Chap. 5 And 6 Level 1 Flashcards
What is a transducer in ultrasound
A device that converts electrical energy into acoustic energy and back to electrical energy
What is the piezoelectric effect
An electric field is created by certain crystal materials getting mechanically deformed through vibrations
What material is commonly used in ultrasound transducers?
Lead zirconate titanate (PZT)
What is poling
Poling is placing the crystal materials into a specialized oven at high temperatures. These high temperatures allow the positive poles to align in one direction and the negative poles the opposite direction
What is the curie point
The temperature at which a crystal loses its poling and efficiency as a piezoelectric material
What is the curie temperature for PZT
300 degrees Celsius
Formula for operating frequency in PW
Fo (mhz) = C(mm/sec)/2 x thickness (mm)
The thickness of the crystal determines what?
- A thinner crystal will imply a shorter period (higher frequency)
- A thicker crystal will imply a longer period (lower frequency)
What determines the frequency in CW?
- The drive voltage frequency of the pulser
- Fo = drive voltage frequency = transmit signal
What is the crystals impulse response
How a crystal responds to a single, short electrical impulse
A long vs short crystal impulse response
- Long, many cycles leading to a long spl
- short, few cycles leading to a short spl
- shorter spl, better axial resolution
What are the 2 physical dimensions of a transducer
Diameter and thickness
What do diameter and thickness determine in terms of a transducer
- diameter determines beam width
- thickness determines operating frequency
What is the beam shape view
The region in the patient that the sound wave propagates
Attenuation occurs at deeper or shallower depths?
Greater attenuation at Deeper depths
What is the shape of an ultrasound beam during CW?
The beam begins at the crystal and gets narrower as it reaches the center then diverges as it increases in depth
What is the natural focus
- The depth at which the beam reaches its narrowest beamwidth.
- the center of the beam
What is the “focal depth” or “near zone length”
The distance from the surface of the transducer to the natural focus
Formula for near zone length
NZL = D^2 x operating frequency/6
NZL is proportional and inverse to what?
-NZL is proportional to diameter squared and operating frequency
- NZL is inversely proportional to 4(wavelength)
What crystals are good for superficial and deeper focus
- Smaller crystals are good for superficial imaging
- large crystals are good for deep imaging
What beams produce better lateral resolution
Narrow beams
Formula for axial resolution
Axial resolution = Spl/2
Why do we have a matching layer
To minimize the acoustic impedance mismatch between the high impedance of the crystal and the low impedance of tissue
What is the matching layer
A thin layer of material attaches to the front of the crystal
Why is the matching layer a 1/4 wavelength
It prevents reverberation artifacts from energy ping ponging within the crystal
What are the 3 imaging planes
Axial, lateral, and elevation
What is lateral resolution
The ability to resolve 2 structures in the lateral dimension
How to identify to separate structures side by side
If the beam is narrower, you can identify each structure individually
What are the techniques for changing focus of the transducer
1) mirrors
2) lenses
3) curved elements
4) electronic focusing
5) retrospective gating
What is elevation resolution
- The elevation resolution is determined by the beamwidth
- the elevation resolution is best where the beam is narrowest
Define transmit power
The amplitude or gain
What is the dynamic range
- Dynamic range is the ratio of the maximum to the minimum amplitude
- also known as the ratio of the biggest signal/smallest signal
What are the 4 types of dynamic range
- input dynamic range: max/min input signal
- output dynamic range: max/min output signal
- display dynamic range: max/min display signal
- gain dynamic range: max/min applicable gain
Difference between receiver gain and transmit power
Both control gain, but transmit power affects the amplitude going into the patient where receiver gain affects the amplitude after it has returned to the receiver
Define signal and noise
- Signal: any phenomenon desired to be measured
- noise: any unwanted signals
Define noise floor and signal-noise-ratio (SNR)
- noise floor: the amplitude level where no signals are visible because of the presence of noise
- SNR: the amplitude of the signal/amplitude of noise
What are the signals for Doppler and ultrasound
- The reflections from the tissues that are converted into images
- signals for spectral Doppler are the Doppler shifts
What is the SNR
SNR is what specifies the signal quality. A higher SNR gives better imaging while lower SNR gives poor imaging
Give an ex: of a good signal to noise ratio and a bad
- Good: a high amplitude and low noise floor
- bad: high amplitude and high noise floor
In terms of SNR, what does increasing amplitude do
Increasing amplitude increases both signal and noise evenly. The true SNR may not change. But the apparent SNR could
Define clutter
Large returning echoes from structures that obliterate weaker signals
Pre processing vs post processing
- pre processing: a real time signal
- post processing: imaging that can be changed after captured
What can be changed in post processing?
Compression, colorization, and reject
What are examples of analog in ultrasound?
Blood flow, EKG’s, pressure waveforms
What is the NyQuist criterion
The minimum rate at which you must sample for an accurate reconstruction of an analog signal
Equation for NyQuist criterion
NyQuist: F(max) = F (sampling)/2
Basic system function
- transducer converts electrical to acoustic and acoustic to electric signals
- receiver receives the echo
- processor converts echo into images
- displays real time image
- image is stored and can perform required measurements
What are the 5 operations the receiver performs
- amplification
- compensation
- compression
- demodulation
- rejection
What is the job of the receiver
To receive signals from the transducer and apply the necessary processing before converting the signals into an image
Compression in dynamic range
The dynamic range must be compressed in order for the human eye to be able to see it.
What is demodulation
The process by which the modulations of the wave are removed or detected (signal detection)
What is rectification
Converts the negative components of a signal into positive components (bipolar to unipolar)
