Ultrasound Physics and Instrumentation - MRD535 PDF

Ultrasound Physics and Instrumentation MRD535 Bioeffects and Safety By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Analyse numerical and visual data related to the physics and...

Ultrasound Physics and Instrumentation MRD535 Bioeffects and Safety By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Analyse numerical and visual data related to the physics and instrumentation in ultrasonography. (C4) Contents 1. Thermal/non-thermal effects on tissue 2. Contrast agent Bioeffects Ultrasound - a mechanical form of energy which interacts with the biological tissue through which it propagates. Ultrasound has at least the potential to produce a biological effect that could constitute risk. Minimize the risk while obtaining the necessary information to achieve the diagnostic benefit - ALARA  Cell studies Bioeffects Plant & animal Epidemiology information studies Mechanisms Cells End points studies with ultrasound exposure of cells is sister- chromatid exchange (SCE). SCE possibly occurred during DNA synthesis either due to some replication error or due to inhibition of DNA replication. SCE formation is an early indicator of chromosome instability. Cells (cont) Confirmation of published positive effect led to the conclusion that the cause for statistically significant effects is unknown. Cells in suspension or in culture are different from those in the intact patient in a clinical environment. Cellular studies are useful in determining mechanisms of the interaction and guiding the design of experimental animal studies and epidemiologic studies Plants and Animal Studies Plants Primary component of plants tissue – stems, leaves, and roots. These contain gas-filled channels between cell walls. Plants are useful for studying the effects of cavitations The sound waves cause the microscopic bubbles present in the natural liquids to expand (during phases A and B of low pressure) and contract until they implode (during phases C and D of high pressure). Plants and Animal Studies Animals In vivo effects include fetal weight reduction, postpartum mortality, fetal abnormalities, tumor regression. Epidemiological Studies 806 children with half of whom had been exposed to diagnostic ultrasound in utero. The study measured Apgar scores, GA, HC, birth weight and length, congenital abnormalities, nerve measurement (hearing, visual). Results: No biologically significant differences between the exposed and unexposed children. Mechanisms Mechanisms of action by which ultrasound could produce biological effects are divided into: Thermal/ heating effect Mechanical/ non thermal effect Thermal Effect Attenuation in tissue primarily due to absorption (conversion of ultrasound to heat). Ultrasound causes tissue molecules to vibrate, result in tissue heating. The extent of the temperature rise depends on the applied intensity and frequency (absorption of ultrasound increases with increasing beam frequency – lesson 2). Thermal Effect Heating increases as intensity or frequency increases. Intensity and exposure time ↑, tissue heating will ↑. Human cells can only survive a relatively small temperature increase above normal. Absorption is higher in bone than in soft tissue. Increase more than 20 is considered significant. Thermal Effect (cont) Biological effect observed depend on: Exposure duration time Type of tissue exposed Cellular proliferation rate Potential for regeneration Adults tissue are more tolerant of temperature increases than fetal and neonatal tissue. Thermal Effect (cont) The temperature increase dependent upon: Output characteristic : frequency, exposure time, power, PRF, pulse duration, field of view, scanning mode. Tissue properties: attenuation, absorption, speed, acoustic impedance Thermal Effect (cont) Calculation of the max temperature increase due to ultrasound exposure in vivo is to estimate the tissue temperature (overestimate/ close) –Thermal Index. The ratio of the acoustic power produced by the transducer (W) to the power required to raise the temperature in tissue by 1 0C (Wdeg) TI = W/Wdeg Thermal Effect (cont) 3 subdivisions: Soft tissue thermal index : TIS Bone thermal index: TIB Adult cranial exposure (TIC) In obstetric : TIS should be used for the first 8 weeks TIB should be monitored thereafter If TIS =2, estimate of max temperature increase (∆Tmax) ≤ 20C Mechanical Effects Related to the phenomenon of cavitation. There are two types of cavitation: Stable cavitation Bubbles that oscillate in diameter with the passing pressure variation of the sound wave. Transient cavitation Bubble oscillation are so large that the bubble collapse. It has the potential for significant destructive effect Schematic representation of cavitation bubbles displaying stable and transient cavitation due to continuous compression and rarefaction of the liquid medium under the propagation of an ultrasound wave Mechanical Effects (cont) Mechanical index (MI) developed to assist user in evaluating the likelihood of cavitation-related adverse biological effect. MI = Pr/ √f Pr -peak rarefactional pressure (Mpascal); f - frequency Mechanical effect depend upon: Tissue characterization Ultrasound parameters: pressure amplitude, pulse duration, frequency Safety No known risk associated with the use of diagnostic ultrasound. Recognizing the possibility bioeffects could be occurring that are subtle, low incidence, or delayed. Safety (cont) TI and MI values are influenced by: Position of focal zone Use of multifocal zone FOV Transmit frequency Output power Mode of operation / combination. Focal Zone Concentration of Energy The focal zone is the area where the ultrasound beam is most concentrated. If the focal zone is set to a specific depth within the tissue, the energy is concentrated in that region. FOV Depth of penetration Deeper penetration means that the ultrasound waves must pass through more tissue, potentially leading to increased absorption of energy and higher TI values, especially if the focal zone is set at a considerable depth. If the FOV includes areas with gas-filled structures (such as the gastrointestinal tract), there may be an increased risk of cavitation (MI) Frequency Higher frequency Higher frequencies are absorbed more readily by tissues, leading to increased local heating. More energy is converted into heat within the tissues. Instruments Output Intensity is the most popular quantity presents to describe instrument output. Spectral Doppler output is the highest Gray-scale imaging output is the lowest M-mode and color flow fall between the two (www.quora.com) Contrast Agent Contrast agent – liquid suspensions that injected into the circulation to increase echogenicity. Contain microbubbles of gas that are stabilized by a shell. Bubbles and particles are small to pass through capillaries. Contrast agent produces strong echoes because the impedance of the suspended particles differs from the impedance of the medium. Contrast Agent (cont) Contrast agent improve sonography when lesion echogenicity is like the surrounding tissue. The enhancement pattern analysed in a similar fashion to contrast enhanced CT and MRI but in real time. Advantages Real time Much higher temporal resolution Enhancement dynamics can be studies No need to predefine scan time points or to perform bolus tracking Excellent tolerance and safety profiles No nephrotoxicity. Safety Consideration No cardio-, hepato- or nephrotoxic effects The incidence of severe hypersensitivity is lower than with current X-ray contrast agent and is comparable to those encountered with MRI contrast agent. Thank you