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What Is an Imaging Agent?

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  • Written By: Maggie J. Hall
  • Edited By: Susan Barwick
  • Last Modified Date: 04 September 2016
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The fluorescing dye that is commonly used in oncology imaging procedures to highlight abnormal growths is known as an imaging agent. Different cancer imaging agents are designed to target tumors based on the unusual growth patterns and metabolic needs of the malignant mass. Some agents not only differentiate between healthy and cancerous cells but also between different types of cancers. Using an imaging agent, oncologists can make diagnoses, monitor treatment regimens, and evaluate the extent of metastasis.

Patients often receive a radiopharmaceutical imaging agent prior to undergoing computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET) procedures. Radiology technicians administer the agent orally, intravenously, or by enema. Depending on the part or structure of the body to be visualized, patients might also receive an imaging agent through an artery or within a specific body cavity. The substances are typically eliminated by the kidneys into urine or in the evacuation of fecal waste. Patients usually receive post procedure instructions that suggest increasing fluid intake to hasten the elimination process.

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Early imaging agents not only targeted and highlighted tumors but passed from the mass after cell damage or death occurred. The imaging agent continued to fluoresce, illuminating a trail to other areas. Scientists devised different compounds that only fluoresce while in a living cell and that stop upon leaving dead or dying cells. One of these later formulations includes boron-dipyrromethene, often called BODIPY, which only fluoresces while within living cells. Scientists further modified the substance by attaching certain molecules that were aimed at targeting specific tumors, enabling BODIPY to illuminate breast cancers with the (human epidermal growth factor receptor2) HER-2 protein.

Oncologists often use the imaging agent known as 18F-fluorodeoxyglucose (18F FDG) for malignant mass detection. Tumor cells grow and reproduce at accelerated rates compared to normal tissue, which means they have a higher rate of nutrition consumption. The compound 18F FDG contains a sugar molecule that tumor cells quickly absorb to satisfy nutritional requirements. The fluorescing compound does not, however, effectively illuminate malignant cell growth in bone tissue. Instead, sodium fluoride-18 (18 NaF) is used to produce illumination in bone tissue, and oncologists often combine this agent with 18F FDG in order to visualize the presence of tumors in soft and bone tissues.

Fluorescing compounds can also be used to monitor tumor growth by analyzing the development of vascular structures in and around malignant tissue. Using vascular endothelial growth factor (VEGF), a protein common to all malignant tissues, physicians track the development of blood vessels created by the tumor. Radiologists might also use certain compounds to identify cancers and determine treatment. The imaging agent 18F-fluoromisonidazole (FMISO), detects tissues that have reduced oxygen levels. These cancers generally have a poor response to chemotherapy or radiation.

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