Post mortem brain temperature and its influence on quantitative MRI of the brain

Objective

MRI temperature sensitivity presents a major issue in in situ post mortem MRI (PMMRI), as the tissue temperatures differ from living persons due to passive cooling of the deceased. This study aims at computing brain temperature effects on the MRI parameters to correct for temperature in PMMRI, laying the foundation for future projects on post mortem validation of in vivo MRI techniques.

Materials and methods

Brain MRI parameters were assessed in vivo and in situ post mortem using a 3 T MRI scanner. Post mortem brain temperature was measured in situ transethmoidally. The temperature effect was computed by fitting a linear model to the MRI parameters and the corresponding brain temperature.

Results

Linear positive temperature correlations were observed for T₁, T₂* and mean diffusivity in all tissue types. A significant negative correlation was observed for T₂ in white matter. Fractional anisotropy revealed significant correlations in all gray matter regions except for the thalamus.

Discussion

The linear models will allow to correct for temperature in post mortem MRI. Comparing in vivo to post mortem conditions, the mean diffusivity, in contrast to T₁ and T₂, revealed additional effects besides temperature, such as cessation of perfusion and active diffusion.

     

Extra-axonal contribution to double diffusion encoding-based pore size estimates in the corticospinal tract

Objective

To study the origin of compartment size overestimation in double diffusion encoding MRI (DDE) in vivo experiments in the human corticospinal tract. Here, the extracellular space is hypothesized to be the origin of the DDE signal. By exploiting the DDE sensitivity to pore shape, it could be possible to identify the origin of the measured signal. The signal difference between parallel and perpendicular diffusion gradient orientation can indicate if a compartment is regular or eccentric in shape. As extracellular space can be considered an eccentric compartment, a positive difference would mean a high contribution to the compartment size estimates.

Materials and methods

Computer simulations using MISST and in vivo experiments in eight healthy volunteers were performed. DDE experiments using a double spin-echo preparation with eight perpendicular directions were measured in vivo. The difference between parallel and perpendicular gradient orientations was analyzed using a Wilcoxon signed-rank test and a Mann–Whitney U test.

Results

Simulations and MR experiments showed a statistically significant difference between parallel and perpendicular diffusion gradient orientation signals (\(\alpha =0.05\)).

Conclusion

The results suggest that the DDE-based size estimate may be considerably influenced by the extra-axonal compartment. However, the experimental results are also consistent with purely intra-axonal contributions in combination with a large fiber orientation dispersion.

     

In vivo diffusion tensor mapping of the brain of squirrel monkey, rat, and mouse using single-shot STEAM MRI

The purpose was to assess the potential of half Fourier diffusion-weighted single-shot STEAM MRI for diffusion tensor mapping of animal brain in vivo. A STEAM sequence with image acquisition times of about 500 ms was implemented at 2.35 T using six gradient orientations and b values of 200, 700, and 1200 s mm^–2. The use of half Fourier phase-encoding increased the signal-to-noise ratio by 45% relative to full Fourier acquisitions. Moreover, STEAM-derived maps of the relative anisotropy and main diffusion direction were completely free of susceptibility-induced signal losses and geometric distortions. Within measuring times of 3 h, the achieved resolution varied from 600×700×1000 m³ for squirrel monkeys to 140×280×720 m³ for mice. While in monkeys the accessible white matter fiber connections were comparable to those reported for humans, detectable fiber structures in mice focused on the corpus callosum, anterior commissure, and hippocampal fimbria. In conclusion diffusion-weighted single-shot STEAM MRI allows for in vivo diffusion tensor mapping of the brain of squirrel monkeys, rats, and mice without motion artifacts and susceptibility distortions.     

