Peter Börnert

Advances in sensitivity encoding with arbitrary k‐space trajectories

New, efficient reconstruction procedures are proposed for sensitivity encoding (SENSE) with arbitrary k‐space trajectories. The presented methods combine gridding principles with so‐called conjugate‐gradient iteration. In this fashion, the bulk of the work of reconstruction can be performed by fast Fourier transform (FFT), reducing the complexity of data processing to the same order of magnitude as in conventional gridding reconstruction. Using the proposed method, SENSE becomes practical with nonstandard k‐space trajectories, enabling considerable scan time reduction with respect to mere gradient encoding. This is illustrated by imaging simulations with spiral, radial, and random k‐space patterns. Simulations were also used for investigating the convergence behavior of the proposed algorithm and its dependence on the factor by which gradient encoding is reduced. The in vivo feasibility of non‐Cartesian SENSE imaging with iterative reconstruction is demonstrated by examples of brain and cardiac imaging using spiral trajectories. In brain imaging with six receiver coils, the number of spiral interleaves was reduced by factors ranging from 2 to 6. In cardiac real‐time imaging with four coils, spiral SENSE permitted reducing the scan time per image from 112 ms to 56 ms, thus doubling the frame‐rate. Magn Reson Med 46:638–651, 2001.

Transmit SENSE: Transmit SENSE

The idea of using parallel imaging to shorten the acquisition time by the simultaneous use of multiple receive coils can be adapted for the parallel transmission of a spatially‐selective multidimensional RF pulse. As in data acquisition, a multidimensional RF pulse follows a certain k‐space trajectory. Shortening this trajectory shortens the pulse duration. The use of multiple transmit coils, each with its own time‐dependent waveform and spatial sensitivity, can compensate for the missing parts of the excitation k‐space. This results in a maintained spatial definition of the pulse profile, while its duration is reduced. This work introduces the concept of parallel transmission with arbitrarily shaped transmit coils (termed “Transmit SENSE”). Results of numerical studies demonstrate the theoretical feasibility of the approach. The experimental proof of principle is provided on a commercial MR scanner. The lack of multiple independent transmit channels was addressed by combining the excitation patterns from two separate subexperiments with different transmit setups. Shortening multidimensional RF pulses could be an interesting means of making 3D RF pulses feasible even for fast T  2* relaxing species or strong main field inhomogeneities. Other applications might benefit from the ability of Transmit SENSE to improve the spatial resolution of the pulse profile while maintaining the transmit duration. Magn Reson Med 49:144–150, 2003.

Three-Dimensional Black-Blood Cardiac Magnetic Resonance Coronary Vessel Wall Imaging Detects Positive Arterial Remodeling in Patients With Nonsignificant Coronary Artery Disease

Background—Direct noninvasive visualization of the coronary vessel wall may enhance risk stratification by quantifying subclinical coronary atherosclerotic plaque burden. We sought to evaluate high-resolution black-blood 3D cardiovascular magnetic resonance (CMR) imaging for in vivo visualization of the proximal coronary artery vessel wall. Methods and Results—Twelve adult subjects, including 6 clinically healthy subjects and 6 patients with nonsignificant coronary artery disease (10% to 50% x-ray angiographic diameter reduction) were studied with the use of a commercial 1.5 Tesla CMR scanner. Free-breathing 3D coronary vessel wall imaging was performed along the major axis of the right coronary artery with isotropic spatial resolution (1.0×1.0×1.0 mm3) with the use of a black-blood spiral image acquisition. The proximal vessel wall thickness and luminal diameter were objectively determined with an automated edge detection tool. The 3D CMR vessel wall scans allowed for visualization of the contiguous proximal right coronary artery in all subjects. Both mean vessel wall thickness (1.7±0.3 versus 1.0±0.2 mm) and wall area (25.4±6.9 versus 11.5±5.2 mm2) were significantly increased in the patients compared with the healthy subjects (both P <0.01). The lumen diameter (3.6±0.7 versus 3.4±0.5 mm, P =0.47) and lumen area (8.9±3.4 versus 7.9±3.5 mm2, P =0.47) were similar in both groups. Conclusions—Free-breathing 3D black-blood coronary CMR with isotropic resolution identified an increased coronary vessel wall thickness with preservation of lumen size in patients with nonsignificant coronary artery disease, consistent with a “Glagov-type” outward arterial remodeling. This novel approach has the potential to quantify subclinical disease.

