Ultrasound Safety and QA PDF

Ultrasound Safety and Quality Assurance 29th January 2025 Justyna Janus NIR Specialist, University Hospitals of Leicester NHS Trust Thanks to colleagues for use of images.  Outline Cl...

Ultrasound Safety and Quality Assurance 29th January 2025 Justyna Janus NIR Specialist, University Hospitals of Leicester NHS Trust Thanks to colleagues for use of images.  Outline Click to edit Master title style 1. Safe use of ultrasound (US) 2. Why we are concerned about its safety 3. Parameters related to biological effects: power and intensity 4. Biological effects of ultrasound 5. Guidelines and regulations for the safe use of ultrasound 6. Exposure controls: ODS and the ALARA principle 7. Epidemiological evidence Ultrasound Safety  Safe use of ultrasound Click to edit Master title style Ultrasound is based on non-ionizing radiation, so it does not have the same risks as X-rays or other types of imaging systems. Generally considered safe when used prudently by appropriately trained staff… Nevertheless….. Ultrasound Safety  Why are we concerned about safety? Click to edit Master title style Ultrasound involves exposure to a form of energy, so there is the potential to initiate biological effects. The number of patients undergoing ultrasound scanning is increasing The range of clinical applications is becoming wider Development of sophisticated and powerful modern diagnostic equipment Ultrasound Safety  Why are we concerned about safety? Click to edit Master title style In 1917, Paul Langevin observed the immediate death of fish swimming near to the acoustic high-intensity US beam for the first time…. …Ten years later, Wood and Loomis published the first paper entitled “The Physical and Biological Effects of High- frequency Sound-waves of Great The coil and reaction chamber used by Wood and Loomis in their studies into the Intensity”. physical and biological effects of high- intensity ultrasound, 1927 From: Shankar H, Pagel PS. Potential adverse ultrasound-related biological Ultrasound Safety effects: a critical review. Anesthesiology. 2011;115(5):1109-24.  Why are we concerned about safety? Click to edit Master title style They reported that: ‘When the frequency was adjusted for resonance the narrow beam of supersonic waves was shot across the tank causing the formation of millions of minute air bubbles and killing small fish that occasionally swam into the beam. If the hand was held in the water an almost insupportable pain was felt, The coil and reaction chamber used by which gave the impression that the Wood and Loomis in their studies into the physical and biological effects of high- bones were being heated.’ intensity ultrasound, 1927 Ultrasound Safety From: Shankar H, Pagel PS. Potential adverse ultrasound-related biological effects: a critical review. Anesthesiology. 2011;115(5):1109-24.  Parameters related title Click to edit Master to US Biological effects style Power output and intensity are directly related to the potential for ultrasound to cause biological effects or tissue damage. Ultrasound Safety  Acoustic Power Click to edit Master title style Power refers to the rate at which the wave delivers energy. Energy, E, is measured in Joules (J), Power is measured in Watts (W), 1 W = 1 J/s Ultrasound Safety  Acoustic Power Click to edit Master title style There are two main sources of acoustic output information: From manufacturers e.g. in the equipment manuals, on- screen indication Have measurements made traceable to NPL, the UK`s national measurement standards laboratory e.g. Ultrasound exerts a force on a target that is directly using a radiation force proportional to the total power absorbed or reflected by the target. balance. Ultrasound Safety  Intensity Click to edit Master title style Intensity, I, is a measure of the power flowing through a specific area of the beam. Intensity of US wave increases with the pressure amplitude, p, wave: I0  p 2 0 Sound beam Cross-sectional area Ultrasound Safety  Intensity Measurements Click to edit Master title style I0  p 2 0 Temporal-peak intensity (ITP) is the maximum value during the pulse Pulse-average intensity (IPA) is the average value over the duration of the pulse Ultrasound Safety From: Safe Use of Ultrasound – Gail ter Haar  Intensity Measurements Click to edit Master title style To assess potential for tissue heating, it is useful to measure the temporal-average intensity (ITA). Value is much lower than pulse-average intensity as it includes the ‘off’ period between pulses Ultrasound Safety From: Safe Use of