Routine clinical brain MRI sequences for use at 3.0 Tesla

Contrast enhancement by combining T1- and T2-weighted structural brain MR Images

PURPOSE

In order to more precisely differentiate cerebral structures in neuroimaging studies, a novel technique for enhancing the tissue contrast based on a combination of T1-weighted (T1w) and T2-weighted (T2w) MRI images was developed.

METHODS

The combined image (CI) was calculated as CI = (T1w - sT2w)/(T1w + sT2w), where sT2w is the scaled T2-weighted image. The scaling factor was calculated to adjust the gray- matter (GM) voxel intensities in the T2w image so that their median value equaled that of the GM voxel intensities in the T1w image. The image intensity homogeneity within a tissue and the discriminability between tissues in the CI versus the separate T1w and T2w images were evaluated using the segmentation by the FMRIB Software Library (FSL) and FreeSurfer (Athinoula A. Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Boston, MA) software.

RESULTS

The combined image significantly improved homogeneity in the white matter (WM) and GM compared to the T1w images alone. The discriminability between WM and GM also improved significantly by applying the CI approach. Significant enhancements to the homogeneity and discriminability also were achieved in most subcortical nuclei tested, with the exception of the amygdala and the thalamus.

CONCLUSION

The tissue discriminability enhancement offered by the CI potentially enables more accurate neuromorphometric analyses of brain structures.

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.

Whole-brain quantitative mapping of metabolites using short echo three-dimensional proton MRSI

BACKGROUND

To improve the extent over which whole brain quantitative three-dimensional (3D) magnetic resonance spectroscopic imaging (MRSI) maps can be obtained and be used to explore brain metabolism in a population of healthy volunteers.

METHODS

Two short echo time (20 ms) acquisitions of 3D echo planar spectroscopic imaging at two orientations, one in the anterior commissure-posterior commissure (AC-PC) plane and the second tilted in the AC-PC +15° plane were obtained at 3 Tesla in a group of 10 healthy volunteers. B1 (+) , B1 (-) , and B0 correction procedures and normalization of metabolite signals with quantitative water proton density measurements were performed. A combination of the two spatially normalized 3D-MRSI, using a weighted mean based on the pixel wise standard deviation metabolic maps of each orientation obtained from the whole group, provided metabolite maps for each subject allowing regional metabolic profiles of all parcels of the automated anatomical labeling (AAL) atlas to be obtained.

RESULTS

The combined metabolite maps derived from the two acquisitions reduced the regional intersubject variance. The numbers of AAL regions showing N-acetyl aspartate (NAA) SD/Mean ratios lower than 30% increased from 17 in the AC-PC orientation and 41 in the AC-PC+15° orientation, to a value of 76 regions of 116 for the combined NAA maps. Quantitatively, regional differences in absolute metabolite concentrations (mM) over the whole brain were depicted such as in the GM of frontal lobes (cNAA = 10.03 + 1.71; cCho = 1.78 ± 0.55; cCr = 7.29 ± 1.69; cmIns = 5.30 ± 2.67) and in cerebellum (cNAA = 5.28 ± 1.77; cCho = 1.60 ± 0.41; cCr = 6.95 ± 2.15; cmIns = 3.60 ± 0.74).

CONCLUSION

A double-angulation acquisition enables improved metabolic characterization over a wide volume of the brain.

Decreased auditory GABA+ concentrations in presbycusis demonstrated by edited magnetic resonance spectroscopy

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central auditory system. Altered GABAergic neurotransmission has been found in both the inferior colliculus and the auditory cortex in animal models of presbycusis. Edited magnetic resonance spectroscopy (MRS), using the MEGA-PRESS sequence, is the most widely used technique for detecting GABA in the human brain. However, to date there has been a paucity of studies exploring changes to the GABA concentrations in the auditory region of patients with presbycusis. In this study, sixteen patients with presbycusis (5 males/11 females, mean age 63.1 ± 2.6 years) and twenty healthy controls (6 males/14 females, mean age 62.5 ± 2.3 years) underwent audiological and MRS examinations. Pure tone audiometry from 0.125 to 8 kHz and tympanometry were used to assess the hearing abilities of all subjects. The pure tone average (PTA; the average of hearing thresholds at 0.5, 1, 2 and 4 kHz) was calculated. The MEGA-PRESS sequence was used to measure GABA+ concentrations in 4 × 3 × 3 cm(3) volumes centered on the left and right Heschl's gyri. GABA+ concentrations were significantly lower in the presbycusis group compared to the control group (left auditory regions: p = 0.002, right auditory regions: p = 0.008). Significant negative correlations were observed between PTA and GABA+ concentrations in the presbycusis group (r = -0.57, p = 0.02), while a similar trend was found in the control group (r = -0.40, p = 0.08). These results are consistent with a hypothesis of dysfunctional GABAergic neurotransmission in the central auditory system in presbycusis and suggest a potential treatment target for presbycusis.

Removal of cerebrospinal fluid partial volume effects in quantitative magnetization transfer imaging using a three-pool model with nonexchanging water component

PURPOSE

Parameters of the two-pool model describing magnetization transfer (MT) in macromolecule-rich tissues may be significantly biased in partial volume (PV) voxels containing cerebrospinal fluid (CSF). The purpose of this study was to develop a quantitative MT (qMT) method that provides indices insensitive to CSF PV averaging.

THEORY AND METHODS

We propose a three-pool MT model, in which PV macro-compartment is modeled as an additional nonexchanging water pool. We demonstrate the feasibility of model parameter estimation from several MT-weighted spoiled gradient echo datasets. We validated the three-pool model in numerical, phantom, and in vivo studies.

RESULTS

PV averaging with the free water compartment reduces all qMT parameters, most significantly affecting macromolecular proton fraction (MPF) and cross-relaxation rate. Monte-Carlo simulations confirmed stability of the three-pool model fit. Unlike the standard two-pool model, the three-pool model qMT parameters were not affected by PV averaging in simulations and phantom studies. The three-pool model fit allowed CSF PV correction in brain PV voxels and resulted in good correlation with standard two-pool model parameters in non-PV voxels.

