Standardized uptake values for [18F] FDG in normal organ tissues: Comparison of whole-body PET/CT and PET/MRI

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Abstract

Purpose

To compare maximum and mean standardized uptake values (SUVmax/mean) of normal organ tissues derived from [18F]-fluoro-desoxyglucose (FDG) positron emission tomography/magnetic resonance imaging (PET/MRI) using MR attenuation correction (MRAC) (DIXON-based 4-segment μ-map) with [18F]-FDG positron emission tomography/computed tomography (PET/CT) using CT-based attenuation correction (CTAC).

Methods and materials

In 25 oncologic patients (15 men, 10 women; age 57 ± 13 years) after routine whole-body FDG-PET/CT (60 min after injection of 290 ± 40 MBq [18F]-FDG) a whole-body PET/MRI was performed (Magnetom Biograph mMR™, Siemens Healthcare, Erlangen, Germany). Volumes of interest of 1.0 cm3 were drawn in 7 physiological organ sites in MRAC-PET and the corresponding CTAC-PET images manually. Spearman correlation coefficients were calculated to compare MRAC- and CTAC based SUV values; Wilcoxon-Matched-Pairs signed ranks test was performed to test for potential differences.

Results

The mean delay between FDG-PET/CT and PET/MRI was 92 ± 18 min. Excellent correlations of SUV values were found for the heart muscle (SUVmax/mean: R = 0.97/0.97); reasonably good correlations were found for the liver (R = 0.65/0.72), bone marrow (R = 0.42/0.41) and the SUVmax of the psoas muscle (R = 0.41). For subcutaneous fat, the correlation coefficient was 0.66 for SUVmean (p < 0.05). Correlations between MRAC and CTAC were non-significant for SUVmean of the psoas muscle, SUVmax of subcutaneous fat, SUVmax and SUVmean of the lungs, SUVmax and SUVmean of the blood-pool. The median SUVmax and SUVmean in MRAC-PET were lower than the respective CTAC values in all organs (p < 0.05) but heart (SUVmax) and the bone marrow (SUVmean).

Conclusion

In conclusion, in oncologic patients examined with PET/CT and PET/MRI SUVmax and SUVmean values generally correlate well in normal organ tissues, except the lung, subcutaneous fat and the blood pool. SUVmax and SUVmean derived from PET/MRI can be used reliably in clinical routine.

Introduction

Recently introduced whole-body integrated positron emission tomography/magnetic resonance imaging (PET/MRI) scanners combine PET and MRI into one imaging modality which enables truly simultaneous acquisition and highly accurate spatial co-registration of PET and MRI data sets [1], [2], [3], [4]. Besides the lack of ionizing radiation as in computed tomography (CT), MRI in combination with PET is expected to provide a new quality in functional cancer imaging [1], [4] mainly due to the combination of high soft-tissue contrast and functional MR with PET. Recently published clinical studies using whole-body PET/MRI in oncologic patients confirmed its feasibility for whole-body cancer staging and reported an image quality comparable to that of PET/CT for lesion detection [5], [6], [7].

The main obstacle to the introduction of integrated PET/MRI was the development of PET detectors capable to operate in the presence of high magnetic fields [8]. The combination of lutetium oxyorthosilicate (LSO) crystals and external avalanche photodiode detectors (APD) fulfills this basic requirement [9]. A first comparative evaluation revealed that PET/MRI systems equal the performance of stand-alone MRI and PET scanners [10].

MR-imaging combined with PET requires accurate correction of detected gamma rays for the attenuation effect caused by different body tissues. In combined PET/CT, CT data provide useful information on tissue density, which is rescaled to the PET emission energy and used for PET attenuation correction (AC) [11]. The integration of PET and MRI into one imaging modality necessitates an MR-based attenuation correction (MRAC). MRAC relies on either automated pattern recognition of anatomical structures, discriminated by differences in pixel gray-values on MR images and fitting of a Pseudo-CT derived by CT-template database (Atlas-based MRAC method) or tissue classification, e.g. by using a Dixon- based in- and out-of-phase separation of fat and water (Segmentation method) [12], [13]. However, the quality of both MRAC methods depends on a correct visualization of the individual anatomy in the MRI source data. The major difficulty of MRAC lies in the fact that the MR-signal is not related to radiodensity of the examined tissue and therefore cannot directly be used for attenuation correction. Hence, the discrepancy in MRAC and CTAC may result in differences in quantitative SUV measurements, potentially leading to a misinterpretation of the PET-MR data. A significant correlation of SUV-values of normal organ tissues assessed by a simultaneous PET-MRI and PET-CT has not been proved in clinical routine yet.

Therefore, the aim of the present study was to quantitatively compare the tracer uptake, as reflected by SUVmax/mean of normal organ tissues derived from FDG-PET/MRI using MRAC (DIXON-based 4-segment μ-map) with FDG-PET/CT using CTAC [13].

Section snippets

Patients

In 25 consecutive oncological patients (15 men, 10 women; age 57 ± 13 years) without history of systemic therapy within 6 month prior to imaging (Table 1), after routine clinical FDG-PET/CT (60 min after injection of 290 ± 40 MBq [18F]-FDG), whole-body FDG-PET/MRI was performed without additional [18F]-FDG injection. Written informed consent was obtained from all participants. This study was performed in accordance with the regulations of the local institutional review board.

PET/CT imaging

Whole-body (WB)

Results

FDG-PET/CT and FDG-PET/MRI acquisitions were completed successfully in all 25 patients. The mean delay between FDG-PET/CT and FDG-PET/MRI was 92 ± 18 min.

Left ventricular myocardium: The SUVmax of the left ventricular myocardium from FDG-PET/MRI and FDG-PET/CT correlated excellent (R = 0.97; p < 0.0001) (Fig. 1A). Also, the SUVmean of the myocardium for FDG-PET/MRI and for FDG-PET/CT had an excellent correlation (R = 0.97, p < 0.0001).

Liver: Good correlations were found for the liver (SUVmax/SUVmean: R = 

Discussion

In contrast to PET/CT SUV-measurements, SUV-measurements of WB PET/MRI using MRAC have not been validated in clinical routine. However, for oncologic purposes such as staging and re-staging and other indications a validated SUV from PET/MRI is essential. Even small SUV changes can be crucial to assess therapy response in follow-up examinations [15]. Therefore, we compared SUVmax and SUVmean of normal organ tissues derived from FDG-PET/MRI using MRAC (DIXON-based 4-segment μ-map) with FDG-PET/CT

Conflict of interest

All listed authors essentially contributed to the design, data acquisition, data interpretation, statistics, manuscript writing and drafting of the presented study. All authors approved the final manuscript version for publication. This article is not under consideration for publication elsewhere.

All authors declare that there is no conflict of interest.

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