2.1.

Invasive Temperature Measurements

Thermocouples, thermistors, and silicon-based sensors are commonly used to measure temperatures invasively. The principle of the sensors is either based on the temperature dependence of contact voltage or resistance between two materials. Their characteristics are described in detail elsewhere.^{4, 5, 6} Small sensors can be positioned invasively underneath the tissue surface to monitor the temperature at one position in time. Accuracy within $0.1°\mathrm{C}$ can be achieved if precautions are taken to shield interference from the environment. Although the sensors can be as small as $100\mathrm{\mu }\mathrm{m}$ , they are still interfering with the environment when used invasively. During laser irradiation, the sensor can absorb light itself, resulting in an error of the temperature reading. By irradiating the tissue intermittently, it is possible to correct for this error.⁷ Invasive sensors provide a temperature reading at one position only, which is a limitation to determine temperature distributions during laser-tissue interaction.

2.2.

Noninvasive Temperature Measurements

Temperature measurements without touching the object are typically performed using infrared sensors detecting the “heat” irradiated by the object. This technique, thermography, has been used in medicine for many years to measure skin temperature distribution providing information on blood perfusion for diagnosing or detecting malignancies.^{8, 9} Infrared cameras have been used for many years and technology has greatly improved the detectors and method of cooling. Smart software packages and libraries enable correction for material characteristics and emissivity to obtain calibrated temperatures. However, only temperatures at a surface can be visualized, which is a limitation of the ir camera when studying laser-tissue interaction.² However, the data at the surface is useful to extrapolate to temperature distribution underneath the surface.

There are three emerging techniques to measure temperatures inside tissue in a noninvasive way: magnetic resonance imaging (MRI), microwave radiometry, ultrasound thermometry.

MRI has proven to be a very promising technique to monitor temperature changes for diagnosis and during minimal invasive treatments.^{10, 11} The proton resonance frequency (PRF) used for MRI has a linear temperature dependency and is relatively independent for tissue type. These characteristics make PRF a perfect technique for temperature measurements in real time. At this moment, a resolution of $1°\mathrm{C}$ , within $1\mathrm{s}$ in a tissue area of $2\mathrm{mm}$ is achieved; however, with evolving techniques, higher resolutions can be expected in near future.

Another technique is based on microwave radiometry.¹² By measuring the microwave electromagnetic thermal noise, temperature information can be obtained. This technique is able to provide temperature information at a depth of up to several centimeters in subcutaneous tissues.

The use of ultrasound for noninvasive thermometry is under investigation.^{13, 14, 15} In theory it should be possible to acquire temperature data by measuring thermally induced modifications in backscattered rf echoes due to thermal expansion and local changes in the speed of sound. However, there are still many problems to solve before reliable temperature data can be obtained.

The temperature measuring techniques described above have advantages and limitations as to invasive/noninvasive use, resolution, or system size and expense.

This paper discusses a relatively simple multipurpose setup for research that enables the visualization of temperature gradients underneath the surface of a (model) tissue with high spatial and temporal resolution in combination with calibrated temperature measurements.

3.1.

Color Schlieren Techniques

An overview of all the components of the setup is shown in Fig. 1. Conceptually, the setup is an optical processor as is schematically shown in Fig. 2 . A continuous or pulsed white light source is coupled into a fiber. The light emitted from a ball-shaped tip is focused, providing a point source and divergences subsequently. The focal point of a collimating lens coincides with the focus of the beam. The diameter of the lens is matched with the divergence of the beam so that all light is collimated. A rectangular tank filled with a medium (e.g., water or tissue) is positioned between the collimating and imaging lens, the “object” plane. The walls are perpendicular to the parallel beam to prevent any optical distortion due to refraction. Within this tank, conditions are created to study laser-tissue interaction. The imaging lens will focus the parallel beam in its focal point on the optical axis. However, due to variations in the refraction index or irregularities in the medium in the object plane induced by, for example, temperature gradients or local stresses, rays will be deflected. These rays will cross the focal plane at a particular distance $d$ from the optical axis (Fig. 2). The nondistorted rays will be focused on the optical axis. By inserting a mask or a filter in the focal plane of the imaging lens, it is possible to block out the nondeflected rays, preventing them from reaching the image plane or to earmark rays crossing the plane at certain positions. This process of modifying the object information in the focal plane is known as spatial filtering. By blocking the rays crossing the optical axis, only refracted and diffracted rays will pass the filter plane and contribute to an image restoration at the image plane. The result is an enormous contrast enhancement of the image due to the subtraction of the background light (Fig. 3 ).

