Optoacoustic tomography: time-gated measurement of pressure distributions and image reconstruction.

Optoacoustic imaging is a potential novel medical imaging technology to image structures in turbid media to depths of several millimeters with a resolution of some tens of micrometers. Thereby short laser pulses generate thermoelastic pressure waves inside a tissue, which are detected on the surface with a wideband ultrasonic transducer. Image reconstruction has the goal of calculating the distribution of the absorbing structures in the tissue. We present a method in which the acoustic field distribution is captured as a two-dimensional snapshot at the sample surface, using an optical-reflectance-based detection principle with a detection resolution of 20 mum. A new image reconstruction is accomplished by backprojection of the detected two-dimensional pressure distributions into the sample volume by use of the delay between the laser pulse and the time the snapshot was taken. Two-dimensional pressure-wave distribution and image reconstruction are demonstrated by simulations and experiments, in which small objects are irradiated with laser pulses of 6-ns duration. The method opens the possibility to irradiate the sample hidden in a light-scattering medium directly through the detector plane, thus enabling front-surface detection of the optoacoustic signals, which is especially important if structures close to the tissue surface have to be imaged. Reconstructed tomography images with a depth resolution of 20 mum and a lateral resolution of 200 mum are presented.

Model study to investigate the contribution of spallation to pulsed laser ablation of tissue.

BACKGROUND AND OBJECTIVE: Absorption of a short laser pulse produces high thermoelastic stress in the irradiated volume. The relaxation of this stress at a free (tissue-air) surface leads to tensile loading, resulting in mechanical spallation. Using model substances, we investigated the role of this effect in tissue ablation. STUDY DESIGN/MATERIALS AND METHODS: Stained water and gelatine were irradiated with short pulses (8 ns duration) from a Nd:YAG laser at 1,064 nm wavelength. The dynamics of the induced effects were observed with laser-flash photography and stress wave detection. RESULTS: Spallation is indicated by the formation of cavitation bubbles below the irradiated surface and is strongly influenced by impurities serving as nucleation sites. Material ejection due to spallation was observed in the liquid sample at a fluence leading to a temperature below the boiling point but needed a temperature in excess of 100 degrees C in gelatine, owing to the small mechanical energy available for this process, estimated to be < 1%. CONCLUSION: The mechanical action of thermoelastic stress waves is characterized by high stress amplitudes but low energetic efficiency. A model combining spallation and vaporization is therefore proposed for efficient tissue ablation.

[Demonstration of biophysical effects of pulsed laser irradiation with histological tissue sections]. / Darstellung biophysikalischer Effekte gepulster Laserstrahlung anhand histologischer Gewebeschnitte.

For therapeutic application of laser light it is necessary to minimize defects in the nonirradiated tissue. These defects depend on the primary mechanism of interaction between tissue and laser light. Three experiments were performed to distinguish between mechanical and thermal effects of nano- and microsecond laser pulses in skeletal muscle of the rat. The light, transmission and scanning electron microscopes were In the ns-experiments the mechanical action of a single ns pulse (8 ns) produced a crater. Only zones I and IV developed. With 50 to 100 pulses all zones can be identified. These results show that a single ns pulse suffices to form a tissue crater by mechanical action. A higher number of ns pulses leads to heat accumulation and produces thermal lesions similar to those seen after application of microseconds-pulses.

A special irrigation liquid to increase the reliability of laser-induced shockwave lithotripsy.

