Ralf Ladebeck

Performance Measurements of the Siemens mMR Integrated Whole-Body PET/MR Scanner

The recently released Biograph mMR is the first commercially available integrated whole-body PET/MR scanner. There are considerable advantages to integrating both modalities in a single scanner that enables truly simultaneous acquisition. However, there are also concerns about the possible degradation of both PET and MR performance in an integrated system. This paper evaluates the performance of the Biograph mMR during independent and simultaneous acquisition of PET and morphologic MR data. Methods: The NEMA NU 2-2007 protocol was followed for studying the PET performance. The following measurements were performed: spatial resolution; scatter fraction, count losses, and randoms; sensitivity; accuracy of the correction for count losses and randoms; and image quality. The quality control manual of the American College of Radiology was followed for studying the MR performance. The following measurements were performed: geometric accuracy, spatial resolution, low-contrast detectability, signal-to-noise ratio, static field (B0) homogeneity, radiofrequency field (B1) homogeneity, and radiofrequency noise. Results: An average spatial resolution of 4.3 mm in full width at half maximum was measured at 1 cm offset from the center of the field of view. The system sensitivity was 15.0 kcps/MBq along the center of the scanner. The scatter fraction was 37.9%, and the peak noise-equivalent count rate was 184 kcps at 23.1 kBq/mL. The maximum absolute value of the relative count rate error due to dead-time losses and randoms was 5.5%. The average residual error in scatter and attenuation correction was 12.1%. All MR parameters were within the tolerances defined by the American College of Radiology. B0 inhomogeneities below 1 ppm were measured in a 120-mm radius. B1 homogeneity and signal-to-noise ratio were equivalent to those of a standard MR scanner. No radiofrequency interference was detected. Conclusion: These results compare favorably with other state-of-the-art PET/CT and PET/MR scanners, indicating that the integration of the PET detectors in the MR scanner and their operation within the magnetic field do not have a perceptible impact on the overall performance. The MR subsystem performs essentially like a standalone system. However, further work is necessary to evaluate the more advanced MR applications, such as functional imaging and spectroscopy.

Simultaneous MR/PET Imaging of the Human Brain: Feasibility Study

The purpose of this study was to apply a magnetic resonance (MR) imaging-compatible positron emission tomographic (PET) detector technology for simultaneous MR/PET imaging of the human brain and skull base. The PET detector ring consists of lutetium oxyorthosilicate (LSO) scintillation crystals in combination with avalanche photodiodes (APDs) mounted in a clinical 3-T MR imager with use of the birdcage transmit/receive head coil. Following phantom studies, two patients were simultaneously examined by using fluorine 18 fluorodeoxyglucose (FDG) PET and MR imaging and spectroscopy. MR/PET data enabled accurate coregistration of morphologic and multifunctional information. Simultaneous MR/PET imaging is feasible in humans, opening up new possibilities for the emerging field of molecular imaging.

Evaluation of the attenuation properties of MR equipment for its use in a whole-body PET/MR scanner

The combination of magnetic resonance imaging (MR) and positron emission tomography (PET) scanners can provide a powerful tool for clinical diagnosis and investigation. Among the challenges of developing a combined scanner, obtaining attenuation maps for PET reconstruction is of critical importance. This requires accounting for the presence of MR hardware in the field of view. The attenuation introduced by this hardware cannot be obtained from MR data. We propose the creation of attenuation models of MR hardware, to be registered into the MR-based attenuation map prior to PET reconstruction. Two steps were followed to assess the viability of this method. First, transmission and emission measurements were performed on MR components (RF coils and medical probes). The severity of the artifacts in the reconstructed PET images was evaluated. Secondly, a high-exposure computed tomography (CT) scan was used to obtain a model of a head coil. This model was registered into the attenuation map of PET/CT scans of a uniform phantom fitted with the coil. The resulting PET images were compared to the PET/CT reconstruction in the absence of coils. The artifacts introduced by misregistration of the model were studied. The transmission scans revealed 17% count loss due to the presence of head and neck coils in the field of view. Important sources of attenuation were found in the lock, signal cables and connectors. However, the worst source of attenuation was the casing between both coils. None of the measured medical probes introduced a significant amount of attenuation. Concerning the attenuation model of the head coil, reconstructed PET images with model-based correction were comparable to the reference PET/CT reconstruction. However, inaccuracies greater than 1-2 mm in the axial positioning of the model led to important artifacts. In conclusion, the results show that model-based attenuation correction is possible. Using a high-exposure scan to create an attenuation model of the coils has been proved feasible. However, adequate registration of the model is mandatory.

