FRCR: Physics Lectures

Diagnostic Radiology

Lecture 4 Film-screen radiography

Dr Tim WoodClinical Scientist

Overview

• Film-screen radiography

• Processing

• Intensifying screens and the film cassette

• The characteristic curve and sensitivity

• Image quality

The story so far…

• We know how X-rays are made in the X-ray tube and how they interact with the patient

• We know how we control the quality and intensity of the X-ray beam, and hence patient dose, with kVp, mAs, filtration and distance

• We discussed the main descriptors of image quality– Contrast– Spatial Resolution– Noise

• Discussed ways to improve contrast by minimising scatter and using contrast agents

• Remember, there is always a balance between patient dose and image quality – fit for the clinical task!

Film-Screen Imaging

• Traditionally, all X-ray image capture has been through X-ray film

Film base

Emulsion

Emulsion

Adhesive layer

Protective layer

Film• Polyester film base gives mechanical strength to the

film – does not react to X rays• Emulsion consists of silver halide grains (AgBr)

– The image is formed by the reaction of AgBr grains to X-ray photons

– The sensitivity of the film depends on number of grains– Must be evenly distribution– Typically each crystal is about 1 μm in size

• larger grains = more sensitive (contrast), • smaller grain = better resolution

• Adhesive layer ensures emulsion stays firmly attached to base

• Protective layer prevents mechanical damage

Film

• Film is actually much more sensitive to visible light and UV than it is to X-rays– Hence, use a fluorescent screen to convert X-ray

photons to light photons– Enables lower patient dose!

• A latent image is formed upon exposure, which cannot be seen unless the film undergoes chemical processing– Mobile silver ions are attracted to electrons liberated

by light photons, forming a speck of silver metal on the surface

Processing

• The invisible latent image is made visible by processing

• There are three stages to this process;– Development– Fixing– Washing

Processing

• First stage is development:– Film is immersed in an alkaline solution of a reducing

agent (electron donor)– Reduces positive silver ions to metallic grain of silver

(black specks)– Unexposed crystals are unaffected by the developer –

bromide ions repel the electron donor molecules– However, given sufficient time, the developer will

penetrate the unexposed crystals– The amount of background fog is dependent upon the

time, strength and temperature of the developer

Processing

• Second stage is fixing:– If the film is exposed to light after the first stage, the

whole film becomes black– To ‘fix’ the film, unaffected grains are dissolved by an

acid solution, leaving the X-ray image in the form of black silver specks

• Final stage is washing:– The film is washed in water and dried with hot air– Inadequate washing would result in a brown/yellow

film over time (from excess acid) and smell

Processing

• Automatic processors use a roller system to transfer the film through the different solutions

• Regular Quality Assurance of the processor is vital for producing good quality radiographs

• Image is then viewed by transmission of light from a light box with uniform brightness– Dark = lots of X-rays– Light = relatively few X-rays e.g. through bone

Production of a RadiographProcess Time What Happens1. Manufacture Crystals of a suitable size

are made and suspended in gelatine

2. Exposure 0.01 – 10 sec Latent image created

3. Wetting 10 sec Wet film so that subsequent development is uniform

4. Development 3 – 10 min Convert latent image to silver

5. (Acid) wash 1 min Stop development and remove excess developer

6. Fixing and hardening 10 – 30 sec Dissolve out remaining AgBr and harden gelatine

7. Washing 30 sec Remove products of developer and fixer

8. Dry 30 sec Remove water

Logarithms• A logarithm is an exponent – the exponent to

which the base must be raised to produce a given number– 104 = 10x10x10x10 = 10,000– = log1010000 = 4– i.e., 4 is the logarithm of 10000 with base 10

• Seen in many applications– Richter earthquake scale– Sound level measurements (decibels = dB)– Optical Densities blackness on film (OD)

• Written as log10x or if no base specified in physics texts as log x it is interpreted as the same

Properties of logs

• log101 = 0

• log1010 = 1

• log10xy = log10x + log10y

• log10x/y = log10x - log10y

Optical Density

• Optical Density: the amount of blackening in the film

• Defined as the log of the ratio of the intensities of the incident and transmitted light– log is used as the eyes

response is logarithmic

Optical Density

• Optical density can be measured with a densitometer

• From the definition, if 1% of light is transmitted, D = 2.0

• If 10% is transmitted, D = 1.0• The density of an area of interest on a

properly exposure film should be about 1.0– Lung field may be ~2.0

• Areas with D>3.0 too dark to see any detail on a standard light box

Contrast

• Contrast is the difference in optical densities

Contrast = OD1 – OD2

• High contrast - e.g. black and whitewhite• Low contrast – e.g. grey and grey!

