Wei-Qiang Shi

Effect of Varying Laser Parameters on Pulsed Ho:YAG Ablation of Bovine Knee Joint Tissues

We investigated the effect of varying laser parameters on ablation of fresh bovine knee joint tissues (fibrocartilage, hyaline cartilage, and bone) with a free-running Ho:YAG laser. Ablation rate was measured in saline for each tissue type as a function of laser fluence (160-950 J/cm2), pulse width (150-450 microseconds, full width at half maximum), and fiber core diameter (400 and 600 microns). A weight that was attached to the fiber end was used to exert a constant pressure of 40 g/mm2 to the tissue underneath to ensure a constant contact between the fiber tip and the tissue throughout the ablation process. All tissues could be efficiently ablated, and the ablation rate increased linearly with the fluence. Change in laser pulse width and fiber core diameter resulted in minor variations in the ablation rate for fibro- and hyaline cartilage. On the other hand, use of longer pulses and/or the larger fiber significantly accelerated bone ablation. Histology analysis revealed that zones of thermal damage in Ho:YAG irradiated bovine knee joint tissues differed by tissue types and ranged between 100 and 400 microns, but were independent of fluence, pulse width, and fiber size within the range tested.

Intraoperative metastases detection by laser-induced fluorescence spectroscopy

The authors studied the ability of Laser Induced Fluorescence Spectroscopy (LIFS) for the intraoperative identification of metastases using a photosensitizing agent Photofrin IIr to enhance spectroscopic detection. A He-Cd laser source (442 nm) was used to produce low-power illumination of tissue via a hand-held 400 micrometers fiberoptic probe. Through the same fiber, reflected and emitted light was returned to an optical multi-channel analyzer (OMA III) for analysis. Spectroscopic signals were displayed on a screen for immediate examination. Lobund Wistar rats, inoculated with Pollard rat adenocarcinoma cells, were used as an animal model. Photofrin IIr was administered intraperitoneal 24 or 48 hours prior to surgical exploration in doses varying from 0.75-7.5 mg/kg. Metastases detection was performed during abdominal exploration directed to ipsilateral and contralateral inguinal, iliac, para-aortic and renal lymph nodes. Nineteen tissue samples, identified as abnormal by LIFS, were removed for histologic analysis; 11 of these samples were larger than 5mm and histologic examination revealed malignancy in all cases. While LIFS signals showed malignancy in 8 tissue samples with dimensions less than 5mm, histology confirmed this in only 3. However, serial histologic sections were not performed. From the initial results, it was concluded that LIFS detection of malignant tissue is feasible and enhanced by the addition of Photofrin IIr. LIFS may be a promising technique for the intraoperative detection of primary malignant and metastatic tissue.

Comparative study of excimer and erbium:YAG lasers for ablation of structural components of the knee

This study was designed to compare the efficiency and thermal effect of a 135 ns pulsed-stretched XeCl excimer laser (308 nm) and a free-running Erbium:YAG laser (2940 nm) with 200 microsecond(s) pulse duration for ablation of knee joint structures (hyaline and fibrous cartilage, tendon and bone). The radiant exposure used for tissue ablation ranged from 2 to 15 J/cm2 for the XeCl excimer and from 33 to 120 J/cm2 for Er:YAG. The excimer and Er:YAG lasers were operated at 4 and 5 Hz respectively. The ablative laser energy was delivered to tissue through fibers. Ablation rates of soft tissues (hyaline and fibrous cartilage, tendon) varied from 8.5 to 203 micrometers /pulse for excimer and from 8.2 to 273 micrometers /pulse for Er:YAG lasers. Ablation rates of soft tissues are linearly dependent on the radiant exposure. Within the range of parameters tested all the tissues except the bone could be rapidly ablated by both lasers. Bone ablation was much less efficient, requiring 15 J/cm2 and 110 J/cm2 radiant exposure for excimer and Er:YAG lasers to ablate 9.5 and 8.2 micrometers tissue per pulse. However, excimer laser ablation produced less thermal damage in the tissues studied compared to Er:YAG at the same laser parameters. The authors conclude that both lasers are capable of efficient knee joint tissue ablation. XeCl excimer laser requires an order of magnitude less energy than Er:YAG laser for comparable tissue ablation.

Tissue effects of Ho:YAG laser with varying fluences and pulse widths

We investigated the effect of varying fluence and pulse width on the ablation rate and consequent thermal damage of the Ho:YAG (2.130 micrometers ) laser. The rate of ablation on fresh bovine knee joint tissues, fibrous cartilage, hyaline cartilage, and bone in saline was determined after varying the fluence (160 - 640 J/cm2) and pulse width (150, 250, 450 microsecond(s) ec, FWHM) at a repetition rate of 2 Hz. A 400/440 micrometers fiber was used. The ablation rate increased linearly with the fluence. In fibrocartilage, different pulse durations generated significant changes in the ablation rates, but showed minor effects on hyaline cartilage and bone. The heat of ablation for all three tissue types decreased after lengthening the pulse.

