Review of Ultrasonic Imaging

Clinical Values of Ultrasound• Visualization of anatomical structures in real time.

Clinical Values of Ultrasound

• Detection of blood flows, including direction, velocity distribution, variance and energy.

Clinical Values of Ultrasound

• Estimation of mechanical properties such as strain, elasticity, attenuation, acoustic backscattering, …etc.

• Treatment of diseased tissue by hyperthermia.

• Treatment of stones by extracorporeal lithotripsy.

Characteristics of Ultrasonic Imaging

• Real-time.

• Reflection mode.

• Non-invasive.

• Access limited.

• Body type dependent.

Factors of Image Quality

• Spatial resolution.

• Contrast resolution.

• Temporal resolution.

• Uniformity.

• Sensitivity.

• Penetration.

Spatial Resolution

• Lateral and elevational : diffraction limited.• Axial resolution : the width of the pulse.• Given limited total bandwidth, there exists a

tradeoff between axial and lateral/elevational resolutions.

Z

Y X

3D sample volume

Lateral Resolution (X)

• Diffraction limited.

• Determined by frequency, active aperture size and depth.

• Fixed transmit and dynamic receive focusing.

• Is dynamic transmit focusing possible?

• Is a bigger aperture always better?

Elevational Resolution (Y)

• Fixed lens (geometric focus).

• Determined by frequency, aperture size and depth.

• 2D array and alternative 1D array designs.

Geometric focus

Axial Resolution (Z)

• Pulse width (absolute bandwidth).

• System and transducer bandwidth.

• Transmit power.

• Attenuation consideration.

• Coded waveform – long pulse + large bandwidth.

Contrast Resolution

• Contrast resolution is determined by both spatial resolution and speckle noise variations.

• Speckle comes from coherent interference of diffuse scatterers. In-coherent processing must be used to reduce speckle noise.

• There exists a tradeoff between contrast and spatial resolutions.

Contrast Resolution

• Contrast-to-Noise Ratio (CNR):

• On a log display

NIII

CNRII A

21

NII

CNRD

2

1log10I1

I2

A

Contrast Resolution

• Contrast resolution is primarily limited by speckle noise.

• Speckle is a multiplicative noise.

• On a logarithmic display,

.34.4 dBD

Spatial vs. Contrast

• Speckle noise is 4.34dB for true speckle, a figure of merit for detectability.

• CNR increases as speckle noise decreases, generally resulting in loss in spatial resolution.

• Both CNR and spatial resolution can be improved by reducing sample volume.

NII

CNR34.4

log102

1

Speckle Reduction Techniques

• Must be done in-coherently.

• Spatial filtering, loss in spatial resolution.

• Compounding:– Compound image of the same object with

different speckle appearance.– Better edge definition.– Sub-optimal spatial resolution.

Frequency Compounding

• In-coherently adding images acquired at different frequencies.

• Loss in axial resolution.

• Maximal reduction is N1/2.

f

Spatial Compounding

• In-coherently adding images from different angles.• Loss in lateral resolution.• Improved edge definition. • Laterally or elevationally (with a 2D array).• Maximal reduction is N1/2.

Temporal Resolution

• Temporal resolution is determined by acoustic frame rate. It is also related to spatial Nyquist criterion.

• Temporal resolution is fundamentally limited by sound velocity but can be improved by signal processing in some cases.

• There exists a tradeoff between temporal and spatial resolutions.

Increasing Frame Rate

• Smaller field of view.

• Reduced transmit line number:– Spatial Nyquist criterion.– Parallel beamformation.

Parallel Beamformation

• Simultaneously transmit multiple beams.

• Interference between beams, spatial ambiguity.

t1/r1 t2/r2

t1/r1

t2/r2

Parallel Beamformation

• Simultaneously receive multiple beams.• Correlation between beams, spatial ambiguity.• Require duplicate hardware (higher cost) or time

sharing (reduced processing time and axial resolution).

r1 r2t

t

r1 r2

Image Uniformity

• Image uniformity is usually referred to as the variations of the system’s point spread function throughout the entire image.

• Factors of image uniformity include depth of field, pulse shapes and variations due to lateral displacements.

• To achieve image uniformity, a sophisticated imaging system is required.

• Image brightness uniformity is also desired.

Sensitivity

• Sensitivity is defined in the context of the detection of weak signals.

• Sensitivity is determined by transducer design and system’s dynamic range.

• Sensitivity is particularly important in Doppler imaging and can be improved by signal processing.

Penetration

• Penetration is determined by acoustic power delivered to the body on transmit and the dynamic range of the system on receive.

• The transmit power is regulated for safety reasons. Hence, penetration must be improved without exceeding regulations.

Performance Issues in Doppler Ultrasound

Fundamental Tradeoffs

• In pulsed modes (PW and color), maximum velocity without aliasing is .4/max PRIv

• In pulsed modes, maximum depth of the Doppler gate is .2/max PRIcR

• Combining the above two equations, we have.8/maxmax cRv

• In Doppler, the lowest acceptable frequency is usually used.

Fundamental Tradeoffs

• The velocity (frequency) resolution is determined by the inverse of the smaller of transit time and observation time.

• It may be preferable to increase the sample volume (i.e., degrade spatial resolution) in order to reduce spectral broadening (i.e., increase velocity resolution).

Sample volume

Scatterer motion

Sample volume

Scatterer motion

Fundamental Tradeoffs

• Longer pulses (Doppler gates) are often used for increased SNR. Thus, axial resolution is degraded.

• Higher frame rates require larger beam spacing. Thus, lateral resolution is not optimal.

Matched Filtering

• A matched filter is a time-delayed version of the time reversed input signal.

• A matched filter on the receiver maximizes the SNR given a transmit waveform.

• By maximizing the SNR, both the frequency and the time errors can be reduced.

• Gaussian signal gives the poorest estimation from this point of view.

Doppler Ambiguity Function

• The Doppler ambiguity function is designed to evaluate the amount of ambiguity in both time and frequency given a transmit waveform. Matched filtering is typically assumed at the receiver end.

• The total potential ambiguity is the same for all signals that possess the same energy. Therefore, the goal of choosing an “optimal”waveform is to distribute the ambiguity in an optimal way based on specific imaging requirements.

Doppler Ambiguity Function

• Typical examples:– CW.– Single pulse.– PW waveform.– Color Doppler waveform.

• Matched filtering may not be implemented in practical systems.

Design Problems

• Adaptive wall filter:– Design a wall filter that adaptively change the

cut-off frequency based on characteristics of the Doppler signal.

– Particularly effective for reducing “flash” artifact.

Design Problems

• High PRF (measuring high velocity at a large depth).– Strong signals from secondary gates at

shallower depths.– The receiver needs to have very large dynamic

range.– Digital system implementation.

Design Problems

• Simultaneous B and Doppler imaging:– Duplex.– Triplex.– B and Doppler interleave.– Recovery of missing samples.– Effects on spectral and audio quality.