Gradient-echo Echo-planar Imaging Research Articles

Referenceless distortion correction of gradient-echo echo-planar imaging under inhomogeneous magnetic fields based on a deep convolutional neural network

Single-shot gradient-echo echo-planar imaging (GE-EPI) plays a significant role in applications where high temporal resolution is necessary. However, GE-EPI is susceptible to inhomogeneous magnetic fields that will cause image distortion. Most existing methods either need additional acquisitions for field mapping or cannot correct the distortion at high field. Here, we propose a new algorithm based on a deep convolutional neural network (CNN) to solve this problem without additional acquisitions. The residual learning and the cascaded structure improved the performance of the CNN on distortion correction. A simulated dataset was used for training. The simulated and experimental results demonstrate that the proposed method can correct the image distortion caused by field inhomogeneity.

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

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

MR-compatibility assessment of MADPET4: a study of interferences between an SiPM-based PET insert and a 7 T MRI system

Compromises in the design of a positron emission tomography (PET) insert for a magnetic resonance imaging (MRI) system should minimize the deterioration of image quality in both modalities, particularly when simultaneous demanding acquisitions are performed. In this work, the advantages of using individually read-out crystals with high-gain silicon photomultipliers (SiPMs) were studied with a small animal PET insert for a 7 T MRI system, in which the SiPM charge was transferred to outside the MRI scanner using coaxial cables. The interferences between the two systems were studied with three radio-frequency (RF) coil configurations. The effects of PET on the static magnetic field, flip angle distribution, RF noise, and image quality of various MRI sequences (gradient echo, spin echo, and echo planar imaging (EPI) at 1H frequency, and chemical shift imaging at 13C frequency) were investigated. The effects of fast-switching gradient fields and RF pulses on PET count rate were studied, while the PET insert and the readout electronics were not shielded. Operating the insert inside a 1H volume coil, used for RF transmission and reception, limited the MRI to T1-weighted imaging, due to coil detuning and RF attenuation, and resulted in significant PET count loss. Using a surface receive coil allowed all tested MR sequences to be used with the insert, with 45–59% signal-to-noise ratio (SNR) degradation, compared to without PET. With a 1H/13C volume coil inside the insert and shielded by a copper tube, the SNR degradation was limited to 23–30% with all tested sequences. The insert did not introduce any discernible distortions into images of two tested EPI sequences. Use of truncated sinc shaped RF excitation pulses and gradient field switching had negligible effects on PET count rate. However, PET count rate was substantially affected by high-power RF block pulses and temperature variations due to high gradient duty cycles.

Altered gray matter volume and disrupted functional connectivity of dorsolateral prefrontal cortex in men with heroin dependence.

Chronic heroin use can cause various neuropathological characteristics that may compromise brain function. The present study evaluated the alteration of gray matter volume (GMV) and its resting-state functional connectivity (rsFC) over the dorsolateral prefrontal cortex (DLPFC) among male heroin users. Thirty heroin-dependent men undergoing methadone maintenance therapy and 30 educational-level- and age-matched male controls were recruited for this study. To assess their GMV and rsFC, the participants were evaluated using spoiled gradient echo and gradient-recalled echo planar imaging sequences with a 3-Tesla General Electric MR scanner under resting state. The heroin-dependent men showed lower GMV over the right DLPFC in comparison with the controls. Further evaluation of the rsFC of the right DLPFC revealed a marked decrease in interhemispheric DLPFC connectivity among those with heroin dependence under control of head movement and GMV of the right DLPFC. Although the mechanism remains unclear, the present study shows that chronic heroin use is associated with alteration of morphology as well as rsFC over the right DLPFC. As the DLPFC plays an imperative role in various domains of cognitive function, service providers for heroin users should consider the impacts of possible DLPFC-related cognitive deficits on treatment effectiveness.

