Improved short tau inversion recovery (iSTIR) for increased tumor conspicuity in the abdomen

Object

To develop an improved short tau inversion recovery (iSTIR) technique with simultaneous suppression of fat, blood vessels and fluid to increase tumor conspicuity in the abdomen for cancer screening.

Materials and methods

An adiabatic spectrally selective inversion pulse was used for fat suppression to overcome the reduced signal to noise ratio associated with chemically non-selective inversion pulse of STIR. A motion-sensitizing driven equilibrium was used for blood vessel suppression and a dual-echo single-shot fast spin echo acquisition was used for fluid suppression. The technique was optimized on four normal subjects and later tested on five patients referred for metastatic tumor evaluation.

Results

A velocity encoding of 2 cm/s achieved effective blood suppression even in small vessels. Subtraction of two images (one with 60 ms and the other with 280 ms echo time) acquired in the same echo train achieved excellent fluid suppression (>70 % reduction). Simultaneous suppression of fat, blood vessels and fluid improved the tumor conspicuity compared to corresponding fat-suppressed (STIR) image.

Conclusion

This technique generated two complementary images from a single scan: one that is equivalent to a STIR image and the other that qualitatively resembles a diffusion-weighted image and may have potential for magnetic resonance imaging cancer screening.

     

Influence of principal component analysis acceleration factor on velocity measurement in 2D and 4D PC-MRI

Objective

The objective of the study was to determine how to optimize 2D and 4D phase-contrast magnetic resonance imaging (PC-MRI) acquisitions to acquire flow velocities in millimetric vessels. In particular, we search for the best compromise between acquisition time and accuracy and assess the influence of the principal component analysis (PCA).

Materials and methods

2D and 4D PC-MRI measurements are conducted within two in vitro vessel phantoms: a Y-bifurcation phantom, the branches of which range from 2 to 5 mm in diameter, and a physiological subject-specific phantom of the carotid bifurcation. The same sequences are applied in vivo in carotid vasculature.

Results

For a vessel oriented in the axial direction, both 2D and axial 4D PC-MRI provided accuracy measurements regardless of the k-t PCA factor, while the acquisition time is reduced by a factor 6 for k-t PCA maximum value. The in vivo measurements show that the proposed sequences are adequate to acquire 2D and 4D velocity fields in millimetric vessels and with clinically realistic time durations.

Conclusion

The study shows the feasibility of conducting fast, high-resolution PC-MRI flow measurements in millimetric vessels and that it is worth maximizing the k-t PCA factor to reduce the acquisition time in the case of 2D and 4D axial acquisitions.

     

Three-dimensional echo-planar cine imaging of cerebral blood supply using arterial spin labeling

Objective

Echo-planar imaging (EPI) with CYlindrical Center-out spatiaL Encoding (EPICYCLE) is introduced as a novel hybrid three-dimensional (3D) EPI technique. Its suitability for the tracking of a short bolus created by pseudo-continuous arterial spin labeling (pCASL) through the cerebral vasculature is demonstrated.

Materials and methods

EPICYCLE acquires two-dimensional planes of k-space along center-out trajectories. These “spokes” are rotated from shot to shot about a common axis to encode a k-space cylinder. To track a bolus of labeled blood, the same subset of evenly distributed spokes is acquired in a cine fashion after a short period of pCASL. This process is repeated for all subsets to fill the whole 3D k-space of each time frame.

Results

The passage of short pCASL boluses through the vasculature of a 3D imaging slab was successfully imaged using EPICYCLE. By choosing suitable sequence parameters, the impact of slab excitation on the bolus shape could be minimized. Parametric maps of signal amplitude, transit time, and bolus width reflected typical features of blood transport in large vessels.

Conclusion

The EPICYCLE technique was successfully applied to track a short bolus of labeled arterial blood during its passage through the cerebral vasculature.