High-resolutation magnetic resonance imaging of the endolymphatic duct and sac

An anatomical study was carried out to determine the extent to which magnetic resonance imaging (MRI) could delineate inner ear structures. Anatomical preparations of human petrous temporal bone were examined and compared with the results of MRI in 20 healthy subjects to see whether the structures of the inner ear could be visualized. Imaging of the subjects was carried out in a 1.0-T MRI scanner (Siemens Magnetom Impact). Two stronglyT ₂*-weighted sequences were used: a 3D-PSIF sequence and a 3D-CISS sequence. The 3D data sets were postprocessed using a Maximum Intensity Projection (MIP) program. Our investigations show that it is possible to obtain accurate visualization of structures with a diameter of under 1 mm. In all 20 subjects it was possible to identify both the endolymphatic duct and the endolymphatic sac.     

Peculiarities of Variation of the Dielectric Permittivity and the Pyroelectric Coefficient with Temperature in PZT-Based Ferrosoft Ceramics Modified with Different Additives of PbGeO3

ABSTRACT

The temperature dependencies of the dielectric permittivity ?( T) the dynamic pyroelectric coefficient γ_d( T) and the quasi-static pyroelectric coefficient γ_st( T) of prepoled samples of the low-coercive materials belonging to the PZT-based multicomponent system modified with 0.5%; 1% and 2% PbGeO₃ and having the Curie temperatures T _c = 207, 215 and 206°C, respectively, were studied. Polarization of the samples was performed under three different conditions. It was been established that in the ?( T) curve one can observe a step-like variation of ?( T) or a slope variation at temperatures below the temperature maximum by 40–50°C. The additional low- and high-temperature maxima are observed in the γ_d( T) and γ_st( T) curves.     

High proton relaxivity for gadolinium oxide nanoparticles

Objective: Nanosized materials of gadolinium oxide can provide high-contrast enhancement in magnetic resonance imaging (MRI). The objective of the present study was to investigate proton relaxation enhancement by ultrasmall (5 to 10 nm) Gd₂O₃ nanocrystals. Materials and methods: Gd₂O₃ nanocrystals were synthesized by a colloidal method and capped with diethylene glycol (DEG). The oxidation state of Gd₂O₃ was confirmed by X-ray photoelectron spectroscopy. Proton relaxation times were measured with a 1.5-T MRI scanner. The measurements were performed in aqueous solutions and cell culture medium (RPMI). Results: Results showed a considerable relaxivity increase for the Gd₂O₃–DEG particles compared to Gd-DTPA. Both T ₁ and T ₂ relaxivities in the presence of Gd₂O₃–DEG particles were approximately twice the corresponding values for Gd–DTPA in aqueous solution and even larger in RPMI. Higher signal intensity at low concentrations was predicted for the nanoparticle solutions, using experimental data to simulate a T₁-weighted spin echo sequence. Conclusion: The study indicates the possibility of obtaining at least doubled relaxivity compared to Gd–DTPA using Gd₂O₃–DEG nanocrystals as contrast agent. The high T ₁ relaxation rate at low concentrations of Gd₂O₃ nanoparticles is very promising for future studies of contrast agents based on gadolinium-containing nanocrystals.     

Evaluation of exposure to (ultra) high static magnetic fields during activities around human MRI scanners

Objective

To assess the individual exposure to the static magnetic field (SMF) and the motion-induced time-varying magnetic field (TVMF) generated by activities in an inhomogeneous SMF near high and ultra-high field magnetic resonance imaging (MRI) scanners. The study provides information on the level of exposure to high and ultra-high field MRI scanners during research activities.

Materials and methods

A three-axis Hall magnetometer was used to determine the SMF and TVMF around human 3- and 7-Tesla (T) MRI systems. The 7-T MRI scanner used in this study was passively shielded and the 3-T scanner was actively shielded and both were from the same manufacturer. The results were compared with the exposure restrictions given by the International Commission on Non-Ionizing Radiation Protection (ICNIRP).

Results

The recorded exposure was highly variable between individuals, although they followed the same instructions for moving near the scanners. Maximum exposure values of B = 2057 mT and dB/dt = 4347 mT/s for the 3-T scanner and B = 2890 mT, dB/dt = 3900 mT/s for 7 T were recorded. No correlation was found between reporting the MRI-related sensory effects and exceeding the reference values.

