Hepatic lipid composition differs between ob/ob and ob/+ control mice as determined by using in vivo localized proton magnetic resonance spectroscopy

Object

Hepatic lipid accumulation is associated with nonalcoholic fatty liver disease, and the metabolic syndrome constitutes an increasing medical problem. In vivo proton magnetic resonance spectroscopy (¹H MRS) allows the assessment of hepatic lipid levels noninvasively and also yields information on the fat composition due to its high spectral resolution.

Materials and methods

We applied ¹H MRS at 9.4T to study lipid content and composition in eight leptin-deficient ob/ob mice as a model of obesity and in four lean ob/+ control mice at 24?weeks of age. PRESS sequence was used. For accurate estimation of signal intensity, differences in relaxation behavior of individual signals were accounted for each mouse individually. Also, in order to minimize spectral degrading due to motion artifacts, respiration gating was applied.

Results

Significant differences between ob/ob and ob/+ control mice were found in both lipid content and composition. The mean chain length was found to be significantly longer in ob/ob mice with a higher fraction of monounsaturated lipids.

Conclusion

¹H MRS enables accurate assessment in hepatic lipids in mice, which is attractive for mechanistic studies of altered metabolism given the large number of genetically engineered mouse models available.     

3D gradient system for two B 0 field directions in earth’s field MRI

Object

A new gradient system for earth’s field magnetic resonance imaging (EFMRI) is presented that can be rotated relatively to the earth’s field direction while maintaining the ability to encode images. Orthogonal components of the gradient field are exploited to reduce the number of gradient coils.

Materials and methods

Two favorable orientations of the gradient system relative to the earth’s magnetic field (parallel and perpendicular) are discussed. We introduce the theory for the magnetic fields of the new gradient system and illustrate the design of the coil geometries which were worked out with the help of simulations and a numerical optimization algorithm. Field mapping measurements and imaging experiments in the two different orientations of the gradient system were carried out.

Results

Orthogonal components of the gradient field take over the role of the additionally needed gradient fields when the gradient system is rotated relative to the earth’s magnetic field. The results from the field mapping and imaging experiments verify the presented theory and show the functionality of the new gradient system.

Conclusion

The presented system demonstrates that gradient coils can be used for image encoding in multiple directions. This fact can be exploited to realize an EFMRI setup for parallel and perpendicular prepolarization with a single set of gradient coils.     

A preliminary study on the effects of acute ethanol ingestion on default mode network and temporal fractal properties of the brain

Object

To study the effect of acute alcohol intoxication on the functional connectivity of the default mode network (DMN) and temporal fractal properties of the healthy adult brain.

Materials and methods

Eleven healthy male volunteers were asked to drink 0.59 g/kg of ethanol. Resting state blood oxygen level dependent (rsBOLD) MRI scans were obtained before consumption, 60 min post-consumption and 90 min post-consumption. Before each rsBOLD scan, pointed-resolved spectroscopy (PRESS) ¹H-MRS (magnetic resonance spectroscopy) scans were acquired to measure ethanol levels in the right basal ganglia.

Results

Significant changes in DMN connectivity were found following alcohol consumption (p < 0.01). Both increased and decreased regional connectivity were found after 60 min, whereas mostly decreased connectivity was found after 90 min. The fractal behaviour of the rsBOLD signal, which is believed to help reveal complexity of small-scale neuronal circuitry, became more ordered after both 60 and 90 min of alcohol consumption (p < 0.01).

Conclusion

The DMN has been linked to personal identity and social behavior. As such, our preliminary findings may provide insight into the neuro-functional underpinnings of the cognitive and behavioral changes observed during acute alcohol intoxication. The reduced fractal dimension implies a change in function of small-scale neural networks towards less complex signaling.     

Ultra-high field MRI for primate imaging using the travelling-wave concept

Object

Ultra-high field (UHF) neuroimaging is usually conducted with volume transmit (Tx) and phased array receive (Rx) coils, both tightly enclosing the object. The travelling-wave (TW) concept allows a remote excitation offering more flexible experimental setups. To investigate the feasibility of primate MRI in horizontal UHF MRI, we first compared the distribution of the electromagnetic fields in an oil phantom and then verified the concept with an in vivo experiment.

