Absolute quantitation of brain metabolites with respect to heterogeneous tissue compositions in 1H-MR spectroscopic volumes

Object

Referencing metabolite intensities to the tissue water intensity is commonly applied to determine metabolite concentrations from in vivo ¹H-MRS brain data. However, since the water concentration and relaxation properties differ between grey matter, white matter and cerebrospinal fluid (CSF), the volume fractions of these compartments have to be considered in MRS voxels.

Materials and methods

The impact of partial volume correction was validated by phantom measurements in voxels containing mixtures of solutions with different NAA and water concentrations as well as by analyzing in vivo ¹H-MRS brain data acquired with various voxel compositions.

Results

Phantom measurements indicated substantial underestimation of NAA concentrations when assuming homogeneously composed voxels, especially for voxels containing solution, which simulated CSF (error: ≤92%). This bias was substantially reduced by taking into account voxel composition (error: ≤10%). In the in vivo study, tissue correction reduced the overall variation of quantified metabolites by up to 35% and revealed the expected metabolic differences between various brain tissues.

Conclusions

Tissue composition affects extraction of metabolite concentrations and may cause misinterpretations when comparing measurements performed with different voxel sizes. This variation can be reduced by considering the different tissue types by means of combined analysis of spectroscopic and imaging data.     

Diffusion time dependence of magnetic resonance diffusion signal decays: an investigation of water exchange in human brain in vivo

Object

To understand the behavior of diffusion signal decays of water in white matter of human brain in vivo and to estimate tissue microstructure parameters such as exchange time of diffusing water molecules in human brain.

Materials and methods

Diffusion decays were measured over an extended range of diffusion weightings (b-values) up to a maximum of 12,500?s/mm² and diffusion times between 19.9 and 53.8?ms in eight healthy human subjects using MRI scans. The diffusion signal decays were all Rician noise corrected and then analyzed using multi-component non-negative least squares (NNLS) data analysis.

Results

Three diffusion coefficients including one at (0.930?±?0.003)?×?10^?3 (80?±?1%)?mm²/s, another at (0.067?±?0.002)?×?10^?3 (19?±?1%)?mm²/s and a small contribution at (1.20?±?0.02)?×?10^?2 (1.00?±?0.01%)?mm²/s were observed in the diffusion decay using the highest b-value. The diffusion decays show diffusion time dependence for the slow diffusion coefficient which has not previously been reported.

Conclusion

This study presents the accurate diffusion parameters by the use of very large b-values along with Rician noise correction and multi-component data analysis. The experimental results are consistent with the theoretical predictions used to estimate the exchange time of diffusing water molecules for a model of human brain tissue.     

Visualization of immune cell infiltration in experimental viral myocarditis by 19F MRI in vivo

Objective

This paper introduces a new approach permitting for the first time a specific, non-invasive diagnosis of myocarditis by visualizing the infiltration of immune cells into the myocardium.

Materials and methods

The feasibility of this approach is shown in a murine model of viral myocarditis. Our study uses biochemically inert perfluorocarbons (PFCs) known to be taken up by circulating monocytes/macrophages after intravenous injection.

Results

In vivo ¹⁹F MRI at 9.4 T demonstrated that PFC-loaded immune cells infiltrate into inflamed myocardial areas. Because of the lack of any fluorine background in the body, detected ¹⁹F signals of PFCs are highly specific as confirmed ex vivo by flow cytometry and histology.

Conclusion

Since PFCs are a family of compounds previously used clinically as blood substitutes, the technique described in our paper holds the potential as a new imaging modality for the diagnosis of myocarditis in man.     

Direct cerebral and cardiac 17O-MRI at 3 Tesla: initial results at natural abundance

Purpose

To establish direct ¹⁷O-magnetic resonance imaging (MRI) for metabolic imaging at a clinical field strength of 3 T.

Methods

An experimental setup including a surface coil and transmit/receive switch was constructed. Natural abundance in vivo brain images of a volunteer were acquired with a radial three-dimensional (3D) sequence in the visual cortex and in the heart with electrocardiogram (ECG)-gating.

Results

In the brain, a signal-to-noise ratio of 36 was found at a nominal resolution of (5.6 mm)³, and a transverse relaxation time of T₂* = (1.9 ± 0.2) ms was obtained. In the heart ¹⁷O images were acquired with a temporal resolution of 200 ms.

Conclusion

Cerebral and cardiac ¹⁷O-MRI at natural abundance is feasible at 3 T.     