Fig. 2

Scheme of optical processor for spatial filtering using a either a block filter or a color filter.

Fig. 3

Example of contrast enhancement using the block filter showing convection of water heated by laser fiber emitting ir light absorbed by water.

Depending on the deflection angle $\alpha$ and the focal length of the transform lens, a ray will pass the filter plane at a deflection distance $d$ from the optical axis (Fig. 2): $d=f\tan \alpha$ . The information on the degree of deflection can be preserved by color coding the rays coming through the filter plane using a color filter (Fig. 4 ). This filter consists of concentric rings of discrete color bands separated by small black rings. The center of the filter is a black dot blocking the background light. Adjacent to the black dot, with increasing distance from the center, the colors shift gradually from blue to red. Rays passing the filter plane will be color coded depending on the deflection distance $d$ and will be reconstructed to an image at the image plane. The generated color image will show, depending on the position, the degree of deflection in the object plane. From each color, the deflection angle $\alpha$ can be determined; $\alpha$ is related to the change in the refractive index of the medium in the object plane. The color image can be interpreted as a thermal image when the relation between refractive index and temperature gradient is known. The black rings in the filter will result in black lines in the image separating the discrete colors giving an impression of “isotherms” (Fig. 5 ). The position of the imaging lens, the filter, and the charge-coupled device (CCD) camera are chosen depending on the magnification desired according to the lens formula. The diameter of the filter determines the dynamical range of temperature gradient that can be visualized. Using an $x\text{-}y$ microtranslator, the filter can be optimally aligned on the optical axis. The CCD camera is positioned in the image plane. Additional filters can be used to filter out scattered light from the primary laser wavelength. To obtain microsecond resolution, a video camera with high-speed mode can be used.

Fig. 4

Example of a "rainbow" color filter.

Fig. 5

Example of a color coded image showing the thermal zone around an ablation crater during ${\mathrm{CO}}_2$ laser irradiation. The black lines can be interpreted as isotherms. The left inset shows the normal view.

3.2.

Absolute Temperature Measurements

The color images give a relative impression of temperature distributions as will be discussed in Secs. 3.4, 7.2. To obtain absolute temperatures, thermocouples are positioned in the field of view. Up to six $K$ -type thermocouples (200 to $600\text{-}\mathrm{\mu }\mathrm{m}$ diameter) can be positioned at strategic positions in the tissue being studied to measure the temperature simultaneously during imaging. However, because the thermocouple wires will disturb the image, they are used mostly for calibration of the temperature distribution in a series of experiments (Fig. 6 ). The readings are acquired with a personal computer (PC) and converted to absolute temperatures.

Fig. 6

Example of a thermal image with additional information. The numbered row of dark lines is the shadow of the thermocouples positioned in the transparent tissue at the level of the thermal field. The temperatures of related thermocouples are displayed at the bottom in real time. The image is tagged by a time code and dimensions can be determined.

3.3.

Video Processing

A sensitive, high resolution CCD camera is used for imaging. The camera has a wide range of controls as to exposure time, white balance, and frame coding. The video frames are captured and processed in real time by a PC at video frames rate (up to $30\mathrm{fs}$ ).

Additional information is mixed in the video frames:

3.4.

Interpretation of Color Schlieren Images as Thermal Images

The color Schlieren setup as described produces real-time false color images that give the impression of a thermal image. However, assigning absolute temperatures to the colors in the images is complicated. It is important to understand that the images show temperature gradients that refract the parallel light rays. The “calibration” of the color Schlieren images is highly dependent on the symmetry of the temperature field. The theoretical description of the relation between the colors and temperature gradients is discussed in the Appendix0.⁶

In summary, if the temperature field is unidirectional (e.g., a hot surface as created by a highly absorbed laser wavelength), the relation between color and temperature can be described by a first order polynomial. If the temperature field has an axial distribution, the temperature is related to a ratio derived from the radius of the temperature gradient field and the position of a point in this field (see Appendix0).

Without being able to ascribe temperatures to colors, the images are very useful for relative comparison and study temperature field dynamics.

4.1.

