An accurate tumor delineation in neurosurgery is still a very challenging problem which we are addressing with optical coherence elastography (OCE). Because of the highly viscoelastic properties of brain tissue, we developed a new Air-Jet based tissue excitation source and evaluated the tissue stiffness with a 3.2 MHz swept-source Optical Coherence Tomography (OCT) system with a line scan rate of 2.45 kHz. The phase based displacement per pixel is measured and stiffness maps are calculated for brain tumor samples. However, certain features in the stiffness maps are seemingly not correlatable to the tissue features in the histological sections. Therefore, the structural properties of the histological sections e.g. fiber orientation, cell nuclei concentration and the “onion structure” with their rotational direction for meningioma were given greater consideration. The structural information are extracted from the histological sections via color deconvolution and structural tensor analysis. First results show that the stiffness transitions correlate with some structures of the histological sections. In summary, the Air-Jet OCE seems to be capable of measuring the stiffness as well as the structural composition of the sample. The long-term aim of this project is to establish OCE to support tumor delineation in the field of neurosurgery.
Objective dose control is currently not possible for laser treatments at the retina. Especially in the case of subvisible irradiations, the assessment of the intended temperature effects by the visual control of the doctors is impossible. Due to the large individual differences in light transmission of the eye and absorption at the RPE, for the same laser power the achieved temperature varies at the RPE. Therefore, an opto acoustic technique to measure the temperature of the retina during laser treatment was developed and applied in a clinical study. A conventional 532 nm cw laser was used during a standard treatment. A microcontroller-based control module was optically coupled between the treatment laser and the slit lamp. This control module is able to measure and automatically control the RPE temperature rise in real time at a rate of 3 kHz and regulates the laser power in such a way that a target temperature specified by the physician is reached within several ms and kept constant until the end of the irradiation time of 100 ms. In the clinical study on patients with CSCR, a target temperature of 51°C was set. So far 7 Patients were treated. Target temperature could be reached and kept constant until the end of irradiation time of 100 ms. A wide range of applied laser power (35 to 95 mW) was used and show the need for active control during retinal laser treatment. It was demonstrated that temperature-controlled retinal laser therapy can be applied safely in patients.
Ultrasonic aspirators are commonly used for volume reduction of neurosurgical tumours. Bleeding occurs occasionally during ultrasonic debulking since ultrasonic aspirators do not coagulate affected vessels. Usually bipolar forceps are used for haemostasis, however requiring a change of instrumentation by the surgeon. Thulium laser emitting at a wavelength of 1940 nm in a strong water absorption band are suitable for tissue and blood vessel coagulation with subsequent haemostasis. Therefore, such laser system was combined with an ultrasonic aspirator by adapting the light transmitting multimode fiber tip to the distal tip of the ultrasonic aspirator. The thulium laser showed very good haemostasis during tumour debulking. Instrumental changes to bipolar forceps were reduced, surgeon’s feedbacks were convincingly positive.
Microscope integrated real time 4D MHz-OCT operating at high scanning densities are capable of capturing additional visual contrast resolving depth and tissue. Even within a plain C-scan en-face projection structures are recognizable, that are not visible in a white light camera image. With advanced post processing methods, such as absorption coefficient mapping, and morphological classifiers more information is extracted. Presentation to the user in an intuitive way poses practical challenges that go beyond the implementation of a mere overlay display. We present our microscope integrated high speed 4D OCT imaging system, its clinical study use for in-vivo brain tissue imaging, and user feedback on the presentation methods we developed. In neurosurgery the de-facto standard contrast agents used for visibly highlighting brain tumors are Fluorescin and ALA, both of which come with certain caveats. As part of a clinical study we developed a microscope integrated real time 4D MHz-OCT system, operating as high scanning densities, with the intent of creating visual tissue contrast without the use of such contrast agents. Advanced post processing methods to classify tissue can be derived from static properties such as light absorption and morphology, and from dynamic properties, such as perfusion and elastography. However we also noticed that even in a plain C-scan en-face projection structures of interest could be recognized, that were not visible in the corresponding white light camera image. As part of a clinical study so far we collected data from 20 patients, used it for machine learning based classifiers and developing data presentation modalities for eventual use in a surgical environment. We present the challenges in implementing our microscope integrated high speed 4D OCT imaging system, a selection of the imaging data we collected so far during brain tumor surgeries, and the avenues toward presenting processed data to the surgeon.
In recent years, it was demonstrated that discrimination between white matter and tumor-infiltrated white matter based on optical coherence tomography (OCT) data is possible with high accuracy. However, gray matter is also present during the tumor resection and shows similar optical properties to tumor infiltration, which aggravates the tumor classification using optical coherence tomography. A semantic segmentation approach based on a convolutional neural network was applied to the problem in order to classify healthy brain tissue from tumor infiltrated brain tissue. A dataset was created, which consisted of ex vivo OCT B-scans, which were acquired by a swept-source OCT system with a central wavelength of 1300 nm. Each OCT B-scan was indirectly annotated by transforming histological labels from a corresponding H&E section onto it. The labels differentiate between white matter, gray matter and tumor infiltration. The output of the network was modeled to a Dirichlet prior distribution, which enabled the capturing of a prediction uncertainty. This approach achieved an intersection over union score of 0.72 for healthy brain tissue and 0.69 for highly tumor infiltrated brain tissue, when only confident predictions were considered.
