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
Transducer Construction and Characteristics
Thickness resonance mode: The positive and negative charge center will mismatch and form a dipole when external force is applied. The dipole direction maybe parallel or perpendicular to the external force direction. For most ultrasound application, the transducer is a plate of piezoelectric material with two sides coated with electrodes. With this structure, the dipole direction will be parallel to the external force, called 3-3 mode or thickness mode. There are also transducers in the other way, the stress and electrical field perpendicular, called 1-3 mode, are common in low frequency application range.
Bandwidth and Q: When it says the transducer has a center frequency of 5MHz, it doesn’t mean the transducer only works at exactly 5.0MHz, and it won’t work at 5.1MHz or 4.9MHz. It always has a range, and it is called spectrum if it drawn with vertical axis as magnitude and horizontal axis as frequency. Most good transducer will have bell or Gaussian shape spectrum curve. It has best response to the input excitation at center frequency, and the response will become weak as the excitation frequency moves away from the center frequency. On the spectrum, with the maximum point marked as 0dB, two points can be found at both sides with magnitude of -3dB, -6dB, or any other number you can name. The frequency range between these two data points is called Bandwidth. It is obvious that bandwidth is always linked with a dB level, such -3dB bandwidth or -6dB bandwidth. On the voltage spectrum, -6dB is often used, and on the power spectrum, -3dB is more commonly used. Q is a simple name of “Quality factor”, is the ratio of center frequency over the bandwidth. The lower the Q, the wider the
bandwidth,
and the pulse will be short. For ultrasound imaging, the transducer need transmit a very short pulse to achieve sharp resolution, and thus a low Q is required for the whole system, or we can say, the imaging system is a wide band width system.
High Q system is for resonant, for example, a crystal watch has a very high Q.
Damping: Damping is to decrease the system Q. For ultrasound transducer, it normally means the backing layer. Heavy damping results in wide bandwidth, short pulse length, but lower sensitivity. Doppler transducer usually has lower damping, and thus a higher sensitivity can be achieved since the Doppler signal is normally weak because it is generated from blood scattering.
Matching layer: Most medical ultrasound transducer is based on piezoelectric ceramic or crystal, having a very high acoustic impedance (about 30MRyl), and human acoustic impedance is only about 1.5MRyls. Without matching layer, the vibration of the ceramics will be bounce back and forth inside itself and gradually die
out,
only a small port of energy can be released to the tissue with each time of bouncing. The final pulse enter the tissue will be long with a lower amplitude. With a proper matching layer, the pulse will enter the tissue with minimal lengthened.