Lasers in Surgery and Medicine 12:482-489 (1992)
Comparison of Holmium and Flashlamp Pumped Dye Lasers for Use in Lithotripsy of Biliary Calculi Mitchell L. Spindel, BA, Ali Moslem, MSEE, Kuldip S. Bhatia, PhD, Bahaeddin Jassemnejad, PhD, Kenneth E. Bartels, DVM, Richard C. Powell, PhD, Ciaran M. O’Hare, MD, and Tim Tytle, MD Center for Laser Research (A.M., K.S.B., R.C.P.) and College of Veterinary Medicine (M.L.S., K.E.B.),Oklahoma State University, Stillwater, Oklahoma 74078; Department of Physics, University of Central Oklahoma (6.J.), Edmond, Oklahoma 73034; Department of Radiology, Oklahoma University Health Sciences Center (C.M.O., T.T ) , Oklahoma City, Oklahoma 74551
The characteristics of laser lithotripsy of biliary calculi are compared for a flashlamp pumped dye laser (A = 640 nm) and a Cr: Tm:Ho-YAG laser ( h = 2.1 pm). Data on fragmentation efficiency with respect to laser power and pulse repetition rate are presented for different types of stones. It is shown that both lasers can produce effective stone fragmentation. The laser power required for efficient fragmentation characteristics is significantly less for the visible wavelength laser. However, the problems associated with damage to the fiber tips of the delivery system during operation were found to be less with the near infrared wavelength. The laser power for efficient fragmentation with the dye laser varies significantly for different types of stones while the power for efficient fragmentation with the holmium laser is the same for all stones. o 1992 Wiley-Liss, Inc. Key words: cholecystectomy, fragmentation threshold, optimum laser power, threshold power
INTRODUCTION
the common bile duct, extracorporeal shockwave lithotripsy requires general anesthesia, expenGallstones remain a major medical problem sive technology, and extensive preliminary and in America today with approximately 500,000 follow-up examinations. Chemical dissolution of cholecystectomies being performed annually [1,21. To reduce hospital stay, the concept of lap- retained calculi is dependent on stone composition, is a lengthy procedure, and success is not aroscopic cholecystectomy, with or without use of guaranteed [4,51. a surgical laser, has been introduced. Since comLaser lithotripsy is a procedure in which lamon bile duct calculi complicate approximately ser energy is used to crack stones into fragments 10% of the cases, development of noninvasive prowhich are small enough to be aspirated or excedures t o remove common bile duct stones is estracted through a T-tube, or pass naturally sential [31. through a sphincterotomy. Pulsed flashlamp Endoscopic papillotomy and the extension of that method through ultrasonic or mechanical lithotripsy will deal with approximately 90% of primary bile duct stones, and T-tube extraction is possible for removal of retained bile duct stones. Accepted for publication May 7 , 1992. However, very large or impacted stones still re- Address reprint requests to Richard C. Powell, Center for quire surgical intervention [4].Although fairly Laser Research, 413 NRC, Oklahoma State University, Stillsuccessful in breaking up impacted stones within water, OK 74078. 0 1992 Wiley-Liss, Inc.
