Semiautomatic Analysis of Phase Contrast Magnetic Resonance Imaging of Cerebrospinal Fluid Flow through the Aqueduct of Sylvius

Objective: Quantification of the cerebrospinal fluid (CSF) flow through the aqueduct of Sylvius by means of magnetic resonance imaging (MRI) is subject to interobserver variability due to the region of interest (ROI) selection. Our objective is to develop a semiautomatic measurement method to achieve reproducible quantitative analysis of CSF flow rate and stroke volume. Material and methods: MR examinations were performed using a 1.5 T scanner with a phase contrast sequence (velocity encoding [V _enc] of 20 cm/s, FOV = 160, 3 mm slice thickness, image matrix size = 256×256, TR = 53 ms, TE = 11 ms, NSA = 2, flip angle = 15° and 23 frames per cardiac cycle with peripheral retrospective pulse gating). Our method was developed using MATLAB R7. Errors introduced by background offset and possible aliased pixels were automatically detected and corrected if necessary in order to calculate the flow parameters that characterize CSF dynamics. The semiautomatic seed method reproducibility was evaluated and compared with the radius method by two observers analysing 21 healthy subjects. Results: The measurements using the semiautomatic seed method reduced the interobservers variability (intra-class correlation [ICC] = 1.0 for stroke volume and for volumetric flow rate) versus the radius method (ICC = 0.46 for stroke volume and 0.65 for flow rate). Normal stroke volume (39.19 ± 20.13 μl/cycle), flow rate (3.81 ± 2.81 ml/min), maximal mean systolic velocity (5.27 ± 1.3 cm/s) and maximal mean diastolic velocity (4.20 ± 1.4 cm/s) were calculated with the half moon and aliasing corrected seed method. Conclusions: Semiautomatic measurements (seed method with half moon background and aliasing correction) allow a generalization of the calculus of flow parameters with great consistency and independency of the operator.     

3D magnetic resonance fingerprinting on a low-field 50 mT point-of-care system prototype: evaluation of muscle and lipid relaxation time mapping and comparison with standard techniques

Objective

To implement magnetic resonance fingerprinting (MRF) on a permanent magnet 50 mT low-field system deployable as a future point-of-care (POC) unit and explore the quality of the parameter maps.

Materials and methods

3D MRF was implemented on a custom-built Halbach array using a slab-selective spoiled steady-state free precession sequence with 3D Cartesian readout. Undersampled scans were acquired with different MRF flip angle patterns and reconstructed using matrix completion and matched to the simulated dictionary, taking excitation profile and coil ringing into account. MRF relaxation times were compared to that of inversion recovery (IR) and multi-echo spin echo (MESE) experiments in phantom and in vivo. Furthermore, B₀ inhomogeneities were encoded in the MRF sequence using an alternating TE pattern, and the estimated map was used to correct for image distortions in the MRF images using a model-based reconstruction.

Results

Phantom relaxation times measured with an optimized MRF sequence for low field were in better agreement with reference techniques than for a standard MRF sequence. In vivo muscle relaxation times measured with MRF were longer than those obtained with an IR sequence (T₁: 182 ± 21.5 vs 168 ± 9.89 ms) and with an MESE sequence (T₂: 69.8 ± 19.7 vs 46.1 ± 9.65 ms). In vivo lipid MRF relaxation times were also longer compared with IR (T₁: 165 ± 15.1 ms vs 127 ± 8.28 ms) and with MESE (T₂: 160 ± 15.0 ms vs 124 ± 4.27 ms). Integrated ΔB₀ estimation and correction resulted in parameter maps with reduced distortions.

Discussion

It is possible to measure volumetric relaxation times with MRF at 2.5 × 2.5 × 3.0 mm³ resolution in a 13 min scan time on a 50 mT permanent magnet system. The measured MRF relaxation times are longer compared to those measured with reference techniques, especially for T₂. This discrepancy can potentially be addressed by hardware, reconstruction and sequence design, but long-term reproducibility needs to be further improved.

