Caitlin Smith has a B.A. in biology from Reed College, a Ph.D. in neuroscience from Yale University, and completed postdoctoral work at the Vollum Institute.
As molecular and cellular biologists study networks of signaling molecules that continue to grow in complexity, it is increasingly important to look at how such phenomena influence the physiology of the whole organism. The ability to put new information in an overall physiological context is becoming possible with in vivo imaging, which lets researchers examine the anatomy or even molecular activity in whole, live animals.
In vivo imaging technologies are akin to the CT scans and MRIs that physicians use for human patients, and they offer the same advantages, namely, being noninvasive and enabling the study of animals in real time and over a period of time. Yet although each type of imaging can give potentially valuable data, combining imaging modalities can yield still further insights. Here is a look at some progress and tools for 3D in vivo imaging of small animals.
What is 3D imaging?
3D imaging refers to the use of tomographic imaging technology, in which a 3D image is reconstructed by a computer using a series of planar imaging sections through the specimen. The tomographic imaging modalities used today include: PET (positron emission tomography), SPECT (single photon emission computed tomography), CT (computed tomography) and MRI (magnetic resonance imaging).
These four imaging modalities provide two main types of information. Both SPECT and PET detect signals from radiolabeled molecules or radioisotopes, sampled in a series of different angles to construct a 3D image. These are particularly useful for imaging molecular tracers or monitoring the pharmacological activity of a compound, such as in toxicology applications.
CT and MRI are excellent for imaging anatomical details (and also, in the case of MRI, the movement of fluids such as blood flow). CT constructs images using a series of high-resolution X-rays, and MRI detects movements of molecules in a magnetic field. For small-animal researchers, microCT systems enable higher-resolution CT studies for subjects such as mice and rats.
Each modality has it pros and cons, says Jens Waldeck, global application manager at Bruker Biospin, a company that supports several imaging modalities. MRI shows contrasts in soft tissues, but CT is better for imaging bone. In addition, “PET is most often used [in] translational approaches, since the major [imaging] compound, 18FDG [18F-deoxyglucose], is already used in clinics,” says Waldeck. “SPECT gives you superior resolution but lacks the sensitivity PET offers.”
All-in-one: Multimodal-imaging systems
Combining imaging modalities can be achieved either by using a multimodalimaging system, or by overlaying (or co-registering) datasets from two different imaging modalities using computer software. The Bruker Albira PET/CT, for instance, combines two human-imaging modalities in a single small-animal, preclinical instrument. Bruker’s MSFXPro is another multimode choice, says Derek Adler, manager of the Molecular Imaging Center at Rutgers University. “Instruments such as the MSFXPro from Bruker have a feature to detect bioluminescence, fluorescence, radioisotope and X-ray, all in one unit,” says Adler, who uses the Albira and MSFXPro systems in addition to VisualSonics’ Vevo® 2100 system, Aspect Imaging's M2™ platform and Revvity’s IVIS® 100 system.
Alexandra DeLille, director of technical applications for in vivo imaging at Revvity, says co-registering molecular imaging data within an anatomical context is an exciting trend in life-science research that is catalyzing new technological developments in imaging, such as Revvity’s IVIS series of instruments. For example, DeLille says, the IVIS SpectrumCT integrates both low-dose microCT and optical (bioluminescence and fluorescence) imaging in one instrument, “making 3D anatomical co-registration of an optical signal a one-click procedure.” (Revvity acquired the IVIS line when it purchased Caliper Life Sciences, which in turn acquired it from Xenogen.)
Co-registering data from different modalities
Co-registering different imaging datasets is possible without a multimodal-imaging system, but two ingredients are required. One is software, such as inviCRO’s VivoQuant™ or FEI Visualization Sciences Group’s Amira®. Another is hardware, such as an animal bed, that keeps the animal in the same physical position while you transfer it from one imager to another. “Technology is improving integration software and hardware so that an animal can be taken from the MRI to the PET-CT to the optical imager without issue,” says Adler. “While the technology exists for co-registration of datasets, they continue to improve the quality and the number of instruments that can be combined per dataset.”
Revvity’s QuantumFX, a microCT system for animals weighing up to 5 kg, incorporates an imaging shuttle for mice or rats with built-in fiducial markers, which enables identification of the animal’s physical position between modalities for easier data co-registration later. “[The] combination with [Revvity’s] Living Image software’s co-registration module allows for automated co-registration with either IVIS Spectrum or FMT 3D data,” says DeLille. Revvity’s FMT system is a high-throughput fluorescent molecular tomography imager “with great capabilities for repeat assays for biomarker, toxicology and biodistribution applications in vivo,” she adds.
Both intrinsic (i.e., using anatomical landmarks from within the animal) and extrinsic (i.e., using fiducial markers on equipment) information can be used to co-register imaging datasets. This information informs software such as with the image-fusion tool in the PMOD software from Swiss company PMOD Technologies, which puts the datasets together. Both methods (using intrinsic vs. extrinsic markers) can work well. But Waldeck says that in some cases, relying on extrinsic markers alone can be problematic, because “animals sometimes reposition themselves (even under anesthetics), and [the method does] not reflect heart and lung movements,” he says. “Thus intrinsic fusions with ECG and respiratory-gated datasets are best, such as [when combining data from] PET and CT.”
Imaging on the horizon
Though not in day-to-day clinical use now, two new imaging methods are engendering interest in the imaging community and may soon be coming to a multimodal imager near you. One is optoacoustics, which combines optical imaging and ultrasound. In optoacoustic imaging, a pulsed laser is absorbed by the tissue. This elevates the tissue’s temperature, which produces waves of increased pressure that are detected from outside the animal’s body. An acoustic transducer then converts the signals to an image that reflects the initial absorption of the photons from the laser pulses.
One advantage of optoacoustic imaging is its resolution: It can detect breast tumors 2-mm in diameter, at greater depths into the tissue, without the use of harmful X-rays.
“Optical imaging, optoacoustics and ultrasound have gained ground in small-animal imaging,” says Waldeck. “[They are] growing and might be competitive players in the future.”
Another emerging technology is magnetic particle imaging (MPI), a noninvasive method that images small animals at high frame rates of 30 to 40 frames per second. “This technique has a very high potential to change the small-animal imaging landscape in the coming years dramatically, since it was never possible [before] to image at such high imaging rates,” says Waldeck.
Bruker recently released its first MPI system, which was developed in collaboration with Royal Phillips. MPI produces live, 3D images of tissues by detecting magnetic responses of iron oxide nanoparticles that are injected into the animal’s bloodstream. MPI can capture images with better time resolution than other methods, enabling the direct study of a beating heart, for example.
In today’s imaging world, every modality has its particular strengths, but according to Waldeck “there is no Swiss Army Knife” of imaging methods. “Those ‘one-fits-all solutions’ most often compromise—at the moment—either the performance or sensitivity, and in most cases both.” Ideally, identifying the type of imaging you need most will let you focus on systems that shine in that modality.
Image: Christian Lackas (link)