Ivan Zimine

Variability of fMRI Activation During a Phonological and Semantic Language Task in Healthy Subjects

Assessing inter‐individual variability of functional activations is of practical importance in the use of functional magnetic resonance imaging (fMRI) in a clinical context. In this fMRI study we addressed this issue in 30 right‐handed, healthy subjects using rhyme detection (phonologic) and semantic categorization tasks. Significant activations, found mainly in the left hemisphere, concerned the inferior frontal gyrus, the superior/middle temporal gyri, the prefrontal cortex, the inferior parietal lobe, the superior parietal lobule/superior occipital gyrus, the pre‐central gyrus, and the supplementary motor area. Intensity/spatial analysis comparing activations in both tasks revealed an increased involvement of frontal regions in the semantic task and of temporo‐parietal regions in the phonologic task. The frequency of activation analyzed in nine regional subdivisions revealed a high inter‐subject variability but showed that the most frequently activated regions were the inferior frontal gyrus and the prefrontal cortex. Laterality indices, strongly lateralizing in both tasks, were slightly higher in the semantic (0.76 ± 0.19) than the phonologic task (0.66 ± 0.27). Frontal dominance indices (a measure of frontal vs. posterior left hemisphere dominance) indicated more robust frontal activations in the semantic than the phonologic task. Our study allowed the characterization of the most frequently involved foci in two language tasks and showed that the combination of these tasks constitutes a suitable tool for determining language lateralization and for mapping major language areas. Hum. Brain Mapping 23:140–155, 2004.

EEG-Triggered Functional MRI in Patients With Pharmacoresistant Epilepsy

Functional magnetic resonance imaging (fMRI) triggered by scalp electroencephalography (EEG) recordings has become a promising new tool for noninvasive epileptic focus localization. Studies to date have shown that it can be used safely and that highly localized information can be obtained. So far, no reports using comprehensive clinical information and/or long‐term follow‐up after epilepsy surgery in a larger patient group have been given that would allow a valuable judgment of the utility of this technique. Here, the results of 11 patients with EEG‐triggered fMRI exams who also underwent presurgical evaluation of their epilepsy are given. In most patients we were able to record good quality EEG inside the magnet, allowing us to trigger fMRI acquisition by interictal discharges. The fMRI consisted of echoplanar multislice acquisition permitting a large anatomical coverage of the patients brain. In 8 of the 11 patients the exam confirmed clinical diagnosis, either by the presence (n = 7) or absence (n = 1) of focal signal enhancement. In six patients, intracranial recordings were carried out, and in five of them, the epileptogenic zone as determined by fMRI was confirmed. Limitations were encountered a) when the focus was too close to air cavities; b) if an active epileptogenic focus was absent; and c) if only reduced cooperation with respect to body movements was provided by the patient. We conclude that EEG‐triggered fMRI is a safe and powerful noninvasive tool that improves the diagnostic value of MRI by localizing the epileptic focus precisely. J. Magn. Reson. Imaging 2000;12:177–185.

Territorial Arterial Spin Labeling in the Assessment of Collateral Circulation Comparison With Digital Subtraction Angiography

Background and Purpose— Collateral circulation plays a vital role in patients with steno-occlusive disease, in particular for predicting stroke outcome. Digital subtraction angiography (DSA) is the gold standard for the assessment of collateral circulation, despite its invasive nature. Recently, the development of a new class of arterial spin labeling (ASL) methods allowed independent measurement of territorial flow information without the need for contrast media injection. Here, we compared combined territorial ASL (TASL) and MR angiography (MRA) against DSA in the assessment of collateral circulation. Methods— Eighteen patients presenting with extra- or intracranial arterial steno-occlusive disease were recruited. All DSA studies were performed using a biplane angiography unit. MR imaging consisted of time-of-flight MRA and TASL, performed at 3T. Collateral circulation on both modalities was evaluated in consensus in a double-blinded manner by 3 neuroradiologists. Results— Good agreement was found between DSA and TASL in the assessment of collateral flow: Cramer coefficient, V=0.53 (P<0.0001) and Contingency coefficient, C=0.67, with kappa=0.70 and kappa=0.72 in the assessment of flow and collaterals, respectively. TASL and DSA successfully evaluated 89% and 98% of the vessels, respectfully. Failure was linked to motion-related artifacts in TASL, and highly tortuous vessels in DSA. Generally, combined MRA–TASL was comparable to DSA in diagnostic quality. Conclusions— TASL provided radiological information comparable to DSA on collateral flow, with the advantage that it could be performed during routine MRI studies. TASL may provide insight on collateral perfusion in patients who may not otherwise be candidates for DSA, and may potentially replace it.