Field of view extension and truncation correction for MR-based human attenuation correction in simultaneous MR/PET imaging

PURPOSE In quantitative PET imaging, it is critical to accurately measure and compensate for the attenuation of the photons absorbed in the tissue. While in PET/CT the linear attenuation coefficients can be easily determined from a low-dose CT-based transmission scan, in whole-body MR/PET the computation of the linear attenuation coefficients is based on the MR data. However, a constraint of the MR-based attenuation correction (AC) is the MR-inherent field-of-view (FoV) limitation due to static magnetic field (B0) inhomogeneities and gradient nonlinearities. Therefore, the MR-based human AC map may be truncated or geometrically distorted toward the edges of the FoV and, consequently, the PET reconstruction with MR-based AC may be biased. This is especially of impact laterally where the patient arms rest beside the body and are not fully considered. METHODS A method is proposed to extend the MR FoV by determining an optimal readout gradient field which locally compensates B0 inhomogeneities and gradient nonlinearities. This technique was used to reduce truncation in AC maps of 12 patients, and the impact on the PET quantification was analyzed and compared to truncated data without applying the FoV extension and additionally to an established approach of PET-based FoV extension. RESULTS The truncation artifacts in the MR-based AC maps were successfully reduced in all patients, and the mean body volume was thereby increased by 5.4%. In some cases large patient-dependent changes in SUV of up to 30% were observed in individual lesions when compared to the standard truncated attenuation map. CONCLUSIONS The proposed technique successfully extends the MR FoV in MR-based attenuation correction and shows an improvement of PET quantification in whole-body MR/PET hybrid imaging. In comparison to the PET-based completion of the truncated body contour, the proposed method is also applicable to specialized PET tracers with little uptake in the arms and might reduce the computation time by obviating the need for iterative calculations of the PET emission data beyond those required for reconstructing images.

MR-based field-of-view extension in MR/PET: B0 homogenization using gradient enhancement (HUGE)

In whole‐body MR/PET, the human attenuation correction can be based on the MR data. However, an MR‐based field‐of‐view (FoV) is limited due to physical restrictions such as B0 inhomogeneities and gradient nonlinearities. Therefore, for large patients, the MR image and the attenuation map might be truncated and the attenuation correction might be biased. The aim of this work is to explore extending the MR FoV through B0 homogenization using gradient enhancement in which an optimal readout gradient field is determined to locally compensate B0 inhomogeneities and gradient nonlinearities. A spin‐echo‐based sequence was developed that computes an optimal gradient for certain regions of interest, for example, the patients arms. A significant distortion reduction was achieved outside the normal MR‐based FoV. This FoV extension was achieved without any hardware modifications. In‐plane distortions in a transaxially extended FoV of up to 600 mm were analyzed in phantom studies. In vivo measurements of the patients arms lying outside the normal specified FoV were compared with and without the use of B0 homogenization using gradient enhancement. In summary, we designed a sequence that provides data for reducing the image distortions due to B0 inhomogeneities and gradient nonlinearities and used the data to extend the MR FoV. Magn Reson Med, 70:1047–1057, 2013.

