Ultrasound Instrumentation - P.ppt fro radiology x

usg instrumentation or physics

1. ULTRASOUND INSTRUMENTATION Presented by:Prativa Khanal BSc.MIT 2 nd year NMCTH

2. INTRODUCTION TO ULTRASOUND Sound waves with frequency greater than range of human hearing (>20000 Hz) Audible sound range-20 to 20,000 Hz The sound with frequency <15Hz - infrasound Ultrasound was first used for clinical purposes in Glasgow in 1956 Obstetrician Ian Donald and engineer Tom Brown developed first prototype systems based on an instrument used to detect industrial flaws in ships

3. Medical diagnostic ultrasound is a modality that uses ultrasound energy and the acoustic properties of the body to produce an image from stationary and moving tissues Along with the use of "pulse echo" technique gray scale image is produced based on the mechanical interaction of the short pulses of ultrasound and their returning echoes

4. Christian Doppler an Austrian physicist

5. Although ultrasound is better known for its diagnostic capabilities, it was initially used for therapy rather than diagnosis In the 1940’s, ultrasound was used to perform services similar to that of radiation or chemotherapy now Ultrasonic waves emit heat that can create disruptive effects on animal tissue and destroy malignant tissue

6. The Ultrasound Machine A basic ultrasound machine has the following parts : Transducer probe - probe that sends and receives the sound waves Central processing unit (CPU) - computer that does all of the calculations and contains the electrical power supplies for itself and the transducer probe Transducer pulse controls - changes the amplitude, frequency and duration of the pulses emitted from the transducer probe Display - displays the image from the ultrasound data processed by the CPU Keyboard/cursor - inputs data and takes measurements from the display Disk storage device (hard, floppy, CD) - stores the acquired images Printer - prints the image from the displayed data

7. CONTROLS IN MACHINE To regulate the intensities of echoes from various depths the machine is incorporated with multiple controls. Various controls are: Intensity control : determines the potential differences across the transducer. With increase in intensity, there is generation of a more energetic pulses thus stronger echoes

8. Overall gain control -use to amplify all signals equally Time Gain Compensation : With time gain compensation, a depth-dependent gain is applied to the echoes.Simply, echo signals from deep structures are amplified more than signals from shallow structures Time gain compensation is controlled in most machines using a set of 6 to 10 gain knobs, each adjusting the receiver gain at a different depth

9. Coarse gain : regulates height of echoes from all depths. increase in gain- enhancement of echoes proportionately Reject control : rejects echoes below a minimum amplitude. Thus prevents the useless signals to enter the image system Near gain control : diminish the near echoes Far gain : enhance all distant echoes Enhancement control : augments a localised portion of TGC curve. i.e. enhances echoes at specified depth

10. Depth : used to adjust the size of the image so that organs and adjacent structures or regions of interest are equally well visualized Focal point (s):This allows the operator to choose the level at which the ultrasound beam is focused to increase the resolution at a specific point or points This control should be set at the most posterior aspect of the organ or structure being imaged

11. Compression :The wide range of amplitudes returning to the transducer are compressed into a range ( dynamic range) which can be displayed on screen Dynamic range is the ratio of highest and lowest amplitudes in decibels that can be displayed In clinical applications the dynamic range may be upto 120 db because the range of reflected signals may vary by a factor 1: 10 12

12. Postprocessing : can be used to change the appearance of echo signals, already stored in memory, on the image Failure to properly adjust the gain control and/or poor placement of focal point during scanning may result in suboptimal image quality and misdiagnosis

13. ULTRASOUND PARTS

14. TRANSDUCER Ultrasound is produced and detected with a transducer, composed of one or moreceramic elements with electromechanical properties The ceramic element converts electrical energy into mechanical energy to produce ultrasound and mechanical energy into electrical energy for ultrasound detection Over the past several decades,the transducer assembly has evolved considerably in design, function, and capability, from a single element resonance crystal to a broadband transducer array of hundreds of individual elements

