Nuclear Medicine—Diagnostic Procedures

I will soon be having a nuclear medicine test done because of a heart condition. I am breast-feeding and wonder whether I should stop or if it is okay to continue. Can you help?

Some nuclear medicine tests involve the administration of radioactive materials for which breast-feeding needs to be stopped. For some radioactive materials there are no restrictions. The hospital where you receive the test will give you specific instructions on this topic. The table below shows what the U.S. Nuclear Regulatory Commission suggests as a breast-feeding interruption period for various radioactive materials (Regulatory Guide 8.39, "Release of Patients Administered Radioactive Materials"). You can use this for some general guidance, but ask your physician for specific guidance.

Suggested breast-feeding interruption times after radionuclide administration1 Radiopharmaceutical Suggested Time Period of No Breast-Feeding 131I sodium iodide complete cessation 123I sodium iodide no interruption2 123mI IBG 24 hours 99mTc DTPA no interruption2 99mTc MAA 13 hours 99mTc pertechnetate 24 hours 99mTc IDA agents no interruption2 99mTc glucoheptonate no interruption2 99mTc HAM no interruption2 99mTc MIBI no interruption2 99mTc MDP no interruption2 99mTc PYP no interruption2 99mTc red blood cells 6 hours 99mTc sulphur colloid no interruption2 99mTc MAG3 no interruption2 99mTc white blood cells 24 hours 67Ga citrate 1 month 51Cr EDTA no interruption2 111In white blood cells 1 week 201Tl chloride 1 week

1The guideline for 99mTc compounds is a 24-hour interruption for >1,110 MBq administered, 12 hours for 444–1,110 MBq, and no interruption for <444 MBq administered.

2The normally administered activity is below activities that require any interruption.

What documentation should be provided to patients who have undergone nuclear imaging and intend to use public transportation or visit high-security facilities?

Guidance is posted on the Society of Nuclear Medicine website under frequently asked questions about nuclear medicine.

You can help your patients and security personnel by providing patients who will be traveling on public transportation (such as airplanes, trains, and rapid transit) or visiting secure facilities with a letter that contains the following information:

• Patient name
• Name of nuclear medicine procedure
• Date of nuclear medicine procedure
• Radionuclide
• Half-life
• Administered activity
• 24-hour contact information

The letter should provide specific details about who should be contacted. Outside of normal working hours, the contact person should have access to an appropriate source of information such as a hospital or radiology information system, so that the information in the letter can be independently confirmed.

Can you tell me why Lugol's solution is given before administration of ¹³¹I and what is the recommended practice on when it should be taken? Does it reduce the exposure to the thyroid and, if so, by how much?

Lugol's solution (potassium iodide) is given to protect the thyroid gland. Lugol's solution is used because of a process called a "mass effect." As iodine is ordinarily taken up efficiently by the thyroid gland, flooding the body with iodine will exhaust the thyroid's ability to take up the iodine. If most of the available iodine is nonradioactive iodine, and only a little is radioactive, the thyroid will take up mostly nonradioactive iodine and little of the radioactive iodine. However, nonradioactive iodine will not block all the radioactive iodine. Instead of taking up 10–25 percent of the available radioactive iodine, the thyroid with Lugol's solution will take up only about 0.1–0.5 percent of the available radioiodine. This will decrease the radiation dose to the thyroid and preclude possible ill effects on the gland. Lugol's solution, at about 10 drops per day by mouth, is usually started a day before the administration of ¹³¹I compounds and is continued for at least a week.

Should patients keep a distance from their small children for a period of time after injection with diagnostic radiopharmaceuticals?

Doses from diagnostic radiopharmaceuticals are quite low and not thought worthy of an extensive dosimetric analysis. If no logistic or other personal issues arise, maintaining distance from others for a few hours after ^99mTc administrations, for example, is not bad practice. However, patients should not be overly concerned about exposures that may occur to family members or others from diagnostic medical procedures with radiopharmaceuticals.

How much is the total estimated cost of a positron emission tomography (PET) brain scan?