Conclusions

According to the results of our single-center study with five subjects, violation of the ICNIRP restrictions for max B in MRI research environments was quite unlikely at 3 and 7 T. Occasions of exceeding the dB/dt limit at 3 and 7 T were almost similar (30% of 60 exposure scenarios) and highly variable among the individuals.

     

Possibilities and limitations for high resolution small animal MRI on a clinical whole-body 3T scanner

Object

To investigate the potential of a clinical 3 T scanner to perform MRI of small rodents.

Materials and methods

Different dedicated small animal coils and several imaging sequences were evaluated to optimize image quality with respect to SNR, contrast and spatial resolution. As an application, optimal grey-white-matter contrast and resolution were investigated for rats. Furthermore, manganese-enhanced MRI was applied in mice with unilateral crush injury of the optic nerve to investigate coil performance on topographic mapping of the visual projection.

Results

Differences in SNR and CNR up to factor 3 and more were observed between the investigated coils. The best grey-white matter contrast was achieved with a high resolution 3D T ₂-weighted TSE (SPACE) sequence. Delineation of the retino-tectal projection and detection of defined visual pathway damage on the level of the optic nerve could be achieved by using a T ₁-weighted, 3D gradient echo sequence with isotropic resolution of (0.2?mm)³.

Conclusions

Experimental studies in small rodents requiring high spatial resolution can be performed by using a clinical 3 T scanner with appropriate dedicated coils.     

Signal losses in diffusion preparation: Comparison between spin-echo, stimulated echo and SEASON

Diffusion-weighted magnetic resonance imaging and spectroscopy commonly apply a spin-echo or stimulated echo preparation including sensitizing field gradients. The article reports on a systematic numerical approach to an optimum diffusion preparation considering undesired signal losses caused by relaxation. A large range of possible applications on whole-body units and animal scanners is covered. Instructions for an optimized type and timing of the diffusion preparation are provided for the readership, based on the desired diffusion weighting (b-value), the available maximum field gradient amplitudes, the RF pulse durations and gradient ramp times, and the relaxation characteristics of the specimen (or tissue) of interest. In addition, a new type of diffusion preparation named SEASON (simultaneous Spin-Echo And Stimulated echO preparatioN) is introduced and compared with spin-echo and stimulated echo diffusion preparation. It is demonstrated that spin-echo preparation is superior to stimulated echo preparation in all cases with T₂ ≈ T₁ and in all cases with relatively low diffusion weighting resulting in short duration of diffusion sensitizing gradients δ « T₂. For tissues with T₂ « T₁ (as musculature or red bone marrow) stimulated echo preparation becomes superior to spin-echo preparation for high ratios b/A² (b-value indicates diffusion weighting,A is the maximum gradient amplitude). The new SEASON technique allows a higher yield in signal intensity compared to spin-echo or stimulated echo preparations in clinically relevant cases. © 1998 Elsevier Science B.V. All rights reserved.     

On the diffusion of the electromagnetic field through a system of coaxial cylindrical conductors

Contents In this paper the diffusion of a three-dimensional electromagnetic field through cylindrical conducting layers is discussed and analytical solutions are obtained for the case that the outermost cylindrical surface is an ideal conductor. —Simple approximate solutions are also obtained and their validity range is investigated. — A computation program based on the search of the poles ot the field functions has been developed and some numerical examples are reported.

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Diffusion des elektromagnetischen Feldes durch ein System koaxialer zylindrischer Leiter

Übersicht in diesem Aufsatz wird die Diffusion eines dreidimensionalen elektromagnetischen Feldes durch zylindrische Leiterplatten untersucht. Für den Fall, daß der äußere Zylinder perfekt leitet, werden analytische Lösungen angegeben. Weiterhin wird der Gültigkeitsbereich von einfachen Näherungslösungen untersucht. — Ein Rechenprogramm, basierend auf der Bestimmung der Pole der Feldfunktionen, wurde erstellt und einige numerische Beispiele werden angegeben.