The recently developed SLIDE microscope enables rapid imaging in nonlinear two-photon microscopy, where frame rates of 4 kHz are achieved. Such fast acquisition speeds coupled with the molecular specificity of fluorescence markers and the high optical resolution in the sub-μm range allow volume scan rates at 40 Hz. A commercially available Fourier Domain Mode Locked Laser system (Optores GmbH, Munich, Germany) was used as the light source emitting at 1060 nm (Bandwidth 15 nm). An electro-optical Modulator (EOM) splits the light of a single sweep duration up into 600 pulses with 30 ps pulse duration each. Each of it is then spatial separated by a diffractive grating. Only one scanner is needed for beam steering to excite the slow axis resulting to a frame rate of 4 kHz. Using a piezo driver for the objective of the microscope at a frequency of 20 Hz, a live 4D volume scan of 40 Hz with 600 x 400 x 100 voxel is possible. Until now, SLIDE systems were bulky and bound onto a fixed optical desk. The Medical Laser Centre Lübeck developed a transportable and reliable SLIDE system, so that this new and highly innovative technology can be made available to various biological laboratories in Europe. This work was conducted in the framework of the EU project “Faircharm.”
Neuro-surgery is challenged by the difficulties of determining brain tumor boundaries during excisions. Optical coherence tomography is investigated as an imaging modality for providing a viable contrast channel. Our MHz-OCT technology enables rapid volumetric imaging, suitable for surgical workflows. We present a surgical microscope integrated MHz-OCT imaging system, which is used for the collection of in-vivo images of human brains, with the purpose of being used in machine learning systems that shall be trained to identify and classify tumorous tissue.
The long-term aim of this project is to establish optical coherence elastography for tumor delineation in the field of neurosurgery. Because of the challenging highly viscoelastic properties of brain tissue, we developed a new Air-Jet based excitation source. With pulse duration of up to 700 ms and real time force measurement, this novel system allows the sample to reach a semi-steady state. In parallel with a 3.2 MHz swept-source optical coherence tomography system over 800 line scans are acquired over the whole sample excitation process. The phase data is extracted, unwrapped and the displacement per pixel is calculated. This system enables the measurement of mechanical properties like stiffness and Young’s modulus, similar to the standard indentation measurement. As well as viscoelastic properties i.e. relaxation times, in non-contact. The first processing step is to split the excitation progression into three main time ranges: the high dynamic, the steady state, and the viscoelastic range. In each range typical features of the displacement curve are extracted for every pixel in the B-scan. For those features, various mechanical parameters are calculated mainly, the stiffness and Young’s modulus and stored as feature matrices. The results are processed, visualized and overlaid with either the OCT intensity image or the histological sections. Strain stress curves are generated for some selected positions in the B-scan leading to a specific viscoelastic hysteresis. The feature matrices will be utilized as a fingerprint for each tissue, and are the first step for an AI based classification of the tissue.
In neurosurgical tumor operations on the central nervous system, intraoperative haptic information often assists for discrimination between healthy and diseased tissue. Thus, it can provide the neurosurgeon with additional intraoperative source of information during resection, next to the visual information by the light microscope, fluorescent dyes and neuronavigation. One approach to obtain elastic and viscoelastic tissue characteristics non-subjectively is phase-sensitive optical coherence elastography (OCE), which is based on the principle of optical coherence tomography (OCT). While phase-sensitive OCE offers significantly higher displacement sensitivity inside a sample than commonly used intensity-based correlation methods, it requires a reliable algorithm to recover the phase signal, which is mathematically restricted in the -π to π range. This problem of phase wrapping is especially critical for inter-frame phase analysis since the time intervals between two referenced voxels is long. Here, we demonstrate a one-dimensional unwrapping algorithm capable of removing up to 4π-ambiguities between two frames in the complex phase data obtained from a 3.2 MHz-OCT system. The high sampling rate allows us to resolve large sample displacements induced by a 200 ms air pulse and acquires pixel-precise detail information. The deformation behavior of the tissue can be monitored over the entire acquisition time, offering various subsequent mechanical analysis procedures. The reliability of the algorithm and imaging concept was initially evaluated using different brain tumor mimicking phantoms. Additionally, results from human ex vivo brain tumor samples are presented and correlated with histological findings supporting the robustness of the algorithm.
