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Glossary of Molecular Imaging Terms

bioluminescence imaging (BLI)

Bioluminescence imaging* uses natural chemicals (such as luciferase, the substance that creates the natural glow of fireflies) to trace the movement of certain cells within the body or to identify cells where specific kinds of chemical reactions are taking place. For example: Heart muscle stem cells genetically engineered to produce luciferase have been injected into the bloodstream of mice. When researchers found glowing cells in damaged heart muscle, they knew that the stem cells had gone to the right place. This technique has also allowed researchers to see how long the stem cells live in the body and if they create new heart muscle cells. (See also fluorescence imaging.)

biomarker

Biomarkers are molecules that indicate a specific biological process is occurring. For example, prostate specific antigen (PSA) is a marker for prostate disease.

contrast agent

A contrast agent is a drug that is used to improve the contrast of medical images, thereby making it easier to see the differences between adjacent tissues. This is helpful for diagnosing changes in anatomy. For example, contrast agents make it easier to see the borders of tumors so that changes in the size or number of tumors can be more accurately measured.

CT imaging, CT scan

Computed tomographic (CT) scanning is a medical imaging technique that uses a computer to build a three-dimensional (3D) image of the body from a series of two-dimensional (2D) cross-sectional x-ray images. The resulting image can be rotated and viewed from any angle. CT images are often displayed as 2D images that display a single "slice" of anatomy, but the displayed image is derived from the 3D image, unlike standard X-rays, which are simple, 2D pictures.

diagnostic scan, diagnostic imaging

Diagnostic images, such as X-rays, CTs, MRIs, PET, and SPECT scans, provide physicians with a way to look at the interior of the body without surgery. They are considered "non-invasive" diagnostic techniques, as opposed to a biopsy or exploratory surgery. PET and SPECT imaging also provide information about how certain tissues are functioning, helping to identify tumors, inflammation, or areas of reduced blood flow.

FDG

Fluorodeoxyglucose (FDG) is a compound is which a radioactive fluoride atom is attached to a molecule of glucose. Once in the body, the FDG molecule is absorbed by various tissues just as normal glucose would be, and the radiation from the fluorine is used to map the distribution of glucose within the patient. (See PET scan.)

fluorescence imaging (also fluorescent molecular tomography, FMT)

Fluorescence imaging is similar to bioluminescent imaging except that the proteins only produce light (fluoresce) when activated by an external light source, for example, a laser.

fusion imaging, fusion scan

Fusion imaging is the result of merging two different kinds of diagnostic scans to create a more complete picture of what is going on within the body. For example, a CT scan can be combined with a PET scan to provide the physician with a combination of metabolic information (from the PET scan) and anatomic information (from the CT scan). The combined scan can provide more detailed information than is available with either of the scans by themselves.

gamma camera

Gamma cameras are the most commonly used nuclear medicine instrument. They detect the gamma radiation that is emitted from radiopharmaceuticals and provide the raw data that allows the reconstruction of the images that are used extensively in nuclear medicine and in nuclear cardiology.

imaging biomarker

Imaging biomarkers are measurable characteristics obtained by imaging that indicate a specific biological process is occurring in the body. They help speed drug development because the biomarkers show effectiveness earlier than anatomic changes. They have been historically difficult to use in drug trials and clinical practice due to a lack of standardized methods and regulatory approval.

ligand

In medical imaging, ligands are molecules that can bind both to a tracer (radioactive or light emitting) and to a molecule of interest within a living system. For example, in radioimmunotherapy a radioisotope is attached to a monoclonal antibody. The antibody is the ligand; it is designed to bind with specific molecules on the surface of cancer cells, so it carries the radioisotope to the tumor. The radiation kills the cancer cell while sparing nearby tissue. In optical imaging, the ligand might carry a bioluminescent protein. The antibody binds to a receptor on the surface of the cell being studied, delivering the bioluminescent protein directly where the researcher wants it.

magnetic resonance imaging (MRI)

Magnetic resonance imaging is a diagnostic scan that uses high-strength magnetic fields rather than radiation. MRI techniques are used primarily to study anatomy, but a special type of MR scan, functional MRI (fMRI) can be used to map blood flow for functional studies.

magnetic resonance spectroscopy (MRS)

Magnetic resonance spectroscopy uses MRI equipment to obtain information about the distribution and concentration of a number of biochemicals within the living body. These chemicals can provide very detailed and specific information about disease processes. For example, an MRI scan may show the location of a tumor, but MRS can, by reading the chemical signatures of the tumor's metabolism, indicate how aggressive the tumor is.

medical imaging

See diagnostic imaging.

microbubbles

Microbubbles are extremely small, hollow structures either containing or attached to therapeutic or imaging molecules. They are used clinically with ultrasound as a contrast agent. Research indicates that they may be effective carriers of drugs and radioisotopes.

