Isotopes: Versions of Atoms make Visions of You

A Isotope Discovery Experiment

HASPI Medical Chemistry Lab

Background

You've been having headaches every day this week. When you go into the doctor they can't find anything in particular that would be causing it, but they want to scan your brain just to be sure there's nothing wrong.

The doctor sends you to get a PET scan (Positron Emission Tomography). When you get there they inject you with something... and they say it is RADIOACTIVE! Is it safe? Will you turn into Spiderman?

Although there are some minor risks when you are injected with a radiotracer, the risks are much lower than the other option... exploratory surgery. The radiotracer is an unstable isotope of an atom. The key to its safety as well as the ability to be imaged is the instability. By using a radioisotope with a short half life, most of the radioactive atoms will decay within the next 24 hours, leaving you with a very short period of time when you were exposed to radiation.

When the atom decays a small particle is emitted, and the gamma camera or PET scanner detects the emission. When all of these emissions are recorded you end up with a map of the body, and you can gain information about the anatomy as well as the physiology (functionality) of the organs. The element left after decay is not radioactive.

These kinds of scans are most often used in the fields of oncology, neurology and cardiology in order to help with diagnosis and choose the right treatment for each patient.

An isotope is just a version of an atom. We identify an element based on the number of protons it has, but sometimes the number of neutrons will vary. Due to the fact that each neutron weighs 1amu, this alters the mass of the atom. This doesn't change the way the atom interacts with other atoms, but it can change the stability of an atom.

For instance, there are a few versions, or isotopes, of carbon. Although only 3 occur naturally, there are a total of 15 that have been created. Carbon-12 and Carbon-13 are the only two isotopes which are stable, meaning that they won't break down over time. Carbon-14 has a VERY long half life of 5, 730 years, so we actually can measure the amount of C-14 in anything that ever lived to find out how much C-14 is left and use that information to figure out how old it is... this is called Radiometric Dating. By using a synthetic (man-made) version of Carbon called Carbon-11 as a radiotracer, the half life is only 20 minutes. This means that after just 1 hour only 1/8 of the original injection remains.

The number following the name represents the mass of that isotope. If you subtract the atomic number you will find the number of neutrons in one atom of the isotope.

The radiotracers usually used in a PET scan include the following. Identify the number of neutrons, protons & electrons in each one as well as the mass.

Isotope / Half Life / Isotopic Notation / Mass / # of Protons / # of neutrons / # of electrons
Carbon-11 / 20 min
Nitrogen-13 / 10 min
Oxygen - 15 / 2 min
Fluorine-18 / 110 min

A Decaying Supply

In order to synthesize these elements in the lab scientists must use enriched Uranium in a nuclear reactor. As this can also be used for weapons, this type of lab is strictly regulated so that about 95% of the radioisotopes used world wide come from just 5 aging reactors, none of which are in the US. This method also produces nuclear waste. A few years ago two of these labs went under renovations, leaving many hospitals with no radioisotopes for months, requiring doctors to use old techniques which are less effective.

A new technique of using a particle accelerator or a cyclotron to synthesize isotopes would make no nuclear waste and not require the dangerous use of enriched uranium. These technologies are new and will take time and cost money to implement in the beginning, but should result in a safer and more accessible way to synthesize medical radioisotopes.

Review Questions
1. What is detected by the PET machine?
2. What fields of medicine most commonly use this technique?
3. What does the term synthetic mean?
4. Why are there so few labs making radioisotopes?

References:

http://en.wikipedia.org/wiki/Positron_emission_tomography

http://www.world-nuclear.org/info/non-power-nuclear-applications/radioisotopes/radioisotopes-in-medicine/

http://www.medicalisotopes.org/faq.html

http://www.hopkinsmedicine.org/healthlibrary/test_procedures/neurological/positron_emission_tomography_pet_scan_92,P07654/

http://cdn.medindia.net/health-images/brain-scan.jpg IMAGE

http://www.akidjustlikeme.com/id92.htm IMAGE

http://science.howstuffworks.com/nuclear-medicine1.htm IMAGE

http://www.triumf.ca/chemistry/radiochemistry-for-pet-imaging IMAGE

http://www.radiochemistry.org/nuclearmedicine/radioisotopes/01_isotopes.shtml

http://theconversation.com/supply-of-medical-isotopes-has-dangerously-decayed-20004

Lab REFERENCES:

http://www.angelfire.com/planet/gathman/Studydownloads/Atom/Isotopepennylab.pdf