Quantification of arterial cerebral blood volume using multiphase-balanced SSFP-based ASL

Effects of a 12-Week Periodized Resistance Training Program on Resting Brain Activity and Cerebrovascular Function: A Nonrandomized Pilot Trial

Resistance training is a promising strategy to promote healthy cognitive aging; however, the brain mechanisms by which resistance training benefits cognition have yet to be determined. Here, we examined the effects of a 12-week resistance training program on resting brain activity and cerebrovascular function in 20 healthy older adults (14 females, mean age 69.1 years). In this single group clinical trial, multimodal 3 T magnetic resonance imaging was performed at 3 time points: baseline (preceding a 12-week control period), pre-intervention, and post-intervention. Along with significant improvements in fluid cognition (d = 1.27), 4 significant voxelwise clusters were identified for decreases in resting brain activity after the intervention (Cerebellum, Right Middle Temporal Gyrus, Left Inferior Parietal Lobule, and Right Inferior Parietal Lobule), but none were identified for changes in resting cerebral blood flow. Using a separate region of interest approach, we provide estimates for improved cerebral blood flow, compared with declines over the initial control period, in regions associated with cognitive impairment, such as hippocampal blood flow (d = 0.40), and posterior cingulate blood flow (d = 0.61). Finally, resistance training had a small countermeasure effect on the age-related progression of white matter lesion volume (rank-biserial = -0.22), a biomarker of cerebrovascular disease. These proof-of-concept data support larger trials to determine whether resistance training can attenuate or even reverse salient neurodegenerative processes.

Noninvasive Quantification of Cerebral Blood Flow Using Hybrid PET/MR Imaging to Extract the [15 O]H2 O Image-Derived Input Function Free of Partial Volume Errors

BACKGROUND

Quantification of cerebral blood flow (CBF) with [¹⁵ O]H₂ O-positron emission tomography (PET) requires arterial sampling to measure the input function. This invasive procedure can be avoided by extracting an image-derived input function (IDIF); however, IDIFs are sensitive to partial volume errors due to the limited spatial resolution of PET.

PURPOSE

To present an alternative hybrid PET/MR imaging of CBF (PMRFlow_IDIF ) that uses phase-contrast (PC) MRI measurements of whole-brain (WB) CBF to calibrate an IDIF extracted from a WB [¹⁵ O]H₂ O time-activity curve.

STUDY TYPE

Technical development and validation.

ANIMAL MODEL

Twelve juvenile Duroc pigs (83% female).

POPULATION

Thirteen healthy individuals (38% female).

FIELD STRENGTH/SEQUENCES

3 T; gradient-echo PC-MRI.

ASSESSMENT

PMRFlow_IDIF was validated against PET-only in a porcine model that included arterial sampling. CBF maps were generated by applying PMRFlow_IDIF and two previous PMRFlow methods (PC-PET and double integration method [DIM]) to [¹⁵ O]H₂ O-PET data acquired from healthy individuals.

STATISTICAL TESTS

PMRFlow and PET CBF measurements were compared with regression and correlation analyses. Paired t-tests were performed to evaluate differences. Potential biases were assessed using one-sample t-tests. Reliability was assessed by intraclass correlation coefficients. Statistical significance: α  = 0.05.

RESULTS

In the animal study, strong agreement was observed between PMRFlow_IDIF (average voxel-wise CBF, 58.0 ± 16.9 mL/100 g/min) and PET (63.0 ± 18.9 mL/100 g/min). In the human study, PMRFlow_DIM (y = 1.11x - 5.16, R²  = 0.99 ± 0.01) and PMRFlow_PC-PET (y = 0.87x + 3.82, R²  = 0.97 ± 0.02) performed similarly to PMRFlow_IDIF, and CBF was within the expected range (eg, 49.7 ± 7.2 mL/100 g/min for gray matter).

DATA CONCLUSION

Accuracy of PMRFlow_IDIF was confirmed in the animal study with the primary source of error attributed to differences in WB CBF measured by PC MRI and PET. In the human study, differences in CBF from PMRFlow_IDIF , PMRFlow_DIM , and PMRFlow_PC-PET were due to the latter two not accounting for blood-borne activity.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1.