Materials and methods

In the phantom experiments an in-house circularly polarized hybrid birdcage coil and a self-developed patch antenna were used for Tx and an eight-element phased array antenna for Rx. B ₁ ⁺ fields were calculated and measured for both approaches. For in vivo experiments the Rx part was replaced with an optimized three-element phased array head coil. The SAR was calculated using field simulation.

Results

In the phantom the field distribution was homogenous in a central volume of interest of about 10 cm diameter. The TW concept showed a slightly better homogeneity. Examination of a female crab-eating macaque led to homogeneous high-contrast images with a good delineation of anatomical details.

Conclusion

The TW concept opens up a new approach for MRI of medium-sized animals in horizontal UHF scanners.     

Correction of <Emphasis Type="Italic">B</Emphasis><Subscript>0</Subscript>-induced geometric distortion variations in prospective motion correction for 7T MRI

Objective

Prospective motion correction can effectively fix the imaging volume of interest. For large motion, this can lead to relative motion of coil sensitivities, distortions associated with imaging gradients and B ₀ field variations. This work accounts for the B ₀ field change due to subject movement, and proposes a method for correcting tissue magnetic susceptibility-related distortion in prospective motion correction.

Materials and methods

The B ₀ field shifts at the different head orientations were characterized. A volunteer performed large motion with prospective motion correction enabled. The acquired data were divided into multiple groups according to the object positions. The correction of B ₀-related distortion was applied to each group of data individually via augmented sensitivity encoding with additionally integrated gradient nonlinearity correction.

Results

The relative motion of the gradients, B ₀ field and coil sensitivities in prospective motion correction results in residual spatial distortion, blurring, and coil artifacts. These errors can be mitigated by the proposed method. Moreover, iterative conjugate gradient optimization with regularization provided superior results with smaller RMSE in comparison to standard conjugate gradient.

Conclusion

The combined correction of B ₀-related distortion and gradient nonlinearity leads to a reduction of residual motion artifacts in prospective motion correction data.

     

Iterative separation of transmit and receive phase contributions and B 1 + -based estimation of the specific absorption rate for transmit arrays

Object

The specific absorption rate (SAR) can be determined from radiofrequency transmit fields measured via magnetic resonance imaging.

Materials and methods

The proposed method estimates the SAR solely from the complex transmit field (B ₁ ⁺ ) by taking into account the particular properties of the electromagnetic field generated by an 8-channel transmit array. It is further based on an iterative consistency check between the measured B ₁ ⁺ magnitude and an appropriate field estimate fulfilling Maxwell’s equations. For testing the method, simulations and phantom experiments were performed for a multi-transmit array at 3T using a cylindrical phantom.

Results

The method’s robustness with respect to the assumptions made about electric tissue properties as well as its stability under different initial conditions regarding the signal phase was shown. A high sensitivity to signal noise was found. Robust reconstruction results were achieved including information from more than two transmit elements. The validity of the experimental results was confirmed by a qualitative comparison to simulated electromagnetic fields.

Conclusions

The method allows the determination of the SAR as well as the transmit phase of the individual channels of a multi-transmit array. With additional B₀ inhomogeneity measurements, a reconstruction of the receive phase is feasible independent of the receive coil type in use.     

Spectroscopic sampling of the left side of long-TE spin echoes: a free lunch?

Objective

Use of spectroscopically-acquired spin echoes typically involves Fourier transformation of the right side of the echo while largely neglecting the left side. For sufficiently long echo times, the left side may have enough spectral resolution to offer some utility. Since the acquisition of this side is “free”, we deemed it worthy of attention and investigated the spectral properties and information content of this data.

Materials and methods

Theoretical expressions for left- and right-side spectra were derived assuming Lorentzian frequency distributions. For left-side spectra, three regimes were identified based upon the relative magnitudes of reversible and irreversible transverse relaxation rates, R ₂′ and R ₂, respectively. Point-resolved spectroscopy (PRESS) data from muscle, fat deposit and bone marrow were acquired at 1.5 T to test aspects of the theoretical expressions.

