Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Thermal and mechanical indices of ultrasound operation are now the acceptedmethod of determining power levels for real-time instruments that provide theoperator with quantitative estimates of power deposition in the patient. Theseindices are selected for their relevance to risks from biologic effects, and are displayedon the monitor during real-time scanning. The sonographer can use theseindices to minimize power deposition to the patient (and fetus) consistent withobtaining useful clinical images in the spirit of the ALARA (as low as reasonablyachievable) concept.The thermal index, TI, is the ratio of the acoustic power produced by the transducerto the power required to raise tissue in the beam area by 1°C. This is estimated bythe ultrasound system using algorithms that take into account the ultrasonic frequency,beam area, and the acoustic output power of the transducer. Assumptionsare made for attenuation and thermal properties of the tissues with long, steadyexposure times. An indicated TI value of 2 signifies a possible 2°C increase in thetemperature of the tissues when the transducer is stationary. TI values are associatedwith the ISPTAmeasure of intensity.On some scanners, other thermal indices that might be encountered are TIS (Sfor soft tissue), TIB (B for bone), and TIC (C for cranial bone). These quantitiesare useful because of the increased heat buildup that can occur at a bone-soft tissueinterface when present in the beam, particularly for obstetrics scanning of late-termpregnancies, and with the use of Doppler ultrasound (where power levels can besubstantially higher).Cavitation is a consequence of the negative pressures (rarefaction of the mechanicalwave) that induce bubble formation from the extraction of dissolved gases in themedium. The mechanical index (MI) is a value that estimates the likelihood of cavitationby the ultrasound beam. The MI is directly proportional to the peak rarefactional(negative) pressure, and inversely proportional to the square root of theultrasound frequency (in MHz). An attenuation of 0.3 (dB/cm)/MHz is assumedfor the algorithm that estimates the MI. As the ultrasound output power (transmitpulse amplitude) is increased, the MI increases linearly, while an increase in thetransducer frequency (say from 2 to 8 MHz) decreases the MI by the square root of4, or by a factor of two. MI values are associated with the ISPPAmeasure of intensity.Biologic Mechanisms and EffectsDiagnostic ultrasound has established a remarkable safety record. Significant deleteriousbioeffects on either patients or operators of diagnostic ultrasound imagingprocedures have not been reported in the literature. Despite the lack of evidencethat any harm can be caused by diagnostic intensities of ultrasound, it is prudentand indeed an obligation of the physician to consider issues of benefit versus riskwhen performing an ultrasound exam, and to take all precautions to ensure maximalbenefit with minimal risk. The American Institute of Ultrasound in Medicine