MAGNIFICATION(MACRORADIOGRAPHY/MICRORADIOGRAPHY)DISTORTION

PARTHA JYOTI DAS

MSc MIT

Contents

Macroradiography

Principles

Magnification with fixed FFD

Magnification with fixed FOD

Unsharpness with macroradiography

Applications

Microradiography

Mass miniature radiography

Distortion

References

The technique of producing an image by direct

magnification is called macroradiography.

Enables fine anatomical details than the original

and accurate diagnosis.

Macroradiograp

hy

Principles

Magnified image can be produced

by increasing the object to film

distance in which the x rays

diverging from the point source

will produce a directly magnified

image.

Relation:

The

relationship

between

size of

object and

it’s image.

FFD

FOD

OFD

Image Size

X-ray Focus

Film

Plane

Object

Plane

Principles

Magnification (M) can be calculated by the

following formula:

focus to film distance (FFD)

focus to object distance(FOD)

. The focus to object distance (or object to film

distance) is taken from the mid level or part.

M =

MAGNIFICATION WITH FIXED

FFD

Magnification is increased by bringing the object nearer to the x ray tube

FOD = FFD/Magnification.

For eg : With a fixed FFD of 100cm, if a magnification factor of 1.6 is required then;

FOD = 100/1.6 = 62.5 cm

MAGNIFICATION WITH FIXED

FOD

The required magnification is obtained by

moving the film away from the object.

The required OFD for a given magnification is

then calculated from

OFD = FOD(M – 1)

Eg. FOD is fixed at 100cm , given magnification

factor 1.6

OFD = 100(1.6 – 1) = 60cm

UNSHARPNESS WITH

MACRORADIOGRAPHY

Movement Unsharpness

Geometric Unsharpness

MOVEMENT UNSHARPNESS

Direct magnification by increased object to

film distance can be carried out if there is

complete immobilization of the patient

Any movement due to lack of immobilization or

involuntary movement will be magnified on the

radiograph due to increased OFD

Movement

unsharpness

Some steps

Use of immobilization devices, e.g. supports,

binders,sandbags and pads.

Instructions should be given to the patient to

remain still.

GEOMETRIC UNSHARPNESS

Occurs because the source of x rays is not a

point source and any distance between the

object and film will cause an image penumbra

in addition to magnifying the image

The geometrical unsharpness for given focal

spot size calculated by

Focal Spot Size x Object-Film

Distance

Ug =

Focus-Object Distance

Geometric unsharpness

How to minimize Ug:-

Focal Spot size:- By reducing Focal Spot

Size.

Object image Distance:- By reducing object

image distence.

Focus object Distance:-By increasing the

Focus object distence.

Relationship between focal spot

size and magnification

For a given focal spot size there is a limit to

magnify the image without significant loss of

details.

APPLICATIONS

Limited use of macroradiography

Dacrocystography of Skull (Lacrimal ducts after injection of contrast)

Imaging the carpal bones in cases of suspected fracture of scaphoid

Localised areas of the lung

Localised areas during angiography eg: cerebral angiography

Macroradiograph of wrist with a magnification

factor of 2

Microradiography

An imaging technique used to examine

very small objects or minute details, often

with the use of high-voltages, very small

focal spot size, and an ultrafine film

emulsion.

Ultrafine Emulsion film

Photo print made by these films performs rich

sharpness, high definition, strong scratch

resistance, as well as long lasting by pigment

Microradiography

It is also called as chest fluoroscopy.

The image on a fluorescent screen is photographed on a small film. Screen and film are separated and the image is focused on the film by a camera lens

Miniature radiographs (70mmx70mm or 100mmx100mm roll or cut films were used).

A permanent record is possible.

Mass Miniature Radiography(MMR)

In order to control tuberculosis, medical research council, London appointed their Committee on Tuberculosis in 1941.

The detection of symptomless or latent pulmonary tuberculosis by mass radiography had been the subject of much careful investigation.

An excellent instrument was designed by the council for this purpose and in 1943 the Ministry of Health announced that a limited number of mobile miniature radiography sets.

The equipment consist of

Fluorescent screen(emits blue or green light)

A mirror optics system on recording exposure

on film

Survey in progress

Control Panel

Apparatus for MMR

1.Power unit:- High tension unit

Four valve rectification

An output of from 100 to 400mA

A maximum kilovoltage of 100 kVp and 91 kVp

respectively

2. X-ray Tube

The rotating anode X-ray tube

Dual focus, Focal-spot sizes are 1mm and

2mm square.

An optical centring device.

Adjustable diaphragm

3.Fluorescent Screen and Grid:

16 by 16 inches fluorescent screen

Covered by a sheet of protective lead glass.

Yellow-blue or green type of screen

A stationary grid is also present which reduces scatter

4.The Camera.:

An electrically operated camera is linked with the X-ray exposure switch.

Lead glass protection is present on the camera side of screen

5.The lens: a number of different lenses

Developing MMR films

Special tanks meant for roll films on a spiral frame

Development time was 7min at 68 degree F

Viewing of MMR FILMS

Projector for viewing miniature radiographs

Magnifying glasses

DISTORTION

DISTORTION

Misrepresentation of the true size and

shape of an object is called distortion.

Distortion results from unequal

magnification of different parts of the

same object.

Foca

l

Spot

Film

•The object AB is

tilted with respect to

the plane of the film.

•The size and shape

of image vary with

the amount of tilting.

Distortion

A

B

A B

Distortion of the image of an object will be

different in different parts of the x-ray beam.

Distortion of thick object occurs if they are

not directly in the central part of the x-ray

beam.

When the part to be imaged does not lay parallel

with the IR (cassette).

If the Central Ray is not perpendicular to the part.

Caused by improper alignment object with relation to tube focus and film .

Distortion of an object will be different in different part of the x-ray beam.

Elongated Foreshortened Normal

GOOD RADIOGRAPHS

MAKES EVERYONE HAPPY

REFERENCES

CHESNEY’S RADIOGRAPHIC IMAGING, 6TH EDITION JOHN BALL, TONY PRICE HDCR

CLARK’S POSITIONING IN RADIOGRAPHY, 12TH

EDITION A. Stewart Whitley Charles Sloane Graham Hoadley Adrian D. Moore

Chrissie W. Alsop

.

Thank You