Clinical Value of Patlak Ki Images Extracted From Dynamic 18F-FDG PET/CT For Evaluation of The Relationships Between Disease Activity and Clinical Events in Cardiac Sarcoidosis

Background: Cardiac sarcoidosis (CS) has a poor prognosis because of frequent complication of atrioventricular block, ventricular tachycardia and congestive heart failure. Qualitative or quantitative 18 F-FDG PET/CT has been used for diagnosing or assessing the disease activity of CS. However, the association between 18 F-FDG myocardial uptake and clinical presentations in CS has not yet been claried, and it is unknown if Patlak Ki images (Ki images) extracted from dynamic 18 F-FDG-PET/CT are useful for evaluating the disease activity or clinical events in CS patients. In this context, this study was performed to investigate the usefulness of SUV and Patlak Ki images extracted from dynamic 18 F-FDG-PET/CT for evaluating the risk of severe clinical events (SCEs) in CS. Methods: The SUV and Ki myocardial images were generated from 30 dynamic 18 F-FDG-PET/CT scans of 21 CS patients including those with cardiac dysfunction and arrhythmic events. SUV parameters and Ki parameters (Ki max, Ki mean, Ki volume) were measured for positive myocardial lesions. The Mann–Whitney U-test or Fisher’s exact test was used appropriately to assess differences between quantitative variables or compare categorical data. The association between each quantitative parameter and presence of SCEs was analyzed by logistic regression analysis. Results: The SUV and Ki mages both were rated as positive in 19 scans and negative in 11 scans with the same incidence of SCEs which were signicantly higher in positive than negative scans [cardiac dysfunction: 78.9% (15/19) vs. 27.2% (3/11), p=0.009; arrhythmic events: 65.5% (10/19) vs. 0% (0/11), p=0.004]. In 19 positive scans, neither SUV nor Ki parameters were signicantly different between scans of patients with cardiac dysfunction (n=15) and those without (n=4) (p>0.05, each), whereas the three Ki parameters were signicantly higher in scans for patients with arrhythmic events (n=10) than in those without (n=9) (p<0.05, each). Logistic regression analysis showed that the Ki volume alone was signicantly associated with the risk of arrhythmic events (Odds ratio: 1.11, p=0.047). Conclusion: Patlak Ki images may add value to SUV images for evaluating the risk of SCEs in CS patients. 2, similar to; 3, somewhat higher than; and 4, noticeably higher than hepatic uptake [19]. To determine the disease activity of CS, scores of 0–1 and 2–4 were assigned as negative and positive, respectively. Thereafter, they visually assessed each Ki image as negative (myocardial visibility was lower than or similar to that of liver) or positive (myocardial visibility was higher than that of liver).


Introduction
Sarcoidosis is a systemic granulomatous disease of unknown etiology. Clinical manifestations of cardiac involvement are estimated to occur in 5% of patients with sarcoidosis, but the prevalence at autopsy has ranged from 25-58% [1,2]. Cardiac sarcoidosis (CS) has a poor prognosis because of frequent complication of atrioventricular block (AVB), ventricular tachycardia (VT) and congestive heart failure [3][4][5]. To prevent adverse outcomes, an accurate and early diagnosis is essential so that anti-in ammatory therapy can be initiated [6,7]. Glucose metabolic activity can be shown by measuring 18 F-uorodeoxyglucose ( 18 F-FDG) uptake during positron emission tomography (PET)/computed tomography (CT) for not only oncological but also in ammatory disorders [8,9]. Qualitative or quantitative 18 F-FDG PET/CT has been used for diagnosing or assessing the disease activity of CS [10,11]. However, the association between 18 F-FDG myocardial uptake and clinical presentations in CS has not yet been clari ed.
The Patlak slope, Ki, which represents the rate of 18 F-FDG uptake is a quantitative index of 18 F-FDG metabolism. Dynamic imaging is required to calculate the Ki, and an arterial input function must be measured and a lesion time-activity curve constructed [12,13]. Although a good correlation has been reported between the standardized uptake value (SUV) and Ki, they are not equivalent [14]. The SUV measures the total activity in the lesion, and includes both metabolized 18 F-FDG and unmetabolized 18 F-FDG (unphosphorylated 18 F-FDG) in the blood, intercellular spaces, and/or cells. Patlak analysis separates these two components, and the Patlak slope is determined only by metabolized 18 F-FDG [15]. Thus, measurements of Ki might contribute to assessments of CS disease activity. However, to our knowledge, only one report has investigated dynamic 18 F-FDG-PET/CT images for diagnosis of CS [16], and it is unknown if Patlak Ki images (Ki images) extracted from dynamic 18 F-FDG-PET/CT are useful for evaluating the disease activity or clinical events in CS patients.
The study aim was to investigate the usefulness of SUV and Patlak Ki images for evaluating the risk of severe clinical events (SCEs) in CS patients.

