Ultrasound Transducers and Scanning Modes

INTRODUCTION OF USS

THE ULTRASOUND BEAM

CHARACTERISTICS

BEAM SHAPE

The path which the ultrasound waves travel from the transducer is called the ultrasound beam. It is divided into two regions.

• The near field (or Fresnel zone) which is shaped like a cylinder.
• The far field (or Fraunhofer zone) where it becomes cone shaped.

NOTE: Increasing the frequency will give a longer near field and less far field divergence.

PULSED ULTRASOUND

Instead of a continuous output of waves from the transducer face, most transducers use a series of short pulses of sound energy. These short pulses form the ultrasound beam.

This is important, as we will see later, in giving good axial resolution.

Beam intensity

The intensity of the beam is the power (measured in watts) flowing through a unit area of the ultrasound beam.

Increasing the output power to the transducer produces high intensity sound pulses, which make returning echo signals from all reflectors appear brighter.

BEAM WIDTH AND SLICE THICKNESS

Beam width means the measurements of the beam in the scan plane. This will vary according to distance from the transducer and what focusing is applied to the beam.

The width of each pulse of sound will therefore change with depth. The beam width affects the spatial resolution of the image (the ability to differentiate small structures). The narrower the beam width the better the spatial resolution.

Slice thickness is the measurement of the beam at 90 degrees to the scan plane

FOCUSING THE BEAM

The ultrasound beam can be focused to improve image quality. This is mostly done electronically.

The depth of the focal zone and the length of the focal zone can be changed by the operator on modern ultrasound machines.

IMAGE RESOLUTION

Spatial resolution is the ability of an ultrasound system to display two closely spaced reflectors as separate structures.

This depends upon the wavelength of the sound used to produce the image.

For example, the wavelength of a 5MHz ultrasound beam is about 0.3mm, so it could not resolve (display separately) objects less than 0.3mm apart

AXIAL AND LATERAL RESOLUTION

Axial resolution is resolution along the axis of the direction of the ultrasound beam.

Lateral resolution is resolution at right angles to the direction of the ultrasound beam.

• Axial resolution depends on the spatial pulse length.

Lateral resolution depends on the ultrasound beam width.

AXIAL RESOLUTION

A short pulse length gives good axial resolution and the best resolution that can be achieved is half the spatial pulse length Therefore if the pulse length is 1mm, then structures along the axis which are less than 0.5 mm will not be resolved.

Axial resolution is affected by the frequency of the ultrasound beam and damping applied to the piezoelectric crystal

SPATIAL PULSE LENGTH

Each ultrasound pulse is about two wavelengths long, and therefore a shorter wavelength will reduce the pulse length. Since wavelength is related to frequency, we can see that a high frequency beam will result in a shorter pulse length and better axial resolution.

Pulse length also depends upon the amount of damping applied to the piezoelectric crystal. Increasing the damping shortens the pulse length and improves axial resolution.

LATERAL RESOLUTION

A narrow ultrasound beam gives best spatial resolution and the best resolution that is achieved is equal to beam width at the focus of the beam.

The beam width determines the size of the echoes displayed and the structures must be separated by a distance greater than the beam width for them to be resolved. That is, the beam must fit into the gap between the two structures so that is can return “no echoes” from that position.

Ultrasound Transducers and Scanning Modes

Reflector separation is less than beam width in fig A and B but greater than beam width in Fig C and D.

Fig A The beam first encounters the left reflector, then both reflectors, then the right reflector.

Fig B This results in continuous reflection from one or both reflectors.

Fig C The beam encounters the left reflector, then fits between both, and then meets the right reflector.

Fig D Separate echoes are shown and the reflectors are resolved.

FACTORS AFFECTING BEAM WIDTH

As the diameter of the piezoelectric crystal increases so does the beam width near the crystal face.

• Higher frequency beams have a longer near field and less divergent far field.
• The amount of focusing and the length of the focal zone.
• Divergence of the beam increases beam width further from the crystal face.

The best lateral resolution is at the focal point of the beam.