Inflow effect correction in fast gradient-echo perfusion imaging.

The purposes of this study were to assess the extent of the inflow effect on signal intensity (SI) for fast gradient‐recalled‐echo (GRE) sequences used to observe first‐pass perfusion, and to develop and validate a correction method for this effect. A phantom experiment with a flow apparatus was performed to determine SI as a function of Gd‐DTPA concentration for various velocities. Subsequently a flow‐sensitive calibration method was developed, and validated on bolus injections into an open‐circuit flow apparatus and in vivo. It is shown that calibration methods based on static phantoms are not appropriate for accurate signal‐to‐concentration conversion in images affected by high flow. The flow‐corrected calibration method presented here can be used to improve the accuracy and robustness of the arterial input function (AIF) determination for tissue perfusion quantification using MRI and contrast media. Magn Reson Med 50:885–891, 2003.

Dual vessel arterial spin labeling scheme for regional perfusion imaging

Regional perfusion imaging (RPI) based on pulsed arterial spin labeling and angulated inversion slabs has been recently proposed. The technique allows mapping of individual brain perfusion territories of the major feeding arteries and could become a valuable clinical tool for evaluation of patients with cerebrovascular diseases. Here we propose a new labeling scheme for RPI where lateral and posterior circulations are labeled simultaneously. Two scans instead of three are sufficient to obtain the same perfusion territories as in the original approach, allowing for a 33% reduction in the total RPI protocol time. Moreover, the position of the inversion slabs with respect to vascular anatomy facilitates the planning and allows potentially better labeling efficiency. The new approach was tested on seven healthy volunteers and compared to the original labeling scheme. The results showed that the same perfusion territories and regional CBF values can be obtained. Magn Reson Med, 2006.

FAST sequences optimization for contrast media pharmacokinetic quantification in tissue

The purpose of this study was to investigate the influence of the fast gradient‐recalled echo (GRE) sequence parameters on the contrast dynamic range and signal sensitivity, to optimize the magnetic resonance (MR) sequence for contrast media pharmacokinetic assessment. Effects of the fast low‐angle shot (FLASH), Fast acquisition at steady rate (FAST), and radiofrequency‐spoiled (RF)‐FAST sequence parameters were studied in vitro. The FAST sequence had the highest sensitivity in low gadolinium (Gd) concentration. The FLASH and RF‐FAST sequences had a larger contrast dynamic range, but the FLASH images contained side band artifacts. Increasing the flip angle to 90° raised the sensitivity of the FAST sequence and the contrast dynamic range of the RF‐FAST sequence. The shortest possible TE was optimal for both contrast dynamics and imaging time. TI had an influence on the sensitivity of the FAST sequence only for small acquisition matrices. This study indicates the optimal parameters for contrast dynamics (RF‐FAST, 90° flip angle, shortest possible TE) and sensitivity (FAST, 90° flip angle, long TIeff). J. Magn. Reson. Imaging 2001;14:771–778.

Inflow effect in first‐pass cardiac and renal MRI

To estimate the effect of the inflow effect on the arterial input function in vivo in cardiac and renal MR perfusion imaging using fast gradient echo (GRE) sequences and contrast media.

Arterial Spin Labeling: a One-stop-shop for Measurement of Brain Perfusion in the Clinical Settings

Arterial spin labeling (ASL) has opened a unique window into the human brain function and perfusion physiology. Altogether fast and of intrinsic high spatial resolution, ASL is a technique very appealing not only for the diagnosis of vascular diseases, but also in basic neuroscience for the follow-up of small perfusion changes occurring during brain activation. However, due to limited signal-to-noise ratio and complex flow kinetics, ASL is one of the more challenging disciplines within magnetic resonance imaging. In this paper, the theoretical background and main implementations of ASL are revisited. In particular, the different uses of ASL, the pitfalls and possibilities are described and illustrated using clinical cases.

Brain activation using triggered event-related fMRI

EEG-triggered fMRI provides a method for localizing the sources of brain electrical activity, such as epileptic discharges. Extending single-image acquisitions, following an event on the EEG, into triggered image series acquisitions may allow BOLD time courses to be obtained, such as those observed in event-related (ER) fMRI experiments. However, in contrast to the standard ER-fMRI, triggered image series are greatly affected by magnetization non-steady-state effects. The purpose of this paper is to show that the BOLD responses can be recovered using subtraction between two triggered image series having different functional contrasts. In order to evaluate this technique, a comparison with standard ER-fMRI using motor cortex activation task was made in 5 volunteers. We conclude that this can be a useful technique for studying brain activation associated with irregularly appearing stimuli.