Three-dimensional assessment of brain arterial compliance: Technical development, comparison with aortic pulse wave velocity, and age effect

PURPOSE

The ability to measure cerebral vascular compliance (VC) is important in the evaluation of vascular diseases. Additionally, quantification of arterial wall pulsation in the brain may be useful for understanding the driving force of the recently discovered glymphatic system. Our goal is to develop an MRI technique to measure VC and arterial wall pulsation in major intracranial vessels.

METHODS

A total of 17 healthy subjects were studied on a 3T MRI system. The technique, called VaCom-PCASL, uses pseudo-continuous arterial spin labeling (PCASL) to obtain pure blood vessel signal, uses a 3D radial acquisition, and applies a golden-angle radial sparse parallel (GRASP) algorithm for image reconstruction. The k-space data were retrospectively sorted into different cardiac phases. The GRASP algorithm allows the reconstruction of 5D (three spatial dimensions, one control/label dimension, and one cardiac-phase dimension) data simultaneously. The proposed technique was optimized in terms of reconstruction parameters and labeling duration. Intracranial VC was compared with aortic pulse wave velocity measured with phase-contrast MRI. Age differences in VC were studied.

RESULTS

The VaCom-PCASL technique using 10 cardiac phases and GRASP sparsity constraints of λ_{label/control} = 0.05 and λ_cardiac = 0.05 provided the highest contrast-to-noise ratio. A labeling duration of 800 ms was found to yield signals comparable to those of longer duration (P > .2), whereas 400 ms yielded significant overestimation (P < .005). A significant correlation was observed between intracranial VC and aortic pulse wave velocity (r = -0.73, P = .038, N = 8). Vascular compliance in the older group was lower than that in the younger group.

CONCLUSION

The VaCom-PCASL-MRI technique represents a promising approach for noninvasive assessment of arterial stiffness and pulsatility.

Arterial Spin Labeling Applications in Pediatric and Adult Neurologic Disorders

Arterial spin labeling (ASL) is a powerful noncontrast magnetic resonance imaging (MRI) technique that enables quantitative evaluation of brain perfusion. To optimize the clinical and research utilization of ASL, radiologists and physicists must understand the technical considerations and age-related variations in normal and disease states. We discuss advanced applications of ASL across the lifespan, with example cases from children and adults covering a wide variety of pathologies. Through literature review and illustrated clinical cases, we highlight the subtleties as well as pitfalls of ASL interpretation. First, we review basic physical principles, techniques, and artifacts. This is followed by a discussion of normal perfusion variants based on age and physiology. The three major categories of perfusion abnormalities-hypoperfusion, hyperperfusion, and mixed patterns-are covered with an emphasis on clinical interpretation and relationship to the disease process. Major etiologies of hypoperfusion include large artery, small artery, and venous disease; other vascular conditions; global hypoxic-ischemic injury; and neurodegeneration. Hyperperfusion is characteristic of vascular malformations and tumors. Mixed perfusion patterns can be seen with epilepsy, migraine, trauma, infection/inflammation, and toxic-metabolic encephalopathy. LEVEL OF EVIDENCE: 4 TECHNICAL EFFICACY STAGE: 3.

Effects of Susceptibility Artifacts on Perfusion MRI in Patients with Primary Brain Tumor: A Comparison of Arterial Spin-Labeling versus DSC

BACKGROUND AND PURPOSE

Our aim was to investigate the effects of intratumoral hemorrhage, calcification, and postoperative changes on the sensitivity of arterial spin-labeling and DSC perfusion MR imaging in patients with primary brain tumors.

MATERIALS AND METHODS

Eighty-six brain tumor lesions were examined with single-phase and multiphase arterial spin-labeling and DSC perfusion MR imaging. The lesions that had no intratumoral bleeding/calcifications and history of surgery were assigned to group 1 (n = 38), and the lesions that had these were assigned to group 2 (n = 48). The relative regional cerebral blood flow was calculated in both perfusion methods, and relative regional cerebral blood volume was calculated in DSC. Imaging results were correlated with histopathology or follow-up.

RESULTS

In the quantitative evaluation, the sensitivity and specificity of relative regional cerebral blood flow in multiphase arterial spin-labeling perfusion were 94.4% and 80% in group 1 and 78.3% and 88% in group 2, respectively. The sensitivity and specificity of relative regional cerebral blood flow in DSC perfusion were 88.9% and 75% in group 1 and 78.3% and 84% in group 2, respectively. The sensitivity and specificity of relative regional cerebral blood volume in DSC perfusion were 66.7% and 100% in group 1 and 69.6% and 96% in group 2, respectively. In the qualitative evaluation, the sensitivities for single-phase and multiphase arterial spin-labeling were 48.2% and 79.3%, respectively, with 100% specificity for both.

