Chapter 12 Sound in Medicine

Transcription

1 Infrasound < 0 Hz Earthquake, atmospheric pressure changes, blower in ventilator Not audible Headaches and physiological disturbances Sound 0 ~ 0,000 Hz Audible Ultrasound > 0 khz Not audible Medical imaging, blood flow measurements, etc 1. General Properties of Sound Sound: Fig. 1.1 Mechanical disturbance or vibration in a gas, liquid, or solid Travels from a source with some definite velocity Vibration local increase (compression) or decrease (rarefaction) of pressure relative to atmospheric pressure Longitudinal wave: pressure changes in the same direction as wave Compression and rarefaction: density changes by displacements of atoms and molecules v= λf (f = 1000 Hz, v = 344 m/s in air at 0 C, λ = m) Energy carried by the wave by K.E. and P. E. Intensity, I : energy passing through 1 m /s or W/m 1 1 P0 = = = Z I ρva ( π f ) Z( Aω) ρ = density of the medium v = sound velocity f = frequency KHU, EI 468

2 ω = angular frequency A = maximal displacement of atoms or molecules from the equilibrium position Z = ρv = acoustic impedance P 0 = maximal change in pressure Table 1.1 and Example 1.1 Units of intensity ratio I log10 I1, bel I P, db 10log10 = 0log10 I1 P1 Hi-fi system with ±3 db (6 db variation) bandwidth of 30 ~ 15,000 Hz pressure variation of (0log 10 = 6) Sound intensity Reference sound intensity and pressure I 0 = W/cm, P 0 = 10-4 dyne/cm Barely audible at 1000 Hz by a person with good hearing Table 1. Maximally intense sound without pain = 10 db Reflection of perpendicular wave (Fig. 1.) R Z Z1 = A Z + Z 0 1 Z Z 1 Z Z 1 = no reflection < phase change in reflected wave Transmission of perpendicular wave (Fig. 1.) T Z = A Z + Z 0 1 Example 1.: sound transmission between air and muscle Impedance matching: maximal sound transmission, Example 1.3 Reflection and refraction (bending): Fig. 1.3 θ = θ : reflection i sinθ v i r sinθ = : refraction v 1 Sound refraction acoustic lens to focus sound - - KHU, EI 468

3 Absorption of sound energy Amplitude at depth of x = Ax ( ) = Ae αx 0 α = absorption coefficient in cm -1 at a particular frequency Table 1.3 and Fig. 1.4 Intensity, I A ( ) = I x Ie αx 0 HVT (half-value thickness): tissue thickness where I = I 0 / Table 1.3 High absorption in the skull Absorption increases as sound frequency increases there is maximal frequency that can be used in the human body (Example 1.4) Divergence (spreading out) of sound Decreases intensity If point source, intensity decreases by the inverse square law (I 1/x ). The Body as a Drum (Percussion in Medicine) Percussion (tapping): Fig. 1.5 L. Auenbrugger, On Percussion of the Chest, The Stethoscope Hearing aid for sounds from the heart and lungs (Fig. 1.7) Ausculation: act of listening sound using a stethoscope Modern stethoscope: bell, tube, ear pieces (Fig. 1.6) Open bell: small air volume is desirable Skin: diaphragm Impedance matcher Tight skin higher frequency Larger bell lower frequency Closed bell with diaphragm: better for lung sound, small air volume is desirable Tube: 0 cm long, 0.3 cm diameter For small volume, short and small diameter KHU, EI 468

4 For low friction, large diameter Ear pieces: must fit snugly in the ear to minimize air leakage 4. Ultrasound Pictures of the Body Bats and porpoises emit ultrasound (30 ~ 100 khz) and listen to the echos to navigate (Fig. 1.9): delay time of echo distance to the object SONAR (SOund NAvigation and Ranging) during World War II Transducer Piezoelectric: convert electrical energy to sound energy (Fig. 1.8) Pulse of ultrasound with 1 ~ 10 MHz Power: a few mw/cm Coupling with the skin by ultrasound gel or paste for maximal transmission Detector The same transducer is used Echo vibration of crystal voltage across the crystal A scan (Fig. 1.10) Pulse transmission: a few µs long pulse with 400 ~ 1000 pulse/s Two medium with different acoustic impedance reflection Delay time of echo and sound velocity distance Multiple echo: Fig Compensation of sound absorption: Fig. 1.1 Resolution is limited by wavelength, λ In water, 1. MHz 1. mm, 3.5 MHz 0.43 mm Higher frequency smaller wavelength better resolution Higher frequency larger absorption shorter imaging depth Echoencephalography (Fig and 14) Applications in ophthalmology (Fig and 16) B scan (Fig. 1.17) Two-dimensional view by moving the transducer Storage oscilloscope is used (Fig. 1.18) Provides internal structure of the body Diagnosis of the eyes, liver, breast, heart, and fetus (Fig and 0) Leading-edge display (Fig. 1.1): equal brightness for all echos Gray-scale display (Fig. 1.1): gray scale mapping of different echo KHU, EI 468