Comparison of Holmium and Dye Lasers
pumped dye lasers, Q-switched ND:YAG lasers, and excimer lasers have been reported to successfully fragment stones within the gallbladder and the common bile duct [61. Although successful, lithotripsy using these lasers is also dependent on stone composition [6,71. In addition, lasers currently used are large, expensive units and this has restricted their acceptance by many physicians and surgical units [81. A need still remains for a portable, inexpensive laser able to meet requirements for both selective lithotripsy and precise laparoscopic tissue dissection. It is known that the mechanism of stone fragmentation is essentially a two step process. Initially, the radiation absorbed by the target generates a plasma which during its expansion phase induces a shock wave causing fragmentation of the stone [9,10,111.There may be a small contribution due t o the superheated steam pressures inside the stone facilitating this fragmentation. The problem of developing the exact hydrodynamic model to explain all aspects of laser lithotripsy remains to be solved. However, one general conclusion drawn from previous studies clearly favors pulsed over continuous lasers. Medical laser systems from UV to IR have been tried for lithotripsy [12,131. The optimum system depends on the size, chemical composition, color and location of the calculi. The choice of optimum laser wavelength and duty cycle remains a controversial issue to be resolved by additional experiments. Most of the work reported thus far on laser lithotripsy has been performed with either a NdYAG laser at a wavelength of 1064 nm or a flash lamp pumped dye laser operating in the red region of the visible spectrum. The recent availability of Ho-YAG lasers operating at a wavelength of 2.1 pm and having a suitable optical fiber delivery system has provided a further impetus t o study laser lithotripsy of urinary and biliary calculi. The portability, ruggedness and ease of operation of the Ho-YAG laser system is an attractive feature for clinical applications, and the absorption of 2.1 p,m radiation in water is better than that of the Nd-YAG laser emission by several orders of magnitude 1141. This decreases the risk of accidental damage of the surrounding tissue through direct laser radiation which is an important consideration in the choice of a laser for lithotripsy. However tissue damage from shock waves produced by water absorption requires further investigations. For surgical procedures under visual control, infrared laser wavelengths al-
483
ways have the advantage over visible laser wavelengths due t o the available eye protection for the infrared that does not inhibit vision and the absence of extremely bright flashes of light associated with pulsed visible lasers. A comparative invitro study of biliary stone fragmentation under various operating parameters with a HoYAG and a flashlamp pumped dye laser tuned to 640 nm is reported in this paper. MATERIALS AND METHODS
A flash lamp pumped dye laser (Candela, Natick, MA Model LFDL-8LP) with a pulse duration of 10 ps was tuned to 640 nm using rhodamine 640 dye. Choice of wavelength was based on the conclusion from reflectivity measurements on tissue and biliary stones by previous workers [ E l . It was shown that at 640 nm, a maximum differential absorption existed between calculi and tissue. This laser operated either single-shot or at repetition rates between 1and 5 Hz. Also, we performed measurements at 504 nm, the clinically used wavelength. Given the uncertainty in the experimental results, for example, nonuniform composition of every stone sample, we did not observe any significant difference compared with the results from the 640 nm. Thus we chose 640nm because of its maximum differential absorption between calculi and tissue feature. The Cr:Tm:Ho:YAG laser (Sunrise Technology, Freemont, CA Model Surgical 210) used in the present study provided a fixed wavelength emission at 2.1 pm in 250 ps pulses with variable energy per pulse up to 1.0 J and pulse repetition rates of 5, 10, or 15 Hz. The optical fibers used for the laser delivery system were 320 km in core diameter and were made from low OH glass with glass cladding. Compared t o quartz fibers used in earlier studies, this fiber seemed less likely to be damaged by the range of powers used in this work [161. A detailed study of the optical properties of these specific fibers, including attenuation, bending losses and non-linear effects has been done and will be reported elsewhere. The distal end of a 3-4 m fiber was brought into contact with the stones inside a Segura-Dretler stone basket (Microvasive, Watertown, MA) immersed in a glass beaker containing 0.9% saline maintained at room temperature. Biliary Stones
Human gallstones supplied by the University of Oklahoma Health Sciences Center were
Spindel et al. distal end of each fiber was cleaved prior t o each measurement and checked by visual inspection of the He-Ne laser spot used as an aiming beam in these experiments. Distortion of the circular spot indicated defective cleaving or fiber damages caused by fragments blown from the stone surface. Laser power was measured before and after stone fragmentation using a power meter (Scientech Model 362). At least 10 stones of each type were used for each set of laser operating parameters. Since no published data on laser lithotripsy with the Ho-YAG laser is available, a wide range of laser pulse energies and repetition rates for this system operating on different types of stones selected randomly were investigated. To compare uL, the results of fragmentation, Student’s t-test (setting statistical significance at P< 0.05) was apBASKET plied t o the percentage weight of different group Fig. 1. Experimental configuration: 1,000 ml glass beaker, of fragments < 2.0 mm. This statistical analysis Segura-Dretler Stone basket, 320 Fm fiber and 0.9% saline. was used in the comparisons of all of the data described below.
