Lesson 05: Intensity Measurements and Bioeffects. This lesson contains 18 slides plus 6 multiple-choice questions.

Transcription

1 Lesson 05: Intensity Measurements and Bioeffects This lesson contains 18 slides plus 6 multiple-choice questions. This lesson was derived from pages 22 through 24 in the textbook:

2 Intensity Measurements and Bioeffects

3 How are intensity measurements made? SATA SPTA SATP (SAPA) SPTP (SPPA) Many different methods may be used to measure the strength of a sound beam. Differences in the results of these measurements are as varied as the number of different measuring methods. Before intensity measurements are performed, two decisions must be made.

4 The first decision is: Where in the beam s cross section will the measurement be made? The Spatial Peak (SP) method of measuring the intensity in an ultrasound beam provides a result that is greater than that obtained when using the Spatial Average (SA) method. I SP > I SA The Beam Uniformity Ratio (B.U.R.) is the ratio of Spatial Peak to Spatial Average.

5 The second decision is: When (during the pulse - receive interval) will the measurement be made? The Temporal Peak (TP), which is more appropriately designated as Pulse Average (PA), is the method of measuring the intensity of the beam that provides a result that is greater than that obtained when using the Temporal Average (TA) method. I TP > I PA > I TA For pulse-echo measurements, the result obtained with the TA method is affected by the duty factor. For CW measurements, the TP, PA and TA methods provide identical results. TA intensity can be measured with a force balance. TP (PA) intensity and beam profile can be measured with a hydrophone that measures acoustic pressure variations.

6 WHERE WHEN SA TA SP TP(PA) SP or SA may be used with either TP (PA) or TA.

7 WHERE - WHEN SATA SPTA SATP(PA) SPTP(PA) SP or SA may be used with either TP (PA) or TA.

8 Which method produces the lowest result? Which method produces the highest result? I SATA I SPTA I SATP (I SAPA ) I SPTP (I SPPA ) Lowest Highest For the four possible combinations, the order of intensity measurement results (from lowest to highest) is: SATA, SPTA, SATP (SAPA), and SPTP (SPPA).

9 BIOEFFECTS HEAT (Thermal) CAVITATION (Mechanical) Bioeffects are the biological effects of ultrasound on tissue. Basic bioeffects of ultrasound energy are heat (thermal) and cavitation (mechanical).

10 BIOEFFECTS HEAT (Thermal) CAVITATION (Mechanical) Heat is the effect that is caused by friction produced as a result of vibration in tissue. Heating is negligible when using pulsed ultrasound for medical diagnostic purposes, but is significant when ultrasound is used for therapeutic purposes.

11 BIOEFFECTS HEAT (Thermal) CAVITATION (Mechanical) Stable Transient Cavitation is the result of the production of gas bubbles in the tissue. It is possible when higher than normal ultrasound intensities are used, particularly due to an increase in peak pressure that accompanies rarefaction.

12 BIOEFFECTS HEAT (Thermal) CAVITATION (Mechanical) Stable Transient Stable cavitation, which could be produced when intensity levels are low may result in some stress to cells, but is relatively minor.

13 BIOEFFECTS HEAT (Thermal) CAVITATION (Mechanical) Stable Transient Transient cavitation, which is common when very high intensities are used, such as with a lithotripter, may cause the bubbles to collapse or burst, thus providing a potential for significant cellular damage.

14 SAFETY INDICES MECHANICAL INDEX THERMAL INDEX Safety indices have been developed to predict the potential for adverse bioeffects.

15 MECHANICAL INDEX MI The mechanical index (MI) value, which is related to the maximum value of negative peak pressure is used as a guide in predicting mechanical bioeffects (mainly cavitation). For a given intensity, lower frequencies produce higher MI values.

16 THERMAL INDEX TI The thermal index (TI) value, which is related to absorption is an estimate of the rise in tissue temperature in C. Three different thermal indices are used for different combinations of soft tissue and bone in the area to be examined. TIS is the thermal index in soft tissue. TIB is the thermal index in bone near the beam s focal zone. TIC is the thermal index in bone near the surface. There have been no adverse biological effects observed due to temperature increases less than or equal to 2 degrees above normal.

