Download digital equipment - El Camino College

DIGITAL EQUIPMENT
MAY 2008
TERMINOLOGY REVIEW
ARRT CONTENT SPECS
2008
ARRT SPECS - DIGITAL
• Image Receptors
• digital image characteristics
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–
spatial resolution
sampling frequency
DEL (detector element size)
receptor size and matrix
size
image signal (exposure
related)
quantum mottle
SNR (signal to noise ratio)
or
CNR (contrast to noise ratio)
• Digital Systems
• electronic collimation
• grayscale rendition or look-up
table (LUT)
• edge enhancement/
– noise suppression
• contrast enhancement
• system malfunctions (e.g.,
ghost image, banding, erasure,
dead pixels, readout
problems, printer distortion)
ARRT SPECS - DIGITAL
• Image Display
–
–
–
–
–
viewing conditions (i.e., luminance,ambient lighting
spatial resolution
contrast resolution/dynamic range
DICOM gray scale function
window level and width function
• Image Acquisition and Readout
• PSP (photo-stimulable phosphor)
• flat panel detectors
– (direct and indirect)
•
•
•
•
•
Noise
Acceptable Range of Exposure
Exposure Indicator Determination
Gross Exposure Error
Image Degradation (mottle, light or dark, low contrast)
ARRT SPECS - DIGITAL
• Recognition of Malfunctions
• Digital Image Receptor Systems
• Digital artifacts
– (grid lines, Moiré effect or aliasing)
– maintenance (e.g., detector fog)
– ( non-uniformity, erasure)
ARRT SPECS - DIGITAL
• PACS
• HIS (hospital information system) - work
list
• RIS (radiology information system)
• DICOM
• Workflow (inappropriate documentation,
lost images, mismatched images, corrupt
data)
• windowing and leveling
Review of Digital
Radiography
and PACS
Key Terms
• Computed radiography
• DICOM (digital imaging and communications in
medicine)
• Digital imaging
• Digital radiography
• Direct capture DR
• Indirect capture DR
• PACS
• Teleradiology
Digital
Radiography
Direct
Capture
Indirect
Capture
Direct-to-Digital
Radiography
(DDR)-Selenium
Computed
Radiography
(CR) - PSL
Direct-to-Digital
Radiography
Silicon Scint.
Laser
Scanning
Digitizers
Image Acquisition and Readout
• PSP (photo-stimulable phosphor)
• flat panel detectors
– (direct and indirect)
Computed Radiography
• Uses storage phosphor
plates
• Uses existing equipment
• Requires special cassettes
• Requires a special cassette
reader
• Uses a computer workstation
and viewing station and a
printer
Computed Radiography
• Storage phosphor plates are similar to
intensifying screens.
• Imaging plate stores x-ray energy for
an extended time.
• Process was first introduced in the
United States by Fuji Medical Systems
of Japan in 1983.
• First system used a phosphor storage
plate, a reader, and a laser printer.
Imaging Plate
• Construction
• Image recorded on a thin sheet of plastic known
as the imaging plate
• Consists of several layers:
Cassette and Imaging Plate
• Cassette contains a window with a barcode
label or barcode sticker on the cassette.
• Label enables technologist to match the image
information with the patient-identifying barcode
on the exam request.
Using the Laser to Read
the Imaging Plate
• The light collection optics direct the released
phosphor energy to an optical filter and then
to the photodetector.
• Although there will be variances between
manufacturers, the typical throughput is 50
cassettes per hour.
• Some manufacturers claim up to 150
cassettes per hour, but based on average
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Process up to 101
cassettes an hour.
• Handle 16 cassettes at
one time:
up to 8 queued for
processing,
and 8 erased and ready
for new
imaging studies.
• Cassette is ready to
reuse in
40 seconds.*
• Review an image in 34
seconds
at a Kodak DirectView
remote
operations panel.*
• “Drop-and go”
workflow virtually
• Based on proven
DirectView CR 850
system design
• ·Process up to 62 35
x 43 cm plates an
hour
• ·Small footprint size
of 25 x 29 inch (63.5 x
73.6 cm
Digital Radiography
• Cassetteless system
• Uses a flat panel detector or
charge-coupled device (CCD)
hard-wired to computer
• Requires new installation of
room or retrofit
Digital Radiography
• DR is hard-wired.
