Reply: Shortened Dynamic ¹⁸F-FDG PET

REPLY: Dr. Disselhorst and colleagues note that most protocols use at least 45 or 60 min for data acquisition. This is absolutely correct if the standard Levenberg–Marquardt algorithm is used to calculate the compartment parameters. In our work (1) we use a predictive algorithm, which predicts the 60-min compartment results from a shortened data acquisition. Only with a predictive algorithm may compartment parameters be calculated accurately from a shortened acquisition series.

With regard to positioning of VOIs, the database contains results from 60-min series, and the VOIs are always placed on the last frame of the series and on the 60-min images (e.g., 20-min series plus 60-min whole-body images). For full dynamic series (60-min dynamic acquisition), the last frame is used for positioning of VOIs. Of course, for the input VOI, the first frames are the most important and are used for positioning. The CT images from PET/CT are usually helpful to support positioning of VOIs. Because whole-body imaging is done in all oncologic patients, the 60-min data are always available, even for a shortened acquisition series. Therefore, lesion contrast is usually not a problem. In the clinical environment, it is not the aim to assess exclusively the highest tracer accumulation but to match a lesion as defined on CT or MR images with the corresponding finding on the PET image in order to estimate the metabolism in that mass. Besides that, additional VOIs may be used, such as to assess maximum uptake.

Indeed, the spatial distribution of maximum tracer concentration may be different on early and late images in a series, but this is not really relevant for clinical purposes although it may be interesting from a scientific point of view. In oncology, we always refer to a certain tumor volume, which usually does not change in spatial location between early and late images. Therefore, repositioning of VOIs within a series is done only in the rare cases in which movement of the patient is observed. If we were to keep track of, for example, just the maximum, this would not reflect the real uptake in the whole mass.

• © 2011 by Society of Nuclear Medicine