CONCLUSION

Quantitative MT imaging based on a three-pool model with a non-exchanging water component yields a set of CSF-insensitive qMT parameters, which may improve MPF-based assessment of myelination in structures strongly affected by CSF PV averaging such as brain gray matter.

Inter-vendor reproducibility of pseudo-continuous arterial spin labeling at 3 Tesla

PURPOSE

Prior to the implementation of arterial spin labeling (ASL) in clinical multi-center studies, it is important to establish its status quo inter-vendor reproducibility. This study evaluates and compares the intra- and inter-vendor reproducibility of pseudo-continuous ASL (pCASL) as clinically implemented by GE and Philips.

MATERIAL AND METHODS

22 healthy volunteers were scanned twice on both a 3T GE and a 3T Philips scanner. The main difference in implementation between the vendors was the readout module: spiral 3D fast spin echo vs. 2D gradient-echo echo-planar imaging respectively. Mean and variation of cerebral blood flow (CBF) were compared for the total gray matter (GM) and white matter (WM), and on a voxel-level.

RESULTS

Whereas the mean GM CBF of both vendors was almost equal (p = 1.0), the mean WM CBF was significantly different (p<0.01). The inter-vendor GM variation did not differ from the intra-vendor GM variation (p = 0.3 and p = 0.5 for GE and Philips respectively). Spatial inter-vendor CBF and variation differences were observed in several GM regions and in the WM.

CONCLUSION

These results show that total GM CBF-values can be exchanged between vendors. For the inter-vendor comparison of GM regions or WM, these results encourage further standardization of ASL implementation among vendors.

Fast water concentration mapping to normalize (1)H MR spectroscopic imaging

OBJECT

To propose a fast and robust acquisition and post-processing pipeline that is time-compatible with clinical explorations to obtain a proton density (ρ) map used as a reference for metabolic map normalization. This allows inter-subject and inter-group comparisons of magnetic resonance spectroscopic imaging (MRSI) data and longitudinal follow-up for single subjects.

MATERIALS AND METHODS

A multi-echo T 2 (*) mapping sequence, the XEP sequence for B 1 (+) -mapping and Driven Equilibrium Single Pulse Observation of T 1-an optimized variable flip angle method for T 1 mapping used for both B 1 (-) -mapping and M 0 calculation-were used to determine correction factors leading to quantitative water proton density maps at 3T. Normalized metabolite maps were obtained on a phantom and nine healthy volunteers. To show the potential use of this technique at the individual level, we also explored one patient with low-grade glioma.

RESULTS

Accurate ρ maps were obtained both on phantom and volunteers. After signal normalization with the generated ρ maps, metabolic concentrations determined by the present method differed from theory by <7 % in the phantom and were in agreement with data from the literature for the healthy controls. Using these normalized metabolic values, it was possible to demonstrate in the patient with brain glioma, metabolic abnormalities in normalized N-acetyl aspartate, choline and creatine levels; illustrating the potential for direct use of this technique in clinical studies.

CONCLUSION

The proposed combination of sequences provides a robust ρ map that can be used to normalize metabolic maps in clinical MRSI studies.

Acoustic noise reduction in pseudo-continuous arterial spin labeling (pCASL)

OBJECT

While pseudo-continuous arterial spin labeling (pCASL) is a promising imaging technique to visualize cerebral blood flow, it is also (acoustically) very loud during labeling. In this paper, we reduced the labeling loudness on our scanner by increasing the interval between the RF pulses from the literature standard of 1.0 ms. We also propose recommendations to reduce the loudness on scanners of the same type at other sites.

MATERIALS AND METHODS

First, the sound pressure level (SPL) was both simulated and measured as a function of the labeling interval (1.0-1.8 ms) and longitudinal position in the scanner (-10 to +10 cm, relative to isocenter). Subsequently, we selected the labeling interval with the lowest overall SPL for the "SPL-optimized" pCASL sequence. Nine volunteers were scanned to compare raw signal intensity, temporal signal-to-noise ratio (tSNR) and labeling efficiency between the SPL-optimized and the standard PCASL sequence.

RESULTS

Sound pressure level measurements on our scanner showed that loudness was reduced by 6.5 dB at the approximate location of the ear by adjusting the labeling interval to 1.4 ms. Furthermore, image quality was not affected, since no significant differences in signal intensity, tSNR and labeling efficiency were observed.

CONCLUSION

By increasing the pCASL labeling interval, acoustic noise in the pCASL sequence was reduced with 6.5 dB, while image quality was preserved.

The interaction between apparent diffusion coefficients and transverse relaxation rates of human brain metabolites and water studied by diffusion-weighted spectroscopy at 7 T

The dependence of apparent diffusion coefficients (ADCs) of molecules in biological tissues on an acquisition-specific timescale is a powerful mechanism for studying tissue microstructure. Unlike water, metabolites are confined mainly to intracellular compartments, thus providing higher specificity to tissue microstructure. Compartment-specific structural and chemical properties may also affect molecule transverse relaxation times (T₂). Here, we investigated the correlation between diffusion and relaxation for N-acetylaspartate, creatine and choline compounds in human brain white matter in vivo at 7 T, and compared them with those of water under the same experimental conditions. Data were acquired in a volume of interest in parietal white matter at two different diffusion times, Δ = 44 and 246 ms, using a matrix of three echo times (T(E)) and five diffusion weighting values (up to 4575 s/mm²). Significant differences in the dependence of the ADCs on T(E) were found between water and metabolites, as well as among the different metabolites. A significant decrease in water ADC as a function of TE was observed only at the longest diffusion time (p < 0.001), supporting the hypothesis that at least part of the restricted water pool can be associated with longer T₂, as suggested by previous studies in vitro. Metabolite data showed an increase of creatine (p < 0.05) and N-acetylaspartate (p < 0.05) ADCs with TE at Δ = 44 ms, and a decrease of creatine (p < 0.05) and N-acetylaspartate (p = 0.1) ADCs with TE at Δ = 246 ms. No dependence of choline ADC on TE was observed. The metabolite results suggest that diffusion and relaxation properties are dictated not only by metabolite distribution in different cell types, but also by other mechanisms, such as interactions with membranes, exchange between "free" and "bound" states or interactions with microsusceptibility gradients.

