Transcript Radiation Protection in Digital Radiology

Radiation Protection in Digital Radiology
Fundamentals of Digital Radiography
L01
IAEA
International Atomic Energy Agency
Educational Objectives
• Explain how ordinary
radiographic images can be
captured in digital form
• Discuss the advantages and
limitations of digital images
• Explain how the dissociation of
acquisition and display in DR
can contribute to unnecessary
radiation exposure to patients
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What is a digital image?
• An approximation of an
analog image, with regard to
• spatial information
• contrast information
• A computer file composed of
discrete picture elements, or
pixels
• location in file (array or matrix)
represents image position
• numeric value represents signal
intensity
Etruscan Roman Mosaic
circa 50BC
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Why would we want digital images?
• Availability
• a digital image can be transmitted electronically to
distant locations and can exist simultaneously at multiple
locations
• Flexibility
• the appearance of a digital image can be modified
• Convenience
• a digital image can be stored electronically without
occupying physical space
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Conventional screen-film radiography
• Radiation strikes intensification screen(s) producing
fluorescence
• Fluorescent light exposes photographic film producing
latent image
• Latent image is chemically developed to produce density in
film
• Film density is viewed by trans-illumination
1895
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Developed film is effectively analogue
• Density is the result of many developed silver grains
• Grains in intensification screen are quite small
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0
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[1,0,0,2,3,4]
[1,0,0,2,3,4]
[1,0,0,2,3,4]
[0,0,0,0,0,0]
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Three approaches to digital radiography
I.
II.
III.
Translate developed film into digital form.
Capture the radiographic projection by nonphotographic method and digitize during
development.
Capture the radiographic projection or its
fluorescence directly in digital form.
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Method I: Film Digitization
• Video of transilluminated radiographs
• “Camera-on-a-stick”
• Low cost, low quality
• LASER film digitisers
• Best quality
• Expensive and involves periodic maintenance
• CCD film digitisers
• Less cost than LASER, less maintenance, better
quality than camera-on-a-stick
• Old problems of drift, noise, non-uniform
illumination, and veiling glare – mostly rectified
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Process of film digitization
• Light is directed
onto a film
• Light passing
through the film is
measured
• Amount of light
attenuated is
converted into a
digital code value
Bushberg, Seibert, Leidholdt, Boone
The Essential Physics of
nd
IAEA Medical Imaging 2 Ed
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L01 Fundamentals of Digital Radiography
Fundamental limitations of film digitization
• Prone to artefacts
• Labour intensive – an extra step
• Best quality achievable is limited by original screenfilm image
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How many of us maintain capability to digitize film?
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Method II: Non-photographic capture
with digital development
• Xeroradiography
• Charged Selenium plate
• Electrostatic latent image
• Charge distribution transferred to paper using toner
• Selenium drum detector
•
•
•
•
Selenium deposited on cylindrical Al drum
Selenium uniformly charged before exposure
X-rays partly neutralize the charge
Charge distribution measured by electrometer array
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Computed Radiography (CR) or Photostimulable
Phosphor (PSP) Radiography
• Latent image of trapped electrons is formed when x-rays
hit the imaging plate
• Latent image is read out physically instead of chemical
process
• As the latent image is read out…
• Stimulated light emitted with the help of LASER is directed to a
Photomultiplier Tube (PMT)
• The PMT signal is digitized using Analog-to-Digital Converter
(ADC)
• The digital image consists of an array of ADC Code
Values
• ADC Code values represent exposure information
• Array locations represent spatial information
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Photostimulable phosphor reader
Rotating polygon mirror
Analog-to-Digital Converter
Photomultiplier tube
?
Light guide
Laser
Amplifier
fast scan
Latent Image
slow scan
Imaging plate
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Characteristics of PSP systems
• Generally, but not exclusively, cassette•
•
•
•
based systems
Moderate initial capital investment
Simple retrofit to existing radiographic
equipment
Individual scanner can serve multiple
acquisition devices
Workflow comparable to daylight loader film
processing
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Method III: X-rays are converted immediately
into digital signals without latent image
• Fluorescent screen with video camera (videofluoroscopy, image intensifiers)
• Fluorescent screen with Charged-Coupled Devices
(CCD) or Complementary Metal Oxide Semiconductor
(CMOS) array
• Optical lens coupling
• Secondary quantum sink
• Fiber optic coupling
“Flat panels”
• Small area
• Hydrogenated Amorphous Silicon (a-Si:H) with Thin Film
Transistors (TFT)
• alternative = switching diode
• requires x-ray converter (Gadolinium oxysulphide or Caesium
Iodide)
• Amorphous Selenium (a-Se) electronically coupled to
TFT
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Characteristics of “Direct” capture systems
+ Rapid acquisition and processing
+ Typically integrated with x-ray
+
—
—
—
?
generator
No mechanical scan mechanism
High initial capital investment
Challenging manufacturing
processes
Limited systems for bedside
radiography
Brief history of clinical operation
? Life cycle issues unknown
(durability?)