• DR is cassetteless.
• Detectors are permanently enclosed inside a
rigid protective housing.
• Thin-film transistor (TFT) detector arrays may
be used in direct- and indirect-conversion
detectors.
Digital Radiography
• Two types of digital radiography
• Indirect capture DR
• Machine absorbs x-rays and converts
them to light.
• CCD or thin-film transistor (TFT)
converts light to electric signals.
• Computer processes electric signals.
• Images are viewed on computer
monitor.
Digital Radiography
• Direct capture DR
• Photoconductor
absorbs x-rays.
• TFT collects signal.
• Electrical signal is
sent to computer for
processing.
• Image is viewed on
computer screen.
Digital Radiography
• DR used CCD technology developed by the
military and then used TFT arrays shortly after.
• CCD and TFT technology developed and
continues to develop in parallel.
• No one technology has proved to be better than
the other.
Flat-Panel Detectors
• Consist of a photoconductor
• Amorphous selenium
• Holds a charge on its surface that can then be read out by a
TFT
Direct Conversion
• X-ray photons are absorbed by the coating
material.
• Photons are immediately converted into an
electrical signal.
• The DR plate has a radiation-conversion
material or scintillator.
Direct Conversion DR Scintillator
• Typically made of amorphous selenium
• Absorbs x-rays and converts them to visible
photons
• Converts photons to electrical charges
• Charges stored in the TFT detectors
Indirect Conversion
• Similar to direct detectors in that the TFT
technology is also used
• Two-step process:
• X-ray photons are converted to light.
• Light photons are converted to an electrical signal.
• A scintillator converts x-rays into visible light.
• Light is then converted into an electrical charge
by photodetectors such as amorphous silicon
photodiode arrays or charge-coupled devices,
or CCDs.
Indirect Conversion
• More than a million pixels can be read and
converted to a composite digital image in under
a second.
Comparison of Film to CR and DR
• For conventional x-ray film and computed
radiography (CR), a traditional x-ray room with
a table and wall Bucky is required.
• For DR, a detector replaces the Bucky
apparatus in the table and wall stand.
• Conventional and CR efficiency ratings are
about the same.
• DR is much more efficient, and image is
available immediately.
Comparison of Film to CR and DR
• CR
• A storage phosphor plate is
placed inside of CR
cassette.
• Most storage phosphor
plates are made of a barium
fluorohalide.
• When x-rays strike the
photosensitive phosphor,
some light is given off.
• Some of the photon energy
is deposited within the
phosphor particles to create
the latent image.
• The phosphor plate is then
fed through the CR reader.
Comparison of CR and DR
• CR, continued
• Focused laser light is scanned over the plate, causing
the electrons to return to their original state, emitting
light in the process.
• This light is picked up by a photomultiplier tube and
converted into an electrical signal.
• The electrical signal is then sent through an analogto-digital converter to produce a digital image that can
then be sent to the technologist review station.
Comparison of CR & DR
• DR
• No cassettes are required.
• The image acquisition device is built into the table
and/or wall stand or is enclosed in a portable device.
• Two distinct image acquisition methods are indirect
capture and direct capture.
• Indirect capture is similar to CR in that the x-ray
energy stimulates a scintillator, which gives off light
that is detected and turned into an electrical signal.
• With direct capture, the x-ray energy is detected by a
photoconductor that converts it directly to a digital
electrical signal.
Amorphous Silicon Detector
• The light photons are then converted into an
electric charge by the photodiode arrays.
• Unlike the selenium-based system used for
direct conversion, this type of indirectconversion detector technology requires a twostep process for x-ray detection.
• The scintillator converts the x-ray beams into
visible light, and light is then converted into an
electrical charge by photodetectors, such as
amorphous silicon photodiodes
Cesium Iodide Detectors
• A newer type of amorphous
silicon detector uses a cesium
iodide scintillator.
• The scintillator is made by
growing very thin crystalline
needles (5 µm wide) that work
as light-directing tubes, much
like fiber optics.
• This allows greater detection of
x-rays, and because there is
almost no light spread, there is
much greater resolution.
Cesium Iodide Detectors
• These needles absorb the x-ray photons and
convert their energy into light, channeling it to
the amorphous silicon photodiode array.
