RAD 354 Chapt. 28 The Digital Image
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Spatial resolution
Contrast resolution
Contrast-detail curve
Pt. dose considerations
Spatial Resolution
• The ability of an imaging system top RESOLVE
and RENDOR on the image a HIGH CONTRAST
object of items in close proximity and similar
atomic mass density (shades of gray
differenced in close objects to one another)
Spatial FREQUENCY
• Refers to LINE PAIR!
– Line pair per millimeter (lp/mm)
• The HIGHER the spatial frequency, the HIGHER
the SPATIAL RESOLUTION!
– In digital imaging, spatial resolution is limited by
PIXEL size.
Line Pair Device
Modulation Transfer Function (MTF)
• The ability of an imaging system to render
objects of DIFFERENT SIZES ONTO AN IMAGE
(RATIO OF IMAGE TO OBJECT DUE TO SPATIAL
RESOLUTION)
Contrast Resolution
• Shades of gray making various atomic mass
densities visible
– The HIGHEST possible contrast is TWO shades of
gray = BLACK AND WHITE!
Dynamic Range (OD)
• Dynamic range is really the OD as shown on
the H & D curve = ranges from base + fog (.2
or lower for film; and “base OD for digital” = 0)
to the solarization point
• The number of shades of GRAY that an
imaging system can produce (remember, the
human eye can only see about 30 different
shades of gray)
NOISE
• Image noise limits contrast resolution (SNR)
• Post processing of the image allows for
manipulation and visualization of shades of
gray (window and leveling – also called
“smoothing”)
Digital Imaging Pt. Dose
• Digital imaging should reduce pt. dose (in
reality, it has done the reverse!) – the “if in
doubt, burn it out” syndrome and post image
manipulation plus “dose creep” have
significantly INCREASED patient dose in digital
imaging
Misc. Information
• Focal spot size determines spatial resolution in
film/screen systems
• Post image manipulation = contrast resolution
in digital REGARDLESS os pt. dose
• Pt. dose in digital SHOULD be LOW because of
DQE (probability of a photon interacting with
an image receptor due to atomic number, etc.)
• DQU = measure of x-ray absorption efficency
RAD 354 Chapt. 29 Viewing the Digital
Image
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Photometric Quantities
Hard/Soft Copy
Active Matrix Liquid Crystal
Display
Pre-Postprocessing digital image
PACS
Photometric Quantities
• Response of the human eye
– Photopic
– Scotopic
Luminous Flux
• Total intensity of light from a source
• Measured in “luminous flux” in LUMENS
Illuminance
• Intensity of light incident on a surface
– Goes back to the measure of “candle light” on a
single square foot of surface
Luminance Intensity
• Amount of light emitted by a light source
– A viewbox is a good example
Luminance
• Measure of the BRIGHTNESS of a source (like
in a digital display)
• Measured in CANDELA per meter squared =
NIT
Cosine Law
• Relate to inverse square law and the amount
of radiation at varied distance
– Same for loight at GREATER distances – like the CR
vs PERIPHERY of the beam (Pathagora’s Theory
– A2 + B2 = C2)
Hard – Soft Copy
• Hard = radiograph on film
• Soft = Viewing image on a cathode ray tube
(computer/TV monitor, etc)
Active Matrix Liquid Crystal Display
• Instead of using a “cathode ray tube” (TV type
tube) viewing system, the active matrix, PIXEL
by PIXEL, viewing monitor is used
• Spatial resolution IMPROVES with larger
matrix (PIXEL) displays
Preprocessing the Image
• YOU can program the computer with what YOU
want the image to look like (convert an analogue
image {radiograph} into a digital image)
• i.e. Do we want a hand with a DYNAMIC range of
2?
– Perhaps a CXR with a dynamic range of 60?
– Shows what SCALE of contrast and shaded of gray to
display
– At the workstation, you tell the computer what
anatomic area you radiographed and the computer
shows it on the screen as it should look like (hand vs
CXR)
Postprocessing
• Annotation (NOT l or R!!!) upright, decube, etc
• Window and leveling (expand or shrink the
shades of grays)
PACS
• Ann – Tell your thing!!!