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

2. 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

3. Evolutionary relationships

4. 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

5. 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

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

7. 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

8. 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)

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

10. 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

11. Planimetric imaging

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

13. Dedicated small animal CT
© SkyScan Inc.

14. 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

15. 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)

16. Ultrasound
© Turnbull, New York

17. Optical imaging

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

19. Disadvantages of FRI Not quantitative
Sensitivity varies with wave length

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

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

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

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

24. Small animal coil

25. 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

26. Quantification of tumour perfusion

27. Dynamic analysis
Parametric display

28. 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

30. 3D data analysis

31. Volumetric analysis

32. Dedicated systems

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

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

35. Positron emission tomography

36. 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

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

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

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

40. 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

41. SPET probes

42. 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

44. SPET

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

46. Single pinhole vs multihole (7)
20 min
5 min

47. 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

48. 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

49. 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!

50. Image fusion (co-registration)

51. 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

52. Co-registration: SPET-MRT

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

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

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