Results

For muscle water or methylene marrow resonances, left-side signals were substantially or moderately larger than right-side signals but were similar in magnitude for muscle choline and creatine resonances. Left- versus right-side spectral-peak amplitude ratios depend sensitively on the relative values of R ₂ and R ₂′, which can be estimated given this ratio and a right-side linewidth measurement.

Conclusion

Left-side spectra can be used to augment signal-to-noise and to estimate spectral R ₂ and R ₂′ values under some circumstances.

     

Effect of motion on the ADC quantification accuracy of whole-body DWIBS

Background and methods

Diffusion-weighted whole-body imaging with background body signal subtraction was introduced as a qualitative approach to detecting metastases in the body. A liver-mimicking phantom with embedded tumours that could be moved to replicate respiratory motion was developed to assess its ability to accurately quantify ADC values.

Results

Mean tumour ADC values were unaltered by the motion; however, a significant (p?Conclusions These findings may be of significance in cancer therapy monitoring where subtle changes in ADC histograms may reveal changes in tumour heterogeneity.     

Spatial phase encoding exploiting the Bloch–Siegert shift effect

Objective

The present work introduces an alternative to the conventional \(B_{0}\) -gradient spatial phase encoding technique. By applying far off-resonant radiofrequency (RF) pulses, a spatially dependent phase shift is introduced to the on-resonant transverse magnetization. This so-called Bloch–Siegert (BS) phase shift has been recently used for \(B_{1}^{ + }\) -mapping. The current work presents the theoretical background for the BS spatial encoding technique (BS-SET) using RF-gradients.

Materials and methods

Since the BS-gradient leads to nonlinear encoding, an adapted reconstruction method was developed to obtain undistorted images. To replace conventional phase encoding gradients, BS-SET was implemented in a two-dimensional (2D) spin echo sequence on a 0.5 T portable MR scanner.

Results

A 2D spin echo (SE) measurement imaged along a single dimension using the BS-SET was compared to a conventional SE 2D measurement. The proposed reconstruction method yielded undistorted images.

Conclusions

BS-gradients were demonstrated as a feasible option for spatial phase encoding. Furthermore, undistorted BS-SET images could be obtained using the proposed reconstruction method.     

Diffusion-weighted echo planar MR imaging of the neck at 3 T using integrated shimming: comparison of MR sequence techniques for reducing artifacts caused by magnetic-field inhomogeneities

Objective

Our objective was to compare available techniques reducing artifacts in echo planar imaging (EPI)-based diffusion-weighed magnetic resonance imaging MRI (DWI) of the neck at 3 Tesla caused by B0-field inhomogeneities.

Materials and methods

A cylindrical fat–water phantom was equipped with a Maxwell coil allowing for additional linear B0-field variations in z-direction. The effect of increasing strength of this superimposed gradient on image quality was observed using a standard single-shot EPI-based DWI sequence (sEPI), a zoomed single-shot EPI sequence (zEPI), a readout-segmented EPI sequence (rsEPI), and an sEPI sequence with integrated dynamic shimming (intEPI) on a 3-Tesla system. Additionally, ten volunteers were examined over the neck region using these techniques. Image quality was assessed by two radiologists. Scan durations were recorded.

Results

With increasing strength of the external gradient, marked distortions, signal loss, and failure of fat suppression were observed using sEPI, zEPI, and rsEPI. These artifacts were markedly reduced using intEPI. Significantly better in vivo image quality was also observed using intEPI compared with the other techniques. Scan time of intEPI was similar to sEPI and zEPI and shorter than rsEPI.

Conclusion

The use of integrated 2D shim and frequency adjustment for EPI-based DWI results in a significant improvement in image quality of the head/neck region at 3 Tesla. Combining integrated shimming with rsEPI or zEPI can be expected to provide additional improvements.

