Screen Film Radiographic Technique

Screen Film Radiographic Technique
Muhammad Arif Afridi
Lecturer In Medical Imaging
Email: drarifafridi@gmail.com
MUHAMMAD ARIF AFRIDI | LECTURER IN MEDICAL IMAGING | DRARIFAFRIDI@GMAIL.COM 1

Exposure Factors
Kilovolt Peak (kVp)
Mill Amperes (mA)
Exposure Time
Distance
2MUHAMMAD ARIF AFRIDI | LECTURER IN MEDICAL IMAGING | DRARIFAFRIDI@GMAIL.COM

Exposure Factors
The factors that influence and determine the quantity and quality of x-radiation to which the patient
is exposed are called exposure factors.

Factors That May Influence X-ray
Quantity and Quality

The four prime exposure factors are:
1. kilovolt peak (kVp),
2. current (mA),
3. exposure time (s), and
4. sourceto-image receptor distance (SID).

Secondary Factors
1. Focal-spot size,
2. distance, and
3. filtration are
 may require manipulation for particular examinations.

Four Prime Exposure are:

1. Kilovolt Peak (kVp)
kVp controls screen-film radiographic contrast.
beam penetrability.
The kVp has more effect than any other factor on image receptor exposure.
kVp increases, less differential absorption occurs. Therefore, high kVp results in reduced image
contrast.

2. Milliamperes
The mA selected determines the number of x-rays produced and therefore the radiation
quantity.
As more electrons flow through the x-ray tube, more x-rays are produced.
With a constant exposure time, mA controls the x-ray quantity and therefore the patient
radiation dose.
X-ray quality remains fixed with a change in mA.
A change in mA does not change the kinetic energy of electrons flowing from cathode to anode.
It simply changes the number of electrons.

3. Exposure Time
Radiographic exposure times usually are kept as short as possible.
The purpose is not to minimize patient radiation dose but rather to minimize motion blur that
can occur because of patient motion.
Short exposure time reduces motion blur.

4. Distance
Distance has no effect on radiation quality.
Distance (SID) affects OD.

Secondary Factors are:

Imaging System Characteristics
1. Focal-Spot Size
Region of the anode target in which electrons interact to produce x-rays.
2. Filtration
Removal of low-energy x-rays from the useful beam with aluminum or another metal. It results
in increased beam quality and reduced patient dose.
3. High-Voltage Generation

1. Focal-Spot Size
Most x-ray tubes are equipped with two focal-spot sizes.
On the operating console, these usually are identified as small and large, 0.5 mm/1.0 mm, 0.6
mm/1.2 mm, or 1.0 mm/2.0 mm.
X-ray tubes used in interventional radiology procedures or magnification radiography may have
0.3 mm/1.0 mm focal spots.
One difference between large and small focal spots is the capacity to produce x-rays.
Many more x-rays can be produced with the large focal spot because anode heat capacity is
higher.
With the small focal spot, electron interaction occurs over a much smaller area of the anode,
and the resulting heat limits the capacity of x-ray production.

Changing the focal spot for a given kVp/mAs setting does not change the x-ray quantity or
quality.
A small focal spot is reserved for fine-detail radiography.

Generators
The radiologic technologist cannot select the type of high-voltage generator to be used for a
given examination.
Three basic types of high-voltage generators are available: single phase, three phase, and high
frequency.
The radiation quantity and quality produced in the x-ray tube are influenced by the type of high-
voltage generator that is used.

Half-wave rectification results in the same radiation quality as is produced by full-wave
rectification, but the radiation quantity is halved.
Half-wave rectification is used rarely today.
Some mobile and dental x-ray imaging systems are half-wave rectified.
Radiation quality does not change when going from half-wave to full-wave rectification;
however, radiation quantity doubles.
Rectification is the conversion of alternating current (AC) to direct current (DC).

Three-phase power results in higher x-ray quantity and quality.
High-frequency generation results in even greater x-ray quantity and quality.

