Impact of gradient timing error on the tissue sodium concentration bioscale measured using flexible twisted projection imaging

Quantitative 7T sodium magnetic resonance imaging of the human brain using a 32-channel phased-array head coil: Application to patients with secondary progressive multiple sclerosis

Apparent tissue sodium concentrations (aTSCs) determined by ²³ Na brain magnetic resonance imaging (MRI) have the potential to serve as a biomarker in pathologies such as multiple sclerosis (MS). However, the quantification is hindered by the intrinsically low signal-to-noise ratio of ²³ Na MRI. The purpose of this study was to improve the accuracy and reliability of quantitative ²³ Na brain MRI by implementing a dedicated postprocessing pipeline and to evaluate the applicability of the developed approach for the examination of MS patients. ²³ Na brain MRI measurements of 13 healthy volunteers and 17 patients with secondary progressive multiple sclerosis (SPMS) were performed at 7 T using a dual-tuned ²³ Na/¹ H birdcage coil with a receive-only 32-channel phased array. The aTSC values were determined for normal appearing white matter (NAWM) and normal appearing gray matter (NAGM) in healthy subjects and SPMS patients. Signal intensities were normalized using the mean cerebrospinal fluid (CSF) sodium concentration determined in 37 separate patients receiving a spinal tap for routine diagnostic purposes. Five volunteers underwent MRI examinations three times in a row to assess repeatability. Coefficients of variation (CoVs) were used to quantify the repeatability of the proposed method. aTSC values were compared regarding brain regions and subject cohort using the paired-samples Wilcoxon rank-sum test. Laboratory CSF sodium concentration did not differ significantly between patients without and with MS (p = 0.42). The proposed quantification workflow for ²³ Na MRI was highly repeatable with CoVs averaged over all five volunteers of 1.9% ± 0.9% for NAWM and 2.2% ± 1.6% for NAGM. Average NAWM aTSC was significantly higher in patients with SPMS compared with the control group (p = 0.009). Average NAGM aTSC did not differ significantly between healthy volunteers and MS patients (p = 0.98). The proposed postprocessing pipeline shows high repeatability and the results can serve as a baseline for further studies establishing ²³ Na brain MRI as a biomarker in diseases such as MS.

Motion reduction for quantitative brain sodium MR imaging with a navigated flexible twisted projection imaging sequence at 9.4 T

Quantitative measurement of the tissue sodium concentration (TSC) provides a metric for tissue cell volume fraction for monitoring tumor responses to therapy and neurodegeneration in the brain as well as applications outside the central nervous system such as the fixed charge density in cartilage. Despite the low detection sensitivity of the sodium MR signal compared to the proton signal and the requirement for a long repetition time to minimize longitudinal magnetization saturation, acquisition time has been reduced to less than 10 min for a nominal isotropic voxel size of 3.3 mm with the improved acquisition efficiency of twisted projection imaging (TPI) at 9.4 T. However, patient motion can degrade the accuracy of the quantification even within these acquisition times. Our goal has been to improve the robustness of quantitative sodium MR imaging by minimizing the impact of motion that may occur even in cooperative patients. We present a method to spatially encode a lower resolution navigator echo after encoding the free induction decay signal for the quantitative image at no time penalty. Both the imaging and navigator data are sampled with flexTPI readout trajectories. Navigator images are generated at a higher temporal resolution (∼1 min) albeit at lower spatial resolution (8 mm) than the quantitative high-resolution images. The multiple volumes of navigator echo images are then aligned to extract the translational and rotational motion parameters assuming rigid-body motion. These parameters are used to align the k-space data during the acquisition of each volume of the quantitative images. Our results show significantly reduced image blurring with this method when the subject's head moved randomly by up to 7° between the navigator acquisitions.

High-resolution quantitative sodium imaging at 9.4 Tesla

PURPOSE

Investigation of the feasibility to perform high-resolution quantitative sodium imaging at 9.4 Tesla (T).

METHODS

A proton patch antenna was combined with a sodium birdcage coil to provide a proton signal without compromising the efficiency of the X-nucleus coil. Sodium density weighted images with a nominal resolution of 1 × 1 × 5 mm(3) were acquired within 30 min with an ultrashort echo time sequence. The methods used for signal calibration as well as for B0, B1, and off-resonance correction were verified on a phantom and five healthy volunteers.

RESULTS

An actual voxel volume of roughly 40 μL could be achieved at 9.4T, while maintaining an acceptable signal-to-noise ratio (8 for brain tissue and 35 for cerebrospinal fluid). The measured mean sodium concentrations for gray and white matter were 36 ± 2 and 31 ± 1 mmol/L of wet tissue, which are comparable to values previously reported in the literature.

CONCLUSION

The reduction of partial volume effects is essential for accurate measurement of the sodium concentration in the human brain. Ultrahigh field imaging is a viable tool to achieve this goal due to its increased sensitivity.

Measurement techniques for magnetic resonance imaging of fast relaxing nuclei

In this review article, techniques for sodium ((23)Na) magnetic resonance imaging (MRI) are presented. These techniques can also be used to image other nuclei with short relaxation times (e.g., (39)K, (35)Cl, (17)O). Twisted projection imaging, density-adapted 3D projection reconstruction, and 3D cones are preferred because of uniform k-space sampling and ultra-short echo times. Sampling density weighted apodization can be applied if intrinsic filtering is desired. This approach leads to an increased signal-to-noise ratio compared to postfiltered acquisition in cases of short readout durations relative to T 2 (*) relaxation time. Different MR approaches for anisotropic resolution are presented, which are important for imaging of thin structures such as myocardium, cartilage, and skin. The third part of this review article describes different methods to put more weighting either on the intracellular or the extracellular sodium signal by means of contrast agents, relaxation-weighted imaging, or multiple-quantum filtering.

PCr/ATP ratio mapping of the human head by simultaneously imaging of multiple spectral peaks with interleaved excitations and flexible twisted projection imaging readout trajectories at 9.4 T

Quantitative (31)P magnetic resonance imaging of the whole human brain is often time-consuming even at low spatial resolution due to the low concentrations, long T(1) relaxation times, and low detection sensitivity of phosphorus metabolites. We report herein the results of combining the increased detection sensitivity of an ultra-high field 9.4 T scanner designed for human imaging with a new pulse sequence termed simultaneously imaging of multiple spectral peaks with interleaved excitations and flexible twisted projection imaging readout trajectories to rapidly sample multiple resonances in the (31)P spectrum. The phosphocreatine and γ-adenosine triphosphate images, obtained simultaneously from the entire human head, are demonstrated at 1.5 cm isotropic nominal resolution in a total acquisition time of 33 min. The phosphocreatine/γ-adenosine triphosphate ratio calculated for brain parenchyma (1-2) and the superficial temporalis muscle (3-5) are in agreement with literature values.

Sodium MRI and the assessment of irreversible tissue damage during hyper-acute stroke

Sodium MRI (sMRI) has undergone a tremendous amount of technical development during the last two decades that makes it a suitable tool for the study of human pathology in the acute setting within the constraints of a clinical environment. The salient role of the sodium ion during impaired ATP production during the course of brain ischemia makes sMRI an ideal tool for the study of ischemic tissue viability during stroke. In this paper, the current limitations of conventional MRI for the determination of tissue viability during evolving brain ischemia are discussed. This discussion is followed by a summary of the known findings about the dynamics of tissue sodium changes during brain ischemia. A mechanistic model for the explanation of these findings is presented together with the technical requirements for its investigation using clinical MRI scanners. An illustration of the salient features of the technique is also presented using a nonhuman primate model of reversible middle cerebral artery occlusion.