Digital Imaging
Review
RT 255
1
Display Workstations
• Conventional film/screen
radiography uses large multiviewer
lightboxes.
• With early PACS, radiologists
thought that they needed 4-6
monitors.
• Now, the number of monitors has
dropped to an average of 2.
• Development of viewing software
and better hardware.
2
• The CRT is the most
popular monitor on the
market.
• It consists of a cathode
and anode within a
vacuum tube.
• Cathode boils off a cloud
of electrons, and then a
potential difference is
placed on the tube.
• A stream of electrons is
sent across to the anode,
which in the case of the
monitor is a sheet of
glass coated with a
phosphor layer.
CRT
3
MONITORS:Display Workstations
• Early PACS reading rooms required
supplemental air-conditioning to offset
the heat from multiple CRTs.
• Resolution and orientation of the monitor
is also a factor in determining which type
of monitor is to be used.
• Most cross-sectional imaging is read on a
1K square monitor.
• Most computed radiography (CR)
and digital radiography (DR)
images are read on at least a
2K portrait monitor
4
LCD
• 1.3 megapixels
• to 5 megapixels.
• mammography
imaging =
• at least 5 megapixel
resolution is
required.
5
• Rads VIEWER
• 4K or higher
6
Physician Review Stations Monitors
• Step-down model of the radiologist’s reading
station (lower res)
• Some functions reduced
• One of the most important features
• is ability to view current and previous reports
with images.
• Many vendors are integrating
• the RIS functions
• with PACS software.
7
Technologist QC Stations
• review images after acquisition
but before sending them to the
radiologist
• May be used to improve or
adjust image-quality
characteristics
• May be used to verify patient
demographic information
• Placed between the CR and DR
acquisition modalities as a passthrough to ensure that the
images have met the
departmental quality standard
8
Technologist QC Station
• Generally has a 1K monitor
• Does not have the resolution capabilities of the
radiologist’s reading station
• Care required of technologist when manipulating
images not to change the appearance too much
from original acquired image
9
Technologist workstation monitors are used
in brightly lighted areas. So monitor
luminance, the brightness of a monitor
display, is an important consideration. Also,
the monitor must allow a technologist to
visualize enough detail to discern motion and
that the recorded lines are sharp and visible.
10
The File Room Workstation
•
•
•
•
•
•
Workstation may be used to
look up exams for a physician
or to print copies of images
for the patient to take to an
outside physician.
Many hospitals are moving away from
printing films because of the cost.
• Hospitals are moving toward burning
compact disks (CDs) with the patient’s
images.
11
Name the 3 types of monitors
• Two major types of monitors with a third
type gaining acceptance:
• CRT
• LCD
• Plasma screen
12
ARRT DEFINITIONS
Image Display= MONITORS
• viewing conditions
– (i.e.,luminance,ambient lighting)
•
•
•
•
spatial resolution
contrast resolution/dynamic range
DICOM gray scale function
window level and width function
13
viewing conditions
luminance,ambient lighting
• How does this affect viewing images?
• Surrounding light impacted what was seen on
image – now :
With different monitors:
• LCD gives more light.
• LCD can be used in areas
with a high amount of ambient light.
“dark rooms” not necessary
14
WINDOW LEVEL / WIDTH
Which one controls
Denisty (brightness) ?
Contrast
What else control these in DIGITAL IMAGING?
15
•
•
•
•
The ability to window is a valuable
feature of all digital images.
Windowing is the process of
selecting some segment of the
total pixel value range (the wide
dynamic range of the receptors)
and then displaying the pixel
values within that segment over
the full brightness (shades of gray)
range from white to black.
Important point...Contrast will be
visible only for the pixel values
that are within the selected
window. All pixel values that are
either below or above the window
will be all white or all black and
display no contrast.
The person controlling the display
can adjust both the center and the
width of the window. The
combination of these two
parameters determine the range
of pixel values that will be
displayed with contrast in the
image.
16
Enhanced Visualization
Image Processing
•
•
•
•
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
17
Nyquist frequency Review
• The highest spatial frequency that can be
recorded by a digital detector.
• is determined by the ________________
• The Nyquist frequency is half the number
of pixels/mm.
18
10 PIXEL DENSITY
WHAT IS THE
NYQUIST FREQUENCY= ?
19
A digital system with a pixel density of 10
pixels/mm would have a Nyquist frequency
of 5 line pair/mm.
10 PIXEL DENSITY
WHAT IS THE
NYQUIST FREQUENCY= ?
20
Sampling Frequency ?
Define ……
21
Sampling Frequency
• The sampling frequency is the rate at
• which the laser extracts the image data
• from the plate.
• What other term does this relate to?
22
Nyquist Frequency
• The Nyquist Frequency will be ½ of the
sampling frequency.
• A plate that is scanned using a sampling
frequency of 10 pixels per millimeter would not
be able to demonstrate more than 5 line pairs
per millimeter based upon the Nyquist
Frequency.