     

FID modulus: a simple and efficient technique to phase and align MR spectra

Object

The post-processing of MR spectroscopic data requires several steps more or less easy to automate, including the phase correction and the chemical shift assignment. First, since the absolute phase is unknown, one of the difficulties the MR spectroscopist has to face is the determination of the correct phase correction. When only a few spectra have to be processed, this is usually performed manually. However, this correction needs to be automated as soon as a large number of spectra is involved, like in the case of phase coherent averaging or when the signals collected with phased array coils have to be combined. A second post-processing requirement is the frequency axis assignment. In standard mono-voxel MR spectroscopy, this can also be easily performed manually, by simply assigning a frequency value to a well-known resonance (e.g. the water or NAA resonance in the case of brain spectroscopy). However, when the correction of a frequency shift is required before averaging a large amount of spectra (due to B ₀ spatial inhomogeneities in chemical shift imaging, or resulting from motion for example), this post-processing definitely needs to be performed automatically.

Materials and methods

Zero-order phase and frequency shift of a MR spectrum are linked respectively to zero-order and first-order phase variations in the corresponding free induction decay (FID) signal. One of the simplest ways to remove the phase component of a signal is to calculate the modulus of this signal: this approach is the basis of the correction technique presented here.

Results

We show that selecting the modulus of the FID allows, under certain conditions that are detailed, to automatically phase correct and frequency align the spectra. This correction technique can be for example applied to the summation of signals acquired from combined phased array coils, to phase coherent averaging and to B ₀ shift correction.

Conclusion

We demonstrate that working on the modulus of the FID signal is a simple and efficient way to both phase correct and frequency align MR spectra automatically. This approach is particularly well suited to brain proton MR spectroscopy.     

Contrast agent derived determination of the total circulating blood volume using magnetic resonance

Object

Knowledge of the total circulating blood volume (TCBV) is essential for the treatment of a variety of medical conditions and blood disorders. To date, blood volume analysis is rarely carried out due to the disadvantages of available methods. Our aim was to develop a widely available, simple, fast, yet accurate method for the determination of the total circulating blood volume.

Materials and methods

Magnetic resonance (MR) is a well-established, non-invasive technique. In this article, we present a method that uses MR contrast agents for the determination of the blood volume. The dependence of MR relaxation times on the concentration of MR contrast agents allows the calculation of the volume the contrast agent has been diluted in.

Results

In phantom and in vivo experiments we could demonstrate that TCBV can be determined with high accuracy and precision.

Conclusion

This work introduces a novel method for the determination of the total circulating blood volume using magnetic resonance contrast agents as tracers.     

Compatibility between 3T 1H SV-MRS data and automatic brain tumour diagnosis support systems based on databases of 1.5T 1H SV-MRS spectra

Object

This study demonstrates that 3T SV-MRS data can be used with the currently available automatic brain tumour diagnostic classifiers which were trained on databases of 1.5T spectra. This will allow the existing large databases of 1.5T MRS data to be used for diagnostic classification of 3T spectra, and perhaps also the combination of 1.5T and 3T databases.

Materials and methods

Brain tumour classifiers trained with 154 1.5T spectra to discriminate among high grade malignant tumours and common grade II glial tumours were evaluated with a subsequently-acquired set of 155 1.5T and 37 3T spectra. A similarity study between spectra and main brain tumour metabolite ratios for both field strengths (1.5T and 3T) was also performed.

Results

Our results showed that classifiers trained with 1.5T samples had similar accuracy for both test datasets (0.87 ± 0.03 for 1.5T and 0.88 ± 0.03 for 3.0T). Moreover, non-significant differences were observed with most metabolite ratios and spectral patterns.

Conclusion

These results encourage the use of existing classifiers based on 1.5T datasets for diagnosis with 3T ¹H SV-MRS. The large 1.5T databases compiled throughout many years and the prediction models based on 1.5T acquisitions can therefore continue to be used with data from the new 3T instruments.     

Phosphorus magnetic resonance spectroscopic imaging using flyback echo planar readout trajectories

Object

To present and evaluate a fast phosphorus magnetic resonance spectroscopic imaging (MRSI) sequence using echo planar spectroscopic imaging with flyback readout gradient trajectories.

