Of What Potential Significance Is the Beam Non-Uniformity Ratio (BNR)

Of what potential significance is the beam non-uniformity ratio (BNR) of an ultrasound transducer?

A) It is the ratio of the power of the ultrasound to the duty factor of the ultrasound.
B) It is the ratio of the peak power to the average power in the cross section of the transducer.
C) It is the ratio to the surface area of the transducer in square centimeters to measurable acoustical energy leaving the crystal in Watts.
D) It is a ratio of the surface area of the transducer and how pulsed the energy is over the average area.