     

Vessel radius mapping in an extended model of transverse relaxation

Objectives

Spin dephasing of the local magnetization in blood vessel networks can be described in the static dephasing regime (where diffusion effects may be ignored) by the established model of Yablonskiy and Haacke. However, for small capillary radii, diffusion phenomena for spin-bearing particles are not negligible.

Material and methods

In this work, we include diffusion effects for a set of randomly distributed capillaries and provide analytical expressions for the transverse relaxation times T2* and T2 in the strong collision approximation and the Gaussian approximation that relate MR signal properties with microstructural parameters such as the mean local capillary radius.

Results

Theoretical results are numerically validated with random walk simulations and are used to calculate capillary radius distribution maps for glioblastoma mouse brains at 9.4 T. For representative tumor regions, the capillary maps reveal a relative increase of mean radius for tumor tissue towards healthy brain tissue of \(128 \pm 23 \%\) (p < 0.001).

Conclusion

The presented method may be used to quantify angiogenesis or the effects of antiangiogenic therapy in tumors whose growth is associated with significant microvascular changes.

     

Non-gadolinium dynamic angiography of the neurovasculature using arterial spin labeling MRI: preliminary experience in children

Objective

We demonstrate the potential clinical utility of a 4D non-gadolinium dynamic angiography technique based on arterial spin-labeling called contrast inherent inflow enhanced multi-phase angiography (CINEMA) in pediatric patients.

Materials and Methods

CINEMA was qualitatively compared to conventional time-of-flight (TOF) angiography in a cohort of 31 pediatric patients at 3 Tesla.

Results

CINEMA data were successfully acquired and reconstructed in all patients with no image artifacts. There were no cases where CINEMA was rated inferior to TOF in depicting intracranial vessel conspicuity. In 19 cases, CINEMA was rated equivalent to TOF and in the 12 remaining cases CINEMA was rated superior to TOF.

Conclusion

There is a steadily rising concern in adults and children over the potential effects of intracranial deposition of gadolinium. CINEMA is therefore a viable alternative in dynamic neurovascular imaging.

     

Accelerated visualization of selected intracranial arteries by cycled super-selective arterial spin labeling

Objective

To accelerate super-selective arterial spin labeling (ASL) angiography by using a single control condition denoted as cycled super-selective arterial spin labeling.

Materials and methods

A single non-selective control image is acquired that is shared by selective label images. Artery-selective imaging is possible by geometrically changing the position of the labeling focus to more than one artery of interest during measurement. The presented approach is compared to conventional super-selective imaging in terms of its labeling efficiency inside and outside the labeling focus using numerical simulations and in vivo measurements. Additionally, the signal-to-noise ratios of the images are compared to non-selective ASL angiography and analyzed using a two-way ANOVA test and calculating the Pearson’s correlation coefficients.

Results

The results indicate that the labeling efficiency is not reduced within the labeled artery, but can increase as a function of distance to the artery of interest when compared to conventional super-selective ASL. In the final images, no statistically significant difference of image quality can be observed while the acquisition duration could be reduced when the major brain feeding arteries are being tagged.

Conclusion

Using super-selective arterial spin labeling, a single non-selective control acquisition suffices for reconstructing selective angiograms of the cerebral vasculature, thereby accelerating image acquisition of the major intracranial arteries without notable loss of information.

     

Insight into the labeling mechanism of acceleration selective arterial spin labeling

Objectives

Acceleration selective arterial spin labeling (AccASL) is a spatially non-selective labeling technique, used in traditional ASL methods, which labels spins based on their flow acceleration rather than spatial localization. The exact origin of the AccASL signal within the vasculature is not completely understood. To obtain more insight into this, the acceleration selective module was performed followed by a velocity selective module, which is used in velocity selective arterial spin labeling (VS-ASL).

