Multimodal Imaging For Detection Of Cancer

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Multimodal Imaging of brain tumor. Image courtesy: http://crmbm.timone.univ-mrs.fr/

Imaging modalities such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) provide information about metabolic and functional changes due to a pathological process, and Magnetic Resonance Imaging (MRI) and X-Ray Computed Tomography (CT) provide information about anatomical or structural changes due to a pathological process. Sometimes it is necessary to obtain both functional and anatomical information for improved diagnostics, and monitoring of therapy.

Each imaging modality is characterized by different spatial and temporal resolution, and their sensitivity for measuring properties related to morphology or function also varies. The integrated multimodal imaging system eliminates the shortcomings of individual imaging modalities and offers a range of imaging possibilities.

Earlier the approach to combine functional and anatomical images was software-based. Two images of the same organ or body part were acquired and superimposed using various computer softwares. This approach was not successful due to uncontrolled movements of body parts during two acquisitions. Thus integration of different imaging modalities was a solution to easily obtain both anatomical and functional information. Integration of PET or SPECT with CT or MR imaging provides an excellent soft-tissue contrast and information about functional, structural and morphological changes due to pathological process.

Combination of PET/SPECT with CT:-

Positron emission tomography (PET) and Single Photon Emission Computed Tomography (SPECT) are important functional imaging modalities due to the high sensitivity and their ability to observe metabolic processes over time and track radioactive tracers. However PET/SPECT lack good spatial resolution and do not provide sufficient anatomical information. To be more accurate in diagnosis of tumor and radiation therapy, morphological information is also necessary. X ray computed tomography (CT) can provide information about morphological changes and a very good spatial resolution.

The association of PET/SPECT, with CT provide information about both morphological and functional changes and has been a major breakthrough in cancer imaging. PET/SPECT, with CT offers great potential in the planning of radiation therapy, when tumor regions are difficult to deﬁne.

Combination of PET/SPECT with MR:-

A limitation of CT is that it cannot provide adequate soft-tissue contrast. While, MRI imaging can provide structural or anatomical information in soft tissues and bones. SPECT-MR/PET-MR imaging modalities could provide sufficient spatial and temporal resolutions and high sensitivity. MR imaging do not require the use of ionizing radiation and therefore the radiation dose required for imaging is less.

Nanotechnology and Multimodality:-

Nanoparticles have large surface area and multiple functional groups on the surface, by appropriate surface modiﬁcations and engineering it is possible conjugate different ligands to produce multifunctional nanoparticles for multimodal molecular imaging.

Weissleder’s group have developed crosslinked iron oxide (CLIO) based Cy5.5 arginyl peptide conjugated nanoparticle for MR and near-infrared ﬂuorescence dual imaging. All biomolecules (hemoglobin, water and lipids) have minimum absorption in the NIR wavelength range (650–900 nm), which minimizes tissue autoﬂuorescence. Iron oxide nanoparticles provide good contrast for MRI while Cy5.5 is responsible for NIR Fluorescence Imaging. They have also developed trimodal imaging probes for PET, MRI, and ﬂuorescence microscopy. A radioactive tracer motif for PET and near-infrared ﬂuorochrome was attached to the iron oxide nanoparticles for multiple imaging detections.

Nanotechnology offers great potential in multimodal imaging. It is possible to incorporate or attach different dye/radioactive/contrast agents to nanoparticles and create a nanosystem with bi or trimodal abilities. By active targeting Nanoparticles can be delivered to specific tissues or organs, thus increasing the contrast and efficiency of imaging techniques. Stealth nanoparticles (by coating of PEG) have increased circulation time, which allows sustained imaging for few days or weeks by single dose of the imaging agent.

References:-

Cherry, S. R., & Mri, P. E. T. (2006). Multimodality In Vivo Imaging Systems : Twice the Power or Double the Trouble ? doi:10.1146/annurev.bioeng.8.061505.095728.
Liu, Z., Kiessling, F., & Jessica, G. (2010). Advanced Nanomaterials in Multimodal Imaging : Design , Functionalization , and Biomedical Applications, 2010. doi:10.1155/2010/894303.