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This paper presents the approach of extending a classical generic event tree (ET) of a Level 2 Probabilistic Safety Analysis to the results of a probabilistic dynamic safety analysis. The example of creep-induced steam generator tube rupture has been chosen. The results of an Analysis of Thermal Hydraulics of Leaks and Transients with Core Degradation (ATHLET-CD)/Monte Carlo Dynamic Event Tree (MCDET) simulation analyzing the failure of reactor coolant system components by creeping in a scenario of a high-pressure core melt accident in a generic pressurized water reactor (PWR) have been implemented in the ET. From the results of these simulations, a set of parameters has been extracted and integrated into the ET along with their probability distributions. The effect of these parameters on both the progression of the severe accident sequence under consideration and the release categories of a generic German PWR plant, respectively, is discussed in this paper. [ABSTRACT FROM AUTHOR]

Purpose: A 32-channel parallel transmit (pTx) add-on for 7 Tesla whole-body imaging is presented. First results are shown for phantom and in-vivo imaging. Methods: The add-on system consists of a large number of hardware components, including modulators, amplifiers, SAR supervision, peripheral devices, a control computer, and an integrated 32-channel transmit/receive body array. B1+ maps in a phantom as well as B1+ maps and structural images in large volunteers are acquired to demonstrate the functionality of the system. EM simulations are used to ensure safe operation. Results: Good agreement between simulation and experiment is shown. Phantom and in-vivo acquisitions show a field of view of up to 50 cm in z-direction. Selective excitation with 100 kHz sampling rate is possible. The add-on system does not affect the quality of the original single-channel system. Conclusion: The presented 32-channel parallel transmit system shows promising performance for ultra-high field whole-body imaging. [ABSTRACT FROM AUTHOR]

Purpose: Current methods for mitigation of transmit field B 1 + inhomogeneities at ultrahigh field (UHF) MRI by multi‐channel radiofrequency (RF) shimming rely on accurate B 1 + mapping. This can be time consuming when many RF channels have to be mapped for in vivo body MRI, where the B1 maps should ideally be acquired within a single breath‐hold. Therefore, a new B 1 + mapping technique (B1TIAMO) is proposed. Methods: The performance of this technique is validated against an established method (DREAM) in phantom measurements for a cylindrical head phantom with an 8‐channel transmit/receive (Tx/Rx) array. Furthermore, measurements for a 32‐channel Tx/Rx remote array are conducted in a large body phantom and the | B 1 + | map reliability is validated against simulations of the transmit RF field distribution. Finally, in vivo results of this new mapping technique for human abdomen are presented. Results: For the head phantom (8‐channel Tx/Rx coil), the single | B 1 + | comparison between B1TIAMO, the direct DREAM measurements, and simulation data showed good agreement with 10–19% difference. For the large body phantom (32‐channel Tx/Rx coil), B1TIAMO matched the RF field simulations well. Conclusion: The results demonstrate the potential to acquire 32 accurate single‐channel B 1 + maps for large field‐of‐view body imaging within only a single breath‐hold of 16 s at 7T UHF MRI. Magn Reson Med 79:2652–2664, 2018. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

Purpose: Conventional saturation pulses cannot be used for 7 Tesla ultra-high-resolution time-of-flight magnetic resonance angiography (TOF MRA) due to specific absorption rate (SAR) limitations. We overcome these limitations by utilizing low flip angle, variable rate selective excitation (VERSE) algorithm saturation pulses. Material and Methods: Twenty-five neurosurgical patients (male n = 8, female n = 17; average age 49.64 years; range 26–70 years) with different intracranial vascular pathologies were enrolled in this trial. All patients were examined with a 7 Tesla (Magnetom 7 T, Siemens) whole body scanner system utilizing a dedicated 32-channel head coil. For venous saturation pulses a 35° flip angle was applied. Two neuroradiologists evaluated the delineation of arterial vessels in the Circle of Willis, delineation of vascular pathologies, presence of artifacts, vessel-tissue contrast and overall image quality of TOF MRA scans in consensus on a five-point scale. Normalized signal intensities in the confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter were measured and vessel-tissue contrasts were calculated. Results: Ratings for the majority of patients ranged between good and excellent for most of the evaluated features. Venous saturation was sufficient for all cases with minor artifacts in arteriovenous malformations and arteriovenous fistulas. Quantitative signal intensity measurements showed high vessel-tissue contrast for confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter. Conclusion: The use of novel low flip angle VERSE algorithm pulses for saturation of venous vessels can overcome SAR limitations in 7 Tesla ultra-high-resolution TOF MRA. Our protocol is suitable for clinical application with excellent image quality for delineation of various intracranial vascular pathologies. [ABSTRACT FROM AUTHOR]

