Serial triple quantum sodium MRI during non-human primate focal brain ischemia

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.

Sodium Triple Quantum MR Signal Extraction Using a Single-Pulse Sequence with Single Quantum Time Efficiency

PURPOSE

Sodium triple quantum (TQ) signal has been shown to be a valuable biomarker for cell viability. Despite its clinical potential, application of Sodium TQ signal is hindered by complex pulse sequences with long scan times. This study proposes a method to approximate the TQ signal using a single excitation pulse without phase cycling.

METHODS

The proposed method is based on a single excitation pulse and a comparison of the free induction decay (FID) with the integral of the FID combined with a shifting reconstruction window. The TQ signal is calculated from this FID only. As a proof of concept, the method was also combined with a multi-echo UTE imaging sequence on a 9.4 T preclinical MRI scanner for the possibility of fast TQ MRI.

RESULTS

The extracted Sodium TQ signals of single-pulse and spin echo FIDs were in close agreement with theory and TQ measurement by traditional three-pulse sequence (TQ time proportional phase increment [TQTPPI)]. For 2%, 4%, and 6% agar samples, the absolute deviations of the maximum TQ signals between SE and theoretical (time proportional phase increment TQTPPI) TQ signals were less than 1.2% (2.4%), and relative deviations were less than 4.6% (6.8%). The impact of multi-compartment systems and noise on the accuracy of the TQ signal was small for simulated data. The systematic error was <3.4% for a single quantum (SQ) SNR of 5 and at maximum <2.5% for a multi-compartment system. The method also showed the potential of fast in vivo SQ and TQ imaging.

CONCLUSION

Simultaneous SQ and TQ MRI using only a single-pulse sequence and SQ time efficiency has been demonstrated. This may leverage the full potential of the Sodium TQ signal in clinical applications.

Multidimensional compressed sensing to advance 23 Na multi-quantum coherences MRI

PURPOSE

Sodium (23 Na) multi-quantum coherences (MQC) MRI was accelerated using three-dimensional (3D) and a dedicated five-dimensional (5D) compressed sensing (CS) framework for simultaneous Cartesian single (SQ) and triple quantum (TQ) sodium imaging of in vivo human brain at 3.0 and 7.0 T.

THEORY AND METHODS

3D 23 Na MQC MRI requires multi-echo paired with phase-cycling and exhibits thus a multidimensional space. A joint reconstruction framework to exploit the sparsity in all imaging dimensions by extending the conventional 3D CS framework to 5D was developed. 3D MQC images of simulated brain, phantom and healthy brain volunteers obtained from 3.0 T and 7.0 T were retrospectively and prospectively undersampled. Performance of the CS models were analyzed by means of structural similarity index (SSIM), root mean squared error (RMSE), signal-to-noise ratio (SNR) and signal quantification of tissue sodium concentration and TQ/SQ ratio.

RESULTS

It was shown that an acceleration of three-fold, leading to less than2 × 10 $$ 2\times 10 $$ min of scan time with a resolution of8 × 8 × 20   mm 3 $$ 8\times 8\times 20\;{\mathrm{mm}}^3 $$ at 3.0 T, are possible. 5D CS improved SSIM by 3%, 5%, 1% and reduced RMSE by 50%, 30%, 8% for in vivo SQ, TQ, and TQ/SQ ratio maps, respectively. Furthermore, for the first time prospective undersampling enabled unprecedented high resolution from8 × 8 × 20   mm 3 $$ 8\times 8\times 20\;{\mathrm{mm}}^3 $$ to6 × 6 × 10   mm 3 $$ 6\times 6\times 10\;{\mathrm{mm}}^3 $$ MQC images of in vivo human brain at 7.0 T without extending acquisition time.

CONCLUSION

5D CS proved to allow up to three-fold acceleration retrospectively on 3.0 T data. 2-fold acceleration was demonstrated prospectively at 7.0 T to reach higher spatial resolution of 23 Na MQC MRI.

Sodium MR Neuroimaging

Sodium MR imaging has the potential to complement routine proton MR imaging examinations with the goal of improving diagnosis, disease characterization, and clinical monitoring in neurologic diseases. In the past, the utility and exploration of sodium MR imaging as a valuable clinical tool have been limited due to the extremely low MR signal, but with recent improvements in imaging techniques and hardware, sodium MR imaging is on the verge of becoming clinically realistic for conditions that include brain tumors, ischemic stroke, and epilepsy. In this review, we briefly describe the fundamental physics of sodium MR imaging tailored to the neuroradiologist, focusing on the basics necessary to understand factors that play into making sodium MR imaging feasible for clinical settings and describing current controversies in the field. We will also discuss the current state of the field and the potential future clinical uses of sodium MR imaging in the diagnosis, phenotyping, and therapeutic monitoring in neurologic diseases.

