Xing Y et al, 2017: Comparison of Broad vs Narrow Focal Width Lithotripter Fields.
Xing Y, Chen TT, Simmons WN, Sankin G, Cocks FH, Lipkin ME, Preminger GM, Zhong P.
Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina.
School of Medicine, Duke University, Durham, North Carolina.
Comprehensive Kidney Stone Center/Urologic Surgery, Duke University, Durham, North Carolina.
OBJECTIVE: To investigate the impact of lithotripter focal width on stone fragmentation.
MATERIALS AND METHODS: A modified reflector was used to reduce -6 dB beam size of the HM3 lithotripter, while increasing concomitantly peak pressure. Fragmentation in vitro was assessed with modified and original reflectors using BegoStone phantoms. A membrane holder was used to mimic lithotripsy in vivo, and a matrix holder was used to assess variations of fragmentation power in the focal plane of the lithotripter field. Stone fragmentation in vivo produced by the two reflectors was further compared in a swine model.
RESULTS: Stone fragmentation in vitro after 500 (or 2000) shocks was ∼60% (or ∼82%) vs ∼40% (or ∼75%) with original and modified reflector, respectively (p ≤ 0.0016). Fragmentation power with the modified reflector was the highest on the lithotripter axis, but dropped rapidly in the lateral direction and became insignificant at radial distances >6.0 mm. Stone fragmentation with the original reflector was lower along the lithotripter axis, but fragmentation power decayed slowly in lateral direction, with appreciable fragmentation produced at 6.0 mm. Stone fragmentation efficiency in vivo after 500 (or 2000) shocks was ∼70% (or ∼90%) vs ∼45% (or ∼80%) with original and modified reflector, respectively (p ≤ 0.04).
CONCLUSIONS: A lithotripter field with broad beam size yields superior stone comminution when compared with narrow beam size under comparable effective acoustic pulse energy both in vivo and in vitro. These findings may facilitate future improvements in lithotripter design to maximize comminution efficiency while minimizing tissue injury.
J Endourol. 2017 May;31(5):502-509. doi: 10.1089/end.2016.0560. Epub 2017 Apr 21. FREE ARTICLE
A very interesting paper produced by a group with long standing experience in research on ESWL. The interested reader should get a reprint.
A few results are not clear, a few were to be expected and a few are difficult to judge on:
Why were 500 shots used in a setting leading to a fragmentation rate of artificial stones of only 60% with the original HM3 source or 40% with the modified HM3 source? What is the experimental or clinical relevance of this setting? Significance of differences for 500 shots (original vs. modified source) was 0,002 and for 2000 shots 0,016.
What is meant by fragmentation rate: % of stones fragmented or % of fragments?
The fragmentation rate within the focal axis of the modified source was 40 % more efficient than that of the original source. It was to be expected that the stone fragmentation power of the broad original source was still 0,1% of the ”normalized power” at 6 mm distance from the focal axis and that of the modified source was close to only 0 %.
Obviously the power of the original source was not stable with a mean normalized fragmentation power of only 0,6 compared to that of the modified source (1,0) and a standard variations of between appr. 0,45 and 0,75. But why were only 350 shocks used in this setting? And why did the authors chose ”to represent an average effect produced at 20 kV” by combining the results of 18 + 22 kV in the form of mean + standard deviation instead of showing the results of the two groups separately?
Parts of the in vivo experiments were performed with an insufficient number of 500 shots. With 2000 shots the difference was marginal.
Like in many other studies done with the old HM3 these results cannot be transferred to other lithotripters and the comparative discussion is not possible or justifiable.
A good example of the problem to compare different lithotripters are the comparative experimental studies on large and small focus lithotripters ( Connors BA, et al. Evaluation of shock wave lithotripsy injury in the pig using a narrow focal zone lithotriptor. BJU Int 2012;110:1376–1385) showing atraumatic performances in animal experiments and a final clinical paper published by Bhojani et al. (Bhojani N et al. Lithotripter outcomes in a community practice setting: comparison of an electromagnetic and an electrohydraulic lithotripter. J Urol. 2015 Mar; 193(3):875-9. doi: 10.1016/j.juro.2014.09.117.) showing no clinical problems and, if any difference at all, slight advantage for the small focus.
Many factors determine the performance of a lithotripter. Positioning of stones, quality of coupling, energy settings, treatment strategies, skills of the operator etc. are decisive amongst others to achieve superior treatment results. The authors of this paper focus on the technical feature of focal size. They compare the unmodified HM3 electro-hydraulic lithotripter with a modification on the same lithotripter. An insert with a slightly modified elliptical surface contour is used in order to generate a modified focus with smaller dimensions while keeping all other parameters such as shock wave generation, primary energy settings etc. unmodified.
