ADC changes in schizophrenia: a diffusion-weighted imaging study

Processing speed impairment in schizophrenia is mediated by white matter integrity

BACKGROUND

Processing speed predicts functional outcome and is a potential endophenotype for schizophrenia. Establishing the neural basis of processing speed impairment may inform the treatment and etiology of schizophrenia. Neuroimaging investigations in healthy subjects have linked processing speed to brain anatomical connectivity. However, the relationship between processing speed impairment and white matter (WM) integrity in schizophrenia is unclear.

METHOD

Individuals with schizophrenia and healthy subjects underwent diffusion tensor imaging (DTI) and completed a brief neuropsychological assessment that included measures of processing speed, verbal learning, working memory and executive functioning. Group differences in WM integrity, inferred from fractional anisotropy (FA), were examined throughout the brain and the hypothesis that processing speed impairment in schizophrenia is mediated by diminished WM integrity was tested.

RESULTS

WM integrity of the corpus callosum, cingulum, superior and inferior frontal gyri, and precuneus was reduced in schizophrenia. Average FA in these regions mediated group differences in processing speed but not in other cognitive domains. Diminished WM integrity in schizophrenia was accounted for, in large part, by individual differences in processing speed.

CONCLUSIONS

Cognitive impairment in schizophrenia was mediated by reduced WM integrity. This relationship was strongest for processing speed because deficits in working memory, verbal learning and executive functioning were not mediated by WM integrity. Larger sample sizes may be required to detect more subtle mediation effects in these domains. Interventions that preserve WM integrity or ameliorate WM disruption may enhance processing speed and functional outcome in schizophrenia.

Assessment of white matter integrity of autistic preschool children with diffusion weighted MR imaging

The purpose was to assess white matter integrity of autistic preschool children with diffusion weighted MR imaging. Prospective study was carried on 19 autistic children (mean age 55.2ms, IQ of 86.5) and 10 sex, age and IQ matched control (mean age 53.2ms, IQ 84.5). The childhood Autism Rating Scale (CARS), social age and language age were calculated. Patients and controls underwent diffusion weighted MR imaging of the brain with b factor of 0, 500 and 1000s/mm(2). The apparent diffusion coefficient (ADC) value at different regions of the white matter were calculated and correlated with CARS, social age and language age. There were significant differences at the ADC value of the white matter between autistic and control children at genu (P=0.043), splenium (P=0.003) of the corpus callosum, frontal white matter (P=0.015) and temporal white matter (P=0.020). There was positive correlation of CARS score with ADC value of the genu (r=0.63, P=0.001), splenium (r=0.59, P=0.005), frontal white matter (r=0.81, P=0.001) and temporal white matter (r=0.74, P=0.001). The social age well correlated with ADC value of the frontal white matter (r=0.81, P=0.001) and language age well correlated with ADC value of the temporal white matter (r=0.78, P=0.001). We concluded that ADC value can be helpful in assessment of integrity of the white matter in autistic preschool children and well correlated with CARS score, social age and language age.

Visualizing non-Gaussian diffusion: clinical application of q-space imaging and diffusional kurtosis imaging of the brain and spine

Recently, non-Gaussian diffusion-weighted imaging (DWI) techniques, including q-space imaging (QSI) and diffusional kurtosis imaging (DKI), have emerged as advanced methods to evaluate tissue microstructure in vivo using water diffusion. QSI and DKI have shown promising results in clinical applications, such as in the evaluation of brain tumors (e.g., grading gliomas), degenerative diseases (e.g., specific diagnosis of Parkinson disease), demyelinating diseases (e.g., assessment of normal-appearing tissue of multiple sclerosis), and cerebrovascular diseases (e.g., assessment of the microstructural environment of fresh infarctions). Representative metrics in clinical use are the full width at half maximum, also known as the mean displacement of the probability density function curve, which is derived from QSI, and diffusional kurtosis, which is derived from DKI. These new metrics may provide information on tissue structure in addition to that provided by conventional Gaussian DWI investigations that use the apparent diffusion coefficient and fractional anisotropy, recognized indices for evaluating disease and normal development in the brain and spine. In some clinical situations, sensitivity for detecting pathological conditions is higher using QSI and DKI than conventional DWI and diffusion tensor imaging (DTI) because DWI and DTI calculations are based on the assumption that water molecules follow a Gaussian distribution, whereas hindrance of the distribution of water molecules by complex and restricted structures in actual neural tissues produces distributions that are far from Gaussian. We review the technical aspects and clinical applications of QSI and DKI, focusing on clinical use and in vivo studies and highlighting differences from conventional diffusional metrics.

