Nguyên tắc vật lý của sóng siêu âm Physical Principles of Ultrasound

Physical Principles of Ultrasound Grateful appreciation to Richard A Lopchinsky, MD, FACS and Nancy H Van Name, RDMS, RTR, and Marlene Kattaron, RDMS ©2000 UIC All Rights Reserved 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 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 Ultrasound: Physical Definition • Sound waves greater than 20,000 Hertz or cycles per second Infrasound 20,000 Hz 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 - 15 MHz 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 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 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 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 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 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 Sector Linear Display Modes • • • • B Mode B Color M Mode D Mode or Doppler – spectral – audio – color • Color/Doppler/PowerAngio slow flow B-Mode M-Mode Color Doppler Power Doppler Duplex and Triplex Imaging 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 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 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 Shadowing 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 Enhancement Bioeffects • Prudent use assures patient safety • Effects at intensities higher than those used in diagnostic medical ultrasound include: cavitation sister chromatid exchange 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.” 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!