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List of Symbols B magnetic field (B ,B ,B _z are its cylindrical components) - E electric field (E ,E ,E _z are its cylindrical components) - J current density - _i conductivity of thei-th layer - _i permeability of thei-th layer - _i thickness of thei-th layer - r _i internal radius of thei-th layer - u, w auxiliary functions related to the electric field - _d diffusion time constant for an infinite plane slab - n unity vector perpendicular to the separation surfaces - E_a input array - X unknowns array - D matrix of the coefficients of the resolutive equation system - , , ,A, B, F, G, H auxiliary functions - l _m, K_m Modified Bessel functions of orderm - time expressed in adimensional form (referred ot _d )     

Assessment of inhibitory potency of antibiotics by MRI: apparent T 2 as a marker of cell growth

A new method to assess the antibiotic potency by MRI has been developed. Correlating ¹H NMR spectra of bacterial cultures with the extracellular parameters T ₂, OD₆₀₀, and pH, a relationship between cell growth and T ₂ variations was established. T ₂ is influenced by chemical exchange that depends on pH, composition, and concentration of the medium. Changes in the medium from bacterial metabolism are reflected in alternating T ₂ values. At 17.6 T, growth curves based on T ₂ values were measured simultaneously of several cultures of Streptococcus vestibularis. From T ₂ growth curves in the presence of varying concentrations of vancomycin, the minimum inhibitory concentration of the antibiotic could be determined to be 0.33 ± 0.08 μM. This value was in good agreement with the result obtained by the conventional broth microdilution. In principle, T ₂ growth curves can be determined on a large number of cultures simultaneously and may potentially be used as a novel tool in high through-put screening of novel anti-infective substances.     

Orientational dependent sensitivities of T2 and T1ρ towards trypsin degradation and Gd-DTPA2? presence in bovine nasal cartilage

Objective

To study the orientational dependencies of T₂ and T_1ρ in native and trypsin-degraded bovine nasal cartilage, with and without the presence of 1?mM Gd-DTPA^2?.

Materials and methods

Sixteen specimens were prepared in two orthogonal fibril directions (parallel and perpendicular), treated using different protocols (native, Gd treated, trypsin-treated, and combination), and imaged using μMRI at 0° and 55° (the magic angle) fibril orientations with respect to the magnetic field B₀. Two-dimensional (2D) T₂ and T_1ρ images were then calculated quantitatively.

Results

Without Gd, native perpendicular tissues demonstrated significant T_1ρ dispersion (including T₂ at the zero spin-lock field) at 0° and less dispersion at 55°, while native parallel specimens exhibited smaller T_1ρ dispersion at both 0° and 55°. Trypsin degradation caused a minimum 50% increase in T_1ρ. With Gd, trypsin degradation caused significant reduction in T_1ρ values up to 60%.

Conclusion

The collagen orientation in nasal cartilage can influence T₂ and T_1ρ MRI of cartilage. Without Gd, T_1ρ was sensitive to the proteoglycan content and its sensitivity was nearly constant regardless of fibril orientation. In comparison, the T₂ sensitivity to proteoglycan was dependant upon fibril orientation, i.e., more sensitive at 55° than 0°. When Gd ions were present, both T₂ and T_1ρ became insensitive to the proteoglycan content.     

Spatially resolved kinetics of skeletal muscle exercise response and recovery with multiple echo diffusion tensor imaging (MEDITI): a feasibility study

Objectives

We describe measurement of skeletal muscle kinetics with multiple echo diffusion tensor imaging (MEDITI). This approach allows characterization of the microstructural dynamics in healthy and pathologic muscle.

