The acoustic output of diagnostic ultrasound scanners 3
used but, in either case, it is preferable to avoid type T thermocouples since the thermal conductivity of copper is very high and distorts the temperature fi eld more than other types. Again, it is often instructive to observe how the temperature varies as the scanner controls are adjusted (Figure 3.10). Thermal phantoms can be made to mimic particular tissue paths and have an important role in evaluating any potential hazard arising from ultrasound-induced heating. Shaw et al. (2011) used a phantom designed to mimic the neonatal head to estimate the temperature rise at several locations in the head due to scanning through the fontanel at typical clinical settings. They found that approximately 35% of the configurations studied gave a temperature increase at the phantom skin surface in excess of 6 °C in less than 10 min. They also found that there was no useful correlation between the displayed TI and the temperature measured in the phantom: the average skin surface temperature on the phantom was 6 times larger than the average TI value. This is because the model for calculating TI completely ignores the self-heating of the transducer, which is actually the dominant factor governing Tsurf. The use of phantoms is not restricted to measurement of surface temperature (Shaw et al., 1998, 1999; IEC62306, 2006).
Tsurf on tissue is usually much higher than the TI value
3.4 Control settings that give the highest output levels Awareness of the control settings that are likely to give the highest output levels is important both for users wishing to avoid high outputs and reduce the MI or TI value for safety reasons, and to measurers who are trying to maximize the output. Those who look for worst-case values must have an understanding of the operating principles of the particular machine, since the number of different possible combinations of control settings can run into millions. The nature and range of controls is constantly changing with the evolution of new scanning features, so provision of a rigid universal protocol is not possible. Controls on some of the newer machines can have quite unexpected effects, as manufacturers often arrange for, say, drive voltages or pulse repetition frequencies to change automatically when controls are set in a way which would otherwise cause a particular safety parameter, such as Ispta or TI, to exceed a predetermined limit. However, B-mode— harmonics off
Move focus from 10 cm to 5 cm
Colour plus PW Doppler
Set image depth to maximum
TIS was 1.9 here Trying to maximize TIS
35
Temperature (°C)
33 31 29 27 25 23 0 B-mode— harmonics on
2
4 Reduce sector width
6 Colour-flow (narrow box)
8
10 Time (min)
12
14
16
18
20
Gate depth from 10 cm to 5 cm
Figure 3.10. Example showing the variation in the surface temperature of a 3 MHz linear array transducer as scanner settings are adjusted. 31
Awareness of the control settings is important for users wishing to avoid high outputs and reduce the MI or TI value for safety reasons