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IEC 61846:1998 — Ultrasonics: Pressure Pulse Lithotripsy
Measurement and Characterization of Lithotripter Acoustic Fields
Key Insight
IEC 61846:1998 establishes standardized measurement methods for characterizing the acoustic pressure fields generated by lithotripters used in extracorporeal shock wave treatment of kidney stones and biliary calculi, defining key parameters including peak pressure, pulse energy, focal zone dimensions, and acoustic dose.
IEC 61846:1998 establishes standardized measurement methods for characterizing the acoustic pressure fields generated by lithotripters used in extracorporeal shock wave treatment of kidney stones and biliary calculi, defining key parameters including peak pressure, pulse energy, focal zone dimensions, and acoustic dose.
1. Scope and Clinical Significance
IEC 61846:1998 specifies measurement methods for the acoustic pressure pulse fields generated by extracorporeal lithotripters — medical devices that use focused shock waves to fragment kidney stones, ureteral stones, and biliary stones. The standard defines the key acoustical parameters that characterize the lithotripter output, including peak positive pressure (p+), peak negative pressure (p-), pulse energy, focal zone dimensions (defined by the -6 dB and -12 dB pressure contours), and the acoustic dose delivered to the target. These parameters are essential for predicting stone fragmentation efficacy, assessing the potential for tissue damage, and comparing the performance of different lithotripter designs. The standard covers all three principal lithotripter shock wave generation technologies: electrohydraulic (spark gap with ellipsoidal reflector), electromagnetic (coil and membrane), and piezoelectric (phased array of elements).
Clinical Context
IEC 61846 addresses measurement of lithotripter output only. Clinical efficacy, patient selection criteria, anesthesia protocols, and treatment outcome assessment are outside the scope. The standard provides the measurement framework that supports clinical studies and device regulatory approval by ensuring consistent characterization of lithotripter acoustic output.
IEC 61846 addresses measurement of lithotripter output only. Clinical efficacy, patient selection criteria, anesthesia protocols, and treatment outcome assessment are outside the scope. The standard provides the measurement framework that supports clinical studies and device regulatory approval by ensuring consistent characterization of lithotripter acoustic output.
2. Key Measurement Parameters
The standard defines a comprehensive set of parameters that characterize the acoustic pressure pulse produced by a lithotripter. These parameters are measured using a calibrated hydrophone positioned at the focal point of the lithotripter and at specific locations within the focal zone.
| Parameter | Symbol | Definition | Typical Range | Measurement Method |
|---|---|---|---|---|
| Peak positive pressure | p+ | Maximum positive pressure in the pulse | 20-150 MPa | Wideband hydrophone at focus |
| Peak negative pressure | p- | Maximum negative (tensile) pressure | 5-15 MPa | Wideband hydrophone at focus |
| Pulse rise time | tr | 10-90 % of leading positive edge | 5-200 ns | Deconvolved hydrophone signal |
| Pulse duration | td | Time enclosing ±50 % of pulse energy | 0.3-3 µs | Energy integration of squared pressure |
| Focal zone -6 dB length | Z(-6) | Axial extent where p+ ≥ 50 % of peak | 10-80 mm | Hydrophone scan along axis |
| Focal zone -6 dB width | R(-6) | Radial extent where p+ ≥ 50 % of peak | 2-15 mm | Hydrophone scan transverse to axis |
| Pulse energy | E | Acoustic energy per pulse at focus | 20-200 mJ | Integral of intensity over focal area |
| Total acoustic dose | D | Cumulative energy over treatment | 20-200 J | E × number of pulses |
2.1 Hydrophone Measurement Requirements
The accuracy of lithotripter characterization depends critically on the hydrophone measurement system. The standard specifies that the hydrophone must have a bandwidth of at least 30 MHz (extending to 100 MHz for the fastest rise-time pulses), a sensitive element diameter no larger than 1 mm (preferably 0.5 mm or smaller) to provide adequate spatial resolution, and a calibrated frequency response from 0.5 MHz to at least 20 MHz. The hydrophone must be able to withstand the high peak pressures without damage — needle-type PVDF hydrophones with a membrane construction are preferred for their robustness and wide bandwidth. The measurement system must include a digital oscilloscope with sampling rate of at least 200 MS/s and bandwidth of at least 100 MHz. The standard emphasizes the importance of proper hydrophone alignment, as a misalignment of 1 mm from the focal point can result in a 50 % or greater error in measured peak pressure.