Study design and patient selection
This retrospective study was approved by institutional review board, which waived the requirement for informed consent. From April 2019 to January 2020, 18 F-FDG-PET/CT was performed in 24 consecutive patients for suspected CS or known CS, and their clinical records were reviewed to identify patients for analysis. The inclusion criterion was diagnosis of CS according to the Japanese Society of Sarcoidosis and Other Granulomatous Disorders guidelines [17]. Patients with a history or coexistence of other cardiac disorders were excluded.
Two patients were excluded because of hypertrophic cardiomyopathy and one for insu cient evidence of CS. Finally, 21 CS patients were enrolled (14 women and seven men; mean ± standard deviation [SD], age 61 years ± 11; age range, 37−76 years). Eight patients underwent 18 F-FDG PET/CT scans before steroid treatment, and two of them underwent scans at the 6-months follow-up after initiation of steroid treatment. The remaining 13 patients received steroid therapy, and seven of them underwent follow-up scans during the study period. Consequently, 21 patients had a total of 30 18 F-FDG-PET/CT scans.

Imaging protocols
All patients were instructed to fast for ³18 h before PET/CT, which resulted in a mean plasma glucose level of 107 mg/dl (range, 68-154 mg/dl) immediately before 18 F-FDG intravenous injection.
All 18 F-FDG PET/CT examinations were performed on a Discovery MI PET/CT (GE Healthcare, Milwaukee, WI). First, low-dose CT covering the entire heart was performed (slice thickness, 3.75 mm; pitch, 1.375 mm; 120 keV; auto mA [40-100 mA depending on patient body mass]; reconstructed matrix size, 512 × 512) with the transaxial and cranio-caudal eld of view (FOV) of 70 cm and 20 cm that was used for attenuation correction of the PET images.
Thereafter, 18 F-FDG (227 MBq ± 28 [range, 179-286 MBq]) was injected, and dynamic PET (single-bed) images covering the same cranio-caudal FOV as that of the above CT were acquired with the following PET frames. The acquisition began at the injection, with scan times of 10s/frame for the rst 2 min, 3 min/frame for the next one frame, and 5 min/frame thereafter for a total of 60 min.

Generation of the Patlak Ki and SUV static images
To determine the 18 F-FDG kinetic parameters within the lesion, a linear approximation of the mathematical representation of the standard twocompartmental model with irreversible trapping was used according to Patlak analysis [12]. From C i (tk), the 18 F-FDG activity concentration in the lesion (Bq mL tissue −1 ) at a given time tk after injection, the analytical solution of the two-compartment model is given as: where Cp(tk) represents the 18 F-FDG activity concentration in blood plasma at time tk (Bq mL blood −1 ) and Vp is the total blood distribution volume (i.e, the unmetabolized fraction of 18 F-FDG in blood and interstitial volume).
The compartmental transfer rates, k1 (from blood to cell), k2 (from cell to blood), and k3 (from 18 F-FDG to 18 F-FDG-6-phophate), were used to calculate Ki, the net in ux rate, as follows: The transfer rate k4 from 18 F-FDG-6-phophate to 18 F-FDG is negligible because the Patlak analysis assumes unidirectional uptake of 18 F-FDG (k4=0). The Ki unit is ml/g/min.
In this study, the non-invasive plasma arterial input function estimation technique using a Patlak graphical plot method was applied to calculate the Ki for generating the Ki images [12]. We characterized the input function by using blood time-activity curves derived from PET image. A region of interest was drawn in the ascending aorta to determine the arterial input function, and the Patlak analysis was performed over the period from 10 to 60 min after injection during a steady state. The data were reconstructed by using a matrix size of 128 × 128 with a Bayesian penalized likelihood (BPL) reconstruction algorithm [18] (with point spread function incorporated as a default setting (Q.Clear, GE Healthcare, Milwaukee, WI) with a beta value (penalization factor) of 700 and transaxial FOV of 50 cm as the Ki image.
The PET frames acquired 53-60 mins after 18 F-FDG injection were corrected for attenuation by using the CT data, and these attenuation corrected images were reconstructed by using a matrix size of 128 × 128 with BPL reconstruction algorithm with a beta value of 350 and transaxial FOV of 50 cm as the static SUV image (SUV image).

