Influence of Gadolinium-Based Contrast Agents on Tissue Sodium Quantification in Sodium Magnetic Resonance Imaging:

Dynamics of Ionic and Cytotoxic Edema During Acute and Subacute Stages of Patients With Ischemic Stroke: Complementarity of 23Na MRI and Diffusion MRI

Cerebral imaging is crucial in the diagnosis and treatment algorithm of acute stroke to determine salvageable brain tissue. While diffusion MRI is commonly used to define the ischemic core, it cannot reliably distinguish irreversibly damaged from salvageable tissue. We investigated the added value of 23Na MRI to define irreversible necrotic tissue after a stroke. Fifteen patients with acute stroke involving medial cerebral artery occlusion were longitudinally explored with conventional and 23Na MRI within 24 h, 70 h following stroke and at 3 months to characterize the necrotic area. Time-courses of sodium accumulations were observed within regions presenting with or spared by cytotoxic/ionic edema and converting or not to necrosis. Dynamics of sodium accumulations were very different across subjects. At the group level, time-courses of sodium signal in cytotoxic edema showed a non-linear increase with an upper asymptote of 59 ± 6%% relative to the contralateral hemisphere. Regions with a larger early increase in 23Na signal (ionic edema) showed a non-linear accumulation during the first 70 h and were associated with subsequent necrosis at month 3. Some of the regions with no ionic edema during the first 70 h became necrotic at month 3, showing that pejorative pathophysiological processes could worsen after 70 h following attack. Final necrotic volume was well predicted by the cytotoxic volume (ADC decrease) during the first 24 h, and by the volume of ionic edema during the subacute period (25-70 h) following attack. The regions showing ionic edema showed a non-linear increase of 23Na signal during the first 70 h, with larger sodium accumulations in regions converting to necrosis at month 3. It may be of interest to consider the role of ionic edema imaging in the 70 h after stroke and reperfusion, with a view to better understand stroke pathophysiology. Sodium MRI could add complementary information about the fate of cell necrosis within low ADC signal regions.

Radiosurgery-induced early changes in peritumoral tissue sodium concentration of brain metastases

BACKGROUND

Stereotactic radiosurgery (SRS) is an effective therapy for brain metastases. Response is assessed with serial 1H magnetic resonance imaging (MRI). Early markers for response are desirable to allow for individualized treatment adaption. Previous studies indicated that radiotherapy might have impact on tissue sodium concentration. Thus, 23Na MRI could provide early quantification of response to SRS.

PURPOSE

We investigated whether longitudinal detection of tissue sodium concentration alteration within brain metastases and their peritumoral tissue after SRS with 23Na MRI was feasible.

STUDY TYPE

Prospective.

POPULATION

Twelve patients with a total of 14 brain metastases from various primary tumors.

ASSESSMENT

23Na MRI scans were acquired from patients 2 days before, 5 days after, and 40 days after SRS. Gross tumor volume (GTV) and healthy-appearing regions were manually segmented on the MPRAGE obtained 2 days before SRS, onto which all 23Na MR images were coregistered. Radiation isodose areas within the peritumoral tissue were calculated with the radiation planning system. Tissue sodium concentration before and after SRS within GTV, peritumoral tissue, and healthy-appearing regions as well as the routine follow-up with serial MRI were evaluated.

STATISTICAL TESTS

Results were compared using Student's t-test and correlation was evaluated with Pearson's correlation coefficient.

RESULTS

We found a positive correlation between the tissue sodium concentration within the peritumoral tissue and radiation dosage. Two patients showed local progression and a differing tissue sodium concentration evolution within GTV and the peritumoral tissue compared to mean tissue sodium concentration of the other patients. No significant tissue sodium concentration changes were observed within healthy-appearing regions.

CONCLUSION

Tissue sodium concentration assessment within brain metastases and peritumoral tissue after SRS with 23Na MRI is feasible and might be able to quantify tissue response to radiation.

