The electric conductivity can potentially be used as an additional diagnostic parameter, e.g., in tumor diagnosis. Moreover, the electric conductivity, in connection with the electric field, can be used to estimate the local SAR distribution during MR measurements. In this study, a new approach, called electric properties tomography (EPT) is presented. It derives the patient's electric conductivity, along with the corresponding electric fields, from the spatial sensitivity distributions of the applied RF coils, which are measured via MRI. Corresponding numerical simulations and initial experiments on a standard clinical MRI system underline the principal feasibility of EPT to determine the electric conductivity and the local SAR. In contrast to previous methods to measure the patient's electric properties, EPT does not apply externally mounted electrodes, currents, or RF probes, thus enhancing the practicality of the approach. Furthermore, in contrast to previous methods, EPT circumvents the solution of an inverse problem, which might lead to significantly higher spatial image resolution.

The k-t broad-use linear acquisition speed-up technique (BLAST) has become widespread for reducing image acquisition time in dynamic MRI. In its basic form k-t BLAST speeds up the data acquisition by undersampling k-space over time (referred to as k-t space). The resulting aliasing is resolved in the Fourier reciprocal x-f space (x ‫؍‬ spatial position, f ‫؍‬ temporal frequency) using an adaptive filter derived from a low-resolution estimate of the signal covariance. However, this filtering process tends to increase the reconstruction error or lower the achievable acceleration factor. This is problematic in applications exhibiting a broad range of temporal frequencies such as free-breathing myocardial perfusion imaging. We show that temporal basis functions calculated by subjecting the training data to principal component analysis (PCA) can be used to constrain the reconstruction such that the temporal resolution is improved. The presented method is called k-t PCA. Magn Reson Med 62:706 -716, 2009.

A novel multislice B₁-mapping method dubbed dual refocusing echo acquisition mode is proposed, able to cover the whole transmit coil volume in only one second, which is more than an order of magnitude faster than established approaches. The dual refocusing echo acquisition mode technique employs a stimulated echo acquisition mode (STEAM) preparation sequence followed by a tailored single-shot gradient echo sequence, measuring simultaneously the stimulated echo and the free induction decay as gradient-recalled echoes, and determining the actual flip angle of the STEAM preparation radiofrequency pulses from the ratio of the two measured signals. Due to an elaborated timing scheme, the method is insensitive against susceptibility/chemical shift effects and can deliver a B₀ phase map and a transceive phase map for free. The approach has only a weak T₁ and T₂ dependence and moreover, causes only a low specific absorption rate (SAR) burden. The accuracy of the method with respect to systematic and statistical errors is investigated both, theoretically and in experiments on phantoms. In addition, the performance of the approach is demonstrated in vivo in B₁-mapping and radiofrequency shimming experiments on the abdomen, the legs, and the head on an eight-channel parallel transmit 3 T MRI system.

Respiratory motion is a major source of artifacts in cardiac magnetic resonance imaging (MRI). Free-breathing techniques with pencil-beam navigators efficiently suppress respiratory motion and minimize the need for patient cooperation. However, the correlation between the measured navigator position and the actual position of the heart may be adversely affected by hysteretic effects, navigator position, and temporal delays between the navigators and the image acquisition. In addition, irregular breathing patterns during navigator-gated scanning may result in low scan efficiency and prolonged scan time. The purpose of this study was to develop and implement a selfnavigated, free-breathing, whole-heart 3D coronary MRI technique that would overcome these shortcomings and improve the ease-of-use of coronary MRI. A signal synchronous with respiration was extracted directly from the echoes acquired for imaging, and the motion information was used for retrospective, rigid-body, through-plane motion correction. The images obtained from the self-navigated reconstruction were compared with the results from conventional, prospective, pencilbeam navigator tracking. Image quality was improved in phantom studies using self-navigation, while equivalent results were Key words: 3D radial; motion detection; self-navigation; wholeheart MRI; coronary angiography Free-breathing, three-dimensional (3D) cardiac MRI has received considerable attention because it enables highresolution imaging and, in contrast to breath-held imaging, does not require patient cooperation. For data acquisition during free breathing with the use of navigator technology (1), an end-expiratory gating window is defined prior to image acquisition, and data that were acquired outside the predefined respiratory gating window are rejected and remeasured in the next RR interval.However, fluctuations of the breathing pattern during prolonged scans may adversely affect the scan efficiency and prolong the scan time. Furthermore, the correlation between the navigator information and the actual position of the heart may be compromised by temporal delays (2) between the navigator signal and the actual image acquisition, which may arise from magnetization preparation pulses and the startup cycles necessary to approach the steady state. Also, the 2D-selective navigator pulses ("pencil beams") are preferably localized at the dome of the right hemidiaphragm to avoid contamination of the magnetization in the region of interest (ROI). While a stable correlation between the superior-inferior (SI) displacement of the diaphragm and the heart over a large number of subjects has been reported (3), hysteretic effects may occur (4), such as those induced by the difference in relative displacement between the diaphragm and the heart that reduce the precision of the motion estimation. Finally, the localization of the navigator beam requires special attention (5), which may decrease the utility and reproducibility of coronary MRI.To address these shortcomings, a self-navigated, 3D radi...

