Absolute arterial cerebral blood volume quantification using inflow vascular-space-occupancy with dynamic subtraction magnetic resonance imaging

On the optimization of 3D inflow-based vascular-space-occupancy (iVASO) MRI for the quantification of arterial cerebral blood volume (CBVa)

PURPOSE

The inflow-based vascular-space-occupancy (iVASO) MRI was originally developed in a single-slice mode to measure arterial cerebral blood volume (CBVa). When vascular crushers are applied in iVASO, the signals can be sensitized predominantly to small pial arteries and arterioles. The purpose of this study is to perform a systematic optimization and evaluation of a 3D iVASO sequence on both 3 T and 7 T for the quantification of CBVa values in the human brain.

METHODS

Three sets of experiments were performed in three separate cohorts. (1) 3D iVASO MRI protocols were compared to single-slice iVASO, and the reproducibility of whole-brain 3D iVASO MRI was evaluated. (2) The effects from different vascular crushers in iVASO were assessed. (3) 3D iVASO MRI results were evaluated in arterial and venous blood vessels identified using ultrasmall-superparamagnetic-iron-oxides-enhanced MRI to validate its arterial origin.

RESULTS

3D iVASO scans showed signal-to-noise ratio (SNR) and CBVa measures consistent with single-slice iVASO with reasonable intrasubject reproducibility. Among the iVASO scans performed with different vascular crushers, the whole-brain 3D iVASO scan with a motion-sensitized-driven-equilibrium preparation with two binomial refocusing pulses and an effective TE of 50 ms showed the best suppression of macrovascular signals, with a relatively low specific absorption rate. When no vascular crusher was applied, the CBVa maps from 3D iVASO scans showed large CBVa values in arterial vessels but well-suppressed signals in venous vessels.

CONCLUSION

A whole-brain 3D iVASO MRI scan was optimized for CBVa measurement in the human brain. When only microvascular signals are desired, a motion-sensitized-driven-equilibrium-based vascular crusher with binomial refocusing pulses can be applied in 3D iVASO.

Cerebral microcirculatory pulse wave propagation and pulse wave amplitude mapping in retrospectively gated MRI

To analyze cerebral arteriovenous pulse propagation and to generate phase-resolved pulse amplitude maps from a fast gradient-echo sequence offering flow-related enhancement (FREE). Brain MRI was performed using a balanced steady-state free precession sequence at 3T followed by retrospective k-space gating. The time interval of the pulse wave between anterior-, middle- and posterior cerebral artery territories and the superior sagittal sinus were calculated and compared between and older and younger groups within 24 healthy volunteers. Pulse amplitude maps were generated and compared to pseudo-Continuous Arterial Spin Labeling (pCASL) MRI maps by voxel-wise Pearson correlation, Sørensen-Dice maps and in regards to signal contrast. The arteriovenous delays between all vascular territories and the superior sagittal sinus were significantly shorter in the older age group (11 individuals, ≥ 31 years) ranging between 169 ± 112 and 246 ± 299 ms versus 286 ± 244 to 419 ± 299 ms in the younger age group (13 individuals) (P ≤ 0.04). The voxel-wise pulse wave amplitude values and perfusion-weighted pCASL values correlated significantly (Pearson-r = 0.33, P < 0.01). Mean Dice overlaps of high (gray) and low (white matter) regions were 73 ± 3% and 59 ± 5%. No differences in image contrast were seen in the whole brain and the white matter, but significantly higher mean contrast of 0.73 ± 0.23% in cortical gray matter in FREE-MRI compared to 0.52 ± 0.12% in pCASL-MRI (P = 0.01). The dynamic information of flow-related enhancement allows analysis of the cerebral pulse wave propagation potentially providing information about the (micro)circulation on a regional level. However, the pulse wave amplitude reveals weaknesses in comparison to true perfusion-weighting and could rather be used to calculate a pulsatility index.

Vascular space occupancy asymmetric spin echo (VASO-ASE) for non-invasive quantification of cerebral oxygen extraction fraction

PURPOSE

Asymmetric spin echo (ASE) MRI is a method for measuring regional oxygen extraction fraction (OEF); however, extravascular tissue models have been shown to under-estimate OEF. The hypothesis investigated here is that the addition of a vascular-space-occupancy (VASO) pre-pulse will more fully suppress blood water signal and provide global OEF values more consistent with physiological expectation and 15 O positron emission tomography (PET)-validated T2 -relaxation-under-spin-tagging (TRUST) OEF measures.

METHODS

Healthy adults (n = 14; age = 27.7 ± 5.2 y; sex = 7/7 male/female) were scanned at 3.0T. Multi-echo ASE without inter-readout refocusing (ASERF- ), multi-echo ASE with inter-readout refocusing (ASERF+ ), and single-echo VASO-ASE were acquired twice each with common spatial resolution = 3.44 × 3.44 × 3.0 mm and τ = 0-20 ms (interval = 0.5 ms). TRUST was acquired twice sequentially for independent global OEF assessment (τCPMG  = 10 ms; effective TEs = 0, 40, 80, and 160 ms; spatial resolution = 3.4 × 3.4 × 5 mm). OEF intraclass-correlation-coefficients (ICC), summary statistics, and group-wise differences were assessed (Wilcoxon rank-sum; significance: two-sided p < 0.05).

RESULTS

ASERF+ (OEF = 36.8 ± 1.9%) and VASO-ASE (OEF = 34.4 ± 2.3%) produced OEF values similar to TRUST (OEF = 36.5 ± 4.6%, human calibration model; OEF = 32.7 ± 4.9%, bovine calibration model); however, ASERF- yielded lower OEF (OEF = 26.1 ± 1.0%; p < 0.01) relative to TRUST. VASO-ASE (ICC = 0.61) yielded lower ICC compared to other ASE variants (ICC >0.89).

CONCLUSION

VASO-ASE and TRUST provide similar OEF values; however, VASO-ASE spatial coverage and repeatability improvements are required.

Separating spin compartments in arterial spin labeling using delays alternating with nutation for tailored excitation (DANTE) pulse: A validation study using T2 -relaxometry and application to arterial cerebral blood volume imaging

PURPOSE

To clarify the type of spin compartment in arterial spin labeling (ASL) that is eliminated by delays alternating with nutation for tailored excitation (DANTE) pulse using T₂ -relaxometry, and to demonstrate the feasibility of arterial cerebral blood volume (CBV_a ) imaging using DANTE-ASL in combination with a simplified two-compartment model.

METHOD

The DANTE and T₂ -preparation modules were combined into a single ASL sequence. T₂ values under the application of DANTE were determined to evaluate changes in T₂ , along with the post-labeling delay (PLD) and the relationship between transit time without DANTE (TT_noVS ) and T₂ . The reference tissue T₂ (T_{2_ref} ) was also obtained. Subsequently, the DANTE module was embedded into the Hadamard-encoded ASL. Cerebral blood flow (CBF) and CBV_a were computed using two Hadamard-encoding datasets (with and without DANTE) in a rest and breath-holding (BH) task.

RESULTS

While T₂ without DANTE (T_{2_noVS} ) decreased as the PLD increased, T₂ with DANTE (T_{2_DANTE} ) was equivalent to T_{2_ref} and did not change with the PLD. Although there was a significant positive correlation between TT_noVS and T_{2_noVS} with short PLD, T_{2_DANTE} was not correlated with TT_noVS nor PLD. Baseline CBV_a values obtained at rest were 0.64 ± 0.12, 0.64 ± 0.11, and 0.58 ± 0.15 mL/100 g for anterior, middle, and posterior cerebral arteries, respectively. Significant CBF and CBV_a elevations were observed in the BH task.

CONCLUSION

Microvascular compartment signals were eliminated from the total ASL signals by DANTE. CBV_a can be measured using Hadamard-encoded DANTE-ASL in combination with a simplified two-compartment model.

Differential detection of metastatic and inflammatory lymph nodes using inflow-based vascular-space-occupancy (iVASO) MR imaging

PURPOSE

To investigate the potential value of inflow-based vascular-space-occupancy (iVASO) MR imaging in differentiating metastatic from inflammatory lymph nodes (LNs).

METHODS

Ten female New Zealand rabbits with 2.5-3.0 kg body weight were studied. VX2 cells and egg yolk emulsion were inoculated into left and right thighs, respectively, to induce ten metastatic and ten inflammatory popliteal LNs. Conventional MRI and iVASO were performed 2 h prior to, and 10, 20 days after inoculation (D0, D10, D20). The short-axis diameter (S), short- to long-axis diameter ratio (SLR), and arteriolar blood volume (BVa) at each time point and their longitudinal changes of each model were recorded and compared. At D20, all rabbits were sacrificed to perform histological evaluation after the MR scan.

