Reducing Radiation Dose in Molecular Imaging
 | |
| Siemens Biograph mCT |
It should be mentioned that several publications that have alarmed the public have used various models for predicting radiation induced cancer risk, but no real-life data on actual risk are available. Thus, it is unknown whether the applied cancer risk models are correct. Moreover, in general, patients who undergo PET, PET/CT or CT studies for various indications are (or should be) in need of high-quality diagnostic tests. Thus, the potential radiation risk needs to be weighed against the risk of suboptimal image quality which might have much more measurably detrimental effects on patient outcome. Nevertheless, discussions about imaging protocols, redundancies among imaging tests that involve radiation and ways to minimize radiation exposure to patients while achieving comparable diagnostic quality are necessary. It will, however, also be necessary to educate the public about the importance of diagnostic image quality and concepts such as risk-benefit ratios.
The FDA’s oversight efforts in this area will go beyond the issue of CT, which has dominated the headlines. Those efforts also will extend to other high-dose types of radiation procedures found within the domain of nuclear medicine. For sure, dose conservation efforts are on the rise from the CT side by means of new dose reduction scanning techniques, better education of technologists to tailor dose to the patient and lowering the tube current (mAs) as well as radiopharmaceutical dose efficiency (mCi) in PET and SPECT.
An assistant professor of radiology at Duke University Medical Center, Robert Reiman, MD, has worked in the radiation safety office of Duke’s Occupation and Environmental Safety Office since 1998. As Reiman explains, PET and SPECT radio-pharmaceuticals, by virtue of their physical decay, and based on the excretion of that material by the body, will be retained in certain organs of the body for a certain amount of time.
“There are a variety of mathematical algorithms, I have available to me, that can help me take all that information and get an internal dose to the patient that involves an actual radiation dose to the different organs,” says Reiman. “And it will be different for each drug since each drug has a different bio-distribution and a potentially different radioactive tag.”
Administering 10 mCi of radioactive material to a patient for a PET or SPECT study, provides for an internal radiation dose, Reiman says, and that dose can be reduced in a number of ways. “If I reduce a drug by a factor of two, and administer only 5 mCi, then the dose will be exactly half,” he says, “so you can reduce the radiation dose to the patient by reducing the administered activity.”
But that reduction has to be balanced against image quality, he points out, and in order to get the same image quality with the reduced dose, you may have to image for twice as long. “The camera would have to sit there and accumulate counts to get the same image quality for exactly twice the amount of time,” Reiman says. “Now, that’s easy enough to do if you are not especially concerned with how long the patient has to wait on the table, or the amount of throughput in your clinic.”
Pharmacologic intervention is another strategy. As an example, for a bone scan, a physician looking to reduce the amount of dose to a patient’s bladder can hydrate the patient either orally or intravenously and move the radioactive material through the patient at a faster pace. “So the bladder would get less of a dose that way,” Reiman says.
Driving down dose in PET/CT and SPECT/CT
David Townsend, PhD, a professor at the National University of Singapore and head of SPECT and PET Development at the Singapore Bioimaging Consortium, has been intimately involved in issues involving radiation exposure and PET/CT ever since he and electrical engineer Ronald Nutt built the first prototype of a PET/CT scanner.As Townsend points out, when the two acquisitions—either SPECT or PET and CT—are completed, the major radiation dose comes from CT. According to the International Atomic Energy Agency, the CT component of a PET/CT exam can produce a radiation dose that runs as low as 7 mSv to as high as 30 mSv, depending on the type of study, the area of the body scanned and the purpose of the test. The typical radiation dose from PET with 10 mCi of activity injected will be somewhere in the range of 5 or 6 mSv, according to Townsend, compared to a full clinical dose of a full body CT of approximately 20 to 25 mSv.
So when it comes to reducing CT dose, says Townsend, many nuclear medicine departments will opt for a low dose or low mAs, CT—thus lowering the tube current to minimize the amount of radiation exposure to the patient.
“In Japan, for instance, where the population in general is smaller in stature than in the U.S., they go very low in mAs—as low as 20 mAs—and they still get reasonably good scans for CT,” says Townsend. “That’s very low dose—probably lower than what you get from the nuclear medicine (PET) scan.”
