Radiochromic Film. David F. Lewis, Ph.D. Senior Science Fellow Advanced Materials Group International Specialty Products

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1 Radiochromic Film David F. Lewis, Ph.D. Senior Science Fellow Advanced Materials Group International Specialty Products October 20, 2010

2 High Resolution Imaging 1984: Could we make a processless film for recording very high resolution images in an electron-beam recorder? Tri-phenyl methane dyes in PVC Photolysis produces HCl and activates a ph-sensitive leucodye No amplification - slow Diacetylenes Solid-state polymerization >>100x amplification - faster

3 Performance of Photosensitive Systems Silver halide fast : 5x10-10 J/cm 2 Silver halide slow: 10-8 J/cm 2 Xerography: 10-6 J/cm 2 Photopolymerization: 10-3 J/cm 2 Photochromism: 1 J/cm 2 Imaging Systems, Jacobson and Jacobson, Focal Press 1976

4 Amplification One photochemical event effecting >1 molecule Silver halide a whole crystal, silver atoms, can become developable after as few as 4 photochemical events Photopolymerization up to thousands of monomer molecules react to form a polymer

5 Processless Electron Recording Media 1988

6 Monochrome color 1996 Monochrome B&W 1998 Polychrome in one active layer

7 Core Radiochromic Film Technology The diacetylene monomer must be in an ordered form to be active Original active component : Pentacosa-10,12-diynoic acid (PCDA) Current active component is lithium pentacosa-10,12-diynoate (LiPCDA)

8 Radiation Dosimetry Applications GAFCHROMIC Films Five products introduced between 1990 and 2002 Primarily for MeV radiation HD-810 transparent, single layer, 5 400Gy MD-55 transparent, laminated double layer, 1 80Gy HS transparent, laminated, single layer, Gy Primarily for kev radiation XR-R opaque, laminated, single layer, Gy XR-T transparent, laminated, double layer, 0.02Gy 10Gy

9 Features of radiochromic film include No film processing No darkroom No chemicals Low energy dependence Fractionation independence Dose-rate independence Quantitative film dosimetry also requires. Film Digitizer Software

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11 Development of High Sensitivity Film Development began in December 2002 GAFCHROMIC RTQA 2-500cGy, radiotherapy QA where use is qualitative Introduced July 2004 GAFCHROMIC EBT 1-800cGy, for quantitative film dosimetry, e.g. IMRT Currently in final clinical evaluations Introduction late September 2004 GAFCHROMIC XRQA kev photons cGy Currently in field evaluations Introduction about December 2004

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13 Active Component is Hair-Like

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15 LiPCDA Active Component in EBT and EBT2

16 GAFCHROMIC Dosimetry Films Radiology/diagnostic Energy Range Dose Range GAFCHROMIC XR- R kv 0.1Gy to 15 Gy GAFCHROMIC XR-CT kv 0.1cGy to 20 cgy GAFCHROMIC XR-M kv 0.1cGy to 20 cgy GAFCHROMIC XR-QA kv 0.1cGy to 20 cgy Radiotherapy Energy Range Dose Range GAFCHROMIC HD-810 MV 5Gy to 500Gy GAFCHROMIC MD-V2-55 MV 1Gy to 100Gy GAFCHROMIC RTQA MV 0.02Gy to 8Gy GAFCHROMIC EBT kv-mv up to 50Gy

17 GAFCHROMIC EBT Coating SURFACE LAYER ~3 microns ACTIVE LAYER ~17 microns CLEAR POLYESTER ~97 microns

18 EBT Film Structure and Composition CLEAR POLYESTER - 97 microns ACTIVE LAYER - 17 microns SURFACE LAYER - 6 microns ACTIVE LAYER - 17 microns CLEAR POLYESTER - 97 microns Layer Active layer (contains about 7.5% water) Surface layer (contains about 15% water) Polyester COMPOSITION (ATOM%) C H O N Li Cl Zeff 25.0% 54.2% 9.6% 10.4% 0.7% 0.1% % 53.3% 11.1% 12.7% 0.2% 0.2% % 36.4% 18.2% 0.0% 0.0% 0.0% 6.64

19 GAFCHROMIC EBT: rgb Color Scanning

20 The End of EBT Film Fortune meets reality Contract coating facility (Polaroid Corp.) closed in 2008 Forced changes in coating technique Active component remained the same at in EBT Binder material changed from natural to synthetic polymer Active layers reduced from two to one Voluntary change Incorporate a marker dye in the active layer

21 GAFCHROMIC EBT2 dosimetry film Polyester Laminate - 50 microns Adhesive Layer - 25 microns Active Layer - 28±3 microns Polyester Base microns