Pulse sequence for operating a nuclear magnetic resonance tomography apparatus for producing images with different T2 contrast

A pulse sequence is disclosed for operating a nuclear magnetic resonance tomography apparatus for producing images having different T2 contrast. A signal for the gradient-echo imaging is first acquired after excitation of the nuclei subsequently, the magnetization, which is already prepared for imaging, is refocused by a 180° radio-frequency pulse, and a second echo is measured in the presence of a readout gradient. Independent images having different T2 contrast can be reconstructed with the gradient echo and with the spin echo. It is thus possible to simultaneously measure a gradient echo image having a significantly lower T2 contrast during the measuring time for a spin echo image.

Nuclear magnetic resonance tomography apparatus operable with a pulse sequence according to the echo planar method

In nuclear magnetic resonance tomography apparatus operable with a pulse sequence according to the echo-planar method, only a part of the k-space is scanned in the phase-coding direction per data acquisition, i.e., per radio-frequency excitation pulse. A phase-coding gradient is used such that regions of the k-space which are interleaved relative to each other are scanned in successive data acquisitions in the phase-coding direction. The number of echoes employed for the raw data matrix, and thus the resolution in phase-coding direction, or the length of the individual pulses of the read-out gradient, and thus the resolution in read-out direction, can thereby be enhanced.

Monte Carlo simulations of the count rate performance of a clinical whole-body MR/PET scanner.

The combination of MR and PET scanners can provide a powerful tool for clinical diagnosis and investigation. Among the existing approaches, the most challenging is that of complete hardware integration of both scanners. Such an integrated tomograph would allow simultaneous acquisition of both modalities, which could help solve issues such as cardiac and respiratory motion. Full integration imposes restrictions on the design of the PET part, such as detector configuration and maximum ring diameter. Furthermore, MR components surrounding the PET detector ring may cause gamma ray interactions, thus affecting PET performance. The purpose of this article is to assess the performance of a hypothetical whole-body integrated MR/PET scanner using Monte Carlo simulation techniques and compare it to state-of-the-art PET/CT devices used in clinical routine. The Monte Carlo simulation toolkit used for this study is the GEANT4 application for emission tomography. A hypothetical whole-body MR/PET tomograph fully integrated at hardware level and positioned between gradient and local coils of the MR scanner has been modeled. The NEMA 2-2001 protocol has been used to configure the simulations in order to measure sensitivity, scatter fraction, count losses, and random detections. Global sensitivity values as a function of the lower-level discriminator (LLD) energy are provided for time resolutions of 5 and 2.25 ns. In addition, the scatter fraction of the system is studied as a function of the LLD for energy resolution values of 10%, 15%, and 20%. Finally, true, scatter, random, and noise equivalent count rate curves as a function of activity concentration are given for dead-time values of 136, 432, and 1150 ns and for time resolution values of 2.25 and 5 ns. The influence on the count rate performance of the integrated PET scanner of the new geometry and interfering MR elements has been measured. The results show that the interference of the MR components has a much lower impact than the reduction in the detector ring diameter. Due to the larger solid angle coverage, the sensitivity is higher than that measured for a clinical PET/CT system (6200-10 900 cps/MBq at the center of the scanner) but not enough to compensate the degradation of the noise equivalent count rate due to increased scatter detection. The simulations prove the viability of an integrated MR/PET system and suggest that priority has to be given to either the improvement of the temporal resolution or the correction of triple coincidences if competitive performance is to be achieved.

Apparatus for the identification of nuclear magnetic spectra from spatially selectable regions of an examination subject

A method for identifying nuclear magnetic spectra from spatially selectable regions of an examination subject obtains an optimally unattenuated FID signal by the steps of cancelling the magnetization present due to the fundamental magnetic field in volume regions of the examination subject which are not to be evaluated, subjecting the examination subject to a non-selective RF read-out pulse which deflects at least the nuclear spins in the examination regions to be selected, and reading out the resulting FID signal following the non-selective RF read-out pulse. The selected examination region is determined by the transmission/reception characteristics of the coil which receives the FID signal minus the selectively saturated volume region.