Book contents
- Frontmatter
- Contents
- List of contributors
- Preface
- Acknowledgments
- Section 1 Brain, head, and neck
- Section 2 Spine
- Section 3 Thorax
- Section 4 Cardiovascular
- Section 5 Abdomen
- Section 6 Pelvis
- Section 7 Musculoskeletal
- Case 78 Pseudofracture from motion artifact
- Case 79 Mach effect
- Case 80 Foreign bodies not visible on radiographs
- Case 81 Accessory ossicles
- Case 82 Fat pad interpretation
- Case 83 Posterior shoulder dislocation
- Case 84 Easily missed fractures in thoracic trauma
- Case 85 Sesamoids and bipartite patella
- Case 86 Subtle knee fractures
- Case 87 Lateral condylar notch sign
- Case 88 Easily missed fractures of the foot and ankle
- Section 8 Pediatrics
- Index
- References
Case 79 - Mach effect
from Section 7 - Musculoskeletal
Published online by Cambridge University Press: 05 March 2013
- Frontmatter
- Contents
- List of contributors
- Preface
- Acknowledgments
- Section 1 Brain, head, and neck
- Section 2 Spine
- Section 3 Thorax
- Section 4 Cardiovascular
- Section 5 Abdomen
- Section 6 Pelvis
- Section 7 Musculoskeletal
- Case 78 Pseudofracture from motion artifact
- Case 79 Mach effect
- Case 80 Foreign bodies not visible on radiographs
- Case 81 Accessory ossicles
- Case 82 Fat pad interpretation
- Case 83 Posterior shoulder dislocation
- Case 84 Easily missed fractures in thoracic trauma
- Case 85 Sesamoids and bipartite patella
- Case 86 Subtle knee fractures
- Case 87 Lateral condylar notch sign
- Case 88 Easily missed fractures of the foot and ankle
- Section 8 Pediatrics
- Index
- References
Summary
Imaging description
When multiple structures overlap or abut on a radiograph, an optical illusion known as the “Mach effect” may simulate light and dark lines. This enhances edges and in some cases makes the structures easier to differentiate. However, the Mach effect may also simulate a fracture. The Mach effect is a normal phenomenon that results from the physiologic process called “lateral inhibition” in the radiologist’s retina [1]. As shown on Figure 79.1, each gray rectangle appears shaded, light at the top and darker at the bottom. Now use a sheet of paper to cover all but one of the gray rectangles – the illusion of a gradient of gray within the remaining rectangle disappears, revealing that each rectangle is, in fact, homogeneous in color.
When the Mach effect is being considered, careful inspection is necessary to avoid misdiagnosis. Additional views may remove the superimposition of structures creating the effect. In other cases, a CT may be necessary to exclude underlying pathology.
- Type
- Chapter
- Information
- Pearls and Pitfalls in Emergency RadiologyVariants and Other Difficult Diagnoses, pp. 267 - 271Publisher: Cambridge University PressPrint publication year: 2013