Materials and methods

Nine healthy volunteers were scanned with various combinations of the control and label conditions in both the acceleration and velocity selective module. The cut-off acceleration (0.59 m/s²) or velocity (2 cm/s) was kept constant in one module, while it was varied over a large range in the other module. With the right subtractions this resulted in AccASL, VS-ASL, combined AccASL and VS-ASL signal, and signal from one module with crushing from the other.

Results

The label created with AccASL has an overlap of approximately 50% in the vascular region with VS-ASL, but also originates from smaller vessels closer to the capillaries.

Conclusion

AccASL is able to label spins both in the macro- and meso-vasculature, as well as in the microvasculature.

     

Denoising of MR spectroscopic imaging data using statistical selection of principal components

Objectives

To evaluate a new denoising method for MR spectroscopic imaging (MRSI) data based on selection of signal-related principal components (SSPCs) from principal components analysis (PCA).

Materials and methods

A PCA-based method was implemented for selection of signal-related PCs and denoising achieved by reconstructing the original data set utilizing only these PCs. Performance was evaluated using simulated MRSI data and two volumetric in vivo MRSIs of human brain, from a normal subject and a patient with a brain tumor, using variable signal-to-noise ratios (SNRs), metabolite peak areas, Cramer-Rao bounds (CRBs) of fitted metabolite peak areas and metabolite linewidth.

Results

In simulated data, SSPC determined the correct number of signal-related PCs. For in vivo studies, the SSPC denoising resulted in improved SNRs and reduced metabolite quantification uncertainty compared to the original data and two other methods for denoising. The method also performed very well in preserving the spectral linewidth and peak areas. However, this method performs better for regions that have larger numbers of similar spectra.

Conclusion

The proposed SSPC denoising improved the SNR and metabolite quantification uncertainty in MRSI, with minimal compromise of the spectral information, and can result in increased accuracy.

     

The traveling heads: multicenter brain imaging at 7 Tesla

Objective

This study evaluates the inter-site and intra-site reproducibility of 7 Tesla brain imaging and compares it to literature values for other field strengths.

Materials and methods

The same two subjects were imaged at eight different 7 T sites. MP2RAGE, TSE, TOF, SWI, EPI as well as B1 and B0 field maps were analyzed quantitatively to assess inter-site reproducibility. Intra-site reproducibility was measured with rescans at three sites.

Results

Quantitative measures of MP2RAGE scans showed high agreement. Inter-site and intra-site reproducibility errors were comparable to 1.5 and 3 T. Other sequences also showed high reproducibility between the sites, but differences were also revealed. The different RF coils used were the main source for systematic differences between the sites.

Conclusion

Our results show for the first time that multi-center brain imaging studies of the supratentorial brain can be performed at 7 T with high reproducibility and similar reliability as at 3T. This study develops the basis for future large-scale 7 T multi-site studies.

     

Mapping fine-scale anatomy of gray matter,white matter,and trigeminal-root region applying spherical deconvolution to high-resolution 7-T diffusion MRI

Objectives

We assessed the use of high-resolution ultra-high-field diffusion magnetic resonance imaging (dMRI) to determine neuronal fiber orientation density functions (fODFs) throughout the human brain, including gray matter (GM), white matter (WM), and small intertwined structures in the cerebellopontine region.

Materials and methods

We acquired 7-T whole-brain dMRI data of 23 volunteers with 1.4-mm isotropic resolution; fODFs were estimated using constrained spherical deconvolution.

Results

High-resolution fODFs enabled a detailed view of the intravoxel distributions of fiber populations in the whole brain. In the brainstem region, the fODF of the extra- and intrapontine parts of the trigeminus could be resolved. Intrapontine trigeminal fiber populations were crossed in a network-like fashion by fiber populations of the surrounding cerebellopontine tracts. In cortical GM, additional evidence was found that in parts of primary somatosensory cortex, fODFs seem to be oriented less perpendicular to the cortical surface than in GM of motor, premotor, and secondary somatosensory cortices.

