Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

In the pulsed mode of ultrasound operation, the instantaneous intensity variesgreatly with time and position. At a particular location in tissue, the instantaneousintensity is quite large while the ultrasound pulse passes through the tissue, but thepulse duration is only about a microsecond or less, and for the remainder of thepulse repetition period the intensity is nearly zero.The temporal peak, hp, is the highest instantaneous intensity in the beam; thetemporal average, hA, is the time-averaged intensity over the pulse repetitionperiod; and the pulse average, IpA,is the average intensity of the pulse (Fig. 16-56B).The spatial peak, Isp, is the highest intensity spatially in the beam, and the spatialaverage, ISA,is the average intensity over the beam area, usually taken to be the areaof the transducer (Fig. 16- 56C).The acoustic power contained in the ultrasound beam (watts), averaged over atleast one pulse repetition period, and divided by the beam area (usually the area ofthe transducer face) is the spatial average-temporal average intensity ISATA.Othermeaningful measures for pulsed ultrasound intensity are determined from IsATA,including the following:1. The spatial average-pulse average intensity, ISAPA= ISATA/ duty cycle, where IrA =hA/duty cycle.2. The spatial peak-temporal average intensity, ISPTA= ISATA(Isp/IsA), which is agood indicator of thermal ultrasound effects.3. The spatial peak-pulse average intensity, ISPPA= ISATA(Isp/IsA)/duty cycle, anindicator of potential mechanical bioeffects and cavitation.For acoustic ultrasound intensity levels, ISPPA> ISPTA> ISAPA> ISATA.Typicalacoustical power outputs are listed in Table 16-9. The two most relevant measuresare the ISPPAand the ISPTA.Both measurements are required by the U.S. Food andDrug Administration (FDA) for certification of instrumentation. Values OfISPTAfordiagnostic imaging are usually below 100 mW/cm 2 for imaging, but for certainDoppler applications ISPTAcan exceed 1,000 mW/cm 2 • ISPPAcan be substantiallygreater than ISPTA,as shown in Table 16-8. For real-time scanners, the combinedintensity descriptors must be modified to consider dwell time (the time the ultrasoundbeam is directed at a particular region), and the acquisition geometry andspatial sampling. These variations help explain the measured differences betweenthe ISPTAand ISPPAvalues indicated, which are much less than the duty cycle valuesthat are predicted by the equations above.TABLE 16-9. TYPICAL INTENSITY MEASURES FOR ULTRASOUND DATACOLLECTION MODESMode Pressure Amplitude (MPa) ISPTA(mW/cm 2 ) ISPPA(W/cm 2 ) Power (mW)B-scan 1.68 19 174 18M-mode 1.68 73 174 4Pulsed doppler 2.48 1,140 288 31Color flow 2.59 234 325 81Adapted from compilation of data presented by the American Institute of Ultrasound in Medicine.Note the difference in units for ISPTA(mWlcm 2 ) versus ISPPA(W/cm 2 ).