As part of a NIST-wide effort to develop a metrological infrastructure for quantitative medical imaging, the Radioactivity Group has recently established a dedicated facility to support its efforts to develop the necessary standards and measurement techniques for quantitative PET-CT.
The cornerstone of the new facility is a new Philips Gemini-TF clinical PET-CT scanner that was acquired under the America Recovery and Reinvestment Act (ARRA) during FY10. The scanner is intended to be used for characterizing new phantoms that the Group is developing for both PET and CT to act as a testbed for the development of data acquisition and analysis methods, as well as to act as a reference for interlaboratory phantom comparisons.
After final acceptance is complete, the first project that will be carried out will be calibration of the scanner for 18F activity measurement on an absolute basis with traceability to National standards for that radionuclide. Additional studies are planned to cross-calibrate the scanner for other positron emitters, such as 68Ge, and investigate its potential use as a transfer standard for calibrating various phantoms for contained activity. A much larger, long-term project will be carried out to identify and quantify the principal components of uncertainty in the imaging process and how they affect the activity determination. Basic imaging physics studies, such as partial volume correction methods and translation of absolute image quantification to the clinic will also be an important part of the program over the next few years.
In addition to the work on PET imaging standards, an similar program will be carried out in the area of CT to provide measurement infrastructure for that modality as well. Some of the near-term tasks that will be carried out include:
· Provide reference standards for CT image qualities that are related to length and contrast impacted by spectral characteristics and noise levels. Our in-house CT machine will greatly aid the current effort with Division 685 on the lung CT phantom development in which various scanning protocols in different clinical centers are compared to assess the Hounsfield Unit (HU) variation. NIST has taken up the role for certifying the appropriate foams for CT characteristics. Transmission measurements have been performed using polychromatic beams (laboratory x-ray machines including the NIST mammography standard beam) and at a monochromatic synchrotron x-ray source to obtain a more definitive HU value for these foams of various densities at various energies.
· Contribute toward addressing the current concern for the lack of uniform standards for patient dose in clinical CT diagnostics. The air kerma is the current diagnostics x-ray standard maintained by the NIST Dosimetry Group. The implementation of this standard is through the AAPM TG-111 that is in the process of improving the protocols to reflect the modern day helical scanning and the deficiency of CTDI (dose index) in representing actual patient dose. A suite of image quality and dose phantoms will be calibrated to assess the optimal image quality and noise level with the goal of minimizing dose.