Module Three: The Ultrasonic Field Flashcards by Brian Chuong

What is divergence?

The spreading out of a sound wave from a small source where the smaller the source the more divergence

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Huygen’s principle taught us that to limit divergence, we can do what?

We can fire a group of elements that together create a wave front

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Near fields have non- uniform Beams as the result of what?

The interference between the wavelets and far fields have uniform beans

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The many frequencies from a low q probe also makes the probe less what?

Less uniform

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Another way to represent the non-uniformity of the near field is to release it from what?

The intensity differences

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What is a grating lobe?

Off axis beams

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Between the main beam and the grating lobes which contains more power?

Grating lobes is weaker

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In mechanical probes the grating lobes are also called what?

Side lobes

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The grating lobes are the result of what?

The length and width vibration of the crystal resulting in crosstalk

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Side lobes are the result of what?

Radial mode vibration in the single disc probes

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What are the alternate names of the near and far fields?

Fresnel (near field)
Fraunhofer (Far field)

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The near field is characterized how?

A nearly constant beam

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The far field is characterized how?

By a divergent beam width

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At one near field length (NZL), even with old flat disc probes, what happens?

There is a natural narrowing of the beam that occurs

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What is the transition zone?

The natural narrowing of a beam that occurs at one near field length

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What is the crystal diameter at one near field length?

1/2

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What is the usable beam shape?

The natural beam shape

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What is the formula for near field length?

NZL = (D^2)/ (4*lambda)

D= diameter of the crystal

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If we were to rewrite the formula for NZL for soft tissue what would it be?

NZL = ((D^2)(F))/6

recall that lamda = C/F and C is tissue 1.54. 41.54 is 6*

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Looking at the formula for NZL what can we say about the relationship of Diameter and Frequency?

It is linear if D or F increase then so does NZL

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What is the Fraunhofer zone or far field?

The divergent part of the beam

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How do we calculate divergence?

Sin(x) = (1.22*lambda)/D

D is the diameter of the crystal

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If we increase the frequency or the diameter of the crystal what will we do to the angle of divergence?

We will decrease the angle of the divergence

Recall just like the NZL formula we should exchange lambda with frequency and Speed of sound to get an alternative formula

sinx = ((1.22*1.54)/((D)(F))

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Here we see the effect of diameter on what?

The NZL and angle of divergence

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Here we see the effect of frequency on what?

NZL and the angle of divergence

What is aperture?

The part of the probe that controls diameter

As the depth of the focus increases what happens to the aperture?

The Apeture increases to maintain a relatively consistent beam width at the focus

We can only focus in which field?

Near

What does this picture show us?

The manipulation of delayed firing to adjust the focus of the beam. Area of focus is the small black dot

The prime reason for focusing is to do what?

Improve our lateral resolution

How would we improve our lateral resolution?

decrease the beam width

Decreasing beam width would do what?

Improve sensitivity

Focusing can be divided into how many different parts?

Mechanical (both internal and external) and electronical

What is internal focusing?

Internal focusing that occurs when a curve is applied to the crystal itself

What is this picture an example of?

Internal focusing

What is this picture an example of?

Internal focusing

External focusing is accomplished with what?

An acoustic lens or mirror

Explain the two diagrams

The left uses internal focusing and the right uses external focusing

With older single crystal probes the focus is fixed so that the only way to change it was to do what?

change probes

Mechanical probes could be categorized into which focal lengths?

1. Short 2. Medium 3. Long

What is the typical range of a short probe?

1-4 cm

What is the range of a medium probe?

4 - 10 cm

What is the range of long probe?

7 - 19 cm

What is transmit focusing?

Electronic focusing for the array transducer which is more variable and operator controlled

If all the elements in an array probe are excited at the same time then they act how?

As a flat single disc

We can apply delay to the crystals in a probe to do what?

Steer and focus the probe

When time delays are used to excite a group of elements then what happens?

Wave fronts converge towards the focal region

When we use time delays to cause convergence of wave fronts towards a focal region what is it similar to?

Using a lens or curved element in a single element transducer

Increase the delays of the probe will do what?

Increase our focus

What is this an example of?

Increasing the delays of the probe to increase the focus

What are some disadvantages of using focus?

The divergence in the far field will increase greatly and the NZL will decrease

What does using focus increase?

Lateral resolution in the near field and focal point

How do we overcome the disadvantages of using focus?2

1. Dynamic aperture 2. Frequency

What is dual focusing?

The use of both mechanical and electronic focusing in the probe

What is the elevational plane?

The third dimension of our probe beam, or the Z axis if you may

Is it possible to have more than one focus on an image?

Yes

Multiple focusing does what?

expands the focal regions of the transducer creating a long focus

What does multiple focus require?

multiple pulses per scan line with each pulse focused at a different depth

What happens as a result of using multiple focuses?

Frame rate is reduced by resolution is optimized

What is receive focus?

When array transducers have the ability to focus the received echo signals

To focus received echo signals what must happen?

Time delays are applied to the received echoes to allow for constructive interference.

How does receive focus effect frame rate?

It does not effect frame rate and is not operator controlled

How is receive focus done?

It is done dynamically as echoes come back from deeper depths

What is the goal of receive focus?

To bring the echoes into phase so they don't cancel each other out

As reflected echoes arrive at various elements they do what?

Are amplified and combined in the beam former

What is dynamic receive focus?

The dynamic process of receiving echoes without canceling each other out

What is this an example of?

Receive focus

What is slice thickness?

Another way to describe the elevational plane?

Another name for slice thickness is what?

Elevation resolution

Slice thickness depends on what?

The beam width perpendicular to the image plane

Axial resolution is dependent on what?

SPL

What is lateral resolution dependent on what?

Beam width

Cystic structures that are smaller then the slice thickness may demonstrate what?

False debris since echoes are picked up from the off center part of the beam as well as the center

Slice thickness is fixed and requires what to help reduce the thickness at a fixed depth?

A curved element or lens to help reduce the thickness at a fixed depth

Label the beam points

A: Target is close on central axis (appears brightest) B: On axis but in the far field so less intense C: Target off axis so weaker than B D: Probably not detected E: Target may scape detection because it is off axis

What controls determine the sensitivity of the system and can change the effective beam shape? 3

1. Gain 2. Power 3. Suppression