Introduction to ultrasounds imaging.pptx

Ultrasound notes

1. Introduction to ultrasound physics by Alamiga G erald Job 0706436277, O761137824 [email protected] SUBMODULE ONE

2. What is ultrasound imaging? Ultrasound imaging or ultrasonography is a medical imaging technique that uses high frequency sound waves to obtain cross sectional images of the body. Also known as ‘pulse echo’ technique.

3. SOUND Sound is a mechanical energy that travels through matter, as a result of vibration of the particles of the medium through which the sound wave is moving.  Vibration of particles  Transport the energy through medium  pressure wave  Change in pressure  sinusoidal waveform

4. SOUND WAVE Sound creates images by sending short bursts into the body. Thus we are concerned with the interaction of sound & media. A type of wave that carries energy, not matter, from place to place. Created by the vibration of a moving object. Rhythmical variations in pressure or density. Comprised of compressions (increases in pressure or density) and rarefactions (decreases in pressure or density). Sound must travel through a medium, cannot travel through a vacuum. Sound is a mechanical, longitudinal wave.

5. Waves Transverse waves Longitudinal waves waves parallel to the direction of energy transfer  Waves perpendicular to the direction of energy transfer 

6. Transverse Wave Particles move in a perpendicular direction (right angles or 90) to the direction of the wave:

7. Longitudinal Wave Particles move in the same direction as the wave:

8. Sound waves longitudinal waves   Compression Space where density and pressure is elevated (positive)  Rarefaction Space where density and pressure is depressed (negative)

9. TYPES OF SOUND Ultrasound A wave with a frequency exceeding 20,000Hz (20 kHz). This frequency is so high that it is not audible. Audible Sound Heard by man, frequencies between 20Hz and 20,000Hz. Infrasound Sound with frequencies less than 20Hz. This frequency is so low that it is not audible.

10. Audible sound 15 – 20 000 Hz < 15 infrasound > 20 000 ultrasound Medical USG 1-20 MHz

11. illustration

12. Parameters used to describe sound wave

13. PERIOD Definition The time required to complete a single cycle. Units – seconds ;any unit of time Typical Values 0.06 to 0.5µs Determined By Sound source Changed by Sonographer? No

14. F requency The number of complete cycles that occur in a particular time duration. Unit of acoustic frequency(number of cycles in a unit of time) - Hertz (Hz)  1 Hz = 1 cycle per second  High frequencies expressed in kHz and MHz  1 kHz = 1000Hz  1 MHz = 1,000kHz Determined by Sound source Changed by Sonographer? No Typical Values for diagnostic ultrasound From 2MHz to 15MHz Note Frequency affects penetration and axial resolution (image quality.)

15. WAVELENGTH Definition The length or distance of a single cycle. Units mm - any unit of length Determined By Both the source and the medium Changed by Sonographer? No Wavelength influences axial resolution (image quality). Typical Values 0.1–0.8mm (in soft tissue)

16. Frequency vs wavelength relationships

17. Wavelengths in Soft Tissue In soft tissue, sound with a frequency of 1MHz has a wavelength of 1.54mm. In soft tissue, sound with a frequency of 2MHz has a wavelength of 0.77mm. Rule In soft tissue, divide 1.54mm by frequency in MHz wavelength (mm) = 1.54mm / frequency (MHz)

18. Intensity (also power and amplitude) Definition The concentration of energy in a sound beam. Units watts/square cm or watts/cm2 Determined By Sound source (initially) Changed by Sonographer Yes Intensity decreases as sound propagates through the body. Amplitude is determined by the length of oscillation of sound wave Greater amplitude

19. Speed of sound Determined by properties of medium i.e . Its resistance to compression  C α -1 COMPRESSIBILITY FASTER IN SOLID SLOWER IN GAS Propagation velocity of sound (c) = f x λ

20. VELOCITY BONE AND METAL FASTER AVERAGE SOFT TISSUE 1540m/s LUNG AND AIR SLOWER

21. REFRACTION REFLECTION ECHO ABSORPTION Interaction of ultrasound with matter

22. REFLECTION Reflection is a phenomenon which occurs when sound wave is bounced off from a surface that it encounters on its path. angle of incidence α -1 amount of reflected sound 

23. Cont …….. Reflection occurs at an interface between two adjacent tissues  having different impedences . Z = ρ c

24. Z1 Z2 The difference in acoustic impedance (z) between the two tissues causes reflection of the sound wave 

25. soft tissue - air interface reflects almost the entire beam, transducer must be directly coupled to the patient skin without an air gap  bone - tissue interface Bone transmits about 70% of sound energy due to its impedance differenc e - hence not possible to image through it  air/gas interface causes total reflection - hence they cast a shadow and structures underneath cannot be imaged E.g Bowel wall can be imaged and not its lumen

26. Specular reflectors Diffuse reflectors

27. Interference If two sound waves of the same wavelength cross each other, the pressure waves combine.  C onstructive interference  waves in step/phase, their amplitudes add up - D estructive interference  waves out of phase, they tend to cancel out each other

28. Refraction When sound passes from a tissue with one acoustic propagation velocity to a tissue with a higher or lower sound velocity, there is a change in the direction of the sound wave . Causes artefacts.

