Jochen Leupold

The molecular basis for gray and white matter contrast in phase imaging

Direct magnetic resonance phase images acquired at high field have been shown to yield superior gray and white matter contrast up to 10-fold higher compared to conventional magnitude images. However, the underlying contrast mechanism is not yet understood. This study demonstrates that the water resonance frequency is directly shifted by water-macromolecule exchange processes (0.040 ppm/mM for bovine serum albumin) and might be a major source of contribution to in vivo phase image contrast. Therefore, magnetic resonance phase imaging based on the proposed contrast mechanism could potentially be applied for in vivo studies of pathologies on a macromolecular level.

Alternating repetition time balanced steady state free precession

A novel balanced SSFP technique for the separation or suppression of different resonance frequencies (e.g., fat suppression) is presented. The method is based on applying two alternating and different repetition times, TR1 and TR2. This RF scheme manipulates the sensitivity of balanced SSFP to off‐resonance effects by a modification of the frequency response profile. Starting from a general approach, an optimally broadened stopband within the frequency response function is designed. This is achieved with a TR2 being one third of TR1 and an RF‐pulse phase increment of 90°. With this approach TR2 is too short (∼1 ms) to switch imaging gradients and is only used to change the frequency sensitivity. Without a significant change of the spectral position of the stopband, TR1 can be varied over a range of values (∼2.5–4.5 ms) while TR2 and phase cycling is kept constant. On‐resonance spins show a magnetization behavior similar to balanced SSFP, but with maximal magnetization at flip angles about 10° lower than in balanced SSFP. The total scan time is increased by about 30% compared to conventional balanced SSFP. The new technique was applied on phantoms and volunteers to produce rapid, fat suppressed images. Magn Reson Med, 2006.

Fast multiecho balanced SSFP metabolite mapping of 1H and hyperpolarized 13C compounds

ObjectTo investigate the feasibility of multiecho balanced steady-state free precession (bSSFP)-based fast chemical shift mapping hyperpolarized 13C metabolites. The overall goal was to reduce total imaging time and to increase spatial resolution compared to common chemical shift imaging (CSI).Materials and methodsA multiecho bSSFP sequence in combination with an iterative reconstruction algorithm was implemented. 1H experiments were performed on phantoms and on a human volunteer in order to investigate the feasibility of the method on a system with metabolite maps that are known beforehand. 13C experiments were performed in vivo on pigs, where CSI images were acquired also for comparison.ResultsChemical shift images of three and four distinct 1H resonance frequencies as well as chemical shift images of up to five hyperpolarized 13C metabolites were successfully obtained.ConclusionFast metabolite mapping based on multiecho balanced SSFP in combination with an iterative reconstruction approach could successfully separate several 1H resonances and hyperpolarized 13C metabolites.

A continuous-flow, high-throughput, high-pressure parahydrogen converter for hyperpolarization in a clinical setting.

Pure parahydrogen (pH2) is the prerequisite for optimal pH2‐based hyperpolarization experiments, promising approaches to access the hidden orders of magnitude of MR signals. pH2 production on‐site in medical research centers is vital for the proliferation of these technologies in the life sciences. However, previously suggested designs do not meet our requirements for safety or production performance (flow rate, pressure or enrichment). In this article, we present the safety concept, design and installation of a pH2 converter, operated in a clinical setting. The apparatus produces a continuous flow of four standard liters per minute of ≈98% enriched pH2 at a pressure maximum of 50 bar. The entire production cycle, including cleaning and cooling to 25 K, takes less than 5 h, only ≈45 min of which are required for actual pH2 conversion. A fast and simple quantification procedure is described. The lifetimes of pH2 in a glass vial and aluminum storage cylinder are measured to be T1C(glass vial) = 822 ± 29 min and T1C(Al cylinder) = 129 ± 36 days, thus providing sufficiently long storage intervals and allowing the application of pH2 on demand. A dependence of line width on pH2 enrichment is observed. As examples, 1H hyperpolarization of pyridine and 13C hyperpolarization of hydroxyethylpropionate are presented. Copyright

Dental MRI: Imaging of soft and solid components without ionizing radiation

To evaluate the ability of conventional and ultra‐short or zero echo time MRI for imaging of soft and solid dental components in and ex vivo.

