Understanding Ultrasound Physics Chapter 9 - Sound Beams

Five terms describe the shape and regions of a sound beam:

1. focus

2. near zone

3.focal length zone or near zone length

4. far zone

5. focal zone

focal point

The location where the beam is the narrowest

Near Zone (Fresnel Zone)

the region or zone in between the transducer and the focus. sound beams converge in the near zone

focal length

Distance from the transducer to the focus.

the focus

focal point

end of the near zone

beginning of the far zone

middle of the focal zone

Far Zone (Far Field) (Fraunhofer Zone)

REGION starting at the FOCUS and extending DEEPER

- At the beginning of the far zone, the beam is only 1/2 as wide as it is at the transducer

- When the beam is TWO NEAR ZONE LENGTHS from the transducer, the beam is again the SAME SIZE as the active element

- At depths MORE THAN 2 near zone lengths, the beam is WIDER than the active element

focal zone

the point at which the sound beam is the narrowest and the resolution is the best

Sound beam (summary)

the sound beam exits the probe and immediately enters the near or fresnel zone.

the beam diameter, or width, is exactly the same as that of the transducer.

the sound beam gets progressively narrower in the near zone. The beam continues to narrow until it reaches its narrowest point, the focus.

the focus marks the end of the near zone. The distance from the transducer to the focus is called the focal length, focal depth, or near zone length.

the focus also marks the beginning of the far or fraunhofer zone. at this depth, the sound beam starts to diverge, or widen. the beam continues to wide. at a depth that is twice the focal length, the sound beam is again as wide as the transducer. at depth more than twice the focal length, the beam's width exceeds that of the transducer.

beam diameter at the transducer

beam diameter equals transducer diameter

Beam diameter at the focus

beam diameter is one-half transducer diameter

Beam diameter at 2 near zone lengths

same as transducer aperture or diameter.

Beam diameter deeper than 2 near zone lengths

wider than transducer diameter

Adjustable focus systems are called what?

phased array

what characteristics of a fixed focus transducer determine the focal depth?

1. transducer diameter

2. frequency of the sound

Transducer diameter and focal depth are ____ related.

directly related

Frequency and focal depth are _____ related.

directly-higher acoustic frequencies create deeper foci and lower frequency crystals produce beams with shallow foci.

shallow focus

-Smaller diameter PZT

-Lower frequency

deep focus

-Larger diameter PZT

-Higher frequency

Increasing the diameter of the transducer results in what?

deeper focus

higher frequency sound creates

deeper focus

Sound Beam Divergence

~describes the spread of the sound beam in the deep far zone.

what characteristics of a transducer determine the spread of the beam in the far field?

1. transducer diameter

2. frequency of the sound

Crystal diameter and beam divergence are ______ related.

inversely related.

Frequency and beam divergence are ______ related.

inversely

higher frequency sound

Improves lateral resolution in the far field

less divergence

~large diameter

~high frequency

more divergence

smaller diameter

lower frequency

larger diameter crystals

improve lateral resolution in the far field

produce beams that diverge less in the far field

spherical waves

Sound waves produced by VERY SMALL sources (tiny pieces of PZT) and diverge in the SHAPE of a "V"

This type of wave is created when the SOURCE is about the size of the sound's WAVELENGTH

known as Huygen's sources.

Huygen's Principle

States that a large active element may be thought of as millions of tiny, distinct sound sources.

explains the shape of an imaging transducer's emitted sound beam based upon in-phase and out-of-phase wavelets interfering with each other.

A deeper focus will result in transducers which have what?

higher frequency

What is the relationship between crystal diameter and beam divergence?

inversely related

Large diameter crystals improve lateral resolution in which field?

far field

The relationship between frequency and beam divergence is what?

inversely related

In the far zone, higher frequencies result in what?

smaller beam diameters

Sound waves produced by very small sources (about the size of the wavelength) diverge in the shape of a V and are called what?

Huygen's wavelets (spherical waves)

The small sources (about the size of the wavelength) that produces spherical waves are called what?

Huygen's sources

The accuracy in imaging in what?

resolution

The ability to distinctly identify two structures that are very close together when the structures are side by side (perpendicular to the sound beam's main axis) is what?

lateral resolution

What units is lateral resolution meausured in?

distance

What determines lateral resolution?

width of the sound beam

Narrower sound beams have better what?

lateral resolution

Other names for lateral resolution are:

lateral, angular, transverse, azimuthal (LATA)

Lateral resolution (mm) = ?

beam diameter (mm)

Because ultrasound pulses are shorter than they are wide, the best resolution in ultrasound imaging is what?

axial resolution

What is a lens used for?

external focusing

What is a curved active element used for?

internal focusing

What are the electronics of the ultrasound system used for?

phased array focusing

What does adjustable or mulitiple focusing describe?

phased array transducers

In which field is the length reduced because of focusing of the beam?

near

What happens to the beam beyond the focus zone during focusing?

widens