Ultrasound Beam Shape

ULTRASOUND BEAM SHAPE AND FOCUSING

J.J. John (DDR-CUHAS)

Beam shape

As a beam of ultrasound travels outwards from the surface of the transducer, the distribution in space of the ultrasonic energy undergoes change.

The ultrasound beam spreads out, or undergoes divergence as it moves away from the transducer.

The term "ultrasound beam shape" is commonly used to describe the manner in which the spatial distribution of the beam changes with distance from the source.

Beam shape

• The beam shape has very significant effects on the
• Quality of the ultrasonic image (Image Resolution)

The tissue depths that can be usefully interrogated We are going to examines the factors which influence ultrasound beam shape and the associated implications for ultrasonic imaging.

General shape of the ultrasound beam

The typical manner in which the ultrasound beam spreads out with increasing distance from the transducer T is shown below

T- Transducer

• d – Beam width
• P – Plane/point where the beam diverge

D – Lenght of near field/fresnel zone

General shape of the ultrasound beam

Initially, between T and the plane P along the beam path, the beam is narrow, with a small beam width d, equal to about the diameter of the piezoelectric crystal. This part of the beam is referred to as the near field, or the Fresnel zone.

Near field/Fresnel zone

General shape of the ultrasound beam

Beyond P, the beam spreads out (diverges) over a larger and larger area, with increasing beam widths which result in a rapid deterioration of spatial resolution of the image. This part of the beam is known as the far field, or the Fraunhofer zone.

Far field/Fraunhofer zone

General shape of the ultrasound beam

The distance from the transducer to the plane P is sometimes called the transition distance (in reference to the change from Fresnel zone to Fraunhofer zone).

General shape of the ultrasound beam

The length D of the Fresnel zone, and the beam width d, at a given plane across the beam, are important parameters The length of the Fresnel zone influence the practical tissue depth that can be interrogated with the beam The beam width influence the spatial resolution in the ultrasonic image.

The narrow beam associated with near field is desirable for good spatial resolution.

Factors affecting shape of the beam

• The shape of the ultrasound beam is affected by:
• The size and shape of the ultrasound source
• The beam frequency

Beam focusing

Effect of source size

• The size of the ultrasound source affects the
• Beam width
• The length of the Fresnel zone (Transition distance)

Angle of divergence beyond the near field With No focusing applied,the Fresnel zone's length (D), is determined by the diameter of the Piezoelectric Crystal (d) and the wavelength of the ultrasound beam according to the relation:

Where r = radius of the transducer, = wavelength, v=Velocity of the ultrasound beam and d = 2r is the diameter of the transducer.

Effect of source size

Within the near field, the beam width is approximately equal to the transducer diameter.

The length of the Fresnel zone (D) increases rapidly as the beam width (or transducer crystal’s diameter, d) is increased.

Conversely, the length of the Fresnel zone diminishes rapidly as the transducer diameter is reduced.

Variation of fresnel zone's length and angle of divergence with source diameter

In addition, a small transducer diameter results in a large angle of divergence beyond the near field

Practical Implication

An important practical implication of these observations is that:

Although a narrow beam gives us good image resolution, narrow beams should not be obtained ONLY by making the transducer smaller, as this would also reduce the depth of tissue interrogation.

It is for this reason that, in multicrystal transducers where many small crystal elements are used:

The crystals are not pulsed individually, but in small groups of neighbouring crystals which then provide an instantaneous beam wide enough to give a sufficiently long length of the Fresnel zone.

Effect of source size: In summary

A small source provides a narrow beam initially, is associated with a short Fresnel zone, and the beam diverges rapidly beyond the near field.

A large source provides a broader beam initially, gives a longer Fresnel zone, and the beam diverges more gradually, thus providing better resolution of deeper structure.

Effect of beam frequency

• substituting the wavelength of the ultrasound beam by = v/f

we get:

From this expression, we conclude that the length of the Fresnel zone increases as the beam frequency is increased.

Also, the angle of divergence beyond the near field diminishes with increasing frequency.

Effect of beam frequency

The effect of higher frequencies is therefore not only improved image resolution but also an increase in the length of the useful near field.