The ill-defined tumor borders of glioblastoma multiforme pose a major challenge for the surgeon during tumor resection, since the goal of the tumor resection is the complete removal, while saving as much healthy brain tissue as possible. In recent years, optical coherence tomography (OCT) was successfully used to classify white matter from tumor infiltrated white matter by several research groups. Motivated by these results, a dataset was created, which consisted of sets of corresponding ex vivo OCT images, which were acquired by two OCT-systems with different properties (e.g. wavelength and resolution). Each image was annotated with semantic labels. The labels differentiate between white and gray matter and three different stages of tumor infiltration. The data from both systems not only allowed a comparison of the ability of a system to identify the different tissue types present during the tumor resection, but also enable a multimodal tissue analysis evaluating corresponding OCT images of the two systems simultaneously. A convolutional neural network with dirichlet prior was trained, which allowed to capture the uncertainty of a prediction. The approach increased the sensitivity of identifying tumor infiltration from 58 % to 78 % for data with a low prediction uncertainty compared to a previous monomodal approach.
Optical coherence elastography (OCE) offers the possibility of obtaining the mechanical behavior of a tissue. When also using a non-contact mechanical excitation, it mimics palpation without interobserver variability. One of the most frequently used techniques is phase-sensitive OCE. Depending on the system, depth-resolved changes in the sub-µm to nm range can be detected and visualized volumetrically. Such an approach is used in this work to investigate and detect transitions between healthy and tumorous brain tissue as well as inhomogeneities in the tumor itself to assist the operating surgeon during tumor resection in the future. We present time-resolved, phase-sensitive OCE measurements on various ex vivo brain tumor samples using an ultra-fast 3.2 MHz swept-source optical coherence tomography (SS-OCT) system with a frame rate of 2.45 kHz. 4 mm line scans are acquired which, in combination with the high imaging speed, allow monitoring and investigation of the sample's behavior in response to the mechanical load. Therefore, an air-jet system applies a 200 ms short air pulse to the sample, whose non-contact property facilitates the possibility for future in vivo measurements. Since we can temporally resolve the response of the sample over the entire acquisition time, the mechanical properties are evaluated at different time points with depth resolution. This is done by unwrapping the phase data and performing subsequent assessment. Systematic ex vivo brain tumor measurements were conducted and visualized as distribution maps. The study outcomes are supported by histological analyses and examined in detail.
The identification of ex vivo brain tumor tissue was investigated with two different optical coherence tomography systems exploiting two optical parameters. The optical parameters were calculated from semantically labelled OCT B-scans.
For enabling haemostasis during brain tumour resection, suitable laser application parameters for the wavelengths 1940 nm and 1480 nm were investigated as an alternative for bipolar forceps in tissue coagulation.
A 1.6 MHz Fourier-domain mode-locked (FDML) optical coherence tomography (OCT) was adapted to an OR-Microscope for clinical application in neurosurgery. 3D-volume scans at video rate are envisaged with approximately 50μm lateral and 20μm axial resolution
A precision air puff excitation system for MHz Optical Coherence Elastography in neurosurgery was developed. It enables non-contact soft-tissue excitation down to μN, with direct, noncontact force determination via gas flow measurement.
The separation of tumorous brain tissue and healthy brain tissue is still a big challenge in the field of neurosurgery, especially when it comes to the detection of different infiltration grades of glioblastoma multiforme at the tumor border. On the basis of a recently created labelled OCT dataset of ex vivo glioblastoma multiforme tumor samples the detection of brain tumor tissue and the identification of zones with varying degrees of infiltration of tumor cells was investigated. The identification was based on the optical properties, which were extracted by an exponential fit function. The results showed that a separation of tumorous tissue and healthy white matter based on these optical properties is possible. A support vector machine was trained on the optical properties to separate tumor from healthy white matter tissue, which achieved a sensitivity of 91% and a specificity of 76% on an independent training dataset.
Optical coherence tomography (OCT) has the potential to become an additional imaging modality for surgical guidance in the field of neurosurgery, especially when it comes to the detection of different infiltration grades of glioblastoma multiforme at the tumor border. Interpretation of the images, however, is still a big challenge. A method to create a labeled OCT dataset based on ex vivo brain samples is introduced. The tissue samples were embedded in an agarose mold giving them a distinctive shape before images were acquired with two OCT systems (spectral domain (SD) and swept source (SS) OCT) and histological sections were created and segmented by a neuropathologist. Based on the given shape, the corresponding OCT images for each histological image can be determined. The transfer of the labels from the histological images onto the OCT images was done with a non-affine image registration approach based on the tissue shape. It was demonstrated that finding OCT images of a tissue sample corresponding to segmented histological images without any color or laser marking is possible. It was also shown that the set labels can be transferred onto OCT images. The accuracy of method is 26 ± 11 pixel, which translates to 192 ± 75 μm for the SS-OCT and 94 ± 43 μm for the SD-OCT. The dataset consists of several hundred labeled OCT images, which can be used to train a classification algorithm.
The treatment of macular diseases requires frequent monitoring by optical coherence tomography (OCT). Home monitoring would reduce the burden of frequent clinical visits and increase therapy adherence. In a pilot study with 47 patients having different macular diseases we tested a proprietary self-examination low-cost full-field OCT (SELFF-OCT). For comparison, scans with a standard clinical spectral domain OCT were taken. Data was graded by a reading center. Patients were able to successfully acquire images that were clinically gradable for 85% of the included eyes. The sensitivity and specificity for an anti-VEGF treatment decision based on the SELFF-OCT was 0.94 and 0.95, respectively.
