Chapter 9

Question 1

Shape of a sound beam

Answer 1

At the starting point the beam width is exactly the same as element/transducer diameter, then it narrows progressively reaching its smallest diameter

Question 2

Focus or focal point

Answer 2

is the location where the beam diameter is the narrowest

Question 3

Near Zone (near field or Fresnel zone)

Answer 3

is the region from the element to the focus

Question 4

Focal Length (focal depth or near zone length)

Answer 4

is the distance from the element to the focus

Question 5

Far Zone (far field or Fraunhofer zone)

Answer 5

is the region starting at the focus and extending deeper

Question 6

Focal Zone

Answer 6

is the region around the focus where the beam is relatively narrow

Question 7

Diameter and focal depth are directly related

Answer 7

diameter ∝ focal depth

Question 8

Frequency and focal depth are directly related

Answer 8

frequency ∝ focal depth

Question 9

Beam divergence

Answer 9

describes the gradual spread of the ultrasound beam in the far field

Question 10

Two factors determine divergence

Answer 10

1. Element diameter
2. Frequency of the sound exiting the element

Question 11

Spherical waves

Answer 11

sound waves produced by very small sound sources (tiny pieces of PZT)

Question 12

Huygens’ Principle

Answer 12

states that a large active element may be thought of as millions of tiny, distinct sound sources each creating a v shaped wavelet; these wavelets interfere constructively and take on the shape of an hourglass