Ultrasound is a longitudinal wave with a frequency greater than 20kHz. It is non-ionising, non-invasive and quick.
Ultrasound used in medical imaging typically has frequencies of 1-15MHz, arriving in pulses with a frequency of 5kHz. It is generated and detected by ultrasound transducers.
Piezoelectric effect: crystals, such as quartz, produce an electromotive force when they are compressed, stretched, twisted or distorted. This is a reversible process, so when an external p.d. is applied the crystal stretches and compresses. Highfrequency alternating p.d. whose frequency matches the natural frequency of oscillation of the crystal causes resonance, permitting production of an intense ultrasound signal. A transducer is a device that emits and receives ultrasound.
Two types of ultrasound scan exist:
- A-scan: a single transducer is used to record along a straight line through the patient, allowing calculation of thickness of bone or distance between the lens and retina. The pulsed voltage at the transducer is displayed on an oscilloscope or computer screen.
- B-scan: the transducer is moved over the patient’s skin with the transducer output connected to a high-speed computer. Each position of the transducer produces a row of dots corresponding to the boundary between two tissues. The brightness of the dot is proportional to the intensity of the reflected ultrasound. The collection of dots makes up a 2D image of a section through the patient.
The acoustic impedance of a medium is the product of the density of the medium and the speed of ultrasound in it.
The units of acoustic impedance is kgm-2s-1. The reflected intensity of a collimated beam of ultrasound incident at the boundary between two substances with different acoustic impedances is calculated by:
Where is the intensity reflection coefficient, 1 is the acoustic impedance of the original medium and 2 is the acoustic impedance of the new medium.
To reduce reflection of ultrasound at the air-skin boundary, a coupling gel with an acoustic impedance very similar to that of the skin is used. The gel fills the gap between the transducer and skin, ensuring that all the ultrasound enters the patient’s body. This is impedance matching.
The Doppler effect also plays a role in ultrasound imaging, allowing doctors to monitor blood flow through major arteries and veins. Analysing the change in frequency between emitted and detected ultrasound indicates the rate at which blood is moving towards or away from the transducer.
Where is the angle between the direction of the ultrasound beam and the direction of blood flow in the vessel.