This paper describes the further investigation into the capabilities of the already established noncontact optoacoustic method to measure temperature profiles in cell cultures during controlled heating. The technic is scalable in spatial and temporal resolution. The intra and extracellular medium is heated by a thulium laser (wavelength 1.94 μm; power up to 25W). With a second Q-switched thulium laser (2.01 μm; up to 3 mJ) the sample medium temperature is simultaneously probed in the dish (20 mm diameter) via the photoacoustic effect. The pressure waves emitted due to the thermoelastic expansion of water are measured with an ultrasonic hydrophone at the side of the dish. The amplitudes of the waves are temperature dependent and are used to calculate the temperature/time course at 10 locations. Temperatures of up to 70°C with a heating power of up to 25 W after 5 s were measured, as well as lateral temperature profiles over time. Measurements in water show temperature fluctuations likely due to thermal convection and water circulation. Since measurements in agar do not show similar temperature fluctuations, this theory seems to be confirmed. In conclusion optoacoustics can serve as a real-time non-contact technique to determine temperature changes in cell and organ cultures as well as in vivo and during hyperthermia based therapies.
Selective retina therapy (SRT) is currently used in clinical studies to treat several chorioretinal diseases. For SRT a laser pulse duration of 1,7 μs is currently used. At this pulse duration the retinal pigment epithelium (RPE) cells are destroyed by transient microbubbles without damaging the neuronal retina. So far it is unclear whether slightly longer laser pulses are still acting thermomechanically or whether thermal effects show responsible for cell damage close above damage threshold. In order to investigate the damage threshold increase with pulse duration, a novel laser with adjustable pulse duration in the range of 2-20 μs was used to investigate RPE damage on ex-vivo porcine RPE explants. The specimen were fixed in an eye model and were exposed to laser pulse energies ranging from 15-150 μJ with a top hat square of 120×120μm2, exhibiting a spatial intensity modulation factor of 1,3. Viability tests using binary evaluation result in threshold values with peak radiant exposures of 233 mJ/cm2 and 389 mJ/cm2 for 2 μs and 20 μs laser durations, respectively. An almost logarithmic increase of the threshold radiant exposure over pulse duration was found.
Selective retina therapy (SRT) is a short pulse (μs-regime) alternative to conventional laser photocoagulation (LPC) for treatment of retinal diseases. LPC leads to collateral damage of retinal layers adjacent to the retinal pigment epithelium (RPE), including healthy, non-regenerative photoreceptors due to the high thermal load, whereas in SRT, RPE cells are destroyed by microbubbles without damaging the neuronal retina. A novel experimental SRT laser operating at 532 nm wavelength can deliver 2 – 20 μs pulse sequences. Its tight integration into an upgraded diagnostic SPECTRALIS system combines beam control for treatment planning with real-time optical coherence tomography (OCT) overexposure protection of the photoreceptors. This “Spectralis Centaurus” system, was built and preliminary tested on porcine ex-vivo samples, reaching an unprecedented accuracy with unique planning and follow-up capabilities for upcoming clinical cellular level micro-surgery. The combination of OCT with SRT selectively limits cell death to the RPE by precisely controlling energy deposition while optically monitoring tissue response.
In order to fully exploit the diagnostic potential of optical coherence tomography (OCT) in contemporary restorative dentistry, an intraorally applicable OCT probe has been constructed. The probe was connected to a commercially available OCT system. The handling of the probe and the quality of the OCT images were optimized and evaluated on human extracted teeth fixed in a patient-equivalent simulation. In addition, the probe was applied intraorally to volunteers. With the intraoral OCT probe hard tooth substances, carious lesions in enamel and dentin and composite restorations could be imaged. In vivo, the probe allowed OCT imaging of all tooth surfaces except the vestibular surfaces of third molars and proximal surface areas of molars within a "blind spot" at a distance of 2.5 mm from the tooth surface. Superficial tissue structures of the marginal gingiva could also be imaged. The intraoral OCT probe is a promising tool for non-invasive imaging and monitoring of healthy and diseased hard tooth substances and tooth-colored restorations. It can be a valuable addition to established methods for caries diagnosis and restoration evaluation.
Optical coherence tomography (OCT) is a non-invasive imaging technique which is currently investigated for intraoperative detection of residual tumor during resection of human gliomas. Three different OCT systems were used for imaging of human glioblastoma in vivo (830nm spectral domain (SD) OCT integrated into a surgical microscope) and ex vivo (940nm SD-OCT and 1310nm swept-source MHz-OCT using a Fourier domain mode locked (FDML) laser). Before clinical data acquisition, the systems were characterized using a three-dimensional point-spread function phantom. To distinguish tumor from healthy brain tissue later on, attenuation coefficients of each pixel in OCT depth profiles are calculated. First examples from a clinical study show that the pixel-resolved calculation of the attenuation coefficient provides a good image contrast and confirm that white matter shows a higher signal and more homogeneous signal structure than tumorous tissue.
