Robert Splinter

Quantifying spatial localization of optical mapping using Monte Carlo simulations

Optical mapping techniques used to study spatial distributions of cardiac activity can be divided into two categories; (1) broad-field excitation method, in which hearts stained with voltage or calcium sensitive dyes are illuminated with broad-field excitation light and fluorescence is collected by image or photodiode arrays; (2) laser scanning method, in which illumination uses a scanning laser and fluorescence is collected with a photomultiplier tube. The spatial localization of the fluorescence signal for these two methods is unknown and may depend upon light absorption and scattering at both excitation and emission wavelengths. We measured the absorption coefficients (/spl mu//sub a/), scattering coefficients (/spl mu//sub s/), and scattering anisotropy coefficients (g) at representative excitation and emission wavelengths in rabbit heart tissue stained with di-4-ANEPPS or co-stained with both Rh237 and Oregon Green 488 BAPTA 1. Monte Carlo models were then used to simulate absorption and scattering of excitation light and fluorescence emission light for both broad-field and laser methods in three-dimensional tissue. Contributions of local emissions throughout the tissue to fluorescence collected from the tissue surface were determined for both methods. Our results show that spatial localization depends on the light absorption and scattering in tissue and on the optical mapping method that is used. A tissue region larger than the laser beam or collecting area of the array element contributes to the optical recordings.

MODIFICATION OF ATRIOVENTRICULAR NODE TRANSMISSION PROPERTIES BY INTRAOPERATIVE NEODYMIUM-YAG LASER PHOTOCOAGULATION IN DOGS

The purpose of this study was to test the feasibility of neodymium-yttrium-aluminum-garnet (Nd-YAG) laser photocoagulation of the atrioventricular (AV) node to control the ventricular rate during rapid atrial rhythms without creating AV block. In 12 dogs on normothermic cardiopulmonary bypass, short laser pulses were delivered to an area between the coronary sinus orifice and the site of the most proximally recorded His deflection until second degree AV block occurred at a paced atrial rate of 200 beats/min. Long-term effects on AV node function were followed up for 3 months. Three animals developed chronic high grade AV block. In nine animals with preserved 1:1 conduction, the mean (+/- SEM) critical atrial cycle length resulting in AV node Wenckebach periodicity increased from 183 +/- 6 to 261 +/- 24 ms (+43%), the mean RR interval during induced atrial fibrillation increased from 248 +/- 14 to 330 +/- 27 ms (+32%) and the shortest RR interval during atrial fibrillation increased from 215 +/- 11 to 275 +/- 20 ms (+28%). Laser effects were not reversed by isoproterenol infusion. Histologic examination of the irradiated area showed fibrotic changes in the AV node and fatty metamorphosis. This study suggests that 1) graded Nd-YAG laser photocoagulation of the AV node region in dogs results in long-term modification of anterograde AV node transmission properties; 2) 1:1 conduction during sinus rhythm usually remains preserved, but ventricular rate during rapid atrial rhythms is chronically reduced; and 3) progression to high grade AV block occurs in a minority of animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature distribution in dental tissue after interaction with femtosecond laser pulses

Algebraic and numerical solutions are presented of the temperature rise in dental tissue due to interaction with ultrashort optical radiation. Results of the studies with femtosecond laser pulses show agreement between theory and experiment. A temperature rise of typically 5 K is found for a 40 millisecond train of 7 nJ, 70 fs laser pulses at a repetition rate of 80 MHz. The peak irradiance in our experimental studies was limited to 3x10(6) W/cm(2). Applications include photoacoustic imaging and tomography of dental tissue.

Catheterization Technique for Laser Photoahlation of Atrioventricular Conduction from the Aortic Root in Dogs

A simple catheterization technique for producing suppression or block of atrioventricular (AV) conduction in dogs by neodymium:YAG (Nd:YAG) laser photocoagulationfrom the aortic root is described. In 14 canines, electrode catheters were percutaneously advanced to the high right atrium, to the His bundle area at the tricuspid valve (right His (RHJ), and the noncoronary cusp of the aortic valve (left His [LH]). A 400μ optical fiber was fed into the lumen of the LH catheter. Continuous wave Nd:YAG lasing at λ= 1,064 nm at 15–20 watts was performed at the site of the largest recorded LH deflection, opposite ta the RH catheter, until complete AV block (CAVB, group I, eight dogs) or persistent A‐H prolongation (AHPr, group 11, four dogs) occurred; in two animaJs, laser photoablation was unsuccessful. CAVB and AHPr were achieved in < 140 seconds of lasing. Mean lasing time was 51 ± 41 seconds, total energy delivered, 776 ± 601 J. There were no complications. Dogs were followed for up to 120 days. In group I, 6/6 dogs followed /or at least I week continued to have CAVB (three AV nodal, five infra‐His). In group II, 1/4 dog progressed to infra‐His CAVB; 3/4 had depressed AV nodal function. AV block was not reversed by isoproterenol. Aortic root angiograms revealed intact valve function in all. Histology showed various degrees of damage and repair of AV functional tissue. There were no valvular perforations. Transcatheter Nd:YAG laser photocoagulation of the AV junction from the aortic root is a fast, simple, and safe method for creating AV conduction block in dogs.

