Pseudo-continuous transfer insensitive labeling technique

Quantitative multiple boli arterial spin labeling

PURPOSE

mbASL relies on the use of a series of adiabatic radio-frequency pulses to label successive boli of blood water. As the sequence is a hybrid of pCASL and PASL, it requires an appropriate kinetic model to accurately describe the signal for quantification purposes.

THEORY AND METHODS

Drawing on the Buxton standard kinetic model, we propose modifications to account for the multiple labeling pulses at variable delays. By varying the number of adiabatic pulses and the thickness of the inversion slab, we maximize SNR and demonstrate the hybrid nature of the sequence.

RESULTS

The mbASL kinetic model is used to produce mbASL cerebral blood flow maps, with average values for mice (110 ml/100 g/min) and rats (96 ml/100 g/min).

CONCLUSION

We have successfully quantified and validated the mbASL kinetic model and demonstrated that the resulting CBF values agree with the existing literature.

Multiple boli arterial spin labeling for high signal-to-noise rodent brain perfusion imaging

PURPOSE

A systematic method is proposed for optimizing a promising preclinical arterial spin labeling (ASL) sequence based on the use of a train of adiabatic radiofrequency pulses labeling successive boli of blood water.

METHODS

The sequence optimization is performed and evaluated using brain imaging experiments in mice and in rats. It involves the investigation of several parameters, ranging from the number of adiabatic pulses and labeling duration to the properties of the adiabatic hyperbolic secant pulses (ie, amplitude and frequency modulation).

RESULTS

Species-dependent parameters are identified, allowing for robust fast optimization protocols to be introduced. The resulting optimized multiple boli ASL (mbASL) sequence provides with significantly higher average signal-to-noise ratios (SNR) per voxel volume than currently encountered in ASL studies (278 mm^-3 in mice and 172 mm^-3 in rats). Comparing with the commonly used flow-sensitive alternating inversion recovery technique (FAIR), mbASL-to-FAIR SNR ratios reach 203% for mice and 725% for rats.

CONCLUSION

When properly optimized, mbASL can offer a robust, high SNR ASL alternative for rodent brain perfusion studies Magn Reson Med 79:1020-1030, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Cerebrovascular MRI: a review of state-of-the-art approaches, methods and techniques

Cerebrovascular imaging is of great interest in the understanding of neurological disease. MRI is a non-invasive technology that can visualize and provide information on: (i) the structure of major blood vessels; (ii) the blood flow velocity in these vessels; and (iii) the microcirculation, including the assessment of brain perfusion. Although other medical imaging modalities can also interrogate the cerebrovascular system, MR provides a comprehensive assessment, as it can acquire many different structural and functional image contrasts whilst maintaining a high level of patient comfort and acceptance. The extent of examination is limited only by the practicalities of patient tolerance or clinical scheduling limitations. Currently, MRI methods can provide a range of metrics related to the cerebral vasculature, including: (i) major vessel anatomy via time-of-flight and contrast-enhanced imaging; (ii) blood flow velocity via phase contrast imaging; (iii) major vessel anatomy and tissue perfusion via arterial spin labeling and dynamic bolus passage approaches; and (iv) venography via susceptibility-based imaging. When designing an MRI protocol for patients with suspected cerebral vascular abnormalities, it is appropriate to have a complete understanding of when to use each of the available techniques in the 'MR angiography toolkit'. In this review article, we: (i) overview the relevant anatomy, common pathologies and alternative imaging modalities; (ii) describe the physical principles and implementations of the above listed methods; (iii) provide guidance on the selection of acquisition parameters; and (iv) describe the existing and potential applications of MRI to the cerebral vasculature and diseases. The focus of this review is on obtaining an understanding through the application of advanced MRI methodology of both normal and abnormal blood flow in the cerebrovascular arteries, capillaries and veins.

