Imaging systems can be grouped by the energy used to derive visual information (X-rays, positrons, photons or sound waves), the spatial resolution that is attained (macroscopic, mesoscopic or microscopic) or the type of information that is obtained (anatomical, physiological, cellular or molecular). Macroscopic imaging systems that provide anatomical and physiological information are now in widespread clinical and preclinical use: these systems include computed tomography (CT), magnetic resonance imaging (MRI) and ultrasound. By contrast, systems that obtain molecular information are just emerging, and only some are in clinical and preclinical use: these systems include positron-emission tomography (PET), single-photon-emission CT (SPECT), fluorescence reflectance imaging, fluorescence-mediated tomography (FMT), fibre-optic microscopy, optical frequency-domain imaging, bioluminescence imaging, laser-scanning confocal microscopy and multiphoton microscopy.
For imaging technologies to be adapted more widely and to be complementary to other types of molecular measurement, the read-outs need to meet certain criteria: they need to be quantitative, high resolution, longitudinal (that is, allow imaging over time), comprehensive, standardized, digital and sensitive to molecular perturbations in the system. In terms of quantification, imaging technologies can provide data that are absolute or relative. Absolute quantification is possible in techniques where signals are independent of position in the sample. CT, FMT, MRI and PET inherently provide quantifiable information. Relative quantification is obtained from image data sets whose signals are depth and sample-type dependent but can be validated by rigorous experimental design. Bioluminescence imaging, fluorescence reflectance imaging and multiphoton microscopy technologies belong in this category. summarizes the spatial resolution, depth penetration, imaging time and cost of available systems.
For routine clinical practice and for testing the efficacy of drugs in clinical trials, CT, MRI, PET and SPECT are useful. Adaptations of these systems with much higher spatial resolutions have become available for use in experimental mouse models, allowing the development of new imaging probes for use in the clinic. By contrast, fluorescence reflectance imaging, FMT, fibre-optic microscopy and optical frequency-domain imaging are still mainly used experimentally, but they have clear potential for translation into the clinic. Because each technology has unique strengths and limitations, platforms that combine several technologies — such as PET-CT, FMT-CT, FMT-MRI and PET-MRI — are emerging, and these multimodal platforms have improved the reconstruction and visualization of data.
Complementary approaches to imaging include the use of portable in vivo
flow cytometers and molecular ‘nanolabs’ to track circulating tumour cells (Box 1
), as well as implantable, miniaturized fibre-optic multiphoton microscopy systems and implantable sensors for imaging molecular information in tumour environments. Other imaging technologies have recently been described, including thermal, electromagnetic and terahertz imaging, but their use in vivo
or in oncology is not as established as the techniques described here.
Box 1 | Quantification of circulating tumour cells
The number of circulating tumour cells (CTCs) is a sensitive biomarker for tumour progression and metastasis94
. Therefore, the quantification of CTCs is emerging as useful for diagnosing and ‘staging’ cancer, for assessing responses to treatment, and for evaluating whether there is residual disease. To be clinically useful, emerging technologies need to be highly sensitive (with detection limits around 1 cell per ml of blood) and highly specific for CTCs. Several such methods have recently been described and are in use experimentally.
Imaging CTCs non-invasively in microvessels of the skin could improve the sensitivity of detection by allowing large volumes of blood to be analysed. Confocal detection of CTCs with a dedicated in vivo
flow cytometer was first demonstrated in 2004 (ref. 95
), and portable systems have been described more recently96
. In the figure, part a
shows circulating green fluorescent protein (GFP)-positive multiple myeloma cells detected by in vivo
flow cytometry in ear arterioles. Measurements were taken over 60 s at various intervals after tumour cells were injected (three separate days are shown). Each signal spike represents a single CTC. Therefore, the data show an increase in the numbers of CTCs over time, and this occurred together with tumour progression. Another technique that has been advocated is using multiphoton microscopy to image the peripheral vasculature after intravenous injection of tumour-cell-specific fluorescent ligands, such as fluorescent folates, which are thought to be internalized by tumour cells97
An alternative blood-screening method recently reported uses a highly sensitive microfluidic platform. The platform consists of an array of microposts that are made chemically functional with antibodies specific for epithelial cell-adhesion molecule (EpCAM) and therefore captures CTCs of epithelial origin 98
Finally, a chip-based diagnostic MRI (DMR) platform with multiple channels allows rapid and quantitative detection of biological targets99
, as shown in parts b
of the figure. Using functionalized magnetic nanoparticles as proximity sensors to amplify molecular interactions100
, the DMR system can carry out highly sensitive and selective measurements on small volumes of unprocessed biological samples, including the profiling of circulating cells and the multiplexed identification of cancer biomarkers99
. Part c
of the figure shows detectable DMR changes (change in the time of magnetic relaxation, ΔT2
) of whole blood as a function of added cancer cells. (Panel a
courtesy of C. Lin, Massachusetts General Hospital, Boston, Massachusetts, and A. Kung and I. Ghobrial, Dana-Farber Cancer Institute, Boston, Massachusetts. Panel b
adapted, with permission, from ref. 99