Transcript 06. Avoiding Artefacts in Computed Radiography

Radiation Protection in Digital Radiology
Avoiding Artefacts in Computed
Radiography
L06
IAEA
International Atomic Energy Agency
Educational Objectives
• Explain how the CR image is created
• Explain how errors in the process can
produce sub-standard images
• Artefacts in CR
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A radiographic projection can be captured
using photostimulable phosphor (PSP) plates
• X-rays contribute energy to electrons by the
photoelectric effect when it strikes the PSP plate
• Electrons can give up energy (violet light)…
• by emitting light immediately (fluorescence)
• by emitting light slowly (phosphorescence)
• Some electrons can retain (store) their energy
• crystal defects can “trap” excited electrons
• electrons can escape the traps when exposed to the proper
wavelength (red) light using LASER source. Photo-stimulated
luminescence)
• electrons can also escape by thermal mechanisms (fading)
PSP fluorescence
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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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Correction of non-uniformity improves
contrast and sharpness
Before
After
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Imaging plate artefact
• PSP Plates
• Scratches in
the plate.
• Abdomen of
a 3rd
trimester
pregnant
woman.
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a
b
c
(a)Chest (PA) obtained as chest Lat (flat-panel detector), 125 kV,
6.2 mAs, 0.54 mGy (patient entrance dose four times higher
than necessary). AEC centre cell used. Saturated image at the
lung area.
(b)Same image displayed with inverted grey scale.
(c)Same image. Isocontour 99% of pixel content. Some lung
areas are saturated without any diagnostic information. The
image was repeated.
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CR Imaging plates do not last forever
Imaging plate artefact
Agfa plate defect
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Same defect,
different orientation
Artefact remedy: The Imaging plate (IP) must
be replaced when cracks occur in clinically
useful areas
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Imaging plate artefact
• PSP Plates
• Artifact due to
humidity
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Imaging plate artefact
Phosphor plate deteriorated.
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Imaging plate artefact
Phosphor plate misused. Impossible to clean.
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Plate reader artefact
• Digitiser
• Problems with the
analog-digital
conversion
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Plate reader artefact
• Digitiser
• Reading: galvo
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Dust on collection optics causes line in
slow-scan dimension (Fuji CR)
• Dirt on the light guide in the
CR reader. The light guide
collects light emitted from the
imaging plate when it is
scanned by the laser.
• Artefact remedy: the light
guide of the photomultiplier
tube was cleaned by service
personnel.
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Plate reader artefact
• Digitiser
• Problems with
the storage of the
image in the
computer
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Plate reader artefact
• Digitiser
• Processing
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Plate reader artefact
Digitiser
• Processing
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Plate reader artefact
• Digitiser
• Processing
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Plate reader artefact
• Digitiser
• Processing
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Plate reader artefact
Digitiser problem
• Artefacts in the
horizontal direction
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Plate reader artefact
•Digitiser problem.
• Artefacts in the
vertical direction.
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Plate reader artefact
• Digitiser problem.
• Saturation in the
lower part of the
image.
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Plate reader artefact
Two exposures in the
same plate.
Humerus and lateral
dorsal spine.
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Incorrect collimation can cause inappropriate
image processing: Agfa CR
Automatic processing
Manual reprocessing
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Incorrect collimation
Bad collimation but
with cropped image
seems good
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Failure to detect radiation field
(exposure recognition failure) : Fuji CR
• Causes:
•
•
•
•
•
nonparallel collimation
multiple fields
poor centering
Implants
violation of collimation rules
• Result: inappropriate
histogram analysis =>
incorrect rescaling
• Artefact remedy: Use proper
collimation and appropriate
positioning
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Over-exposure should cause a strong signal,
less noise, less mottle
8 mAs
24 mAs
But extra scatter => Contrast loss
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Over-exposure should cause a strong signal,
less noise, less mottle
S = 252
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Barry Burns,
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S = 68
~ 4 X Overexposure
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What’s wrong with this CR image?