Accuracy and precision of pseudo-continuous arterial spin labeling perfusion during baseline and hypercapnia: a head-to-head comparison with ¹⁵O H₂O positron emission tomography

Measurements of the cerebral blood flow (CBF) and cerebrovascular reactivity (CVR) provide useful information about cerebrovascular condition and regional metabolism. Pseudo-continuous arterial spin labeling (pCASL) is a promising non-invasive MRI technique to quantitatively measure the CBF, whereas additional hypercapnic pCASL measurements are currently showing great promise to quantitatively assess the CVR. However, the introduction of pCASL at a larger scale awaits further evaluation of the exact accuracy and precision compared to the gold standard. (15)O H₂O positron emission tomography (PET) is currently regarded as the most accurate and precise method to quantitatively measure both CBF and CVR, though it is one of the more invasive methods as well. In this study we therefore assessed the accuracy and precision of quantitative pCASL-based CBF and CVR measurements by performing a head-to-head comparison with (15)O H₂O PET, based on quantitative CBF measurements during baseline and hypercapnia. We demonstrate that pCASL CBF imaging is accurate during both baseline and hypercapnia with respect to (15)O H₂O PET with a comparable precision. These results pave the way for quantitative usage of pCASL MRI in both clinical and research settings.

Three-dimensional whole-brain perfusion quantification using pseudo-continuous arterial spin labeling MRI at multiple post-labeling delays: accounting for both arterial transit time and impulse response function

Measurement of the cerebral blood flow (CBF) with whole-brain coverage is challenging in terms of both acquisition and quantitative analysis. In order to fit arterial spin labeling-based perfusion kinetic curves, an empirical three-parameter model which characterizes the effective impulse response function (IRF) is introduced, which allows the determination of CBF, the arterial transit time (ATT) and T(1,eff). The accuracy and precision of the proposed model were compared with those of more complicated models with four or five parameters through Monte Carlo simulations. Pseudo-continuous arterial spin labeling images were acquired on a clinical 3-T scanner in 10 normal volunteers using a three-dimensional multi-shot gradient and spin echo scheme at multiple post-labeling delays to sample the kinetic curves. Voxel-wise fitting was performed using the three-parameter model and other models that contain two, four or five unknown parameters. For the two-parameter model, T(1,eff) values close to tissue and blood were assumed separately. Standard statistical analysis was conducted to compare these fitting models in various brain regions. The fitted results indicated that: (i) the estimated CBF values using the two-parameter model show appreciable dependence on the assumed T(1,eff) values; (ii) the proposed three-parameter model achieves the optimal balance between the goodness of fit and model complexity when compared among the models with explicit IRF fitting; (iii) both the two-parameter model using fixed blood T1 values for T(1,eff) and the three-parameter model provide reasonable fitting results. Using the proposed three-parameter model, the estimated CBF (46 ± 14 mL/100 g/min) and ATT (1.4 ± 0.3 s) values averaged from different brain regions are close to the literature reports; the estimated T(1,eff) values (1.9 ± 0.4 s) are higher than the tissue T1 values, possibly reflecting a contribution from the microvascular arterial blood compartment.

Quantitative magnetic susceptibility mapping without phase unwrapping using WASSR

The magnetic susceptibility of tissue within and around an image voxel affects the magnetic field and thus the local frequency in that voxel. Recently, it has been shown that spatial maps of frequency can be used to quantify local susceptibility if the contributions of surrounding tissue can be deconvolved. Currently, such quantitative susceptibility mapping (QSM) methods employ gradient recalled echo (GRE) imaging to measure spatial differences in the signal phase evolution as a function of echo time, from which frequencies can be deduced. Analysis of these phase images, however, is complicated by phase wraps, despite the availability and usage of various phase unwrapping algorithms. In addition, lengthy high-resolution GRE scanning often heats the magnet bore, causing the magnetic field to drift over several Hertz, which is on the order of the frequency differences between tissues. Here, we explore the feasibility of applying the WAter Saturation Shift Referencing (WASSR) method for 3D whole brain susceptibility imaging. WASSR uses direct saturation of water protons as a function of frequency irradiation offset to generate frequency maps without phase wraps, which can be combined with any image or spectroscopy acquisition. By utilizing a series of fast short-echo-time direct saturation images with multiple radiofrequency offsets, a frequency correction for field drift can be applied based on the individual image phases. Regions of interest were delineated with an automated atlas-based method, and the average magnetic susceptibilities calculated from frequency maps obtained from WASSR correlated well with those from the phase-based multi-echo GRE approach at 3T.

Cerebral blood flow response to acute hypoxic hypoxia

Hypoxic hypoxia (inspiratory hypoxia) stimulates an increase in cerebral blood flow (CBF) maintaining oxygen delivery to the brain. However, this response, particularly at the tissue level, is not well characterised. This study quantifies the CBF response to acute hypoxic hypoxia in healthy subjects. A 20-min hypoxic (mean P(ETO2) = 52 mmHg) challenge was induced and controlled by dynamic end-tidal forcing whilst CBF was measured using pulsed arterial spin labelling perfusion MRI. The rate constant, temporal delay and magnitude of the CBF response were characterised using an exponential model for whole-brain and regional grey matter. Grey matter CBF increased from 76.1 mL/100 g/min (95% confidence interval (CI) of fitting: 75.5 mL/100 g/min, 76.7 mL/100 g/min) to 87.8 mL/100 g/min (95% CI: 86.7 mL/100 g/min, 89.6 mL/100 g/min) during hypoxia, and the temporal delay and rate constant for the response to hypoxia were 185 s (95% CI: 132 s, 230 s) and 0.0035 s(-1) (95% CI: 0.0019 s(-1), 0.0046 s(-1)), respectively. Recovery from hypoxia was faster with a delay of 20 s (95% CI: -38 s, 38 s) and a rate constant of 0.0069 s(-1) (95% CI: 0.0020 s(-1), 0.0103 s(-1)). R2*, an index of blood oxygenation obtained simultaneously with the CBF measurement, increased from 30.33 s(-1) (CI: 30.31 s(-1), 30.34 s(-1)) to 31.48 s(-1) (CI: 31.47 s(-1), 31.49 s(-1)) with hypoxia. The delay and rate constant for changes in R2 * were 24 s (95% CI: 21 s, 26 s) and 0.0392 s(-1) (95% CI: 0.0333 s(-1), 0.045 s(-1)), respectively, for the hypoxic response, and 12 s (95% CI: 10 s, 13 s) and 0.0921 s(-1) (95% CI: 0.0744 s(-1), 0.1098 s(-1)/) during the return to normoxia, confirming rapid changes in blood oxygenation with the end-tidal forcing system. CBF and R2* reactivity to hypoxia differed between subjects, but only R2* reactivity to hypoxia differed significantly between brain regions.

