spatial resolution and distortion

what is spatial resolution

the sharpness or accuracy of an anatomical structure lines

what are other words for spatial resolution

sharpness, definition, recorded detail, detail

what does visibility of SR refer to

brightness or density of the image with image contrast

how is accuracy of structural lines achieves

high SR while minimizing distortion

what is the film-screen unit for resolution

line pairs per mm (Ip/mm) or cycles per mm

how are resolution test tools arranged

pairs of lines set at a distance from one another

how are resolution test tools read

based on the viewer can differ the close line pairs from each other represents the Ip/mm reading

what is Ip/mm most people can read up to

5 Ip/mm

what is the relationship between Ip/mm and resolution

the higher the Ip/mm, closer the line pairs, the higher the resolution (direct relationship)

what determines spatial resolution

matrix size, pixel size, and grayscale bit depth

spatial resolution is commonly described in terms of

spatial frequency

what is the relationship between SR and spatial frequency

when SR is high, there is a shorter wavelength signal, so SF is also high for fine details only (direct relationship)

what is the relationship between line pairs and spatial frequency

higher the SF the closer the line pairs

what is lack of sharp definitions of fine details caused by

penumbra

regarding information from images,

some information is always lost

what also increases unsharpness of an image regarding a patient

patient movement

how is SR for digital radiographic images described

x-axis, y-axis, and z-axis (grayscale bit depth)

what does the z-axis represent

the different shade of gray represent the added depth of the image or the info by each pixel

what is a matrix

a combo of rows and columns of pixels

what are pixels

small squares that makes up the matrix

what does each pixel represent

a number representing the amount of brightness

what does the pixel location on the matrix represent

the area within the patient or the volume of tissue

what is field of view (FOV)

the dimensions of an anatomical area

what is the relationship bw matrix size, pixels, and resolution

the greater the matrix size, the greater amount of pixels, and the better the resolution (direct relationships)

what is the equation for pixel size

pixel size = FOV/matrix size

what is the relationship between the matrix side and the pixel size w/ fixed FOV size

when matrix size increase, the pixel size decreases (inverse relationship)

what is the relationship between the FOV and the pixel size w/ fixed matrix size

the greater the FOV, the greater the pixel size (direct relationship)

what does each pixel have

bit depth

what is the relationship between bit depth and resolution

the larger the bit depth, the greater number of shades of gray, the better the resolution

what is point spread function (PSF)

measuring penumbra by using a single point from an extended line

what is the relationship between PSF, penumbra, and resolution

when PSF increases, penumbra increases, and resolution decreases

what was line spread function (LSF)

a narrow slit in a sheet of lead to measure penumbra

what is edge spread function (ESF)

uses a sharp edge instead of a line or point to measure penumbra

how is PSF measured in other modalities (xray, NM< CT, US, MRI)

X-ray: tiny hole in lead sheet then producing x-ray image of the hole

NM: point source of radioactivity

CT: a thin metal wire

US: a monofilament

MRI: water filled hole in a phantom

what does a PSF graph look like it

the narrow part of the graph has better resolution

what is modulation transfer function (MTF)

the ability of the system to record spatial frequencies

what does MTF measure

the accuracy of an image compared to the original object

what is the desired relationship between MTF and SR

high MTF at high SR

what is the relationship between MTF and spatial frequency

when MTF increases, spatial freq decreases

what is system noise

noise from the digital equipment

what is ambient noise

noise from background radiation

what is quantum noise

noise caused my insufficient photons hitting the IR due to improper kVp and mAs (quantum mottle)

what is signal to noise ratio (SNR)

the amount of radiation exposed to the detector (signal) compared to the noise

what is the relationship between the SNR and noise

high SNR means stronger signal than noise and vice versa

what is the relationship between SNR and SR

higher the SNR the better the SR

what is contrast to noise ratio (CNR)

how big of a difference bw the contrast ad the noise

what is the relationship between CNR and noise

when CNR is high, the contrast is better than the noise and vice versa

what is the nyquist criterion

the requirement for systems to sample spatial frequency signals twice per cycle

why is sampling twice required

the more sampling during a cycle taken, the more information will be taken and the more detail of the image will be seen

