Kay Nehrke

k-t PCA: temporally constrained k-t BLAST reconstruction using principal component analysis.

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, 2009.

Determination of Electric Conductivity and Local SAR Via B1 Mapping

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 patients 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 patients 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.

Free-breathing whole-heart coronary MRA with 3D radial SSFP and self-navigated image reconstruction

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 self‐navigated, 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, pencil‐beam navigator tracking. Image quality was improved in phantom studies using self‐navigation, while equivalent results were obtained with both techniques in preliminary in vivo studies. Magn Reson Med 54:476–480, 2005.

Novel prospective respiratory motion correction approach for free-breathing coronary MR angiography using a patient-adapted affine motion model.

A novel technique is presented which enables the calibration of a 3D affine respiratory motion model to the individual motion pattern of the patient. The concept of multiple navigators and precursory navigators is introduced to address nonlinear properties and hysteresis effects of the model parameters with respect to the conventional diaphragmatic navigator. The optimal combination and weighting of the navigators is determined on the basis of a principal component analysis (PCA). Thus, based on a given navigator measurement the current motion state of the object can be predicted by means of the calibrated motion model. The 3D motion model is applied in high‐resolution coronary MR angiography examinations (CMRA) to prospectively correct for respiration‐induced motion. The basic feasibility of the proposed calibration procedure was shown in 16 volunteers. Furthermore, the application of the calibrated motion model for CMRA examinations of the right coronary artery (RCA) was tested in 10 volunteers. The superiority of a calibrated 3D translation model over the conventional 1D translation model with a fixed correction factor and the potential of affine prospective motion correction for CMRA are demonstrated. Magn Reson Med 50:122–131, 2003.

DREAM—a novel approach for robust, ultrafast, multislice B1 mapping

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. Magn Reson Med, 2012.

On the performance and accuracy of 2D navigator pulses.

The purpose of this study was to investigate and to optimize the performance of two-dimensional spatially selective excitation pulses used for navigator applications on a clinical scanner. The influence of gradient imperfections, off-resonance effects, and incomplete k-space covering on the pencil beam-shaped spatial excitation profile of the 2D RF pulse was studied. The studies involved experiments performed on phantoms and in vivo. In addition, simulations were carried out by numerical integration of the Bloch equations. The accuracy of positioning of the pencil beam was increased by a factor of three by employing a simple correction scheme for the compensation of gradient distortions. The spatial selectivity of the 2D RF pulse was improved by taking sampling density corrections into account. The 2D RF pulse performance was found to be sufficient to monitor the diaphragm motion even at moderate gradient strength. For applications, where a high spatial resolution is required or a less characteristic contrast is present a strong gradient system is recommended.

On the steady-state properties of actual flip angle imaging (AFI)

AFI (actual flip angle imaging) represents an interesting approach to map the B1 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. Furthermore, numerical simulations were performed to predict the steady‐state signals and, hence, the accuracy of the AFI technique as a function of the sequence and tissue parameters. It is shown that the spoiling properties of the sequence are mainly defined by the phase shift increment ϕ of the RF pulses and the diffusion sensitivity resulting from the unbalanced gradients of the sequence. Adequate spoiling may be achieved for a reasonable range of tissue parameters and flip angles for moderate spoiling gradients if a favorable value for ϕ is chosen. Phantom and in vivo head imaging experiments show an excellent agreement with the theoretical predictions, indicating that the proper operating range of the approach may be reliably predicted by the theory. Magn Reson Med 61:84–92, 2009.

Respiratory motion in coronary magnetic resonance angiography: a comparison of different motion models.

To assess respiratory motion models for coronary magnetic resonance angiography (CMRA). In this study various motion models that describe the respiration‐induced 3D displacements and deformations of the main coronary arteries were compared.

A specific absorption rate prediction concept for parallel transmission MR

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. Magn Reson Med, 2012.

Prospective correction of affine motion for arbitrary MR sequences on a clinical scanner

The concept of prospective 3D affine motion correction was generalized, based on the Bloch equations, for signal excitation and sampling using arbitrary MR sequences. The technique was implemented on a clinical MRI scanner for Cartesian, radial, and spiral imaging sequences, as well as for 2D spatially selective RF excitation pulses. A patient‐specific motion model steered by real‐time navigators was employed to account for the additional degrees of freedom provided by the affine motion model. Different navigator concepts (multiple spatial and temporal navigators, quadratic navigators and other motion sensors) were investigated, with the aim of improving the correlation between navigator information and the motion model. Experiments on moving phantoms are presented to prove the technical feasibility of the approach. In vivo experiments on coronary MRA and renal MRI show the potential of the method for cardiac and abdominal applications hampered by respiratory motion. Magn Reson Med, 2005.