A radiologic investigation that utlises high frequency sound waves & their echoes off different tissue boundaries to generate a 2 dimensional image which we refer to as an ultrasound.
Echogenicity. Ultrasound images are described as hyperechoic, isoechoic, hypoechoic & anechoic.
Generation of image
A crystal is made to oscillate at a high frequency by passage of an electric current (the piezoelectric effect). This produces a sound wave. Short pulses of sound lasting around one millionth of a second are transmitted around 500 times/second (500Hz). High frequency probes of 7.5-10 megahertz provide the best resolution but have a short focusing distance. There is a trade off between the depth of penetration & the axial resolution (ability to distinguish between objects as being separate when they lie directly over each other) e.g. 7.5MHz depth 8cm & spacial resolution 0.2mm vs. 10MHz having a depth of 6cm & resolution 0.15mm. Deep structures such as the hip & knee use 5MHz transducers.
The intensity of the ultrasound is 1 to 50 mW/cm2, which is around 5 orders of magnitude less than the levels used for surgical ultrasound (eg ablation of cement, fragmentation of calculi); these use around 5 to 300 W/cm2.
The sound waves enter the body & are either reflected or refracted at interfaces of different acoustic impedance. The time taken for the reflected sound wave to return is proportional to the distance from the transducer. The refracted waves are not received. The brighter the echo, the larger the differences in acoustic impedance.
The crystal receives the reflected sound waves & converts them to electrical signals which are displayed on a screen.
Bone & calcified tissues reflect nearly all of an ultrasound beam.
Cysts produce large echoes from their walls but none from their contents; there is also an ↑ echo from the tissues behind the cyst, known as acoustic enhancement.
A calcified structure produces an acoustic shadow behind it.
Doppler ultrasound uses principle that blood flowing towards the transducer will reflect a higher frequency than the original frequency; blood flowing away will produce a lower frequency, & the change in frequency is proportional to the speed of blood flow.
Ultrasound properties of various tissues
Tendons & ligaments are hyperechoic & demonstrate a fibrillar pattern.
Muscle is hypoechoic with respect to tendon fibres, but has hyperechoic fascial & aponeurotic fibres within it.
Cortical bone is highly echogenic, as is abnormal soft tissue calcification & ossification.
Hyaline cartilage is hypoechogenic.
Fibrocartilage is hyperechoic.
Simple fluid (i.e. unloculated cyst) is anechoic.
- No ionizing radiation
- Relatively inexpensive
- Images can be obtained in any plane by simply moving the transducer
- Can be performed at the bed side or in theatre
- Useful in guiding aspiration or biopsy
- Good imaging of soft tissues; particularly good at distinguishing between solid & fluid filled structures
- Major disadvantage is the technique is highly user dependent, & ultrasound is also a dynamic technique, so the operator is the best person to interpret the results
- Two dimensional (but multiple passes can provide three dimensional information)
- Poor imaging of bones
What to say when confronted by an ultrasound
- Echogenic cortical surface
- Posterior acoustic shadowing
- Transverse sonogram
- Thin stripe of hypoechoic
- Bright echogenic cortical surface
- Longitudinal sonogram