Chapter 12, 13, 14 Flashcards
To create a 2-D image slice, the
probe sends out multiple scan
lines right next to each other. What issues does this resolve?
The issues that sound travels in a straight line and beams must be narrow to improve lateral resolution.
What are characteristics of a mechanical transducer
Single crystal, sector shaped image, mechanical steering and focusing, and a damaged PZT loses the entire image.
What does array mean in context of modern transducers?
a transducer with multiple active
elements or crystals
How does an array transducer work?
Each active element is connected to its own wire
What are characteristics of a linear phased array transducer
100-300 crystals, sector shaped, electronic beam steering and focusing, damaged crystals cannot be predicted.
How does electronic steering/focusing work? What causes this?
Crystals do not move or change, the order they fire can be altered which is called phasing. The electronics that allow phasing is the beam former.
What is transmit focusing?
When we focus the beam by using electronic means to send out the beam using an array.
How do we focus a beam? Is the steepness of the curve directly or inversely related to the focus?
To focus the beam, the crystals are fired in a curved pattern. Steep curves create a shallow focus- they are inversely related.
What happens with multiple transmit focal zones? How is this related to frame rate?
Multiple sound beams are sent down each line. More focal zones decreases the frame rate.
What is dynamic receive focusing
The beam hits the transducer and the information is processed using the electronic spike lines. The information is virtually focused using delay patterns
What are characteristics of annular phased array?
Multiple ring shaped crystals, mechanical steering, electronic focusing, sector shape, horizontal dropout when PZT is damaged.
What are characteristics of linear sequential array?
120-250 rectangle crystals, electronic steering and focusing, rectangle image, damaged PZT leads to vertical dropout.
What are the differences between linear phased and linear sequential?
Sequential has a larger footprint and crystals. Each crystal is about 1 wavelength width whereas a phased crystal’s width is 1/4-1/2 a wavelength. Sequential is rectangular and linear is sector shaped.
What is a linear sequential array?
These transducers fire a sequence of groups of crystals to improve frame rate, axial resolution, and so there is no divergence (good lateral resolution).
What are characteristics of a convex sequential array?
120 to 250 large rectangle crystals, electronic beam steering and focusing, blunted sector, vertical drop out if PZT is damaged.
What are characteristics of a vector array?
120-250 large rectangle crystals, electronic steering and focusing, trapezoidal shape.
Disc shaped crystals are found in mechanical and annular phased array transducers. What kind of beam do these produce?
They produce beams with equal lateral and elevational resolution
What is elevational resolution? Where is it measured?
Another word for slice thickness. It is 3 dimensional and It is measured perpendicular to the imaging plane.
phased array, linear array, and convex array transducers have rectangular elements. What effect does this have on the beam?
the beam is narrowed side-to-side, not top to bottom. This improves lateral resolution, but does not impact slice thickness.
What do 1½ dimensional arrays do?
They decrease the slice thickness by decreasing the thickness of the crystal. They have multiple crystals stacked on one aonther.
What is a side lobe?
Side lobes happen when single element transducers create extra lobes off to the side, these degrade lateral resolution.
What is a grating lobe?
Grating lobes are essentially side lobes that are created by array transducers and can be fixed by apodization
What is apodization?
Apodization alters the electrical spike voltages. Stronger spikes excite the inner crystals and weaker spikes excite the crystals on the
edge of the transducer.
What does subdicing mean?
The crystal is divided into a group of
smaller crystals, these are all electrically joined so they act as one crystal.
What should you change when your ultrasound imager displays reflectors far from the transducer but no reflectors close to the transducer? What is the purpoe of this button?
Compensation (TGCs). They compensate for attenuation.
What is real time imaging
Each frame is created and displayed quickly so that we have an impression of real-time scanning
What is a frame rate and what unit do we use to describe it.
the number of frames per second. Measured in Hz or Frames per Second
What is frame rate determined by?
It is mostly determined by depth but is also determined by speed of sound in tissue.
What is temporal resolution? Is good TR many or few frames/second?
Accuracy in time- how quickly a system can display frames to appear real-time. Good TR is many frames/second
What is temporal resolution determined by and what units do we use to measure it?
It is determined by frame rate and measured in Hz or Frames per second.
Frame rate and T-frame are inversely related. Why is this?
When a system creates one image in 1/100 of a second, the frame rate is 100 frames per second.
What is a T-frame?
The time it takes to display a single frame.
How would a sonographer adjust the frame rate?
Changing depth or the number of pulses in each picture
How does depth impact frame rate and temporal resolution?
The farther away the beam travels, the more the system must process.
What factors affect the number of pulses needed to create an image
Number of focal zones, sector size, and line density
What happens when you increase the number of focal zones
More pulses get sent down each scan line. This creates more work for the machine and decreases the frame rate.
What are the two main functions of an ultrasound system?
Preparation/Transmission and the Reception of electrical signals.
What does a pulser do with transmission?
The pulser creates electrical signals that excite the transducer’s PZT crystal and create sound beams
What is SNR
Signal/Noise ratio . It’s the ratio of important to non important information.
Low pulser output power is best for ALARA principles, when would we want high output power?
I creating output power improves and increases the SNR
What does the beam former do
Received the pulser’s electrical spike and distributes the voltage to the PZT
What affect does bean former have on side lobes?
It reduces them with apodization
Is a switch used for transmitting or receiving? Why?
Both. It protects components from the signal and also directs electrical signals
What does a receiver do?
After pulses return and strike the PZT, an electrical signal is received. the receiver prepares the information from the received signals.
What are the 5 steps of receiving a pulse?
Amplification, compensation, compression, demodulation, and reject
What is amplification? Is it adjustable and does it affect SNR?
Each signal is made larger. It is adjustable but does not affect SNR
What is compensation? What button is it?
It is the TGC button and it compensates for attenuation
What is compression?