Conclusion

With 7-T MRI being introduced into clinical routine, high-resolution dMRI and derived measures such as fODFs can serve to characterize fine-scale anatomic structures as a prerequisite to detecting pathologies in GM and small or intertwined WM tracts.

     

Non-contrast MRI methods as a tool for the pre-operative assessment and surveillance of the arterio-venous fistula for haemodialysis

Objective

To compare non-contrast enhanced MRI with ultrasound (US) for measurement of arm blood vessel geometries and flow velocities in volunteers and patients with end-stage renal disease.

Materials and methods

Subjects were scanned using US (reference standard), and MRI 2D time-of-flight (ToF), 2D phase contrast (PC), and 3D multi-echo data image combination (MEDIC). Patients were also scanned after arteriovenous fistula (AVF) surgery.

Results

For mean vessel diameters (radial and brachial arteries; cephalic vein) MEDIC measurements were similar to US (p?>?0.05). However, ToF underestimated the mean diameter of the cephalic vein relative to US (p?<?0.05). For arterial velocity measurements, the mean values derived by PC-MR and US were similar (p?>?0.05). Post-operatively, the intra-luminal signal intensity was hypo-intense at the anastomosis site using ToF and MEDIC. At the same site the outer boundary of the vessel was consistently lost on ToF, but remained clearly delineated on the MEDIC images.

Discussion

With the exception of ToF, the MRI data demonstrated excellent agreement with US for measurements of vessel geometry and flow velocity. Further, the ability to clearly delineate the post-surgery vessel edges with MEDIC MRI suggests that the technique may be useful for surveillance after AVF creation or for patient-specific modelling studies.

     

Theory of MRI contrast in the annulus fibrosus of the intervertebral disc

Objective

Here we develop a three-dimensional analytic model for MR image contrast of collagen lamellae in the annulus fibrosus of the intervertebral disc of the spine, based on the dependence of the MRI signal on collagen fiber orientation.

Materials and methods

High-resolution MRI scans were performed at 1.5 and 7 T on intact whole disc specimens from ovine, bovine, and human spines. An analytic model that approximates the three-dimensional curvature of the disc lamellae was developed to explain inter-lamellar contrast and intensity variations in the annulus. The model is based on the known anisotropic dipolar relaxation of water in tissues with ordered collagen.

Results

Simulated MRI data were generated that reproduced many features of the actual MRI data. The calculated inter-lamellar image contrast demonstrated a strong dependence on the collagen fiber angle and on the circumferential location within the annulus.

Conclusion

This analytic model may be useful for interpreting MR images of the disc and for predicting experimental conditions that will optimize MR image contrast in the annulus fibrosus.

     

In vivo functional connectome of human brainstem nuclei of the ascending arousal,autonomic, and motor systems by high spatial resolution 7-Tesla fMRI

Objective

Our aim was to map the in vivo human functional connectivity of several brainstem nuclei with the rest of the brain by using seed-based correlation of ultra-high magnetic field functional magnetic resonance imaging (fMRI) data.

Materials and methods

We used the recently developed template of 11 brainstem nuclei derived from multi-contrast structural MRI at 7 Tesla as seed regions to determine their connectivity to the rest of the brain. To achieve this, we used the increased contrast-to-noise ratio of 7-Tesla fMRI compared with 3 Tesla and time-efficient simultaneous multi-slice imaging to cover the brain with high spatial resolution (1.1-mm isotropic nominal resolution) while maintaining a short repetition time (2.5 s).

Results

The delineated Pearson’s correlation-based functional connectivity diagrams (connectomes) of 11 brainstem nuclei of the ascending arousal, motor, and autonomic systems from 12 controls are presented and discussed in the context of existing histology and animal work.

Conclusion

Considering that the investigated brainstem nuclei play a crucial role in several vital functions, the delineated preliminary connectomes might prove useful for future in vivo research and clinical studies of human brainstem function and pathology, including disorders of consciousness, sleep disorders, autonomic disorders, Parkinson’s disease, and other motor disorders.