Steady pulsed imaging and labeling scheme for noninvasive perfusion imaging

PURPOSE

A steady pulsed imaging and labeling (SPIL) scheme is proposed to obtain high-resolution multislice perfusion images of mice brain using standard preclinical MRI equipment.

THEORY AND METHODS

The SPIL scheme repeats a pulsed arterial spin labeling (PASL) module together with a short mixing time to extend the temporal duration of the generated PASL bolus to the total experimental time. Multislice image acquisition takes place during the mixing times. The mixing time is also used for magnetization recovery following image acquisition. The new scheme is able to yield multislice perfusion images rapidly. The perfusion kinetic curve can be measured by a multipulsed imaging and labeling (MPIL) scheme, i.e., acquiring single-slice ASL signals before reaching steady-state in the SPIL sequence.

RESULTS

When applying the SPIL method to normal mice, and to mice with unilateral ischemia, high-resolution multislice (five slices) CBF images could be obtained in 8 min. Perfusion data from ischemic mice showed clear CBF reductions in ischemic regions. The SPIL method was also applied to postmortem mice, showing that the method is free from magnetization transfer confounds.

CONCLUSION

The new SPIL scheme provides for robust measurement of CBF with multislice imaging capability in small animals.

Taking the pulse of aging: mapping pulse pressure and elasticity in cerebral arteries with optical methods

Cerebrovascular support is crucial for healthy cognitive and brain aging. Arterial stiffening is a cause of reduced brain blood flow, a predictor of cognitive decline, and a risk factor for cerebrovascular accidents and Alzheimer's disease. Arterial health is influenced by lifestyle factors, such as cardiorespiratory fitness (CRF). We investigated new noninvasive optical measures of cerebrovascular health, which provide estimates of arterial pulse parameters (pulse pressure, transit time, and compliance/elasticity) within specific cerebral arteries and cortical regions, and low-resolution maps of large superficial cerebral arteries. We studied naturally occurring variability in these parameters in adults (aged 55-87), and found that these indices of cerebrovascular health are negatively correlated with age and positively with CRF and gray and white matter volumes. Further, regional pulse transit time predicts specific neuropsychological performance.

Cardiorespiratory fitness mediates the effects of aging on cerebral blood flow

The brain's vasculature is likely to be subjected to the same age-related physiological and anatomical changes affecting the rest of the cardiovascular system. Since aerobic fitness is known to alleviate both cognitive and volumetric losses in the brain, it is important to investigate some of the possible mechanisms underlying these beneficial changes. Here we investigated the role that estimated cardiorespiratory fitness (eCRF) plays in determining the relationship between aging and cerebral blood flow (CBF) in a group of older adults (ages 55–85). Using arterial spin labeling to quantify CBF, we found that blood flow in the gray matter was positively correlated with eCRF and negatively correlated with age. Subsequent analyses revealed that eCRF fully mediated the effects of age on CBF in the gray matter, but not in the white matter. Additionally, regional measures of CBF were related to regional measures of brain volume. These findings provide evidence that age-related effects on cerebrovascular health and perfusion in older adults are largely influenced by their eCRF levels.

Localized blood flow imaging using quantitative flow-enhanced signal intensity

Flow-enhanced signal intensity (FENSI) was previously introduced as a novel functional imaging method for measuring changes in localized blood flow in response to a stimulus. However, FENSI was limited to a qualitative functional MRI tool, due to magnetization transfer effects and different tagging plane profiles between tag and control images. In this work, a revised FENSI acquisition is proposed to enable quantitative imaging, which is capable of providing absolute localized blood flow maps free from magnetization transfer and slice profile errors. The feasibility and accuracy of measuring microvascular (arteriole, capillary, and venule) blood flow by using quantitative FENSI was validated by our phantom studies. Additionally, localized cerebral blood flow, 366 ± 45 μL/min/cm(2) in gray matter and 153 ± 23 μL/min/cm(2) in white matter, was measured in healthy subjects during resting state, whereas a flow change of 73 ± 13% was detected during a visual task.