5 intensities 5. Ultrasound to Measure Motion M (motion) scan Motion of the heart and heart valves A scan (fixed transducer) + B scan (display as a function of time): Fig. 1. Positioning of the transducer: Fig. 1.3 M scan of the mitral valve: Fig. 1.4 M scan of pericardial effusion: Fig. 1.5 Doppler technique Blood flow Motion of fetal heart, umblical cord, and placenta Doppler effect: Fig. 1.6 fv 0 Doppler frequency shift for moving blood, fd = cosθ in Fig. 1.7 v Fetal heart sound in Fig. 1.8 (fetal monitor) Sounds from the pregnant uterus in Fig Physiological Effects of Ultrasound in Therapy Low intensity ultrasound (0.01 W/cm average power and 0 W/cm peak power) no harmful effects are observed Continuous 1 W/cm deep heating effect Temperature rise due to the absorption of acoustic energy Ultrasound diathermy 10 cm transducer with gel Several W/cm for 3 ~ 10 min Once or twice a day ~ three times a week Transducer is moved to avoid forming hot spot Effective for bones and joints (Fig. 1.30) Continuous 10 3 W/cm tissue destroying effect Region of compression and rarefaction pressure differences stretching over elastic limits of tissue tissue tearing KHU, EI 468

6 1 ~ 10 W/cm, 1 MHz Amplitude displacement, A = about 10-6 cm Maximal pressure amplitude, P 0 = about 5 atm λ/ = 0.7 mm 35 W/cm 10 atm over a very short distance molecule cannot disperse the energy to its surrounding by vibration breakage of chemical bonds Intense ultrasound water changes into H and H O rupture DNA molecules Negative pressure during rarefaction dissolved gas bubble cavitation (forming of bubbles) Cavitation break molecule bonds between the gas and tissue Collapse of bubbles energy release break bonds Free radicals during the breaking of bonds oxidation reaction Focused 10 3 W/cm selective destroy of deep tissue Harmonic scalpel 7. The Production of Speech (Phonation) Modulation of outward flow of air speech Stream of air in voiced sound: lungs vocal folds (cords, glottis) several vocal cavities mouse and nostrils (Fig. 1.31) Unvoiced sounds are produced in the oral portion of the vocal tract without the use of vocal folds: air flow through constrictions or past edges formed by the tongue, teeth, lips, and palate Plosive sound: p, t Fricative sound: s, f, th Combination sound: ch Source-filter model of vocal tract (Fig. 1.3) Vocal folds Within the larynx or adam s apple (Fig and 34) Normal respiration: large triangular openning Speech production: drawn close together by muscles air pressure below the vocal folds rises closed folds are forced apart rapid upward air flow decrease in pressure between folds elastic forces in the tissue move folds together partial blockage of passage reduced air velocity increase the KHU, EI 468

7 air pressure below the vocal folds recycle complex vibration Fundamental frequency of vibration: mass and tension of the vocal folds Men: longer and heavier vocal folds 15 Hz Women: 50 Hz Lowest frequency by base singer: 64 Hz Highest frequency by a soprano:,048 Hz Vocal cavities: pharyngeal (throat), oral, and nasal cavities Throat and nasal cavity: well fixed to each individual and determine the sound of voice Oral cavity: can be changed by tongue, palate, and cheeks Production of speech: Fig Speaking Joe took Father s workbench out Energy: ~ J Time: s Average power: 10 ~ 0 µw Energy of continuous talking for a year < energy to boil a cup of water Energy in vowel sound >> energy in consonant sound vowels are easier to understand Homework Review questions: #, #4, #8-7 - KHU, EI 468