US Imaging Flashcards
The pulse-echo technique
Describe
What’s an A-mode trace? Where would you use it?
Short ultrasonic pulse generated by transducer, assume it travels through tissue at 1540ms-1, echo recieved after time, t = 2d/co = 13us/cm.
A-Mode traces used to be used to show amplitude as fun of time down scan line. Echoes produced via reflection and backscatter are displayed at delays proportional to their depth. Still might use it in Opthamology (need high f, good spat res).
M-mode
How doesit work?
Where would you use it?
Take scanner and translate across surface, Accquire multiple A-Mode traces and plots how the intensity of each reflection moves. M stands for motion.
Used in ECG, low frequency (high penetration) phased array transducer accquires data at same time as recording image information. Used to show how heart expands and contracts over time.
B-mode image construction
How does it work?
What different types of transducer can you use, which require steering?
Typical number of array elements & lines in image.
B = brightness. Sitching tgether multiple A-mode traces, where the brightness os proportional to strenght of echo.
Curved, linear and radial transducers can give you a different field of view, and do not need to be streared.
Trapezoidal and sector need steering and have worse image quality.
Each transducer has 80-256 array elements, with 128-256 lines in each image.
B-mode image construction: Time Gain Compensation
Why do we need it?
There is a two-way attenuation of beam e.g. 1dB/cm/Mhz. Echoes that come from deeper are attenuated more, need to up the gain to compensate for this. Normally done with sliders on kit.
GIVES SIGNAL WITH ROUGHLY UNIFORM AMPLITUDE
B-mode image construction: A or B-mode demodulation
What information is held in low and high frquencies?
Which one do you want to get rid of and how do you do it?
Lower frequencies contain the anatomical info, higher frequencies contain the ultrasonic info.
Demodulation: Remove the high frequency ultrasonic information by rectifying (magnitude) and smoothing.
GIVES GREY-SCALE IMAGE WITH ANATOMICAL INFORMATION ONLY
B-mode image construction: A or B-mode log compression
Why do you compress?
What’s the dynamic range of a modern scanner in dB?
Dynamic range of unmodulated signal is too large to display on scanner. Have to log-compress it for the eye to see it.
Modern video display has 20log10(256) = 48dB
COMPRESSION ALLOWS WEAKER ECHOES TO BE DISPLAYED.
Linear array transducer
Main components
Main components are
- Leads from machine to each element
- cylindrical lens - Si rubber for slice thickness focussing
- matching layer -
- narrow rectangular elements (192-256 elements)
array transducer
How do you adjust the focus in the
elevation direction
image plane
Why is the beam profile considered to be complex?
Can’t adjust focus in elevation direction, it is set by the Si lens.
You can have adjustable electronic focusing in the image plane by using active aperatures. Multiple elements (20-40) are fired at the same time, and swept across the array NB// need to wait for echo before you fire another group off.
Beam profile is complec because the azimuthal and slice thickness foci are at different depths (rectangular aperature).
spatial resolution - axial
What determines axial resolution, eqn?
How do you get best resolution, penetration, sensitivity?
How is bandwidth related to pulse duration, how do we minimise bandwidth?
6C1 transducer explain notation
Which has a narrower bandwidth, pulse or transudcer? WHy is this important?
Axial (up-down in images) is determined by the pulse duration. Need to be able to resolve the echoes from two closely spaced targets one above the other. Pulse duration = T. Axial res = coT/2.
Best resolution needs shortest pulses which are generated by high-frequency, highly damped pulses. NB// high damping = less sensitive. NB// Low frequencies are good for penetration.
Bandwith is inversely proportional to pulse length. Broadest bandwith = short, well damped pulse. Narrower bandwidth = worse damping. Narrowest bandwith = PW doppler toneburst .Pulsed has narrower bandwith, means you can use several different transmit frequencies. 6C1 = Centre frequency of 6MHz, pm 1MHz either side.
spatial resolution - lateral
What determines
Lateral resolution (lefty-righty in images) is determined by beam width in image plane. If two separate beams overlap, you couldn’t tell where a reflection came from. Lateral res = beam width.
Frame Rate/ Time resolution
What 3 things determine?
Time resolution is affected by the depth , the sector width, and the line density.
- Time to acquire a line, t=2D/co = 13us/cm
- Time taken for a full B-Mode image = Nt
- N depends on sector width, line density, number of foci
