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Radiotracer dose reduction in 18F-FDG whole-body PET/MR: Effects on image quality and quantification
Maike E. Lindemann1, Vanessa Stebner2, Alexander Tschischka3, Julian Kirchner3, Lale Umutlu4, and Harald H. Quick1,5

1Highfield- and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, 2Department of Nuclear Medicine, University Hospital Essen, Essen, Germany, 3Department of Diagnostic and Interventional Radiology, University Hospital Düsseldorf, Düsseldorf, Germany, 4Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany, 5Erwin L. Hahn Institute for Magnetic Resonance Imaging, Essen, Germany

Synopsis

The study goal is to investigate how the simulated reduction of injected radiotracer affects PET image quality and quantification in whole-body PET/MR in patients with oncologic findings. PET data of fifty-one patients was reconstructed with 4, 3, 2 and 1 minute/bed time interval. Image quality parameters were analyzed. As expected, the image quality decreases with shorter PET image acquisition times. Besides the two key factors acquisition time and injected activity, the image quality is influenced by the BMI. A lower BMI results in better image quality parameters. 2 minutes acquisition time per bed is sufficient to provide accurate lesion detection.

Purpose

In contrast to positron emission tomography/computed tomography (PET/CT) integrated PET/magnetic resonance (PET/MR) inherently reduces the overall patient radiation dose by replacing the ionizing CT modality by nonionizing MR imaging [1]. Moreover, the simultaneous data acquisition in PET/MR provides latitude for further reduction of the radiation dose. The rather long PET acquisition times to match the prolonged MR examinations may allow for a decrease in applied radiotracer activity [2]. The study goal, thus, is to investigate how the simulated reduction of injected radiotracer affects PET image quality and quantification in whole-body PET/MR in patients with oncologic findings.

Methods

Fifty-one patients with different oncologic findings underwent a clinical whole-body 18F-Fluorodesoxyglucose PET/MR examination with 4 min acquisition time per bed position. Detailed patient information is listed in Tab.1. PET data in list-mode format was reconstructed with 4, 3, 2, and 1 min/bed time intervals for each patient. The 4-minute PET reconstructions served as reference standard. All whole-body PET data sets with four time intervals for each patient were reconstructed and analyzed regarding image quality, lesion detectability, PET quantification and standardized uptake values. Region-of-interests-analyses were generated in all detectable lesions, the liver, and in the mediastinal blood pool. Signal-to-noise ratio, contrast-to-noise ratio and image noise for each timeframe were calculated. An image quality score was defined (0 = non-diagnostic, 1 = poor, 2 = moderate, and 3 = good).

Results

A total of 91 lesions were detected in the 4-minute PET reconstructions. The same number of congruent lesions was also noticed in the 3 and 2 mpb reconstructed images. A total of 2 lesions in 2 patients was not detected in the 1 minute PET data reconstructions due to poor image quality. The results of image quality parameters are shown in Fig.1. The image noise in the liver increased from 18.2 % in the 4 min timeframe to 32.9 % in the 1 min timeframe. The image quality score decreased from 2.4 at 4 mpb to 1.8 at 1 mpb. The signal-to-noise ratio also declined with shorter timeframes from 6.4 (4 mpb) to 3.5 (1 mpb). The contrast-to-noise ratio decreased from 20.9 at 4 mpb to 16.5 at 1 mpb. SUVmean and SUVmax showed no significant changes between 4 and 1 mpb reconstructed timeframes. Fig.2 depicts the overview of SUVs and SNR of detected lesion in four different body regions. In the head/neck region the differences in SUVs and SNR between 4 mpb and shorter reconstructed acquisition times is highest. The impact of reduced PET acquisition times on image quality and quantification parameters is lower for the thorax and pelvic body regions. Table 2 shows, that a lower BMI, a short time interval between activity injection and PET/MR examination and a higher activity are optimal conditions to obtain high image quality regardless of different PET reconstruction times. Besides the two key factors acquisition time and injected activity, the image quality is influenced by the BMI (Figure 3). A lower BMI results in better image quality parameters. Fig.4 depicts a patient example of consistently high image quality scores even in shorter imaging duration. The lesion is detectable in all image reconstructions. Fig.5 shows a patient example of significantly decreasing image quality with shorter PET acquisition times. The marked lesion is not detectable in in the 1 mpb reconstruction.