RESULTS

The mean values of S, SLR and BVa showed no significant difference between the two groups at D0 (P = 0.987, P = 0.778, P = 0.975). The BVa of the metastatic group was greater than that of the inflammatory at both D10 and D20 (P < 0.05; P < 0.001), whereas the S and SLR of the metastatic group were greater only at D20 (P < 0.001; P = 0.001). Longitudinal analyses showed that the BVa of the metastatic group increased at both D10 and D20 (P = 0.004; P = 0.001), while that of the inflammatory group only increased at D10 (P = 0.024).

CONCLUSION

The BVa measured with iVASO has the potential to detect early metastatic LNs.

Non-Invasive Evaluation of Cerebral Microvasculature Using Pre-Clinical MRI: Principles, Advantages and Limitations

Alterations to the cerebral microcirculation have been recognized to play a crucial role in the development of neurodegenerative disorders. However, the exact role of the microvascular alterations in the pathophysiological mechanisms often remains poorly understood. The early detection of changes in microcirculation and cerebral blood flow (CBF) can be used to get a better understanding of underlying disease mechanisms. This could be an important step towards the development of new treatment approaches. Animal models allow for the study of the disease mechanism at several stages of development, before the onset of clinical symptoms, and the verification with invasive imaging techniques. Specifically, pre-clinical magnetic resonance imaging (MRI) is an important tool for the development and validation of MRI sequences under clinically relevant conditions. This article reviews MRI strategies providing indirect non-invasive measurements of microvascular changes in the rodent brain that can be used for early detection and characterization of neurodegenerative disorders. The perfusion MRI techniques: Dynamic Contrast Enhanced (DCE), Dynamic Susceptibility Contrast Enhanced (DSC) and Arterial Spin Labeling (ASL), will be discussed, followed by less established imaging strategies used to analyze the cerebral microcirculation: Intravoxel Incoherent Motion (IVIM), Vascular Space Occupancy (VASO), Steady-State Susceptibility Contrast (SSC), Vessel size imaging, SAGE-based DSC, Phase Contrast Flow (PC) Quantitative Susceptibility Mapping (QSM) and quantitative Blood-Oxygenation-Level-Dependent (qBOLD). We will emphasize the advantages and limitations of each strategy, in particular on applications for high-field MRI in the rodent's brain.

Impaired response of cerebral oxygen metabolism to visual stimulation in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease caused by a CAG triplet repeat expansion in the Huntingtin gene. Metabolic and microvascular abnormalities in the brain may contribute to early physiological changes that subserve the functional impairments in HD. This study is intended to investigate potential abnormality in dynamic changes in cerebral blood volume (CBV) and cerebral blood flow (CBF), and cerebral metabolic rate of oxygen (CMRO2) in the brain in response to functional stimulation in premanifest and early manifest HD patients. A recently developed 3-D-TRiple-acquisition-after-Inversion-Preparation magnetic resonance imaging (MRI) approach was used to measure dynamic responses in CBV, CBF, and CMRO2 during visual stimulation in one single MRI scan. Experiments were conducted in 23 HD patients and 16 healthy controls. Decreased occipital cortex CMRO2 responses were observed in premanifest and early manifest HD patients compared to controls (P < 0.001), correlating with the CAG-Age Product scores in these patients (R2 = 0.4, P = 0.001). The results suggest the potential value of this reduced CMRO2 response during visual stimulation as a biomarker for HD and may illuminate the role of metabolic alterations in the pathophysiology of HD.

Decreased Muscular Perfusion in Dermatomyositis: Initial Results Detected by Inflow-Based Vascular-Space-Occupancy MRI

OBJECTIVE. This study aimed to determine whether inflow-based vascular-space-occupancy (iVASO) MRI could reproducibly quantify skeletal muscle perfusion and differentiate patients with dermatomyositis (DM) from healthy subjects. MATERIALS AND METHODS. A total of 25 patients with DM and 22 healthy volunteers underwent iVASO MRI in a 3-T MRI scanner. Maximum and mean arteriolar muscle blood volume (MBV) values of four subgroups of muscles (normal muscles, morphologically normal-appearing muscles, edematous muscles, and atrophic or fat-infiltrated muscles) were obtained. Maximum and mean arteriolar MBV values were compared among the different subgroups, and repeat testing was performed in 20 subjects to assess reproducibility. RESULTS. Compared with normal muscles in healthy subjects, morphologically normal-appearing muscles, edematous muscles, and atrophic or fat-infiltrated muscles in patients with DM showed a significant decrease of both maximum and mean arteriolar MBV (p < .001). Both parameters were significantly lower in atrophic or fat-infiltrated muscles than in morphologically normal-appearing and edematous muscles (p < .001). ROC AUCs for discriminating patients with DM from healthy volunteers were 0.842 and 0.812 for maximum and mean arteriolar MBV values, respectively. As a measure of test-retest studies, the intraclass correlation coefficients (ICCs) were 0.990 (95% CI, 0.986-0.993) and 0.990 (95% CI, 0.987-0.993) for maximum and mean arteriolar MBV, respectively. For interobserver reproducibility, the ICCs were 0.989 (95% CI, 0.986-0.991) and 0.980 (95% CI, 0.975-0.983), respectively. CONCLUSION. iVASO MRI can reproducibly quantify arteriolar MBV in the thigh and discriminate between healthy volunteers and patients with DM.

The Added Value of Inflow-Based Vascular-Space-Occupancy and Diffusion-Weighted Imaging in Preoperative Grading of Gliomas

OBJECTIVES

The present study aimed to study whether combined inflow-based vascular-space-occupancy (iVASO) MR imaging (MRI) and diffusion-weighted imaging (DWI) improve the diagnostic accuracy in the preoperative grading of gliomas.

METHODS

Fifty-one patients with histopathologically confirmed diffuse gliomas underwent preoperative structural MRI, iVASO, and DWI. We performed 2 qualitative consensus reviews: (1) structural MR images alone and (2) structural MR images with iVASO and DWI. Relative arteriolar cerebral blood volume (rCBVa) and minimum apparent diffusion coefficient (mADC) were compared between low-grade and high-grade gliomas. Receiver operating characteristic (ROC) curve analysis was performed to compare the tumor grading efficiency of rCBVa, mADC, and the combination of the two parameters.

RESULTS

Two observers diagnosed accurate tumor grade in 40 of 51 (78.4%) patients in the first review and in 46 of 51 (90.2%) in the second review. Both rCBVa and mADC showed significant differences between low-grade and high-grade gliomas. ROC analysis gave a threshold value of 1.52 for rCBVa and 0.85 × 10-3 mm2/s for mADC to provide a sensitivity and specificity of 88.0 and 81.2% and 100.0 and 68.7%, respectively. The area under the ROC curve (AUC) was 0.87 and 0.85 for rCBVa and mADC, respectively. The combination of rCBVa and mADC values increased the AUC to 0.92.

CONCLUSION

The combined application of iVASO and DWI may improve the diagnostic accuracy of glioma grading.

High-resolution magnetic resonance imaging-based atlases for the young and adolescent domesticated pig (Sus scrofa)

BACKGROUND

Neurodevelopmental studies utilize the pig as a translational animal model due to anatomical and morphological similarities between the pig and human brain. However, neuroimaging resources are not as well developed for the pig as they are for humans and other animal models. We established a magnetic resonance imaging-based brain atlas at two different ages for biomedical studies utilizing the pig as a preclinical model.

NEW METHOD

Twenty artificially-reared domesticated male pigs (Sus scrofa) and thirteen sow-reared adolescent domesticated male pigs (Sus scrofa) underwent a series of scans measuring brain macrostructure, microstructure, and arterial cerebral blood volume.

RESULTS

An atlas for the 4-week-old and 12-week-old pig were created along with twenty-six regions of interest. Normative data for brain measures were obtained and detailed descriptions of the data processing pipelines were provided.

COMPARISON WITH EXISTING METHOD

Atlases at the two different ages were created for the pig utilizing newer imaging technology and software. This facilitates the performance of longitudinal studies and enables more precise volume measurements in pigs of various ages by appropriately representing the neuroanatomical features of younger and older pigs and accommodating the proportion differences of the brain over time.