     

Monte Carlo Simulations of Hole Dynamics in Si/SiGe Quantum Cascade Structures

We report the first detailed ensemble Monte Carlo simulation of hole dynamics in cascaded p-Si/SiGe quantum wells. The hole subband structure is calculated using the 6 × 6 k · p model. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. The scattering mechanisms included are the alloy disorder, acoustic and optical phonon scattering. Results are presented for prototype Si/SiGe cascade structures.     

Slice acquisition order and blood oxygenation level dependent frequency content: an event-related functional magnetic resonance imaging study

Many event-related functional magnetic resonance imaging paradigms performed so far have been designed to study a limited part of the brain with high temporal resolution. However, event-related paradigms can be exploratory, therefore requiring whole brain scans and so repetition times (TR) of several seconds. For these large TR values, the slice acquisition order may have an important effect on the detection of event-related activation. Indeed, when the scanning is interleaved, the temporal delay between the acquisition of two contiguous slices can reach a few seconds. During this time, the subject is likely to move, and the haemodynamic response will vary significantly. In this case, the interpolation applied between contiguous slices for motion correction induces a temporal smoothing between voxels that are spatially close but temporally sampled a few seconds apart. This should modify the frequency structure of the response and may impair the detection of short events. We, therefore tested the effect of three acquisition schemes (sequential, sequential with gap and interleaved. INT) at two repetition times (TR = 3 and 6 s on six and seven subjects, respectively) on activation detection and frequency content in a visual motion event-related paradigm. Unexpectedly, tor large TR (6 s), results were found in favour of the INT acquisition scheme(P <0.05). For smaller TR. no strong bias could be found. Generally, intra-subject variability (across acquisition schemes) is found to be much smaller than inter-subject variability, confirming the importance of multi-subjects analyses. Our study also shows that important physiological information is carried by high frequency components that should not be filtered out.     

A simulator for teaching MR image contrast behavior

A computer program to create magnetic resonance images simulating the main features of a magnetic resonance imaging (MR) machine was developed on a personal computer. The simulated MR images are calculated using the relevant equations describing a chosen pulse sequence on the basis of proton density,T ₁ andT ₂ images acquired from whole-body MR equipment. The imaging parameters (i.e., TR, TE, TI, etc.) are provided by the user. This program produces highly realistic simulated images and includes features such as choice of echo time, inversion time, repetition time and flip angle, as well as standard features of image display programs such as setting of window level/width, zooming and region of interest definition. A novel additional feature is the ability to define artificial pathologies in user-defined areas by specifying changes in the relaxation times and proton density. The simulator has been developed for teaching and training purposes. A prototype version has been used for teaching MRI with extremely positive reactions from students.     

Comparison of 2D simultaneous multi-slice and 3D GRASE readout schemes for pseudo-continuous arterial spin labeling of cerebral perfusion at 3 T

Objective

In this perfusion magnetic resonance imaging study, the performances of different pseudo-continuous arterial spin labeling (PCASL) sequences were compared: two-dimensional (2D) single-shot readout with simultaneous multislice (SMS), 2D single-shot echo-planar imaging (EPI) and multishot three-dimensional (3D) gradient and spin echo (GRASE) sequences combined with a background-suppression (BS) module.

Materials and methods

Whole-brain PCASL images were acquired from seven healthy volunteers. The performance of each protocol was evaluated by extracting regional cerebral blood flow (rCBF) measures using an inline morphometric segmentation prototype. Image data postprocessing and subsequent statistical analyses enabled comparisons at the regional and sub-regional levels.

Results

The main findings were as follows: (i) Mean global CBF obtained across methods was were highly correlated, and these correlations were significantly higher among the same readout sequences. (ii) Temporal signal-to-noise ratio and gray-matter-to-white-matter CBF ratio were found to be equivalent for all 2D variants but lower than those of 3D-GRASE.

Discussion

Our study demonstrates that the accelerated SMS readout can provide increased acquisition efficiency and/or a higher temporal resolution than conventional 2D and 3D readout sequences. Among all of the methods, 3D-GRASE showed the lowest variability in CBF measurements and thus highest robustness against noise.

     

The advantages of radial trajectories for vessel-selective dynamic angiography with arterial spin labeling

Objectives

To demonstrate the advantages of radial k-space trajectories over conventional Cartesian approaches for accelerating the acquisition of vessel-selective arterial spin labeling (ASL) dynamic angiograms, which are conventionally time consuming to acquire.

