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

ReporterBlack box

Transparent box

HistopathologyPCR etc.

Dissected tissueTime delay

Less predictive

Intact animalReal timePredictive

Physiology

Species Weight Blood volume

Heart rate Resp. rate

Human 70 000 g 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 mm1 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:– Radiography– Computed tomography (CT)

Ultrasound (US) Optical imaging Magnetic resonance tomography (MRI) Positron emission tomography (PET) Single photon emission tomography (SPET)

Basic principle

Planar– 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 FMTof 9L gliosarkomasin 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

Parametric display

Dynamic analysis

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

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 No limit Sec

US <50µm mMol mm Sec

PET 1-2mm p-nMol No limit Min

SPET < 1mm p-nMol No limit Min

FRI 1-2mm p-nMol < 1cm Sec

FMT 1-2mm p-nMol < 10cm Sec

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