Review of Ultrasonic Imaging

Clinical Values of Ultrasound Visualization of anatomical structures in real time. Review of Ultrasonic Imaging

Clinical Values of Ultrasound Detection of blood flows, including direction, velocity distribution, variance and energy. Review of Ultrasonic Imaging

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. Review of Ultrasonic Imaging

Characteristics of Ultrasonic Imaging Real-time. Reflection mode. Non-invasive. Access limited. Body type dependent. Review of Ultrasonic Imaging

Factors of Image Quality Spatial resolution. Contrast resolution. Temporal resolution. Uniformity. Sensitivity. Penetration. Review of Ultrasonic Imaging

Review of Ultrasonic Imaging 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 Review of Ultrasonic Imaging

Lateral Resolution (X) Diffraction limited. Fourier transform of the aperture function (in focus, CW). Determined by frequency, active aperture size and depth. Fixed transmit focus and dynamic receive focus. Review of Ultrasonic Imaging

Elevational Resolution (Y) Fixed lens (geometric focus). Determined by frequency, aperture size and depth. Geometric focus Review of Ultrasonic Imaging

Review of Ultrasonic Imaging Axial Resolution (Z) Pulse width (absolute bandwidth). System and transducer bandwidth. Transmit power. Attenuation consideration. Review of Ultrasonic Imaging

Review of Ultrasonic Imaging 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. Review of Ultrasonic Imaging

Review of Ultrasonic Imaging Contrast Resolution Contrast-to-Noise Ratio (CNR): On a log display I1 I2 A Review of Ultrasonic Imaging

Review of Ultrasonic Imaging Contrast Resolution Contrast resolution is primarily limited by speckle noise. Speckle is a multiplicative noise. On a logarithmic display, Review of Ultrasonic Imaging

Review of Ultrasonic Imaging Spatial vs. Contrast Speckle noise 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. Review of Ultrasonic Imaging

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. Review of Ultrasonic Imaging

Frequency Compounding In-coherently adding images acquired at different frequencies. Loss in axial resolution. Maximal reduction is N1/2. f Review of Ultrasonic Imaging

Review of Ultrasonic Imaging 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. Review of Ultrasonic Imaging

Review of Ultrasonic Imaging 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. Review of Ultrasonic Imaging

Review of Ultrasonic Imaging Increasing Frame Rate Smaller field of view. Reduced transmit line number: Spatial Nyquist criterion. Parallel beamformation. Review of Ultrasonic Imaging

Parallel Beamformation Simultaneously transmit multiple beams. Interference between beams, spatial ambiguity. t1/r1 t2/r2 Review of Ultrasonic Imaging

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 r2 t Review of Ultrasonic Imaging

Review of Ultrasonic Imaging 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. Review of Ultrasonic Imaging

Review of Ultrasonic Imaging 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. Review of Ultrasonic Imaging

Review of Ultrasonic Imaging 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. Review of Ultrasonic Imaging