10. Assessing Distortion Flashcards
is the second of the 2 geometric properties affecting radiographic image quality
Assessing Distortion
is a misrepresentation of the size or shape of
the structures being examined
Distortion
It creates a misrepresentation of the size and/or shape of
the anatomical part being imaged
Distortion
This misrepresentation can be classified as either ____ or
_____ distortion
size,
shape
Distortion can be _____ to determine even when normal sizes and shapes are known
difficult
Size distortion can be only magnified with ______
radiography
film/screen
Since it is not possible to reflect or refract x-ray photons by ordinary methods, they can only diverge from their
______
point source
In all instances, ______ magnification size distortion
______ the resolution of recorded detail
reduced,
increases
Magnification size distortion is controlled by positioning
the body part and tube to maximize ____ while
minimizing ____
SID (max),
OID (min)
The ____ has a major effect on magnification
SID
The ____ is the critical distance for magnification and resolution
OID
Although _______ has developed as the current
routine SID, this was not always so.
40” (100 cm)
The first x-ray techniques book in the United States is generally recognized to be the ____ “Extract from the United States Army X-ray manual” – ______ is discussed as a reasonable SID
1918,
20 in or 50 cm
In ____ “Modern X-ray Technic”, ______
recommended distances from 25” (63 cm) to 36” (90 cm)
1928,
Ed Jerman
In 1928 “Modern X-ray Technic”, Ed Jerman
recommended distances from
25” (63 cm) to 36” (90 cm)
_______ book, “Medical Radiographic Technic” – ______
Glenn Files’ 1945,
40’ (100cm) SID
Some institutions are adopting the ______ as a routine distance and the movement appears to be growing
48” (120 cm)
For many, many years chest radiography has been
performed at _______ because the erect positioning arrangement permits horizontal beam to be used and the increased SID effectively minimizes the
magnification of the heart shadow
72” (180cm)
Another critical factor in both magnification and
resolution
Object-to-Image Receptor Distance
When describing objects, it is important to remember
the size and distance relationship in a radiographically
projected image is the ______ of that perceived
visually
opposite
is the misrepresentation by unequal magnification of the actual shape of the structure being examined
Shape distortion
projects the object so it appears to be longer
than it really is
Elongation
projects the image so it appears shorter than it really is
Foreshortening
often results because structures lie normally at different levels within the body. It also occurs due the divergence of the xray beam
Shape distortion
Shape distortion can be caused or avoided by careful alignment of the _____ with the anatomical part
and the image receptor
central ray
When the position of the body part or object within the body does not permit this alignment, _______ must be utilized
creative positioning
is the theoretical photon that exits from the
exact center of the focal spot
Central ray
Central ray ideally is intended to be projected ______ to both the anatomical part & the image receptor
perpendicular
Whenever the central ray is not _______, some
degree of shape distortion will result
perpendicular
Any structure that will is not positioned at the central
ray will be distorted because of the divergence of the beam – the _____ from the central ray, the _____ the
distortion
farther,
greater
When a part is superimposed, central ray _______ can be a useful tool to provide a projection that would otherwise be impossible to differentiate from overlying
structures
angulation
The long axis of the anatomical part, or object, is
intended to be positioned _______ to the central
ray and ______ to the image receptor.
perpendicular (central ray),
parallel (IR)
occurs when there is poor alignment of the
tube and/or image receptor
Elongation
occurs only when there is poor alignment
of the part
Foreshortening
The anatomical part is normally positioned with its long axis _____ to the central ray and _____ to the image receptor.
perpendicular (central ray),
parallel (IR)
Refers to the direction and degree the tube is moved
from its normal position perpendicular to the image
receptor
Angulation
The angulation of the tube is designed to cause a
controlled or expected amount of shape distortion to avoid ________
superimpositions
The most common direction of the tube angle is ______
longitudinal
Longitudinal angulations are usually termed ______
when the tube is angled toward the head of the patient
cephalad
Longitudinal angulations are usually termed ______
when the tube is angled toward the patient’s feet
caudad
Some radiographic tubes can also be angled
transversely
transversely (sometimes referred to as “___”)
roll
is simply a method of describing the exact
amount of angulation and is usually stated as the angle
between the central ray and the image receptor plane
from the standard reference point of perpendicularity
Degree
is a more subjective evaluation than size
Shape distortion
It is much more difficult to assess because there is no effect that can be calculated, as in the magnification factor for size
distortion
Shape distortion
is generally a matter of
magnification
Size distortion
involves both elongation and
foreshortening and is a serious alteration in the
projection image
Shape distortion
