A primer on functional magnetic resonance imaging

Abstract In this manuscript, basic principles of functional magnetic resonance imaging (fMRI) are reviewed. In the first section, two intrinsic mechanisms of magnetic resonance image contrast related to the longitudinal and transverse components of relaxing spins and their relaxation rates, T 1 and T 2 , are described. In the second section, the biophysical mechanisms that alter the apparent transverse relaxation time, T * 2 , in blood oxygenation level dependent (BOLD) studies and the creation of BOLD activation maps are discussed. The physiological complexity of the BOLD signal is emphasized. In the third section, arterial spin labeling (ASL) measures of cerebral blood flow are presented. Arterial spin labeling inverts or saturates the magnetization of flowing spins to measure the rate of delivery of blood to capillaries. In the fourth section, calibrated fMRI, which uses BOLD and ASL to infer alterations of oxygen utilization during behavioral activation, is reviewed. The discussion concludes with challenges confronting studies of individual cases. Keywords Functional magnetic resonance imaging . Perfusion magnetic resonance imaging . Regional blood flow . Cerebral oxygen metabolism G. G. Brown ( ) Psychology Service (MC 116B), VA San Diego Healthcare System, 3350 La Jolla Village Drive

Functional perfusion imaging using pseudocontinuous arterial spin labeling with low‐flip‐angle segmented 3D spiral readouts

Arterial spin labeling (ASL) provides quantitative and reproducible measurements of regional cerebral blood flow, and is therefore an attractive method for functional MRI. However, most existing ASL functional MRI protocols are based on either two‐dimensional (2D) multislice or 3D spin‐echo and suffer from very low image signal‐to‐noise ratio or through‐plane blurring. 3D ASL with multishot (segmented) readouts can improve the signal‐to‐noise ratio efficiency relative to 2D multislice and does not suffer from T 2 ‐blurring. However, segmented readouts require lower imaging flip‐angles and may increase the susceptibility to temporal signal fluctuations (e.g., due to physiology) relative to 2D multislice. In this article, we characterize the temporal signal‐to‐noise ratio of a segmented 3D spiral ASL sequence, and investigate the effects of radiofrequency phase cycling scheme and flip‐angle schedule on image properties. We show that radiofrequency‐spoiling is essential in segmented 3D spiral ASL, and that 3D ASL can improve temporal signal‐to‐noise ratio 2‐fold relative to 2D multislice when using a simple polynomial (cubic) flip‐angle schedule. Functional MRI results using the proposed optimized segmented 3D spiral ASL protocol show excellent activation in the visual cortex. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96313/1/24261_ftp.pd

Complex-valued analysis of arterial spin labeling–based functional magnetic resonance imaging signals

Cerebral blood flow-dependent phase differences between tagged and control arterial spin labeling images are reported. A biophysical model is presented to explain the vascular origin of this difference. Arterial spin labeling data indicated that the phase difference is largest when the arterial component of the signals is preserved but is greatly reduced as the arterial contribution is suppressed by postinversion delays or flow-crushing gradients. Arterial vasculature imaging by saturation data of activation and hypercapnia conditions showed increases in phase accompanying blood flow increases. An arterial spin labeling functional magnetic resonance imaging study yielded significant activation by magnitude-only, phase-only, and complex analyses when preserving the whole arterial spin labeling signal. After suppression of the arterial signal by postinversion delays, magnitude-only and complex models yielded similar activation levels, but the phase-only model detected nearly no activation. When flow crushers were used for arterial suppression, magnitude-only activation was slightly lower and fluctuations in phase were dramatically higher than when postinversion delays were used. Although the complex analysis performed did not improve detection, a simulation study indicated that the complex-valued activation model exhibits combined magnitude and phase detection power and thus maximizes sensitivity under ideal conditions. This suggests that, as arterial spin labeling imaging and image correction methods develop, the complex-valued detection model may become helpful in signal detection. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/64527/1/22106_ftp.pd

Arterial input function and gray matter cerebral blood volume measurements in children

