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Medical Physics - INIR: Ultrasound Y2 > Array Transducers > Flashcards

Flashcards in Array Transducers Deck (21)
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1

How are the far-field directivity, and distance tot he axial maxima and minima related to the frequency?

Far field directivity is inversely proportional to frequency.
Axial max and min a directly proportional to frequency.

2

After the last axial maxima, how does the axial pressure profile change?

Drops off as 1/wavelength.

3

What is the equation for the axial pressure?

p=2p_0*|sin{(k/2)*(sqrt(z^2+a^2)-z)}|
where p0 is the pressure at z=0,
z is the depth in tissue,
a is the radius of the transducer,
k is the wavenumber.

4

What is the equation for the location os the last axial maxima and minima?

z=a^2/(n*lambda)
n=odd, max
n=even, min

5

What is the equation for the far-field directivity of a circular transducer?

D(0,k)=|[2*J_1*(kasin0)]/kasin0|
where k is the wavenumber,
a is the half aperture, and
J_1 is a first order Bessel function.

6

How does the axial profile of a pulsed wave differ from that of a continuous wave?

- The final axial maximum is further out.
- Fewer axial minima
- Shallower minima.

7

How does the axial profile of a square transducer differ from that of a circular one?

- The final axial maximum is shifted outwards.
- Incomplete destructive interference (non-zero minima)

8

What is the equation for the far-field directivity of a square transducer?

D(0, phi, k)=|sinc{0.5*kL_xsin(0}*sinc{0.5kL_ysin(phi)}|

9

What is the equation for the axial pressure of a circular focused transducer?

p=2p_0*(A/A-z)*|sin{(k/2)*(B(z)-z)}|
Where B(z)=sqrt(z^2+2b(A-z)),
and b = A-sqrt(A^2-a^2)
A is the radisu of curvature of the spherical trnasducer.

10

What is the equation for the focal gain?

G=(pi*a^2)/F*lambda

11

How does the last axial maxim change with increased focal length?

Gets further out and lower in amplitude.

12

What is the equation for the -6dB beam width at the focus?

w=0.7F*lambda/a

13

Describe the construction of a mechanical sector transducer?

- Mechanically scanned single element transducer.
- The element is on a motor and is waggled side-to-side to produce the image.
- Only used in simple devices today (e.g. bladder scanners)

14

Describe the construction of an annular array transducer?

- Mechanically scanned annular array transducers.
- Typically 4-8 equal area, concentric, ring-shaped elements.
- One electrical connection for each element.

15

What are the advantages and disadvantages of an annular array transducer?

+ Beam is radially symmetrical.
+ Scan plane focusing and slice thickness focusing are identical.
- Moving parts make them less reliable.

16

Describe the construction of a simple linear array transducer.

- Flexible printed circuit attaches to lower surface of the crystal.
- Flexible ground plate bonded to the upper surface.
- Backing material pedestal with crystal and matching layers bonded to it.
- Cut into a row of individual elements leaving air kerfs.
- Cylindrical silicone rubber lens bonded on top.

17

What is the ideal array geometry?

- The pitch should be lambda/2 in tissue.
- The thickness should be lambda/2 in the crystal.
- w/d should be 0.7 for "width expanders".

18

What are the properties of a mechanically scanned array?

- Curved array with a silicone rubber lens in an oil bath.
- Used in private clinics for antenatal "bonding" scanning.
- 4D frame rate limited to about 4Hz due to sweep speed.

19

What are the properties of a biplane transrectal transducer?

- Two separate curved arrays for sagittal and transverse views.
- Used widely in urology clinics for prostate biopsies.

20

What are the properties of a radial transrectal transducer?

- Aperture size is limited by the array diameter (11mm).
- Used in some urology clinics to endoanal scanning.

21

What are the properties of an intravascular transducer?

- Two types rotating a single element, and radial arrays.
- The diameter must be less than 8mm to fit in a 10mm trocar or less than 1mm for intracoronary imaging.