Infographics / Ultrasound Physics

Q factor, damping and bandwidth

A side-by-side comparison of two transducers driven at the same centre frequency. On the left, a high Q (lightly damped) element rings for many cycles giving a long pulse and a tall, narrow frequency spectrum. On the right, a low Q (heavily damped) element rings for only a couple of cycles giving a short pulse and a low, broad spectrum. Q equals resonant frequency divided by bandwidth: narrow bandwidth means high Q, suited to continuous-wave and Doppler; broad bandwidth means low Q, giving short pulses and better axial resolution for imaging.

Q factor and damping in ultrasound transducers A side-by-side comparison of two transducers driven at the same centre frequency. On the left, a high Q (lightly damped) element rings for many cycles giving a long pulse and a tall, narrow frequency spectrum. On the right, a low Q (heavily damped) element rings for only a couple of cycles giving a short pulse and a low, broad spectrum. Q equals resonant frequency divided by bandwidth: narrow bandwidth means high Q, suited to continuous-wave and Doppler; broad bandwidth means low Q, giving short pulses and better axial resolution for imaging. High Q transducer lightly damped — long ring-down Low Q transducer heavily damped — short ring-down Emitted pulse — both driven at the same centre frequency time time long spatial pulse length → poorer axial resolution short SPL → better axial resolution Frequency spectrum (Q = f0 / Δf) frequency frequency f0 f0 narrow Δf broad Δf high Q = narrow band low Q = broad band efficient transmitter; suits continuous-wave / Doppler good broadband receiver; suits pulsed imaging (axial resolution)
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