Chapter 1 Intro to Imaging Sciences

Chapter 1 Intro to Imaging Sciences

RADIOGRAPHIC IMAGING SCIENCES

Introduction to Radiographic and

Imaging Sciences

• What is an image?
• An image is a recognizable pattern carrying useful
• information.
• An images can be;
• Real images; those having a real, physical
• existence e.g. photographic and radiographic
• images (x-ray images).
• Mental image; those generated as mental
• pictures.
• Production of an x-ray image
• Production of a radiograph (x-ray image on a film)
• involves three stages; The formation of an invisible x-ray image
• (latent image)
• The conversion of the invisible x-ray image
• into a visible light image.
• ‒ This is done through chemical processing
• of the film.
• The viewing, perception and interpretation of
• the visible image.
• ‒ This is done by the aid of a lightbox.
• How is an image recorded on a film?
• The traditional way in which x-rays images are recorded
• is by using photographic effect of X-radiation on
• sensitive emulsion.What is photographic effect?
• Photographic effect refers to the structural changes of
• some chemical compounds when they are exposed to
• electromagnetic radiation.
• The electromagnetic radiation can be visible light,
• ultraviolet light or x-rays.
• These chemical changes are usually invisible but they
• are made visible through chemical processing.
• NB; Photographic development; is the chemical
• processing necessary to make the invisible changes
• visible on a photosensitive material.What is a photosensitive material?
• Photosensitive material refers to any material which
• undergoes chemical changes when exposed to
• electromagnetic radiation.
• The photosensitive materials used in radiography are

known as silver halides.

Silver Halides

• The silver halides are a group of chemical compounds
• consisting of atoms of the element silver combined with
• atoms of the halogen elements; bromine, iodine and
• chlorine.
• The compounds thus formed are silver bromide, silver
• iodide and silver chloride respectively.
• These compounds are used because they are sensitive
• to light and x-rays.
• When they are exposed to radiations they undergo
• changes which enable them to form a photographic or
• radiographic image.
• Physical properties of silver halides
• They are white or pale yellow crystalline salts similar in
• appearance to common salt.
• The links between silver and halogen atoms are ionic
• bonds.
• NB; Ionic bonds occur when positive (+) and negative (-)
• ions are locked together by electric forces.
• In silver halides, the silver ions are positively charged

(+) and the halide ions are negatively charged (-).The electric forces fix the positions of silver and halide ions in a regular three dimensional crystal structure or lattice.Chemical properties of silver halides

• Pure silver halide crystals are chemically stable and do
• not easily suffer chemical breakdown.
• Under certain conditions, it is possible to neutralize all
• the positively charged silver ions in a crystal and
• convert them to atoms of metallic silver.
• This conversion is achieved by donating –vely charged
• electrons to the silver ions.
• A chemical which provide a supply electrons for this
• purpose is called a reducing agent.
• NB; A reducing agent; is a chemical which provides a
• supply of electrons in order to reduce silver halide to
• metallic silver.
• The chemical reduction of a silver halide sample which
• has been exposed to radiation takes place much more
• rapidly (because of weakened bonds) than the
• reduction of an unexposed sample.NB; In photographic development, what actually takes
• place is chemical reduction of silver halide to metallic
• silver.
• ⸫ Photographic development is primarily a process of
• chemical reduction.
• What happens when a silver halide is irradiated?
• Invisible structural changes occur in silver halide
• crystals when exposed to radiation.
• The following image shows a regular arrangement of
• silver and bromine ions in a crystal.
• What is a sensitivity speck?
• A sensitivity speck refers to a low energy electron trap
• in a silver halide crystal.
• It is where electrons collect after exposing the halide
• crystal to radiation.
• NB; Sensitivity specks are formed by deliberate
• introduction of ‘impurities’ into the crystal during its
• manufacture.
• What is a latent image?
• A latent image is an invisible image formed on a film
• after exposure to radiation.
• A latent image is later made visible through
• photographic development.
• In a latent image, very few exposed silver halide
• crystals have been completely reduced to metallic
• silver. To complete the process, photographic development is
• required. Only after completing this process can the
• image become visible.
• Differences between silver halides and metallic
• silver
• NB; the conversion from silver halide to metallic silver in
• exposed part is important because the two substances
• have different properties;
• Silver halides are sensitive to light while metallic silver
• is not.
• Metallic silver is opaque to light and is seen as a
• darkened area. The blackened parts of a radiographic
• image are due to metallic silver. Silver halides can be converted into soluble compounds
• by the action of chemicals known as fixing agents.
• NB; an image which contains silver halide can undergo
• changes whenever exposed hence it cannot be a
• permanent image. An image formed only by metallic
• silver cannot undergo any changes even when exposed
• again hence it is a permanent image.
• Light sensitive and X-ray sensitive films
• Light sensitive films are films in which production of an
• image is through exposure to visible light.
• X-ray sensitive films are films in which production of an

image is through exposure to x-rays.