Robust intra-individual estimation of structural connectivity by Principal Component Analysis

Fiber tractography based on diffusion-weighted MRI provides a non-invasive characterization of the structural connectivity of the human brain at the macroscopic level. Quantification of structural connectivity strength is challenging and mainly reduced to "streamline counting" methods. These are however highly dependent on the topology of the connectome and the particular specifications for seeding and filtering, which limits their intra-subject reproducibility across repeated measurements and, in consequence, also confines their validity. Here we propose a novel method for increasing the intra-subject reproducibility of quantitative estimates of structural connectivity strength. To this end, the connectome is described by a large matrix in positional-orientational space and reduced by Principal Component Analysis to obtain the main connectivity "modes". It was found that the proposed method is quite robust to structural variability of the data.

Modelfree global tractography

Tractography based on diffusion-weighted MRI investigates the large scale arrangement of the neurite fibers in brain white matter. It is usually assumed that the signal is a convolution of a fiber specific response function (FRF) with a fiber orientation distribution (FOD). The FOD is the focus of tractography. While in the past the FRF was estimated beforehand and was usually assumed to be fix, more recent approaches estimate the response function during tractography. This work proposes a novel objective function independent of the FRF, just aiming for FOD reconstruction. The objective is integrated into global tractography showing promising results.

Imaging with positive T1-contrast using superstimulated echoes

This article presents the basic principles of the superstimulated echo mechanism and shows preliminary results of its application to T1-weighted imaging with positive T1-contrast. A superstimulated echo scheme uses a preparation of square-wave modulated, periodically inverted z-magnetization, which after signal evolution during the mixing time TM is fully converted into transverse magnetization. This avoids the 50% signal loss of a conventional stimulated echo. Furthermore, its implementation as a preparation module for standard turbo spin echo (TSE) imaging allows producing images with positive T1-contrast.

Reduced interhemispheric structural connectivity between anterior cingulate cortices in borderline personality disorder

Functional and structural alterations of the anterior cingulate cortex (ACC), a key region for emotional and cognitive processing, are associated with borderline personality disorder (BPD). However, the interhemispheric structural connectivity between the left and right ACC and between other prefrontal regions in this condition is unknown. We acquired diffusion-tensor imaging data from 20 healthy women and 19 women with BPD and comorbid attention-deficit hyperactivity disorder (ADHD). Interhemispheric structural connectivity between both sides of the ACC, dorsolateral prefrontal cortices and medial orbitofrontal cortices was assessed by a novel probabilistic diffusion tensor-based fiber tracking method. In the BPD group as compared with healthy controls, we found decreased interhemispheric structural connectivity between both ACCs in fiber tracts that pass through the anterior corpus callosum and connect dorsal areas of the ACCs. Decreased interhemispheric structural connectivity between both ACCs may be a structural correlate of BPD.

Extraction of prefronto-amygdalar pathways by combining probability maps

Many recent studies reported altered functional connectivity within the frontolimbic circuitry in a wide range of neuropsychiatric disorders. However, functional connectivity must rely on structural connections. In this study we applied a novel probabilistic fiber tracking method to assess the structural connectivity between the amygdala and different prefrontal brain regions in vivo. Twenty healthy subjects were investigated with diffusion tensor imaging. Probabilistic fiber tracking was started from the amygdala and different prefrontal brain regions. Resulting probability maps were combined using an extended multiplication of probabilistic maps to identify the most probable anatomical pathways connecting these structures. We found one ventral pathway through the uncinate fascicle, connecting the amygdala and the medial and lateral orbitofrontal cortices. In addition to this ventral pathway, we depicted distinct dorsal pathways (medial and lateral), which connect the amygdala with the anterior cingulate cortex and the dorsolateral prefrontal cortex. The dorso-medial pathway proceeds through the inferior thalamic peduncle, while the dorsolateral pathway travels through the external capsule. We believe that our approach provides a promising tool to assess the integrity of specific structural connections in patients with neuropsychiatric disorders.

Connecting and merging fibres: pathway extraction by combining probability maps

Probability mapping of connectivity is a powerful tool to determine the fibre structure of white matter in the brain. Probability maps are related to the degree of connectivity to a chosen seed area. In many applications, however, it is necessary to isolate a fibre bundle that connects two areas. A frequently suggested solution is to select curves, which pass only through two or more areas. This is very inefficient, especially for long-distance pathways and small areas. In this paper, a novel probability-based method is presented that is capable of extracting neuronal pathways defined by two seed points. A Monte Carlo simulation based tracking method, similar to the Probabilistic Index of Connectivity (PICo) approach, was extended to preserve the directional information of the main fibre bundles passing a voxel. By combining two of these extended visiting maps arising from different seed points, two independent parameters are determined for each voxel: the first quantifies the uncertainty that a voxel is connected to both seed points; the second represents the directional information and estimates the proportion of fibres running in the direction of the other seed point (connecting fibre) or face a third area (merging fibre). Both parameters are used to calculate the probability that a voxel is part of the bundle connecting both seed points. The performance and limitations of this DTI-based method are demonstrated using simulations as well as in vivo measurements.