Selective coagulation necrosis of canine adventitia and media induces extracellular matrix accumulation without neointima formation

Endothelial cell injury, the disruption of the internal elastic membrane and medial damage represent important stimuli for the development of a neointima. It is unclear whether selective adventitial and medial injury also induce neointima formation. Incremental argon laser energies (11.4-180 J/cm2) were applied to the external surface of dog femoral arteries to evaluate the vascular repair of acute adventitial or medial necrosis without injury of the intima. The animals were sacrificed either one hour after the initial procedure or after an 8 week follow up period for histologic examination. Acute, and mild to moderate necrosis of the arterial wall was found above 50 J/cm2. Ablation of the internal elastic membrane or mural thrombi was not detected. Eight weeks after photocoagulation with laser energies above 50 J/cm2, a significant increase in mean wall thickness of the media was observed. The medial thickening was characterised by an accumulation of extracellular matrix and a loss of smooth muscle cells. Necrosis of adventitia and media resulted in arterial wall thickening without neointima formation. It is concluded that, in dogs, an acute, selective injury of adventitia and media stimulates the production of extracellular matrix and not the proliferation of cells. Smooth muscle cell migration and subsequently neointima formation are induced by viable smooth muscle cells when blood-borne stimuli are available.

Heat-induced changes in optical properties of human whole blood in vitro

The effect of anomalous optical behavior of biological tissue at high-intensity laser irradiation can be caused by heat- induced changes in optical properties of consisting components, mainly muscle tissue and blood. We registered the spectral transmission of fresh human whole blood and serum samples in the wavelength range of 300 - 700 nm at the heating of samples in the temperature range of 35 - 65 degrees Celsius. The results showed an increase of 10 - 15% in the transmission of blood serum at the temperature rising up to 50 - 60 degrees Celsius. In the case of diluted whole blood a sharply enhanced transmission was observed at the temperature of 56 - 60 degrees Celsius, while further heating resulted in a decreased transmission down to the initial level. The significant changes (of a three orders of magnitude) in the transmission of whole blood at the wavelength of Nd:YAG laser (1064 nm) were observed. The obtained results can be considered as one of the possible explanations of the anomalous light distribution in certain tissues.

Selective elimination of retrograde conduction by intraoperative neodymium: YAG laser photocoagulation in dogs

OBJECTIVES The purpose of this study was to test the feasibility of selective elimination of ventriculoatrial (VA) conduction by limited laser photocoagulation of the atrioventricular (AV) node, and to analyze the histologic substrate of unidirectional retrograde block. BACKGROUND Atrioventricular node reentry requires intact retrograde conduction. METHODS Neodymium:yttrium-aluminum-garnet laser photocoagulation was performed during cardiopulmonary bypass through a right atriotomy in 15 dogs that had intact retrograde conduction before operation. Short laser pulses were delivered to an area between the coronary sinus orifice and the proximal His bundle. The end point of lasing was second-degree AV node block at a paced atrial cycle length of 250 ms. RESULTS Complete retrograde block developed immediately in 11 of the 15 dogs (group I), while AV conduction persisted in all 11. In 4 of the 15 dogs (group II), both AV and VA conduction remained intact. During a 3-month follow-up period, retrograde conduction remained absent in all group I dogs. Retrograde block was not reversed by isoproterenol. Anterograde AV node characteristics (Wenckebach cycle length, functional refractory period, ventricular rate during atrial fibrillation) were unchanged in five dogs and modified in six. Complete AV block did not develop. In four control dogs (group III, sham operation), anterograde and retrograde AV node characteristics were unchanged. The anterograde Wenckebach cycle lengths in groups I, II and III at 3 months measured 192 +/- 15 ms, 195 +/- 6 ms and 170 +/- 22 ms, respectively, whereas the retrograde Wenckebach cycle lengths in groups II and III measured 345 +/- 62 ms and 278 +/- 25 ms, respectively. Histologic study at 3 months in cases with unidirectional VA block showed the compact part of the AV node intact with destruction of the atrial approaches and the superficial layers of the proximal end of the node on the right side. CONCLUSIONS 1) With limited laser photocoagulation of the proximal AV node area, VA conduction can be eliminated and anterograde AV node transmission maintained. 2) Destruction of the atrial approaches on the right side with preservation of the compact part of the AV node may result in unidirectional retrograde block.

