ultrasound physics
TRANSCRIPT
Ultrasound Physics
Characteristics of Sound Requires medium for its transmission. Compression and rarefaction. Propagation.
Ultrasound waves Range Longitudinal waves Length of wave Wave once generated continue in original direction until it is either reflected, refracted or absorbed
Velocity of Sound Independent of frequency medium
How velocity is related to: Compressibility Density Intensity
Transducers Converts an electrical signals into ultrasonic energy that can be transmitted to tissues & vice versa Constituents :1. Piezoelectrical crystals 2. Two electrodes 3. Backing block 4. Acoustic insulator(rubber ) 5. Plastic housing
Two basic modes of transducers 1. Continuous mode doppler studies 2. Pulse mode
Curie temperature Resonant frequency Transducer Q factor Two characteristicspurity of sound & length of sound High Q transducer Low Q transducer
Advantage of High frequency over low frequency Depth resolution is superior & near zone is long Dis-advantage Tissue absorption increases with increased frequency
Interaction between ultrasound & matter1. Reflection 2. Refraction 3. Absorption
Reflection Reflected wave sound produces image It depends on 1. Acoustic impedance 2 Angle of incidence
Acoustic impedance Impedance is product of tissue density with velocity of sound in the material.
Angle of incidenceHigher the amount of angle less is the amount of sound reflected Specular interface Examples diaphragm,walls of the vessels boundaries of many organs
Refraction Bending of waves as they pass from one medium to another is called refraction Interfaces that are either smaller than the wavelength or not smooth are nonspecular interface Example Rbc,liver parenchyma
Absorption It means conversion of ultrasonic to thermal energy It depend on 1. Frequency of sound 2. Viscosity of medium 3. Relaxation time
Ultrasonic display A Mode displayed as spikes projecting from baseline. Spike height is proportional to echo intensity. It is static onedimensional mode. TM Mode echoes produced by moving structures as dots. B Mode produces a picture of a slice of tissue. Echoes displayed as dots. Gray scale Imaging Its a B mode type displaying large amplitude of echoes arising from tissues as varying shades of gray
Real time imaging produces multiple images in a very short period Two types 1. Mechanical Oscillating & Rotating wheel 2. Electronic array Linear array & phased or steered array
PRINCIPLES OF DOPPLER ULTRASOUND
BASIC PRINCIPLE Doppler ultrasound is a technique for making non-invasive velocity measurements of blood flow. Christian Doppler was the first to describe the frequency shift that occurs when sound or light is emitted from a moving source. Ultrasound is transmitted into a vessel and the sound that is reflected from the blood is detected. Because the blood is moving, the sound undergoes a frequency (Doppler) shift.
Uses of doppler Determination of anatomy. Determination of small vessel flow distribution. Detection of vascular dilatation, obstruction. Evaluation of intravascular disease and evaluation. Detection of extra vascular flow.
1) Continuous wave doppler Without B MODE imaging 2) Pulsed wave doppler Pulsed Doppler ultrasound is a technique for measuring the velocity of blood in a small sample volume Used in general & obstetric ultrasound & also used to provide data for doppler sonograms & colour flow images.
Disadvantage
Aliasing
3) Color Doppler
DOPPLER INDICES