Materials and methods

Vessel-encoded pseudocontinuous ASL was combined with time-resolved balanced steady-state free precession (bSSFP) and spoiled gradient echo (SPGR) readouts to obtain dynamic vessel-selective angiograms arising from the four main brain-feeding arteries. Dynamic 2D protocols with acquisition times of one minute or less were achieved through radial undersampling or a Cartesian parallel imaging approach. For whole-brain dynamic 3D imaging, magnetic field inhomogeneity and the high acceleration factors required rule out the use of bSSFP and Cartesian trajectories, so the feasibility of acquiring 3D radial SPGR angiograms was tested.

Results

The improved SNR efficiency of bSSFP over SPGR was confirmed for 2D dynamic imaging. Radial trajectories had considerable advantages over a Cartesian approach, including a factor of two improvements in the measured SNR (p < 0.00001, N = 6), improved distal vessel delineation and the lack of a need for calibration data. The 3D radial approach produced good quality angiograms with negligible artifacts despite the high acceleration factor (R = 13).

Conclusion

Radial trajectories outperform conventional Cartesian techniques for accelerated vessel-selective ASL dynamic angiography.

     

Parallel acquisition for effective density weighted imaging: PLANED imaging

Objective Density weighted phase-encoding has proven to be a highly efficient method for k-space sampling as it improves the localization properties and increases the signal-to-noise ratio for extended samples at the same time. But either density weighted imaging lengthens the minimum scan time or, if the Nyquist criterion is violated in parts of the sampled k-space, undersampling artefacts occur. Purpose of this work was to combine density weighted imaging and parallel imaging techniques to improve the spatial response function and consequently the signal-to-noise ratio without spoiling image quality by undersampling artefacts. Materials and Methods Images were acquired with parallel acquisition for effective density weighted imaging (PLANED imaging) and compared to results sampled with conventional Cartesian phase-encoding with the same spatial resolution and the same number of excitations. Results Both in vivo and phantom measurements recorded with the PLANED method revealed a considerable enhancement of the signal-to-noise ratio and a remarkable reduction of Gibbs artefacts compared to standard Cartesian imaging. Conclusion It has been demonstrated that PLANED improves image quality by suppressing truncation artefacts and increasing the SNR without lengthening the measurement time.     

Left ventricular quantification with breath-hold MR imaging: comparison with echocardiography

To evaluate the reproducibility of measurements of left ventricular (LV) dimensions, function, and myocardial mass, segmentedk-space gradient-recalled-echo (GRE) magnetic resonance (MR) imaging was performed on two occasions on 12 healthy volunteers. To compare the MR data, all volunteers underwent a two-dimensional echocardiography with determination of LV dimensions and function. The left ventricle was imaged during breath-hold by consecutive, contiguous short-axis views at end-diastole and end-systole. An average of eight short-axis views was needed to encompass the whole left ventricle. This fast MR sequence limited the total acquisition time to 12 min. LV volumes and masses were calculated after manual delineation of epicardial and endocardial surfaces by two observers in a blinded fashion. Interstudy variability varied between 4.1% and 10.3% for LV end-diastolic volume and end-systolic volume, respectively. Differences in interobserver variability were smaller and varied between 3.6% and 7.3% for LV ejection fraction and end-diastolic volume, respectively. Intraobserver variabilities ranged between 2.0% and 7.0% for LV ejection fraction and end-systolic volume, respectively. These variability percentages agree very well with other studies in literature using other MR sequences. No significant differences in LV dimensions or function were found between MR imaging and echocardiography. In conclusion, this MR sequence allows fast and reproducible LV quantification.     

