Sensitometry

Describing photographic performance

Objectives

The student should be able to: Describe the importance of the study Define the relevant terms Describe the methods of producing a sensitometric strip Plot the characteristic curve Describe the Features of the characteristic curve Describe the importance & practical applications

Why is the study important?

To produce images with optimum contrast that reveal high details of the object examined.

Optimum contrast?

Low contrast (long scale contrast) High contrast (Short scale contrast)

Photographic images

Optimum contrast

High contrast

Low contrast

X-ray images

Introduction

Photosensitive materials (x-ray films) are used to record the invisible x-ray image It is required to reproduce all the characteristics of the invisible image visible form.

in The films ability to do so depends on its sensitometric properties.

It is advantages to have a sound knowledge of sensitometric properties of x-ray films.

X-ray tube Plot of incident x-ray beam intensity Invisible x-ray image Object Plot of transmitted x-ray beam intensity

Invisible x-ray image

kV mA Sec FFD E B2 Supporting tissue (m) T1 T2 T3 Air E M E B1 E B2 E T1 E M E T2 E T3 E A

Invisible X-ray image

What is Sensitometry?

The scientific study of the response of photosensitive material to different levels of exposures.

How is it done?

Producing a sensitometric strip and plotting a characteristic curve.

What is Response of exposure & How is it manifested?

 The response is the change that takes place, after exposure to electromagnetic radiation (light or x-rays), in the photosensitive emulsion on the film  The response of the film to exposures is manifested as a degree of blackening produced after chemical processing

How is degree of blackening quantified/measured?

 In Radiography the degree of film blackening is quantitatively indicated by the term ‘Optical Density’.

 The optical density describes how much a certain area of the film is opaque to light incident upon it .

Optical Density

The optical density is expressed quantitatively as, Optical Density = Log 10 Opacity It is measured by using the ‘Densitometer’ (The densitometer works on the following principle)

Consider the light transmitted through an area of a film Incident light intensity (I

i

) Transmitted light intensity (I

t

) Transmittance (Transmission ratio)= I

t

/ I

i

Opacity = 1 / Transmittance = I

i

/ I

t

Density = Log 10 Opacity = Log 10

I i

/ I

t

Range of densities on a film

I i 100 0.01

0.1

1.0

10.0

100 I t Transmit tance 0.0001

0.001

0.01

0.1

1 4 3 2 1 0 Opacity 10000 1000 100 10 1 Density 4 3 2 1 0

Sensitometric strip

A film containing number of areas with different optical densities from white (fully transparent) to black (fully opaque)

How to produce a sensitometric strip and the characteristic curve?

1.Expose a film to different amounts of known exposures starting from a minimum and increasing at a known rate (ratio / wedge factor) up to a maximum.

2. Process the film 3. Measure the densities 4. Tabulate the result & Study the response by plotting a curve (density Vs Log relative exposure

Methods of exposing

Time scale method (using x-ray exposure) Intensity scale method i. Using x-ray exposure ii. Using light exposure

Time scale method

The film is exposed to different quantities exposures using constant intensity and variable duration.

of (Quantity = Intensity x Time) Equipment required X-ray machine Cassette with film inside Lead sheets Processor Densitometer Graph paper

Procedure (time scale method)

X-ray tube Constants kV, mA, FFD Variable Time Loaded cassette Loaded cassette Areas to be exposed Lead sheets

Exposure selection

Minimum exposure Low enough not to produce a measurable density Maximum exposure High enough to produce a density around 3.0

Increment Wedge factor (Ratio between two exposures) of 2 is adequate. (2 ½ can be used to get more levels) Alternatively the time steps available in the machine may be used to get more points on the graph

Intensity scale method 1

Making a single exposure using a calibrated wedge step X-ray machine a loaded cassette (The intensity of x-rays passing through the steps are different & the duration of exposure is the same)

Intensity scale method 2

The film is exposed to a series of different intensities of light for the same duration using an instrument called the “ Sensitometer ”. The wedge factor is usually 2 ½ The colour of light should match the spectral sensitivity of the film

Film obtained using time scale method

Plotting the curve

Area Exposure (Time) A 0.01

B 0.02

Relative exposure 1 Log relative exposure Density 0 0.25

2 0.3

0.3

C D 0.04

0.08

4 0.6

8 0.9

0.4

0.9

Plotting the curve

Area Exposure (Time) 0.16

0.32

0.64

1.28 2.56

Relative exposure Log relative exposure Density E 16 1.2

1.45

F 32 1.5

2.1

G 64 1.8

2.5

H 128 256 2.1

2.7

I 2.4

2.9

GF 1 4 3 2 Characteristic

curve

Shoulder

D Max Straight line portion

Toe 1 2 3 4

Log relative exposure GF = Gross Fog

1.50

1.00

0.50

GF 0.00

3.00

2.50

Shoulder 2.00

Characteristic curve D-Max Straight line portion Toe

Log Relative exposure

(Features) Information obtainable

Gross fog (Basic fog) Threshold Contrast Latitude (film latitude & exposure latitude) Speed & Sensitivity Maximum density Reversal

Gross fog (Basic fog) & net density

This is the density of the horizontal part of the curve at the minimum exposure level

Gross fog

= Base Density +

Fog

Base Density :- Density produced by the base material

Fog

:- Density produced by the development of silver halide crystals which have not received an intentional exposure Net density = Gross density – Gross fog

4 3 2 Characteristic curve (with net density) Shoulder

Net density = gross density – gross fog

1 0 Toe 1 2 3 Log relative exposure 4

Threshold

The region where the film emulsion begins to respond to the exposure

Contrast

The rate of change of density for a given change of log relative exposure Contrast = ΔD / ΔE It is given by the slope (gradient) of the straight line portion of the curve. If it is a true straight line then the contrast is called Gamma.

