2 The propagation of ultrasound through tissue
Bone attenuates much more than soft tissue
Attenuation in bone is much greater than in soft tissue. Attenuation coefficients in the range 10–20 dB cm⁻1 have been reported at 1 MHz for cortical and skull bone. Attenuation in trabecular bone is highly variable, probably due to the contribution from scatter.
2.2.4 Beam structure and frequency content Diagnostic pulses are typically shorter than 1μs and contain a spectrum of frequencies
In practice, a number of other characteristics of beams of sound are significant for the complete description of the transmission of ultrasound through tissues. The structure of a beam of ultrasound close to its source can be highly complex (Humphrey and Duck, 1998). Of particular practical interest are the beams from the pulsed transducers that are widely used in medical diagnostic applications. Such sources emit very short pulses, being typically only two or three cycles, about 0.5 μs, in duration. The energy in these pulses of ultrasound is contained in a band of frequencies extending both above and below the resonant frequency of the ultrasound transducer that forms the source.
Focusing increases the intensity by up to 50 times, excluding attenuation effects
Diagnostic beams are also focused. This is done to reduce the beam width in order to improve imaging resolution. Focussing has the additional effect of increasing the acoustic pressure and intensity (see below) in the focal zone. The degree of focussing is weak, however, giving an increase in pressure amplitude of no more than about a factor 7, equivalent to a gain in intensity of about 50. In tissue, this increase is reduced because of attenuation of the tissue lying between the transducer and the focus.
2.2.5 Acoustic power and intensity Acoustic power is a measure of the rate of energy flow
The total acoustic power emitted by the transducer is of central importance when considering its safe use. Acoustic power is a measurement of the rate at which energy is emitted by the transducer measured in watts: that is, joules per second. Acoustic powers in diagnostic beams vary from less than 1 mW to several hundred milliwatts. All this power is absorbed by the tissue, and, as a result, the temperature of the tissue is raised slightly. Although the power is delivered in very short pulses, it is more relevant to heating to average out the effects and to consider only the average power over many seconds.
Maps of acoustic intensity describe the spatial distribution of power
Whilst acoustic power is important, it is also relevant to describe how that power is distributed throughout the beam and across a scanning plane, so that local “hot-spots” may be quantified. This variation in “brightness” is measured as acoustic intensity, which is obtained by averaging the power over an area. The practical unit of measurement is milliwatt per square centimetre, mW cm⁻2. The area may cover the whole beam, or a very local part of the beam. A commonly quoted intensity is the “spatial-peak temporalaverage intensity, Ispta”, which is the greatest intensity in the beam, where the beam is “brightest”. For an unscanned beam, such as that used for pulsed Doppler or M-mode, this will be in the focal zone: for a scanned beam, it may occur much closer to the transducer, particularly for sector scan formats. Acoustic power and spatial-peak time-average intensity only give information about energy deposition when averaged over extended periods of time. Other acoustic quantities are used when it is necessary to describe the magnitude of the pulse itself; for example, 10