In vivo chlorine and sodium MRI of rat brain at 21.1 T

Object

MR imaging of low-gamma nuclei at the ultrahigh magnetic field of 21.1 T provides a new opportunity for understanding a variety of biological processes. Among these, chlorine and sodium are attracting attention for their involvement in brain function and cancer development.

Materials and methods

MRI of ³⁵Cl and ²³Na were performed and relaxation times were measured in vivo in normal rat (n = 3) and in rat with glioma (n = 3) at 21.1 T. The concentrations of both nuclei were evaluated using the center-out back-projection method.

Results

T ₁ relaxation curve of chlorine in normal rat head was fitted by bi-exponential function (T _1a = 4.8 ms (0.7) T _1b = 24.4 ± 7 ms (0.3) and compared with sodium (T ₁ = 41.4 ms). Free induction decays (FID) of chlorine and sodium in vivo were bi-exponential with similar rapidly decaying components of $ T_{{2{\text{a}}}}^{*} = 0.4 $  ms and $ T_{{2{\text{a}}}}^{*} = 0.53 $  ms, respectively. Effects of small acquisition matrix and bi-exponential FIDs were assessed for quantification of chlorine (33.2 mM) and sodium (44.4 mM) in rat brain.

Conclusion

The study modeled a dramatic effect of the bi-exponential decay on MRI results. The revealed increased chlorine concentration in glioma (~1.5 times) relative to a normal brain correlates with the hypothesis asserting the importance of chlorine for tumor progression.     

Numerical and experimental evaluation of RF shimming in the human brain at 9.4 T using a dual-row transmit array

Objective

To provide a numerical and experimental investigation of the static RF shimming capabilities in the human brain at 9.4 T using a dual-row transmit array.

Materials and methods

A detailed numerical model of an existing 16-channel, inductively decoupled dual-row array was constructed using time-domain software together with circuit co-simulation. Experiments were conducted on a 9.4 T scanner. Investigation of RF shimming focused on B₁ ⁺ homogeneity, efficiency and local specific absorption rate (SAR) when applied to large brain volumes and on a slice-by-slice basis.

Results

Numerical results were consistent with experiments regarding component values, S-parameters and B₁ ⁺ pattern, though the B₁ ⁺ field was about 25 % weaker in measurements than simulations. Global shim settings were able to prevent B₁ ⁺ field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B₁ ⁺ standard deviations of below 10 % without field dropouts could be achieved in axial, sagittal and coronal orientations across the brain, even with phase-only shimming, but decreased B₁ ⁺ efficiency and SAR limitations must be considered.

Conclusion

Dual-row transmit arrays facilitate flexible 3D RF management across the entire brain at 9.4 T in order to trade off B₁ ⁺ homogeneity against power-efficiency and local SAR.     

Exploring and enhancing relaxation-based sodium MRI contrast

Object

Sodium MRI is typically concerned with measuring tissue sodium concentration. This requires the minimization of relaxation weighting. However, ²³Na relaxation may itself be interesting to explore, given an underlying mechanism (i.e. the electric-quadrupole-moment–electric-field-gradient interaction) that differs from ¹H. A new sodium sequence was developed to enhance ²³Na relaxation contrast without decreasing signal-to-noise ratio.

Materials and Methods

The new sequence, labeled Projection Acquisition in the steady-state with Coherent MAgNetization (PACMAN), uses gradient refocusing of transverse magnetization following readout, a short repetition time, and a long radiofrequency excitation pulse. It was developed using simulation, verified in model environments (saline and agar), and evaluated in the brain of three healthy adult volunteers.

Results

Projection Acquisition in the steady-state with Coherent MAgNetization generates a large positive contrast-to-noise ratio (CNR) between saline and agar, matching simulation-based design. In addition to enhanced CNR between cerebral spinal fluid and brain tissue in vivo, PACMAN develops substantial contrast between gray and white matter. Further simulation shows that PACMAN has a ln(T _2f/T ₁) contrast dependence (where T _2f is the fast component of ²³Na T ₂), as well as residual quadrupole interaction dependence.

Conclusion

The relaxation dependence of PACMAN sodium MRI may provide contrast related to macromolecular tissue structure.     

Evaluation of an attenuation correction method for PET/MR imaging of the head based on substitute CT images

Object

The aim of this study was to evaluate MR-based attenuation correction of PET emission data of the head, based on a previously described technique that calculates substitute CT (sCT) images from a set of MR images.

