Measuring the change in CBV upon cortical activation with high temporal resolution using look-locker EPI and Gd-DTPA

Magnetic Particle Imaging in Vascular Imaging, Immunotherapy, Cell Tracking, and Noninvasive Diagnosis

Magnetic particle imaging (MPI) is a new tracer-based imaging modality that is useful in diagnosing various pathophysiology related to the vascular system and for sensitive tracking of cytotherapies. MPI uses nonradioactive and easily assimilated nanometer-sized iron oxide particles as tracers. MPI images the nonlinear Langevin behavior of the iron oxide particles and has allowed for the sensitive detection of iron oxide-labeled therapeutic cells in the body. This review will provide an overview of MPI technology, the tracer, and its use in vascular imaging and cytotherapies using molecular targets.

Quantification of cerebral blood volume changes caused by visual stimulation at 3 T using DANTE-prepared dual-echo EPI

PURPOSE

We investigate the influence of moving blood-attenuation effects when using "delay alternating with nutation for tailored excitation" (DANTE) pulses in conjunction with blood oxygen level dependent (BOLD) of functional MRI (fMRI) at 3 T. Based on the effects of including DANTE pulses, we propose quantification of cerebral blood volume (CBV) changes following functional stimulation.

METHODS

Eighteen volunteers in total underwent fMRI scans at 3 T. Seven volunteers were scanned to investigate the effects of DANTE pulses on the fMRI signal. CBV changes in response to visual stimulation were quantified in 11 volunteers using a DANTE-prepared dual-echo EPI sequence.

RESULTS

The inflow effects from flowing blood in arteries and draining vein effects from flowing blood in large veins can be suppressed by use of a DANTE preparation module. Using DANTE-prepared dual-echo EPI, we quantitatively measured intravascular-weighted microvascular CBV changes of 25.4%, 29.8%, and 32.6% evoked by 1, 5, and 10 Hz visual stimulation, respectively. The extravascular fraction (∆S/S)_extra at TE = 30 ms in total BOLD signal was determined to be 64.8 ± 3.4%, which is in line with previous extravascular component estimation at 3 T. Results show that the microvascular CBV changes are linearly dependent on total BOLD changes at TE = 30 ms with a slope of 0.113, and this relation is independent of stimulation frequency and subject.

CONCLUSION

The DANTE preparation pulses can be incorporated into a standard EPI fMRI sequence for the purpose of minimizing inflow effects and reducing draining veins effects in large vessels. Additionally, the DANTE-prepared dual-echo EPI sequence is a promising fast imaging tool for quantification of intravascular-weighted CBV change in the microvascular space at 3 T.

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.

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.

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.

Inflow-based vascular-space-occupancy (iVASO) MRI

Vascular-space-occupancy (VASO) MRI, a blood nulling approach for assessing changes in cerebral blood volume (CBV), is hampered by low signal-to-noise ratio (SNR) because only 10-20% of tissue signal is recovered when using nonselective inversion for blood nulling. A new approach, called inflow-VASO (iVASO), is introduced in which only blood flowing into the slice has experienced inversion, thereby keeping tissue and cerebrospinal fluid (CSF) signal in the slice maximal and reducing CSF partial volume effects. SNR increases of 198% ± 12% and 334% ± 9% (mean ± SD, n = 7) with respect to VASO were found at TR values of 5 s and 2 s, respectively. When using inflow approaches, data interpretation is complicated by the fact that signal changes are affected by vascular transit times. An optimal TR-range (1.5-2.5 s) was derived in which the iVASO response during activation predominantly reflects arterial/arteriolar CBV (CBV(a)) changes. In this TR-range, perfusion contributions to the signal change are negligible because arterial label has not yet undergone capillary exchange, and arterial and precapillary blood signals are nulled. For TR = 2 s, the iVASO signal change upon visual stimulation corresponded to a CBV(a) increase of 58% ± 7%, in agreement with arteriolar CBV changes previously reported. The onset of the hemodynamic response for iVASO occurred 1.2 ± 0.5 s (n = 7) faster than for conventional VASO.

