Thermal contribution of compact bone to intervening tissue-like media exposed to planar ultrasound

Abstract

The presence of bone in the ultrasound beam path raises concerns, both in diagnostic and therapeutic applications, because significant temperature elevations may be induced at nearby soft tissue-bone interfaces due the facts that ultrasound is (i) highly absorbed in bone and (ii) reflected at soft tissue-bone interfaces in various degrees depending on angle of incidence. Consequently, in ultrasonic thermal therapy, the presence of bone in the ultrasound beam path is considered a major disadvantage and it is usually avoided. However, based on clinical experience and previous theoretical studies, we hypothesized that the presence of bone in superficial unfocused ultrasound hyperthermia can actually be exploited to induce more uniform and enhanced (with respect to the no-bone situation) temperature distributions in superficial target volumes. In particular, we hypothesize that the presence of underlying bone in superficial target volume enhances temperature elevation not only by additional direct power deposition from acoustic reflection, but also from thermal diffusion from the underlying bone. Here we report laboratory results that corroborate previous computational studies and strengthen the above-stated hypothesis. Three different temperature measurement techniques, namely, thermometric (using fibre-optic temperature probes), thermographic (using an infrared camera) and magnetic resonance imaging (using proton resonance frequency shifts), were used in high-power short-exposure, and in low-power extended-exposure, experiments using a 19 mm diameter planar transducer operating at 1.0 and 3.3 MHz (frequencies of clinical relevance). The measurements were performed on three technique-specific phantoms (with and without bone inclusions) and experimental set-ups that resembled possible superficial ultrasound hyperthermia clinical situations. Results from all three techniques were in general agreement and clearly showed that significantly higher heating rates (greater than fourfold) were induced in soft tissue-like phantom materials adjacent (within approximately 5 mm) to a bovine bone as compared to similar experiments without bone inclusions. For low-power long-exposure experiments, where thermal conduction effects are significant, the thermal impact of bone reached at distances > 10 mm from the bone surface (upstream of the bone). Therefore, we hypothesize that underlying bone exposed to planar ultrasound hyperthermia creates a high-temperature thermal boundary at depth that compensates for beam attenuation, thus producing more uniform temperature distribution in the intervening tissue layers. With appropriate technology, this finding may lead to improved thermal doses in superficial treatment sites such as the chest wall and the head/neck.

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“…Contrary, some researchers believe that the ability of LIPUS to stimulate changes in tissues and cells may be related to the temperature rising effects induced by energy absorption. [51][52][53][54] High intensities (1000-3000 mW/cm 2 ) have temperature effects and can cause considerable heating of tissues. Whereas the heating effect from LIPUS (20 to 50 mW/cm 2 ) is estimated <11°C, [54] some enzymes such as matrix metalloproteinase or interstitial collagenase have been shown to be very sensitive to small variations in temperature.…”

O ultra-som terapêutico de 1 MHz, na dose de 0,5 W cm-2, sobre o tecido ósseo de cães avaliado por densitometria óptica em imagens radiográficas

“…Proteínas são as que mais absorvem as ondas sonoras. Esta absorção resulta em aquecimento do tecido, então tecidos com alto conteúdo protéico (ossos e tendões) se aquecem muito mais que tecidos adiposos ou epiteliais (KOTTKE & LEHMANN, 1994;LONGO, 1996;MOROS et al, 2004).…”

“…Isso também causa ondas transversais no periósteo, as quais também ajudam a aumentar a temperatura local. Como no periósteo não existe o efeito de resfriamento proporcionado pela corrente sangüínea, esta situação levará a um superaquecimento do local e conseqüentemente a dor ao paciente (NCRP, 1983;KOTTKE & LEHMANN, 1994;LONGO, 1996;MOROS et al, 2004).…”

“…A presença de osso no caminho da irradiação de ultra-som sempre aumentou as preocupações, tanto em diagnóstico como em aplicações terapêuticas (MOROS et al, 2004) elevações de temperatura significativas próximo à interface tecido mole/osso, devido ao fato de o ultrasom ser altamente absorvido no tecido ósseo e de haver a reflexão de ondas irradiadas. Além disso, a interferência destas ondas pode resultar em um aumento brusco da intensidade irradiada (NCRP, 1983;MOROS et al, 2004).…”

“…Além disso, a interferência destas ondas pode resultar em um aumento brusco da intensidade irradiada (NCRP, 1983;MOROS et al, 2004). Por conseqüência, na terapia ultra-sônica térmica, a presença de osso no caminho das ondas de ultra-som é considerada a principal desvantagem e normalmente é evitada (MOROS et al, 2004).…”