? Image rendering unknown
? Exposure factor issues
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Courtesy JA Seibert, UC-Davis
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How good an approximation does the digital
image make?
• Spatial information depends on …
• Dimensions of the pixels (matrix size)
• Blur
• Contrast information depends on …
• Grayscales (Code values) per pixel (i.e.
quantization)
• Characteristic function (Code values vs.
exposure)
• Noise
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Effects of Pixel size and Bit-depth
Larger
pixels
1024 x 1024
64 x 64
32 x 32
16 x 16
More
bit-depth
1 bit/pixel
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2 bits/pixel
3 bits/pixel
8 bits/pixel
For an image of M x N matrix, k bytes/pixel,
the memory needed to store the image is k x M x N bytes
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Practical resolution is less than the
Nyquist frequency
• Factors besides sampling
compromise sharpness; depends on
• X-ray focal spot dimensions
• Blur in Indirect DR and CR
• Optical and mechanical imprecision in
IDR and CR
• Afterglow in fast-scan dimension in CR
• Limit of resolution is where
Modulation Transfer Function (MTF)
has decreased to 10%
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MTF of DR depends on more than just
sampling
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Noise interferes with our ability to detect
contrast
σ = √N
SNR = N/σ = √N
Air KERMA
Photons
Noise
(µGy)
/100mX100µm
(%)
9.0
1333
2.7
0.9
133
8.6
0.09
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27.4
Bushberg, Seibert, Leidholdt, Boone The Essential
Physics of Medical Imaging 2nd Ed
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Combination of quantum noise and
anatomic noise limits low contrast detection
DR Image
CT Image
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Detective Quantum Efficiency (DQE) of DR
Combines SNR and Resolution
Ideal
detector
(200 um)
(143 um)
(200 um)
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DR has wide dynamic range (latitude)
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10000
1000
2
1.5
100
1
10
Intensity (rel)
Density (OD)
2.5
Film/screen
PSL
0.5
0
0.1
1
10
100
1
1000
1023
Air kerma (μGy)
High kV
L=2.2, S=50
Over-Exposed
Histogram re-scaling
00.1 µGy
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Raw Plate Exposure
1000 µGy
Low kV, L=1.8, S=750
Under-Exposed
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Rescaling of DR images is a doubleedged sword
• Variations in exposure factor selection are
automatically compensated by rescaling for
consistent appearance
• Inappropriately high or low exposures are not
immediately apparent from the appearance of the
image until the range-of-adjustment is exceeded
• Underexposure makes noisy images
• Overexposure makes crisp, noiseless images, that are
preferred by radiologists
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There is a documented tendency to
overexpose in CR and DR
• Oversight of exposure factor selection is impossible without
an exposure indicator
Freedman et al. SPIE 1897 (1993),472-479.
Gur D et al. Proc 18th European Congress of Radiology. Vienna Sep 12-17.(1993)154.
Barry Burns, UNC
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What would be the appropriate exposure?
• Seibert, et al Acad Radiol (1996) 4: 313-318
• QA based on exposure indicator reduces doses
• Willis Ped Radiol (2002) 32: 745-750
• 33% dose reduction if exposure indicator target followed
• AAPM Task Group #116 is effort to standardize indicators
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Five ways of making a radiographic
projection with lower dose to the patient
• Each method has consequences with respect to the
five aspects of image quality
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Important information about DR acquisition
and processing is in metadata
• CR vs. DX object
• Mandatory vs. optional
vs. private tags
• Automatic vs. manual
entry of data
• PACS interpretation of
metadata
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New artefacts from the
discrete nature of DR
• Interference pattern
between fixed grid
lines and downsampling rate for
display
• Disappeared on
zoom
• Bad choices
• Display default
magnification
factor
• Line rate of grid
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X-ray energy sensitivity differs among
detectors
Martin Yaffe/Tony Seibert
1.0
Gd2O2S:Tb 120 mg/cm2 (Lanex)
BaFBr 100 mg/cm² (CR)
A-Selenium 25 mg/cm2
0.1
CsI:Tl 45 mg/cm2 (a-Si/CsI)
0.01
0
20
40
60
80
100
120
140
Photon Energy (keV)
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Would you expect to use the same kVp?
