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Multimodality small animal imaging: registration of functional EPR images with

MRI anatomy

Supported by grants DAMD17-02-1-0034 (DoD) and P41EB002034(NIBIB)

Chad R. Haney, Adrian Parasca, Charles A. Pelizzari, Greg S. Karczmar*, Howard J. Halpern

Department of Radiation and Cellular Oncology and *Department of RadiologyThe University of Chicago

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In Vivo EPR Imaging – Topic of NIBIB Research Resource

(PI: Howard Halpern, MD, PhD)

• Long term goal - develop EPR imaging techniques which provide functional information that can be of use in designing, delivering, and assessing cancer therapy.

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Biological imaging to enhance targeting of radiation therapy: oxygen imaging

• Intensity modulated radiation therapy allows sophisticated control over spatial distribution of radiation dose

• Areas of hypoxia could be given extra dose if we could identify them

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Why EPR Imaging?• Spectroscopic Imaging: Specific quantitative

sensitivity to Oxygen, Temperature, Viscosity, pH, Thiol

• No water background obscures spectrum of interest (vs MRI)

• ~600 times stronger coupling to magnetic field, environment (vs MRI)

• Deep sensitivity at lower frequency (vs optical)• Noninvasive (vs probes)

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EPR in vivo oximetry techniques• Localized spectroscopy with implanted

particulate probes (Dartmouth)

• Spectroscopic imaging with stepped fixed gradients, water soluble probes– CW (Chicago, OSU, Aberdeen, L’Aquila)– pulsed (NCI, Chicago)

• OMRI (NCI, Aberdeen)– dynamic nuclear polarization using EPR spin

probes

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EPR is analogous to NMR:

Fix RF frequency, sweep field or fix field, sweep frequency:

figure 2.7 field modulation and phase sensitive detection

Zeeman splitting of electron spin energy states in magnetic field

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EPRI is not identical to MRI:

• Relaxation times ~10-6 as long

• pulsed gradient techniques not applicable

• FID correspondingly short → demanding of pulsed measurement techniques π/2 pulse ~50 ns long, FID lasts few μs

• have to introduce spin probe – no endogenous signal

• frequency ~660 times higher for given field (or, field 660 times lower for given frequency)

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RF penetration favors lower frequency

250 MHz ~6 T MRI,90 G EPR

S/N

N~ 1.2

IN LOSSY,CONDUCTIVETISSUE

proton Larmor frequency = 4258 Hz/gauss42.6 MHz at 1 Tesla

electron Larmor frequency = 2.80 MHz/gauss28 GHz at 1 Tesla

ratio meas/calc

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Continuous wavespectral spatial imaging:

each voxel yields a spectrum whose linewidth increases linearly with local oxygen concentration

fixed stepped field gradients, sweptmagnetic field

EPR line broadening for current narrow line spin probes: approximately 0.5 mG/torr O2

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Line width pO2 calibration

Oxygen dependence of lorentzian line width obtained in a series of homogenous solutions of OX31spin probe

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Spectral-spatial projection(a) With no gradient, a field sweep integrates over all spatial locations. This is a pure spectral projection.

(b) A gradient along the x direction couples the spatial and spectral coordinates (the spectrum is shifted linearly with position).

(c) A field sweep now corresponds to a projection along a direction rotated in the spectral-spatial plane. Larger gradients correspond to larger rotation angles. Pure spatial projection would require infinite gradient.

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250 MHz Spectrometer Magnetsvarying diameter homogeneous field regions (90 G)

Small8 cmdiam.

Intermediate 15 cm diam.

Large30 cm diam.

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Mouse Image using OX063 spin probe

PC3 human prostate cancer xenograft on nude mouse hind limb

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Registration of EPR with MRI for anatomically aided analysis

Registration based on

- Fiducials

- Surfaces

- Intensity distribution

Note high intensity due to poor clearance of spin probe from tumor, and low oxygen tension in same region

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Early fiducial markers filled with dilute spin probe solution.Problem: need to remove during 4D image to avoid artifacts

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Immobilization cast, fiducial markers for serial and intermodality registration

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Alignment of MRI and EPRI (red)

fiducial surfaces

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Manual refinement of initial registration estimatebased on fiducials

19Application: radiation inducible antivascular gene therapy

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PC3 tumor treated with Ad.CMV.null virus (control)

Pre treatment: mean pO2 in tumor 44.6 torr, std 35.1, SEM 1.62.

tumor volume from MRI: 0.160 mL

4 days post treatment (right): mean pO2 in tumor 28.7 torr, std 29.1, SEM 1.065.

tumor volume from MRI: 0.422 mL

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4 days post treatment: mean pO2 in tumor 31.7 torr, std 17.1, SEM 0.472.

tumor volume from MRI: 0.417 mL

PC3 tumor treated with Ad.EGR-TNFα virus + 10 Gy

Pre treatment: mean pO2 in tumor 27.3, std 36.1, SEM 1.122 tumor volume from MRI: 0.524 mL

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Conclusions• 4D EPR Images can be obtained with ~1 mm spatial

resolution and ~1.5mG (~3 torr pO2) spectral resolution

• Preliminary images of increased and decreased regional oxygenation levels following radiation + adeno-EGR-TNF anti-vascular therapy have been seen.

• These images may have potential for biologically-based planning and assessment of radiation therapy

• Registration of these functional images with anatomic images such as MRI is key to accurate interpretation and to eventual clinical applications

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Chicago EPRI Lab:

Howard HalpernMartyna ElasColin MailerChad HaneyCharles PelizzariKazuhiro IchikawaGene BarthBen WilliamsKang-Hyun AhnAdrian ParascaVS Subramanian

Chicago MRI Lab:

Greg KarczmarJonathan RiverXiaobing FanMarta Zamora

EGRF-TNF radiation therapy:

Ralph WeichselbaumHelena MauceriMichael Beckett

Denver EPR Lab: Gareth Eaton Sandra Eaton Richard Quine George Rinard