Introduction

Most diagnostic conclusions about both normal and abnormal ultrasound appearances are based on four key observations:

• The spatial definition of tissue boundaries
• Relative tissue reflectivity (brightness)
• Echo-texture
• The effect of tissue on the through transmission of sound

All of these observations are based upon appearances that are the result of how sound waves interact with tissue. A great deal of potentially useful diagnostic information can be inferred from these observations, but some understanding of how sound behaves is required to appreciate why the appearances arise.