Ultrasound – Gail ter Haar  Intensity Measurements Click to edit Master title style The value of intensity parameters vary with position in the beam. The highest value is the Spatial-Peak Intensity (ISP). The average value over the beam in the Spatial Average Intensity (ISA). Ultrasound Safety From: Safe Use of Ultrasound – Gail ter Haar  Intensity Measurements Click to edit Master title Symbol Parameter style Description Spatial-peak The highest intensity measured at any Isppa pulse-average intensity point in the US beam averaged over the temporal duration of the pulse Spatial-peak The highest intensity measured at any Ispta temporal-average intensity point in the US beam averaged over the pulse repetition period Spatial-average The average intensity over a selected Isata temporal-average intensity area, averaged over the pulse repetition period Spatial-average The average intensity over a selected Isapa pulse-average intensity area, averaged over the temporal duration of pulse Isppa > Ispta > Isapa > Isata Ultrasound Safety From: Safe Use of Ultrasound – Gail ter Haar  Intensity Measurements Click to edit Master title style Free field: acoustic pressure and intensity measured using hydrophone in water (no attenuation) De-rated: free fields measurements are altered to estimate the values expected in tissue: 0.3 dBcm-1MHz-1 Ultrasound Safety From: Safe Use of Ultrasound – Gail ter Haar  Biological effects of ultrasound Click to edit Master title style Thermal effects (heating) - related to heating and the changes heat can cause. Non-thermal (mechanical) effects: cavitation -determined by the negative pressure peak or rarefaction of the US beam. other gas body mechanisms Ultrasound Safety  Thermal effects Click to edit Master title style The absorption of energy within tissue leads primarily to a rise in temperature. The most rapid heating is in the centre of the beam near the focus, where ITA is highest. The amount of energy absorbed depends on the composition of the tissue and the frequency of the ultrasound beam. Ultrasound Safety From: Safe Use of Ultrasound – Gail ter Haar  Thermal effects Click to edit Master title style ultrasound heating transducer heating DT distance Not all the electrical energy supplied to the transducer is converted into ultrasound energy. Some electrical energy is converted into heat due to inefficiency in the transducer. The temperature of the transducer face can rise, heating nearby tissues. The transducer gets hotter if it is left radiating into air. Ultrasound Safety  Thermal effects Click to edit Master title style Transducer surface temperature limited to less than 50°C in air and less than 43°C when touching the patient either internally or externally. Thermal image showing the pattern of skin temperature rise immediately after removal of a transducer (F A Duck 2003) Ultrasound Safety  Thermal effects Click to edit Master title style Some tissues are particularly sensitive such as those in developing embryo and fetus. Organogenesis (up to 8 weeks after conception) Mineralization of developing bones Development of the brain and spinal cord Eyes in fetus and adult Ultrasound Safety From: https://www.medicinenet.com  Thermal effects Click to edit Master title style A diagnostic exposure that produces a maximum temperature rise of no more than 1.5 °C above normal physiological levels (37 °C) may be used clinically without reservation on thermal grounds. (WFUMB, 1998) Temperature rises > 1.5 °C are unlikely to occur in soft tissues at diagnostic intensity levels. A diagnostic exposure that elevates embryonic and fetal in situ temperature above 41°C (4°C above normal temperature) for 5 minutes or more should be considered potentially hazardous. (WFUMB, 1998) Temperature rises of 4 °C or more have been recorded in animal models adjacent to a bone surface and in thermal phantoms. Ultrasound Safety  Click to edit Master title effects Non-thermal style Non-thermal effects of ultrasound can be considered in the following categories: Stable cavitation Transient (inertial) cavitation Other mechanisms Ultrasound Safety  Stable cavitation Click to edit Master title style repetitive oscillation around the equilibrium radius of a bubble in a bubble liquid exposed to an acoustic field without large changes in volume may result in heat generation, microflow of fluid near the bubble, and localized shear forces (shear stress) in streaming the cell wall. currents cell Shear stresses can give rise to membrane rupture and sonoporation. It is unlikely to occur with diagnostic