Materials and methods

Images from eight patients, examined with ¹⁸F-FLT PET/CT and MRI, were included. sCT images were calculated and co-registered to the corresponding CT images, and transferred to the PET/CT scanner for reconstruction. The new reconstructions were then compared with the originals. The effect of replacing bone with soft tissue in the sCT-images was also evaluated.

Results

The average relative difference between the sCT-corrected PET images and the CT-corrected PET images was 1.6 % for the head and 1.9 % for the brain. The average standard deviations of the relative differences within the head were relatively high, at 13.2 %, primarily because of large differences in the nasal septa region. For the brain, the average standard deviation was lower, 4.1 %. The global average difference in the head when replacing bone with soft tissue was 11 %.

Conclusion

The method presented here has a high rate of accuracy, but high-precision quantitative imaging of the nasal septa region is not possible at the moment.     

The size of cytoplasmic lipid droplets varies between tumour cell lines of the nervous system: a 1H NMR spectroscopy study

Object

Cytoplasmic lipid droplets (LDs) are dynamic cellular organelles; their accumulation is associated with several cellular processes, such as cell proliferation, apoptosis and necrosis. ¹H Nuclear Magnetic Resonance (NMR) spectroscopy detects resonances from lipids present in cytoplasmic (LDs); an understanding of the relationship between LD characteristics and NMR lipid signals is important.

Materials and methods

In this study, five nervous system cancer cell lines were investigated. Nile red staining was used to measure the diameter of LDs. High-resolution magic angle spinning NMR (HR-MAS) was performed on harvested cell pellets to quantify the patterns of lipid signals.

Results

LDs were present in all five cell lines with different morphology. An average LD diameter of approximately 0.2???m was found in all cell types. Diameter of the largest LDs varied across the cell lines. The intensity of NMR lipid signals varied greatly between cell types, and a good correlation was found between total volume of LDs and the proton NMR lipid signal intensity at 0.9 and 1.3?ppm.

Conclusion

The correlation implied that little NMR signal is detected from LDs of diameters less than approximately 0.34???m, most likely due to restriction of rotational motion of the lipids.     

Quantitative T 2 * assessment of acute and chronic myocardial ischemia/reperfusion injury in mice

Object

Imaging of myocardial infarct composition is essential to assess efficacy of emerging therapeutics. T ₂ ^* mapping has the potential to image myocardial hemorrhage and fibrosis by virtue of its short T ₂ ^* . We aimed to quantify T ₂ ^* in acute and chronic myocardial ischemia/reperfusion (I/R) injury in mice.

Materials and methods

I/R-injury was induced in C57BL/6 mice (n?=?9). Sham-operated mice (n?=?8) served as controls. MRI was performed at baseline, and 1, 7 and 28?days after surgery. MRI at 9.4?T consisted of Cine, T ₂ ^* mapping and late-gadolinium-enhancement (LGE). Mice (n?=?6) were histologically assessed for hemorrhage and collagen in the fibrotic scar.

Results

Baseline T ₂ ^* values were 17.1?±?2.0?ms. At day 1, LGE displayed a homogeneous infarct enhancement. T ₂ ^* in infarct (12.0?±?1.1?ms) and remote myocardium (13.9?±?0.8?ms) was lower than at baseline. On days 7 and 28, LGE was heterogeneous. T ₂ ^* in the infarct decreased to 7.9?±?0.7 and 6.4?±?0.7?ms, whereas T ₂ ^* values in the remote myocardium were 14.2?±?1.1 and 15.6?±?1.0?ms. Histology revealed deposition of iron and collagen in parallel with decreased T ₂ ^* .

Conclusion

T ₂ ^* values are dynamic during infarct development and decrease significantly during scar maturation. In the acute phase, T ₂ ^* values in infarcted myocardium differ significantly from those in the chronic phase. T ₂ ^* mapping was able to confirm the presence of a chronic infarction in cases where LGE was inconclusive. Hence, T ₂ ^* may be used to discriminate between acute and chronic infarctions.     

Sodium (23Na) ultra-short echo time imaging in the human brain using a 3D-Cones trajectory

Object

Sodium magnetic resonance imaging (²³Na-MRI) of the brain has shown changes in ²³Na signal as a hallmark of various neurological diseases such as stroke, Alzheimer’s disease, Multiple Sclerosis and Huntington’s disease. To improve scan times and image quality, we have implemented the 3D-Cones (CN) sequence for in vivo ²³Na brain MRI.

