J.A. de Zwart

Real-time shimming to compensate for respiration-induced B0 fluctuations.

In MRI of human brain, the respiratory cycle can induce B0‐field fluctuations through motion of the chest and fluctuations in local oxygen concentration. The associated NMR frequency changes can affect the MRI data in various ways and lead to temporal signal fluctuations, and image artifacts such as ghosting and blurring. Since the size of the effect scales with magnetic field strength, artifacts become particularly problematic at fields above 3.0T. Furthermore, the spatial dependence of the B0‐field fluctuations complicates their correction. In this work, a new method is presented that allows compensation of field fluctuations by modulating the B0 shims in real time. In this method, a reference scan is acquired to measure the spatial distribution of the B0 effect related to chest motion. During the actual scan, this information is then used, together with chest motion data, to apply compensating B0 shims in real time. The method can be combined with any type of scan without modifications to the pulse sequence. Real‐time B0 shimming is demonstrated to substantially improve the phase stability of EPI data and the image quality of multishot gradient‐echo (GRE) MRI at 7T. Magn Reson Med 57:362–368, 2007.

Pittfalls of MRI measurement of white matter perfusion based on arterial spin labeling

Although arterial spin labeling (ASL) MRI has been successfully applied to measure gray matter (GM) perfusion in vivo, accurate detection of white matter (WM) perfusion has proven difficult. Reported literature values are not consistent with each other or with perfusion measured with other modalities. In this work, the cause of these inconsistencies is investigated. The results suggest that WM perfusion values are substantially affected by the limited image resolution and by signal losses caused by the long transit times in WM, which significantly affect the label. From gadolinium diethylenetriamine pentaacetic acid (Gd‐DTPA) bolus‐tracking experiments (N = 6), it is estimated that the transit time can be several seconds long in deep WM. Furthermore, simulations show that even at a spatial resolution of 7 μl voxel size, contamination by the GM signals can exceed 40% of the actual WM signal. From 10‐min long flow‐sensitive alternating inversion recovery ASL (FAIR‐ASL) measurements at 3T in normal subjects (N = 7), using highly sensitive detectors, it is shown that single‐voxel (7 μl) deep WM perfusion values have an signal‐to‐noise ratio (SNR) less than 1. The poor sensitivity and heterogeneous transit time limit the applicability of ASL for measurement of perfusion in WM. Magn Reson Med 59:788–795, 2008.

fMRI study of effort and information processing in a working memory task.

It is unclear how effort translates into brain function. In this study we endeavored to identify the activity in a working memory task that is related to the allocation of mental resources. Such activity, if present, would be a likely candidate to explain how effort works in terms of brain function. Eleven healthy participants performed a Sternberg task with a memory‐set of one, three, or five consonants in an fMRI study. Probe stimuli were either one consonant or one digit. We expected digits to be processed automatically and consonants to require working memory. Because the probe type was unpredictable and subjects had to respond as fast as possible, we expected subjects to allocate mental resources on the basis of the memory‐set size, not the probe type. Accordingly, we anticipated that activity in regions involved in effort would be a function of the size of the memory‐set, but independent of the type of probe. We found that the reaction‐time for digits increased in line with our expectation of automatic processing and the reaction time for letters increased in line with our expectation of controlled processing. fMRI revealed that activity in the right ventral‐prefrontal cortex changed as a function of effort. The ventral anterior cingulate cortex and hypothalamus showed reduced activity as a function of effort. Activity in regions regarded as pivotal for working memory (among others, the left dorsolateral prefrontal cortex, anterior cingulate cortex) appeared to be predominantly related to information processing and not involved in effort. Hum Brain Mapp, 2007.

Optimization of a high sensitivity MRI receive coil for parallel human brain imaging

Two eight-channel MRI receive-only coils were developed to provide whole-brain coverage at 1.5 T and 3.0 T field strength, respectively. Objectives were an image signal-to-noise ratio superior to standard designs throughout the human brain, as well as high parallel imaging performance. Electromagnetic field simulations were used to determine array diameter and inter-element coil gap. Low mutual inductive coupling was achieved at 1.5 and 3.0 T using high-impedance pre-amplifiers. Coils show an average SNR improvement over commercial birdcage coils of 2.4 and 2.3 for the 1.5 T and 3.0 T design, respectively. The mean of the noise-amplification factor related to reconstruction of under-sampled data (g-factor) was 1.03 for 2-fold under-sampled data (rate-2) and 1.22 for rate-3 at 1.5 T. For data acquired with the 3.0 T coil array, these values were respectively 1.06 for rate-2 and 1.37 for rate-3.

Effects of spatial fMRI resolution on the classification of naturalistic movies

&NA; Studies involving multivariate pattern analysis (MVPA) of BOLD fMRI data generally attribute the success of the information‐theoretic approach to BOLD signal contrast on the fine spatial scale of millimeters facilitating the classification or decoding of perceptual stimuli. However, to date MVPA studies that have actually explored fMRI resolutions at less than 2 mm voxel size are rare and limited to small sets of unnatural stimuli (like visual gratings) as well as specific sub‐regions of the brain, notably the primary somatosensory cortices. To investigate what spatial scale best supports high information extraction under more general conditions this study combined naturalistic movie stimuli with high‐resolution fMRI at 7 T and linear discriminant analysis (LDA) of global and local BOLD signal patterns. Contrary to predictions, LDA and similar classifiers reached a maximum in classification accuracy (CA) at a smoothed resolution close to 3 mm, well above the 1.2 mm voxel size of the fMRI acquisition. Maximal CAs around 90% were contingent upon global fMRI signal patterns comprising 4 k–16 k of the most reactive voxels distributed sparsely throughout the occipital and ventro‐temporal cortices. A Searchlight analysis of local fMRI patterns largely confirmed the global results, but also revealed a small subset of brain regions in early visual cortex showing limited increases in CA with higher resolution. Principal component analysis of the global and local fMRI signal patterns suggested that reproducible neuronal contributions were spatially auto‐correlated and smooth, while other components of higher spatial frequency were likely related to physiological noise and responsible for the reduced CA at higher resolution. Systematic differences between experiments and subjects suggested that higher CA was significantly correlated with more consistent behavior revealed by eye tracking. Thus, the optimal resolution of fMRI data for MVPA was mainly limited by physiological noise of high spatial frequency as well as behavioral (in‐)consistency. HighlightsfMRI spatial resolution affects classification of naturalistic visual stimuli.A relatively low (3 mm) resolution was found to be optimal for global LDA classifiers.Principal components of reproducible BOLD signal are spatially smooth, unlike noise.

Evolution of Instrumentation for Functional Magnetic Resonance Imaging

Since the discovery of functional MRI (fMRI) early in the 1990s, major developments in MRI instrumentation have provided particular benefits for human fMRI. Improvements in system stability and signal detection hardware, as well as an increase in magnetic field strength and rises in the amplitude and switching speed of gradients, have significantly increased fMRI spatial resolution and contrast-to-noise ratio. These technological improvements and their specific impact on fMRI research are reviewed here.