CONCLUSION

Two high-resolution MRI brain atlases specific to the domesticated young and adolescent pig were created using defined image acquisition and data processing methods to facilitate the generation of high-quality normative data for neurodevelopmental research.

Inflow-based vascular-space-occupancy (iVASO) might potentially predict IDH mutation status and tumor grade in diffuse cerebral gliomas

PURPOSE

The aim of the study is to assess the diagnostic performance of inflow-based vascular-space-occupancy (iVASO) MR imaging for differentiating glioblastomas (grade IV, GBM) and lower-grade diffuse gliomas (grade II and III, LGG) and its potential to predict IDH mutation status.

METHODS

One hundred and two patients with diffuse cerebral glioma (56 males; median age, 43.5 years) underwent iVASO and dynamic susceptibility contrast (DSC) MR imaging. The iVASO-derived arteriolar cerebral blood volume (CBVa), relative CBVa (rCBVa), and the DSC-derived relative cerebral blood volume (rCBV) were obtained, and these measurements were compared between the GBM group (n = 43) and the LGG group (n = 59) and between the IDH-mutation group (n = 54) and the IDH-wild group (n = 48).

RESULTS

Significant correlation was observed between rCBV and CBVa (P < 0.001) or rCBVa (P < 0.001). Both CBVa (P < 0.001) and rCBVa (P < 0.001) were higher in the GBM group. Both CBVa (P < 0.001) and rCBVa (P < 0.001) were lower in the IDH-mutation group compared to the IDH-wild group. Receiver operating characteristic analyses showed the area under curve (AUC) of 0.95 with CBVa and 0.97 with rCBVa in differentiating GBM from LGG. The AUCs were 0.82 and 0.85 for CBVa and rCBVa in predicting IDH gene status, respectively, which were lower than that of rCBV (AUC = 0.90). Combined rCBV and rCBVa significantly improved the diagnostic performance (AUC = 0.95).

CONCLUSIONS

iVASO MR imaging has the potential to predict IDH mutation and grade in glioma.

Differential Changes in Arteriolar Cerebral Blood Volume between Parkinson's Disease Patients with Normal and Impaired Cognition and Mild Cognitive Impairment (MCI) Patients without Movement Disorder - An Exploratory Study

Cognitive impairment amongst Parkinson's disease (PD) patients is highly prevalent and associated with an increased risk of dementia. There is growing evidence that altered cerebrovascular functions contribute to cognitive impairment. Few studies have compared cerebrovascular changes in PD patients with normal and impaired cognition and those with mild-cognitive-impairment (MCI) without movement disorder. Here, we investigated arteriolar-cerebral-blood-volume (CBVa), an index reflecting the homeostasis of the most actively regulated segment in the microvasculature, using advanced MRI in various brain regions in PD and MCI patients and matched controls. Our goal is to find brain regions with altered CBVa that are specific to PD with normal and impaired cognition, and MCI-without-movement-disorder, respectively. In PD patients with normal cognition (n=10), CBVa was significantly decreased in the substantia nigra, caudate and putamen when compared to controls. In PD patients with impaired cognition (n=6), CBVa showed a decreasing trend in the substantia nigra, caudate and putamen, but was significantly increased in the presupplementary motor area and intracalcarine gyrus compared to controls. In MCI-patients-without-movement-disorder (n=18), CBVa was significantly increased in the caudate, putamen, hippocampus and lingual gyrus compared to controls. These findings provide important information for efforts towards developing biomarkers for the evaluation of potential risk of PD dementia (PDD) in PD patients. The current study is limited in sample size and therefore is exploratory in nature. The data from this pilot study will serve as the basis for power analysis for subsequent studies to further investigate and validate the current findings.

Quantitative transport mapping (QTM) of the kidney with an approximate microvascular network

Purpose

Proof‐of‐concept study of mapping renal blood flow vector field according to the inverse solution to a mass transport model of time resolved tracer‐labeled MRI data.

Theory and Methods

To determine tissue perfusion according to the underlying physics of spatiotemporal tracer concentration variation, the mass transport equation is integrated over a voxel with an approximate microvascular network for fitting time‐resolved tracer imaging data. The inverse solution to the voxelized transport equation provides the blood flow vector field, which is referred to as quantitative transport mapping (QTM). A numerical microvascular network modeling the kidney with computational fluid dynamics reference was used to verify the accuracy of QTM and the current Kety’s method that uses a global arterial input function. Multiple post‐label delay arterial spin labeling (ASL) of the kidney on seven subjects was used to assess QTM in vivo feasibility.

Results

Against the ground truth in the numerical model, the error in flow estimated by QTM (18.6%) was smaller than that in Kety’s method (45.7%, 2.5‐fold reduction). The in vivo kidney perfusion quantification by QTM (cortex: 443 ± 58 mL/100 g/min and medulla: 190 ± 90 mL/100 g/min) was in the range of that by Kety’s method (482 ± 51 mL/100 g/min in the cortex and 242 ± 73 mL/100 g/min in the medulla), and QTM provided better flow homogeneity in the cortex region.

Conclusions

QTM flow velocity mapping is feasible from multi‐delay ASL MRI data based on inverting the transport equation. In a numerical simulation, QTM with deconvolution in space and time provided more accurate perfusion quantification than Kety’s method with deconvolution in time only.

Mutant G2019S-LRRK2 Induces Abnormalities in Arteriolar Cerebral Blood Volume in Mouse Brains: An MRI Study

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease and the most common movement disorder characterized by motor impairments resulting from midbrain dopamine neuron loss. Abnormalities in small pial arteries and arterioles, which are the primary pathways of local delivery of nutrients and oxygen in brain tissue, have been reported in many neurodegenerative diseases including PD. Mutations in LRRK2 cause genetic PD and contribute to sporadic PD. The most common PD-linked mutation LRRK2 G2019S contributes 20-47% of genetic forms of PD in Caucasian populations. The human LRRK2 G2019S transgenic mouse model displays PD-like movement impairment and was used to identify novel LRRK2 inhibitors, which provides a useful model for studying microvascular abnormalities in PD.

OBJECTIVES

To investigate abnormalities in arteriolar cerebral blood volume (CBVa) in various brain regions using the inflow-based vascular-space occupancy (iVASO) MRI technique in LRRK2 mouse models of PD.

METHODS

Anatomical and iVASO MRI scans were performed in 5 female and 7 male nontransgenic (nTg), 3 female and 4 male wild-type (WT) LRRK2, and 5 female and 7 male G2019S-LRRK2 mice of 9 months of age. CBVa was calculated and compared in the substantia nigra (SN), olfactory cortex, and prefrontal cortex.

RESULTS

Compared to nTg mice, G2019S-LRRK2 mice showed decreased CBVa in the SN, but increased CBVa in the olfactory and prefrontal cortex in both male and female groups, whereas WT-LRRK2 mice showed no change in CBVa in the SN (male and female), the olfactory (female), and prefrontal (female) cortex, but a slight increase in CBVa in the olfactory and prefrontal cortex in the male group only.

CONCLUSIONS

Alterations in the blood volume of small arteries and arterioles (CBVa) were detected in the G2019S-LRRK2 mouse model of PD. The opposite changes in CBVa in the SN and the cortex indicate that PD pathology may have differential effects in different brain regions. Our results suggest the potential value of CBVa as a marker for clinical PD studies.

Association of Glioma Grading With Inflow-Based Vascular-Space-Occupancy MRI: A Preliminary Study at 3T

BACKGROUND

Inflow-based vascular-space-occupancy (iVASO) MRI is a noninvasive perfusion technique that does not require administration of exogenous contrast agents. Arteriolar cerebral blood volume (CBVa) obtained from iVASO MRI is hypothesized to be an indicator of tumor microvasculature.

PURPOSE

To assess the diagnostic performance of iVASO MRI implemented at 3T in predicting histologic grades of cerebral gliomas.

STUDY TYPE

Retrospective.

SUBJECTS

Forty-five patients (31 males) consisting of 14 WHO grade IV glioblastoma multiformes, 14 grade III, and 17 grade II gliomas.

FIELD STRENGTH/SEQUENCE

At 3T we acquired CBVa data using an iVASO sequence.

ASSESSMENT

The maximum and mean CBVa (CBVa_max and CBVa_mean) values were calculated in the tumor and normalized to the contralateral thalamus (nCBVa_max and nCBVa_mean).

STATISTICAL TESTS

Kruskal-Wallis test, Mann-Whitney test, and receiver operating characteristics (ROC) curve were used for statistical analysis.

