Transcript Spatial Resolution - El Camino College

ARRT & Other
DIGITAL
Terms
Defined
Supplement
to HW assignment
2014 255
Lect 4
CRT vs LCD
• CRT
• Luminance higher in the
center
• Lower measurable black
levels
• Phosphor granularity
adds to spatial noise
• Viewable area smaller
than stated size
• Better color reproduction
• More responsive on
redraw
• More rugged
• Aspect ratio 4:3
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LCD
Less veiling glare
Consumes less energy
Increased spatial
resolution
Larger viewing area by
described size
Display limited to
designed resolution
Can position screen
Smaller footprint and
lighter
Widescreen aspect ratio
16:9
Image Display= MONITORS
• viewing conditions
– (i.e.,luminance,ambient lighting)
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spatial resolution
contrast resolution/dynamic range
DICOM gray scale function
window level and width function
ARRT DEFINITIONS
MONITORS: 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.
MONITOR RESOLUTION
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.
ARRT definitions
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)
Spatial Resolution
• A radiograph typically
does not show soft
tissue structures
• Digital image shows
not only the soft tissue
but also the edge of
the skin.
• Giving the
appearance of more
detail.
MONITORS
contrast resolution
/dynamic range
• Appearance of more detail is due to the
wider dynamic recording range / contrast
resolution
• and does not mean that there is additional
detail
• Because so many more densities are
recorded in CR (wide dynamic range),
images appear more detailed.
contrast resolution
• The contrast resolution of a monitor is the
difference between the maximum and
minimum luminance of the display.
Exposure Latitude
or Dynamic Range
• CR and DR
• Contain a detector that
can respond in a linear
manner
• as compared to S
shape of H&D Curve
• Exposure latitude is
wide, allowing the
single detector to be
sensitive to a wide
range of exposures.
Why do digital systems have
significantly greater latitude?
• Linear response give the imaging plates
greater latitude
• Area recieving little radiation can be
enhanced by the computer
• Higher densities can be separated and
brought down to the visibile density ranges
• (Brightness in DR replaces density)
Monitors - RESOLUTION
• Pixel is a basic picture element on a display.
• A pixel is “any of the small discrete elements that
together constitute an image.”
• Resolution -# of pixels contained on a display
• Relationship:
• More pixels in an image, the higher the
resolution & more information that can be
displayed.
• Resolution also is defined as the process or
capability of distinguishing between individual
parts of an image that are adjacent.
Nyquist frequency ?
10 PIXEL DENSITY
WHAT IS THE
NYQUIST FREQUENCY= ?
The Nyquist Theorem
• Theorem states that when sampling a signal, the
sampling frequency must be greater than twice
the bandwidth of the input signal so that the
reconstruction of the original image will be
nearly perfect.
• At least twice the number of pixels needed to
form the image must be sampled.
• If too few pixels are sampled, the result is a lack
of resolution.
Nyquist frequency
• The highest spatial frequency that can be
recorded by a digital detector.
• is determined by the pixel pitch.
• The Nyquist frequency is half the number
of pixels/mm.
• A digital system with a pixel density of 10
pixels/mm would have a Nyquist frequency
of 5 line pair/mm.
Sampling Frequency ?
Define ……
ARRT definitions
sampling frequency
• The frequency that a data sample is acquired
from the exposed detector.
• It is expressed in pixel pitch and pixels per mm.
• Sampling frequency may be determined by
receptor size depending on the vendor.
• KODAK 8x10 better detail than 14x17
• Use of the smallest imaging plate possible for
each exam results in the highest sampling rate.
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When the smallest possible imaging plate is selected, a corresponding matrix is used
by the computer algorithm to process the image
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Pixel “picture element,”
• the smallest area represented in a digital
image.
• A digital radiography image consists of a
matrix of pixels which is typically several
thousand pixels in each direction.
• Pixel density – A term that describes the
number of pixels/mm in an image. Pixel
density is determined by the pixel pitch.
DEL (detector element size)
receptor size and matrix size
• a pixel or picture element.
• The typical number of pixels in a matrix
range from about 512 × 512 to 1024 ×
1024 and can be as large as 2500 × 2500.
• The more pixels there are, the greater the
image resolution.
• The image is digitized by position (spatial
location) and by intensity (gray level).
DELs – detector elements
• DELs collect electrons
that are extracted from
the detector assembly
and converted into a
digital value by an ADC.
That process creates the
image that displays on
our monitor.
• DEL size controls the
recorded detail, or spatial
resolution, for the flatpanel device. The
technologist can’t change
the size of the DEL,
which is fixed for that
piece of equipment.
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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
• Known as the fill factor, the larger the area
of the TFT photodiodes, the more radiation
can be detected and the greater amount of
signal generated.
• Consequently, the greater the area of the
TFT array, the higher the DQE.
• Over 1 million pixels are read & converted
FILL FACTOR
• A field-effect transistor (FET) or silicon TFT
• Isolates each pixel element
• Reacts like a switch to send the electrical charges to
the image processor
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.