In magnetic resonance imaging, there has been a constant drive to higher static magnetic field strengths ( B0) to achieve a higher signal-to-noise ratio and new or enhanced contrasts. In today's high-field systems, severe problems regarding the homogeneity of the transmission field are encountered. Recently, an acquisition scheme called Time-Interleaved Acquisition of Modes has been proposed to tackle the inhomogeneity problems in high-field magnetic resonance imaging. The basic premise is to excite two (or more) different B [ABSTRACT FROM AUTHOR]

Purpose: Current methods for mitigation of transmit field B1+ inhomogeneities at ultrahigh field (UHF) MRI by multi-channel radiofrequency (RF) shimming rely on accurate B1+ mapping. This can be time consuming when many RF channels have to be mapped for in vivo body MRI, where the B 1 maps should ideally be acquired within a single breath-hold. Therefore, a new B1+ mapping technique (B1TIAMO) is proposed., Methods: The performance of this technique is validated against an established method (DREAM) in phantom measurements for a cylindrical head phantom with an 8-channel transmit/receive (Tx/Rx) array. Furthermore, measurements for a 32-channel Tx/Rx remote array are conducted in a large body phantom and the |B1+| map reliability is validated against simulations of the transmit RF field distribution. Finally, in vivo results of this new mapping technique for human abdomen are presented., Results: For the head phantom (8-channel Tx/Rx coil), the single |B1+| comparison between B 1 TIAMO, the direct DREAM measurements, and simulation data showed good agreement with 10-19% difference. For the large body phantom (32-channel Tx/Rx coil), B1TIAMO matched the RF field simulations well., Conclusion: The results demonstrate the potential to acquire 32 accurate single-channel B1+ maps for large field-of-view body imaging within only a single breath-hold of 16 s at 7T UHF MRI. Magn Reson Med 79:2652-2664, 2018. © 2017 International Society for Magnetic Resonance in Medicine., (© 2017 International Society for Magnetic Resonance in Medicine.)

When performing non-enhanced time-of-flight MR angiography of the lower extremity arteries at 7 T with cardiac triggering, the acquisition time is a crucial consideration. Therefore, in previous studies, saturation RF pulses were applied only every second TR. In the axial source images a slight artifact with an appearance similar to aliasing could be observed. The purpose of this study was to investigate the origin of this artifact. The reason for the artifact is supposed to be related to the two effective TRs during acquisition caused by the sparsely applied saturation RF pulse. Several sequence variants were simulated and implemented within the sequence source code to examine this hypothesis. An adaptation of the excitation flip angles for each TR as well as a correction factor for the k-space data was calculated. Additionally, a different ordering of the k-space data during acquisition was implemented as well as the combination of the latter with the k-space correction factor. The observations from the simulations were verified using both a static and a flow phantom and, finally, in a healthy volunteer using the same measurement setup as in previous volunteer and patient studies. Of all implemented techniques, only the reordering of the k-space was capable of suppressing the artifact almost completely at the cost of creating a ringing artifact. The phantom measurements showed the same results as the simulations and could thus confirm the hypothesis regarding the origin of the artifact. This was additionally verified in the healthy volunteer. The origin of the artifact could be confirmed to be the periodic signal variation caused by two effective TRs during acquisition.