exposures as the pulses are too short. Ultrasound Safety  Transient (inertial) cavitation Click to edit Master title style The bubble can expand more than twice its initial radius and then rapidly collapse. This may produce a strong shock wave, which is Large changes accompanied by extremely high local in diameter temperature values that are associated with the release of free radicals. Free radicals can cause undesirable biochemical reactions between tissues. Local shear stresses may cause lysis of adjacent cells. Gas bubble Occurs only above a threshold acoustic pressure and in the presence of suitable gas bubbles. The pressure threshold is lower at low frequency. Ultrasound Safety  Contrast agent safety Click to edit Master title style Contrast agent shells may be ruptured at relatively low acoustic pressure (p- < 1 MPa) releasing free gas bubbles which can then produce inertial cavitation. There is evidence of micro-vascular damage at diagnostic pressures in the presence of contrast agents. Sonoporation and lysis of cells have been observed at diagnostic pressure levels. Ultrasound Safety From: Erasmus MC, Rotterdam  Lung capillary bleeding Click to edit Master title style Alveolar bleeding has been observed in small mammals exposed to acoustic pressures > 1 MPa. Similar small haemorrhages have been observed in the intestines of small animals. These effects appear to be related to the presence of gas in Blood-filled tubules generated within these organs. the scan plane on the surface of a rat kidney, insonated in the These effects have not been presence of ultrasound contrast observed in humans. agents: scale bar 5 mm. Ultrasound Safety From: Safe Use of Ultrasound – Gail ter Haar  Exposure controls Click to edit Master title style In 1992, the AIUM and the NEMA defined and published the “Output Display Standard” (ODS). Diagnostic US machines display on the screen a thermal index (TI) and a mechanical index (MI) as safety indices to standardize diagnostic US examinations and provide information AIUM - American Institute of to the user related to safety. Ultrasound in Medicine NEMA - National Electrical Manufacturers Association Ultrasound Safety  FDA US exposure controls Click to edit Master title style In 1993, this standard has become a part of the FDA's guidance to manufacturers who were given the option to market systems that exceed previously allowed application‐specific levels of ultrasound exposure as long as the levels of exposure are displayed on screen. FDA - Food and Drug Administration Ultrasound Safety  FDA US exposure controls Click The FDA to edit application-specific removed Master title style acoustic output limits but retained a maximum overall output limit of 720 mW/cm2 except ophthalmic as long as the ODS is displayed for every possible setting of transducer type, output setting, focus, frame rate and pulse rate. This change created the possibility for nearly an eightfold increase in acoustic output capabilities in the case of obstetric imaging! Ultrasound Safety  Output Display Standard Click to edit Master title style Thermal Index (TI) and Mechanical Index (MI) must be displayed on the screen. *No requirement to display : TI when < 0.4 MI when < 0.4 *No requirement to display ODS at all if equipment cannot exceed TI = 1 MI = 1 Ultrasound Safety  Thermal Index (TI) Click to edit Master title style Estimates maximum tissue temperature rise likely to be reached with current control settings. TI = Total acoustic power (with current settings) [W] Power required to raise tissue temperature by 1 °C [W] Not a real temperature measurement!!! TI does not include temperature rises due to probe heating. In some circumstances, TI can underestimate temperature rise by a factor of 2. Ultrasound Safety  Thermal Index cont. Click to edit Master title style Thermal Index is defined for 3 different models for use in different circumstances TIS assumes uniform soft tissue TIB assumes bone where pulse energy is greatest TIC assumes bone at the surface (cranium) TIS TIB TIC Maximum Maximum temperature temperature Maximum temperature Soft tissue bone bone Transducer Ultrasound Safety  Mechanical Index (MI) Click Indicates to edit potential Masterfortitle style cavitation bioeffects pr.3 ( z sp ) MI = fc pr.3(zsp) = the peak de-rated negative pressure (MPa) measured at the point in the beam where it is greatest. fc = the centre frequency (MHz) of the pulse spectrum. Theoretically, inertial cavitation should not occur in water if MI < 0.7 The model suggests that cavitation is more likely : at