Radiographic Imaging- Chapter 2

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Processing the Latent Image


The Latent Image:

It is the invisible image formed as a result of exposure to radiation and
which may be visible by photographic development.


Film Processing:

Before the routine use of automatic film processing in radiography, x
-
ray
films were hand processed. In hand processing, the expose film is first
immersed in a tank containing
developer

and then immersed in
a stop bath
,
followed by immersed in
a fixer
. The

film is
washing

in running water and
hung to drip
dry
.




Developer


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Sequence of events in a radiograph:

Films can be processed with manual dipping or with automatic film
processors.




Automatic

Manual

Purpose

Step

---

15 s

Swells the emulsion for better chemical
penetration.


Wetting

22 s

5 min

Produces a visible image from the latent
image.


Developing


---

30 s

Terminates development and removes

excess chemicals from emulsion.


Stop bath


22 s

15 min

Removes remaining silver halide and

hardens gelatin.


Fixing


20 s

20 min

Removes excess chemicals


Washing


26 s

30 min

Removes water and prepares film for

viewing


Drying




The Developer Solution:

It is a chemical solution converts the invisible latent image to a visible
manifest image.


Developer Components:

1
-

Solvent:

Water is the solvent used to wet the emulsion.

2
-

Developing agent:

Most developer is formed of phenidone and hydroquinone.

-

Hydroquinone develops the dark areas of the film.

-

Phenidone develops the grays.

3
-

Accelerator:

It is alkali provides an alkaline medium in which the developing agent can
operate.

4
-

Buffers:

This is a chemical compound that has the effect of maintaining the PH of a
solution within close limits.e.g

(Sodium Carbonate)

5
-

Restrainers:

It can modify the behavior of developing agent to be more selective.

It reduces the tendency of developing agent

for un
-
exposed silver halide
crystals.

6
-

Preservative:

It reduces the oxidation of developing agent e.g. (Sodium Sulfite).

7
-

Hardener:

Its main value is to toughen the gelatin e.g. gluteraldhyde.

8
-

Sequestrating agent:

It is a chemical substance that
prevents precipitation of insoluble mineral
salts.


It is composed of

Fixer solution:

1
-

Solvent

Water acts as a solvent and diluted at the same time.

2
-

Fixing agent:

It is a chemical that combines with the insoluble silver halides in the film
emulsion

to from soluble compound that can be easily washed out of the
emulsion.

3
-

Acid:

A weak acid, acetic acid, is sufficient ( pH= 4


4.5)

-

ensure that development ceases when the film enter the fixer

-

provide a suitable environment for hardening agent

4
-

Buffers:

To ensure neutralization of developer

To maintain optimum hardener activity

5
-

Preservative:

It retards the decomposition of thio
-
sulphates e.g. sodium sulphate

6
-

Hardener:

To limit water uptake by the emulsion, reducing the drying time and
prev
ents physical damage.

Aluminum chloride and aluminum sulphate is appropriate

7
-

Anti
-
sludging agent:

It is a chemical substance that reduces the formation of sludge e.g. boric
acid.






Automatic Film Processor




Developer

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Components of an Automatic Processor


Function

Component

Moves film

Transport system

Controls Developer temperature

Temperature Control

Agitates chemicals

Circulation

Maintains concentration

Replenishment

Removes chemicals

Wash

Removes moisture
vents exhaust

Dry

Fused power

Electrical




Transport System Functions:

Moves film through processor at the correct speed.


1
-

Entrance Rollers

Activates replenishment of developer and fixer.

2
-

Crossover Racks

Move film from on tank to the next tank
and remove chemicals from
film.

3
-

Turn around

Also called master rollers, turn the film around at the bottom of the
tanks.

4
-

Motor drives gears

that turn the rollers.




Replenishment System

-

Each time a film passes through the chemicals; fresh chemic
als are
pumped into the tank.

-

This maintains the proper concentration and level of chemicals in
the tanks.

-

Developer replenishment is 60 to 70 ml for each 14 x 17.

-

Fixer replenishment is 100 to 110 ml for a 14 x 17.





Dryer System

-

Dryer removes
all of the moisture from the film.

-

Consists of a heat coils, thermostat, ducts and blower. Heat should
be exhausted to the return air system of the dark room.

-

Some processors used Infrared Heater to dry the film.


Importance of Proper Development

-

De
velopment is a chemical reaction governed by:

1
-

Time

2
-

Temperature

3
-

Concentration of the developer

-

Long time with low temperature or high temperature with short time
will work.


