Appendix: Partial Volume Correction Methods
To describe partial volume effects and the correction techniques used, we define a CT Volume of Interest (VOI), and a PET threshold VOI. The CT VOI is defined by the organ boundaries visible on CT images only. A threshold VOI is drawn such that it encompasses all pixel values above some threshold PET activity, expressed as a percent of the peak value within a defined CT VOI. Due to finite spatial resolution, PET images will blur the measured activity concentration outside a given CT VOI. This has the effect of lowering the measured activity concentration in organs whose dimension approaches the resolution of the PET scanner. To determine the true activity in these organs, partial volume losses must be corrected.
PET 40-80 Threshold
Partial volume correction in small organs (<2 x FWHM resolution) including the kidneys, heart wall and thyroid, was accomplished using a technique based on PET threshold VOIs. This method relies mainly on the PET data and is therefore not prone to PET/CT co-registration issues. First, a CT VOI is drawn to encompass the entire organ, and the maximum PET activity is measured. Next, the organ volume is defined as those regions above 40% of the maximum PET activity, to avoid spill-in from any adjacent organs. The measured organ activity then is defined as the average value in those regions above 80% of the maximum PET activity. The corrected PET activity is obtained as the product of the measured organ activity times the organ volume,thus the volume at a 40% threshold value was multiplied by the average activity concentration at an 80% threshold. This is called the 40-80 method. This method is applicable for tissues ranging in thickness from 4 mm to 80 mm thick, provided the PET activity is high enough above background to apply a threshold value at 40%. An analytical simulation of partial volume correction for various tissue thicknesses was done. This simulation shows that for all tissue thicknesses above 10 mm the 40-80 method corrects for partial volume losses with a maximum overestimate of ≈ 2% and gives no underestimate. The limitation of this method is that the whole organ must be present in the scan and a 40% threshold must be well defined and separable from other organs, since it is the whole volume at the 40% threshold which is required (see Figure A1). As a result, the 40-80 method is often best used in later time frames when the background is sufficiently below the areas of high uptake.
Some organs are either equal to or just above the background PET activity either due to very low uptake or extremely small size. In these cases a partial volume correction is not possible using this method. Thus no partial volume correction is performed for small organs with activity either equal to or slightly above background. Since the scanning area may not capture the whole volume of an organ. The organ volume in these cases is taken from the organ volumes in the ICRP 110 phantom.[14]
CT Volume Erosion
Partial volume correction in large organs was accomplished by reducing the size of the CT VOI to avoid partial volume losses which occur near the organ boundaries. This method is referred to as the VOI-Erosion technique, and was used where the exact delineation of an organ could be made on CT. The limitation of this method is that the organ must be larger than twice the FWHM of the spatial resolution point spread function of the scanner.