Imaging Systems

X-Rays

Imaging Systems: Shared Elements

1. Where did the energy come from?

2. What happens when the energy interacts with matter?

3. How is the energy collected after that interaction?

4. How is the collected energy captured?

5. How is the signal manipulated?

6. What do you do with the information?

7. How is the information shown?

8. How does the brain interpret that information?

Source

X rays are a form of electromagnetic radiation.

They have enough energy to liberate electrons from the atoms that bind them ionizing radiation.

X rays can be produced by different methods: Synchrotron, free electron lasers, x-ray tubes that emit bremsstrahlung and characteristic x rays.

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1. “Where did the energy come from?”

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Energetic electrons may be attracted into partial orbit around the nucleus of a target atom. They slow down, losing kinetic energy.

The energy lost is converted into a photon of electromagnetic radiation, called bremsstrahlung radiation.

The energy lost by the incoming electron may be of any amount, up to the total kinetic energy carried by the electron. Similarly, the photon emitted may have any value up to the total kinetic energy of the electron.

Source

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Source

www.mcw.edu/medphys/learning

Object

2. “What happens when the energy interacts with matter?”

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Interaction between X rays and matter usually results in the deposition of energy.

Different types of interactions include: Photoelectric effect, Rayleigh scattering, Compton scattering, pair and triplet production.

These mechanisms combine to produce attenuation of the incident x-ray photon beam as it passes through matter.

Object

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Penetrate

ScatterCompton interaction

Photoelectric interaction

Release kinetic energy

Collection and Detection

3. “How is the energy collected after that interaction?”

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There are two basic ways to create x-ray images:

a) Pass an x-ray beam through a body section and project a shadow image onto a receptor.

b) Computer tomography: use a computer to reconstruct an image from the penetration data.

Detection

4. “How is the collected energy captured?”

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In conventional radiography the receptor consists of the

film mounted in contact with an intensifier screen.

The contrast that appears in the image is determined by

several factors: penetration characteristics, scattered

radiation, contrast characteristics of the film.

Processing

5. “How is the signal manipulated?”

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a) The most important effect of the intensifying screens

on image quality is that they blur the image.

b) Motion blur

c) Finite size of x-ray tube focal spot E ffec tive fo ca l sp o t s ize

O b jec t po in t

F ilm

B lu r

Processing

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5. “How is the signal manipulated?”

If the image is stored in computer memory processing for noise reduction is possible.

Contrast can be manipulated.

Images may be subtracted form each other to show a difference or change between them.

Compression/Storage/Transmission

6. “What do you do with the information?”

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In conventional radiography hospitals store films

In digital radiography images are stored in computer

memory.

Image files may be compressed for transmission.

Display

7. “How is the information shown?”•MRI•PET scan•Film photography•Digital photography•Television•VCR•DVD•Microscopes•Telescopes•CRT•LCD•Plasma display•Thermal imagers•Passive IR sensors•Radar•Movies•Overhead projector•Slide projector•Holography•Copiers•Scanners•Printers (inkjet)•Printers (laser)•Printing (lithography)•Printing (screen)•Printing (intaglio)•Aerial imaging•Astronomy •Airborne telescopes•Orbiting telescopes•Satellite imaging•CCD•CMOS•Color cameras•Color displays•Computer vision•Robotic vision•Nightlight•Image compression•Human visual system •Glasses•Ultrasound•Contact lenses•Laser surgery•…

In conventional radiography the image is a photographic

film.

Digital radiography can be displayed in either a CRT

monitor or a laser film printer. But these are not able to

display full dynamic range. One way to overcome

limitations is to use image processing techniques to

enhance lesion visibility.

Perception

8. “How does the brain interpret that information?”

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Human observers are integral part of imaging system.

Relationship between physical image quality and

diagnostic performance is known as psychophysics.

Use of ROC (receiver operating characteristics) is one of

the major statistical analytical tools to characterize

human performance.

The “Imaging Chain”

• The common thread in these imaging systems can be described in terms of the component ‘black boxes’ that make them up.

1. Source

2. Object

3. Collection

4. Detection

5. Processing

6. Compression/storage/transmission

7. Display

8. Perception