CONCLUSIONS

The sensitivity and specificity of multiphase arterial spin-labeling were similar to those of DSC perfusion irrespective of bleeding and calcification in primary brain tumors. Thus, we suggest that noncontrast multiphase arterial spin-labeling can be used instead of DSC perfusion MR imaging in the diagnosis and follow-up of intracranial tumors.

Cerebral blood volume mapping using Fourier-transform-based velocity-selective saturation pulse trains

PURPOSE

Velocity-selective saturation (VSS) pulse trains provide a viable alternative to the spatially selective methods for measuring cerebral blood volume (CBV) by reducing the sensitivity to arterial transit time. This study is to compare the Fourier-transform-based velocity-selective saturation (FT-VSS) pulse trains with the conventional flow-dephasing VSS techniques for CBV quantification.

METHODS

The proposed FT-VSS label and control modules were compared with VSS pulse trains utilizing double refocused hyperbolic tangent (DRHT) and 8-segment B1-insensitive rotation (BIR-8). This was done using both numerical simulations and phantom studies to evaluate their sensitivities to gradient imperfections such as eddy currents. DRHT, BIR-8, and FT-VSS prepared CBV mapping was further compared for velocity-encoding gradients along 3 orthogonal directions in healthy subjects at 3T.

RESULTS

The phantom studies exhibited more consistent immunity to gradient imperfections for the utilized FT-VSS pulse trains. Compared to DRHT and BIR-8, FT-VSS delivered more robust CBV results across the 3 VS encoding directions with significantly reduced artifacts along the superior-inferior direction and improved temporal signal-to-noise ratio (SNR) values. Average CBV values obtained from FT-VSS based sequences were 5.3 mL/100 g for gray matter and 2.3 mL/100 g for white matter, comparable to literature expectations.

CONCLUSION

Absolute CBV quantification utilizing advanced FT-VSS pulse trains had several advantages over the existing approaches using flow-dephasing VSS modules. A greater immunity to gradient imperfections and the concurrent tissue background suppression of FT-VSS pulse trains enabled more robust CBV measurements and higher SNR than the conventional VSS pulse trains.

Quantification of intracranial arterial blood flow using noncontrast enhanced 4D dynamic MR angiography

PURPOSE

Noncontrast enhanced dynamic magnetic resonance angiography delineates the pattern of dynamic blood flow of the cerebral vasculature. A model-free solution was proposed to quantify arterial blood flow (aBF) by using the monotonic property of the residual function.

THEORY AND METHODS

Analytical simulations and in-vivo studies were performed to evaluate the performance of the proposed method by comparing the aBF values generated from the proposed and conventional singular value decomposition methods. The aBF values were compared with blood flow velocity measured by 2D phase contrast MRI, and compared between balanced steady-state free precession-based radial and spoiled GRE-based Cartesian acquisitions. Hemodynamic parametric maps were generated in 1 patient with arteriovenous malformation.

RESULTS

The proposed method generates reliable aBF measurement at different signal-to-noise ratio levels, whereas overestimation/underestimation of aBF was observed when a high/low threshold was applied in the singular value decomposition method. Average aBF in large vascular branches was 214.4 and 214.5 mL/mL/min with radial and Cartesian acquisitions, respectively. Significant correlations were found between aBF and blood flow velocity measured by phase contrast MRI (P = 0.0008), and between Cartesian and radial acquisitions (P < 0.0001). Altered hemodynamics were observed at the lesion site of the arteriovenous malformation patient.

CONCLUSION

A robust analytical solution was proposed for quantifying aBF. This model-free method is robust to noise, and its clinical value in the diagnosis of cerebrovascular disorders awaits further evaluation.

