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The Role of Imaging in IMRTThe Role of Imaging in IMRT

James F. Dempsey, Ph.D.James F. Dempsey, Ph.D.Department of Radiation OncologyDepartment of Radiation Oncology

University of Florida College of MedicineUniversity of Florida College of MedicineGainesville, Florida USAGainesville, Florida USA

AcknowledgementsAcknowledgements

nn Omar A. Zeidan, Ph.D.Omar A. Zeidan, Ph.D. Univ. of FloridaUniv. of Floridann Tony Tony StellStell Univ. of FloridaUniv. of Floridann ChihrayChihray Liu, Ph.D.Liu, Ph.D. Univ. of FloridaUniv. of Floridann Jonathan G. Li, Ph.D.Jonathan G. Li, Ph.D. Univ. of FloridaUniv. of Floridann Jatinder R. Palta, Ph.D. Jatinder R. Palta, Ph.D. Univ. of FloridaUniv. of Floridann Daniel A. Low, Ph.D.Daniel A. Low, Ph.D. Washington UniversityWashington Universitynn Clifford K.S. Chao, M.D.Clifford K.S. Chao, M.D. M.D. Anderson M.D. Anderson nn Jeffrey F. Williamson, Ph.D.Jeffrey F. Williamson, Ph.D. Washington UniversityWashington Universitynn Sasa Mutic, M.S.Sasa Mutic, M.S. Washington UniversityWashington Universitynn Robert Robert MalyapaMalyapa, M.D., Ph.D. , M.D., Ph.D. Washington UniversityWashington Universitynn Perry W. Grigsby, M.D.Perry W. Grigsby, M.D. Washington UniversityWashington University

OutlineOutlinenn Why IMRT Needs Quantitative ImagingWhy IMRT Needs Quantitative Imagingnn The “Theory” of Quantitative ImagingThe “Theory” of Quantitative Imaging

nn Sampling & ReconstructionSampling & Reconstructionnn Filters for Linear SystemsFilters for Linear Systems

nn Survey of the Roles of Quantitative Imaging in Survey of the Roles of Quantitative Imaging in IMRTIMRTnn Imaging for Simulation & Treatment PlanningImaging for Simulation & Treatment Planningnn Imaging for Target Delineation Imaging for Target Delineation nn Imaging for Dose MeasurementImaging for Dose Measurementnn Imaging for Delivery ValidationImaging for Delivery Validation

nn Methods of Delivery Validation via ImagingMethods of Delivery Validation via Imaging

Clinical Motivation: Clinical Motivation: ConformalityConformality => Better Sparing=> Better Sparing

Screen Capture from Nomos Peacock Tx Planning System MIR Washington Univ.

Parotid Sparing in the Presence of Parotid Sparing in the Presence of Systematic Setup ErrorSystematic Setup Error

0 1 0 20 30 40 50 60 7 0 800

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Effect of Lateral Shift on Left Parotid Dose

Fra

ctio

n V

olum

e

Dose (Gy)

15 mm

10 mm

5 mmCorrect But Random Set-

Up Errors will “wash out” any overdosing of the parotid, right?

Systematic Set-up errors can have a significant impact on critical structure sparing

Parotid Sparing in the Presence of Parotid Sparing in the Presence of Random SetRandom Set--Up ErrorUp Error

Well No,

With IMRT we have purposely placed the parotid in a high gradient

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Questions for IMRTQuestions for IMRT

nn How Do We Ensure Our IMRT Dose Calculations How Do We Ensure Our IMRT Dose Calculations are Accurate?are Accurate?

nn How Do We Ensure That Can Achieve Our How Do We Ensure That Can Achieve Our Clinical Goals?Clinical Goals?

nn How can we best employ IMRT to the greatest How can we best employ IMRT to the greatest efficacy?efficacy?

Could Quantitative Imaging Be the Answer?Could Quantitative Imaging Be the Answer?