Materials and Methods

Waveforms were designed and implemented using a 3 Tesla MRI system. ³¹P spectra were acquired with 2 × 2 cm² and 3 × 3 cm² resolution over a 20- and 21-cm field of view and spectral bandwidths up to 1923 Hz. The sequence was first tested using a 20-cm-diameter phosphate phantom, and subsequent in vivo tests were performed on healthy human calf muscles and brains from five volunteers.

Results

Flyback EPSI achieved 10× and 7× reductions in acquisition time, with 68.0 ± 1.2 and 69.8 ± 2.2% signal-to-noise ratio (SNR) per unit of time efficiency (theoretical SNR efficiency was 74.5 and 76.4%) for the in vivo experiments, compared to conventional phase-encoded MRSI for the 2 × 2 cm² and 3 × 3 cm² resolution waveforms, respectively. Statistical analysis showed no difference in the quantification of most metabolites. Time savings and SNR comparisons were consistent across phantom, leg and brain experiments.

Conclusion

EPSI using flyback readout trajectories was found to be a reliable alternative for acquiring ³¹P-MRSI data in a shorter acquisition time.

     

Absolute quantitation of brain metabolites with respect to heterogeneous tissue compositions in 1H-MR spectroscopic volumes

Object

Referencing metabolite intensities to the tissue water intensity is commonly applied to determine metabolite concentrations from in vivo ¹H-MRS brain data. However, since the water concentration and relaxation properties differ between grey matter, white matter and cerebrospinal fluid (CSF), the volume fractions of these compartments have to be considered in MRS voxels.

Materials and methods

The impact of partial volume correction was validated by phantom measurements in voxels containing mixtures of solutions with different NAA and water concentrations as well as by analyzing in vivo ¹H-MRS brain data acquired with various voxel compositions.

Results

Phantom measurements indicated substantial underestimation of NAA concentrations when assuming homogeneously composed voxels, especially for voxels containing solution, which simulated CSF (error: ≤92%). This bias was substantially reduced by taking into account voxel composition (error: ≤10%). In the in vivo study, tissue correction reduced the overall variation of quantified metabolites by up to 35% and revealed the expected metabolic differences between various brain tissues.

Conclusions

Tissue composition affects extraction of metabolite concentrations and may cause misinterpretations when comparing measurements performed with different voxel sizes. This variation can be reduced by considering the different tissue types by means of combined analysis of spectroscopic and imaging data.     

Diffusion simulation-based fiber tracking using time-of-arrival maps: a comparison with standard methods

Object

We propose a new tracking method based on time-of-arrival (TOA) maps derived from simulated diffusion processes.

Materials and methods

The proposed diffusion simulation-based tracking consists of three steps that are successively evaluated on small overlapping sub-regions in a diffusion tensor field. First, the diffusion process is simulated for several time steps. Second, a TOA map is created to store simulation results for the individual time steps that are required for the tract reconstruction. Third, the fiber pathway is reconstructed on the TOA map and concatenated between neighboring sub-regions. This new approach is compared with probabilistic and streamline tracking. All methods are applied to synthetic phantom data for an easier evaluation of their fiber reconstruction quality.

Results

The comparison of the tracking results did show severe problems for the streamline approach in the reconstruction of crossing fibers, for example. The probabilistic method was able to resolve the crossing, but could not handle strong curvature. The new diffusion simulation-based tracking could reconstruct all problematic fiber constellations.

Conclusion

The proposed diffusion simulation-based tracking method used the whole tensor information of a neighborhood of voxels and is, therefore, able to handle problematic tracking situations better than established methods.     

Characterization of the impact to PET quantification and image quality of an anterior array surface coil for PET/MR imaging

Object

The aim of this study was to determine the impact to PET quantification, image quality and possible diagnostic impact of an anterior surface array used in a combined PET/MR imaging system.

Materials and methods

An extended oval phantom and 15 whole-body FDG PET/CT subjects were re-imaged for one bed position following placement of an anterior array coil at a clinically realistic position. The CT scan, used for PET attenuation correction, did not include the coil. Comparison, including liver SUV_mean, was performed between the coil present and absent images using two methods of PET reconstruction. Due to the time delay between PET scans, a model was used to account for average physiologic time change of SUV.