Intracellular Sodium Changes in Cancer Cells Using a Microcavity Array-Based Bioreactor System and Sodium Triple-Quantum MR Signal

The sodium triple-quantum (TQ) magnetic resonance (MR) signal created by interactions of sodium ions with macromolecules has been demonstrated to be a valuable biomarker for cell viability. The aim of this study was to monitor a cellular response using the sodium TQ signal during inhibition of Na/K-ATPase in living cancer cells (HepG2). The cells were dynamically investigated after exposure to 1 mM ouabain or K+-free medium for 60 min using an MR-compatible bioreactor system. An improved TQ time proportional phase incrementation (TQTPPI) pulse sequence with almost four times TQ signal-to-noise ratio (SNR) gain allowed for conducting experiments with 12–14 × 106 cells using a 9.4 T MR scanner. During cell intervention experiments, the sodium TQ signal increased to 138.9 ± 4.1% and 183.4 ± 8.9% for 1 mM ouabain (n = 3) and K+-free medium (n = 3), respectively. During reperfusion with normal medium, the sodium TQ signal further increased to 169.2 ± 5.3% for the ouabain experiment, while it recovered to 128.5 ± 6.8% for the K+-free experiment. These sodium TQ signal increases agree with an influx of sodium ions during Na/K-ATPase inhibition and hence a reduced cell viability. The improved TQ signal detection combined with this MR-compatible bioreactor system provides a capability to investigate the cellular response of a variety of cells using the sodium TQ MR signal.

Protein conformational changes affect the sodium triple-quantum MR signal

The aim of this study was to investigate possible sodium triple-quantum (TQ) signal dependence on pH variation and protein unfolding which may happen in vivo. The model system, composed of bovine serum albumin (BSA), was investigated over a wide pH range of 0.70 to 13.05 and during urea-induced unfolding. In both experimental series, the sodium and BSA concentration were kept constant so that TQ signal changes solely arose from an environmental change. The experiments were performed using unique potential to detect weak TQ signals by implementing a TQ time proportional phase increment pulse sequence. At a pH of 0.70, in which case the effect of the negatively charged groups was minimized, the minimum TQ percentage relative to single-quantum of 1.34% ± 0.05% was found. An increase of the pH up to 13.05 resulted in an increase of the sodium TQ signal by 225%. Urea-induced unfolding of BSA, without changes in pH, led to a smaller increase in the sodium TQ signal of up to 40%. The state of BSA unfolding was verified by fluorescence microscopy. Results of both experiments were well fitted by sigmoid functions. Both TQ signal increases were in agreement with an increase of the availability of negatively charged groups. The results point to vital contributions of the biochemical environment to the TQ MR signals. The sodium TQ signal in vivo could be a valuable biomarker of cell viability, and therefore possible effects of pH and protein unfolding need to be considered for a proper interpretation of changes in sodium TQ signals.

Elevated cerebrospinal fluid sodium in hypertensive human subjects with a family history of Alzheimer's disease

High salt (sodium) intake leads to the development of hypertension despite the fact that plasma sodium concentration ([Na⁺]) is usually normal in hypertensive human patients. Increased cerebrospinal fluid (CSF) sodium contributes to elevated sympathetic activity and high blood pressure (BP) in rodent models of hypertension. However, whether there is an increased accumulation of sodium in the CSF of humans with chronic hypertension is not well defined. Here, we investigated CSF [Na⁺] from hypertensive and normotensive human subjects with family histories of Alzheimer's disease in samples collected in a clinical trial, as spinal tap is not a routine clinical procedure for hypertensive patients. The [Na⁺] and osmolality in plasma and CSF were measured by flame photometry. Daytime ambulatory BP was monitored while individuals were awake. Participants were deidentified and data were analyzed in conjunction with a retrospective analysis of patient history and diagnosis. We found that CSF [Na⁺] was significantly higher in participants with high BP compared with normotensive participants; there was no difference in plasma [Na⁺], or plasma and CSF osmolality between groups. Subsequent multiple linear regression analyses controlling for age, sex, race, and body mass index revealed a significant positive correlation between CSF [Na⁺] and BP but showed no correlation between plasma [Na⁺] and BP. In sum, CSF [Na⁺] was higher in chronic hypertensive individuals and may play a key role in the pathogenesis of human hypertension. Collectively, our findings provide evidence for the clinical significance of CSF [Na⁺] in chronic hypertension in humans.