2.2 Measurement Conditions and Water Tank Requirements
All measurements are performed in degassed water at 22 ± 3 °C, with dissolved oxygen content less than 4 mg/L. Degassing is critical because dissolved gas nucleates cavitation bubbles that scatter and attenuate the pressure pulse, reducing the measured pressure amplitude. The water tank must be of sufficient size to avoid reflections from the tank walls during the measurement window — typically a minimum of 1 m × 1 m × 1 m for clinical lithotripters. Sound-absorbing material should be placed on tank walls to reduce reflections. The hydrophone positioning system must provide three-axis positioning with accuracy of at least ±0.1 mm and the ability to scan through the focal region. The lithotripter must be operated at the same energy settings used clinically, with measurements taken at the beginning of operation (after 10 warm-up pulses) and after extended operation to characterize output stability.
Engineering Best Practice
For reliable lithotripter characterization, perform a focus alignment procedure before each measurement session using the lithotripter’s integrated imaging system (ultrasound or X-ray) to position the hydrophone at the acoustic focus. After alignment, fine-tune the hydrophone position by measuring the pressure at small increments (±0.5 mm steps) in all three axes to confirm that the maximum pressure is centered on the hydrophone. This xyz alignment scan should be repeated after every 100 measurement pulses to account for potential drift.
For reliable lithotripter characterization, perform a focus alignment procedure before each measurement session using the lithotripter’s integrated imaging system (ultrasound or X-ray) to position the hydrophone at the acoustic focus. After alignment, fine-tune the hydrophone position by measuring the pressure at small increments (±0.5 mm steps) in all three axes to confirm that the maximum pressure is centered on the hydrophone. This xyz alignment scan should be repeated after every 100 measurement pulses to account for potential drift.
3. Characterization and Reporting Requirements
The standard specifies the data processing methods for extracting the key characterization parameters from the raw hydrophone measurements and the format for reporting results. These procedures ensure that lithotripter specifications from different manufacturers and test laboratories are directly comparable.
| Reported Quantity | Data Source | Analysis Method | Presentation Format |
|---|---|---|---|
| Focus pressure waveform | Hydrophone at focal point | Ensemble average of 16-64 pulses | Pressure vs. time plot with p+, p-, tr, td |
| Focal zone axial profile | Hydrophone scan along beam axis | p+ vs. z position (peak values) | Normalized pressure vs. distance plot |
| Focal zone transverse profile | Hydrophone scan perpendicular to axis | p+ vs. x,y position (peak values) | Contour plot at -6 dB and -12 dB levels |
| Pulse energy distribution | Integrated pressure squared data | Numerical integration over focal plane | Energy per pulse in mJ |
| Output stability | Sequential pulses over operation | Mean and standard deviation of p+ | p+ vs. pulse number, CV % |
| Beam non-linearity | Frequency analysis of waveform | FFT, harmonic content up to 10th order | Frequency spectrum plot |
3.1 Data Processing and Deconvolution
The raw hydrophone signal must be deconvolved with the hydrophone’s frequency response to obtain the true pressure waveform. The standard specifies the deconvolution procedure, which uses the hydrophone’s calibrated sensitivity vs. frequency data to correct both amplitude and phase response. After deconvolution, the pressure waveform is analyzed to extract p+, p-, rise time (10-90 % of the leading positive edge), and pulse duration (time window containing 95 % of the pulse energy). The ensemble average of 16 to 64 consecutive pulses is recommended to reduce pulse-to-pulse variability inherent in shock wave generation, particularly for electrohydraulic lithotripters where spark gap erosion causes pulse amplitude fluctuations. The standard defines acceptance criteria for pulse-to-pulse stability: the coefficient of variation (CV) of p+ over 100 consecutive pulses should be less than 20 % for electrohydraulic types and less than 10 % for electromagnetic and piezoelectric types.