15. TRANSDUCER DESIGN Maching layer Piezoelectric crystal Backing block Acoustic absorber Metal shield Signal cable

16. MATCHING LAYER It minimizes the acoustic impedence differences between transducer and the patient Its impedence is intermediate to that of the soft tissue and the transducer consist of layer of materials with acoustic impedance that are intermediate to soft tissue and its transducer material Its thickness is equal to one-fourth of the wavelength, which is known as quarter wave matching Matching layer is made of Perspex or plexiglass loaded with aluminium powder

17. PIEZOELECTRIC CRYSTALS A piezoelectric material (often a crystal or ceramic)-functional component of the transducer Converts electrical energy into mechanical (sound) energy by physical deformation of the crystal structure Conversely, mechanical pressure applied to its surface creates electrical energy

18. Piezoelectric materials are characterized by a well-defined molecular arrangement of electrical dipoles When mechanically compressed by an externally applied pressure,the alignment of the dipoles is disturbed from the equilibrium position to cause an imbalance of the charge distribution A potential difference (voltage) is created across the element with one surface maintaining a net positive charge and one surface a net negative charge

19. Surface electrodes measure the voltage, which is proportional to the incident mechanical pressure amplitude Conversely, application of an external voltage through conductors attached to the surface electrodes induces the mechanical expansion and contraction of the transducer element

20. There are natural and synthetic piezoelectric materials An example of a natural piezoelectric material is quartz crystal, commonly used in watches and other time pieces to provide a mechanical vibration source at 32.768 kHz for interval timing Ultrasound transducers for medical imaging applications employ a synthetic piezoelectric ceramic, most often lead-zirconate-titanate (PZT) The piezoelectric attributes are attained after a process of molecular synthesis, heating, orientation of internal dipole structures with an applied external voltage, cooling to permanently maintain the dipole orientation,and cutting into a specific shape

21. DAMPING BLOCK Located on the backside of the crystal, made up to tungsten particles suspended in epoxy resin It absorbs backward US pulse and attenuated stray US signals Transducer and damping block are separated from the causing by an insulator (rubber cork) It dampens the transducer vibration to create an ultrasound with a short Spatial Pulse Length (SPL) which is necessary for axial resolution SPL: Number of cycle in a pulse of US x wavelength of pulse

22. Dampening of the vibration (also k/a ring down) lessens the purity of the resonance frequency and introduce a broadband frequency spectrum With ring down an increase in the bandwidth of the ultrasound pulse occur by introducing higher and lower frequencies above and below the center (resonance) frequency

23. Function of damping block In B-mode operation, It must stop the vibration within a microsecond so that the transducer becomes ready to immediately receive the reflected echoes from the body

24. Q-FACTOR It describes the bandwidth of the sound arising from the transducer Q = f /bandwidth (where f is the center frequency and the bandwidth is the width of frequency distribution) For high Q transducer: Narrow bandwidth little damping Long SPL Low spatial resolution

25. For low Q transducer: Wide bandwidth High damping Short SPL High Spatial resolution

26. Effect of damping block on the frequency spectrum

27. TRANSDUCER ARRAY Majority of ultrasound systems employ transducers with many individual rectangular piezoelectric elements arranged in linear or curvilinear arrays Typically 128 to 512 individual recTangular element comprise the transducer assembly Each elements has a width typically less than ½ wavelength and a length of several millimeter

28. TYPE OF TRANSDUCERS Unfocused Focused Desirable for most imaging application because it produces pulse of small diameter Best detail within focal zone Produce a beam with 2 distinct region: Near field or Fresnel Zone and Far field or Fraunhofer Zone

