IMAGE QUALITY REVIEW RT 244 2007

What affects DENSITY on the radiographic image?

Factors Affecting mAs & Density Patient factors: size of pt., density / pathology of tissue kVp Collimation Distance Grids Film/Screen Combinations Processing

Factors Affecting mAs Patient factors: size of pt., density of tissue, pt. compliance kVp Distance Grids Film/Screen Combinations Processing

Influences technique & density on image Patient’s Body Habitus (size) Influences technique & density on image

PATHOLOGY ?

pneumothorax Lung collapses No tissue in space Easy to penetrate with x-ray photons pneumothorax

LUNG Cancer

DON’T ! WE WANT YOU HEALTHY & NOT SMELLY !

LUNG CANCER

Creating the Image Scatter Creates fog Lowers contrast (more grays) Increases as kV increases Field size increases Thickness of part increases

Effects of collimation on scatter

Collimate to area of interest -reduces scatter and radiation dose to the patient

Grids A device with lead strips that is placed between the patient and the cassette Used on larger body parts to reduce the number of scattering photons from reaching the image

Grid is placed between patient (behind table or upright bucky) & cassette If placed BACWARDS CAN CAUSE GRID ERRORS

GRIDS CAN LEAVE LINES ON THE IMAGE

DETAIL & Quality : How well we can see something on the image

The degree of sharpness in an object’s borders and structural details. How “clear” the object looks on the radiograph

Recorded Detail The degree of sharpness in an object’s borders and structural details. Other names: -sharpness of detail -definition -resolution -degree of noise

2 principal characteristics of any image are Spatial & Contrast Resolution Spatial resolution Resolution is the ability to image two separate objects and visually distinguish one from the other Spatial resolution is the ability to image small objects that have high subject contrast (eg. bone-soft tissue interface, calcified lung nodules)

2 principal characteristics of any image are Spatial & Contrast Resolution Spatial resolution Determined by focal-spot size and other factors that contribute to blur Diagnostic x-ray has excellent spatial resolution. It is measured in line pairs per mm. (CT measured in cm)

Factors that affect the detail of an image

Factors that affect Recorded Detail Geometric unsharpness OID SID SIZE SHAPE Motion unsharpness (blurring) Intensifying Screens Film Speed / Compostion Film – Screen contact Kvp & Mas (density / visibility)

Main Factors Affecting Recorded Detail kVp & mAs Motion Object Unsharpness Focal Spot Size SID (Source to Image Distance) OID (Object to Image Distance) Material Unsharpness

GEOMETRIC QUALITIES DETAIL DISTORTION MAGNIFICATION

RESOLUTION TEST TOOLS LINE PAIRS/ MM Depits how well you can see the differences in structures More lines=more detail

POOR DETAIL GOOD DETAIL

Motion Can be voluntary or involuntary Best controlled by short exposure times Use of careful instructions to the pt. Suspension of pt. respiration Immobilization devices

Decrease Motion Unsharpness Instruct patient not to move or breath Use Immobilization devices Use Short exposure times Lock equipment in place

Blurring of image due to patient movement during exposure.

Object Unsharpness Main problem is trying to image a 3-D object on a 2-D film. Human body is not straight edges and sharp angles. We must compensate for object unsharpness with factors we can control: focal spot size, SID & OID

SID Source to Image Distance The greater the distance between the source of the x-ray (tube) and the image receptor (cassette), the greater the image sharpness. Standard distance = 40 in. most exams Exception = Chest radiography 72 in. *See page 74 in your book

The position of the tube (SID) to IR Will influence how the structures appear on the image The farther away – the less magnified ↑SID ↓ MAGNIFICATION

SID Shine a flashlight on a 3-D object, shadow borders will appear “fuzzy” -On a radiograph called Penumbra Penumbra (fuzziness) obscures true border – umbra Farther the flashlight from object = sharper borders. Same with radiography.

OID Object to Image Distance The closer the object to the film, the sharper the detail. OID , penumbra , sharpness  OID , penumbra , sharpness  Structures located deep in the body, radiographer must know how to position to get the object closest to the film. *See page 74 in your book

The position of the structure in the body will influence how magnified it will be seen on the image The farther away – the more magnified

Focal spot size – determined by filament in cathode & surface area used at anode

Distortion Misrepresentation of the true size or shape of an object -MAGNIFICATION (size distortion) -TRUE DISTORTION (shape distortion)

MAGNIFICATION TUBE CLOSE TO THE PART (SID) PART FAR FROM THE CASSETTE (OID)

In terms of recorded detail and magnification, the best image is produced with a small OID and a large SID.

