Effects of image noise, respiratory motion, and motion compensation on 3D activity quantification in count-limited PET images

The aims of this study were to evaluate the effects of noise, motion blur, and motion compensation using quiescent-period gating (QPG) on the activity concentration (AC) distribution - quantified using the cumulative AC volume histogram (ACVH) - in count-limited studies such as ⁹⁰Y-PET/CT. An International Electrotechnical Commission phantom filled with low ¹⁸F activity was used to simulate clinical ⁹⁰Y-PET images. PET data were acquired using a GE-D690 when the phantom was static and subject to 1-4 cm periodic 1D motion. The static data were down-sampled into shorter durations to determine the effect of noise on ACVH. Motion-degraded PET data were sorted into multiple gates to assess the effect of motion and QPG on ACVH. Errors in ACVH at AC₉₀ (minimum AC that covers 90% of the volume of interest (VOI)), AC₈₀, and AC_mean (average AC in the VOI) were characterized as a function of noise and amplitude before and after QPG. Scan-time reduction increased the apparent non-uniformity of sphere doses and the dispersion of ACVH. These effects were more pronounced in smaller spheres. Noise-related errors in ACVH at AC₂₀ to AC₇₀ were smaller (<15%) compared to the errors between AC₈₀ to AC₉₀ (>15%). The accuracy of AC_mean was largely independent of the total count. Motion decreased the observed AC and skewed the ACVH toward lower values; the severity of this effect depended on motion amplitude and tumor diameter. The errors in AC₂₀ to AC₈₀ for the 17 mm sphere were -25% and -55% for motion amplitudes of 2 cm and 4 cm, respectively. With QPG, the errors in AC₂₀ to AC₈₀ of the 17 mm sphere were reduced to -15% for motion amplitudes <4 cm. For spheres with motion amplitude to diameter ratio >0.5, QPG was effective at reducing errors in ACVH despite increases in image non-uniformity due to increased noise. ACVH is believed to be more relevant than mean or maximum AC to calculate tumor control and normal tissue complication probability. However, caution needs to be exercised when using ACVH in post-therapy ⁹⁰Y imaging because of its susceptibility to image degradation from both image noise and respiratory motion.