High-Speed Schlieren Imaging

In the setup, the Schlieren techniques can be combined with high-speed imaging. First of all, motion in the field of view can be frozen using a high-speed shutter of the video camera itself normally going down to $100\mathrm{\mu }\mathrm{s}$ . However, a continuous wave white light source will usually not provide sufficient light for a good exposure. High intensity white light pulses from a flash, for example used in photography, can be used if sufficient light can be coupled into the illumination fiber. While using normal video rate exposure of $20\mathrm{ms}$ , the flash will “freeze” motion with pulse times down to microseconds. For imaging thermal processes, this time range is adequate and can be used to study thermal relaxation times of small structures like blood vessels.

To study the mechanical effects of pulsed lasers, Schlieren techniques can be used for contrast enhancement. To image the microexplosions and shockwaves that are induced by pulsed lasers in tissue, exposure times down to nanoseconds are needed. The light source suitable to provide light flashes down to the nanosecond or even picosecond region, with efficient light coupling into the illumination fiber, is a pulsed laser in the visible range. Because lasers are monochromatic, the color technique to code the degree of deflection of the parallel rays cannot be used. Instead, quantitative data can be obtained using special design spatial filters or using a multiple wavelength laser, a broadband dye laser, or laser-induced fluorescence light. By controlling the time between the laser pulse interacting with tissue and the light flash for exposure, it is possible to study the dynamics of the interaction in time with a temporal resolution down to nanoseconds. A trigger box drives the light flash for exposure after a preset time delay from the start of the laser pulse interacting with tissue (Fig. 1). By increasing the delay time, a sequence of images of individual laser pulses can be obtained showing the dynamics of the interaction.¹⁸ Using a laser source with a high repetition rate as a flash, it is possible to make stroboscopic images using multiple exposures within one video frame. Figure 7 shows such an image obtained using a copper vapor laser as a light source that emits $10\mathrm{ns}$ pulses of green-yellow light with a repetition frequency of $10\mathrm{kHz}$ . During the $1\mathrm{ms}$ exposure time, 10 flashes, each $100\mathrm{\mu }\mathrm{s}$ apart, captured the formation of a cavity at a water surface induced by water vapor created by a holmium laser pulse.

Fig. 7

Multiple exposure of an expanding cavity at a water surface induced by water vapor created by a holmium laser pulse. It shows seven contours, each $100\mathrm{\mu }\mathrm{s}$ apart.

4.2.

Combination of High-Speed Imaging and Color Schlieren Imaging

Besides imaging in “Schlieren mode,” a separate flash, directly illuminating the target, can be used for “normal” high-speed imaging. The same triggering and time delay technique can be applied to capture images during interaction of laser pulses with tissue using a white light flash of several microseconds. Arc lamps in stroboscopes provide $1\text{-}\mathrm{\mu }\mathrm{s}$ light flashes with a repetition rate up to several hundred hertz. This enables flash pulse repetition at video rate or multiple exposures within one video frame.

In the setup (Fig. 1), the high-speed and color Schlieren imaging technique can be combined to visualize thermal and mechanical effects during laser-tissue interaction simultaneously in real time. Figure 8 shows an explosive vapor bubble above the surface of a tissue being heated by previous pulses.

Fig. 8

Combined high-speed and Schlieren image of an explosive vapor bubble during a holmium laser pulse above a tissue area heated by previous pulses.

6.1.

Orthopedic Surgery: Simulation of Cartilage Reshaping during Arthroscopy

The holmium laser is used for remodeling defects of the cartilage in the knee. The thermal effect of the number of pulses at one position (Fig. 10 ) and the angle of irradiation (Fig. 11 ) was studied to obtain the optimum and safe settings for treatment.¹⁹ The imaging showed that irradiating the tissue near perpendicular angles resulted in extensive thermal and mechanical damage. Using the fiber at an angle around $30\mathrm{deg}$ to the tissue, the explosive vapor bubbles act as a shaver remodeling an irregular surface while the thermal energy molds and seals it.

Fig. 10

Orthopedic surgery, cartilage reshaping. Accumulation of heat in tissue after a number of holmium pulses of $0.5\mathrm{J}$ at $5\mathrm{Hz}$ .

Fig. 11

Orthopedic surgery, cartilage reshaping. Bottom row: Effect of angle or irradiation on the extent of thermal damage in tissue after exposure with 10 holmium pulses of $0.5\mathrm{J}$ . Top row: High-speed image of explosive vapor bubbles inducing mechanical damage (cracks) to the tissue.

6.2.

