Xavier Golay

Determining the longitudinal relaxation time (T1) of blood at 3.0 Tesla.

It is important to determine the longitudinal relaxation time of blood for black blood imaging, as well as for quantifying blood flow by arterial spin labeling (ASL). In this study a circulation system was used to measure blood T1 under physiological conditions at the new clinical field strength of 3.0T. It was found that 1/T1 in s−1 was linearly dependent (P < 0.05) on hematocrit (Hct) within a normal range of 0.38–0.46. The relationships were 1/T1 = (0.52 ± 0.15) · Hct + (0.38 ± 0.06) and 1/T1 = (0.83 ± 0.07) · Hct + (0.28 ± 0.03) for arterial (oxygenation = 92% ± 7%) and venous blood (69% ± 8%), respectively, which led to estimated T1 values of 1664 ± 14 ms (arterial) and 1584 ± 5 ms (venous) at a typical human Hct of 0.42. The temperature dependencies of blood T1 were 22.3 ± 0.6 ms/°C and 19.8 ± 0.8 ms/°C for Hct values of 0.42 and 0.38, respectively. When a head coil transmit/receive setup was used, radiation damping caused a slight reduction (19 ms) of the measured T1 values. Magn Reson Med 52:679–682, 2004.

Functional magnetic resonance imaging based on changes in vascular space occupancy.

During brain activation, local control of oxygen delivery is facilitated through microvascular dilatation and constriction. A new functional MRI (fMRI) methodology is reported that is sensitive to these microvascular adjustments. This contrast is accomplished by eliminating the blood signal in a manner that is independent of blood oxygenation and flow. As a consequence, changes in cerebral blood volume (CBV) can be assessed through changes in the remaining extravascular water signal (i.e., that of parenchymal tissue) without need for exogenous contrast agents or any other invasive procedures. The feasibility of this vascular space occupancy (VASO)‐dependent functional MRI (fMRI) approach is demonstrated for visual stimulation, breath‐hold (hypercapnia), and hyperventilation (hypocapnia). During visual stimulation and breath‐hold, the VASO signal shows an inverse correlation with the stimulus paradigm, consistent with local vasodilatation. This effect is reversed during hyperventilation. Comparison of the hemodynamic responses of VASO‐fMRI, cerebral blood flow (CBF)‐based fMRI, and blood oxygenation level‐dependent (BOLD) fMRI indicates both arteriolar and venular temporal characteristics in VASO. The effect of changes in water exchange rate and partial volume contamination with CSF were calculated to be negligible. At the commonly‐used fMRI resolution of 3.75 × 3.75 × 5 mm3, the contrast‐to‐noise‐ratio (CNR) of VASO‐fMRI was comparable to that of CBF‐based fMRI, but a factor of 3 lower than for BOLD‐fMRI. Arguments supporting a better gray matter localization for the VASO‐fMRI approach compared to BOLD are provided. Magn Reson Med 50:263–274, 2003.

Amide proton transfer imaging of human brain tumors at 3T

Amide proton transfer (APT) imaging is a technique in which the nuclear magnetization of water‐exchangeable amide protons of endogenous mobile proteins and peptides in tissue is saturated, resulting in a signal intensity decrease of the free water. In this work, the first human APT data were acquired from 10 patients with brain tumors on a 3T whole‐body clinical scanner and compared with T1‐ (T1w) and T2‐weighted (T2w), fluid‐attenuated inversion recovery (FLAIR), and diffusion images (fractional anisotropy (FA) and apparent diffusion coefficient (ADC)). The APT‐weighted images provided good contrast between tumor and edema. The effect of APT was enhanced by an approximate 4% change in the water signal intensity in tumor regions compared to edema and normal‐appearing white matter (NAWM). These preliminary data from patients with brain tumors show that the APT is a unique contrast that can provide complementary information to standard clinical MRI measures. Magn Reson Med, 2006.

SENSE-DTI at 3 T

While holding vast potential, diffusion tensor imaging (DTI) with single‐excitation protocols still faces serious challenges. Limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low signal‐to‐noise ratio (SNR) may be considered the most prominent limitations. It is demonstrated that all of these shortcomings can be effectively mitigated by the transition to parallel imaging technology and high magnetic field strength. Using the sensitivity encoding (SENSE) technique at 3 T, brain DTI was performed in nine healthy volunteers. Despite enhanced field inhomogeneity, parallel acquisition permitted both controlling geometric distortions and enhancing spatial resolution up to 0.8 mm in‐plane. Heightened SNR requirements were met in part by high base sensitivity at 3 T. A further significant increase in SNR efficiency was accomplished by SENSE acquisition, exploiting enhanced encoding speed for echo time reduction. Based on the resulting image data, high‐resolution tensor mapping is demonstrated. Magn Reson Med 51:230–236, 2004.

Routine clinical brain MRI sequences for use at 3.0 Tesla.

To establish image parameters for some routine clinical brain MRI pulse sequences at 3.0 T with the goal of maintaining, as much as possible, the well‐characterized 1.5‐T image contrast characteristics for daily clinical diagnosis, while benefiting from the increased signal to noise at higher field.

Comparison of the dependence of blood R2 and R2* on oxygen saturation at 1.5 and 4.7 Tesla.

