Answer #16

The correct answer is C: Pulse repetition period. Having good working knowledge of all of these variables is important for understanding basic principles of ultrasound physics. Doing many questions on these topics should help your understanding.

Let's go through them one by one. Axial resolution is defined as spatial pulse length or SPL divided by 2. Spatial pulse length is just the distance of the all of the cycles in a pulse and is calculated as cycle wavelength x number of cycles in a pulse. Since changing the frequency changes the wavelength (they are inversely related) then SPL is also affected. The bottom line is that axial resolution is affected when we change frequency. Increasing frequency, decreases SPL and decreases axial resolution. Choice A is therefore not correct.

Pulse duration (PD) is the period of one cycle x the number of cycles. Since frequency and period are inversely related, changing frequency definitely changes PD. Choice D is therefore not correct.

Pulse repetition period (PRP) is based only on the depth/and velocity of propagation. Neither of these depend on the frequency. The depth can be set by the user and the velocity of propagation is a property of the medium. Hence choice C is correct.

Duty Factor is the PD/PRP or the fraction of time spent transmitting a pulse. For PW Doppler this number is usually very low. For CW Doppler this number if 1 (always transmitting). Since Duty Factor depends on PD and PD depends on frequency, this Duty Factor also depends on frequency and so choice B is not correct.