Correction of scan time dependence of standard uptake values in oncological PET

Abstract

BackgroundStandard uptake values (SUV) as well as tumor-to-blood standard uptake ratios (SUR) measured with [ 18F-]fluorodeoxyglucose (FDG) PET are time dependent. This poses a serious problem for reliable quantification since variability of scan start time relative to the time of injection is a persistent issue in clinical oncological Positron emission tomography (PET). In this work, we present a method for scan time correction of, both, SUR and SUV.MethodsAssuming irreversible FDG kinetics, SUR is linearly correlated to Km (the metabolic rate of FDG), where the slope only depends on the shape of the arterial input function (AIF) and on scan time. Considering the approximately invariant shape of the AIF, this slope (the ‘Patlak time’) is an investigation independent function of scan time. This fact can be used to map SUR and SUV values from different investigations to a common time point for quantitative comparison. Additionally, it turns out that modelling the invariant AIF shape by an inverse power law is possible which further simplifies the correction procedure. The procedure was evaluated in 15 fully dynamic investigations of liver metastases from colorectal cancer and 10 dual time point (DTP) measurements. From each dynamic study, three ‘static scans’ at T=20,35,and 55 min post injection (p.i.) were created, where the last scan defined the reference time point to which the uptake values measured in the other two were corrected. The corrected uptake values were then compared to those actually measured at the reference time. For the DTP studies, the first scan (acquired at (78.1 ± 15.9) min p.i.) served as the reference, and the uptake values from the second scan (acquired (39.2 ± 9.9) min later) were corrected accordingly and compared to the reference.ResultsFor the dynamic data, the observed difference between uncorrected values and values at reference time was (-52±4.5)% at T=20 min and (-31±3.7)% at T=35 min for SUR and (-30±6.6)% at T=20 min and (-16±4)% at T=35 min for SUV. After correction, the difference was reduced to (-2.9±6.6)% at T=20 min and (-2.7±5)% at T=35 min for SUR and (1.9% ± 6.2)% at T=20 min and (1.7 ± 3.3)% at T=35 min for SUV. For the DTP studies, the observed differences of SUR and SUV between late and early scans were (48 ± 11)% and (24 ± 8.4)%, respectively. After correction, these differences were reduced to (2.6 ± 6.9)% and (-2.4±7.3)%, respectively.ConclusionIf FDG kinetics is irreversible in the targeted tissue, correction of SUV and SUR for scan time variability is possible with good accuracy. The correction distinctly improves comparability of lesion uptake values measured at different times post injection.

Prognostic Value of Pretherapeutic Tumor-to-Blood Standardized Uptake Ratio in Patients with Esophageal Carcinoma

“…In the latter case, it has only to be assumed that the AIF can be described by some power law starting early after bolus passage, which empirically is fulfilled to a good degree (14). For scan-time correction of SUV it is necessary to assume that all AIFs can be described with the same power law-that is, the exponent b in Equation 1 is identical for all AIFs and its value directly enters the correction formula Equation 1.…”

“…In the clinical context, variability of the uptake period is unavoidable, which directly translates into a corresponding variability of the measured tracer uptake. But as has been shown recently, it is possible to reliably correct SUR (and somewhat less reliably SUV) for variations of the 18 F-FDG uptake period (14) by converting the measured uptake values to a preselected fixed scan time point. This scan-timenormalized SUR removes several of the shortcomings of SUV, leading to a much improved linear correlation between this uptake parameter and the actually targeted quantity, namely the metabolic rate of 18 F-FDG.…”

Comparison of prone versus supine 18F‐FDG‐PET of locally advanced breast cancer: Phantom and preliminary clinical studies

“…12 Methods for modifying PET data to account for differences in 18F-FDG uptake time have been proposed. 10,[13][14][15][16] A straightforward method for comparing uptake values acquired at different times after injection was introduced by Beaulieu and colleagues. 10 Importantly, this method was developed and prospectively validated in a breast cancer patient population.…”

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

“…This theoretical prediction has in fact been verified in patient data [9,11]. Two causes (beyond the elimination of all calibration related issues that is the principal advantage of any ratio method) are operational here: SUR corrects for the inherent variability of actual arterial tracer supply (present even after SUV normalization of the image data) [9] as well as for the practically unavoidable substantial variability of uptake time prior to scanning [8].…”

“…So far, in oncological PET, especially the tumour to liver ratio (TLR) has been used (one example of this approach is the work of Huang et al [7]). More recently, we have proposed the imagederived tumour to arterial blood standard uptake ratio (SUR) as a promising alternative [8,9] and investigated its properties and performance compared to SUV [5,10,11]. We also addressed the question to what extent the healthy liver parenchyma might act as a substitute for actual arterial tracer supply [12].…”