Their conclusion is:
A lithotripter field with broad beam size yields superior stone comminution when compared with
narrow beam size under comparable effective acoustic pulse energy both in vivo and in vitro. These findings may facilitate future improvements in lithotripter design to maximize comminution efficiency while minimizing tissue injury.
Does this paper prove the conclusion?
Some questions and comments may be allowed:
1. It is not clear, why the insert produces a narrow beam focus with higher peak pressure compared to the unmodified HM3 ellipsoid. For physical reasons (smaller aperture and splitting of the reflector surface), we expect a broader focus with lower peak pressure and less power due to energy dissipation by additional deflecting structures. Accordingly, the comparison would turn around with a broad focus at the modified (with insert) vs. a narrow focus at the unmodified (without insert) ellipsoid.
Under these assumptions, a reduction of fragmentation efficiency of the modified ellipsoid is expected. This is in agreement with another publication of the same group:
Suppression of large intraluminal bubble expansion in shock wave lithotripsy without compromising stone comminution:
Refinement of reflector geometry
Yufeng Zhou and Pei Zhong, J. Acoust. Soc. Am. 110 (6), December 2001
The same insert was used in an “upgraded” reflector leading to the conclusion:
In vitro phantom tests have shown that stone comminution produced by the upgraded reflector at 24 kV is comparable to that produced by the original HM-3 reflector at 20 kV. However, the upgraded reflector significantly reduces the potential for vascular injury.
This seems to contradict the conclusion of the current paper (see above).
Our guess: The “upgraded” reflector is the one with a broader focus requiring significant more energy (24kV vs. 20kV) to achieve fragmentation efficiency comparable with the original HM3 reflector.
2. Why does this paper report a peak pressure of ~80 MPa at 20 kV energy level for the upgraded reflector compared to ~50 MPa of the original HM3 when a similar insert (published in the previous paper) requires 24 kV for the upgraded reflector to come close to 50 MPa of the original HM3 reflector (see graphs from the previous paper below). It seems that the data were mixed up and show the opposite of claimed result, namely that the narrower focus of the original HM3 turns out more effective, because the upgraded reflector featured a broader focus than the original HM3 reflector.
3 . The presented pressure wave form of the original HM3 has a double peak structure not known from other HM3 papers and other electro-hydraulic lithotripters. Is it a unique feature of the device under investigation by the group?
4. Instead, due to the insert, the modified reflector would be expected to feature a double peak structure and not the single peak as displayed. Is there any reason for this unexpected clear single peak? The stepped reflector surface is expected to shift the upper part of the reflected wave that arrives a couple of microseconds earlier at the focal point, thereby a double peak structure would appear.
5. Manipulation of the ideal focussing geometry to reduce precision of focussing usually reduces the amount of energy collected at a focal area and requires additional power to reach equivalent fragmentation. This was published by the group itself in another paper modifying an acoustic lens in order to generate a broader focus. The driving power had to be increased from 13 – 16 kV to 16 – 19 kV.
Improving the lens design and performance of a contemporary electromagnetic shock wave lithotripter Andreas Neisiusa,b,1, Nathan B. Smithc,1, Georgy Sankinc, Nicholas John Kuntza, John Francis Maddend, Daniel E. Fovarguee, Sorin Mitrane, Michael Eric Lipkina, Walter Neal Simmonsc, Glenn M. Premingera, and Pei Zhonga,c,2 “new lens must operate at a higher source voltage (i.e., 16–19 kV) than the one used for the original lens (i.e., 13–16 kV) to compensate for the acoustic energy loss caused by destructive in situ pulse superposition”
6. The type of stone holder used in this paper which allows fragments to spread out laterally does not reflect clinical conditions. Usually stones are trapped in kidney calices, in the pelvis or in the ureter and don`t have ample space to spread.
7. Obviously, a smaller focus requires careful targeting which is not always easy in clinical routine. The target position should be fixed at the longer lasting expiration phase of the patient in order to catch a longer time period of the stone in the target position.
For this and other reasons the conclusion of the paper is not convincing.
Our conclusion is:
The performance of a lithotripter depends on multiple factors. To restrict it to the focal size is not sufficient. A narrow focus – at least – has an advantage to deliver the energy for fragmentation to a confined area around the target stone and avoid critical energy levels anywhere else. This helps to reduce pain and tissue lesions in remote areas while providing sufficient fragmentation efficiency only at the location of interest, the target stone.