What is Quantitative Imaging?!What is Quantitative Imaging?!

nn An Oxymoron?...An Oxymoron?...nn Most “Imaging Science” is concerned with Most “Imaging Science” is concerned with

Qualitative Feature Extraction Qualitative Feature Extraction nn e.g. for diagnosise.g. for diagnosis

nn Radiation Therapy Attempts to Perform Radiation Therapy Attempts to Perform QuantitativeQuantitative Measurements Measurements Using Using Imaging TechniquesImaging Techniques

nn Lets Look at the applications for IMRTLets Look at the applications for IMRT

Where Do We Need Quantitative Where Do We Need Quantitative Imaging Techniques in IMRT?Imaging Techniques in IMRT?

nn Imaging for Simulation & Treatment PlanningImaging for Simulation & Treatment Planningnn CT based Heterogeneity CorrectionsCT based Heterogeneity Correctionsnn BreathBreath--Hold Gated CTHold Gated CTnn CT or MR Cine’ for Organ MotionCT or MR Cine’ for Organ Motion

nn Imaging for Target Delineation Imaging for Target Delineation nn Pet Registration & FusionPet Registration & Fusionnn MR Registration & FusionMR Registration & Fusion

nn Imaging for Delivery ValidationImaging for Delivery Validationnn Fluence Map ValidationFluence Map Validation

nn Imaging for Dose MeasurementImaging for Dose Measurementnn Film DosimetryFilm Dosimetrynn Gel DosimetryGel Dosimetrynn Exit DosimetryExit Dosimetry

Theory of “Quantitative” ImagingTheory of “Quantitative” Imaging

nn All of these applications attempt to extract All of these applications attempt to extract quantitative information from imaging quantitative information from imaging

nn We must 1We must 1stst ask, how do we know that ask, how do we know that the imaging devices are capable of the imaging devices are capable of measuring the distributions that we seek?measuring the distributions that we seek?

nn Secondly, If the device fails to accurately Secondly, If the device fails to accurately measure our distribution can we correct or measure our distribution can we correct or recover the information?recover the information?

How to Perform Quantitative How to Perform Quantitative ImagingImaging

nn 1) Sample Data with High Enough Frequency1) Sample Data with High Enough Frequencynn 2) Characterize the Linearity and Spatial 2) Characterize the Linearity and Spatial

Independence of the imaging systemIndependence of the imaging systemnn 3) Determine Line Spread and or Point Spread 3) Determine Line Spread and or Point Spread

functions and Modulation Transfer Functionsfunctions and Modulation Transfer Functionsnn 4) Evaluate the Ability to Make Quantitative 4) Evaluate the Ability to Make Quantitative

MeasurementsMeasurementsnn 5) Apply Filters to Recover Information if 5) Apply Filters to Recover Information if

necessarynecessary

ShannonShannon--NyquistNyquist Sampling Sampling Theorem (1)Theorem (1)

nn If a function has all of its frequency components If a function has all of its frequency components below some frequency n, then sampling that below some frequency n, then sampling that function with frequency 2function with frequency 2νν allows for the exact allows for the exact reconstruction of that functionreconstruction of that function

F(x) F(ω)

ν

ω [freq.]x [dist]

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ShannonShannon--NyquistNyquist Sampling Sampling Theorem (1)Theorem (1)

nn With this theorem and your knowledge of With this theorem and your knowledge of Medical Physics you can determine the temporal Medical Physics you can determine the temporal and spatial resolutions at which distributions and spatial resolutions at which distributions should be sampled to prevent aliasing!should be sampled to prevent aliasing!

nn For example, if we know a priori that a dose For example, if we know a priori that a dose distribution takes on a functional form we can distribution takes on a functional form we can determine the required sampling as follows …determine the required sampling as follows …

Example of the Use of Sampling Example of the Use of Sampling Theory (1)Theory (1)

A fitted 1x1 cm2 profile from 0.1 mm pixel RCF measurements in solid water

Practical Sampling Theory (1)Practical Sampling Theory (1) Linear Systems Theory (2)Linear Systems Theory (2)

nn A system (a transformation T) is considered A system (a transformation T) is considered “linear” if it have the following properties:“linear” if it have the following properties:

( )[ ] ( ) )()(^

ϖϖ GbFaxgbxfa ×+×=×+×Τ

In other words, superposition and scaling hold under the transformation

Here a system is though to be an imaging system of course!

