Hanzhang Lu

Diffusional kurtosis imaging: The quantification of non‐gaussian water diffusion by means of magnetic resonance imaging

A magnetic resonance imaging method is presented for quantifying the degree to which water diffusion in biologic tissues is non‐Gaussian. Since tissue structure is responsible for the deviation of water diffusion from the Gaussian behavior typically observed in homogeneous solutions, this method provides a specific measure of tissue structure, such as cellular compartments and membranes. The method is an extension of conventional diffusion‐weighted imaging that requires the use of somewhat higher b values and a modified image postprocessing procedure. In addition to the diffusion coefficient, the method provides an estimate for the excess kurtosis of the diffusion displacement probability distribution, which is a dimensionless metric of the departure from a Gaussian form. From the study of six healthy adult subjects, the excess diffusional kurtosis is found to be significantly higher in white matter than in gray matter, reflecting the structural differences between these two types of cerebral tissues. Diffusional kurtosis imaging is related to q‐space imaging methods, but is less demanding in terms of imaging time, hardware requirements, and postprocessing effort. It may be useful for assessing tissue structure abnormalities associated with a variety of neuropathologies. Magn Reson Med 53:1432–1440, 2005.

Recommended implementation of arterial spin-labeled Perfusion mri for clinical applications: A consensus of the ISMRM Perfusion Study group and the European consortium for ASL in dementia

This review provides a summary statement of recommended implementations of arterial spin labeling (ASL) for clinical applications. It is a consensus of the ISMRM Perfusion Study Group and the European ASL in Dementia consortium, both of whom met to reach this consensus in October 2012 in Amsterdam. Although ASL continues to undergo rapid technical development, we believe that current ASL methods are robust and ready to provide useful clinical information, and that a consensus statement on recommended implementations will help the clinical community to adopt a standardized approach. In this review, we describe the major considerations and trade‐offs in implementing an ASL protocol and provide specific recommendations for a standard approach. Our conclusion is that as an optimal default implementation, we recommend pseudo‐continuous labeling, background suppression, a segmented three‐dimensional readout without vascular crushing gradients, and calculation and presentation of both label/control difference images and cerebral blood flow in absolute units using a simplified model. Magn Reson Med 73:102–116, 2015.

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.

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.

Quantitative evaluation of oxygenation in venous vessels using T2-relaxation-under-spin-tagging MRI

Noninvasive measurement of cerebral venous oxygenation can serve as a tool for better understanding fMRI signals and for clinical evaluation of brain oxygen homeostasis. In this study a novel technique, T2‐Relaxation‐Under‐Spin‐Tagging (TRUST) MRI, is developed to estimate oxygenation in venous vessels. This method uses the spin labeling principle to automatically isolate pure blood signals from which T2 relaxation times are determined using flow‐insensitive T2‐preparation pulses. The blood T2 is then converted to blood oxygenation using a calibration plot. In vivo experiments gave a baseline venous oxygenation of 64.8 ± 6.3% in sagittal sinus in healthy volunteers (n = 24). Reproducibility studies demonstrated that the standard deviation across trials was 2.0 ± 1.1%. The effects of repetition time and inversion time selections were investigated. The TRUST technique was further tested using various physiologic challenges. Hypercapnia induced an increase in venous oxygenation by 13.8 ± 1.1%. On the other hand, caffeine ingestion resulted in a decrease in oxygenation by 7.0 ± 1.8%. Contrast agent infusion (Gd‐DTPA, 0.1 mmol/kg) reduced venous blood T2 by 11.2 ms. The results of this study show that TRUST MRI is a useful technique for quantitative assessment of blood oxygenation in the brain. Magn Reson Med 60:357–363, 2008.

