Image-derived input function for human brain using high resolution PET imaging with [¹¹C](R)-rolipram and [¹¹C]PBR28

Background: The aim of this study was to test seven previously published image-input methods in state-of-the-art high resolution PET brain images. Images were obtained with a High Resolution Research Tomograph plus a resolution-recovery reconstruction algorithm using two different radioligands with different radiometabolite fractions. Three of the methods required arterial blood samples to scale the image-input, and four were blood-free methods. Methods: All seven methods were tested on twelve scans with [¹¹C](R)-rolipram, which has a low radiometabolite fraction, and on nineteen scans with [¹¹C]PBR28 (high radiometabolite fraction). Logan V_T values for both blood and image inputs were calculated using the metabolite-corrected input functions. The agreement of image-derived Logan V_T values with the reference blood-derived Logan V_T values was quantified using a scoring system. Using the image input methods that gave the most accurate results with Logan analysis, we also performed kinetic modelling with a two-tissue compartment model. Results: For both radioligands the highest scores were obtained with two blood-based methods, while the blood-free methods generally performed poorly. All methods gave higher scores with [¹¹C](R)-rolipram, which has a lower metabolite fraction. Compartment modeling gave less reliable results, especially for the estimation of individual rate constants. Conclusion: Our study shows that: 1) Image input methods that are validated for a specific tracer and a specific machine may not perform equally well in a different setting; 2) despite the use of high resolution PET images, blood samples are still necessary to obtain a reliable image input function; 3) the accuracy of image input may also vary between radioligands depending on the magnitude of the radiometabolite fraction: the higher the metabolite fraction of a given tracer (e.g., [¹¹C]PBR28), the more difficult it is to obtain a reliable image-derived input function; and 4) in association with image inputs, graphical analyses should be preferred over compartmental modelling.