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ultrasound safety

Aboubakr Elnashar

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ultrasound safety

  1. 1. ULTRASOUND SAFETY Prof. Aboubakr Elnashar Benha university Hospital, Egypt ABOUBAKR ELNASHAR
  2. 2. CONTENTS 1.INTRODUCTION 2.FACTORS AFFECTING 3.BIOEFFECTS OF US 4.SAFETY INDICES 5.HOW TO LIMIT FETAL EXPOSURE  CONCLUSION ABOUBAKR ELNASHAR
  3. 3. 1. INTRODUCTION  Ultrasound  Form of energy  Lack of knowledge of US clinical users on  output  bioeffects  safety both in the USA and abroad ABOUBAKR ELNASHAR
  4. 4.  US examinations/ pregnancy  Most pregnant women 2–3  Some countries (Egypt): 10 whether there is a cumulative dose effect? ABOUBAKR ELNASHAR
  5. 5.  Epidemiological studies  No harmful effects in human fetuses.  Most, based on information obtained with pre-1991 machines.  Around that time, FDA allowed the acoustic output of US machines for fetal use to be increased from  94 to  720 mW/cm2, a factor of almost 8. ABOUBAKR ELNASHAR
  6. 6. ABOUBAKR ELNASHAR
  7. 7. 2. FACTORS AFFECTING SAFETY OF ULTRASOUND 1. US exposure  The ultrasound energy or  Total acoustic output power (w) emitted by equipment. 2. US settings  Type of transducer  Depth of penetration  Overlying layers of tissue  alter the acoustic output to the particular target.  E.g. US exposure to the fetus in 1st T differs significantly between TA and TV probes. ABOUBAKR ELNASHAR
  8. 8. 3. Tissue composition  determines the acoustic absorption coefficients  more proteinaceous tissue: more susceptible to thermal injury  higher fluid and gas content: more susceptible to cavitational activity. 4. Tissue susceptibility  Fetal or embryonic tissues:  Rapidly proliferating : more susceptible to US effects.  Adult tissues have:  static cell population  safety features such as the hyperaemic reflex {an increase in blood flow through the tissue that carries the heat away}. ABOUBAKR ELNASHAR
  9. 9. 3. BIO EFFECTS OF ULTRASOUND  When performing diagnostic US  2 major mechanisms are operative:  Thermal  Nonthermal. ABOUBAKR ELNASHAR
  10. 10. 1. THERMAL EFFECTS  Increase in tissue temp is the most worrying bioeffect associated with diagnostic US in obstetrics  Resulting from  transformation of acoustic energy into heat  an indirect effect. ABOUBAKR ELNASHAR
  11. 11.  Thermal Effects  Hyperthermia is teratogenic for numerous species, including humans.  Most at risk is CNS {lack of compensatory growth of undamaged neuroblasts} ABOUBAKR ELNASHAR
  12. 12.  Temperature threshold:  Temp increases of 1 °C are easily reached in routine US.  An increase of 2.5 °C and above is possible with 1h of exposure to US  Temp elevations ≤1.5 C: no hazard, including a human embryo or fetus, even if maintained indefinitely.  A 1.5°C temp elevation above the normal value is considered a universal threshold.  Some scientists : any positive temp differential for any period of time has some effect.  Fetal temp is 0.5–1.0C higher than maternal temp: caution in a febrile mother. ABOUBAKR ELNASHAR
  13. 13. In early pregnancy the entire fetus is within the US beam. Gestational age of 12 w ABOUBAKR ELNASHAR
  14. 14.  Factors affecting thermal effect 1. Type of probe  Abdominal probe Vs vaginal probe  Abd:  the skin surface is close to room temp and heat is removed by air convection  Vag:  tissues are at an average temp of 37 °C and there is very little heat removal  A fixed transducer and target: higher than expected temp rise.  The active fetus in 3nd &3rd T: TAS escapes this effect. ABOUBAKR ELNASHAR
  15. 15. 2. Type of mode:  B-mode, M-mode & 3D US less likely to give rise to thermal injury in routine practice  Doppler US  can cause significant temp rises.  Temp rise to above 41°C lasting for 5 mins or more  potentially hazardous to a fetus or embryo  possible with spectral Doppler and colour Doppler imaging ABOUBAKR ELNASHAR
  16. 16. 3. Beam characteristics  Temp elevation is proportional to the  wave amplitude  length of the pulse  pulse repetition frequency.  manipulation of any of these will alter the in situ conditions. ABOUBAKR ELNASHAR
  17. 17. 4. Gestational age  Milder exposure during the preimplantation period:  similar consequences to more severe exposures during embryonic and fetal development  death and abortion or  structural and functional defects. ABOUBAKR ELNASHAR