484
thoroughly washed with water, dried for 24 hours and weighed before rehydration for 24 hours in physiologic saline. The stones were sorted into three groups according t o color. Black stones commonly known as “pigmented stones” and composed primarily of calcium bilirubinate are porous and easy to fragment. White or slightly tanned “cholesterol stones” composed of cholesterol monohydrate were the hardest to shatter. The third group, called “brown” or “mixed stones”, had a black and brown surface appearance and are a variable mixture of pigmented and cholesterol stones plus calcium salts, bile acids, bile pigments, fatty acids, proteins, and phospholipids [171. After ablation, the stone fragments were dried for 24 hours and separated using U S . standard brass sieves (Fisher Scientific, Springfield, NJ) with mesh diameters of 1.0 mm, 2.0 mm, 2.8 mm, and 4.0 mm. Each group of fragments was then weighed and percentages were calculated.
RESULTS AND DISCUSSION
There is no consistency in the literature regarding the definitions of terms and units of measurement used in laser lithotripsy studies. For the quantitative description of stone fragmentation, we have used the following units and definitions throughout this paper: Threshold Power
This is the minimum average power required to ablate the “tiniest” flake from the surface or to produce visible damage on the surface as seen through a 6 x magnifying lens. Fragmentation Efficiency or Fragmentation Rate
In the literature, fragmentation efficiencies are measured by measuring size distribution of stone fragments verses a fixed value of the dose, or by measuring the dose required to fragment a given stone mass so all pieces fall through a mesh basket of a standard size [18]. The “normalized Experiment residual mass” defined as stone fragments reA schematic diagram of the experimental maining in the basket after a fixed number of configuration is shown in Figure 1. A sample pulses has also been used for measuring fragmenstone was held in a Segura-Dretler basket inside tation rate [3]. Fragmentation efficiency has also a glass beaker submerged in physiologic saline to been defined as the relative size distribution of a depth of approximately 5 cm. The optical fiber fragments produced by a single pulse [191. Following another study we have used fragtip was brought into contact with the stone. For threshold power measurements, interaction of la- mentation efficiency as the energy required per ser radiation with the stone was visually moni- mass of stone in grams for obtaining fragments tored through a lighted 6 x magnifying lens. The d . 0 mm in size [ZO]. Total dose delivered to each
Comparison of Holmium and Dye Lasers 80
I
0114 J/cm2
70 -
270 J/cm2 450 J/an;?,
60 -
70
700 J/cm
50 -
T
485
,
i
05 H z
m
I
10 HZ 15Hr
1
1
40 30
~
20 -
10 -
0d
Comparison of Holmium and Flashlamp Pumped Dye Lasers for Use in Lithotripsy of Biliary Calculi Mitchell L. Spindel, BA, Ali Moslem, MSEE, Kuldip S. Bhatia, PhD, Bahaeddin Jassemnejad, PhD, Kenneth E. Bartels, DVM, Richard C. Powell, PhD, Ciaran M. O’Hare, MD, and Tim Tytle, MD Center for Laser Research (A.M., K.S.B., R.C.P.) and College of Veterinary Medicine (M.L.S., K.E.B.),Oklahoma State University, Stillwater, Oklahoma 74078; Department of Physics, University of Central Oklahoma (6.J.), Edmond, Oklahoma 73034; Department of Radiology, Oklahoma University Health Sciences Center (C.M.O., T.T ) , Oklahoma City, Oklahoma 74551
The characteristics of laser lithotripsy of biliary calculi are compared for a flashlamp pumped dye laser (A = 640 nm) and a Cr: Tm:Ho-YAG laser ( h = 2.1 pm). Data on fragmentation efficiency with respect to laser power and pulse repetition rate are presented for different types of stones. It is shown that both lasers can produce effective stone fragmentation. The laser power required for efficient fragmentation characteristics is significantly less for the visible wavelength laser. However, the problems associated with damage to the fiber tips of the delivery system during operation were found to be less with the near infrared wavelength. The laser power for efficient fragmentation with the dye laser varies significantly for different types of stones while the power for efficient fragmentation with the holmium laser is the same for all stones. o 1992 Wiley-Liss, Inc. Key words: cholecystectomy, fragmentation threshold, optimum laser power, threshold power
INTRODUCTION
the common bile duct, extracorporeal shockwave lithotripsy requires general anesthesia, expenGallstones remain a major medical problem sive technology, and extensive preliminary and in America today with approximately 500,000 follow-up examinations. Chemical dissolution of cholecystectomies being performed annually [1,21. To reduce hospital stay, the concept of lap- retained calculi is dependent on stone composition, is a lengthy procedure, and success is not aroscopic cholecystectomy, with or without use of guaranteed [4,51. a surgical laser, has been introduced. Since comLaser lithotripsy is a procedure in which lamon bile duct calculi complicate approximately ser energy is used to crack stones into fragments 10% of the cases, development of noninvasive prowhich are small enough to be aspirated or excedures t o remove common bile duct stones is estracted through a T-tube, or pass naturally sential [31. through a sphincterotomy. Pulsed flashlamp Endoscopic papillotomy and the extension of that method through ultrasonic or mechanical lithotripsy will deal with approximately 90% of primary bile duct stones, and T-tube extraction is possible for removal of retained bile duct stones. Accepted for publication May 7 , 1992. However, very large or impacted stones still re- Address reprint requests to Richard C. Powell, Center for quire surgical intervention [4].Although fairly Laser Research, 413 NRC, Oklahoma State University, Stillsuccessful in breaking up impacted stones within water, OK 74078. 0 1992 Wiley-Liss, Inc.
