The Physics of Ultrasound. The Physics of Ultrasound. Claus G. Roehrborn. Professor and Chairman. Ultrasound Physics

Transcription

1 The Physics of Ultrasound Pipe Organ Emission Dog Man Spectrum Bat 10, ,000 Porpoise ,000 Claus G. Roehrborn Professor and Chairman Cycles per second Reception 100 Dog 15-50,000 Man 20-20, K Bat , K 500K 3 M Moth ,000 Porpoise ,000 Ultrasound 3 MHz-12 MHz The Physics of Ultrasound Ultrasound waves are mechanical waves Like other mechanical waves, ultrasound waves need a medium to be transmitted The ultrasound waves used for medical imaging are in the frequency range of 1 million cycles per second or in the megahertz (MHz) range The Physics of Ultrasound The most commonly used transducers range from 3.5 MHz to 10 MHz depending on the application Ultrasound waves are created by applying alternating current to a piezoelectric crystal For CME Use Only 1

2 Piezoelectric Effect Transducer CONTRACTED THICKNESS A B C NORMAL THICKNESS EXPANDED THICKNESS Transverse Waves Longitudinal Waves (A) Looking down on surface (A) Source Particle Motion (B) Particle Motion Direction of travel Particle motion in and out of paper plane Looking across surface (B) Direction of Travel For CME Use Only 2

3 Characteristics of Sound Wave Wave length Wave length Distance The Physics of Ultrasound The physical description of ultrasound waves follows the standard nomenclature of all naturally occurring waves. Frequency = number of cycles per second 1 cycle per second = 1 Hertz 1,000,000 cycles per second = 1 MHz Amplitude is the maximal excursion in the + and - direction Period is the time it takes for one complete cycle ( peak and valley ) The Physics of Ultrasound Velocity is a constant, and thus as frequency changes, wavelength must change Velocity = frequency x wavelength This has important consequences regarding choice of transducer for medical imaging The Physics of Ultrasound The transducer (from latin transducere = to convert) in medical imaging has a dual function as a sender and receiver Electrical energy is converted into mechanical sound wave energy Sound waves are at least partially reflected For CME Use Only 3

4 The Physics of Ultrasound Reflected mechanical sound waves are received by the transducer and converted back into electrical energy Pulse duration or pulse ON time Pulsed-wave Output The electrical energy is converted into a picture on the screen Listen time or pulse OFF time TIME The scan head acts as receiver > 99% of the time Total cycle time or pulse repetition period Diagram of Ultrasound Imaging Pulse generator Clock Mechanisms of Attenuation Attenuation refers to the weakening of ultrasound waves as they travel through the body Object Transducer / receiver Monitor Image memory/ scan converter Time-gain compensation Amplifier Attenuation is due to the following interactions: reflection interference absorption (conversion to heat) scattering divergence For CME Use Only 4

5 Reflection Reflection and Refraction Sono Source Mechanisms of Attenuation Incident wave Incident Reflected Transmitted Medium 1 Θ i Θr Reflected wave Medium 1 Medium 2 Medium 2 Θ t Transmitted and refracted wave Reflection and Refraction Critical Angle Refraction Simple Refraction Critical Angle Total Reflection Edging.avi For CME Use Only 5

6 Reflection and Impedance Reflection Impedance 1 x 0.5 Impedance 2 x 1.5 Source Source intensity Incident intensity Transmitted intensity Received intensity Reflected intensity 1.5 cm soft tissue Tissue boundary Received intensity is 6.3% of source intensity Interference Scattering Sound is reinforced Scattering particles Sound is diminished Sound is cancelled Spherical scatter waves For CME Use Only 6

7 Absorption Conversion of mechanical energy to heat Directly proportional to frequency Most pronounced at interfaces with large impedance differences HIFU High-intensity focused ultrasound Ultrasound Ranging TIME=0 TIME=1/2 t Transmitted TIME=t Reflected 1 cm For CME Use Only 7

8 = Distance to Reflector (cm) round trip burst travel time (µs) 13 Velocity = 1 cm / 13 µs Axial Resolution Axial resolution refers to the ability to identify (as separate) two objects in the direction of the traveling sound wave Axial resolution is dependent on the frequency of sound waves The higher the frequency, the better the axial resolution Increase in frequency and decrease in wavelength limit penetration of tissue planes due to greater loss of energy (attenuation) A B Axial Resolution Unresolved Resolved LF - Poor HF - Good Lateral Resolution Lateral resolution refers to the ability to identify (as separate) objects which are equidistant from the transducer but spaced apart Lateral resolution is a function of the focused width of the sound wave beam The more focused the beam, the better the lateral resolution (i.e. even closely spaced objects can be differentiated) Most transducers have a focal point (producing the best lateral resolution) and a focal range (producing adequate resolution) A narrow focal range limits the ability to image large organs For CME Use Only 8

9 Lateral Resolution Clinical Importance of Focal Range A Poor Focal Range Focal Point Good B Effective Focal Range vs Frequency 3 mhz 7 mhz 10 mhz Transducer Frequency vs Range Maximum Range in cm For CME Use Only 9

10 Artifacts in Ultrasound Reverberation Edging artifact Axial distortion (refraction artifact) Propagation velocity artifact Reverberation Artifact Testicular Prosthesis Edging Artifact (Refraction) Edging.avi For CME Use Only 10

11 Refraction Artifact Axial Distortion Transducer Image Transducer Image Target Refracted beam Image of target... Correct location of target Low velocity Linear Structure Distorted linear structure Tissue Density and Impedance Differences in impedance refer to differences in the mechanical and acoustical properties of tissue The boundaries between different tissues in the body can be seen because of impedance differences. If the difference in impedance is large, significant amount of ultrasound energy will be reflected back and not through-transmitted Tissue Density and Impedance If the difference in impedance is very large, all ultrasound energy will be reflected, and no through-transmission will occur (shadowing behind the object with high or low impedance, loss of any imaging capability) In general, the relatively small difference in impedance between soft tissues allows tissue differentiation For CME Use Only 11

12 Porcine Kidney Tissue Density and Impedance Density Impedance Air & other gases Water & other clear liquids Avg of soft tissues Muscle Liver Fat tissue Bone & other calcified objects Tissue Appearance The appearance of tissue in the body is a consequence of: the tissue composition the various mechanisms of attenuation the impedance difference between the target tissue and the surrounding tissues Tissue Appearance The liver is used as a benchmark for echogenicity Hypoechoic = darker and black Hyperechoic = bright and white Isoechoic = similar to reference point of liver High water content makes tissue appear hypoechoic High fat content makes tissue appear hyperechoic For CME Use Only 12

13 Description of Ultrasound Images Description of Ultrasound Images Description of Ultrasound Images Description of Ultrasound Images For CME Use Only 13