Comparison of triple quantum (TQ) TPPI and inversion recovery TQ TPPI pulse sequences at 9.4 and 21.1 T

PURPOSE

Both sodium T1 triple quantum (TQ) signal and T1 relaxation pathways have a unique sensitivity to the sodium molecular environment. In this study an inversion recovery time proportional phase increment (IRTQTPPI) pulse sequence was investigated for simultaneous and reliable quantification of sodium TQ signal and bi-exponential T1 relaxation times.

METHODS

The IRTQTPPI sequence combines inversion recovery TQ filtering and time proportional phase increment. The reliable and reproducible results were achieved by the pulse sequence optimized in three ways: (1) optimization of the nonlinear fit for the determination of both T1-TQ signal and T1 relaxation times; (2) suppression of unwanted signals by assessment of four different phase cycles; (3) nonlinear sampling during evolution time for optimal scan time without any compromises in fit accuracy. The relaxation times T1 and T2 and the TQ signals from IRTQTPPI and TQTPPI were compared between 9.4 and 21.1 T. The motional environment of the sodium nuclei was evaluated by calculation of correlation times and nuclear quadrupole interaction strengths.

RESULTS

Reliable measurements of the T1-TQ signals and T1 bi-exponential relaxation times were demonstrated. The fit parameters for all four phase cycles were in good agreement with one another, with a negligible influence of unwanted signals. The agar samples yielded normalized T1-TQ signals from 3% to 16% relative to single quantum (SQ) signals at magnetic fields of both 9.4 and 21.1 T. In comparison, the normalized T2-TQ signal was in the range 15%-35%. The TQ/SQ signal ratio was decreased at 21.1 T as compared with 9.4 T for both T1 and T2 relaxation pathways. The bi-exponential T1 relaxation time separation ranged from 15 to 18 ms at 9.4 T and 15 to 21 ms at 21.1 T. The T2 relaxation time separation was larger, ranging from 28 to 35 ms at 9.4 T and 37 to 40 ms at 21.1 T.

CONCLUSION

The IRTQTPPI sequence, while providing a less intensive TQ signal than TQTPPI, allows a simultaneous and reliable quantification of both the T1-TQ signal and T1 relaxation times. The unique sensitivities of the T1 and T2 relaxation pathways to different types of molecular motion provide a deeper understanding of the sodium MR environment.

Standardized brain tumor imaging protocols for clinical trials: current recommendations and tips for integration

Standardized MRI acquisition protocols are crucial for reducing the measurement and interpretation variability associated with response assessment in brain tumor clinical trials. The main challenge is that standardized protocols should ensure high image quality while maximizing the number of institutions meeting the acquisition requirements. In recent years, extensive effort has been made by consensus groups to propose different "ideal" and "minimum requirements" brain tumor imaging protocols (BTIPs) for gliomas, brain metastases (BM), and primary central nervous system lymphomas (PCSNL). In clinical practice, BTIPs for clinical trials can be easily integrated with additional MRI sequences that may be desired for clinical patient management at individual sites. In this review, we summarize the general concepts behind the choice and timing of sequences included in the current recommended BTIPs, we provide a comparative overview, and discuss tips and caveats to integrate additional clinical or research sequences while preserving the recommended BTIPs. Finally, we also reflect on potential future directions for brain tumor imaging in clinical trials.

Fibrosis imaging with multiparametric proton and sodium MRI in pig injury models

Chronic kidney disease (CKD) is common and has huge implications for health and mortality. It is aggravated by intrarenal fibrosis, but the assessment of fibrosis is limited to kidney biopsies, which carry a risk of complications and sampling errors. This calls for a noninvasive modality for diagnosing and staging intrarenal fibrosis. The current, exploratory study evaluates a multiparametric MRI protocol including sodium imaging (²³ Na-MRI) to determine the opportunities within this modality to assess kidney injury as a surrogate endpoint of fibrosis. The study includes 43 pigs exposed to ischemia-reperfusion injury (IRI) or unilateral ureteral obstruction (UUO), or serving as healthy controls. Fibrosis was determined using gene expression analysis of collagen. The medulla/cortex ratio of ²³ Na-MRI decreased in the injured kidney in the IRI pigs, but not in the UUO pigs (p = 0.0180, p = 0.0754). To assess the combination of MRI parameters in estimating fibrosis, we created a linear regression model consisting of the cortical apparent diffusion coefficient, ΔR2*, ΔT1, the ²³ Na medulla/cortex ratio, and plasma creatinine (R²  = 0.8009, p = 0.0117). The ²³ Na medulla/cortex ratio only slightly improved the fibrosis prediction model, leaving ²³ Na-MRI in an ambiguous place for evaluation of intrarenal fibrosis. Use of multiparametric MRI in combination with plasma creatinine shows potential for the estimation of fibrosis in human kidney disease, but more translational and clinical work is warranted before MRI can contribute to earlier diagnosis and evaluation of treatment for acute kidney injury and CKD.