In the U.S., with “larger patients,” says Townsend, low dose CTs may go as low as 50 mAs, which compares to a full body scan that can range from 160 to 200 mAs. While this obviously radically reduces radiation exposure, Townsend says, it results in scans that are not considered to have been done at clinical levels (standard of care for CT), will not generally be read by radiologists and will only be used for localization and attenuation correction.
“There’s a lot of debate about this,” Townsend says. “If you look at lower dose CT scans, they most likely look like state-of-the-art scans five or 10 years ago. So although they are declared by radiologists not to be state-of-the-art, they are, nevertheless, pretty good.” Consequently, he adds, “the question becomes whether it can be argued that just because a radiologist doesn’t formally read the scan doesn’t mean he can claim it is non-diagnostic.”
Thomas Beyer, PhD, who is a teaching professor at the University Hospital Essen and CEO of cmi-experts, a Zurich-based company specializing in cross-modality imaging, says that turning down the CT dose parameters when performing a PET/CT to achieve an absolute minimum radiation dose is not always advisable. “It’s just not that easy,” he says, because you can end up with a much diminished image quality. They might suffice for a very coarse anatomical localization of PET findings, but if you want to do a full staging scan—and in selected cases, a follow up—you need a radiologically-equivalent CT. It’s just not wise to turn down those CT parameters to minimize patient exposure without seriously considering the clinical indication for the scan.”
Just as importantly, Beyer argues, radiologists and nuclear medicine physicians have to recognize that patients are probably undergoing diagnostically equivalent CT exams prior to, or just after undergoing a PET/CT, “so, even though you are reducing the dose on the PET/CT scan, the patient is still suffering from excessive exposure because he or she is just getting too many scans.”
Beyer says a number of steps can be taken to try to prevent this. “We can educate referring physicians that times have changed and that ordering a CT scan the moment a patient steps into a hospital may not be a wise thing to do.” Beyer also points out that it would be helpful to improve the line of communication between referring physicians, radiologists and nuclear medicine physicians.
“A big problem in imaging today is that nuclear medicine and radiology do not talk to each other,” Beyer says. “It’s not so much a problem in the states, but it is a real problem in Europe. If they don’t talk to each other, then both professions typically perform CT and PET/CT independently. Or, whatever physician has the PET/CT in his or her possession doesn’t consult with the other physician on optimum scan parameters.”
As to the PET parameters of a PET/CT exam, both Townsend and Beyer believe “time of flight” is an optimal way of reducing patient radiation exposure. Not a new concept, recent technological improvements have enabled it to be included in new scanners.
In PET imaging, when the nucleus of the injected radioactive material decays, a positron is released that immediately collides with an electron, creating an annihilation that releases a pair of photons. The photons move away from each other in opposite directions and the PET scanner can calculate where the radioactive agent is concentrated and produce an image localizing the affected area. Time of flight can pinpoint the origination of annihilation more accurately, which can improve the accuracy of the imaging. Improved event localization reduces noise in image data, resulting in higher image quality, shorter imaging times, and lower dose to the patient.
Townsend and colleagues at the University of Tennessee and Siemens Molecular Imaging have published several studies, including one in the February issue of the Journal of Nuclear Medicine on time of flight. “We’ve been able to demonstrate that you can approximately halve the FDG activity you inject and still get the same signal-to-noise, the same quality FDG scans, if you reconstruct the data using time-of-flight information,” he says, adding that it can be particularly useful in improving image quality for large patients.
And patient size is always going to be a consideration when it comes to PET/CT, particularly in a country like the U.S. with its burgeoning obesity problem. “A CT image is likely to be compromised when you have a patient who is 350 pounds,” Townsend says. “And the major factor that affects the quality of PET imaging is the size of the patient, simply because size can now vary by a factor of four, and even higher.”
Whether a physician is imaging a patient who weighs 100 pounds or 400 pounds, Townsend points out, the amount of radioactive material injected into that patient will not vary greatly—it will always range from about 10 to 15 mCi. That’s a difference in patient weight of up to 400 percent, compared to a potential change in injected activity of only 50 percent. “Size is the biggest factor that affects image quality, and, to some extent, it’s the same with CT,” Townsend says.