Contribution of diffusion tensor MR imaging in detecting cerebral microstructural changes in adults with neurofibromatosis type 1

BACKGROUND AND PURPOSE

After an early progression of signal intensity changes in T2-weighted MR images, also known as "neurofibromatosis bright objects," in patients with neurofibromatosis type 1 (NF-1), there is a tendency toward regression or even disappearance in early adulthood. The purpose of this study was to investigate whether adult patients with NF-1 exhibit generalized microstructural alterations even in normal-appearing brain regions.

MATERIALS AND METHODS

Conventional and diffusion tensor MR imaging of the brain was obtained in 10 adult patients with NF-1 and 10 age-matched healthy volunteers. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were measured in brain stem, basal ganglia, thalamus, corpus callosum, and frontal and parietooccipital white matter regions.

RESULTS

Significantly increased ADC and decreased FA values were found in all regions of interest and in all patients with NF-1, irrespective of their scholastic achievement and subsequent professional performance, compared with control subjects (P < .001). There were no significant correlations with the age (P > .1) or with the lateralization between brain hemispheres (P > .05).

CONCLUSION

Diffusion tensor imaging reveals globally elevated FA and decreased ADC values in the mature brains of patients with NF-1, which is most likely a consequence of diffuse and basic alterations in cerebral microstructure that result from the underlying gene mutation.

Inferior frontal white matter microstructure and patterns of psychopathology in women with borderline personality disorder and comorbid attention-deficit hyperactivity disorder

BACKGROUND

Inferior frontal white matter circuits are likely to be relevant to dysfunctional affect regulation, aggression, dissociative symptoms, neuropsychological functioning and psychopathology in women with borderline personality disorder (BPD) and comorbid attention-deficit hyperactivity disorder (ADHD).

METHODS

20 women with BPD and comorbid ADHD and 20 healthy women were investigated, and groups were matched for age, education and premorbid intelligence. Mean diffusivity and fractional anisotropy were measured bilaterally in the inferior frontal white matter by diffusion tensor imaging (DTI). Conflict resolution was assessed by the Attention Network Test, sustained vigilance by the Degraded-Stimulus Continuous Performance Test.

RESULTS

Among women with BPD and comorbid ADHD, increased mean diffusivity in inferior frontal white matter was associated with higher levels of dysfunctional affect regulation, anger-hostility, dissociative symptoms, and general psychopathology (p<.05, uncorrected). Poor conflict resolution during the attention task was positively associated with anger-hostility, psychopathology, dissociation and the number of ADHD-symptoms, but not related to DTI variables. Both groups did not differ significantly with respect to DTI variables or neuropsychological performance. In the BPD group, a lifetime history of major depression or a current eating disorder were associated with impaired inferior frontal white matter integrity, while a history of sexual abuse or a current posttraumatic stress disorder were not.

CONCLUSIONS

Inferior frontal white matter microstructural abnormalities may be linked to key aspects of psychopathology in women with BPD and comorbid ADHD and add to alterations in orbitofrontal and limbic areas. The relationship between neuropsychological functioning and white matter structure remains unclear.

Is the “biexponential diffusion” biexponential?

Diffusion-weighted signal from the brain, S, deviates from monoexponential dependence on the b-factor. This property is often referred to as biexponential diffusion, since the corresponding model fits data well. The aim of this study is to examine the necessity of using the biexponential model in homogeneous voxels under isotropic diffusion weighting up to b = 2.5 ms/microm(2). The model is compared to the cumulant expansion of ln S in a power series in b, which takes its origin in fundamental properties of the diffusion-weighted signal, but diverges at large b. The absence of statistically significant evidences for the biexponential diffusion is demonstrated in gray matter. The cumulant expansion terminated after the term b(2) describes data equally well with fewer adjustable parameters. The biexponential model is preferable in voxels with a partial volume of CSF.

