Low doses of morphine, the most commonly used opioid analgesic, have been shown to significantly reduce the affective but not the sensory intensive dimension of pain. This suggests differential dose-response relationships of opioid analgesia on the sensory and affective components of pain. We investigated the effects of different alfentanil plasma concentration levels (0, 19.6+/-2.7, 47.2+/-7.6, and 76.6+/-11.3 ng/ml) on pain-related brain activation achieved by short pulses of gaseous CO(2) delivered to the nasal mucosa, using functional magnetic resonance imaging (fMRI) on a 3.0 T MRI scanner in 16 non-carriers and 9 homozygous carriers of the mu-opioid receptor gene variant OPRM1 118A>G. Increasing opioid concentrations had differential effects in brain regions processing the sensory and affective dimensions of pain. In brain regions associated with the processing of the sensory intensity of pain (primary and secondary somatosensory cortices, posterior insular cortex), activation decreased linearly in relation to alfentanil concentrations, which was significantly less pronounced in OPRM1 118G carriers. In contrast, in brain regions known to process the affective dimension of pain (parahippocampal gyrus, amygdala, anterior insula), pain-related activation disappeared at the lowest alfentanil dose, without genotype differences.
Purpose:To evaluate the parallel acquisition techniques, generalized autocalibrating partially parallel acquisitions (GRAPPA) and modified sensitivity encoding (mSENSE), and determine imaging parameters maximizing sensitivity toward functional activation at 3T.
Materials and Methods:A total of eight imaging protocols with different parallel imaging techniques (GRAPPA and mSENSE) and reduction factors (R ϭ 1, 2, 3) were compared at different matrix sizes (64 and 128) with respect to temporal noise characteristics, artifact behavior, and sensitivity toward functional activation.Results: Echo planar imaging (EPI) with GRAPPA and a reduction factor of 2 revealed similar image quality and sensitivity than full k-space EPI. A higher incidence of artifacts and a marked sensitivity loss occurred at R ϭ 3. Even though the same eight-channel head coil was used for signal detection in all experiments, GRAPPA generally showed more benign patterns of spatially-varying noise amplification, and mSENSE was also more susceptible to residual unfolding artifacts than GRAPPA.
Conclusion:At 3T and a reduction factor of 2, parallel imaging can be used with only little penalty with regard to sensitivity. With our implementation and coil setup the performance of GRAPPA was clearly superior to mSENSE. Thus, it seems advisable to pay special attention to the employed parallel imaging method and its implementation.
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