PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Measurement Parameters for the characterization of unfocused
Extracorporeal Pressure Pulse Sources - Standardization of
Biomedical Equipment
Friedrich Ueberle1
1

Hamburg University of Applied Sciences, Germany

ABSTRACT
Extracorporeal Pressure Pulse Sources (EPPS) are in regular clinical use to treat a growing variety of tissue
and pain therapy conditions. Since 1999, second-generation devices mostly using ballistic generation
methods are available, often claiming to provide “radial shock waves”. A Chinese national standard from2015 describes few measurement methods for ballistic EPPS, mainly overall energy and impulse of thesources, but no field parameters. Up to now, no international standard for the characterization of the signals
and the acoustic fields of EPPS is in place. Clinical trial reports of EPPS therapies reveal contradictory results,
which seems connected to a lack of knowledge on the physical EPPS signal and field parameters. Even worse,
measurements for the certification process are carried out according to the IEC standard 61846, which was
originally developed for the characterization of focused shockwave-lithotripters. Aside from lacking thecharacteristics of physical shock waves, many of those parameters are not applicable or even misleadingwhen used to characterize non-focused (“radial”) EPPS. As a result, there is a significant lack of knowledge
how to relate reported parameters to observed tissue and health effects and how to apply the devices properly
for optimum treatment results. This paper reports on the development of the international standard on acoustic
EPPS measurements, IEC 63045.
Keywords: Pressure Pulse, Biomedical Equipment, Standardization, Shockwave

1. INTRODUCTION
Extracorporeal Pressure Pulse Sources (EPPS) are in use in a variety of tissue and pain therapy
applications, e.g. for the treatment of shoulder, heel spur and elbow pain, erectile dysfunction, cardiac
pain, cellulitis etc. Today, a variety of second-generation devices, mostly using ballistic generation
methods and claiming to provide “radial shock waves”, is available. Nevertheless, a review of the
publications on the EPPS shows that neither the characteristics of the sound field nor the therapeutic
effects in biological tissue are fully understood. Furthermore, clinical trial reports of EPPS therapies
reveal contradictory results, mainly connected to a lack of knowledge on the physical properties ofthe pressure pulse EPPS field. As a proper measurement standard for unfocused EPPS devices is notin place, measurements for the certification process are carried out according to the IEC standard
61846, which was originally developed for the characterization of focused shockwave-lithotripters.
Many of the parameters standardized in IEC 61846 are not applicable or even misleading for thecharacterization of non-focused EPPS.
A Chinese national standard YY 0950-2015 [1] describes measurement methods for ballistic EPPS
driven by air pressure, and mainly overall energy and impulse of the sources, but no field parameters.
This paper reports on the development of IEC 63045 [2], the international standard on acoustic EPPS
measurements.

2. PARAMETERS IN IEC 63045
2.1 Background
The IEC technical committee TC87 has the scope to create measurement standards for ultrasonic1

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friedrich.ueberle@haw-hamburg.de