Galvanometer
bearing failure
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Artifact observed after radiographer
frees jam (Agfa CR)
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Also observed on multiple
subsequent
images
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Faulty ground on scanner control board
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CR and film-screen from dose efficiency
point of view
• Typically CR systems are initially adjusted to work
with the same level of dose as the screen film
systems.
• Later, with the optimisation programmes, it is
possible to work with lower doses.
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CR and film-screen. Latitude advantage.
• The wide dynamic range of the CR, allows more
information to be shown on the images (modifying
the window and level for a good visualisation).
• In addition, if same mistake occurs in the selection
of the radiographic technique, this wide dynamic
range avoids in most of the cases, the need to
repeat the exposure.
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a
b
(a) Relative exposure index 1.15, image too noisy.
(b) Relative exposure index 1.87, image with sufficient quality
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CR lumbar spine adult 42 y; Dose Level (Agfa): 0.85 (too low) image too noisy
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CR lumbar spine adult 42 y; Dose Level (Agfa): 1.41 (normal) image good for diagnosis
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Dose level
(Agfa): 2.36
(higher than
necessary)
Good image
quality
Patient 10 months
old.
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Dose level
(Agfa): 2.95
(too high)
Saturated
image
Patient 10 months
old.
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DL= 2.95
DL = 2.36
The first image was not valid for diagnosis (saturated due to the very high dose) and
repeated.
The first image required 4 times the dose used for the second one, and 18 times more
dose than necessary.
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Deletion of image files at the viewing or workstation
of apparently non diagnostic images
• Sometimes, radiographers can assume that some
images are unuseful for diagnosis and they could
proceed to delete the files.
• This could be a bad practice because they avoid
the possible use of some of the information
contained in the bad image and in addition, they
could prevent the appropriate analysis of retakes.
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Practical advice
•
•
Poor monitor setup (e.g. low illumination,
contrast or spatial resolution) could lead to
unnecessary repeated exposures, due to
apparent non diagnostic image.
All users of the monitor should fully understand
how to use correct settings.
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Postprocessing
• ICRP 93 advice:
• Image quality can be compromised by
inappropriate levels of data compression and/or
postprocessing techniques. This is dependent on
the modality.
• Data compression and postprocessing
requirements should be defined by the modality
and the medical imaging task.
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Influence of postprocessing in image quality
Figure from ICRP 93 (courtesy of R. Loose).
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Effect of the postprocessing
• The standard postprocessing parameters offered
in some CR workstations includes the noise
reduction and the edge enhancement.
• The basic principle for Agfa postprocessing tool
called “MUSICA” (Multi scale Image Contrast
Enhancement):
• contrast enhancement irrespective of feature size.
• difference with respect to spatial frequency band
filtering.
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Standard image
Noise reduction
Edge enhancement
Examples of different postprocessing using Agfa CR software
(MUSICA)
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Standard image
Noise reduction
Edge enhancement
Examples of different postprocessing using
Agfa CR software (MUSICA)
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Some key points:
• Use of different
post processing
(sometimes could
avoid repetitions).
• In this case using
different postprocessing
means that the
lung tissue and
mediastinum can
be visualised.
Figure from ICRP 93
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Practical advice
• Many problems with, eg, the digitiser, printer, local
hard disk, faults in electrical power supply, network
problems can influence patient dose.
• All of these elements occasionally can lead to
problems with image quality resulting in a repeated
exposure.
• Correct maintenance of all these elements is a key
aspect of the quality assurance programmes.
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Some key points:
• Lost of images in the network or in the PACS due
to improper identification or other reasons.
• Sometimes, images not properly identified are lost, or
archived with the wrong identity, requiring some
examinations to be repeated.
• Existence of false lesions or pathologies due to
artefacts introduced by incorrect digital post
processing
• Sometimes, improper post processing (trying to reduce
the noise or using a too high edge enhancement could
simulate the appearance of false lesions.
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Some key points:
• Images stored in some PACS cannot be
(sometimes) post-processed.