29. Absorption As the ultrasound pulse moves through matter, it continuously loses energy - attenuation  There is absorption of the ultrasound energy by the material and gets converted into HEAT.    Scattering and refraction interactions also remove some of the energy from the pulse and contribute to its overall attenuation , but absorption is the most significant. Reduction of intensity of beam as it traverses through matter.

30. Attenuation The amplitude of a the sound waves decreases with increasing depth of penetration in the body. This is due loss of sound energy since it is absorbed by media to produce heat. The loss of energy and thus attenuation is directly related to the frequency of the ultrasound beam. Thus, the greater frequency, more attenuation, and lesser is the penetration of the ultrasound wave. The amplitude/strength of the wave decreases with increasing depth  Unit >Decibel  

31. Cont.,…. Attenuation of sound energy influences the depth in tissue, from which useful information can be obtained  Affects transducer selection and operator controlled instrument settings  The deeper the wave travels in the body, the weaker it becomes 

32. TISSUE absorption + reflection + scattering   Attenuation

33. Cont ……. Attenuation determines the efficiency with which ultrasound penetrates a specific tissue.   Attenuation coefficient is a measure of caused by each tissue as function of the ultrasound wave frequency.

34. scattering This the redirection of sound waves in different directions caused due to interaction with a rough surface or a small reflector. When the reflecting surface is irregular in surface, and its dimensions are smaller than the diameter of the ultrasound beam, the incident ultrasound beam is scattered.

35. Piezoelectricity Discovered by Pierre and Jacques Curie in 1880   Greek word “ piezein ” - to press Piezoelectric effect – application of an electric field to certain materials causes a change in their physical dimensions Piezoelectric material - innumerable dipoles arranged in a geometric pattern Each dipole – positive charge at one end and negative charge at other

36. Compression force and associated voltage – piezoelectric effect In response to a compressive force on a piezoelectric slab, a net electric dipole moment  detection of a voltage in response to this strain  Reverse piezoelectricity -  In response to the application of an electric field, a deformation of the piezoelectric slab detected.

37. Explain the principles of the generation and detection of ultrasound waves. An ultrasound transducer is made up of a piezoelectric crystal and electrodes which produce an alternating p.d . The crystal Is heavily damped, usually with epoxy resin, to stop the crystal from vibrating too much. This produces short pulses and increases the resolution of the ultrasound device An alternating p.d . is applied across a piezoelectric crystal, causing it to change shape The alternating p.d . causes the crystal to vibrate and produce pulses of ultrasound waves The crystal vibrates at the frequency of the alternating p.d ., so, the crystal must be cut to a specific size in order to produce resonance Detection: The ultrasound pulse is reflected at the boundary of the tissue and returns to the transducer When the ultrasound wave returns, the crystal vibrates which produces an alternating p.d . across the crystal. This received signal can then be processed and used for medical diagnosis

38. Piezoelectric material Quartz – original transducer material Ferroelectrics ( artificial piezoelectric material ) – barium titanate , lead zirconate titanate To produce polarization the ceramic is heated at high temperature in a strong electric field   At high temperature, the dipoles are free to move, and the electric field brings them into the desired alignment The crystal is then gradually cooled while subjected to a constant high voltage  

39. Curie temperature It is the temperature above which this polarization is lost PZT - 365˚ C Quartz - 573˚ C

40. Resonant frequency Frequency at which the wavelength = 2 x thickness of piezoelectric disc  Constructive interference / resonance of waves emitted from forwards and back face of disc occurs  Transducer most efficient as transmitter and receiver of sound  Depends on thickness of disc and material of disc (determines velocity ) 

41. Cont The thicker the piezoelectric element, the lower the resonant frequency Conversely thinner the element higher the operating frequency   Range of frequencies produced by a transducer – bandwidth Broad band technology produces medical transducers that contain more than one operating frequencies

42. 2.5 MHz deep abdomen, obstetric and gynaecological imaging 3.5MHz general abdomen, obstetric and gynaecological imaging 5.0MHz vascular, breast, pelvic imaging 7.5MHz breast, thyroid 10.0MHz breast, thyroid, superficial veins, superficial masses, musculoskeletal imaging. 15.0MHz superficial structures, musculoskeletal imaging. Frequency settings .

43. Pulse duration/length Transducer continues to vibrate for a short time after it is stimulated  ultrasound pulse will be several cycles long Damping block is used to stop the crystal vibration so that transducer is ready to receive the reflected waves from tissue interfaces within the body    Damping materials used – tungsten and rubber powder suspended in epoxy resin(backing block) 

44. Q factor Mechanical coefficient ( Q ) – mean frequency : bandwidth    Greater Q – narrower bandwidth, lesser damping, longer ring down time/ pulse duration  Lesser Q – broader bandwidth, heavier damping, smaller ring down time/ pulse duration