Two papers in J. Radioanalytical and Nuclear Chemistry were just published by Dr. Metzger and his colleagues. The first paper covers the measurement and the dosimetry of long-lived contaminants in cyclotron produced radiopharmaceuticals. The second describes the interaction these contaminants have with the ubiquitous homeland security radiation monitors and their potential impact for patients who have had diagnostic studies.
The links to the papers are:
There has been a tremendous technological advancement in life-saving cardiac and interventional radiology procedures performed under fluoroscopic guidance. These procedures, such as TAVRS (see https://www.youtube.com/watch?v=csxJYTLXNJY), provide life-saving benefits to the patients, but can be long and complex, and result in high radiation doses to the cardiologists and radiologists performing the procedures. Doses to the head and neck, which are outside of the protective lead apron, are monitored with a collar badge attached to the top of the apron. These badge results are now over 1 rad per month in many institutions, and are climbing due to the demand for these new procedures.
The most sensitive organ in the head and neck is the lens of the eye, where radiogenic cataract formation can occur after a threshold dose is received. The current limit for eye dose is 15 rads per year, which is based on a threshold dose of 500 rads for cataract formation. Recent epidemiological studies suggest that the actual threshold is less than 500 rads, and the International Commission on Radiological Protection (ICRP) has recommended a lower threshold dose, and head and neck limit. Forcing the dose lower by regulatory means is not desirable, as any adversarial regulatory action that reduced the availability or accessibility to the new life-saving procedures would necessarily cost human life in the vulnerable patient population. A shielding plan to protect the cardiologists and interventional radiologists is needed.
The only shielding available to the cardiologists and radiologists are various types of shadow shields which must be positioned correctly to reduce the head and neck dose. While essentially all hospitals and outpatient clinics equipped with cath labs to perform these procedures offer some kind of shadow shielding, it is only rarely positioned and used optimally.
Dr. Metzger has produced a Monte Carlo model of a cath lab and a special procedures room that allows the efficacy of the various shadow shields available in a facility to be evaluated prior to a specific procedure. The dose reduction possible with each shadow shield and any combination of the shields can be modeled and determined before the start of the procedure.
The paper describing this model can be found here, and will be presented at the American Nuclear Society Radiation Protection and Shielding Division meeting later this year in Sante Fe, New Mexico (http://rpsd2018.ans.org/).
Ga-68 for PET imaging is now available in a generator form from Eckert and Ziegler. As with all generators, there is always a possibility of some of the radionuclide held on the column (Ge-68) breaking through into the eluate (Ga-68) needed for imaging. A breakthrough test is needed with a limit of 0.001% or less of the Ge-68 parent in the Ga-68 eluate. We have recently developed an inexpensive test using high resolution gamma spectroscopy. Call for more details.
Long-lived contaminants are introduced into cyclotron produced radiopharmaceuticals from the activation and spallation of elements in the HAVAR target window into the target water. They are removed during the synthesis of the final imaging agent. Below, are a series of papers on the measurement of these contaminants, their retention in the body, and the dose delivered to the patient from them. The quantity of the contaminants is strictly controlled by the FDA which requires all drugs be 99.5% pure. The vast majority of prepared radiopharmaceuticals have only a tiny fraction of the allowed levels in them. The radiation dose these contaminants deliver to the patient, if present, is trivial.
They do, however, outlive the imaging isotope (typically F-18) and may alarm Homeland Security detectors found in many locations. The length of time these isotopes can alarm the detectors will be determined and published in the near future.
New method to detect and quantify contaminants
Keith Eckerman and Richard Leggett have provided uptake retention functions for soluble injected forms of each possible contaminating isotope and these tables are also presented in the link above.
Dr. Robert Metzger of Radiation Safety Engineering, Inc. is pleased to be presenting 3 papers at next weeks ICRS-13 / RSPD-2016 joint conference in Paris, France.
Below are links to each of the 3 papers.
