Cancer imaging can be conducted by planar X-ray, computed tomography (CT), ultrasound (US), or magnetic resonance imaging (MRI) scans. Molecular imaging can be defined as "the non-invasive visualization of molecular processes." Molecular imaging uses X-ray, CT, MRI, and US in addition to Xuorescence microscopy and endoscopy as well as nuclear scanning techniques like positron-emission tomography (PET) and single-photon emission tomography (SPECT). Cancer is initiated by and progresses through genetic changes. Initial genetic changes allow cancer cells to evade biologic programs that regulate and limit cellular growth under normal conditions. As tumor growth continues, the unique physiologic microenvironment that cancer cells are exposed to in solid tumors influences tumor progression, aggressiveness, and response to treatment. Molecular imaging can be applied to visualize gene expressionrelated processes such as promoter activity or transcriptional activity, among other processes, in living organisms in vivo. Cancer is a multifaceted disease that requires individual characterization for each tumor but also shares common characteristics. It is important to be able to identify and visualize molecular markers that occur either collectively or individually in various cancers. Such ability is valuable in the quest for the development and application of more potent molecular-targeted cancer therapies that kill malignant tissue while sparing normal tissue. As multimodality imaging instruments become increasingly available, a combined molecular-functional-anatomic imaging approach will become more commonplace for preclinical and clinical investigations and will play an integral role in characterizing tumors to select and validate treatment, screen for sensitivity, and monitor treatment.
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