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.

Differential diagnosis of intracranial ring enhancing cystic mass lesions--role of diffusion-weighted imaging (DWI) and diffusion-tensor imaging (DTI).

OBJECTIVE: To evaluate the contribution of DWI and DTI to the differential diagnosis of cerebral ring enhancing lesions by describing DWI and ADC (apparent diffusion coefficient) findings and measuring the two DTI parameters mean diffusivity (MD) and fractional anisotropy (FA). MATERIALS AND METHODS: A total of 17 patients presenting with 26 rim enhancing cysts were investigated with DWI and DTI. Parameter maps of the DTI metrics MD and FA were calculated and quantified using regions of interest (ROIs). RESULTS: Five patients suffered from abscesses with a total of 10 cysts, 9 from glioblastomas with 10 cysts, and 3 from metastases with 6 cysts. All abscess cavities showed hyperintense DWI signal intensity compared to normal appearing white matter (NAWM), low ADC, low MD and high FA. Eight out of 10 glioblastoma cysts and all 6 metastatic cysts revealed hypointensity on DWI, high ADC, high MD and low FA (p<0.0001 compared to abscess group). DWI findings of 2/10 glioblastoma cysts overlapped with those of abscesses showing hyperintensity on DWI, low ADC and low MD and hence mimicked abscesses. FA of these 2 glioblastoma cysts was significantly lower than in abscess cavities (p=0.032). CONCLUSION: The findings of reduced diffusion compared to NAWM and increased FA within a ring enhancing cyst strongly indicate a cerebral abscess. In contrast, the majority of neoplastic cysts revealed high diffusion and low FA. Reduced diffusion is also found in a very small number of tumour cysts, but in these low FA refers to a non-infectious origin and thus helps distinguishing from infectious abscess.

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.

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.

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.