Image analysis
Two nuclear medicine radiologists, who were aware of the study purpose but blinded to clinical information, interpreted the SUV and Ki images together to reach in a consensus. First, the radiologists visually scored the 18 F-FDG myocardial uptake by using a ve-point scale for each SUV image as follows, 0, no visible uptake; 1, 18 F-FDG uptake lower than; 2, similar to; 3, somewhat higher than; and 4, noticeably higher than hepatic uptake [19]. To determine the disease activity of CS, scores of 0-1 and 2-4 were assigned as negative and positive, respectively. Thereafter, they visually assessed each Ki image as negative (myocardial visibility was lower than or similar to that of liver) or positive (myocardial visibility was higher than that of liver).
The following quantitative parameters were obtained in the interpreted positive (visible) myocardium; the maximum SUV (SUVmax), mean SUV (SUVmean), cardiac metabolic volume (CMV), and cardiac metabolic activity (CMA) for SUV images, and maximum and mean Ki values (Ki max and K imean) and Ki volume for Ki images. The third nuclear medicine radiologist set the volumes of interest (VOIs) separately for SUV images and Ki images. He placed the VOIs manually on a suitable reference fused axial image and then de ned the cranio-caudal and mediolateral extent encompassing the entire positive myocardial lesion, excluding any avid extracardiac structures, to obtain SUVmax and Ki max. He next set a 40% threshold of SUVmax [20] or of Ki max to automatically delineate a VOI equal to or greater than the 40% threshold of SUVmax or Ki max to calculate the SUVmean, CMV, and CMA or Ki mean and Ki volume, respectively. The CMA was de ned as the SUVmean x CMV. Workstations (Xeleris or Advantage Windows Workstation 4.5; GE Healthcare) calculated the SUVmax, SUVmean, CMV, CMA and the Ki max, Ki mean and Ki volumes automatically.
Parameters for assessing the risk of SCEs (Tables 3 and 4) The AUCs for the ability to assess the risk of the cardiac dysfunction were 0.80 for CMV (p = 0.031) and 0.82 for CMA (p = 0.020) ( Table 3). The logistic analysis revealed that no parameters were signi cantly associated with cardiac dysfunction (p > 0.05, each) ( Table 4).

Ascertainment of SCEs
Echocardiography was performed within 1 month of the 18 F-FDG-PET/CT (mean ± SD, 6 days ± 8; range, −26 to +26 days), and the echocardiography report was used as the reference standard for cardiac function. Cardiac dysfunction was de ned as a left ventricular ejection fraction (LVEF) <50% [21].
Patients were also assessed to determine if arrhythmic events, including sustained VT or AVB were presented within 1 month of the 18 F-FDG-PET/CT (mean ± SD, 8 days ± 8; range, −24 to +31 days). AVB was characterized as either second-or third-degree AVB or trifascicular block documented on 12lead or Holter echocardiography [21,22].

Statistical analysis
The incidence of each SCE was compared between the positive and negative images by using Fisher's exact test. Quantitative variables were compared between scans with and without SCEs by using the Mann-Whitney U test. The Spearman rank correlation was used to assess the relationship between two quantitative variables. Receiver operating characteristic (ROC) curve analysis was performed to examine the diagnostic performance of each parameter between the presence and absence of SCEs, and the Youden index was used to determine the best cutoff point for each parameter [23]. The statistical signi cance of differences between the areas under the ROC curves (AUCs) were analyzed by using the DeLong method [24]. The association between each parameter and presence of SCEs was analyzed by logistic regression analysis.
Data are presented as medians and interquartile ranges (IQRs). A value of p<0.05 was considered to be indicative of statistical signi cance, and all P values were two-tailed. MedCalc Statistical Software (MedCalc Software Ltd, Acacialaan 22, 8400 Ostend, Belgium) was used for the statistical analyses.

