Simultaneous measurement of cerebral blood flow and transit time with turbo dynamic arterial spin labeling (Turbo-DASL): application to functional studies

Oxygen metabolism MRI - A comparison with perfusion imaging in a rat model of MCA branch occlusion and reperfusion

The penumbra is sustained by an increased extraction of oxygen (OEF) from blood to brain tissue. Metabolic imaging may improve penumbra specificity when examining stroke patients with wake-up stroke and a long time between admission and symptom onset. We used MRI to examine OEF, and compared the volume of regions with elevated OEF to the volume of regions with perfusion deficit in a M2 occlusion model (M2CAO) with preserved collateral blood flow. OEF was calculated from BOLD MRI examining tissue R2', with ASL perfusion imaging employed to determine cerebral blood flows (CBF) and volumes. Diffusion imaging was used to identify the ischemic core (IC). Examinations were performed during and after transient M2CAO in rats. The IC-OEF mismatch was significantly smaller than the IC-CBF mismatch during M2CAO. The penumbra OEF was significantly increased during M2CAO, and decreased significantly after reperfusion. The IC-OEF mismatch may provide increased penumbra specificity compared to IC-CBF mismatch regimens. Results strongly indicate the potential of metabolic MRI for thrombectomy patient selection in cases with a long time from symptom onset to admission. Results demonstrate the effectiveness of reperfusion in alleviating metabolic disturbances in ischemic regions, emphasizing fast treatment to achieve significant neurological recovery in stroke patients.

MRI techniques to measure arterial and venous cerebral blood volume

The measurement of cerebral blood volume (CBV) has been the topic of numerous neuroimaging studies. To date, however, most in vivo imaging approaches can only measure CBV summed over all types of blood vessels, including arterial, capillary and venous vessels in the microvasculature (i.e. total CBV or CBVtot). As different types of blood vessels have intrinsically different anatomy, function and physiology, the ability to quantify CBV in different segments of the microvascular tree may furnish information that is not obtainable from CBVtot, and may provide a more sensitive and specific measure for the underlying physiology. This review attempts to summarize major efforts in the development of MRI techniques to measure arterial (CBVa) and venous CBV (CBVv) separately. Advantages and disadvantages of each type of method are discussed. Applications of some of the methods in the investigation of flow-volume coupling in healthy brains, and in the detection of pathophysiological abnormalities in brain diseases such as arterial steno-occlusive disease, brain tumors, schizophrenia, Huntington's disease, Alzheimer's disease, and hypertension are demonstrated. We believe that the continual development of MRI approaches for the measurement of compartment-specific CBV will likely provide essential imaging tools for the advancement and refinement of our knowledge on the exquisite details of the microvasculature in healthy and diseased brains.

Estimation of perfusion properties with MR Fingerprinting Arterial Spin Labeling

In this study, the acquisition of ASL data and quantification of multiple hemodynamic parameters was explored using a Magnetic Resonance Fingerprinting (MRF) approach. A pseudo-continuous ASL labeling scheme was used with pseudo-randomized timings to acquire the MRF ASL data in a 2.5 min acquisition. A large dictionary of MRF ASL signals was generated by combining a wide range of physical and hemodynamic properties with the pseudo-random MRF ASL sequence and a two-compartment model. The acquired signals were matched to the dictionary to provide simultaneous quantification of cerebral blood flow, tissue time-to-peak, cerebral blood volume, arterial time-to-peak, B₁, and T_1. A study in seven healthy volunteers resulted in the following values across the population in grey matter (mean ± standard deviation): cerebral blood flow of 69.1 ± 6.1 ml/min/100 g, arterial time-to-peak of 1.5 ± 0.1 s, tissue time-to-peak of 1.5 ± 0.1 s, T₁ of 1634 ms, cerebral blood volume of 0.0048 ± 0.0005. The CBF measurements were compared to standard pCASL CBF estimates using a one-compartment model, and a Bland-Altman analysis showed good agreement with a minor bias. Repeatability was tested in five volunteers in the same exam session, and no statistical difference was seen. In addition to this validation, the MRF ASL acquisition's sensitivity to the physical and physiological parameters of interest was studied numerically.

Preclinical Arterial Spin Labeling Measurement of Cerebral Blood Flow

Magnetic resonance imaging has been utilized as a quantitative and noninvasive method to image blood flow. Arterial spin labeling (ASL) is an MRI technique that images blood flow using arterial blood water as an endogenous tracer. Herein we describe the use of ASL to measure cerebral blood flow completely noninvasively in rodents, including methods, analysis, and important considerations when utilizing this technique.

Wedge-shaped slice-selective adiabatic inversion pulse for controlling temporal width of bolus in pulsed arterial spin labeling

PURPOSE

In pulsed arterial spin labeling (PASL) methods, arterial blood is labeled by inverting a slab with uniform thickness, resulting in different temporal widths of boluses in vessels with different flow velocities. This limits the temporal resolution and signal-to-noise ratio (SNR) efficiency gains in PASL-based methods intended for high temporal resolution and SNR efficiency, such as turbo-ASL and turbo-QUASAR.