Recent progress in ASL

This article aims to provide the reader with an overview of recent developments in Arterial Spin Labeling (ASL) MRI techniques. A great deal of progress has been made in recent years in terms of the SNR and acquisition speed. New strategies have been introduced to improve labeling efficiency, reduce artefacts, and estimate other relevant physiological parameters besides perfusion. As a result, ASL techniques has become a reliable workhorse for researchers as well as clinicians. After a brief overview of the technique's fundamentals, this article will review new trends and variants in ASL including vascular territory mapping and velocity selective ASL, as well as arterial blood volume imaging techniques. This article will also review recent processing techniques to reduce partial volume effects and physiological noise. Next the article will examine how ASL techniques can be leveraged to calculate additional physiological parameters beyond perfusion and finally, it will review a few recent applications of ASL in the literature.

Evaluation of 2D Imaging Schemes for Pulsed Arterial Spin Labeling of the Human Kidney Cortex

A number of imaging readout schemes are proposed for renal arterial spin labeling (ASL) to quantify kidney cortex perfusion, including gradient echo-based methods of balanced fast field echo (bFFE) and gradient-echo echo-planar imaging (GE-EPI), or spin echo-based schemes of spin-echo echo-planar imaging (SE-EPI) and turbo spin-echo (TSE). Here, we compare these two-dimensional (2D) imaging schemes to evaluate the optimal imaging scheme for pulsed ASL (PASL) assessment of human kidney cortex perfusion at 3 T. Ten healthy volunteers with normal renal function were scanned using each 2D multi-slice imaging scheme, in combination with a respiratory triggered flow-sensitive alternating inversion recovery (FAIR) ASL scheme on a 3 T Philips Achieva scanner. All volunteers returned for a second identical scan session within two weeks of the first scan session. Comparisons were made between the imaging schemes in terms of perfusion-weighted image (PWI) signal-to-noise ratio (SNR) and perfusion quantification, temporal SNR (tSNR), spatial coverage, and repeatability. For each imaging scheme, the renal cortex perfusion was calculated (bFFE: 276 ± 29 mL/100g/min, GE-EPI: 222 ± 18 mL/100g/min, SE-EPI: 201 ± 36 mL/100g/min, and TSE: 200 ± 20 mL/100g/min). Perfusion was found to be higher for GE-based readouts when compared with SE-based readouts, with significantly higher measured perfusion for the bFFE readout when compared with all other schemes (p < 0.05), attributed to the greater vascular signal present. Despite the PWI-SNR being significantly lower for SE-EPI when compared with all other schemes (p < 0.05), the SE-EPI readout gave the highest tSNR, and was found to be the most reproducible scheme for the assessment of kidney cortex, with a coefficient of variation (CoV) of 17.2%, whilst minimizing variability of the perfusion-weighted signal across slices for whole-kidney perfusion assessment. For the assessment of kidney cortex perfusion using 2D readout schemes, SE-EPI provides optimal tSNR, minimal variability across slices, and repeatable data acquired in a short scan time with low specific absorption rate.

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.

Quantitative susceptibility mapping-based cerebral metabolic rate of oxygen mapping with minimum local variance

PURPOSE

The objective of this study was to demonstrate the feasibility of a cerebral metabolic rate of oxygen (CMRO₂ ) mapping method based on its minimum local variance (MLV) without vascular challenge using quantitative susceptibility mapping (QSM) and cerebral blood flow (CBF).

METHODS

Three-dimensional multi-echo gradient echo imaging and arterial spin labeling were performed in 11 healthy subjects to calculate QSM and CBF. Minimum local variance was used to compute whole-brain CMRO₂ map from QSM and CBF. The MLV method was compared with a reference method using the caffeine challenge. Their agreement within the cortical gray matter (CGM) was assessed on CMRO₂ and oxygen extraction fraction (OEF) maps at both baseline and challenge states.

RESULTS

Mean CMRO₂ (in µmol/100 g/min) obtained in CGM using the caffeine challenge and MLV were 142 ± 16.5 and 139 ± 14.8 µmol/100 g/min, respectively; the corresponding baseline OEF were 33.0 ± 4.0% and 31.8 ± 3.2%, respectively. The MLV and caffeine challenge methods showed no statistically significant differences across subjects with small ( < 4%) biases in CMRO₂ and OEF values.

CONCLUSIONS

Minimum local variance-based CMRO₂ mapping without vascular challenge using QSM and arterial spin labeling is feasible in healthy subjects. Magn Reson Med 79:172-179, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Quantitative measurement of cerebral blood volume using velocity-selective pulse trains

PURPOSE

To develop a non-contrast-enhanced MRI method for cerebral blood volume (CBV) mapping using velocity-selective (VS) pulse trains.