Contrast-enhanced abdominal echo planar imaging: Dynamic signal-intensity changes following intravenous injection of gadolinium-DOTA

Objectives: After I.V. administration of gadolinium-DOTA, the early contrast enhancement pattern and related signal-intensity (SI) changes in normal abdominal organs (kidney, spleen, liver) are evaluated over the first 4 min by using ultrafast spin-echo echo planar imaging (SE-EPI). Methods: On a 1.5-T magnetic resonance unit ultrafast EPI of the upper abdomen was performed in 12 patients in order to show the contrast enhancement pattern and related measurable SI changes onT ₁ andT ₂-weighted (w) images over the first 4 min after I.V. bolus injection of 0.1 mmol kg^–1 gadolinium (Gd)-DOTA in the spleen, liver, renal cortex, and renal medulla. A TR/TE of 500/44 or 45 ms inT ₁w SE-EPI and a TR/TE of 2000/80 or 100 ms inT ₂-w SE-EPI were used. Results: Typical time-dependent SI changes were noticed onT ₁w images: Subsequent to a SI increase in the renal cortex (starting 7 s after the I.V. injection of Gd-DOTA) SI increased first in the outer renal medulla (6 s later) and then in the inner renal medulla (21 s later). A SI increase was observed in the spleen (starting after 15 s) and in the liver (starting 7 s later). OnT ₂-w images, a SI decrease in the renal cortex (starting after 14 s) was followed by migration of a dark band from the outer (after 46 s) to the inner medulla (after 70 s). Only minimal changes were noticed in the spleen and liver. Conclusions: Ultrafast SE-EPI following I.V. bolus injection of Gd-DOTA enables the observation of the very early contrast agent kinetics in various abdominal organs. The associated SI changes onT ₁- andT ₂- SE EPI are related to organ perfusion and contrast agent tissue concentration and biodistribution.Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Penn Plaza, New York, NY 10119.     

Simulation-based evaluation of SAR and flip angle homogeneity for five transmit head arrays at 14 T

Introduction

Various research sites are pursuing 14 T MRI systems. However, both local SAR and RF transmit field inhomogeneity will increase. The aim of this simulation study is to investigate the trade-offs between peak local SAR and flip angle uniformity for five transmit coil array designs at 14 T in comparison to 7 T.

Methods

Investigated coil array designs are: 8 dipole antennas (8D), 16 dipole antennas (16D), 8 loop coils (8D), 16 loop coils (16L), 8 dipoles/8 loop coils (8D8L) and for reference 8 dipoles at 7 T. Both RF shimming and k_T-points were investigated by plotting L-curves of peak SAR levels vs flip angle homogeneity.

Results

For RF shimming, the 16L array performs best. For k_T-points, superior flip angle homogeneity is achieved at the expense of more power deposition, and the dipole arrays outperform the loop coil arrays.

Discussion and conclusion

For most arrays and regular imaging, the constraint on head SAR is reached before constraints on peak local SAR are violated. Furthermore, the different drive vectors in k_T-points alleviate strong peaks in local SAR. Flip angle inhomogeneity can be alleviated by k_T-points at the expense of larger power deposition. For k_T-points, the dipole arrays seem to outperform loop coil arrays.

     

Apparent diffusion coefficients of 31P metabolites in the human calf muscle at 7 T

Purpose

In this study, we aimed to measure the apparent diffusion coefficients (ADCs) of major phosphorous metabolites in the human calf muscle at 7 T with a diffusion-weighted (DW)-STEAM sequence.

Methods

A DW-STEAM sequence with bipolar gradients was implemented at 7 T, and DW MR spectra were acquired in three orthogonal directions in the human calf muscle of six healthy volunteers (TE/TM/TR = 15 ms/750 ms/5 s) at three b-values (0, 800, and 1200 s/mm²). Frequency and phase alignments were applied prior to spectral averaging. Averaged DW MR spectra were analyzed with LCModel, and ADCs of ³¹P metabolites were estimated.

Results

Four metabolites (phosphocreatine (PCr), adenosine triphosphate (ATP), inorganic phosphate (Pi) and glycerol phosphorylcholine (GPC)) were quantified at all b-values with mean CRLBs below 10%. The ADC values of PCr, ATP, Pi, and GPC were (0.24 ± 0.02, 0.15 ± 0.04, 0.43 ± 0.14, 0.40 ± 0.09) × 10^–3 mm²/s, respectively.

Conclusion

The ADCs of four ³¹P metabolites were successfully measured in the human calf muscle at 7 T, among which those of ATP, Pi and GPC were reported for the first time in humans. This study paves the way to investigate ³¹P metabolite diffusion properties in health and disease on the clinical MR scanner.