Neurosurgery: Treatment of Hydrocephalus

For the treatment of too high fluid pressure in the brain (hydrocephalus), an alternative passage is created to drain the fluid from the brain by fenestration of the floor of the third ventricle.²⁰ This procedure was simulated to prove the optimal setting and safety of a specially designed ball-shaped fiber tip with a carbon coating that converts all laser light to thermal energy at the tip.²¹ A comparison was made between coated and noncoated fiber tips at various power settings in the brain tissue of a rabbit and imaged using the color Schlieren technique (Fig. 12 ). This confirmed the efficiency and safety of the coated tips to penetrate through tissue membranes at low power settings.²² This procedure has been performed successfully in over 300 patients using these “black tip” fibers.

Fig. 12

Neurosurgery, treatment of hydrocephalus. The extent of thermal damage imaged with the color Schlieren technique (top row) and confirmed in vivo by $\mathrm{Nd}:\mathrm{YAG}$ laser exposure of rabbit brain tissue (bottom row) at the settings shown in the images. The first three frames from the left show the effects of an uncoated ball tip. There is no tissue ablation, only undesired deep coagulation. In contrast, the two frames from the right show the effect of a coated ball tip. Already at powers of a few watts, tissue is ablated with minimal thermal damage in the tissue underneath.

6.3.

Urology: Treatment of Benign Prostate Hyperplasia

Benign prostate hyperplasia (BPH) is a common syndrome in elderly males that hinders voiding the bladder. It can be treated effectively by resecting the obstructive part of the prostate within the urethra. Many devices have been developed over the years based on laser and rf energy sources. These devices have been studied extensively using the color Schlieren setup^{23, 24, 25} and comparing the thermal effects at various parameters such as power, scanning speed, probe design, etc. (Figs. 13 and 14 ). The images show the ablation area and distribution and dynamics of the thermal energy dissipated in the tissue. Most laser devices proved to be effective for the treatment; however, due to the long learning curve and expensive equipment, most urologists turned back to the “golden standard,” transurethral resection of the prostate (TURP), using high power rf generators.

Fig. 13

Urology. This thermal image shows the simulation of BPH treatment, tracking a fiber over the tissue surface using $50\mathrm{W}$ of cw thulium laser light. The dark area shows the volume of tissue ablated. The extent of thermal effects can be appreciated along this area.

Fig. 14

Urology. Similar to Fig. 13, BPH treatment is simulated comparing two power settings of a rf generator coupled to a ball-shaped metal electrode in contact with the tissue. While rolling the ball over the tissue surface at a speed of $2\mathrm{mm}∕\mathrm{s}$ a hot “tail” is left behind inside the tissue that is larger for the higher power setting, as can be expected.

6.4.

Urology: Safety for Lithotripsy in Ureter

The explosive vapor bubbles created by holmium laser pulses can be effectively applied to break stones in the field of urology. During lithotripsy in the bladder, there is plenty of space for the vapor to expand without damaging the bladder wall. However, when treating a stone in the ureter, the explosive vapor bubble can easily expand beyond the lumen of the ureter overstretching the wall. Using the combined high-speed and thermal imaging setup,²⁶ it was determined that pulse energy should not exceed $0.5\mathrm{J}$ and only a train of three pulses at the time should be given to prevent damage to the ureter wall (Fig. 15 ).

Fig. 15

Urology. Simulation of lithotripsy in ureter: a fiber is positioned above a urinary stone in a water-filled lumen with a 3-mm diameter within transparent tissue. After a train of 5 holmium pulses of $0.5\mathrm{J}$ at $5\mathrm{Hz}$ , thermal effects can be observed along the channel wall.

6.5.

Cardiac Surgery: Reviving Hibernating Myocardium by Transmyocardial Revascularization to Relieve Angina Complaints

For several years, reviving hibernating myocardium has been attempted by drilling holes in the tissue to stimulate the formation of new vasculature feeding the myocardium in need of oxygen. Various lasers were used for this goal and the mechanism of action of the lasers was studied using the combined high-speed and thermal imaging setup.²⁷ It was shown that the mechanical and thermal damage varied significantly between the various lasers and settings (Figs. 16 and 17 ). However, for the patients the outcome relieved angina complaints dramatically after surgery. After 1 year, complaints returned and the method was consequently abandoned by most cardiac surgeons.

Fig. 16

Cardiac surgery. The thermal effects resulting from the drilling of a long channel in tissue using either a fiber-delivered holmium laser pulse ( $1\mathrm{J}$ , $5\mathrm{Hz}$ , left frame) or a pulse from a focused ${\mathrm{CO}}_2$ beam ( $500\mathrm{mJ}$ , right frame). The channel created by the ${\mathrm{CO}}_2$ beam is smooth with minimal thermal effects. The wall of the channel created with holmium pulses is irregular and the thermal effects are significant.