RESULTS

Both CBVa_max and nCBVa_max increased with tumor grade (P < 0.001). Grade II gliomas showed CBVa_max <0.78 ml / 100 ml in 10/17 cases and nCBVa_max <1.20 in 11/17 cases. Grade III gliomas showed both CBVa_max >0.78 ml / 100 ml and nCBVa_max >1.20 in 13/14 cases, and CBVa_max <2.06 ml / 100 ml in 13/14 cases and nCBVa_max <2.33 in 11/14 cases. Grade IV gliomas showed CBVa_max >2.06 ml / 100 ml in 9/14 cases and nCBVa_max >2.33 in 13/14 cases. The areas under the ROC curve, sensitivity, and specificity were 0.839 (P < 0.001), 92.9% (26/28), and 64.7% (11/17) for CBVa_max, and 0.883 (P < 0.001), 92.9% (26/28), and 70.6% (12/17) for nCBVa_max in the discrimination between grade II and high-grade (grade III and grade IV) tumors, respectively. DATA CONCLUSION: iVASO MRI might be used to help determine and predict glioma grade.

LEVEL OF EVIDENCE

4 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1817-1823.

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.

Increased cerebral blood volume in small arterial vessels is a correlate of amyloid-β-related cognitive decline

The protracted accumulation of amyloid-β (Aβ) is a major pathologic hallmark of Alzheimer's disease and may trigger secondary pathological processes that include neurovascular damage. This study was aimed at investigating long-term effects of Aβ burden on cerebral blood volume of arterioles and pial arteries (CBVa), possibly present before manifestation of dementia. Aβ burden was assessed by 11C Pittsburgh compound-B positron emission tomography in 22 controls and 18 persons with mild cognitive impairment (MCI), [ages: 75(±6) years]. After 2 years, inflow-based vascular space occupancy at ultra-high field strength of 7-Tesla was administered for measuring CBVa, and neuropsychological testing for cognitive decline. Crushing gradients were incorporated during MR-imaging to suppress signals from fast-flowing blood in large arteries, and thereby sensitize inflow-based vascular space occupancy to CBVa in pial arteries and arterioles. CBVa was significantly elevated in MCI compared to cognitively normal controls and regional CBVa related to local Aβ deposition. For both MCI and controls, Aβ burden and follow-up CBVa in several brain regions synergistically predicted cognitive decline over 2 years. Orbitofrontal CBVa was positively associated with apolipoprotein E e4 carrier status. Increased CBVa may reflect long-term effects of region-specific pathology associated with Aβ deposition. Additional studies are needed to clarify the role of the arteriolar system and the potential of CBVa as a biomarker for Aβ-related vascular downstream pathology.

Hemodynamic mechanisms underlying elevated oxygen extraction fraction (OEF) in moyamoya and sickle cell anemia patients

Moyamoya is a bilateral, complex cerebrovascular condition characterized by progressive non-atherosclerotic intracranial stenosis and collateral vessel formation. Moyamoya treatment focuses on restoring cerebral blood flow (CBF) through surgical revascularization, however stratifying patients for revascularization requires abilities to quantify how well parenchyma is compensating for arterial steno-occlusion. Globally elevated oxygen extraction fraction (OEF) secondary to CBF reduction may serve as a biomarker for tissue health in moyamoya patients, as suggested in patients with sickle cell anemia (SCA) and reduced oxygen carrying capacity. Here, OEF was measured (TRUST-MRI) to test the hypothesis that OEF is globally elevated in patients with moyamoya (n = 18) and SCA (n = 18) relative to age-matched controls (n = 43). Mechanisms underlying the hypothesized OEF increases were evaluated by performing sequential CBF-weighted, cerebrovascular reactivity (CVR)-weighted, and structural MRI. Patients were stratified by treatment and non-parametric tests applied to compare study variables (significance: two-sided P < 0.05). OEF was significantly elevated in moyamoya participants (interquartile range = 0.38-0.45) compared to controls (interquartile range = 0.29-0.38), similar to participants with SCA (interquartile range = 0.37-0.45). CBF was inversely correlated with OEF in moyamoya participants. Elevated OEF was only weakly related to reductions in CVR, consistent with basal CBF level, rather than vascular reserve capacity, being most closely associated with OEF.

Effect sizes of BOLD CVR, resting-state signal fluctuations and time delay measures for the assessment of hemodynamic impairment in carotid occlusion patients

Background and purpose

The BOLD signal amplitude as a response to a hypercapnia stimulus is commonly used to assess cerebrovascular reserve. Despite recent advances, the implementation remains cumbersome and alternative ways to assess hemodynamic impairment are desirable. Resting-state BOLD signal fluctuations (rsBOLD) have been proposed however data on its sensitivity and dependence on baseline venous cerebral blood volume (vCBV) is limited. The primary aim of this study was to compare the effect sizes of resting-state and hypercapnia induced BOLD signal changes in the detection of hemodynamic impairment. The second aim of the study was to assess the dependence of BOLD signal variability on vCBV.

Materials and methods

Fifteen patients with internal carotid artery occlusive disease and 15 matched healthy controls were included in this study. The BOLD signal was derived from a dual-echo gradient-echo echo-planar sequence during hypercapnia (HC) and hyperoxia (HO) gas modulations. BOLD (fractional) amplitude of low frequency fluctuations ((f)ALFF) was compared to HC-BOLD, BOLD response delays derived from time delay analysis and ΔBOLD in response to progressively increasing HC. Effect sizes (i.e. the standard mean difference between patients and controls) were calculated. HO-BOLD was used to estimate vCBV, and its contribution to the variability in rsBOLD signal was evaluated.

Results

The effect sizes of ALFF and fALFF (0.61 and 0.72) were lower than the effect sizes related to hypercapnia-based hemodynamic assessment analysis; 1.62, 1.56 and 0.90 for HC-BOLD, BOLD response delays and ΔBOLD in response to progressively increasing HC. A moderate relation was found between (f)ALFF and HC-BOLD in controls (R² of 0.61 and 0.42), but this relation decreased in patients (R² of 0.33 and 0.15). (f)ALFF did not differ between patients and controls whereas HC-BOLD did (p < 0.005). The ΔBOLD response to progressively increasing HC was significantly different in between patients and controls for ΔEtCO₂ values ≥ 2 mmHg (at +2  mmHg F(1, 18) = 5.85, p = 0.026). Up to 31% and 53% of the variance in the ALFF and HC-BOLD spatial distribution could be explained by HO-BOLD.

Conclusion

ALFF and fALFF demonstrated a moderate effect size to detect hemodynamic impairment whereas the effect size was large for methods employing a hypercapnia-based vascular stress stimulus. Based on our analysis of BOLD signal change as a response to a progressively increasing hypercapnia stimulus we can argue that a hypercapnia stimulus of at least 2 mmHg above baseline EtCO₂ is necessary to evaluate hemodynamic impairment. We also demonstrated that a substantial amount of information imbedded in the rsBOLD and HC-BOLD was explained by HO-BOLD. HO-BOLD can serve as a proxy for vCBV and this thus indicates that one should be careful when adopting these techniques in disease cases with compromised CBV.

Noninvasive imaging of cerebral blood volume in piglets with vascular occupancy MR imaging and inflow vascular space occupancy with dynamic subtraction

Accurate quantitative non-invasive assessments of arterial cerebral blood volume (aCBV) can greatly benefit the study of cerebral vascular health in both humans and in animal models. In recent years, progress has been made in the techniques available to quantify CBV with magnetic resonance imaging (MRI). Here, we compared a non-invasive technique, measuring inflowing vascular space occupancy with dynamic subtraction (iVASO-ds) with a contrast-based vascular space occupancy measurement in piglets. In addition, we measured how the iVASO-ds derived aCBV changed with piglet development from 4 weeks to 8 weeks. Our results indicate that there is a significant correlation between the non-invasive iVASO-ds derived aCBV and CBV quantified using a gadolinium contrast agent, despite the contrast-based method providing significantly higher estimates of CBV resulting from challenges inherent to using the contrast-based technique. In addition, it was possible to see significant increases in blood volume across 4 weeks to 8 weeks in pig development with the non-invasive technique. Our results suggest that the non-invasive technique, iVASO-ds can assess aCBV in the developing piglet, both cross-sectionally and longitudinally, and has significant advantages over the contrast-based quantification method.

