Thomas J. Pacala

Laser ablation of human atherosclerotic plaque without adjacent tissue injury

Seventy samples of human cadaver atherosclerotic aorta were irradiated in vitro using a 308 nm xenon chloride excimer laser. Energy per pulse, pulse duration and frequency were varied. For comparison, 60 segments were also irradiated with an argon ion and an Nd:YAG (neodymium:yttrium aluminum garnet) laser operated in the continuous mode. Tissue was fixed in formalin, sectioned and examined microscopically. The Nd:YAG and argon ion-irradiated tissue exhibited a central crater with irregular edges and concentric zones of thermal and blast injury. In contrast, the excimer laser-irradiated tissue had narrow deep incisions with minimal or no thermal injury. These preliminary experiments indicate that the excimer laser vaporizes tissue in a manner different from that of the continuous wave Nd:YAG or argon ion laser. The sharp incision margins and minimal damage to adjacent normal tissue suggest that the excimer laser is more desirable for general surgical and intravascular uses than are the conventionally used medical lasers.

Pulsed ultraviolet lasers and the potential for safe laser angioplasty.

Endoscopic laser ablation of atheroma using continuous wave lasers is limited by imprecise control of thermal ablation, resulting in a crater that expands in width and depth, with thermal damage to adjacent normal tissue. We compared the gross and histologic effects of pulsed 308 mm excimer irradiation to continuous-wave Nd:YAG and Argon Ion laser irradiation, and pulsed 1,060 nm, 532 nm, 355 nm, and 266 nm laser irradiation in 205 atherosclerotic aortic segments. In contrast to the continuous-wave Nd: YAG, Argon Ion, and pulsed 1,060 nm, 532 nm, and 355 nm laser irradiation, which produced gross and histologic evidence of uncontrolled ablation, the 308 nm and 266 nm pulsed lasers induced incisions that conformed precisely to the beam configuration without gross evidence of thermal injury. The incision edges from these two lasers were histologically smooth and comparable to a scalpel incision. Our histologic findings suggest that rapid, precise endoscopic ablation of vascular and nonvascular tissue can be performed at these shorter pulsed wavelengths with very high precision with relatively little damage or risk to adjacent tissue.

Effect of hematoporphyrin derivative and photodynamic therapy on atherosclerotic rabbits

This study was performed to demonstrate selective uptake of hematoporphyrin derivative (HPD) within actively developing atheroma, to localize the site of uptake of HPD within the atheroma, and to determine the potential for photodynamic therapy (PDT) of atherosclerosis in the rabbit model. Fifteen rabbits were rendered atherosclerotic. Five rabbits received neither HPD nor PDT and 2 rabbits received HPD, 10 mg/kg intravenously, without subsequent irradiation. Eight other rabbits received 5 to 20 mg of HPD intravenously and subsequent intravascular 636-nm laser radiation to either the thoracic aorta or the aortic arch. A total of 32 to 288 J of laser energy was delivered through a 300-mu quartz fiber. All rabbits that received in vivo HPD had red fluorescence of their aortas when placed under ultraviolet light. The pattern of fluorescence corresponded precisely to the pattern of atheroma. In segments that received PDT, light microscopic examination revealed an accumulation of smooth muscle cells at the intimal surface. Fluorescence microscopy revealed a diminishing concentration gradient of HPD from intimal surface layers towards the media. Assessment of treated thoracic aortic segments revealed quantitative and qualitative differences compared with control segments. In the arch-treated segments, however, no changes were seen. It is concluded that HPD localizes within rabbit atheroma, can be detected by fluorescence and is deposited in a diminishing concentration gradient from lumen toward media. Irradiation with 636-nm light may induce qualitative and quantitative changes in atheroma.

Ultranarrow linewidth, magnetically switched, long pulse, xenon chloride laser

A spectral linewidth of <7×10−4 A and diffraction‐limited beam divergence has been obtained from a long pulse, electric discharge xenon chloride laser with intracavity Fabry–Perot etalons. A gain duration of 100 ns provided for multipass operation of the etalons, significantly improving both contrast and finesse. The electrical discharge circuit required to produce this long gain duration was comprised of a pulse forming network, saturable inductor magnetic switch, and a tapered, constant impedance, interface transmission line.

Single longitudinal mode operation of an XeCl laser

Single longitudinal mode output from a directly tuned, long pulse, magnetically switched XeCl laser is reported for the first time. Mode control was achieved using spatial aperatures and intracavity etalons. The narrow bandwidth laser output was diffraction limited with an optical pulse width of 30 ns, which would yield a transform limited bandwidth of 30 MHz.

A wavelength scannable XeCl oscillator—ring amplifier laser system

A holographic grating at grazing angle of incidence was used to achieve tunable, narrow bandwidth (0.005 nm) operation of a XeCl oscillator for injection locking of a ring amplifier. The amplifier’s narrow bandwidth output energy was constant and equal to the untuned, broadband output (∼15 mJ) in regions where injection locking was achieved. Scanning was provided by use of a stepping motor‐driven differential micrometer on the tuning mirror. This system was used to produce a laser excitation spectrum of hydroxyl radicals (OH) in a flame.

Electric‐discharge‐pumped nitrogen ion laser

A preionized transverse electric‐discharge‐pumped N+2 charge‐transfer laser oscillating on the (B2Σ+u−X2Σ+g) transition of N+2 is reported. Peak power of 180 kW in a 8‐nsec FWHM laser pulse has been obtained with a 0.1% mixture of N2 in helium at a total pressure of 3 atm. The most intense laser oscillation occurs on the (0,1) vibrational transition at 427.8 nm.

New technological developments for the remote detection of atmospheric hydroxyl radicals

New developments in the areas of narrowband tunable excimer lasers, high-resolution high-rejection optical filters, and wavelength measurement devices are considered for application to the remote sensing of atmospheric hydroxyl radicals. The conclusion is that an increase in the SNR of 104 could readily be gained through the use of these new devices. Also, considerable reductions in size and electrical energy consumption could be realized.

Applications Of Excimer Lasers For Atmospheric Species Measurements

Excimer lasers are proven sources of high pulse energy and high average power coherent radiation at several wavelength regions throughout the ultraviolet spectrum. If designed properly, excimer lasers can be operated with both low divergence and narrow spectral band-width, and usingoan oscillator - injection-locked amplifier configuration they can be tuned over a 10 to 20 A bandwidth without appreciable loss in energy. Additional wavelength coverage from excimer lasers can be obtained by stimulated Raman scattering. With the development of magnetic switching for the high voltage, high power switches, the electric discharge exci-mer laser can also be operated reliably for extended periods of time at high pulse rates. All these features make excimer lasers ideal sources of ultraviolet radiation for a variety of remote and in-situ atmospheric species measurements.

Thyratron-switched, LC inverter, prepulse-sustainer, laser discharge circuit

A prepulse-sustainer discharge circuit for excitation of a xenon chloride laser is described. A single thyratron switch is used to initiate an electrical sequence which preionizes, prepulses and sustains the electric discharge in the laser. Important parameters within each of the circuits are calculated and the methods of implementation are given.