Tissue- and column-specific measurements from multi-parameter mapping of the human cervical spinal cord at 3 T

The aim of this study was to quantify a range of MR parameters [apparent proton density, longitudinal relaxation time T1, magnetisation transfer (MT) ratio, MT saturation (which represents the additional percentage MT saturation of the longitudinal magnetisation caused by a single MT pulse) and apparent transverse relaxation rate R2*] in the white matter columns and grey matter of the healthy cervical spinal cord. The cervical cords of 13 healthy volunteers were scanned at 3 T using a protocol optimised for multi-parameter mapping. Intra-subject co-registration was performed using linear registration, and tissue- and column-specific parameter values were calculated. Cervical cord parameter values measured from levels C1-C5 in 13 subjects are: apparent proton density, 4822 ± 718 a.u.; MT ratio, 40.4 ± 1.53 p.u.; MT saturation, 1.40 ± 0.12 p.u.; T1 = 1848 ± 143 ms; R2* = 22.6 ± 1.53 s(-1). Inter-subject coefficients of variation were low in both the cervical cord and tissue- and column-specific measurements, illustrating the potential of this method for the investigation of changes in these parameters caused by pathology. In summary, an optimised cervical cord multi-parameter mapping protocol was developed, enabling tissue- and column-specific measurements to be made. This technique has the potential to provide insight into the pathological processes occurring in the cervical cord affected by neurological disorders.

T₁ mapping using variable flip angle SPGR data with flip angle correction

PURPOSE

To improve the calculation of T1 relaxation time from a set of variable flip-angle (FA) spoiled gradient recalled echo images.

MATERIALS AND METHODS

The proposed method: (a) uses a uniform weighting of all FAs, (b) takes into account global inaccuracies in the generation of the prescribed FAs by estimating the actual FAs, and (c) incorporates data-driven local B1 inhomogeneity corrections. The method was validated and its accuracy tested using simulated data, phantom, and in vivo experiments. Results were compared with existing analysis methods and to inversion recovery (IR). Consistency was assessed by means of repeated scans of two subjects. Reference values were obtained from eight healthy subjects from various brain regions and compared with literature values.

RESULTS

The method accurately and consistently estimated T1 values in all cases. The method was more robust, in comparison with the standard method, to the choice of FA set; to inaccuracies in generation of the prescribed FAs (in simulated data, T1 estimation error was 12.1 ms versus 235.5 ms); demonstrated greater consistency (in vivo study showed interscan T1 difference of 80 ms versus 356 ms); and achieved a better agreement with IR on phantom (median absolute difference of 123.8 ms versus 790 ms). Reference T1 values were 883/801 ms for female/male in white matter and 1501/1349 ms in gray matter, within the range previously reported.

CONCLUSION

The proposed method overcomes some inaccuracies in FA production, providing more accurate estimation of T1 values compared with standard methods, and is applicable for currently available data.

3T MRI improves the detection of transmantle sign in type 2 focal cortical dysplasia

PURPOSE

Type 2 focal cortical dysplasia (FCD2) is one of the main causes of refractory partial epilepsy, but often remains overlooked by MRI. This study aimed to elucidate whether 3T MRI offers better detection and characterization of FCD2 than 1.5T, using similar coils and acquisition time.

METHODS

Two independent readers reviewed the 1.5T and 3T MR images of 25 patients with histologically proven FCD2. For both magnetic fields, the ability to detect a lesion was analyzed. We compared the identification of each of the five criteria typical of FCD2 (cortical thickening, blurring, cortical signal changes, subcortical signal changes, and "transmantle" sign) and artifacts, using a four-point scale (0-3). Interobserver reliability for lesion detection was calculated.

KEY FINDINGS

Seventeen lesions (68%) were detected at 3T, two of which were overlooked at 1.5T. Interobserver reliability was better at 3T (κ = 1) than at 1.5T (κ = 0.83). The transmantle sign was more clearly identified at 3T than 1.5T (mean visualization score: 1.72 vs. 0.56; p = 0.002).

SIGNIFICANCE

The use of 3T MRI in patients suspected of type 2 FCD improves the detection rate and the lesion characterization owing to the transmantle sign being more clearly seen at 3T. This point is of interest, since this feature is considered as an MR signature of FCD2.

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.

Quantification of subcortical gray-matter vascularization using 7 Tesla time-of-flight angiography

Background The integrity of subcortical brain nuclei is associated with maintenance of regular cognitive performance levels and has been shown to be particularly affected by aging-related vascular pathology. This study aims to demonstrate applicability of high field strength magnetic resonance angiography at 7 Tesla (7T) for assessment of interindividual variation in subcortical vascularization. Methods Two healthy female subjects without known history of cerebrovascular disease or malformation, aged 43 and 86 years, respectively, were administered three-dimensional (3D) high-resolution time-of-flight (TOF) magnetic resonance angiography at 7T. The FreeSurfer software package was used for automated parcellation and assessment of subcortical volumes. For each volume, mean regional intensities were calculated based on the TOF contrast as a quantitative reflection of regional subcortical gray-matter vascularization. Results While volumes of the subcortical brain region assessed did not differ significantly (30.2 and 27.8 mL, P = 0.78), mean intensities were significantly reduced in the older participant (10%, P = 0.004). Mean intensities could be assessed for each participant for 14 subcortical structures, strongest differences were observable for the left and right Thalamus (T [left, right] = 3.85, 3.82; P [left, right] = 0.002, 0.003). Conclusions High-resolution TOF magnetic resonance angiography may be used in combination with automated volume-based parcellation to quantify regional subcortical vascularization and to assess interindividual differences. Additional studies are necessary to assess its potential use in clinical trials on cerebrovascular integrity in a context of aging-related brain change.