Since, in practice, the curve is not an exact straight line, the average gradient is taken as the contrast.

3.00

2.50

2.00

1.50

ΔD 1.00

0.50

0.00

Characteristic curve ΔE Contrast = ΔD/ ΔE

Average gradient

4 3 DY 2 D X 1 B

Average gradient = BC/AC D log E Y Y - D X = ---------- – log E X How to select points A and B ?

A C Log E

X

1 Log E

Y

2 3 4 Log relative exposure

Point A ? Point B ?

A : D X = 0.25 above Basic fog B : D Y  = 2.0 above Basic fog The densities from 0.25 to 2.0 is called the Useful Density range.



Useful density range is the density range in which the differences can be identified by the human eye .

 The densities which represents different structures on a radiograph should lie within this range of densities.

Useful density range Average Gradient

=

Range of log relative exposures that produces the useful range of densities  Useful density range is the range of densities within which the human eye can recognize the small differences  That is the range of net densities from 0.25 to 2.0

Latitude Latitude is an expression of the tolerance of a system to extreme conditions of exposure.

It refers to the ability of a film or film-screen system to record successfully a wide range of exposure . (considered in two parts) Film latitude & Exposure latitude Film Latitude The difference between the upper and lower limits of log relative exposure which produce densities within the useful range

Significance of film latitude

The range of x-ray intensities transmitted through the body part should lie within the film latitude, if they are to be viewed as useful densities on the radiograph.

Any x-ray intensity that falls out side the film latitude will not reveal any information & a useful piece of information might be lost

Film latitude & Average gradient

4 3 DY 2 D X 1 Log E X A 1 Log E Y Film latitude C B

When Dx = 0.25+BF & Dy = 2.0 +BF D Y Average gradient = ---------- becomes log E Y - D X – log E X

2

1.75

= ----------------- Film latitude

3 4 Log relative exposure

Exposure latitude

This refers to the freedom of the radiographer to select slightly different exposures (to make room for errors) for a particular examination so that the resulted densities remains within the useful density range.

Exposure latitude = film latitude – subject contrast ( log relative exposure range transmitted from a particular body part)

Subject contrast

kV mA Sec FFD E Air E 1 E 2 E 3 E 4 E 5 E 6 E 7

Transmitted intensities Relative Intensity Log relative intensity

E 1 RE 1 RE 2 e 1 E e 2 2 E 3 RE 3 e 3 E 4 RE 4 e 4 E 5 RE 5 e 5 E 6 RE 6 E 7 RE 7 e 6 e 7

e 1 e 7 Subject contrast

Image contrast

e 1 e 7

D1 D2 D3 D4 D5 D6 D7

3.00

Characteristic curve Exposure latitude = film latitude – Subject contrast e 1 ≥ X ; e 7 ≤ Y 2.50

2.00

1.50

1.00

0.50

0.00

X Film latitude Y Subject contrast Exposure latitude

Speed & Sensitivity

 Sensitivity refers to the exposure required by a film or film-screen system to produce a net density of 1.  Sensitivity is expressed in miliroentgens (mR)  A high sensitive (have low mR value for sensitivity) or high speed system requires less exposure than that of a low sensitive or low speed system.

Speed Numerically the Speed is proportional to the reciprocal of the sensitivity (mR) and is expressed as 128 Speed = ------------- Sensitivity (mR) 1200 800 400 200 100 50 25 12 Sensitivity (mR) 0.1

0.16

0.32

0.64

1.28

2.56

5.0

10.0

BF 4 Comparison of Speeds of two films (film screen systems) A B 3 2 1 1+BF

Speed A > Speed B Speed A α 1/ E SA Speed B α 1/ E SB Speed A E Speed B E SB ------------- = ----- SA

1 Log E SA 2 3 Log E SB 4 Log relative exposure

As Log (E SB / E SA ) = Log E SB - Log E SA Taking logarithms on both sides Log(

Speed A / Speed B)

= Log E SB - Log E SA If Log E SA = a , and Log E SB = b Log(

Speed A / Speed B)

= b – a Then

Speed A / Speed B = antilog (b-a)

Maximum Density (D

Max

)

The density produced when all the silver bromide crystals in the emulsion is exposed and developed

Reversal

This is the region where the density reduces with the increasing exposure greater than that produce D Max

BF 1 4 3 2

D

Max

& Reversal

D Max Reversal 1 2 3 4 Log relative exposure

Spectral Sensitivity

This is the term used to describe 1

Monochromatic film

Orthochro matic film the response of the film to the range of wavelengths (spectrum of colours) of light 0.5

300 400 500 600 700

Wavelength (nm)

Uses of the characteristic curve

Information

Gross fog (Basic fog) Threshold Contrast Latitude (film latitude & exposure latitude) Speed & Sensitivity Maximum density Reversal

Uses

Selection of films Assessing processor performance (Quality control) Selection of exposure factors Comparison of film screen systems Duplication of radiographs

Summary

Definitions of terms Methods of producing a sensitometric strip Plotting the Characteristic curve Features of the characteristic curve The practical applications

Thank You

V.G.Wimalasena, Principal, School of Radiography