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IMAGE FORMATION
Introduction
The Invisible and Visible Image
Image Characteristics
Learning objectives
By the end of this Lecture the student will be able to:
Define the terms, image and identify its types and methods of viewing
Identify the stages of image formation
 Differentiate between the invisible (latent) and visible image
List and define the basic image characteristics
Differentiate between density and contrast and state their relationship
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IMAGE FORMATION
References
• John Ball& Tony Price; Chesney's Radiographic Imaging
Websites
• http://www.e-radiography.net/
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IMAGE FORMATION
What is an Image?

The term image describe a recognizable pattern carrying information
Other meaning
An optical appearance
 A mental representation
 A form or semblance
 An idea or conception
Visual images are of two types
 Real images : Those have real existence
such as photographic or radiographic
images
-Static ( RADIOGRAPH OR PHOTOGRAPH
- Dynamic ( On a television screen)

Mental images: Pictures those generated
within our minds
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IMAGE FORMATION
Medical imaging is essentially the extraction of
anatomical and physiological information from the
patient and the interpretation of these information
for the purpose of diagnosing diseases
The flow of the information from the patient to the
observer is through three stages:



The formation of the invisible image
The conversion of the invisible image into a visible light image
Interpretation of the visible image
IMAGE FORMATION
Stage.1 The formation of the invisible image


During radiographic examination X-ray passes through the patient
As x-rays penetrate through the body tissues it become modified
each part of the beam is attenuated in a degree which depend on
 The tissue type
 The intensity of the beam
 Thickness of the tissue
Transmit
Absorb
Scatter
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IMAGE FORMATION
Primary Radiation – The beam of photons, interacts
with the pt’s body.
Remnant Radiation – The resulting beam that is
able to exit from the patient.
Scatter Radiation – Radiation that interacts with
matter & only continues in a different direction – not
useful for image production.
Attenuation – Primary radiation that is changed
(partially absorbed) as it travels through the pt.
Path & Attenuation of
X-ray Beam
IMAGE FORMATION
Stage.1 The formation of the invisible image


During radiographic examination X-ray passes through the patient
As x-rays penetrate through the body tissues it become modified
each part of the beam is attenuated in a degree which depend on

The tissue type
 The intensity of the beam
 Thickness of the tissue
Film is the simplest image receptor, but it is usually coupled with
intensifying screens, which help reduce the dose to the patient.
 X-ray film responds to a range of wavelengths and is in fact far more
sensitive to light than it is to x-rays.
 After exposure to x-rays an invisible image known as the latent image
is formed.

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IMAGE FORMATION
Stage.2 The conversion of the invisible image into
visible
Types of Conversion
Using
the photographic effect of x-rays on a sensitive
sheet ( x-ray film)
Using
xeroradiographic effect process through exposing
an electrically charged imaging plate
Using
the fluorescent effect of x-ray on a sensitive screen
where the x-rays is converted into light when touches
these screens
Using the process of photon stimulated luminescence where
a phosphor coated imaging plate is exposed to x-ray , the
absorbed energy is stored in the phosphor layer until
stimulated by laser photons to form the visible image
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IMAGE FORMATION
Why you see what you see…
The images have different levels of density –
different shades of gray
X-rays show different features of the body in
various shades of gray.
The gray is darkest in those areas that do not
absorb X-rays well – and allow it to pass through
The images are lighter in dense areas (like
bones) that absorb more of the X-rays.
IMAGE FORMATION
Stage.3 (Viewing) Interpretation of the visible image
The final image whether it be on a film sheet or on a television screen
should be viewed under specific conditioned to evaluate its quality and
then passes for diagnosis

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IMAGE FORMATION
Stage.3 (Viewing) Interpretation of the visible image
Types of viewing
Viewing by reflected light from a surface
- Examples( texts, illustrations in books etc)
Viewing by transmitted light
- Through a semitransparent layer (e.g. Radiographs)
Viewing by emitted light from fluorescent layer.
- Examples( fluoroscopic image on the TV screen)
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IMAGE FORMATION
Image characteristics
Noise
Contrast
Sharpness
Resolution
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IMAGE FORMATION
Image characteristics
Noise
Real images consist of 2 components
- Meaningful pattern (Signal)
- Chaotic pattern ( Noise)
The details of the structures is affected by noise
+Noise = - Details
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IMAGE FORMATION
Image characteristics
Signal to Noise Ratio (SNR)
Under optimal condition the magnitude of signal
is greater than the magnitude of the noise
- SNR is Saied to be high ( More details)
When the magnitude of nearest the magnitude of
the noise
- SNR is Saied to be Low ( Less details)
Low SNR
High SNR
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IMAGE FORMATION
Image characteristics
Density and Contrast
Density
The amount of blackening
“darkness” on the radiograph
Contrast
The differences between
the blacks to the whites
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IMAGE FORMATION
Image characteristics
Contrast Changes
High
Optimal
Low
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IMAGE FORMATION
Image characteristics
Sharpness
Describe the blackening changes at the
boundaries between adjacent parts
Element of Image Sharpness
Recorded detail : the sharpness of the lines of
the image.
Distortion : the misrepresentation of the true
size or shape of the image compared to the
object.
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IMAGE FORMATION
Image characteristics
Sharpness (SHAPE DISTORTION – TERMS)
width
length
True Shape
Elongated
Foreshortened
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IMAGE FORMATION
Image characteristics
Resolution
The resolution of a system Is its ability to
demonstrate closely spaced structures in the
subject as separate entities in the image
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IMAGE FORMATION
Summary
Image characteristics
Visibility
density
contrast
Sharpness
detail
distortion
size
shape
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Unit III
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