Results

On phantom data, neglecting the coil caused a mean bias of ?8.2 % for non-TOF/PSF reconstruction, and ?7.3 % with TOF/PSF. On clinical data, the liver SUV neglecting the coil presence fell by ?6.1 % (±6.5 %) for non-TOF/PSF reconstruction; respectively ?5.2 % (±5.3 %) with TOF/PSF. All FDG-avid features seen with TOF/PSF were also seen with non-TOF/PSF reconstruction.

Conclusion

Neglecting coil attenuation for this anterior array coil results in a small but significant reduction in liver SUV_mean but was not found to change the clinical interpretation of the PET images.     

Fitting interrelated datasets: metabolite diffusion and general lineshapes

Objective

Simultaneous modeling of true 2-D spectroscopy data, or more generally, interrelated spectral datasets has been described previously and is useful for quantitative magnetic resonance spectroscopy applications. In this study, a combined method of reference-lineshape enhanced model fitting and two-dimensional prior-knowledge fitting for the case of diffusion weighted MR spectroscopy is presented.

Materials and methods

Time-dependent field distortions determined from a water reference are applied to the spectral bases used in linear-combination modeling of interrelated spectra. This was implemented together with a simultaneous spectral and diffusion model fitting in the previously described Fitting Tool for Arrays of Interrelated Datasets (FiTAID), where prior knowledge conditions and restraints can be enforced in two dimensions.

Results

The benefit in terms of increased accuracy and precision of parameters is illustrated with examples from Monte Carlo simulations, in vitro and in vivo human brain scans for one- and two-dimensional datasets from 2-D separation, inversion recovery and diffusion-weighted spectroscopy (DWS). For DWS, it was found that acquisitions could be substantially shortened.

Conclusion

It is shown that inclusion of a measured lineshape into modeling of interrelated MR spectra is beneficial and can be combined also with simultaneous spectral and diffusion modeling.

     

k-t BLAST and SENSE accelerated time-resolved three-dimensional phase contrast MRI in an intracranial aneurysm

Objective

The objective of this study was to investigate the performance of k-t BLAST (Broad-use Linear Acquisition Speed-up Technique) accelerated time-resolved 3D PC-MRI compared to SENSE (SENSitivity Encoding) acceleration in an in vitro and in vivo intracranial aneurysm.

Materials and methods

Non-accelerated, SENSE and k-t BLAST accelerated time-resolved 3D PC-MRI measurements were performed in vivo and in vitro. We analysed the consequences of various temporal resolutions in vitro.

Results

Both in vitro and in vivo measurements showed that the main effect of k-t BLAST was underestimation of velocity during systole. In the phantom, temporal blurring decreased with increasing temporal resolution. Quantification of the differences between the non-accelerated and accelerated measurements confirmed that in systole SENSE performed better than k-t BLAST in terms of mean velocity magnitude. In both in vitro and in vivo measurements, k-t BLAST had higher SNR compared to SENSE. Qualitative comparison between measurements showed good similarity.

Conclusion

Comparison with SENSE revealed temporal blurring effects in k-t BLAST accelerated measurements.     

An MR-compatible stereoscopic in-room 3D display for MR-guided interventions

Background and methods

A commercial three-dimensional (3D) monitor was modified for use inside the scanner room to provide stereoscopic real-time visualization during magnetic resonance (MR)-guided interventions, and tested in a catheter-tracking phantom experiment at 1.5 T. Brightness, uniformity, radio frequency (RF) emissions and MR image interferences were measured.

Results and discussion

Due to modifications, the center luminance of the 3D monitor was reduced by 14 %, and the addition of a Faraday shield further reduced the remaining luminance by 31 %. RF emissions could be effectively shielded; only a minor signal-to-noise ratio (SNR) decrease of 4.6 % was observed during imaging. During the tracking experiment, the 3D orientation of the catheter and vessel structures in the phantom could be visualized stereoscopically.