Page 92 - Libro vascular I

Chap-07.qxd 29~8~04 14:42 Page 83
OPTIMIZING THE SCAN
83
mechanical index (MI). These indices are displayed on the screen of modern scanners in real time and will demonstrate any changes in the potential risk as the scanner modalities or controls are altered.
The TI has been developed to indicate the potential risk of producing thermal effects during the scan. It is the ratio of acoustic power emitted at the time to the power required to heat the tissue by 1°C. A TI of 1 would therefore indicate the potential to heat the tissue in the beam by 1°C. A TI of 2 indicates a potential rise of 2° C, and so forth. An ultrasound exposure that does not pro- duce a temperature rise of greater than 1.5°C above normal body temperature of 37°C is not thought to pose any risk of producing thermal damage. The power required to heat the tissue will depend greatly on what tissue is lying in the path of the ultrasound beam and is especially affected by the presence of bone, as bone is a strongly absorb- ing medium. For this reason, three models for the TI have been developed:
● soft tissue (TIS)
● tissue with bone present at the focus (TIB)
● the cranial thermal index (TIC), used in tran-
scranial Doppler.
The appropriate TI should be displayed depend- ing on the scanner examination set-up selected. The development of these indices suffers from some limitations, as it is not straightforward to estimate the heat lost from the various regions of the body that are scanned.
The mechanical index (MI) indicates the likeli- hood of the onset of inertial cavitation. It is related to the peak negative pressure of the ultrasound pulses being used at the time. For an MI 0.7, the physical conditions probably cannot exist for bubble growth and collapse to occur (Duck & Shaw 2003). How- ever, if this threshold is exceeded, it does not mean that bioeffects due to cavitation will occur. The higher the value of MI above this threshold, the greater the potential risk. There is currently no evi- dence that diagnostic ultrasound causes cavitation in the soft tissue, except in the presence of gas, such as in the lung and intestines, and in the presence of contrast agents.
Another potential thermal hazard that the sonog- rapher should be aware of is heating of the trans- ducer itself, which may occur if the transducer has
been damaged. Malfunction of the scanner may potentially lead to a higher than expected output power.
User’s responsibility
Diagnostic ultrasound has been used for many years with no reported evidence of harmful effects. However, it is prudent to keep patient exposure to the minimum required to obtain an optimal diagnostic result. This can be done by keeping the time of the examination of a particular area to a minimum, especially when using color and spectral Doppler ultrasound, as these modes are more likely to cause heating. Controls such as the gain should be optimized before increasing the output power. Changes in the TI and MI with changes in scanner set-up should be monitored. It is important to keep up-to-date with current guidelines on the safe use of diagnostic ultrasound (BMUS Safety Group 2000, EFSUMB).
Probably the biggest risk of ultrasound is misdi- agnosis, and it is therefore important to obtain an adequate scan. The sonographer should be aware of new technologies and new developments in scanning techniques. If the sonographer is in any doubt of the result at the end of the scan, the lim- itations of the scan should be reported.
Infection control
Cross-infection of patients by the ultrasound trans- ducer is a possible risk, therefore the transducer should be cleaned between each examination. The front face of transducers can be made of delicate material and the use of strong cleaning fluids is often not recommended. Consult the operating manual or manufacturer for advice on suitable cleaning pro- cedures. The best method to overcome this problem, when there is a known risk, is to use a disposable probe cover. If the scan is to be performed near an open wound, a sterile probe cover and sterile gel should be used. Alternatively, a sterile transparent plastic dressing may be used to cover the wound, ensuring no air bubbles are trapped under the dress- ing that would prevent imaging. Disposable gloves should always be worn if scanning infected or dis- charging regions.