• As the light hits the array, the charge on each of
the photodiodes decreases in proportion to the
light received.
Charge-Coupled Devices
• The oldest indirect-conversion DR system is
based on CCDs.
• X-ray photons interact with a scintillation
material, such as photostimulable phosphors,
and this signal is coupled or linked by lenses or
fiber optics, which act like cameras.
Charge-Coupled Devices
• These cameras reduce the size of the projected
visible light image and transfer the image to one
or more small (2 to 4 cm2) CCDs, which convert
the light into an electrical charge.
• This charge is stored in a sequential pattern and
released line by line and sent to an analog-todigital converter.
Charge-Coupled Devices
• Even though CCD-based detectors require
optical coupling and image size reduction, they
are widely available and relatively low in cost.
Summary
• There are two types of cassetteless digital
imaging systems: direct and indirect.
• Direct sensors are TFT arrays of amorphous
silicon coated with amorphous selenium.
• Direct sensors absorb x-ray photons and
immediately convert them to an electrical signal.
Summary
• Indirect-conversion detectors use a scintillator
that converts x-rays into visible light, which is
then converted into an electrical charge.
• CCDs act as miniature cameras that convert
light produced by x-ray interaction with
photostimulable phosphors into an electrical
charge.
Image Display
– viewing conditions (i.e., luminance,ambient
lighting
– DICOM gray scale function
– window level and width function
– spatial resolution
– contrast resolution/dynamic range
MONITOR RESOLUTION
DICOM gray scale function
window level and width function
• Depending on modalities
such as CT, CR, MRI,
resolution requirements
can range
• from 1.3 megapixels to 5
megapixels.
• Generally, 3 megapixel
and higher class displays
are used for softcopy
interpretation.
• Where higher accuracy
and a subtle reproduction
of grayscale are critical in
applications such as
• mammography imaging,
5 megapixel resolution is
required.
viewing conditions
luminance,ambient lighting
DICOM gray scale function
window level and width function
• A photometer to a
monitor screen in a
check of the monitor's
conformance with the
DICOM Grayscale
Standard Display
Function.
DICOM gray scale function
window level and width function
Grayscale or color monitors
Digital Systems
electronic collimation
grayscale rendition or look-up table (LUT)
edge enhancement/
noise suppression
contrast enhancement
Detective Quantum Efficiency
• How efficiently a system converts the x-ray input
signal into a useful output image is known as
detective quantum efficiency, or DQE.
• DQE is a measurement of the percentage of xrays that are absorbed when they hit the
detector.
Detective Quantum Efficiency
• In other words, CR records all of the phosphor
output. Systems with higher quantum efficiency
can produce higher-quality images at a lower
dose.
• Indirect and direct DR capture technology has
increased DQE over CR.
• However, DR direct capture technology, because
it does not have the light conversion step and
consequently no light spread, increases DQE
the most.
Image Display
• spatial resolution
contrast resolution/dynamic range
Spatial Resolution
• Spatial resolution refers to the amount of detail
present in any image.
• Phosphor layer thickness and pixel size
determines resolution in CR.
• The thinner the phosphor layer is, the higher
resolution.
• Film/screen radiography resolution at its best is
limited to 10 line pairs per millimeter (lp/mm).
• CR resolution is 2.55 lp/mm to 5 lp/mm, resulting
in less detail.
Spatial Resolution
• CR dynamic range, or the number of recorded
densities, is much higher, and lack of detail is
difficult to discern.
• More tissue densities on the digital radiograph
are seen, giving the appearance of more detail.
SPATIAL RESOLUTION
Spatial Resolution determined by:
􀁹 Pixel size.
• CR- sampling frequency
• DR – DEL size
• 􀁹 There are relationships between
• Pixel size
• Receptor size
• Matrix size
• 􀁹 pixel size = larger matrix
• 􀁹 receptor size = larger matrix
• Spatial resolution is not related the amount of exposure
Spatial Resolution
• knee radiograph
typically does not
show soft tissue
structures.
• A digital image shows
not only the soft tissue
but also the edge of
the skin. This is due to
the wider dynamic
recording range and
does not mean that
there is additional
detail.
Spatial Resolution
• Depending on the physical characteristics of the
detector, spatial resolution can vary a great deal.
