TTC post-processing is beneficial for functional MRI at low magnetic field: a comparative study at 1 T and 3 T

Abstract

This study aimed to broaden the diagnostic possibilities of lowfield MRI systems (i) by examining the feasibility of functional MRI of human brain activation at 1 T, and (ii) by assessing its reliability in comparison with acquisitions at 3 T. Eight subjects were studied at 1 T and 3T using standard echo-planar-imaging sequences at 3-mm isotropic spatial resolution. Paradigms included silent word generation, sequential finger-tothumb opposition, and passive finger movements. Image post-processing was carried out either with statistical parametric mapping (SPM5, singlesubject and group analysis) or with a two-threshold correlation (TTC, single-subject analysis only) analysis. Single-subject analysis with SPM5 resulted in 3-5 times more activated pixels at 3 T than at 1 T in the examined Broca and sensorimotor regions. By comparison, the TTC single-subject analysis yielded the same amount of activated pixels at 3 T and 1 T. Moreover, this number was identical to that obtained with SPM at 3 T. The group analysis with SPM5 resulted in very similar numbers of activated pixels at both field strengths. The present findings suggest that a field strength of 1 T combined with adequate post-processing allows for reliable functional MRI studies of human brain activation. High-field advantages are therefore best invested in higher spatial resolution.

Does obstetric brachial plexus injury influence speech dominance?

“…Functional studies used an echo planar imaging sequence as supplied by the manufacturer with the following parameters: TR/TE, 2,500/80 milliseconds; 90‐degree flip angle, 752Hz receiver bandwidth, 192 × 192cm field of view, and 64 × 64 matrix yielding 3 × 3mm in‐plane resolution, and 16 sections with a thickness of 3mm and a 1mm gap. The mapping of language centers was accomplished with the use of a block design alternating active and passive periods, each lasting for 50 seconds, involving seven cycles of internal word generation and resting state inactivity as validated earlier 23, 24. This paradigm has been routinely used in our practice and in most studies to determine language lateralization 12, 20, 23, 24.…”

“…The mapping of language centers was accomplished with the use of a block design alternating active and passive periods, each lasting for 50 seconds, involving seven cycles of internal word generation and resting state inactivity as validated earlier 23, 24. This paradigm has been routinely used in our practice and in most studies to determine language lateralization 12, 20, 23, 24. In the beginning of an active period, a certain letter was given to the subject via a magnetic resonance–compatible headphone.…”

“…A retrospective motion correction was performed as offered by the manufacturer. All data sets were evaluated by using a two‐threshold correlation analysis on a single‐subject basis 23, 25–27. In brief, in the first step, activation maps were calculated as maps of correlation coefficients (CCs) with the use of a boxcar reference function derived from the task protocol and shifted by 6 seconds to account for the delayed hemodynamic response.…”

“…Thus, the upper and lower thresholds of the two‐threshold correlation approach correspond to p values of less than 0.001 and less than 0.05, respectively. The CCs corresponding to these p values can be rigorously determined from the Gaussian fit of the CC distribution 23, 25–27. The method is user independent and does not require subjective thresholding either in the statistical score or the number of concurrently activated neighboring pixels.…”

An iterative two-threshold analysis for single-subject functional MRI of the human brain

“…Accordingly, the TT method employs a first upper threshold to identify significant activation centres -ensuring specificity -while a second lower threshold limits the iterative addition of directly neighbouring voxels to these centresmaximising sensitivity. The robustness and reliability of the TT method has previously been demonstrated for both single-subject [19] and group analyses [20] of the fine-scale functional organisation of the finger areas in the human primary somatosensory cortex [21], as well as for clinical applications [22][23][24] including the visualisation of epileptic activity [25] and the use of other techniques such as SPECT [22,26].…”