Fog Film:

-

Any deviation from those parameters will result in a loss of

image
quality, usually resulting in fog.

-

Fog causes an increase in base fog and a drop in contrast. A fogged
image is gray with poor contrast.

-

There are three ways to fog film:

1
-

Chemical fog:

contaminated developer, high temperature, slow
transport
.

2
-

Radiation fog:

unintentional exposure to radiation.

3
-

Improper storage:

wrong safelight or storage in high heat and
humidity, expired or out of date film.

Factors affecting processing

Factors affecting Developing

1
-

Constitution of developing solutio
ns:

-

Choice of developing agent

-

Concentration of developing agent

-

The pH of developer solution

-

Concentration of restrainer


2
-

Developer Temperature:

If high



image density &


chemical fog but


image contrast

If low


image density with possibility

of loss of contrast


3
-

Developing Time:

It is determined by developer activity and type of film emulsion.


Factors affecting fixation:

1
-

Constitution of fixer solutions:

-

Choice of fixing agent

-

Concentration of hardeners

-

Presence of hardeners

-

The

pH of fixing solution

2
-

Fixer Temperature

3
-

Fixing Time


Factor affecting washing efficiency

1
-

Film emulsion

2
-

Condition of wash water

3
-

Temperature of wash water

4
-

Washing time


Factor affecting drying time

-

Wetness of emulsion (thickness and har
dener)

-

Drying condition (air humidity and air temperature)

















Processor Maintenance


Daily

-
1

1
-

Remove cross
-
over assemblies and clean rollers under warm
(38˚C) and wipe dry.

2
-

Wipe down the entry rollers.

3
-

Wipe off chemical deposited.

4
-

Wipe all top rollers above solution level.


Weekly

-
2

1
-

Repeat the daily cleaning.

2
-

Put splashguard between developers. And fixer tank and remove
deep racks and clean it.

3
-

Operate each rack manually

4
-

Re
-
install racks and replace cross
-
over
assemblies

5
-

Check water supply filters.

6
-

Clean drier section air tubes and rollers


Monthly

-
3

1
-

Repeat daily and weekly maintenance

2
-

Drain main tanks and clean

3
-

Close drain valve and fill tanks with water and switch on.

4
-

Turn off processor and
drain tanks and re
-
fill with fresh chemical


The processing area



This area is known as the dark room which is very important in any radiology
department



Location of darkroom

It should be centrally placed relative to the x
-
ray room, which it serves



Dark room size and shape

A long dark room is more convenient than a square one because:

o

More wall space is available for equipment

o

No wasted floor space in the center of the room


Construction of dark room

1
-

Wall should have a hard smooth finish

2
-

Ceilings sho
uld be finished in oil
-
smooth finish

3
-

Floor should be water proof, resistant to photographic chemical and
staining

4
-

The entrance should allow easy access, yet prevent passage of light

5
-

Cassette hatch should be installed in one wall of the dark room

6
-

Water serv
ice in the form of adequate plumbing and drainage together
with hot and cold water supply

7
-

Electricity service in the dark room

8
-

Ventilation and heating



Dark room accessories

o

Film hangers

o

Stainless steel clips

o

A ringing timer

o

A thermometer



Beam
-
Restricting Devices


Three factors contribute to an increase in scatter radiation:



Increased kVp



Increased Field Size



Increased Patient or Body Part Size.


X
-
ray Interactions






a


some interact with the patient and are scattered away from the
patient.



b


some are absorbed



c
-

some pass through without interaction



d


some are scattered in the patient



c & d are image forming x
-
rays.



There are two principal means to reduce scatter radiation:



Beam Restricting Devices

limit the field size to red
uce scatter and primary
radiation.



Grids

to absorb scatter before it reached the image receptor.





There are three principal types of beam restricting devices:

1
-

Aperture Diaphragm


Aperture diaphragms are basically lead

or lead lines metal devices placed in


the beam to restrict the x
-
rays emitted from the tube.


2
-

Cones & Cylinders

Cones and cylinders are modifications to the aperture.

Cones are typically used in dental radiography.


3
-


Collimators

The light localizing

variable aperture collimator is the most common beam
restricting device in diagnostic radiography.




Not all of the x
-
rays are emitted precisely from the focal spot.



These rays are called off
-
focus radiation and they increase image blur.



