Medical ultrasound transducer crystal structure

Structure of the medical ultrasound transducer crystal:

1. Matching Layer:

An acoustic material layer applied to the radiating surface of the transducer crystal to match the acoustic impedance between the transducer crystal and the transmission medium, allowing sound energy to pass through well. The layer thickness is generally one-quarter wavelength, and the acoustic impedance is the geometric mean of the acoustic impedances of the crystal and the transmission medium. It is also known as a transition layer, acoustic lens, or protective film. In complex transducers, the matching layer can consist of multiple quarter-wavelength-thick layers with appropriate acoustic impedance.

2. Probe Backing:

A sound-absorbing block bonded to the back surface of the piezoelectric crystal used in the probe. Its purpose is to absorb the useless sound waves radiated from the back of the crystal, reducing the Q value of the probe and improving its pulse response. In general, this will be accompanied by a corresponding decrease in probe sensitivity. Commonly used sound-absorbing blocks are made of epoxy resin, tungsten powder epoxy with appropriate curing agents and plasticizers, formulated according to a specific recipe.

Practical Selection and Design of Backing Material:

Considering both acoustic and process characteristics, among the various existing single-phase materials, almost none can be directly used as an ultrasonic transducer backing; only specially formulated composite materials, combining multiple useful characteristics, are suitable.

High-impedance backing: Those with acoustic impedance exceeding 10*105 Pa.s/m can be classified as high-impedance backing. Two methods have been proven effective for achieving an acoustic impedance close to that of the piezoelectric element. One is to mix tungsten powder with high-ductility metal powder, then apply high-pressure processing to squeeze the ductile metal into the gaps between the tungsten powder particles and form bonding force at the interface between the two; or to mix tungsten powder and thermoplastic plastic powder thoroughly before hot-pressing it into shape. Another method is the impregnation method, which involves compressing tungsten powder into a tightly packed state and then immersing it in liquid thermosetting resin. After complete impregnation, it is heated and cured. The technical route adopted includes:

(1) Impregnation epoxy mixture (IEM for short)

3.5 parts of cyclohexene oxide

6.5 parts of poly(propylene glycol) diglycerol ether

10 parts of nonyl succinic anhydride (NSA)

0.1 ml of dimethylaminoethanol per 10g of NSA

(2) First impregnation step

The pressed tungsten powder block is immersed in propylene oxide, and then the concentration of IEM is gradually increased. The specific process is as follows: 100% propylene oxide, 24 hours; 75% propylene oxide + 25% IEM, 24 hours; 50% propylene oxide + 50% IEM, 24 hours; 25% propylene oxide + 75% IEM, 24 hours.

(3) Second impregnation step

The tungsten powder block after the first impregnation step is placed in 100% IEM for 96 hours, with the IEM liquid level slightly below the top of the tungsten powder block.

(4) Heating and curing

After the two impregnation steps are completed, the tungsten powder block is placed in an 80°C environment for 17.5 hours for curing.

This method is characterized by the addition of propylene oxide, a butyl diluent, to the epoxy resin to achieve complete impregnation. Furthermore, for greater rigor, G.G. LOW et al. always cut the lower end of the impregnated tungsten powder block and attached it to the transducer.

Another technical difficulty with high-impedance backing is bonding it to the piezoelectric crystal. To avoid damaging the crystal and to make the low-viscosity epoxy resin adhesive layer as thin as possible, E.P. Papadak used a non-uniform pressurizing device with rubber hemispheres.

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