     

2D and 3D texture analysis to differentiate brain metastases on MR images: proceed with caution

Objective

To find structural differences between brain metastases of lung and breast cancer, computing their heterogeneity parameters by means of both 2D and 3D texture analysis (TA).

Materials and methods

Patients with 58 brain metastases from breast (26) and lung cancer (32) were examined by MR imaging. Brain lesions were manually delineated by 2D ROIs on the slices of contrast-enhanced T1-weighted (CET1) images, and local binary patterns (LBP) maps were created from each region. Histogram-based (minimum, maximum, mean, standard deviation, and variance), and co-occurrence matrix-based (contrast, correlation, energy, entropy, and homogeneity) 2D, weighted average of the 2D slices, and true 3D TA were obtained on the CET1 images and LBP maps.

Results

For LBP maps and 2D TA contrast, correlation, energy, and homogeneity were identified as statistically different heterogeneity parameters (SDHPs) between lung and breast metastasis. The weighted 3D TA identified entropy as an additional SDHP. Only two texture indexes (TI) were significantly different with true 3D TA: entropy and energy. All these TIs discriminated between the two tumor types significantly by ROC analysis. For the CET1 images there was no SDHP at all by 3D TA.

Conclusion

Our results indicate that the used textural analysis methods may help with discriminating between brain metastases of different primary tumors.

     

Application of the limited-memory quasi-Newton algorithm for multi-dimensional,large flip-angle RF pulses at 7T

Objective

Ultrahigh field MRI provides great opportunities for medical diagnostics and research. However, ultrahigh field MRI also brings challenges, such as larger magnetic susceptibility induced field changes. Parallel-transmit radio-frequency pulses can ameliorate these complications while performing advanced tasks in routine applications. To address one class of such pulses, we propose an optimal-control algorithm as a tool for designing advanced multi-dimensional, large flip-angle, radio-frequency pulses. We contrast initial conditions, constraints, and field correction abilities against increasing pulse trajectory acceleration factors.

Materials and methods

On an 8-channel 7T system, we demonstrate the quasi-Newton algorithm with pulse designs for reduced field-of-view imaging with an oil phantom and in vivo with scans of the human brain stem. We used echo-planar imaging with 2D spatial-selective pulses. Pulses are computed sufficiently rapid for routine applications.

Results

Our dataset was quantitatively analyzed with the conventional mean-square-error metric and the structural-similarity index from image processing. Analysis of both full and reduced field-of-view scans benefit from utilizing both complementary measures.

Conclusion

We obtained excellent outer-volume suppression with our proposed method, thus enabling reduced field-of-view imaging using pulse trajectory acceleration factors up to 4.

     

Segmentation and texture analysis of structural biomarkers using neighborhood-clustering-based level set in MRI of the schizophrenic brain

Objective

Schizophrenia (SZ) is a psychiatric disorder that especially affects individuals during their adolescence. There is a need to study the subanatomical regions of SZ brain on magnetic resonance images (MRI) based on morphometry. In this work, an attempt was made to analyze alterations in structure and texture patterns in images of the SZ brain using the level-set method and Laws texture features.

Materials and methods

T1-weighted MRI of the brain from Center of Biomedical Research Excellence (COBRE) database were considered for analysis. Segmentation was carried out using the level-set method. Geometrical and Laws texture features were extracted from the segmented brain stem, corpus callosum, cerebellum, and ventricle regions to analyze pattern changes in SZ.

Results

The level-set method segmented multiple brain regions, with higher similarity and correlation values compared with an optimized method. The geometric features obtained from regions of the corpus callosum and ventricle showed significant variation (p < 0.00001) between normal and SZ brain. Laws texture feature identified a heterogeneous appearance in the brain stem, corpus callosum and ventricular regions, and features from the brain stem were correlated with Positive and Negative Syndrome Scale (PANSS) score (p < 0.005).