T143. NOT A NUISANCE ANY MORE: GLOBAL FMRI SIGNAL AT REST, PROCESSING SPEED AND SYMPTOM SEVERITY IN SCHIZOPHRENIA

BackgroundBefore data from resting-state functional magnetic resonance imaging (rs-fMRI) is analyzed, the global signal (GS) - average blood-oxygen level dependent (BOLD) signal across all voxels in the brain - is normally removed through global signal regression (GSR). This convention arose in order to control for changes in brain activity that are usually of no interest but may be caused by non-neuronal factors, including changes in respiratory rate, arterial CO2 levels or cardiac pulsation. However, recent studies have indicated that GS may systematically vary between patients with schizophrenia and healthy controls. In addition to testing this notion, we also studied if the dynamic variance of global signal across time carried any meaningful information that relates to overall symptom severity and information processing speed in patients with schizophrenia.MethodsData was collected from 39 in a clinically stable, medicated early stage of schizophrenia (median duration of illness= 6.5 years), and 34 sex, age and parental socioeconomic-status matched healthy controls over a 10-minute period of eyes-open rest at TR=2.5s (3T Philips Achieva, 240 time points, dual-echo, gradient-echo EPI). Scores were obtained from the Signs and Symptoms of Psychotic Illness (SSPI) scale and the Digit Symbol Substitution Test (DSST). Rs-fMRI time-series data were motion-corrected (using 6 parameters), slice-time corrected, reoriented with structural images, band-pass filtered (0.01–0.1 Hz), scrubbed using ArtRepair for framewise displacement and transformed to MNI space. SPM8 and the advanced version of the Data Processing Assistant for resting-state fMRI (DPARSFA) were used for this purpose. GS mean was computed from all grey matter voxels using a template mask in MNI space, using grand mean scaling to a base of 1000, averaged across all time-points. The variance of GS across time (dynamic GS variance) was computed from the entire 10-minutes of acquisition (240 time points).ResultsIndependent-sample t-tests used to compare GS mean between controls (mean[sd] = 3135.1[1244.4]) and the SCZ group (mean[sd] = 3207.8[1191.7]) yielded no significant results [t(71) = -0.14, p = .89]. The temporal variance of GS did not differ between controls (mean[sd] = 113.05[59.41]) and the SCZ group (mean[sd] = 118.02[57.93]) [t(71) = -0.36, p = .72]. In the SCZ group there was a significant correlation between the total SSPI score reflecting overall illness severity (□ = -.322, p < 0.05) and the mean GS. This relationship was especially pronounced for the syndrome of Reality Distortion (rho = -.344, p < 0.05) and Disorganization (rho = -.303, p = 0.065), where higher symptom severity was seen in patients with lower mean GS.Dynamic variance in GS was higher in healthy controls with lower mean DSST (r = -.364, p = 0.04), but no such relationship was seen in the SCZ group (r = .066, p = .694). Notably, when compared to controls (mean[sd] = 57.4[9.40]), patients (mean[sd]=42.4[9.97]) had significantly lower DSST scores [t(69) = 6.47, p < 0.001]. Neither GS nor GS variance related to root mean square of framewise displacement in the 2 groups.DiscussionWe did not find an overall increase or reduction in the global signal in patients with schizophrenia. Nevertheless, the strength of global signal obtained from resting fMRI is related to severity of persisting symptoms of schizophrenia, whereas the dynamic variance of this signal relates to the speed of processing ability assessed outside the scanner in healthy subjects. With emerging evidence relating global signal to cognitive vigilance and overall brain connectivity, our results indicate that global signal is a parameter of interest that should not be automatically discarded in resting fMRI studies of schizophrenia

Fiber-optic implant for simultaneous fluorescence-based calcium recordings and BOLD fMRI in mice.

