USG PHYSICS

1. USG PHYSICS

2. CHARACTERISTICS OF SOUND Properties of sound : • Requires medium for its transmission. • Form of energy. • Longitudinal wave : passes as Compressions and rarefactions. • Propagation. • Wavelength of sound wave. • Velocity of sound-Independent of frequency Depends on transmitting medium- compressibility, density.

3. Parameters of ultrasound beam: • Frequency :1Mhz to 20 Mhz. • Velocity: in soft tissue: average taken by machine as 1540 cm/sec. • Intensity: can be changed by gain settings.

5. Features OF ULTRASONIC BEAM • The intensity of US varies longitudinally along the length of the beam. • Wavefront diverges beyond some distance. • The parallel component - near or Fresnel zone. • The diverging portion of beam – far or Fraunhoffer zone.

6. The length of near / Fresnel zone/ parallel component of beam increases with : • Diameter of transducer. • Frequency of sound. • Advantage of High frequency Probe superior depth resolution, near zone is long, good near resolution Dis-advantage : Tissue absorption increases with increased frequencyhence penetration decreases. • Larger transducer : ADV : Longer near / Fresnel / parallel zone • Disadvantage : Deterioration of lateral resolution.

8. INTERACTION BETWEEN ULTRASOUND AND MATTER • Reflection: depends upon type of reflector and acoustic impedance. • Refraction and transmittance: • Absorption

9. INTERACTION BETWEEN ULTRASOUND AND MATTER : Reflection, refraction, absorption. REFLECTION • In US beam, 3 to 5% of the sound is reflected. (forms speckles at level of tissues. These speckles are identified specific for tissue.) So reflected waves produce image. It depends on 1. Acoustic impedance of tissue 2. Reflector type 3. Angle of incidence.

10. Acoustic Impedance • It is the product of tissue density with velocity of sound in the material. Z = density x velocity. • As the sound wave passes from one tissue plane to another acoustic impedance changes as their change in density. • More the difference in the impedance, more is the reflection.

11. Reflector Type Display depends upon angle of insonation. If sound beam at 90 degrees to the interface only then it will return echoes to transducer otherwise it will be reflected away. Ex ; diaphragm, vessel wall, UB wall, endometrial stripe.

12. Medium reflectors: Muscle.

13. Diffuse reflectors : • It includes solid organs such as liver which has interfaces much small as compared to wavelength of sound used for imaging. • Scatters sound in all directions and only portion returns to transducer to produce image. • Constructive and destructive interference of sound produces USG speckle, characteristic of sonograms of solid organs- liver, spleen, kidney.

14. Angle of Incidence • Angle between the sound beam and reflecting surface. • Higher the angle of incidence, less is the reflection.

15. REFRACTION • Bending of waves as they pass from one medium to another is called refraction. • Refraction artifacts cause spatial distortion and loss of resolution .

16. ABSORPTION • It means conversion of ultrasonic energy into thermal energy. • It depends on 1.Viscosity of conducting medium 2.Frequency of sound 3.Relaxation time of medium. Absorption increases with viscosity, longer relaxation time, increased frequency. Time taken by molecule to return its original position after it has been displaced. Hence selection of low or high frequency for scanning is compromise between high and low freq to optimize resolution n tissue penetration.

17. Instrumentation in USG:

18. TRANSMITTER • Transducer is energized by application of precisely timed high amplitude voltage. • It also controls the rate of pulses emitted by the transducer (PRF). • PRF in turn determines time interval between USG pulses that is important in determining depth of the data in both USG and DOPPLER. • Maximum voltage : high intensity. That can be applied is determined by federal regulations as it implies more exposure to the patient.

20. TRANSDUCERS • Ultrsonic transducers converts an electrical signals into ultrasonic energy that can be transmitted to tissues & • convert ultrasonic energy reflected back from the tissue into an electric signal. Constituents : • 1. Piezoelectrical crystals • 2. Two electrodes • 3. Backing block • 4. Acoustic insulator(rubber ) • 5. Plastic housing • 6. Cable

21. PRINCIPLE OF PIEZOELECTRICITY Piezoelectric crystals • It means “pressure electricity”. • Some materials change in physical dimension on application of electric field and vice versa. This effect is called piezoelectric effect. • First described by Pierre and Jacques Curie in 1880.

22. These materials are made up of electrical dipoles arranged in geometric pattern. • The positive and negative ends are randomly aligned before application of electric field. • When an electric field is applied will cause them to realign • thus changing the dimension of the crystal. • When the voltage is applied in a pulse, the crystal vibrates like cymbal and generates a sound wave.

24. PIEZOELECTRIC MATERIALS • CRYSTALS Quartz SiO2, Berlinite AlPO4 Gallium orthophosphate GaPO4, Tourmaline • CERAMICS Barium titanate  BaTiO3 Lead zirconate titanate PZT • Other materials Zinc oxide ZnO, Aluminum nitride AlNPolyvinylidenefluoridePVDF. • Artificial piezoelectric materials are known as “ferroelectrics”. • Lead zirconatetitanate PZT is most commonly used ferroelectrics now.

25. These crystals can formed into different shapes depending on their uses. • They can be designed to vibrate into radial mode and thickness mode. In medical ultrasound- thickness mode is used.

26. Terminologies related to transducers. • Curie temperature • Resonant frequency • Transducer Q factor • PRF

27. CURIE TEMPERATURE • Curie temperature To attain piezoelectricity ceramic crystals are heated at a high temperature in a strong electric field so that the dipoles in it arrange in a particular geometric configuration. If they are heated to the curie temp. is the temp again this polarization is lost.

