Iterative reconstruction of radially-sampled 31P bSSFP data using prior information from 1H MRI.

The purpose of this study is to improve direct phosphorus (31P) MR imaging. Therefore, 3D density-adapted radially-sampled balanced steady-state free precession (bSSFP) sequences were developed and an iterative approach exploiting additional anatomical information from hydrogen (1H) data was evaluated. Three healthy volunteers were examined at B0=7T in order to obtain the spatial distribution of the phosphocreatine (PCr) intensities in the human calf muscle with a nominal isotropic resolution of 10mm in an acquisition time of 10min. Three different bSSFP gradient schemes were investigated. The highest signal-to-noise ratio (SNR) was obtained for a scheme with two point-reflected density-adapted gradients. Furthermore, the conventional reconstruction based on a gridding algorithm was compared to an iterative method using an 1H MRI constraint in terms of a segmented binary mask, which comprises prior knowledge. The parameters of the iterative approach were optimized and evaluated by simulations featuring 31P MRI parameters. Thereby, partial volume effects as well as Gibbs ringing artifacts could be reduced. In conclusion, the iterative reconstruction of 31P bSSFP data using an 1H MRI constraint is appropriate for investigating regions where sharp tissue boundaries occur and leads to images that represent the real PCr distributions better than conventionally reconstructed images.

Sodium MRI in Multiple Sclerosis is Compatible with Intracellular Sodium Accumulation and Inflammation-Induced Hyper-Cellularity of Acute Brain Lesions.

The cascade of inflammatory pathogenetic mechanisms in multiple sclerosis (MS) has no specific conventional MRI correlates. Clinicians therefore stipulate improved imaging specificity to define the pathological substrates of MS in vivo including mapping of intracellular sodium accumulation. Based upon preclinical findings and results of previous sodium MRI studies in MS patients we hypothesized that the fluid-attenuated sodium signal differs between acute and chronic lesions. We acquired brain sodium and proton MRI data of N = 29 MS patients; lesion type was defined by the presence or absence of contrast enhancement. N = 302 MS brain lesions were detected, and generalized linear mixed models were applied to predict lesion type based on sodium signals; thereby controlling for varying numbers of lesions among patients and confounding variables such as age and medication. Hierarchical model comparisons revealed that both sodium signals average tissue (χ(2)(1) = 27.89, p < 0.001) and fluid-attenuated (χ(2)(1) = 5.76, p = 0.016) improved lesion type classification. Sodium MRI signals were significantly elevated in acute compared to chronic lesions compatible with intracellular sodium accumulation in acute MS lesions. If confirmed in further studies, sodium MRI could serve as biomarker for diagnostic assessment of MS, and as readout parameter in clinical trials promoting attenuation of chronic inflammation.

Evaluation of adaptive combination of 30-channel head receive coil array data in 23Na MR imaging.

PURPOSE: The aim was to optimally combine multichannel coil array data in sodium ((23) Na) MRI. METHODS: (23) Na MRI was conducted on a 3 Tesla MR system using a 30-channel head receive coil array. The parameters used for the adaptive combination (ADC) reconstruction of the low signal-to-noise ratio (SNR) dataset have been optimized by finding the maximum mean SNR. A pseudo multiple-replica approach has been used to obtain SNR maps of the combined images. To prove reproducibility of the combination algorithm, the procedure was repeated for several measurements. RESULTS: For low SNR data, sum-of-squares (SOS) reconstruction leads to high background noise and a signal bias in the imaged object. The ADC reconstruction clearly reduces noise in the image and leads to an increase of the mean SNR in the range of 8% to 50%, compared to weighted SOS depending on the absolute SNR of the image. The evaluation of the effects of different noise scans showed that a small number of projections can be used to estimate noise statistics of the coil array without substantially decreasing the resulting SNR. CONCLUSION: (23) Na MRI can be markedly improved by using a 30-channel receive array and ADC reconstruction. The ADC reconstruction showed robust results for all measurements without the need for sensitivity maps.

Partial volume correction for in vivo (23)Na-MRI data of the human brain.

