In recent decades ultrasonic applications in medicine have grown dramatically and now cover a wide range of techniques in diagnosis and therapy. The widespread diffusion of these techniques and the introduction of new ones have triggered the need for a metrological support to improve knowledge of the quantity describing ultrasound emitted by a medical device and the interaction between ultrasound and biological tissue.
In recent years INRIM, like other national metrological institutes, has set up a new laboratory for ultrasound field characterization.
The aim of this research activity is to offer a metrological support to medicine in order to improve both safety and efficiency in treatments.
The most important quantity in order to characterise the emission of an ultrasonic transducer is acoustic output power.
For this purpose at INRIM in recent years a primary level system (fig. 1) has been developed based on the radiation force balance principle. The system measures the force exerted on a specific target (reflecting or absorbing ultrasonic radiation) and the temperature of the transmission mean of ultrasounds and calculates the acoustic power emitted by the transducer under investigation.

Fig. 1 Radiation force balance apparatus realized at INRIM.

The reference transmission mean for medical ultrasound is degassed high purity water.
To check the radiation force balance, a series of metrological transducers has been developed with the purpose of becoming a reference transfer standard.
The apparatus covers a range of power between 10 mW and 15 W in a frequency range from 2 MHz to 10 MHz. And the uncertainties associated with the measurement results are comparable with those of the main European laboratory.
After a bilateral comparison on ultrasonic power INRIM obtained the CMC (Calibration and Measurement Capability) in the range above mentioned.
INRIM will participate in the early months of 2009 in the CCAUV comparison for ultrasonic power.
While power measurement gives information regarding integral energy released, another important aspect of the metrological problem related to ultrasonic emissions is to identify local and temporal variations in pressure and intensity in the ultrasonic field emitted.
At INRIM PC controlling a scanning system with three motorized micrometric linear actuators that makes it possible to investigate the sound field, has been implemented. Measurements are made in a 200 L water tank where the transducer under inspection is immersed (fig. 2). The scanning system moves the hydrophone in the volume of water affected by the sound field.

Fig. 2 Example of measurement results of spatial distribution of intensity in a plane transversal to the beam axis. Intensity is expressed in dB relative to the spatial peak level.

Hydrophones give a voltage signal related to the pressure on their sensitive area through a calibration curve. A digital storage oscilloscope is used to read the voltage signals that are acquired and processed with dedicated software. This system makes it possible to measure all the parameters related to the local energy released by the ultrasonic transducer and the main indices (Mechanical Index and Thermal Index) pointing out physical effects on tissue insonated.
With the development of these measurement devices the interest is to study new techniques to characterize ultrasound probes with very high power emissions. Particular interest lies in studying transducers used in the newest and most promising therapeutic devices for cancer treatment based on High Intensity Focused Ultrasound (HIFU).
Research activity in this field is integrated in a Joint Research Project under the European project iMERA+ (Implementing the Metrology European Research Area) in which INRIM is involved at present.
Techniques are under study to extend the power measurement capability of INRIM up to 200 W, in the range where no established metrological standard already exists. The techniques used are based on the radiation force balance principle with new targets suitable for high energy released and a new system based on load cells that allow very fast time response. A new fluid based target has been realised and tested with HIFU at a power level up to 70 W.
Measurements to characterize HIFU with proper hydrophones are under investigation and a new optic fibre hydrophone has been acquired by the laboratory. This instrument will allow to measure high pressure fields as in HIFU focus and find methods to understand energy released in biological tissue.
A related area of recent improvement concerns the realization of a gelatine phantom with acoustic properties mimicking those of biological tissues.
A very promising prototype has been developed on polyacrylamide base and different chemical recipes has been studied to obtain proper ultrasound attenuation coefficient. An experimental apparatus has been developed to measure the speed of sound of samples and their attenuation coefficient as a function of the frequency components of the signal.
The characterized tissues mimicking material are used to study the physical and in particular the thermal effects of HIFU (fig. 3).

Fig. 3 Tissue mimicking gel insonated with a HIFU transducer. The gel shows a lesion in the region of the ultrasonic focus due to the temperature rise.

As HIFU therapy is based on the ablation of cancer due to a precise localized rise in temperature (up to 90 °C), it is crucial for the efficiency of the treatment to quantify accurately the temperature generated in the focus region of the ultrasound field and determine its position. Simulation on a phantom is really promising for this purpose.
Activity in the field of ultrasounds in medicine at INRIM gives rise to cooperation with other research groups of Università degli Studi and Politecnico di Torino that in the last year led to the project for developing a reference centre for ultrasound in medicine. The aim is to join in research and create and spread checking procedures for ultrasonic devices used in hospitals and medical services where periodic control of this kind of device is not usual.