29. FOCUSED TRANSDUCER A focused ultrasound transducer produces a beam that is narrower at some distance from the transducer face than its dimension at the face of the transducer In the region where the beam narrows (termed the focal zone of the transducer), the ultrasound intensity may be heightened by 100 times or more compared with the intensity outside of the focal zone Because of this increased intensity, a much larger signal will be induced in a transducer from a reflector positioned in the focal zone The distance between the location for maximum echo in the focal zone and the element responsible for focusing the ultrasound beam is termed and the focal length of the transducer An ultrasound beam also may be focused with mirrors and refracting lenses or phasing the linear array electronically

30. REAL TIME TRANSDUCER Real time imaging systems are those that have frame rates enough to allow movement to be followed in no time during scanning At least 15 frames per second is required for real time image perception There are two basic techniques for producing real-time ultrasonic images: Mechanical scanners Electronic array technique

31. MECHANICAL TRANSDUCER Seldom used now Early US systems relied on the operator to manually change the position and orientation of the transducer and scan the ultrasound beam through a plane in the patient to obtain the echo data necessary for each image It is of three type: Oscillating transducer(unclosed crystal) Oscillating transducer(closed crystal) Rotating wheel transducer

32. OSCILLATING TRANSDUCER(UNCLOSED CRYSTAL) Transducer crystal oscillates through an angle Frame rate depends upon the rate of oscillation and varies from 15 - 30 frame/s A sector shape image is produced Also k/a Wobbler sector scanner Angle of wobbler can be varied from about 15˚ to 60˚

33. OSCILLATING TRANSDUCER (CLOSED CRYSTAL) Transducer are enclosed in an oil or water filled container Castor oil is commonly used Another design attaches a permanent magnet to the back of transducer and mounts the combination between the poles of electromagnet Change in the direction of current flow through the coils of electromagnet causes the magnet and transducer to oscillate

34. ROTATING WHEEL TRANSDUCER Employs 3 or 4 transducer that are mounted 120˚ to 90˚ apart on a wheel. Wheel diameter: 2 to 5 cm Motor rotates the wheel in only one direction Ultrasound beam emerges through a acoustically transparent window Either a sector or trapezoid shaped image may be produced depending upon design

35. ELECTRONIC ARRAY TECHNIQUE Current technology Are under electronic control Use an array of small rectangular piezoelectric elements arranged either in a straight line or an annular design that do not move It is of 3 type Linear array Curved array Phased array

36. LINEAR ARRAY Typically it consist of 256 to 512 elements In operation the simultaneous firing of a small group of approx. 20 adjacent element produces the ultrasound beam The simultaneous activation produces a synthetic aperture (effective transducer width) defined by number of active element A rectangular FOV is produced with linear array of transducer arrangement

37. Often used with high frequencies i.E. 7MHZ This has the advantage of good near field resolution There disadvantage is artifacts when applied to a curved part of the body creating gaps between skin and transducer

38. CURVED ARRAY Its linear array that have been shaped into convex curves It gives sector display with relatively larger field of view It is used in Abdominal scan, Obstetric scan, Trans-abdominal pelvic scan High frequency curved array transducers are often used in trans-vaginal and trans-rectal probes and for pediatric imaging

39. Often with frequencies of 2-5 MHz (to allow for a range of patients from obese to slender)

40. SECTOR / PHASED ARRAY (freq 1 to 2MHz in adult, in pediatric sector probes upto 8 MHz) Produces a fan like image that is narrow near the transducer and increase in width with deeper penetration It is useful when scanning between the ribs as it fits in the intercostals space. The disadvantage is poor near field resolution

41. ANNULAR ARRAY A series of concentric elements nested within one another in a circular piece of piezoelectric crystal to produce an annular array Use of multiple concentric elements enables precise focusing

42. LINEAR & PHASED ARRAY Linear array-individual elements or groups are fired in sequence resulting parallel US beams Phasic array: produce a sector field of view by firing multiple transducer elements in precise sequence to generate interference of acoustic wave fronts

43. TRANSDUCER SELECTION For general purpose- convex with 3.5 MHz For obstetric purpose- convex or linear with 3.5 MHz For superficial structure- linear with 5MHz For pediatric or in thin people- 5 MHz