BOLD-specific cerebral blood volume and blood flow changes during neuronal activation in humans

To understand and predict the blood-oxygenation level-dependent (BOLD) fMRI signal, an accurate knowledge of the relationship between cerebral blood flow (DeltaCBF) and volume (DeltaCBV) changes is critical. Currently, this relationship is widely assumed to be characterized by Grubb's power-law, derived from primate data, where the power coefficient (alpha) was found to be 0.38. The validity of this general formulation has been examined previously, and an alpha of 0.38 has been frequently cited when calculating the cerebral oxygen metabolism change (DeltaCMRo(2)) using calibrated BOLD. However, the direct use of this relationship has been the subject of some debate, since it is well established that the BOLD signal is primarily modulated by changes in 'venous' CBV (DeltaCBV(v), comprising deoxygenated blood in the capillary, venular, and to a lesser extent, in the arteriolar compartments) instead of total CBV, and yet DeltaCBV(v) measurements in humans have been extremely scarce. In this work, we demonstrate reproducible DeltaCBV(v) measurements at 3 T using venous refocusing for the volume estimation (VERVE) technique, and report on steady-state DeltaCBV(v) and DeltaCBF measurements in human subjects undergoing graded visual and sensorimotor stimulation. We found that: (1) a BOLD-specific flow-volume power-law relationship is described by alpha = 0.23 +/- 0.05, significantly lower than Grubb's constant of 0.38 for total CBV; (2) this power-law constant was not found to vary significantly between the visual and sensorimotor areas; and (3) the use of Grubb's value of 0.38 in gradient-echo BOLD modeling results in an underestimation of DeltaCMRo(2).

Early brain swelling in acute hypoxia

Acute mountain sickness (AMS) and high-altitude cerebral edema share common clinical characteristics, suggesting cerebral swelling may be an important factor in the pathophysiology of AMS. Hypoxia and hypocapnia associated with high altitude are known to exert strong effects on the control of the cerebral circulation, yet how these effects interact during acute hypoxia, and whether AMS-susceptible subjects may have a unique response, is still unclear. To test if self-identified AMS-susceptible individuals show altered brain swelling in response to acute hypoxia, we used quantitative arterial spin-labeling and volumetric MRI to measure cerebral blood flow and cerebrospinal fluid (CSF) volume changes during 40 min of acute hypoxia. We estimated changes in cerebral blood volume (CBV) (from changes in cerebral blood flow) and brain parenchyma swelling (from changes in CBV and CSF). Subjects with extensive high-altitude experience in two groups participated: self-identified AMS-susceptible (n = 6), who invariably experienced AMS at altitude, and self-identified AMS-resistant (n = 6), who almost never experienced symptoms. During 40-min hypoxia, intracranial CSF volume decreased significantly [-10.5 ml (SD 6.9), P < 0.001]. There were significant increases in CBV [+2.3 ml (SD 2.5), P < 0.005] and brain parenchyma volume [+8.2 ml (SD 6.4), P < 0.001]. However, there was no significant difference between self-identified AMS-susceptible and AMS-resistant groups for these acute-phase changes. In acute hypoxia, brain swelling occurs earlier than previously described, with significant shifts in intracranial CSF occurring as early as 40 min after exposure. These acute-phase changes are present in all individuals, irrespective of susceptibility to AMS.

Method for rapid calculation of quantitative cerebral perfusion

PURPOSE

To evaluate an algorithm based on algebraic estimation of T1 values (three-point estimation) in comparison with computational curve-fitting for the postprocessing of quantitative cerebral perfusion scans.

MATERIALS AND METHODS

Computer simulations were performed to quantify the magnitude of the expected error on T1 and consequently cerebral perfusion using the three-point estimation technique on a Look-Locker (LL) EPI scan. In 50 patients, quantitative cerebral perfusion was calculated using the bookend method with three-point estimation and curve-fitting. The bookend method, a novel approach for calculating quantitative cerebral perfusion based on changes in T1 values after a contrast injection, is currently being validated. The number of computations was used as a measure of computation speed for each method. Student's paired t-test, Bland-Altman, and correlation analyses were performed to evaluate the accuracy of estimation.

RESULTS

There was a 99.65% reduction in the number of computations with three-point estimation. Student's t-test showed no significant difference in cerebral perfusion (P=0.80, 0.49, paired t-test N=50, quantitative cerebral blood flow-white matter [qCBF-WM], qCBF-gray matter [qCBF-GM]) when compared to curve-fitting. The results of the two techniques were strongly correlated in patients (slope=0.99, intercept=1.58 mL/(100 g/minute), r=0.86) with a small systemic bias of -0.97 mL/(100 g/minute) in Bland-Altman analysis.

CONCLUSION

The three-point estimation technique is adequate for rapid calculation of qCBF. The estimation scheme drastically reduces processing time, thus making the method feasible for clinical use.

Vascular space occupancy-dependent functional MRI by tissue suppression

PURPOSE

To measure the cerebral blood volume (CBV) dynamics during neural activation, a novel technique named vascular space occupancy (VASO)-based functional MRI (fMRI) was recently introduced for noninvasive CBV detection. However, its application is limited because of its low contrast-to-noise ratio (CNR) due to small signal change from the inverted blood.

MATERIALS AND METHODS

In this study a new approach-VASO with tissue suppression (VAST)-is proposed to enhance CNR. This technique is compared with VASO and blood oxygenation level-dependent (BOLD) fMRI in block-design and event-related visual experiments.