In practice, however, some of this advantage is taken away by increased beam attenuation at higher frequencies

Focusing of the ultrasound beam

The shape of the ultrasound beam can be influenced to varying extents by applying different focusing methods In general the main methods are:

• Mechanical methods of focusing

Electronic methods of focusing

The Mechanical methods of focusing includes the use of Acoustic lenses and Shape of the crystal element while The Electronic methods of focusing archived by appllication of a pulsing programme with carefully controlled time delays between different crystal elements

Mechanical methods of focusing

Shape of the crystal element

The crystal element can be suitably shaped by concave curvature to focus the ultrasound beam This is an internal focusing method, because it is effected in the crystal itself.

The degree of focusing will depend on the extent of curvature (radius of curvature) of the crystal.

Mechanical methods of focusing

Acoustic lenses

Acoustic lenses made from materials which propagate ultrasound at velocities different from that in soft tissue can be used to focus the beam by refraction.

The lens will have concave curvature and the degree of focusing will be determined by the radius of curvature of the lens.

Acoustic lenses provide external focusing.

Electronic beam focusing

Electronic focusing is employed in multicrystal transducers In such transducers, movement of the ultrasound beam across the plane of interest in the subject is effected electronically by pulsing small groups of crystal elements at a time By applying a pulsing programme with carefully controlled time delays between different crystal elements, ultrasound waves from all the crystals in the array can be made to arrive in phase at one particular point (the focus) where they reinforce to produce a high intensity zone

Electronic beam focusing

Electronic beam focusing. Pulsing sequence is controlled so that wavefronts from each of the crystal elements arrive in phase at the transducer focal point. F.

Electronic beam focusing

The time delay programme can also be applied during reception of echoes.

Electronic focusing offers the advantage of providing variable focus, or dynamic focus, as opposed to the other methods which provide fixed focus.

Variable focusing is achieved by altering the time delay programme.

Focus of a transducer

The focus, F, of a transducer is that point along the central axis of the beam which is equidistant in time from all points on the surface of the transducer.

The times of flight of the ultrasound waves are equal for all linear paths between the surface of the transducer and F.

The waves therefore arrive at F in phase and reinforce each other by constructive interference.

The beam has its narrowest width, greatest intensity, and best spatial resolution.

Focal zone

The focus of a transducer is not sharply defined.

Areas within the beam close to F will have properties which will closely match those at F itself.

The region around F over which these conditions prevail is called the focal zone of the transducer

Focusing and Image resolution

Classification of focusing

• The degree of focusing may be classified into three categories as follows:
• Strong focusing (or short focusing)
• Medium focusing

Weak focusing (or long focusing)

Strong focusing

Strong focusing brings the focal point very close to the transducer, typically 2 – 4 cm.

It achieves a high degree of beam narrowing, but the beam diverges rapidly beyond the focal distance.

It can only be applied to transducers for high resolution examinations of small parts.

Weak focusing

Weak focusing gives a focal point further away from the transducer – typically more than 8 cm – and a gentle divergence of the beam beyond the focus.

Weak focusing is preferred in diagnostic applications because it provides an extended useful, narrow beam.

Summary

It has been shown that the shape of the ultrasound beam is of great significance in ultrasonic imaging.

Deliberate efforts are therefore required during transducer design to control the beam shape to suit the desired applications Generally, a narrow beam would be desirable to maximize spatial resolution of the image, as would be an extended length of the near field in order to facilitate imaging to adequate tissue depths To achieve these goals,It requires that the size and shape of the ultrasound source, the beam frequency, and focusing of the transducer, be suitably chosen

Summary

In ultrasonic imaging, efforts to enhance one desirable feature quite often works in opposition to another desirable feature. Thus, we have seen that Increasing the beam frequency improves image resolution, but also reduces beam penetration due to increased attenuation.

A large source of ultrasound at the transducer may extend the useful range of the beam, but it will diminish resolution in the near field.

Summary

The optimum choice of frequency would be the highest frequency compatible with the tissue depth requirements.

An interesting development in this connection has been the introduction of broad band transducers which offer mixed frequency beams to exploit a bit of the advantages of both low and high frequencies

focus.