Method for measuring speed of sound in tissue and tissue assessment apparatus

A method for measuring the speed of sound in a tissue wherein said tissue is held between a pair of transducer units, ultrasound is transmitted and received by said units, and each of said units comprises an ultrasonic transducer, a transducer cover of which at least part deforms freely and which covers said ultrasonic transducer, and an acoustic matching material filling the space between said cover and said transducer, said method comprising the steps below:

• (a) a preparing step wherein a reference phantom having a predetermined thickness is held between said pair of transducer units under a predetermined pressure, ultrasound is transmitted and received, the propagation time of said ultrasound from one to the other of said transducers is measured and the temperature of said reference phantom is measured;
• (b) a step wherein the speed of sound in said reference phantom is calculated based on the temperature of said reference phantom;
• (c) a measuring step wherein a tissue is held between said pair of transducer units under said predetermined pressure, ultrasound is transmitted and received, the propagation time of said ultrasound from one to the other of said transducers is measured, and the distance between said transducers is measured;
• and
• (d) a calculating step wherein the speed of sound in said tissue is computed based on the propagation time measured in said preparing step, the propagation time and distance between said ultrasonic transducers measured in said measuring step, and the speed of sound in said reference phantom.

A tissue assessment apparatus for computing assessment values of tissue based on a received signal obtained when ultrasound is passed through a tissue and received, said apparatus comprising:

• a pair of transducer units wherein each of said units comprises a transducer, a transducer cover of which at least part deforms freely and which covers said transducer, and an acoustic matching material filling the space between said cover and said transducer,
• a transducer unit moving mechanism for moving said pair of transducer units closer together or further apart,
• a torque limiter for limiting the force with which said transducer units press against said tissue when said tissue is held between said pair of transducer units,
• a memory for storing a propagation time of ultrasound between said transducers when the transducer covers of said pair of transducer units are pressed to each other with a predetermined force,
• a time measuring device for measuring the propagation time of ultrasound between said transducers based on a received signal from said transducer units,
• a distance measuring device for measuring the distance between said transducers, and
• a processor for computing the speed of sound in said tissue based on the propagation time stored in said memory, the propagation time found by said time measuring device, and the distance measured by said distance measuring device.