Quantitative diffusion tensor MR imaging of the brain: field strength related variance of apparent diffusion coefficient (ADC) and fractional anisotropy (FA) scalars

The objectives were to study the "impact" of the magnetic field strength on diffusion tensor imaging (DTI) metrics and also to determine whether magnetic-field-related differences in T2-relaxation times of brain tissue influence DTI measurements. DTI was performed on 12 healthy volunteers at 1.5 and 3.0 Tesla (within 2 h) using identical DTI scan parameters. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) values were measured at multiple gray and white matter locations. ADC and FA values were compared and analyzed for statistically significant differences. In addition, DTI measurements were performed at different echo times (TE) for both field strengths. ADC values for gray and white matter were statistically significantly lower at 3.0 Tesla compared with 1.5 Tesla (% change between -1.94% and -9.79%). FA values were statistically significantly higher at 3.0 Tesla compared with 1.5 Tesla (% change between +4.04 and 11.15%). ADC and FA values are not significantly different for TE=91 ms and TE=125 ms. Thus, ADC and FA values vary with the used field strength. Comparative clinical studies using ADC or FA values should consequently compare ADC or FA results with normative ADC or FA values that have been determined for the field strength used.

Multitensor approach for analysis and tracking of complex fiber configurations

A multidiffusion-tensor model (MDT) is presented containing two anisotropic and one isotropic diffusion tensors. This approach has the ability to detect areas of fiber crossings and resolve the direction of crossing fibers. The mean diffusivity and the ratio of the tensor compartments were merged to one independent parameter by fitting MDT to the diffusion-weighted intensities of a two-point data acquisition scheme. By an F-test between the errors of the standard single diffusion tensor and the more complex MDT, fiber crossings were detected and the more accurate model was chosen voxel by voxel. The performance of crossing detection was compared with the spherical harmonics approach in simulations as well as in vivo. Similar results were found in both methods. The MDT model, however, did not only detect crossings but also yielded the single fiber directions. The FACT algorithm and a probabilistic connectivity algorithm were extended to support the MDT model. For example, a mean angular error smaller than 10 degrees was found for the MDT model in a simulated fiber crossing with an SNR of 80. By tracking the corticospinal tract the MDT-based tracks reached a significantly greater area of the gyrus precentralis.

Fast quantitative diffusion-tensor imaging of cerebral white matter from the neonatal period to adolescence

We investigated the isotropic diffusion coefficient (D') and fractional anisotropy (FA) in white matter (WM) during brain development, using an optimised diffusion-tensor imaging (DTI) method with whole brain coverage in a clinically acceptable time. We images 52 children with no evident neurological abnormality (30 boys, 22 girls aged 1 day-16 years) using high-angle DTI with optimised temporal gradient performance. D' and FA were calculated in 10 regions of interest in white matter. We saw that the age-related reduction in D' and increase in FA follow a mono- or biexponential model in white matter, probably depending on the compactness and myelination rate of the fibre tracts. In contrast to other areas, in which adult values were reached during the third year, there is a trend to continuous increase in FA in all deep white-matter areas, suggesting continuing maturation and organisation of deep tracts not detected on conventional MRI.

Diffusion tensor imaging in cases of adrenoleukodystrophy: preliminary experience as a marker for early demyelination?

BACKGROUND AND PURPOSE

Diffusion tensor imaging measures the proton diffusivity and preferential orientation of the diffusion tensor. X-linked adrenoleukodystrophy is a demyelinating disease for which therapy depends on the onset and extension of demyelination. We investigated the ability of diffusion tensor imaging to detect changes in the demyelinated lesions and in the normal appearing white matter.

METHODS

Diffusion tensor imaging of three related boys with X-linked adrenoleukodystrophy and seven age-matched control participants was performed. Isotropic diffusion (D') and fractional anisotropy (FA) values were determined in 18 regions of interest in the white matter of both hemispheres.

RESULTS

In all the demyelinated white matter areas, a pattern with increased D' and loss of FA was found. For example, mean D' was 1.772 x 10(-3)mm(2)/s in patient 2 with blindness and extensive demyelination of the occipital white matter and was 0.693 x 10(-3)mm(2)/s in control participants (P =.01). In the same region, mean FA was 0.103 (0.464 in control participants, P <.0001). Significant alterations of D' and FA were also observed in normal appearing white matter. For example, mean D' was 0.802 x 10(-3)mm(2)/s in the parietal white matter of patient 1 with no visible alterations on T2-weighted images (0.715 x 10(-3) mm(2)/s in control patients, P =.03), whereas mean FA was 0.320 (0.400 in control participants, P =.003).

CONCLUSION

Elevated D' and loss of FA revealed by diffusion tensor imaging are consistent with severe demyelination in patients with X-linked adrenoleukodystrophy. Significant alterations of D' and FA in normal appearing white matter may indicate early demyelination in areas that are not yet visibly altered on conventional MR images. Further evaluation in a larger series of patients and long-term study are needed.