     

Quantitative in vivo 1H spectroscopic imaging of metabolites in the early postnatal mouse brain at 17.6 T

Object  Early postnatal brain maturation is closely connected to local changes of metabolite levels. Spatially resolved in vivo ¹H NMR spectroscopic imaging is applied to follow absolute changes of brain metabolites in early postnatal mouse brain. Materials and methods  A short echo time semi LASER (localization by adiabatic selective refocusing) chemical shift imaging (CSI) sequence incorporating weighted k-space averaging was implemented at high magnetic field (17.6 T). In vivo measurements were carried out on postnatal days 5, 8, 12, 16, and 20. In vivo relaxation times T ₁ and T ₂ were measured using variable repetition times or a CPMG sequence, respectively, combined with LASER single voxel localization. Results  Spectra were obtained with a spatial resolution of (1 × 1) mm² in a 1.5 mm slice as early as postnatal day 5. Maturational changes of absolute metabolite concentrations of major metabolites were calculated in four different brain regions. A significant increase of N-acetylaspartate (NAA), total creatine (tCr), and glutamate/glutamine (Glx) concentration was paralleled by a decrease of taurine (Tau) concentration with age (P < 0.05). Differences between brain regions were found for NAA, tCr, and Tau (P < 0.05). Furthermore, in vivo T ₁ and T ₂ of the four major brain metabolites in adult mice are reported. Conclusion  The implemented semi LASER CSI sequence allows following regional changes of metabolite levels. It is suitable for investigation of local differences in brain metabolism and development.     

Functional MRI at 3T using intermolecular double-quantum coherence (iDQC) with spin-echo (SE) acquisitions

Object To reinvestigate the dependence of the signal and contrast on sequence parameters and tissue relaxation times for intermolecular double-quantum coherence (iDQC) signals, and to explore the possibility to use a spin-echo (SE)-iDQC sequence for detecting activation signals at 3T. Materials and methods Brain activations were detected in five human volunteers in a visual simulation study using a SE-iDQC sequence, in addition to a GE-iDQC and a conventional single-quantum coherence (SQC) blood-oxygenation-level-dependent (BOLD) sequence. A brain phantom was also used for some quantitative measurements. Results By choosing an optimal echo time TE (~T2) and iDQC evolution time τ(~20 ms), robust brain activations were detected using the SE-iDQC sequence, in addition to the GE-iDQC and a conventional single-quantum coherence (SQC) BOLD sequence. A higher percentage signal change due to activation was observed for both the iDQC-based measurements in comparison to the conventional SQC acquisition. Conclusion Even though a phenomenological analysis consistent with the experimental results was provided, a detailed model is still needed for the contrast mechanism at microscopic level to guide potential applications of brain functional imaging based on the SE-iDQC.     

Quantitative T 2 measurement of a single voxel with arbitrary shape using pinwheel excitation and CPMG acquisition

Objective The aim of this study is to present a new approach for making quantitative single-voxel T ₂ measurements from an arbitrarily shaped region of interest (ROI), where the advantage of the signal-to-noise ratio (SNR) per unit time of the single-voxel approach over conventional imaging approach can be achieved. Materials and methods Two-dimensional (2D) spatially selective radiofrequency (RF) pulses are proposed in this work for T ₂ measurements based on using interleaved spiral trajectories in excitation k-space (pinwheel excitation pulses), combined with a summed Carr–Purcell Meiboom–Gill (CPMG) echo acquisition. The technique is described and compared to standard multi-echo imaging methods, on a two-compartment water phantom and an excised brain tissue. Results The studies show good agreement between imaging and our method. The measured improvement factors of SNR per unit time of our single-voxel approach over imaging approach are close to the predicted values. Conclusion Measuring T ₂ relaxation times from a selected ROI of arbitrary shape using a single-voxel rather than an imaging approach can increase the SNR per unit time, which is critical for dynamic T ₂ or multi-component T ₂ measurements.     

