1725 Spatial Resolution

Spatial Resolution

• Resolution is the ability to image two separate objects and visually distinguish one from the other

  • The ability to image small objects that have high subject contrast

    • Ex: Bones and soft tissue

Detail or Recorded Detail

• Less precise older terms for spatial resolution

• These terms refer to the ability to visualize recorded detail when image contrast and optical density are optimized

Geometric Properties of the Image Quality

• Spatial Resolution

• Distortion

• One of the easiest factors to evaluate and adjust

• Control detail itself

ARRT Definition of Spatial Resolution

• The sharpness of structural edges recorded in the image

• The imaging process makes it impossible to produce a radiographic image without some degree of un-sharpness

What determines spatial resolution in digital imaged

• Primarily by Pixel Size

  • Small usually square picture element

  • Smallest component of the matrix

• Then by Matrix Size

  • A digital image is recorded as a matrix or combination of rows and columns of pixels

Pixel Pitch

The space between two pixels (Smaller pixel pitch = smaller pixels = higher spatial resolution)

Matrix Affect on Spatial Resolution

Matrix size: Larger the matrix = Increased or improved resolution

Pixel Affect on Spatial Resolution

• Pixel Size: Small and thinner the pixel = Increased or improved resolution

• Pixel Amount: More pixels = Increased/Improved resolution

Geometric Properties Relationship

• Distortion and Spatial Resolution

  • Shares an inverse relationship

• Diagnostic quality is achieved by maximizing spatial resolution and minimizing the amount of image distortion

Factors Affecting Spatial Resolution

• Motion - Voluntary, Involuntary, Equipment

• Distance - OID, SID

• Focal Spot - Small, Large

What’s the first approach to a resolution problem

Eliminate motion

What’s the second approach to a resolution problem

Reduce OID

What’s the third approach to a resolution problem

Reduce Focal Spot Size

What’s the fourth approach to a resolution problem

Increase SID

Motion unsharpness or motion blur

• Resolution affects the image appearance by demonstrating fine detail structures

• When fine details are lacking, the image will often appear blurred

• Motion unsharpness is often not perceived by the beginning student because they must leanr to critically examine each image

• The assessment of motion is obtaining sufficient clinical experience

What causes Motion Unsharpness

• Patient *most common

  • Voluntary and involuntary

• Equipment

  • Due to grid bucky mechanism (reciprocating grid vibration) table, tube, & image receptor (Not balanced - drift or vibrate)

  • Very hard to detect

Voluntary Patient Motion

• Due to intentional patient movement

• Can be controlled by the technologist

  • Explanation of exam

  • Short exposure times

  • Immobilizing devices/positioning sponges

Involuntary Patient Motion

• Heart beat

• Peristalsis

• Can be controlled only by short exposure times

Equipment Motion

• Due to grid bucky mechanism (reciprocating grid vibration)

• Table, tube, and Image receptor (Not balanced - drift or vibrate)

• Very hard to detect

Methods to reduce motion if patient is able to cooperate

• Patient communication

• Use clear, concise, and understandable instructions

• Use positioning aids such as foam pads, angle sponges, and sandbags

Methods to reduce motion if patient is unable to cooperate

A reduction in exposure time with corresponding increase in mA to maintain sufficient mAs and film density/IR exposure

Distance

The distances between the source or focal spot (S), object or part (O), and the image receptor (I) are critical in establishing recorded detail

How to calculate SID

SOD + OID = SID

Object to image receptor distance (OID)

• OID is the critical factor

• OID is the first distance evaluated

• Minimum OID is the goal

• Resolution improves when OID is decreased

OID TableTop vs. Bucky Tray

• OID can be minimized by considering distance between surface supporting the part and IR

• Less OID with tabletop

• Need for grid supersedes the resolution that is lost by using a bucky tray

Source to Image Receptor Distance (SID)

• Once OID has been minimized, resolution increases when SID Increases

• Positioning routines were established to take advantage of this

• Ex: Lateral Cervical Spine

• Lateral C-Spine 72” and AP C-Spine at 40”

Geometry of beam-beam divergence

• It is the most important factor in establishing the level of resolution desired

• X-rays come from a small point (Focal Spot), the further the photons move from their source, the further they diverge

Focal Spot Size

• When all other factors remain the same - the smaller the focal spot, the sharper the image or the increase in spatial resolution

  • Inversely proportional

• Focal spot size selection is done at the control panel

• By doing so, the radiographer is selecting the size of the filament used

What controls the focal spot size

The line focus principle: used to reduce the effective area of the focal spot - This permits the best resolution of detail while permitting as large an actual area as possible

Line Focus Principle

• The effective focal spot size is controlled by the size of the actual focal spot (which is controlled by the length of the filament) and the anode target angle

• As the actual focal spot increases, the effective focal spot also increases

Umbra

• Distinctly sharp area of a shadow or the region of complete shadow

• Also know as edge gradient

Penumbra

• Imperfect, unsharp shadow surrounding the umbra

• Invades the umbra shadow

• Umbra will shrink because of increasing penumbra

When penumbra grows, it not only expands ___ but also ___

Outward; Inward

Focal Spot

• Major controller of image resolution because it controls penumbra

• As focal spot decreases, penumbra decreases therefore increasing resolution

• Focal spots are not usually capable of imaging structures smaller than the focal spots themselves

Small focal spot (relative to penumbra)

• Produces less penumbra than a large focal spot

• Produces a greater resolution

Reducing OID (relative to penumbra)

Reduces penumbra and creates greater recorded detail

Increasing SID (Relative to penumbra)

Reduces penumbra and creates greater recorded detail (SID)

The formula to calculate width of penumbra

P = Focal Spot Size x OID/ SOD

Attenuation or absorption unsharpness

Increases penumbra size

What shape object would have perfectly sharp edge

Trapezoidal (Due to beam divergence)

Line Pairs Per Millimeter (LP/MM)

• Unit of resolution is measured using lp/mm

• The resolution tool is composed of pairs of lines a set distance from one another

• The point at which a viewer can detect the closest pair of lines from each other represent the lp/mm reading

Noise

• Image noise is simply background info that the IR receives

  • Random fluctuation in optical density of the image

  • Contributes no useful diagnostic information and only detracts from the qualities of the image

  • Radiographic noise is inherent in the imaging system

  • Lower noise results in a better radiographic image because it improves resolution

Quantum Noise or Mottle

• Refers to the lack of sufficient data for processing

  • Results in blotchy or mottled image

  • Also called quantum mottle

• The fewer the x-ray photons reaching the image receptor to form the image the greater the visibility of quantum noise

  • To resolve this increase mAs