New approaches in small animal imaging H Alfke Department of Radiology University of Marburg

Background Animal models widely used in biomedical research More than 90% of animals used are mice Disease models for longitudinal studies more efficient Demand for phenotyping of transgenic disease models Non-invasive imaging studies very valuable tool

Evolutionary relationships http://informatics.jax.org/silver/contents.shtml

Molecular imaging in animals In vivo Ex vivo Data Histopathology PCR etc. Dissected tissue Time delay Less predictive Intact animal Real time Predictive Black box Reporter Transparent box

Physiology Species Weight Blood volume Heart rate Resp. rate Human 7000 ml 60 bpm 20 pm Rat 500 g 30 ml 350 bpm 100 pm Mice 20 g 5 ml 600 bpm 160 pm

Voxel size and signal to noise ratio 10 x 10 mm 1 x 1 mm  SNR x 1000!

What resolution do we need? To have the same spatial resolution as in clinical imaging: mm3 -> 100µm3 For more functional analysis the Basic Functional Unit (BFU) is important: BFU = the smallest aggregation of cells within an organ that functions like the organ Size: 100µm3

Imaging modalities X-rays: Ultrasound (US) Optical imaging Radiography Computed tomography (CT) Ultrasound (US) Optical imaging Magnetic resonance tomography (MRI) Positron emission tomography (PET) Single photon emission tomography (SPET)

Basic principle Planar Tomography Volumetric acquisition Fast, small data sets Tomography Internal structures Quantitative Volumetric acquisition Volumetry

X-rays Widely used in clinical routine Fastest imaging method 3D data acquisition and quantification possible with computed tomography (CT) High resolution and sensitivity limited by use of ionising radiation Low intrinsic tissue contrast

Planimetric imaging

CT rat lung Clinical scanner (Siemens Volume Zoom), slice thickness: 500µm

Dedicated small animal CT © SkyScan Inc.

Possible improvements X-ray source Reduced focal spot size Quasi monochromatic X-rax X-ray imaging detectors Larger arrays of smaller detectors Spiral CT Better reconstruction algorithm

Ultrasound Widely used in clinical routine No ionising radiation Very high spatial resolution in small objects possible Real time imaging Guidance of intervention Functional information (heart pulsation, blood flow)

Ultrasound © Turnbull, New York

Optical imaging

Fluorescence reflectance imaging (FRI) Fast imaging technique Good for near surface structures Sensitivity dependend on absorption and background fluorescence

Disadvantages of FRI Not quantitative Sensitivity varies with wave length

NIRF Imaging within the „NIRF-window“ (700 – 900 nm) Tissue penetration in the cm range

Fluorescent-mediated molecular tomography (FMT) © Ntziachristos, CMIR

FMT In vivo FMT of 9L gliosarkomas in mice brain © Ntziachristos, CMIR

Magnetic resonance imaging Best overall imaging method High intrinsic tissue contrast Morphologic, functional, and molecular imaging Relatively low sensitivity

Small animal coil

In vivo MRI Resolution down to 100 µm possible with clinical scanner Imaging time from seconds to minutes Easy adaption of animal models to the clinical situation

Quantification of tumour perfusion

Dynamic analysis Parametric display

Targeted MRI contrast media Sipkins et al. Detection of tumor angiogenesis in vivo by a2ß3-targeted magnetic resonance imaging. Nat Med 1998;4:623–626

3D data analysis

Volumetric analysis

Dedicated systems

Heart-MRI of new born mice High field MR Heart-MRI of new born mice

MR-Spectroscopy Auricchio A et al. PNAS 2001;98:5205-10

Positron emission tomography

Characteristics of PET „Electronic“ collimation (coincidence) Short halfe lifes of nuklids High costs (PET camera, cyclotron) Physical limitation of resolution: 1 mm for 18F Some mm for other nuklid Advantage: Organic elements like 11C, 15O

Radionuclides Nuklid T ½ 18F 2h 11C 20min 15O 2min 124I 4d 86Y 15h 68Ga 68min

Transgene expression imaging with PET Chatziioannu AF Eur J Nucl Med 29 (2002) 98

Single photon emission tomography (SPET or SPECT) Absorptive collimator Szintillation detector and photo multiplyer Rotation of detector or object necessary

Radionuclides Nuklid T ½ Energy 99mTc 6h 140keV 111In 2.8d 245/171keV 67Ga 3.3d 93/185keV 123I 13h 159keV 131I 8d 364/284keV

SPET probes

Single Pinhole SPET Spatial resolution ~ de + de (b/l) < 1mm achievable for near (1-2cm) subjects Sensitivity = de cos3  / (16b2) Best close to the pinhole King MA et al. J Cell Biochem S39 (2002) 221

SPET

Possible improvements Higher sensitivity: Multi-hole designs Better detectors Higher resolution: Small pinhole designs resolution (< 0,1 mm)

Single pinhole vs multihole (7) 20 min 5 min

Comparison of imaging technologies Technique Resolution Sensitivity Depth Time MRI 10-100µm µ-mMol No limit Min CT 50µm m-cMol Sec US <50µm mMol mm PET 1-2mm p-nMol SPET < 1mm FRI < 1cm FMT < 10cm

Recent developments and future directions Multi-modality imaging or image co-registration Iterative image reconstruction algorithms Technical improvements Better detectors für SPET and PET Improvements in coil design for MRI Sequence adaptation for small animals for MRI CT and MRI for high throughput screening New reporter probes (contrast agents) New animal models

Multi-modality imaging CT/PET MRI/PET CT/SPET FMT/MRT SPET/CT: 125I labeled Herceptin®, © Iwata K et al. No one imaging modality can provide all the information (structure, function, molecular processes) in one image!

Image fusion (co-registration)

Co-registration: MRT-PET Comparison of 18F-FDG-PET and MRI in hamster After i.p injection of human GW39 colon cancer cells Lewis JS et al. Cancer Res 62 (2002) 445

Co-registration: SPET-MRT

Image reconstruction Chatziioannu AF Eur J Nucl Med 29 (2002) 98

New contrast agents Aime S et al. JMRI 16 (2002) 394

New animal models Giana P et al. Nat Med 9 (2003) 82