44. COUPLING AGENTS Commonly known as “GEL” Fluid medium needed to provide a link between the transducer and the patient Composition: Carbomer- 10.0gm Propylene glycol- 75.0gm (72.4ml) EDTA- 0.25gm Distilled water- upto 500gm or 500ml

45. DISPLAY SYSTEM Ultrasound information received by the transducer crystal are converted into an electric signal which can be displayed on ultrasound instrument in following ways: Mode display system A mode B mode M mode Scanner converter Analog design Digital design

46. A MODE It is also k/a Amplitude mode Echoes returning from the body are displayed as signal on an oscilloscope Echoes displayed as spikes from a baseline. Baseline identifies the central axis of the beam Spike height is proportional to echo intensity It represents the echo amplitude only along the single line of sight of the mid-beam of the transducer at any point of time

47. It contains information about the depth of structure and the amplitude of returning echoes

48. APPLICATIONS OF A-MODE Opthalmology-distance measurements Echoencephalography Echocardiography Detecting a cyst in breast Studying midline displacement in brain

49. B MODE It is also k/a Brightness mode A series of dots appears along the line of sight Brightness of each dot proportional to amplitude of returning echoes Its electronic conversion of A-mode and A-line information into brightness modulated dots on the display screen B mode display is used for M mode and 2D gray scale imaging

50. When an ultrasound image is displayed on a black background, signals of greatest intensity appear as white, absence of signal is shown as black (anechoic) and signals of intermediate intensity appear as shades of grey

51. B MODE REAL TIME IMAGING In real-time imaging, the image is also built up as the sound beam scans across the patient, but the scanning is performed automatically and quickly, and one image follows another in quick succession. (as many as 30-60 complete images per second) Real-time B-mode images are useful in the display of moving structures such as heart valves

52. M-MODE ULTRASOUND (MOTION MODE) Is the motion mode displaying moving structures along a single line in the ultrasound beam A single beam in an ultrasound scan can be used to produce an M-mode picture where movement of a structure such as heart valve can be depicted in a wavelike manner Because of its high sampling frequency (upto 1000 pulses per second) This is useful in assessing rates and motion and is still used extensively in cardiac and fetal cardiac imaging

53. The function of the scan converter is to create 2D images from echo information and distinct beam directions. It perform scan conversion to enable image to be viewed on video Analog design : It uses cathode ray tubes to capture data These device drifted easily and were unstable over time Electron beam strikes the target and leaves a small electric charge on each of display monitors the silicon oxide element

54. Digital design : It is extremely stable and allows subsequent image processing by the application of variety of mathematical function It stores information in a solid state semiconductor device It provides flicker free scanning

55. IMAGE STORAGE Ultrasound are typically composed of 640 x 480 or 520 x 512 pixel matrices Each pixel typically has a depth of 8 bits (1 byte) of digital data, providing upto 256 level of gray scale Image storage (without compression) is typically 0.25MB image For real time imaging amount can be hundred of Megabyte

56. DOPPLER MODE Doppler mode is based on the shift of frequency in an ultrasound wave, caused my moving reflector, eg. blood cells Austrian physicist John Doppler helped to discover the doppler mode

57. DOPPLER INSTRUMENTATION TRANSDUCER FOR CONTINUOUS WAVE DOPPLER Two elements in a single head One as a transmitter and next as receiver The two elements are inclined with each other slightly to get a region of beam overlapping which gives maximum sensitivity for detecting echoes. Use of high Q transducer to get narrow range of frequencies as the concern is with frequency shift Use of low impedance backing material (air) and quarter wave matching which help to transmit the maximum power generated up to the patient Broad beam to encompass the fetal heart even after minor fetal movements

58. Pulse doppler Same transducer as transmitter and receiver Transducer is same as array transducer Pulse repetition frequency in the doppler ultrasound must be variable

59. REFERENCES The Essential Physics of Medical Imaging Radiopedia Various internet sources

60. Thank you….