Chapter 8, 9, 10 Flashcards
What is axial resolution?
In order to show two seperate items in line with the the beam, the space between them needs to be documented. If a pulse’s length is too long they will be blurred.
What is axial resolution determined by? Is it adjustable.
It is not adjustable and is determined by SPL and PD
What are the equations for axial resolution?
SPL/2
What is the equation for SPL? Is it directly or inversely related to axial resolution
wavelength x cycles. it is inversely related to axial resolution
What are the two ways to create a shorter pulse with axial resolution?
- Increase frequency to create a shorter wavelength
2.Use backing material to dampen the pulse/ decrease the number of cycles in the pulse.
What are the other terms for axial resolution? (what does LARRD stand for)
Longitudinal, Axial, Range, Raidal, Depth
What is lateral resolution?
distinguishing between two objects that in line perpendicularly to the beam
What are the other terms for lateral resolution? (what does LATA stand for?)
Lateral, Angular, Transverse, Azimuthal
What is lateral resolution determined by?
The width of the sound beam.
Why is lateral resolution related to depth?
The beam’s width changes with the depth
Do higher frequency beams have increased lateral or axial resolution? Why?
Both. Axial resolution increases in the entire image because the wavelength is shorter. Lateral resolution increases in the far field only, as the beam will diverge less.
What is the equation for lateral resolution?
lateral resolution (mm) = beam width (mm)
What is focusing?
Concentration of the beam to narrow it and improve lateral resolution
What is the difference between internal and external focusing?
Internal is achieved with a curved PZT crystal. External involves focusing with an acoustic lens
What is fixed/ conventional/ mechanical focusing
When the focal zone cannot be changed. It is created when the probe is manufactured.
What does phased array mean?
focusing by using electronic means to change the crystals and therefore the beam.
Are phased array transducers adjustable? Why or why not?
Yes. Electronic focusing is adjustable- you can use a focus button to change the focusing depth.
When you focus, the focal zone is smaller. What happens to the near field, far field, and depth?
The depth is shallower, the near field decreases while the far field increases
What is a transducer
A device that transfers one form of energy to another.
How is SPL related to axial resolution? What about the axial resolution value?
Inversely related to AR, directly related to the value.
What does Piezoelectric material do? What does it allow for?
Piezoelectric material allows for sound energy to be both transmitted and received. It allowes for pulsed wave ultrasound to be achieved with one transducer.
what does Piezoelectric mean? when do we use it?
pressure makes electricity. this happens when we convert sound to electrical energy.
What is reverse Piezoelectric? when do we use it?
energy makes pressure. we do this when sending out a signal to create sound energy.
what is PZT?
Man-made Piezoelectric material. also called the crystal, ceramic, or active element.
How does PZT work?
The crystals have molecular structures arranged in a latice formation with dipoles. When the crystal heats, the molecules move freely and when it cools it freezes them.
What is a dipole
seperate regions of + and - charges
in a molecule
What is the purpose of the matching layer?
The impedance mismatch between the skin and the ultrasound beam is huge, and gel is not enough. the matching layer helps the beam enter the body.
How thick is the matching layer in the transducer? How thick is the active element?
The matching layer is 1/4 the wavelength. The active element is 1/2 the wavelength
How does the backing material decrease sensitivity
Reduces active element vibration during both transmission and reception
What is a bandwidth? What kind of bandwidth does pulsed ultrasound have?
the range of frequencies we can hear. Pulsed wave has a wide bandwidth.
what is the Q factor equation?
frequency/bandwidth
what is a q value? Why do we have one?
the ringing time of the transducer. We get this as a result of dampening the pulse.
How are Q factor, SPL, and bandwidth related?
Q value and SPL are Inversely related to bandwidth. A low Q value has short SPL and a wide bandwidth.
What characteristics of the PZT is pulsed wave dependent on?
The speed of sound and thickness of the PZT
How are the PZT speed and thickness related to frequency?
Frequency is directly related to PZT speed and inversely related to thickness of the crystal.
What is the equation for frequency using the speed/thickness of a crystal?
Frequency = speed of PZT / (Thickness x 2)
Explain Huygens Principle
The beam is created by a disc shaped crystal. Many particles in the crystal create Huygens wavelets which interfere and create an hourglass beam.
What is the width of the focus point of the beam?
1/2 the transducer width
What is the Fresnel zone
the near field from the transducer to the focus.
What is the Fraunhofer zone
the far field past the focus
How is transducer diameter related to the focal depth?
They’re directly related. A larger diameter transducer’s beam will travel farther into the body.
What is divergence? What is it related to?
the spreading out of the beam
after the focus in the far field. It is inversely related to crystal diameter and frequency
Lateral resolution is dependent on the width of the beam. What frequency or crystal width are best for lateral resolution?
Larger diameter and higher frequency
Frequency is determined by crystal thickness. What is this equation?
thickness = wavelength / 2
Focal zone is determined by crystal diameter. What is this equation?
focal zone = 1/2 crystal diameter
Electrical energy is converted into sound during…?
Transmission
A sound pulse is converted into electrical energy during…?
Reception
What does the electrical shield do? Where in the transducer is it?
Prevents electrical signals in the air from entering the transducer and contaminating the important diagnostic electrical signals. It is the middle layer between the case and insulator.
What is the acoustic insulator? Where is it?
A barrier of cork or rubber that prevents vibrations in the case from the PZT. It is the inner most layer.
What does the case do? Where is it?
It protects the inner components of the transducer from damage. It also protects the patient from electric shock. It is the outermost layer.
How much greater than the impedance of the skin is the impedance of the PZT?
20 times greater
Why do we need backing material for the quality of an image?
Backing material reduces the vibrating time of the transducer and creates a shorter pulse. This shorter pulse creates a better axial resolution.
What are characteristics of backing material?
High sound absorption, high acoustic impedance similar to PZT.
What are the consequences of using backing material
Decreased sensitivity, wide bandwidth, low Quality factor
What happens if PZT is heated above the curie point (350• C)
The piezoelectric properties are destroyed. This is called depolarization of PZT.
Imaging transducers have low sensitivity. Will a lower sensitivity transducer create a short or long pulse? Will this create a wide or narrow bandwidth.
Lower sensitivity transducers create short pulses. wide bandwidth.
What determines frequency of sound in a continuous wave transducer
The frequency of the electrical signal = the frequency of sound