Discussion and Conclusion

The study setup with retrospective reconstruction of PET data in list-mode format into different time-intervals allowed for a systematic and controlled evaluation of image quality and quantitative parameters. This allows eliminating numerous confounding factors that would have been present when multiple tracer injections and PET re-examinations would have been performed [3]. As expected, the image quality decreases with shorter PET image acquisition times. Besides the two key factors acquisition time and injected activity, the image quality is influenced by the BMI. A lower BMI results in better image quality parameters. Reconstruction of whole-body PET data with different time intervals has shown that 2 minutes instead of 4 minutes acquisition time per bed position is sufficient to provide accurate lesion detection, high image quality, SNR and CNR despite the trends to lower image quality with shorter PET acquisition times [4]. This result provides the foundation for the potential to reduce the PET acquisition time in fast 18F-FDG whole-body PET/MR imaging protocols [5, 6]. Another conclusion of this study is that radiotracer dose may be further reduced in 18F-FDG whole-body PET/MR in patients with oncologic findings while maintaining high image quality and accurate PET quantification.

Acknowledgements

No acknowledgement found.

References

1. Drzezga A, Souvatzoglou M, Eiber M, et al. First clinical experience with integrated whole-body PET/MR: comparison to PET/CT in patients with oncologic diagnoses. J. Nucl. Med. 2012; 53:845-855.

2. Oehmigen M, Ziegler S, Jakoby BW, Georgi JC, Paulus DH, Quick HH. Radiotracer dose reduction in integrated PET/MR: implications from national electrical manufacturers association phantom studies. J. Nucl. Med. 2014; 55:1361-1367.

3. Gatidis S, Würslin C, Seith F, et al. Towards tracer dose reduction in PET studies: Simulation of dose reduction by retrospective randomized undersampling of list-mode data. Hell. J. Nucl. Med. 2016; 19:15-18.

4. Hartung-Knemeyer V, Beiderwellen KJ, Buchbender C, et al. Optimizing positron emission tomography image acquisition protocols in integrated positron emission tomography/magnetic resonance imaging. Invest. Radiol. 2013; 48:290-294.

5. Grueneisen J, Schaarschmidt BM, Heubner M, et al. Implementation of FAST-PET/MRI for whole-body staging of female patients with recurrent pelvic malignancies: A comparison to PET/CT. Eur J Radiol. 2015 Nov;84(11):2097-102

6. Kirchner J, Sawicki LM, Suntharalingam S, et al. Whole-body staging of female patients with recurrent pelvic malignancies: Ultra-fast 18F-FDG PET/MRI compared to 18F-FDG PET/CT and CT. PLoS One. 2017 Feb 22;12(2):e0172553.

Figures

Fig. 1: Boxplots of the image quality score (IQS), the image noise (liver), the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) against PET acquisition time in minutes per bed (mpb).

Fig. 2: Overview over the average SUVmean, SUVmax and SNR for four different body regions (head/neck, thorax, abdomen, pelvis/upper legs). The relative difference in % is given for 3, 2 and 1 min acquisition time per bed (mpb) PET in comparison to 4 mpb serving as the reference standard.

Fig. 3: Correlation graphs of the image quality parameters image noise (liver) and SNR against the BMI of each patient compared for 1 to 4 minutes PET acquisition time. The image noise increases with shorter PET examination times, while SNR decreases.

Fig. 4: Example of consistently good image quality. MR images, PET/MR fusion images and PET images of patient #23 after 4, 3, 2 and 1 minute acquisition time per bed position in coronal and axial orientation with marked lesion. In addition an axial slice of the liver is shown. Note that the lesion is well detectable in all reconstructed time frames.

Fig. 5: Example of decreasing image quality. MR, fusion and PET images of patient #4 after 4, 3, 2 and 1 mpb. In addition an axial slice of the liver is shown. Note that the marked lymph node lesion is not well detectable in PET images with 1 min timeframe.

Tab. 1: Statistically relevant data of the patient population.

Tab. 2 Image quality parameters (mean ± SD) for 4, 3, 2 and 1 minutes acquisition time per PET bed position. The impact on SNR, CNR and image noise for different BMIs, applied activity, and time interval between tracer injection and PET examination are shown. Note that a lower BMI, short time interval between activity injection and PET/MR examination and a higher activity are optimal conditions to obtain high image quality regardless of different PET reconstruction times.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
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