Systematic investigation of balanced steady-state free precession for functional MRI in the human visual cortex at 3 Tesla.

Previous studies have applied balanced steady‐state free precession (bSSFP) to functional brain imaging. Methods that exploit the strong frequency dependence of the MR signal in the bSSFP transition band are strongly affected by field inhomogeneity and frequency drifts. Recent bSSFP studies using “on‐resonance” (in the bSSFP passband) acquisition claimed that higher sensitivity was achieved compared to traditional fMRI methods. However, the contrast mechanism that generates activation‐related signal changes in bSSFP imaging is not yet fully understood. We performed a systematic study of on‐resonance bSSFP signal behavior using a multiecho balanced SSFP sequence with different TRs at 3 Tesla. We conclude that intravoxel dephasing, or the off‐resonance averaged steady state, dominates the bSSFP signal decay and determines the bSSFP fMRI contrast. Experimental findings were confirmed by simulations based on existing theories for signal formation around blood vessels in inhomogeneous tissues. The activation‐induced signal change in on‐resonance bSSFP increases with TE, and the TE dependence of the contrast‐to‐noise ratio (CNR) in bSSFP is similar to that in gradient echo‐planar imaging (GE‐EPI). However, GE‐EPI has a significantly higher CNR efficiency. Magn Reson Med 57:67–73, 2007.

Gradient echo imaging

Magnetic resonance imaging (MRI) based on gradient echoes is used in a wide variety of imaging techniques and clinical applications. Gradient echo sequences form the basis for an essential group of imaging methods that find widespread use in clinical practice, particularly when fast imaging is important, as for example in cardiac MRI or contrast‐enhanced MR angiography. However, the term “gradient echo sequence” is somewhat unspecific, as even images acquired with the most common sequences employing the gradient echo for data acquisition can significantly differ in signal, contrast, artifact behavior, and sensitivity to, eg, flow. This is due to the different use of sequence timing and basic sequence building blocks such as spoiler gradients or specific radiofrequency (RF) pulse phase patterns. In this article the basic principles of gradient echo formation compared to spin echo imaging are reviewed and the properties of gradient echo imaging in its simplest form (TR ≫ T2) are described. Further, the most common three variants of fast gradient echo sequences (TR < T2), namely, unbalanced gradient echo, RF spoiled gradient echo, and balanced steady state free precession; are discussed. For each gradient echo sequence type, examples of applications exploiting the specific properties of the individual technique are presented. J. Magn. Reson. Imaging 2012;35:1274–1289.

Double average parallel steady-state free precession imaging: optimized eddy current and transient oscillation compensation.

Balanced steady‐state free precession (SSFP) imaging is sensitive to off‐resonance effects, which can lead to considerable artifacts during a transient phase following magnetization preparation or steady‐state interruption. In addition, nonlinear k‐space encoding is required if contrast‐relevant k‐space regions need to be acquired at specific delays following magnetization preparation or for transient artifact reduction in cardiac‐gated k‐space segmented CINE imaging. Such trajectories are problematic for balanced SSFP imaging due to nonconstant eddy current effects and resulting disruption of the steady state.

Moment and direction of the spoiler gradient for effective artifact suppression in RF-spoiled gradient echo imaging

Radiofrequency (RF) ‐spoiled gradient echo sequences were developed with the aim to produce images with T1 weighted contrast within short acquisition time. Over the past two decades, this type of sequence has proven to be a robust technique and represents a reliable workhorse in clinical MRI. This study presents an analysis of ghost artifacts, which appear occasionally in RF‐spoiled gradient echo images. It is demonstrated that the artifacts result from intrinsically emerging signal oscillations, which can be damped down by the application of spoiler gradients. Magn Reson Med 60:119–127, 2008.

Magnetic properties of materials for MR engineering, micro-MR and beyond.

We present the results of a systematic measurement of the magnetic susceptibility of small material samples in a 9.4 T MRI scanner. We measured many of the most widely used materials in MR engineering and MR micro technology, including various polymers, optical and substrate glasses, resins, glues, photoresists, PCB substrates and some fluids. Based on our data, we identify particularly suitable materials with susceptibilities close to water. For polyurethane resins and elastomers, we also show the MR spectra, as they may be a good substitute for silicone elastomers and good casting resins.