Physical Principles of Ultrasound

Transcription

1 Physical Principles of Ultrasound Grateful appreciation to Richard A. Lopchinsky, MD, FACS and Nancy H. Van Name, RDMS, RTR, and MarleneKattaron, RDMS 2000 UIC All Rights Reserved.

2 Course Objectives Identify history & define ultrasound Define piezoelectric effect Define frequency & wavelength; identify their relationship Define bandwidth Define attenuation; identify relationship to frequency Define resolution & its components; identify relationship to frequency Identify basic transducer types Define electronic array Differentiate between sector & linear array Identify types of image display Identify artifacts useful to diagnosis Discuss safety of medical ultrasound

3 History of Ultrasound Piezoelectricity discovered by the Curies in 1880 using natural quartz SONAR was first used in 1940 s war-time Diagnostic Medical applications in use since late 1950 s

4 Ultrasound: Physical Definition Sound waves greater than 20,000 Hertz or cycles per second Infrasound Ultrasound <20 Hz >20,000 Hz

5 Ultrasound: Medical Definition Diagnostic Medical Ultrasound is the use of high frequency sound to aid in the diagnosis and treatment of patients. Frequency ranges used in medical ultrasound imaging are 2-15 MHz

6 Piezoelectric Effect Definition: The principle of converting energy by applying pressure to a crystal. The reverse of the piezoelectric effect converts the energy back to its original form.

7 Piezoelectric Effect and Ultrasound Transducers A transducer converts one type of energy into another. Based upon the pulse-echo principle occurring with ultrasound piezoelectric crystals, ultrasound transducers convert: Electricity into sound = pulse Sound into electricity = echo

8 Pulse Pulse of sound is sent to soft tissues Sound interaction with soft tissue = bioeffects Pulsing is determined by the transducer or probe crystal(s) and is not operator controlled

9 Echo Echo produced by soft tissues Tissue interaction with sound = acoustic propagation properties Echoes are received by the transducer crystals Echoes are interpreted and processed by the ultrasound machine

10 Frequency Number of complete cycles per unit of time Man-made transducer frequency is predetermined by design Ultrasound transducers are referred to by the operating, resonant or main frequency

11 Frequency Units One cycle per second = one Hertz (Hz) One thousand Hertz = One kilohertz (KHz) One million Hertz = One megahertz (MHz) Example: a 7.5 MHz transducer operates at 7,500,000 cycles per second

12 Wavelength Definition: The distance between consecutive cycles of sound. Transducer frequency Transducer wavelength

13 Transducer Frequencies 2.5 MHz 3.5MHz 5.0 MHz 7.5 MHz 10.0 MHz Deep abdomen, OB/Gyn General abdomen, OB/Gyn Vascular, Breast, Gyn Breast, Thyroid Breast, Thyroid, Superficial veins, Superficial masses

14 Bandwidth All ultrasound transducers contain a range of frequencies, termed bandwidth Broad bandwidth technology produces medical transducers that contain more than one operating frequency, for example: MHz for general abdominal imaging MHz for superficial imaging

15 Attenuation Definition: The reduction in power and intensity as sound travels through a medium. Transducer frequency Depth of penetration Higher frequencies attenuate, or are absorbed, faster than lower frequencies

16 Attenuation

17 Time Gain Compensation Operator controlled adjustment to compensate for the attenuation of sound as it travels into the tissue Must be adjusted manually for each tissue type examined and may be manipulated throughout an exam to optimize the image

18 RESOLUTION The ability to differentiate between structures that are closely related, both in terms of space and echo amplitude Wavelength (frequency) dependent Gray Scale Resolution Axial Resolution Lateral Resolution

19 Frequency vs. Resolution Transducer frequency Resolution and image detail Higher frequency transducers provide better image resolution better gray scale resolution improved ability to distinguish fine detail

20 Frequency and Resolution 3.5 MHz 7.5 MHz

21 Gray Scale Resolution Adequate gray scale resolution allows for the differentiation of subtle changes in the tissues Dynamic Range determines how many shades of gray are demonstrated on an image

22 Dynamic Range Decreased DR Increased DR

23 Axial & Lateral Resolution Spatial Resolution describes how physically close two objects can be and displayed separately. Axial: along the beam path Lateral: perpendicular to beam path All current equipment has an overall spatial resolution of 1.0 mm or less.

24 Frequency Summary High frequency improved resolution Low frequency poorer resolution depth of penetration loss full depth of penetration higher frequency transducers for superficial uses lower frequency transducers for general abdominopelvic uses

25 Machine Components Transducer Beam Former Receiver Memory Display

26 Transducer Types Mechanical Oscillating Rotating Electronic Linear Arrays Curved Arrays Phased Arrays

27 Electronic Arrays Groups of piezoelectric material working singly or in groups Transducer

28 Electronic Transducers Sector Array crystals are placed parallel or in concentric rings transducer face is curved produces sector or pie-shaped image Linear Array crystals are placed parallel transducer face is flat produces rectangular image

29 Display Field of View Field Of View -- the display of the echo amplitudes shape dependent on transducer type and function

30 Field of View Shapes SECTOR FOV LINEAR FOV produced by oscillating rotating curved arrays phased arrays typically used in abdominal application produced by linear arrays typically used in superficial application

31 Sector Linear

32 Display Modes B Mode B Color M Mode D Mode or Doppler spectral audio color Color/Doppler/PowerAngio -- slow flow

33 B-Mode M-Mode

34 Color Doppler Power Doppler

35 Duplex and Triplex Imaging

36 Artifacts Portions of the display which are not a true representation of the tissue imaged Medical Diagnostic Ultrasound imaging utilizes certain artifacts to characterize tissue

37 Artifacts The ability to differentiate solid vs. cystic tissue is the hallmark of ultrasound imaging Acoustic Shadowing and Acoustic Enhancement are the two artifacts that provide the most useful diagnostic information

38 Shadowing Diminished sound or loss of sound posterior to a strongly reflecting or strongly attenuating structure Strong reflectors large calcifications, bone Strong attenuators solid tissue, significantly dense or malignant masses

39 Shadowing

40 Enhancement Increased through transmission of the sound wave posterior to a weakly attenuating structure Gain curve expected a certain loss or attenuation with depth of travel Occurs posterior to simple cysts or weakly attenuating masses

41 Enhancement

42 Bioeffects Prudent use assures patient safety Effects at intensities higher than those used in diagnostic medical ultrasound include: cavitation sister chromatid exchange

43 AIUM Statement No confirmed biological effects on patients or operators caused by exposure at intensities typical of diagnostic ultrasound...current data indicate that the benefits outweigh the risks.

44 Summary Ultrasound > 20,000 Hz Piezoelectric Effect = pulse-echo principle Frequency & wavelength are inversely proportional Broad bandwidth enables multihertz probes Attenuation & frequency are inversely related Resolution determines image clarity Electronic Arrays may be sector or linear Display mode chosen determines how image is registered Shadowing & Enhancement are the artifacts most used in ultrasound diagnosis Diagnostic Medical Ultrasound is safe!