Thermosensitive paramagnetic liposomes for temperature control during MR imaging-guided hyperthermia: in vitro feasibility studies

RATIONALE AND OBJECTIVES

Magnetic resonance (MR) imaging-based temperature monitoring has gained interest for use in general hyperthermia treatment of tumors. Such therapy requires an accurate control of the temperature, which should range from 41 degrees to 45 degrees C. A novel type of thermosensitive MR agent is proposed: liposome-encapsulated gadolinium chelates whose temperature response is linked to the phase-transition properties of the liposome carrier. In vitro relaxometry and MR imaging were used to evaluate the thermosensitivity of the contrast properties of liposomal gadolinium diethylenetriaminepentaacetic acid bis(methylamide) (Gd-DTPA-BMA).

MATERIALS AND METHODS

T1 relaxivity (rl) measurements of liposomal Gd-DTPA-BMA were undertaken at 0.47 T and at temperatures of 20 degrees-48 degrees C. MR imaging was performed at 2.0 T with a gel phantom containing inserts of liposomes. Diffusion-weighted and T1-weighted gradient-recalled echo images were acquired as the phantom was heated from 22 degrees to about 65 degrees C.

RESULTS

At ambient temperature, the r1 of liposomal Gd-DTPA-BMA was exchange limited due to slow water exchange between the liposome interior and exterior. A sharp, marked increase in r1 occurred as the temperature reached and exceeded the gel-to-liquid crystalline phase-transition temperature (Tm) of the liposomes (42 degrees C). The relaxation enhancement was mainly attributable to the marked increase in transmembrane water permeability, yielding fast exchange conditions. There was good correlation between the relaxometric and imaging results; the signal intensity on T1-weighted gradient-recalled echo images increased markedly as the temperature approached Tm. The temperature sensitivity of the diffusion-weighted technique differed from that of the liposome-based T1-weighted approach, with an apparent water diffusion coefficient increasing linearly with temperature.

CONCLUSION

Since the transition from low to high signal intensity occurred in the temperature range of 38 degrees - 42 degrees C, the investigated paramagnetic liposomes have a potential role as "off-on" switches for temperature control during hyperthermia treatment.

Assessment of arterial blood flow characteristics in normal and atherosclerotic vessels with the fast Fourier flow method

The purpose of this study was to scrutinize the ability of magnetic resonance imaging (MRI)-performed measurements to compare arterial flow patterns in patients with peripheral arterial occlusive disease (PAOD), healthy volunteers (HV) and endurance athletes (EA). MRI blood flow data were partially repeated with Doppler ultrasound (DUS) with a view to a methodical comparison. Additionally, pulse wave velocity was assessed with the MUFF technique. For this purpose, MRI-performed flow measurements were performed in the common femoral artery in 21 patients with PAOD, in 34 HV and in 12 EA. The analysis included maximum flow velocities (MFV), velocity/time profile (VTP), pulse wave velocity (Vpulse), and vessel diameter (VD). In addition, MFV and VD were observed by DUS in most individuals. The results revealed a significant change regarding arterial blood flow characteristics in patients compared with HV and EA, with respect to the span between the peak positive and negative blood flow velocity in the femoral artery. The pulse wave velocity in patients was markedly elevated compared with healthy individuals. Furthermore, a complete, characteristic change in the VTP could be observed in patients. The methodical comparison between DUS and MRI showed a good correlation. Multi-slice Fourier flow data have indicated markedly increased pulse wave velocity in PAOD patients. Changes in the arterial blood flow can be clearly observed with MRI. In the future, this might offer a noninvasive possibility not only for the evaluation of the stage of the disease, but also for the detection of early, pre-clinical stages of atherosclerosis.

Single-shot diffusion-weighted RARE sequence: application for temperature monitoring during hyperthermia session

A diffusion-sensitive single-shot RARE (rapid acquisition with relaxation enhancement) sequence was implemented on a 2T whole-body MRI system. The sequence was optimized for diffusion-based MR thermometry, both on a conventional whole-body gradient system and on a high-performance gradient insert. The use of spin-echo versus stimulated-echo diffusion weighting is discussed as a function of gradient performance. Diffusion-based temperature mapping was used to observe the effect of the geometry of the antenna used for radiofrequency (RF) hyperthermia on the temperature distribution. Temperature changes of +/-.5 degrees C in gel and +/-2 degrees C in a muscle sample in vitro could be detected within 16 seconds (gel) or 1 minute (muscle) at a spatial resolution of 2 x 2 x 8 mm. Temperature changes in vivo were also observed on human muscle cooled with ice with comparable sensitivity for the measured apparent diffusion coefficient (ADC) values.