Automatic temperature controlled retinal photocoagulation

Abstract

Laser coagulation is a treatment method for many retinal diseases. Due to variations in fundus pigmentation and light scattering inside the eye globe, different lesion strengths are often achieved. The aim of this work is to realize an automatic feedback algorithm to generate desired lesion strengths by controlling the retinal temperature increase with the irradiation time. Optoacoustics afford non-invasive retinal temperature monitoring during laser treatment. A 75 ns/523 nm Q-switched Nd:YLF laser was used to excite the temperature-dependent pressure amplitudes, which were detected at the cornea by an ultrasonic transducer embedded in a contact lens. A 532 nm continuous wave Nd:YAG laser served for photocoagulation. The ED50 temperatures, for which the probability of ophthalmoscopically visible lesions after one hour in vivo in rabbits was 50%, varied from 63°C for 20 ms to 49°C for 400 ms. Arrhenius parameters were extracted as ΔE=273 J mol(-1) and A=3 x 10(44) s(-1). Control algorithms for mild and strong lesions were developed, which led to average lesion diameters of 162 ± 34 μm and 189 ± 34 μm, respectively. It could be demonstrated that the sizes of the automatically controlled lesions were widely independent of the treatment laser power and the retinal pigmentation.

Temperature-Controlled Retinal Photocoagulation Reliably Generates Uniform Subvisible, Mild, or Moderate Lesions

“…The linear correlations of treatment laser power with ophthalmoscopic diameter review the advantage of automatically controlled TTC photocoagulation. 18,19 In standard, 200 ms lesions, the lesion diameters grows linearly with power (R 2 ¼ 0.5, left), while this correlation is lost in TTC lesions (R 2 ¼ 0.0, right, here for TTC subgroup two lesions of the present study). As TTC lesions are independent of power, they will not require power titration in a clinical setting and be uniform throughout patients and fundus locations, largely irrespective of local pigmentation and transmission.…”

“…The treatment device measures fundus temperature increments noninvasively in realtime with 1-kHz sampling rate 16 and stops the treatment laser, when a predefined TTC criterion is achieved. 18 Five different TTC intensities were evaluated clinically in order to define adequate TTC criteria for subvisible and ETDRS intensity treatments.…”

“…The characteristic curve for a 95% probability to create visible lesions of the desired size (expected dose 95/ED95) was determined previously. 18,19 The characteristic curve was fitted using the damage integral from the Arrhenius theory, which describes the relationship between thermal denaturation of proteins over time and the temperature course, 18,23 by stepwise time integration over the expected denaturation or damage, respectively. The Figure 2.…”

“…[15][16][17] Based on temperature measurements, we implemented automatic exposure time control that produces homogenous lesions widely independently of laser power or local absorption, respectively. 18,19 The treatment device measures fundus temperature increments noninvasively in real-time with 1-kHz sampling rate and stops the treatment laser, when a predefined timetemperature characteristic (TTC) criterion is achieved. While in standard lesions with fixed exposure time, the ophthalmoscopic lesion diameter grows linearly with laser power, automatically TTC controlled lesions lose the correlation of power and lesion diameter (Fig.…”

Two-dimensional micro-displacement measurement for laser coagulation using optical coherence tomography

“…For example, optoacoustic signals are being used to measure the laser-induced temperature increase [5], which has enabled temperature controlled photocoagulation [6,7]. Another example is the use of optical coherence tomography (OCT) [8].…”