Faculty Bibliography

The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed. Magn Reson Med, 2011. (c) 2011 Wiley Periodicals, Inc

Specific absorption rate management and excitation fidelity are key aspects of radiofrequency pulse design for parallel transmission at ultra-high magnetic field strength. The design of radiofrequency pulses for multiple channels is often based on the solution of regularized least-squares optimization problems for which a regularization term is typically selected to control the integrated or peak pulse waveform amplitude. Unlike single-channel transmission, the specific absorption rate of parallel transmission is significantly influenced by interferences between the electric fields associated with the individual transmission elements, which a conventional regularization term does not take into account. This work explores the effects upon specific absorption rate of incorporating experimentally measurable electric field interactions into parallel transmission pulse design. Results of numerical simulations and phantom experiments show that the global specific absorption rate during parallel transmission decreases when electric field interactions are incorporated into pulse design optimization. The results also show that knowledge of electric field interactions enables robust prediction of the net power delivered to the sample or subject by parallel radiofrequency pulses before they are played out on a scanner. Magn Reson Med, 2011. (c) 2011 Wiley-Liss, Inc

There is substantial interpatient variation in recovery from upper limb impairment after stroke in patients with severe initial impairment. Defining recovery as a change in the upper limb Fugl-Meyer score (DeltaFM), we predicted DeltaFM with its conditional expectation (i.e., posterior mean) given upper limb Fugl-Meyer initial impairment (FM(ii)) and a putative functional magnetic resonance imaging (fMRI) recovery measure. Patients with first time, ischemic stroke were imaged at 2.5 +/- 2.2 days poststroke with 1.5-T fMRI during a hand closure task alternating with rest (fundamental frequency = 0.025 Hz, scan duration = 172 s). Confirming a previous finding, we observed that the prediction of DeltaFM by FM(ii) alone is good in patients with nonsevere initial hemiparesis but is not good in patients with severe initial hemiparesis (96% and 16% of the total sum of squares of DeltaFM explained, respectively). In patients with severe initial hemiparesis, prediction of DeltaFM by the combination of FM(ii) and the putative fMRI recovery measure nonsignificantly increased predictive explanation from 16% to 47% of the total sum of squares of DeltaFM explained. The implications of this preliminary negative result are discussed.

OBJECTIVE: To determine whether functional magnetic resonance imaging activation obtained in the first few days after stroke correlates with subsequent motor recovery. METHODS: Twenty-three patients with hemiparesis after first-time stroke were scanned at 2.0 +/- 0.9 days while performing a simple motor task. We defined recovery as the change in Fugl-Meyer score from time of scan to approximately 3 months later (90 +/- 8 days). We performed three different tests to assess correlations between brain activation and change in Fugl-Meyer score: (1) multivariate (most sensitive to spatially diffuse activation); (2) voxel-wise Statistical Parametric Mapping (most sensitive to focal activation), and (3) primary motor cortex region-of-interest analysis (most sensitive to average activation within this region). All tests controlled for initial stroke severity and lesion volume, as well as other established clinical variables. RESULTS: The multivariate test was significant [F (595, 4,934) = 1.93; p < 0.001]. The Statistical Parametric Mapping test detected two small clusters of focal activity located in the ipsilesional postcentral gyrus and cingulate cortex (p < 0.05, corrected). The region-of-interest test was not significant. INTERPRETATION: There is a pattern of brain activation present in the first few days after stroke, of which the postcentral gyrus and cingulate cortex are a part, that correlates with subsequent motor recovery. This result suggests that there are recovery processes engaged early after stroke that could provide a target for intervention.