Application of infrared focal plane arrays for temperature monitoring in laser surgery

Preliminary analysis of the applicability of IR FPAs to monitor temperature changes during various laser therapies has been performed at the Beckman Laser Institute. Temperature measurements have been made implementing a high frame rate staring FPA. Extensive measurement of skin surface temperature during pulsed laser exposure of port wine stain was performed. Cursory experiments were performed on lung, teeth, and eye tissue during laser exposure.


INTRODUCTION
Lasers have been applied in medicine since their invention.In the early 1960's, medical researchers quickly implemented the advantages the laser presents as a unique light source.The first applications were in the treatment of retinal disease.Lasers are now applied in numerous medical tasks ranging from unclogging obstructed arteries, fracturing kidney stones, removing secondary cataracts and altering genetic material.
Many medical treatments implement laser energy to create conirolled heat.In fact, the thermal effects created by the light's interaction with tissue are the most widely exploited phenomena associated with medical lasers.This heat results when the wavelength of the laser is matched closely with the absorption band of the target structure.This wavelength dependence, combined with the pulse duration control offered by medical lasers, offers researchers and practitioners a controlled heat source for numerous applications.
Imaging of heat caused by laser-tissue interaction in PWS is the primary motivation for experiments involving JR FPAs.Today, most laser tissue interactions occur on a controlled and short time scale.Thus, the high frame rates offered by FPA cameras allow time-dependent sequential imaging of the thermal events shortly following delivery of the laser pulse.Formerly.with only serial scan IR camera systems, fast events were impossible to image due to the inherently slow framing rates.

PORT WINE STAIN LASER THERAPY
Pws is a congenital.progressive vascular malformation of the dermis hat occurs in an estimated S children per 1,000 live births.Although PWS may occur anywhere on the body, most lesions appear on the face and are noted to occur over the dermatome distribution of the first and second trigeminal nerves.PWS potentially results in considerable psychological and physical complications.Personality development is adversely influenced in virtually all patients by the negative reaction of others to a "marked" person .Detailed studies have documented these adverse psychological effects of PWS.
In early childhood, PWS are often faint pink markings on the skin.These markings tend to darken to red-purple, and by middle age, they often become raised as a result of the development of vascular papules or nodules, and, occasionally, tumors.Effects on underlying bone and soft tissue that occurs in approximately two-thirds of the patients with PWS further disfigures the facial features of many children.
Historical treatments for PWS included scalpel surgery, ionizing radiation, skin grafting, dermabrasion, cryosurgery, tattooing, and electrotherapy.Results from these therapies proved unsatisfactory due to the cosmetically unacceptable scarring after treatment.More recent treatment options using the argon or flashlamp-pumped pulsed dye laser (FLPPDL) have offered a superior approach in therapy due to their ability to selectively destroy cutaneous blood vessels.Light passing through the epidermis is preferentially absorbed by hemoglobin (the major chromophore in blood) in the ectatic capillaries in the upper dermis.The radiant energy is converted to heat causing thermal damage and thrombosis in the targeted PWS vessels.Currently, only 10-20% of PWS patients obtain 100% fading of their marking even after multiple laser treatments.
The principal reason for these poor clinical results or treatment failure has been inadequate heat generation in large PWS blood vessels.In the ideal PWS, all vessels have a uniform diameter.Each of these vessels would then have a uniform thermal relaxation constant tr.tr is defined as the time required for the core temperature, produced by the absorbed light energy within the target blood vessel, to cool to one-half the original value immediately after the laser pulse.Ideally, the laser pulse duration (tr) should closely match the vessel's thermal relaxation constant.If longer pulse durations are employed (tr>tr), heat diffuses outside the target structure, resulting in permanent scaring.Too short of a pulse results in a high-peak intravascular temperature rise which can produce explosive vaporization of tissue water, or photoacoustic transients which can result in vessel rupture.In most cases, the PWS vessels revascularize.
In the real world, vessel diameter varies greatly from patient to patient, and from site to site on the same patient.Improved theraputic outcomes are expected should a method to determine tr be developed.The relationship between vessel diameter and the vessel's thermal relaxation time has been determined, thus a method to determine the vessels diameter would aid in the selection of the correct pulse duration.

EXPERIMENTAL GOALS
Infrared tomography (IRT) implements the fast frame rate of an IR FPA to detect the time dependent temperature rise of a targeted tissue structure.IRT has been implemented in a number of materials science and non-destructive testing applications where defects or structures under the test object's surface are to be imaged.The use of IRT to image subsurface PWS blood vessels in human skin is novel.
For the purposes of IRT, PWS in human skin can be modeled as a plexus of subsurface absorbing structures.In this model, if a pulsed laser source is used to irradiate the skin, an immediate thermal wave can be detected on the skin surface due to laser-heated hemoglobin in the PWS blood vessels.The infrared emission from the skin surface degraded due to lateral diffusion.The motivation for using a high frame rate JR FPA is obvious since the image with least thermal diffusion will be integrated and observed immediately after the laser pulse.

IR emission
JR Emission from model PWS skin: a) immediately after laser exposure and b) some time later.
The broad objective of this research is the determination of the initial space-dependent temperature increase in the individual PWS blood vessels by a tomagraphic reconstruction algorithm, immediately following exposure to a diagnostic laser pulse, given only the recorded JR image sequence.Analysis of the time sequence of JR frames by lateral deconvolution and longitudinal inversion algorithms provides a means to determine directly the diameter and depth of individual PWS vessels.Given this information, the correct pulse duration of laser exposure can be selected, thus matching the thermal relaxation time of the PWS vessels, with predicted improvement in therapy.

FUTURE WORK
Future experiments are planned to confirm the applicability of the vessel deconvolution algorithms, and suitability of this technique for the determination of PWS vessel diameters.Subsequent work will include a novel laser which allows for adjustable pulse widths.Also planned are additional experiments in dermatology, and novel experiments to test the applicability of JR EPA imaging in laser treatment in dentistry, ophthalmology, and a variety of other medical disciplines.

CONCLUSIONS
The JR FPA imaging system has been demonstrated as a reasonable tool for the acquisition of fast time sequences to monitor temperature during laser exposure.Further work needs to be performed to refine the applicability of this technology in medical laser therapies.

Figure 5 .
Figure 5.Sequence of JR images of human PWS.
Figure 3.IRT image of phantom PWS together with resultant image after lateral deconvolution algorithm.