METHODS

The new pulse sequence applied velocity-sensitive gradient waveforms in the VS label modules and velocity-compensated ones in the control scans. Sensitivities to the gradient imperfections (e.g., eddy currents) were evaluated through phantom studies. CBV quantification procedures based on simulated labeling efficiencies for arteriolar, capillary, and venular blood as a function of cutoff velocity (Vc) are presented. Experiments were conducted on healthy volunteers at 3T to examine the effects of unbalanced diffusion weighting, cerebrospinal (CSF) contamination and variation of Vc.

RESULTS

Phantom results of the used VS pulse trains demonstrated robustness to eddy currents. The mean CBV values of gray matter and white matter for the experiments using Vc = 3.5 mm/s and velocity-compensated control with CSF-nulling were 5.1 ± 0.6 mL/100 g and 2.4 ± 0.2 mL/100 g, respectively, which were 23% and 32% lower than results from the experiment with velocity-insensitive control, corresponding to 29% and 25% lower in averaged temporal signal-to-noise ratio values.

CONCLUSION

A novel technique using VS pulse trains was demonstrated for CBV mapping. The results were both qualitatively and quantitatively close to those from existing methods. Magn Reson Med 77:92-101, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Feasibility of Quantifying Arterial Cerebral Blood Volume Using Multiphase Alternate Ascending/Descending Directional Navigation (ALADDIN)

Arterial cerebral blood volume (aCBV) is associated with many physiologic and pathologic conditions. Recently, multiphase balanced steady state free precession (bSSFP) readout was introduced to measure labeled blood signals in the arterial compartment, based on the fact that signal difference between labeled and unlabeled blood decreases with the number of RF pulses that is affected by blood velocity. In this study, we evaluated the feasibility of a new 2D inter-slice bSSFP-based arterial spin labeling (ASL) technique termed, alternate ascending/descending directional navigation (ALADDIN), to quantify aCBV using multiphase acquisition in six healthy subjects. A new kinetic model considering bSSFP RF perturbations was proposed to describe the multiphase data and thus to quantify aCBV. Since the inter-slice time delay (TD) and gap affected the distribution of labeled blood spins in the arterial and tissue compartments, we performed the experiments with two TDs (0 and 500 ms) and two gaps (300% and 450% of slice thickness) to evaluate their roles in quantifying aCBV. Comparison studies using our technique and an existing method termed arterial volume using arterial spin tagging (AVAST) were also separately performed in five subjects. At 300% gap or 500-ms TD, significant tissue perfusion signals were demonstrated, while tissue perfusion signals were minimized and arterial signals were maximized at 450% gap and 0-ms TD. ALADDIN has an advantage of visualizing bi-directional flow effects (ascending/descending) in a single experiment. Labeling efficiency (α) of inter-slice blood flow effects could be measured in the superior sagittal sinus (SSS) (20.8±3.7%.) and was used for aCBV quantification. As a result of fitting to the proposed model, aCBV values in gray matter (1.4-2.3 mL/100 mL) were in good agreement with those from literature. Our technique showed high correlation with AVAST, especially when arterial signals were accentuated (i.e., when TD = 0 ms) (r = 0.53). The bi-directional perfusion imaging with multiphase ALADDIN approach can be an alternative to existing techniques for quantification of aCBV.

Whole-brain perfusion imaging with balanced steady-state free precession arterial spin labeling

Recently, balanced steady-state free precession (bSSFP) readout has been proposed for arterial spin labeling (ASL) perfusion imaging to reduce susceptibility artifacts at a relatively high spatial resolution and signal-to-noise ratio (SNR). However, the main limitation of bSSFP-ASL is the low spatial coverage. In this work, methods to increase the spatial coverage of bSSFP-ASL are proposed for distortion-free, high-resolution, whole-brain perfusion imaging. Three strategies of (i) segmentation, (ii) compressed sensing (CS) and (iii) a hybrid approach combining the two methods were tested to increase the spatial coverage of pseudo-continuous ASL (pCASL) with three-dimensional bSSFP readout. The spatial coverage was increased by factors of two, four and six using each of the three approaches, whilst maintaining the same total scan time (5.3 min). The number of segments and/or CS acceleration rate (R) correspondingly increased to maintain the same bSSFP readout time (1.2 s). The segmentation approach allowed whole-brain perfusion imaging for pCASL-bSSFP with no penalty in SNR and/or total scan time. The CS approach increased the spatial coverage of pCASL-bSSFP whilst maintaining the temporal resolution, with minimal impact on the image quality. The hybrid approach provided compromised effects between the two methods. Balanced SSFP-based ASL allows the acquisition of perfusion images with wide spatial coverage, high spatial resolution and SNR, and reduced susceptibility artifacts, and thus may become a good choice for clinical and neurological studies. Copyright © 2015 John Wiley & Sons, Ltd.