Despite the growing popularity of blood oxygen level-dependent (BOLD) functional MRI (fMRI), understanding of its underlying principles is still limited. This protocol describes a technique for simultaneous measurement of neural activity using fluorescent calcium indicators together with the corresponding hemodynamic BOLD fMRI response in the mouse brain. Our early work using small-molecule fluorophores in rats gave encouraging results but was limited to acute measurements using synthetic dyes. Our latest procedure combines fMRI with optical detection of cell-type-specific virally delivered GCaMP6, a genetically encoded calcium indicator (GECI). GCaMP6 fluorescence, which increases upon calcium binding, is collected by a chronically implanted optical fiber, allowing longitudinal studies in mice. The chronic implant, placed horizontally on the skull, has an angulated tip that reflects light into the brain and is connected via fiber optics to a remote optical setup. The technique allows access to the neocortex and does not require adaptations of commercial MRI hardware. The hybrid approach permits fiber-optic calcium recordings with simultaneous artifact-free BOLD fMRI with full brain coverage and 1-s temporal resolution using standard gradient-echo echo-planar imaging (GE-EPI) sequences. The method provides robust, cell-type-specific readouts to link neural activity to BOLD signals, as emonstrated for task-free ('resting-state') conditions and in response to hind-paw stimulation. These results highlight the power of fiber photometry combined with fMRI, which we aim to further advance in this protocol. The approach can be easily adapted to study other molecular processes using suitable fluorescent indicators.

P.2.049 - Fronto-parietal connectivity and its relation to frontal glutamate in patients suffering from schizophrenia

Cognitive deficits like working memory impairment in schizophrenia are of great importance for clinical outcome, but the underlying neurobiology is not fully understood. During working memory (WM) altered connectivity patterns in the fronto-parietal network are present in patients suffering from schizophrenia [1–3]. One candidate biochemical marker for the integrity of connectivity is glutamate [4]. Here we tested for group differences in fronto-parietal connectivity and it’s relation to possible glutamatergic influences. During a functional magnetic resonance imaging (fMRI) scan, a sample of 42 medicated patients (SZ) and 41 age and gender matched healthy controls (HC) were asked to perform a numeric n-back working memory task, which consisted of two conditions “2-back” and “0-back”. FMRI was conducted on a 3T Siemens Trio scanner with a 12 channel head coil using gradient-echo echo-planar imaging. Data were further preprocessed using the statistical parametric mapping (SPM 8; Welcome Department of Imaging Neuroscience, London, UK; htt://www.fil.ion.ucl.ac.uk/spm) in MATLAB 2009b. We performed dynamic causal modeling [5] (DCM) on a model space comprising regions of interest for a visual input, parietal (PC) and dorsolateral prefrontal cortex (DLPFC). We calculated Bayesian model averages to obtain weighted connectivity parameters. One-sample t-tests were calculated on parameters for PC->DLPFC and DLPFC->PC connectivity within each group in order to evaluate modulatory effects of working memory on the executive network. Glutamate levels were measured in left DLPFC using magnetic resonance spectroscopy. We used LCmodel (Linear Combination of Model spectra, a commercial spectral fitting package) to estimate local glutamate concentration. Absolute glutamate concentrations were adjusted for grey and white matter volume. We performed group comparisons (two-sample t-test) on connectivity parameters as well as on glutamate levels. Due to non-normality two-sided Spearman correlation analysis were calculated between connectivity parameters and glutamate levels. Working memory dependent connectivity effects (between PC and DLPFC) could be observed in the left hemisphere on backward connections (PC->DLPFC, HC: t=2.77, p=0.008; SZ: t=2.62, p=0.012), whereas no significant working memory effects were present on DLPFC->PC connectivity. We found no group difference in fronto-parietal connectivity parameters and no difference in glutamate levels. Nonetheless numerically controls show higher glutamate levels as compared to patients. To explore possible effects of local Glutamate levels on working memory dependent connectivity, we correlated parameters for PC->DLPFC connectivity (where we found significant working memory dependent effects) with glutamate levels. We found a significant negative association between parieto-frontal connectivity and glutamate in DLPFC in patients (rho=-0.47, p=0.0035). Controls did not show any significant association (rho=-0.12, p=0.53). Comparing correlation coefficients, we found a trend-wise significant difference between groups (z=1.55, p=0.06). Although our data neither showed a difference in connectivity nor in glutamate levels, our findings suggest that glutamate is differentially related to working memory dependent connectivity from parietal to frontal areas in patients as compared to controls.