Fig. 17

Cardiac surgery. High-speed sequence of the formation of a channel during a focused ${\mathrm{CO}}_2$ beam at the surface of a transparent tissue, each frame is $100\mathrm{\mu }\mathrm{s}$ apart. The explosive vapor creates an opening in the tissue surface enabling the beam to penetrate to deeper layers during a pulse with $500\mathrm{mJ}$ energy and $1\mathrm{ms}$ duration.

6.6.

General Surgery: Working Mechanism of Ultrasound Cavitational Devices

In this study, a hollow titanium needle was used as a resection device for soft tumor tissue, for example, in the liver, was imaged with the high-speed Schlieren setup²⁸ while the tip was vibrating at ultrasonic velocities in water (Fig. 18 ). Cavitation bubbles were observed that pulverize soft tissues during their collapse. At the moment of implosion, shock waves were observed that had not been noticed before.

Fig. 18

Neurosurgery, tumor resection. High-speed Schlieren imaging of an instrument creating cavitation bubbles and shock waves [cavitational ultrasonic surgical aspirator (CUSA), Valley Lab, Boulder, CO]. The sequence of frames shows the phases through the duty cycle of $50\mathrm{\mu }\mathrm{s}$ . Cavitation bubbles are formed underneath the tip of a hollow titanium needle vibrating at ultrasonic speed. At the moment the cavitation bubbles implode, shock waves are visible using Schlieren techniques.

6.7.

General Surgery: Coagulation of Large Tumor Areas in the Liver

Secondary tumors affect large volumes over $10\mathrm{cm}$ in diameter in the liver. Especially when multiple areas are involved, resection is not possible. Instead, the tumors are coagulated by using laser (Nd:YAG, diode) or rf as the thermal energy source. Multiple diffusing fibers or needles with rf electrodes are positioned at strategic positions in the liver to destroy the tumor while preserving the organ structure. The treatment was simulated using the color Schlieren imaging to see the development of the heat distribution in time depending on the power settings and activation sequences of the multiple fibers or electrodes (Fig. 19 ).

Fig. 19

General surgery. Simulation of rf coagulation of liver tumors. The image of the temperature field between the electrodes of two needles on each side of the frame activated in turn with a rf generator.

7.1.

Development of the Setup

The components to build a color Schlieren setup are relatively straightforward and the first images obtained can be impressive. However, it takes more effort to improve the setup. Over the years, the setup was enhanced by using better optics and by matching the focal lengths, magnification, and size of the filter with the degree of refraction of rays in the object plane. The originally photographic rainbow filters were replaced by computer-generated filters, in which black circles were added to separate the colors to enable possibilities for quantification. In the images, these black rings provide an impression of “isotherms.”

The video camera was upgraded in time and combined with PC-based video mixing techniques adding information like dimensions, time codes, and temperatures (Fig. 6).

The high-speed imaging option was also added in a later phase. It takes some practice to align the flash light to the optimal position to combine the exposures of the “thermal” image with the “still flash” in one video frame.

7.2.

Quantification of Temperature Distributions Visualized

It is important to realize that the images produced are a result of the defection of rays due to a temperature gradient. This gradient is largest during the heating of tissue as long as the source is active. The temperature decrease is usually exponentially away from the source. The largest gradient is near the source, which is reflected by the color coding in the image. As soon as the energy source is turned off, the steepness of the gradient fades away. When the temperature gradient decays, it becomes by approximation linear resulting in just one color in the image across the thermal field. This could be interpreted mistakenly as a thermal field of a constant temperature.

In the Appendix0, it is shown how the colors can be related to temperatures if the temperature distribution is symmetrical with a presumed geometry that is planar, cylindrical, or spherical. For most conditions, the geometry can be approximated by one of these geometries. The heat source is usually a point, a disk, or half dome. Consequently, the direction of the heat conduction is either unidirectional or radial. For a unidirectional temperature gradient (planar), the relation can be approximated by the first order polynomial. Using a smart distribution of the color rings in the Schlieren filter with the width adjusted for this relationship, it should be possible to produce color images in which the colors represent calibrated temperature ranges.

