Chapter 2: Ultrasound Transducers Flashcards
What factors affect the frequency at which a piezoelectric element resonates?
Thickness of the piezoelectric element and the propagation speed of the element itself.
Thicker element = lower frequency
What is Huygen’s principle?
An ultrasound beam starts out as small wavelets at the face of the transducer. Then the wavelets produce a propagation sound beam which travels perpendicular to the wavefront.
What is the backing layer of the transducer?
Tungsten based material provides damping of the element. This shortens the length of the pulse by decreasing the # of cycles in the pulse.
Increases bandwidth, reduces quality factor, decreases sensitivity of the transducer, increases axial resolution.
What is the matching layer of a transducer?
Layers of material that lies between the piezoelectric element and the patient’s skin to decrease impedance.
What is a Q factor?
What is the formula to calculate Q factor?
Quality factor: quantitates the purity of a beam.
Q = frequency / bandwidth
What is spatial resolution?
The ability to distinguish between closely spaced objects.
What is axial resolution?
AKA: longitudinal, radial, range, depth
The minimum distance two reflectors can be, parallel to the beam, and still appear on the screen as two dots.
(smaller number is better resolution)
Determined by the spatial pulse length.
What is lateral resolution?
AKA angular, transverse, azimuthal
The ability to accurately identify 2 reflectors that are arranged perpendicular to the US beam.
What is contrast resolution?
The ability to differentiate one shade of gray from another.
What is the formula for axial resolution?
1/2 spatial pulse length
SPL = wavelength x number of cycles in a pulse Wavelength = propagation speed / frequency
What is the Near Zone Length?
What affects the NZL?
The distance between the aperture and the focal point. The distance at which the beam diameter is 1/2 the element diameter.
The width of the aperture ( smaller = shorter) & the frequency (lower = shorter)
