Upgrade to the Advanced Lab (PHY 243): Positon Tomography Teaching Laboratory

062508: By far, the course that our undergraduates like the most is the Advanced Lab (PHY 243), which they take in the fall of senior year. This course is a centerpiece of the curriculum leading to a BS in Physics, enabling students to perform sophisticated experiments, where they apply everything they've learned.

Thanks largely to Physics alumnus Dr. Chris Lirakis, a board member of the Donaldson Trust, the Department is adding an interdisciplinary experiment to the Advanced Lab in the emerging frontier of bio-medical physics. Because Dr. Lirakis enjoyed the Advanced Lab during his undergraduate years at the University of Rochester, he has enabled the Department to purchase a high-resolution germanium detector for use in the study of positron tomography. Future upgrades are also in the works.

The Advanced Lab has been heavily focused on optics experiments for years, having been run by quantum optics specialists Chair and Professor Nicholas Bigelow and Assistant Professor John Howell. Professor Frank Wolfs, who is in charge of our undergraduate program, has always wanted to give the Advanced Lab a medical twist because, as he says, "a lot of physics students want to do graduate work in medical applications. This is a burgeoning field." After talking with Dr. Lirakis during Meliora Weekend, Professor Wolfs devised a new experiment, one that focuses on nuclear radiation.

Positron emitters are radioactive nuclei that decay with the emission of a positron. The positron, the anti-particle of the electron, annihilates when it encounters an electron and creates a characteristic pair of 511 keV photons, emitted back to back. If the positron emitter is located in the human body, the positron annihilation will occur within a few µm from the position of the emitter. By detecting the intensity distribution of the 511 keV photons and/or using the back-to-back nature of coincident 511 keV photons, the location of the emitter can be determined accurately.

The first Positron Emission Tomography (PET) machine was developed in 1950, and since then PET scans have increased in importance in health care. The technique complements other imaging techniques such as Magnetic Resonance Imaging (MRI). An example of images of the brain of a patient with Huntington's disease, obtained with different imaging techniques, is shown in the following figure.

Image: Comparison of brain images of a patient with Huntington's disease obtained with MRI and PET techniques (from http://neurosurgery.mgh.harvard.edu/pet-hp.htm).

Patients who are scheduled for a PET scan are administered a substance that is labeled with a positron emitter. Usually, the positron emitter is attached to a compound that occurs naturally in the human body, for example, glucose. The type of compound can be adjusted based on the part of the body to be examined. The interpretation of the results of a PET scan relies on the fact that different tissue types collect the compound at different rates; for example, cancerous tissue has a much higher rate of glucose absorption than healthy tissue. Since the PET scan not only provides information about the location of the emitting source, but also about the intensity of the source, it is a very powerful tool to detect cancer. PET scans are also used to examine the health of the heart tissue. In this application, the difference in collection rates of glucose in healthy and unhealthy tissue is used to identify the areas of the heart that show decreased functionality, for example, as a result of a heart attack.

Modern PET scan imaging machines use hundreds of small scintillation crystals to detect the coincident 511 keV photons. Based on the detection location of many pairs of coincident photons, the location of the emitter can be determined very accurately.

In the Advanced Lab, we will focus on the principle of positron tomography using one or two gamma ray detectors. We plan to create an object that can hold one or more Na22 sources at different locations. The students will only see the outside of the object and not the location of the sources, but they will be able to move the gamma ray detectors to different locations around the object. By measuring the gamma ray intensity distribution around the object and/or the coincidence efficiency as a function of the angle between the gamma ray detectors, the students will try to determine the location of the source. Initially, the students will work with one source. When the position of the single source is accurately determined, we will increase the complexity of the analysis by adding more sources to the object. In essence, the new experiments will simulate what happens in the medical community on a daily basis.

The Advanced Lab currently uses sodium iodide (NaI) detectors. Ideally, we would like to use two germanium detectors in the new experiment; these detectors can measure the gamma ray energy with much higher resolution than, for example, NaI crystals, and their excellent signal-to-noise capabilities allows the use of low-intensity gamma ray sources. However, in the initial development, we will carry out the experiment with a single germanium detector, complemented by one of our NaI detectors to capture the second gamma ray. In this manner, the students will explore the differences in imaging accuracy between single and coincident photon detection techniques. In the future, we hope to upgrade the experiment with the addition of a second germanium detector.

It is critical, though expensive, to continually upgrade the Advanced Lab. It is the hope of Professor Frank Wolfs and Chair Nicholas Bigelow to bring PHY 243 fully into the future with at least four new sophisticated experiments. We thank Dr. Chris Lirakis and the Donaldson Trust for enabling us to offer the first of a series of new experiments, the Positron Tomography Teaching Laboratory. (lhg)