By Kevin Murnane, Nikki T. Sawyer and Chris Glielmi
Originally published February 2008.
Positron Emission Tomography, Kevin Murnane
Positron Emission Tomography (PET) is a technique that allows a researcher to localize radioactive emissions in space. A PET scan requires a radioactive tracer. This tracer is a ligand that binds to a protein and is attached to a heavy atomic isotope. This tracer travels
though the body and attaches to its target protein. As the heavy atomic isotope undergoes
radioactive decay it emits a positron (the antimatter equivalent of an electron – yes, there is such a thing as antimatter). When this positron comes in contact with an electron they will annihilate each other. The annihilation event produces gamma rays that travel away from the source. The PET scanner measures these gamma rays through a ring of scintillation crystals. By measuring the speed and geometry of the gamma rays’ movement,
the PET scanner can triangulate the source. The scanner picks up millions of annihilation events during a scan and uses the combined information to create a spatial map of these radioactive decays.
The seemingly simple idea of localizing radioactive emissions in space has a surprisingly large number of biological applications. PET can be used to indirectly
measure neuronal activity by measuring its correlate cerebral blood flow. This is done by localizing the source of emissions from water containing a heavy isotope of oxygen that is flowing with the blood. Alternatively, radioactively labeled glucose can be used to measure the metabolic state of the neurons (this same technique is commonly used by Oncologists
to locate metabolically active tumors). By injecting a radioactively labeled drug, such as cocaine, you can measure its kinetics and distribution. By competing a radioactively labeled (hot) ligand with a non-radioactively labeled (cold) drug you can calculate the proportion of its target receptors or transporters the cold drug occupies. PET is a useful technique with widespread applications in pharmacology and neuroscience, however, it does have some significant safety concerns primarily due to the use of radioactive material.
1. Exposure to radioactive material – One of the core responsibilities of the Emory
University Environmental Health and Safety Office is radiation safety (http://www.ehso.emory.edu/). Each person that works with radiation, using
PET or any other technique, must first complete a safety course. Furthermore,
all people working with radioactive material must wear monitoring badges.
These badges are tested every three months to ensure that exposure for any
researcher is within the acceptable safety limits. Individuals working with
very high levels of radiation or particularly dangerous forms of radiation will
also be routinely monitored by bioassay.
2. Long half-lives – The half-life of radioactive material is the amount of time it
takes for half of the substance to decay. This varies by isotope. For example, the
half-life of C11 is on the order of 20 minutes whereas F18 is on the order of 110
minutes. It is required that 10 half-lives go by before the radiation is considered eliminated. For F18 this takes about 20 hours. Therefore, anything that has come in contact with the radioactive material (including the subject) is considered a safety hazard for potentially long periods of time.
3. Radioactive spread – Since radiation can last hours and contaminates anything that
comes in contact with it, it is important to ensure the minimum spread of the material. Therefore, anything that comes in contact with the radiation must be carefully and properly disposed of. Reasonable efforts must be made to limit the subject’s contact with others during the 10 half-life window. In the case of laboratory animals, the subject is returned to its colony but the colony must be labeled to warn people of the hazard from the animals, as well as, any bodily fluids.
4. Long terms effect of radioactive exposure– Chronic exposure to low levels of radiation has the potential to be dangerous. Therefore, there are yearly limits imposed on radioactive exposure. In order to put the danger in perspective, the EHSO office has made Table 1.
5. Spiders – The bite from a spider exposed to high levels of radiation can have unpredictable consequences. In summary, Positron Emission Tomography is a useful technique with widespread applications in pharmacology and neuroscience. Due to the use of radioactive material, there are some significant safety concerns. However, Emory University has actively addressed the safety of personnel utilizing this technology.
Of Mice and Men… ,Nikki T. Sawyer
Men have always had a hate-hate relationship with mice (think mouse traps, rat poison, and even the Black Plague). But as scientists, we love rodents because of their usefulness in answering scientific questions. Rodents are an ideal mammalian model system: they are small, easily cared for in the laboratory setting, have a short generation time, and can be genetically manipulated. As mammals, they share high homology with humans; in fact, almost every mouse gene has a human homolog. For these reasons, rodents are used quite extensively in neuroscience research. From studying embryonic development to drug addiction…from learning more about intelligence to epilepsy… from creating model systems for sleep disorders to Alzheimer’s Disease… rodents have made innumerable contributions to our knowledge of neuroscience.
As with the use of all living organisms, there are important safety concerns to consider when working with mice.