high p and low frequency (negative half cycle is longer so more time for bubble growth). Ultrasound Safety  Safety Advice - TI Click to edit Master title style Advice on acoustic safety: (British Medical Ultrasound Society 2000) TI < 0.7 No restriction except if probe selfheating is noticeable or maternal temperature elevated. TI > 0.7 The overall exposure time (including pauses) of an embryo or fetus should be restricted (e.g. 1.0 Eye scanning is not recommended, other than as part of a fetal scan. TI 3.0 Scanning of an embryo or fetus is not recommended, however briefly. Monitor TIB if bone is in the insonated area. Ultrasound Safety  Safety Advice - MI Click to edit Master title style Advice on acoustic safety: (British Medical Ultrasound Society 2000) MI < 0.3 No restriction MI > 0.3 There is a possibility of minor damage to neonatal lung or intestine. If such exposure is necessary, try to reduce the exposure time as much as possible. MI > 0.7 There is a risk of cavitation if an ultrasound contrast agent containing gas micro-spheres is being used. There is a theoretical risk of cavitation without the presence of ultrasound contrast agents. The risk increases with MI values above this threshold. Ultrasound Safety  Advice on acoustic safety Click to edit Master title style British Medical Ultrasound Society 2000- “Guidelines for the safe use of diagnostic ultrasound equipment” Ultrasound Safety  Advice on acoustic safety Click to edit Master title style American Institute of Ultrasound in Medicine (AIUM) (www.AIUM.org) International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) (www.ISUOG.org) The British Medical Ultrasound Sociaty (BMUS) (https://www.bmus.org/) ❑ The uncontrolled use of US without medical benefit should be avoided (e.g., souvenir images of the fetus) ❑ Based on evidence currently available, routine clinical scanning of every woman during pregnancy using real time B-mode imaging is not contraindicated. Its use therefore appears to be safe, for all stages of pregnancy Ultrasound Safety  Advice on acoustic safety (cont.) Click to edit Master title style ❑ Caution is recommended when using Color and Spectral Dopper (particularly in the vicinity of bone) with a very small region of interest- this mode produces the highest potential for bioeffects. Close monitoring of safety indices is recommended. ❑ The risk of damage to the fetus by teratogenic agents is particularly great in the first trimester. ❑ Use M-mode for Fetal HR at first because time averaged acoustic intensity is lower than with spectral Doppler. ❑ Spectral Doppler may be used only briefly (e.g., 4-5 heart beats) and keep the thermal index (TIS for soft tissues in the first trimester, TIB for bones in second and third trimesters) as low as possible, preferably below 1 in accordance with the ALARA principle. Ultrasound Safety  ALARA principle Click to edit Master title style Users should regularly check both indices while scanning and should adjust the machine controls to keep them as low as reasonably achievable (ALARA principle) without compromising the diagnostic value of the examination. Where low values cannot be achieved, examination times should be kept as short as possible. Ultrasound Safety  ALARA principle cont. Click to edit Master title style ▪ Set power output levels to lowest levels which give adequate signals ▪ Do not exceed manufacturer’s guidelines for the particular application ▪ Start with the B-mode image Receiver Gain has NO (which has the lowest power) and effect on heating or progress to colour and PW cavitation So ….. use it! Doppler if required ▪ Do not scan for longer than is necessary Ultrasound Safety  Epidemiological Evidence Click to edit Master title style ▪ Several large epidemiological studies have been performed to search for effects following exposure to ultrasound in utero. ▪ Available evidence doesn`t support an association between the use of US for foetal imaging with cancer or with adverse effects on birth weight, growth, or neurodevelopment during childhood. Ultrasound Safety  US Safety – QUIZ 1. Click Can youto edit Master determine titlewould which model stylebe recommended for each of the following situations? a) Scanning a fetus after 10 weeks of gestation: TIS TIC TIB √ b) Transcranial monitoring in adults and neonates: TIS TIC √ TIB 2. True or False: a) To minimize exposure to ultrasound, sonographers should use high transmit power and low receiver gain setting X b) Both TI and MI must always be displayed on the ultrasound screen X c) Follow ALARA (as low as reasonably