Revealing the three-dimensional murine brain microstructure by contrast-enhanced computed tomography

To improve our understanding of the brain microstructure, high-resolution 3D imaging is used to complement classical 2D histological assessment techniques. X-ray computed tomography allows high-resolution 3D imaging, but requires methods for enhancing contrast of soft tissues. Applying contrast-enhancing staining agents (CESAs) ameliorates the X-ray attenuating properties of soft tissue constituents and is referred to as contrast-enhanced computed tomography (CECT). Despite the large number of chemical compounds that have successfully been applied as CESAs for imaging brain, they are often toxic for the researcher, destructive for the tissue and without proper characterization of affinity mechanisms. We evaluated two sets of chemically related CESAs (organic, iodinated: Hexabrix and CA4+ and inorganic polyoxometalates: 1:2 hafnium-substituted Wells-Dawson phosphotungstate and Preyssler anion), for CECT imaging of healthy murine hemispheres. We then selected the CESA (Hexabrix) that provided the highest contrast between gray and white matter and applied it to a cuprizone-induced demyelination model. Differences in the penetration rate, effect on tissue integrity and affinity for tissue constituents have been observed for the evaluated CESAs. Cuprizone-induced demyelination could be visualized and quantified after Hexabrix staining. Four new non-toxic and non-destructive CESAs to the field of brain CECT imaging were introduced. The added value of CECT was shown by successfully applying it to a cuprizone-induced demyelination model. This research will prove to be crucial for further development of CESAs for ex vivo brain CECT and 3D histopathology.

Comprehensive Account of Sodium Imaging and Spectroscopy for Brain Research

Sodium (²³Na) is a vital component of neuronal cells and plays a key role in various signal transmission processes. Hence, information on sodium distribution in the brain using magnetic resonance imaging (MRI) provides useful information on neuronal health. ²³Na MRI and MR spectroscopy (MRS) improve the diagnosis, prognosis, and clinical monitoring of neurological diseases but confront some inherent limitations that lead to low signal-to-noise ratio, longer scan time, and diminished partial volume effects. Recent advancements in multinuclear MR technology have helped in further exploration in this domain. We aim to provide a comprehensive description of ²³Na MRI and MRS for brain research including the following aspects: (a) theoretical background for understanding ²³Na MRI and MRS fundamentals; (b) technological advancements of ²³Na MRI with respect to pulse sequences, RF coils, and sodium compartmentalization; (c) applications of ²³Na MRI in the early diagnosis and prognosis of various neurological disorders; (d) structural-chronological evolution of sodium spectroscopy in terms of its numerous applications in human studies; (e) the data-processing tools utilized in the quantitation of sodium in the respective anatomical regions.

Longitudinal Sodium MRI of Multiple Sclerosis Lesions: Is there Added Value of Sodium Inversion Recovery MRI

BACKGROUND

Sodium enhancement has been demonstrated in multiple sclerosis (MS) lesions.

PURPOSE

To investigate sodium MRI with and without an inversion recovery pulse in acute MS lesions in an MS relapse and during recovery.

STUDY TYPE

Prospective.

SUBJECTS

Twenty-nine relapsing-remitting MS patients with an acute relapse were included.

FIELD STRENGTH/SEQUENCE

A 3D density-adapted radial sodium sequence at 3 T using a dual-tuned (²³ Na/¹ H) head coil.

