Diagnostic Ultrasound
During an ultrasonographic examination sound waves are sent out from the transducer head to detect tissue structures. Sound waves that are reflected back to the transducer head by a very dense solid structure such as bone appear as a white line on the ultrasound image. Fluids (such as blood and joint fluid) permit sound waves to travel very easily through them – the waves are not reflected back and on the ultrasound image they appear as black structures. Tissues that reflect some sound waves back to the transducer, and allow some to travel through show on the image as various shades of grey.
The ability of certain tissues to reflect or allow passage of sound waves through them is known as ‘echogenicity’. Fluid is seen as a hypoechoic or anechoic structure on the ultrasound image, because very few sound waves return to the transducer head. Bone is seen as a hyperechoic structure, because almost all the sound waves return to the transducer head. Areas of injury may have abnormal echogenicity, an irregular or abnormal outline or a change in size of the structure may be seen.