Imaging of sodium in the brain: a brief review

Sodium-based MRI plays a vital role in the study of metabolism and can unveil valuable information about emerging and existing pathology--in particular in the human brain. Sodium is the second most abundant MR active nucleus in living tissue and, due to its quadrupolar nature, has magnetic properties not common to conventional proton MRI, which can reveal further insights, such as information on the compartmental distribution of intra- and extracellular sodium. Nevertheless, the use of sodium nuclei for imaging comes at the expense of a lower sensitivity and significantly reduced relaxation times, making in vivo sodium studies feasible only at high magnetic field strength and by the use of dedicated pulse sequences. Hybrid imaging combining sodium MRI and positron emission tomography (PET) simultaneously is a novel and promising approach to access information on dynamic metabolism with much increased, PET-derived specificity. Application of this new methodology is demonstrated herein using examples from tumour imaging.

Sodium-23 magnetic resonance imaging has potential for improving penumbra detection but not for estimating stroke onset time

Tissue sodium concentration increases in irreversibly damaged (core) tissue following ischemic stroke and can potentially help to differentiate the core from the adjacent hypoperfused but viable penumbra. To test this, multinuclear hydrogen-1/sodium-23 magnetic resonance imaging (MRI) was used to measure the changing sodium signal and hydrogen-apparent diffusion coefficient (ADC) in the ischemic core and penumbra after rat middle cerebral artery occlusion (MCAO). Penumbra and core were defined from perfusion imaging and histologically defined irreversibly damaged tissue. The sodium signal in the core increased linearly with time, whereas the ADC rapidly decreased by >30% within 20 minutes of stroke onset, with very little change thereafter (0.5-6 hours after MCAO). Previous reports suggest that the time point at which tissue sodium signal starts to rise above normal (onset of elevated tissue sodium, OETS) represents stroke onset time (SOT). However, extrapolating core data back in time resulted in a delay of 72 ± 24 minutes in OETS compared with actual SOT. At the OETS in the core, penumbra sodium signal was significantly decreased (88 ± 6%, P=0.0008), whereas penumbra ADC was not significantly different (92 ± 18%, P=0.2) from contralateral tissue. In conclusion, reduced sodium-MRI signal may serve as a viability marker for penumbra detection and can complement hydrogen ADC and perfusion MRI in the time-independent assessment of tissue fate in acute stroke patients.

Sodium imaging as a marker of tissue injury in patients with multiple sclerosis

Recent studies have suggested that intra-axonal sodium accumulation contribute to axonal degeneration in patients with MS. Advances in MRI hardware and software allow acquisition of brain sodium signal in vivo. This review begins with a summary of the experimental evidence for impairment of sodium homeostasis in MS. Then, MRI methods for sodium acquisition are reviewed and the application of the techniques in patients with MS is discussed. Sodium imaging and ultra-high field MRI have the potential to provide tissue-specific markers of neurodegeneration in MS.

Evaluation of B0-inhomogeneity correction for triple-quantum-filtered sodium MRI of the human brain at 4.7 T

Off-resonance can result in signal loss on triple-quantum-filtered (TQF) sodium images. Three correction methods have been proposed to mitigate this problem, but their effectiveness and necessity has not yet been evaluated for human brain. This evaluation is warranted given the doubling or quadrupling of scan length without the expected signal-to-noise ratio (SNR) benefit. First, simulations and agar gel experiments showed that the off-resonance effects on signal loss were asymmetric about on-resonance. Second, the two scan length doubling correction methods were tested for two sets of TQF acquisition parameters in 10 healthy volunteers at 4.7 Tesla. Using only manual shimming on the sodium signal and a 3-pulse TQF sequence with an optimal preparation time value of 6 ms, the majority of brain tissue voxels (87-94% depending on sequence parameters) experienced B0 inhomogeneity amounting to less than 10% signal losses. Relative signal intensities of 0.96 ± 0.04 and 0.98 ± 0.02 were measured in these voxels relative to on-resonant voxels for SNR-optimized and standard TQF parameters. The remaining brain voxels in regions with known susceptibility problems suffered more substantial signal losses, which were partially recovered with the correction methods. At field strengths below 4.7T, at similar ranges of offset frequencies at higher fields and in typical volunteers, B0 correction appears unnecessary for TQF analysis in most of the brain. In many cases where regions with known susceptibility issues are not of concern, a doubling of scan time may be better spent to either improve SNR or spatial resolution in the TQF sodium images.