MR image reconstruction algorithms for sparse k-space data: a Java-based integration

We have worked on multi-dimensional magnetic resonance imaging (MRI) data acquisition and related image reconstruction methods that aim at reducing the MRI scan time. To achieve this scan-time reduction we have combined the approach of ’increasing the speed’ ofk-space acquisition with that of ‘deliberately omitting’ acquisition ofk-space trajectories (sparse sampling). Today we have a whole range of (sparse) sampling distributions and related reconstruction methods. In the context of a European Union Training and Mobility of Researchers project we have decided to integrate all methods into one coordinating software system. This system meets the requirements that it is highly structured in an object-oriented manner using the Unified Modeling Language and the Java programming environment, that it uses the client-server approach, that it allows multi-client communication sessions with facilities for sharing data and that it is a true distributed computing system with guaranteed reliability using core activities of the Java Jini package.     

MR CAT scan: a modular approach for hybrid imaging

In this study, a modular concept for NMR hybrid imaging is presented. This concept essentially integrates different imaging modules in a sequential fashion and is therefore called CAT (combined acquisition technique). CAT is not a single specific measurement sequence, but rather a sequence design concept whereby distinct acquisition techniques with varying imaging parameters are employed in rapid succession in order to coverk-space. The power of the CAT approach is that it provides a high flexibility toward the acquisition optimization with respect to the available imaging time and the desired image quality. Important CAT sequence optimization steps include the appropriate choice of thek-space coverage ratio and the application of mixed bandwidth technology. Details of both the CAT methodology and possible CAT acquisition strategies, such as FLASH/EPIRARE/EPI-and FLASH/BURST-CAT are provided. Examples from imaging experiments in phantoms and healthy volunteers including mixed bandwidth acquisitions are provided to demonstrate the feasibility of the proposed CAT concept.     

Monitoring the development of hepatocellular carcinoma in woodchucks using 31P-MRS

The woodchuck is one of the only lab animal models of chronic viral hepatitis infection and the development of hepatocellular carcinoma. Using this model, changes in tissue energetics in the liver due to the development of hepatocellular carcinoma can be monitored by repeated magnetic resonance imaging and localized phosphorus spectroscopy. Age- and sex-matched control (n=5) and chronically infected (n=5) adult woodchucks were imaged four times in a six-month period in a 7-T horizontal-bore magnet. Using a custom-built doubly tunable quadrature volume coil, sagittal and axial FLASH images (128×128, slice thickness = 5 mm, TR/TE=1000/4.1, 8 averages) were acquired to locate the largest portion of the liver with the least amount of signal contamination from surrounding abdominal muscle. Two-dimensional 31P chemical-shift imaging (2D-CSI) was acquired (16×16 data matrix, 24×24×2 cm³, 1024 data points, 16 averages) for all animals. The extent of liver injury was determined using serum gamma glutamyltransferase (GGT). The livers of infected woodchucks showed a significant increase (p=0.01) in phosphomonoesters (PME):β-adenosine triphosphate (NTP). Chronically infected woodchucks had higher levels of serum GGT compared to uninfected woodchucks (p=0.002). An increase in the PME:β-NTP ratio indicates cellular proliferation within the malignant tumor.     

ECG-gated <Superscript>23</Superscript>Na-MRI of the human heart using a 3D-radial projection technique with ultra-short echo times

Pathological changes in tissue often manifest themselves in an altered sodium gradient between intra- and extracellular space due to a malfunctioning Na⁺–K⁺ pump, resulting in an increase in total sodium concentration in ischaemic regions. Therefore, ²³Na-MRI has the potential to non-invasively differentiate viable from non-viable tissue by detecting concentration changes of intra- and extracellular sodium. As the in vivo sodium signal shows a bi-exponential T₂ decay, with a short component of less than 1 ms, the accurate quantification of the total sodium content requires imaging techniques with ultra-short echo times (TE) below 0.5 ms. A 3D-radial projection technique has been developed which allows the acquisition of ECG-triggered sodium images of the human heart with a TE of 0.4 ms. With this pulse sequence ²³Na-MRI volunteer measurements of the head or the heart were performed in less than 18 min on a 1.5-T clinical scanner with an isotropic resolution of 10 mm³. The signal to noise ratio of the radial projection technique is twofold higher than that of a Cartesian gradient echo pulse sequence (TE = 3.2 ms). Radial ²³Na-MRI provides a tool for clinical studies, aiming at the differentiation of viable and non-viable tissue.