**2. Main body**

#### **2.1. Physical bases, technical approach and quality assurance of ultrasonic equipment**

Ultrasound (US) is a mechanical non‐ionizing radiation, which propagates in a medium transferring energy from one particle to another by molecular oscillation. The longitudi‐ nal waves (compression) can propagate in any medium, while the transverse waves are observed only in solids, because of the weak links that are established between atoms and molecules in the tissue fluids. The ultrasonic wave is mainly longitudinal in biological tis‐ sues and characterized by alternating compression and rarefaction of the medium in which it propagates, with variations in pressure within it. Related to the pressure amplitude, which describes the degree of compression and rarefaction and thus the strength per unit area to which the material is subjected in unit of Pascal (Pa = N/m2 ) and its multiples (e.g. MPa), also the concept of power of an ultrasound beam, i.e. the energy transmitted in the time unit measured in Watt and that of wave intensity, that is the amount of energy flowing in the time unit through a surface of unit area, perpendicular to the direction of wave propagation (mea‐ sured in W/cm2 ) are important. The intensity varies over time both in the case of continuous wave (CW) or pulsed wave (PW): in particular, the presence of a pulsed field introduces a temporal variation, defining a duty cycle (DC) as the ratio between the pulse duration ultrasound (in time units) and the length of the period, calculated as a percentage. Therapies that employ the US can be divided into two groups: 'high power' and 'low power' [1]. The high power applications include HIFU (high‐intensity focused ultrasounds) and lithotripsy, while low‐power applications include physical therapy, sonophoresis, sonoporation and gene therapy. When a US wave proceeds from one medium to another, it is partly reflected and partly transmitted, according to the laws of classical mechanics. Each medium is inher‐ ently characterized by a complex quantity, the impedance *Z*, which synthesizes the acoustic characteristics of the medium and quantifies the resistance that the medium itself opposes the passage of sound waves. The acoustic impedance is defined as the product of the density *ρ* of the medium (kg/m3 ) for the propagation velocity *c* (m/s)

led to the need for a thorough understanding of the interaction between ultrasonic waves and biological matter. Scientific evidence of US therapeutic effectiveness in rehabilitation fields requires more attention on the technical specifications of devices used, and the exact parameters applied in the treatment of selected patients in order to avoid standardized

(2) Towards physiotherapeutic US dosimetry: treatment planning by the assessment of thermal and mechanical effects. Quantitative assessment of thermal and mechanical effects, and their dependence on the US parameters (frequency, emitted power, pulsed or continue waves) and the treatment modalities (fixed field or massage, duration of the treatment) may be quantitatively investigated on joint mimicking phantoms made of muscle‐equivalent agar‐based material and bone disks. '*In vitro*' temperature increases can be predicted, which are the 'asymptotical values' in the absence of blood perfusion

(3) Customized 'in‐patient' assessment of clinical effects: clinical, functional and sonogra‐ phycal evaluation can/should be performed before and at the end of the US therapy us‐ ing Numeric Rating Scale, Constant Score, DASH questionnaire and sonographic images

**2.1. Physical bases, technical approach and quality assurance of ultrasonic equipment**

Ultrasound (US) is a mechanical non‐ionizing radiation, which propagates in a medium transferring energy from one particle to another by molecular oscillation. The longitudi‐ nal waves (compression) can propagate in any medium, while the transverse waves are observed only in solids, because of the weak links that are established between atoms and molecules in the tissue fluids. The ultrasonic wave is mainly longitudinal in biological tis‐ sues and characterized by alternating compression and rarefaction of the medium in which it propagates, with variations in pressure within it. Related to the pressure amplitude, which describes the degree of compression and rarefaction and thus the strength per unit area to

also the concept of power of an ultrasound beam, i.e. the energy transmitted in the time unit measured in Watt and that of wave intensity, that is the amount of energy flowing in the time unit through a surface of unit area, perpendicular to the direction of wave propagation (mea‐

wave (CW) or pulsed wave (PW): in particular, the presence of a pulsed field introduces a temporal variation, defining a duty cycle (DC) as the ratio between the pulse duration ultrasound (in time units) and the length of the period, calculated as a percentage. Therapies that employ the US can be divided into two groups: 'high power' and 'low power' [1]. The high power applications include HIFU (high‐intensity focused ultrasounds) and lithotripsy, while low‐power applications include physical therapy, sonophoresis, sonoporation and

) are important. The intensity varies over time both in the case of continuous

) and its multiples (e.g. MPa),

US treatments using 'protocols' and non‐specific parameter settings.

and other heat dissipation mechanisms.

supporting clinical and functional data.

