Molecular Imaging
The past century has seen medical imaging emerge as a powerful method for diagnosing diseases and for monitoring treatments. Molecular imaging allows molecular processes to be visualized, quantified, and tracked over time - demonstrating, in ways not previously possible, the underlying causes of disease and providing better treatment monitoring. These images often result in earlier detection of disease, better estimation of disease staging, and an accelerated drug development process.
Molecular imaging has two basic applications. The first is diagnostic imaging to determine the location and levels of specific targets associated with the disease being assessed. The second is for targeting therapies to specific disease-associated molecules, such as in the treatment of certain cancers.
As a diagnostic tool, molecular imaging is used to measure and characterize specific molecules, cells, and their processes. For example, Alseres’ ALTROPANE® molecular imaging agent binds specifically to a cellular structure or receptor called the Dopamine Transporter (DAT) found on certain central nervous system (CNS) cells. ALTROPANE is coupled with a radionuclide and an imaging procedure called Single Photon Emission Computed Tomography (SPECT) to produce an image of the brain that can potentially aid in the diagnosis of Parkinson’s Disease. Medical imaging allows trained radiologists to obtain clear images of various parts of the body that cannot otherwise be easily seen. Molecular imaging is most commonly used for neurological and cardiovascular conditions, as well as cancer detection.
As additional knowledge of genetics, biomarkers, and the human genome is gained, the combination of new molecular imaging agents with diagnostic tools will continue to evolve. The ability to evaluate molecular pathways in the body, particularly those that are key targets in the disease process, will provide doctors and researchers with a means to diagnose and treat disease.
What are molecular imaging agents?
Molecular imaging agents are created using the techniques of molecular and cellular biology and diagnostic imaging. The synergy of these technologies may make it possible to diagnose and treat disease, track the effectiveness of pharmaceuticals, and monitor the response to therapies in novel ways.
The basic principle in creating a molecular imaging agent is to identify a molecule that is specific for a target associated with the disease being studied. The molecule is labeled with a radioactive substance or other modality that allows detection by an imaging device. When administered, the molecular imaging agent locates and binds to the target in high enough concentration to be detected by a sensor and provides an image intensity that is proportional to the amount of target present.
What are the typical radio-labeling modalities?
Nuclear imaging procedures are techniques that use radio-labeled pharmaceutical agents. These agents, often referred to as radionuclides or tracers, are chemical compounds that contain a radioactive element such as 123I, which allows certain structures in the body to be visualized.
Iodine-123 (123I) and Technetium-99m (99mTc) are the most commonly used radio labels in SPECT imaging. Both 123I and 99mTc have proven to be convenient radioactive tracers. However, 99mTc is relatively inexpensive, has a short but useful half-life, and is readily available from portable generators at nuclear pharmacies or the imaging site.
Tracers used in Positron Emission Tomography (PET) scanning are typically isotopes with short half lives such as 11C (~20 min), 13N (~10 min), 15O (~2 min), and 18F (~110 min). Due to their short half lives, the radionuclides must be produced in a cyclotron at or near the site of the PET scanner.
To obtain images, tiny amounts of a tracer are introduced into the patient's body. These tracers emit a certain type of energy called gamma rays, which are detected by special devices.
Molecular Imaging Procedures
These non-invasive procedures are routinely used to look inside the body without the need for invasive surgery. Two of these, SPECT and PET, can measure biological activity inside the skull and image functional activity in the brain. Each technique has its own advantages and each provides different information about brain function. SPECT and PET both rely on the properties of radioisotopes and are primarily used to show physiological function of a system or organ.
SPECT Imaging
SPECT imaging is an acronym for Single Photon Emission Computed Tomography. This diagnostic imaging procedure is noninvasive and used to create clear, accurate, computer-reconstructed images of major organs, such as the brain. Energy from a radionuclide (tracer) in the body is detected by a gamma camera, which takes pictures that are reconstructed by a computer into two- or three-dimensional images used for diagnosis.
SPECT is a standard imaging procedure and widely available in most hospitals and outpatient clinics. There are approximately 14,500 SPECT cameras in the United States.
PET Imaging
PET imaging is an acronym for Positron Emission Tomography. PET is a noninvasive, diagnostic imaging procedure that produces an image of functional processes in the body. Once a PET agent is injected into the body, the PET scanner detects the tracer as the compound accumulates in different regions. A computer uses the data gathered by the sensors to construct precise two- or three-dimensional images. In the brain, this may allow doctors and clinical researchers to identify the sites where drugs and naturally occurring neurotransmitters localize, how quickly drugs reach and activate a neural receptor, how long drugs occupy these receptors, and how long they take to leave the brain.1
1Mathias, Robert. [The Basics of Brain Imaging] National Institute on Drug Abuse. November 1996; Vol ll, No 5