making up for the spaces between the pixels

what is aliasing

when there was not enough sampling causing the data to be misinterpreted

often due to the spacing between the pixels

what factors affect spatial resolution

motion

SID

OID

focal spot size

intensifying screens

phosphor size

phosphor concentration

what happens to photons coming out of the focal spot

diverge from the source like a cone shape

what is the relationship between SID and SR

as SID increases, SR increases

what is the relationship between OID and SR

as OID increase, SR decrease

what is the relationship between focal spot size and SR

when focal spot size increase, penumbra increase, SR decrease

what is the formula for finding the width of penumbra

P= (focal spot size x OID)/ SOD

what is penumbra

the unsharp edges around an image

what is umbra

the sharp area of the image often surrounded by the penumbra

what is attenuation absorption unsharpness

when the beam diverges due to the the object being attenuated causing penumbra

what are DR detectors made of

silicone or selenium

what limits DR detectors

the size of the detector elements

what does DR image processing system also limited by

matrix size, pixel pitch, and grayscale bit depth

what are CR systems limited by

phosphor size, layer thickness, concentration, and IRD

what is the relationship between film screen speed and SR

as speed increases SR decreases (inverse)

what are the part of a film

the base and the emulsion

what is the base of the film

a polyester sheet that is between the emulsions

its able to keep its size and shape through any processing condition and can be tinted blue to help with images

what is the emulsion of the film

gelatin substance that contains bromine crystals

the gelatin is colloid and amphoteric

what is the relationship between density and SR

when density increases, sensitivity increases and SR decreases (inverse)

what are intensifying screens made up of

a base and active layer

what happens when the xray beam hits the phosphor crystals

the phosphor emits light which creates the image

what are some problems of intensifying screens

light diffusion, screen speed, and poor screen contact

what is the relationship between light diffusion and SR and why

when light diffusion increases, SR decreases bc the space between the film and the phosphor cause the light to spread out on the film surface

what is the relationship between the crystal size and SR

when phosphor crystal size increase, light diffusion increase, and SR decrease

what is the relationship between intensifying screen speed and SR and why

when speed increases, more light emits causing SR to decrease

fast screens also have bigger crystals or more crystal layers making SR decrease

how does poor screen contact decrease SR

poor screen contact cause more light diffusion which decreases SR

what is the relationship of phosphor size and resolution, pt dose, and film density/IR expo

when phosphor size increase

resolution decrease, pt dose decrease, density increase

what is the relationship of phosphor layer thickness and resolution, pt dose, and film density/IR expo

when phosphor layer thickness increase

resolution decrease, pt dose decrease, density increase

what is the relationship of phosphor concentration and resolution, pt dose, and film density/IR expo

when phosphor concentration increase

resolution increases, pt dose decrease, density increase

what types of motion affect SR

voluntary, involuntary, equipment

what is voluntary movement and what is the best way to minimize it

patient has direct control over movements

communication is key or sometimes immobilization is required

what is involuntary movement and how can it be minimized

movement that’s out of the patient’s control

lowering mAs and increasing kVP (15% rule)

what is equipment motion

vibrations in the machine or xray tube suspension system

unable to use machine till fixed

what is distortion

misinterpretation of the true size and shape of the anatomy being examined

what is magnification

when the object being radiographed is bigger than the object itself

what is the only size distortion possible

magnification

what controls magnification

SID or OID

what is the relationship between size distortion and SID

when SID increases, size distortion decreases

what is the relationship between OID and size distortion

when OID increases, size distortion increases

what is ideal for the least amount of magnification

short OID with long SID

calculating magnification (formula)

M= SID/SOD

tells you how much longer and wider the image is compared to the original object

calculating actual object size (formula)

O= I/M

I: image size

M: magnification factor

O: object size

calculating % of magnification from original

M% = ((I-O)/O) x 100

what is size distortion

misrepresentation of the true size of the anatomical part on the image

what is shape distortion

misinterpretation of the true shape of the anatomical part on an image

what can be considered shape distortion

elongation or foreshorten

when does elongation occur

when the CR and the IR aren’t properly aligned while the object and the IR are parallel

i.e CR is angled or IR w/ object isn’t perpendicular with CR

how does elongated images look like

long and thin (stretched out)

what causes foreshortening

when the object isn’t properly aligned as the CR is perpendicular to the IR

how does foreshortening look

anatomical part squash together with unequal magnification

what factors affect shape distortion

when an anatomical part isn’t at the same level entirely (think as if it was diagonal rather than horizontal) causing beam divergence