Simultaneous measurement of noise and spatial resolution in PET phantom images

As an aid to evaluating image reconstruction and correction algorithms in positron emission tomography, a phantom procedure has been developed that simultaneously measures image noise and spatial resolution. A commercially available Ge-68 cylinder phantom (20 cm diameter) was positioned in the center of the field-of-view and two identical emission scans were sequentially performed. Image noise was measured by determining the difference between corresponding pixels in the two images and by calculating the standard deviation of these difference data. Spatial resolution was analyzed using a Fourier technique to measure the extent of the blurring at the edge of the phantom images. This paper addresses the noise aspects of the technique as the spatial resolution measurement has been described elsewhere. The noise measurement was validated by comparison with data obtained from multiple replicate images over a range of noise levels. In addition, we illustrate how simultaneous measurement of noise and resolution can be used to evaluate two different corrections for random coincidence events: delayed event subtraction and singles-based randoms correction. For a fixed number of iterations of the maximum-likelihood expectation-maximization algorithm, the singles-based correction gave rise to higher noise than delayed event subtraction. However, when noise and resolution were measured simultaneously it was shown that singles-based randoms correction gave rise to lower noise than delayed event subtraction for a fixed spatial resolution. The proposed method of simultaneously measuring image noise and spatial resolution is useful for evaluating reconstruction algorithms and may aid standardization of data collection between centers.