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physics exam notes 3


A-MODE amplitude modulation in diagnostic ultrasonography. It represents the time required for the ultrasound beam to strike a tissue interface and return its signal to the transducer.
A-mode (amplitude mode) The greater the reflection at the tissue interface, the larger the signal amplitude on the A-mode screen
In both A-mode and static B-mode the data (echoes) are acquired one frame at a time. • Depth and time are interchangeable because they are directly proportional.
X axis (horizontal) of an A-mode display represents reflector depth which is derived from time of flight
Y axis (vertical) represents amplitude • Used only in ophthalmology where accurate measurements of depth are critical
M-mode is a presentation of changing reflector position (motion) versus time.
M-mode AKA TM -MODE • Used in echocardiography, also called TM-mode (time-motion mode) and PM mode (position-motion mode.
• The X-axis (horizontal) of an M-mode display represents time,
the y-axis (vertical represents reflector depth which is derived from the time of flight information of the sound pulse.
B-mode (Brightness mode) was the first form of grayscale imaging
B-mode using manual scanners, single element transducer was attached to a mechanical arm. Each amplitude is mapped to a grayscale level or brightness.
B-mode (Brightness mode) 1 • Weaker signals are mapped to a gray, very weak signals are mapped to dark grays and the absence of signal is mapped to almost black.
• Amplitude of the returning echo is represented by the brightness of a dot. (brightness mode), ›
(brightness mode), The position of the dot represents the depth (time) of the interface from the transducer. For static B-mode you acquire the image by moving the transducer as it acquires one line of sight at a time
B-mode reflections • Weaker reflections appear as darker gray dots whereas stronger reflections appear as brighter white dots.
B-Mode disadvantages • Has poor temporal resolution • One major problem with B-mode scanners was the registration arm could easily become out of alignment and the displayed images were not accurate
B-mode technology is used to create two-dimensional imaged, a process referred to as B-scanning, static scanning, or compound B-mode.
C-mode “Gate Mode” (Constant depth mode) the US system electronics employs the distance equation to turn the receiver electronics on and off (gating) as to listen to echoes returning only from a specific depth
Pulse Wave (PW) is the most closely related to C-Mode • This gives information only form the area of interest the gate
Neither c-mode nor ECG • Neither c-mode nor ECG (ElectroCardioGraph) mode scanning are used much anymore.
Transmission mode scanning is the only ultrasound method that detects the transmitted beam through the patient. Good for breast and scrotal scanning
Transmission mode scanning The transmitting and receiving transducers are separated by an angle of 180 degrees and move in concert, This is comparable to CT scanning
Reflux transmission imaging was developed for use in, Lithotripsy: like a shock wave it breaks up kidney stones.
Registration accuracy is the ability of the system to place reflections in proper positions while imaging from different orientations. The accuracy of reflector depth positioning is called depth calibration.
• B-Mode has an advantage of a Large Field of View
Water Path scanners or liquid scanners are The other First generation scanners
liquid path transducer Liquid path transducer the crystals are put in a liquid, usually water and alcohol
Two kinds: Liquid path transducers non-reflecting and reflecting path
Crystals (liquid path) Crystals can be single or multiple in either sector or linear format. Creates lower pulse repetition frequency.
The advantage of liquid path transducers of these transducers is that the loss of image from the first mm of scan data is not lost like in the contact scanners
The advantage of liquid path transducers design • the design allows strongly focused large diameter crystals to improve lateral resolution
• Mechanical Transducers are Shaped like a sector, steered mechanically and fixed focus
• Linear switched transducers Rectangular in shape, electronically focused and steered.
Phased array transducers: Image is pie shaped, electronically steered and focused
• Annular phased array Sector shaped, mechanically steered and electronically focused
• Convex: Blunted sector, steered in many directions, focused electronically
• Vector: Trapezoidal shaped, electronically steered and focused
• A major problem with electronic array transducers is the formation of secondary lobes of sound waves. This produces artifacts.
two main types of secondary lobes Side Lobes and Grating Lobes.
Side Lobes and Grating Lobes. • They are the result of width and length mode vibrations, vibrations at crystal tissue interfaces, and interference phenomena. They are usually low intensity compared to the main beam
Side Lobes intensity of side lobes is less than that of main lobe in the center • Produce artifact and clutter that effects sensitivity, Side lobes are possible with any type of transducer system, both single element and array.