THEORY AND METHODS

A novel wedge-shaped (WS) adiabatic inversion pulse is developed by adding in-plane gradient pulses to a slice-selective (SS) adiabatic inversion pulse to linearly modulate the inversion thicknesses at different locations while maintaining the adiabatic properties of the original pulse. A hyperbolic secant (HS)-based WS inversion pulse was implemented. Its performance was tested in simulations and in phantom and human experiments and compared with an SS HS inversion pulse.

RESULTS

Compared with the SS inversion pulse, the WS inversion pulse was capable of inducing different inversion thicknesses at different locations. It could be adjusted to generate a uniform temporal width of boluses in arteries at locations with different flow velocities.

CONCLUSION

The WS inversion pulse can be used to control the temporal widths of labeled boluses in PASL experiments. This should benefit PASL experiments by maximizing labeling duty cycle and improving temporal resolution and SNR efficiency. Magn Reson Med 76:838-847, 2016. © 2015 Wiley Periodicals, Inc.

Comparison of retinal and cerebral blood flow between continuous arterial spin labeling MRI and fluorescent microsphere techniques

PURPOSE

To compare basal retinal and cerebral blood flow (BF) values using continuous arterial spin labeling (CASL) MRI and fluorescent microspheres.

MATERIALS AND METHODS

A total of 41 animals were used. BF was measured using an established microsphere technique (a mixture of 2.5 million 8 μm green and 0.5 million 10 μm blue fluorescent microspheres) and CASL MRI blood flow measurement in the rat retina and brain at 7 Tesla (T) and 11.7T, respectively.

RESULTS

Retinal BF by MRI was 1.18 ± 0.57 mL/g/min and choroidal BF was 8.14 ± 1.8 mL/g/min (n = 6). Microsphere retinal BF was 9.12 ± 2.8 μL/min per tissue and choroidal BF was 73.38 ± 44 μL/min per tissue (n = 18), corresponding to a retinal BF value of 1.22 ± 0.36 mL/g/min by means of a wet weight conversion. The wet-weight of the choroid could not be determined. To corroborate our findings, cerebral BF (CBF) by MRI was also analyzed. In the cerebral cortices, CBF was 0.91 ± 0.29 mL/g/min (n = 14) by CASL MRI and 1.09 ± 0.37 mL/g/min (n = 6) by microspheres. There were no significant differences found between MRI and microsphere blood flow in the retina and brain.

CONCLUSION

BF values in the rat retina and cerebral cortex by MRI are in agreement with those obtained by the microsphere technique.

Clinical applications of arterial spin labeling

MR arterial spin labeling is primarily applied as a neuroimaging method to measure cerebral blood flow. As this technique becomes more widely available, a basic understanding of the clinical applications is necessary for optimal utilization in the setting of patient care. This review focuses on the use of arterial spin labeling imaging for the evaluation of cerebrovascular disease, brain tumors and neuropsychiatric illness.

Blood oxygenation level dependent, blood volume, and blood flow responses to carbogen and hypoxic hypoxia in 9L rat gliomas as measured by MRI

PURPOSE

To study vascular responsiveness to hypoxia and hypercarbia together with vessel size index (VSI) in a 9L rat glioma (n = 11) using multimodal MRI.

MATERIALS AND METHODS

VSI was determined using T2 and T2* MRI following AMI-227 contrast agent. Blood oxygenation level dependent (BOLD) signal response was determined using T2 EPI MRI, blood volume changes using AMI-227 and blood flow by means of continuous arterial spin labeling.

RESULTS

VSI in the cortex, tumor rim, and core of 2.2 ± 1.0, 18.2 ± 5.4, and 23.9 ± 14.7 μm, respectively, showing a larger average vessel size in glioma than in the brain parenchyma. BOLD and blood volume signal changes to hypoxia and hypercapnia were much more profound in the tumor rim than the core. Hypoxia led to rim BOLD signal change that was larger in amplitude and it attained the low value much faster than either core or brain cortex. The vasculature in the rim appears more responsive to respiratory challenges in terms of volume adaptation than the core. Blood flow values within the gliomas were much lower than in the contralateral brain. Neither hypercarbia nor hypoxia had an effect on the tumor blood flow.

CONCLUSION

Vascular responses of 9L gliomas to respiratory challenge, in particular hypoxia, are heterogeneous between the core and rim zones, potentially offering a means to classify and separate intratumor tissues with differing hemodynamic characteristics.

Cine-ASL: a steady-pulsed arterial spin labeling method for myocardial perfusion mapping in mice. Part II. Theoretical model and sensitivity optimization

In small rodent myocardial perfusion studies, the most widely used method is based on Look-Locker measurements of the magnetization recovery after FAIR preparation, which bears limitations regarding acquisition efficiency due to the pulsed arterial spin labeling nature of the sequence. To improve efficiency, this two-article set proposes a new steady-pulsed arterial spin labeling scheme using a cine readout incorporating one tagging pulse per heart cycle. In this part, we derive a theoretical description of the magnetization time evolution in such a scheme. The combination of steady-pulsed labeling and cine readout drives tissue magnetization into a stationary regime that explicitly depends on perfusion. In comparison with dedicated experiments on the mouse heart, the model is discussed and validated for perfusion quantification. The model predicts that in this regime, signal is independent of irregular dynamics occurring during acquisition, such as heart rate variations or arterial input function. Optimization of the sequence offers the possibility to increase the signal to noise ratio by efficient signal averaging. The sensitivity of this new method is shown to be more than three times larger than previously used techniques.