Intensifying screens• Film is relatively insensitive to X-rays directly

– Only about 2% of the X-rays would interact with the emulsion

– Requires unacceptably high doses to give a diagnostic image

• An intensifying screen is a phosphor sheet the same size as the film, which converts the X-rays to a pattern of light photons

• The intensity of the light is proportional to the intensity of X-rays

• The pattern of light is then captured by the film– One exception is intraoral dental radiography, where

screens are not practical

Intensifying screens

• Modern intensifying screens use rare earth materials, which emit light that is matched to the sensitivity of the film being used– Spectral match between the emission of the screen

and the absorption in the film e.g. blue or green– K-edges clinically relevant (39-61 keV)

• Rare earth screens used as they very efficient at converted absorbed X-ray energy into light– Results in a ‘faster’ (more sensitive) system

• The sensitive emulsion of the film must be in close contact with the screen

Intensifying screens

• General radiography film usually double coated with emulsion on each side of the base

• The front screen absorbs ~1/3 of X-rays and ~1/2 light travels forward and is absorbed by front layer of emulsion

• Rear screen absorbs ~1/2 of X-rays transmitted through the front and exposes the rear emulsion

• ~2/3 of total X-ray fluence absorbed in screens• Mammography only uses a single screen to

maximise spatial resolution (more on this later)• Screen materials chosen to have no

phosphorescence (delayed fluorescence) to avoid ghost images

The film-cassette

• Flat, light tight box with pressure pads to ensure film in good contact with the screen(s) mounted on the front (and back)

• The tube side of the cassette is low atomic number material (Z~6) to minimise attenuation

• Rear of cassette often lead backed to minimise back scatter (not in mammo)

The characteristic curve

• Plotting OD against log exposure gives the Characteristic Curve of the X-ray film

• Different types of film – subtle differences but all basically the same

Log exposure

Optical density

Fog

Linear region,gradient = gamma

Saturation

Solarisation

The characteristic curve

• Depends on type of film, processing and storage• Fog: Background blackening due to

manufacture and storage (undesirable)– Generally in the range 0.15-0.2

• Linear portion: useful part of the curve in which optical density (blackening) is proportional to the log of X-ray exposure

• The gradient of the linear portion determines contrast in an image and patient exposures must lie within this region– Need to match this to the clinical task!

• Hence, film suffers from a limited and fixed dynamic range

The characteristic curve• Gradient of linear region =

Gamma, = OD2 – OD1

log E2-log E1

• Gamma depends on– Emulsion– Size and distribution of

grains– Film developing

• Gamma ~ Contrast• Latitude = useful range of

exposures

Linear region

Latitude

Log exposure

Optical density

The characteristic curve

• Gamma and latitude are inversely related– High gamma = low latitude– Wide latitude (low gamma) for chests– High gamma (low latitude) for mammography

• At doses above the shoulder region, the curve flattens off at D~3.5– Saturation, whereby all silver bromide crystals have

been converted to silver

• At extremely high exposures density will begin to fall again due to solarization– Not relevant to radiography

Film Speed

• Definition: 1 / ExposureB+F+1

• Reciprocal of Exposure to cause an OD of 1 above base plus fog

• Speed of film = sensitivity = amount of radiation required to produce a radiograph of standard density

• Speed shifts H-D curve left and right• Fast film requires less radiation (lower patient

dose)• Speed is generally used as a relative term

defined at a certain OD; one film may be faster than another at a certain point on the curve

Factors affecting speed

• Size of grains – larger means faster– This is the main factor and conflicts with the need for

small crystals to give good image sharpness.– Fast films are grainier but reduce patient dose

• Thickness of emulsion – Double layers of emulsion give faster films

• Radiosensitisers added• (X-ray energy)

Effect of developing conditions

• Increasing developer temperature, concentration or time increases speed at the expense of fog