3.2 Focal Zone Characterization
The focal zone is characterized by scanning the hydrophone through the focal volume in three dimensions. The axial scan extends along the beam axis from 50 mm proximal to 50 mm distal to the focus, with a step size of 0.5 mm in the focal region and 2 mm outside. Transverse scans are performed in two orthogonal directions (x and y) through the focal point, extending to ±20 mm with 0.2 mm steps near the focus. The -6 dB focal zone is defined as the region where p+ exceeds 50 % of the peak value. The -12 dB zone is also reported for comparison. For asymmetric focal zones, the dimensions are reported separately for each axis. The focal zone dimensions directly affect clinical performance — a larger focal zone facilitates targeting but may cause more pain and tissue trauma, while a smaller focal zone concentrates energy more effectively but requires more precise stone positioning.
Critical Safety Consideration
The peak negative pressure (p-) is a critical safety parameter as it correlates with cavitation-induced tissue damage. Values of p- exceeding 10 MPa in water are associated with significant cavitation activity that can cause renal hemorrhage, petechiae, and tissue necrosis. The standard requires that p- be reported explicitly, and designers should aim to minimize p- while maintaining sufficient p+ for effective stone fragmentation. Modern lithotripters typically achieve p+/p- ratios of 10:1 to 15:1 for optimal efficacy-to-safety balance.
The peak negative pressure (p-) is a critical safety parameter as it correlates with cavitation-induced tissue damage. Values of p- exceeding 10 MPa in water are associated with significant cavitation activity that can cause renal hemorrhage, petechiae, and tissue necrosis. The standard requires that p- be reported explicitly, and designers should aim to minimize p- while maintaining sufficient p+ for effective stone fragmentation. Modern lithotripters typically achieve p+/p- ratios of 10:1 to 15:1 for optimal efficacy-to-safety balance.
4. Frequently Asked Questions
Q1: How does IEC 61846 relate to other lithotripsy standards?
IEC 61846 is the primary standard for lithotripter acoustic output measurement. It is complemented by IEC 60601-2-36, which specifies the safety and essential performance requirements for lithotripters as medical electrical equipment, including labeling requirements, output control, and protection against hazards. The two standards are used together for regulatory compliance.
Q2: Why is the water degassing requirement so important for lithotripter measurement?
Dissolved gas in water forms cavitation bubbles under the high tensile stress (p-) of the lithotripter pulse. These bubbles scatter the acoustic wave, reducing the measured pressure at the focus. Even a small increase in dissolved oxygen from 4 mg/L to 8 mg/L can reduce measured p+ by 20-30 %. Degassed water (below 4 mg/L dissolved oxygen) is essential for reproducible measurements that reflect the true lithotripter output.
Q3: What is the significance of the rise time parameter?
The rise time of the leading positive pressure edge is a key determinant of stone fragmentation efficiency. Faster rise times (shorter than 100 ns) produce stronger stress waves within the stone that propagate more effectively to the posterior stone surface, enhancing fragmentation through spallation. Slower rise times (greater than 200 ns) result in less efficient energy coupling to the stone. The rise time also affects the harmonic content of the pulse, with faster rise times producing more high-frequency components that are preferentially absorbed in tissue.
Q4: Can IEC 61846 measurements predict clinical stone fragmentation success?
The measured acoustical parameters provide important but not sufficient predictors of clinical outcome. Stone fragmentation depends not only on the acoustic pressure field but also on stone composition, size, location, the coupling efficiency between the lithotripter and the patient, and the number of pulses delivered. However, clinical studies have shown statistically significant correlations between p+ values measured according to IEC 61846 and stone-free rates, with studies suggesting that a p+ of at least 40-50 MPa at the stone location is required for reliable fragmentation.