• Spatial resolution of amorphous selenium for
direct detectors and cesium iodide for indirect
detectors is higher than CR detectors but lower
than film/screen radiography.
Spatial Resolution
• Excessive image processing, in an effort to alter
image sharpness, can lead to excessive noise.
• Digital images can be processed to alter
apparent image sharpness; however, excessive
processing can lead to an increase in perceived
noise.
• The best resolution is achieved by using the
appropriate technical factors and materials.
Speed
• In conventional radiography, speed is
determined by the size and layers of crystals in
the film and screen.
• In CR, speed is not exactly the same because
there is no intensifying screen or film.
• The phosphors emit light according to the width
and intensity of the laser beam as it scans the
plate, resulting in a relative speed that is roughly
equivalent to a 200-speed film/screen system.
Speed
• CR system speeds are a reflection of the
amount of photostimulable luminescence
given off by the imaging plate while being
scanned by the laser.
• For example, Fuji Medical Systems reports that
a 1-mR exposure at 80 kVp and a source-toimage distance of 72 inches will result in a
luminescence value of 200, hence the speed
number.
Speed
• In CR, most cassettes have the same speed;
however, there are special extremity or chest
cassettes that produce greater resolution.
• These are typically 100 relative speed.
• Great care must be taken when converting to a
CR system from a film/screen system to adjust
technical factors to reflect the new speed.
Exposure Latitude
or Dynamic Range
• Conventional radiography
• Based on the characteristic response of the film,
which is nonlinear.
• Radiographic contrast is primarily controlled by
kilovoltage peak.
• Optical density on film is primarily controlled by
milliampere-second setting.
CR Cassettes
• Because so many more densities are recorded
in CR (wide dynamic range), images appear
more detailed.
• Because energy stored in the imaging plate is
lost over time, imaging plates should be read as
quickly as possible to avoid image information
loss.
• Imaging plates are erased by exposing them to
bright light such as fluorescent light.
Exposure Latitude
or Dynamic Range
• CR and DR
• Contain a detector that can respond in a linear
manner.
• Exposure latitude is wide, allowing the single detector
to be sensitive to a wide range of exposures.
• Kilovoltage peak still influences subject contrast, but
radiographic contrast is primarily controlled by an
image processing look-up table. LUT
• Milliampere-second setting has more control over
image noise, whereas density is controlled by imageprocessing algorithms.
Density
• .25 TO -2.5
• The straight line of
the H&D curve
Optical Density
• A numerical value indicating the degree of blackening on
the film.
(average OD seen on a radiograph = 1.2 Range is 0.21 –
2.5)
# of photons coming through film =
# of photons hitting film
OD=
1 =0
1
1
100
1 = 1 101
10
OD #
2
3
1 = 2 102 1 = 3 103
100
1000
Why do digital systems have
significantly greater latitude?
• Linear response give the imaging plates
greater latitude
• Area receving little radiation can be
enhanced by the computer
• Higer densities can be separated and
brought down to the visibile density ranges
Note
It is important to note that just because a
• digital imaging system has the capacity to
• produce an image from gross underexposure
• or gross overexposure it does not equate to
• greater exposure latitude.
• The reason the system is capable of producing
an image when significant exposure errors occur
is through a process called automatic rescaling.
• In a digital system, underexposure of
• 50% or greater will result in a mottled
• image.
• 􀁹 In a digital system, overexposure
• greater than 200% of the ideal will result
• in loss of image contrast.
Look-Up Table
• The look-up table (LUT) is a reference
histogram.
• LUT is used as a cross-reference to transform
the raw information.
• LUT is used to correct values.
• LUT has a mapping function:
• All pixels are changed to a new gray value.
• Image will have appropriate appearance in
brightness and contrast.
• LUT is provided for every anatomic part.
Look-Up Table
• LUT can be graphed as follows:
• Plotting the original values ranging from 0 to 255
on the horizontal axis
• Plotting new values, also ranging from 0 to 255 on
the vertical axis
• 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.
Histogram Analysis
• It is important to choose the correct anatomic
region on the menu before exposing the patient.
• Raw data used to form the histogram are
compared with a “normal” histogram of the same
body part by the computer.
Image Receptors
digital image characteristics
– spatial resolution
– sampling frequency
– DEL (detector element size)
– receptor size and matrix size
– image signal (exposure related)
– quantum mottle
– SNR (signal to noise ratio) or
– CNR (contrast to noise ratio)
Matrix size is determined by . . .
