Formation of the Ultrasound Beam

ULTRASOUND PHYSICS LECTURE 2

FORMATION OF THE ULTRASOUND BEAM

THE PIEZOELECTRIC EFFECT

This is the ability of a material to generate an electric charge in response to applied pressure. When a piece of piezoelectric material is compressed a potential difference is generated across opposite faces- the one becomes positive, the other negative. Conversely, if an electric field is applied across the crystal, it changes its shape.

This property is natural to certain crystals such as quartz and man made materials such as zirconate titanate.

Piezoelectric materials are crystals composed of dipolar molecules, which are positive at one end and negative at the other

Dipolar Molecules

Normally dipolar molecules have a random arrangement within the material, and they are unable to align themselves with an electric field. However, when heated to a high temperature in the presence of an electric field the molecules align themselves with that field, and will maintain that alignment when they have cooled. The piezoelectric crystal can now be used as an ultrasound transducer to transmit or detect sound.

The piezoelectric crystal as a transmitter of sound

In this case the crystal is converting electrical energy into mechanical energy. A voltage is applied across the conducting plates causing the molecules to twist in the direction of the electric field, and this causes the crystal to become thicker.

If the voltage is reversed, the molecules will twist back in the opposite direction making the crystal thinner. Applying an alternating voltage to the crystal will cause it to expand and contract (oscillate) at the same frequency as the voltage, producing a continuous sound wave of that frequency.

The amplitude of the sound wave will depend upon voltage applied and the natural resonant frequency of the crystal.

The piezoelectric crystal as a receiver of sound

In this case the crystal is converting mechanical energy into electrical energy.

When a sound wave makes contact with the piezoelectric crystal the changing pressures cause the crystal to contract and expand. This twists the dipolar molecules, and causes the conducting plates to become charged.

These small signals from the conducting plates are used to create a two dimensional image of the returning echoes. The strength of an echo determines the size of the voltage signal produced, which in turn determines the brightness of the echo display on the image

TRANSDUCERS

Ultrasound transducers act as both a sender and receiver of sound. The essential parts of a transducer assembly are:

• Piezoelectric element
• Electrical connections
• Backing material
• Acoustic lens
• Impedence matching layer

Physical housing assembly

Components of an electronic array transducer

ELECTRONIC ARRAY TRANSDUCERS

Electronic array transducers consist of a row of rectangular piezoelectric crystals. They all have backing material, to shorten the pulse length, and matching layers on the front surface to improve the sound transmission into the patient.

Normally there are between 128 to 256 piezoelectric elements across the face of these transducers.

Each element is connected so that small groups of elements can be fired together and in sequence.

FIRING ONE GROUP OF ELEMENTS

CONTROLLING THE ULTRASOUND BEAM

Each single element in an electronic array transducer can produce a small wave front which joins with other small wave fronts to form a larger ultrasound beam.

By using time delays, and activating each element in a sequence, the beams shape and direction can be controlled to focus and direct the beam.

Focusing the ultrasound beam improves the image quality (resolution) by making the beam thinner within the focal zone.

Directing the ultrasound beam

ELECTRONIC BEAM STEERING

The ultrasound beam can also be steered (directed) using time delays. An image is formed as each group of elements is directed to form a wider wave front, or wide sector field of view.

• Transducers used today are of three main types:
• Linear array transducers
• Curvilinear array ( also called curved linear array or sector) transducers.

Phased linear array transducers

TYPES OF ELECTRONIC ARRAY TRANSDUCER

Linear array transducers transmit parallel scan lines at right angles to the transducer face producing a rectangular field of view. These are high frequency transducers used for superficial structures Curved linear (curvilinear) array is similar but the face of the transducer is curved giving a wider, diverging field of view. These are low frequency transducers used to image deep lying structures.

Phased array transducers are similar to linear array with a rectangular, flat face. However, the beam is electronically steered to produce a wide field of view similar to a curved linear array. These have a smaller area of contact with the advantage of a wide field of view, making them suitable for applications with a small acoustic window such as cardiac scans . They also use lower frequencies like the curved array.

Linear Array: elements are arranged in a straight line allowing for a rectangular field of view

Curvilinear Array: elements are arranged in a curved line resulting in a wider field of view Sector Array: elemnts are arranged in a sector shape, providing a wider field of view at the expense of a smaller footprint