The axial geometry (spherical or cylindrical) is more difficult to calibrate since it is related to the ratio derived from the radius the temperature gradient field and the position of a point in this field. The result is best illustrated by Fig. 20 where the beam of a $\mathrm{Nd}:\mathrm{YAG}$ laser penetrates deep into the tissue heating a cylindrical volume of tissue. The thermal image shows two lobes formed from the center. Although, the central axis is hottest, its appearance in color distributions suggests that it is cooler. This example clearly shows one of the pitfalls of this imaging method to interpret the color zones as temperature zones.

Fig. 20

Quantification. Thermal image of effect of a $\mathrm{Nd}:\mathrm{YAG}$ beam penetrating deep into the tissue heating a cylindrical volume. Although, the area around the central axis is expected to be hottest, the image shows two hot lobes beside the central axis. This image illustrates that the colors cannot directly be interpreted as temperatures.

To obtain a better understanding of the refraction of light rays due to a temperature gradient in a medium, the path of the rays was simulated by ray tracing.²⁹ The ray tracing was performed under the symmetrical conditions unidirectional or radial. These conditions were also simulated in the color Schlieren setup so the results could be compared. Figure 21 shows the results from the ray tracing next to an image of a unidirectional temperature gradient above a hot surface. The match between both color bands is significant. Similar results can be obtained for the radial symmetry. However, these ideal temperate distributions can easily be disturbed by convection and turbulence. From that moment, it becomes too complex to relate colors to temperatures. Due to the high contrast and detail, the qualitative images obtained with this visualization technique are still useful for studying thermal transport and fluid dynamics.

Fig. 21

Quantification. Result from a ray-trace simulation through a unidirectional temperature gradient above a hot surface (left side) compared to “real” color Schlieren image of the same condition (right side). The match between both color bands is significant.

7.3.

Thermocouple Measurements

To overcome the limitation of relating colors to real temperatures, thermocouples can be positioned at representative positions in the field of view, such as the axis of symmetry of the expected temperature field. However, the thermocouples are invasive and disturb the field of view. Pushing the wires into the model tissue creates small cracks that are clearly visible in the image since this imaging technique is very sensitive for small disturbances. The temperatures from the thermocouples are superposed in real time onto the captured images. Showing calibrated temperatures of some positions in the color images is a useful addition that makes the need for calibration of the colors less necessary.

7.4.

Qualitative Comparison using Color Schlieren Imaging

Although the use of color Schlieren imaging is limited for quantitative temperature imaging, it is useful for conducting comparative studies. For example, thermal distributions and relaxation times can be visualized comparing various systems or system settings without the need for absolute temperatures. Many studies have been conducted this way as is shown in Table 1.

7.5.

Tissue Model

It is essential that the tissue to be transparent for this imaging method. The only biological tissue that would meet these conditions would be the vitreous humor of the eyeball. However, due to its jelly substance, it is difficult to work with and also hard to obtain in large quantities. Any transparent model will lack the scattering properties of tissue. Even the smallest quantity of scattering material in the model tissue would blur the image and make it useless. The high sensitivity of the technique for optical “irregularities” is in this sense a disadvantage. To overcome this limitation, a slab of real biological tissue can be used with a thickness limited to the effective penetration depth for the wavelength studied, sandwiched between slabs of the transparent model tissue (Fig. 9, right column). The initial scattering and absorption takes place in this tissue layer and the heat generated will be conducted to the gel and become visible by the Schlieren color coding. If the absorption is high and dominates scattering, one can use an absorber dissolved in the model tissue as long as it is transparent for the visual range of wavelengths. Such absorbers are available both the uv and near ir $(\mathrm{Nd}:\mathrm{YAG})$ .

7.6.

Educational Tool

Despite several limitations of the color Schlieren technique to study laser-tissue interaction as discussed above, the most important advantage is its value for education. The movie clips obtained of the dynamics during laser-tissue interaction are self-explaining even for novices in this area of research. Although, the laser-tissue interactions cannot be simulated in detail, the essence can be mimicked to show a good representation of reality. The imaging has proven to be very effective for education and training of students, scientists, and physicians. Even professionals in this area of research have shown their appreciation for this imaging technique as it shows the basic essentials of laser-tissue interactions.

7.7.

Areas for Application

As shown in the overview of applications (Table 1), this imaging technique is not limited to studying laser interactions with tissue. Any energy source generating heat or mechanical effects in tissue can be studied. As shown, imaging was obtained of rf devices and ultrasound devices generating cavitation bubbles.