Linear Systems and Imaging (2)Linear Systems and Imaging (2)

nn If an imaging system can be approximated If an imaging system can be approximated as a linear system …as a linear system …

nn And any process that degrades the output And any process that degrades the output of the imager is a linear process …of the imager is a linear process …

nn And the degrading process is spatially or And the degrading process is spatially or temporally invariant …temporally invariant …

nn Then… Then…

Spread Functions (3)Spread Functions (3)

nn Convolution with Spread Functions can be used Convolution with Spread Functions can be used to characterize the degradation of the imaging to characterize the degradation of the imaging data.data.

True Signal Imaged Signal

Spread Function

)()()( xtionSpreadFucnxTrueSignalxSignal ⊗=

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Spread Functions (3)Spread Functions (3)

nn Spread functions plot the redistribution of data Spread functions plot the redistribution of data in a pixel due to a in a pixel due to a degradatingdegradating processprocess

nn Line Spread Functions (LSF) characterize 1D Line Spread Functions (LSF) characterize 1D processprocess

nn Point Spread Functions (PSF) characterize 2D (or Point Spread Functions (PSF) characterize 2D (or 3D) process3D) process

nn If a PSF is isotropic it can be related to the LSF If a PSF is isotropic it can be related to the LSF by the Abel Transformby the Abel Transform

Modulation Transfer Function Modulation Transfer Function (MTF) (4)(MTF) (4)

The Fourier Transform of the LSF or the HankelTransform of the PSF produce the MTF: The MTF needs to be very close to a value of unity for quantitative systems !

The Fourier Convolution Theorem The Fourier Convolution Theorem Can Be Used to Recover Data (5)Can Be Used to Recover Data (5)

)(/)()(~~~

ϖϖϖ tionSpreadFucnSignalTrueSignal =

Quantitative Imaging SummaryQuantitative Imaging Summary

nn 1) Sample Data with High Enough Frequency1) Sample Data with High Enough Frequencynn 2) Characterize the Linearity and Spatial 2) Characterize the Linearity and Spatial

Independence of the imaging systemIndependence of the imaging systemnn 3) Determine Line Spread and or Point Spread 3) Determine Line Spread and or Point Spread

functions and Modulation Transfer Functionsfunctions and Modulation Transfer Functionsnn 4) Evaluate the Ability to Make Quantitative 4) Evaluate the Ability to Make Quantitative

MeasurementsMeasurementsnn 5) Apply Filters to Recover Information if 5) Apply Filters to Recover Information if

necessarynecessary

Imaging for Simulation & Imaging for Simulation & Treatment PlanningTreatment Planning

Brief Survey of Imaging in IMRTBrief Survey of Imaging in IMRTVirtual simulation for Virtual simulation for IMRT Using XIMRT Using X--Ray CTRay CT

nn Due to it’s inherent Due to it’s inherent spatial integrity Xspatial integrity X--ray CT remains the ray CT remains the “Gold Standard” “Gold Standard” for Tx Planningfor Tx Planning

nn CT Data can allow CT Data can allow dose calculations dose calculations to account for to account for Tissue Tissue Heterogeneities Heterogeneities

nn We are relying We are relying on accurate on accurate CT#sCT#s

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Heterogeneities in IMRTHeterogeneities in IMRT

Heterogeneities can cause large discrepancies in the presences of air cavities for IMRT.

Shown: CCC w/ and w/o heterogeneities for nasopharyngeal target

CT ArtifactsCT Artifacts•Artifactual CT Numbers can be produced in the presence of object that can

•Spatial Integrity of the Data is not compromised

•Assigning Bulk Densities to Structures can overcome Heterogeneity corrections to Artifactual CT Number regions

•IEEE Trans Med. Imaging 2001 20(10) 1009-1017 Snyder and Williamson et al.

High Quality DRR and Portal Films High Quality DRR and Portal Films are Critical to IMRT are Critical to IMRT

Port Films are required to verify isocenter to within a few mm

Spatial Integrity is of Utmost Importance: Feature Extraction is qualitative

Serial Tomotherapy DRR and Port Film

Adaptive Radiotherapy & IMRTAdaptive Radiotherapy & IMRTXX--Ray Cone Beam Reconstruction Ray Cone Beam Reconstruction Cone Beam Imaging at The Accelerator can Provide Daily Setup Based on the Imaging of Soft Tissues!

