Jessica Schulz

Prospective slice-by-slice motion correction reduces false positive activations in fMRI with task-correlated motion

OBJECTIVE We aimed to test the hypothesis that slice-by-slice prospective motion correction at 7T using an optical tracking system reduces the rate of false positive activations in an fMRI group study with a paradigm that involves task-correlated motion. MATERIALS AND METHODS Brain activation during right leg movement was measured using a block design on 15 volunteers, with and without prospective motion correction. Clearly erroneous activations were compared between both cases, at the individual level. Additionally, conventional group analysis was performed. RESULTS The number of falsely activated voxels with T-values higher than 5 was reduced by 48% using prospective motion correction alone, without additional retrospective realignment. In the group analysis, the statistical power was increased - the peak T-value was 26% greater, and the number of voxels in the cluster representing the right leg was increased by a factor of 9.3. CONCLUSION Slice-by-slice prospective motion correction in fMRI studies with task-correlated motion can substantially reduce false positive activations and increase statistical power.

Fully automated calculation of image-derived input function in simultaneous PET/MRI in a sheep model

BackgroundObtaining the arterial input function (AIF) from image data in dynamic positron emission tomography (PET) examinations is a non-invasive alternative to arterial blood sampling. In simultaneous PET/magnetic resonance imaging (PET/MRI), high-resolution MRI angiographies can be used to define major arteries for correction of partial-volume effects (PVE) and point spread function (PSF) response in the PET data. The present study describes a fully automated method to obtain the image-derived input function (IDIF) in PET/MRI. Results are compared to those obtained by arterial blood sampling.MethodsTo segment the trunk of the major arteries in the neck, a high-resolution time-of-flight MRI angiography was postprocessed by a vessel-enhancement filter based on the inertia tensor. Together with the measured PSF of the PET subsystem, the arterial mask was used for geometrical deconvolution, yielding the time-resolved activity concentration averaged over a major artery. The method was compared to manual arterial blood sampling at the hind leg of 21 sheep (animal stroke model) during measurement of blood flow with O15-water. Absolute quantification of activity concentration was compared after bolus passage during steady state, i.e., between 2.5- and 5-min post injection. Cerebral blood flow (CBF) values from blood sampling and IDIF were also compared.ResultsThe cross-calibration factor obtained by comparing activity concentrations in blood samples and IDIF during steady state is 0.98 ± 0.10. In all examinations, the IDIF provided a much earlier and sharper bolus peak than in the time course of activity concentration obtained by arterial blood sampling. CBF using the IDIF was 22 % higher than CBF obtained by using the AIF yielded by blood sampling.ConclusionsThe small deviation between arterial blood sampling and IDIF during steady state indicates that correction of PVE and PSF is possible with the method presented. The differences in bolus dynamics and, hence, CBF values can be explained by the different sampling locations (hind leg vs. major neck arteries) with differences in delay/dispersion. It will be the topic of further work to test the method on humans with the perspective of replacing invasive blood sampling by an IDIF using simultaneous PET/MRI.

Center-out echo-planar spectroscopic imaging with correction of gradient-echo phase and time shifts

A procedure to prevent the formation of image and spectral Nyquist ghosts in echo‐planar spectroscopic imaging is introduced. It is based on a novel Cartesian center‐out echo‐planar spectroscopic imaging trajectory, referred to as EPSICO, and combined with a correction of the gradient‐echo phase and time shifts. Processing of homogenous sets of forward and reflected echoes is no longer necessary, resulting in an optimized spectral width. The proposed center‐out trajectory passively prevents the formation of Nyquist ghosts by privileging the acquisition of the center k‐space line with forward echoes at the beginning of an echo‐planar spectroscopic imaging dwell time and by ensuring that all k‐space lines and their respective complex conjugates are acquired at equal time intervals. With the proposed procedure, concentrations of N‐acetyl aspartate, creatine, choline, glutamate, and myo‐inositol were reliably determined in human white matter at 3 T. Magn Reson Med, 2013.

Optically detunable, inductively coupled coil for self‐gating in small animal magnetic resonance imaging

An inductively coupled coil concept is presented, which improves the compensation of physiological motion by the self‐gating (SG) technique. The animal is positioned in a conventional volume coil encompassing the whole animal. A small, resonant surface coil (SG‐coil) is placed on the thorax so that its sensitive region includes the heart. Via inductive coupling the SG‐coil amplifies selectively the MR signal of the beating heart. With an optical detuning mechanism, this coupling can be switched off during acquisition of the MR image information, whereas it is active during SG data sampling to provide the physiological information. In vivo experiments on a mouse show an amplification of the SG signal by at least 40%. Magn Reson Med, 2011.