Gradient‐echo (GRE) blood oxygen level‐dependent (BOLD) effects have both intra‐ and extravascular contributions. To better understand the intravascular contribution in quantitative terms, the spin‐echo (SE) and GRE transverse relaxation rates, R2 and R u20092* , of isolated blood were measured as a function of oxygenation in a perfusion system. Over the normal oxygenation saturation range of blood between veins, capillaries, and arteries, the difference between these rates, R′2 = R u20092* − R2, ranged from 1.5 to 2.1 Hz at 1.5 T and from 26 to 36 Hz at 4.7 T. The blood data were used to calculate the expected intravascular BOLD effects for physiological oxygenation changes that are typical during visual activation. This modeling showed that intravascular ΔR u20092* is caused mainly by R2 relaxation changes, namely 85% and 78% at 1.5T and 4.7T, respectively. The simulations also show that at longer TEs (>70 ms), the intravascular contribution to the percentual BOLD change is smaller at high field than at low field, especially for GRE experiments. At shorter TE values, the opposite is the case. For pure parenchyma, the intravascular BOLD signal changes originate predominantly from venules for all TEs at low field and for short TEs at high field. At longer TEs at high field, the capillary contribution dominates. The possible influence of partial volume contributions with large vessels was also simulated, showing large (two‐ to threefold) increases in the total intravascular BOLD effect for both GRE and SE. Magn Reson Med 49:47–60, 2003.

Sustained Poststimulus Elevation in Cerebral Oxygen Utilization after Vascular Recovery

The brains response to functional activation is characterized by focal increases in cerebral blood flow. It is generally assumed that this hyperemia is a direct response to the energy demands of activation, the so-called flow-metabolism coupling. Here we report experimental evidence that increases in oxygen metabolism can occur after activation without increases in flow. When using multimodality functional MRI (fMRI) to study visual activation in human brain, we observed a postactivation period of about 30 seconds during which oxygen consumption remained elevated, while blood flow and volume had already returned to baseline levels. The finding of such a prolonged and complete dissociation of vascular response and energy metabolism during the poststimulus period indicates that increased metabolic demand needs not per se cause a concomitant increase in blood flow. The results also show that the postactivation undershoot after the positive blood-oxygen-level-dependent hemodynamic response in fMRI should be reinterpreted as a continued elevation of oxygen metabolism, rather than a delayed blood volume compliance.

Measurement of tissue oxygen extraction ratios from venous blood T2: Increased precision and validation of principle

It has recently been shown that parenchymal oxygen extraction ratios (OERs) can be quantified using the absolute T2 of venous blood draining from this tissue (Oja et al., J Cereb Blood Flow Metab 1999;19:1289–1295). Here, a modified Carr‐Purcell‐Meiboom‐Gill (CPMG) multiecho experiment was used to increase the efficiency and precision of this approach and to test the applicability of the two‐compartment exchange model for spin‐echo BOLD effects in pure venous blood. Relaxation measurements on bovine blood as a function of CPMG interecho spacing, oxygen saturation, and hematocrit provided the baseline relaxation and susceptibility shift parameters necessary to directly relate OER to T2 of venous blood in vivo. Using an interecho spacing of 25 ms, the results on visual activation studies in eight volunteers showed T2(CPMG) values increasing from 128 ± 9 ms to 174 ± 18 ms upon activation, corresponding to local OER values of 0.38 ± 0.04 and 0.18 ± 0.05 during baseline activity and visual stimulation, respectively. These OER values are in good agreement with literature data on venous oxygenation and numbers determined previously using a single‐echo approach, while the measured T2s are about 20–40 ms longer. Magn Reson Med 46:282–291, 2001.

Measurements of cerebral perfusion and arterial hemodynamics during visual stimulation using TURBO-TILT

Estimation of cerebral blood flow (CBF) in functional perfusion imaging could benefit from a method capable of separating effects of arterial arrival time and trailing edge. To accomplish this, the transfer insensitive labeling technique (TILT) was combined with a train of 13 consecutive acquisitions, called TURBO‐TILT. Visual activation maps obtained at 13 postlabeling delay times (TI) showed a spatial shift from regions surrounding the arterial vasculature at short TI to brain parenchyma at longer delay times. High baseline CBF and short arrival times were found for the voxels with maximum activation at short TI (<1200 ms), while CBF values (43 ml / 100 g tissue/min) and its increase upon activation (55%) at longer TI were in agreement with literature data on regional cerebral perfusion. Magn Reson Med 50:429–433, 2003.

fMRI Evidence for Multisensory Recruitment Associated With Rapid Eye Movements During Sleep

We studied the neural correlates of rapid eye movement during sleep (REM) by timing REMs from video recording and using rapid event‐related functional MRI. Consistent with the hypothesis that REMs share the brain systems and mechanisms with waking eye movements and are visually‐targeted saccades, we found REM‐locked activation in the primary visual cortex, thalamic reticular nucleus (TRN), ‘visual claustrum’, retrosplenial cortex (RSC, only on the right hemisphere), fusiform gyrus, anterior cingulate cortex, and the oculomotor circuit that controls awake saccadic eye movements (and subserves awake visuospatial attention). Unexpectedly, robust activation also occurred in non‐visual sensory cortices, motor cortex, language areas, and the ascending reticular activating system, including basal forebrain, the major source of cholinergic input to the entire cortex. REM‐associated activation of these areas, especially non‐visual primary sensory cortices, TRN and claustrum, parallels findings from waking studies on the interactions between multiple sensory data, and their ‘binding’ into a unified percept, suggesting that these mechanisms are also shared in waking and dreaming and that the sharing goes beyond the expected visual scanning mechanisms. Surprisingly, REMs were associated with a decrease in signal in specific periventricular subregions, matching the distribution of the serotonergic supraependymal plexus. REMs might serve as a useful task‐free probe into major brain systems for functional brain imaging. Hum Brain Mapp 2009.