The following are a few to consider:
1. Rodent-Transmitted Diseases – Modern laboratory mice and rats are bred to exclude all zoonotic agents, so the chances of catching a disease from lab rodents are rare. Furthermore, incoming rodents from new sources are kept in quarantine before being used. The only risk of catching a rodent-transmitted disease occurs if you handle wild populations, or if your lab rodents are exposed to wild populations. Wild mice and rats can
carry diseases such as hantavirus.
2. Rodent Bites – There is a risk of acquiring a secondary infection through rodent bites and scratches. Only handle rodents while wearing the correct personal protection equipment (PPE). If you are bitten or scratched, thoroughly wash the wound with soap and water for 15 minutes and inform your supervisor.
3. Allergies – People who work extensively around rodents are at risk for developing
rodent allergies. For mild cases, over-the-counter allergy medicines can lessen the symptoms. If allergies are a continued problem for you, limit your exposure to the rodents’ bedding, as the allergens are primarily located in the urine and feces. If you have continued and persistent asthmatic symptoms, be sure to inform your health care worker
that you work with rodents.
4. Surgery Concerns – When operating on rodents, it is essential to follow guidelines and procedures for aseptic surgical techniques to prevent the rodents from acquiring microbial infections. Having healthy, disease-free rodents is important in generating quality research data.
5. Identification – Finally, make sure that you never, ever mistake your laboratory rodent for the device that you use with your computer. Attempting to plug the tail of Mus musculus into your USB port will certainly lead to a bite (see #2).
As long you are careful and follow the established rules and regulations for the handling of rodents, you can expect to have a useful, non-hateful relationship with these little critters who have been much maligned over the centuries.
MRI Safety: How Safe is your Grandmother’s Tattoo in the Scanner?, Chris Glielmi Magnetic Resonance Imaging (MRI) has emerged as a leading imaging tool for research and clinical applications for the past 20 years. MRI is commonly used for neuroscience applications because it enables noninvasive visualization of structure and function with superior spatial resolution relative to other imaging modalities. Neurologically, MRI is typically used to study anatomical structure, functional correlates of neural activity (fMRI), functional connectivity networks and metabolite levels in brain tissue using Magnetic Resonance Spectroscopy (MRS). While MRI is increasingly used for medical diagnosis and research, safety precautions are extremely important. The magnetic field of Emory’s 3 Tesla MRI scanner, commonly used for human research, is 60,000 times the strength of the Earth’s magnetic field. This power makes seemingly innocuous metallic objects act like projectile weapons. For patients with risk factors, clinicians determine if the benefit of scanning outweighs risk but research studies should omit potential participants with any risk at all. Here are some of the safety issues that we consider prior to scanning:
1. Metallic surgical implants and pacemakers – While surgical implants in the past 15 years are typically MRI-compatible, older implants could be very dangerous in the scanner. Radiofrequency fields used in MRI scanning can also be detrimental to pacemakers.
2. Accidental and forgotten metallic components – It is critical to probe the participant about potential bullets, pellets, or shrapnel that could be forgotten or unknown. Furthermore, subjects often forget hair clips or pins, so repeated checks are important.
3. Dental braces and body piercings – In addition to traditional braces, wiring behind teeth is often hidden from view but potentially dangerous. Also, all piercings and jewelry should be removed because metallic qualities of jewelry are sometimes unknown.
4. Pregnancy – While there is no conclusive evidence about adverse effects of MRI on fetuses, there also has not been proof that it’s safe. For this reason, you should never conduct MRI research on a subject who might be pregnant.
5. Empty your pockets – It is critical that you do not have any loose change, keys or wallets when you enter the scanner room. Metal objects could be pulled into the scanner and your credit cards could be wiped out.
6. “But I’ve been scanned before…” – Safety screens might ask about prior scans but this does not preclude risk from future scans. A variety of variables including magnetic field strength, imaging acquisition technique, scanner hardware and region being scanned could pose risk even if no adverse effects resulted from past scans.
7. Are your Grandmother’s tattoos safe? – While adverse interaction of tattoos and MRI are extremely rare, old tattoo ink (before regulation) can contain metallic fragments. The Discovery Channel’s MythBusters “busted” the notion that a tattoo could “explode” but acknowledged potential burning or discomfort. Other studies have shown rare cases where skin irritation resulted only from old, homemade tattoos with cheap ink. Although these cases are very rare, it is worth consideration with tattoos from more than 25 years ago.
When safely used, MRI enables noninvasive insight to the brain’s function and structure. To continue safe applications of MR research, it is critical to strictly screen subjects to avoid even minor risks. The general rule is to weigh the benefit vs. the risk for clinical cases but to take absolutely no safety risks for research initiatives.