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.

Optimization of scan parameters for T₁-FLAIR imaging at 1.5 and 3T using computer simulation

PURPOSE

We attempted to optimize scan parameters for T₁-weighted fluid-attenuated inversion recovery (T₁-FLAIR) sequence at 3 and 1.5 tesla (T) using computer simulation.

METHODS

We measured the T₁ and T₂ relaxation time values (T₁v and T₂v) of gray (GM) and white matter (WM) at 3 and 1.5T, generated computer-simulated T₁-FLAIR (CS-T₁-FLAIR) images using those values, and compared the simulated and actual T₁-FLAIR images to verify the contrast reliability of our computer simulation. We mathematically and visually evaluated CS-T₁-FLAIR images at various repetition times (TR) and echo times (TE).

RESULTS

At 3T, the measured relaxation values for GM were T₁v, 1524 ms, and T₂v, 85 ms, and for WM, T₁v, 750 ms, and T₂v, 65 ms. At 1.5T, the measured relaxation values for GM were T₁v, 1251 ms, and T₂v, 99 ms, and for WM, T₁v, 623 ms, and T₂v, 75 ms. Contrast of CS-T₁-FLAIR and actual T₁-FLAIR images was identical. An optimal TR of 3140 ms was determined for T₁-FLAIR at 3T and 2440 ms at 1.5T based on mathematical evaluation. The optimal TR ranges were 2400 to 3900 ms at 3T and 1800 to 3200 ms at 1.5T based on visual assessment of CS-T₁-FLAIR. A shorter TE provided better T₁ contrast.

CONCLUSION

We optimized T₁-FLAIR by focusing on its most important scan parameters using computer simulations and determined that a longer TR was suitable at 3T than at 1.5T. Our computer simulation was useful for determining the optimal scan parameters.

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.

Life-long aerobic exercise preserved baseline cerebral blood flow but reduced vascular reactivity to CO2

PURPOSE

To examine the potential benefits of life-long aerobic exercise on brain health, in particular cerebrovascular function.

MATERIALS AND METHODS

Ten Masters athletes (MA) (seven males, three females; 74.5 ± 5.8 years) and 10 sedentary elderly individuals (SE) (eight males, two females; 75.4 ± 5.6 years) were recruited and baseline cerebral blood flow (CBF) and cerebral vascular reactivity (CVR) to CO2 were measured on a 3T MRI scanner. Nine sedentary young subjects were also recruited to serve as a control group to verify the age effect.

RESULTS

When compared to the SE group, MA showed higher CBF in posterior cingulate cortex/precuneus, which are key regions of the default-mode-network and are known to be highly sensitive to age and dementia. CVR in the MA brains were paradoxically lower than that in SE. This effect was present throughout the brain. Within the MA group, individuals with higher VO2max had an even lower CVR, suggesting a dose-response relationship.

CONCLUSION

Life-long aerobic exercise preserved blood supply in the brain's default-mode-network against age-related degradation. On the other hand, its impact on the cerebral vascular system seems to be characterized by a dampening of CO2 reactivity, possibly because of desensitization effects due to a higher lifetime exposure.

Optimal visualization of multiple brain metastases for gamma knife radiosurgery

BACKGROUND

Optimal management of metastatic brain disease requires precise detection and detailed characterization of all intracranial lesions.

METHODS

We analyzed an experience with 3200 brain MRI investigations performed at 1.5 T and 3.0 T for identification and/or evaluation of intracranial metastases. Usually axial T1- and T2-weighted images and contrast-enhanced T1-weighted images in axial and coronal and/or sagittal projections were obtained. Fluid-attenuated inversion recovery and diffusion-weighted imaging were sometimes used as well. Routinely, 0.2 mmol/kg of gadoteridol (ProHance®) was administered intravenously, but the dose was reduced to 0.1 mmol/kg in elderly patients or in patients with mild renal dysfunction.

FINDINGS

Magnetic resonance imaging (MRI) provided excellent information on tumor location; interrelations with functionally important intracranial structures; type of growth; vascularity; recent, old or multiple hemorrhages within or in the vicinity of the mass; presence of peritumoral edema; necrotic changes; subarachnoid dissemination; meningeal carcinomatosis. However, without administration of gadoteridol or without contrast enhancement, small metastatic tumors could not be reliably distinguished from brain lacunes. Some metastases (malignant melanoma, thyroid cancer, endocrine carcinoma, small cell lung carcinoma) may demonstrate specific neuroimaging features. Non-metastatic -multiple brain lesions caused by vascular, inflammatory, demyelinative or lymphoproliferative diseases require a thorough differential diagnosis with metastatic brain tumors based not only on neuroimaging but on additional analysis of various clinical data.

CONCLUSION

Contemporary MRI techniques provide excellent options for detection, detailed characterization, and differential diagnosis of metastatic brain tumors, which is extremely important when choosing the optimal treatment strategy, particularly with Gamma Knife radiosurgery.

Model-based Acceleration of Parameter mapping (MAP) for saturation prepared radially acquired data

A reconstruction technique called Model-based Acceleration of Parameter mapping (MAP) is presented allowing for quantification of longitudinal relaxation time and proton density from radial single-shot measurements after saturation recovery magnetization preparation. Using a mono-exponential model in image space, an iterative fitting algorithm is used to reconstruct one well resolved and consistent image for each of the projections acquired during the saturation recovery relaxation process. The functionality of the algorithm is examined in numerical simulations, phantom experiments, and in-vivo studies. MAP reconstructions of single-shot acquisitions feature the same image quality and resolution as fully sampled reference images in phantom and in-vivo studies. The longitudinal relaxation times obtained from the MAP reconstructions are in very good agreement with the reference values in numerical simulations as well as phantom and in-vivo measurements. Compared to available contrast manipulation techniques, no averaging of projections acquired at different time points of the relaxation process is required in MAP imaging. The proposed technique offers new ways of extracting quantitative information from single-shot measurements acquired after magnetization preparation. The reconstruction simultaneously yields images with high spatiotemporal resolution fully consistent with the acquired data as well as maps of the effective longitudinal relaxation parameter and the relative proton density.