RESULTS

Based on acquired T(1) maps, 75.3% of the activated pixels detected by VAST are located in the cortical gray matter. Temporal characteristics of functional responses obtained by VAST were consistent with that of VASO. Although the baseline signal was decreased by the tissue suppression, the CNR of VAST was about 43% higher than VASO.

CONCLUSION

With the improved sensitivity, VAST fMRI provides a useful alternative for mapping the spatial/temporal features of regional CBV changes during brain activation. However, the technical imperfectness of VAST, such as the nonideal inversion efficiency and physiological contaminations, limits its application to precise CBV quantification.

The post-stimulation undershoot in BOLD fMRI of human brain is not caused by elevated cerebral blood volume

Functional magnetic resonance imaging (fMRI) based on blood oxygenation level dependent (BOLD) contrast is the most widely used technique for imaging human brain function. However, the dynamic interplay of altered cerebral blood flow (CBF), cerebral blood volume (CBV), and oxidative metabolism (CMRO2) is not yet fully understood. One of the characteristics of the BOLD response is the post-stimulation undershoot, that is increased deoxyhemoglobin, which has been suggested to originate from a delayed recovery of elevated CBV or CMRO2 to baseline. To investigate the CBV contribution to the post-stimulation BOLD undershoot, we performed bolus-tracking experiments using a paramagnetic contrast agent in eight healthy subjects at 3 T. In an initial BOLD experiment without contrast agent, we determined the individual hemodynamic responsiveness. In two separate experiments, we then evaluated the relative CBV (rCBV) during visual stimulation and the post-stimulation undershoot, respectively. The results confirm a pronounced rCBV increase during stimulation (31.4+/-8.6%), but reveal no change in rCBV relative to baseline in the post-stimulation phase (0.7+/-7.2%). This finding renders a CBV contribution to the BOLD MRI undershoot unlikely and--in conjunction with a rapid post-stimulation return of CBF to baseline--supports the idea of a prolonged elevation of oxidative metabolism.

Modeling dynamic cerebral blood volume changes during brain activation on the basis of the blood-nulled functional MRI signal

Recently, vascular space occupancy (VASO) based functional magnetic resonance imaging (fMRI) was proposed to detect dynamic cerebral blood volume (CBV) changes using the blood-nulled non-selective inversion recovery (NSIR) sequence. However, directly mapping the dynamic CBV change by the NSIR signal change is based on the assumption of slow water exchange (SWE) around the capillary regime without cerebral blood flow (CBF) effects. In the present study, a fast water exchange (FWE) model incorporating with flow effects was derived from the Bloch equations and implemented for the quantification of dynamic CBV changes using VASO-fMRI during brain activation. Simulated results showed that only subtle differences in CBV changes estimated by these two models were observed on the basis of previously published VASO results. The influence of related physiological and biophysical factors within typical ranges was evaluated in steady-state simulations. It was revealed that in the transient state the CBV curves could be delayed in comparison with measured NSIR curves owing to the imbalance between the inflowing and outflowing blood signals.

Vascular space occupancy weighted imaging with control of residual blood signal and higher contrast-to-noise ratio

It has been recently proposed that the local cerebral blood volume change during brain activation can be measured by a series of images whose contrast is dependent on vascular space occupancy (VASO). VASO takes advantage of the inversion recovery sequence to acquire images when the longitudinal magnetization (Mz) of blood is relaxing through zero. The degree of blood suppression, however, is not always well controlled as a consequence of spatial variations in inversion efficiency and blood T1. Furthermore, while blood is eliminated, the Mz of other tissues is also small, which makes the contrast-to-noise ratio inherently low in VASO. In this paper, diffusion gradients were applied to demonstrate residual intravascular signal in the original VASO. An alternative VASO-weighted imaging was then proposed using a longer inversion time at which the Mz difference between blood and gray matter was optimized. A global saturation immediately after image acquisition was employed to eliminate the Mz disparity between inflowing blood and the residual in-plane blood from previous acquisition. Feasibility was evaluated by numerical simulation and functional experiments. In human visual cortex, the fractional VASO signal and cerebral blood volume changes were found to be -0.6% and 44%, respectively (voxel size = 3.4 x 3.4 x 5.0 mm3). As compared to the original VASO, the presented method provided a largely comparable activation map and hemodynamic curve but was not confounded by the existence of blood. Results also demonstrated its advantages of 1.6-fold higher CNR and insensitivity to variant tissue/blood T1 as well as inversion efficiency.