Fast measurement of blood T1 in the human carotid artery at 3T: Accuracy, precision, and reproducibility

PURPOSE

To develop a fast protocol for measuring T1 values in the internal carotid artery (ICA), to validate this technique with in vitro measurements, and to evaluate its reproducibility.

METHODS

A modified Look-Locker sequence was optimized to enable rapid determination of T1 in the ICA at 3T. T1 values from the ICA were compared with in vitro measurements on individually sampled venous blood oxygenated to arterial levels. A test-retest reproducibility study was also conducted.

RESULTS

The group-averaged arterial blood T1 value was 1908 ± 77 ms for six women (hematocrit = 0.39 ± 0.03) and 1785 ± 55 ms for seven men (hematocrit = 0.45 ± 0.02), which is 100-200 ms longer than the widely adopted value obtained from bovine blood experiments. The arterial T1 value per subject correlated significantly with individual hematocrit values. The intrasession and intersession coefficients of variation were 1.1% and 2.1%, respectively, indicating good precision and reproducibility of our method. Reasonable agreement was observed between the in vivo and in vitro results with a correlation coefficient of 0.78.

CONCLUSION

The proposed method can provide fast arterial T1 measurement on individual subjects. When not performing such a subject-specific measurement, we recommend the use of 1908 ms and 1785 ms for healthy women and men, respectively, or 1841 ms for adults in general. Magn Reson Med 77:2296-2302, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Abnormal Grey Matter Arteriolar Cerebral Blood Volume in Schizophrenia Measured With 3D Inflow-Based Vascular-Space-Occupancy MRI at 7T

Metabolic dysfunction and microvascular abnormality may contribute to the pathogenesis of schizophrenia. Most previous studies of cerebral perfusion in schizophrenia measured total cerebral blood volume (CBV) and cerebral blood flow (CBF) in the brain, which reflect the ensemble signal from the arteriolar, capillary, and venular compartments of the microvasculature. As the arterioles are the most actively regulated blood vessels among these compartments, they may be the most sensitive component of the microvasculature to metabolic disturbances. In this study, we adopted the inflow-based vascular-space-occupancy (iVASO) MRI approach to investigate alterations in the volume of small arterial (pial) and arteriolar vessels (arteriolar cerebral blood volume [CBVa]) in the brain of schizophrenia patients. The iVASO approach was extended to 3-dimensional (3D) whole brain coverage, and CBVa was measured in the brains of 12 schizophrenia patients and 12 matched controls at ultra-high magnetic field (7T). Significant reduction in grey matter (GM) CBVa was found in multiple areas across the whole brain in patients (relative changes of 14%-51% and effect sizes of 0.7-2.3). GM CBVa values in several regions in the temporal cortex showed significant negative correlations with disease duration in patients. GM CBVa increase was also found in a few brain regions. Our results imply that microvascular abnormality may play a role in schizophrenia, and suggest GM CBVa as a potential marker for the disease. Further investigation is needed to elucidate whether such effects are due to primary vascular impairment or secondary to other causes, such as metabolic dysfunction.

Perfusion information extracted from resting state functional magnetic resonance imaging

It is widely known that blood oxygenation level dependent (BOLD) contrast in functional magnetic resonance imaging (fMRI) is an indirect measure for neuronal activations through neurovascular coupling. The BOLD signal is also influenced by many non-neuronal physiological fluctuations. In previous resting state (RS) fMRI studies, we have identified a moving systemic low frequency oscillation (sLFO) in BOLD signal and were able to track its passage through the brain. We hypothesized that this seemingly intrinsic signal moves with the blood, and therefore, its dynamic patterns represent cerebral blood flow. In this study, we tested this hypothesis by performing Dynamic Susceptibility Contrast (DSC) MRI scans (i.e. bolus tracking) following the RS scans on eight healthy subjects. The dynamic patterns of sLFO derived from RS data were compared with the bolus flow visually and quantitatively. We found that the flow of sLFO derived from RS fMRI does to a large extent represent the blood flow measured with DSC. The small differences, we hypothesize, are largely due to the difference between the methods in their sensitivity to different vessel types. We conclude that the flow of sLFO in RS visualized by our time delay method represents the blood flow in the capillaries and veins in the brain.

Measurement of arteriolar blood volume in brain tumors using MRI without exogenous contrast agent administration at 7T

PURPOSE

Arteriolar cerebral-blood-volume (CBVa) is an important perfusion parameter that can be measured using inflow-based vascular-space-occupancy (iVASO) MRI without exogenous contrast agent administration. The purpose of this study is to assess the potential diagnostic value of CBVa in brain tumor patients by comparing it with total-CBV (including arterial, capillary and venous vessels) measured by dynamic-susceptibility-contrast (DSC) MRI.

MATERIALS AND METHODS

Twelve brain tumor patients were scanned using iVASO (on 7T as part of a research project) and DSC (on 3T as part of routine clinical protocols) MRI. Region-of-interest analysis was performed to compare the resulting perfusion measures between tumoral and contralateral regions, and to evaluate their associations with tumor grades.

RESULTS

CBVa measured by iVASO MRI significantly correlated with WHO grade (ρ = 0.37, P = 0.04). Total-CBV measured by DSC MRI showed a trend of correlation with WHO grade (ρ = 0.28, P = 0.5). The signal-to-noise ratio was comparable (P > 0.1) between the two methods, while the contrast-to-noise ratio between tumoral and contralateral regions was higher in iVASO-CBVa than DSC-CBV in WHO II/III patients (P < 0.05) but comparable in WHO IV patients (P > 0.1). A trend of positive correlation between DSC-CBV and iVASO-CBVa was observed (R2 = 0.28, P = 0.07).

CONCLUSION

In this initial patient study, CBVa demonstrated a stronger correlation with WHO grade than total-CBV. Further investigation with a larger cohort is warranted to validate whether CBVa can be a better classifier than total-CBV for the stratification of brain tumors, and whether iVASO MRI can be a useful alternative method for the assessment of tumor perfusion, especially when exogenous contrast agent administration is difficult in certain patient populations. J. Magn. Reson. Imaging 2016;44:1244-1255.

Hippocampal arterial cerebral blood volume in early psychosis

Recent studies of patients in the early stage of psychosis have revealed increased cerebral blood volume (CBV) in specific subfields of the anterior hippocampus. These studies required injection of a contrast agent to measure steady state CBV. Here we used a novel, non-invasive method, inflow-based-vascular-space-occupancy with dynamic subtraction (iVASO-DS), to measure the arterial component of CBV (aCBV) in a single slice of the hippocampus. Based on evidence from contrast-enhanced CBV studies, we hypothesized increased aCBV in the anterior hippocampus in early psychosis. We used 3T MRI to generate iVASO-derived aCBV maps in 17 medicated patients (average duration of illness = 7.6 months) and 25 matched controls. We did not find hemispheric or regional group differences in hippocampal aCBV. The limited spatial resolution of the iVASO-DS method did not allow us to test for aCBV differences in specific subfields of the hippocampus. Future studies should investigate venous and arterial CBV changes in the hippocampus of early psychosis patients.

Noise concerns and post-processing procedures in cerebral blood flow (CBF) and cerebral blood volume (CBV) functional magnetic resonance imaging

Functional neuroimaging with blood oxygenation level-dependent (BOLD) contrast has emerged as the most popular method for evaluating qualitative changes in brain function in humans. At typical human field strengths (1.5-3.0T), BOLD contrast provides a measure of changes in transverse water relaxation rates in and around capillary and venous blood, and as such provides only a surrogate marker of brain function that depends on dynamic changes in hemodynamics (e.g., cerebral blood flow and volume) and metabolism (e.g., oxygen extraction fraction and the cerebral metabolic rate of oxygen consumption). Alternative functional neuroimaging methods that are specifically sensitive to these constituents of the BOLD signal are being developed and applied in a growing number of clinical and neuroscience applications of quantitative cerebral physiology. These methods require additional considerations for interpreting and quantifying their contrast responsibly. Here, an overview of two popular methods, arterial spin labeling and vascular space occupancy, is presented specifically in the context of functional neuroimaging. Appropriate post-processing and experimental acquisition strategies are summarized with the motivation of reducing sensitivity to noise and unintended signal sources and improving quantitative accuracy of cerebral hemodynamics.

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.

Non-invasive quantification of absolute cerebral blood volume during functional activation applicable to the whole human brain

PURPOSE

Cerebral blood volume (CBV) changes in many diverse pathologic conditions, and in response to functional challenges along with changes in blood flow, blood oxygenation, and the cerebral metabolic rate of oxygen. The feasibility of a new method for non-invasive quantification of absolute cerebral blood volume that can be applicable to the whole human brain was investigated.