Spectra acquired on high resolution Germanium crystals are analyzed by standard software packages produced by Canberra (Genie), or Ortec (Gammavision). We have both software products and both identify gamma peaks in the spectrum and associate them with the gamma lines from known radionuclides in the gamma library. Spectra from natural sources that contain uranium, radium and other nuclides from the natural chains are difficult to analyze as the gamma peaks are heavily overlapped as shown above. The photopeaks shown above are from an expanded section of the 90 keV peak complex from natural uranium in soils. While the information on the uranium and radium concentrations in the soils is plainly there, the current software cannot deconvolute the overlapped peaks to provide the results. So Radium and Uranium concentrations in soils are normally determined by putting a small quantity of the soil into solution by a pyrosulfate or NaOH fusion, and then separating the uranium and thorium by extraction chromatography and counting. This is time consuming and expensive.
Recently, George Lasche and Bob Caldwell developed a new spectrum analysis package that takes a different approach to the analysis of a HPGE acquired gamma spectrum. In their approach, an isotope, once identified in the spectrum, is stripped from the spectrum with all of its known gamma peaks. This process continues interactively until all of the lines in the spectrum have been resolved and the residuals are structureless. Then the spectrum is rebuilt and the contribution of each isotope in the overlapped peaks can be easily determined (see above).
Then, using soil specific efficiency curves at the soil density being analyzed, the concentrations of the uranium and radium in the sample can be quantified. Testing we have been doing with known soil standards and intercomparisons with the separations lab on routine samples indicate excellent correlation for the new approach. The minimum detectable activity for U and Ra is comparable to or below traditional methods if sufficient volumes of soil are analyzed. Please look back for papers and presentations being developed on this method for the RPSD 2016 meeting.
The end result is a fast, inexpensive measurement of isotopic uranium and radium in soils at a fraction of the time and cost required for traditional techniques.
This technique has been used for analysis of soils for NORM concentrations in several states, but has not been approved for analysis of drinking water or radiopharmaceuticals.
A MCNP model of a PET/CT scanner
with a dosed patient in the scanner.
The vast majority of shield designs are performed by simple point kernel methods using the bulk attenuation properties of the shielding material and a known source term. All of the NCRP manuals use this method. However, when the geometries are complex (see above), or the shields are layered, the point kernel techniques perform poorly and can result in significant overshielding at great cost for the construction, or undershielding, requiring shielding retrofits, also at great cost.
For these complex designs, Monte Carlo (e.g. MCNP)or discreet ordinate (e.g. 3DANT or Mercurad) methods are preferable. While desirable, the implementation of these methods can be difficult. The three dimensional geometry of the shielded room must be described to the code, and the run times for Monte Carlo methods can run days to weeks before the results converge. Previously we have rented time on one of several supercomputers for these designs.
Recently, we have acquired two supercomputers from Thinkmate and Kingstar computers and have installed MCNP on them. With the new installations of these dedicated supercomputers, we have been able to completely eliminate the computer charge for advanced designs and have reduced the time it takes to perform the analysis significantly. This makes these approaches to shield designs far more affordable, and usable for smaller projects.
Areas where the Monte Carlo or discreet ordinate methods can contribute include facilities where:
- There is significant potential for streaming of radiation around barriers.
- Layered shields (e.g. Concrete over lead)
- Mixed field sources (e.g. protons, and photons)
- Multiple sources impacting a single area.
- Complex geometries where the source term is hard to define for point kernel methods.
Please call us for more information if these new capabilities could be of use to you.
We’re pleased to announce that the Journal of Radioanalytical and Nuclear Chemistry has published our article A Monte Carlo approach to food density corrections in gamma spectroscopy in their September, 2015 edition.
A PDF version of the article is available here and can also be found linked from our Papers page. The article fully citeble and can also be found online by those with access to SpringerLink here.
Dr. Robert Metzger’s Powerpoint presentation for the upcoming Methods of Analytical Radiochemistry Conference in Hawaii is now available online for download and review. In his presentation, Dr Metzger details a new approach to accurately measuring the radionuclides in food samples of different densities. This new approach is robust and less expensive than the normal method of using many standards of different densities.
Download the PDF version here:A Monte Carlo Approach to Food Density Corrections in Gamma Spectroscopy