First stage shutt
ers protrude into the tube housing to control the off
-
focus
radiation.



Adjustable second stage shutter pairs are used to restrict the beam.



Light localization is accomplished by a small projector lamp and mirror to
project the setting of the shutters on th
e patient.






The Grid




Collimation reduces scatter radiation but that alone is not sufficient for
larger body parts.



In 1913, Gustave Bucky demonstrated that strips of lead interspaced with
radiolucent material is

an effective means to reduce scatter radiation
reaching the film.



Only rays that travel in a relatively straight line from the source are allowed
to reach the film.



The others are absorbed by the lead.



There are three important dimensions on a g
rid.

1
-

Width of the grid strip (T)

2
-

Width of the interspace material (D)

3
-

Height of the grid (h)






There are three important aspects of grid construction;

1
-

Grid Ratio

2
-

Grid Frequency

3
-

Grid material

Grid Ratio



High ratio grids are more effective in cleaning up
scatter radiation because
the angle of scatter allowed by the high ratio is less than permitted to pass by
low ratio grids.



Ratios range from 5:1 to 16:1



8:1 and 10:1 grids are the most popular ratios in general radiography.




Grid Frequency



The number o
f grid lines per inch or centimeter is called the Grid Frequency.



Grid frequency range from 25 to 45 lines per centimeter or 60 to 145 lines
per inch.



The advantage of high frequency grids is there are no objectionable grid
lines on the image.


Grid Materi
al



The most common grid material is lead because of its cost and ease of
forming the strips.



The interspace material is used to maintain a precise separation of the lead
strips.



Plastic fiber or aluminum is used as the interspace material.



Aluminum is used

as the cover for the grid to protect it from damage and
moisture.



Three types of grids

1
-

Parallel Linear Grids

2
-

Crossed Grids

3
-

Focused Linear Grids


Parallel Grid



Cheap and easy to manufacture.



Problem: Grid cutoff at the outer edge of the 14”X17”
film.



Cut off is most pronounced at short SID.








Crossed Grid



Two parallel grids can be sandwiched together with the lines running across
the long axis and short axis of the film.



More efficient than parallel grid.



Grid cut off is the primary disadvantage of a crossed grid.



Tube can not be angled.



Focused Grids



Focused grids are designed to minimize grid cut off.



The grid lines are angled to match the divergence of the beam.



Focused grids are marked with an inte
nded focal range and the side that
should be towards the tube.



If the tube is improperly aligned or the SID is under the focal range, grid cut
off will occur.



The Bucky Grid



If the grid moves during the exposure, the grid lines can be blurred out. This
was discovered by Hollis Potter in 1920.



There are two types used today, reciprocating and oscillating.



The reciprocating design is moved by a motor during the exposure.



The oscillating design is moved by an electromagnet in a circular pattern.



The mechani
sm adds space between the patient and the film.



The motion can move the film resulting in image blur.



When they fail, the lines appear.




Grid Problems










Radiographic image defects



1
-

Un
-
sharpness:

Un
-
sharp image means blurred image and this may be
due to:


-

Geometric un sharpness(U
G
)

-

Motional un
-
sharpness(U
M
)

-

Photographic un
-
sharpness(U
P
)


2
-

Over
-
under penetration:

The abnormal density of radiographic image is caused by


-

Radiographic errors such as

a
-

Poor choice of exposure factors

b
-

Failure to match
exposure factors to the film
-
screen system

c
-

Use of non
-
standard focus film distance


-

Equipment errors such as

a
-

Reduced x
-
ray output

b
-

Premature termination of exposure

c
-

Inadequate mains electrical supply


Processing errors such as abnormal developer temperature


3
-

Poor contrast

Contrast: is the difference in density between two adjacent area of the image


Factors affecting contrast:

-

Subject contrast

-

Screen contrast

-

Fogging



4
-

Graininess

5
-

Double image

6
-

Image artifacts

7
-

Distortion






exposure factors :



Definition:

These factors that affect the quantity and distribution of radiation energy to which
the image receptor is exposed


1
-

Kilo
-
voltage

Raising the tube kilo
-
voltage increase the density of x
-
rays produced
and consequently the image density


2
-

Milli
-
ampere
-
second

A
s general rule in radiography, we almost prefer to use a maximum
mA and a minimum time combination


3
-

Focal spot


4
-

Filtration


5
-

Focus Film Distance


6
-

Collimation


7
-

Table
-
top attenuation


8
-

Grid