Amplitude of low-frequency fluctuations in childhood-onset schizophrenia with or without obsessive-compulsive symptoms: a resting-state functional magnetic resonance imaging study.

IntroductionChildhood-onset schizophrenia with obsessive-compulsive symptoms (COSO) and without obsessive-compulsive symptoms (COS) share considerable overlap in clinical features and genetic risk factors. However, the extent of brain functional abnormalities in COSO and COS is poorly understood.Material and methodsA total of 51 first-episode childhood schizophrenic patients and 30 healthy age- and sex-matched controls were recruited. We used the Yale-Brown Obsessive Compulsive Scale to divide patients into COSO (n = 21) and COS (n = 30) groups. Resting-state functional magnetic resonance imaging images were obtained using a gradient-echo echo-planar imaging sequence. Voxel-based analysis of amplitude of low-frequency fluctuation (ALFF) maps between the two groups was performed.ResultsThe COS group showed significantly increased ALFF in the right caudate body, middle temporal gyrus and inferior parietal lobule (p < 0.05), while showing decreased ALFF in the left cerebellum posterior lobe (p < 0.05). The COSO group showed significantly increased ALFF in the left and right frontal lobe, cerebellum posterior lobe and precuneus (p < 0.05). When comparing the two groups, COSO showed significantly higher ALFF in the left cerebellum posterior lobe, frontal lobe, supramarginal gyrus, precuneus, right inferior frontal gyrus and medial frontal gyrus (p < 0.05). Interestingly, significantly lower ALFF was found in the right fusiform gyrus, corpus callosum and inferior parietal lobule in the group of patients with obsessive-compulsive symptoms (p < 0.05).ConclusionsOur findings increase the understanding of the pathophysiology of schizophrenia and may provide imaging evidence for early diagnosis of COSO or COS.

Real-time correction of respiration-induced distortions in the human spinal cord using a 24-channel shim array.

PurposeTo reduce respiration‐induced magnetic field distortions and the attendant image artifacts in echo‐planar imaging (EPI) of the human spinal cord.MethodsUsing a custom‐designed 24‐channel shim array, shim updates were issued in real time based on a concurrently monitored respiratory trace and a pair of gradient echo (GRE) field maps acquired during an initial training phase. Proof‐of‐concept application in GRE–EPI was conducted in 6 subjects.ResultsOver the thoracic spinal cord, real‐time shimming reduced respiration‐induced distortions in the EPI by 48.2% ± 12.2% and increased the mean temporal signal‐to‐noise ratio by 15.7% ± 7.9%.ConclusionReal‐time shim adjustment substantially reduces spatiotemporal B0 field variation, opening the door to more robust imaging and spectroscopy investigations of the spinal cord. Magn Reson Med 80:935–946, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Detection of fast oscillating magnetic fields using dynamic multiple TR imaging and Fourier analysis.

Neuronal oscillations produce oscillating magnetic fields. There have been trials to detect neuronal oscillations using MRI, but the detectability in in vivo is still in debate. Major obstacles to detecting neuronal oscillations are (i) weak amplitudes, (ii) fast oscillations, which are faster than MRI temporal resolution, and (iii) random frequencies and on/off intervals. In this study, we proposed a new approach for direct detection of weak and fast oscillating magnetic fields. The approach consists of (i) dynamic acquisitions using multiple times to repeats (TRs) and (ii) an expanded frequency spectral analysis. Gradient echo echo-planar imaging was used to test the feasibility of the proposed approach with a phantom generating oscillating magnetic fields with various frequencies and amplitudes and random on/off intervals. The results showed that the proposed approach could precisely detect the weak and fast oscillating magnetic fields with random frequencies and on/off intervals. Complex and phase spectra showed reliable signals, while no meaningful signals were observed in magnitude spectra. A two-TR approach provided an absolute frequency spectrum above Nyquist sampling frequency pixel by pixel with no a priori target frequency information. The proposed dynamic multiple-TR imaging and Fourier analysis are promising for direct detection of neuronal oscillations and potentially applicable to any pulse sequences.

