Bernd Aldefeld

Improvements in spiral MR imaging.

The basic principles of spiral MR image acquisition and reconstruction are summarised with the aim to explain how high quality spiral images can be obtained. The sensitivity of spiral imaging to off-resonance effects, gradient system imperfections and concomitant fields are outlined and appropriate measures for corrections are discussed in detail. Phantom experiments demonstrate the validity of the correction approaches. Furthermore, in-vivo results are shown to demonstrate the applicability of the corrections under in-vivo conditions. The spiral image quality thus obtained was found to be comparable to that obtainable with robust spin warp sequences.

On spatially selective RF excitation and its analogy with spiral MR image acquisition.

The basic principles of the design of spatially selective RF pulses are described, and their analogy with MR image acquisition and reconstruction is shown. The paper focuses on RF-pulse design and imaging schemes in which spiral k-space trajectories are used. The sensitivity of RF excitation to gradient-system imperfections and to spatially varying off-resonance are analyzed, and suitable measures of correction are discussed. The spatial resolution obtainable with selective RF pulses and the consequences of the linearity of the pulse-design problem are examined. Phantom experiments showing the performance of multidimensional spatially selective RF pulses further illustrate the analogy with MR image acquisition.

Reversed spiral MR imaging

Reversed spiral imaging is discussed as an approach that provides strong intrinsic T *2 contrast without the need for long repetition times. In comparison to the conventional forward spiral method, the T *2 contrast achieved by reversing the spiral k‐space trajectory is similar and differs only for very fast relaxing species. The flow and motion sensitivity of the reversed approach is the same if flow compensation is applied, except for a flow‐dependent voxel shift and the sign of the artifact pattern. By simulations as well as phantom and in vivo experiments, it is shown that the image quality in reversed spiral imaging is comparable to that obtained with the forward spiral method. Magn Reson Med 44:479–484, 2000.

Principles of whole‐body continuously‐moving‐table MRI

Continuously‐moving‐table (CMT) imaging is a new and promising approach to virtually extend the field‐of‐view (FOV) in currently available MRI systems. It shows high potential to improve a number of applications that require a large FOV, such as whole‐body contrast‐enhanced angiography and contrast‐optimized whole‐body head‐to‐toe imaging. In this work, an overview of the different approaches to CMT imaging is given. Basic principles of two‐ and three‐dimensional (2D and 3D) techniques are discussed, with emphasis on performance and image‐artifact issues. Potential clinical applications and further desirable improvements are outlined. J. Magn. Reson. Imaging 2008;28:1–12.

Improved 3D spiral imaging for coronary MR angiography.

Thin‐slab 3D spiral imaging has been used for MR angiography to image selected coronary arteries. Improved scan efficiency was achieved using a train of multiple spiral interleaves within each single R‐R interval acquired in the late diastole. Data acquisition was performed during free breathing, using navigator gating. Additionally, prospective slice tracking was applied to further reduce the sensitivity to motion. The application of a T2‐preparation pulse and fat suppression increased the contrast between blood and myocardium. Experiments performed on healthy volunteers are presented to show the feasibility of this approach, which allows coronary artery imaging of selected vessels within a few minutes. Magn Reson Med 45:172–175, 2001.

Whole-body 3D water/fat resolved continuously moving table imaging†

To study the feasibility of three‐dimensional (3D) whole‐body, head‐to‐toe, water/fat resolved MRI, using continuously moving table imaging technology.

Continuously moving table SENSE imaging

A combination of continuously moving table imaging and parallel imaging based on sensitivity encoding (SENSE) is presented. One specific geometry is considered, where the receiver array is fixed to the MR magnet and does not move with the table, which allows for head‐to‐toe imaging with a small total number of coils. Sensitivity maps are defined for the enlarged virtual field of view and are composed according to the k‐space sampling scheme such that established parallel reconstruction techniques are applicable to good approximation. In vivo experiments show the feasibility of this approach, and simulations determine the application range. Three‐dimensional head‐to‐toe imaging of volunteers is performed in 77 s with a SENSE reduction factor of 2 in a virtual field of view of 1800 × 460 × 100 mm3. Magn Reson Med 53:217–220, 2005.

Spatial excitation using variable-density spiral trajectories

To examine the usefulness of variable‐density k‐space trajectories for the design of multi‐dimensional spatially selective RF pulses.

Continuously moving table 3D MRI with lateral frequency-encoding direction

A method is presented for 3D MRI in an extended field of view (FOV) based on continuous motion of the patient table and an efficient acquisition scheme. A gradient‐echo MR pulse sequence is applied with lateral (left–right (L/R)) frequency‐encoding direction and slab selection along the direction of motion. Compensation for the table motion is achieved by a combination of slab tracking and data alignment in hybrid space. The method allows fast k‐space coverage to be achieved, especially when a short sampling FOV is chosen along the direction of table motion, as is desirable for good image quality. The method can be incorporated into different acquisitions schemes, including segmented k‐space scanning, which allows for contrast variation with the use of magnetization preparation. Head‐to‐toe images of volunteers were obtained with good quality using 3D spoiled gradient‐echo sequences. As an example of magnetization‐prepared imaging, fat/water separated images were acquired using chemical shift selective (CHESS) presaturation pulses. Magn Reson Med, 2006.