Overview
This lecture covers shape distortion in radiographic imaging, focusing on its causes, effects, and how to minimize or use it deliberately for optimal diagnoses.
Shape Distortion in Radiography
- Shape distortion occurs when anatomy appears altered in length on a radiograph compared to its actual shape.
- Misalignment of the X-ray tube, image receptor, or anatomy causes shape distortion.
- Shape distortion can misrepresent the size or position of anatomy or pathology, affecting diagnosis.
Types of Shape Distortion
- Two main types are foreshortening and elongation.
- Foreshortening makes anatomy appear shorter than it is.
- Elongation makes anatomy appear longer than it is.
Causes and Correction of Foreshortening
- Foreshortening happens when anatomy is not parallel to the image receptor.
- Angling the anatomy increases foreshortening by superimposing the anatomy's shadow.
- Patient positioning, such as rotating the femur during hip imaging, helps avoid foreshortening.
- The law of isometry states the central ray should be angled at half the angle between the object and image receptor to minimize foreshortening.
Causes and Correction of Elongation
- Elongation results from X-ray tube or image receptor angulation.
- Occurs when the X-ray tube is not perpendicular to the image receptor, even if the image receptor and anatomy are parallel.
- Increased angulation of the tube or receptor increases elongation.
Intentional Use of Shape Distortion
- Shape distortion is sometimes used to visualize anatomy better, such as oblique sternum views or the town's view of the skull.
- Intentional angulation can prevent superimposition or highlight structures.
Key Terms & Definitions
- Shape Distortion — alteration in the appearance (length/shape) of anatomy on a radiograph.
- Foreshortening — anatomy appears shorter on the radiograph due to misalignment.
- Elongation — anatomy appears longer on the radiograph due to angulation.
- Law of Isometry — central ray angled at half the angle between the object and image receptor to reduce foreshortening.
Action Items / Next Steps
- Practice identifying and correcting shape distortion in radiographic images.
- Review the law of isometry and apply it to positioning scenarios.