Materials and methods

In a Siemens 3-T Skyra scanner, MEDITI was used to collect dynamic DTI with a combination of rapid diffusion encoding, radial imaging, and compressed sensing reconstruction in a multi-compartment agarose gel rotation phantom and within in vivo calf muscle. An MR-compatible ergometer (Ergospect Trispect) was employed to enable in-scanner plantar flexion exercise. In a HIPAA-compliant study with written informed consent, post-exercise recovery of DTI metrics was quantified in eight volunteers. Exercise response of DTI metrics was compared with that of T2-weighted imaging and characterized by a gamma variate model.

Results

Phantom results show quantification of diffusivities in each compartment over its full dynamic rotation. In vivo calf imaging results indicate larger radial than axial exercise response and recovery in the plantar flexion-challenged gastrocnemius medialis (fractional response: nT2w?=?0.385?±?0.244, nMD?=?0.163?±?0.130, nλ₁?=?0.110?±?0.093, nλ_rad?=?0.303?±?0.185). Diffusion and T2-weighted response magnitudes were correlated (e.g., r?=?0.792, p?=?0.019 for nMD vs. nT2w).

Conclusion

We have demonstrated the feasibility of MEDITI for capturing spatially resolved diffusion tensor data in dynamic systems including post-exercise skeletal muscle recovery following in-scanner plantar flexion.

     

Imaging lung function using rapid dynamic acquisition of <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>-maps during oxygen enhancement

This paper describes imaging of lung function with oxygen-enhanced MRI using dynamically acquired T ₁ parameter maps, which allows an accurate, quantitative assessment of time constants of T ₁-enhancement and therefore lung function. Eight healthy volunteers were examined on a 1.5-T whole-body scanner. Lung T ₁-maps based on an IR Snapshot FLASH technique (TE = 1.4 ms, TR = 3.5 ms, FA = 7 ^∘) were dynamically acquired from each subject. Without waiting for full relaxation between subsequent acquisition of T ₁-maps, one T ₁-map was acquired every 6.7 s. For comparison, all subjects underwent a standard pulmonary function test (PFT). Oxygen wash-in and wash-out time course curves of T ₁ relaxation rate (R ₁)-enhancement were obtained and time constants of oxygen wash-in (w _in) and wash-out (w _out) were calculated. Averaged over the whole right lung, the mean w _out was 43.90 ± 10.47 s and the mean (w _in) was 51.20 ± 15.53 s, thus about 17% higher in magnitude. Wash-in time constants correlated strongly with forced expired volume in one second in percentage of the vital capacity (FEV₁ % VC) and with maximum expiratory flow at 25% vital capacity (MEF25), whereas wash-out time constants showed only weak correlation. Using oxygen-enhanced rapid dynamic acquisition of T ₁-maps, time course curves of R ₁-enhancement can be obtained. With w _in and w _out two new parameters for assessing lung function are available. Therefore, the proposed method has the potential to provide regional information of pulmonary function in various lung diseases.     

YBa2Cu3O7 films grown onto SrTiO3 and YSZ substrates by chemical solution deposition of trifluoroacetates

We report the preparation and characterization of YBa₂Cu₃O₇ (YBCO) films grown onto SrTiO₃ and YSZ substrates by the trifluoroacetates chemical solution deposition method and following sintering with oxygen atmosphere at 860 °C. The X-ray diffraction (XRD) reveals (00?) – oriented crystallites indicating epitaxial growth of the films in the c-direction. Despite granular morphology and the presence of Y₂BaCuO₅ and CuO as minor secondary phases, the technique shows the successful formation of the superconducting YBCO and preventing the formation of the unwanted BaCO₃ phase. Rocking curve measurements of the (005) reflection for the YBCO/SrTiO₃ was fitted with one Gaussian function with full width at the half maximum (FWHM) of 0.44° confirming that it consists of YBCO crystallites with different texture. For the sample grown on YSZ, the rocking curve was fitted with two Gaussian functions, one corresponding to the YBCO layer (FWHM?=?0.4°) and another to the substrate (FWHM?=?0.3°). The magnetic measurements taken in zero field cooling and field cooling modes confirm the formation of the superconducting YBCO with critical temperatures (T_C) 91.8 and 85.7 K for the samples grown onto YSZ and SrTiO₃, respectively. The critical current density (J_C) curves indirectly calculated by using the Bean´s model from the M(T) loops were J_C?~?10⁹ A/cm² for the sample deposited onto YSZ and J_C?~?10⁷A/cm² for the YBCO deposited onto SrTiO₃. Overall, the difference in T_C and J_C values between both samples could be related to their difference in oxygen content, porosity, hole concentration per Cu ion and the presence of secondary phases.