achievable) √ Ultrasound Safety  Summary of US Safety Major Clickmechanisms to edit Master title for bioeffects arestyle thermal and non-thermal TI and MI indicate the potential for thermal and cavitation effects Machine acoustic outputs have increased over the years Recent changes to legislation put more on the user to make risk/benefit judgements. Particular care is needed in scanning foetal tissues and the premature infant lung. Care is required also in scanning eyes and when using contrast agents. The lack of an observed effect doesn`t implicitly suggest that there is no risk of bioeffect but only implies that we have been unable to detect one. Follow ALARA The greatest risk to the patient remains misdiagnosis. Ultrasound Safety Click to edit MasterAssurance Quality title style  Click to edit Master title style Outline 1. Quality assurance (QA) Guidelines 2. User tests 3. Tissue Equivalent Test Object (TETO) 4. QA tests using TETO 5. Doppler phantoms Quality Assurance  US Quality Assurance (QA) Click to edit Master title style How to measure ultrasound system performance? Nature of an US image is unique! range of possible settings range of transducer types and imaging applications Aim is to design a QA program that will monitor aspects of the performance of the US system which are considered likely to change or deteriorate and which will impact on the clinical efficacy of the system. Quality Assurance  QA Guidelines Click to edit Master title style IPEM Report 102 recommends: Baseline testing: Full set of performance tests for new equipment / upgrades to establish test data. Carried out by Medical Physics. User tests: Simple tests carried out by user at 1 – 4 week intervals. Routine quality assurance tests: Provided annually by Medical Physics Department or company service engineer to www.ipem.org.uk demonstrate consistent performance. Quality Assurance  US Quality Assurance (QA) Click to edit Master title style Who? Ultrasound Physicist (Medical Physics or Clinical Engineering) Users – you! Why? Identify whether new equipment is functioning properly Check image quality and measurement accuracy are suitable for the desired clinical application Monitor degradation of the system over time Identify faults to help justify replacement When? On acceptance or after repairs by Med Phys Every week or month by users Annually (or 6 monthly for breast scanners) by Med Phys Electrical safety testing should also be performed annually by Clinical Engineering Quality Assurance  US Quality Assurance (QA) Click to edit Master title style Tests that user (you!) should do on weekly/month basis Visual inspection Air filters Uniformity test Quality Assurance  Tissue Equivalent Test Object (TETO) Click to edit Master title style Imaging performance can be assessed by Ultrasound Physicist using ultrasound test objects. These contain targets designed to measure imaging parameters. The targets are embedded in tissue mimicking material (TMM) with similar acoustic properties of soft tissue. The targets are imaged via an acoustic window. CIRS TETO currently used by the Trust for its US QA program Quality Assurance  Tissue Equivalent Test Object (TETO) Click to edit Master title style TMM : Agar, Urethane, TPE, Zerdine, Other (Liquids, Natural materials) Properties: Speed of Sound 1540 m/s Attenuation Coefficient 0.5 dB/cm-MHz 0.7 dB/cm-MHz Quality Assurance  QA tests Click to edit Master title style What to measure when using TETO? Spatial Resolution ( Axial, Lateral and Elevational ) Depth of Penetration Measurement accuracy Lesion Detectability: high contrast (anechoic objects) and low contrast (grey scale objects) Image Uniformity Quality Assurance  Imaging performance – spatial resolution Click to edit Master title style (slice thickness) (lateral) axial Spatial resolution is measured in 3 orthogonal directions. These are: lateral resolution – along the scan plane axial resolution – along the beam axis slice thickness – at 90° to the scan plane Quality Assurance  Lateral resolution Click to edit Master title style Lateral resolution can be estimated using wire pairs. These are arranged in groups say 1.25, 2.5 and 5 mm apart. The smallest separation which can be imaged is a measure of the lateral resolution at that depth. Changes significantly with depth due to beam shape and focussing → measure at each available target depth with the focal setting → more likely to spot deterioration in performance than if using just one focal depth Quality Assurance  Lateral resolution Click to edit Master title style