  18. 18.  Recommendations  ALARA (As Low As Reasonably Achievable) principle: keep the exposure  as low as possible,  for the least amount of time possible,  enough to get adequate diagnostic images.  World Federation for Ultrasound in Medicine and Biology (WFUMB)  Temp elevation of no more than 1.5 °C above normal physiological levels may be used without reservation on thermal ground ABOUBAKR ELNASHAR
  19. 19. 2. Non thermal Effects  These are interactions between US wave and the tissue that do not cause a significant degree of temp increase (<1°C above physiological temp).  Included are  Mechanical  acoustic cavitation  radiation torque  force and acoustic streaming secondary to propagation of US waves.  Physical (shock wave)  Chemical (release of free radicals) effects. ABOUBAKR ELNASHAR
  20. 20. 1. Cavitation  The major factor in mechanical effects  To occur, gas bubbles must be present in the tissues.  No gas bubbles in fetal lungs or bowel: risk from mechanical effect is minimal  US contrast agents can act as source of cavitation, when injected into the body before US examination.  No indication for its use in fetal US ABOUBAKR ELNASHAR
  21. 21.  US Doppler: no relation to cavitation  No harmful effects of diagnostic US, secondary to nonthermal mechanisms have been reported in human fetuses. ABOUBAKR ELNASHAR
  22. 22. 2. Acoustic streaming and torque  = twisting or spinning forces  US wave tend to push target tissue away from the transducer:  acoustic streaming in fluids  cell distortion and lysis  demonstrated in experimental models  unlikely to be significant with diagnostic US in soft tissues in vivo {in situ adhesiveness is high}. ABOUBAKR ELNASHAR
  23. 23. 4. SAFETY INDICES  An on-screen display to guide the user to extent of thermal & mechanical effect (American Institute of Ultrasound in Medicine (AIUM) National Electrical Manufacturers Association (NEMA).)  Called the Output Display Standard (ODU)  First reported in 1992.  FDA allow manufacturers to increase power outputs by up to 8–10 times, provided there is a display of safety indices on the screen.  The aim to  keep these indices as low as possible  While obtaining the best possible diagnostic images ABOUBAKR ELNASHAR
  24. 24. The TI and MI acoustic indices as demonstrated on the monitor screen during US examination. MI is 0.9 and the TIS, 0.1ABOUBAKR ELNASHAR
  25. 25.  Thermal index  An indicator of the temp elevation possible at a particular equipment setting.  The ratio of  acoustic power emitted by the transducer To  acoustic power required to produce a 1C rise in temp at a particular equipment setting  3 subdivisions:  Soft tissues (TIS)  Bone (TIB)  Adult cranial exposure (TIC).  In obstetric:  TIS should be used for the first 8 w  TIB should be monitored thereafter. ABOUBAKR ELNASHAR
  26. 26.  The acoustic power of an US scanner depends on: 1. focus, 2. pressure 3. intensity 4. scan depth 5. mode and transducer characteristics.  Various combinations of these parameters: varying levels of acoustic power output with significant variations in the temp levels ABOUBAKR ELNASHAR
  27. 27. Safety indices  Mechanical index (MI)  Thermal index  soft tissues (TIS);  bone (TIB)  adult cranial exposure or bone (TIC) ABOUBAKR ELNASHAR
  28. 28. B-mode ultrasound (TIB and MI are displayed in the top right hand corner) TIB = 0.2, MI = 1.1 ABOUBAKR ELNASHAR
  29. 29. Doppler mode. Note the change in TIB and MI when the settings are changed from B mode to Doppler mode TIB = 1.4, MI = 0.55 ABOUBAKR ELNASHAR
  30. 30. Umbilical artery Doppler. TIB (solid arrow) is displayed in the top right hand corner TIB = 1.1, MI = 0.55 Depth: 5.8 cm ABOUBAKR ELNASHAR
  31. 31. Umbilical artery Doppler. TIB (solid arrow) is displayed in the top right hand corner. Note how an increase in depth from 5.8 cm (Fig a) to 13 cm (Fig b) almost triples the TIB (1.1 to 3.1)ABOUBAKR ELNASHAR
  32. 32.  Mechanical index  an indicator of the Likelihood of cavitation events.  Definition:  „Maximum estimated in situ rarefaction pressure or  Maximum negative pressure (in mpa) divided by the square root of the frequency (in mhz)  inversely proportional to the frequency.  MI: 0.3 is the threshold value for hges to occur in the mouse lung. ABOUBAKR ELNASHAR
  33. 33.  Mechanical bioeffects  in humans  not reported from currently used diagnostic US  In animals: reported: raising the concern that there is potential for similar injury in humans.  MI Should be less than 1.9. ABOUBAKR ELNASHAR