• Developer conditions should be optimised for maximum gamma, and minimum fog

• Automatic processor has temperature controls and time maintained by roller speed

• Concentration is controlled by automatic replenishment of the chemicals

Film-screen sensitivity

• Intensification factor– Each X-ray photon generates ~1000 light photons– Just under half of these will reach the film– ~100 light photons to create a latent image– Hence, more efficient process– Intensification factor is the ratio of air KERMA to

produce D = 1 for film alone, to that with a screen– Intensification factor typically 30-100

• Speed class– Most common descriptor of sensitivity– Speed = 1000/K, where K is air KERMA (in μGy) to

achieve D = 1– Typically 400 speed (K = 2.5 μGy)

Image quality

• Contrast– Contrast in film-screen radiography is due to both

subject contrast, scatter and gamma– Remember, high gamma = high contrast = low

latitude (and vice-versa)– Contrast is fixed for any given film and processing

conditions– Image detail may be lost if contrast is too high as it

may be lost in the saturated or fog regions– Hence, vital to match gamma to the clinical task– Ambient light conditions and viewing box uniformity

may also impact on the subjective contrast presented to the Radiologist

• Use a darkened room, mask off unused areas of lightbox, etc

Image quality

• Screen-unsharpness– The film-screen system has inherent unsharpness

additional to geometric, motion and absorption– Only partly due to finite size of the emulsion crystals– Most significant effect is due to spread of light from

the point of X-ray absorption in the phosphor, to detection by the film

– Depends on the point in the phosphor where the interaction occurs

– Thicker phosphor layers more sensitive (absorb more X-rays), but result in more blurring – allow lower patient doses

Screen-unsharpness

Film

Phosphor

Object

Screen unsharpness

• Speed class should be chosen carefully to match the application– e.g. 400-speed (thick phosphor) for thick sections of

the body (abdo/pelvis), – e.g. 100-150-speed (thin phosphor) for extremeties

(require detail) • Also may have reflective layer on top of

phosphor to increase sensitivity (reflect light photons back to the film) at the expense of resolution

• Colour dyes to absorb light photons at wider angles (longer path lengths) – at the expense of sensitivity

Screen unsharpness

• Crossover – light photons from the front screen may be absorbed by the rear emulsion (and vice-versa)– Crossover is a significant contributor to overall

unsharpness– Reason for only using one screen in mammography

where resolution is critical

• Minimise screen-unsharpness by ensuring good contact between the screen and film– Poor contact may result from damage to the film

cassette

Film-screen in clinical practice

• Kilovoltage: Increased kV gives…– Increased penetration = lower patient dose– Increased exposure latitude = larger range of tissues

displayed, BUT lower radiographic contrast– Reduction in mAs = shorter exposures = less motion

blur

• mAs– Correct mAs must be chosen to ensure the

correct level of blackening on the film – avoid under or overexposing the film

• Too much = saturation, too little = ‘thin’ image– Produce standard protocols that can be adapted for

patient size

Exposure Control

• For an acceptable image, require a dose at the image receptor of about 3 μGy for film-screen radiography

• This is the exit dose from the patient after attenuation

• Entrance surface dose (ESD) is much higher than this;– ~10 times greater than exit dose for PA chest– ~100 times greater for skull– ~1000 times greater for AP pelvis– ~5000 times greater for lateral lumbar spine

Automatic Exposure Control (AEC)

• Limited latitude of film makes it difficult to choose correct mAs – skill and experience of radiographer

• Alternative is to use an AEC to terminate the exposure when enough dose has been delivered to the film

• AEC is a thin radiation detector (ionisation chamber) behind the grid, but in front of the film (though in mammo it is behind to avoid imaging the chamber on the film)

• Usually three chambers that can be operated together or individually

Automatic Exposure Control (AEC)

• When a predetermined level of radiation is detected, the exposure terminates

• Choice of chambers determined by clinical task– e.g. left and right for lungs in PA chest, but central if

looking at spine

• Also has a density control that can increase or decrease exposure where necessary

• AEC limited to exposures in the Bucky system

Modern Day

• Film is dying out• Across most (but not all) of the country film

is no longer used for General X-ray imaging

• Only mammography (breast imaging), where very high resolution specialist film is used– This Trust no longer uses film for

mammography, and is on the verge of being fully digital…