J.H. Duyn

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.

Reproducibility of Proton Magnetic Resonance Spectroscopic Imaging in Patients with Schizophrenia

Using proton magnetic resonance spectroscopic imaging (1H-MRSI) we found in a previous study a specific pattern of neuronal pathology in patients with schizophrenia as determined by relative loss of signal from N-acetyl-containing compounds (NAA). The purpose of the present study was to assess the reproducibility of the results of 1H-MRSI both in patients with schizophrenia and in normal controls. We studied twice 10 patients and 10 controls on 2 days separated by, on average, 3 months. Reproducibility was assessed with several statistical procedures including ANOVA, coefficients of variation (CVs) and intra-class correlation coefficients (ICC). Patients showed significant reductions of NAA/creatine-phosphocreatine (CRE) and NAA/choline-containing compounds (CHO) selectively in the hippocampal region (HIPPO) and in the dorsolateral prefrontal cortex (DLPFC) on both experimental days. A repeated measures ANOVA showed no effect of time on metabolite ratios in all subjects. CVs were fairly low (especially for NAA/CRE and CHO/CRE) and did not differ significantly between patients and controls. The ICCs of the ROIs reached statistical significance only in a few instances. The present multislice 1H-MRSI study shows that: (1) patients with schizophrenia, when compared as a group to normal controls, show a consistent 1H-MRSI pattern of group differences, i.e., bilateral reductions of NAA/CRE and NAA/CHO in HIPPO and DLPFC; (2) 1H-MRSI data in both patients and controls do not show significant changes over this 90-day period; however, absolute metabolite ratios in individuals show low predictability over this time interval; (3) 1H-MRSI data show relatively low variability (as measured by the CVs) both in patients and normal controls, especially for NAA/CRE and CHO/CRE.

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.

Study of brain anatomy with high-field MRI: recent progress

Recent developments in high-field magnetic resonance imaging technology have led to improved contrast and resolution and are opening up new possibilities for the study of human brain anatomy. In particular, techniques sensitized to magnetic susceptibility contrast provide particular advantages at high field that have allowed visualization of brain structures that have been difficult to detect with conventional technology. In this review, some of these developments and techniques will be discussed, and an attempt will be made to interpret magnetic susceptibility contrast based on recent studies.

Method for functional MRI mapping of nonlinear response

Nonlinear systems analysis combining blood oxygen level dependent (BOLD), functional magnetic resonance imaging (fMRI) and m-sequence stimulation paradigms are proposed as a new method for exploring neuronal responses and interactions. Previous studies of electrical activity in the human visual cortex have observed significant nonlinearities of task-induced activity with temporal dynamics on a timescale of 10-20 ms. Despite the confounding effect of the seconds-long hemodynamic response, it is demonstrated that BOLD fMRI can be used to probe neuronal interactions on a time scale of tens of ms. Visual activation experiments were performed with various stimuli, and amplitude maps of first and second order kernel coefficients were generated using correlation analysis. Second order nonlinearities in BOLD fMRI were observed and attributed to temporal contrast caused by transitions in the stimulus sequence. In addition, the kernel maps showed significant differences between second order nonlinearities of foveal and peripheral vision. By including a reference experiment with a slightly modified stimulus presentation, a distinction could be made between (fast) neuronal nonlinearities and hemodynamic effects on the time scale of the seconds. The results indicate that BOLD fMRI can probe fast neuronal nonlinearities.

The PRESTO technique for fMRI.

In the early days of BOLD fMRI, the acquisition of T(2)(*) weighted data was greatly facilitated by rapid scan techniques such as EPI. The latter, however, was only available on a few MRI systems that were equipped with specialized hardware that allowed rapid switching of the imaging gradients. For this reason, soon after the invention of fMRI, the scan technique PRESTO was developed to make rapid T(2)(*) weighted scanning available on standard clinical scanners. This method combined echo shifting, which allows for echo times longer than the sequence repetition time, with acquisition of multiple k-space lines per excitation. These two concepts were combined in order to achieve a method fast enough for fMRI, while maintaining a sufficiently long echo time for optimal contrast. PRESTO has been primarily used for 3D scanning, which minimized the contribution of large vessels due to inflow effects. Although PRESTO is still being used today, its appeal has lessened somewhat due to increased gradient performance of modern MRI scanners. Compared to 2D EPI, PRESTO may have somewhat reduced temporal stability, which is a disadvantage for fMRI that may not outweigh the advantage of reduced inflow effects provided by 3D scanning. In this overview, the history of the development of the PRESTO is presented, followed by a qualitative comparison with EPI.