• This will be known in advance and appropriate post
processing should be applied before the image is
archived.
• Allowing easy access to PACS and teleradiology to
look at previous images can save exposures.
• Except of the “for presentation” images, PACS save
also images “for processing”. These may
retrospectively be processed to recover information
preventing re-exposures
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Effect of incorrect calibration or misuse or lack
of use of automatic exposure system
• The automatic exposure control (AEC) system
should be calibrated to finish the exposure when
the appropriate dose is received by the imaging
detector.
• If this device is mis-calibrated or used in a non
appropriate way, patients can be over irradiated
and with digital technology this could be unnoticed
by radiographers, thus, the indication of patient
dose is crucial.
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Deteriorated storage-phosphor plates
• The use of deteriorated phosphor plates (PP)
could mean the repetition of some exposures.
• The quality control programme should include the
periodic test of the PP.
• In some of the CR systems (e.g. Agfa), the number
of exposures in the PP are stored in the DICOM
header of the image.
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(0008,0020) : Study Date
: 04/12/2003
(0008,0022) : Acquisition Date
: 04/12/2003
(0008,0060) : Modality
: CR
(0008,0070) : Manufacturer
: AGFA
(0008,0080) : Institution Name
: HCSC
(0008,1010) : Station Name
: ADCC2
(0008,103E) : Series Description
: lumbar AP
(0010,1010) : Patient's Age
: 020Y
(0018,0015) : Body Part Examined
: LSPINE
(0018,1004) : Plate ID
: U13-35
(0018,1401) : Acquisition Device Processing : 60025Ia712Ra
(0018,1403) : Cassette Size
: 35CMX43CM
(0018,1404) : Exposures on Plate
: 342
(0018,5101) : View Position
: AP
(0018,6000) : Sensitivity
: 4.00000000E+02
(0019,1010) : Image processing parameters :
MENU=60025 CC=0 MC=3.00 EC=0.00 LR=2.00 NR=4.00
(0019,1013) : Sensitometry name
: NK5
(0019,1015) : Dose monitoring list
: 1.54
(0020,0013) : Image Number
:1
(0020,1002) : Images in Acquisition
:1
(0028,0010) : Rows
: 3730
(0028,0011) : Columns
: 3062
(0028,0100) : Bits Allocated
: 16
(0028,0101) : Bits Stored
: 12
(0028,0102) : High Bit
: 11
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Example of some of the DICOM tags in the header
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Possibility of reducing the number of images
per procedure with digital imaging
• Due to the wide dynamic range of digital detectors
and the post-processing and visualization tools
(including magnification, change of window and
level, change in contrast, etc), it is possible to
reduce in some cases the number of exposures
per examination.
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Possibility of reducing the number of images
per procedure with digital imaging
• For the lumbar spine examinations, sometimes in
conventional film-screen imaging, a lumbosacral
joint image is required in addition to the AP and
LAT projections.
• With digital techniques the lumbosacral image can
be avoided managing properly the other two
projections.
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Availability of a workstation for post
processing to avoid some retakes
• Sometimes, especially during the transition from
conventional to digital radiology, managers decide
to buy only the detectors (e.g. the CR plates and
digitisers) and to continue with the film printing.
• A delay in the purchase of the workstations means
a lack in the profit of one of the most important
advantages of digital radiology: to modify the
visualization of the image and the postprocessing.
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Impact of image compression
• Image compression increases the transmission
speed in the network and reduces the waiting time
to retrieve images, but it can also mean a certain
loss of image information (image quality).
• ICRP 93 alerts on this risk with the following
statement:
• Image quality can be compromised by inappropriate
levels of data compression and/or post-processing
techniques. This is dependent on the modality. Data
compression and post-processing requirements should
be defined by modality and the medical imaging task.
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Impact of image compression
• A wide study on the effect of compression has been done
in cardiology:
• Tuinenburg JC et al. American College of
Cardiology/European Society of Cardiolgoy International
Study of Angiographic Data Compression Phase II: the
effects of varying JPEG data compression levels on the
quantitative assessment of the degree of stenosis in
digital coronary angiography. Joint Photographic Experts
Group. J Am Coll Cardiol. 2000 Apr;35(5):1380-7.