• The Nyquist Frequency allows the
• determination of the spatial resolution for
• a given sampling frequency.
23
LUT
• In the typical digital
radiographic system, a
variety of LUTs are
installed.
• The appropriate LUT is
then automatically
selected to give the
desired contrast
characteristics to match
the type of procedure
(chest, extremity, etc) that
is designated by the
operator.
a histogram of the luminance values derived during image acquisition
used as a reference to evaluate the raw information and correct the
luminance values.
a mapping function in which all pixels are changed to a new gray value.
24
LOOK UP TABLE (LUT)
Linear LUT
Black
Saturation
White
Saturation
Black Shirt
Facial Tones
* No Detail in Black Areas
* High Contrast
* Only Detail in White
Areas can be seen
* No Detail in White Areas
* Low Contrast
* Only Detail in Black
Areas can be seen
25
26
ARRT definitions
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)
27
image signal (exposure related)
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.
28
Digital artifacts
• Grid Lines: Appear as grid cutoff.
• Moire (Aliasing)
– wavy artifact occurs because the grid lines and the
scanning laser are parallel.
– When the spatial frequency is greater than the
Nyquist frequency
• Maintenance (e.g., detector fog): When errors
occur in equipment performance, corrective
action must occur. These corrections will
generally be done by service personnel
employed by the vendor.
• non-uniformity, erasure - blub problems
29
How else can Morie OCCUR?
30
ARRT definitions
quantum mottle
failure of an imaging system to record
densities usually caused by a lack of xray photons.
“PHOTON STARVED”
KVP & MAS HOW IS THIS AFFECTED
IN DR / CR?
31
PREPROCESSING ARTIFACTS
-
dead detector elements
dead columns or rows
nonuniform response
heel effect (fixed systems)
light guide variations
ghosting
32
33
34
ARRT definitions
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)
35
Image Acquisition and Readout
• PSP (photo-stimulable phosphor)
• flat panel detectors
– (direct and indirect)
36
CR Imaging Plate
• Construction
• Image recorded on a thin sheet of plastic known
as the imaging plate - PSP
• Consists of several layers:
• Phosphor?
37
Imaging Plate
• Phosphor?
• BARIUM FLUORO
• HALIDE WITH
• A EUROPIUM BASE
38
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.
39
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.
40
Image Acquisition and Readout
flat panel detectors
Phosphors?
– direct = Am SELENIUM
– indirect = Am SILICON
41
• CR
• Indirect
array
• Direct
array
• CCD/CMOS
CCD/CMOS
PSL
light guide
PMT/PD
CsI/GOS contact layer PD/TFT
a-Se
none
Cs/GOS
lens/fiber
TFT
42
CMOS ?
43
CMOS ?
• Complimentary metal-oxide semiconductor (CMOS) – A photographic
detector.
44
• CCD and CMOS
systems – both use a
scintillator. These
systems are cameralike,
• they both use lenses
to focus the light onto
a detector.
45
•
•
•
•
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)
46
Image Data Recognition
and Preprocessing shutter
• Agfa uses the term collimation, Kodak uses the
term segmentation, and Fuji uses the phrase
“exposure data recognition.”
• All systems use a region of interest to define the
area where the part to be examined is
recognized and the exposure outside the region
of interest is subtracted.
47
•So in essence, rescaling provided an acceptable image, despite
an excess level of exposure to the receptor.
What about the dose to the patient?
•
Excessive exposure to
receptor without rescaling.
•
Excessive exposure to
receptor with rescaling.
48
Rescaling
• image pixel values to appear appropriate,
display properly, can lead to overexposing a
patient.
• The visual cue to the technologist that
overexposure has occurred isn’t present.
• With an analog system, a technologist would
have seen the image on the left as it came out of
the processor and used the excessive density of
the image as a visual cue to repeat the image.
• Rescaling forces a technologist to look
elsewhere for signs that a proper exposure was
used to produce an image.
49
Dose creep
• refers to the potential
to gradually increase
patient exposure over
time.
• However, a
technologist lacks
visual feedback that
additional radiation is
being used to
produce the images
50
DAP
• The dose area product
(DAP) meter is a device
that may be interlinked
with the x-ray unit to
determine the actual
patient entrance skin
exposure dose with
accurately calibrated
equipment. Currently, no
standards are established
for using a dose area
product meter
These two radiographs
show the difference in
entrance skin exposure
measured by the DAP
meter. The area with a
smaller exposure field size
carries a lower exposure
without reducing image
quality
51
Exposure Latitude
• The analog receptor exposure latitude
ranges from approximately
• 30% underexposed
• to 50% overexposed relative to
• the “ideal” exposure level.
52
Exposure Latitude
The digital image receptor
• exposure latitude ranges from
• approximately
• 50% underexposed
• to 100% over exposure
• relative to the “ideal” exposure level.
53
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.
54
• 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.