instrumentation. Thus, IEC 63045 is designed as a measurement standard, where the main scope is todescribe measurement parameters and methods how to measure them in a reliable and repeatable way.
It is not intended to serve as a safety standard for pressure pulse therapy, this task may be assigned byan international voting to a different technical committee, IEC SC62D, which is responsible for thecreation and maintenance of biomedical equipment safety standards.
During the drafting of 63045, concerns were raised that “too many parameters were defined, whichleads to a large number of required measurements”. It needs to be stressed that the intent of ameasurement standard is to state definitions and measurement methods for all technical parameterswhich might be useful to characterize the parameters of a device according to best technicalknowledge. This may also include requirements necessary to gain proper measurement results, e.g.
the definition of minimal technical specifications for measurement probes and instruments andmethods for calibrating those, material parameters and important construction details of test benches
and consumables (like using degassed water), and minimal requirements for proper technicaldocumentation of the results. In contrast, it is not intended to define which ones of the given
parameters need to be measured. These requirements can be given in appropriate safety standards, orthey need to be assessed during the development process of the device according to the rules ofnational and international regulations and standards, namely the medical device directives, usually
pointing to the IEC 601 series of safety standards. It is usual practice, that the devices need to beapproved by notified bodies and / or governmental authorities for the clinical use. The necessary steps
also include a risk management process, which considers all possible hazards for the patient and theuser. If no sufficient previous experience, e.g. documented by literature, is available, this risk
management process may also require clinical studies. During this process, all parameters necessary
to provide the safe use of the device needs to be stated by the manufacturers. IEC 63045 gives a firmbase of the internationally accepted technical state-of-the art.
2.2 Scope
IEC 63045 is applicable to therapy equipment using or producing extracorporeally induced nonfocused or weakly focused mechanical energy (pressure pulses), where the pressure pulses are
released as single events of duration up to25 microseconds.
The application to therapy equipment using focused pressure pulse sources and to extracorporeal
lithotripsy equipment (to which IEC 61846 is dedicated), or to therapy equipment using other acoustic
waveforms like physiotherapy equipment, low intensity ultrasound equipment and HIFU / HITU
equipment is not in the scope.
IEC 63045 specifies measurable parameters which should be used in the declaration of the acoustic
output of extracorporeal non-focused or weakly focused pressure pulse sources, and methods ofmeasurement and characterization of the pressure field generated by non-focused or weakly focusedpressure pulse equipment.
The parameters do not – at the present time – allow quantitative statements to be made about clinicalefficacy and possible hazard. Particularly it is not possible to make a statement about the limits forthese effects.
2.3 Pressure pulse sources
Pressure pulse sources were originally developed for the extracorporeally induced disintegration of
kidney stones. Since the first successful treatment in 1980 kidney lithotripsy is the standard treatment
for a wide variety of stones. The first pressure pulse source used the electrohydraulic principle, which
employs a spark gap, which is placed at one focus of an elliptical mirror. The stones to be treated areplaced in the other focus of this mirror by ultrasound or x-ray imaging. Since 1984, electromagnetic
sources are applied, which use a flat or cylindrical coil and a metal membrane, which is repelled by
the interaction of a pulsed primary magnetic field created by the coil, and a secondary field in themembrane, generated by the eddy current due to the primary field. The third type of pressure pulse
source employs large numbers of piezoelectric elements, which are arranged on a spherical surface,and emit an ultrasonic wave when excited by a high voltage impulse. In general, the first devices forpressure pulse (pain) therapy were smaller versions of the lithotripter sources, so the pressure pulse
fields were generally focused.
Since 1999, many devices on the market use the ballistic source technology, which is based on
accelerating a projectile (e.g. of 30 g mass) in a tube of e.g. 20…30 mm length towards the rear sideof a metallic block (e.g. …. g mass, typical diameter 6 to 15, up to 30 mm), which is called applicator[3] [4]. The other (front) side of the applicator is in contact with the patient. When the projectile hits1025

the rear side of the applicator, a longitudinal density wave propagates through the block and istransmitted at the front side into the patient. The manufacturers claimed that these sources weregenerating “radial” pressure pulses, thus implying that the devices were non-focused. Today, besides
the ballistic source type, there are also non-focused or weakly focused designs for the “classical”
electrohydraulic, electromagnetic or piezoelectric sources available.
2.4 Non-focused, Weakly Focused and Focused Fields
In this paper, only a few aspects shall be discussed in detail, starting with the definitions of focusing.
“In a non-focused pressure pulse field, the pressure pulse amplitude in the field is nowhere larger
than the peak-positive pressure pulse amplitude at any place at the source aperture, and the pressure
pulse amplitude is decreasing with increasing distance from the source aperture…” (Fig. 1a; 4a). “…ina weakly focused pressure pulse field, a local acoustic pressure maximum occurs, which has anamplitude less than the peak-positive pressure amplitude at the source aperture” (Fig. 1b,c; 4b,c).
“In a focused pressure pulse field, a beam pressure maximum point occurs, which is more distantfrom the source aperture than the beam pressure maximum -6 dB extent. In a focused pressure pulse
field, the pressure pulse amplitude adjacent to the beam pressure maximum is larger than the pressure
pulse amplitude at any place at the source aperture” [2].

Figure 1: a) unfocused field,b) c) weakly focused fields(drawings start 19 mm from applicator), source aperture at +20 mm

2.5 Applicator, Source Aperture, Measurement Center Point and Target Location
The concept of the source aperture enables the unambiguous application of all definitions of 63045,
independent from the physical nature and the mechanical construction of the source (e.g. ballistic,
electrohydraulic, electromagnetic, piezoelectric etc.).