Selective retina therapy (SRT) targets the retinal pigment epithelium (RPE) with pulsed laser irradiation by inducing microbubble formation (MBF) at the intracellular melanin granula, which leads to selective cell disruption. The following wound healing process rejuvenates the chorio-retinal junction. Pulse energy thresholds for selective RPE effects vary intra- and interindividually. We present the evaluation of an algorithm that processes backscattered treatment light to detect MBF as an indicator of RPE cell damage since these RPE lesions are invisible during treatment. Eleven patients with central serous chorioretinopathy and four with diabetic macula edema were treated with a SRT system, which uses a wavelength of 527 nm, a repetition rate of 100 Hz, and a pulse duration of 1.7 μs. Fifteen laser pulses with stepwise increasing pulse energy were applied per treatment spot. Overall, 4626 pulses were used for algorithm parameter optimization and testing. Sensitivity and specificity were the metrics maximized through an automatic optimization process. Data were verified by fluorescein angiography. A sensitivity of 1 and a specificity of 0.93 were achieved. The method introduced in this paper can be used for guidance or automatization of microbubble-related treatments like SRT or selective laser trabeculoplasty.
Selective retina therapy (SRT) is an ophthalmological laser technique, targeting the retinal pigment epithelium (RPE) with repetitive microsecond laser pulses, while causing no thermal damage to the neural retina, the photoreceptors as well as the choroid. The RPE cells get damaged mechanically by microbubbles originating, at the intracellular melanosomes. Beneficial effects of SRT on Central Serous Retinopathy (CSR) and Diabetic Macula Edema (DME) have already been shown. Variations in the transmission of the anterior eye media and pigmentation variation of RPE yield in intra- and inter- individual thresholds of the pulse energy required for selective RPE damage. Those selective RPE lesions are not visible. Thus, dosimetry-systems, designed to detect microbubbles as an indicator for RPE cell damage, are demanded elements to facilitate SRT application. Therefore, a technique based on the evaluation of backscattered treatment light has been developed. Data of 127 spots, acquired during 10 clinical treatments of CSR patients, were assigned to a RPE cell damage class, validated by fluorescence angiography (FLA). An algorithm has been designed to match the FLA based information. A sensitivity of 0.9 with a specificity close to 1 is achieved. The data can be processed within microseconds. Thus, the process can be implemented in existing SRT lasers with an automatic pulse wise increasing energy and an automatic irradiation ceasing ability to enable automated treatment close above threshold to prevent adverse effects caused by too high pulse energy. Alternatively, a guidance procedure, informing the treating clinician about the adequacy of the actual settings, is possible.
The relative occurrence of volatile organic compounds in the human respiratory gas is disease-specific (ppb range). A prototype of a gas analysing device using two tuneable laser systems, an OPO-laser (2.5 to 10 μm) and a CO2-laser (9 to 11 μm), and an optoacoustic measurement cell was developed to detect concentrations in the ppb range. The sensitivity and resolution of the system was determined by test gas measurements, measuring ethylene and sulfur hexafluoride with the CO2-laser and butane with the OPO-laser. System sensitivity found to be 13 ppb for sulfur hexafluoride, 17 ppb for ethylene and <10 ppb for butane, with a resolution of 50 ppb at minimum for sulfur hexafluoride. Respiratory gas samples of 8 healthy volunteers were investigated by irradiation with 17 laser lines of the CO2-laser. Several of those lines overlap with strong absorption bands of ammonia. As it is known that ammonia concentration increases by age a separation of people <35 und >35 was striven for. To evaluate the data the first seven gas samples were used to train a discriminant analysis algorithm. The eighth subject was then assigned correctly to the group >35 years with the age of 49 years.
Aiming for laser-assisted resection of calcified aortic valve structures for Transcatheter Aortic Valve Implantation (TAVI), a Q-switched Tm:YAG laser emitting at a wavelength of 2.01 μm was used to evaluate the cutting efficiency on highly calcified human aortic leaflets in-vitro. The calcified aortic leaflets were examined regarding ablation rates and debris generation, using a pulse energy of 4.3 mJ, a pulse duration of 0.8-1 μs and a repetition rate of 1 kHz. The radiation was transmitted via a 200 μm core diameter quartz fiber. Resection was performed in a fiber-tissue contact mode on water-covered samples in a dish. The remnant particles were analyzed with respect to quantity and size by light microscopy. Additionally, soft tissue of porcine aortic vessels was examined for histologically detectable thermo-mechanical damage after continuous wave and Q-switched 2μm laser irradiation. An ablation rate of 36.7 ± 25.3 mg/min could be realised on highly calcified aortic leaflets, with 85.4% of the remnant particles being <6 μm in diameter. The maximum damaged area of the soft tissue was < 1 mm for both, cw and pulsed laser irradiation. This limits the expected collateral damage of healthy tissue during the medical procedure. Overall, the Q-switched Tm:YAG laser system showed promising results in cutting calcified aortic valves, transmitting sufficient energy through a small flexible fibre.