Concern for the pediatric population
As Reiman points out, adjustments need to be made for pediatric patients. Two recent studies demonstrate that the development of patient-specific protocols can significantly reduce radiation exposure for pediatric patients.In the most recent study in the February issue of the Journal of Nuclear Medicine, Roberto Accorsi, PhD, former research assistant professor of radiology in the department of radiology at Children’s Hospital of Philadelphia in Pennsylvania and colleagues found that pediatric PET studies of constant image quality can be performed with time or dose savings of as much as 50 percent for the lightest patients.
The researchers acquired and analyzed data from 73 patients whose weight ranged from 25 pounds to 200 pounds. “When following an injection protocol proportional to weight, the NECD [noise equivalent count density] of PET images were found to improve for decreasing weight from the reference case of an adult-sized (70 kg) patient. As compared with height, girth and body mass index, weight was found to be the patient statistic correlating best with the dose necessary for imaging at constant
NECD and time,” wrote Accorsi and colleagues. “These findings mean that PET can be used in children with methods that are even more patient-specific than those currently employed.”
In the October 2009 issue of the Journal of Nuclear Medicine, Adam Alessio, PhD, from the department of radiology at University of Washington and the department of radiology at Seattle Children’s Hospital and colleagues describe an 11-category protocol they developed for pediatric PET/CT. They used the Broselow-Luten color-coded weight scale designed for emergency pediatrics to create 11 weight-based categories for PET/CT exams. For each category, the authors developed protocols to vary the amount of injected FDG activity and length of PET acquisition times, as well as weight-based tube-current protocols for the CT acquisition.
According to Alessio, the protocols he and his team developed allow them to perform pediatric PET/CT exams where radiation dose is as much as three times lower than with an adult protocol. “The important message from the study is that of having a system in place that is logical and patient specific allows you to make changes in a logical and patient-specific way,” says Alessio. “And that should be the thrust of any pediatric PET/CT protocol—you want it to be tailored for the individual child.”
Staff exposure
In nuclear medicine the issue of radiation exposure goes beyond that of the patient to strategies necessary to mitigate the exposure risk associated with technical personnel who are tasked with handling and administering radioactive material.At the University Hospital Zurich in Switzerland, Thomas Berthold, division of nuclear medicine, and colleagues and his colleagues infused 400 patients with 200-300 MBq 18F-FDG using an automatic FDG infusion system. The total injection time of 30 seconds allowed technical personnel to move away from the patient reducing radiation exposure.
The researchers found that the total radiation burden for the technologist, including injection and relocation of the patient to the uptake room was 0.4 mSv. According to Berthold, this is a significant reduction in the total radiation burden compared to manual injection during which the average radiation burden for a technologist was 2 mSv.
“The system proved to be very practical, easy to operate and delivered the radioactive activity with higher accuracy,” Berthold says.
There are regulatory limits for all occupationally exposed personnel when it comes to radiation, and, according to Reiman, people who are generally expected to become exposed to more than 10 percent of that limit should be issued radiation dosimeters. At Duke, he says, dosimeter badges are changed on a monthly basis so that he always has a good indication of what exposure levels are so that staff members do not come close to approaching that occupational limit.
Reiman adds that staff education is important when it comes to reducing radiation exposure. “What we stress to our people here,” says Reiman, “is that most of the dose that they will accumulate during a PET activity will be due to their interaction with the patient.”
That radiation exposure not only comes from actually injecting the radioactive material, but even more so when staff escort patients to the scanner. According to Reiman, the radiation dose rate drops by a factor of the inverse square of the distance away from the patient, so that if technologists double the distance away from a patient, they decrease their radiation exposure by a factor of four. “We emphasize time and distance,” says Reiman. “Staff should keep as much distance as possible away from the patient for as long as possible, while still doing their job.”
But, when talking about the risks associated with nuclear medicine—as well as efforts to reduce those risks—as they relate to staff, children, as well as the adult patient population, Alessio reminds observers that the benefits of all of these modalities always have to be considered as well. “There’s a reason why dedicated children’s hospitals have large radiology departments,” he says. “And that’s because they are an essential part of pediatric care. So theirs is definitely an established value to these modalities.”