     

Investigation of blood-brain barrier permeability to magnetite-dextran nanoparticles (MD3) after osmotic disruption in rats

The permeability of experimentally disrupted blood-brain barrier (BBB) to superparamagnetic nanoparticles (MD3) was studied in rats. BBB opening was induced by intracarotid injection of mannitol. One hundred eighty rats were used for the study. Rats were examined at two time points, 30 minutes and 12 hours after intracarotid mannitol injection. Different preparations intravenously injected 30 minutes before rat sacrifice were used for characterization of BBB disruption. BBB integrity was determined with^99mTc-diethylenetriamine pentaacetic acid (DTPA) and^99mTc-albumin. Iron oxide-glucose particles (12-nm mean diameter),^99mTc-labeled lecithin-cholesterol liposomes of three different sizes (50, 100, and 200 nm), and polyethylene glycol (PEG)-coated^99mTc liposomes (50 nm) were used for investigations of the dependence of BBB permeability on particle system size or surface. Magnetite-dextran nanoparticles (MD3) were evaluated as superparamagnetic contrast agent to monitor with magnetic resonance imaging (MRI) the BBB breakdown.In vitro T₁ and T₂ relaxation times of the brain tissue were measured at 40 MHz and 37°C, and T₂-weighted MR images were acquired at 0.5 T. After intracarotid mannitol infusion, as expected, the BBB breakdown was immediate and temporary as judged by soluble molecule diffusion. MD3 nanoparticles crossed the BBB 12 hours after intravenous mannitol injection, at a time when brain permeability for molecules or small particles returns to normal. Magnetite crystals were found in cytoplasmic vesicles of glial cells. On MRI, signal intensity decreased after injection of MD3, even 12 hours after mannitol injection. This particularity could be useful in the study of focal pathological lesions accompanied by BBB permeability modifications. In such conditions, superparamagnetic particle contrast agents could be caught by the BBB, allowing the observation of impaired BBB areas without detectable cellular lesions. V. Rousseau was supported by a grant from the city of Angers.     

Volumetric analysis of mice lungs in a clinical magnetic resonance imaging scanner

Small animal models are widely used to study various pathologies. Magnetic resonance imaging (MRI) allows investigation of these animals in a non-invasive way. Therefore, the aim of our study was to develop and evaluate a low-cost approach to measure lung volumes in small animal MRI using a clinical scanner and a specially designed RF coil. Five mice (three of an established emphysema model and two controls) were investigated in a 1.0-T clinical scanner using a specially built small animal saddle coil and three different three-dimensional sequences; overall imaging time was approximately 16 min. Lung volumes were calculated from these images using an interactive watershed transform algorithm for semi-automatic image segmentation. The gold standard for the volume measurement was water displacement after surgical explantation. MRI measured volumes correlated significantly with ex vivo measurements on the explanted lungs (r = 0.99 to 0.89; p < 0.05). Mean lung volume in emphysema model mice was larger than in controls. High-resolution, small animal MRI using a clinical scanner is feasible for volumetric analysis and provides an alternative to a dedicated small animal scanner.     

