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PULSED ULTRASOUND

US PULSES

Most US relies on pulses of US waves passing into the tissues followed by a pause during which the machine listens to the echoes and translates these echoes into a grey scale image on the screen. Usually one pulse is 2-3 cycles (or wavelengths) of US. 

PULSE DURATION (PD)

Pulse duration is the amount of time taken for each pulse to occur and is measured in microseconds. As you can imagine, in a high frequency US wave with very short wavelengths, the PD is shorter than for a low frequency US wave in which each wavelength is much longer. Further, the more cycles per pulse, the longer the PD. 

PULSE REPETITION FREQUENCY (PRF) and FRAME RATE (FR)

Pulse repetition frequency (PRF) is the number of pulses which occur per second and is measured in kHz.

The concept of PRF is very important for your understanding of some artefacts and resolution. A machine sends out a pulse of US and in the pause, it assumes that all returning echoes are from that pulse. From the time delay of the returning echoes (and knowing the average speed of US in tissues - 1540m/s), the machine can calculate the depth of the object which reflected the US wave to create the echo, Further, from the amplitude of the returning echo, it can calculate the brightness (echogenicity) of the object. However, if a new pulse has occurred before some of the echoes from the previous pulse return, you can see how the machine could get confused and create artefacts.

PRF is also important for the resolution of moving objects. If more pulses are generated per unit of time, the machine receives more information per unit of time. This allows the machine to update the information on the screen more frequently. This called the frame rate. The higher the frame rate, the more accurate the depiction of moving structures. 

SPATIAL PULSE LENGTH (SPL)

If the number of pulses per unit of time is the PRF, the spatial pulse length is the length of each pulse. This is calculated by multiplying the number of wavelengths in each pulse by the length of each wave.

The shorter the SPL, the better the resolution of the image on the screen. A shorter SPL means that the machine can read that two objects which are close together in a vertical orientation as seperate. Because high frequency US waves have shorter wavelengths, the SPL of high frequency US is intrinsically shorter. This is another reason for better resolution with high frequency US. 

So just to recap if you want better resolution of the image, use the highest frequency US transducer possible (allowing shorter wavelengths) and decrease the pulse duration by decreasing the number of cycles per pulse (these will both decrease the SPL) and increase the PRF.

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