Quantification: there is more to worry about than good scanner hardware and reliable calibration

Abstract

Positron emission tomography (PET) is Ban analytical imaging technology developed to use compounds labelled with positron-emitting radioisotopes as molecular probes to image and measure biochemical processes of mammalian biology in vivo^ [1]. One outstanding feature of the PET technology is the ability to perform absolute quantification of regional perfusion, metabolism, and function [2]. There are clinical demands for quantification regarding description of biodistribution, dosimetry, intra-and inter-individual comparisons, and setup of age-and gender-specific (normal) databases. Notably, FDG PET allows diagnosis, differential diagnosis, assessment of prognosis, and patient stratification in malignant disease. Moreover, image guided therapy has been proven to improve tumour delineation and irradiation field definition regarding protection of normal tissue and dose escalation on tumour tissue [3]. After initial assessment, followup investigations describe the effect of therapy and influence therapeutic management regarding continuation or change of modality and intensification or de-escalation of therapy. In addition to qualitative description and quantification of tracer uptake or uptake changes during follow-up, more sophisticated kinetic modelling and analysis may be applied. However, reliability and significance of all derived numbers is influenced by technical factors and biological processes. Methodological aspects of quantificationFormally, the principal goal in quantitative oncological FDG PET is to measure a surrogate of the tumour's metabolic rate of glucose consumption. Ideally, the surrogate parameter should be proportional to the latter quantity but, of course, a more general (linear or nonlinear) monotonic relation between surrogate and target parameter would suffice, too. But in any case, the respective relation has to be universally valid (i.e., invariant) across different scans, scanners, and patients. Otherwise, comparisons between different investigations are affected by spurious variability of the measured surrogate that is unrelated to actual changes in tumour metabolism.Currently, the standardized uptake value (SUV) is accepted as a suitable surrogate parameter derivable from static investigations, although it is widely recognized that its properties are far from ideal. Consequently, much effort has been invested to improve the reliability of SUV measurements by means of addressing the recognized technical issues, e.g., by focusing on strict calibration procedures and SOPs on how to perform measurement and data evaluation [4].Despite the unquestionable value of these efforts, test-retest stability of SUV remains rather unsatisfactory: even under highly controlled study conditions with nearly perfectly observed constant uptake times, test-retest variability is of the order of ±(30-40)% [5,6]. Consequently, SUV is not able to reliably detect (or exclude) moderate changes of the tumours' metabolic rate.In this context it is interesting to observe that there exist assorted reports of superior p...

Comparison of image quality and spatial resolution between 18F, 68Ga, and 64Cu phantom measurements using a digital Biograph Vision PET/CT

“…The same study revealed that the maximum standardized uptake value (SUV max ) of lesions attributed to benign origin was higher for 18 F-PSMA-1007-PET than for 68 Ga-PSMA-11-PET [ 2 ]. These differences between PET scans of different radionuclides might be caused by a combination of different effects: Radiotracers differ in their biokinetics and affinities [ 5 ]. In addition, state-of-the-art digital PET/CT scanners allow PET measurements with higher sensitivity and spatial resolution.…”

Comparison of Image Quality and Spatial Resolution Between 18F, 68Ga and 64Cu Phantom Measurements Using a Digital Biograph Vision PET/CT

“…lesions attributed to benign origin was higher for 18 F-PSMA-1007-PET compared to 68 Ga-PSMA-11-PET (2). It is currently unclear whether these differences are due to different biokinetics and a nities of the tracers (5), different counting statistics in the respective measurements or whether different physical properties increase detection probability. The maximum positron energy and thus the mean range in tissue differ between 18 F and 68 Ga: positrons arising from the decay of 68 Ga feature a three times higher endpoint energy than that of 18 F and therefore have a much greater mean range in tissue (3.5 mm compared to 0.6 mm) (Table 1).…”