Principles of Imaging

Scatter Radiation

Field Size

It affects the level of Scatter Radiation and is controlled by the radiologic technologist.

  • increased filed size, increased SR

Patient thickness

thicker (denser areas) results in more SR.

  • normally, patient thickness is not controlled by the RT.

CONTROL OF SR

Effects of SR on image contrast

CONTRAST

  • the visible difference between the light and dark areas of an image.

  • is the degree of difference in optical density(OD) between areas of a radiographic image.

CONTRAST RESOLUTION

  • is the ability to image and distinguish soft tissues.

Beam Restrictors

3 types of beam-restricting devices are:

  • Aperture Diaphragm

  • Cones or cylinders

  • variable-aperture collimator

Aperture diaphragm

  • is the simplest of all beam-restricting devices.

  • it is basically a lead or lead-lined metal diaphragm that is attached to the x-ray tube head.

Cones and Cylinders

  • a radiographic extension

  • are considered modifications of the aperture diaphragm.

  • most commonly used type is the cylinder.

  • cones are routinely used in dental radiography.

Variable- aperture collimator

  • is the most commonly used beam restricting device.

  • COLLIMATION REDUCES THE PATIENT RADIATION DOSE AND IMPROVES CONTRAST RESOLUTION

  • LIGHT LOCALIZING APERTURE COLLIMATOR - is a typical variable - aperture collimator and is accomplished with a small lamp and mirror.

  • COLLIMATION FILTRATION - may be necessary to produce high quality radiographs with minimum patient radiation dose.

  • AL (ALUMINUM) IS THE COMMON FILTRATION (1mm)

  • TOTAL FILTRATION = INHERENT FILTRATION+ ADDED FILTRATION

Radiographic Grids

An extremely effective device for reducing the level of scatter radiation that reaches the image receptor.

  • opaque material (grid strips)

  • radiolucent material (interspace material)

GRID- is positioned between the patient and the image receptor.

GRID RATIO - is the height of the grid divided by the interspace width

GRID FREQUENCY - is the number of grid strips per centimeter

  • specially designed grids are used for mammography.

LEAD

  • is most widely used because it is easy to shape and is relatively inexpensive.

  • its high atomic number and high mass density make lead the material of choice in the manufacturer of grids.

GRID TYPES

PARALLEL GRID

  • the simplest type of grid

  • all lead strips are parallel

  • is the easiest to manufacture, but it has some properties that are clinically undesirable, namely GRID CUT OFF, the undesirable absorption of primary x-rays by the grid

  • most pronounced when the grid is used at a short SID(source-to-image receptor-distance) or with a large-area image receptor.

  • clean up scatter radiation in only one direction along the axis of the grid

CROSSED GRID

  • have lead grid strips that run parallel to the long and short axes of the grid.

  • usually fabricated by SANDWICHING TWO PARALLEL GRIDS together with their grid strips perpendicular to one another.

  • more efficient than parallel grids in cleaning up SR.

MAIN DISADVANTAGE OF PARALLEL AND CROSSED GRIDS IS GRID CUT OFF

FOCUSED GRID

  • designed to minimize grid cut off

  • lead grid strips of a focused grid lie on the imaging radial lines of a circle centered at the focal spot, so they coincide with the divergence of the x-ray beam.

MOVING GRID

  • Potter-Bucky diaphragm (bucky for short)

  • an obvious and annoying shortcoming of the grids is that they can produce GRID LINES on the image.

  • GRID LINES- are the images made when primary x-rays are absorbed within the grid strips are very small, their image is still observable.

  • are placed in a holding mechanism that begins moving just before x-ray exposure and continues moving after the exposure ends.

Screen film Radiography

Radiographic film

Fog- radioactive fallout

2 parts of radiographic film:

  • base

  • emulsion

DOUBLE-EMULSION FILM- coated both sides

ADHESIVE LAYER

  • between emulsion and the base is a thin coating material.

  • ensures uniform adhesion of the emulsion to the base.

  • allows the emulsion and the base to maintain proper contact and integrity during use and processing

OVERCOAT

  • a gelatin enclosed by a protective covering

  • protects the emulsion from scratches, pressures, and contamination during handling, processing, and storage.

BASE

  • is the foundation of radiographic film.

  • provide a rigid structure onto which the emulsion can be coated.

  • is flexible and fracture resistant

EMULSION

  • is the heart of radiographic film.

  • it is the material with which x-rays or light photons from radiographic I.S. interact.

  • consists of a homogeneous mixture of gelatin and silver halide crystals (SHC)

SILVER HALIDE CRYSTAL

  • is the active ingredient of the radiographic emulsion.

  • 98% of silver halide is silver bromide

  • and the remaining is usually silver iodide.

Radiographic film is manufactured in total darkness, meaning from the moment the emulsion ingredients are brought together until final packaging, no light is present.

types of film

SCREEN FILM

  • most commonly used film that is customarily

  • is the type of film used with radiographic I.S.