which the material is subjected in unit of Pascal (Pa = N/m2

**2. Main body**

214 Clinical Physical Therapy

sured in W/cm2

$$\mathbf{Z} = \rho \mathbf{c} \tag{1}$$

Its unit of measure is Pa s/m or Rayl, named after the famous British scientist Lord Rayleigh, which is equivalent to kg/m2 s. Propagating in a medium, the acoustic wave is subject to a pro‐ gressive loss of energy and, more properly, it causes a decrease in the intensity as a function of distance from the source. This is due both to the absorption, where the mechanical energy of the waves is partially converted into heat, and to the scattering, where the interaction between the wave and any inhomogeneous structure in the medium determines a partial diffusion of energy along directions different from that of direct wave propagation.

The overall effect, in a homogeneous medium, is such that a field of wave initial intensity *I* 0 after a certain distance *z*, has an intensity, which decreases exponentially according to the equation:

$$I = -I\_0 \exp(-2az) \tag{2}$$

where *I* 0 is the initial intensity *I α* and the absorption coefficient of the medium (cm‐1 ).

The energy absorption of ultrasound within the medium (and the biological tissues as well) depends on the frequency of the waves, being the coefficient of attenuation *α* inversely depen‐ dent on the square of the frequency. This is responsible for the fact that most of the tissues crossed by US exhibit an increase in the absorption coefficient of at least three times when the frequency is increased from 1 to 3 MHz (e.g. from 0.14 to 0.42 cm‐1 in fat, from 1.12 to 3.36 cm‐1 in tendons, from 0.76 to 2.28 cm‐1 in muscle).

Lower absorption (and therefore higher penetration) of the ultrasonic wave is observable in water and in fact as tissue rich in water, and therefore the local heating is not significant. On the contrary, the absorption is much higher in the bone tissue and tendons [2]. In gen‐ eral, soft tissues absorb about 10–20% of the emitted power per centimetre, while adult bone completely absorbs the ultrasound beam in short distances. US at the frequency of 1 MHz is mainly absorbed by tissues that are 3–5 cm from the probe, and precisely for this property they are recommended for deeper lesions and in patients with subcutaneous fat [3].

Note that 3 MHz frequency is instead recommended for more superficial lesions, e.g. 1–2 cm deep [3, 4]. All the above parameters contribute to the effects of US in biological tissues, which are normally accounted for as 'thermal' and 'non‐thermal' effects.

(i) Thermal effects: When US loose energy and the beam is attenuated due to the absorption and dissipation of the ultrasonic energy, heat is produced by vibration, shock, and friction with the cellular and intercellular structures of the crossed tissues. The temperature increase that occurs in the medium can cause chemical or structural changes in biopolymers.

This phenomenon is influenced by both the characteristics of the ultrasound beam (intensity and frequency), the duration of exposure and the characteristics of the crossed tissues (acous‐ tic impedances). Heating is established quickly; however, a thermal equilibrium due to the heat dissipation due to blood flow is reached in longer times.

The thermal effect is most evident at the interface between tissues and in particular at the interface between fat and muscle and at the level of the periosteum. The periosteum, for its anatomical structure and for the continuity with the bone, absorbs a large amount of energy and is therefore easily heated. The thermal elevation generates, as secondary effects, increase in cellular metabolism and vasodilation; in particular, the latter property is important in the use of therapeutic US in physiotherapy, promoting the wash‐out of pro‐inflammatory sub‐ stances and pro‐algogenic tissues.

(ii) Non‐thermal effects: They include cavitation, which consists in the formation, growth and implosion of gas bubbles within the fluid subjected to an ultrasonic field. In general, the cavi‐ tation can be seen as the 'break' of a liquid and the consequent formation inside the same of 'cavities' (bubbles) of the liquid containing dissolved gas or vapour itself. This phenomenon occurs in many situations, for example, in boiling water or in proximity to the propeller in rotation of a ship, and in any case when liquids are subject to high and rapid changes in pres‐ sure and can occur in the use of therapeutic US or in Doppler ultrasound [5].

The almost instantaneous variations of density, pressure and temperature of the fluid in which propagates the ultrasonic wave can also produce the so‐called shock waves or pressure waves which can also be extremely intense. The ultrasonic irradiation of water leads to the formation of the hydroxyl radical and hydrogen radical, which give as the main final products H2 O2 (hydrogen peroxide) and H<sup>2</sup> .

At the cellular level, the production of radicals induced by exposure to the US can also produce biological effects on DNA; theoretical models and experimental studies have shown that the effects of the US on biopolymers especially relate to the degradation pattern of structures [6].

