Ultrasound Physics
Welcome
Ultrasound Basics
Vibration and Wave
Ultrasound Parameters
Medium Acoustic Property
Ultrasound Reflection
Ultrasound Refraction
Ultrasound Scattering
Ultrasound Attenuation
Ultrasound Application
Ultrasound Transducer
Piezoelectric Effect
Transducer Cosntruction
Array Transducer
Beamforming
Ultrasound Beamformation
Beam Focus
Beam Steering
Imaging
Pulse-echo Method
Imaging Method
Imaging Resolution
Ultrasound Imaging Artifacts
Signal and Circuit
Unipolar Transmitter
Bipolar Transitter
Transceiverg
Time Gain Control
Conditioning
Preprocessing and Postprocessing
Flow Dection
Doppler Effect
Continue Wave Doppler (CW)
Pulse wave Doppler(PW)
Color Flow Imaging
Safety
Intensity
Mechanical Index
Thermal Index
Cavitation
Regulations
Ultrasound Image Resolution
Axial resolution: Axial resolution is the minimal distance in depth, or ultrasound propagation direction that the imaging system can distinguish. Because ultrasound imaging using pulse-echo method, the pulse length determines the axial resolution. In ideal situation, the pulse is a Gaussian shape sinusoidal wave.
The echoes from two point targets on the beam path will be totally separated when their distance is larger enough, for example, larger than half the -40dB pulse length. The echoes will be get closer when the targets distance decrease, and will merge together when they are close enough, such as the distance is smaller than half of the -3dB pulse length. Since the pulse length is related with bandwidth, the shorter the pulse, the wider the bandwidth, and thus the wide bandwidth system is required to achieve higher axial resolution. For Gaussian shape sinusoidal pulse, it need contain minimal one cycle of sine wave, and thus the higher is the frequency, the shorter is the pulse length when bandwidth is fixed. For this reason, high frequency system will give better axial resolution.
Lateral: In ultrasound imaging, ultrasound pulse travel in depth
direction,
and perpendicular to the depth direction, the beam scan direction is called lateral direction. Lateral direction is also parallel to the transducer surface.
Point spread function: In a perfection imaging system, a point target will have a point correspond to it on the image. However, for ultrasound imaging, the ultrasound pulse has irregular 3D spatial shape, and thus, the image correspond
to
a
point target will be spread out, also called Point Spread Function (PSF). A typical ultrasound PSF looks like a flying bird.
Lateral resolution from a focused aperture: The lateral resolution is determined by the beam width, and the higher the frequency, the thinner the focused beam width. To achieve higher lateral resolution, high frequency and strong focus is required.
Slice Thickness Resolution (
Elevational
Resolution): Perpendicular to the depth and beam scan
direction,
is called elevation direction. And the resolution in this direction is called elevation resolution. For a round or square transducer, the beam is symmetry in lateral and elevation. However, if the aperture is rectangle, or other asymmetry shape, elevation resolution and lateral resolution is different. Normally, elevation or slice thickness resolution is worse than lateral.