micro- (PET, MR, CT, SPECT)

medical imaging instruments or techniques specifically designed for medical research using small animals. Microimaging studies are useful in preclinical investigations of new imaging agents or with existing imaging agents to evaluate new therapies using exiting agents in animal models.

molecular imaging (MI)

Molecular imaging is an array of non-invasive, diagnostic imaging technologies that can create images of both physical and functional aspects of the living body. It can provide information that would otherwise require surgery or other invasive procedures to obtain. Molecular imaging differs from microscopy, which can also produce images at the molecular level, in that microscopy is used on samples of tissue that have been removed from the body, not on tissues still within a living organism. It differs from X-rays and other radiological techniques in that molecular imaging primarily provides information about biological processes (function) while CT, X-rays, MRI and ultrasound, image physical structure (anatomy).

Since molecular imaging works at the cellular and molecular level, it can provide accurate information about disease processes at a very early stage. Molecular imaging provides personalized information about an individual's specific disease and how that disease may react or has reacted to specific treatments.

Molecular imaging technologies include traditional nuclear medicine, optical imaging, magnetic resonance Spectroscopy, PET and SPECT. Ultrasound, traditionally an anatomical imaging technique, uses microbubbles to create molecular images.

molecular imaging probe

A molecular imaging probe is a molecule used in molecular imaging to deliver a tracer to a specific organ or tissue. A probe typically consists of a ligand and a tracer. The tracer provides the signal (light, radiation, etc.) that can be picked up by a detector, and the ligand carries the tracer to the site of interest.

nanotechnology

Nanotechnology is both a science and an art. The term refers to the study of extremely small physical structures (100 nanometers or smaller). At that size, many physical interactions do not follow the same rules as larger structures. The art of nanotechnology involves building nanoscale strutures that can be extremely useful in the medical field.

nanoparticle

Nanoparticles are unique nanoscale structures that are designed for specific purpose. Their extremely small size makes them very useful in medical applications where specifically designed particles scan deliver drugs directly into targeted cells and evenacross the blood-brain barrier.

nuclear medicine/nuclear imaging

Nuclear medicine uses very small amounts of radioactive materials (called radiopharmaceuticals) to diagnose and treat disease. In diagnostic imaging, the distribution of a radiopharmaceutical in the body is determined using a gamma cameras or PET scanners. For therapeutic procedures, the treatment is designed to deliver the radioactive material to specific areas of the body. For example, radioactive iodine is used to treat thyroid cancer because the thyroid gland is the only tissue that accumulates iodine. The amount of radiation in a typical nuclear imaging procedure is similar to that received from a diagnostic x-ray or CT scan. The amount of radiation received in a typical treatment procedure is higher than that received from an imaging procedure but is similar to or less than that received from other types of radiation therapy.

Nuclear medicine procedures can often identify abnormalities very early in the progress of a disease—long before many medical problems are apparent with other diagnostic tests.

nuclear cardiology

Nuclear cardiology uses radiopharmaceuticals to trace the flow of blood through the chambers of the heart, through the arteries that supply blood to heart, and through the muscles of the heart. Tracing blood flow through the heart can indicate how well the heart is pumping. Tracing blood flow through the arteries can reveal blockages that might cause (or have caused) a heart attack. Tracing blood through the muscles of the heart after a heart attack shows which areas of heart muscle were damaged by the attack. New radiopharmaceuticals are being developed that bind to plaque and can demonstrate the extent of plaque build-up in arteries.

optical imaging

Optical imaging, as a medical imaging technique, refers to the use of molecules that produce light. These molecules are designed bind to specific cells (i.e. cancer cells, inflammatory cells) or molecules (i.e., brain chemicals) thereby making them detectable with an external imaging device. See bioluminescent imaging and fluorescent imaging.

PET scan, PET scanner

Positron emission tomography (PET) is a medical imaging technique that uses radiopharmaceuticals that emit positrons (positively charged electrons). A radiopharmaceutical such as FDG is injected into the patient. The fluorine emits positrons which react with the first electron they come in contact with, annihilating both and producing energy according to Einstein's famous E=MC² formula. This energy takes the form of two photons (particles of light) with a very specific energy level that shoot off in opposite directions. When these photon pairs are detected by the PET scanner, the location of the original fluorine atom can be extrapolated. Although positron/electron annihilation is one of the most powerful reactions known to science, the amount of mass involved is so small that the actual energy produced is not harmful to the patient, and the fluorine decays rapidly into harmless oxygen.

A PET scanner consists of a circular array of detectors tuned to detect photons at the specific energy level created by the positron/electron annihilation. Tomographic reconstruction software assembles these signals into images that show the location and concentration of the radiopharmaceutical inside the patient. When scanning with FDG, the rapidly dividing cancer cells use a lot of glucose to fuel their growth; therefore, they show up as "hot spots" on the PET image. PET is useful in diagnosing certain cardiovascular and neurological diseases as well because it highlights areas of increased, diminished, or absent metabolic activity.

