Q-switched (QS) Tm:YAG laser ablation mechanisms on urinary calculi are still unclear to researchers. Here, dependence of water content in calculus phantom on calculus ablation performance was investigated. White gypsum cement was used as a calculus phantom model. The calculus phantoms were ablated by a total 3-J laser pulse exposure (20 mJ, 100 Hz, 1.5 s) and contact mode with N=15 sample size. Ablation volume was obtained on average 0.079, 0.122, and 0.391 mm3 in dry calculus in air, wet calculus in air, and wet calculus in-water groups, respectively. There were three proposed ablation mechanisms that could explain the effect of water content in calculus phantom on calculus ablation performance, including shock wave due to laser pulse injection and bubble collapse, spallation, and microexplosion. Increased absorption coefficient of wet calculus can cause stronger spallation process compared with that caused by dry calculus; as a result, higher calculus ablation was observed in both wet calculus in air and wet calculus in water. The test result also indicates that the shock waves generated by short laser pulse under the in-water condition have great impact on the ablation volume by Tm:YAG QS laser.
Calculus migration is a common problem during ureteroscopic laser lithotripsy procedure to treat urolithiasis. A conventional experimental method to characterize calculus migration utilized a hosting container (e.g. a “V” grove or a test tube). These methods, however, demonstrated large variation and poor detectability, possibly attributing to friction between the calculus and the container on which the calculus was situated. In this study, calculus migration was investigated using a pendulum model suspended under water to eliminate the aforementioned friction. A high speed camera was used to study the movement of the calculus which covered zero order (displacement), 1st order (speed) and 2nd order (acceleration). A commercialized, pulsed Ho:YAG laser at 2.1 um, 365-um core fiber, and calculus phantom (Plaster of Paris, 10×10×10mm cube) were utilized to mimic laser lithotripsy procedure. The phantom was hung on a stainless steel bar and irradiated by the laser at 0.5, 1.0 and 1.5J energy per pulse at 10Hz for 1 second (i.e., 5, 10, and 15W). Movement of the phantom was recorded by a high-speed camera with a frame rate of 10,000 FPS. Maximum displacement was 1.25±0.10, 3.01±0.52, and 4.37±0.58 mm for 0.5, 1, and 1.5J energy per pulse, respectively. Using the same laser power, the conventional method showed <0.5 mm total displacement. When reducing the phantom size to 5×5×5mm (1/8 in volume), the displacement was very inconsistent. The results suggested that using the pendulum model to eliminate the friction improved sensitivity and repeatability of the experiment. Detailed investigation on calculus movement and other causes of experimental variation will be conducted as a future study.
Q-switched Tm:YAG laser ablation mechanisms on urinary calculi are still unclear to researchers. In this study, dependence of water content in calculus phantom on calculus ablation performance was investigated. White gypsum cement was used as a calculus phantom model. The calculus phantoms were ablated at single pulse and contact mode in three different conditions: dry calculus in air, wet calculus in air, and wet calculus in water. Ablation volume was obtained on average 0.006, 0.008, and 0.008 mm3 in dry calculus in air, wet calculus in air, and wet calculus in water groups, respectively. There were three proposed ablation mechanisms that could explain the effect of water content in calculus phantom on calculus ablation performance, including shock wave due to bubble collapse, spallation, and microexplosion. Shock wave generation due to bubble collapse in wet calculus in water condition had negligible effect on calculus ablation as captured by a needle hydrophone and cannot be a primary mechanism for calculus ablation in this study. Increased absorption coefficient of wet calculus can cause stronger spallation process compared with that caused by dry calculus; and as a result, higher calculus ablation was observed in both wet calculus in air and wet calculus in water. Vaporization of interstitial water in porous calculus phantom can also help enhance calculus ablation efficiency. There were some limitations in this study including use of small sample size and lack of employing real urinary calculus, which should be addressed in future experiment.
Photoselective vaporization of the prostate (PVP) is considered a minimally invasive procedure to treat benign prostatic
hyperplasia (BPH). During the PVP, the prostate gland is irradiated by the 532-nm laser and the fiber is swept and
dragged along the urethra. In this study the speed of sweeping fiber during the PVP is being investigated. In vitro
porcine kidney model was used (N=100) throughout the experiment. A Q-switched 532-nm laser, equipped with sidefiring
750-Um fiber, was employed and set to power levels of 120 and 180 W. The speed of fiber sweeping was the only
variable in this study and varied at 0 (i.e. no sweeping), 0.5, 1.0, 1.5, and 2.0 sweep/s. Ablation rate, depth, and
coagulation thickness were quantified. Based on the current settings, ablation rate decreased as sweeping speed increased
and was maximized between 0 to 1.0 sweep/s for 120-W power level and between 0 to 0.5 sweep/s for 180-W power
level. Ablation rate at 180 W was higher than that at 120 W, regardless of sweeping speed. Ablation depth at both 120
and 180 W was maximized at 0 sweep/s and decreased 35% at 0.5 sweep/s. The overall coagulation thickness was less
than 1.5 mm and comparable from 0 to 1.5 sweep/s (0.8~0.9 mm) and increased at 2.0 sweep/s (~1.1 mm). This study
demonstrated that tissue ablation performance was contingent upon sweeping speed and maximized at slow sweeping
speed due to longer laser-tissue interaction time and larger area coverage by the 532-nm light.
Photoselective vaporization of the prostate (PVP) has been widely used to treat benign prostatic hyperplasia (BPH). It is
well regarded as a safe and minimally invasive procedure and an alternative to the gold standard transurethral resection
of the prostate (TURP). Despite of its greatness, as well aware of, the operative procedure time during the PVP is still
prolonged. Such attempts have been tried out in order to shorten the operative time and increase its efficacy. However,
scientific study to investigate techniques used during the PVP is still lacking. The objective of this study is to investigate
how sweeping angle might affect the PVP performance. Porcine kidneys acquired from a local grocery store were used
(N=140). A Q-switched 532-nm GreenLight XPSTM (American Medical Systems, Inc., MN, USA), together with 750-
μm core MoXyTM fiber, was set to have power levels of 120 W and 180 W. Treatment speed and sweeping speed were
fixed at 2 mm/s and 0.5 sweep/s, respectively. Sweeping angles were varied from 0 (no sweeping motion) to 120 degree.
Ablation rate, depth, and coagulation zone were measured and quantified. Tissue ablation rate was peaked at 15 and 30
degree for both 120- and 180-W power levels and dramatically decreased beyond 60 degree. At 180 W, ablation rate
increased 20% at 30 degree compared to 0 degree. This study demonstrated that ablation rate could be maximized and
was contingent upon sweeping angle.