lecture 1 the radiographic image - xraykamarul · lecture 1 the radiographic image ... •any...

LECTURE 1 The Radiographic Image

Prepared by:- KAMARUL AMIN ABDULLAH @ ABU BAKAR

UiTM

Faculty of Health Sciences Medical Imaging Department

11/23/2011 KAMARUL AMIN (C) 1

Lesson Objectives

At the end of the lesson, the student should able to explain:-

• The definition of density, contrast, unsharpness, and distortion.

• The factors affecting them.


DENSITY


DENSITY

• Represents the degree of darkening of an exposed x-ray film (white to black).

• The overall density of the film affects the diagnostic value of the film (ideal density, too light, too dark).


Cont’d..

• Black areas on a developed radiograph are produced by deposits of black metallic silver in the film emulsion that result from exposure to x-rays and their subsequent processing.

• Degree of blackness on a radiograph depends on the amount of x-rays reaching the film.


Cont’d..

• Also known as the logarithm of opacity or optical density (OD).

• OD: the ratio of the amount of light incident on the film to the amount of light transmitted through the film. 1. Light incident – the light striking the radiograph from the back

(viewing box). 2. Light transmitted – the light which is seen coming through the

radiograph while being viewed either by human eye or densitometer.

OD = LOG10 light incident

light transmitted



http://radiography.files.wordpress.com/2008/02/hand-xray.jpg


LOW DENSITY HIGH DENSITY

Factors Affecting Density

Exposure Factors Patient Size

Distance Object Density Film Processing

Film/Screen Combination Collimation

Grids Anode Heel Effect

Filtration


Cont’d..

• Exposure Factors (mAs, kVp).

• An unnecessary increase in any of these factors results in an increase in film density.

A. mAs

– The chief controlling factor of exposure and density.

– Directly control the total number or amount of x-ray photons reaching the films.

– Increasing mA or time increases the density and Decreasing mA or time will decrease the density.


Cont’d..

B. kVp

– kVp affects density differently than mAs. In order for there to be a significant increase in density a 15% change in kVp must be made.

– Directly control the penetration or quality of x-ray photons.

– There is a peak or optimal kVp for each body part.


Cont’d..

• Patient Size: the larger the patient’s body, the more x-rays that are needed to produce an ideal film density.

i.e.: Increase in thickness, means that the patient can absorb more x-rays which will results in a lighter image.


Cont’d..

Distance • Distance is inversely related to density.

• A decrease of distance of the source of x-rays to film increases the density and vice versa.

• Known as the Inverse Square Law.

• e.g.:If distance is doubled, density decreases four times.


Cont’d..

• Object Density: determined by type of material (metals, structure (e.g. bone, tissue), composite, etc.) and by amount of material.

• Types of tissue affects density. For example, higher density tissues will cause less x-rays to reach film, therefore have lighter areas on the film at that place.

• Density (darkness) decreases when object density increases.


RADIOLUCENT

Dark Gray to Black

Low object density results in high film density OR

allows x-ray to easily penetrate (e.g. air, soft tissue)


RADIOPAQUE

White to Light Gray

High object density results in low film density OR

x-rays do not easily penetrate (e.g. bone, metals)


Activities..


Instruction: Please indicate which parts are the radiolucent and radiopaque for each images.

Cont’d..

Film Processing: due to light, mishandling and automatic film processor.

The developing time – increase time will increase density

The temperature of the developer – increase temperature will increase density

Mishandling and light can occur due to wet hands, open cassettes in light areas, high beam safelight, leakage, etc.


Cont’d..

Film/Screen Combination • Screens were used to enhance the radiation so

that fewer x-ray photons (less dose) / more light photons are used to create a radiographic image with optimum density.

• The color of the glow of the intensifying screen must match the color sensitivity of the film (spectral matching).

• Good film/screen combination = GOOD Density


Cont’d..

Collimation • Limiting the size or increase collimation will

decrease density.

• i.e. it reduces the scattered radiation that will adds density to the film.


Cont’d..

Grids • Grids also absorb scattered radiation before

reaching the film.

• It will reduce the density.


Cont’d..

Anode Heel Effect

• Density is greater near cathode end.

• Density is lesser near the anode end.


Cont’d..

Filtration • Improve quality of beam by absorbing low

photon energies.

• Good quality of beam has high penetration and improve the density.

• More penetration = more interaction with matter = more x-ray photons interact with film.


CONTRAST


CONTRAST

• Defined as visible difference between two adjacent radiographic densities.

• Primary function is to visible image details.

• Is divided into

Radiographic Contrast

Subject Contrast


Radiographic Contrast

• Density between two adjacent areas on a radiograph.

Many black and white areas means you have contrast.

If a radiograph has many grays and a small density difference between 2 adjacent areas, then it has low contrast.

Need right amount of contrast. (Include grays, blacks and whites so eye can easily see details).