TO GET RID OF LOBES More Elements (larger aperture) reduces intensity of the lobes, Higher frequency transducers reduce the number and intensity of side lobes,
• Apodization is a method for reducing side lobes (lateral array elements) in some arrays. It gradually decreases the vibration of the transducer surface with distance from its center by improving the directivity.
dynamic aperture can be used to make a sound beam narrow over a greater range of depths and optimize lateral resolution
• Grating Lobes 1 are secondary lobes of beam energy that occur because the transducer is not a continuous surface but diced into small elements.
Grating lobes 2 • Side lobes are possible with any type of transducer system, both single element and array. Grating lobes, on the other hand, are found only with array transducers
Grating lobes 3 • Can add ghost images of the object being scanned. Also diminishes image quality by degrading lateral resolution. CONT>>
• TO GET RID OF Grating LOBES Grating lobes can be reduced in strength and importance by designing the array so that there is no greater than ½ wavelength between the centers of individual elements
• Dynamic receive focusing 1 is introducing varying electrical delays in the signals returning from the transducer to the US system during return echo reception.
Dynamic receive focusing 2 the electric outputs of the elements can be timed so that the array listens in a particular direction with a listening focus at a particular depth.
Dynamic receive focusing 3 • Each independent element, delay, and amplifier path constitutes a channel. An increased number of channels allows improved phase correction in dynamic focusing Greatly improves detail resolution over large depth ranges in images.
Dynamic receive focusing 4 is applied at all depths, but does not slow down the frame rate, for array transducers, the transmit focal zone is selected by the operator. Receive focusing is applied, but the operator does not control this.
• SUBDICING: is a technique used to overcome grating lobes. With subdicing each major transducer element is divided into smaller parts, each one being a half wave length.
multiple element instead on single-element. An array consists of a group of closely spaced piezoelectric elements each with its own electrical connection to the US instrument. This enables the elements to be excited individually or in groups to produce a beam.
multiple element instead on single-element. The number of crystals in the array determines the number of lines of sight for each image. If the array contains 130 individual crystals that are fired (excited) one at a time in sequence then 130 lines of sight are acquired.
line density 1 The line density describes the number of acoustic scan lines per sector in a 2D black and white or color sector image.
line density 2 as the time it takes to build each line in the image for any given depth that is desired, the number of beams in an image limits the frame rate.
line density 3 And if a greater sector width is desired without reducing the frame rate, the line density is reduced (same number of lines over a wider angle).
line density itself is limited by other factors as well: Frame rate (including the depth). Sector width. Increasing sector width has to be compensated either by lower frame rate or by lower line density (keeping the same number of lines in a wider sector) as shovn below.
line density itself is limited by other factors as well: And in addition, as the sector width is increasing by depth, the line density falls correspondingly.
Analog Scan Converter 1 Temporary image storage is an obvious role of the scan converter •›Most US scanners write image data to a memory device called a “Scan Converter”.
Analog Scan Converter 2 ›The scan converter plays 2 important roles during imaging: 1.It stores images during scan build-up for viewing and recording 2.It performs scan conversion enabling image data to be viewed on video monitors
• Limitations of analog: Image fade- stored charges on the silicon wafer dissipate over time Image flicker- switching between read and writes modes Instability- too many factors depend on image quality Deterioration-image degrades as device ages
analog scan Image fade- stored charges on the silicon wafer dissipate over time Image flicker- switching between read and writes modes Instability- too many factors depend on image quality Deterioration-image degrades as device ages
image fade Image fade- stored charges on the silicon wafer dissipate over time Image flicker- switching between read and writes modes Instability- too many factors depend on image quality Deterioration-image degrades as device ages
Digital Scan Converter • Current ultrasound instruments use digital technology for storage and manipulation of data, Digital devices are extremely stable • Digital scan converters use computer technology to convert images into numbers a process called digitizing.
• Advantages of digital scan converters: 1.Uniformity- consistent gray scale throughout image 2.Stability- does not fade or drift 3. Durability- not affected by age or heavy use 4.Speed- nearly instant processing 5.Accuracy- error free 6.Important elements of digital: Pixel and bits
• Apodization applies a variable-strength voltage pulse to the crystals across the aperture during delay line focusing, During reception of the returning echoes, the machine will further reduce the intensity of the echoes coming from the outer edges of the scan line.
Created by: 100001592513232