Determination of optical properties in situ from radial backscattering profile using a CCD camera

Local tissue optical parameters are of principal importance in determining the ideal dosage of laser energy and choosing the most suitable therapeutic wavelength for a specific organ. We have developed a non-invasive method for the determination of optical parameters of whole tissue in-situ. Backscattered laser light from a pencil beam incident on the tissue surface was measured using a CCD camera interfaced with a personal computer. Backscatter measurements were performed on in-vitro canine myocardium and on phantoms, with optical properties predetermined by the integrating sphere method. Backscattering radiance proved to be a function of the distance to the incident beam. Regression analysis of the backscatter showed that the backscatter can be expressed as an exponential function of the distance to the beam with a regression coefficient ((mu) r) resembling the tissue diffusion coefficient ((mu) eff), for tissue thickness much greater than optical free path. The CCD derived backscatter radiance exponent showed excellent agreement with the diffusion coefficient, with a correlation coefficient of 0.91. The backscattering technique might prove to be a simple, non-invasive, accurate new technique in the determination of in-vivo optical properties.

Electrophysiologic characteristics of manifest and latent retrograde conduction in dogs

Atrioventricular (AV) nodal reentry requires intact retrograde ventriculoatrial (VA) conduction. The purpose of this study was to assess the contribution of various pacing and pharmacologic techniques to uncover VA conduction during apparent unidirectional VA block, and to evaluate the role of several biologic and electrophysiologic factors in concealment of retrograde conduction. Forty healthy dogs underwent catheter-electrophysiologic studies of AV and VA conduction. Group I (20 animals) had intact VA conduction. Group II (six animals) had VA dissociation with ventricular pacing initiated during sinus rhythm, but the presence of VA conduction was confirmed by isoproterenol infusion or by premature ventricular stimulation. In group III (14 animals), the above techniques failed to uncover VA conduction. Eight of 14 group III animals underwent thoracotomy and crushing or freezing of the sinoatrial (SA) node. Ventricular pacing initiated during sinus standstill was associated with 1:1 VA conduction in each experiment. VA conduction time and retrograde Wenckebach cycle length, both in the baseline state and during isoproterenol infusion, were significantly longer in the eight animals in group III than in those in group I. Age, gender, weight, breed, sinus cycle length, and anterograde AV conduction properties were not significantly different between groups I, II, and III. The data suggest that (1) in normal dogs, complete unidirectional VA block probably does not exist; (2) in the presence of anterograde input to the AV node, even sophisticated pacing and pharmacologic maneuvers may fail to uncover the presence of VA conduction; (3) although anterograde input is essential for concealment of VA conduction, the phenomenon is more closely associated with depressed retrograde than with anterograde AV nodal characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

High Resolution Biological Visualization Techniques

Imaging with ionizing radiation is an integral part in today’s medicine. For example, breast tissue is more radiosensitive during puberty and exposures to the chest during that time would impose a higher risk of radiation-induced breast cancer (Doody et al., 2000). The use of ionizing radiation for medical imaging has significantly increased in the past 20 years, which has caused the effective dose per individual in the US to increase from 0.53 mSv in the early 1980s to 3 mSv in 2006 as reported by the National Council on Radiation Protection and Measurement. [NCRP No. 160, 93] This has naturally raised concerns about the possible risks that are associated with multiple exposures from diagnostic sources of radiation. There is a huge effort towards creating imaging modalities that can visualize tissue with a reduced dose to the patient without loss of image quality. Therefore, it would be advantageous to develop visualization techniques that employ non-ionizing radiation and that provide comparable high-resolution image quality as the currently available modalities. This is one reason why Magnetic Resonance Imaging (MRI) technology is gaining so much popularity in spite of the cost and the length of the exams. The non-invasive nature of light in the near-infrared region and its reduced attenuation in tissue are the reasons why optical imaging is gaining popularity. However, tissue scatter is a major limitation in optical imaging techniques. It is possible to combine optical imaging and ultrasound to produce images. This is accomplished by the use of ultra-short laser pulses that are absorbed in the medium, which cause the temperature of the material to rapidly rise and fall. Consequently, the irradiated volume becomes an internal source of ultrasonic waves, which can be used to obtain diagnostic information. Photo-acoustic spectroscopy has been shown to be a great tool to successfully visualize and characterize: • blood vessels (Zhang et al. 2009; Hoelen et al. 1998; Sethuraman, S. et al. 2006); • breast cancer (Ermilov et al. 2009); • brain in vivo (Wang, X. D. et al. 2003; Yang, X. et al. 2007; Lingu et al. 2007); • prostate lesions (Yaseen et al. 2010). Photo-acoustic spectroscopy has also been considered for detection of caries in its early stage (Kim et al. 2006), analysis of dental materials (Coloiano et al. 2005) and potentially for visualizations under porcelain crowns. (Pike et al. 2007) New clinical demands for imaging are continuously emerging, placing different constraints on the imaging requirements and