The specific absorption rate (SAR) is a limiting factor in high-field MR. SAR estimation is typically performed by numerical simulations using generic human body models. However, SAR concepts for single-channel radiofrequency transmission cannot be directly applied to multichannel systems. In this study, a novel and comprehensive SAR prediction concept for parallel radiofrequency transmission MRI is presented, based on precalculated magnetic and electric fields obtained from electromagnetic simulations of numerical body models. The application of so-called Q-matrices and further computational optimizations allow for a real-time estimation of the SAR prior to scanning. This SAR estimation method was fully integrated into an eight-channel whole body MRI system, and it facilitated the selection of different body models and body positions. Experimental validation of the global SAR in phantoms demonstrated a good qualitative and quantitative agreement with the predictions. An initial in vivo validation showed good qualitative agreement between simulated and measured amplitude of (excitation) radiofrequency field. The feasibility and practicability of this SAR prediction concept was shown paving the way for safe parallel radiofrequency transmission in high-field MR.

The specific absorption rate (SAR) is a limiting constraint in sequence design for high-field MRI. SAR estimation is typically performed by numerical simulations using generic human body models. This entails an intrinsic uncertainty in present SAR prediction. This study first investigates the required detail of human body models in terms of spatial resolution and the number of soft tissue classes required, based on finite-differences timedomain simulations of a 3 T body coil. The numerical results indicate that a resolution of 5 mm is sufficient for local SAR estimation. Moreover, a differentiation between fatty tissues, water-rich tissues, and the lungs was found to be essential to represent eddy current paths inside the human body. This study then proposes a novel approach for generating individualized body models from whole-body water-fat-separated MR data and applies it to volunteers. The SAR hotspots consistently occurred in the arms due to proximity to the body coil as well as

The respiratory motion of several anatomic regions (right hemidiaphragm, left ventricle of the heart, chest wall, abdominal wall) was investigated during free breathing in 10 healthy volunteers by using multinavigator technology and real-time magnetic resonance (MR) imaging. The respiratory motion shows hysteretic effects, which are strongly subject dependent and might have some effect on the quality of cardiac MR images.

AFI (actual flip angle imaging) represents an interesting approach to map the B 1 transmit fields by measuring the spatial variations of the effective flip angle. However, the accuracy of the technique relies on the adequate spoiling of transverse magnetization. In the present work configuration theory was employed to develop a proper RF and gradient spoiling scheme for the AFI technique, making the sequence robust against off-resonance without the need of large spoiling gradients. In MRI applications, spatially varying sensitivity profiles of the RF transmit coils and local variations of the conductive and dielectric properties in the tissue may result in a spatially inhomogeneous B 1 transmit field. Therefore, accurate B 1 mapping is required for a variety of applications such as quantitative image processing (1) and multielement transmit applications (2,3) like RF-shimming (4), supported by appropriate high-field MRI systems. Most existing B 1 mapping approaches employ multiple acquisitions with different flip angles (5-9) or stimulated echoes in multipulse sequences (10 -13). Despite recent advances (1,8,9,13), many of these techniques are still too complicated and time-consuming for routine use. This is especially the case for RF shimming applications based on multiple RF transmit coil elements, where B 1 maps have to be acquired for each particular coil element.Recently, a fast steady-state B 1 mapping approach dubbed AFI (actual flip angle imaging) was introduced (14,15), consisting of two interleaved spoiled gradient echo acquisitions with different repetition times. In this approach, the flip angle is estimated by a robust and simple approximation, allowing an automatic derivation of the B 1 maps for a broad range of T 1 values. Therefore, this technique potentially enables fast in vivo B 1 mapping, making, e.g., in vivo B 1 shimming of multiple transmit coil elements possible.However, this approach is based on the assumption that the transverse magnetization is completely spoiled prior to each RF pulse. In previous studies RF spoiling has been proposed for this purpose (14 -16), but a thorough validation of this concept has not yet been presented for the AFI sequence. Moreover, large spoiling gradients were applied to overcome the off-resonance sensitivity of the sequence (14).In the present study a comprehensive analysis of the steady-state properties of the AFI approach was performed both qualitatively and quantitatively in the framework of configuration theory (17,18). Thus, a proper description of the steady-state conditions is developed, resulting in a novel RF and gradient spoiling scheme for the AFI sequence. Furthermore, numerical simulations are conducted, predicting the accuracy of the method dependent on the sequence and tissue parameters. Finally, experimental results on phantoms and volunteers will be presented for validation. THEORY AFI SequenceThe AFI approach employs a dual TR steady-state sequence. It may be regarded as a conventional spoiled gradient echo sequence, where an additional...