Lateral resolution is the resolution of reflectors located in the plane perpendicular to the long axis of the ultrasound beam.
Lateral resolution is also known as angular, transverse or azimuthal resolution.
Lateral resolution depends on:
– beam width
– beam frequency
– scan line density
Axial resolution is the ability to distinguish two objects with different positions along the long axis of the ultrasound beam.
Axial resolution depends on the spatial pulse length (SPL). Two reflectors have to be separated by at least half the SPL to be resolved:
Axial resolution = 0,5 x SPL
This means that the resolution is increased with shorter spatial pulse length. One pulse is typically 2-3 wave cycles. As frequency and wavelength is inversely related, SPL is shortened proportional with higher frequency.
Axial resolution is also improved by the damping layer of the transducer because it reduces the number of wave cycles per pulse.
When a bell is struck it continues to ring if it is not stopped mechanically.
When a transducer is excited by the electrical voltage it vibrates 2-3 wavelengths until stopped by the damping layer.
The vibrations that excite an ultrasound crystal generate an ultrasound beam.
One vibration of the crystal is a short pulse comprising only 2-3 cycles because of the damping layer of the transducer. The wavelength remains constant but the amplitude decreases with time – just like the bell.
Between two consecutive pulses is a pause of silence where the transducer crystal serves as a receiver of the echoes reflected from tissue interfaces struck by the ultrasound beam.
The duration of the pulse = wave cycle period (i.e. duration) multiplied by the number of wave cycles per pulse.
Spatial pulse length (SPL) = wavelength multiplied by the number of wave cycles per pulse.
The rate of pulse generation is called the pulse repetition frequency (PRF). In other words the number of pulses emitted from the transducer crystal per second. It is NOT the vibration frequency of the transducer.
This module presents an overview of the basic functional groups and the operating modes of the ultrasound system.
The learning objective is to obtain a theoretical understanding of how the ultrasound system controls imaging.
The ultrasound system consists of the following functional groups:
– transmitter
– transducer
– beamformer
– receiver
– scan converter
– display device
– master synchronizer
– user interface
Generally speaking a transducer is any device that converts one form of energy into another.
An ultrasound transducer converts electrical energy to ultrasound waves and converts reflected sound waves back to electrical pulses.
An ultrasound transducer contains piezoelectric crystals that convert electrical energy to sound.
When an alternating voltage is applied to each piezoelectric crystal in the transducer, the crystal vibrates and emits ultrasound waves with a specific wave length and frequency determined by its type and size. In this way, electrical energy is converted into sound energy. This is known as the piezoelectric effect.
The reverse piezoelectric effect signifies that reflected sound waves cause charge shifts in the crystals that can be measured as alternating voltage signals. The same crystal can therefore be used as both emitter and receiver.
The piezoelectric effect and the reverse piezoelectric effect are responsible for the ability to both generate and receive sound in an ultrasound probe.
Each piezoelectric crystal is connected by an electrode conducting the pulsing electricity and the returning piezoelectric impulses after conversion back to electricity. A crystal is typically pulsed by electrical current for less than 100 nanoseconds (1 nanosecond = 10-9 second).
All modern transducers are electronic. Electronic is also called
To generate ultrasound waves the piezoelectric crystals of the transducer have to be excited by electrical voltage.
The transmitter (syn: pulser or pulse transmitter) consists of a pulse generator and a clock.
The pulse generator produces high voltage spikes of typically 100-900 V. The clock controls the duration of the pulse which is typically 1-2 microseconds.