Probing myelin and axon abnormalities separately in psychiatric disorders using MRI techniques

In this manuscript we present novel MRI approaches to dissecting axon vs. myelin abnormalities in psychiatric disorders. Existing DTI approaches are not able to provide specific information on these subcellular elements but novel approaches are beginning to do so. We review two approaches (magnetization transfer ratio—MTR; and diffusion tensor spectroscopy—DTS) and the theoretical framework for interpreting data derived from these approaches. Work is ongoing to collect data that will answer some relevant questions using these techniques in schizophrenia and related conditions.

Review of functional and anatomical brain connectivity findings in schizophrenia

PURPOSE OF REVIEW

We review diffusion tensor imaging (DTI) studies that investigate white matter abnormalities in patients with chronic schizophrenia, first episode schizophrenia, and those who are at genetic risk for developing schizophrenia. Additionally, we include studies that combine DTI and functional MRI (fMRI) to investigate brain connectivity abnormalities.

RECENT FINDINGS

Schizophrenia is a complex mental disorder with a peak age of onset in early adulthood. Abnormalities in white matter tracts, which connect brain regions into functional networks, are most likely relevant for understanding structural and functional brain abnormalities in schizophrenia. Dysconnectivity between brain regions, in fact, is thought to underlie cognitive abnormalities in schizophrenia but little is known about how alterations at the functional level relate to abnormalities in anatomical connectivity. DTI has become one of the most popular tools in brain research to address such questions. Here we review white matter abnormalities using DTI with the aim of understanding dysconnectivity of brain regions and their implications in schizophrenia.

SUMMARY

Advances in DTI and in combining DTI with fMRI provide new insight into anatomical and functional connections in the brain, and for studying dysconnectivity in schizophrenia.

Tract-specific analysis of white matter integrity disruption in schizophrenia

Several studies have suggested that white matter integrity is disrupted in some brain regions in patients with schizophrenia. The purpose of this study was to assess the white matter integrity of the cingulum, uncinate fasciculus, fornix, and corpus callosum using diffusion tensor imaging (DTI). Participants comprised 39 patients with schizophrenia (19 males and 20 females) and 40 age-matched normal controls (20 males and 20 females). We quantitatively assessed the fractional anisotropy (FA) and apparent diffusion coefficient (ADC) of the anterior cingulum, body of the cingulum, uncinate fasciculus, fornix, and corpus callosum on a tract-specific basis using diffusion tensor tractography (DTT). Group differences in FA and ADC between the patients and normal controls were sought. Additional exploratory analyses of the relationship between the FA or ADC and four clinical parameters (i.e., illness duration, positive symptom scores, negative symptom scores, and medication dosage) were performed. Results were analyzed in gender-combined and gender-separated group comparisons. FA was significantly lower on both sides of the anterior cingulum, uncinate fasciculus, and fornix in the schizophrenia patients irrespective of gender group separation. In the gender-combined analyses, significantly higher ADC values were demonstrated in the schizophrenia patients in both sides of the anterior cingulum, body of the cingulum and uncinate fasciculus, the left fornix, and the corpus callosum, compared with those of the normal controls. In the gender-separated analyses, the male patients showed higher ADC in the left anterior cingulum, the bilateral cingulum bodies, and the bilateral uncinate fasciculi. The female patients showed higher ADC in the right anterior cingulum, the left fornix, and the bilateral uncinate fasciculus. In correlation analyses, a significant negative correlation was found between illness duration and ADC in the right anterior cingulum in the gender-combined analyses. The gender-separated analyses found that the male patients had a significant negative correlation between negative symptom scores and FA in the right fornix, a positive correlation between illness duration and FA in the right anterior cingulum, and a negative correlation between illness duration and FA in the left uncinate fasciculus. Our DTI study showed that the integrity of white matter is disrupted in patients with schizophrenia. The results of our sub-analyses suggest that changes in FA and ADC may be related to negative symptom scores or illness duration.