55
Image Evaluation:
Brightness and Contrast in Images
•
•
•
•
•
•
•
Exposure Field Recognition Error
Gross Overexposure
Excessive Scatter Striking the Receptor
Excessive Fog on the Receptor
Grid Cutoff
Intra-Field or Off-Focus Radiation
Wrong Menu Selection
56
EDR
• Exposure Data Recognition
• When laser scans it is looking for area of
plate that has exposure
• Some read from center out and look for
two sides of collimation
• Works best when image centered
57
58
59
S# 8,357
S# 12,361 lat CXR
60
Exposure Numbers
• The exposure numbers can only be used if
all other parameters are correct
– Centering to plate
– Collimation
• Position over AEC, look at mAs readout to
determine if poor positioning caused light
or dark image
61
Same technique, different centering
and collimation
S# 592
S# 664
62
ARRT SPECS - DIGITAL
• PACS
• HIS (hospital information system) - work
list
• RIS (radiology information system)
• DICOM
• Workflow (inappropriate documentation,
lost images, mismatched images, corrupt
data)
63
PACS
• Image is stored on a computer: retrieval and
viewing system for digital imaging examinations.
• The PACS software provides “tools” that enable
a PACS operator to manipulate images.
• The basic image manipulations possible are:
magnification and minification of an image;
altering image brightness and contrast; and
annotating the image using text, symbols, lines
and arrows.
64
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
65
HIS – RIS INTERFACE
66
RIS Data Storage
• Less ERRORS
• A radiology information system, or RIS, is a data system for
patient-related functions in the radiology department.
Examples of functionality a RIS provides within a radiology
department include (but are not limited to):
• scheduling
• appointments;
• collecting and displaying orders for radiologic examinations;
• storing and displaying patient data;
• tracking patients
• providing patient and order data to a PACS; storing and
distributing radiology reports; providing billing services;
and providing a database to track and project trends.
67
HIS
• A hospital information
system, or HIS, is a
paper and/or data
system that manages
the administrative,
financial and clinical
information necessary
to operate a hospital
or health care system
68
Health Level 7 (HL7)
• HL7 are the software standards
established for exchanging electronic
information in health care. PACS is mainly
concerned with images and data
associated with images, whereas HL7 sets
standards for transmitting text-based
information throughout a medical center.
69
Cassette-based
systems
PSP plates
• Turbid phosphors ?
• Structured
phosphors ?
70
• Turbid Phosphor – A phosphor layer with a
random distribution of phosphor crystals
within the active layer.
• Structured (needle) phosphor – A
phosphor layer with columnar phosphor
crystals within the active layer. Resembles
needles lined up on end and packed
together.
71
72
Exposure index
• Cassette based– represents exposure level to plate
• a. Vendor specific values
• 1). Sensitivity “S” (Fuji, Philips, Konica) inversely related
to exposure- 200 S# =1mR to the plate – optimal range
250-300 for trunk, 75-125 for extremities
• 2). Exposure Index (EI)- (Kodak) – directly related to
exposure has a logarithmic component (change of 300 in
EI = factor of 2; i.e. 1800 is exposed twice as much as
1500) optimal range 1800-1900.
• 3). Log Mean (LgM) - (Agfa) – directly related to
exposure has a logarithmic component (change of 0.3 in
LgM = factor of 2, ie 2.3 is exposed twice as much as
2.0) optimal range 1.9-2.1.
73
• MTF ?
74
MODULATION TRANSFER FUNCTION - MTF
• A measure of the ability of the imaging system to preserve signal
contrast as a function of the spatial resolution.
• Every image can be described in terms of the amount of energy
for each of its spatial frequency components.
• MTF often is regarded as the ideal expression of the image
quality provided by a detector.
75
Exposure myths associated with digital
systems
1. mAs – myth: digital is mAs driven.
Truth: digital is exposure driven. The digital
detector is unable to discriminate whether the
exposure change was mAs or kVp. The
only thing that matters is exposure to pixels.
2. kVp – myth: digital is kVp driven.
Truth: see above
3. Collimation – myth: you cannot collimate.
Truth: you can and should collimate.
Inappropriate collimation will cause a histogram
analysis error.
76
Exposure myths associated with
digital systems
4. Grid – myth: cannot use grids and don’t need
them.
Truth: digital systems are sensitive to scatter just
like film; in fact they are more sensitive.
Appropriate grid useis even more important. A
grid should be used when the remnant beam is
more than 50% scatter, chest larger than 24cm
and anything else larger than 12cm.
5. SID – myth: magnification doesn’t occur with
digital so SID is unimportant.
Truth:Geometric rules of recorded detail and
distortion are unchanged from film to digital.
77
Exposure myths associated with
digital systems
7. Fog – myth: digital systems can’t be
fogged by scatter or background radiation.
Truth:digital systems are more sensitive to
both.
8. Myth: fluorescent lights fog PSP plates.
Truth: that is not true.
78
Next
• ODIA - Summer Review
• Prep for Registry Exam!
79