not classified as flat-panel digital
detector
CCD (Charged-coupled Device) digital detectors
The main technical components of a CCD-based DR detector includes:
- X-ray absorber
- Light optics
- CCD - sensor (chip) for capturing the
light electrical charge readout device
A CCD digital detector consists of several CCD’s
in order to _____ the size of the detection
area
increase
3 OTHER NOTEWORTHY
COMPONENTS:
- Scintillation screen – detects X-rays &
converts them into light - Light collection optics
- Array of CCDs – CCD camera
Other
systems
used:
- Fiberoptic-coupled CCD System
- Lens-coupled CCD
- Fiberoptic-coupled Scanning Array
System
based on an indirect conversion process
Indirect flat-panel
TFT (Thin Film Transistor) digital detectors
uses several physical components to convert x-rays into light that is subsequently converted
into electrical charges
Indirect flat-panel
TFT (Thin Film Transistor) digital detectors
The main technical components of a TFT digital detector includes:
- X-ray scintillator (X-ray conversion layer) CsI,
Gd2O2S2 - Amorphous silicon (a-Si) photodiode flat-panel
layer with a thin-film transistor (TFT) array for
readout of the electrical charges by the
photodiode array.
DIRECT DIGITAL DETECTORS: technical components
- Source of high voltage
- Top electrode
- Dielectric layer
- Photoconductor
- Collection electrode
- TFT
- Storage capacitor
- Glass substrate
SOURCE
OF
PHOTOCONDUCTORS:
Amorphous Selenium (a-Se)
excellent X-ray detection properties & a very
high spatial resolution
Amorphous Selenium (a-Se)
detect x-ray photons from the patient &
converts them directly into electrical charges
Amorphous Selenium (a-Se)
Other photoconductors used:
- Lead Oxide
- Lead Iodide
- Thallium Bromide
- Gadolinium
designed as a matrix of detector elements, each of which can be regarded as a pixel & constructed as:
design is called: “______”
Configuration of the Flat-Panel,
large area integrated circuit
also referred to as an active matrix array
Matrix
consists of rows & columns that play a role in
addressing & readout of the signal from each
pixel
Matrix
Each pixel contains a TFT (switch), a storage
capacitor & a sensing area, referred to as the
_____
sensing/storage element
The sensing area will detect the light from the
___ scintillator
CsI
A concept that examines the response of the image
receptor to the radiation falling upon it
Exposure Latitude
Exposure latitude for CR/DR detectors
0.1-1000 µGy
Detectors with wide dynamic range can respond to ____ of exposure (low to high) and still provide an image that appears acceptable to the observer
different levels
Typical detector dimensions:
43 cm x 43 cm
30 x 40 cm
18 x 18 cm
Typical matrix sizes:
1760 x 2140 2000 x 2500 2736 x 2736 2560 x 3072 2688 x 2688 3121 x 3121
The pixel size & spacing determine the _____ of the image
spatial
resolution
the distance from the
midpoint of one pixel to the midpoint of the
adjacent pixel
Pixel Pitch
Pixel sizes in current detectors can be
139 µm,
143 µm, 160 µm, 162 µm, 167 µm, and 200
µm
The ratio of sensing area of the pixel to the area
of the pixel itself
Fill Factor of the Pixel
Image Quality Descriptors
- Spatial Resolution
- Density Resolution
- Noise
- Quantum Detective Efficiency
- Artifacts
related to the size of the pixels on the image
matrix
Spatial Resolution
Pixel size of 35 cm x 43 cm (24“ x 17“)
0.2 mm
Pixel size of 23 cm x 30 cm (10” x 12”)
0.14 mm
Pixel size of 18 cm x 24 cm (8” x 10”)
0.1 mm
The smaller the pixel size the ____ the spatial
resolution of the image
better
The pixel size can be calculated using the
relationship:
PS = FOV/ Matrix Size
Same FOV, the _____ the matrix size, the _____ the pixels, the better the ____
greater,
smaller,
image sharpness
A typical CR image matrix size
2048 x 2048
linked to bit depth
Density Resolution
range of gray levels per pixel
Density Resolution
An image with a bit depth of 8 will have _____
shades of gray per pixel
256 (2^8)
refers to the color
information stored in an image
Bit Depth
The ___ the bit depth of an image, the
more colors it can store
higher
1 bit
Two values: 0 white, 1 black
8 bit
256 colors
24 bit
16 million
Noise
- Electronic Noise (System)
2. Quantum Noise (Quantum Mottle)
determined by the number of X-ray photons
(signal) falling upon the detector to create
the image
Quantum Noise
Low exposure factors will produce few photons at the detector (less signal, more noise)
Results in a noisy image, (Grainy/Poor image)
Higher exposure factors will generate more
photons at the detector (more signal, less noise)
Produce a better image, increase patient dose
___ Noise, ___ Detector Exposure
More,
Less
The detector receives an input exposure &
converts it into a useful output image