PET is used predominately in determining the presence and severity of cancers, neurological conditions, and cardiovascular disease. It is also used to identify and stage cancers in the initial diagnosis and to check for recurrences. An example of how PET can be uniquely useful is in following patients after radiation therapy. The radiation may create scar tissue at the cancer site. Other medical imaging techniques can only identify the scar tissue as a "mass," i.e. it looks the same before and after therapy, while PET can indicate whether or not the mass is still malignant.

PET/CT

PET/CT is a fusion scanning technique that combines information from a CT scan and a PET scan into a single image. PET is superior at differentiating malignant from non-malignant masses such as scar tissue, infection or inflammation. However, CT is superior in visualizing the details of anatomy. Fusion techniques such as PET/CT take advantage of each modality's strengths. The CT scan provides a detailed three-dimensional image, and the integrated PET image identifies the location of cancerous cells.

pharmacodynamics

Pharmacodynamics is the study of the way drugs affect a living organism including the relationship between size of dose and the effect of the drug. For example, molecular imaging measures pharmacodynamics when assessing response to therapy by measuring changes in the amount of FDG used by tumors before and after treatment.

pharmacokinetics

Pharmacokinetics is the study of how living tissues process drugs, i.e. alter their chemical make-up as a drug is absorbed, distributed, metabolized and excreted. By tagging a drug with a tracer, molecular imaging allows researcher to follow a drug as it progresses through a living system.

pharmacogenetics

Pharmacogenetics is the study of how a living system reacts to a drug based on an individual patients' genetic make-up.

radioimmunotherapy (RIT)

Radioimmunotherapy uses specially designed antibodies to deliver radioisotopes to targeted cells (usually cancerous). The radiation then destroys those cells. Because the antibodies are designed to attach only to very specific types of cells, radioimmunotherapy maximizes the radiation that can be delivered to the diseased tissue and minimizes the amount of radiation to which healthy tissue is exposed.

radioisotope

A radioisotope is a radioactive version of an element. Radioactive elements differ from the stable versions of the same element in that they have either more or fewer neutrons. For example all forms of carbon have the same number on protons (six), but the most common form has six neutrons as well. Forms with five (¹¹C), seven (¹³C) and eight (the famous ¹⁴C) neutrons are radioactive. Some radioisotopes have very long half-lives. For example, the half-life of ¹⁴C is 5700 years, making it useful for dating organic materials. And some have very short half-lives: the half-live of ¹¹C is only 20 minutes. There is no chemical difference between the way the radioactive and non-radioactive versions of an element react. The fact that there is no difference allows the radioactive versions of the element to be substituted for the non-radioactive versions to produce a tracer. This principal can also be applied to therapy. Iodine, which was the first element used in nuclear medicine, is used exclusively by the thyroid gland. If a radioactive isotope of iodine is introduced into the body, it will be taken up by the thyroid gland in exactly the same way as non-radioactive iodine. A gamma camera can then be used to determine how well the thyroid is working. Similarly, a large dose of radioactive iodine can be used to treat thyroid cancer by delivering a tumor-killing radiation dose directly to the cancerous tissue.

radiopharmaceutical

Any of the drugs used in nuclear medicine for imaging or therapy that include a radioisotope.

radiotracer, tracer

A radiotracer may be a simple radioactive element (a radioisotope like radioactive iodine) or a combination of a radioisotope and a drug (such as glucose in FDG) that delivers the radioisotope to a specific type of tissue. The radiotracer produces small amounts of radiation that can be detected by imaging devices outside the body and used to create medical images.

Non-radioactive tracers include bioluminescent and fluorescent molecules and microbubbles.

SPECT scan

SPECT stands for "single-photon emission-computed tomography." A SPECT scan uses a gamma camera to detect radioisotopes that emit high-energy radiation. The gamma camera works with a computer to create three-dimensional images of the distribution of the tracer in the body. SPECT is most often used in cardiology to provide information about blood flow through the heart muscle that can be used to diagnose heart disease. It is also used for brain and bone scans and to detect infection and certain types of tumors.

tomography, tomographic reconstruction

Tomographic reconstruction is a technique that uses a series of two-dimensional images to create a three-dimensional image. For example, a CT (computed tomography) scanner acquires a series of cross-sectional x-rays, which are combined (using tomographic reconstruction algorithm software) into a three-dimensional image of the body that can be viewed from any direction.

translational medicine

Translational medicine is the process of moving basic laboratory research into mainstream medical practice. Translational medicine focuses on the necessary steps—including patient testing and clinical trials—that will ensure safety before a technique can be used on patients in clinical practice.

ultrasound (US)

Ultrasound is essentially an anatomical imaging technology that uses sound waves to create images of tissue within the body. It can be a molecular imaging technique when used in conjunction with targeted microbubbles.