Sound Beams

Question 0

What is the Near Zone?

Answer 0

It is the distance between the transducer and the focus of the beam (the narrowest part of the waist in the hourglass shaped beam) also called the focal length, focal depth, and fresnel zone.

Note: this applies to old fixed focus transducers

Question 1

Describe Huygen’s principle

Answer 1

Shows that a beam is made up of many V shaped waves called wavelets which interfere both constructively and destructively forming an hour glass sound beam (wide-narrow-wide), the narrowest part of the waist being the focus.

Huygen’s principle allows for the focus of the beam to be to be moved deeper or more shallow, changing the shape and direction of the beam by time delays of the wavelets.

Question 2

How is the diameter of the transducer, beam density, and frequency related to the focus?

Answer 2

⬆️ the diameter = deeper the focus (they are directly related)
⬇️ diameter = shallower the focus
Diameter of transducer = 1/2 the width of the focus

⬆️ the frequency = deeper focus (directly related)
⬇️ the frequency = shallower focus

Question 3

What does density and divergence have to do with the sound beam?

Answer 3

Beam density converges in the near zone and is the most dense at the focus where there is better lateral resolution and dissipates in the far field, this is called beam divergence.

⬆️ beam density= higher lateral resolution

Question 4

What is the Far zone and Divergence?

Answer 4

The far zone is the distance of sound beam after the focus, this is where the beam diverges (spreading of the beam), so there is less density.

⬆️ Divergence in the far zone = ⬇️ frequency and diameter of transducer (inversely related)

So our focus is best at or just below the structure being scanned

Question 5

What is Lateral Resolution?

Answer 5

As axial resolution is a sound beam’s ability to see 2 close objects, posterior/anterior. Lateral resolution is the beam’s ability to resolve 2 objects transversely with out seeing them as 1 object.

AR is always superior to lateral resolution.

LR also called side by side, transverse, or azimuthal

best at focus, with ⬆️ frequency ⬇️ diameter of transducer

At focus: LR = diameter (mm) Otherwise LR = beamwidth
2

Question 6

How to focus a fixed beam and non fixed?

Answer 6

Fixed

• Placing a lence in front of crystals
• Curved crystal element (PZT)

Non-fixed

-Phased fired beams (Time delays with pulses)

Question 7

What are the changes that occur in Focusing?

Answer 7

Focal Zone narrows

Focus zone reduces

Focus moves closer to transducer

Divergence increases

Question 8

Name the display modes

Answer 8

A-mode: Amlitude
B-mode: Brightness
M-mode: Motion

Question 9

What is A- mode?

Answer 9

Old style of ultrasound, measures returning amplitudes on a spiked line

Question 10

What is B-mode?

Answer 10

This is what we use today, it takes all those varying amplitude strengths of the reflections and show them as brightness on the screen.

It is rephered to as 2D = B + real time

Question 11

What is M-mode?

Answer 11

Maps the changes in motion down one crystals line.

Question 12

Mechanical Tansducer?

Answer 12

One Crystal on a motor that steers the crystal in sweeping motion it back and forth making a fan shaped image.

fixed focus depth, (old school)

Question 13

Phasing and sequencing transducers?

Answer 13

Timed firing of multiple crystals, types:
	Annular array~ 
	Linear Array ~ 
	Curvilinear or convex array
Can change depth and focus of beam
Can have Multiple-foci (phasing)

Question 14

What is an annular array?

Answer 14

Old school round transducer with multiple rings of crystals (target like) turn outer rings off to narrow focus

They had mechanicle steering by inner motor producing that fan shaped image

Question 15

What is a Linear phased array?

Answer 15

Linear array, is made up of 100 to 300 crystals with a square foot print and a fan shaped image

Steered by electronicly by phasing

Question 16

What is Phaseing?

Answer 16

Using small delays in voltage to direct beam, controlled by the Beam Former which decide which controls what which crystal will fire when, which determines the direction and focus of the beam.

Multiple Focus = multiple beams of different shapes projected

Question 17

Time delays?

Answer 17

Phasing uses time delays to send out beams, the dynamic receive focus uses the return time to determined focal area/depth, (The machine listens for echoes coming back from certain times).

The time delays allows the transducer to “listen” for the certain depths.

Question 18

What are Linear Sequential arrays?

Answer 18

A transducer with rectangular crystal and a rectangle image which can be “steered” to a trapezoid.

Crystals fire in small groups, each group firing one beam, can be steered electroniclly by using phasing with in small crystal groups, using Dynamic Receive Focus to determine which group fires when for desired focus.

This is the probe used for scanning small parts!

Question 19

Define a Curvilinear Array.

Answer 19

Line of 120-250 crystals (not directly adjacent from each other like linear array), which use sequencing to fire different groups of crystals to be able to use phasing to electronically focus, using Dynamic to direct firing of crystals.

This is the transducer with the curved top that we use constantly to view deep structures.

Question 20

What are Vector Arrays?

Answer 20

A combination of linear sequential and linear phased array, used to better get in between rib spaces. Its a line of 120-250 crystals (lower F then linear), giving a trapezoid shaped image. Uses phasing to focus and Dynamic Receive focus to direct phasing sequence.

Question 21

What is thickness Resolution?

Answer 21

Slice Thicknesss or elevation resolution. The old modules that was best at this were disk, mechanicle, and annular were best (but we don’t use these anymore). Old way of focusing this way is using lenses front and back to get better focus or use multiple rows of crystals to phase in the thickness damention.

With modern transducers we adjust the focus with lateral resolution not the thickness resolution

Question 22

Apodization?

Answer 22

Applying less voltage to crystals contributing to to grating lobes (multiple areas of beam in a array transdusers that degrade lateral resolution,)

In single element transducers there areas in beam that degenerate lateral resolution in far field by widening it