Purpose To investigate how arterial input functions (AIFs) vary with age in children and compare the use of individual and population AIFs for calculating gray matter CBV values. Quantitative measures of cerebral blood volume (CBV) using dynamic susceptibility contrast (DSC) magnetic resonance imaging (MRI) require measurement of an AIF. AIFs are affected by numerous factors including patient age. Few data presenting AIFs in the pediatric population exists. Materials and Methods Twenty‐two previously treated pediatric brain tumor patients (mean age, 6.3 years; range, 2.0–15.3 years) underwent DSC‐MRI scans on a 3T MRI scanner over 36 visits. AIFs were measured in the middle cerebral artery. A functional form of an adult population AIF was fitted to each AIF to obtain parameters reflecting AIF shape. The relationship between parameters and age was assessed. Correlations between gray matter CBV values calculated using the resulting population and individual patient AIFs were explored. Results There was a large variation in individual patient AIFs but correlations between AIF shape and age were observed. The center (r = 0.596, P < 0.001) and width of the first‐pass peak (r = 0.441, P = 0.007) were found to correlate significantly with age. Intrapatient coefficients of variation were significantly lower than interpatient values for all parameters (P < 0.001). Differences in CBV values calculated with an overall population and age‐specific population AIF compared to those calculated with individual AIFs were 31.3% and 31.0%, respectively. Conclusion Parameters describing AIF shape correlate with patient age in line with expected changes in cardiac output. In pediatric DSC‐MRI studies individual patient AIFs are recommended

Post-stimulus fMRI and EEG responses: evidence for a neuronal origin hypothesised to be inhibitory

Post-stimulus undershoots, negative responses following cessation of stimulation, are widely observed in functional magnetic resonance (fMRI) blood oxygenation level dependent (BOLD) data. However, the debate surrounding whether the origin of this response phase is neuronal or vascular, and whether it provides functionally relevant information, that is additional to what is contained in primary response, means that undershoots are widely overlooked. We simultaneously recorded electroencephalography (EEG), BOLD and cerebral blood-flow (CBF) [obtained from arterial spin labelled (ASL) fMRI] fMRI responses to hemifield checkerboard stimulation to test the potential neural origin of the fMRI post-stimulus undershoot. The post-stimulus BOLD and CBF signal amplitudes in both contralateral and ipsilateral visual cortex depended on the post-stimulus power of the 8-13 Hz (alpha) EEG neuronal activity, such that trials with highest EEG power showed largest fMRI undershoots in contralateral visual cortex. This correlation in post-stimulus EEG-fMRI responses was not predicted by the primary response amplitude. In the contralateral visual cortex we observed a decrease in both cerebral rate of oxygen metabolism (CMRO2) and CBF during the post-stimulus phase. In addition, the coupling ratio (n) between CMRO2 and CBF was significantly lower during the positive contralateral primary response phase compared with the post-stimulus phase and we propose that this reflects an altered balance of excitatory and inhibitory neuronal activity. Together our data provide strong evidence that the post-stimulus phase of the BOLD response has a neural origin which reflects, at least partially, an uncoupling of the neuronal responses driving the primary and post-stimulus responses, explaining the uncoupling of the signals measured in the two response phases. We suggest our results are consistent with inhibitory processes driving the post-stimulus EEG and fMRI responses. We therefore propose that new methods are required to model the post-stimulus and primary responses independently, enabling separate investigation of response phases in cognitive function and neurological disease

7. Réseaux, pots-de-vin et dons à des partis

Lorsque l’on pense aux scandales de corruption en République fédérale d’Allemagne, ce sont probablement les années 1980 et 1990 qui viennent d’abord à l’esprit : l’affaire Flick, le scandale autour de la société de logements « Neue Heimat », le scandale des dons à la CDU (Christlich Demokratische Union Deutschlands) et de nombreux autres cas de corruption ont fait la une des journaux durant cette période. En revanche, les premières années de la RFA sont habituellement considérées comme une pé..