Test Review Flashcards
What is a transducer
The device that generates and recieves US
A transducer converts one form of energy to another; for ultrasound they convert electric energy into sound energy and vice versa
How does piezoelectric effect work
The piezoelectric effect is the formation of an electrical change on the surfaces of the crystal when pressure (mechanical energy) is applied. In pulse echo imaging, this effect occurs when echos return to the transducer and are converted into electric signals
The reverse or converse piezo electric effect is the production of US when an electric signal is applied to the crystal
Give an example of the material used for our crystals
Lead zirconate titanate= PZT. Ceramic material formulated to obtain piezoelectric properties when placed in the presence of a strong electric filed while immersed in a high temp bath
What type of voltage is the transducer driven by
Alternating current is the voltage that drives our transducers, which is the current that runs through residential homes
Whats another name for natural vibrational frequency
Natural freq
Operating freq
Resonance freq
What determines operating freq
The thickness of the piezoelectric crystal determines the transducers natural freq
What freq will a thick element operate at? High or low?
Low freq
What is a dipole
Molecules with a positive charge at one end and a negative charge at the other end
How does the ceramic cystral get its piezoelectric properties
The ceramic material obtains its piezoelectric properties when placed in the presence of a strong electric field while immersed in a high temperature bath, and then cooled down while still in the presence of the electrical field
What happens if the transducer is heated above the curie point
The dipoles re- orient into their random state and lose their piezoelectric properties
Where is the damping material found
Attached to rear face of the element
What is the job of the damping layer
To reduce the number of cycles in each pulse. Its known as the mechanical pulse damper as it serves primarily to limit SPL and PD
What parameters are effected by damping layer and how
Reduces PD and SPL and improves resolution
Also reduces sensitivity and efffectiveness
Why is the matching layer required
Transducer element is 20x the impedance of the tissues, by itself, this would create a large reflection at the skin and very little waves would be transmitted into the body.
Where is the matching layer located
At the surface of the transducer
What is “self focusing effect” or “natural focus” seen in the beam profile
Self focusing effect is the natural narrowing of the sound beam in a non focused single element transducer
What is the beam profile? Describe it
The ceramic material obtains its piezoelectric properties when placed in the presence of a strong electric field while immersed in a high temperature bath, and then cooled down while still in the presence of the electrical field
What is the near zone? What happens there?
The NZ is the region extending from the element out to the narrowest portion of the sound beam and is characterized by beam convergence
What other names does the near zone go by
Near field and frensel zonw
What determines the Near zone length
The size of the aperature and the operating freq of the element
What is the far zone and what is it characterized by
It sthe sound bea, beyond the narrowest point and is characterized by divergence
Other names of far zone
Far field and fraunhofer zone
What size is the width of the beam at 2x the near zone length
The size of the aperature
Why is focusing of the beam required
Focusing improves resolution. Increases the intensity of the sound beam
What are the two methods of focusing
Mechanical- lens or curved element
Electronic - phasing
What is the focal length
The distance from the transducer to the focal point — same as NZL
What are two methods of real time scannign
Mechanical and electronic/automatic
What is the major difference between mechanical and electronic trasnducers
A mechanical transducer has a moving part to steer the beam the electronic transducer does not
What are the 2 modes of activation to produce a beam
Sequencing
Phasing
How do sequenced arrays wokr
A sequenced array applies voltage to a group of elements in succession to form scan lines
How does a phased array transducer work?>
A phased array applies voltage pulses to all the elements with short time delays to steer the beam
How is the beam directed to the right
Voltage is applied with time delays in a rapid progression from lt to rt directing the beam to the right, based on Huygen’s principle
What is a vector array and why is it useful
A vector array is a linear phased array - converts a linear rectangular format into a sector like feild of view
How is electronic focusing achieved
By using a curved pattern of phased delays; an increase or decrease in the curvature of the delay pattern moves the focus shallower or deeper, respectively.
WHAT IS THE LIMIT TO MULTOPLE FOCI
Temporal resolution is reduced (multiple pulses per scan line = frame rate reduced)
What is variable aperture, focusing?