Considerations for Gradient Echo Echo Planar Imaging of Skeletal Muscle.

Adequate blood circulation to muscles is important for good health. In recent years, researchers have increasingly used magnetic resonance imaging (MRI) to study temporal skeletal muscle physiological changes using gradient echo (GRE) echo planar imaging (EPI). These studies, typically involving exercise or ischemic challenges, have differentiated healthy subjects from athletic or unhealthy populations, such as those with peripheral vascular disease. The T*2-weighted GRE EPI signal is sensitive to changes in blood flow and oxygenation in muscle. Furthermore, the signal can be weighted differently by adjusting the echo time (TE), achieving either blood volume (BV) or blood oxygenation level dependent (BOLD) imaging. This paper comprehensively reviews the muscle GRE EPI literature to date, with a particular emphasis on studies that have also used other modalities in an attempt to elucidate the GRE EPI signal characteristics. Finally, the systemic and muscle physiological factors that are thought to influence the GRE EPI signal during and after exercise are described. These include temperature, blood flow, blood oxygen saturation, myoglobin, and postural effects. The summary attempts to predict, based on the published literature, the temporal dynamics of these signal-influencing parameters in a hypothetical exercise experiment.

Using multi-echo simultaneous multi-slice (SMS) EPI to improve functional MRI of the subcortical nuclei of the basal ganglia at ultra-high field (7T)

The nuclei of the basal ganglia pose a special problem for functional MRI, especially at ultra-high field, because T2* variations between different regions result in suboptimal BOLD sensitivity when using gradient-echo echo-planar imaging (EPI). Specifically, the iron-rich lentiform nucleus of the basal ganglia, including the putamen and globus pallidus, suffers from substantial signal loss when imaging is performed using conventional single-echo EPI with echo times optimized for the cortex. Multi-echo EPI acquires several echoes at different echo times for every imaging slice, allowing images to be reconstructed with a weighting of echo times that is optimized individually for each voxel according to the underlying tissue or T2* properties. Here we show that multi-echo simultaneous multi-slice (SMS) EPI can improve functional activation of iron-rich subcortical regions while maintaining sensitivity within cortical areas. Functional imaging during a motor task known to elicit strong activations in the cortex and the subcortex (basal ganglia) was performed to compare the performance of multi-echo SMS EPI to single-echo SMS EPI. Notably within both the caudate nucleus and putamen of the basal ganglia, multi-echo SMS EPI yielded higher tSNR (an average 84% increase) and CNR (an average 58% increase), an approximate 3-fold increase in supra-threshold voxels, and higher t-values (an average 39% increase). The degree of improvement in the group level t-statistics was negatively correlated to the underlying T2* of the voxels, such that the shorter the T2*, as in the iron-rich nuclei of the basal ganglia, the higher the improvement of t-values in the activated region.

Estimation of physiological sources of nonlinearity in blood oxygenation level-dependent contrast signals

Blood oxygenation level-dependent (BOLD) contrast appears through a variation in the transverse relaxation rate of magnetic resonance signals induced by neurovascular coupling and is known to have nonlinear characteristics along echo time (TE) due to the intra-vasculature. However, the physiological causes of this nonlinearity are unclear. We attempted to estimate the physiological information related to the nonlinearity of BOLD signals by using a two-compartment model. For this purpose, we used a multi-echo gradient-echo echo-planar imaging sequence and developed a computational method to estimate the physiological information from the TE-dependent BOLD signals. The results showed that the average chemical exchange time in the intra-vasculature varied during stimulation, which might be the essential source of the nonlinearity.

Slice-accelerated gradient-echo echo planar imaging dynamic susceptibility contrast-enhanced MRI with blipped CAIPI: effect of increasing temporal resolution.

