Medizinisches Laserzentrum Lübeck (Germany)

When illuminated with green light, tissue shows negligible autofluorescence in comparison to urinary stones. In automatically controlled lithotripsy, this property is utilized to prevent the laser from being triggered if the fiber is mispositioned: the fluorescence signal is compared to a set threshold before each pulse. However, previous studies have shown that tissue damage cannot be completely ruled out. We are investigating this phenomenon and its impact on fluorescence guidance. An experiment with porcine calyx (with the automatic control switched off) shows that single Ho:YAG laser pulses are sufficient to coagulate tissue, resulting in an increase in autofluorescence. During lithotripsy of fluorescent artificial stones embedded in renal cortex, thermal damage occurs despite automatic laser control. Maximum fluorescence values measured on those tissue places were above the control's set threshold for laser emission. Therefore, an increase in autofluorescence in the event of denaturation must be considered when using automatically controlled lithotripsy.

In intraoperative optical coherence tomography (OCT), the dynamic range (DNR) and its roll-off are critical parameters to simultaneously image weak scattering and highly reflective surfaces and maintain image fidelity across the entire axial imaging range. This study demonstrates improved DNR roll-off performance using a high-finesse Fabry-Pérot tunable filter (FFP-TF) in a Fourier domain mode-locked (FDML) laser. Comparative measurements between high- and low-finesse filters reveal that increased finesse reduces intensity noise and flattens the roll-off of a 60.5-dB dynamic range from 3.71 dB/mm to 1.33 dB/mm, highlighting the critical role of FFP-TF finesse in FDML laser optimization.

The technical development of diode lasers with regard to wavelengths, output powers and pulse durations in the order of microseconds is expanding the range of medical applications. The 445 nm wavelength, in particular, offers a promising approach. Such a laser system is now available, and its technical and optical properties are introduced here. To characterize the diode laser with a wavelength of 445 nm, laser settings were recorded in both CW and pulse modes in the range µs to ms, with duty-cycles of 1% up to 40% (partially also up to 90%). At the same time, the temporal behaviour of the laser pulses was documented. In addition, the influence of different irradiation parameters for pulse durations of 10 µs, 100 µs and 1 ms at duty-cycles of 50%, 25% and 15% on porcine tissue was investigated. With regard to the cutting depth and thermal damage zones, cuts were carried out in the contact mode using an optical fiber with a core diameter of 375 µm and a cut speed of 3 mm/s. The average power was set between 2 W and 8 W. Output powers of up to 12.7 W in CW mode and peak powers > 30 W, with pulse durations of 5 and 10 µs, were achieved. Cuts with a depth of up to 1 mm could be created using 100 µs pulses with different duty-cycles and also in CW mode. The width of the thermal damage zone was between 200 and 350 µm for all settings. The 445 nm wavelength is characterized by a large number of possible advantages in surgical applications. Pulse durations down to the µs-range with peak power approximately three times higher than average power could open up new applications.

Laser treatments of the retina lack objective dosing and dosing control, especially for non-damaging thermally stimulating irradiations without any visible endpoint. Over the last years, a real-time optoacoustic temperature determination has been developed and has been successfully demonstrated to determine the temperature course during retinal photocoagulation in clinical study on diabetic macula oedema (DME). In recent work, the technique has been extended towards an automatic closed loop temperature control, which is now been used in a first clinical study on patients suffering from central serous chorioretinopathy (CSCR). A standard commercial ophthalmic 532 nm laser was used for treatment with a standard laser link at a slitlamp. In between the ophthalmic laser and the slitlamp a separate control unit was implemented, containing a pulsed probe laser (523 nm, 75 ns, 3 kHz) for optoacoustic temperature determination and a PID-controller steering an acoustooptic modulator to control the power of the treatment laser with a rate of 3 kHz. For therapy, laser spots around the feeder vessel with a transient temperature rise to 51 °C were placed. So far 8 patients were treated, all patients will be observed over a period of 3 months. No adverse effects owing to the treatment has been observed. As interim results, temperature control could be achieved in most laser spots. 6 patients showed a complete or partially resolved subretinal fluid and all patients an increased or stable visual acuity.