  34. 34.  Limitations of the thermal and mechanical indices  do not consider factors such as  Duration of examination  Patient temp  Presence of contrast agents.  There is probably an underestimation of temp rise by the thermal Index.  Not perfect  The most practical measurements available. ABOUBAKR ELNASHAR
  35. 35.  Recommendations:  Mechanical index  ≥0.3: minor damage to neonatal lung or intestine is possible  ≥0.7 have a propensity for cavitation injury, especially with use of contrast agents.  Thermal index  ≥0.7 the overall exposure to embryo or fetus should be restricted to less than 60 mins ABOUBAKR ELNASHAR
  36. 36. Maximum recommended exposure times for an embryo or fetus (British Medical Ultrasound Society.) ABOUBAKR ELNASHAR
  37. 37.  Doppler effects on the fetus in the first trimester  Doppler is different  1. Acoustic Output is much higher in Doppler than in B-mode:  34 mW/cm2 for the ISPTA in B-mode versus  1080 mW/cm2 for spectral Doppler  35-fold difference.  2. Dwell time (duration of exposure) is important:  The average duration was 27 min (the longest 4 h!). ABOUBAKR ELNASHAR
  38. 38. Very high TI (5.7) may be obtained in Doppler mode (not an actual clinical examination). Note that this is a general obstetrics settingABOUBAKR ELNASHAR
  39. 39.  Precautions 1. Clear indication 2. Limit time and acoustic output  Excellent, diagnostic images can be obtained at low outputs, as defined by the TI values of 0.5 or even 0.1.  Therefore, the switch-on default should be set up such that a low acoustic output power is initiated for each new patient, when starting an examination.  Only if images are not satisfactory from a diagnostic standpoint, should the output be increased ABOUBAKR ELNASHAR
  40. 40. 3. To have the transducer as steady as possible. {blood vessels or heart valves are small in comparison to the general organ or body size being scanned and even small movements will have more undesired effects on the resulting image}. 4. Using Doppler to “listen” to the fetal heart should be discouraged and replaced by M-mode assessment.  If Doppler is used, it is sufficient to “hear” 3–4 heart beats and thus limit the exposure ABOUBAKR ELNASHAR
  41. 41. Doppler velocimetry in the umbilical artery. (a) TIB is 2.4. (b) TIB is 0.4 and the image is equally diagnostic ABOUBAKR ELNASHAR
  42. 42.  3D/4D ULTRASOUND  Characteristics  short acquisition time and post processing analysis: decreased exposure.  TI and MI, acoustic output are comparable to the TI during the B-mode scanning ABOUBAKR ELNASHAR
  43. 43. 5. HOW TO LIMIT FETAL EXPOSURE AND SAFETY STATEMENTS 1. Perform US only with a clear indication 2. keep exposure to a minimum power and time, compatible with an adequate diagnosis  application of the ALARA principle 3. Watch the TI (and, to a lesser degree) the MI on- screen 4. Begin your exam with a low power output and increase only if necessary. ABOUBAKR ELNASHAR
  44. 44. 3-D acquisition with 3 orthogonal planes and reconstructed volume. The output power is determined by the acquisition plane (in general plane A), since the 2 other planes (B, C) and the reconstructed volume are computer-generated. In this acquisition, TIS was 0.5 ABOUBAKR ELNASHAR
  45. 45. 5. Pulsed Doppler (spectral, power, and color flow imaging) ultrasound 1. should not be used routinely, may be used for clinical indications such as to refine risks form trisomies. 2. When performing Doppler US  TI should be less than or equal to 1.0  Exposure time should be kept as short as possible  usually no longer than 5–10 min and not exceed 60 min. ABOUBAKR ELNASHAR
  46. 46. CONCLUSIONS  The early fetal period is a time of increased susceptibility to external factors, such as hyperthermia, a recognized teratogen, with CNS being most at risk.  Bioeffects of US may be secondary to 2 major mechanisms: 1. thermal (indirect, resulting from conversion of acoustic energy into heat) 2. non-thermal (also known as mechanical, direct effects caused by bubble cavitation and other mechanical phenomena). ABOUBAKR ELNASHAR
  47. 47.  The application of safety indices and on-screen display is important.  To limit exposure and potential harmful effects,  use US only when indicated,  keep the exam as short as possible,  at lowest possible output for diagnostic accuracy (ALARA principle)  keep TI and MI below 1.  Diagnostic US is safe in pregnancy  both for the mother and fetus  no substantiated long-term effects have been demonstrated. ABOUBAKR ELNASHAR
  48. 48. Thanks ABOUBAKR ELNASHAR
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