Optical method using fluence or radiance measurements to monitor ...

394 Rev. Sci. Instrum., Vol. 74, No. 1, January 2003 Chin et al.FIG. 1. Simulation geometry inset and Monte Carlo results showing predictedchanges in fluence, , forward radiance, L f (10°), and backwardradiance, L a (170°), at 2 mm away from the source during LITT employinga spherically emitting source fiber. The fluence behavior has been verifiedexperimentally see Ref. 5. Symbols are Monte Carlo results while the linesare fitted.dimensionally spherically. This geometry allows for the basicphysics of the optical monitoring problem to beinvestigated. However, for practical application the methodneeds to be extended to other geometries such as cylindricalsources source which are more commonly used in ITT.This article extends our previous MC simulation to includeradiance information. We also present preliminary experimentaldata demonstrating fluence monitoring of rf thermaltherapy of ex vivo porcine kidney. Based on our MCresults, we discuss why radiance might prove advantageousover fluence data for this complex geometry.L f . In contrast, the fluence and L a decrease less noticeablyindicating that at 2 mm, the majority of interstitial light intensityis traveling forward away from the source fiber. Asthe coagulation radius passes 2 mm, a light trapping effectcauses an increase in the fluence 150%, and a more significantchange in the L a value 450%. Here, L f remainsalmost constant as opposed to L a in the previous case. Here,the implication is that the increase in fluence is due to photonstraveling back towards the source fiber.The differences between the two radiances are a result ofthe highly forward scattering nature of light in tissue. Forsmall coagulation radii, the increased scattering properties ofcoagulated tissue will have minimal effect on the bulk radiancedistribution since photons scatter forward away fromthe source fiber and readily escape the coagulation volume.Still, there is sufficient scattering to cause collimated photonsto diverge from their original straight-line path. Hence, as thecoagulation front approaches 2 mm, it is primarily L f that isaffected. After passing 2 mm, the increase in fluence iscaused by photons that have scattered numerous times,enough to reverse their initial forward direction. Consequently,L a increases significantly while L f remains relativelyconstant following passing of the coagulation front.The MC results show that compared to fluence, L f and L aoffer increased sensitivity to an approaching and passing coagulationboundary, respectively.The fluence results of the MC model have been validatedexperimentally in well-characterized tissue mimicking albumenphantoms 6 and demonstrated excellent qualitative andquantitative agreement 20%, 5 experiments are underwayto validate the new radiance results.II. PRINCIPLES OF OPTICAL MONITORINGFor a spherical source of optical energy positionedwithin homogeneous tissue, our MC algorithm models thermaldamage as concentric spheres of coagulated and nativeoptical properties centered at the source inset of Fig. 1. Itisassumed that the index of refraction of coagulated tissue isthe same as native tissue. To mimic the ‘‘growth’’ of thedamaged region during heating, separate MC simulations arerun for increasing radii of the inner ‘‘coagulated’’ sphere.Next, changes in light intensity at a given detector positionare plotted versus the inner coagulation radius. Since theextent of damage increases as a function of heating time, thex axis damage radius is effectively a measure of heatingtime with the horizontal scaling of the graph expanding orcontracting depending on the rate of heating for a giventreatment. Additional details of the MC simulation can befound in Ref. 5.Figure 1 shows MC simulation results showing relativechanges at 2 mm away from a spherical source in fluence,radiance facing the source, L f L(10°), and radiance facingaway from the source, L a L(170°). Native ( a0.5 cm 1 , s 2.67 cm 1 ) and coagulated ( a0.7 cm 1 , s 13.1 cm 1 ) optical properties were chosento simulate albumen phantoms, which have similar propertiesas soft tissue. 6 Prior to the coagulation boundary reachingthe 2 mm point, there is a significant decrease 70% inIII. MONITORING RADIOFREQUENCY THERMALTHERAPYRf ablation has shown promise for the minimally invasivetreatment of small solid renal masses. 7,8 We are currentlyinvestigating a system produced by Radio TherapeuticsCorp. Sunnyvale, CA, which delivers rf energy at 460kHz via a LeVeen electrode Radio Therapeutics Corp. consistingof an array of evenly spaced wire electrodes enclosedby a metal cannula. When deployed, the array appears similarin shape to an umbrella Fig. 2 and produces a sphericalthermal lesion in tissue. A recent study in patients that underwentradical nephrectomy following rf ablation showedthat areas of viable tumor were present at the margins of thetreated volume. 8 The untreated areas were attributed to heatsink effects, abnormal tumor vasculature, and uneven rf energydistribution. 8 We envision that optical methods mightplay a role in rf thermal therapy monitoring to ensure completecoagulation of the tumor volume.We performed fluence monitoring of the coagulationfront in fresh ex vivo porcine kidney during rf thermaltherapy using a previously developed protocol that deliveredcontinuous rf energy until a coagulation-induced impedanceof 400 was reached, which caused a cessation of powerdelivery. 7 Figure 2 shows a schematic of our experimentalgeometry. The optical fibers and rf applicator were fixed inan acrylic jig for accurate positioning. A 2 mm diam diag-Downloaded 10 Feb 2003 to 142.150.192.30. 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