Tissue-specific imaging is a robust methodology to differentiate in vivo T1 black holes with advanced multiple sclerosis-induced damage.

BACKGROUND AND PURPOSE: Brains of patients with multiple sclerosis (MS) characteristically have “black holes” (BHs), hypointense lesions on T1-weighted (T1W) spin-echo (SE) images. Although conventional MR imaging can disclose chronic BHs (CBHs), it cannot stage the degree of their pathologic condition. Tissue-specific imaging (TSI), a recently introduced MR imaging technique, allows selective visualization of white matter (WM), gray matter (GM), and CSF on the basis of T1 values of classes of tissue. We investigated the ability of TSI-CSF to separate CBHs with longer T1 values, which likely represent lesions containing higher levels of destruction and unbound water. MATERIALS AND METHODS: Eighteen patients with MS, who had already undergone MR imaging twice (24 months apart) on a 1.5T scanner, underwent a 3T MR imaging examination. Images acquired at 1.5T included sequences of precontrast and postcontrast T1W SE, T2-weighted (T2W) SE, and magnetization transfer (MT). Sequences obtained at 3T included precontrast and postcontrast T1W SE, T2W SE, T1 inversion recovery prepared fast spoiled gradient recalled-echo (IR-FSPGR) and TSI. A BH on the 3T-IR-FSPGR was defined as a CBH if seen as a hypointense, nonenhancing lesion with a corresponding T2 abnormality for at least 24 months. CBHs were separated into 2 groups: those visible as hyperintensities on TSI-CSF (group A), and those not appearing on the TSI-CSF (group B). RESULTS: Mean MT ratios of group-A lesions (0.22 ± 0.06, 0.13–0.35) were lower (F1,13 = 60.39; P < .0001) than those of group-B lesions (0.32 ± 0.03, 0.27–0.36). CONCLUSIONS: Group-A lesions had more advanced tissue damage; thus, TSI is a potentially valuable method for qualitative and objective identification.

Simultaneous BOLD/perfusion measurement using dual-echo FAIR and UNFAIR: sequence comparison at 1.5T and 3.0T

Functional MRI (fMRI) studies designed for simultaneously measuring Blood Oxygenation Level Dependent (BOLD) and Cerebral Blood Flow (CBF) signal often employ the standard Flow Alternating Inversion Recovery (FAIR) technique. However, some sensitivity is lost in the BOLD data due to inherent T1 relaxation. We sought to minimize the preceding problem by employing a modified UN-inverted FAIR (UNFAIR) technique, which (in theory) should provide identical CBF signal as FAIR with minimal degradation of the BOLD signal. UNFAIR BOLD maps acquired from human subjects (n = 8) showed significantly higher mean z-score of approximately 17% (p < 0.001), and number of activated voxels at 1.5T. On the other hand, the corresponding FAIR perfusion maps were superior to the UNFAIR perfusion maps as reflected in a higher mean z-score of approximately 8% (p = 0.013), and number of activated voxels. The reduction in UNFAIR sensitivity for perfusion is attributed to increased motion sensitivity related to its higher background signal, and, T2 related losses from the use of an extra inversion pulse. Data acquired at 3.0T demonstrating similar trends are also presented.

PRESTO, a Rapid 3D Approach for Functional MRI of Human Brain

Ogawa et al. [1, 2] proposed in 1990 that physiological information related to neuronal activity can be incorporated in functional magnetic resonance imaging (fMRI) based on changes in the concentration of deoxyhemoglobin in blood (blood oxygenation level dependent, or BOLD effect). As compared to positron emission tomography (PET) and single photon emission computed tomography (SPECT), BOLD fMRI offers substantial advantages: minimal discomfort, no exposure to ionizing radiation and excellent spatial and temporal resolution. Several studies have now demonstrated that sensory and language functions can be mapped with fMRI.