Advances in multimodal neuroimaging: hybrid MR-PET and MR-PET-EEG at 3 T and 9.4 T

Multi-modal MR-PET-EEG data acquisition in simultaneous mode confers a number of advantages at 3 T and 9.4 T. The three modalities complement each other well; structural-functional imaging being the domain of MRI, molecular imaging with specific tracers is the strength of PET, and EEG provides a temporal dimension where the other two modalities are weak. The utility of hybrid MR-PET at 3 T in a clinical setting is presented and critically discussed. The potential problems and the putative gains to be accrued from hybrid imaging at 9.4 T, with examples from the human brain, are outlined. Steps on the road to 9.4 T multi-modal MR-PET-EEG are also illustrated. From an MR perspective, the potential for ultra-high resolution structural imaging is discussed and example images of the cerebellum with an isotropic resolution of 320 μm are presented, setting the stage for hybrid imaging at ultra-high field. Further, metabolic imaging is discussed and high-resolution images of the sodium distribution are presented. Examples of tumour imaging on a 3 T MR-PET system are presented and discussed. Finally, the perspectives for multi-modal imaging are discussed based on two on-going studies, the first comparing MR and PET methods for the measurement of perfusion and the second which looks at tumour delineation based on MRI contrasts but the knowledge of tumour extent is based on simultaneously acquired PET data.

Simultaneous single-quantum and triple-quantum-filtered MRI of 23Na (SISTINA)

The low MR sensitivity of the sodium nucleus and its low concentration in the human body constrain acquisition time. The use of both single-quantum and triple-quantum sodium imaging is, therefore, restricted. In this work, we present a novel MRI sequence that interleaves an ultra-short echo time radial projection readout into the three-pulse triple-quantum preparation. This allows for simultaneous acquisition of tissue sodium concentration weighted as well as triple-quantum filtered images. Performance of the sequence is shown on phantoms. The method is demonstrated on six healthy informed volunteers and is applied to three cases of brain tumors. A comparison with images from tumor specific O-(2-[18F]fluoroethyl)-L-tyrosine positron emission tomography and standard MR images is presented. The combined information of the triple-quantum-filtered images with single-quantum images may enable a better understanding of tissue viability. Future studies can benefit from the evaluation of both contrasts with shortened acquisition times.

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.

Magnetic alignment and quadrupolar/paramagnetic cross-correlation in complexes of Na with LnDOTP5-

The observation of a double-quantum filtered signal of quadrupolar nuclei (e.g. (23)Na) in solution has been traditionally interpreted as a sign for anisotropic reorientational motion. Ling and Jerschow (2007) have found that a (23)Na double-quantum signal is observed also in solutions of TmDOTPNa(5). Interference effects between the quadrupolar and the paramagnetic interactions have been reported to lead to the appearance of double-quantum coherences even in the absence of a residual quadrupolar interaction. In addition, such processes lead to differential linebroadening effects between the satellite transitions, akin to effects that are well known for dipolar-CSA cross-correlation. Here, we report experiments on sodium in the presence of LnDOTP compounds, where it is shown that these cross-correlation effects correlate well with the pseudo-contact shift. In addition, anisotropic g-values of the lanthanide compounds in question, can also lead to alignment within the magnetic field, and consequently to the appearance of line splitting and double-quantum coherences. The two competing effects are demonstrated and it is concluded that both cross-correlated relaxation and alignment in the magnetic field must be at work in the systems described here.

Multimodal MRI of nonhuman primate stroke

Stroke is the fourth leading cause of death. Despite decades of research, no neuroprotective drug has proven to be effective clinically. One widely accepted view to account for this negative outcome is that the rodent stroke model simply does not adequately reflect the complexity of human stroke. Recent failures of several high-profile neuroprotective drugs for stroke treatment in phase III clinical trials further underscore the importance of developing adequate animal models for stroke research. The brain organization and vascular circuitry of nonhuman primates (NHPs) are more homologous with humans than the widely used rodent for stroke modeling. The Stroke Therapy Academic Industry Roundtable, a national committee commissioned by the American Heart Association, recommended that clinically relevant NHP stroke models be established for developing and assessing neuroprotective drugs. The aim of this article is to review the challenges and applications of magnetic resonance imaging studies of NHP stroke models.

