Brian Saltzman MD. Beth IsraelDeaconess Medical Center Harvard Medical School, Boston, MA. and Josef Hochman, BSc. EE.Direx Systems Corporation, Natick, MA
American Urology Associaton Congress
Focal CrossSection, the Truncated Focal Area and the Truncated Volume:
Abstract
The Peak Pressure at F2 and the Focal Area are the traditional parameters used to compare the performance and effectiveness of the Shock Wave produced by different lithotripters.
Lately, new electromagnetic lithotripters were introduced, some with higher Focal Peak Pressure. This fact may lead to believe that they are more efficient than the traditional spark gap systems.
At the same time all electromagnetic systems have very thin focal areas, much smaller than the typical stone size, and therefore the available energy is not optimized for stone fragmentation, usually requiring much more shocks compared to a traditional Electrohydraulic lithotripter.
The Focal Cross Section at F2, the Truncated Focal Area and Volume are 3 new tools which allow a more accurate evaluation of the Shock Wave characteristics and efficiency of different lithotripters.
Eleven currently used lithotripters including the Dornier HM3 were compared:
The results show two categories of Lithotripters:
a) Large Focus: Dornier HM3, Medstone STST, Direx Tripter Compact and Medispec Econolith.
b) Small Focus: All electromagnetic lithotripters, plus Edap Praktis and the Healthronics Lithotron.
The Average Focal Cross Section for Large Focus lithotripters is 5 times bigger than the small ones.
The Average Truncated Area is 2.35 times bigger and the Average Truncated Volume is 5 times bigger in Big Focus Lithotripters compared to Small Focus ones.
This may help to explain why usually the electromagnetic lithotripters require much more shocks to break stones and have larger retreatment rates.
Introduction
Various lithotripters using different Shock Wave technologies are currently offered to treat stones in the urinary tract.
In order to compare the various systems offered, Urologists analyze their technical specifications to evaluate their performance. (Ref 1)
Traditionally the Peak Bar Pressure at F2 is the first parameter considered as an indicator of the available energy of a lithotripter and, therefore, has served as a first indicator of the efficiency of the system.
Some confusion existed in the past regarding the numerical value of this Pressure at F2.
Due to specific conditions of the Shock Waves, the measurements done with the older Piezoelectric Crystal sensors lead to erroneous high values of pressure (above thousand bars).
During the last years a new precise sensor made of a membrane of Poly Vinyl Duo Fluoride (PVDF) was developed and adopted by FDA as the only one to be used in Pressure measurements (Ref 2).
Using PVDF, the pressure values recorded are "smaller" compared to old Piezoelectric Crystal sensors, but obviously this is are more accurate and "real" values.
Recently, new Electromagnetic lithotripters were introduced some of them with higher Peak Bar Pressure. This fact leads one to believe that they are more efficient than the traditional spark gap systems.
Looking carefully we can see that this may be misleading.
A correct analysis requires one to look not only at the Peak Bar pressure but also at the focal area geometric dimensions.
All Electromagnetic systems have very thin focal areas  much smaller than the typical stone sizeand, therefore the available energy is not optimized for stone fragmentation.
The Total Focal Area which is also used sometimes to compare different lithotripters may be misleading too, because it does not take into account the fact that the typical stone size is much smaller than the long axis of the Focal ellipse and therefore a big portion of the energy is not applied to the stone.
In order to clarify this issue three new tools were developed and are presented:
a) The Focal Cross Section
b) The Truncated Ellipse Focal Area
c) The Truncated Focal Volume.
They will allow a more precise geometrical comparison of the Focal Areas of different lithotripters and hence their effectiveness.
Materials and Methods
Specifications of 11 currently used lithotripters were used from published references (Ref 1).
The distribution of pressure of a lithotripter is centered at the
focal point F2 and includes all points whose pressure is between 100% (
f2) and 50% of the Peak Power( 6 dB).
The shape of this focal volume is approximately an ellipsoid (a "cigar" or "watermelon" shape) (Ref 2).
This ellipsoid volume is obtained by rotating the focal ellipse around the long axis.
The geometric specifications of the focal area are the Long
Dimension (LD) and Short Dimension (SD) of the ellipse and ellipsoid.
The (a) Long radius and (b) Short Radius equal half of the previous
values respectively.
1) Focal Area Cross Section (FACS)
The easiest way of visualizing how much of the stone is subjected to
pressure is to look at the Cross Section of the ellipsoid at F2 (Like
"cutting" the ellipsoid/cigar at F2 and looking at the circle that
originated).
We can calculate the Cross Section area using the formula of the circle area

b = Focal Short Radius =SD/2
SD=Focal Short Dimension (diameter) 
2) Ellipse shape geometric function
3) Ellipse Focal Area (EFA)
Using the ellipse Long Radius a , and the Short Radius b, we can calculate the full Ellipse area using the formula
4) Truncated Ellipse Focal Area (TEFA)
Using the Long Radius a, and the Short Radius b, we can calculate the
Truncated Ellipse area using the formula5) Ellipsoid Focal Volume (EFV)
5) Ellipsoid Focal Volume (EFV)
6) Truncated Ellipsoid Focal Volume (TEFV)
Results
Calculations and graphs were made using the Excel (Microsoft) Program 1)
1) Focal Area Cross Section
Focal Cross Sections at F2