ARRT definitions
SNR (signal to noise ratio) or
CNR (contrast to noise ratio)
• SNR (signal to noise ratio): there is always a
very small electric current flowing in any circuit is called background electronic noise.
• It is similar to the fog on a radiograph in that it
conveys no information and serves only to
obscure the electronic signal.
• CNR (contrast to noise ratio): measure for
assessing the ability of imaging an procedure to
generate clinically useful image contrast.
• gives an objective measure of useful contrast.
Image Display
• spatial resolution
contrast resolution/dynamic range
• What is a 3-D array of Pixels ?
• A voxel (a volumetric pixel) is a volume
element, representing a 3-D value space.
A pixel which represents 2D image data.
Pixel Pitch
• The space from the
center of a pixel to the
center of the adjacent
pixel. It is measured in
microns (μm).
• Pixel pitch is determined
by sampling frequency for
cassette-based PSP
systems and by DEL
spacing for TFT flat
panel.
Monitors: Display Workstations
• Pixels are arranged in a matrix.
• Common screen resolutions found on today’s
monitors are the following:
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1280 × 1024 (1K)
1600 × 1200 (2K)
2048 × 1536 (3K)
2048 × 2560 (5K)
Monitors – DOT PITCH
• Dot pitch is the measurement of how close the
dots are located to one another within a pixel.
• The smaller the dot pitch of a display, the finer the
resolution.
• Dot pitch may be expressed as aperture grille pitch or
slot pitch.
Monitors – REFRESH RATE
• Refresh rate or vertical scanning rate
• Refresh rate is a measure of how fast the monitor
rewrites the screen or the number of times that the
image is redrawn on the display each second.
• Refresh rate helps to control the flicker seen by the
user.
• The higher the refresh rate, the less flicker will be seen.
• Most refresh rates on today’s computer are set
between 60 and 75 Hz; the image is redrawn 60 to 75
timers per second.
Image Display= MONITORS
• viewing conditions
– (i.e.,luminance,ambient lighting)
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spatial resolution
contrast resolution/dynamic range
DICOM gray scale function
window level and width function
MONITOR RESOLUTION
DICOM gray scale function – calibration of
monitors to the same standard &
communication of images
•Window level affects brightness (density) - B
•Window width function affects contrast/gray scale - A
“windowing and level”
MONITOR Image Manipulation and
Enhancement Functions
• Window/level
• This is a default function of the left mouse button.
• By depressing and holding down the mouse button
and moving the mouse up and down and left and
right, the window and level can be adjusted.
• Window (width) represents the range of gray values.
• Level represents the center value of the range.
• A change in the window and level appears to change
the brightness and contrast of the image.
Image Manipulation
and Enhancement Functions
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Other Tools: Annotations
Annotations are NOT to be used
to label left or right to indicate the patient’s side.
Annotations are used to indicate prone or
supine, 30 minutes, upright or flat.
• Any other image information is appropriate.
• Radiologist will place arrows or circles around
pathologic or questionable areas.
MONITOR Image Manipulation
and Enhancement Functions
• Pan, zoom, and magnify
• Tools are used primarily by the
radiologist to increase the size of an area on the image.
• Magnify usually magnifies a square area of the image
• Pan and zoom functions are usually used together.
• Image is first zoomed up to the desired magnification
level then Pan icon is activated.
• Zoomed image can be moved around to view the
different areas of the image.
Image Manipulation
and Enhancement Functions
• Measurements
• Size of a pixel is a known
so the software
has the ability to measure
structures on the image
based on this.
• Angle measurement.
– Can give an angle measurement between two
structures
– Commonly used when reading spine studies
Image Manipulation
and Enhancement Functions
• Measurements
– Region of interest
– Measurement tool determines the pixel intensity of a
certain area.
– Each type of tissue or fluid has a different intensity of
reading.
– Radiologist can make a determination whether
something is solid or fluid.
– Each pixel can have a gray
level between 0 (20) and 4096 (212).
The gray level will be a factor
in determining the quality of the image
DR
Monitor :Navigation Functions
• Hanging protocols
Can be viewed:
1:1 4:1 etc
• Protocol can also be specified to show the
previous exam on one monitor and the
current exam on the other
• Once set, the most efficient study navigation
is determined.
Image Management
Functions
• Patient demographics
• Patient demographics
• must be correct.
– If demographics are not correct at the archive level,
the images could be lost.
• Changes should only be made when the
information is absolutely known to be wrong.
• Many hospitals allow only certain persons the
access to change demographics just to keep the
errors to a minimum.
Image Management Functions
• Query/retrieve icon
• Used to retrieve on demand
any studies from the archive
• Allows user to query a study
on multiple fields
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Patient’s name or identification
Date of service
Modality
Diagnosis code or comment
field
WINDOW LEVEL / WIDTH
Which one controls
Denisty (brightness) ?
Contrast
What else control these in DIGITAL IMAGING?