Vision and perception - Ch 14
# of rods
120 million
the length of time the human visual system can integrate
.2 second
Viewing distance
Due to the anatomy of the eye, changing viewing distance may make perception easier. Changes in intensity due to the inverse square law and because of the physiological processing of the image changes when the angle of the incident light photons change. The fovea centralis creates a blind spot at a viewing distance of about 9". Radiologists often vary their viewing distances when addressing areas of perceptual difficulty.
Rods are responsible for
Scotopic vision (night vision): sensitive to low light cannot perceive colors no sharp detail in the retina of the eye - spaced further apart than cones
Cone cells contains
3 different light-sensitive pigments.
# of cones
7 million
Mach effect
A perceptual illusion of a lighter edge just before the edge of the next darker step. The retina contains neural connections that inhibit impulses under certain conditions. Bright light and impulse intensity. When the eye perceives a change in density, there is a change in the intensity of the impulses sent to the brain. Causes brain to identify a boundary.
Are dim or bright objects best viewed peripherally
Dim
Why do our eyes take a while to adjust when going from light to dark?
It takes time for the rods to take over
nondestructive testing
NDT radiography - assists in the examination and assessment of materials such as castings, welds and others through noninvasive techniques.
why does a person experience temporary blindness in bright light
Rhodopsin is oversensitized and becomes bleached out.
edge enhancement
This is not something your eye does. It is part of the equipment, post-processing algorithm of equipment of a digital system. Compresses the entire density scale while making the boundary appear more distinct than it really is. increases in contrast due to post-processing algorithms, creates a more distinct boundary subtle changes in density may be missed
Contrast perception is better in
bright light - cones
Rods function better when the image information is
changing.
Color blindness
condition that occurs when there is a lack of cones sensitive to a particular color or colors
which can detect changes in brightness better? Rods or cones
cones - allows for greater recognition of density differences (contrast)
photopic vision is controlled by
cones - require bright light to function - a minimum of 100 light photons - ability to see fine detail
misperceptions of any type can be
dangerous in clinical radiography
photopic
daylight
visual acuity is improved in
daylight
A radiograph lacks the dimension of
depth
When a PA and lateral still show superimposition
two oblique positions at 90° angles from one another can be used to achieve the same effect
Rod cells cannot distinguish
wavelength, but are more sensitive to green light
eye motion
when the eye uses a scanning motion, contrast perception is dramatically increased. Because the photosensitive cells in the eye can integrate a limited amount of info, eye movement maintains a constantly changing neurological signal, avoiding saturation of the optical nerves.
Only an improvement in the image quality
will improve visualization
Cones are most sensitive to
yellow light
The post processing application of boundaries is called
edge enhancement
Why do we need to take 2 radiographs as close to 90° as possible
because we need to see the part as if it were in 3D
when adjacent to each other, small changes in density are easier or harder to see
easier
How to overcome misperception
eliminate preconceived ideas about an image
What affects the amount of scatter radiation reaching the IR
filtration, kVp, grid and collimation
which influences amount of scatter reaching the IR
filtration, kVp, grid, collimation (NOT focal spot size)
location of cones
fovea centralis
the human eye is anatomically designed to
gather light, focus it, convert it to nervous impulses and transmit it to the brain for processing
If not enough information is available to see,
more information needs to be provided on the IR
scotopic
night vision
boundary effect
occurs b/c the visual system has difficulty perceiving contrast differences that are distant from one another. When a boundary exists b/w 2 densities, differences as small as 2% may be perceived. If no boundary exists, a 20% difference may be required.
veil glare
occurs when the intensely bright light from a viewbox floods the eye directly. Occurs in unexposed areas of the image and b/w images. Light blindness.
surrounding densities have an effect
on the perception of nearby densities
Dim objects are best viewed
peripherally
Cones are responsible for
photopic vision (daylight vision): require bright light improved visual acuity - (see fine detail) Greater contrast perception color perception are in the fovea centralis of the eye
Rod and cone cells contain
photosensitive pigments that create an electrical signal when stimulated by light
Rod and cone cells contain
photosensitive pigments that respond to light by sending an electrical potential to specialized nerve cells that emit neurological impulses when excited by light photons
A challenge the radiographer faces is
placing the pt so the area imaged is in a position that enhances the quality of the image. The radiographer must manipulate the object in relation to the x-ray beam and IR. = controlling the image in space
contrast perception
recognition of density differences - assisted by cones - they are able to detect changes in brightness far better than rods
Most common problems with corneal malfunction
requiring corrective lenses - a distinct loss of detail is perceived because the incoming light is not properly focused on the retina
location of rods
retina
human image conversion occurs in the
rod and cone cells in the retina
Why can't humans perceive colors in extremely low light
rod cells cannot distinguish wavelength
scotopic vision is controlled by the
rods - sensitive to low light levels - may respond to just 15 photons. Cannot function in bright light.
Difficulty perceiving contrast differences that are distant from one another is called
the boundary effect
Positioning requires a solid knowledge of
the shape and location of skeletal and soft-tissue structures and their anatomical relationship to one another. Superimposition can be avoided when positioning a part so overlying structures don't overlap
If sufficient info is not processed in .2 seconds,
the visual system resets and acquires for another .2 seconds.
A radiographer must be able to think
three dimensionally
pattern recognition
Comparing mental images of patterns - anatomical, physiological, pathological and histological - to arrive at a diagnosis. Perceiving combinations of details that can be defined and classified toward a diagnosis. Often the area studied the hardest. Involves comparing mental images of patterns - anatomical, physiological, pathological and histological - to arrive at a diagnostic opinion. After knowing what's normal, you can begin to see what's wrong.
threshold detection
a visual phenomenon involving the perception of extremely small or faint details.
Dimensional view required are
anterior-posterior medial-lateral superior-inferior
To provide a 3-d view
at least 2 images as close to 90° from on one another are required
density change should be
at least 25-33% in order to be visible.
why is peripheral vision poor in daylight
because there is sparse concentration of cones in the retina outside the fovea centralis.