The typical cost for a PET brain scan would be about $2,000.

I have to undergo a bone scan with radioactive material. Please fill me in with the pros and/or cons of such an injection and any or all contraindications.

A bone scan involves the administration of 55–925 MBq of ^99mTc-labeled methyl diphosphonate (MDP) into a vein and imaging of the photon emissions from the radionuclide. This is a routine procedure, which has been around in its present form for more than 30 years. Following the intravenous injection of the radioactive drug, you wait two to three hours and are asked to drink lots of water. During the scan, you lie down on a table for 20 to 30 minutes. The radiation dose is very low and is not a hazard to you or others. You will be radioactive for a day or two, which can be measured if you are near any typical radiation detectors. This radioactive drug is not like contrast material, which can cause an allergic reaction in some individuals. It is given in tiny amounts, with no adverse reactions.

What is a V/Q scan?

A V/Q scan (sometimes also referred to as a V/P or ventilation/perfusion scan) is a radionuclide test performed in nuclear medicine to look for a pulmonary embolism. The V/Q scan has been used for over 35 years as a tool to diagnose pulmonary embolisms and, even with the use of newer computerized tomography scans (CT scans—x-ray images of the inside of the body), it remains the gold standard—nothing has been found to be as good.

Usually in a V/Q scan, two radionuclides are administered. One is ^99mTc. It is used for the perfusion part of the study. If there is decreased uptake of radioactivity in an area of the lungs, that corresponds to a decreased blood flow to the area where the embolization has occurred. The other radionuclide commonly used is ¹³³Xe gas. This is used for the ventilation portion of the test. This test, as the name implies, requires the patient to "ventilate" or breathe the radioactive particles into his or her lungs. Decreased areas of radioactivity correspond to some irregular lung function.

I recently had a nuclear medicine test done. On my return to work, some colleagues pointed a Geiger counter at me and the instrument went off scale and the alarm signal came on. This worried me.

It is often surprising to people that the radioactivity from a particular procedure does not immediately disappear from their body. You probably had a scan with a radionuclide known as ^99mTc, which has a physical half-life of six hours. This means that every six hours, one-half of the radioactive material is no longer present. Radioactivity is also eliminated as the compound onto which the radionuclide was tagged is eliminated from the body. Typically, this elimination is via urine.

Obviously, the radioactive emissions had to be detected outside the body or there would be no ability to perform the scan. However, detectability does not mean hazardous. There simply is enough radiation still left in the body to be detected by a very sensitive radiation detector.

I have two patients and they each received a 740 MBq injection of ^99mTc for diagnostic imaging. One guy had a brain scan and I estimated the effective dose to be 7 mSv. Another patient, who underwent a bone scan, had an effective dose estimate of 4 mSv. How or why would the effective doses differ even though the amount of activity injected in both cases was about the same?

What you are really asking is a fundamental, and very good, question—with the same amount of activity, how could the calculated values be so different? The short answer is that different radioactive drugs (radiopharmaceuticals) have different behavior in the body. Actually, this is the whole design of nuclear medicine—how can we design different drugs that will go to different parts of the body and give us images of the structure and/or function of the different organs (bone, brain, liver, etc.)? Even though there are many different drugs labeled with ^99mTc, their internal behavior is quite different, so the radiation doses that they deliver can be quite different as well. The effective dose is a "risk-weighted" average of the dose to all individual organs of the body—different organs have different weighting factors. So when two drugs go to different parts of the body, and have different rates of uptake and elimination, you can easily get different individual organ doses, and different effective doses, even when the nuclide (for example, ^99mTc) is the same.

For a patient receiving a ^99mTc scan, how much shielding does a patient's own body tissue provide to protect her family?

^99mTc emits 140 keV gamma rays. When a source is spread out (as it would be inside a patient), the shielding effect from tissue is about 12 percent per centimeter and the amount of tissue required for a reduction of one-half is about 6 centimeters. One way of estimating the shielding effect is to assume that the activity is located at the center of the body. In this case, even a thin person's body will minimally reduce the intensity of the radiation by at least one half. The other factors that influence the amount of radiation dose are the distance from the source and the time that someone is exposed to the source. 