Application of LCModel for quality control and quantitative in vivo1H MR spectroscopy by short echo time STEAM sequence

The linear combination of model spectra (LCModel) calculation of a parameter for long-term quality control,k _T, was introduced, representing the ratio of the temporal and nominal intensities of CH₃ groups of lactate and acetate in a quality control phantom. This procedure is a part of the quality assurance of the scanner using fully automatic measurement and calculation ofk _T parameters, and utilizing Shewhart regulation control charts for continuous evaluation of the magnetic resonance (MR) scanner setting. The application of thek _T parameter for the correction of in vivo data increases the precision of molar concentration determination by about 4%. This was tested by the quantitative in vivo MR determination of the molar concentrations of 13 prominent metabolites (N-acetylaspartate (NAA),N-acetylaspartylgutamate, creatine and phosphocreatine (Cr), choline-containing compounds (Cho), myo-inositol, scyllo-inositol., γ-aminobutyric acid, glutamine, glutamate, glucose, lactate, alanine, taurine) in the white matter and hippocampus of the brain in groups of volunteers, using a short echo time stimulated echo acquisition mode sequence (echo time =10 ms) and the LCModel technique. The repeatability of the measurement of prominent metabolites such as NAA, Cr and Cho was found to be around 10% (relative standard deviation,n=6); precision in a group of volunteers (n=20 and 28, respectively) was in the range of approximately 13–20%. For other metabolites, which are measured with a lower signal-to-noise ratio, the precision can be much lower.     

Evaluation of the pulmonary vasculature with three-dimensional magnetic resonance imaging techniques

Cardiac and respiratory motion during the long acquisition times required in 3d magnetic resonance imaging (MRI) can lead to excessive image degradation and consequently poor diagnosis and interpretation in the thoracic region. This paper addresses the issue of obtaining good image quality with 3d gradient echo (GE) imaging in the study of the pulmonary vascular system and its diseases. To study the pulmonary vascular system two approaches have been considered. First, a proton density/inflow weighted scan consisting of a syncopated 3d FLASH acquisition is used to provide a 3d pulmonary angiogram. Secondly, aT ₁ weighted scan using 3d IR-FLASH (inversion recovery FLASH) helps in determining the presence of pulmonary emboli. Multiple acquisitions and rectangular field of view are utilized to pseudogate to the respiratory period in order to reduce motion artifacts while keeping reasonable imaging times. Technical aspects on data collection during the approach to equilibrium and acquisition strategies in the presence of thoracic motion and its impact on vessel resolution are addressed. The method has proven successful for imaging volunteers and, more recently, in obtaining useful clinical information.     

High-resolution imaging of the excised porcine heart at a whole-body 7 T MRI system using an 8Tx/16Rx pTx coil

Introduction

MRI of excised hearts at ultra-high field strengths (\({\mathrm{B}}_{0}\)≥7 T) can provide high-resolution, high-fidelity ground truth data for biomedical studies, imaging science, and artificial intelligence. In this study, we demonstrate the capabilities of a custom-built, multiple-element transceiver array customized for high-resolution imaging of excised hearts.

Method

A dedicated 16-element transceiver loop array was implemented for operation in parallel transmit (pTx) mode (8Tx/16Rx) of a clinical whole-body 7 T MRI system. The initial adjustment of the array was performed using full-wave 3D-electromagnetic simulation with subsequent final fine-tuning on the bench.

Results

We report the results of testing the implemented array in tissue-mimicking liquid phantoms and excised porcine hearts. The array demonstrated high efficiency of parallel transmits characteristics enabling efficient pTX-based B₁⁺-shimming.

Conclusion

The receive sensitivity and parallel imaging capability of the dedicated coil were superior to that of a commercial 1Tx/32Rx head coil in both SNR and T₂*-mapping. The array was successfully tested to acquire ultra-high-resolution (0.1 × 0.1 × 0.8 mm voxel) images of post-infarction scar tissue. High-resolution (isotropic 1.6 mm³ voxel) diffusion tensor imaging-based tractography provided high-resolution information about normal myocardial fiber orientation.