Intro to Radiography & Projection Geometry

Define Radiography

Conventional technique/process of using x-rays to produce a static, 2D imge of internal structures of the body (radiograph; plain or projection view)

Define Imaging Chain

Components that contribute to radiographic image formation and display

What are the components of an imaging chain?

X-ray source (produces x-rays), Image receptor (receives/detects x-rays), image display device (for digital imaging)

The entire volume of tissue between source and receptor image is

Projected onto a 2D image

What is radiograph formation?

The path of radiation (x-ray) beam, must include anatomy of interest and receptorIn

For the anatomy to be imaged, what must occur?

Beam must travel through the anatomy and then also hit the receptor

The radiographic image is essentially a map of beam

AttenuationDe

Define attenuation

Reduction in x-ray beam intensity as it travels through the anatomyThe

The thicker and denser the structure,

The more x-rays are absorbed and the more the beam is attenuated

What are some key features of x-ray beams?

Consist of many x-ray photos, x-rays travel in straight lines, x-ray beam is divergent

Incident beam is differentially attenuated by structures of different

Dentisty

What is the order of decreasing density?

Metal > enamel > dentin and cementum > bone > muscle > fat > air

What carries the attenuating information to the receptor?

Transmitted beam

What color would you see in radiolucency?

Black

What does black represent on x-ray images?

Less attenuating structures, more intense transmitted beam

What color would you see in radiopacity?

White

What does white represent on x-ray images?

More attenuating structures, less intense transmitted beam

Define radiodensity

Refers to an objects ability to attenuate or absorb x-rays

What are some characteristics of radiodensity?

They are dense and attenuate more x-rays, and appear radiopaque on the image

Define optical density

Degree of darkening or opacity of an exposed filmOp

Optical density depends on

The number of x-ray photons absorbed by the film

More x-ray photons (more intense transmitted beam) =

Greater film exposure = greater optical density = darker image

Define contrast

Range of densities on an image defined as difference in densities between light and dark regionsW

What does high contrast show?

Both light and dark areas with few shades of gray in-between

What does low contrast show?

Various light and dark shades of gray

What are some types of dental radiographs?

Intraoral and extraoral

What are some types of intraoral radiographs?

Periapical, bitewing, occlusal

What are some types of extraoral radiographs?

Panoramic, cephalometric and skull projectionswh

What is the fundamental difference between intraoral and extraoral dental radiography?

The location of the x-ray sensor

Define image quality

Reliability of an image in its representation of the true state of anatomy examinedWh

What are the parameters of radiographic image quality

• Image sharpness

• Spatial resolution

• contrast resolution

• Magnification

• Distortion

Images should have

Minimal magnification and distortion and adequate contrast and spatial resolution for the diagnostic task

Define sharpness

Measures how well a boundary between two areas of differing radiodensity is revealed

Define spatial resoluation

Measures how well an image reveals small objects that are close together

What is image size distoration?

The different between object size on image and actual object sizeD

Define magnificaition

Increase in size of object on image compared to actual size of objectW

What causes magnification?

Divergent paths of x-ray photons in a beam

What id image shape distortion?

The difference in appearance of object shape on image compared to actual object shape

What does image shape distortion result in?

Unequal magnification of different parts of the same object

The focal spot should be

As small as possible

The source-receptor distance should be

As long as possible

The object-receptor distance should be

As small as possible

In which orientation should the receptor be to the long axis of the object?

Parallel

In which orientation should the central beam be to the object and receptor?

Perpendicular

Define focal spot

The area on the target of the x-ray tube where x-rays are producedW

Where do x-rays originate from?

All points within the area of the focal spot

What is the range of the focal spot size for dental, panoramic, and cone beam CT?

0.04-0.08 nm

A small focal point

Yields a sharper image

What is geometric unsharpness in radiography?

The blurring of object edges that occurs when x-rays projected at different points in the focal spot pass through the same point on an object but hit different spots on the receptor

What causes geometric unsharpness?

x-rays being projected from different points in the focal spot to the same point on an object, resulting in variations in where the x-rays hit the receptor

What are other terms for geometric unsharpness?

penumbra or adumbration

A larger focal spot creates

A wider zone of geometric unsharpness, which results in a loss of image sharpness

What happens if you increase the distance between the x-ray source and the object (move the x-ray tube further from the tooth)?

The x-ray beam spreads out less, the rays in the center of the beam are almost parallel which makes the image sharper, and the image has less blurriness and isn’t as magnified

Why is the x-ray tube recessed in the tube head housing?

To help increase distance and improve image quality

What happens when you decrease the object-to-receptor distance?

Reduce x-ray beam divergence, x-ray phons in the center travel nearly parallel, reduces geometric unsharpness (sharper image), and minimizes image magnification

How do you get a sharper and more accurate x-ray image?

1. Increase source-to-object distance

2. Decrease object to receptor distance

Object- tooth

Receptor- sensory

Source- x-ray tube

What is the magnification equation?

Image size / object size = source-receptor distance / source-object distance

What is the source to image distance (source-receptor distance)?

X-ray to sensor

What is the source to object distance?

X-ray tube to toothIn

Intraoral radiographic image magnification is

Usually less than 10%

How can magnification from a large object-to-receptor distance be reduced?

By increasing the source-to-object distance (e.g., from 8" to 12" to 16"), which reduces image magnification and improves sharpness.

What causes image shape distortion in dental x-rays?

Uneven source-object or object-receptor distances, which distort the shape of the image.

How can you minimize image shape distortion?

• Align the object and receptor parallel

• Aim the x-ray beam perpendicular to both

Why might some shape distortion be unavoidable?

Because the natural shape of teeth (like maxillary molar roots) makes perfect alignment impossible

What is foreshortening in a dental x-ray?

The image looks shorter than the real object because the tooth isn’t parallel to the receptor, and the x-ray beam is aimed at the receptor, not the tooth

What is elongation in a dental x-ray?

The image looks longer than the real object because the tooth isn’t parallel to the receptor, and the x-ray beam is aimed at the tooth, not the receptor

What causes both foreshortening and elongation in dental imaging?

When the tooth is not parallel to the receptor, and the central ray isn’t aimed correctly at both

What are three ways to maximize image sharpness in dental radiography?

• Use a small focal spot

• Increase source-to-object distance

• Minimize object-to-receptor distance

Can the focal spot size be changed by the operator?

No, it’s determined by the x-ray unit manufacturer

Why does increasing the source-to-object distance improve image sharpness?

It reduces beam divergence and geometric unsharpness

Why does minimizing the object-to-receptor distance improve image sharpness?

It reduces magnification and keeps image detail more accurate.

Factors affecting image sharpness and image magnification