DA Jaffray, DG Drake, M Moreau, AA Martinez, and JW. Wong, Int. J. Radiation Oncology Biol. Phys., Vol. 45, No. 3, pp. 773–789, 1999

Adaptive Radiotherapy & IMRTAdaptive Radiotherapy & IMRTMegaVoltageMegaVoltage CTCT

Helical Tomotherapy and Megavoltage CT Imaging at The Accelerator

K J Ruchala, G H Olivera , E A Schloesser and T R Mackie

Phys. Med. Biol. 44 (1999) 2597–2621.

Imaging for Target DelineationImaging for Target Delineation

Brief Survey of Imaging in IMRTBrief Survey of Imaging in IMRT

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Virtual Simulation Software QAVirtual Simulation Software QAMULTIMODALITY IMAGE REGISTRATIONMULTIMODALITY IMAGE REGISTRATION & FUSION& FUSION

nn Validation of the Validation of the Complete Clinical Complete Clinical Process is Important:Process is Important:

nn Mutic et al. Mutic et al. Multimodality image Multimodality image registration quality registration quality assurance for assurance for conformal threeconformal three--dimensional treatment dimensional treatment planning. Int J Radiat planning. Int J Radiat Oncol Biol Phys, 2001Oncol Biol Phys, 2001

a) b)

c) d)

MR PET

CT2CT1

MRIMRI--CT Image FusionCT Image Fusion--RegistrationRegistration

•• Magnetic Resonance Imaging (MRI)Magnetic Resonance Imaging (MRI)

•• Excellent soft tissue contrast allows Excellent soft tissue contrast allows better differentiation between better differentiation between normal tissues and many tumorsnormal tissues and many tumors

•• It is not limited to imaging in axial It is not limited to imaging in axial planes planes

•• Disadvantages:Disadvantages:

–– Susceptible to spatial Susceptible to spatial distortionsdistortions

–– Image intensity values Image intensity values do not relate to physical do not relate to physical or electron densityor electron density

MR

CT

CUCU--ATSM PETATSM PET--CT Image Registration & CT Image Registration & Target DefinitionTarget Definition

Chao et al. Int J Radiat Oncol Biol Phys 2001 Mar 15;49(4):1171-82

FDG-PET Scan CT Scan

Para-aortic L.N.

Kidney

Target Delineation By PET – CT Registration & Target DefinitionRegistration & Target Definition

R. Malyapa et al. MO-D-517A-07

Imaging for Lung IMRT?Imaging for Lung IMRT?

nn Attractive IMRT Site Due to Need for Attractive IMRT Site Due to Need for Conformal Dose Distributions and Target Conformal Dose Distributions and Target Encompassing Critical StructuresEncompassing Critical Structures

nn Significant Concern Over Loss of Significant Concern Over Loss of Superposition Due to Breathing Motion Superposition Due to Breathing Motion and Significant Heterogeneity Correctionsand Significant Heterogeneity Corrections

Spirometer Gated Spirometer Gated MultisliceMultislice CTCT

D. Low et al. WED-C-517D

Multi -slice CT gated on tidal lung volume as measured by digital spirometry during free breathing

Reconstruction is performed using sinograms that are assembled by the coincident spirometer reading

Produces time dependent CT data to map out breathing motions

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Imaging for Dose MeasurementImaging for Dose Measurement

Brief Survey of Imaging in IMRTBrief Survey of Imaging in IMRTRadiographic Film Dosimetry for Radiographic Film Dosimetry for

Patient Specific QAPatient Specific QA

Radiochromic Film Dosimetry for Radiochromic Film Dosimetry for Commissioning IMRT Treatment Commissioning IMRT Treatment

Planning SystemsPlanning Systems600

900

1200

1400

300

Polymerizing Gel Dosimetry in Polymerizing Gel Dosimetry in IMRTIMRT

Imaging for IMRT Delivery Imaging for IMRT Delivery ValidationValidation

nn How Do We Validate the Complex How Do We Validate the Complex Orchestration of MLC Leaf Motion and Orchestration of MLC Leaf Motion and Dose Delivery That Occurs in SMLCDose Delivery That Occurs in SMLC-- and and DMLCDMLC--Based IMRT?Based IMRT?

nn Imaging of the integral dose or fluence Imaging of the integral dose or fluence with film or EPIDwith film or EPID

nn Time Resolved Imaging of the Fluence or Time Resolved Imaging of the Fluence or DoseDose

MLC Delivery Device MLC Delivery Device Characterization via FilmCharacterization via Film

•Radiographic meas. of accuracy of leaf travel/ offsetAnd Integral Fluence Maps

LoSasso T et al. Med. Phys. 2001; 28(11):2209-2219.