Partial volume correction in arterial spin labeling using a Look-Locker sequence

PURPOSE

Partial volume (PV) effects are caused by limited spatial resolution and significantly affect cerebral blood flow investigations with arterial spin labeling. Therefore, accurate PV correction (PVC) procedures are required. PVC is commonly based on PV maps obtained from segmented high-resolution T1 -weighted images. Segmentation of these images is error-prone, and it can be difficult to coregister these images accurately with the single-shot ASL images such as those created by echo-planar imaging (EPI). In this paper, an alternative method for PV map generation is proposed.

METHODS

The Look-Locker EPI (LL-EPI) acquisition is used for analyzing the T1 -recovery curve and for subsequent PV map generation. The new method was evaluated in five healthy volunteers (mean age 30 ± 3.7 years).

RESULTS

By applying a linear regression method for PVC, a 12% decrease in regression error was reached with the new method.

CONCLUSION

PV maps extraction from LL-EPI is a viable, possibly superior alternative to the standard approach based on segmentation of high-resolution T1 -weighted images.

Comparison of spin-echo T1- and T2-weighted and gradient-echo T1-weighted images at 3T in evaluating very preterm neonates at term-equivalent age

Term-equivalent imaging can assess myelination status in very preterm infants (<30 weeks' gestational age at birth). However, myelination assessment has yet to be compared among GRE-T1WI, SE-T1WI, and SE-T2WI at 3T. We aimed to compare the rates of myelination among those 3 sequences in 11 very preterm neonates who underwent 3T MR imaging at term-equivalent age and subsequently had normal neurologic development. On each sequence, 2 neuroradiologists individually assessed 22 structures. SE-T2WI depicted a higher myelination rate (present in 58.2%-66.4% of all structures) than either GRE-T1WI (51.6%-63.9%) or SE-T1WI (20.5%-38.5%), while GRE-T1WI had the highest interobserver agreement (κ, 0.56; P < .0001). Myelination was present in 90%-100% of patients within the corpus callosum splenium, DSCP, ICP, lateral lemniscus, and spinal tract/nucleus of cranial nerve V on SE-T2WI, and in the DSCP, ICP, lateral lemniscus, medial lemniscus, pyramidal decussation, PLIC, and superior cerebellar peduncle on GRE-T1WI, occurring in similar structures as previously shown at 1.5T and 1T. However, it is not clear whether these findings represent true myelination versus precursors to myelination.

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.

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.

Comparison of relative cerebral blood flow maps using pseudo-continuous arterial spin labeling and single photon emission computed tomography

Pseudo-continuous arterial spin labeling (PCASL) MRI is a relatively new arterial spin labeling technique and has the potential to extend the cerebral blood flow (CBF) measurement to all tissue types, including white matter. However, the arterial transit time (δ(a)) for white matter is not well established and a limited number of reports using multi-delay methods have yielded inconsistent findings. In this study, we used a different approach and measured white matter δ(a) (mean ± standard deviation, 1541 ± 173  ms) by determining the arrival times of exogenous contrast agent in a bolus tracking experiment. The data also confirmed δ(a) of gray matter to be 912 ± 209  ms. In the second part of this study, we used these parameters in PCASL kinetic models and compared relative CBF (rCBF, with respect to the whole brain) maps with those measured using a single photon emission computed tomography (SPECT) technique. It was found that the use of tissue-specific δ(a) in the PCASL model was helpful in improving the correspondence between the two modalities. On a regional level, the gray/white matter CBF ratios were 2.47 ± 0.39 and 2.44 ± 0.18 for PCASL and SPECT, respectively. On a single-voxel level, the variance between the modalities was still considerable, with an average rCBF difference of 0.27.

Neuroradiology back to the future: brain imaging

The beginning of neuroradiology can be traced to the early 1900s with the use of skull radiographs. Ventriculography and pneumoencephalography were introduced in 1918 and 1919, respectively, and carotid angiography, in 1927. Technical advances were made in these procedures during the next 40 years that lead to improved diagnosis of intracranial pathology. Yet, they remained invasive procedures that were often uncomfortable and associated with significant morbidity. The introduction of CT in 1971 revolutionized neuroradiology. Ventriculography and pneumoencephalography were rendered obsolete. The imaging revolution continued with the advent of MR imaging in the early 1980s. Noninvasive angiographic techniques have curtailed the use of conventional angiography, and physiologic imaging gives us a window into the function of the brain. In this historical review, we will trace the origin and evolution of the advances that have led to the quicker, less invasive diagnosis and resulted in more rapid therapy and improved outcomes.

Magnetisation transfer effects of Q2TIPS pulses in ASL

OBJECT

In pulsed arterial spin labelling (ASL), Q2TIPS saturation pulses are used to actively control the temporal width of the labelled bolus. However, these Q2TIPS pulses also induce magnetisation transfer (MT) effects in the adjacent tissue. In this work, we investigated how Q2TIPS-related MT alters tissue signal in pulsed ASL and, consequently, CBF quantification.

MATERIALS AND METHODS

Seven volunteers were studied at 3 tesla using a multi-TI FAIR sequence and 3D-GRASE readout with background suppression. Q2TIPS pulses were used and the spacing between RF pulses was varied to modulate MT effects. Computer simulations were designed to mimic in-vivo signals at multiple TI values.

RESULTS

Q2TIPS-associated MT was found to reduce tissue T1 and M0 values by up to 42 and 50% respectively; leading to a reduction of up to 40% in the effectiveness of background suppression and, therefore, increased sensitivity to motion for the longest TI values. In addition, greater MT effects were associated with reduced grey matter CBF estimates of up to 15%.

