Alignment & Motion - Chapter 24 Flashcards
What are the 4 fundamental principles of alignment?
- Anatomical part should be parallel to the IR
- Tube should be perpendicular to the IR
- Thicker objects show greater distortion
- Sperical anatomy experience more distortion than flat objects
What are the two types of distortion?
Foreshortening and Elongation
What is foreshortening?
When the recorded image appears shorter than the actual object
What causes foreshortening in an image?
When a flat, wedge or tubular object is tilted in relation to the plate and tube angle remains perpendicular
What is elongation?
When the recorded image appears stretched or longer than the actual object
What causes elongation in an image?
When a flat, wedge or tubular object is tilted in relation to the plate and the CR is angled to place it perpendicular to the long axis of the object
When is angling useful in an image?
To reduce or remove superimposition of anatomy on top of each other
- IE angling tube for a lateral knee
What effect does off centering the part have on an image?
It puts the anatomy in the path of the diverging beams vs central ray, which causes elongation
Which anatomy exhibit greater distortion?
Cranium, head of shoulders and head of femur
How are objects with long axis impacted by angling the tube/CR?
They generally resist distortion because even with the angle there is no distortion of the SOD/SID ratio
What are examples of objects with distinct long axis?
Flat, wedge or tubular objects
How are objects without a long axis impacted by angling the CR?
Angling the beam creates an artificial long axis
What are examples of objects without a long axis?
Spherical objects
What type of distortion will appear on a spherical object when the tube is angled?
Elongation distortion
What is Ceisynski’s law of isometry?
An isometric angle equal to half the angled formed by the long axis of the object with the IR will eliminate or reduce distortion
Based on Ceiszynski’s law of isometry, if a sacrum is angled 30 degrees to the IR, what should the tube angle be?
15 degrees
If a flat coin is tilted 45 degrees to the IR and the tube remains perp, what distortion will be reflected in the image?
Foreshortening
If a flat coin is tilted 45 degrees to the IR and the CR is also angled 45 degrees, what distortion will be reflected in the image?
Elongation
If a flat coin is angled 45 degrees to the IR and the CR is angled 22.5 degrees, what distortion will be reflected in the image?
No distortion because of Ceizynski’s law
If a flat coin is kept parallel to the IR and the CR is angled 30 degrees, what distortion will be reflected in the image?
No distortion because the object is flat
What is the gneral rule of beam divergence?
Even if you don’t have angulation of the tube, if you off center the object and place it in the diverging beam you will see distortion
At 40 inches SID, how much divergence is expected per inch?
2 degrees divergence per inch from central ray
At 72 inches SID, how much divergence is expected per inch?
1 degree divergence per inch from central ray
What is the general rule of thumb to maintain 40” SID/consistent exposure when angling the tube?
For every 5 degrees anglulation, reduce the tube to tabletop distance by 1 inch
What will occur if you don’t compensate for angulation and reduce the SID?
The object will appear smaller in the image
