Echo Display Modes

A ‐Mode• 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

• 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.

M‐Mode• M‐mode (M for motion) is a technique

that uses B‐mode information to displaythe echoes from a moving organ, such asthe myocardium and valve leaflets, froma fixed transducer position and beamdirection on the patient.

• The echo data from a single ultrasoundbeam passing through moving anatomyare acquired and displayed as a functionof time, represented by reflector depthon the vertical axis (beam pathdirection) and time on the horizontalaxis.

• M‐mode can provide excellent temporalresolution of motion patterns, allowingthe evaluation of the function of heartvalves and other cardiac anatomy.

• Only anatomy along a single linethrough the patient is represented bythe M‐mode technique.

B‐Mode                &                     M‐Mode

Digital Scan Convertors• 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

• 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• 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 

• 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 incidentand received frequency and extracts thedoppler shift frequency.

• Doppler signal contains very low frequencysignals from vessel walls and other movingspecular reflectors that a "wall filter"selectively removes.

• An audio amplifier amplifies the Dopplersignal to an audible sound level, and arecorder tracks spectrum changes as afunction of time for analysis of transientpulsatile flow.

• Quadrature Detection:‐Detects thedirection of the flow by comparing thereal and imaginary part of the signalreceived.

Pulsed Doppler Operation

• Echo signals sampled – five times.Pulse –transmitted ,Echo ‐ Received

• Sample and hold circuit detects the phasechanges.

• A wall filter removes the low‐frequencydegradations caused by transducer and patientmotion.

Pulsed Doppler cont’d• Aliasing occurs when the frequencies in the

sampled signal are greater than one‐half thePRF.

• In this example, a signal of twice the frequencyis analysed as if it were the lower frequency,and thus "masquerades“ as the lowerfrequency.

• The maximum Doppler shift that isunambiguously determined in the pulsedDoppler acquisition follows directly from theDoppler equation by substituting Vmax for V:

• Rearranging:

• Doppler shift frequencies exceeding one‐halfthe PRF, aliasing will occur, causing apotentially significant error in the velocityestimation of the blood

Pulsed Doppler Operation

• The spatial pulse length is longer (a minimum of 5 cycles per pulse up to 25 cycles per pulse)

Duplex Scanning

• Duplex scanning refers to the combination of2D B‐mode imaging and pulsed Doppler dataacquisition.

• A visual guidance to vessel of interest.

• Electronic array transducers switch between agroup of transducers used to create a B‐modeimage & and one or more transducers used forthe Doppler information.

• Velocities mapped with a colour scale visuallyseparate the flow information from the gray‐scale image, and a real‐time colour flowDoppler ultrasound image indicates thedirection of flow through colour coding.

Doppler Spectral Interpretation

Colour Flow Imaging

• 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