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Producing An Image
RVT: Chapter 6
Learning Objectives: Chapter 6
• Understand the 4 factors of radiographic exposure
and how each impacts the production of a
diagnostic image.
• Understand how milliamperage (quantity) and
kilovoltage (quality) of x-rays can impact a
radiograph.
• Define scatter radiation and understand its impact
on radiographic quality and radiation exposure of
personnel.
• Define the 15% rule, and be able to use it to finetune a radiographic image.
• Understand how to create and use a technique
chart to produce diagnostic radiographs.
The Four Factors of Radiographic Exposure
These must be manipulated so tissue absorption of
radiation is exactly correct to demonstrate
anatomy/pathology and minimize artifacts on the
image.
The 4 factors:
• Kilovoltage (kV) – “quality” or contrast
• Milliamperes (mA) – “quantity” or darkness
• Time (seconds) – exposure time
• Distance
1st Factor of Radiographic Exposure: Distance
The Inverse Square Law
• The intensity of radiation at a
location is inversely
proportional to the square of
its distance from the radiation
source.
The distance between the xray tube and image receptor
is fixed at 40 inches.
2nd Factor : Kilovoltage
Kilovoltage Peak (kVp) - the maximum value of x-ray tube
voltage during x-ray production
• This is relating to the power
• The energy at which the x-rays will penetrate the
body
• Need this to be as low as possible to decrease
______________ radiation
• Directly affects the contrast in the anatomy
• More contrast =
• More black and white
• Fewer shades of gray in-between
• Can distinguish between structures easier
*Can impact density too (kVp too high will darken film)
Optimizing Kilovoltage
Range on the generator: 40-125 kV (may vary)
• Use the lowest setting that will penetrate region of
interest, enhance tissue contrast, and minimize
scatter radiation.
• Extremities will have the lowest kV
• Abdomen and thorax views will have high kV
• Calculate a starting point, but be ready to adjust
Calculating Kilovoltage
Sante’s Rule
kV= 2 x thickness (cm) + 40
Example:
Body Part = 8 cm
(2 X 8) + 40 = 56 kVp
Anatomical Measuring
Technical factors are determined based on the
thickness of the anatomy being radiographed
• Measure using calipers
• Unit must be in centimeters
• Measure the animal in the position they will be in
during the radiograph!
Sante’s Rule: Why 40?
• Represents the distance
from the x-ray tube focal
spot to the image receptor
(film)
• In inches
• This distance can be
referred to as the Focal Film
Distance (FFD) or Source
Image Distance (SID).
Optimizing Kilovoltage
The 15% Rule
Used to optimize kV’s and enhance contrast
• Doesn’t impact density
• To increase penetration – increase kV’s 15%
• To decrease penetration – decrease kV’s by 15%
Must also adjust mA if you change kVp:
• Increasing kV = Divide mA by 2
• Decreasing kV = Multiply mA by 2
Adjusting kVp’s
Not enough Contrast
Optimized Contrast
What is Scatter Radiation?
• “Secondary radiation”
• Lower-energy x-ray photons that have undergone a
change in direction after interacting with structures
in a patient’s body
• Is of concern because:
1. Decreases image quality
2. Increases radiation exposure
• The primary source of exposure for technicians!
3. Darkens radiograph & decreases contrast
Managing Scatter Radiation
•Is directly impacted by increases in:
• Kilovoltage
• Size of the field
•Can be managed by:
• Reducing kVp’s as low as
possible
• Correctly collimating
• Avoiding retakes
• Use of a grid
3rd Factor : Milliamperage
•mA
•Quantity of x-rays produced with each exposure
•Impacts density (darkness)
•Directly proportional
• Doubling mA doubles density
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Advantages of High mA
• Allows for examination of thicker anatomic areas
• Allows for shorter exposure time setting with the
same number of x-rays produced =
1. Possibility of motion is decreased
2. Decreases exposure for restraining personnel
Technical Factors of Exposure Analogy
Imagine x-ray photons as pool balls:
• kVp = Power/energy that the cue hits the white ball
• Ma = Number of balls on the table
Energy is transferred as balls bounce off each
other and the table
4th Factor : Time
• Adds in time factor
• mA X seconds = mAs
• Suitable mA setting depends on the thickness and
type of tissue being radiographed
• mA & time are inversely proportional
• Often combined settings on the x-ray machine
• Changes in mAs:
• Increased = x-ray becomes blacker in color overall
• Decreased = x-ray becomes lighter in color overall
mAs: Too High & Too low
Troubleshooting the Technical Factors
If image isn’t diagnostic:
• Reposition & re-measure
• Adjust mA’s first, as long as tissue is penetrated
• An easy change to make & measure
• Adjust kVp’s using 15% rule
• If film is still light, check temperature of chemicals
• If image is dull/gray, look for light leaks in darkroom or
the cassette
• Check for improperly exposed or expired film in box
• Have the unit serviced & recalibrated
Anatomic Considerations
•Skull & Cervical Spine
• High contrast bone & tissue densities already
• Higher kVp not required
•Chest, thorax, abdomen, lumbar spine, pelvis
• Thickened areas with similar densities, so scatter
likely if high kVp’s used
• Keep KVp as low as possible and increase mAs
•Extremities
• Body parts thin, and tissue-to-bone ratio high
• Low kVp’s indicated
•Birds & pocket pets (guinea pigs, ferrets, etc)
• Similar technical factors to extremities
Evaluation of Radiographic Technique
Two basic questions…
• Is the film too light or too dark?
• More exposure = blacker film overall
• Increase mAs to darken
• Decrease mAs to lighten
• Is there proper penetration/differentiation?
• If cannot see contrast between structures, adjust
kVp’s
• Increase kVp to darken
• Decrease kVp to lighten
What Determines Adequate Penetration?
• Abdominal radiograph:
• Can see outlines of liver, spleen, kidneys and bowel
• Thoracic:
• Heart clearly outlined, diaphragm boundary evident, bone
differentiation clear
• Extremities:
• Bones white, soft tissue can easy be distinguished
Inadequate penetration = areas are the same shade of
grey, and organs/bones cannot distinguished from one
another
Radiograph Too Dark?
• If bone tissue is gray, with too little contrast
between the bone and adjacent soft tissue, there
was too much penetration.
• Decrease kVp by 10-15%
• If bone tissue is relatively white, compared to
surrounding tissues, then penetration is adequate.
• Decrease mAs 30-50%
*Evaluating radiographs is an art, and often several changes
can be made to improve the radiograph equally*
Radiograph Too Light?
Is the film under penetrated?
• If no: Increase mAs 30-50%
• If yes: Increase the kVp 10-15%
Is the film over penetrated?
• If yes: Decrease kVp by 10-15%
• If no: Decrease mAs by 30-50%.
Exposure Factors & the X-ray Tube
X-ray generation:
• mAs (current) is applied to the filament in the
cathode.
• mAs control the quantity of x-rays
• Generates an electron cloud
• The electrons are directed to the anode target
by kVp’s.
• kVp’s control the quality/penetration of x-rays
• The collision produces heat and x-radiation.
Viewing a Radiograph
•Viewed on an evenly lit
view box in a semidarkened room.
•View box should be
clean, and all light bulbs
should be in working
order.
Positioning the Radiograph
Film position on the illuminator
matters:
• V/D or D/V:
• _________ region at the top
• Handshaking position
• Lateral:
• Head at viewer’s _______
• Spine on top
• Limbs:
• ___________ end up
Radiography Log Book
•Record of every radiograph taken
•Must include:
• Patient’s name/identifiers
• Measurement
• View taken
• Technical factors
• kVp, mAs