• 􀁹 Receptor size (Field of View: FOV)
• 􀁹 Pixel size
• CR - Sampling frequency
• DR - DEL size (Dector ELement)
DIGITAL: MATRIX SIZE
• The number of rows and columns of
• pixels in the image representation.
• 7X7
Digital - Grayscale
Bit depth.
􀁹
Number of gray shades available for display
• 8 bit 256
• 10 bit 1024
• 12 bit 4096
• 14 bit 16384
Digitizing the Signal
• So, how bright a pixel is determines where it will
be located in the matrix in conjunction with the
amount of gray level or bit depth.
• Some CR systems have bit depths of 10 or 12,
resulting in more shades of gray.
• Each pixel can have a gray level between 0 (20)
and 4096 (212). The gray level will be a factor in
Summary
• Pixel and matrix size are important in
determining the amount of resolution and the
size of the image to be stored in the PACS
system. In TFT technology, pixel and matrix size
are determined by the amount of area available
to “fill” with photons.
ARRT SPECS - DIGITAL
• PACS
• HIS (hospital information system) - work
list
• RIS (radiology information system)
• DICOM
• Workflow (inappropriate documentation,
lost images, mismatched images, corrupt
data)
• windowing and leveling
Picture Archival and
Communication Systems
• Networked group of computers,
servers, and archives to store
digital images
• Can accept any image that is in
DICOM format
• Serves as the file room, reading
room, duplicator, and courier
• Provides image access to multiple
users at the same time, ondemand images, electronic
annotations of images, and
specialty image processing
PACS
PACS Uses
• Made up of different components
•
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Reading stations
Physician review stations
Web access
Technologist quality control stations
Administrative stations
Archive systems
Multiple interfaces to other hospital and radiology
systems
DICOM
• stands for digital imaging and
communications in medicine, and it is a
universally accepted standard for
exchanging medical images between
networked medical devices.
HIS RIS
• The HIS holds the patient’s full medical
information from hospital billing to the inpatient
ordering system.
• The RIS holds all radiology-specific patient data
from the patient scheduling information to the
radiologist’s dictated and transcribed report.
HIS – RIS INTERFACE
Image Acquisition and Readout
• PSP (photo-stimulable phosphor)
• flat panel detectors
– (direct and indirect)
•
•
•
•
•
Noise
Acceptable Range of Exposure
Exposure Indicator Determination
Gross Exposure Error
Image Degradation (mottle, light or dark, low
contrast)
Exposure Indicators
• The amount of light given off by the imaging
plate is a result of the radiation exposure that the
plate has received.
• The light is converted into a signal that is used to
calculate the exposure indicator number, which
is a different number from one vendor to
another.
Exposure Indicators
• The base exposure indicator number for all
systems designates the middle of the detector
operating range.
• For Fuji, Phillips, and Konica systems, the
exposure indicator is known as the S, or
sensitivity, number.
• The S number is the amount of luminescence
emitted at 1 mR at 80 kVp, and it has a value of
200.
Exposure Indicators
• The higher the S number with these systems,
the lower the exposure.
• For example, an S number of 400 is half the
exposure of an S number of 200, and an S
number of 100 is twice the exposure of an S
number of 200.
Exposure Indicators
• The numbers have an inverse relationship to the
amount of exposure so that each change of 200
results in a change in exposure by a factor of 2.
• Kodak uses exposure index, or EI, as the
exposure indicator.
• A 1 mR exposure at 80 kVp combined with
aluminum/copper filtration yields an EI number
of 2000.
Exposure Indicators
• An EI number plus 300 (EI + 300) is equal to a
doubling of exposure, and an EI number of
minus 300 (EI − 300) is equal to a halving of
exposure.
• The numbers for the Kodak system have a direct
relationship to the amount of exposure so that
each change of 300 results in change in
exposure by a factor of 2.
Exposure Indicators
• This is based on logarithms, only instead of
using 0.3 (as is used in conventional
radiographic characteristic curves) as a change
by a factor of 2, the larger number 300 is used.
• This is also a direct relationship; the higher the
EI, the higher the exposure.
Exposure Indicators
• The term for exposure indicator in an Agfa system is the
lgM, or logarithm of the median exposure.