CONCLUSIONS

The MT effect associated with the Q2TIPS pulse train has a significant effect on tissue signal. It is recommended that MT effects are characterised and both background suppression and Q2TIPS schemes are accordingly optimised to reduce the effects of MT on accuracy and precision of CBF estimation.

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.

The relationship of resting cerebral blood flow and brain activation during a social cognition task in adolescents with chronic moderate to severe traumatic brain injury: a preliminary investigation

Alterations in cerebrovascular function are evident acutely in moderate to severe traumatic brain injury (TBI), although less is known about their chronic effects. Adolescent and adult patients with moderate to severe TBI have been reported to demonstrate diffuse activation throughout the brain during functional magnetic resonance imaging (fMRI). Because fMRI is a measure related to blood flow, it is possible that any deficits in blood flow may alter activation. An arterial spin labeling (ASL) perfusion sequence was performed on seven adolescents with chronic moderate to severe TBI and seven typically developing (TD) adolescents during the same session in which they had performed a social cognition task during fMRI. In the TD group, prefrontal CBF was positively related to prefrontal activation and negatively related to non-prefrontal, posterior, brain activation. This relationship was not seen in the TBI group, who demonstrated a greater positive relationship between prefrontal CBF and non-prefrontal activation than the TD group. An analysis of CBF data independent of fMRI showed reduced CBF in the right non-prefrontal region (p<.055) in the TBI group. To understand any role reduced CBF may play in diffuse extra-activation, we then related the right non-prefrontal CBF to activation. CBF in the right non-prefrontal region in the TD group was positively associated with prefrontal activation, suggesting an interactive role of non-prefrontal and prefrontal blood flow throughout the right hemisphere in healthy brains. However, the TBI group demonstrated a positive association with activation constrained to the right non-prefrontal region. These data suggest a relationship between impaired non-prefrontal CBF and the presence of non-prefrontal extra-activation, where the region with more limited blood flow is associated with activation limited to that region. In a secondary analysis, pathology associated with hyperintensities on T2-weighted FLAIR imaging over the whole brain was related to whole brain activation, revealing a negative relationship between lesion volume and frontal activation, and a positive relationship between lesion volume and posterior activation. These preliminary data, albeit collected with small sample sizes, suggest that reduced non-prefrontal CBF, and possibly pathological tissue associated with T2-hyperintensities, may provide contributions to the diffuse, primarily posterior extra-activation observed in adolescents following moderate to severe TBI.

The nature of memory failure in mild cognitive impairment: examining association with neurobiological markers and effect of progression

The main goal of this study was to assess vulnerability to proactive interference and memory binding capacity, the ability to combine different information into a single coherent memory event, in persons with mild cognitive impairment (MCI). We also examined whether hippocampal atrophy and vascular burden were differentially related to these memory capacities in MCI. We further assessed whether memory performance and brain changes differ as a function of later development (or not) of dementia and whether they can predict progression to dementia. The study included 77 participants, 49 meeting the criteria for MCI and 28 healthy older adults. Results showed binding deficits and greater vulnerability to proactive interference in persons with MCI compared with healthy older adults. Hippocampal volume was associated with binding capacity, whereas vascular burden was associated with resistance to interference in persons with MCI. Follow-up analyses indicated that binding deficits predict progression from MCI to dementia. In conclusion, binding deficits and vulnerability to proactive interference are present in persons with MCI and are associated with different brain markers. However, only binding deficits predict progression to dementia.

Reduced resolution transit delay prescan for quantitative continuous arterial spin labeling perfusion imaging

Arterial spin labeling perfusion MRI can suffer from artifacts and quantification errors when the time delay between labeling and arrival of labeled blood in the tissue is uncertain. This transit delay is particularly uncertain in broad clinical populations, where reduced or collateral flow may occur. Measurement of transit delay by acquisition of the arterial spin labeling signal at many different time delays typically extends the imaging time and degrades the sensitivity of the resulting perfusion images. Acquisition of transit delay maps at the same spatial resolution as perfusion images may not be necessary, however, because transit delay maps tend to contain little high spatial resolution information. Here, we propose the use of a reduced spatial resolution arterial spin labeling prescan for the rapid measurement of transit delay. Approaches to using the derived transit delay information to optimize and quantify higher resolution continuous arterial spin labeling perfusion images are described. Results in normal volunteers demonstrate heterogeneity of transit delay across different brain regions that lead to quantification errors without the transit maps and demonstrate the feasibility of this approach to perfusion and transit delay quantification.

Gray matter nulled and vascular space occupancy dependent fMRI response to visual stimulation during hypoxic hypoxia

Two cerebral blood volume (CBV)-weighted fMRI techniques, gray matter nulled (GMN) and vascular space occupancy (VASO)-dependent techniques at spatial resolution of 2 × 2 × 5 mm(3), were compared in the study investigating functional responses in the human visual cortex to stimulation in normoxia (inspired O(2) = 21%) and mild hypoxic hypoxia (inspired O(2) = 12%). GMN and VASO signals and T(2)* were quantified in activated voxels. While the CBV-weighted signal changes in voxels activated by visual stimulation were similar in amplitude in both fMRI techniques in both oxygenation conditions, the number of activated voxels during hypoxic hypoxia was significantly reduced by 72 ± 22% in GMN fMRI and 66 ± 23% in VASO fMRI. T(2)* prolonged in GMN and VASO activated voxels in normoxia by 1.6 ± 0.5 ms and 1.7 ± 0.5 ms, respectively. In hypoxia, however, T(2)* shortened in GMN-activated voxels by 0.7 ± 0.6 ms (p < 0.001 relative to normoxia), but prolonged in VASO-activated ones by 1.1 ± 0.6 ms (p < 0.05 relative to normoxia). The data show that the hemodynamic responses to visual stimulation were not affected by hypoxic hypoxia, but T(2)* increases by both CBV-weighted fMRI techniques were smaller in activated voxels in hypoxia. The mechanisms influencing GMN fMRI signal in both oxygenation conditions were explored by simulating effects of the oxygen extraction fraction (OEF) and partial voluming with cerebral spinal fluid (CSF) and white matter in imaging voxels. It is concluded that while GMN fMRI data point to increased, rather than decreased OEF during visual stimulation in hypoxia, partial voluming by CSF is likely to affect the CBV quantification by GMN fMRI under the experimental conditions used.