The influence of carbon dioxide on brain activity and metabolism in conscious humans

A better understanding of carbon dioxide (CO2) effect on brain activity may have a profound impact on clinical studies using CO2 manipulation to assess cerebrovascular reserve and on the use of hypercapnia as a means to calibrate functional magnetic resonance imaging (fMRI) signal. This study investigates how an increase in blood CO2, via inhalation of 5% CO2, may alter brain activity in humans. Dynamic measurement of brain metabolism revealed that mild hypercapnia resulted in a suppression of cerebral metabolic rate of oxygen (CMRO 2 ) by 13.4%±2.3% (N=14) and, furthermore, the CMRO 2 change was proportional to the subjects end-tidal CO2 (Et-CO2) change. When using functional connectivity MRI (fcMRI) to assess the changes in resting-state neural activity, it was found that hypercapnia resulted in a reduction in all fcMRI indices assessed including cluster volume, cross-correlation coefficient, and amplitude of the fcMRI signal in the default-mode network (DMN). The extent of the reduction was more pronounced than similar indices obtained in visual-evoked fMRI, suggesting a selective suppression effect on resting-state neural activity. Scalp electroencephalogram (EEG) studies comparing hypercapnia with normocapnia conditions showed a relative increase in low frequency power in the EEG spectra, suggesting that the brain is entering a low arousal state on CO2 inhalation.

Estimation of labeling efficiency in pseudocontinuous arterial spin labeling

Pseudocontinuous arterial spin labeling MRI is a new arterial spin labeling technique that has the potential of combining advantages of continuous arterial spin labeling and pulsed arterial spin labeling. However, unlike continuous arterial spin labeling, the labeling process of pseudocontinuous arterial spin labeling is not strictly an adiabatic inversion and the efficiency of labeling may be subject specific. Here, three experiments were performed to study the labeling efficiency in pseudocontinuous arterial spin labeling MRI. First, the optimal labeling position was determined empirically to be approximately 84 mm below the anterior commissure‐posterior commissure line in order to achieve the highest sensitivity. Second, an experimental method was developed to utilize phase‐contrast velocity MRI as a normalization factor and to estimate the labeling efficiency in vivo, which was founded to be 0.86 ± 0.06 (n = 10, mean ± standard deviation). Third, we compared the labeling efficiency of pseudocontinuous arterial spin labeling MRI under normocapnic and hypercapnic (inhalation of 5% CO2) conditions and showed that a higher flow velocity in the feeding arteries resulted in a reduction in the labeling efficiency. In summary, our results suggest that labeling efficiency is a critical parameter in pseudocontinuous arterial spin labeling MRI not only in terms of achieving highest sensitivity but also in quantification of absolute cerebral blood flow in milliliters per minute per 100 g. We propose that the labeling efficiency should be estimated using phase‐contrast velocity MRI on a subject‐specific basis. Magn Reson Med 63:765–771, 2010.

Flow territory mapping of the cerebral arteries with regional perfusion MRI.

Background and Purpose— Conventional contrast-enhanced angiography is the gold standard for visualization of the vascular tree supplied by the major cerebral arteries and assessment of collateral flow. Thus far, however, no methods are available to assess the actual flow territories of the individual cerebral arteries. In the present study, we evaluate a noninvasive arterial spin labeling MRI method for selective mapping of the flow territories of the left and right internal carotid arteries and posterior circulation (basilar artery and vertebral arteries). Methods— A spatially selective labeling approach, regional perfusion imaging, was developed on the basis of selective slab inversion of the arterial water with a pulsed arterial spin labeling sequence. The selectivity of this method was demonstrated. Results— Regional perfusion imaging enables assessment of the perfusion territories of the major cerebral arteries. With selective labeling of an internal carotid artery, signal is present in both the ipsilateral anterior cerebral artery and ipsilateral middle cerebral artery flow territory. With labeling of the basilar artery, perfusion-weighted signal is symmetrically present in both posterior cerebral artery flow territories. Cerebral blood flow values measured with regional perfusion imaging in the complete hemisphere (40.1 mL · min−1 · 100 g−1 tissue), white matter (22.1 mL · min−1 · 100 g−1 tissue), and gray matter (65.8 mL · min−1 · 100 g−1 tissue) are in agreement with data in the literature. Conclusions— We present the first imaging method capable of evaluating both quantitatively and qualitatively the flow territories of the individual brain-feeding arteries in vivo.

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.