To assess the influence of high temporal resolution on the perfusion measurements and image quality of perfusion maps, by applying simultaneous-multi-slice acquisition (SMS) dynamic susceptibility contrast-enhanced (DSC) magnetic resonance imaging (MRI). DSC-MRI data using SMS gradient-echo echo planar imaging sequences in 10 subjects with no intracranial abnormalities were retrospectively analyzed. Three additional data sets with temporal resolution of 1.0, 1.5, and 2.0s were created from the raw data sets of 0.5s. Cerebral blood flow (CBF), cerebral blood volume, mean transit time (MTT), time to peak (TTP), and time to maximum tissue residue function (T max) measurements were performed, as was visual perfusion map analysis. The perfusion parameter for temporal resolution of 0.5s (reference) was compared with each synthesized perfusion parameter. CBF, MTT, and TTP values at temporal resolutions of 1.5 and 2.0s differed significantly from the reference. The image quality of MTT, TTP, and T max maps deteriorated with decreasing temporal resolution. The temporal resolution of DSC-MRI influences perfusion parameters and SMS DSC-MRI provides better image quality for MTT, TTP, and T max maps.

Comparison between multi-shot gradient echo EPI and balanced SSFP in unenhanced 3T MRA of thoracic aorta in healthy volunteers

PurposeThe purpose of this study was to compare scan time and image quality between magnetic resonance angiography (MRA) of the thoracic aorta using a multi-shot gradient echo planar imaging (MSG-EPI) and MRA using balanced steady-state free precession (b-SSFP). Materials and methodsHealthy volunteers (n=17) underwent unenhanced thoracic aorta MRA using balanced steady-state free precession (b-SSFP) and MSG-EPI sequences on a 3T MRI. The acquisition time, total scan time, signal-to-noise ratio (SNR) of the thoracic aorta, and the coefficient of variation (CV) of thoracic aorta were compared with paired t-tests. Two radiologists independently recorded the images’ contrast, noise, sharpness, artifacts, and overall quality on a 4-point scale. ResultsThe acquisition time was 36.2% shorter for MSG-EPI than b-SSFP (115.5±14.4 vs 181.0±14.9s, p<0.01). The total scan time was 40.4% shorter for MSG-EPI than b-SSFP (272±78 vs 456±144s, p<0.01). There was no significant difference in mean SNR between MSG-EPI and b-SSFP scans (17.3±3.6 vs 15.2±4.3, p=0.08). The CV was significantly lower for MSG-EPI than b-SSFP (0.2±0.1 vs. 0.5±0.2, p<0.01). All qualitative scores except for image noise were significantly higher in MSG-EPI than b-SSFP scans (p<0.05). ConclusionThe MSG-EPI sequence is a promising technique for shortening scan time and yielding more homogenous image quality in MRA of thoracic aorta on 3T scanners compared with the b-SSFP.

Improving fMRI in signal drop-out regions at 7T by using tailored radio-frequency pulses: application to the ventral occipito-temporal cortex.

Signal drop-off occurs in echo-planar imaging in inferior brain areas due to field gradients from susceptibility differences between air and tissue. Tailored-RF pulses based on a hyperbolic secant (HS) have been shown to partially recover signal at 3T, but have not been tested at higher fields. The aim of this study was to compare the performance of an optimized tailored-RF gradient-echo echo-planar imaging (TRF GRE-EPI) sequence with standard GRE-EPI at 7 T, in a passive viewing of faces or objects fMRI paradigm in healthy subjects. Increased temporal-SNR (tSNR) was observed in the middle and inferior temporal lobes and orbitofrontal cortex of all subjects scanned, but elsewhere tSNR decreased relative to the standard acquisition. In the TRF GRE-EPI, increased functional signal was observed in the fusiform, lateral occipital cortex, and occipital pole, regions known to be part of the visual pathway involved in face-object perception. This work highlights the potential of TRF approaches at 7T. Paired with a reversed-gradient distortion correction to compensate for in-plane susceptibility gradients, it provides an improved acquisition strategy for future neurocognitive studies at ultra-high field imaging in areas suffering from static magnetic field inhomogeneities.

