3: radiographic image characteristics

Factors Controlling X Ray Beam

- Exposure Time (S)
- Tube Current (mA)
- Tube Voltage ( kVp)

mAs (quantity or quality?)

quantity of x rays produced

kVp (quantity or quality?)

quality of x rays produced

What are the imaging goals when taking x rays?

to produce an image with optimum diagnostic quality with minimal patient radiation exposure

Radiolucent

darkest area (black)

Radiopaque

brightest area (white)

A structure that appears radiolucent....

has less density
allows passage of more x ray beams through it

a structure that appears radiopaque...

has more density and absorbs more x rays

Factors influencing image quality

Radiation Characteristics
Visual Characteristics
Geometric Characteristics

Radiation Characteristics

X-ray beam quality, quantity, and intensity

Visual Characteristics

density and contrast

Geometric Characteristics

1. Sharpness
2. Magnification
3. Distortion
4. Resolution

Radiation Characteristics that Influence the Quality of the Image

Quality
Quantity
Intensity

Quality

Voltage
kVp

Quantity

Amperage, Milliamperage
Milliampere, Seconds

Intensity

Quality
Quantity
Distance
Inverse Square Law
Half Value Layer

Radiographic Density

the overall degree of blackness/darkness of an exposed film

Increased density results in

dark image

Decreased density results in

light image

Factors influencing density of image

exposure factors (mA, kVp, S)
subject thickness

increased kVp

increased density

Decreased kVp

decreased density

Increased mA

Increased density

Decreased mA

decreased density

increased time

increased density

decreased time

decreased density

relationship between kVp and density

non linear

increase in 15kVp causes...

two times increase in density

Factors in Radiographic Density

1) milliamperage
2) exposure time
3) distance

total exposure formula

mA x seconds = mAs

as long as mAs are equal, film density will be...

maintained

what should be adjusted according to a patient's size?

exposure factors : kVp or time

what happens to radiation when the subject is thicker?

less radiation is able to reach the image receptor

do children require less radiation than adults?

yes

contrast

a difference in the degree of blackness (densities) between adjacent areas of a dental radiograph.

an image that shows both dark and light areas has

high contrast
short grayscale

an image that shows only shades of gray has

low contrast
long grayscale

factors influencing contrast

Subject contrast
Radiation quality
Fog/scatter

subject contrast

the difference in the intensity of the beam caused by an object

most important contribution to subject contrast

attenuation of x ray beam by photoelectric effect

photoelectric effect is increased in...

substances with high atomic number

what attenuates more x rays? Bone or muscle?

bone

low kVp produces

high subject contrast

what happens if the kVp is too low?

almost all x rays are attenuated in patient and never reach film

step wedge

- demonstrates short and long scale of contrast
- made of aluminum
- 2mm increments
- placed on top of image receptor and exposed

magnification

when an object is placed in the path of a beam it will cast a shadow on receptor that will show some degree of enlargement

magification is due to...

divergence of x ray beam

factors influencing magnification

source to image receptor distance
object to image receptor distance

source of image receptor distance is determined by...

length of PID

increasing source object distance results in images with...

increased sharpness and less magnification

relationship between length of cone and divergence

long cone = less divergence

decreasing object and image receptor distance...

increases sharpness and less magnification

distortion

variation in true size and shape of object

distortion results from...

unequal magnification of different parts of the same object

factors influencing distortion

object receptor alignment (x ray beam must be parallel)

elongation occurs with

decreased vertical angulation

foreshortening occurs with

increased vertical angulation

radiographic sharpness

ability of the radiograph to define an edge precisely

penumbra

fuzzy/unclear area that surrounds the radiographic image
blurring of edges

small focal spot leads to

increased image sharpness and resolution

large focal spot leads to

loss of image clarity

radiographic resolution

the ability of the radiograph to record separate structures that are close together

high resolution

radiographs which resolve two adjacent high contrast objects as discrete entities

the smaller the structures visible the __________ the resolution of the image/system

higher