Selective Retina Therapy (SRT) targets the Retinal Pigment Epithelium (RPE) without effecting neighboring layers as the photoreceptors or the choroid. SRT related RPE defects are ophthalmoscopically invisible. Owing to this invisibility and the variation of the threshold radiant exposure for RPE damage the treating physician does not know whether the treatment was successful or not. Thus measurement techniques enabling a correct dosing are a demanded element in SRT devices. The acquired signal can be used for monitoring or automatic irradiation control. Existing monitoring techniques are based on the detection of micro-bubbles. These bubbles are the origin of RPE cell damage for pulse durations in the ns and μs time regime 5μs. The detection can be performed by optical or acoustical approaches. Monitoring based on an acoustical approach has already been used to study the beneficial effects of SRT on diabetic macula edema and central serous retinopathy. We have developed a first real time feedback technique able to detect micro-bubble induced characteristics in the backscattered laser light fast enough to cease the laser irradiation within a burst. Therefore the laser energy within a burst of at most 30 pulses is increased linearly with every pulse. The laser irradiation is ceased as soon as micro-bubbles are detected. With this automatic approach it was possible to observe invisible lesions, an intact photoreceptor layer and a reconstruction of the RPE within one week.
To reduce unwanted collateral thermal damage to surrounding tissue and organs during laparoscopic laser dissection (cw, wavelength: 1.9μm) of porcine liver water spray was used. Size and amount of the produced water droplets of the water spray were photographed by short time imaging and analyzed by imaging software. At in vivo measurements on fresh porcine liver the depth of thermal damage was reduced by 85 % with water spray and the lateral size of thermal damage at the tissue surface could be reduced by 67%. This results show that especially for laparoscopic laser surgery water spray application might be a useful tool to avoid unwanted collateral thermal damage.
Dissection of liver tissue can be performed by different techniques (ultrasound, mono and bipolar dissection,
water jet dissection and by stapler). In this animal study the potential of a Thulium fiber laser system was
investigated for open parenchyma dissection.
Based on a cw Thulium fiber laser (IPG laser GmbH, Burbach, Germany), emitting a wavelength at 1.9 μm
and a maximal power at 50 W, a surgical dissection device was developed at the Medical Laser Centre
Luebeck. Cw laser radiation (40 Watt) was transmitted via a 365 μm fiber with a polished distal fiber tip.
Procedure was performed in contact mode; irradiance at the distal fiber tip was 38.2 kW/cm2. After general
anesthesia and a median laparotomy an atypical laser resection of the liver was performed in 3 pigs. Healing
process was controlled after 2-3 weeks by histological analysis (H&E staining). The final evaluation data
included total resection time, blood loss, bile leakage and mass of dissected tissue. All animals treated in this
study were cared for in accordance to the European convention on animal care.
In general the dissection with the 1.9 μm laser radiation was easily performed. Hemostasis was highly
sufficient so blood loss and bile leakage was negligible. Total resection time including hemostasis of the
remaining tissue was 26 ± 12 min. Weight of resected tissue was 17 ± 8 g. During survival period no
complications (bleeding or inflammation) occurred. After 2 weeks histology showed ongoing scar formation
about 1 - 2 mm in depth of the dissected area.
A cw Laser system (Medical Laser Centre Lübeck GmbH, Germany) emitting a wavelength at 1.92μm and a laser power up to 50 W was used for partial porcine renal parenchyma resection and hemi nephrectomy. Laser radiation (30 Watt) was transmitted via 365μm fiber with polished distal fiber tip which was fixed in a stainless steel tube. Power density at the distal fiber tip was 28.6 kW/cm2. On 3 kidneys a partial renal parenchyma resection without opening of the renal pelvis was performed (Group A). On 8 kidneys a hemi nephrectomy with opening of the renal pelvis was performed
(Group B). Total resection time including haemostasis of the remaining tissue was 501 ± 103 s in group "A" and 730 ± 415 s in group "B". Blood lost was 28 ± 3 ml in group "A" and 98 ± 73 ml in group "B". The ischemic time for the kidneys in group "B" was 157 s in average, while for group "A" no ligation was necessary at all. Healing process was observed over 3 weeks, survival rate was 100 %, no inflammation or renal fistulas were found. In conclusion, the first experiments show that the 1.92 μm Laser-System is a very promising device for bloodless and fast kidney resection.
The technique of nephron sparing surgery has matured significantly over the past
decade and is emerging as an oncologically sound procedure for the management of
renal tumors. Methods of tumor excision as well as parenchymal reconstruction in a
hemostaticallly controlled field have evolved to make this procedure safer. In an
attempt to find an improoved hemostatic cutting instrument we developed a 1.94
micrometer Laser-Scalpel system in a porcine model.
We evaluated data for partial porcine kidney resection performed by a 1.94
micrometer Laser-Scalpel and compared the data to those of a standard HF- (High-
Frequency) dissection device. In 12 pigs general anesthesia and a median
laparotomy was performed to expose both kidneys. In each pig one kidney was
partially resected with the Laser-Scalpel and the other side with the HF-dissection
device. The first 6 pigs were euthanized immediately after the procedure. The
following 6 pigs were allowed to recover and underwent 2-3 weeks later euthanasia.