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Emerging digital technologies
The EOS Imaging system
Charpak Detector: Detector amplification by photon gaz cascade
 High gain signal, sensitivity maximized
Detectors
X ray tubes
Two simultaneous digital planar radiographs (PA and LAT) in the
standing position by linear scanning of a fan-shaped collimated X ray
beam from 5 cm to 180 cm (whole body).
EOS allows for a dose reduction up to 10 times compared to CR
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Advantages of digital radiology
• Digital imaging has practical technical
advantages compared with film techniques:
• wide contrast dynamic range,
• post-processing functionality,
• multiple image viewing options,
• electronic transfer,
• electronic archiving possibilities.
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How to move from film-screen to digital?
• Training should be planned with anticipation.
• Selection of equipment, connectivity and quality
control requires good advice (not only from the
manufacturers) and visits to other installations.
• Patient dose and image quality should be audited
carefully during the transition. The risk of
increasing patient doses exists.
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How to move from film-screen to digital?
• Typically the initial setting for the automatic
exposure control using CR, should be the same or
similar to the existing with film-screen.
Optimisation (including possible kV changes) and
potential dose reduction should be initiated later
once radiologists become familiar with digital.
• Audit DICOM header. It contains a lot of useful
information.
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Checklist of practical advice (ICRP 93)
• When introducing a new digital system into clinical
practice, the system should be set up to achieve
the best balance between image quality and
patient dose.
• Avoid deleting non diagnostic images at the
workstation and carry out a statistical rejection rate
analysis periodically.
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Checklist of practical advice (ICRP 93)
• Be familiar with your workstation capabilities (postprocessing capabilities, options in the monitor to
visualise the images, etc).
• Identify correctly all the images to avoid their loss
in the PACS.
• Ask for a calibration of the automatic exposure
control that is appropriate for the sensitivity range
of the digital system and the selected post
processing.
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Checklist of practical advice (ICRP 93)
• Avoid manual exposures if the automatic exposure
control is usable. However, ensure it is used
correctly, as only some possible errors are
correctable by post processing.
• Control the number of images per examination to
maintain it at a number similar (or lower) than that
for conventional radiology.
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Checklist of practical advice (ICRP 93)
• Make available a workstation for radiographers to
post-process images in order to avoid retakes.
• Pay attention to the dose indication on the panel of
the x-ray system or in the in-room monitors and
utilise the information to manage patient doses.
• Establish easy access to the PACS to review
previous images, in order to avoid retakes.
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Checklist of practical advice (ICRP 93)
• Pay attention to proper collimation for the desired
anatomical area. Once the image is acquired,
numerical methods (i.e. software) may
automatically crop part of the image, and when the
image is received for reading the radiologist will
not be aware that a larger anatomical area than
necessary was irradiated.
• Select the correct pre-programmed technique e.g.
using an abdominal technique (70-80 kV) for chest
imaging (120-130 kV) will result in a higher
entrance surface dose.
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Summary
• A digital radiographic image is a matrix of numbers with discrete physical
pixel dimensions and gray-levels.
• The discrete nature of the DR image is the source of its advantages
and limitations
• Digital radiographs can be produced by three methods:
• Digitization of screen-film radiographs
• Non-photographic capture and digitization
• Direct capture with or without conversion to light
• DR technologies have advantages of availability, flexibility, and
convenience over conventional screen-film
• The utility of the DR image is enhanced by demographic, exam, and
processing information in the DICOM header
• Except for digitized radiography, DR has the potential for unnecessary
patient radiation exposure
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Answer True or False
•
•
•
A digital image contains numeric value
representing signal intensity.
In computed radiography (CR), the electronic
latent image is developed by chemical process
Amorphous selenium is used in direct capture flat
panel detectors
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Answer True or False
• True. A digital image is a computer file composed
of discrete elements or pixels representing signal
intensity.
• False. The electronic latent image in the CR plate
is read out using LASER
• True. Amorphous selenium (aSe) is electronically
coupled to the thin film transistor (TFT) in a direct
capture flat panel detector.
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References
• Managing patient dose in Digital Radiology
ICRP Publication 93 Ann ICRP 2004
Elsevier.
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