17 BIOEFFECTS Nonfocused: SPTA < 100 mw / cm 2 (0.1 Watt per square centimeter) Focused: SPTA < 1 W / cm 2 Bioeffects have been determined experimentally, through animal and invitro studies, but have not been confirmed for nonfocused SPTA intensities below 100mW/cm 2 (0.1 watt per square centimeter) or focused SPTA intensities below 1W/cm 2. Although many transducer measurements are performed using temporal average methods, temporal peak values are important when assessing cavitation. Assuming that there could be a minimal risk of bioeffects when using diagnostic ultrasound, the best course of action is to use ultrasound when the expected benefit outweighs the potential risk. Additionally, a reduction in the time of an examination will reduce the chances of potential bioeffects.

18 TYPICAL METHODS FOR DISPLAY OF RELATIVE INTENSITIES decibel (db): a parameter used to compare the relative powers, intensities, or amplitudes of two ultrasound energy levels OUTPUT POWER AT MAXIMUM OUTPUT POWER AT ONE - HALF ALARA: the guiding principle regarding output intensity, defined as as low as reasonably achievable Depending on the manufacturer, the ultrasound system s display may indicate the relative output power or intensity as a percentage of the system s maximum, as a decibel (db) difference, the estimated SPTA intensity, or as mechanical index and thermal index values. Any displayed values should be interpreted as relative information to help achieve the ALARA principle.

19 INTENSITY power: the rate of doing work INTENSITY = POWER DIVIDED BY AREA POWER BEAM DIMENSIONS AREA INTENSITY 100 mw 1 cm x 1 cm 1 cm mw / cm mw cm X cm 2 cm 2 50 mw / cm mw 2 cm x 2 cm 4 cm 2 25 mw / cm 2 50 mw cm x cm 2 cm 2 25 mw / cm 2 Sound energy is typically measured in watts, which is a parameter of power. Although the unit for power is the watt, the milliwatt (mw) is normally used. Intensity also represents the amount of sound energy, but the cross-sectional area of the beam is specified.

20 Answers to the following SIX practice questions were derived from material in the textbook:

21 Question 1 Which intensity measuring method provides the highest result? SPTA SPTP SATA SAPA SATP Page 22

22 Question 1 Which intensity measuring method provides the highest result? SPTA SPTP SATA SAPA SATP Page 22

23 Question 2 A hydrophone probe can be used to measure the ultrasound intensity beamformer voltage bandwidth ultrasound frequency axial resolution Page 22

24 Question 2 A hydrophone probe can be used to measure the ultrasound intensity beamformer voltage bandwidth ultrasound frequency axial resolution Page 22

25 Question 3 Thermal bioeffects of ultrasound energy have been determined experimentally by using epidemiological and cavitation studies cavitation detectors animal and in vitro studies hydrophones statistical predictions Page 23

26 Question 3 Thermal bioeffects of ultrasound energy have been determined experimentally by using epidemiological and cavitation studies cavitation detectors animal and in vitro studies hydrophones statistical predictions Page 23

27 Question 4 Potential bioeffects of ultrasound can be minimized by using high pulse repetition frequencies increasing the mechanical index increasing the thermal index using highly focused transducer reducing the time of the examination Page 23

28 Question 4 Potential bioeffects of ultrasound can be minimized by using high pulse repetition frequencies increasing the mechanical index increasing the thermal index using highly focused transducer reducing the time of the examination Page 23

29 Question 5 Since very little is known about bioeffects and assuming that there could be a minimal risk of bioeffects when using diagnostic ultrasound, what would be the best course of action? Perform fewer ultrasound examinations but increase the time of each examination Perform ultrasound examinations only when a crash cart is available. Use pulsed Doppler only for obstetrical studies. Use ultrasound when the expected benefit outweighs the potential risk Page 23

30 Question 5 Since very little is known about bioeffects and assuming that there could be a minimal risk of bioeffects when using diagnostic ultrasound, what would be the best course of action? Perform fewer ultrasound examinations but increase the time of each examination Perform ultrasound examinations only when a crash cart is available. Use pulsed Doppler only for obstetrical studies. Use ultrasound when the expected benefit outweighs the potential risk Page 23

31 Question 6 Sound power is measured in units of db/cm mw/cm watts/m 3 watts db Page 24

32 Question 6 Sound power is measured in units of db/cm mw/cm watts/m 3 watts db Page 24

33

34 END OF LESSON 05