For the laser-induced shockwave lithotripsy (LISL) the laser-pulses of a Q-switched Nd:YAG laser produce an optical breakdown in the irrigation liquid surrounding the urinary stone. Subsequently high-pressure shockwaves are emitted causing stone fragmentation. Since the LISL is an endoscopic technique, problems arise from the transmission of the laser pulses through optical fibers. The intensity threshold for an optical breakdown in commonly used saline solution amounts to 21 GW/cm2, in optical silica fibers, to about 3 GW/cm2. Therefore bare fibers cannot be used without being destroyed by a breakdown. So we have developed an irrigation liquid by adding small quantities of metal ions to saline solution to lower the threshold intensity. The most suitable ion was Fe3+ in a concentration of 0.02 mmol/l, which shows a lowering to 5 GW/cm2. In combination with a spherically shaped fiber exit the intensities that have to be transmitted are below the threshold of the fiber material. Using this irrigation liquid the overall reliability of the method could be significantly increased and several stone fragmentations can be performed with a single optical fiber.

[Laser lithotripsy with the neodymium YAG laser]. / Laserlithotripsie mit dem Neodymium-YAG Laser.

Laser-induced shock wave lithotripsy (LISL) with a Q-switched neodymium-YAG laser depends on the generation of a laser-induced breakdown in the fluid surrounding the stone. An oscillating plasma bubble is created, directing shock waves towards the stone. These cavitational effects fragment the calculus into small particles. A new bifunctional laser is introduced: this allows both nanosecond pulses for shock wave generation and disintegration of urinary calculi and millisecond pulses for biliary stones and tissue coagulation. It can be supplied with 320-, 400-, and 600-micron fibers. We have treated 189 ureteric stones in 185 patients with laser lithotripsy utilizing flexible ureteroscopes (n = 26) or rigid ureteroscopes (n = 159). It proved possible to fragment 179 stones into small pieces. In eight patients LISL was not successful. A rigid cystoscope that can be dismantled into an upper and lower hemisheath for the introduction of flexible endoscopes into the ureter without prior dilatation of the ureteral orifice was used in 15 patients.

Laser-induced shock wave lithotripsy. Influence of laser pulse energy and irrigation solutions on stone disintegration.

With a high intensity Q-switched Nd-YAG laser shock waves can be generated in a liquid close to the calculus. Up to 80 mJ single pulse energy with 8 nsec pulse duration can be transmitted through flexible quartz fibers. Energy conversion and enhancement can be accomplished at the fiber tip with optical focussing of the light at the quartz tip, with irrigation solutions and with high pulse energies. Iron-III-dextran solutions (1 mg Fe3+/1) and magnesium chloride (50 mmol/l) increased the pressure in the laser induced breakdown up to ten times (8,000-10,000 bar). Smaller stone particles and higher efficacy in stone fragmentation could be achieved.

Contact probes for intravascular laser recanalization. Experimental evaluation.

Experimental laser ablation of atheromatous plaques was performed with a bare quartz glass fiber, a metal probe, and a sapphire probe. The tissue response after irradiation with increasing energies was evaluated by means of light and scanning electron microscopy. Laser emission through the bare fiber caused a narrow, deep crater with an irregular surface surrounded by a zone of thermal necrosis. After tissue ablation with the metal probe, a disproportion between the small tissue defect and the large zone of thermal necrosis was observed. The largest tissue defect was vaporized by the sapphire contact probe. A small zone of thermal necrosis surrounded the laser crater.

First clinical experience with a Q-switched neodymium:YAG laser for urinary calculi.

Animal studies using a high intensity nanosecond pulsed neodymium:YAG laser did not reveal any serious tissue damage. Following these investigations patient treatment was begun in June 1987. Laser energy of a neodymium:YAG laser with an 8 nsec. pulse duration and a repetition rate of up to 50 Hz. was coupled into a flexible 600 resp. 400 micron. quartz fiber. Laser-induced breakdown was created with 35 to 50 mJ. at the fiber tip, resulting in a shock wave that disintegrated the calculus into tiny fragments. A total of 56 patients with 58 calculi (54 ureteral and 4 kidney stones) was treated from June 1987 to March 1988. Of the calculi 48 could be fragmented completely, while 6 others were reduced to a size small enough to be removed with forceps. Four stones composed of calcium oxalate monohydrate could not be disintegrated. The combination of laser stone disintegration with flexible ureterorenoscopy implies the possibility of an atraumatic, 1-step procedure for fragmentation of ureteral and kidney calculi.

