**1. Introduction**

The purpose of this review is to provide a state-of-the-art overview of factors influencing common quantitative image parameters in positron emission tomography (PET) as well as image interpretation, which is usually not quantitative. To address this dichotomy, the chapter on "quantification" relates to factors with a bearing on quantitative accuracy, while the second chapter "interpretation" focusses on variables that affect the subjective, reader-dependent, mostly visual interpretation of images and their effects on diagnostic accuracy and response assessment. Some sections of the article put special emphasis on [18F]fluorodeoxyglucose (FDG), owing both to the unique clinical importance of [18F]FDG and its vast literature as well as the issue of dietary preparation and influences of blood glucose levels on quantification in [18F]FDG-PET.

PET quantification, as defined in this review article, comprises primarily those methodological factors that determine how accurately the radiopharmaceutical with its biodistribution in an individual patient is depicted. It focusses on those aspects that are potentially relevant for daily routine clinical care (Figure 1).

**Citation:** Rogasch, J.M.M.; Hofheinz, F.; van Heek, L.; Voltin, C.-A.; Boellaard, R.; Kobe, C. Influences on PET Quantification and Interpretation. *Diagnostics* **2022**, *12*, 451. https://doi.org/10.3390/ diagnostics12020451

Academic Editors: Malene Barbara Fischer and Lars C. Gormsen

Received: 9 November 2021 Accepted: 8 February 2022 Published: 10 February 2022

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**Figure 1.** Article structure: From patient preparation to image interpretation. Every step of preparing the patient, acquiring and processing PET images, and choosing criteria to quantify and interpret the data potentially affects quantitative and diagnostic accuracy. Each of these steps is addressed by successive subsections of this article.

#### **2. Factors Affecting PET Quantification**

The essence of PET quantification is lesion contrast recovery (CR), which describes the relative recovery of the true focal activity concentration. Figure 2 illustrates the most relevant factors influencing lesion CR in PET.

**Figure 2.** The thin line of quantitative accuracy in PET. Quantitative accuracy of PET in lesions (i.e., recovery of the true activity concentration) can be imagined as a pair of balances between factors that promote either under- or overestimation of the true activity. Additionally, the point at which the combination of these contrasting factors achieves quantitative accuracy is influenced by lesionspecific and methodological factors (e.g., the choice of standardized uptake value (SUV) parameter). **\*** Reported lesion SUVs in PET/MRI are lower than those in PET/CT; however, this may not be true for every lesion in every tissue. PSF, point spread function; PL, penalized likelihood.