Materials and methods

Using signal-to-noise (SNR) as a measurement of sequence performance, CN is compared against more established 3D-radial k-space sampling schemes featuring cylindrical stack-of-stars (SOS) and 3D-spokes kooshball (KB) trajectories, on five healthy volunteers in a clinical setting. Resolution was evaluated by simulating the point-spread-functions (PSFs) and experimental measures on a phantom.

Results

All sequences were shown to have a similar SNR arbitrary units (AU) of 6–6.5 in brain white matter, 7–9 in gray matter and 17–18 AU in cerebrospinal fluid. SNR between white and gray matter were significantly different for KB and CN (p = 0.046 and <0.001 respectively), but not for SOS (p = 0.1). Group mean standard deviations were significantly smaller for CN (p = 0.016). Theoretical full-width at half-maximum linewidth of the PSF for CN is broadened by only 0.1, compared to 0.3 and 0.8 pixels for SOS and KB respectively. Actual image resolution is estimated as 8, 9 and 6.3 mm for SOS, KB and CN respectively.

Conclusion

The CN sequence provides stronger tissue contrast than both SOS and KB, with more reproducible SNR measurements compared to KB. For CN, a higher true resolution in the same amount of time with no significant trade-off in SNR is achieved. CN is therefore more suitable for ²³Na-MRI in the brain.     

Compatibility between 3T 1H SV-MRS data and automatic brain tumour diagnosis support systems based on databases of 1.5T 1H SV-MRS spectra

Object

This study demonstrates that 3T SV-MRS data can be used with the currently available automatic brain tumour diagnostic classifiers which were trained on databases of 1.5T spectra. This will allow the existing large databases of 1.5T MRS data to be used for diagnostic classification of 3T spectra, and perhaps also the combination of 1.5T and 3T databases.

Materials and methods

Brain tumour classifiers trained with 154 1.5T spectra to discriminate among high grade malignant tumours and common grade II glial tumours were evaluated with a subsequently-acquired set of 155 1.5T and 37 3T spectra. A similarity study between spectra and main brain tumour metabolite ratios for both field strengths (1.5T and 3T) was also performed.

Results

Our results showed that classifiers trained with 1.5T samples had similar accuracy for both test datasets (0.87 ± 0.03 for 1.5T and 0.88 ± 0.03 for 3.0T). Moreover, non-significant differences were observed with most metabolite ratios and spectral patterns.

Conclusion

These results encourage the use of existing classifiers based on 1.5T datasets for diagnosis with 3T ¹H SV-MRS. The large 1.5T databases compiled throughout many years and the prediction models based on 1.5T acquisitions can therefore continue to be used with data from the new 3T instruments.     

The response to rapid infusion of fentanyl in the human brain measured using pulsed arterial spin labelling

Objective

We evaluated the sensitivity of pulsed Arterial Spin Labelling (pASL) for the detection of changes in regional cerebral blood perfusion (CBP) during and after intra-venous (i.v.) infusion of an opioid agonist (fentanyl) and an opioid antagonist (naloxone).

Materials and methods

Twenty-three subjects were scanned four times, receiving i.v. infusion of fentanyl, naloxone, placebo and a second fentanyl administration, in four separate scanning sessions in randomised order. End-tidal CO₂, respiration rate and heart rate were recorded continuously throughout each scan. pASL time series were collected using single shot EPI for 15?min (including 5?min of baseline prior to infusion).

Results

Significant increases in CBP were detected during and after administration of fentanyl, (when compared to placebo and naloxone), in most areas of high concentration of mu-opioid receptors (thalamus, lingual gyrus, para-hippocampal gyrus, and insula); near-significant increases were also observed in the insula. No increases in perfusion were observed during or after naloxone infusion. No correlation was found between regional rCBF changes and end-tidal CO₂, respiration rate or heart rate. Good reliability was found between the first and second fentanyl sessions but the regions of high reliability did not overlap completely with those of highest perfusion change.

Conclusion

pASL is a suitable method for examining rapid, dynamic effects of opioid administration on brain physiology.     

Evaluation of a mobile NMR sensor for determining skin layers and locally estimating the T2eff relaxation time in the lower arm

Object

The nuclear magnetic resonance (NMR) mobile-universal-surface-explorer (MOUSE) was evaluated in a pilot study to determine its ability to detect physiological changes in human skin caused by physical or pharmacological interventions.

