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hobson contrast > Distortion/ Recorded detail (spatial resolution) > Flashcards

Distortion/ Recorded detail (spatial resolution) Flashcards

1
Q

Shape distortion

A

unequal magnification of structure
Elongation (tube an IR not aligned)
foreshortening (body part not properly aligned)

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2
Q

Factors Affecting Size Distortion

digital systems

A

Magnification – post processing

Minification – post processing

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3
Q

2 geometric properties

A

-recorder detail (definition, sharpness, spatial
resolution)
-distortion

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4
Q

Spatial resolution/recorder detail is determined by

in digital

A

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

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5
Q

Unit of resolution

A

line pairs per millimeter
(lp/mm) or cycles per mm
measure by resolution test tool

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6
Q

Resolution test tool

A

how many lines you see it in image

human eye = 5 lp/mm

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7
Q

clinical evaluation

A

trabecular pattern

how sharp patters of bone look

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8
Q

Unsharpness

A

penumbra (outline of shadow)

point spread function (PSF) measures penumbra

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9
Q

Spatial Resolution definition

A

-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

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10
Q

Good detail/resolution may exist even if

A

you can’t see it due to poor visibility.

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11
Q

Spatial Frequency

A

-High or low frequency signal
-Determined by measuring distance between pairs of
lines distinct from one another

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12
Q

High frequency would represent an image with

A

better resolution/detail

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13
Q

point spread function (PSF)

A

complex mathematical measurement of the image produced from a single point

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14
Q

line spread function (LSF)

A

would be measured using a narrow slit in a sheet of lead

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15
Q

edge spread function (ESF)

A

uses a sharp edge instead of a line or point

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16
Q

Modulation transfer function (MTF)

A

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

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17
Q

noise

A

Background information received by image receptor

cause by Quantum noise, Quantum mottle (not properly exposed image) noise affects spatial resolution.

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18
Q

Imaging Noise

A

total noise the IR receives. Includes quantum noise, system noise, and ambient noise

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19
Q

Signal-to-Noise Ratio(SNR)

A

measures strength of the signal to noise. Depends on amount of radiation exposure(signal) to the detector and detector’s quantum efficiency.

20
Q

digital sampling

A

not as much info as film

21
Q

Spatial resolution of a digital system is equal to

A

½ the Nyquist frequency.

22
Q

Nyquist frequency or criterion is the

A

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

23
Q

Aliasing (Moire Pattern)

A

-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

24
Q

Primary factors affecting spatial resolution in digital systems are

A

the detector geometric properties and the processing system

25
Q

Primary limitation:

digital

A

Size of detector element (system can only show objects the size of the DEL)

26
Q

CR imaging plates limitations

A

similar to intensifying screens

Also affected by image reader device (IRD

27
Q

Indirect DR (flat panel systems)

A 
-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
28
Q

Direct DR

A

direct converting photons to electronic signal
uses amorphous Selenium (photoconductor) and TFT
NO scintillator (no light conversion process so they have better spatial resolution)

29
Q

Images lacking fine detail Appear blurry Assessment of motion
Factors Affecting Recorded Detail:

A

-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

30
Q

Geometry

A 
-Distance
   SID= tube to IR
   OID= object to IR 
   SOD= source to IR
-Focal spot size
31
Q

Voluntary motion

A

under patient control

control with Communication

32
Q

Involuntary motion

A

unable to control (heartbeat, Parkinson, etc.)
Exposure time reduction (manipulate mA, time relation or 15% rule)
Immobilization devices

33
Q

grayscale bit depth

A

of shades of gray each pixel in that matrix is capable of recording

34
Q

only things that have an impact in recorded detail

A

OID
SID
focal spot size
maybe also intensifying screen

35
Q

the lower the speed of the intensifying screen …

A

the better the recorded detail

36
Q

digital systems image processing system limits on spatial res/recorded detail

A

Acquisition and display matrix
Pixel size
Grayscale bit depth (how many shades of gray)

larger matrix=smaller pixels=better spatial resolution

37
Q

umbra

A

distinctly sharp area of shadow or the region of complete shadow

38
Q

distortion

A

misrepresentation of the size and shape .

39
Q

magnification

A

equal shape distortion only possible with size distortion, controlled by distances SID,OID reducing magnification increases spatial resolution

40
Q

minification

A

divergent property of x-rays photons

41
Q

when OID is increase and cannot compensate u need to..

A

increase SID

42
Q

to find magnification factor

A

(always come up to 1.something) M=SID/SOD

to find SOD = SID-OID

43
Q

to find object size

A

O= I/M

44
Q

factors affecting shape distortion

A 
Alignment
   Central ray
   Anatomical part
   Image receptor
Angulation
   Degree
   Direction
45
Q

angulation

A

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.

hobson contrast (3 decks)

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