22 Why is EBT2 film yellow? Contains a yellow dye a marker dye But why? Allows for correction of film non-uniformities U.S. Patent 6,285,031 September, 2001 Original intent to use red and blue color channels Red signal dominated by dose information Blue signal dominated by uniformity information

23 Absorbance Absorbance Spectra of EBT2 components Visible Spectrum of Active Component after Exposure 1.2 Active component Signal in red channel Wavelength, nm Visible Spectrum of Marker Dye 3 Marker dye Signal in blue channel 2 Before exposure After exposure to 50Gy Wavelength

24 Absorbance Spectra of EBT2 Before and after exposure Wavelength, nm

25 Handcoating with Yellow Marker-dye: rgb pre-exposure

26 Handcoating with Yellow Marker-dye: Red channel pre-exposure

27 Handcoating with Yellow Marker-dye: Blue channel pre-exposure

28 Dose/profile Before Correction

29 Dose/profile After Correction

30 Protocol for using the marker dye in EBT2 Step 1: Calibration OD R = -log 10 (PV R /65535) OD B = -log 10 (PV B /65535)

31 Dual channel dosimetry Red channel Composite channels Blue channel

32 There s a better way, a much better way! Use all the color channels Multi-channel film dosimetry with non-uniformity correction, A. Micke, et al. submitted to Med. Physics August, 2010.

33 Anisotropy Active material crystals are: Rod shaped approx. 2 μm x 15μm Preferentially aligned in downweb direction (parallel to coating direction) Light polarizers (after exposure) Cross web direction Coating direction (downweb)

34 Dose, cgy Scan Orientation and Response of EBT2 600 Dose Response EBT2, lot# Landscape orientation Red density

35 Lateral response dependence Menegotti et al 2008 Med Phys

36 Lateral Response Dependence Why? Light scattering? No Path length of rays transmitted through the active layer? No Path length of rays transmitted through the CCD filters? Yes, but minor Polarization of light transmitted through the film

37 Polarization and Radiochromic Film Diacetylene polymers are highly anisotropic Light transmitted by the polymer is polarized Vibrational component perpendicular to the backbone is absorbed If the particles in radiochromic film were randomly oriented the transmitted light would be: Lower intensity (equivalent to crossed polarizers) Unpolarized However particle orientation in is induced by fluid flow in the coating process Therefore the transmitted light is polarized Note: The particles in the original radiochromic film were square in cross-section. Therefore the particles were randomly oriented and the transmitted light was unpolarized

38 Crossed Polarizers R: G: B: R: G: B: 0.739

39 Optical Elements in a Flatbed Scanner

40 Dose, Gy Single channel dosimetry in extreme lateral position Side scan - red channel dosimetry, no lateral correction Position, x mm

41 Dose, Gy Lateral response scanner calibration red channel dosimetry, no lateral correction red channel dosimetry, with lateral correction Position, x mm

42 Dose, Gy Triple channel dosimetry in extreme lateral position Side scan - red channel dosimetry, no lateral correction side scan - 3-channel dosimetry, no lateral correction Position, x mm

43 Triple channel film dosimetry: Features and advantages Separates Dose and Dose-independent effects Allows compensation for film thickness variation Allows smart noise reduction (not yet implemented in FilmQA Pro software) Enable the use of full film dose sensitivity of all channels RGB without transition error Significant improvement of dose map accuracy Allows to sense calibration errors Attenuates the lateral response artifact

44 Dose, Gy Lateral response scanner calibration channel dosimetry with lateral correction red channel dosimetry, no lateral correction 3-channel dosimetry, no lateral correction 0.40 red channel dosimetry, with lateral correction Position, x mm

45 Polymerization and Post-Exposure Growth hν Intermolecular distance = D Intermolecular distance = (D - ΔD) Gap = D + N.ΔD Diacetylene monomer Diacetylene polymer - N monomer units

46 Polymerization and Post-exposure Changes Polymerization starts within 100 μsec of exposure At low exposures (<100 Gy) polymerization proceeds as a first order reaction for about 30 msec The polymer shrinks relative to the monomer The distance between the end of a growing chain and the next monomer increases as polymerization increases Within a second the initial fast phase converts to a slow phase where changes in absorption are proportional to log(time) Protocols for using radiochromic film must accommodate the post-exposure changes

47 Absorbance Spectral Changes Post-exposure GAFCHROMIC EBT: Post-Exposure Growth minutes 45 minutes 10 minutes 7 hours Wavelength, nm

48 Density Post-exposure Change Post-exposure change Red Green Blue y = x R² = y = x R² = y = x R² = Log 10 (time after exposure, min)