Smaller regional gray matter volume in homeless african american cocaine-dependent men: a preliminary report

Models of addiction include abnormalities in parts of the brain involving executive function/inhibitory control. Although previous studies have reported evidence of structural abnormalities in cocaine-dependent individuals, none have specifically targeted the homeless. The present preliminary study investigated brain structure in such an understudied group, homeless, crack-cocaine-dependent African American men (n = 9), comparing it to that in healthy controls (n = 8). Structural data were analyzed using voxel based morphometry (VBM) and a regions of interest (ROI) analysis. Homeless cocaine-dependent individuals had smaller gray matter volume in dorsolateral prefrontal cortex, anterior cingulate, the cerebellum, insula, and superior temporal gyrus. Most of these areas subserve executive function or inhibitory control. These results are similar to those found in most previous studies of non-homeless cocaine-dependent individuals. Reduced gray matter in executive function/inhibitory control regions of the brain in cocaine-dependent individuals may be a preexisting risk factor for the development of addiction and/or a consequence of drug abuse.

Algebraic T2 estimation improves detection of right temporal lobe epilepsy by MR T2 relaxometry

Seizure related abnormalities may be detected with T2 relaxometry, which involves quantitative estimation of T2 values. Accounting for the partial-volume effect of cerebrospinal fluid (CSF) is important, especially for voxel-based relaxometry, VBR. With a mono-exponential decay model, this can be accomplished by including a baseline constant. An algebraic calculation, which accommodates this constant, offers improved T2 estimation speed over the commonly used non-linear fitting approach. Our objective was to compare the algebraic approach against three fitting approaches for the detection of seizure related abnormalities. We tested the performance of the four methods in the presence of noise using simulated data as well as real data acquired at 3 T with a Carr-Purcell-Meiboom-Gill sequence from 45 healthy subjects and 24 patients with confirmed right temporal lobe epilepsy. A quantitative analysis was performed on spatially normalized data by measuring T2 in various regions and with a whole brain tissue segmentation analysis. The detection rate of hippocampal T2 changes in patients was assessed by comparing the regional T2 measurements from each patient against the control data with a z-score threshold of 2.33. The algebraic method yielded high sensitivity for detection of hippocampal abnormalities in the epileptic patients in regional assessment and in follow-up single-subject VBR. This can be attributed to the relatively small variance across healthy subjects and improved precision in the presence of CSF and noise in simulation. In conclusion, the algebraic method is better than fitting based on faster calculation speed and better sensitivity for detecting seizure-related T2 changes.

Sensitivity calibration with a uniform magnetization image to improve arterial spin labeling perfusion quantification

Quantification of perfusion with arterial spin labeling MRI requires a calibration of the imaging sensitivity to water throughout the imaged volume. Since this sensitivity is affected by coil loading and other interactions between the subject and the scanner, the sensitivity must be calibrated in the subject at the time of scan. Conventional arterial spin labeling perfusion quantification assumes a uniform proton density and acquires a proton density reference image to serve as the calibration. This assumption, in the form of an assumed constant brain-blood partition coefficient, incorrectly adds inverse proton density weighting to the perfusion image. Here, a sensitivity calibration is proposed by generating a uniform magnetization image whose intensity is highly independent of brain tissue type. It is shown that such a uniform magnetization image can be achieved, and brain tissue perfusion values quantified with the sensitivity calibration agree with those quantified with a proton density image when segmentation of brain tissues is performed and appropriate partition coefficients are assumed. Quantification of brain tissue water density is also demonstrated using this sensitivity calibration. This approach can improve and simplify quantification of arterial spin labeling perfusion and may have broader applications to measurement of edema and sensitivity calibration for parallel imaging.

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.

Identification of signal bias in the variable flip angle method by linear display of the algebraic Ernst equation

A novel linear parameterization for the variable flip angle method for longitudinal relaxation time T₁ quantification from spoiled steady state MRI is derived from the half angle tangent transform, τ, of the flip angle. Plotting the signal S at coordinates x = Sτ and y = S/τ, respectively, establishes a line that renders signal amplitude and relaxation term separately as y-intercept and slope. This representation allows for estimation of the respective parameter from the experimental data. A comprehensive analysis of noise propagation is performed. Numerical results for efficient optimization of longitudinal relaxation time and proton density mapping experiments are derived. Appropriate scaling allows for a linear presentation of data that are acquired at different short pulse repetition times, TR << T1 thus increasing flexibility in the data acquisition by removing the limitation of a single pulse repetition time. Signal bias, like due to slice-selective excitation or imperfect spoiling, can be readily identified by systematic deviations from the linear plot. The method is illustrated and validated by 3T experiments on phantoms and human brain. Magn Reson Med, 2011. © 2011 Wiley-Liss, Inc.

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.

[Evaluation of crosstalk effect on spin-echo images at 1.5 and 3 T]

The purpose of this study is to evaluate the crosstalk effect on spin-echo (SE) images at 1.5 and 3 T MRI. We examined the influence of crosstalk by comparing the full width at half-maximum (FWHM) and slice profile of images of a wedge-shaped phantom for various slice gaps. We also assessed crosstalk effect in the brain by comparing image contrast among healthy volunteers (n=8). Among the subjects, the shapes of the slice profiles at 1.5 T were similar to those at 3 T for long repetition times (TRs); however, at shorter TRs, differences in slice profiles were observed among the subjects and were more apparent at 3 than at 1.5 T. The relative contrast between white matter and gray matter on T(1)-weighted images was lower at 3 than at 1.5 T. The crosstalk effect was strongest when the TR of the excitation pulse was short. The influence of the adjacent excitation pulse is important in the process of T(1) relaxation because T(1) values are greater at 3 T. In conclusion, the influence of crosstalk on SE T(1)-weighted images is greater at 3 than at 1.5 T.