Abstract CT068: Tumor treating fields in patients with glioblastomas: Evaluation of treatment response using advanced mr imaging techniques

Abstract Introduction: Tumor treating fields (TTFields) is a new modality for the treatment of patients with newly diagnosed and recurrent glioblastomas (GBMs). TTFields system delivers low intensity, intermediate frequency alternating electric field directly to brain causing neoplastic cell death with minimal effect on the normal dividing cells. The purpose of present investigator sponsored trial was to evaluate the effects of TTFields in GBM patients using diffusion tensor imaging (DTI), perfusion weighted imaging (PWI) and 3D-echoplanar spectroscopic imaging (EPSI). Methods: Two patients with newly diagnosed GBM and six patients with recurrent GBMs previously treated with standard of care maximal safe resection and chemo-radiation therapy received TTFields. Patients underwent baseline (prior to TTFields) and two follow-up (one and two months post initiation of TTFields) MR imaging on a 3T MR system. DTI data were acquired using 30 directions with a single-shot spin-echo EPI sequence. After motion and eddy current correction of raw DTI data, parametric maps [mean diffusivity (MD), fractional anisotropy (FA)] were generated using in-house developed algorithm. For PWI, T2* weighted gradient-echo EPI sequence was acquired with a temporal resolution of 2.1s. Leakage corrected cerebral blood volume (CBV) maps were constructed. 3D-EPSI was acquired using a spin-echo based sequence. EPSI data were processed using metabolic imaging and data analysis system (MIDAS) package. DTI (MD, FA), EPSI [choline (Cho)/creatine(Cr)], CBV maps and FLAIR images were co-registered to post-contrast T1-weighted images and a semi-automated algorithm was used to segment the contrast-enhancing region of neoplasms. Median values of MD, FA, relative CBV (rCBV) and Cho/Cr were computed at each time point. The 90th percentile rCBV (rCBVmax) values were also measured. Percent changes of each parameter between baseline and follow-up time points were evaluated. Results and Discussion: In general, there was an increasing trend in MD (~3%) and steadily decline trend in FA (~8%) at the 2 month follow-up relative to baseline. Additionally, from baseline to post- TTFields, reductions in Cho/Cr and rCBVmax were also observed from most of the patients. Clinically, all these patients were stable at 2 month follow-up. The inhibited cellular growth may account for large increase in MD and decrease in FA and Cho/Cr as observed in the current study. Reducing trends in rCBVmax at the follow-up may indicate anti- angiogenetic effects of TTFields and tissue perfusion within the tumor bed after the therapy. Conclusion: Our preliminary data suggest that advanced MR imaging may be useful in evaluating treatment response to TTFields in patients with GBMs. Inclusion of more patients is warranted to validate our findings. Acknowledgement: Support of NovoTTF-100A system (Novocure Ltd., Haifa, Israel) is gratefully acknowledged. Citation Format: SANJEEV CHAWLA, Sumei Wang, Gaurav Verma, Aaron Skolnik, Lauren Karpf, Lisa Desiderio, Steven Brem, Katherine Peters, Harish Poptani, Suyash Mohan. Tumor treating fields in patients with glioblastomas: Evaluation of treatment response using advanced mr imaging techniques [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT068. doi:10.1158/1538-7445.AM2017-CT068

Correction of metal-induced susceptibility artifacts for functional MRI during deep brain stimulation

