IMAGE FORMATION Introduction The Invisible and Visible Image Image Characteristics.

IMAGE FORMATION

IntroductionThe Invisible and Visible ImageImage Characteristics

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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

Learning objectives

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References• John Ball& Tony Price; Chesney's Radiographic Imaging

Websites

• http://www.e-radiography.net/

IMAGE FORMATION

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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

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

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Scatter

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

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.

IMAGE FORMATION

Path & Attenuation of X-ray Beam

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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.

IMAGE FORMATION

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Stage.2 The conversion of the invisible image into

visible

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 screensUsing 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

IMAGE FORMATION

Types of Conversion

Why you see what you see…

The images have different levels of density – different shades of grayX-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 throughThe images are lighter in dense areas (like bones) that absorb more of the X-rays.

IMAGE FORMATION

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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

IMAGE FORMATION

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Stage.3 (Viewing) Interpretation of the visible image

IMAGE FORMATION

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)

Types of viewing

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IMAGE FORMATION

Image characteristics

Noise

Contrast

Sharpness

Resolution

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IMAGE FORMATION

Image characteristics

NoiseReal 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)

High SNR

When the magnitude of nearest the magnitude of the noise

- SNR is Saied to be Low ( Less details)

Low 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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High Optimal Low

Contrast Changes

IMAGE FORMATION

Image characteristics

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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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True Shape

Elongated

Foreshortened

lengthwidth

IMAGE FORMATION

Image characteristics

Sharpness (SHAPE DISTORTION – TERMS)

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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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Visibility Sharpness

density contrast detail distortion

size shape

Image characteristics

IMAGE FORMATION

Summary

Unit III 23