Time-resolved noncontrast enhanced 4-D dynamic magnetic resonance angiography using multibolus TrueFISP-based spin tagging with alternating radiofrequency (TrueSTAR)

PURPOSE

The goal of this study was to introduce a new noncontrast enhanced 4D dynamic MR angiography (dMRA) technique termed multibolus TrueFISP-based spin tagging with alternating radiofrequency (TrueSTAR).

METHODS

Multibolus TrueFISP-based spin tagging with alternating radiofrequency was developed by taking advantage of the phenomenon that the steady-state signal of TrueFISP is minimally disturbed by periodically inserted magnetization preparations (e.g., spin tagging) that are sandwiched by two α/2 RF pulses. Both theoretical analysis and experimental studies were carried out to optimize the proposed method which was compared with both pulsed and pseudo-continuous arterial spin labeling-based dMRA in healthy volunteers. Optimized multibolus dMRA was also applied in a patient with arteriovenous malformation to demonstrate its potential clinical utility.

RESULTS

Multibolus dMRA offered a prolonged tagging bolus compared to the standard single-bolus dMRA, and allowed improved visualization of the draining veins in the arteriovenous malformation patient. Compared to pseudo-continuous arterial spin labeling-based dMRA, multibolus dMRA provided visualization of the full passage of the labeled blood with the flexibility for both static and dynamic magnetic resonance angiography.

CONCLUSION

By combining the benefits of pulsed and pseudo-continuous arterial spin labeling-based dMRA, multibolus TrueFISP-based spin tagging with alternating radiofrequency can prolong and enhance the tagging bolus without sacrificing imaging speed or temporal resolution.

Assessing intracranial vascular compliance using dynamic arterial spin labeling

Vascular compliance (VC) is an important marker for a number of cardiovascular diseases and dementia, which is typically assessed in the central and peripheral arteries indirectly by quantifying pulse wave velocity (PWV), and/or pulse pressure waveform. To date, very few methods are available for the quantification of intracranial VC. In the present study, a novel MRI technique for in-vivo assessment of intracranial VC was introduced, where dynamic arterial spin labeling (ASL) scans were synchronized with the systolic and diastolic phases of the cardiac cycle. VC is defined as the ratio of change in arterial cerebral blood volume (ΔCBV) and change in arterial pressure (ΔBP). Intracranial VC was assessed in different vascular components using the proposed dynamic ASL method. Our results show that VC mainly occurs in large arteries, and gradually decreases in small arteries and arterioles. The comparison of intracranial VC between young and elderly subjects shows that aging is accompanied by a reduction of intracranial VC, in good agreement with the literature. Furthermore, a positive association between intracranial VC and cerebral perfusion measured using pseudo-continuous ASL with 3D GRASE MRI was observed independent of aging effects, suggesting loss of VC is associated with a decline in perfusion. Finally, a significant positive correlation between intracranial and central (aortic arch) VC was observed using an ungated phase-contrast 1D projection PWV technique. The proposed dynamic ASL method offers a promising approach for assessing intracranial VC in a range of cardiovascular diseases and dementia.

Physiological and Functional Magnetic Resonance Imaging Using Balanced Steady-state Free Precession

Balanced steady-state free precession (bSSFP) is a highly efficient pulse sequence that is known to provide the highest signal-to-noise ratio per unit time. Recently, bSSFP is getting increasingly popular in both the research and clinical communities. This review will be focusing on the application of the bSSFP technique in the context of probing the physiological and functional information. In the first part of this review, the basic principles of bSSFP are briefly covered. Afterwards, recent developments related to the application of bSSFP, in terms of physiological and functional imaging, are introduced and reviewed. Despite its long development history, bSSFP is still a promising technique that has many potential benefits for obtaining high-resolution physiological and functional images.