Figure 2:

Left: Applicator, directly coupled to the patient. Right: Pressure pulse source, distant from the patient, 1 - source

aperture, 2 - source aperture width Dx, 3 - source aperture width, orthogonal Dy, 4 - measurement center point O where (x, y,z) = (0, 0, 0), 5 - beam axis (z), 6 - pressure pulse source, 7 - acoustic standoff, e.g. coupling pad or water [2]1026The source aperture is not necessarily the front of the applicator, it is the area where the wave
propagates into the patient. It is characterized by its width in both directions x- and y- perpendicular
to the beam axis z (Fig. 2). Depending on the design of the source, there may be a fluid filled spaceor another coupling means between the source emitting the pressure pulses and the source aperture.
The center-of-mass of the source aperture is the measurement center point, which is the origin of thecoordinate system used to describe positions inside the pressure pulse field. The z-axis isperpendicular to the source aperture at the measurement center point, x- and y- axes are orthogonal to
the z-axis.
For each intended therapy, the target location describes the location in space of the biological tissue
to be treated2. It differs depending on the therapy (e.g. superficial structures like tendons, or deeper
positions like trigger points etc.), but also on the size and body mass of the patient. The manufacturer
shall state the intended target locations as three-dimensional position vectors relative to themeasurement center point.
2.6 Pulse and Field Parameters
For the description of focused sources (IEC 61846) many field parameters were linked to the peak
positive pressure at the focus3. In a weakly focused field, there is a local maximum (called beampressure maximum) lower than the peak pressure at the source, which defines the beam pressure
maximum point. In the field of an unfocused source, the beam pressure maximum point is located at
the source aperture and is usually identical to the measurement center point.
In order to give the user reliable knowledge on the possible locations of intended and unintended
effects, both area and beam widths in x- and y- as well as beam extensions in the z- directions need
to be stated. Up today, the correlation between physical values (e.g. positive pressure pc , rarefactionalpressure p r, “energy flux density” or derived pulse intensity integral PII etc.) and the observed
biomedical effects is not commonly known. IEC 63045 calls upon manufacturers to define andcommunicate limits of regions by an appropriate risk assessment process, which may e.g. be based onliterature or own clinical studies. Limits can be defined based on absolute values (e.g. n MPa, n
mJ/mm² etc.) or relative values (e.g. –n dB relative to the beam pressure maximum etc.). These values
then are used to describe the boundaries and positions of relevant areas and volumes and calculate therespective beam extents, areas, or energies.
If a calculation requires time limits for integration, the same procedure for the definition of limit
values is used (positive temporal MPa threshold integration limits, total temporal MPa thresholdintegration limits etc.). Other parameters are based on commonly established scientific definitions,e.g. rise time (10%...90%).

Figure 3: Dry test bench, including: Hydrophone and tissue-mimicking pad chamber, hand-piece holder, rear holder including(adjustable) retention-force spring, are fixed to a base plate [2]2Usually the position of the target tissue’s center-of-mass if larger areas are treatedSince the first standard IEC 61846 on the measurement of pressure pulse parameters was published, aconstant point of discussion was the large uncertainty of those parameters directly linked to the peak focus
pressure. This uncertainty both is heavily influenced by source fluctuations (Pulse-to-pulse variation) andpositional errors of the focus determination.
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2.7 Test Benches and Measurement Conditions
Measurements of the pressure pulse temporal (Table 1) and field parameters (Tables 2, 3) mostly
require a water tank, filled with degassed water. A well-known problem of high intensity sources
measurements in water is the development of cavitation, which makes measurements at higher repeat
rates prone to error. As the EPPS devices for pain therapy are working at pulse repeat rates of morethan 2 to 20 per second, important parameters like the pulse-to-pulse stability cannot be measured
reliably in water. Therefore IEC 63045 also proposes a dry test bench for some measurements (Fig.
3). It can also be used as a quick measurement tool for quality control and service. With the dry test
bench, measurements on the beam axis at a fixed distance from the source aperture can be made at
higher pulse-repetition rates. Parameters which can be measured in the dry test bench are given in
Table 1, where Lp is the thickness of a silicon pad mimicking patient tissue. L p should be 5 mm, which
was chosen as typical measurement distance from the source aperture.
Table 1 – Temporal pressure pulse parameters [2] (Figure 6)
Water

Dry test bench

Unfoc.