Alternatively, lateral resolution can be estimated at a particular depth by measuring the width of the image of a filament. The measurement is more accurate if a large scale is used. Quality Assurance  Axial resolution Click to edit Master title style Axial resolution can be estimated by measuring the axial extent of a single wire displayed on a large scale. The alternative method uses groups of 5 or 6 filaments whose axial separation reduces with depth from 5 mm to 1 mm. The smallest visible separation gives an estimate of the axial resolution Quality Assurance  Slice thickness Click to edit Master title style The slice thickness is usually greater than the beam width in the scan cylindrical lens plane and can lead to infilling of small cystic slice thickness structures in the image, cyst especially outside the focal region. Quality Assurance  Slice Thickness Click to edit Master title style Targets set in a plane at 45º to the imaging plane Appear in the image with an axial or lateral extent, which is equal to the slice thickness Quality Assurance  Penetration Depth Click to edit Master title style The echo signal received from scattering targets reduces with depth due to attenuation. The maximum depth at which real ultrasonic information from small scattering targets can be identified is the penetration depth. It depends on frequency and is affected by transmit power, focal depth, transducer efficiency and system noise level. A change in penetration depth can indicate a fault in any of these parameters. Quality Assurance  Measurement Accuracy Click to edit Master title style Crown-rump length measurement Nuchal Translucency Measurement Ultrasound imaging systems are used widely to make measurements, especially in obstetric applications. It is important that the system is calibrated to give the correct answer. Quality Assurance  Measurement Accuracy cont. Click to edit Master title style Ultrasound is used in many applications for measuring internal dimensions. Linear measurements normally checked in axial and lateral directions. For axial measurements, the measured distance should agree with the distance separating the filament targets to within ± 1% or 1 mm (whichever is greatest). Check of axial measurement accuracy Quality Assurance  Lesion Detectability and Image Uniformity Click to edit Master title style Lesion Detectability- The targets should appear circular in shape and vary in the degree of brightness ranging from low to high levels of contrast. Image uniformity -The image uniformity is defined as the equipment ability to give ultrasound echoes with the same amplitude and deepness when the brightness is fixed. Important to verify if the transducer crystals have minor dropouts. Quality Assurance  Doppler Testing Click to edit Master title style Mimicking the complexity of blood flow patterns and velocity within the phantom accurately is challenging! Types of Doppler phantoms: ▪ Flow phantoms ▪ String phantoms Quality Assurance  Doppler Flow Phantoms Click to edit Master title style Material Properties Tissue mimic (density, speed of sound, attenuation) Blood mimic (speed of sound, attenuation, density, scatterer size / concentration, viscosity, stability) Vessel mimic (density, speed of sound, attenuation) Quality Assurance  Doppler string phantoms Click to edit Master title style Moving string simulates moving blood. String scattering characteristics need to match blood (O-ring rubber) Accurate measurement of velocity as velocity of string is known. Spectral broadening Quality Assurance  QUIZ Click to edit Master title style Regarding electrical safety and quality assurance (QA) in ultrasound: 1. Electrical safety tests of the ultrasound unit should be √ performed annually 2. QA of the ultrasound unit should be performed semi- X annually 3. The resolution in B-mode imaging may be tested with a X string phantom 4. Phantoms used for B-mode imaging are usually filled X with distilled water Quality Assurance  QA Summary Click to edit Master title style 1. It is responsibility of the user and ultrasound physicist to check the machine regularly 2. Various aspects of the ultrasound equipment should be routinely checked to ensure it is safe to operate and that it provides reliable information (images) 3. Ultrasound imaging system performance can be assessed using TETO test objects. 4. Test object measurements: may not relate directly to clinical performance may be subjective and have limited repeatability are most useful for consistency checks Quality Assurance Click to edit Master title style