Three-dimensional radial ultrashort echo-time imaging with T2 adapted sampling

The application of 3D radial sampling of the free‐induction decay to proton ultrashort echo‐time (UTE) imaging is reported. The effects of T2 decay during signal acquisition on the 3D radial point‐spread function are analyzed and compared to 2D radial and 1D sampling. It is found that in addition to the use of ultrashort TE, the proper choice of the acquisition‐window duration TAQ is essential for imaging short‐T2 components. For 3D radial sampling, a maximal signal‐to‐noise ratio (SNR) with negligible decay‐induced loss in spatial resolution is obtained for an acquisition‐window duration of TAQ ≈ 0.69 T2. For 2D and 1D sampling, corresponding values are derived as well. Phantom measurements confirm the theoretical findings and demonstrate the impact of different acquisition‐window durations on SNR and spatial resolution for a given T2 component. In vivo scans show the potential of 3D UTE imaging with T2‐adapted sampling for musculoskeletal imaging using standard MR equipment. The visualization of complex anatomy is demonstrated by extracting curved slices from the isotropically resolved 3D UTE image data. Magn Reson Med, 2006.

Resampling of data between arbitrary grids using convolution interpolation

For certain medical applications resampling of data is required. In magnetic resonance tomography (MRT) or computer tomography (CT), e.g., data may be sampled on nonrectilinear grids in the Fourier domain. For the image reconstruction a convolution-interpolation algorithm, often called gridding, can be applied for resampling of the data onto a rectilinear grid. Resampling of data from a rectilinear onto a nonrectilinear grid are needed, e.g., if projections of a given rectilinear data set are to be obtained. In this paper the authors introduce the application of the convolution interpolation for resampling of data from one arbitrary grid onto another. The basic algorithm can be split into two steps. First, the data are resampled from the arbitrary input grid onto a rectilinear grid and second, the rectilinear data is resampled onto the arbitrary output grid. Furthermore, the authors like to introduce a new technique to derive the sampling density function needed for the first step of their algorithm. For fast, sampling-pattern-independent determination of the sampling density function the Voronoi diagram of the sample distribution is calculated. The volume of the Voronoi cell around each sample is used as a measure for the sampling density. It is shown that the introduced resampling technique allows fast resampling of data between arbitrary grids. Furthermore, it is shown that the suggested approach to derive the sampling density function is suitable even for arbitrary sampling patterns. Examples are given in which the proposed technique has been applied for the reconstruction of data acquired along spiral, radial, and arbitrary trajectories and for the fast calculation of projections of a given rectilinearly sampled image.

Compressed sensing reconstruction for magnetic resonance parameter mapping

Compressed sensing (CS) holds considerable promise to accelerate the data acquisition in magnetic resonance imaging by exploiting signal sparsity. Prior knowledge about the signal can be exploited in some applications to choose an appropriate sparsifying transform. This work presents a CS reconstruction for magnetic resonance (MR) parameter mapping, which applies an overcomplete dictionary, learned from the data model to sparsify the signal. The approach is presented and evaluated in simulations and in in vivo T1 and T2 mapping experiments in the brain. Accurate T1 and T2 maps are obtained from highly reduced data. This model‐based reconstruction could also be applied to other MR parameter mapping applications like diffusion and perfusion imaging. Magn Reson Med, 2010.

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.

3D coronary vessel wall imaging utilizing a local inversion technique with spiral image acquisition

Current 2D black blood coronary vessel wall imaging suffers from a relatively limited coverage of the coronary artery tree. Hence, a 3D approach facilitating more extensive coverage would be desirable. The straightforward combination of a 3D‐acquisition technique together with a dual inversion prepulse can decrease the effectiveness of the black blood preparation. To minimize artifacts from insufficiently suppressed blood signal of the nearby blood pools, and to reduce residual respiratory motion artifacts from the chest wall, a novel local inversion technique was implemented. The combination of a nonselective inversion prepulse with a 2D selective local inversion prepulse allowed for suppression of unwanted signal outside a user‐defined region of interest. Among 10 subjects evaluated using a 3D‐spiral readout, the local inversion pulse effectively suppressed signal from ventricular blood, myocardium, and chest wall tissue in all cases. The coronary vessel wall could be visualized within the entire imaging volume. Magn Reson Med 46:848–854, 2001.

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.