Kodak in vivo imaging system: precise coregistration of molecular imaging with anatomical X-ray imaging in animals
Kodak Molecular Imaging Systems introduces a line of small-animal
in vivo imaging instruments that provide a new level of molecular
signal localization in live animals. The Image Station In-Vivo FX allows
precise multi-modal coregistration of optical or radioisotopic molecular
images with high-resolution anatomical X-ray images in animals
Traditional research on disease mechanisms using animal models has relied
mainly on the detection of morphological changes of the diseased tissues,
with physical measurements and anatomical imaging or on the excision and
pathological study of the tissues of interest. These methods often require
long time periods for measurable changes to occur and require a large
number of animal cohorts as multiple animals are often sacrificed at each
time point for histological testing.
Over the last few years, exciting new molecular imaging agents have emerged
from research laboratories that allow highly specific fluorescence-, luminescence-
and radioisotope-based imaging of disease processes at the molecular level
within living animals. These in vivo molecular imaging agents provide
the potential for rapid detection of specific molecular and metabolic
changes within target tissues in animals (or humans) long before morphologic
changes can be detected. In addition, these molecular changes can be monitored
in vivo without sacrificing the animal, resulting in lower cost,
time savings and improved data by using the same live animal for continued
studies.
The need for multimodal imaging
One major advantage of optical molecular imaging over anatomical imaging
is the use of ‘dark-field’ imaging methods that allow high levels of target
signal over the surrounding background signal. Dark-field contrast, however,
does not typically provide the appropriate contextual anatomic information
for useful localization of the molecular imaging signals within the animal.
Limited anatomic context of darkfield agents has been provided using digital
imaging overlay techniques in which the dark-field contrast is superimposed
on a reflection image of an experimental animal.
Although the overlay methods are beneficial, repeated imaging of the
same animal in different imaging sessions often results in misinterpretation
of the signal localization as animal repositioning is difficult. The white-light
reference image may be suitable for localization of large tumor masses,
but lacks the anatomical context required for repeatedly localizing smaller
signals of interest and/or mapping the molecular signals to bones or other
anatomical structures within the animal.
Hoping to realize the full potential of the dark-field molecular imaging
agents, researchers are beginning to apply multimodal instrumentation
that combines dark-field contrast with penetrating radiographic anatomical
imaging in one system.
Kodak Image Station In-Vivo F/FX
Kodak Molecular Imaging Systems has recently introduced a line of
in vivo small-animal imaging systems, including a model that allows
the capture of X-ray images. These X-ray images provide the detailed penetrating
anatomical guideposts that greatly enhance the localization of the in
vivo optical or radioisotopic molecular imaging agents.
The product line consists of the Kodak Image Station In-Vivo F and the
Kodak Image Station In-Vivo FX. The In-Vivo F allows for very high resolution,
multi-wavelength fluorescence, luminescence and radioisotopic imaging
in small animals. The In-Vivo FX includes all of the capabilities of the
In-Vivo F and the high-resolution X-ray Imaging Module using a Radiographic
(X-ray) Imaging Screen.
For both radiographic and radioisotopic imaging, patented Kodak phosphor
screens coupled to speed-enhancing interference optics efficiently convert
the ionizing radiation into light. The light is emitted by the screens
and captured by the charge-coupled device (CCD) camera to form the image.
Two different screen assemblies are available. One is optimized for the
high-energy radioisotopes such as 111In, 99Tc and
18F, and the other is optimized for the low-energy, high-resolution
requirements of X-ray imaging.
Multimodal imaging operation
The Kodak Image Station In-Vivo F and FX systems use the same operation
and hardware for optical imaging of an animal (or multiple animals) immobilized
and positioned in the animal chamber directly above the imaging chamber
window (Fig. 1a). For fluorescence, excitation light from a high-intensity
lamp is directed through the selected excitation filter to the animal.
