Chapters 9-12 Santanna Flashcards
As sound travels
the width of the beam changes
Narrow beams create
better images
the location at which the beams= reaches its minimum diameter
focus or focal point
the distance from the transducer to the focus
focal depth
focal length or near zone length
near zone (fresnel zone)
the zone between the transducer and the focus
far zone ( fraunhofer zone)
the zone deeper than the focus, beyond the near zone
sound beams ____ in the near zone
coverge
sound beams ___ in the far zone
diverge
what is focal zone
the region surrounding the focus where the beam is kind of narrow and the picture is relatively good
PZT diameter
transducer aperture (another name for the transducer diameter)
at the transducer
beam diameter equals transducer diamter
at the focus
beam diameter is one-half transducer diameter
at 2 near zone lengths
beam diameter equals transducer diameter
deeper than 2 near zone lengths
beam diameter is wider than transducer diameter
focal depth is determined by
transducer diameter and frequency of the ultrasound
shallow focus
small diameter, low frequency
deep focus
large diameter, high frequency
divergence of the beam describes
the spread of the sound beam in the deep far zone
less divergence
larger aperture, high frequency, narrower beam in far field, improved lateral resolution in the far field
more divergence
smaller aperture, low frequency, wider beam in far field, degraded lateral resolution in far field
frequency (CW) is determined by
electronic frequency
frequency (pulsed) is determined by
thickness in ceramic and speed of sound in ceramic
divergence
aperture of active element and frequency of sound
each tiny part of the surface of the transducer face may be considered
an individual sound source, this creates a huygens wavelet
when all of these wavelets combine, they create an hourglass shaped beam
huygens principle
axial resolution
the ability to distinguish two structures that are close to each other front to back, parallel to, or along the beam’s main axis
units of axial resolution
can it be changed by sonographer
distance, mm, cm
no
changing axial resolution means
changing transducers
SPL (mm) / 2
equation for axial resolution
Shorter pulses mean
shorter SPL and PD… meaning images are more accurate
axial resolution is BEST with
highest frequency and fewest number of cycles per pulse
axial resolution is WORST with
lower frequency and larger number of cycles per pulse
If we add more pulses, we
degrade our images
Better axial resolution
shorter SPL
shorter PD
higher frequencies
fewer cycles per pulse
lower numerical values