Materials and methods

The left lower arm skin thicknesses of ten male subjects were measured five times using a Profile NMR-MOUSE^? (¹H, 19?MHz) before and after a venous occlusion manoeuvre. In five of the subjects, the T_2eff relaxation times were derived from a bi-exponential fitting and were determined in the dermis and subcutis before and after applying a salve containing capsaicin.

Results

The dermis (including the epidermis) showed rather homogeneous signal amplitudes. The subcutis was characterised by higher and more variable amplitudes. The full-skin thickness values were affirmed by ultrasound imaging. The NMR profiles did not show significant skin swelling due to venous occlusion. In the dermis, capsaicin caused significant (p?<?0.05) decreases in both components of T _2eff (100?±?19?ms?C19?±?10?ms; 9.5?±?0.5?ms?C7.2?±?1.6?ms). In the subcutis, the T _2eff was not affected.

Conclusion

In principle, NMR-MOUSE profiles are capable of detecting skin structure. However, precise measurements are jeopardised by poor reproducibility, long acquisition times, and incompatibility between the geometries of the sensitive area of the instrument and the non-planar structure of the skin. In the dermis, T _2eff contrast could be used to detect the changes in tissue composition caused by inflammatory reactions.     

Is NMR metabolic profiling of spent embryo culture media useful to assist in vitro human embryo selection?

Object

The prediction of embryo viability by usual morphological analysis is currently unsatisfactory. New non-invasive techniques such as high-resolution nuclear magnetic resonance (¹H-NMR) spectroscopy that allows assessment of metabolic profiling in spent culture media might help embryologists to predict embryo development.

Materials and methods

Individual microdrops of culture media were analysed after 24 h of embryo culture (from day 3 to day 4) by spectroscopy using a 1 mm microliter probe allowing analysis without sample dilution. Embryos were divided into two groups on day 5: non-arrested embryos (n = 19) and arrested embryos unable to reach the blastocyst stage (n = 20). Multivariate analysis techniques such as Principal Component Analysis (PCA) and Orthogonal Partial Least Square Discriminant Analysis (OPLS-DA) were performed to compare extracellular metabolite balance.

Results

¹H-NMR used in combination with a 1 mm probe suggested that in vitro cultured human embryos that have a high developmental potential modify their environment slightly compared to embryos that cease to develop. However, differences between the two groups did not reach statistical significance and multivariate statistical analysis did not allow clustering of the two groups.

Conclusion

This study indicated that this technique would not be sufficiently powerful alone to provide information that might help to assess the developmental potential of individual embryos for in vitro fertilisation (IVF).     

Iterative separation of transmit and receive phase contributions and B 1 + -based estimation of the specific absorption rate for transmit arrays

Object

The specific absorption rate (SAR) can be determined from radiofrequency transmit fields measured via magnetic resonance imaging.

Materials and methods

The proposed method estimates the SAR solely from the complex transmit field (B ₁ ⁺ ) by taking into account the particular properties of the electromagnetic field generated by an 8-channel transmit array. It is further based on an iterative consistency check between the measured B ₁ ⁺ magnitude and an appropriate field estimate fulfilling Maxwell’s equations. For testing the method, simulations and phantom experiments were performed for a multi-transmit array at 3T using a cylindrical phantom.

Results

The method’s robustness with respect to the assumptions made about electric tissue properties as well as its stability under different initial conditions regarding the signal phase was shown. A high sensitivity to signal noise was found. Robust reconstruction results were achieved including information from more than two transmit elements. The validity of the experimental results was confirmed by a qualitative comparison to simulated electromagnetic fields.

Conclusions

The method allows the determination of the SAR as well as the transmit phase of the individual channels of a multi-transmit array. With additional B₀ inhomogeneity measurements, a reconstruction of the receive phase is feasible independent of the receive coil type in use.     

Initial clinical application of modified Dixon with flexible echo times: hepatic and pancreatic fat assessments in comparison with 1H MRS

Objects

Hepatic and pancreatic fat content become increasingly important for phenotyping of individuals with metabolic diseases. This study aimed to (1) evaluate hepatic fat fractions (HFF) and pancreatic fat fractions (PFF) using ¹H magnetic resonance spectroscopy (MRS) and the recently introduced fast mDixon method, and to examine body fat effects on HFF and PFF, (2) investigate regional differences in HFF and PFF by mDixon.

Materials and methods

HFF and PFF were quantified by mDixon with two flexible echo times and by single voxel ¹H MRS in 24 healthy subjects. The regional differences of PFF within the pancreas were assessed with mDixon. Abdominal visceral and subcutaneous fat was assessed by T1-weighted MRI at 3T.