49 Net Visual Density Energy Independent Dose Response kVp, 2mm Al 100kVp, 2mm Al 150kVp, 2mm Al Cobalt Dose (cgy)

50 Energy Dependence of EBT2 Film EBT2 Lot # Ratio OD 105kVp/6MV Cl, ppm* Br, ppm* ± ± ± ± ±90 43± ± ±41 532±20 Alexandra Rink and Pamela Lindsey, AAPM Annual Meeting, 2009 * Proportions of Cl and Br in the entire film

51 Elemental Composition and Z eff of EBT2 Thickness*** microns Approximate density g/cm2 COMPOSITION (ATOM%) Layer H Li C N O Na S Cl Br Polyester film base* % 0.0% 45.5% 0.0% 18.2% 0.0% 0.0% 0.0% 0.0% Adhesive* % 0.0% 33.3% 0.0% 9.5% 0.0% 0.0% 0.0% 0.0% Active layer (assumes 7.5% moisture)** % 0.8% 29.6% 0.1% 10.8% 0.1% 0.0% 0.2% 0.1% Polyester film base* % 0.0% 45.5% 0.0% 18.2% 0.0% 0.0% 0.0% 0.0% Overall Composition 40.57% 0.09% 42.68% 0.01% 16.62% 0.01% 0.00% 0.02% 0.01% Zeff = [ αi (Zi) a ] 1/a * The composition of these layers is a good faith estimate based on the manufacturer's identification of the constituents. The composition should not be used as a specification. ** The composition of these layers is a good faith estimate based on the proportion of the chemical constituents. The composition should not be used as a specification. *** The thicknesses are approximate and are not specifications.

52 Penetration, mm EBT2 is Usable in Water Phantoms Penetration of Water into Radiochromic Film GAFCHROMIC EBT-2 GAFCHROMIC EBT Time, hours Profile Magnification Water front at position 365 Edge of film at position 388

53 What can go wrong? Changes to the signal from the marker dye The active component changes sensitivity The alignment of the active component differs

54 Performance Issues and Corrective Actions Coating uniformity Installed precision coating dies (ex-polaroid) (3/09) Installed direct-drive motors for web transport (3/09) Optimized fluid formulations and machine parameters Uniformity of particle alignment Restricted coating width to <18 (from 21½ ) (6/10) Changes in absorbance of marker dye induced by moisture Eliminated K+ ion (5/09) Adjusted marker dye and Na+ ion concentrations (5/10)

55 Relative response Uniformity of EBT Film Typical Cross-Web Uniformity: EBT Film Lot# H σ/mean: 0.8% Measurement error: ±0.45% Position

56 Relative response, blue channel Uniformity of EBT2 Film Crossweb Uniformity: EBT 2 Coating σ/mean: 0.5% Measurement error: ±0.25% Distance, mm

57 Density Crossweb Profiles Particle Alignment is not Uniform Red Blue Crossweb position, inches

58 Is there an EBT3?

59 EBT2 Improvement Projects Change to a pigment dye Greater chemical stability, will not diffuse Water lamination and symmetric structure Re-create the structure used in the original EBT film Using a polyester substrate with microscopic silica on the surface Eliminates Newton s Rings Cross lamination Energy independence Making the film energy independent from 10 kv into the MV range Enhancing scanner response with selected optical filters

60 Hypothesis We could approximate the random orientation condition by crossing two pieces of film This approaches the state of crossed polarizers Perfect polarizer transmits 50% of incident unpolarized light Two perfect polarizers aligned transmit 50% of incident unpolarized light Two perfect polarizers crossed transmit no light

61 Polarization in Radiochromic Film Crystal alignment is the source of polarization effects in EBT2 Particle alignment is very difficult to control In EBT2 there is only a partial (preferential) alignment Would be easier to deal with if: Alignment was 100% perfect Alignment was 100% imperfect # crystals/unit volume large and crystals randomly oriented

62 In-line lamination

63 Cross lamination

64 Density Density Crossweb Profiles In-line lamination Red Blue Cross lamination Crossweb position, inches Red Blue Crossweb distance, inches

65 Advantages of Cross-lamination Improves sensitivity/contrast at doses >50 cgy Assists the proper function of the marker dye Eliminates orientation effects on the scanner Reduces noise level

66 Density In-line, red In-line, green In-line, blue Cross-laminate, red Cross laminate, green Cross-laminate, blue Dose, cgy

67 Cross-lamination: No orientation 0 90 degrees

68 Cross-lamination: No orientation dependence Cross-laminated EBT2 : Red channel response Original orientation Flipped Turned 90 degrees Turned 45 degrees

69 Noise Reduction In-line laminate Cross-laminate

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79 Thank you