• The conclusion is that compression ratios of 10:1 and 16:1
affected the quantitative coronary arteriography results
negatively and therefore should not be used in clinical
research studies.
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Impact of image compression
• Dynamic flat detectors (FD) allow at present, to acquire
images with a matrix size of 1024 x 1024 and 184
micron/pixel (for cardiology configuration, FD 25 x 25
cm) and 2480 x 1820 and 154 micron/pixel (Axiom
Artis dTA, FD of 30 x 40 cm).
• Typically, images are still archived in the PACS with an
important compression factor (matrix of 512 x 512).
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Different compression levels
• Chest CR without compression 22.3 MB
• Matrix: 3062 (horizontal)x 3730 (vertical); Bits stored : 12
• Transmission of 10 images in a network of 100Mb/s,
requires typically 25 s.
• Chest CR with compression QF 50; 3.0 MB
• Transmission of 10 images in a network of 100Mb/s,
requires typically 4 s.
• Chest CR with compression QF 10; 0.8 MB
• Transmission of 10 images in a network of 100Mb/s,
requires typically 1.6 s
• Chest CR with compression QF 1;
64 kB
• Transmission of 10 images in a network of 100Mb/s,
requires typically < 1 s
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22.3 MB
3.0 MB
0.8 MB
64 kB
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22.3 MB
3.0 MB
0.8 MB
64 kB
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Summary
• CR is based on the PSP process
• The CR image is subject to fading, fogging, and physical
•
•
•
•
•
and chemical phosphor defects.
The CR image must be corrected for non-uniformity in
collection efficiency across one dimension
The CR image is subject to interferences in mechanical
motion of the imaging plate and laser scan mechanism.
The CR image is subject to interference in the optical path
of the laser and the stimulated luminescence.
Double exposures are possible with CR but not DR.
Failure to follow collimation rules compromises algorithms
for determining VOI and can cause loss of contrast.
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Answer True or False
•
•
Image processing artefact can be due to
poor collimation
CR readers can be used even in a dusty
environment
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Answer True or False
• True. Owing to lack of primary beam
collimation, the amount of unattenuated
radiation beam striking the image plate can
alter the histogram leading to image
processing artefact.
• False. Dirt or dust on the light guide of CR
reader can create artefacts in the image.
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References
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Hammerstrom K, Aldrich J, Alves L, and Ho A. Recognition and Prevention of
Computed Radiography Image Artifacts. J Digit Imag. 2006; 19(3) (September),
226-239.
Willis CE, Thompson SK and Shepard SJ. Artifacts and Misadventures in
Digital Radiography. Applied Radiology 33(1):11-20, January 2004.
Willis CE. Chapter 7. Computed Radiographic Imaging and Artifacts. In: Seigel
EL and Kolodner RM (eds). Filmless Radiology New York: Springer-Verlag;
1999:137-154.
Volpe JP, Storto ML, Andriole KP and Famsu G. Artifacts in chest radiography
with a third generation computed radiography system AJR. 1996;166:653-657.
Burns, C. Using computed digital radiography effectively. Seminars in
Radiologic Technology, 1(1) November, 1993; 24-36.
Chotas HG, Ravin CE. Digital radiography with photostimulable storage phosphor: control
of detector latitude in chest imaging. Invest Radiol. 1992;27:822-828.
Oestmann JW, Prokop M, Schaefer CM, and Galanski M. Hardware and
software artifacts in storage phosphor radiography. Radiographics.
1991;11:795-805.
Solomon SL, Jost RG, Glazer HS, Sagel SS, Anderson DJ, and Molina PL
Artifacts in computed radiography. AJR. 1991;157(1):181-185.
Managing patient dose in Digital Radiology ICRP Publication 93 Ann ICRP
2004 Elsevier
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