ASSESSMENT

Full-brain images of the tissue sodium concentration (TSC1, n = 29) and a sodium inversion recovery sequence (SIR1, n = 20) at the beginning of the anti-inflammatory therapy and on medium-term follow-up visits (days 27-99, n = 12 [TSC], n = 5 [SIR]) were measured. Regions of interest (RoIs) with contrast enhancement (T₁ CE+) and without change in T1-weighted imaging (FL + T1n) were normalized (nTSC and nSIR). To gain insight on the origin of the TSC enhancement at time point 1, it is investigated whether the nTSC enhancement of the lesions is accompanied by a change of the respective nSIR. Potential prognostic value of nSIR1 is examined referring to the nTSC progression. STATISTICAL TESTS: nTSC and nSIR were compared regarding the type of lesion and the time point using a one-way ANOVA. Pearson's correlation coefficient was calculated for nTSC over nSIR and for nTSC1-nTSC2 over nSIR1. A P-value <0.05 was considered statistically significant.

RESULTS

At the first measurement, all lesion types showed increased nTSC, while nSIR was decreased in the FL + T₁ n and the T₁ CE+ lesions in comparison to the normal-appearing white matter. For acute lesions, the difference between nTSC at baseline and nTSC at time point 2 showed a significant correlation with the baseline nSIR.

DATA CONCLUSION

At time point 1, nTSC is increased, while nSIR is unchanged or decreased in the lesions. The mean sodium IR signal at baseline correlates with recovery or progression of an acute lesion.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 4.

Evaluation of Sodium (23Na) MR-imaging as a Biomarker and Predictor for Neurodegenerative Changes in Patients With Alzheimer's Disease

BACKGROUND/AIM

Sodium (²³Na) MR imaging is a noninvasive MRI technique that has been shown to be sensitive to visualize biochemical information about tissue viability, their cell integrity, and cell function in various studies. The aim of this study was to evaluate differences in regional brain ²³Na signal intensity between Alzheimer's disease (AD) and healthy controls to preliminarily evaluate the capability of ²³Na imaging as a biomarker for AD.

PATIENTS AND METHODS

A total of 14 patients diagnosed with AD were included: 12 in the state of dementia and 2 with mild cognitive impairment (MCI), and 12 healthy controls (HC); they were all scanned on a 3T clinical scanner with a double tuned ¹H/²³Na birdcage head coil. After normalizing the signal intensity with that of the vitreous humor, relative tissue sodium concentration (rTSC) was measured after automated segmentation in the hippocampus, amygdala, basal ganglia, white matter (WM) and grey matter (GM) in both cerebral hemispheres.

RESULTS

Patients with AD showed a significant increase in rTSC in comparison to healthy controls in the following brain regions: WM 13.6%; p=0.007, hippocampus 12.9%; p=0.003, amygdala 18.9%; p=0.0007.

CONCLUSION

²³Na-MRI has the potential to be developed as a useful biomarker for the diagnosis of AD.

MRI Detection of Changes in Tissue Sodium Concentration in Brain Metastases after Stereotactic Radiosurgery: A Feasibility Study

BACKGROUND AND PURPOSE

To date, treatment response to stereotactic radiosurgery (SRS) in brain metastases (BM) can only be determined by MRI evaluation of contrast-enhancing lesions in a long-time follow-up. Sodium MRI has been a subject of immense interest in imaging research as the measure of tissue sodium concentration (TSC) can give valuable quantitative information on cell viability. We aimed to analyze the longitudinal changes of TSC in BM measured with ²³ Na MRI before and after SRS for assessment of early local tumor effects.

METHODS

Seven patients with a total of 12 previously untreated BM underwent SRS with 22 Gy. In addition to a standard MRI protocol including dynamic susceptibility-weighted contrast-enhanced perfusion, a ²³ Na MRI was performed at three time points: (I) 2 days before, (II) 5 days, and (III) 40 days after SRS. Nine BMs were evaluated. The absolute TSC in the BM, the respective peritumoral edemas, and the normal-appearing corresponding contralateral brain area were assessed and the relative TSC were correlated to the changes in BM longest axial diameters.