Subject Contrast

• Defined as the difference in density and mass between two adjacent anatomic structures.

• Depends on:

Thickness of the anatomic part.

Density of the anatomic part.


High Contrast


Short Scale Black and White (Few shades of gray)

Low Contrast


Long Scale Many shades of gray


Question: Which one is Low Contrast and High Contrast?

Factors Affecting Contrast

• Controlling Factor: kVp Level

• Grids

• Beam Restriction

• Filtration

• Subject Contrast

• Film Processing


Cont’d..

kVp LEVEL • The controlling factor for contrast.

• kVp affects energy (penetrating ability) of x-rays.

• The penetrability will control the differential absorption of x-ray.


Cont’d..

• As HIGH kVp = more penetration = more scatter = low contrast = long scale

• As LOW kVp = less penetration = less scatter = high contrast = short scale


Cont’d..


Cont’d..

Grids • Reduce the amount of scattered radiation

reaching the film.

• And this will improve the contrast as less density has been added to the film.


Cont’d..

• Increase Scattered Radiation = Low Contrast = HIGH kVp = High Penetrability

• Decrease Scattered Radiation = High Contrast = LOW kVp = Low Penetrability


Cont’d..

Beam Restriction • It limits the area being irradiated.

• Close or Increase collimation will reduce scattered by reducing Compton’s Interaction.

• Less scattered reduces the number shades of gray, thereby increasing contrast.


Cont’d..

Filtration • As filtration is increased, the penetrability of

the beam will also increase.

• The increase of high average penetration/energies will produce low contrast as many gray tones will produce.


Cont’d..

Subject Contrast • Results from varying object densities within

patient.

• For example, BONE will have more white on the film than SOFT TISSUE.

Bone HIGH contrast low kVp

Soft tissue LOW contrast high kVp


Cont’d..

Film Processing • Inadequate processing degrades the

radiographic contrast.

• Optimum temperature, time, replenishment chemicals, water flow and etc. will produce GOOD contrast.


RECORDED DETAILS


RECORDED DETAILS

• The distinct representation of an object’s true borders or edges.

• It is often called sharpness of detail, definition or resolution.

• Terms used to describe image sharpness, clarity, distinctness, and perceptibility.


Factors Affecting Details

Motion

Focal Spot Size

Source-Image-Distance (SID)

Object-Image-Distance (OID)

Film/Screen Combination


Cont’d..

Motion • The most common cause.

Voluntary motion • Motion caused by the movement of the patient and

equipment.

• Best controlled by good patient instructions

Involuntary motion • Motion caused by uncontrolled motion of the body

such as the heart beat or peristalsis

• Best controlled by short exposure times


Which one is blurred image? 11/23/2011 KAMARUL AMIN (C) 46

Cont’d..

Focal Spot Size A small focal spot is used when fine detail is needed.

A large focal spot can cause unsharpness of recorded details. However, using of large focal spot size can lengthen the x-ray tube life.

It is because the x-ray produced by larger focal spot will cause bigger divergence of the beam.


Cont’d..

Source-Image-Distance (SID) • As SID increases detail increase.

• Longer SID can reduce penumbra effect.

• Penumbra

A fuzzy border of an object that is obscure.

• Umbra

The true border.


Cont’d..

Object Image Distance (OID) • Distance from anatomical part to the film.

• Decrease the OID will increase details.

• Increase the OID will decrease the details.

• Increase of OID cause the magnification to occur.


Target 16”

Target 16”

Increase OID, Increase magnification, Decrease Details.

Decrease OID, Decrease magnification, Increase Details

OID


Cont’d..

Film-Screen Combination • Slow Speed = Increase Details

• High Speed = Decrease Details

• Speed primarily affected by Phosphors Size of Intensifying Screen and Crystals Size of Film.

Larger Size = Poor Details

Smaller Size = Greater Details


DISTORTION


DISTORTION

• Any misrepresentation of anatomic structure on an image that has changing of its size or shape.

• There are TWO (2) types of distortion :

A) Size

B) Shape


Factors Affecting Distortion

A) SIZE

• Magnification

• Caused by excessive OID

• Caused by insufficient SID

• Causes anatomic structures to appear larger from an original size.



SOURCE-IMAGE-DISTANCE (SID)


OBJECT-IMAGE-DISTANCE (OID)

Cont’d..

B) SHAPE

Caused by:-


1. Film-Object relationship

- Improper structure with the film.

2. Beam alignment

- Improper angulations of tube with the structure/part.

Cont’d..

• It has TWO (2) types:-

1. Elongation

Causes anatomic structure to appear longer than original size.

2. Fore shortening

Causes anatomic structure to appear shorter than original size.


~END OF LECTURE 1~


lecture 1 the radiographic image - xraykamarul · lecture 1 the radiographic image ... •any...

Documents