Michael Graham, MD, PhD, director of the Nuclear Medicine Division at UI Hospitals and Clinics and leader of the Holden Cancer Center Tumor Imaging Program, says nuclear medicine is a technique for studying the function of virtually any organ system using radioactive tracers .
"Some of the studies we do include: blood flow to the heart and brain, gastric emptying (radioactive scrambled eggs), kidney, thyroid, and gallbladder function, sites of infection with labeled while blood cells, bone metabolism, and sites of tumors using several agents including fluorodeoxyglucose and positron tomography (PET).
"We generally inject or sometimes the patient swallows the radio nuclei. The different labels we have ensure that it goes to different places so we can study the blood flow to the heart or brain; or how the biliary system works; or how the stomach works. It uses very small doses of radiation and a gamma camera to image where the activity has deposited.
"Probably the most common study we do is myocardio-profusion, or heart blood flow study, which show what the blood flow is to the heart when the patient is stressed, running on a treadmill or stressed with a drug, to see how the blood flow is at maximum stress versus at rest.
"This is a very sensitive way to detect if there's coronary artery disease. We also do quite a few studies looking for cancer, either to see if cancer is present and to see if it's spread. We use bone scans to see if there's injury to bone. We use PET scan using fluorodioxyglucose that gives us a map of glucose metabolism and shows tumors. It is becoming more and more widespread as a way to help in the care of cancer patients," says Graham.
For most of the procedures, Graham said, there really isn't any preparation required, but there are a few where it's important. "If we do gastric emptying studies, for instance, it's important that you haven't had a large meal just before we do the study because we use radioactive scrambled eggs. For PET imaging, because we're looking a glucose metabolism, it's important that the patient not eat for at least four hours prior to the study because that will raise the glucose levels of the blood and compete with the fluorodioxyglucose for uptake. But most of the studies, the patient actually doesn't have to have any special preparation.
The two main instruments used are the gamma camera and the PET scanner. The gamma camera looks like a large box that's about two and one-half feet square by one foot thick that is mounted on a gantry that allows it to rotate around the patient taking pictures of the patient from various angles. When it moves around the patient in a circle, taking images over 360 degrees, it is called SPECT (single photon emission computed tomography).
A PET scanner is a big machine about six-feet square with a hole in the middle. The patient lies on a table and is advanced into the hole where pictures are taken. "We also have two PET-CT systems, where the CT is attached to the PET scanner and we can take the two sets of images one right after the other.
"Nuclear medicine works by using specific compounds. For instance, we have a specific compound that's taken up by the liver and excreted into the bile and that allows us to look specifically at the gall bladder and liver functions. We have another agent that is specifically taken up by the kidney and excreted into the urine and this allows us to see how well the kidneys are working and whether there's any obstruction. We have about 20 t9o 30 different tracers for various organ systems that allow us to look specifically at how they work," he says.
When you have a nuclear medical procedure, Graham says, generally you are greeted by one of our technologists. Technologists are the people who actually do the study. They explain what is going to happen. "For a few of the studies, the agent is taken by mouth (gastric emptying and thyroid activity). For most of the studies we inject the tracer into a vein. After uptake, sometimes immediately or after a delay of up to three hours (for bone scans) the patient lies on a table and pictures are taken. For most studies imaging takes less than one hour. Gastric emptying and heart and PET imaging usually take about two hours.
The results from a nuclear medicine examination are interpreted by nuclear medicine physicians or a radiologists. "It's important for the public to recognize that nuclear medicine is a separate specialty from radiology. We have our own residency program that trains the residents for three years. Radiologists rotate through nuclear medicine and have up to six months of training in nuclear medicine when they learn to interpret the studies."
The benefits of nuclear medicine are enormous says Graham. "The main benefit is that we can often determine if there is abnormal function of a specific organ non-invasively. There are a number of conditions that can't be evaluated by other methods that are done with nuclear medicine, including gastric emptying with the radioactive scrambled eggs, the renal function that can be established with that, and certainly PET scanning for looking for early spread of tumors. We can detect tumors very sensitively and help guide therapy.
"The only risk, which is very small, is from the radiation associated with the tests. For a typical test the patient gets about half a rad of radiation. This is one tenth of what a radiation worker is allowed each year and has no measurable risk.
"The risks are actually quite minimal. There's a risk of bruising when you get the IV. There's a small amount of radiation associated with each of tests, but it's actually a very small amount, either comparable to an x-ray or even less than an x-ray.
"Some of the studies, for instance in children that we do looking at bladder reflux where the urine flushes back into the kidney, is done with a small dose of radioactivity that's put into the bladder and then it immediately leaves the bladder after the test and the dose of radiation is equivalent to just a few days of background radiation. So the risk overall for our studies is so low we really can't measure it, but it's very small."
"The main limitation is that our images are not as sharp as the other radiology images from CT or MRI systems. This is because of the way the data is collected and used to make images. This means our images are not very good for finding small structural.
We have several research projects underway, particularly in PET imaging. We are beginning studies with two new PET tracers, F18-fluorothymidine to assess tumor response to therapy and C11 acetate to help identify prostate cancer metastases. We are working with scientists in psychiatry and neurology in studies of brain blood flow and how it changes with various stimulations. We are also collaborating with many other medical scientists in projects that involve imaging normal organ or tumor metabolism and function," Graham says. |