METHODS

Multi-slice data were acquired at 3 T using a novel inversion recovery echo planar imaging (IR-EPI) pulse sequence with varying contrast weightings and an efficient rotating slice acquisition order, at rest and during visual activation. A biophysical model was used to estimate absolute cerebral blood volume at rest and during activation, and oxygenation during activation, on data from 13 normal human subjects.

RESULTS

Cerebral blood volume increased by 21.7% from 6.6 ± 0.8 mL/100 mL of brain parenchyma at rest to 8.0 ± 1.3 mL/100 mL of brain parenchyma in the occipital cortex during visual activation, with average blood oxygenation of 84 ± 2.1% during activation, comparing well with literature.

CONCLUSION

The method is feasible, and could foster improved understanding of the fundamental physiological relationship between neuronal activity, hemodynamic changes, and metabolism underlying brain activation; complement existing methods for estimating compartmental changes; and potentially find utility in evaluating vascular health.

Impact of image-based motion correction on dopamine D3/D2 receptor occupancy-comparison of groupwise and frame-by-frame registration approaches

BACKGROUND

Image registration algorithms are frequently used to align the reconstructed brain PET frames to remove subject head motion. However, in occupancy studies, this is a challenging task where competitive binding of a drug can further reduce the available signal for registration. The purpose of this study is to evaluate two kinds of algorithms-a conventional frame-by-frame (FBF) registration and a recently introduced groupwise image registration (GIR), for motion correction of a dopamine D3/D2 receptor occupancy study.

METHODS

The FBF method co-registers all the PET frames to a common reference based on normalised mutual information as the spatial similarity. The GIR method incorporates a pharmacokinetic model and conducts motion correction by maximising a likelihood function iteratively on tracer kinetics and subject motion. Data from eight healthy volunteers scanned with [11C]-(+)-PHNO pre- and post-administration of a range of doses of the D3 antagonist GSK618334 were used to compare the motion correction performance.

RESULTS

The groupwise registration achieved improved motion correction results, both by visual inspection of the dynamic PET data and by the reduction of the variability in the outcome measures, and required no additional steps to exclude unsuccessfully realigned PET data for occupancy modelling as compared to frame-by-frame registration. Furthermore, for the groupwise method, the resultant binding potential estimates had reduced variation and bias for individual scans and improved half maximal effective concentration (EC50) estimates were obtained for the study as a whole.

CONCLUSIONS

These results indicate that the groupwise registration approach can provide improved motion correction of dynamic brain PET data as compared to frame-by-frame registration approaches for receptor occupancy studies.

Inflow-vascular space occupancy (iVASO) reproducibility in the hippocampus and cortex at different blood water nulling times

PURPOSE

Inflow-vascular space occupancy (iVASO) measures arterial cerebral blood volume (aCBV) using accurate blood water nulling (inversion time [TI]) when arterial blood reaches the capillary, i.e., at the arterial arrival time. This work assessed the reproducibility of iVASO measurements in the hippocampus and cortex at multiple TIs.

METHODS

The iVASO approach was implemented at multiple TIs in 10 healthy volunteers at 3 Tesla. aCBV values were measured at each TI in the left and right hippocampus, and the cortex. Reproducibility of aCBV measurements within scans (same day) and across sessions (different days) was assessed using the intraclass correlation coefficient (ICC).

RESULTS

Overall hippocampal aCBV was significantly higher than cortical aCBV, likely due to higher gray matter volume. Hippocampal ICC values were high at short TIs (≤914 ms; intrascan values = 0.80-0.96, interscan values = 0.61-0.91). Cortically, high ICC values were observed at intermediate TIs of 914 (intra: 0.93, inter: 0.87) and 1034 ms (intra: 0.96, inter: 0.86). The ICC values were comparable to established contrast-based CBV measures.

CONCLUSION

iVASO measurements are reproducible within and across sessions. TIs for iVASO measurements should be chosen carefully, taking into account heterogeneous arterial arrival times in different brain regions. Magn Reson Med 75:2379-2387, 2016. © 2015 Wiley Periodicals, Inc.

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.

Noninvasive assessment of arterial compliance of human cerebral arteries with short inversion time arterial spin labeling

A noninvasive method of assessing cerebral arterial compliance (AC) is introduced in which arterial spin labeling (ASL) is used to measure changes in arterial blood volume (aBV) occurring within the cardiac cycle. Short inversion time pulsed ASL (PASL) was performed in healthy volunteers with inversion times ranging from 250 to 850 ms. A model of the arterial input function was used to obtain the cerebral aBV. Results indicate that aBV depends on the cardiac phase of the arteries in the imaging volume. Cerebral AC, estimated from aBV and brachial blood pressure measured noninvasively in systole and diastole, was assessed in the flow territories of the basal cerebral arteries originating from the circle of Willis: right and left middle cerebral arteries (RMCA and LMCA), right and left posterior cerebral arteries (RPCA and LPCA), and the anterior cerebral artery (ACA). Group average AC values calculated for the RMCA, LMCA, ACA, RPCA, and LPCA were 0.56%±0.2%, 0.50%±0.3%, 0.4%±0.2%, 1.1%±0.5%, and 1.1%±0.3% per mm Hg, respectively. The current experiment has shown the feasibility of measuring AC of cerebral arteries with short inversion time PASL.

Three-dimensional acquisition of cerebral blood volume and flow responses during functional stimulation in a single scan

In addition to the BOLD scan, quantitative functional MRI studies require measurement of both cerebral blood volume (CBV) and flow (CBF) dynamics. The ability to detect CBV and CBF responses in a single additional scan would shorten the total scan time and reduce temporal variations. Several approaches for simultaneous CBV and CBF measurement during functional MRI experiments have been proposed in two-dimensional (2D) mode covering one to three slices in one repetition time (TR). Here, we extended the principles from previous work and present a three-dimensional (3D) whole-brain MRI approach that combines the vascular-space-occupancy (VASO) and flow-sensitive alternating inversion recovery (FAIR) arterial spin labeling (ASL) techniques, allowing the measurement of CBV and CBF dynamics, respectively, in a single scan. 3D acquisitions are complicated for such a scan combination as the time to null blood signal during a steady state needs to be known. We estimated this using Bloch simulations and demonstrate that the resulting 3D acquisition can detect activation patterns and relative signal changes of quality comparable to that of the original separate scans. The same was found for temporal signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). This approach provides improved acquisition efficiency when both CBV and CBF responses need to be monitored during a functional task.

Noninvasive MRI measurement of the absolute cerebral blood volume-cerebral blood flow relationship during visual stimulation in healthy humans

PURPOSE

The relationship between cerebral blood volume (CBV) and cerebral blood flow (CBF) underlies blood oxygenation level-dependent functional MRI signal. This study investigates the potential for improved characterization of the CBV-CBF relationship in humans, and examines sex effects as well as spatial variations in the CBV-CBF relationship.

METHODS

Healthy subjects were imaged noninvasively at rest and during visual stimulation, constituting the first MRI measurement of the absolute CBV-CBF relationship in humans with complete coverage of the functional areas of interest.

RESULTS

CBV and CBF estimates were consistent with the literature, and their relationship varied both spatially and with sex. In a region of interest with stimulus-induced activation in CBV and CBF at a significance level of the P < 0.05, a power function fit resulted in CBV = 2.1 CBF(0.32) across all subjects, CBV = 0.8 CBF(0.51) in females and CBV = 4.4 CBF(0.15) in males. Exponents decreased in both sexes as ROIs were expanded to include less significantly activated regions.

CONCLUSION

Consideration for potential sex-related differences, as well as regional variations under a range of physiological states, may reconcile some of the variation across literature and advance our understanding of the underlying cerebrovascular physiology.