• An exposure of 20 µGy at 75 kVp with copper filtration
yields an lgM number of 2.6.
• Each step of 0.3 above or below 2.6 equals an exposure
factor of 2.
• An lgM of 2.9 equals twice the exposure of 2.6 lgM, and
an lgM of 2.3 equals an exposure half that of 2.6.
• The relationship between exposure and lgM is direct
Image Receptors
digital image characteristics
– spatial resolution
– sampling frequency
– DEL (detector element size)
– receptor size and matrix size
– image signal (exposure related)
– quantum mottle
– SNR (signal to noise ratio) or
– CNR (contrast to noise ratio)
Stiching an image
Portrait vs landscape mode
Edge enhancement & post
processing
Enhanced Visualization Image
Processing
•
•
•
•
•
Kodak
Takes image diagnostic quality to a new level
Increases latitude while preserving contrast
Process decreases windowing and leveling
Virtually eliminates detail loss in dense tissues
Grid Selection
• Digital images are
displayed in tiny rows of
picture elements or
pixels.
• Grid lines that are
projected on the
imaging plate when
using a stationary grid
can interfere with the
image, resulting in a
wavy artifact known as a
moiré pattern.
• This pattern occurs
because the grid lines
Collimation
• Although the use of a grid decreases the amount
of scatter that exits the patient from affecting
latent image formation, properly used collimation
reduces the area of irradiation and reduces the
volume of tissue in which scatter can be created.
Collimation
• This results in increased contrast because of the
reduction of scatter as fog and reduces the amount of
grid cleanup necessary for increased resolution.
• Through postexposure image manipulation known as
shuttering, a black background can be added
around the original collimation edges, virtually
eliminating the distracting white or clear areas.
Collimation
• However, this technique is not a replacement for
proper preexposure collimation.
• Shuttering is an image aesthetic only and does
not change the amount or angles of scatter
created.
• There is no substitute for appropriate
collimation, for collimation reduces patient dose.
Automatic Data Recognition
• Collimation is automatically recognized, and a
complete histogram analysis occurs.
• Good collimation practices are critical because
overcollimation or undercollimation leads to data
recognition errors that affect the histogram.
Mis registration – needs
reprocessing
Common CR Image Acquisition
Errors
• As with film screen, artifacts can detract and
degrade images.
– Imaging plate artifacts
•
•
•
•
Plate reader artifacts
Image processing artifacts
Printer artifacts
Operator errors
Imaging Plate Artifacts
• As the imaging plate ages, it becomes prone
to cracks from the action of removing and
replacing the imaging plate within the reader.
• Cracks in the imaging plate appear as areas
of lucency on the image.
Imaging Plate Artifacts
• Adhesive tape used to secure lead markers
to the cassette can leave residue on the
imaging plate.
• If static exists because of low humidity, hair
can cling to the imaging plate.
Imaging Plate Artifacts
• Backscatter created by x-ray photons
transmitted through the back of the cassette
can cause dark line artifacts.
• Areas of the lead coating of the cassette that
are worn or cracked allow scatter to image
these weak areas. Proper collimation and
regular cassette inspection helps to eliminate
this problem.
Plate Reader Artifacts
• The intermittent appearance of extraneous
line patterns can be caused by problems in
the electronics of the plate reader.
• Reader electronics may have to be replaced
to remedy this problem.
Plate Reader Artifacts
• Incorrect erasure settings result in a residual image
left in the imaging plate before the next exposure.
• Results vary depending on how much residual image
is left and where it is located.
• Orientation of a grid so that the grid lines are parallel
to the laser scan lines of the plate reader results in
the moiré pattern error. Grids should be high
frequency, and the grid lines should run
perpendicular to the laser scan lines of the plate
reader.
Operator Errors
• Insufficient collimation results in unattenuated
radiation striking the imaging plate.
• The resulting histogram is changed so that it
is outside the normal exposure indicator
range for the body part selected.
• Using the smallest imaging plate possible and
proper collimation, especially on small or thin
patients, eliminates this error.
Operator Errors
• If the cassette is exposed with the back of a
cassette toward the source, the result is an
image with a white grid-type pattern and white
areas that correspond to the hinges.
• Care should be taken to expose only the tube
side of the cassette.