Differential photoacoustic spectroscopy (DPAS) cells are usually excited on the first longitudinal ring mode, with a microphone situated in the middle of each of the two resonator tubes. However, it is known from other photoacoustic spectroscopy cell designs that connecting the microphones via a capillary can lead to signal enhancement. By means of finite element method (FEM) simulations, we compared such a photoacoustic spectroscopy (PAS) cell with a capillary to a DPAS cell with a capillary attached to each of the two resonators and showed that the behavior of both systems is qualitatively the same: In both the PAS and the DPAS cell, in-phase and anti-phase oscillations of the coupled system (resonator-capillary) can be excited. In the DPAS cell, capillaries of suitable length also increase the pressure signal at the microphones according to the FEM simulations. For different capillary diameters (1.2 mm/1.7 mm/2.2 mm), the respective optimal capillary length (36-37.5 mm) and signal amplification was determined (94%, 70%, 53%). According to the results of these FEM simulations, a significant increase in sensitivity can, therefore, also be achieved in DPAS cells by expanding them with thin tubes leading to the microphones.

Circumferential scanning in endoscopic imaging is crucial across various disciplines, and optical coherence tomography (OCT) is often the preferred choice due to its high-speed, high-resolution, and micron-scale imaging capabilities. Moreover, real-time and high-speed 3D endoscopy is a pivotal technology for medical screening and precise surgical guidance, among other applications. However, challenges such as image jitter and non-uniform rotational distortion (NURD) are persistent obstacles that hinder real-time visualization during high-speed OCT procedures. To address this issue, we developed an innovative, low-cost endoscope that employs a brushless DC motor for scanning, and a sensorless technique for triggering and synchronizing OCT imaging with the scanning motor. This sensorless approach uses the motor's electrical feedback (back electromotive force, BEMF) as a virtual Hall sensor to initiate OCT image acquisition and synchronize it with a Fourier Domain Mode-Locked (FDML)-based Megahertz OCT system. Notably, the implementation of BEMF-triggered OCT has led to a substantial reduction in image jitter and NURD (<4 mrad), thereby opening up a new window for real-time visualization capabilities. This approach suggests potential benefits across various applications, aiming to provide a more accurate, deployable, and cost-effective solution. Subsequent studies can explore the adaptability of this system to specific clinical scenarios and its performance under practical endoscopic conditions.

To investigate fluorescence lifetime of mouse models of age-related macular degeneration (AMD) by fluorescence lifetime imaging ophthalmoscopy (FLIO). Two AMD mouse models, apolipoprotein E knockout (ApoE-/-) mice and NF-E2-related factor-2 knockout (Nrf2-/-) mice, and their wild-type mice underwent monthly ophthalmic examinations including FLIO from 3months of age. After euthanasia at the age of 6 or 11months, blood plasma was collected to determine total antioxidant capacity and eyes were enucleated for Oil red O (ORO) lipid staining of chorioretinal tissue. In FLIO, the mean fluorescence lifetime (τm) of wild type shortened with age in both spectral channels. In short spectral channel, τm shortening was observed in both AMD models as well, but its rate was more pronounced in ApoE-/- mice and significantly different from the other strains as months of age progressed. In contrast, in long spectral channel, both model strains showed completely opposite trends, with τm becoming shorter in ApoE-/- and longer in Nrf2-/- mice than the others. Oil red O staining at Bruch's membrane was significantly stronger in ApoE-/- mice at 11months than the other strains. Plasma total antioxidant capacity was highest in ApoE-/- mice at both 6 and 11months. The two AMD mouse models exhibited largely different fundus fluorescence lifetime, which might be related to the different systemic metabolic state. FLIO might be able to indicate different metabolic states of eyes at risk for AMD. This animal study may provide new insights into the relationship between early AMD-associated metabolic changes and FLIO findings.

During neuro-oncologic surgery, phase-sensitive optical coherence elastography (OCE) can be valuable for distinguishing between healthy and diseased tissue. However, the phase unwrapping process required to retrieve the original phase signal is a challenging and critical task. To address this issue, we demonstrate a one-dimensional unwrapping algorithm that recovers the phase signal from a 3.2 MHz OCE system. With a processing time of approximately 0.11 s per frame on the GPU, multiple 2π wraps are detected and corrected. By utilizing this approach, exact and reproducible information on tissue deformation can be obtained with pixel accuracy over the entire acquisition time. Measurements of brain tumor-mimicking phantoms and human ex vivo brain tumor samples verified the algorithm's reliability. The tissue samples were subjected to a 200 ms short air pulse. A correlation with histological findings confirmed the algorithm's dependability.