40” SID VS 72” SID

MAGNIFICATION PROBLEMS SID SOD SID – OID = SOD

Size Distortion & SID Major influences: SID & OID As SID , magnification  Standardized SID’s allow radiologist to assume certain amt. of magnification factors are present Must note deviations from standard SID

Size Distortion & OID If source is kept constant, OID will affect magnification As OID , magnification  The farther the object is from the film, the more magnification

A = good B & C = shape distortion (elongation of part)

D & E = shape distortion (foreshortening of part)

Shape Distortion Misrepresentation of the shape of an object Controlled by alignment of the beam, part (object), & image receptor Influences: Central ray angulation & body part rotation

Image Distortion When the part to be imaged – does not lay parallel with the IR (cassette) If the Central Ray is not perpendicular to the part

Elongation Foreshortened Normal

Distortion (x-ray beam not centered over object & film) Distortion (object & film not parallel)

Central Ray Radiation beam diverges from the tube in a pyramid shape. Photons in the center travel along a straight line – central ray Photons along the beam’s periphery travel at an angle When central ray in angled, image shape is distorted.

Distortion of multiple objects in same image (right) due to x-ray beam not being centered over objects.

Central Ray Angulation Body parts are not always 90 degrees from one another Central ray angulation is used to demonstrate certain details that can be hidden by superimposed body parts. Body part rotation or obliquing the body can also help visualize superimposed anatomy.

Main Factors Affecting Recorded Detail kVp & mAs Motion Object Unsharpness Focal Spot Size SID (Source to Image Distance) OID (Object to Image Distance) Material Unsharpness/ Film Screen Combo

Factors Affecting mAs Patient factors: size of pt., density of tissue, pt. compliance kVp Distance Grids Film/Screen Combinations Processing

Focal Spot Size Smaller x-ray beam width will produce a sharper image. Fine detail = small focal spot (i.e. small bones) General radiography uses large focal spot Beam from penlight size flashlight vs. flood light beam *See page 73 in your book

ANODE ANODE

THE SMALLER THE BEAM TOWARDS THE PATIENT - THE BETTER THE DETAIL OF THE IMAGE PRODUCED

FOCAL SPOT ANGLE SMALLER ANGLE – SMALLER BEAM AT PATIENT

REVIEW Intensifying Screens and Film

“Fast” Screen Cassettes Equipment used can contribute to image unsharpness Fast film/screen combinations = decrease in image sharpness Slower film/screen combinations = increase in image sharpness

Fast screen vs Slower screen

QUANTUM MOTTLE Not enough PHOTONS – can create a mottled or grainy image - MORE COMMON IN CR SYSTEMS

SAME TECHNIQUE CHANGE IN SCREEN SPEED SLOWER FASTER

CASSETTES with Intensifying Screens The CASSETTE holds the film in a light tight container It consist of front and back intensifying screens

Intensfying screens Lower patient dose (less photons needed) Changes resolution of image Slow screens less LIGHT = better detail Faster – less detail (more blurring on edges)

Intensifying Screens: Located inside the cassette (film holder) Contains Phosphors: Calcium Tungstate Blue to purple light Rare Earth Green & Ultraviolet light

CHANGING CR SPEED

F/S SPEED CHANGES

CR SPEED CLASS

WIDER LATITUDE & DYNAMIC RANGE WITH CR

POOR SCREEN CONTACT FOAM BACKING HELPS TO PLACE INTENSIFYING SCREENS IN DIRECT CONTACT WITH THE FILM – NO GAPS IF GAPS – MORE LIGHT CAN BE EMITTED IN SPACE, CAUSING THE IMAGE TO BE OF POOR DETAIL

Tight contact needed between film & screens

WIRE MESH SCREEN CONTACT TEST

When there is a space between the contact of the film to the intensifying screens, a larger amount of light is allowed to reach the film – causing “more density” on fim

Lack of contact between film and cassette can cause “blurring” of the image

LOADING FILM IN CASSETTE

IMAGE ON FILM SINGLE EMULSION = BETTER DETAIL DOUBLE EMULISON = LESS DETAIL PARALLAX With double emulsion – an image is created on both emulsions – then superimposed – slight blurring of edges

PARALLAX – each emulsion has an image single image overlaped edges edge sharp less sharp

Film Characteristics (more in week 9) Film contains silver halide crystals 2 layers – emulsion & base emulsion thickness determine speed of film and degree of resolution Speed – the response to photons Resolution – the detail seen

Film Speed / Crystal size Larger crystals or Thicker crystal layer Faster response= less detail, and less exposure (chest x-ray) Finer crystals / thinner crystal layer =Slower response, greater detail, more exposure (extremity)

Processing Film (wk 10) Film contains silver crystals If crystals exposed to photons – will convert to black after placed in processing chemicals If not exposed – will remain clear on film

Goal : Produce Optimal Images for diagnosis

Too dark too light Out of focus good image