WeakF

measurement

measurement

SourceSourceInstantaneous acoustic pressurep (x,y,z,t)p(0,0,Lp,t)

Fig. 6aFig. 6c

MPa,μspeak-positive acoustic pressure

pc(x,y,z)pc(0,0,Lp)

13

11

MPa

peak-negative acoustic pressure

pr(x,y,z)pr(0,0,Lp)

-11

-11

MPa

compressional pulse durationtFWHMpc(x,y,z)tFWHMpc(0,0,Lp)

0,74

0,66

μs

tr(x,y,z)tr(0,0,Lp)

0,46

0,52

μs

pulse-pressure-squared integral

ppsi(x,y,z,t)ppsi(0,0,Lp)

-

-

MPa²s

(derived) instantaneous intensity

pi(x,y,z,t)I(0,0,Lp,t)

Fig. 6aFig. 6c

mW/mm²derived pulse-intensity integral

PII(x,y)PII(x,y,Lp)

0,4

0,31

mJ/mm²

Parameterrise time

Unit

In the water tank, all pressure pulse (see Table 1) and field parameters (see Table 2) can be measured
at single pulse setting with sufficient pauses between pulses to allow for complete cavitation recess.
The integrated parameters relating to intensity at any position (x,y,z) are calculated from themeasured pressure-time signal p(t) The temporal limits T over which integration is performed shall be
stated and can be either the duration of the positive signal tPt,lim, the total signal duration tTot,lim (usedin the PII in Table 1), or signal durations delimited by n MPa threshold tPtnMPa,lim or tTot,nMPa,lim.
From these parameters the energy parameters in the field (Tables 2,3; Fig. 5) can be calculated.
Beam areas, volumes and extents are stated either as -n dB re. beam pressure maximum value at theindexed z position, or as isobar values based on n MPa limits. The choice of appropriate values for n
is left to the risk analysis of the manufacturer. Besides a required position of z = 5 mm, other z
positions can be chosen depending on different target locations.

3. APPLYING THE PARAMETERS
3.1 Materials and methods
In order to demonstrate how to derive the parameters from a practical measurement, a ballisticpressure pulse source was simulated using MatLab® and the k-Wave toolbox [5]. This allows thecreation of a hypothetical EPPS with a 13 mm diameter applicator, independent of actual
manufacturers. From the simulations, parameters were retrieved as from a real source. In order tomimic real measurements as close as possible, pressure-time signals were calculated at pre-defined
positions in the pressure pulse field, i.e. at 1 and 5 mm from the source aperture. As replacement forthe mechanical projectile, a velocity signal shaped as exponentially decaying sinusoid was generated
at the rear end of the applicator. During the simulation runs, the propagation of waves in the simulated
fluid can be observed as a video.
As weakly focused fields, strictly speaking, do not have a focus, but local maxima, there is anequivalent parameter “beam pressure maximum”, for weakly focusing sources only (Table 3)1028

Table 2 – Parameters for non-focused and weakly focused fields [2] (Figures 1a,1c,4a,4c,5)Parameter

Unfoc.

WeakF

SourceSource

162

88

mm²

~ 60

~ 60

mm

9720
14,4
10,5
14,4
10,5

4999
10,3
9,0
-

mm³

98,5

95

mm²

52

n.a.

mm

n MPa delimited n=6n MPa isobar delimited,largest extent, z=5, n=6n MPa isobar delimited,largest extent, z=5, n=2

5123

n.a.

mm³

11,2

11,0

mm

13,4

mm

w max,y,z,nMPa

n MPa isobar delim n=6

11,2

11,0

mm

ER

PII integrated overradius R (R=20, z=5)

12

14

mJ

Symbolbeam -n dB crosssectional area

A z,ndB

beam -n dB extent

z b,ndB

beam -n dB volume
beam -n dB width,
maximum
beam -n dB width,
orthogonal
beam isobar crosssectional area
beam isobar extent

V b,ndB

beam isobar volume
beam isobar width,
maximum
beam isobar width,
maximum
beam isobar width,
orthogonal
derived acoustic pulse
energy4

V b,nMPa

Comment-n dB re.beam pressure,n=6dB, z=5mm
maximum value at x=0(see Fig. 4)-n dB re. b.press n=6dB
largest -6dB z= 5mm:
largest -6dB z= 15mm:
orth. -6dB z= 5mm:
orth. -6dB z= 15mm:
n MPa delimited,
z=5, n=6n MPa delimited, n=6

w max,x,z,ndB
w max,y,z,ndB
A nMPa,zz be,nMPa

w max,x,z,nMPa
w max,x,z,nMPa

Unit

mm
mm

Table 3 – Additional parameters for weakly focused fields and equivalences to focused fields [2](Figures 1c, 4c, 5b)
Parameter weakly focused5

Weakly Foc.