The final evaluation data included total resection time, blood loss, mass of dissected
tissue, total ischemic time and histological examination.
Mean resected kidney tissue mass was 4.75 g with the laser system and 5.57 g for
the HF-dissector, respectively. Mean estimated blood loss was 22 ml for the Laser-
Scalpel and 78.2 ml for the HF-dissection device. Resection time was 9.45 min for the Laser-scalpel compared to 10.16 min. No complications, specifically no
postoperative bleeding, occured in any of the animals. Histological evaluation with
H&E staining showed a carbonized zone of about 0.57 mm directly at the dissected
edge followed by a thermal damaged zone of about 1.25 mm in width. Thereafter
healthy tissue was found in all histological samples.
Partial kidney resection was easily and fast performed by the use of a 1.94
micrometer Laser-Scalpel system. Hemostasis was highly sufficient, so blood loss
was minimal compared to conventional HF-dissection device. Therefore the 1.94
micrometer Laser-Scalpel system is a very promising dissection device for urological
surgery.
A 1.94 μm Laser-Scalpel system (Starmedtec, Starnberg, Germany) emitting at a wavelength at 1.94 μm and a max. laser
power 18 W (cw mode) was used for partial resection of porcine kidney. Partial resection was performed on 9 kidneys.
Six kidneys were extracted directly after resection and pigs were sacrificed. The remaining three pigs were kept alive for
2 weeks post resection in order to investigate the healing process. No complications during the operation and during the 2
weeks survival period were found. Total resection time including haemostasis of the remaining tissue was 10.2±6.5 min,
blood lost was 14.7±29 ml. Haemostatic property of the 1.94 μm Laser-Scalpel was sufficient. Histological evaluation
with H&E stained tissue samples showed medium carbonization and a thermal damage zone of 1mm. In conclusion, the
first experiments show that the 1.94 μm Laser-Scalpel is a very promising device for bloodless and fast kidney resection.
Background: Laparoscopic surgery especially relies on hemostatic dissection. In this study, a new laser scalpel suitable
for laparoscopic surgery due to its rapid dissection and adequate hemostatic properties is presented. Methods: A
Thulium YAG laser system (λ= 2.01 μm) was used at power levels of 15- 20 W transmitted via 365μm core diameter
quartz fiber. Arteries and veins of 0.25-3.2 mm inner diameter were coagulated and transected with the laser scalpel in
anesthetized pigs, followed by ex vivo measurement of resistance to pressures up to 375 mmHg. Results: Small vessels
(<0.5 mm) were successfully occluded up to 100% (arteries) and 89% (veins), larger vessels (1- 2.3 mm) to 74%
(arteries) and 65% (veins). At 375 mmHg, 30% of veins and 35% of arteries stayed closed during external pressure
measurement. Conclusion: The laser scalpel proved satisfying hemostatic properties for small vessels, for medium-sized
vessels alternative coagulating devices must serve as possible back-up.
The potential of a new continuous wave Thulium YAG laser is investigated for tissue ablation and cutting focusing on applications in minimally invasive surgery. The laser emits at a wavelength of 2.01μm, which is well suited for tissue ablation due to its high absorption by water. The laser power can be tuned up to 60 W output through a 365 μm core diameter quartz fibre. For the ablation studies, the quartz fibre was placed in contact under various pressures (20 to 90mN) to porcine liver under saline solution in vitro at angles varying between 30° to 60°. The influence of different powers (10 to 60W) and cutting velocities (2 to 10mm/s) on the incision depth and coagulation zones of the tissue were investigated. A maximum incision depth of 3.3 mm was found with a power of 60W, a cutting velocity of 2mm/s and a fibre-tissue angle of 45°. The incisions were surrounded by coagulated tissue between 0.4 and 0.8mm in thickness, sometimes with an inner zone of carbonization of 0.2mm on average. In conclusion, the first experiments show that a cw Thulium laser is very well suited for tissue dissection as required in minimally invasive surgery.
Background. Endocardial laser revascularization (ELR) is a new technique to treat patients with severe coronary artery disease (CAD) in a percutaneous approach. The results show a significant improvement of symptoms, but the mechanism of action is still unknown. One main theory is the angiogenesis for which Vascular Endothelial Growth Factor (VEGF) is the keypromotor. We investigated immunohistochemically the VEGF-expression after ELR in porcine hearts over a timeperiod of four weeks. Methods. ELR was performed with a single-pulse Thulium:YAG laser. 15 pigs were treated with ELR and the hearts were harvested at five timeperiods: directly (group I), 3 days (group II), 1 week (group III), 2 weeks (group IV) and 4 weeks (group V) after ELR. Each group consisted of three pigs. Immunohistochemically the VEGF-expression was assessed by staining with a polyclonal antibody against VEGF and cellcounting using an expression index (VEGF-EI) Results. A maximum of VEGF-expression was found three days (group II) after ELR with a VEGF-EI of 97%. At 1 week (group III) the VEGF-EI was similar high with 93%. Along the timecourse the index decreased to 22% at 4 weeks (groupV). Conclusions. Our findings show that ELR leads to an local upregulation of VEGF around the channels. The resulting angiogenesis could be the mechanism for the relief of angina.
