1372 IEEE TRANSACTIONS ON MEDICAL IMAGING, VOL. 31, NO. 7, JULY 2012 Analysis and Correction of Count Rate Reduction During Simultaneous MR-PET Measurements With the BrainPET Scanner Christoph Weirich*, Daniel Brenner, Jürgen Scheins, Étienne Besançon, Lutz Tellmann, Hans Herzog, Member, IEEE, and N. Jon Shah Abstract—In hybrid magnetic resonance-positron emission to- mography (MR-PET) studies with the Siemens 3T MR-BrainPET scanner an instantaneous reduction of the PET sensitivity was observed during execution of certain MR sequences. This inter- ference was investigated in detail with custom-made as well as standard clinical MR sequences. The radio-frequency pulses, the switched gradient fields and the constant magnetic field were examined as the relevant parameters of the magnetic resonance imaging (MRI) system as well as the air temperature within the PET detectors. Our investigation comprised the analysis of the analog PET signals, the total count rates, the geometric distri- bution of the count rate reduction within the BrainPET detector as well as reconstructed images. The fast switching magnetic field gradients were identified to distort the analog PET detector signals. The measured count rate reduction was found to be less than 3%, but only up to 2% in the case of echo planar imaging sequences, as applied in functional MRI. For clinical sequences routinely used in hybrid MR-BrainPET measurements, a correc- tion method has been designed, implemented, and evaluated . Index Terms—Correction, count rate reduction, positron emis- sion tomography (PET), magnetic resonance imaging (MRI), mu- tual interference. I. INTRODUCTION T HE SUCCESS of combined imaging modalities in med- ical diagnostics such as single-photon emission computed tomography/computed tomography (SPECT/CT) and positron emission tomography/CT (PET/CT) has shown the advantage of complementary imaging of anatomy and function. The emerging technology of magnetic resonance-PET (MR-PET) combines the high spatial resolution and the soft tissue contrast of MR images with the superior sensitivity of PET for metabolic imaging. Besides the sequential arrangement of PET and MRI scanners [1], [2], integrated hybrid MR-PET scanners have been developed for preclinical research [3]–[10] and human studies [11]. After successful tests of a 3T MR-BrainPET pro- totype [12], [13] developed by Siemens Healthcare (Erlangen, Manuscript received November 23, 2011; revised January 25, 2012; accepted February 12, 2012. Date of publication February 24, 2012; date of current ver- sion June 26, 2012. C. Weirich and D. Brenner have equally contributed to this work. Asterisk indicates corresponding author. *C. Weirich is with the Institute of Neuroscience and Medicine, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany (e-mail: c.weirich@fz-juelich.de). D. Brenner, J. Scheins, É. Besançon, L. Tellmann, H. Herzog, and N. J. Shah are with the Institute of Neuroscience and Medicine, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany. Digital Object Identifier 10.1109/TMI.2012.2188903 Germany) for simultaneous MR-PET human brain studies, the first integrated hybrid MR-PET scanners for whole-body imaging have been recently installed [14]. Simultaneous measurements of PET and MRI lead to potentially shorter measurement times and to exact spatial and temporal co-regis- tration of the acquired images. To allow for integrated, hybrid MR-PET imaging, different technical issues and mutual sources of interference have to be considered [2], [10], [15]. The PET detector is exposed to the static and time varying magnetic field of the MR system. Thus, the PET detector electronics is based on nonmagnetosensitive avalanche photo diodes (APDs) and shielded to avoid interference with the MR signal receiver electronics. Furthermore, the PET detector may be influenced by temperature changes and vibrations produced by the MR gradients. All these challenges have been overcome such that the first human MR-PET scanners produced satisfactory results [16], [17]. In further detailed analysis with our hybrid MR-BrainPET scanner, it was found that the MR measurement influences the PET acquisition [18]. While the PET data were not remarkably affected by a standard T1-weighted sequence, the PET count rate showed an instantaneous, reproducible and reversible reduction, when MR sequences with fast switching gradients such as echo planar imaging (EPI) were applied. Similar observations were reported by Catana et al. [19]. Such influences might affect MR-PET studies in which functional MRI (fMRI) is performed simultaneously with, for example, neuroreceptor PET [20], [21]. In this paper, we report detailed analyses of the relationship between the PET count rate and gradients applied with custom-made test MR protocols as well as a set of routinely used clinical protocols, such as MP-RAGE and EPI sequences, for the 3T MR-BrainPET scanner. Further, we provide a correction for the aforementioned effect. II. MATERIAL AND METHODS A. 3T MR-PET Hybrid Scanner The MR-PET scanner (Fig. 1) consists of a BrainPET de- tector and an MRI scanner which is essentially the commer- cially available Siemens 3T MAGNETOM TIM Trio (Siemens Healthcare) with minor modifications, such as a vertically fixed patient bed and an adapted head coil with low attenuation for 511 keV photons. The MRI scanner is equipped with a gra- dient system capable of producing a combined gradient field vector with a modulus of 40 mT/m at a maximum combined 0278-0062/$31.00 © 2012 IEEE