Noninvasive functional imaging of cerebral blood volume with vascular-space-occupancy (VASO) MRI

Functional MRI (fMRI) based on changes in cerebral blood volume (CBV) can probe directly vasodilatation and vasoconstriction during brain activation or physiologic challenges, and can provide important insights into the mechanism of blood oxygenation level-dependent (BOLD) signal changes. At present, the most widely used CBV fMRI technique in humans is called vascular-space-occupancy (VASO) MRI, and this article provides a technical review of this method. VASO MRI utilizes T1 differences between blood and tissue to distinguish between these two compartments within a voxel, and employs a blood-nulling inversion recovery sequence to yield an MR signal proportional to 1 - CBV. As such, vasodilatation will result in a VASO signal decrease and vasoconstriction will have the reverse effect. The VASO technique can be performed dynamically with a temporal resolution comparable with several other fMRI methods, such as BOLD or arterial spin labeling (ASL), and is particularly powerful when conducted in conjunction with these complementary techniques. The pulse sequence and imaging parameters of VASO can be optimized such that the signal change is predominantly of CBV origin, but careful considerations should be taken to minimize other contributions, such as those from the BOLD effect, cerebral blood flow (CBF) and cerebrospinal fluid (CSF). The sensitivity of the VASO technique is the primary disadvantage when compared with BOLD, but this technique is increasingly demonstrating its utility in neuroscientific and clinical applications.

Elevated arteriolar cerebral blood volume in prodromal Huntington's disease

BACKGROUND

Neurovascular alterations have been implicated in the pathophysiology of Huntington's disease (HD). Because arterioles are most responsive to metabolic alterations, arteriolar cerebral blood volume (CBVa) is an important indicator of cerebrovascular regulation. The objective of this pilot study was to investigate potential neurovascular (CBVa ) abnormality in prodromal-HD patients and compare it with the widely used imaging marker: brain atrophy.

METHODS

CBVa and brain volumes were measured with ultra-high-field (7.0-Telsa) magnetic resonance imaging in seven prodromal-HD patients and nine age-matched controls.

RESULTS

Cortical CBVa was elevated significantly in prodromal-HD patients compared with controls (relative difference, 38.5%; effect size, 1.48). Significant correlations were found between CBVa in the frontal cortex and genetic measures. By contrast, no significant brain atrophy was detected in the prodromal-HD patients.

CONCLUSIONS

CBVa may be abnormal in prodromal-HD, even before substantial brain atrophy occurs. Further investigation with a larger cohort and longitudinal follow-up is merited to determine whether CBVa could be used as a potential biomarker for clinical trials.

Magnetic resonance imaging to visualize stroke and characterize stroke recovery: a review

The global burden of stroke continues to grow. Although stroke prevention strategies (e.g., medications, diet, and exercise) can contribute to risk reduction, options for acute interventions (e.g., thrombolytic therapy for ischemic stroke) are limited to the minority of patients. The remaining patients are often left with profound neurological disabilities that substantially impact quality of life, economic productivity, and increase caregiver burden. In the last decade, however, the future outlook for such patients has been tempered by movement toward the view that the brain is capable of reorganizing after injury. Many now view brain recovery after stroke as an area of scientific research with large potential for therapeutic advances, far into the future (Broderick and William, 2004). As a probe of brain anatomy, function and physiology, magnetic resonance imaging (MRI) is a non-invasive and highly versatile modality that promises to play a particularly important role in such research. Here we provide a basic review of MRI physical principles and applications for assessing stroke, looking toward the future role MRI may play in improving stroke rehabilitation methods and stroke recovery.

Combined arterial spin labeling and diffusion-weighted imaging for noninvasive estimation of capillary volume fraction and permeability-surface product in the human brain

A number of two-compartment models have been developed for the analysis of arterial spin labeling (ASL) data, from which both cerebral blood flow (CBF) and capillary permeability-surface product (PS) can be estimated. To derive values of PS, the volume fraction of the ASL signal arising from the intravascular space (v(bw)) must be known a priori. We examined the use of diffusion-weighted imaging (DWI) and subsequent analysis using the intravoxel incoherent motion model to determine v(bw) in the human brain. These data were then used in a two-compartment ASL model to estimate PS. Imaging was performed in 10 healthy adult subjects, and repeated in five subjects to test reproducibility. In gray matter (excluding large arteries), mean voxel-wise v(bw) was 2.3±0.2 mL blood/100 g tissue (all subjects mean±s.d.), and CBF and PS were 44±5 and 108±2 mL per 100 g per minute, respectively. After spatial smoothing using a 6-mm full width at half maximum Gaussian kernel, the coefficient of repeatability of CBF, v(bw) and PS were 8 mL per 100 g per minute, 0.4 mL blood/100 g tissue, and 13 mL per 100 g per minute, respectively. Our results show that the combined use of ASL and DWI can provide a new, noninvasive methodology for estimating v(bw) and PS directly, with reproducibility that is sufficient for clinical use.

Visualization of altered neurovascular coupling in chronic stroke patients using multimodal functional MRI

Evaluation of cortical reorganization in chronic stroke patients requires methods to accurately localize regions of neuronal activity. Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is frequently employed; however, BOLD contrast depends on specific coupling relationships between the cerebral metabolic rate of oxygen (CMRO(2)), cerebral blood flow (CBF), and volume (CBV), which may not exist following stroke. The aim of this study was to understand whether CBF-weighted (CBFw) and CBV-weighted (CBVw) fMRI could be used in sequence with BOLD to characterize neurovascular coupling mechanisms poststroke. Chronic stroke patients (n=11) with motor impairment and age-matched controls (n=11) performed four sets of unilateral motor tasks (60 seconds/30 seconds off/on) during CBFw, CBVw, and BOLD fMRI acquisition. While control participants elicited mean BOLD, CBFw, and CBVw responses in motor cortex (P<0.01), patients showed only mean changes in CBF (P<0.01) and CBV (P<0.01), but absent mean BOLD responses (P=0.20). BOLD intersubject variability was consistent with differing coupling indices between CBF, CBV, and CMRO(2). Thus, CBFw and/or CBVw fMRI may provide crucial information not apparent from BOLD in these patients. A table is provided outlining distinct vascular and metabolic uncoupling possibilities that elicit different BOLD responses, and the strengths and limitations of the multimodal protocol are summarized.

Evaluating quantitative approaches to dynamic susceptibility contrast MRI among carotid endarterectomy patients

PURPOSE

To evaluate two dynamic susceptibility contrast (DSC) quantification methods in symptomatic carotid artery disease patients undergoing carotid endarterectomy (CEA) surgery by comparing methods directly and assessing the reliability of each method in the hemisphere contralateral to surgery.

MATERIALS AND METHODS

Absolute cerebral blood flow (CBF) and volume (CBV) was calculated in putamen and sensorimotor gray matter of 17 patients using two methods: 1) The Bookend method that scales relative DSC images to CBV values calculated from the ratio of pre- and postcontrast T1-weighted images, and 2) the Tail-scaling method that uses the ratio of area under the tails of the venous and arterial concentration time-courses to scale the DSC images.

RESULTS

There was a positive correlation between the methods with significant correlation post-CEA (P < 0.035). Intersession correlation was greater when using the Tail-scaling method contralateral to surgery (P < 0.004).

CONCLUSION

We have demonstrated correlation between methods that is significant after surgery and have found that the Tail-scaling method produces better test-retest reliability than our implementation of the Bookend method. Results from this study suggest that DSC has the potential to measure hemodynamic changes after endarterectomy and future work is required to establish clinical value.

Cerebral blood volume changes during brain activation

Cerebral blood volume (CBV) changes significantly with brain activation, whether measured using positron emission tomography, functional magnetic resonance imaging (fMRI), or optical microscopy. If cerebral vessels are considered to be impermeable, the contents of the skull incompressible, and the skull itself inextensible, task- and hypercapnia-related changes of CBV could produce intolerable changes of intracranial pressure. Because it is becoming clear that CBV may be useful as a well-localized marker of neural activity changes, a resolution of this apparent paradox is needed. We have explored the idea that much of the change in CBV is facilitated by exchange of water between capillaries and surrounding tissue. To this end, we developed a novel hemodynamic boundary-value model and found approximate solutions using a numerical algorithm. We also constructed a macroscopic experimental model of a single capillary to provide biophysical insight. Both experiment and theory model capillary membranes as elastic and permeable. For a realistic change of input pressure, a relative pipe volume change of 21±5% was observed when using the experimental setup, compared with the value of approximately 17±1% when this quantity was calculated from the mathematical model. Volume, axial flow, and pressure changes are in the expected range.