Comparison of Holmium and Dye Lasers
pumped dye lasers, Q-switched ND:YAG lasers, and excimer lasers have been reported to successfully fragment stones within the gallbladder and the common bile duct [61. Although successful, lithotripsy using these lasers is also dependent on stone composition [6,71. In addition, lasers currently used are large, expensive units and this has restricted their acceptance by many physicians and surgical units [81. A need still remains for a portable, inexpensive laser able to meet requirements for both selective lithotripsy and precise laparoscopic tissue dissection. It is known that the mechanism of stone fragmentation is essentially a two step process. Initially, the radiation absorbed by the target generates a plasma which during its expansion phase induces a shock wave causing fragmentation of the stone [9,10,111.There may be a small contribution due t o the superheated steam pressures inside the stone facilitating this fragmentation. The problem of developing the exact hydrodynamic model to explain all aspects of laser lithotripsy remains to be solved. However, one general conclusion drawn from previous studies clearly favors pulsed over continuous lasers. Medical laser systems from UV to IR have been tried for lithotripsy [12,131. The optimum system depends on the size, chemical composition, color and location of the calculi. The choice of optimum laser wavelength and duty cycle remains a controversial issue to be resolved by additional experiments. Most of the work reported thus far on laser lithotripsy has been performed with either a NdYAG laser at a wavelength of 1064 nm or a flash lamp pumped dye laser operating in the red region of the visible spectrum. The recent availability of Ho-YAG lasers operating at a wavelength of 2.1 pm and having a suitable optical fiber delivery system has provided a further impetus t o study laser lithotripsy of urinary and biliary calculi. The portability, ruggedness and ease of operation of the Ho-YAG laser system is an attractive feature for clinical applications, and the absorption of 2.1 p,m radiation in water is better than that of the Nd-YAG laser emission by several orders of magnitude 1141. This decreases the risk of accidental damage of the surrounding tissue through direct laser radiation which is an important consideration in the choice of a laser for lithotripsy. However tissue damage from shock waves produced by water absorption requires further investigations. For surgical procedures under visual control, infrared laser wavelengths al-
483
ways have the advantage over visible laser wavelengths due t o the available eye protection for the infrared that does not inhibit vision and the absence of extremely bright flashes of light associated with pulsed visible lasers. A comparative invitro study of biliary stone fragmentation under various operating parameters with a HoYAG and a flashlamp pumped dye laser tuned to 640 nm is reported in this paper. MATERIALS AND METHODS
A flash lamp pumped dye laser (Candela, Natick, MA Model LFDL-8LP) with a pulse duration of 10 ps was tuned to 640 nm using rhodamine 640 dye. Choice of wavelength was based on the conclusion from reflectivity measurements on tissue and biliary stones by previous workers [ E l . It was shown that at 640 nm, a maximum differential absorption existed between calculi and tissue. This laser operated either single-shot or at repetition rates between 1and 5 Hz. Also, we performed measurements at 504 nm, the clinically used wavelength. Given the uncertainty in the experimental results, for example, nonuniform composition of every stone sample, we did not observe any significant difference compared with the results from the 640 nm. Thus we chose 640nm because of its maximum differential absorption between calculi and tissue feature. The Cr:Tm:Ho:YAG laser (Sunrise Technology, Freemont, CA Model Surgical 210) used in the present study provided a fixed wavelength emission at 2.1 pm in 250 ps pulses with variable energy per pulse up to 1.0 J and pulse repetition rates of 5, 10, or 15 Hz. The optical fibers used for the laser delivery system were 320 km in core diameter and were made from low OH glass with glass cladding. Compared t o quartz fibers used in earlier studies, this fiber seemed less likely to be damaged by the range of powers used in this work [161. A detailed study of the optical properties of these specific fibers, including attenuation, bending losses and non-linear effects has been done and will be reported elsewhere. The distal end of a 3-4 m fiber was brought into contact with the stones inside a Segura-Dretler stone basket (Microvasive, Watertown, MA) immersed in a glass beaker containing 0.9% saline maintained at room temperature. Biliary Stones
