Beam Measurements Intensity Significance of Intensity Intensity Complication Intensity

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Beam Measurements Intensity Significance of Intensity Intensity Complication Intensity

Verini, Bob, Features Writer has reference to this Academic Journal, PHwiki organized this Journal Beam Measurements Intensity intensity = power / beam cross sectional area beam area changes with depth as long as constant beam power, intensity increases with decreasing area Significance of Intensity safety bioeffect considerations

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Intensity Complication intensity changes across beam’s cross section water in a pipe does not all flow at same speed Intensity Changes across beam’s cross section Non-uni as long as mity makes it difficult to quantify intensity Quantifying Intensity: Peak spatial peak (SP) peak intensity across entire beam at a particular depth Peak Establish a measurement convention peak value Peak

Quantifying Intensity: Average spatial average (SA) average intensity across entire beam at a particular depth Average Establish a measurement convention average Average Pulsed Intensity Pulsed ultrasound beam on as long as small fraction of time 1/1000 typical duty factor when beam is off, intensity is zero Challenge: quantifying intensity that is changing over time Pulsed Intensity SP = 60 when beam is on SP = 0 when beam is off How do we define pulsed intensity in a single number 60 0

Pulsed Intensity Conventions Pulse average intensity (PA) beam intensity averaged only during sound generation ignore silences PA Intensity Pulsed Intensity Conventions Temporal average intensity (TA) beam intensity averaged over entire time interval sound periods in addition to silence periods averaged What is weighted average of intensities here in addition to here TA Intensity TA = PA Duty Factor Temporal Average Equation Duty Factor: fraction of time sound is on DF = Pulse Duration / Pulse Repetition Period

Who Cares Temporal peak more indicative of instantaneous effects (heating) Temporal average more indicative of effects over time (heating) Complication: Non-constant pulses intensity does not remain constant over duration of pulse X Non-constant Pulse Parameters PA = pulse average average intensity during production of sound TP = temporal peak highest intensity achieved during sound production TP PA

Combination Intensities Abbreviations Individual SA = spatial average SP = spatial peak PA = pulse average TA = temporal average TP = temporal peak Combinations SATA SAPA SATP SPTA SPPA SPTP The following abbreviations combine to as long as m 6 spatial & pulse measurements Ultrasound Phantoms Gammex.com Per as long as mance Parameters detail resolution contrast resolution penetration & dynamic range compensation (swept gain) operation range (depth or distance) accuracy

Tissue-equivalent Phantom Objects echo-free regions of various diameters thin nylon lines (.2 mm diameter) measure detail resolution distance accuracy cones or cylinders contain material of various scattering strengths compared to surrounding material Gammex.com Doppler Test Objects String test objects moving string used to calibrate flow speed stronger echoes than blood no flow profile Doppler Test Objects Flow phantoms (contain moving fluid) closer to physiological conditions flow profiles & speeds must be accurately known bubbles can present problems expensive

Ultrasound Safety & Bioeffects Sources of Knowledge experimental observations cell suspensions & cultures plants experimental animals humans epidemiological studies study of interaction mechanisms heating cavitation Cavitation Production & dynamics of bubbles in liquid medium can occur in propagating sound wave

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Plant Bioeffects irreversible effects cell death reversible effects chromosomal abnormalities reduction in mitotic index growth-rate reduction continuous vs. pulsed effects threshold as long as some effects much higher as long as pulsed ultrasound Heating Depends on intensity heating increases with intensity sound frequency heating increases with frequency heating decreases at depth beam focusing tissue perfusion Heating (cont.) Significant temperature rise >= 1oC AIUM Statement thermal criterion is potential hazard 1oC temperature rise acceptable fetus in situ temperature >= 41oC considered hazardous hazard increases with time at elevated temperature

Ultrasound Risk Summary No known risks based on in vitro experimental studies in vivo experimental studies Thermal & mechanical mechanism do not appear to operate significantly at diagnostic intensities Animal Data risks as long as certain intensity-exposure time regions physical & biological differences between animal studies & human clinical use make it difficult to apply experimentally proven risks warrants conservative approach to use of medical ultrasound Fetal Doppler Bioeffects high-output intensities stationary geometry fetus may be most sensitive to bioeffects No clinical bioeffects to fetus based upon animal studies maximum measured output values

Statements to Patients no basis that clinical ultrasound produces any harmful effects unobserved effects could be occurring

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