Implementation of vascular-space-occupancy MRI at 7T

Vascular-space-occupancy (VASO) MRI exploits the difference between blood and tissue T1 to null blood signal and measure cerebral blood volume changes using the residual tissue signal. VASO imaging is more difficult at higher field because of sensitivity loss due to the convergence of tissue and blood T1 values and increased contamination from blood-oxygenation-level-dependent (BOLD) effects. In addition, compared to 3T, 7T MRI suffers from increased geometrical distortions, e.g., when using echo-planar-imaging, and from increased power deposition, the latter especially problematic for the spin-echo-train sequences commonly used for VASO MRI. Third, non-steady-state blood spin effects become substantial at 7T when only a head coil is available for radiofrequency transmit. In this study, the magnetization-transfer-enhanced-VASO approach was applied to maximize tissue-blood signal difference, which boosted signal-to-noise ratio by 149% ± 13% (n = 7) compared to VASO. Second, a 3D fast gradient-echo sequence with low flip-angle (7°) and short echo-time (1.8 ms) was used to minimize the BOLD effect and to reduce image distortion and power deposition. Finally, a magnetization-reset technique was combined with a motion-sensitized-driven-equilibrium approach to suppress three types of non-steady-state spins. Our initial functional MRI results in normal human brains at 7T with this optimized VASO sequence showed better signal-to-noise ratio than at 3T.

Novel MRI approaches for assessing cerebral hemodynamics in ischemic cerebrovascular disease

Changes in cerebral hemodynamics underlie a broad spectrum of ischemic cerebrovascular disorders. An ability to accurately and quantitatively measure hemodynamic (cerebral blood flow and cerebral blood volume) and related metabolic (cerebral metabolic rate of oxygen) parameters is important for understanding healthy brain function and comparative dysfunction in ischemia. Although positron emission tomography, single-photon emission tomography, and gadolinium-MRI approaches are common, more recently MRI approaches that do not require exogenous contrast have been introduced with variable sensitivity for hemodynamic parameters. The ability to obtain hemodynamic measurements with these new approaches is particularly appealing in clinical and research scenarios in which follow-up and longitudinal studies are necessary. The purpose of this review is to outline current state-of-the-art MRI methods for measuring cerebral blood flow, cerebral blood volume, and cerebral metabolic rate of oxygen and provide practical tips to avoid imaging pitfalls. MRI studies of cerebrovascular disease performed without exogenous contrast are synopsized in the context of clinical relevance and methodological strengths and limitations.

A review of the development of Vascular-Space-Occupancy (VASO) fMRI

Vascular-Space-Occupancy (VASO) fMRI is a non-invasive technique to detect brain activation based on changes in Cerebral Blood Volume (CBV), as opposed to conventional BOLD fMRI, which is based on changes in blood oxygenation. This technique takes advantage of the T1 difference between blood and surrounding tissue, and uses an inversion recovery pulse sequence to null blood signal while maintaining part of the tissue signal. The VASO signal intensity can thus be considered proportional to 1-CBV. When neural activation causes CBV to increase, the VASO signal will show a decrease, allowing the detection of activated regions in the brain. Activation-induced changes in VASO signal, ∆S/S, are in the order of -1%. Absolute quantification of ∆CBV requires additional assumptions on baseline CBV and water contents of the parenchyma and blood. The first VASO experiment was conducted approximately 10 years ago. The original goal of nulling the blood signal was to isolate and measure extravascular BOLD effects, thus a long TE of 50 ms was used in the inversion recovery experiment. Instead of a positive signal change, a slight decrease in signal was observed, which became more pronounced when TE was shortened to 10 ms. These findings led to the hypothesis of a CBV signal mechanism and the development of VASO fMRI. Since its discovery, VASO has been validated by comparison with MION-CBV studies in animals and has been used in humans and animals to understand metabolic and hemodynamic changes during brain activation and physiologic challenges. With recent development of more sensitive VASO acquisitions, the availability of arterial-based VASO sequences, and improvement in spatial coverage, this technique is finding its place in neuroscience and clinical studies.

Measurement of absolute arterial cerebral blood volume in human brain without using a contrast agent

Arterial cerebral blood volume (CBV(a) ) is a vital indicator of tissue perfusion and vascular reactivity. We extended the recently developed inflow vascular-space-occupancy (iVASO) MRI technique, which uses spatially selective inversion to suppress the signal from blood flowing into a slice, with a control scan to measure absolute CBV(a) using cerebrospinal fluid (CSF) for signal normalization. Images were acquired at multiple blood nulling times to account for the heterogeneity of arterial transit times across the brain, from which both CBV(a) and arterial transit times were quantified. Arteriolar CBV(a) was determined separately by incorporating velocity-dependent bipolar crusher gradients. Gray matter (GM) CBV(a) values (n=11) were 2.04 ± 0.27 and 0.76 ± 0.17 ml blood/100 ml tissue without and with crusher gradients (b=1.8 s/mm(2) ), respectively. Arterial transit times were 671 ± 43 and 785 ± 69 ms, respectively. The arterial origin of the signal was validated by measuring its T(2) , which was within the arterial range. The proposed approach does not require exogenous contrast agent administration, and provides a non-invasive alternative to existing blood volume techniques for mapping absolute CBV(a) in studies of brain physiology and neurovascular diseases.

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

A new technique is introduced in this study for in vivo measurement of arterial cerebral blood volume by combining arterial spin labeling with a segmented multiphase balanced steady-state free precession (bSSFP) readout sequence. This technique takes advantage of the phenomenon that the longitudinal magnetization of flowing blood is not or only marginally disturbed (besides T(1) relaxation) by the bSSFP ± α pulse train. When the blood water exchanges into tissue, it becomes quickly saturated by the bSSFP pulse train due to 0 velocity and reduced T(1), T(2) relaxation times. Therefore, labeled blood water behaves like an intravascular contrast agent in multiphase bSSFP scans, and can be used to quantify arterial cerebral blood volume in a similar way as dynamic susceptibility contrast MRI. Both Bloch equation simulation and in vivo experiments were carried out to demonstrate the feasibility for quantifying cerebral blood volume in arteries, arterioles, and capillaries using two variants of the proposed method. Functional MRI of visual cortex stimulation was further performed using multiphase bSSFP-based arterial spin labeling and compared with vascular-space occupancy contrast. The proposed multiphase bSSFP-based arterial spin labeling technique may allow separation of cerebral blood volume of different vascular compartments for functional MRI studies and clinical evaluation of the cerebral vasculature.

Quantitative MRI of cerebral arterial blood volume

Baseline cerebral arterial blood volume (CBV_a) and its change are important for potential diagnosis of vascular dysfunctions, the determination of functional reactivity, and the interpretation of BOLD fMRI. To quantitative measure baseline CBV_a non-invasively, we developed arterial spin labeling methods with magnetization transfer (MT) or bipolar gradients by utilizing differential MT or diffusion properties of tissue vs. arteries. Cortical CBV_a of isoflurane-anesthetized rats was 0.6 – 1.4 ml/100 g. During 15-s forepaw stimulation, CBV_a change was dominant, while venous blood volume change was minimal. This indicates that the venous CBV increase may be ignored for BOLD quantification for a stimulation duration of less than 15 s. By incorporating BOLD fMRI with varied MT effects in a cat visual cortical layer model, the highest ΔCBV_a was observed at layer 4, while the highest BOLD signal was detected at the surface of the cortex, indicating that CBV_a change is highly specific to neural activity. The CBV_a MRI techniques provide quantified maps, thus, may be valuable tools for routine determination of vessel viability and function, as well as the identification of vascular dysfunction.

Determination of spin compartment in arterial spin labeling MRI

A major difference between arterial-spin-labeling MRI and gold-standard radiotracer blood flow methods is that the compartment localization of the labeled spins in the arterial-spin-labeling image is often ambiguous, which may affect the quantification of cerebral blood flow. In this study, we aim to probe whether the spins are located in the vascular system or tissue by using T2 of the arterial-spin-labeling signal as a marker. We combined two recently developed techniques, pseudo-continuous arterial spin labeling and T2-Relaxation-Under-Spin-Tagging, to determine the T2 of the labeled spins at multiple postlabeling delay times. Our data suggest that the labeled spins first showed the T2 of arterial blood followed by gradually approaching and stabilizing at the tissue T2. The T2 values did not decrease further toward the venous T2. By fitting the experimental data to a two-compartment model, we estimated gray matter cerebral blood flow, arterial transit time, and tissue transit time to be 74.0 ± 10.7 mL/100g/min (mean ± SD, N = 10), 938 ± 156 msec, and 1901 ± 181 msec, respectively. The arterial blood volume was calculated to be 1.18 ± 0.21 mL/100 g. A postlabeling delay time of 2 s is sufficient to allow the spins to completely enter the tissue space for gray matter but not for white matter.