Effects of pulsed mid-IR lasers on bovine knee joint tissues

We investigated the effect of varying Tm:YAG (2.014 micrometers ) and Ho:YAG (2.130 micrometers ) laser parameters on ablation rate and consequent thermal damage. Mid-infrared wavelengths are strongly absorbed by most biological tissues due to the tissues high water content. The ablation rate of fresh bovine knee joint tissues (fibrous cartilage, hyaline cartilage, and bone) in saline was assessed as a function of radiant exposure (160 - 950 J/cm2), at pulse widths of 200 microsecond(s) ec for Tm:YAG and 250 microsecond(s) ec for Ho:YAG and a repetition rate of 2 Hz. All tissues used in this study could be efficiently ablated using two micron lasers. The mechanism of action is likely related to the formation and collapse of cavitation bubbles, associated with mid-infrared lasers. We concluded that the Tm:YAG and Ho:YAG lasers are capable of effective knee joint tissue ablation.

Effects Of Pulsed Mid-infrared Lasers On Bovine Knee Joint Tissues

We investigated the effect of varying laser parameters on ablation rate and consecutive thermal damage of Tm:YAG (2.014pm) and Ho:YAG (2.130pm) lasers. Mid-infrared wavelengths are strongly absorbed by most biological tissues due to high water content. The water vapor filled cavitation bubbles created and sustained by a mid-infrared laser pulse enables the laser beam to penetrate a depth far beyond the absorption depth in aqueous fluids and to reach remote targets (1,2). Collapse and rebound of cavitation bubbles are capable of generating high pressure and temperature transients (3). Ablation rate was defined as the ratio of number of laser pulses required to perforate a tissue sample to its thickness, micron/pulse(pm/p). The ablation rate of fresh bovine knee joint tissues (fibrous cartilage, hyaline cartilage, and bone) in saline was assessed as a function of radiant exposure (160-640 J/cm2), pulse width (150, 200, 250, 450usec, FWHM) and a repetition rate of 2Hz. The delivery system used consisted of a low-OH silica core/cladding diameters 400/440pm and 600/720pm, respectively. The laser fiber tip was held perpendicular to the tissue and weighted to exert a constant pressure of 40g/m2. The irradiated tissue sample6 were placed in formalin for subsequent histologic evaluation. The dimension of the thermal damage zone was evaluated by measurement of the damage area surrounding the craters with grid ruler eyepiece. All tissues used in this study could be efficiently ablated using two micron lasers (Table I & 11).

In-vitro fragmentation of biliary calculi with a 308-nm excimer laser

We report the use of a 308 mu XeC1 exciiuer laser for- biliary stone fragnientation. The 130 nsec laser pulses are delivered through tJV grade fused silica fibers to the target stones inmiersed in normal saline solution and placed in direct contact with the fiber. Sixty biliary calculi, 20 cholesterol and 40 pigment, were fragmented in vitro. The effect of laser repetition rate, energy fluence, and fiber core size on stone fragmentation was studied. Fragmentation thresholds for biliary calculi of different compositions were measured. It was found that higher fragmentation efficiency was obtained with larger fluence, lower repetition rate and fiber of larger core. Our study suggests that the long pulse 308 nm excimer laser may be an effective device for laser lithotripsy with low threshold and good efficiency for biliary stone fragmentation.

Optical Fiber Aided Monitoring Of Arterial Wall Ablation By Time Resolved Laser Induced Fluorescence

The primary goal of this study was to devise an Optical Fiber based monitoring scheme of the arterial wall ablation by the use of laser induced fluorescence. Preliminary results indicate that the time evolution of the fluorescent light follows complex exponential decays. A comparison of the time characteristics of the decays with the findings of other investigators suggests that these decays may be related to the ablation of the successive arterial wall main layers.

Effect of varying Nd:YAG laser fiber tips on porcine dermal tissue

We evaluated the effect of the 600 micrometers bare (flat tip) and 600/100 micrometers conical tip fibers on porcine skin. We compared their effect in both the continuous-wave (cw) and pulsed (P) modes (20 ms ON/OFF) at 20 W to that of the electrocautery at 100 W in the pure and blend 3 modes. On 11 farmer pigs, 6 cuts were made for each parameter combination. The samples were processed for histological evaluation. The cutting depth, cutting width, and total thermal damage (carbonization, coagulation and denaturation) were recorded for each parameter combination. The results show that the 600/100 micrometers conical fiber provides deeper resective capabilities than the 600 micrometers bare and electrocautery. In addition, the 600/100 micrometers conical tip fiber allows for narrower cuts with significantly less thermal necrosis when compared to the 600 micrometers bare and electrocautery. The results indicate that there is an advantage to using the sculptured tip fiber for creating incisions when compared to the bare fiber and electrocautery.

Laser effects in nonpigmented versus pigmented tissues using FiberTomeTM, Nd:YAG, and KTP

This study evaluated the effects of the FiberTomeTM (10, 20, 30 W), conventional Nd:YAG (10, 20, 30 W), and KTP (5, 10, 15 W) surgical laser systems, with respect to pigmented and nonpigmented tissues. The cutting width, cutting depth, and thermal damages were measured on the skin and liver of farmer pigs. The results show that there was no significant difference in the cutting width and depth, for nonpigmented versus pigmented tissues. KTP showed the most significant thermal damage reduction in pigmented tissue; 318 micrometers in nonpigmented vs 94 micrometers in pigmented tissue.