Results

Both methods correlated well for quantification of HFF (r = 0.98, p < 0.0001) and PFF (r = 0.80, p < 0.0001). However, mDixon showed a higher low limit in HFF and PFF. PFF showed no regional differences using mDixon. In addition, both visceral and subcutaneous fat correlated with pancreatic fat, while only visceral fat correlated with liver fat, employing both ¹H MRS and mDixon.

Conclusion

The novel and fast two-point mDixon exhibits a good correlation with the gold-standard ¹H MRS for assessment of HFF and PFF, with limited sensitivity for assessing lower fat content.     

Hepatic lipid composition differs between ob/ob and ob/+ control mice as determined by using in vivo localized proton magnetic resonance spectroscopy

Object

Hepatic lipid accumulation is associated with nonalcoholic fatty liver disease, and the metabolic syndrome constitutes an increasing medical problem. In vivo proton magnetic resonance spectroscopy (¹H MRS) allows the assessment of hepatic lipid levels noninvasively and also yields information on the fat composition due to its high spectral resolution.

Materials and methods

We applied ¹H MRS at 9.4T to study lipid content and composition in eight leptin-deficient ob/ob mice as a model of obesity and in four lean ob/+ control mice at 24?weeks of age. PRESS sequence was used. For accurate estimation of signal intensity, differences in relaxation behavior of individual signals were accounted for each mouse individually. Also, in order to minimize spectral degrading due to motion artifacts, respiration gating was applied.

Results

Significant differences between ob/ob and ob/+ control mice were found in both lipid content and composition. The mean chain length was found to be significantly longer in ob/ob mice with a higher fraction of monounsaturated lipids.

Conclusion

¹H MRS enables accurate assessment in hepatic lipids in mice, which is attractive for mechanistic studies of altered metabolism given the large number of genetically engineered mouse models available.     

Association of white matter deficits with clinical symptoms in antipsychotic-naive first-episode schizophrenia: an optimized VBM study using 3T

Object

To examine the whole brain white matter morphology in antipsychotic-naive patients with first-episode schizophrenia (FES) and its correlations with symptom severity.

Materials and methods

High-resolution T1-weighted images of 64 drug-naive FES patients and 64 matched healthy controls were acquired using a 3 T MR imaging system. Then, optimized voxel-based morphometry was performed to compare the group differences. Finally, correlation analyses were conducted between the white matter volume (WMV) changes and clinical symptoms.

Results

The FES showed significantly decreased WMV in the bilateral posterior limb of the internal capsule (PLIC) and right subgyral frontal white matter. The volume of the bilateral PLIC was negatively correlated with the Positive and Negative Syndrome Scale positive scores. Positive correlations were observed between all of the changed WMV measures and the Global Assessment of Functioning scores.

Conclusion

The current findings provide further evidence to support internal capsule and subgyral frontal white matter deficits at the early stage of schizophrenia that are potentially related to the core pathophysiology of the disease. Furthermore, these anatomical alterations were related to the clinical symptoms but not the untreated illness duration, suggesting that these deficits are related to aberrations in the neurodevelopmental process and may be relatively stable during the early course of schizophrenia.     

Comparison of functional connectivity in default mode and sensorimotor networks at 3 and 7T

Object

The objective of this work was to assess functional connectivity measurements at ultra-high field (7T), given BOLD contrast to noise ratio increases with magnetic field strength but physiological noise also increases.

Materials and methods

Resting state BOLD data were acquired at 3 and 7T to assess connectivity in the sensorimotor network (SMN) and default mode network (DMN) at different spatial smoothing levels.

Results

At 3 and 7T positive correlation is observed between a right sensorimotor seed and left sensorimotor cortex. For the DMN, a seed in posterior cingulate cortex results in a high correlation in inferior parietal lobes and medial prefrontal cortex. We show higher temporal correlation coefficients for both the SMN and DMN at 7T compared to 3T for all smoothing levels. A spatial correlation between connectivity maps revealed no significant differences for the SMN, whilst the DMN showed increased spatial correlation dependent on SNR. The maximum physiological noise contribution was found to be higher at 7T, but noise in both seed and network nodes was not significantly increased, as shown by no significant difference in the spatial correlation of maps following physiological correction.

Conclusion

7T can improve spatial specificity of connectivity maps and facilitate measurement of connectivity in areas of lower intrinsic network correlation.