Functional magnetic resonance imaging (fMRI) is an emerging tool for investigating brain activation associated with, or modulated by, deep brain stimulation (DBS). However, DBS-fMRI generally suffers from severe susceptibility to artifacts in regions near the metallic stimulation electrodes, as well as near tissue/air boundaries of the brain. These result in strong intensity and geometric distortions along the phase-encoding (PE) (i.e., blipped) direction in gradient-echo echo-planar imaging (GE-EPI). Distortion presents a major challenge to conducting reliable data analysis and in interpreting the findings. A recent study showed that the point spread function (PSF) mapping-based reverse gradient approach has a potential to correct for distortions not only in spin-echo EPI, but also in GE-EPI acquired in both the forward and reverse PE directions. In this study, we adapted that approach in order to minimize severe metal-induced susceptibility artifacts for DBS-fMRI, and to evaluate the performance of the approach in a phantom study and a large animal DBS-fMRI study. The method combines the distortion-corrected GE-EPI pair with geometrically different intensity distortions due to the opposing encoding directions. The results demonstrate that the approach can minimize susceptibility artifacts that appear around the metallic electrodes, as well as in the regions near the tissue/air boundaries in the brain. We also demonstrated that an accurate geometric correction is important in improving BOLD contrast in the group dataset, especially in regions where strong susceptibility artifacts appear.

Single-Breath-Hold Whole-heart Unenhanced Coronary MRA Using Multi-shot Gradient Echo EPI at 3T: Comparison with Free-breathing Turbo-field-echo Coronary MRA on Healthy Volunteers.

Purpose:We investigated the feasibility of single breath hold unenhanced coronary MRA using multi-shot gradient echo planar imaging (MSG-EPI) on a 3T-scanner.Methods:Fourteen volunteers underwent single breath hold coronary MRA with a MSG-EPI and free-breathing turbo field echo (TFE) coronary MRA at 3T. The acquisition time, signal to noise ratio (SNR), and the contrast of the sequences were compared with the paired t-test. Readers evaluated the image contrast, noise, sharpness, artifacts, and the overall image quality.Results:The acquisition time was 88.1% shorter for MSG-EPI than TFE (24.7 ± 2.5 vs 206.4 ± 23.1 sec, P < 0.01). The SNR was significantly higher on MSG-EPI than TFE scans (P < 0.01). There was no significant difference in the contrast on MSG-EPI and TFE scans (1.8 ± 0.3 vs 1.9 ± 0.3, P = 0.24). There was no significant difference in image contrast, image sharpness, and overall image quality between two scan techniques. The score of image noise and artifact were significantly higher on MSG-EPI than TFE scans (P < 0.05).Conclusion:The single breath hold MSG-EPI sequence is a promising technique for shortening the scan time and for preserving the image quality of unenhanced whole heart coronary MRA on a 3T scanner.

Characterization of high-resolution Gradient Echo and Spin Echo EPI for fMRI in the human visual cortex at 7 T

The increased signal-to-noise ratio (SNR) offered by functional Magnetic Resonance Imaging (fMRI) at 7T allows the acquisition of functional data at sub-millimetric spatial resolutions. However, simply reducing partial volume effects is not sufficient to precisely localize task-induced activation due to the indirect mechanisms that relate brain function and the changes in the measured signal.In this work T2* and T2 weighted Echo Planar Imaging (EPI) schemes based on Gradient Recalled Echo (GRE) and Spin Echo (SE) were evaluated in terms of temporal SNR, percent signal change, contrast to noise ratio (CNR), activation volume, and sensitivity and specificity to gray matter. Datasets were acquired during visual stimulation at in-plane resolutions ranging between 1.5×1.5mm2 and 0.75×0.75mm2 targeting the early visual cortex.While similar activation foci were obtained in all acquisitions, at in-plane resolutions of 1.0×1.0mm2 and larger voxel sizes the T2 weighted contrast of SE-EPI allowed the identification of the activation site with better spatial accuracy. However, at sub-millimetric resolutions the decrease in temporal SNR significantly hampered the sensitivity and the extent of the activation site. On the other hand, high resolution T2* weighted data collected with GRE-EPI provided higher CNR and sensitivity, benefiting from the decreased physiological and partial volume effects. However, spurious activations originating from regions of blood drainage were still present in GRE data, and simple thresholding techniques were found to be inadequate for the removal of such contributions. The combination of 2-class and 3-class automated segmentations, performed directly in EPI space, allowed the selection of active voxels in gray matter. This approach could enable GRE-EPI to accurately map functional activity with satisfactory CNR and specificity to the true site of activation.