Results
SCEs at 18  Relationship between visual ndings of SUV or Ki images and SCEs (Table 1) The Relationship between SUV or Ki parameters and SCEs ( Table 2) No signi cant differences between the scans of patients with cardiac dysfunction (n = 15) and normal cardiac function (n = 4) in the SUVmax, SUVmean, and CMV were found (p > 0.05, each). The median CMA was higher in cardiac dysfunction than in normal cardiac function, but this difference was not signi cant (p = 0.057). No signi cant correlations were observed between any of the SUV parameters and LVEF (SUVmax, ρ = 0.018, p = 0.94; SUVmean, ρ = 0.037, p = 0.88; CMV, ρ= -0.13, p = 0.60; CMA, ρ= -0.16, p = 0.52, respectively). The median SUVmean was signi cantly higher in the presence of arrhythmic events (n = 10) than in the absence of arrhythmic events (n = 9) (p = 0.041).
The median SUVmax, CMA and CMV were higher in the presence of arrhythmic events than in the absence of arrhythmic events, but these differences were not signi cant (p = 0.060, 0.066, 0.060, respectively). No signi cant differences in the Ki max, Ki mean and Ki volume between cardiac dysfunction and normal cardiac function were found (p > 0.05, each). No signi cant correlations were observed between each Ki parameter and LVEF (K imax, ρ = 0.21, p = 0.39; Ki mean, ρ = 0.24 p = 0.32; Ki volume, ρ= -0.16, p = 0.52).

Discussion
SUV images for evaluating the risk ofSCEs Previous studies have reported that 18 F-FDG SUV images have been important in the assessment of CS [10,11]. A recent meta-analysis reported a sensitivity of 89% and speci city of 78% for detection of CS on 18 F-FDG SUV images [25]. The relationships between 67 Ga scintigraphy or 18 F-FDG SUV images and clinical events in CS patients have been reported previously [26][27][28]. Banba et al. [26] performed 67 Ga scintigraphy in 15 CS patients, and gallium uptake has been documented in 80% of AVB patients. McArdle et al. [27] examined the degree of 18 F-FDG myocardial uptake assessed by SUV parameters, and higher SUVmax and SUVmean were observed in CS patients with VT than in those with AVB or who were clinically silent. Ahmadian et al.
[28] reported that CMA calculated by the SUV threshold using the 1.5 LV blood pool SUVmax was signi cantly higher in CS patients with an EF < 50% than in those with an EF > 50%, but SUVmax was not signi cantly different between them when only 18 F-FDG positive cases were analyzed.
In our study, the rate of cardiac dysfunction or arrhythmic events was signi cantly higher in the visually analyzed positive SUV images than in the negative images, suggesting that visual assessment of SUV images might be useful for evaluating the risk of SCEs. On the other hand, in quantitative analyses of the SUV positive scans, although each parameter was higher in the presence of cardiac dysfunction than in its absence and higher in the presence of arrhythmic events than in their absence, the difference was only ssigni cant for the SUVmean for arrhythmic events (p = 0.041). The discrepancy among the study results of SUV parameters for evaluating the clinical events may be because of the differences in the analysis methods. We performed the quantitative analyses only for visual 18  We found only one study that used dynamic 18 F-FDG-PET/CT images to diagnose CS [16], and the researchers reported that heterogeneous myocardial glucose metabolism assessed by using a normalized coe cient of variation of the Ki could be useful for diagnosis of CS. However, no study has examined the relationship between Ki images and clinical events in CS patients.
In our study, on visual analyses of Ki images, the rate of cardiac dysfunction or arrhythmic events was signi cantly higher in the positive Ki scans than in the negative Ki scans, suggesting that the visual assessment of Ki images might be useful for evaluating the risk of SCEs.
On quantitative analyses of Ki positive scans, although none of the Ki parameter were signi cantly different between cardiac dysfunction and normal cardiac function, they were signi cantly higher with the arrhythmic events than without them. Moreover, Ki volume alone was signi cantly associated with arrhythmic events, suggesting that Ki volume might be the most useful parameter for evaluating the risk of arrhythmic events. Despite the signi cant correlations between SUV and Ki parameters, there were discrepancies in the results of these parameters concerning the association with clinical events in CS. Thus, Ki images may have the potential to provide additional value to the SUV images for evaluation of the clinical events in CS patients.
There were some study limitations that should be considered when interpreting our results. First, this was a retrospective study with a small sample. Therefore, a prospective study with a large sample is needed to con rm the validity of the present ndings. Second, the different treatment statuses at the time of the dynamic 18 F-FDG-PET/CT scans could have affected the positivity of the SUV and Ki images, which may have led to biased quantitative analysis. Third, we did not compare other useful imaging techniques, such as cardiac magnetic resonance imaging (MRI) or myocardial perfusion imaging with SUV or Ki images because only a minority of patients underwent cardiac MRI (n = 6) or myocardial perfusion imaging (n = 2). Finally, in the absence of guidelines on optimal reconstruction parameters for Ki images, we used different beta values of 350 and 700 for reconstruction of the SUV and Ki images, respectively. Further investigations of optimal reconstruction protocols for Ki imaging are warranted.