Detective Quantum Efficiency (DQE)
DQE
Detective Quantum Efficiency
is a measure of the efficiency & fidelity
with which the detector can perform this task
Detective Quantum Efficiency (DQE)
DQE =
SNR^2out / SNR^2in
DQE for a perfect digital detector is
1 or 100%
This means that there is no loss of information
A distortion or error in an image that is
unrelated to the subject being studied
Image Artifacts (Morgan, 1983)
An ____ is a feature in an image that masks
or mimics a clinical feature
artifact, (Willis, 2004)
can be disturbing to
radiologists and may even result in an
inaccurate diagnosis
Artifacts
Sources of Artifacts
- Imaging hardware (equipment)
- Image processing software
- Objects that are imaged & linked to
the operator errors
ensures that every employee plays a
role in creating a quality product
CQI (Continuous Quality Improvement)
CQI
Continuous Quality Improvement
are essential not only
for optimizing the assessment and
evaluation of patient care, but also for
monitoring the performance of equipment
QA & QC programs
Describe systems & procedures for assuring quality
patient care, quality assessment, continuing
education, the usefulness of quality control
procedures & assessment of outcomes
Quality Assurance
deals with the administrative aspects of
patient care & quality outcomes
QA (Quality Assurance)
Component of QA that refers to the monitoring of
important variables that affect image quality &
radiation dose
Quality Control
deals with technical aspects of equipment
performance
QC (Quality Control)
Purpose of the procedures & techniques of CQI, QA & QC:
1. To ensure optimum image quality for the purpose of enhancing diagnosis 2. To reduce the radiation dose to both patients and personnel 3. To reduce costs to the institution
involves a number of activities that are of
significance to the technologist, particularly if
the technologist is in charge of the QC program
Quality Control
Quality Control Activities:
- Acceptance Testing
- Routine Performance
- Analysis of Reject Rates
- Error Correction
Ensures that the equipment meets the
specifications set by the manufacturer
Acceptance Testing
Involves conducting a QC test on the equipment on a regular basis with varying degrees of frequency
Annually, semiannually, monthly, weekly or daily
Routine Performance
Ensures that equipment not meeting the performance
criteria or tolerance limit established for specific QC
tests must be replaced or repaired to meet
specifications
Error Correction
has recommended a number of
tools for CR Quality Control
AAPM (American Association of Physicists in Medicine)
TOOLS FOR CR QC TESTING
- Densitometer
- Copper & aluminum filters
- Calibrated ion chambers
- Screen contact wire mesh patterns
- Anti-scatter grid
- High contrast resolution line pair phantoms
- Low contrast phantoms
- Anthropomorphic phantoms
Common Beam Filtering Materials
Brass, Copper, Aluminum, Lead, Tin, Molybdenum, Tungsten, Titanium, Zirconium
Film-Screen contact test tool
Wire mesh tool/placed on screen & exposed
QC TEST 1: Dark Noise
Purpose: To assess the level of noise present in
the system
Exposure condition: No exposure. Erase a single screen and read it without exposing it
Process in the image
reader: Use the appropriate QC image
processing tool.
Qualitative criterion
for acceptance: Uniform image without artifacts.
QC TEST 2: CR Imaging Plate Test = Uniformity
Purpose: To assess the uniformity of the
recorded signal from a uniformly exposed imaging plate
Exposure condition: Expose imaging plate using appropriate
exposure factors
Process in the image
reader: Use the appropriate QC image
processing tool
Qualitative criterion
for acceptance: Uniform image without artifacts
QC TEST 3: Erase Thoroughness
Purpose: To test the minimal residual signal (ghosting)
on a CR imaging plate after readout &
exposure
Exposure condition: Place a step-wedge at the center of a 14X17
CR IP and expose using appropriate exposure
technique & process the in the image reader.
:Re-expose the same IP a second time without
the step-wedge using the appropriate exposure technique. Collimate in by about 5 cm on each side of the CR IP
Process in the image reader: Use the appropriate QC image processing tool
Qualitative criterion for
acceptance: Absence of a ghost image of the step-wedge from the first exposure in the re-exposed image
refers to the persistence of the image, that is, charge is still being produced after
the radiation beam from the X-ray tube has been
turned off
Memory effect
Charge has been trapped in the metastable band-gap
states in the a-Si and a-Se material during exposure &
is only released slowly over time
Image Lag
detects X-rays &
converts them into light
Scintillation screen