To focus deeper, you need a bigger aperture and for a shallow focus, you need a smaller aperture. Therefore, the transducer fires only the elements needed depending on the distance of the focus. For closer natural focusing, smaller groups of elements are fired
What is dynamic aperture
As the depth changes, the aperture changes to maintain a constant focal width. This is called a dynamic aperture.
What is huygens principle
Huygen’s principle is that every point on a wavefront can be considered a source for secondary wavelets. These wavelets combine to form a ‘wavefront’ that heads in a direction perpendicular to the combined wavelets.
What is spatial compounding? What is it used for
This is the use of phasing to strike objects from multiple angles. These images are compounded to produce an average image which will reduce artifacts and sharpen borders. (eg. Within cysts
What is the limit for spatial compounding
Temporal resolution
What is the difference between grating lobes and side lobes
Grating lobes involve multi-element transducers, side lobes are for disk transducers.
What can we do to reduce grating lobe artifact
Apodization – reducing the amplitude of the outside elements decreases the intensity of returning echoes
Subdicing
THI – grating lobes do not produce harmonics, they are too weak; using THI filters out any grating lobe artifact
What is resolution
The ability to distinguish echoes in terms of space (detail), time (temporal) and strength (contrast).
What are the three aspects of image resolution
- Detail (aka spatial)
- temporal
- contrast
What are the 3 types of spatial resolution
Axial
Lateral
Evelation
What is considered as poor detail resolution
Poor resolution is when 2 separate reflectors close together appear as one on the display screen.
What is axial resolution
The ability to separate interfaces that lie along the beam axis (one on top of the other
How can the operator improce axial resolution
By reducing the SPL (either with wavelength or # of cycles in a pulse), we reduce SPL by decreasing the wavelength with a higher frequency.
SPL = 𝜆 X # cycle in a pulse
SO, we improve axial resolution by INCREASING FREQUENCY.
What other parameters will be affected and how ?
With the higher frequencies we lose the ability to penetrate (attenuation). Therefore, we are choosing resolution over penetration.
How is the transducer built to improve axial resolution
The damping layer decreases the number of cycles per pulse
What is the lateral resolution
Lateral resolution is the ability to separate interfaces that lie perpendicular to the beam.
How can the operator improve lateral resolution, explain?
The more narrow the beam, the better the lateral resolution. So, we can reduce the beam diameter by applying the focus to the area of interest.
What other parameters will be affected and how ?
Increased intensity due to focusing
Decreased ability to penetrate due to higher frequency
What is the equation for lateral resolution
LR= beam width
What is elevation resolution
Elevational resolution is the ability to separate interfaces that lie perpendicular to the beam axis (one in front of the other)
How can the operator improce elevational resolution and how ?
Elevational resolution can be improved by applying focusing in the sectional plane thickness (which the operate cannot due this is done at the manufacturing level)
Or again with a higher frequency/THI to narrow the beam
What artifact can elevational resolution contribute to
Section thickness artifact
Explain how this artifact appears
It is the filling in of a structure that should appear anechoic
In soft tissue, given a 3 pulse calculate the axial resolution for a 3 MHz transducer
1) λ = c/f - λ = 1.54/3 = 0.51 mm
2) SPL = n x λ = 3 x 0.51 = 1.53 mm
3) A.R = SPL/2 = 0.77 mm
A 5MHz transducer generates a 3 cycle pulse and is operated at a pulse repetition frequency of 5000. The media is soft tissue. Beam width is 10 mm at the transducer surface, 3mm at the focal point, and 8mm at the distance of 12cm. What is the best lateral resolution of this transducer
The best lateral resolution is simply equal to the narrowest beam width. In this case, this value is 3mm and not surprisingly is at the focal distance. The frequency, pulse length and PRF are distractors
Which type of focusing cannot be used with a single element transducer
Only arrays may be electronically focused. Single-element transducers are fixed focused by mechanical means which include crystal shaping or the use of an acoustic lens or mirror
What is temporal resolution
Temporal resolution is the ability to seperate closely spaced events in time