28. RESONANT FREQUENCY • Max. sensitive frequency of transducer is called natural transducer frequency. • Resonant frequency is a natural frequency that produces wavelengths that are twice the thickness of the crystal. • Hence crystals are designed such that its thickness is exactly half the wavelength of US to be produced by the transducer.

29. TRANSDUCER Q FACTOR The Q factor : Interval between initiation of the wave and complete cessation of vibration. Depends upon Two characteristics 1. purity of sound 2. Length of time sound persists Types • High Q transducer • Low Q transducer

30. High Q transducer : Produces nearly pure sound Range of frequency is narrow. Ring down time is high. This type of transducer is useful for doppler ultrasound examination.

31. Low Q transducer: It produces a whole spectrum of wider range of frequency. Ring down time is low. This type is particularly useful for organ imaging because it gives short ultrasound pulses and responds to a broad range of returning frequencies.

32. BACKING BLOCKS • Backing blocks are incorporated to quench the vibrations and to shorten the sonic pulse. • Q factor of piezoelectric crystals can be altered by changing characteristics of backing block. • Transducers used in US are pulsed for 2-3 cycles with 500-3000pulses per second. • Ideal backing block material should absorb all the sound waves that reach it and should not reflect any back into crystal. For this their characteristic impedance should match with impedance of piezoelectric crystals. • Backing blocks are made up of combination of tungsten and rubber powder in an epoxy resin. • Ratio of tungsten to resin is adjusted to determine its impedance requirements. • Rubber powder increase attenuation of sound in the backing blocks.

33. Matching Layer • Matching layer placed in front of the element to overcome the large impedance mismatch between the element material and the tissues. • Optimal thickness is ¼ of the wavelength. • Multiple matching layers can be used to match multiple frequencies.

34. Receiver: Receives data from probe. Returning echoes strike probe- piezoelectric crystals vibrate- induce voltage across them. Amplifies signal from deep tissues : site of action of TGC. Sends data to processor.

35. Computer localization of Transducer alignment: Uses arm containing 3 joints, Computer calculates the angle or alignment of the baseline by the number of degrees in these three joints.

36. Processor :

37. ULTRASONIC DISPLAY • The ultrasonic image is an electronic representa-tion of data generated from returning echoes and displayed on TV monitor. • Scan lines: Consists of 525 horizontal display scan lines that produce 1 TV frame of the dynamic image. Horizontal lines. • Line density: is no. of vertical lines per field of view No. of scan lines increased by increasing no. of elements in the transducer and thus increasing the resolution • Frame rate : This picture is updated 30 times/sec to get a real time display.

38. T.V. MONITOR

39. Types of ultrasonic display • A mode • TM –mode • B- mode • Grey scale imaging

40. A Mode or Amplitude Mode • Echoes are displayed as spikes projecting from baseline. • The baseline identifies the central axis of the beam. • Spike ht. is proportional to echo intensity. • Image not stored, replaced by next image. • If to store- photograph it. • Used in Ophthalmology, Echoencephalography, Echocardiography.

41. TM Mode or Time motion Mode • Spikes are converted into dots. • This line of dots is recorded from top to bottom against time. • Motion of the structures are noted in this format over a period of time. • A strip chart is recorded like an ECG. • Mostly used in echocardiography.

42. B Mode or Brightness Mode • It produces picture of a slice of tissue. • Image is made of points. • Localization of one point echo from another is done by a computer that is fed information by an arm containing 3 joints and depth is determined by the time delay required for waves to reach receiver. • Transducer is kept in contact with skin of patient and moved in various angle and direction called compound contact scanning motion . • When transducer Is moved from one position to another the computer recalculates the angle and time delay for returning of signals. • Image is displayed on modified cathode ray tube called ‘storage’ cathode ray tube. Disadvantage: limited ability to show shades of grey. • Hence this is also called “bistable image”. (showing only dark and light areas).

43. Added scan lines Few Scan lines Previous image washed Frame rate say around 1/30 sec, 30 images are formed in 1 sec. Full image formed after display of data of all detectors B MODE REAL TIME SEQUENTIAL IMAGE

44. Grey Scale Imaging • It shows amplitude of echoes as various shades of grey. • Image is displayed with the help of scan converter tube (SCT). • Unlike direct cathode ray tube, SCT does not display image itself but converts the info. received from transducer into image that is visible on TV monitor. • Electron beam is used to write, read and erase the info. on the TV. Most imp to remember in gray scale imaging is SCT / scan convertor tube

45. T.V. MONITOR

46. Scan converter tube Types- Analog scan converter Digital scan converter • Analog scan converter • Are obsolete now. • Grey scale level tend to drift with time hence not useful for comparison. • Slower writing speed. • Image can be viewed for 10 mins ,before deterioration starts.

47. Digital scan converter • Principle: Converts variation in echo amplitude signals into binary numbers (0 and 1). • Need: computer understands only binary language. • ADV: Free of grey scale drift. • Much Faster writing speed. • Image can be stored and viewed indefinitely. • Suitable for advanced computer processing and real time imaging.

48. REAL TIME ULTRASOUND • Real time imaging system are those that have frame rates fast enough to allow movement to be followed.

49. FRAME RATE • To follow moving tissues a sufficiently large number of frames must be scanned / second . • At least 16 frames/s reqd for flicker free display. • To increase frame rate : Compromise between line density &frame rate is required . • It depends on number of lines per frame and is increased by increasing the PRF. • High frame rate Is reqd . to image fast moving str. • Frame rate (P.R.F.) is kept low when visualizing deeper str.

50. Instruments used in Real time ultrasound • Mechanical Scanners A single element or a group of single element transducers is moved mechanically to form real time image. • Electronic Scanners Uses an array of transducers. These transducers don’t move but activated electronically so as sweep the US beam.