SPECTRAL RESPONSE

  • radiologic technologist must particularly careful to use a film whose sensitivity to various colors of light

  • properly matched to the spectrum of light emitted by the screen (spectral matching)

CALCIUM TUNGSTATE SCREEN (MONOCHROMATIC)

  • emit blue and blue-violet light

ORTHOCHROMATIC

  • green-sensitive film

PANCHROMATIC FILM

  • used in photography and is sensitive to the entire visible light spectrum (ALL COLORS)

SAFELIGHT

  • provide enough light to illuminate the darkroom while ensuring that the film remains unexposed

DIRECT- EXPOSURE FILM

  • certain films were manufactured for use without screens

  • direct x-ray interactions

MAMMOGRAPHY FILM

  • all currently available mammography screen film systems use green emitting terbium-doped GADOLINIUM oxysulfide screens with green sensitive film

  • ANTIHALATION - surface of the base opposite the screen is coated with a special light absorbing dye to reduce reflection of screen light

FORMATION OF LATENT IMAGE

formation of the latent image, sometimes called the photographic effect.

  • Gurney-Mott theory, explains the latent image formation

SILVER HALIDE CRYSTALS

  • the silver, bromine, and iodide atoms are fixed in the CRUSTAL LATTICE in ion form

Radiographic I.S.

Less than 1% o the xrays incident on radiographic film interacts with the fikm and contributes to the latent image.

PROTECTIVE COATING

  • the layer of the radiographic I.S. closets to the radiographic film

  • thick and is applied to the face of the screen to make the screen resistant to the abrasion and damage caused by handling

  • helps to eliminate the buildup of static electricity

  • is transparent to light

PHOSPHOR

  • active layer of the radiographic I.S.

  • emits light during stimulation by xrays

  • 1980 CALCIUM TUNGSTATE embedded in a polymer matrix

  • RARE EARTH ELEMENTS : GADOLINIUM, LANTHANUM, AND YTTRIUM are the phosphor material in newer, faster screens.

REFLECTIVE LAYER

  • between the phosphor and the base

  • made of shiny substance such as magnesium oxide or titanium dioxide

  • when x-rays interact with the phosphor, light is emitted ISOTROPICALLY

ISOTROPICALLY- refers to radiation emmited with equal intensity in all directions

BASE

  • layer farthest from the radiographic film

  • serves principally as a mechanical support for the active phosphor layer

POLYESTER - is the popular base material

Image Noise

  • appears on a radiograph as a speckled background

  • QUANTUM MOTTLE - a principal component of image noise

SPATIAL RESOLUTION

Radiographers often use the term IMAGE DETAIL or VISIBILITY OF DETAIL when describing image quality

  • refers to how small and object can be imaged

CONTRAST RESOLUTION

  • refers to the ability to image similar tissues, such as the liver and the pancreas or gray matter and white matter

CASSETTE

  • is the rigid holder that contains the film and the radiographic I.S.

COMPRESSION DEVICE

  • between each screen and the cassette cover

  • suchh as radiolucent plastic foam, which maintains close screen film contact when the cassette is closed and latched.

CARE OF SCREENS

- ANTISTATIC - special screen cleaning materials

FILM PROCESSING

  • approximately 1 hour to prepare a completely dry and ready to read radiograph

  • PAKO introduced the first automatic processor (1942) and could process 120 films per hour with the use of special film hangers.

  • 1956 EASTMAN KODAK COMPANY introduced the first roller transport system

WETTING

  • first step in processing

  • to swell the emulsion

DEVELOPING

  • second step

  • stage of processing during which the latent image is converted to a visible image

  • very short and highly critical

  • after developing, the film is RINSED in an acid solution designed to stop the developing process. STOP BATH

FIXING

  • to fix the silver halide that was not exposed to radiation is the process of clearing it from the emulsion and hardening the emulsion to preserve the image.

WASHING

  • to remove any remaining chemicals from the previous processing steps

DRYING

  • to remove the water used to wash it and to make the film acceptable for handling and viewing

X-RAY TUBE

X-ray tube

  • a component of the x-ray imaging system rarely seen by the RT

DIODE

  • vacuum tube with two electrodes — a cathode and an anode

  • x-ray tube is special type of diode

EXTERNAL COMPONENTS

CEILING SUPPORT SYSTEM

  • probably the most frequently used

  • consists of two perpendicular sets of ceiling mounted rails.

FLOOR- TO- CEILING SUPPORT SYSTEM

  • has a single column with rollers at each end, one attached to a ceiling mounted rail and the other attached to a floor mounted rail.

C-ARM SUPPORT SYSTEM

  • interventional radiology often are equipped with C-arm support system, so called because the system is shaped like a C.

PROTECTIVE HOUSING

  • X-rays are produced, ISOTROPICALLY, with equal in all directions

  • WINDOW- a special section of the x-ray tube where only x-rays emitted through.

  • USEFUL BEAM- these are the x-rays emitted through the window

  • LEAKAGE RADIATION - where x-rays escapes through protective housing

INTERNAL COMPONENTS

CATHODE

FILAMENT

  • is a coil of wire similar to that in a kitchen toaster, but it is smaller.

  • approximately 2mm in diameter and 1-2 cm long

  • usually made of THORIATED TUNGSTEN

  • is embedded in a metal shroud called FOCUSING CUP

THERMIONIC EMISSION

  • a phenomenon which atoms are “boiled off” and ejected from the filament.

ANODE

2 types of anode:

  • stationary

  • rotating

STATIONARY ANODE

  • used in dental x-ray imaging systems, some portable imaging systems and other special-purpose units in which high tube current and power are not required

ROTATING ANODE

  • allows electron beam to interact with a much larger target area

INDUCTION MOTOR

  • it is an electromagnetic and is used to turn the anode