The specificity of the effects stresses the need for accurate quality assessment, by specific acoustic measurements on the clinical equipment.

The performances of the equipment have been investigated, especially on a local basis, by many authors (see Refs. [7–10]), and recommendations have been proposed [11]. For exam‐ ple, the Italian National Institute of Metrological Research (INRIM) settled a protocol [12] for evaluating the ultrasonic power produced by clinical equipment using the Radiation Force Balance (RFB) method. The ultrasonic power is actually determined by the measurement of the force exerted on a target by the sound field generated from an ultrasonic source. The absorbing, connected to the load cell, measures the apparent mass variation due to the ultra‐ sonic field when the source is alternately switched on and off.

#### **2.2. Towards therapeutic US dosimetry: treatment planning by** *'in vitro***' parameter evaluation**

Most of the therapeutic applications of US induce heating in insonated tissues to obtain some beneficial effect. By increasing temperature a few degrees above the normothermic levels, it is possible to induce temporary vasodilation and increase blood inflow in the affected area, performing a sort of 'thermotherapy'. This mechanism has been proposed as the principal one to explain the therapeutic effects obtained in physiotherapy applications: the analgesic effect in the joints and muscles is mediated by vasodilation and by the subsequent wash‐out of pro‐algogenic substances and pro‐inflammatory with local edema reduction. In addition, the increase of tissue temperature promotes a higher extensibility of soft tissue, and a relax‐ ing effect on the muscles; the increase of cell activity and of the local metabolism caused by the heat stimulates the accelerated wound healing and repair of tendon injuries, ligament, muscle, etc.

This phenomenon is influenced by both the characteristics of the ultrasound beam (intensity and frequency), the duration of exposure and the characteristics of the crossed tissues (acous‐ tic impedances). Heating is established quickly; however, a thermal equilibrium due to the

The thermal effect is most evident at the interface between tissues and in particular at the interface between fat and muscle and at the level of the periosteum. The periosteum, for its anatomical structure and for the continuity with the bone, absorbs a large amount of energy and is therefore easily heated. The thermal elevation generates, as secondary effects, increase in cellular metabolism and vasodilation; in particular, the latter property is important in the use of therapeutic US in physiotherapy, promoting the wash‐out of pro‐inflammatory sub‐

(ii) Non‐thermal effects: They include cavitation, which consists in the formation, growth and implosion of gas bubbles within the fluid subjected to an ultrasonic field. In general, the cavi‐ tation can be seen as the 'break' of a liquid and the consequent formation inside the same of 'cavities' (bubbles) of the liquid containing dissolved gas or vapour itself. This phenomenon occurs in many situations, for example, in boiling water or in proximity to the propeller in rotation of a ship, and in any case when liquids are subject to high and rapid changes in pres‐

The almost instantaneous variations of density, pressure and temperature of the fluid in which propagates the ultrasonic wave can also produce the so‐called shock waves or pressure waves which can also be extremely intense. The ultrasonic irradiation of water leads to the formation of the hydroxyl radical and hydrogen radical, which give as the main final products

At the cellular level, the production of radicals induced by exposure to the US can also produce biological effects on DNA; theoretical models and experimental studies have shown that the effects of the US on biopolymers especially relate to the degradation pattern of structures [6]. The specificity of the effects stresses the need for accurate quality assessment, by specific

The performances of the equipment have been investigated, especially on a local basis, by many authors (see Refs. [7–10]), and recommendations have been proposed [11]. For exam‐ ple, the Italian National Institute of Metrological Research (INRIM) settled a protocol [12] for evaluating the ultrasonic power produced by clinical equipment using the Radiation Force Balance (RFB) method. The ultrasonic power is actually determined by the measurement of the force exerted on a target by the sound field generated from an ultrasonic source. The absorbing, connected to the load cell, measures the apparent mass variation due to the ultra‐

**2.2. Towards therapeutic US dosimetry: treatment planning by** *'in vitro***' parameter** 

Most of the therapeutic applications of US induce heating in insonated tissues to obtain some beneficial effect. By increasing temperature a few degrees above the normothermic levels, it

sure and can occur in the use of therapeutic US or in Doppler ultrasound [5].

.

heat dissipation due to blood flow is reached in longer times.

stances and pro‐algogenic tissues.

216 Clinical Physical Therapy

(hydrogen peroxide) and H<sup>2</sup>

acoustic measurements on the clinical equipment.

sonic field when the source is alternately switched on and off.

