Physicist Considerations for PET-CT and SPECT-CT

As technology improves, the radiopharmaceutical doses recommended in the published guidelines decrease. For example, for hepatobiliary imaging using HIDA derivatives labeled with 99mTc, the lowest adult dose recommended by SNM in 2009 was 1.5 mCi, while the 2008 ACR guideline recommended a minimum dose of 3 mCi. Many nuclear medicine departments still have protocols listing 4-5 mCi as the minimum dose to be injected. Thus, a practical way to reduce radiation dose is by keeping the department protocols updated to the latest version of published guidelines. Formal or informal audits of dose administered compared to patient weight or body mass index may pinpoint to additional opportunities for reducing the radiopharmaceutical dose administered to patients, thereby reducing radiation dose.

Dose reduction for CT can be addressed with altered acquisition parameters. Reduced kVp and mAs, faster gantry rotation, larger pitch for spiral CT, and shorter scan lengths can all potentially reduce radiation dose. Of course, these choices are constrained by physical and biological considerations, availability of particular tracers or imaging systems, and the desired diagnostic goals of the imaging study.

PET-CT Implementation, Application, and Radiation Dose

The typical design for a PET-CT system combines a CT scanner gantry into the same “box” as a ring-geometry PET system, with a common bed shared between the two systems. Generic descriptions of PET-CT scanner design and operation can be found as follows:

The CT system is usually a modern multi-slice helical design, identical to any stand-alone CT scanner. This is typically referred to as a “diagnostic” quality scanner; that is, the scanner is capable of generating routine CT scans, as well as scans with altered acquisition settings used specifically for attenuation correction of the PET data. These CT-based attenuation correction (CTAC) scans are usually lower quality (smaller mAs, possibly reduced kVp, usually reduced dose) than a “diagnostic” CT scan.

While a CTAC scan is used for attenuation correction and fused PET-CT image display, it typically should not be used for other purposes. A “diagnostic” quality CT, however, could be used for attenuation correction and image fusion as well as stand-alone decisions based on the CT, radiation treatment planning, or other uses. The decision to use CTAC versus “diagnostic” CT parameters will depend on whether CT data is needed for other tasks besides attenuation correction and image fusion. If no additional CT scan is needed, it seems sensible to use a low-dose CTAC with the PET acquisition. If a diagnostic CT is needed, then one could conceivably use this CT for the PET attenuation correction, thus avoiding the radiation dose from a separate CTAC scan.

For additional resources about radiation dose in PET-CT, access the links to published literature below, paying particularly attention to the references.