The concentration of sodium is a functional cell parameter and absolute quantification can be interesting for diagnostical purposes. The accuracy of sodium magnetic resonance imaging ((23)Na-MRI) is strongly biased by partial volume effects (PVEs). Hence our purpose was to establish a partial volume correction (PVC) method for (23)Na-MRI. The existing geometric transfer matrix (GTM) correction method was transferred from positron emission tomography (PET) to (23)Na-MRI and tested in a phantom study. Different parameters, as well as accuracy of registration and segmentation were evaluated prior to first in vivo measurements. In vivo sodium data-sets of the human brain were obtained at B0=7T with a nominal spatial resolution of (3mm)(3) using a density adapted radial pulse sequence. A volunteer study with four healthy subjects was performed to measure partial volume (PV) corrected tissue sodium concentration (TSC) which was verified by means of an intrinsic correction control. In the phantom study the PVC algorithm yielded a good correction performance and reduced the discrepancy between the measured sodium concentration value and the expected value in the smallest compartments of the phantom by 11% to a mean PVE induced discrepancy of 5.7% after correction. The corrected in vivo data showed a reduction of PVE bias for the investigated compartments for all volunteers, resulting in a mean reduction of discrepancy between two separate CSF compartments from 36% to 7.6%. The absolute TSC for two separate CSF compartments (sulci, lateral ventricles), gray and white brain matter after correction were 129±8mmol/L, 138±4mmol/L, 48±1mmol/L and 43±3mmol/L, respectively. The applied PVC algorithm reduces the PV-bias in quantitative (23)Na-MRI. Accurate, high-resolution anatomical data is required to enable appropriate PVC. The algorithm and segmentation approach is robust and leads to reproducible results.

Two-pulse biexponential-weighted 23Na imaging.

A new method is proposed for acquiring 3D biexponential-weighted sodium images with two instead of three RF pulses to allow for shorter repetition time at high magnetic fields (B0≥7 T) and reduced SAR. The second pulse converts single- into triple-quantum coherences in regions containing sodium ions which are restricted in mobility. Since only single-quantum coherences can be detected, an image acquired after the second pulse is intrinsically single-quantum-filtered and can be used to generate a biexponential-weighted sodium image by a weighted subtraction with the spin-density-weighted image acquired between the pulses. The proposed sequence generates biexponential-weighted sodium images of in vivo human brain with 140% higher SNR than triple-quantum-filtered sodium images and 4% higher SNR than a biexponential-weighted sequence with three RF pulses at equal acquisition time and with 1/3 lower SAR. As SAR is reduced, accordingly repetition time can be spared to obtain even higher SNR-time efficiency. In comparison to a difference image generated from two images of a double-readout sequence, the proposed two-pulse sequence yields about 14% higher SNR. Our new two-pulse biexponential-weighted sequence allows for acquisition of full 3D data sets of the human brain in vivo with a nominal resolution of (5 mm)(3) in about 10 min.

Three-dimensional biexponential weighted (23)Na imaging of the human brain with higher SNR and shorter acquisition time.

A new method is presented for acquiring 3D biexponential weighted sodium images of the in vivo human brain with up to three times higher signal-to-noise ratio compared with conventional six-step phase-cycling triple-quantum-filtered imaging. To excite and detect multiple-quantum coherences, a three-pulse preparation is used. During the pulse train, two images are obtained. The first image is acquired with ultrashort echo time (0.3 ms) during preparation between the first two pulses to yield a spin-density-weighted image. After the last pulse, a single-quantum-filtered image is acquired with an echo time of 11 ms that maximizes the resulting signal. The biexponential weighted image is calculated by subtracting the single-quantum-filtered image from the spin-density-weighted image. The resulting image mainly shows signal from sodium ions with biexponential quadrupolar relaxation behavior. In isotropic environments, the resulting image mainly contains triple-quantum-filtered signal. The four-step phase cycling yields similar signal-to-noise ratio in shorter acquisition time compared with six-step phase-cycling biexponential weighted imaging.