Unit

Focused sources

beam pressure maximum point (bpm)beam pressure maximum … extent(here: only 9,8 MPa, see Fig. 4c)(0,0,17,5)

mm

focus

33,2

mm

focal extent

mm²

focal cross-sectional area

mm³

focal volume

beam pressure max… crosssectional area at6 z (here:z bpm=17,5mm)
beam pressure maximum … volume
beam pressure maximum … width,
maximum (here: zbpm=17,5mm)
bpm … width, orthogonal
derived beam –n dB pressure
maximum acoustic pulse energy7
E bpm,-ndB(here: n=-6dB)

4-6dB: 71
9,8MPa:27
6MPa: 64
2MPa: 230
9,8MPa:907-6dB: 9,5
9,8MPa:5,9
6MPa: 9,02MPa:17,1
Dto.

mm

focal width, maximummm

focal width, orthogonal

12,5

mJ

focal energy in FWHM

S is a circular cross-sectional area of radius R around a z position on the beam axis

5 The “…” stands for n MPa or –n dB
6
7(0,0,z) is the coordinate of the beam pressure maximum point on the beam axis
Derived from the beam pressure maximum –n dB cross-sectional area1029

4. RESULTS
Results of the simulations are shown in the Figures 1 and 2. Tables 1 to 3 contain the values derived
from the simulated measurements.

Figure 4: Axial distributions of simulated ballistic sources with 13 mm source aperturea) unfocused field, b), c) weakly focused fields (drawings start at applicator), source aperture (0,0,0) at +21 mma) unfocused field at z = Lp = 5 mm and 15 mm, pressure distributions, and PII and energy ER at z = 1 mm and z = Lp = 5 mmb) weakly focused field, z=Lp=5 and zbpm=17 mm, pressure and PII distributions and energy ER
Figure 5: Lateral distributions , z=Lp=5 mm and z2 mm from source aperture

Figure 6: Pressure signals on axis: a) unfocused source signal at z = Lp = 5 mm; b) unfocused source signal atc) weakly focused source signal at z = Lp = 5 mm distance from source aperture

Figure 7: Measured pressure pulse signal of an unfocused EPPS, at z = Lp = 5 mm (from [4])1030

z = 15 mm;

5. CONCLUSIONSThe standard IEC 63045 is intended to cover today’s state-of-scientific measurement methods andparameters for unfocused and weakly focused pressure pulse sources. It could be shown by theapplication to simulated sources, that the parameters can be applied both to unfocused and to weakly
focused EPPS sources.
The simulation of the sources with k-Wave was chosen to check the applicability of the draft standard
IEC 63045, independent from any commercial products. Figure 7 proves the close similarity of thesimulated pressure pulse p(t) signals to measured signals at 5 mm from the source aperture from a 15mm diameter ballistic EPPS [4] with three different types of hydrophones: fiberoptic, light spot andpiezoelectric.
Once introduced, IEC 63045 will likely serve as a flexible basis for reliable, traceable reporting of
the properties and fields of biomedical pressure pulse equipment to notified bodies, customers, users
and researchers. It can be expected that, based on these parameters, the knowledge about the links andcorrelations between physical parameters, and the observed biomedical effects can be further clarified.
This will lead to more dependable and reproducible therapy results and further new, efficient
applications of pressure pulses.

ACKNOWLEDGEMENTS
The author wishes to thank all members and observers of IEC TC87 and the ISMST technicalworking group for the fruitful discussions and comments, and particularly my colleagues in Working
Group 7 (Ultrasound surgical instruments) for the vast amount of work on the standard. More thanks
go to my students and assistants at the HAW Hamburg for supporting this work by projects and theses.

6. REFERENCES[1] Chinese Standardization Authority, „YY-0950-2015: Extracorporeal pressure wave devices bycompressed air,“ www.ChineseStandard.net, China, 2015.
[2] IEC, 63045 (CD2), Geneva: IEC, 2019.
[3] F. Ueberle und A. Rad, „Characterization of Unfocused / Weakly Focused Pressure Pulse Sources for
Extracorporeal Pain Therapy ("Radial Shock Wave Therapy" Sources),“ Biomed Tech (Berl).58 Suppl
1. , Aug 2013 .
[4] F. Ueberle und A. Rad, „Unfocused/Weakly Focused Pressure Pulse Sources for Pain Therapy:
Measurements in Water and in a Dry Test Bench,“ ACTA PHYSICA POLONICA Vol. 127 No.1, pp. 135137, 2015.
[5] C. B. T. Treeby B.E., „k-Wave: MATLAB toolbox for the simulation and reconstruction of photoacoustic
wave-fields,“ J. Biomed. Opt., vol. 15, no. 2, p. p. 021314, 2010.
[6] B. T. C. B. E. Treeby, „k-Wave: MATLAB toolbox for the simulation and reconstruction of photoacoustic
wave-fields,“ 2010. [Online]. Available: https://www.k-Wave.org.

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