Triple-quantum-filtered sodium imaging of the human brain at 4.7 T

The limited signal-to-noise ratio of triple-quantum-filtered MRI of sodium is a major hurdle for its application clinically. Although it has been shown that short 90° radiofrequency pulses in combination with sufficiently long repetition time for full T(1) recovery (labelled "standard" parameters) produce the maximum signal through the triple-quantum-filter, and in this work, simulation and images of agar phantoms and human brain demonstrate that the use of longer radiofrequency pulses and reduced repetition time (optimized parameters to accommodate more averages for a constant specific absorption rate, reducing noise variance for a given scan length) results in signal-to-noise ratio improvement (22 ± 5% in brain tissue of five healthy volunteers--images created in 11 min with nominal resolution of 8.4 mm isotropic). However, residual intensity was observed in the ventricular space on triple-quantum-filtered images acquired with either optimized or standard parameters, contrary to the expectation of complete single-quantum signal suppression. Further simulation and experimentation suggest that this is likely due to the combination of triple-quantum-passed signal from surrounding brain tissue being spatially smeared into the ventricular space and single-quantum-signal breakthrough from sodium nuclei in the fluid space. It is shown that the latter can be eliminated with judicious first flip angle selection.

Sodium MRI

Sodium ((23)Na) imaging has a place somewhere between (1)H-MRI and MR spectroscopy (MRS). Like MRS it potentially provides information on metabolic processes, but only one single resonance of ionic (23)Na is observed. Therefore pulse sequences do not need to code for a chemical shift dimension, allowing (23)Na images to be obtained at high resolutions as compared to MRS. In this chapter the biological significance of sodium in the brain will be discussed, as well as methods for observing it with (23)Na-MRI. Many vital cellular processes and interactions in excitable tissues depend on the maintenance of a low intracellular and high extracellular sodium concentration. Healthy cells maintain this concentration gradient at the cost of energy. Leaky cell membranes or an impaired energy metabolism immediately leads to an increase in cytosolic total tissue sodium. This makes sodium a biomarker for ischemia, cancer, excessive tissue activation, or tissue damage as might be caused by ablation therapy. Special techniques allow quantification of tissue sodium for the monitoring of disease or therapy in longitudinal studies or preferential observation of the intracellular component of the tissue sodium. New methods and high-field magnet technology provide new opportunities for (23)Na-MRI in clinical and biomedical research.

B₀ inhomogeneity-insensitive triple-quantum-filtered sodium imaging using a 12-step phase-cycling scheme

Triple-quantum-filtered (TQF) sodium MRI can be used to separate sodium NMR signals from different physiological compartments. Although three-pulse triple-quantum filtering has been demonstrated to be better suited for in vivo imaging, the absence of the refocusing pulse in the filter increases its sensitivity to magnetic field inhomogeneities. Therefore, several TQF cycles have been developed previously to correct image distortions caused by B(0) inhomogeneities. In this paper, we present a new 12-step phase-cycling TQF scheme based on three radiofrequency pulses which allows the compensation of B(0) variations both with and without ancillary B(0) map information. The method offers 40% higher signal-to-noise-ratio efficiency compared with the previously developed B(0)-correcting phase-cycling schemes.

Sodium long-component T(2)(*) mapping in human brain at 7 Tesla

Sodium ((23)Na) MRI may provide unique information about the cellular and metabolic integrity of the brain. The quantification of tissue sodium concentration from (23)Na images with nonzero echo time (TE) requires knowledge of tissue-specific parameters that influence the single-quantum sodium signal such as transverse (T(2)) relaxation times. We report the sodium ((23)Na) long component of the effective transverse relaxation time T(2) (*) values obtained at 7 T in several brain regions from six healthy volunteers. A two-point protocol based on a gradient-echo sequence optimized for the least error per given imaging time was used (TE(1) = 12 ms; TE(2) = 37 ms; averaged N(1) = 5; N(2) = 15 times; pulse repetition time = 130 ms). The results reveal that long T(2)(*) component of tissue sodium (mean +/- standard deviation) varied between cerebrospinal fluid (54 +/- 4 ms) and gray (28 +/- 2 ms) and white (29 +/- 2 ms) matter structures. The results also show that the long T(2)(*) component increases as a function of the main static field B(0), indicating that correlation time of sodium ion motion is smaller than the time-scale defined by the Larmor frequency. These results are a prerequisite for the quantification of tissue sodium concentration from (23)Na MRI scans with nonzero echo time, will contribute to the design of future measurements (such as triple-quantum imaging), and themselves may be of clinical utility.