Arterial cerebral blood volume-weighted functional MRI using pseudocontinuous arterial spin tagging (AVAST)

PURPOSE

Neurovascular regulation, including responses to neural activation that give rise to the blood oxygenation level-dependent (BOLD) effect, occurs mainly at the arterial and arteriolar level. The purpose of this study is to develop a framework for fast imaging of arterial cerebral blood volume (aCBV) signal suitable for functional imaging studies.

METHODS

A variant of the pseudocontinuous arterial spin tagging technique was developed in order to achieve a contrast that depends on aCBV with little contamination from perfusion signal by taking advantage of the kinetics of the tag through the vasculature. This technique tailors the tagging duration and repetition time for each subject. The proposed technique, called AVAST, is compared empirically with BOLD imaging and standard (perfusion-weighted) arterial spin labeling (ASL) technique, in a motor-visual activation paradigm.

RESULTS

The average Z-scores in the activated area obtained over all the subjects were 4.25, 5.52, and 7.87 for standard ASL, AVAST, and BOLD techniques, respectively. The aCBV contrast obtained from AVAST provided 80% higher average signal-to-noise ratio and 95% higher average contrast-to-noise ratio compared with that of the standard ASL measurements.

CONCLUSION

AVAST exhibits improved activation detection sensitivity and temporal resolution over the standard ASL technique, in functional MRI experiments, while preserving its quantitative nature and statistical advantages. AVAST particularly could be useful in clinical studies of pathological conditions, longitudinal studies of cognitive function, and studies requiring sustained periods of the condition.

Noncontrast enhanced four-dimensional dynamic MRA with golden angle radial acquisition and K-space weighted image contrast (KWIC) reconstruction

PURPOSE

To explore the feasibility of 2D and 3D golden-angle radial acquisition strategies in conjunction with k-space weighted image contrast (KWIC) temporal filtering to achieve noncontrast enhanced dynamic MRA (dMRA) with high spatial resolution, low streaking artifacts and high temporal fidelity.

METHODS

Simulations and in vivo examinations in eight normal volunteers and an arteriovenous malformation patient were carried out. Both 2D and 3D golden angle radial sequences, preceded by spin tagging, were used for dMRA of the brain. The radial dMRA data were temporally filtered using the KWIC strategy and compared with matched standard Cartesian techniques.

RESULTS

The 2D and 3D dynamic MRA image series acquired with the proposed radial techniques demonstrated excellent image quality without discernible temporal blurring compared with standard Cartesian based approaches. The image quality of radial dMRA was equivalent to or higher than that of Cartesian dMRA by visual inspection. A reduction factor of up to 10 and 3 in scan time was achieved for 2D and 3D radial dMRA compared with the Cartesian-based counterparts.

CONCLUSION

The proposed 2D and 3D radial dMRA techniques demonstrated image quality comparable or even superior to those obtained with standard Cartesian methods, but within a fraction of the scan time.

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.

Feasibility of MR perfusion-weighted imaging by use of a time-spatial labeling inversion pulse

Perfusion-weighted imaging (PWI) by use of arterial spin labeling (ASL) has been introduced to the clinical setting. However, it is not widely available because it requires specialized pulse sequences. Imaging using a time-spatial labeling inversion pulse (time-SLIP), which is a magnetic resonance angiography (MRA) technique that is based on ASL, can be used in various situations. In this study, we examined the feasibility of time-SLIP PWI. Two types of time-SLIP sequences were evaluated: (1) a single inversion recovery (IR) pulse sequence, which is the same as that used in conventional time-SLIP MRA except for the timing of data acquisition, and (2) a dual IR pulse sequence, where a second, non-selective, IR pulse was added during the inflow time to suppress background signals. Subtraction processing is performed between the "on" and "off" settings of the first IR pulse (time-SLIP tag) to obtain PWI. The average signal intensity was measured in a uniform phantom as the residual of the background, and in five healthy subjects as the perfusion signal. The average signal-to-noise ratio (SNR) was also measured in the five subjects. All imaging was performed with a 1.5-T MR scanner. Images using the dual IR method showed lower background signals and higher perfusion signals compared with images using the single IR method. However, the SNR was lower in images with the dual IR method. These results demonstrate that a time-SLIP, which is an MRA method, can be used for obtaining cerebral PWI simply by adjusting the imaging parameters.