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EPID and IMRTEPID and IMRTDelivery Validation Delivery Validation

EPID Records Integral or Time Resolved Exit Dose to Validate Fluence Map Delivery: Most Time Resolved Systems <10 FPS

Partridge M et al., Med. Phys. 2000; 27(7):1601-1609.

Accelerator Log Files ~20 FPS Accelerator Log Files ~20 FPS (~50 ms sample rate)(~50 ms sample rate)

LoSasso T et al. Med. Phys. 2001; 28(11):2209-2219.

Xia P et al. Med. Phys. 2002; 29(3):412-423.

Ezzell and Chungbin J Appl Clin Med Phys 2001; 2(3):138-148.

Litzenberg et al. J Appl Clin Med Phys 2002; 3(2):63-72.

ScintillationScintillation--CCD Camera Validation CCD Camera Validation of MLC Deliveryof MLC Delivery

ScintillationScintillation--CCD CameraCCD CameraCharacterizationCharacterization

nn Fast Gd2O2S:Tb Scintillation Plate coupled to a Fast Gd2O2S:Tb Scintillation Plate coupled to a CCD or CMOS Camera for high frame speed CCD or CMOS Camera for high frame speed capture.capture.

n Only 1.24 g/cm2 stainless steel, 0.411 g/cm2 Gd2O2S:Tb, and 0.008 g/cm2 aluminized mylarin the beam path: allows verifcation upstream of the patient

nn CCD Camera up to 30 FPSCCD Camera up to 30 FPSnn CMOS Camera up to 1000 FPS (200 FPS is good CMOS Camera up to 1000 FPS (200 FPS is good

enough)enough)

ScintillationScintillation--CCD CameraCCD CameraCharacterizationCharacterization

nn Sampling in Time & SpaceSampling in Time & Spacenn Demonstration of Linearity as well as Demonstration of Linearity as well as

Spatial and Temporal Invariance Spatial and Temporal Invariance nn Spatial PSFs,Temporal Spatial PSFs,Temporal LSFsLSFs, and , and

associated MTFsassociated MTFs

Scintillation Camera Validation of Scintillation Camera Validation of IMRT DeliveryIMRT Delivery

Gd2O2S:TB Scintillation plate

Camera:

30 fps CCD Camera

1000 fps CMOS Camera

Observation of Leaf Motion During SMLC Delivery

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30 FPS30 FPS Comparison of Log Files, Camera, Comparison of Log Files, Camera, and the Intended Deliveryand the Intended Delivery

100 FPS100 FPS Much Ado About Nothing?Much Ado About Nothing?

Feedback Treatment Planning Feedback Treatment Planning

nn Imaged MLC Delivery Errors are accounted Imaged MLC Delivery Errors are accounted for in a recalculated Treatment Planfor in a recalculated Treatment Plan

What Should I Do when What Should I Do when Quantitative Imaging is used In My Quantitative Imaging is used In My

ClinicClinicnn Understand the IssuesUnderstand the Issuesnn Understand the TheoryUnderstand the Theorynn Insist that vendors of a device that is intended Insist that vendors of a device that is intended

for quantitative imaging measurement for quantitative imaging measurement demonstratedemonstratenn LinearityLinearitynn LSFsLSFs, , PSFsPSFs, and MTFs where appropriate, and MTFs where appropriate

nn Use your knowledge of dosimetry to evaluate Use your knowledge of dosimetry to evaluate the practical limits of the device the practical limits of the device

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One should ask:

“Does my Imaging system accurately measure

...with high enoughresolution?

… in high gradients?

Where are the LSF, PSF, or MTF?

You should not assume it will be correct just

because I paid a lot of money for it!