PATIENT FACTORS
The first group includes patient factors, such as anatomical thickness and body composition.
The second group consists of imagequality factors, such as OD contrast, detail, and distortion.
Also of importance is how these image-quality factors are influenced by the patient.
The final group includes the exposure technique factors, such as kVp, milliamperage, exposure
time, and SID, as well as grids, screens, focal-spot size, and filtration.
An understanding of each of these factors is essential for the production of high-quality images.

The general size and shape of a patient is called body habitus
Sthenic—meaning “strong, active”
Hyposthenic patients are thin but healthy
appearing
Hypersthenic patients are big in frame and
usually overweight.
Asthenic patients are small, sometimes often
elderly
Radiographic technique charts are based on sthenic patients.

Thickness
The thicker the patient, the more x-radiation is required to penetrate the patient to expose the
image receptor.
Composition
The chest has high subject contrast; the abdomen has low subject contrast.
Pathology
The type of pathology, its size, and its composition influence radiographic technique.

IMAGE-QUALITY FACTORS
image-quality factors refers to characteristics of the radiographic image; these include OD,
contrast, image detail, and distortion.
Image-quality factors are considered the “language” of radiography

Optical Density
Optical density is the degree of blackening of the finished radiograph.
In medical imaging, many problems involve an image being “too dark” or “too light.” A
radiograph that is too dark has a high OD caused by overexposure.
This situation results when too much x-radiation reaches the image receptor.
A radiograph that is too light has been exposed to too little x-radiation, resulting in
underexposure and a low OD.
Optical density can be controlled in radiography by two major factors: mAs and SID.

Contrast
The function of contrast in the image is to make anatomy more visible.
Contrast is the difference in OD between adjacent anatomical structures, or the variation in OD
on a radiograph.
Contrast, therefore, is perhaps the most important factor in radiographic quality.
kVp is the major factor used in controlling radiographic contrast.

Detail
Detail describes the sharpness of appearance of small structures on the radiograph. With
adequate detail, even the smallest parts of the anatomy are visible, and the radiologist can more
readily detect tissue abnormalities.
Sharpness of image detail refers to the structural lines or borders of tissues in the image.
Sharpness of image detail is best measured by spatial resolution.

Distortion
the misrepresentation of object size and shape on the radiograph. Because of the position of the
x-ray tube, the anatomical part, and the image receptor, the final image may misrepresent the
object.
Distortion is reduced by positioning the anatomical part of interest in a plane parallel to that of
the image receptor.

EXPOSURE TECHNIQUE CHARTS
kVp, mA, exposure time, and SID are the principal exposure technique factors.
It is important for radiologic technologists to know how to manipulate these exposure technique
factors to produce the desired OD, radiographic contrast, image detail, and distortion on the finished
radiograph.
It is not necessary, however, to become creative with each new patient.
For each radiographic imaging system, a chart should be available that describes standard methods
for consistently producing high-quality images.
Such an aid is called a radiographic technique chart.

AUTOMATIC EXPOSURE TECHNIQUES
Automatic control x-ray systems are not completely automatic.
Radiologic technologist does not have to select kVp and mAs settings and time for each
examination.
Patient positioning must be accurate because the specific body part must be placed over the
photo timing device to ensure proper exposure.

Factors to Consider When Constructing an
Exposure Chart for Automatic Systems

Automatic Exposure Control
Radiation exposure in most x-ray imaging systems
is determined by an automatic exposure control
(AEC) system.
AEC incorporates a device that senses the amount
of radiation incident on the image receptor.
Through an electronic feedback circuit, radiation
exposure is terminated when a sufficient number
of x-rays has reached the image receptor to
produce an acceptable OD.

To produce an image, the radiologic technologist
simply touches an icon or a written description of
the anatomical part to be imaged and the body
habitus.
The microprocessor selects the appropriate kVp
and mAs settings automatically.

Thank you!

Screen Film Radiographic Technique

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