Human gallstones supplied by the University of Oklahoma Health Sciences Center were
Spindel et al. distal end of each fiber was cleaved prior t o each measurement and checked by visual inspection of the He-Ne laser spot used as an aiming beam in these experiments. Distortion of the circular spot indicated defective cleaving or fiber damages caused by fragments blown from the stone surface. Laser power was measured before and after stone fragmentation using a power meter (Scientech Model 362). At least 10 stones of each type were used for each set of laser operating parameters. Since no published data on laser lithotripsy with the Ho-YAG laser is available, a wide range of laser pulse energies and repetition rates for this system operating on different types of stones selected randomly were investigated. To compare uL, the results of fragmentation, Student’s t-test (setting statistical significance at P< 0.05) was apBASKET plied t o the percentage weight of different group Fig. 1. Experimental configuration: 1,000 ml glass beaker, of fragments < 2.0 mm. This statistical analysis Segura-Dretler Stone basket, 320 Fm fiber and 0.9% saline. was used in the comparisons of all of the data described below.
484
thoroughly washed with water, dried for 24 hours and weighed before rehydration for 24 hours in physiologic saline. The stones were sorted into three groups according t o color. Black stones commonly known as “pigmented stones” and composed primarily of calcium bilirubinate are porous and easy to fragment. White or slightly tanned “cholesterol stones” composed of cholesterol monohydrate were the hardest to shatter. The third group, called “brown” or “mixed stones”, had a black and brown surface appearance and are a variable mixture of pigmented and cholesterol stones plus calcium salts, bile acids, bile pigments, fatty acids, proteins, and phospholipids [171. After ablation, the stone fragments were dried for 24 hours and separated using U S . standard brass sieves (Fisher Scientific, Springfield, NJ) with mesh diameters of 1.0 mm, 2.0 mm, 2.8 mm, and 4.0 mm. Each group of fragments was then weighed and percentages were calculated.
RESULTS AND DISCUSSION
There is no consistency in the literature regarding the definitions of terms and units of measurement used in laser lithotripsy studies. For the quantitative description of stone fragmentation, we have used the following units and definitions throughout this paper: Threshold Power
This is the minimum average power required to ablate the “tiniest” flake from the surface or to produce visible damage on the surface as seen through a 6 x magnifying lens. Fragmentation Efficiency or Fragmentation Rate
In the literature, fragmentation efficiencies are measured by measuring size distribution of stone fragments verses a fixed value of the dose, or by measuring the dose required to fragment a given stone mass so all pieces fall through a mesh basket of a standard size [18]. The “normalized Experiment residual mass” defined as stone fragments reA schematic diagram of the experimental maining in the basket after a fixed number of configuration is shown in Figure 1. A sample pulses has also been used for measuring fragmenstone was held in a Segura-Dretler basket inside tation rate [3]. Fragmentation efficiency has also a glass beaker submerged in physiologic saline to been defined as the relative size distribution of a depth of approximately 5 cm. The optical fiber fragments produced by a single pulse [191. Following another study we have used fragtip was brought into contact with the stone. For threshold power measurements, interaction of la- mentation efficiency as the energy required per ser radiation with the stone was visually moni- mass of stone in grams for obtaining fragments tored through a lighted 6 x magnifying lens. The d . 0 mm in size [ZO]. Total dose delivered to each
Comparison of Holmium and Dye Lasers 80
I
0114 J/cm2
70 -
270 J/cm2 450 J/an;?,
60 -
70
700 J/cm
50 -
T
485
,
i
05 H z
m
I
10 HZ 15Hr
1
1
40 30
~
20 -
10 -
0d