Conclusion

A framework of geometric and Laws texture features obtained from brain subregions can be used as a supplement for diagnosis of psychiatric disorders.

     

The effect of diffusion gradient direction number on corticospinal tractography in the human brain: an along-tract analysis

Objectives

We evaluated diffusion imaging measures of the corticospinal tract obtained with a probabilistic tractography algorithm applied to data of two acquisition protocols based on different numbers of diffusion gradient directions (NDGDs).

Materials and methods

The corticospinal tracts (CST) of 18 healthy subjects were delineated using 22 and 66-NDGD data. An along-tract analysis of diffusion metrics was performed to detect possible local differences due to NDGD.

Results

FA values at 22-NDGD showed an increase along the central portion of the CST. The mean of partial volume fraction of the orientation of the second fiber (f2) was higher at 66-NDGD bilaterally, because for 66-NDGD data the algorithm more readily detects dominant fiber directions beyond the first, thus the increase in FA at 22-NDGD is due to a substantially reduced detection of crossing fiber volume. However, the good spatial correlation between the tracts drawn at 22 and 66 NDGD shows that the extent of the tract can be successfully defined even at lower NDGD.

Conclusions

Given the spatial tract localization obtained even at 22-NDGD, local analysis of CST can be performed using a NDGD compatible with clinical protocols. The probabilistic approach was particularly powerful in evaluating crossing fibers when present.

     

Radial MRI with variable echo times: reducing the orientation dependency of susceptibility artifacts of an MR-safe guidewire

Objectives

Guidewires are indispensable tools for intravascular MR-guided interventions. Recently, an MR-safe guidewire made from a glass-fiber/epoxy compound material with embedded iron particles was developed. The size of the induced susceptibility artifact, and thus the guidewire’s visibility, depends on its orientation against B ₀. We present a radial acquisition scheme with variable echo times that aims to reduce the artifact’s orientation dependency.

Materials and methods

The radial acquisition scheme uses sine-squared modulated echo times depending on the physical direction of the spoke to balance the susceptibility artifact of the guidewire. The acquisition scheme was studied in simulations based on dipole fields and in phantom experiments for different orientations of the guidewire against B ₀. The simulated and measured artifact widths were quantitatively compared.

Results

Compared to acquisitions with non-variable echo times, the proposed acquisition scheme shows a reduced angular variability. For the two main orientations (i.e., parallel and perpendicular to B ₀), the ratio of the artifact widths was reduced from about 2.2 (perpendicular vs. parallel) to about 1.2 with the variable echo time approach.

Conclusion

The reduction of the orientation dependency of the guidewire’s artifact via sine-squared varying echo times could be verified in simulations and measurements. The more balanced artifact allows for a better overall visibility of the guidewire.

     

Improving the clinical potential of ultra-high field fMRI using a model-free analysis method based on response consistency

Objective

To develop an analysis method that is sensitive to non-model-conform responses often encountered in ultra-high field presurgical planning fMRI. Using the consistency of time courses over a number of experiment repetitions, it should exclude low quality runs and generate activation maps that reflect the reliability of responses.

Materials and methods

7 T fMRI data were acquired from six healthy volunteers: three performing purely motor tasks and three a visuomotor task. These were analysed with the proposed approach (UNBIASED) and the GLM.

Results

UNBIASED results were generally less affected by false positive results than the GLM. Runs that were identified as being of low quality were confirmed to contain little or no activation. In two cases, regions were identified as activated in UNBIASED but not GLM results. Signal changes in these areas were time-locked to the task, but were delayed or transient.

Conclusion

UNBIASED is shown to be a reliable means of identifying consistent task-related signal changes regardless of response timing. In presurgical planning, UNBIASED could be used to rapidly generate reliable maps of the consistency with which eloquent brain regions are activated without recourse to task timing and despite modified hemodynamics.

     

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.