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Echo Display Modes
A ‐ Mode
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A‐mode (A for amplitude) is the display of the processed information from the
receiver versus time.
One "A‐line" of data per pulse repetition period is the result.
The earliest uses of ultrasound in medicine used A‐mode information to determine the midline position of the brain for revealing possible mass effect of brain tumours. A‐mode and A‐line information is currently used in ophthalmology applications for precise distance measurements of the eye. Otherwise, A‐mode display by itself is seldom used.
B ‐ Mode
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B‐mode (B for brightness) is the electronic conversion of the A‐mode and A‐line
information into brightness‐modulated dots on a display screen.
The brightness of the dot is proportional to the echo signal amplitude.
The B‐mode display is used for M‐mode and 2D gray‐scale imaging.
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M‐mode (M for motion) is a technique
that uses B‐mode information to display
the echoes from a moving organ, such as
the myocardium and valve leaflets, from
a fixed transducer position and beam
direction on the patient.
The echo data from a single ultrasound
beam passing through moving anatomy
are acquired and displayed as a function
of time, represented by reflector depth
on the vertical axis (beam path
direction) and time on the horizontal
axis.
M‐mode can provide excellent temporal
resolution of motion patterns, allowing
the evaluation of the function of heart
valves and other cardiac anatomy.
Only anatomy along a single line
through the patient is represented by
the M‐mode technique.
M‐Mode
B‐Mode & M‐Mode
Digital Scan Convertors
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Digitizing the echo information.
Scan controller receives echo intensity, position information, ultrasound velocity information which is fed into the digital memory.
Most ultrasound instruments have a ~500 X 500 pixel matrix.
Transducer beam orientation and echo delay times determine the correct pixel addresses (matrix coordinates) in which to deposit the digital information. The final image is most often recorded with 512 X 512 X 8 bits per pixel, representing about ¼ megabyte of data. For colour display, the bit depth is often as much as 24 bits (3 bytes per primary colour).
Frame Grabbers
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Frame grabbers are used to deliver images from a machine vision camera’s output to the memory of a computer to be further processed and/or displayed. The incoming signal from the vision camera is sampled at an rate specified by a fixed frequency pulse, which can be generated in the frame grabber itself or received from the camera.
If the signal is not already digital it passes through an analogue to digital converter, and stored in the buffer until a full image has been converted/received.
Doppler Operation
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When the sound waves and blood cells are not moving in parallel directions, the equation must be modified to account for less Doppler
shift.
The doppler shift equation modified
v is the velocity of blood, c is the speed of sound in soft tissue.
• The velocity can be calculated by •
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Selected cosine values are cos 0 degrees = 1, cos 30 degrees = 0.87, cos 45 degrees = 0.707, cos 60 degrees = 0.5, and cos 90 degrees= 0. At a 60‐degree Doppler angle, the measured Doppler frequency shift is one half the actual Doppler frequency, and at 90 degrees the measured frequency shift is 0. The preferred Doppler angle ranges from 30 to 60 degrees.
The Doppler frequency shifts for moving blood occur in the audible range. Continuous doppler operation
• The Demodulator compares the incident
and received frequency and extracts the
doppler shift frequency.
• Doppler signal contains very low frequency
signals from vessel walls and other moving
specular reflectors that a "wall filter"
selectively removes.
• An audio amplifier amplifies the Doppler
signal to an audible sound level, and a
recorder tracks spectrum changes as a
function of time for analysis of transient
pulsatile flow.
• Quadrature
Detection:‐Detects
the
direction of the flow by comparing the
real and imaginary part of the signal
received.
Pulsed Doppler Operation
• Echo signals sampled – five times.
Pulse –transmitted ,Echo ‐ Received
• Sample and hold circuit detects the phase
changes.
• A wall filter removes the low‐frequency
degradations caused by transducer and patient
motion.
Pulsed Doppler cont’d
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Aliasing occurs when the frequencies in the
sampled signal are greater than one‐half the
PRF.
In this example, a signal of twice the frequency
is analysed as if it were the lower frequency,
and thus "masquerades“ as the lower
frequency.
The maximum Doppler shift that is
unambiguously determined in the pulsed
Doppler acquisition follows directly from the
Doppler equation by substituting Vmax for V:
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Rearranging:
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Doppler shift frequencies exceeding one‐half
the PRF, aliasing will occur, causing a
potentially significant error in the velocity
estimation of the blood
Pulsed Doppler Operation
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The spatial pulse length is longer (a minimum of 5 cycles per pulse up to 25 cycles per pulse)
Duplex Scanning
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Doppler Spectral Interpretation
Duplex scanning refers to the combination of
2D B‐mode imaging and pulsed Doppler data
acquisition.
A visual guidance to vessel of interest.
Electronic array transducers switch between a
group of transducers used to create a B‐mode
image & and one or more transducers used for
the Doppler information.
Velocities mapped with a colour scale visually
separate the flow information from the gray‐
scale image, and a real‐time colour flow
Doppler ultrasound image indicates the
direction of flow through colour coding.
Colour Flow Imaging
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Colour flow imaging provides a 2D visual display of moving blood in the vasculature, superimposed upon the conventional gray‐scale image.
Phase shift autocorrelation or time domain correlation techniques are used instead of doppler shift.
Comparison of the two A‐line data by auto correlation or Time domain correlation.
Measured Velocity = displacement between the echo /Time between the pulses.
Time domain correlation methods are less prone to aliasing effects
Colour Flow Imaging