Ultrasound
Medical Imaging - OCR A-Level Physics
Key Definition
Ultrasound
Sound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz.
Sound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz.
- UltrasoundSound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz. is produced and detected using a piezoelectric transducerA crystal that converts electrical signals to ultrasound pulses (and vice versa) via the piezoelectric effect..
- The piezoelectric effectThe property of certain crystals to generate a voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. when mechanically deformed, and to deform when a voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. is applied. Used to produce and detect ultrasoundSound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz..: certain crystals (e.g. quartz, PZT) generate a voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. when mechanically deformed, and deform when a voltage is applied.
- An alternating voltage applied to the crystal makes it vibrate at the driving frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz)., producing ultrasoundSound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz. pulses.
- Reflected ultrasound pulses return to the same crystal, causing it to vibrate and generate a voltage - this is the detected signal.
- A coupling gelA gel applied between the ultrasound transducer and the skin to eliminate the air gap and prevent reflection. is applied between the transducer and skin to eliminate the air gap. Air has very different acoustic impedanceThe product of the densityMass per unit volume of a material. Measured in kg m⁻³. of a medium and the speed of sound in that medium. Determines how much ultrasound is reflected at a boundary. from skin, so without gel, almost all the ultrasound would be reflected at the skin surface.