Exploring 2 µm Laser Sources for Lithotripsy

Kidney stones is a common disease in developed countries affecting around 10% of the population. Laser lithotripsy, predominantly via advanced ureteroscopy, has become a major technique for the minimally invasive surgical ablation of ureteral and kidney stones. The flashlamp-pumped solid-state Holmium:YAG laser has been the dominant technology in laser lithotripsy for over the past two decades. However, this mature technology has some fundamental limitations. Alternative technologies, such as Thulium Fiber lasers, Thulium:YAG lasers and Erbium:YAG lasers, have also been explored for laser lithotripsy.

What is laser lithotripsy?

Laser lithotripsy is a minimally invasive endoscopic technique (ureteroscopy) that uses a laser to fragment and remove stones from the urinary tract. Once the stone is localized in the bladder, ureter or kidney, an optical fiber is inserted into the working channel of the ureteroscope, and then the laser is activated to fragment the stone into smaller pieces. Stone fragmentation is primarily achieved by photothermal ablation. The direct absorption of the laser radiation leads to heat generation and subsequent melting and ablation of the stone. A secondary ablation mechanism results from the water absorption of the light. Water in pores rapidly vaporizes or expands creating a high localized pressure resulting also in ablation. In general, the surgeon removes the bigger pieces through the urethra with a small basket, and smaller pieces can be passed later with urination. However, the surgeon can proceed in a different way depending on the type of laser used and its parameters.

• Pulse energy:

The pulse energy available depends on the laser system used, but it can vary between 0.2 to 6.0 J, although typical settings during kidney stone ablation ranges from 0.2 to 2.0 J.

• Frequency or pulse rate

The typical frequency values range from 5 Hz to 80 Hz because of the technical constraints of the Holmium laser.

• Pulse duration

Typical pulse duration values for the Holmium lasers are between 150 and 500 µs.

• Laser wavelength

The main reason why Holmium:YAG lasers are currently the standard clinical lasers for lithotripsy is because its emission wavelength at 2120 nm. Light at this wavelength is strongly absorbed by the water contained within the stone pores and pockets leading to thermal expansion and vaporization of the water that results in enhanced ablation. In addition to this, Ho:YAG lasers has shown success ablation of a wide range of stone compositions.

The number of laser lithotripsy modes used for ablation of urinary stones have increased greatly in the recent years. However, these modes can be grouped into three main techniques:

• Fragmentation
• Dusting
• Popcorning

In fragmentation, the ablation of the stone into multiple pieces is achieved using a high pulse energy at a low pulse rate. The pieces with higher diameter (> 2 mm) are retrieved using a basket. In dusting, the kidney stone is broken into small pieces with a diameter of less than 1 mm, in which active basket retrieval is not needed. Another technique is known as popcorning, where the fiber is held fix in a place and a high pulse energy is used to create turbulent flow and iteratively ablate the stones into small pieces. Table 1 summarizes the typical pulse energy and pulse rate values for each of the laser lithotripsy modes.

Table 1. Most typical laser operation modes used with Holmium:YAG laser lithotripsy

What are the future trends of laser lithotripsy?

Solid-state Holmium:YAG (Ho:YAG) laser has become the most used laser for lithotripsy over the past decades. Its main benefits are its proven high success ratio in fragment many different stone types, and its relatively low cost for low power lasers. However, this technology has also its limitations and disadvantages. The most critical parameters are:

• Wavelength

The emission wavelength of Ho:YAG lasers do not match exactly the water peak absorption around 2 µm. Thulium fiber lasers have closer emission wavelength to the water absorption peak, but Monocrom offers the possibility to tune the emission wavelength by design to perfectly match the water peak absorption, which has been proven to result in a more efficient stone ablation.

• Pulse rate

The maximum pulse rate or pulse frequency is typically in between the range from 20-80 Hz. This limits the possible treatment strategies especially in dusting mode operation. The latest Ho:YAG laser systems offer 100-120 Hz in repetition rate, showing how the trend is to continue increasing this value.

What Monocrom offers is a direct diode system that is electronically pulsed to a frequency that can exceed 1000 Hz. On top of this, our solder-free laser bar mounting technology (Clamping^TM) is insensitive to the CTE mismatch between the semiconductor and the electrodes, allowing improved lifetime in hard pulsed applications.

• Average power

Similar trend is observed with the maximum average power with the latest high-power laser systems offering an output power of 120-140 W. Monocrom @FLEX laser achieves 105 W in average power, with the possibility to continue increasing this parameter in the next generation series.

• Wall-plug efficiency

The wall-plug efficiency of Ho:YAG lasers is rather low with values around 1-2%. The highest wall-plug efficiency has been shown by the thulium fiber laser, with maximum values being reported around 10%. A direct diode laser source  from Monocrom can achieve a maximum wall-plug efficiency of about 5%, which is at least 4 times what can be achieved by a Ho:YAG laser.

Another important aspect to be considered relates to the fact that Monocrom direct diode laser solution can be effectively used in CW and pulsed operation. This benefit in combination with our ability to perfectly match the emission wavelength with low and high temperature water peak absorption makes it an ideal choice to use also in treatments that require soft tissue ablation and/or coagulation such as Benign Prostatic Hyperplasia (BPH). Table 2 summarizes the advantages and disadvantages of Ho:YAG laser sources and direct diode laser sources offered by Monocrom at 2 µm. Table 3 gathers some technical specifications of the state-of-the-art laser technology used in lithotripsy (clinical and experimental) and Monocrom direct diode lasers offered for the same application.

Table 2. Advantages and disadvantages of holmiun:YAG laser and direct diode laser

Table 3. Specifications of low and high power, clinical and experimental 2 µm laser lithotripsy sources.

* Weight and dimensions for experimental 1908 nm Thulium fiber laser do not include separate recirculating chiller.
** Multiple manufacturer exist for the low and high power Holmium:YAG and Thulium:YAG lasers. These specifications are representative of the standard in the field.

How is Monocrom supporting your application?

Monocrom supplies fiber coupled laser modules in the wavelength range from 808 nm to 1060 nm but more important, from 2 µm to 3 µm as well. All our offerings are laser bar-based laser modules coupled into different fiber cores (optionally equipped with HP-SMA, D80 or QBH connectors).

The wavelength range between 2 µm – 3 µm is covered by our @FLEX Series where we provide ANY wavelength between 2 µm and 3 µm achieving up to 105 W output power at continuous wave (CW). So e.g., if your application needs 2520 nm but other laser technologies – like fiber or solid-state lasers – cannot offer this wavelength, we CAN!

The technology used in our @FLEX laser systems is a unique combination technique called “Rectified Polarization Beam Combining”. It is wavelength agnostic and allows to follow the moving gain of semiconductor laser diodes, which can be caused by injection current and p-n-junction temperature in- or decrease.

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