I am concerned about the amount of radiation my two children could have received from me after I had a nuclear medicine scan.

While your concern is understandable, please be assured that the radiation doses to your children were very small. This conclusion is based on extensive data in the peer-reviewed scientific literature in which radiation doses and dose rates from patients who have undergone diagnostic nuclear medicine procedures have been measured. Based on such measurements, the physicians and scientists who have authored these publications have unanimously concluded that such radiation doses from patients are extremely small and do not warrant any undue concern or any radiation precautions.

I will be receiving ¹²³I for a whole-body scan. What precautions do I need to take because of this dosage? I teach third grade. Is there any threat to the students?

The activity administered is low and the half-life is short, about 13 hours. There is no risk to you and absolutely none to others. There is nothing you have to do. (Note: For information on the half-life of a radionuclide, see the Health Physics Society's radiation terms and definitions.)

What are the long- and short-term biological effects of receiving ¹¹¹In and ¹³¹I (by injection) for diagnostic nuclear medicine scans?

Radiation exposures to patients from diagnostic doses of ¹¹¹In and ¹³¹I depend on the chemical form of the radiopharmaceutical. In general, they are below those that are known to cause any harmful effects. Short-term effects require rather large doses—typically at least 100 times that from diagnostic radiopharmaceuticals. In theory, there may be reason to believe that long-term effects (cancer and mutations) could occur from smaller doses; none have been observed with doses in the diagnostic range. In fact, there is evidence that these small doses may be associated with beneficial effects, such as stimulation of the immune system.

I am scheduled for a HIDA (hepatobiliary iminodiacetic acid) scan/gallbladder test. I have two risk factors that may increase my susceptibility to cancer. One: My grandmother died of breast cancer. Two: I have been a type-one diabetic for 42 years and, if I am not mistaken, diabetes does impair the immune system. I am now 52 years old. My question is, considering my increased risk factors for cancer, should I go through the HIDA scan test?

The expected health benefit from a justified x-ray or radioisotope procedure outweighs any potential risk. It is not possible to provide medical advice for a specific procedure and a given individual; that is a matter that must be decided by the patient and the physician.

I'm wondering about the doses of radiation that are used in PET (positron emission tomography) scans and what types of damage to tissue these might cause, even if they're very slight.

There are a number of different types of PET scans using different radioactive tracers and molecular compounds; you'd have to be more specific for me to give you specific numbers that I know would apply to your case. A popular example, however, is ¹⁸F FDG, which is a glucose analog used to study the heart and brain for various reasons. The effective whole-body dose for this is about 7 mSv, which is comparable to that for a chest or abdominal CT and is roughly equivalent to two years' exposure to natural background radiation. Such levels of radiation will not cause any direct damage to tissue. Current models suggest that, like all exposure to ionizing radiation (including background), there might be a small increase in the long-term risk of inducing cancer, but this is thought to be far outweighed by the potential health benefits of the exam.

About five years ago I had an x-ray test done to see if I had kidney stones. They put the dye in the IV, and about five seconds later I started breaking out from head to toe and my throat started to close off. I was given Benedryl and IV fluids. My doctor wants to do a HIDA (hepatobiliary iminodiacetic acid) scan now. Will I have the same reaction to it? Should I have it done?

From your description about the procedure performed five years ago, it appears that you had a reaction to the iodinated (not radioactive) contrast agent or dye. The HIDA scan is performed without contrast. We cannot tell you whether or not to have the procedure done—that's a decision between you and your physician. All we can say is that the contrast agent used for your procedure five years ago is not used for a HIDA scan.

I had a nuclear medicine test and by one to two hours afterward my voice started fading and I had difficulty speaking. Could any part of the scan cause this?

There are no data showing that symptoms such as yours or any others, for the most part, are caused by the radiation received from a diagnostic nuclear medicine test. It seems an unlikely coincidence, but it appears that your symptoms and your scan were unrelated.

Ask the Experts is posting information using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.