Fluorescence from the imaging agent inside the animal is then emitted
and separated from the excitation light as it passes through the patented
Kodak Wide Angle emission filter. The fluorescence enters the 10× zoom
lens and is focused onto a 4 million pixel, cooled CCD. The digitized
read–out is efficiently interfaced to a personal computer (Windows or
Mac).
Multiple optical images of different molecular entities with different
fluorescent tags can be captured in the same animal by simply selecting
different filters and capturing additional images. In Time-Lapse mode,
multiple images can be captured in the same session to track the bio-distribution
of the imaging agent.
Once the desired optical images are captured, the radiographic (Xray)
phosphor screen can be moved into the imaging field by simply sliding
the screen under the animal chamber (Fig. 1b). The phosphor screen
comes into close contact with the thin plastic sheet that supports the
animal in the animal chamber, placing the screen essentially at the same
focal plane setting used with the optical images. The image capture setting
in software is switched to X-ray and the microfocus X-ray generator emits
a maximum energy of 35 Kvp for the desired imaging time (typically <30
s). The X-rays are differentially absorbed by bone and soft tissue, creating
a projection of the animal’s anatomical structure on the phosphor screen.
The bright-field image of the phosphor screen is captured and digitized
in the camera and read into the computer.
As the images of each modality are captured without movement of the animal
and with no change in optical focus or zoom, the images can easily be
merged or overlayed in the Kodak MI software for precise coregistration.
Multimodal imaging examples
Demonstration of the coregistration of fluorescence and X-ray imaging
is shown in Figure 2. The mouse was injected with OsteosenseTM
750, a near-infrared fluorescent diphosphonate probe that binds to bone.
This high-resolution image of the animal’s paw shows the fluorescent signals
coming from the probe attached to the digits in the paw (Fig. 2a).
The X-ray image details the bones in the digits of the animal paw (Fig.
2b), and the overlay image demonstrates the expected colocalization
and the precise coregistration of these two modalities in the Kodak instrument
(Fig. 2c).
Combined multiwavelength fluorescence and X-ray imaging is shown in Figure
3. We injected three different fluorescently tagged imaging agents
subcutaneously into different regions of the mouse abdomen. Fluorescence
imaging with different filter sets appropriate for each fluorochrome was
followed by X-ray imaging. The four images, representing the three different
fluorescent channels and the X-ray image, were easily contrasted and pseudocolored
in Kodak MI 4.0 software and merged in Adobe Photoshop™.
The image in Figure 4 demonstrates the combination of radioisotope
18F imaging (typically used in positron emission tomography
(PET) imaging) with X-ray imaging. The mouse was injected with [18F]
fluordeoxyglucose (FDG) PET and imaged with the Kodak Image Station In-Vivo
FX using the radioisotopic imaging screen for 8 min with the camera in
the highest-binning state. The image was contrasted and overlayed on the
subsequent X-ray image to show the localization of the isotope in the
heart of the animal.
Conclusion
Kodak has developed and commercialized powerful multimodal in vivo
imaging systems that greatly enhance the localization of molecular signals
in live animals. These systems are now used by top academic, biotechnology
and pharmaceutical research institutes worldwide. The flexibility of the
system allows the combination and coregistration of multiple wavelengths
and multiple modalities of imaging including optical, radioisotopic and
radiographic imaging. Several studies are now in progress that will further
detail the utility of combining, coregistering and performing the appropriate
analysis of the multiple imaging modalities provided by the Kodak Image
Station In-Vivo F/FX systems.
1. Mahmood, U. & Weissleder, R. Near-infrared optical imaging of proteases
in cancer. Mol. Cancer Ther. 2, 489–496 (2003).
For more information contact: Kodak Molecular Imaging Systems 4 Science
Park New Haven, CT 06511 william.mclaughlin@kodak.com
William McLaughlin & Douglas Vizard
Kodak Molecular Imaging Systems, 4 Science Park, New Haven, Connecticut 06511, USA.
Correspondence should be addressed to W.M. (william.mclaughlin@kodak.com).
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