exam 4 -- introduction to digital radiography

conventional films

film, intensifying screen/film, not reusable

film system

• image detector: film inside holder (cardboard)

• different sizes, different features

• includes: image requisition (film exposure), wet processing (dark room), lightbox for displaying, and physical storages

photosensitive film

• first image receptor chosen by Rontgen

• no longer most common image receptor because of predominance of digital imaging in 21st century

• basic knowledge of film/screen technology still required of radiographers

• original base: glass

  • plates coated with emulsion — light sensitive material

    • difficult maintenance

• exposure factors with plates very high

• origin of term “flat plate” of abdomen

• base of today’s modern film

  • modern base is thin sheet of polyester

film as a photographic medium

• very long history in world history timeline

• radiographic and photographic films similar

• photosensitive

  • that is why we must handle films in dark room for loading and unloading before fully processed

photosensitive

be able to respond to visible light and x-ray

radiographic film construction

base, adhesive layer, emulsion (mixture) with crystals, supercoat

radiographic film base

• foundation of radiographic film

  • flexible, yet durable

  • uniformly radiolucent

  • film must retain its size and shape throughout use and processing

  • modern base is thin sheet of polyester

specializations of film base

• blue tint:

  • reduces eyestrain

  • increases diagnostic accuracy for reading

• other coats:

  • minimizing background noise

emulsion

• “heart” of film

• composed of silver halide crystals suspended in gelatin

  • active ingredient photosensitive

    • silver halide crystals

      • silver bromide

      • silver iodide

      • silver chloride

    • allows for collection of many silver atoms in one area after being exposed to x-ray or light photons

latent image

raw information before processed and displayed

• invisible before wet processing

latent image formation

• x-ray and light photons deposit energy into silver halide crystals

  • both x-rays and lights affect films

• silver are formed after exposed to visible light or x-ray

  • the amount of silver is proportional to x-ray exposure

intensify screen with film

• film sandwiched between two intensifying screens in a light-tight cassette

• creates a film/screen combination and works as a system, using photographic properties of film

• film is the same as film only systems

intensify screen

• fluorescent screen: phosphor coated screen

• amplify beam to reduce patient dose

• x-rays convert to light

  • 99% of image comes from light conversions

  • 1% of image comes from direct interaction between x-rays and film

• need to consider the type of phosphor (type of screen

  • light sensitivity and image quality

• developed by Edison (1896)

construction of intensify screen

base, reflective layer, phosphor layer, protective coat

phosphor layer of intensify screen

• active layer of intensifying screen

• contained crystals that absorb x-rays and give off light

• anatomic information is transferred to light from transmitted x-ray

screen speed classification

• relative-speed number (RS)

  • par speed

  • high speed

  • fine/detail

• also need to consider film speed

  • based on sensitivity to light

purpose of film processing

• called wet processing

• make latent image visible

• hard copy:

  • physical films including copies

• soft copy:

  • images displayed on monitors

film processing

film processing involves 4 steps:

1. developing

2. fixing

3. washing

4. drying

storage during film processing

• humidity

• length of storage dependent on state law and institution

  • usually 5-7 years

  • minor and legal cases indefinite

automatic processors

• first introduced by Kodak (1957)

• utilizes subsystems:

  • transport

  • dryer

  • replenishment

  • circulation

  • temperature control

densitometer

• uniform light source and optical sensor

• calibration control allows for easy calibration

• film placed over optical sensor and optical density measured

  • displayed as a number

• reads on scale of 0-4

  • 0 means all light transmitted

  • 4 means no light transmitted through film

• typical diagnostic densities ranges between 0.25 and 2.5

optical density (OD) numbers

if 100% of light transmitted through film, OD is 0

if none of light transmits through film, OD is 4

• opacity is the ability of film to stop light transmission

D log E curve

• describes relationship between film density and exposure

• done through sensitometry

• also known as:

  • sensitometric curve

  • characteristic curve

  • hurter and driffield (H & D) curve

• sensitometry’s roots lie in analysis of photographic film

sensitometric curve

• shape: sigmoid shape (s-shape)

• plots OD versus log relative exposure (LRE)

• LRE

  • allows large range of exposure displayed in a few numbers

parts of the curve

• base + fog

• Toe-D_min

  • where density becomes light

• straight line portion

  • used to demonstrate relationship to film’s exposure versus density transmitted

• shoulder-D_max

  • where film density becomes dark

  • blacks are truly black and cannot get blacker

exposure

• film property

• the total darkness of the film

  • in digital system: it is not accurate definition: we use IR exposure

  • inherited

    • dark = more exposure

    • white = less exposure

contrast

• film property

• the difference between adjacent densities

  • in digital imaging: different densities — different grayscales

    • can be quantified — sometimes is done by radiologists for comparison

    • dark = more exposure

    • white = less exposure

    • EI is more accurate reading