Laser induced shock wave lithotripsy--biologic effects of nanosecond pulses.

Laser energy of a Nd-YAG laser (1064 nm. wave length, 8 nsec pulse duration) was directed against various tissue cultures and the urothelium of the ureter, bladder and kidney parenchyma in pigs. Single pulse energy was 50 to 120 mJ with a repetition rate of 20 Hz. Urothelium and kidney parenchyma were irradiated in seven pigs. Tissue samples were examined histologically and electron microscopically directly, two, four, eight and 12 days after irradiation. No macroscopic lesion could be found. Maximum energy caused a small 'rupture cone' of 40 micron. depth. No thermic effects or necrosis resulted, so that no harm is to be expected with unintentional irradiation during laser stone disintegration.

[Morphologic examination of the urothelium after irradiation with laser energy]. / Morphologische Untersuchungen des Urothels nach Einwirkung intensiver Nanosekunden-Laserpulse.

The energy of a Nd-YAG laser (1,064 nm wave length, 8 ns pulse duration) was used to irradiate the urothelium of the ureter or bladder and kidney parenchyma in pigs. Single pulse energy was 50-120 mJ with a 20-Hz repetition rate. The horizontal laser beam was reflected 90 degrees down by a 100% mirror and with a specially designed apparatus focussed on the surface of the tissue. Laser light from a quartz glass fiber was also focussed directly onto the tissue. Urothelium and kidney parenchyma were irradiated in 7 pigs. Tissue samples were examined histologically and raster electron microscopically 2, 4, 8 and 12 days after irradiation. No macroscopic lesion could be found. Maximum energy caused a small cone of 40 micron depth. No thermic effects or necrosis resulted, so that no harm is to be expected with unintentional irradiation.

The influence of vegetative stimuli on the human nasal mucous membrane.

Measuring probes were inserted into the inferior nasal meatus in humans to record the effects of certain defined vegetative stimuli on the cavernous state and the temperature of the mucous membrane. An unilateral carotis compression induces a bilateral reactivity of the mucous membrane in the sense of a sympathicus stimulus. An unilateral bulbus pressure causes a bilateral reaction of the mucous membrane opposite to that of carotis compression. Blocking of the stellate ganglion produces a tonus reduction of the nervus sympathicus as was also found in animal experiments by other authors. Trigeminus stimulation induces a swelling of the nasal mucous membrane, whereas a voluntary breathing stop causes decongestion. Facial blushing, the only undefined and involuntary stimulus, is followed by an unswelling and a decrease of mucous membrane temperature. The results of our investigations are in agreement with analogous animal experiments. This is not surprising as man has a vegetative nervous system which is essentially unchanged from the beginning of evolutionary development. Only blushing is an expression of a reaction behaviour characteristic of human beings only.

The nose as an indicating organ of vegetative controlling mechanisms.

Long-term examinations on test persons of an average age of 28 years were carried out by means of thermistors for measuring the mucous membrane temperature and by means of photo transistors for measuring the state of cavernous tissues. The tactile stimulus during the insertion of the probe regularly leads to a temperature increase and a swelling of the nasal mucous membrane. Pronounced changes in the temperature do not take a linear course but are interrupted by intercurrent temperature inversions. The changes in the state of cavernous tissues take a course synchronous to this. After the first tactile stimulus there adjusts a preliminary temperature equilibrium in the mucous membrane temperature and in the cavernous state. Then there follow temperature oscillations taking a completely different course, of changing amplitudes and frequencies. These temperature oscillations may take courses in the same or in opposite directions in the two nose halves. The causes proposed for discussion, on the one hand for the intercurrent temperature inversions in case of considerable temperature changes, and on the other hand for the differing temperature oscillations after the attainment of the preliminary temperature equilibrium, are central regulating mechanisms, while at the same time analogous animal experiments are taken into consideration.