The objective of this study is to compare the ablation sites induced by two different laser and application systems for myocardial laser revascularization. One system used was an 800 W CO2 laser, which is clinically established for transmyocardial laser revascularization (TMR). The second system was a self-designed Holmium laser emitting single high energy pulses for the minimal invasive approach of endocardial laser revascularization (ELR), whereby the laser light is transmitted via optical fiber into the left ventricle to ablate the myocardial channels from the inside. The laser energy was applied to Polyacrylamide (PAA) as transparent tissue phantom and in water as blood phantom. The ablation dynamics were investigated by high speed flash photography recording a picture series of a single event. Reperfused ex- vivo porcine hearts were treated to quantify differences in the thermal-mechanical damage ranges by polarization light microscopy. Ablation dynamics in water revealed oscillatory changes of the axial length of the steam bubbles between 3 mm and 12 mm during the CO2 laser pulse. For the Holmium laser pulse a maximal axial and lateral length of 5 mm was observed. The lateral dimensions of the bubbles were maximal 1 mm with the CO2- and 3.5 mm with the Holmium laser system. In PAA bubbles also collapse during the laser pulse which affects the size of the ablated channels. Using 12 J Holmium laser pulses for ablation of PAA, channel depths around 7 mm were found. Single Holmium laser pulses demonstrate ablations comparable in size and thermal- mechanical collateral damage to those achieved with the standard CO2 laser. The results are very encouraging for single pulse ELR and demonstrate the potential of a catheter based minimal invasive procedure for laser heart reperfusion.
The influence of spherical aberrations on laser-induced plasma formation in water by 6-ns Nd:YAG laser pulses of 1064 nm wavelength was investigated. Experiments and numerical calculations were carried out for focusing angles similar to those used for intraocular microsurgery. Wave form distortions of 5.5 lambda and 18.5 lambda between the optical axis and the 1/e2 irradiance values of the laser beam were introduced by replacing laser achromats in the delivery system by plano- convex lenses. Aberrations of 18.5 lambda led to an increase of the energy threshold by a factor of 8.5. The threshold irradiance calculated using the diffraction limited spot size was 10 times increased as compared to the case of minimized aberrations and 48 times larger than the actual threshold. The threshold calculated with the measured focus diameter was, on the other hand, reduced by a factor of 35. This reduction is due to the presence of hot spots in the focal region of the aberrated laser beam. In these hot spots, the threshold irradiance is probably unchanged, but the threshold value is reduced when averaged over the whole measured diameter of the beam waist. The determination of breakdown threshold in the presence of aberrations leads, hence, to strongly erroneous results. In the presence of aberrations, the plasmas are up to 3 times longer and the transmitted energy is 17 - 20 times larger than without aberrations. Aberrations can thus strongly compromise the precision and safety of intraocular microsurgery where they may arise through the use of inappropriate contact lenses, tilting of the lens, and oblique light passage through the ocular media. They can further account for a major part of the differences in breakdown threshold and plasma transmission values reported in previous investigations.
Nonlinear absorption through laser-induced breakdown (LIB) offers the possibility of localized energy deposition in linearly transparent media and thus of non-invasive surgery inside the eye. The general sequence of events--plasma formation, stress wave emission, cavitation--is always the same, but the detailed characteristics of these processes depend strongly on the laser pulse duration. The various aspects of LIB are reviewed for pulse durations between 80 ns and 100 fs, and it is discussed, how their dependence on pulse duration can be used to control the efficacy of surgical procedures and the amount of collateral effects.
During optical breakdown, the energy delivered to the sample is either transmitted, reflected, scattered, or absorbed. The absorbed energy can be further divided into the energy required to evaporate the focal volume, the energy radiated by the luminescent plasma, and the energy contributing to the mechanical effects such as shock wave emission and cavitation. The partition of the pulse energy between these channels was investigated for 4 selected laser parameters (6 ns pulses of 1 and 10 mJ, 30 ps pulses of 50 (mu) J and 1 mJ, all at 1064 nm). The results indicated that the scattering and reflection by the plasma is small compared to plasma transmission. The plasma absorption can therefore be approximated by A approximately equals (1-T). The ratio of the shock wave energy and cavitation bubble energy was found to be approximately constant (between 1.5:1 and 2:1). For a more comprehensive study of the influence of pulse duration and focusing angle on the energy partition, we therefore restricted our measurements to the plasma transmission and the cavitation bubble energy. The bubble energy was used as an indicator for the total amount of mechanical energy produced. We found that the plasma absorption first decreases strongly with decreasing pulse duration, but increases again for pulses shorter than 3 ps. The conversion of the absorbed energy into mechanical energy is approximately equal to 90% with ns-pulses at large focusing angles. It decreases both with decreasing focusing angle and pulse duration (to less than or equal to 15% for fs-pulses). The disruptive character of plasma-mediated laser surgery is therefore reduced with ultrashort laser pulses.
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