6 Radiation force and its possible biological effects
6.4.2 Effects of radiation force on soft tissue A number of papers report observations of radiation force effects on soft tissues. These may be grouped into two categories, as either physical effects or sensory effects. 6.4.2.1 Physical effects on tissue Radiation force has been associated with some physical effects in tissue
Several papers in the literature report physical effects on tissue which the authors believe may be better explained as radiation force effects than thermal or cavitation eff ects. Lizzi et al. (1981) have reported blanching of the choroid of the eye prior to the onset of thermal damage. It has been suggested that this occurred due to radiation force causing compression of the blood vessels. Dalecki et al. (1997a) used an experimental lithotripter to deliver ultrasound pulses to the abdomen of pregnant mice. Pulse amplitudes were in the diagnostic range, but the pulse powers used were higher. The foetal tissue showed evidence of haemorrhage, but only where the soft tissue was near to developing bone or cartilage. The authors suggest that this could result from the relative motion between ossified bone and surrounding soft tissue, caused by radiation force on the bone.
The accelerated tissue repair observed in an acoustic field is probably due to mechanical forces
There have been a number of reports of accelerated healing of bone fractures in vivo using low intensity pulsed ultrasound (Kristiansen et al., 1997; Heckman et al., 1994). Although the precise biophysical mechanism is unknown, it has been suggested that it arises from the application of mechanical force to the cellular system. An associated alteration in gene expression has been reported (Yang et al., 1996; Parvisi et al., 1999; see also Chapter 7). Enhancement of soft-tissue regeneration has also been reported using low intensity therapeutic ultrasound (Dyson et al., 1968, 1970). The eff ect, which is greatest during the early stages of regeneration, was not attributed to heating due to the low intensities employed. 6.4.2.2 Sensory effects on tissue
Radiation forces can alter neurosensory responses
Preliminary observations of altered neuronal migration have been attributed to radiation force
Several papers have suggested radiation force as the biophysical mechanism for neurosensory responses. Dalecki et al. (1995) have demonstrated that it is possible to feel the radiation forces that are exerted on the skin by an ultrasound beam. Also reported by Dalecki et al. (1997b) was a decrease in aortic pressure caused by ultrasound insonation of frog hearts. The authors demonstrated that radiation force was responsible by showing an equivalent effect when the beam was incident on a total absorber in contact with the surface of the heart. A number of papers have reported that the auditory nerve may be directly stimulated by ultrasound (Magee and Davies, 1993). The mechanism is unknown but we can speculate that it is the direct effect of the varying force field across the neural structures. 6.4.2.3 Developmental effects Ang et al. (2006) reported evidence that exposure to ultrasound from a clinical scanner caused partial inhibition of neural migration in the embryonic cerebral cortex of mice. In the absence of evidence suggesting heating or cavitation as the cause, radiation force was proposed as being the most likely mechanism. 86