Final Review: Spacial Resolution and Distortion Flashcards by Melissa Unknown

Two geometric properties:

-spacial resolution
-distortion

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Spacial resolution is referred to several terms:

-definition
-sharpness
-recorded detail

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A misrepresentation in an image of an objects size or shape is called

distortion

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what happens to recorded detail when there is less distortion

recorded detail increases

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what happens to the recorded detail as the magnification increases

recorded detail decreases

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what are the two types of distortion

size and shape distortion

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size distortion is also called:

magnification

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is called elongation when a structure appears longer in an image than the structure really is, and foreshortened when the structure appears shorter

Shape distortion

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How can magnification be kept minimal?

increase in SID and decrease in OID

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what law describes how the xray beam diverges, the principle that accounts for magnification and unsharpness

Divergence or penumbra law

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what is kept minimized as possible to reduce magnification

OID

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increases the OID about 4in and increases the magnification of the image

use of a grid

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What kind pf distortion is present on every radiograph?

Magnification- because there will always be OID

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Ability of imaging system to image structures as separate and distinct

Spacial Resolution

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Dependent on matrix size, pixel size, and grayscale bit depth

Spacial Resolution

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what happens to spacial resolution when there is an increases matrix and decreased pixel size

increased spacial resolution

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what wavelength is high spacial frequency

short wavelength

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what wavelength is low spacial frequency

long wavelength

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what represents your rows and columns

matrix

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what represents your individual squares

pixels

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Compare digital and film to dynamic range

Digital has more dynamic range and film has less dynamic range

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unit of resolution

lp/mm

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Penumbra

-unsharpness or blur

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high spacial resolutions is expressed as

high frequency

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More shades of gray, better spacial resolution(more detail)

Gray Scale

noise

Unwanted on image

Assessing Recorded Detail

Spacial Resolution Spacial Frequency Noise

-Ability of imaging system to accurately display objects in two dimensions -penumbra vs. umbra

Spacial Resolution

-High frequency signal -determined by measuring distance between pairs of lines distinct from one another

spacial frequency

what happens to signal when noise is decreased

increases

umbra

sharpness

Measures accuracy of image to actual image -range from 0-1 - 1 represents "perfect" reproduction of original

MTF

As higher spacial frequencies occurs what happens to MTF

falls dramatically

Bit Depth

how many shades of gray that pixel can hold

Ability to capture and display subtle energy differences in area of interest -responsible for low energy, shades of gray on image

Low Contrast Resolution (LCR)

Background information received by image receptor

noise

Types of Noise:

System Noise Ambient Noise Quantum Noise

Best Spacial Resolution

small focal spot increased SID decreased OID

Resolution as a function of exposure time and dynamic motion of body part

Temporal Resolution Extremely low exposure times optimize TR **I dont think we went over this but just put it in here just in case lolll

-artifact if sampling frequency inst working -occurs when Nyquist criterion not met

Aliasing(moire pattern)

Factors affecting recorded detail

Eliminate motion reduce oid reduce focal spot size use detector with smallest detector element (DEL) reduce intensifying screen phosphor size and concentration (film) increase SID

-line focus principle -creates a penumbra and inherent loss of sharpness due to focal spot geometry and distance -a major factor in spacial resolution

Focal Spot Size

-some degree of image unsharpness occurs due to shape and size of patient anatomy -related to beam divergence and incongruence with anatomical structures

Attenuation/Absorption Unsharpness

Image processing system limits recorded detail dependent on:

-matrix(large) -pixel (small) -bit depth (large)

Motion

-voluntary (communication) -Involuntary motion( exposure time reduction and immobilization) -equipment motion

how to reduce voluntary motion

best reduced through effective communication

how to reduce involuntary motion

-best reduced through short exposure time -immobilization

Anatomic details must be recorded accurately and with the greatest amount of sharpness

Recorded Detail

Refers to the distinctness or sharpness of the structural lines that make up the recorded film image

Recorded Detail

What affects distortion?

CR alignment of the tube, part, and image receptor

Radiographic misrepresentation of either the size or shape of the anatomic part

Distortion

an increase in the objects image size compared to its true or actual size -SID and OID affect magnification

Size distortion

A misrepresentation of an objects image shape -elongation and foreshortening

Shape Distortion

Radiographic distances

-controlled by SID and OID -Magnification only

electronic magnification/minification

post processing can resize image

SID is a major factor for magnification longer SID yields less magnification Longer SID's advocated by experts Decreases entrance skin exposure

SID

OID is a critical distance for magnification and spacial resolution OID should be minimized, whenever possible, to minimize magnification and improve resolution

OID

Unwanted exposure to the image receptor resulting in fog -a result of Compton interactions -provides no useful information -scatter of fog decreases image contrast

Scatter

in medical radiography, image size is always larger than object size O=I/M

O=object size I=image size M=magnification factor

M= SID/SOD

magnification factor

Role of central ray

Alignment -central ray -anatomical part -Image Receptor Angulation -Degree -Direction

if an angle is needed, what is better to angle for better spacial resolution

better spacial resolution is achieved by angling body part rather than angling central ray

what is an example of purposeful distortion

Radiographic positioning

SOD=

SID-OID

As the primary beam passes through the patient it will lose some of its orginal energy. This reduction in the energy of the primary beam is known as :

Beam Attenuation

To maintain exposure to IR:

increase mA and proportionally decrease time increase time and proportionally decrease mA increase kvp 15% and decrease mAs by a half Decrease kvp 15% and increase mAs by two times

To increase or decrease exposure to IR what do you do to mA, exposure time or kVp

Increase: increase mA, exposure time or kVp Decrease: decrease mA, exposure time and lVp

increases penetration and decreases absorption

increasing kvp

decreases penetration and increases absorption

decreasing kVp

what type of wavelength is there when the kvp is higher

shorter the wavelength and lower the kvp, longer the wavelength

Doing what to the OID will increase the exposure to the IR, decrease contrast and magnification, and increase recorded detail/ spacial resolution

Decreasing OID

Distance between the anatomic part and IR will affect:

-radiation intensity reaching the ir -amount of scatter radiation reaching the ir -magnification -recorded detail/spacial resolution

Practice: penumbra equation magnification factor