H2 O2

**evaluation**

Even non‐thermal effects (cavitation, emulsion, streaming and sonoporation) may play a role in the physiotherapy field: they, in fact, generate a sort of 'micro‐massage' in tissues, promote the increase of local blood flow; furthermore, at cellular and intracellular level, US induces an increase of membrane permeability, calcium uptake, protein synthesis, mast cell degranula‐ tion, production of growth factors, angiogenesis, increased fibroblast motility and orientation modification of the fibres collagen and shift the type of collagen (type III to type I) in tissue repair [13].

Controindications to the therapeutic US are possibly due to interference with other electronic devices (e.g. cardiac pacemaker) or are related to the possible effects induced from heat and cavitative phenomena.

US can cause damage to eyes, gonads, encephalon and ears, and the presence of growing cartilage remains the most substantial downside.

It is also recommended not to use the US in body regions where there are implants or metal‐ lic synthesis; in fact, given the large difference in impedance between these materials and human tissues, areas of friction and heat accumulation can be formed, with unpredictable consequences.

It is also not recommended for any kind of treatment with US in the presence of malignancies, to avoid the spread in a circle of pathogenic cell lines. In order to get the therapeutic effects described above, it is mandatory to know how US may increase local temperature depending on the setting of the main parameters (frequency, power, etc.) and the modality of treatment (CW, PW, etc.). Although in living tissues many biological mechanisms may dissipate heat, preliminary investigations on '*in vitro*' phantoms may help in finding the 'asymptotic' values of the thermal increase locally induced by the US. Many approaches have been proposed in the literature [see 14–16], based on different test materials exhibiting the same mechani‐ cal and thermal properties of homogeneous tissues. Also, numerical simulations have been proposed [17].

The use of 'tissue‐mimicking phantoms', coupling different tissues (e.g. muscle‐equivalent and bone), as the two presented in **Figure 1** to simulate a superficial (A) and a deep (B) joint, respectively, may be useful to evaluate the temperature at different depth depending on the choice of the parameter values of the equipment and the different treatment protocols [18].

A cartoon cylinder filled with homemade agar‐based gel, prepared using bi‐distilled water (86.5%), glycerine (5.5%), graphite (2%), agar (2.5%) and salicylic acid (traces) was produced.

**Figure 1.** Scheme of the phantoms mimicking a superficial (A) and a deep (B) joint indicating the position of the bone insert and of the temperature probes.

The phantom contains at one end a bovine bone disk 2 (±1) mm thick inserted at 3 (±0.5) cm depth (simulating a 'deep joint') and at the other end a bovine bone disk 1 (±0.5) mm thick inserted at 1 (±0.5) cm (simulating a 'superficial joint') (see **Figure 1A** and **B**). The two disks were always fixed approximately in the centre of the phantom, and their diameter was always smaller than one‐half of the phantom diameter. Deep and superficial 'joints' were treated at 1 and 3 MHz, respectively, using the equipment Enraf Nonius SonoPlus. Thermal probes, inserted at different depth, perform temperature measurements before, during and after sonications lasting 5 min and performed using the most diffused clinical treatment modalities (e.g. selecting 'continuous' (CW) or 'pulsed' (PW) wave on the apparatus and keeping fixed or massaging the probe on the phantom surface). Such massaging is per‐ formed by small circular movements where the probe is freely and randomly moved on the phantom cross section. The temperature increases detected into the phantoms are different in superficial and deep joints, and mainly depends on the operating mode (CW or PW) and on the fixed or massage modality selected for the probe application. The (min‐max) tempera‐ ture increases detected at different positions (see **Figure 1**) and with different modalities are given in **Table 1**.


**Table 1.** (Min‐max) temperature increase (in °C).

PW modality is always safer, while CW can induce temperature peaks (hot‐spots) at different depth inside the joint.

The local characteristics of the thermal field are not easily predictable based only on wave attenuation, because the bone insertions influence the temperature distribution. In Ref. [18] also the mechanical effects may be detected by inserting contrast agents (e.g. nanobubbles [19]), which can be exploded when the US pressures exceed some threshold values, but no significant differences could be detected in the nanobubbles diameter distribution before and after US sonication. As a consequence, very accurate and anatomically based experimental and numerical models are required to predict the thermal field inside any particular joint or non‐ homogeneous body region. The starting points should be joints which most benefit from phys‐ iotherapeutic US [20, 21], and in particular, shoulders, which often suffer from muscle‐skeleton diseases treated with US [22]. On any specific pathologies, such specifically those affecting the shoulders, the previous investigations on phantoms may be useful to perform some 'treatment planning' based on the different anatomical and functional features [23], as shown in **Table 2**.


**Table 2.** 'Treatment planning' based on the different anatomical and functional features in shoulders.
