Transverse relaxometry with transmit field-constrained stimulated echo compensation.

OBJECTIVE: Purely exponential decay is rarely observed in conventional mono-exponential T2 mapping due to transmit field inhomogeneity and calibration errors, which collectively introduce stimulated and indirect echo pathways. Stimulated echo correction (SEC) requires an additional fit parameter for the transmit field, resulting in greater uncertainty in T2 relative to mono-exponential fitting. The aim of this study was to develop an accurate and precise method for T2 mapping using SEC. METHODS: The proposed method, called two-step SEC (tSEC), leverages spatial correlations in the transmit field to reduce the number of fully independent fitting parameters from three to two. The method involves a two-pass fit: the first pass involves a fast but standard SEC fit. The initially estimated transmit field is smoothed and provided as a fixed input to the second pass. RESULTS: Simulations and in vivo experiments demonstrated up to 38% and 27% decreases in relative T2 variance with tSEC relative to SEC. Average T2 values were unchanged between tSEC and SEC fits. The proposed method uses the same input data as SEC and exponential fits, so it is applicable to existing data. DISCUSSION: The proposed method generates reliable and reproducible quantitative T2 maps and should be considered for future relaxometry studies.

A multiparametric analysis of white matter maturation during late childhood and adolescence.

White matter development has been well described using diffusion tensor imaging (DTI), but the microstructural processes driving development remain unclear due to methodological limitations. Here, using neurite orientation dispersion and density imaging (NODDI), inhomogeneous magnetization transfer (ihMT), and multicomponent driven equilibrium single-pulse observation of T1/T2 (mcDESPOT), we describe white matter development at the microstructural level in a longitudinal cohort of healthy 6-15 year olds. We evaluated age and gender-related trends in fractional anisotropy (FA), mean diffusivity (MD), neurite density index (NDI), orientation dispersion index (ODI), quantitative ihMT (qihMT), myelin volume fraction (VFm ), and g-ratio. We found age-related increases of VFm in most regions, showing ongoing myelination in vivo during late childhood and adolescence for the first time. No relationship was observed between qihMT and age, suggesting myelin volume increases are driven by increased water content. Age-related increases were observed for NDI, suggesting axonal packing is also occurring during this time. g-ratio decreased with age in the uncinate fasciculus, implying changes in communication efficiency are ongoing in this region. FA increased and MD decreased with age in most regions. Gender effects were present in the left cingulum for FA, and an age-by-gender interaction was found for MD in the left uncinate fasciculus. These findings suggest that FA and MD remain useful markers of gender-related processes, and gender differences are likely driven by factors other than myelin. We conclude that white matter development during late childhood and adolescence is driven by a combination of axonal packing and myelin volume increases.

Glutamate alterations associated with transcranial magnetic stimulation in youth depression: a case series.

OBJECTIVE: We hypothesized an increase in dorsolateral prefrontal cortex (DLPFC) glutamate levels would occur after 3 weeks of repetitive transcranial magnetic stimulation (rTMS) treatment and a decrease in major depressive disorder (MDD) symptoms. METHODS: We report 6 patients (4 females) 15 to 21 years of age with treatment-resistant MDD. Participants had a mean (SD) age of 18.7 (1.95) years and a mean (SD) IQ of 102.3 (3.39). Short echo proton magnetic resonance spectroscopy (¹H-MRS) was used to quantify glutamate levels in the left DLPFC (4.5 cc) before and after rTMS treatment. Repetitive transcranial magnetic stimulation was localized to the left DLPFC and applied for 15 consecutive weekdays (120% resting motor threshold; 40 pulses over 4 seconds [10 Hz]; intertrain interval, 26 seconds; 75 trains; 3000 pulses). Treatment response was defined as a greater than 50% reduction in Hamilton Depression Rating Scale scores. Short echo proton magnetic resonance spectroscopy data were analyzed with LCModel to determine glutamate concentration. RESULTS: After rTMS, treatment responders (n = 4) showed an increase (relative to baseline) in left DLPFC glutamate levels (11%), which corresponded to an improvement in depressive symptom severity (68% Hamilton Depression Rating Scale score reduction). Treatment nonresponders (n = 2) had elevated baseline glutamate levels compared to responders in that same region, which decreased with rTMS (-10%). Procedures were generally well tolerated with no adverse events. CONCLUSIONS: Repetitive transcranial magnetic stimulation is feasible and possibly efficacious in adolescents with MDD. In responders, rTMS may act by induced elevations in elevating DFPLC glutamate levels in the left DLPFC, thereby leading to symptom improvement.

Highly accelerated dynamic contrast enhanced imaging.

PURPOSE: Dynamic contrast-enhanced imaging provides unique physiological information, notably the endothelial permeability (K(trans)), and may improve the diagnosis and management of multiple pathologies. Current acquisition methods provide limited spatial-temporal resolution and field-of-view, often preventing characterization of the entire pathology and precluding measurement of the arterial input function. We present a method for highly accelerated dynamic imaging and demonstrate its utility for dynamic contrast-enhanced modeling. METHODS: We propose a novel Poisson ellipsoid sampling scheme and enforce multiple spatial and temporal l1-norm constraints during image reconstruction. Retrospective and prospective analyses were performed to validate the approach. RESULTS: Retrospectively, no mean bias or diverging trend was observed as the acceleration rate was increased from 3× to 18×; less than 10% error was measured in K(trans) at any individual rates in this range. Prospectively accelerated images at a rate of 36× enabled full brain coverage with 0.94 × 0.94 × 1.9 mm(3) spatial and 4.1 s temporal resolutions. Images showed no visible degradation and provided accurate K(trans) values when compared to a clinical population. CONCLUSION: Highly accelerated dynamic MRI using compressed sensing and parallel imaging provides accurate permeability modeling and enables full brain, high resolution acquisitions.