You can indirectly tell the quality of the crystal in the Ultrasound (US) transducer head by the value of the beam nonuniformity (BNR) ratio which is indicated on the US unit. (Tip; the higher the BNR value, the poorer the quality of the crystal and vice versa).
The BNR is the maximal intensity divided by the average intensity (in w/cm2) found in the ultrasound field. Each transducer produces sound waves in response to the vibration of the crystal. This vibration has different intensities at points on the transducer head, with peaks and valleys of intensity. The smaller the ratio of highest intensity/average intensity, the more homogeneous the beam. This ensures patients are getting uniform and consistent ultrasound treatment. Conversely, the greater the ratio difference in the BNR, the more likely the transducer will have hot spots. High intensities have been shown to retard tissue repair or cause unstable cavitation effects such as microvessel trauma. It is recommended that the BNR be kept as low as possible. Low BNRs are more comfortable for the patient and have a more focused treatment area.
A useful guide is that ultrasound treatment with a high BNR (>5:1) would be painful, whereas a low BNR (<4:1) treatment would be comfortable. A completely homogeneous wave is represented by a 1:1 BNR.
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