Spatial resolution

Spatial resolution aka

sharpness, definition, recorded detail, or detail

Spatial resolution

degree of sharpness or accuracy of the anatomical structure lines in an image

Film-screen unit of resolution

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

Resolution test tools

set of lines that have some distances from one another

the closer the lines are to one another, the better the spatial resolution

the point where the viewer can see the close line pairs apart is the Ip/mm reading (limited to range of 5)

systems cannot provide this lvl of resolution

the greater the Ip/mm, the higher the resolution

when Ip/mm increase

resolution increase

reciprocal of line pairs per mm

number of line pairs (usually 1) / number of lines

spatial resolution determined by

matrix size, pixel size, and grayscale bit depth (aka spatial frequency)

higher the spatial resolution/frequency

pairs of lines are very close together

when two objects can be seen smaller and closer together

spatial resolution high

when image lacks sharp definitions of fine details

poor resolution, caused by penumbra

realistically regarding information from images

some information in images are always lost

factors like patient movement increase unsharpness

assessing spatial resolution

expressed in DR images as x-axis, y-axis, and grayscale bit depth (z-axis)

z-axis (aka grayscale bit depth)

the different shades of gray in an image represents the added depth to an image

comes from combining the gray levels of each pixel

digital imaging characteristics

recorded as a matrix (combo of rows and columns of small squares called pixels)

each pixel has a number representing the amount of brightness

pixel location on the matrix represents area within patient or volume of tissue

field of view (FOV)

dimensions of anatomic area

greater the matrix size

greater amount of pixels, better resolution

relationship between pixel size, FOV and matrix size (equation)

pixel size = FOV/matrix size

bit depth

each pixel has bit depth

uses system expressed as 2^n (n= # of bits)

large bit depth, greater number of shades of gray, better resolution

point spread function (PSF)

measures penumbra

quantify digital system spatial resolution

uses a single point from an extended line

smaller PSF, less blur, better resolution

Line spread function (LSF)

uses a narrow slit in a sheet of lead

Edge spread function (ESF)

uses sharp edge instead of a line or point

line spread function (LSF), edge spread function (ESF) and PSF

express the boundaries/edges of the image

penumbra or blur

PSF in other modalities

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

PSF graph

narrower the peak of the graph, better spatial resolution

spatial frequency

high freq signals, high spatial resolution, better image of smaller objects

modulation transfer function (MTF)

measures the accuracy of an image compared to the original object from a scale of 0-1

fidelity (or trueness) of an image

record available spatial freq

the overall values of PSF, LSF, and ESF

All system components must be working or else fidelity of patient is less accurate and MTF decrease

High MTF at high spatial resolution are desired

MTF decrease

when spatial freq increase but spatial resolution is high

system noise

noise coming from digital equipment

Ambient noise

noise coming from radiation in the background

quantum noise

lack of quantity of photons causing quantum mottle

improper kvp and mas

signal to noise ratio (SNR)

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

high SNR means signal stronger than noise

low SNR means noise and signal about the same (not good)

higher the SNR, better image will be, better spatial resolution

contrast to noise ratio (CNR)

how big the differences bw contrast resolution and noise are

high CNR, stronger contrast than noise, better resolution

low CNR, contrast weak/ too much noise, low resolution/blurry image

Digital sampling (nyquist criterion)

bc digital systems collect signals at discrete points, spatial resolution freq signals must be sampled twice per cycle

think of it as snapshots for a moving image (more snapshots taken of the image, more detailing)

more sampling, less details missing

uses the spacing between detector elements (or pixels) to figure out how often to take samples so that info is captured correctly

Aliasing

not enough sampling causing misinterpretation of the data

also occurs when the spacing between the detector elements (pixels) are too far apart

Factors affecting spatial resolution

motion

OID

focal spot size

intensifying screen

phosphor size

concentration

SID

Geometry of recorded detail

focal spot size and distance (SID and OID)

bc xray coming out of small area (focal spot) photons will diverge from source

(think cone shape and the tip of the cone is the x-ray tube)

distance affects on spatial resolution

SID increase, spatial resolution increase

OID increase, spatial resolution decrease

focal spot size affects on spatial resolution

controlled by line focus principle

controls penumbra

focal spot size increase, penumbra increase, spatial resolution decrease

width of penumbra (formula)

penumbra= (focal spot size x OID)/ SOD

penumbra

unsharpness around the edges of an image

umbra is the sharp area of a shadow

penumbra surrounds umbra

attenuation absorption unsharpness

increases penumbra due to beam divergence as its attenuated by an object

too much space between the beam divergence and the object sides cause penumbra increase

IR of recorded detail

based off film screen systems or digital systems

digital systems: DR

uses silicon or selenium detectors

silicon limited by fill factor (quantity of photons that gets registered by detector)

high fill factor, high resolution

selenium detectors are thicker and can’t be small like silicon

both are limited by the size of the detector element

image processing also limits: matrix size, pixel size, and grayscale bit depth (xyz)

digital systems: CR

limitations similar to intensifying screens

phosphor size, layer thickness, and concentration, and image reader device (IRD)

film screen systems

classified by speed

when film screen speed increases, resolution decreases

intensifying screens cause poor resolution, to make it up, 20-100x mAs required

resolving power depend on phosphor size, phosphor layer thickness, and phosphor concentration

radiographic film construction

made up of the base and the emulsion

the base

polyester sheet

between two emulsions

dimensional stability: keeps size and shape through all processing conditions (temperature, pH, etc.)

blue tint added to base to enhance contrast, make image more pleasing, and absorb 15% of the light

the emulsion

gelatin substance surrounding the base

crystals of silver bromine in gelatin

gelatin characteristics: colloid (capable of suspending the crystals and coating each to separate them) amphoteric (can be used with acids or alkalis)

speed of film

relationship between exposure a film receives and amount of density called film speed

when film speed increase, resolution decrease

when density increase, sensitivity increase, resolution decrease

more exposure latitude allows more wiggle room

intensifying screens

made up of base and active layer

phosphor emits light when crystal hit with xray photons

light from intensifying screen exposes the film and make an image

problem with intensifying screen: light diffusion

decreases resolution bc of the distance between the film and phosphor, causing the light to spread out on the film surface

when light diffusion increase, resolution decrease

when crystal size increase, light diffusion increase, resolution decrease

problem with intensifying screen: screen speed

higher screen speed, more light emitted, resolution decrease

fast screens have larger phosphor crystals or have more phosphor layers, which produce more light and cause less resolution

problem with intensifying screen: poor screen contact

when distance between the film and screen increase, light diffusion increase, and resolution decrease

caused by dropping cassettes or bending them

when phosphor size increase

resolution decrease, patient dose decrease, film density increase

when layer thickness of phosphor increase

resolution decrease, patient dose decrease, film density increase

when phosphor concentration increases

resolution increases (bc crystals are getting packed closer together and decreases light diffusion), patient dose decreases, film density increases

quantum mottle and intensifying screens

when mAs is not increased when using high speed intensifying screens, not enough photons will reach the film causing quantum mottle

motion affecting spatial resolution

voluntary, involuntary, equipment

voluntary motion

patient has direct control of movement

communication is key

sometimes requires immobilization

involuntary motion

patient not in control of motion

decreasing exposure time or increasing kVp with mAs decrease (15% rule)

equipment motion

vibrations in the machine or x ray tube suspension system

report problem and don’t use machine till fixed

effects of changing factors on spatial resolution chart