Distortion/ Recorded detail (spatial resolution) Flashcards
Shape distortion
unequal magnification of structure
Elongation (tube an IR not aligned)
foreshortening (body part not properly aligned)
Factors Affecting Size Distortion
digital systems
Magnification – post processing
Minification – post processing
2 geometric properties
-recorder detail (definition, sharpness, spatial
resolution)
-distortion
Spatial resolution/recorder detail is determined by
in digital
degree of geometric sharpness, structure lines.
easy to adjust (detail)
Dependent on matrix size, pixel size, and grayscale bit depth
Correspond to to the x and y axes of the digital image
Unit of resolution
line pairs per millimeter
(lp/mm) or cycles per mm
measure by resolution test tool
Resolution test tool
how many lines you see it in image
human eye = 5 lp/mm
clinical evaluation
trabecular pattern
how sharp patters of bone look
Unsharpness
penumbra (outline of shadow)
point spread function (PSF) measures penumbra
Spatial Resolution definition
-Ability of an imaging system to accurately display
objects in two dimensions
-smallest object that can be detected in an image
-film-screen system have better spatial resolution than
digital
Good detail/resolution may exist even if
you can’t see it due to poor visibility.
Spatial Frequency
-High or low frequency signal
-Determined by measuring distance between pairs of
lines distinct from one another
High frequency would represent an image with
better resolution/detail
point spread function (PSF)
complex mathematical measurement of the image produced from a single point
line spread function (LSF)
would be measured using a narrow slit in a sheet of lead
edge spread function (ESF)
uses a sharp edge instead of a line or point
Modulation transfer function (MTF)
Measures accuracy of image compared to actual object
Measures the percentage of object contrast that is absorbed
scale 0 to 1
0=no signal, therefore no image;
1=records image perfectly
noise
Background information received by image receptor
cause by Quantum noise, Quantum mottle (not properly exposed image) noise affects spatial resolution.
Imaging Noise
total noise the IR receives. Includes quantum noise, system noise, and ambient noise
Signal-to-Noise Ratio(SNR)
measures strength of the signal to noise. Depends on amount of radiation exposure(signal) to the detector and detector’s quantum efficiency.
digital sampling
not as much info as film
Spatial resolution of a digital system is equal to
½ the Nyquist frequency.
Nyquist frequency or criterion is the
highest spatial frequency that a digital detector can record and is determined by the sampling frequency of a CR system and the spacing of the DELs of DR systems
Aliasing (Moire Pattern)
-Occurs when Nyquist Criterion not met
-Low-frequency image wraps around high-frequency
image
-Visual appearance of two images slightly out of
alignment, scan lines an grid lines in same direction
Primary factors affecting spatial resolution in digital systems are
the detector geometric properties and the processing system
Primary limitation:
digital
Size of detector element (system can only show objects the size of the DEL)
CR imaging plates limitations
similar to intensifying screens
Also affected by image reader device (IRD
Indirect DR (flat panel systems)
-2 types: TFT and charge couple device (CCD) both need a scintillator to change x-rays to light TFT uses amorphous Silicon (photodetector), amorphous requires scintillator such as cesium iodine or gadolinium oxysulfide Fill factor(number of photons that can be registered within a single detector) High fill factor=high resolution and vice versa
Direct DR
direct converting photons to electronic signal
uses amorphous Selenium (photoconductor) and TFT
NO scintillator (no light conversion process so they have better spatial resolution)
Images lacking fine detail Appear blurry Assessment of motion
Factors Affecting Recorded Detail:
-Eliminate motion
-Reduce OID
-Reduce focal spot size (wires)
-Reduce intensifying screen phosphor size(the larger
the faster speed in intensifying screen=loss of detail)
hhand concentration, means we can use less radiation
for same density
-Increase SID
Geometry
-Distance SID= tube to IR OID= object to IR SOD= source to IR -Focal spot size
Voluntary motion
under patient control
control with Communication
Involuntary motion
unable to control (heartbeat, Parkinson, etc.)
Exposure time reduction (manipulate mA, time relation or 15% rule)
Immobilization devices
grayscale bit depth
of shades of gray each pixel in that matrix is capable of recording
only things that have an impact in recorded detail
OID
SID
focal spot size
maybe also intensifying screen
the lower the speed of the intensifying screen …
the better the recorded detail
digital systems image processing system limits on spatial res/recorded detail
Acquisition and display matrix
Pixel size
Grayscale bit depth (how many shades of gray)
larger matrix=smaller pixels=better spatial resolution
umbra
distinctly sharp area of shadow or the region of complete shadow
distortion
misrepresentation of the size and shape .
magnification
equal shape distortion only possible with size distortion, controlled by distances SID,OID reducing magnification increases spatial resolution
minification
divergent property of x-rays photons
when OID is increase and cannot compensate u need to..
increase SID
to find magnification factor
(always come up to 1.something) M=SID/SOD
to find SOD = SID-OID
to find object size
O= I/M
factors affecting shape distortion
Alignment Central ray Anatomical part Image receptor Angulation Degree Direction
angulation
Angling tube is designed to cause a controlled or expected amount of shape distortion, usually to avoid superimposition.
Angulation also changes SID which needs to be compensated for. In general, decrease SID 1 inch for every 5 degrees of tube angle or increase exposure factors to compensate for new SID.