Circle #

Manufacturer

Model

Diameter

Cross Section




Short
Dimension 
Area 



SD(mm) 
(mm 2) 
1 
1 Dornier 
Doli S

5 
20

2 
Siemens

Lithostar 
5

20 
3 
Edap

Praktis 
5

20 
4 
Storz

Modulith 
6

28 
5 
Siemens

Modularis 
6

28 
6 
Dornier 
Compact Delta 
7.7 
47

7 
Healthronics 
Lithotron 
8

50 
8 
Medispec 
Econolith 
13

133

9 
Direx 
Tripter
Compact 
13.5 
143

10 
Medstone 
STST 
15

177

11 
Dornier 
HM3 
15 
177



Average Large Focal Areas
Standard Deviation
Average Small Focal Areas
Standard Deviation 

157



23



30



13


Small Focal Areas 
Large Focal Areas 
Table and Graph # 1 
2) The Truncated Ellipse Focal Area
The Graph below represents all focal ellipses and their truncation.
Truncation of Focal Ellipses 

Table and Graph # 2 
3) Truncated Ellipse Focal Area

Table and Graph # 3

4) Truncated Ellipsoid Focal Volume

Table and Graph #4 
Discussion
Analyzing the results shown in Tables #1 through #4, we may distinguish two categories of Lithotripters:
 Large Focus: 4 lithotripters are in this category:
Dornier HM3, Medstone STST, Direx Tripter Compact, and Medispec Econolith.
 Small
Focus: 7 Lithotripters are in this category:Storz Modulith, Dornier
Doli S, Dornier Compact Delta, Siemens Lithostar, Siemens Modularis
(All electromagnetic lithotripters), Edap Praktis, and the Healthronics
Lithotron (Spark Gap)

Cross Section 
Cross Section 
Truncated Area 
Truncated Area 
Truncated Volume 
Truncated Volume 

Average 
Standard Deviation 
Average 
Standard Deviation 
Average 
Standard Deviation 
a)Large Focus 
157

23 (15%) 
209 
16 (8%) 
2306 
343 (15%) 
b) Small Focus 
30 
13 (43%) 
89 
19 (21%) 
435 
191 (44%) 
Ratio a/b 
5.23 

2.35 

5.3 
 The 2 categories of Lithotripters are clearly
differentiated, the ration of their Cross Sections, Areas and Volumes
are between 2.35 and 5.3.
 The Large Focus
group is more homogeneous (Standard Deviation 8 to 15 %) , whereas the
Small Focus is less (Standard Deviation 21% to 44 %). This is due to
the fact that the Dornier Delta and Healthronics Lithotron have
relatively bigger dimensions than the rest of the group, but still far
form the Large Focus group.
 ALL Large Focus Lithotripters use the Spark Gap technology.
 ALL Electromagnetic units fall into the Small Focus category.
 Two Spark Gap units are also in the Small Focus category: Edap Praktis and Healthronics Lithotron.
The Edap Praktis, although basically a Spark Device, uses a variation of what is called the Electroconductive Technology.
The purpose of this technology is to reduce the pressure fluctuation
between shocks. In order to achieve this, the system uses a special
electrode in a highly conductive liquid, with a very small gap and as a
result, the focal volume is much smaller than conventional Spark Gap
devices.
It can be seen on Graph # 1, that the Large Focus Lithotripters will
" cover" most of the stone areas at F2 ( diameter 13 to 15 mm) whereas
the Small Focus ones will cover only a fraction of the typical stone.
This may explain why the electromagnetic devices typically require
significantly more shocks to adequately fragment kidney stones and also
may result in higher retreatment rates.
Recently, concerns have been raised ( Ref 5) regarding the fact that
some new Electromagnetic Lithotripters that have very small focal areas
and extremely high peak positive pressures are reporting higher
clinically significant hematoma rates of 3 to 12% (Ref 6,7 and 8). A
trend that is worrisome.
It is becoming clear that the electromagnetic devices with very long
and thin focal area/volumes are not suited to fragment stones.
The Truncated Areas and Volumes are intended to advance the discussion relative to the effectiveness of various lithotripters.
References
1. J. Stuart Wolf, Jr. M.D. Issues in choosing a Lithotriptor: Concepts in Design and use. AUA, 2001.
2. Lewin P.A. and Schafer M.E. "Shock Wave sensors: Requirements and
Design. J. Lithotripsy and Stone Disease vol. 3 pp 317, 1991.
3. IEC International Standard pressure Pulse LithotriptersCharacteristics of Fields. 1998 04 Annex C, page 21.
4. FDA Guidance for the Content of Premarket Notifications (510 k)
for Extracorporeal Shock Wave Lithotripters Indicated for the
Fragmentation of Kidney and Ureteral Calculi. August 9, 2000. Page 6.
5. 1st International Consultation on Stone Disease Committee 8:
Bioeffects and Physical Mechanisms of SW Effects in SWL. Chairman:
James E. Lingeman, M.D. et al.
6. Kohrmann KU, Rassweiler JJ, Manning M, et al. The clinical
introduction of a third generation lithotriptor Modulith SL 20. Journal
of Urology, 1995; 153:13791383.
7. Stefan T, Thorsten B, Chaussy C. Reduced retreatment rate by
anatomy related shockwave (SW) energy. Journal of Urology, 1998;
159:S34 (abstract).
8. Piper NY, Dalrymple N, Bishoff JT. Incidence of renal hematoma
formation after ESWL using the new Dornier DoliS lithotriptor. Journal
of Urology, 2001; 165:S377 (abstract).