RESULTS

TSC was elevated in nine BM at baseline before SRS with a mean of 73.4 ± 12.3 mM. A further increase in TSC was observed 5 days after SRS in all the nine BM with a mean of 86.9 ± 13 mM. Eight of nine BM showed a mean 60.6 ± 13.3% decrease in the longest axial diameter 40 days after SRS; at this time point, the TSC also had decreased to a mean 65.1 ± 7.9 mM. In contrast, one of the nine BM had a 13.4% increase in the largest axial diameter at time point III. The TSC of this BM showed a further TSC increase of 80.1 mM 40 days after SRS.

CONCLUSION

Changes in TSC using ²³ Na MRI shows the possible capability to detect radiobiological changes in BM after SRS.

23 Na-T1 quantification with saturation recovery TrueFISP and variable flip angle GRE at 3T: A phantom study

PURPOSE

The aim of the current study was to compare the reproducibility of sodium (²³ Na)-T₁ estimation using a centric-reordered saturation recovery (SR) true fast imaging with steady-state precession (TrueFISP) and a variable flip angle (VFA) spoiled gradient echo (GRE). Additionally, we evaluated the effect of spatial averaging on ²³ Na-T₁ estimation by the two methods.

METHODS

Measurements were performed in the phantom, consisting of 10 dm³ volume rectangular polyethylene container filled with distilled water solution of 0.6% NaCl + 0.004% CuSO₄ , using a dual-tunable ²³ Na/¹ H coil at 3 Tesla. ²³ Na images were acquired for FOV = 384 × 384 mm² and voxel size = 6 × 6 × 6 mm³ using: (1) TrueFISP: TR/TE = 900/1.5 ms, flip angle = 90°, bandwidth = 450 Hz/px, and (2) GRE: TR/TE = 30/1.5 ms, bandwidth = 350 Hz/px. ²³ Na-T₁ weightings were obtained with nonselective saturation prepulses delayed from the center of the k-space acquisition by 25/40/60/130/280 ms (SR-TrueFISP) and by applying different nominal flip angles: 10°/30°/50°/70°/90° (VFA-GRE). Both sequences were acquired twice, applying 20 and 30 spatial averages. The resulting images were B₁ -corrected with a double-angle GRE method.

RESULTS

Image acquisition varied from 5:41 to 9:37 for TrueFISP and from 12:48 to 19:12 min for GRE using 20 and 30 spatial averages, respectively. Higher averaging increased the acquisition time by 53% and mean SNR at scan < 10%, without an effect on ²³ Na-T₁ estimations with both methods (SR-Truefisp |Δ| = 1.58 ms, VFA-GRE |Δ| = 0.53 ms; for SNR P < .001). Overall, mean ± SD of ²³ Na-T₁ was found as 51 ± 3 ms with SR-TrueFISP and 53 ± 2 ms with VFA-GRE.

CONCLUSION

Both SR-TrueFISP and VFA-GRE provided similar ²³ Na-T₁ estimates based on the phantom measurements with isotropic resolution.

Potential of Sodium MRI as a Biomarker for Neurodegeneration and Neuroinflammation in Multiple Sclerosis

In multiple sclerosis (MS), experimental and ex vivo studies indicate that pathologic intra- and extracellular sodium accumulation may play a pivotal role in inflammatory as well as neurodegenerative processes. Yet, in vivo assessment of sodium in the microenvironment is hard to achieve. Here, sodium magnetic resonance imaging (²³NaMRI) with its non-invasive properties offers a unique opportunity to further elucidate the effects of sodium disequilibrium in MS pathology in vivo in addition to regular proton based MRI. However, unfavorable physical properties and low in vivo concentrations of sodium ions resulting in low signal-to-noise-ratio (SNR) as well as low spatial resolution resulting in partial volume effects limited the application of ²³NaMRI. With the recent advent of high-field MRI scanners and more sophisticated sodium MRI acquisition techniques enabling better resolution and higher SNR, ²³NaMRI revived. These studies revealed pathologic total sodium concentrations in MS brains now even allowing for the (partial) differentiation of intra- and extracellular sodium accumulation. Within this review we (1) demonstrate the physical basis and imaging techniques of ²³NaMRI and (2) analyze the present and future clinical application of ²³NaMRI focusing on the field of MS thus highlighting its potential as biomarker for neuroinflammation and -degeneration.