Inhomogeneous sodium accumulation in the ischemic core in rat focal cerebral ischemia by 23Na MRI

PURPOSE

To test the hypotheses that (i) the regional heterogeneity of brain sodium concentration ([Na(+)](br)) provides a parameter for ischemic progression not available from apparent diffusion coefficient (ADC) data, and (ii) [Na(+)](br) increases more in ischemic cortex than in the caudate putamen (CP) with its lesser collateral circulation after middle cerebral artery occlusion in the rat.

MATERIALS AND METHODS

(23)Na twisted projection MRI was performed at 3 Tesla. [Na(+)](br) was independently determined by flame photometry. The ischemic core was localized by ADC, by microtubule-associated protein-2 immunohistochemistry, and by changes in surface reflectivity.

RESULTS

Within the ischemic core, the ADC ratio relative to the contralateral tissue was homogeneous (0.63 +/- 0.07), whereas the rate of [Na(+)](br) increase (slope) was heterogeneous (P < 0.005): 22 +/- 4%/h in the sites of maximum slope versus 14 +/- 1%/h elsewhere (here 100% is [Na(+)](br) in the contralateral brain). Maximum slopes in the cortex were higher than in CP (P < 0.05). In the ischemic regions, there was no slope/ADC correlation between animals and within the same brain (P > 0.1). Maximum slope was located at the periphery of ischemic core in 8/10 animals.

CONCLUSION

Unlike ADC, (23)Na MRI detected within-core ischemic lesion heterogeneity.

MAP2 immunostaining in thick sections for early ischemic stroke infarct volume in non-human primate brain

The delineation of early infarction in large gyrencephalic brain cannot be accomplished with triphenyl-tetrazolium chloride (TTC) due to its limitations in the early phase, nor can it be identified with microtubule-associated protein 2 (MAP2) immunohistochemistry, due to the fragility of large thin sections. We hypothesize that MAP2 immunostaining of thick brain sections can accurately identify early ischemia in the entire monkey brain. Using ischemic brains of one rat and three monkeys, a thick-section MAP2 immunostaining protocol was developed to outline the infarct region over the entire non-human primate brain. Comparison of adjacent thick and thin sections in a rat brain indicated complete correspondence between ischemic regions (100.4mm(3)+/-1.2%, n=7, p=0.44). Thick sections in monkey brain possessed the increased structural stability necessary for the extensive MAP2 immunostaining procedure permitting quantification of the ischemic region as a percent of total monkey brain, giving infarct volumes of 11.4, 16.3, and 19.0% of total brain. Stacked 2D images of the intact thick brain tissue sections provided a 3D representation for comparison to MRI images. The infarct volume of 16.1cm(3) from the MAP2 sections registered with MRI images agreed well with the volume calculated directly from the stained sections of 16.6 cm(3). Thick brain tissue section MAP2 immunostaining provides a new method for determining infarct volume over the entire brain at early time points in a non-human primate model of ischemic stroke.

Parallel imaging with 3D TPI trajectory: SNR and acceleration benefits

Three-dimensional (3D) twisted projection imaging (TPI) trajectory has a unique advantage in sodium ((23)Na) imaging on clinical MRI scanners at 1.5 or 3 T, generating a high signal-to-noise ratio (SNR) with a short acquisition time (approximately 10 min). Parallel imaging with an array of coil elements transits SNR benefits from small coil elements to acquisition efficiency by sampling partial k-space. This study investigates the feasibility of parallel sodium imaging with emphases on SNR and acceleration benefits provided by the 3D TPI trajectory. Computer simulations were used to find available acceleration factors and noise amplification. Human head studies were performed on clinical 1.5/3-T scanners with four-element coil arrays to verify simulation outcomes. In in vivo studies, proton ((1)H) data, however, were acquired for concept-proof purpose. The sensitivity encoding (SENSE) method with the conjugate gradient algorithm was used to reconstruct images from accelerated TPI-SENSE data sets. Self-calibration was employed to estimate coil sensitivities. Noise amplification in TPI-SENSE was evaluated using multiple noise trials. It was found that the acceleration factor was as high as 5.53 (corresponding to acceleration number 2 x 3, ring-by-rotation), with a small image error of 6.9% when TPI projections were reduced in both polar (ring) and azimuthal (rotation) directions. The average noise amplification was as low as 98.7%, or 27% lower than Cartesian SENSE at that acceleration factor. The 3D nature of both TPI trajectory and coil sensitivities might be responsible for the high acceleration and low noise amplification. Consequently, TPI-SENSE may have potential advantages for parallel sodium imaging.