Noninvasive measurement of arterial cerebral blood volume using Look-Locker EPI and arterial spin labeling

This paper describes a method of noninvasively measuring regional arterial cerebral blood volume fractions (CBV(a)) in vivo using the combination of Look-Locker echo-planar imaging (LL-EPI) with arterial spin labeling (ASL). Using this technique the arterial inflow curve is rapidly sampled and the regional CBV(a) is measured, while tissue perfusion signals are suppressed. Two methods of spin labeling (LL-EPI flow-sensitive alternating inversion recovery (LL-EPI-FAIR) and LL-EPI signal targeting using alternating radiofrequency (LL-EPI-STAR)) are assessed and their advantages discussed. The application of vascular crushing to LL-EPI-FAIR is described and used to validate the insensitivity of the sequence to the perfusion difference signal. LL-EPI-STAR is used to assess changes in CBV(a) in response to a finger-tapping task. LL-EPI-STAR signal difference curves are shown to have a shortened vascular transit delay and increased peak signal change on activation. A 33 +/- 14% increase in CBV(a) on activation is found. CBV(a) is measured with a 6-s temporal resolution and the temporal response is compared with the BOLD signal change. CBV(a) is shown to increase more rapidly and return to baseline significantly faster than the BOLD signal change, which supports the suggestion that a change in CBV(a) is an input to the BOLD response.

Principles of magnetic resonance assessment of brain function

MRI has advanced to being one of the major tools for the assessment of brain function. This review article examines the basic principles that underpin these measurements. The main emphasis is on the characteristics and detection of blood oxygen level dependent (BOLD) contrast. In the first part of the article the relationship between BOLD, blood flow, blood oxygen, and the rate of metabolic consumption of oxygen is described. The four contrast mechanisms that contribute to the BOLD signal change, namely extravascular static and dynamic dephasing, intravascular T2-like changes, and the intravascular frequency offset effect are described in terms of their spatial localization and relative contributions to the BOLD signal. The current model of changes in blood flow being an indirect consequence of synaptic input to a region is presented. The second section of the article deals with the imaging characteristics of BOLD in terms of the attainable spatial resolution and linear system characteristics. In the third section, practical BOLD imaging is examined for choice of pulse sequence, resolution, echo time (TE), repetition time (TR), and flip angle. The final section touches on other MRI approaches that are relevant to cognitive neuroimaging, in particular the measurement of blood flow, blood volume, resting state fluctuations in the BOLD signal, and measures of connectivity using diffusion tensor imaging and fiber-tracking.

Analysis of near-infrared spectroscopy and indocyanine green dye dilution with Monte Carlo simulation of light propagation in the adult brain

Near-infrared spectroscopy (NIRS) combined with indocyanine green (ICG) dilution is applied externally on the head to determine the cerebral hemodynamics of neurointensive care patients. We applied Monte Carlo simulation for the analysis of a number of problems associated with this method. First, the contamination of the optical density (OD) signal due to the extracerebral tissue was assessed. Second, the measured OD signal depends essentially on the relative blood content (with respect to its absorption) in the various transilluminated tissues. To take this into account, we weighted the calculated densities of the photon distribution under baseline conditions within the different tissues with the changes and aberration of the relative blood volumes that are typically observed under healthy and pathologic conditions. Third, in case of NIRS ICG dye dilution, an ICG bolus replaces part of the blood such that a transient change of absorption in the brain tissues occurs that can be recorded in the OD signal. Our results indicate that for an exchange fraction of Delta=30% of the relative blood volume within the intracerebral tissue, the OD signal is determined from 64 to 74% by the gray matter and between 8 to 16% by the white matter maximally for a distance of d=4.5 cm.

Venous refocusing for volume estimation: VERVE functional magnetic resonance imaging

A novel, noninvasive magnetic resonance imaging-based method for measuring changes in venous cerebral blood volume (CBV(v)) is presented. Venous refocusing for volume estimation (VERVE) exploits the dependency of the spin-spin relaxation rate of deoxygenated blood on the refocusing interval. Interleaved CPMG EPI acquisitions following a train of either tightly or sparsely spaced hard refocusing pulses (every 3.7 or 30 msec, respectively) at matched echo time were used to isolate the blood signal while minimizing the intravascular blood oxygenation level dependent (BOLD) signal contribution. The technique was employed to determine the steady-state increase in the CBV(v) in the visual cortex (VC) in seven healthy adult volunteers during flickering checkerboard photic stimulation. A functional activation model and a set of previously collected in vitro human whole blood relaxometry data were used to evaluate the intravascular BOLD effect on the VERVE signal. The average VC venous blood volume change was estimated to be 16 +/- 2%. This method has the potential to provide efficient and continuous monitoring of venous cerebral blood volume, thereby enabling further exploration of the mechanism underlying BOLD signal changes upon physiologic, pathophysiologic, and pharmacologic perturbations.