Burst – As the name suggests, a burst is a short segment of ultrasound “pulse” that is sent into the tissue. For general knowledge, B-mode has the shortest burst to get the best image resolution, while transcranial doppler uses a long burst due to attenuated echos.
Duty Factor – DF, or duty cycle is the fraction of time the ultrasound probe is transmitting, displayed in percentages. If you send a ultrasound burst of 5 μsec and the pulse repetition period (PRP) is 250 μsec, the DF is 5/250 = 0.02 or 2%.
Attenuation – Attenuation is the amount of ultrasound energy absorbed by the tissue. It is measured in half-value layers. To simplify things further, the half-value layers in wavelengths, or HVLW, expresses half-value layers in wavelength terms. A potential exam question may ask you to calculate how much energy is left after going through certain HVLWs. To do this, simply multiple the initial energy by 1/2 raised to the number of HVLWs.
Remaining energy = initial energy x (1/2)#HVLW.
Just a word of caution when calculating energy loss, by going through 2 HVLWs, remaining ultrasound does not equal to zero. If you start with 100J, after 2 HVLWs, your remaining energy is 100 x (1/2)2 = 100 x 1/4 = 25J.
Ultrasound Frequency In Relation to Tissue Tickness
The calculations of ultrasound frequency F and tissue type and thickness is convoluted.
F = 2/T x 1/(alpha/10) x 1/ln(10)
just remember that frequency F is inversely related to tissue thickness, and that the smaller the T, the higher the frequency. The deeper and thicker the tissue, the lower the frequency is required to provide the best doppler echo strength.
Aliasing, Pulse Repetition Frequency (PRF), and Nyquist Limit
Pulse Repetition Frequency is calculated by ultrasound speed in tissue C and distance of the sample volume d.
PRF = C/2d
PRF must be high enough to obtain 2 samples from each cycle of doppler signal. Therefore, the returning doppler signal can only be as high as half of the PRF. This is called the Nyquist limit.
Nyquist Limit = 1/2(PRF)
For example, to sample a blood vessel at a depth of 5 cm. PRF is calculated by sound speed C = 1540 m/s or 154,000 cm/s.
PRF = 154000/(2×5) = 15400, or 15.4 kHz.
The Nyquist limit is therefore:
Nyquist = 15.4kHz/2 = 7.7 kHz.
If the blood velocity is so high such that the doppler shift exceeds 7.7 kHz, then PRF must be increased to avoid aliasing.
Transducers:
In transmit focus, the focus is electronically controlled by separate pulse-echo cycles. When receiving, the focus is also electronically controlled by the timing of the receiver getting signals back from various depths.