Deep flow-net for EPI distortion estimation.

INTRODUCTION: Geometric distortions along the phase encoding direction caused by off-resonant spins are a major issue in EPI based functional and diffusion imaging. The widely used blip up/down approach estimates the underlying distortion field from a pair of images with inverted phase encoding direction. Typically, iterative methods are used to find a solution to the ill-posed problem of finding the displacement field that maps up/down acquisitions onto each other. Here, we explore the use of a deep convolutional network to estimate the displacement map from a pair of input images. METHODS: We trained a deep convolutional U-net architecture that was previously used to estimate optic flow between moving images to learn to predict the distortion map from an input pair of distorted EPI acquisitions. During the training step, the network minimizes a loss function (similarity metric) that is calculated from corrected input image pairs. This approach does not require the explicit knowledge of the ground truth distortion map, which is difficult to get for real life data. RESULTS: We used data from a total of Ntrain â€‹= â€‹22 healthy subjects to train our network. A separate dataset of Ntest â€‹= â€‹12 patients including some with abnormal findings and unseen acquisition modes, e.g. LR-encoding, coronal orientation) was reserved for testing and evaluation purposes. We compared our results to FSL's topup function with default parameters that served as the gold standard. We found that our approach results in a correction accuracy that is virtually identical to the optimum found by an iterative search, but with reduced computational time. CONCLUSION: By using a deep convolutional network, we can reduce the processing time to a few seconds per volume, which is significantly faster than iterative approaches like FSL's topup which takes around 10min on the same machine (but using only 1 CPU). This facilitates the use of a blip up/down scheme for all diffusion-weighted acquisitions and potential real-time EPI distortion correction without sacrificing accuracy.

Improved cortical boundary registration for locally distorted fMRI scans.

With continuing advances in MRI techniques and the emergence of higher static field strengths, submillimetre spatial resolution is now possible in human functional imaging experiments. This has opened up the way for more specific types of analysis, for example investigation of the cortical layers of the brain. With this increased specificity, it is important to correct for the geometrical distortions that are inherent to echo planar imaging (EPI). Inconveniently, higher field strength also increases these distortions. The resulting displacements can easily amount to several millimetres and as such pose a serious problem for laminar analysis. We here present a method, Recursive Boundary Registration (RBR), that corrects distortions between an anatomical and an EPI volume. By recursively applying Boundary Based Registration (BBR) on progressively smaller subregions of the brain we generate an accurate whole-brain registration, based on the grey-white matter contrast. Explicit care is taken that the deformation does not break the topology of the cortical surface, which is an important requirement for several of the most common subsequent steps in laminar analysis. We show that RBR obtains submillimetre accuracy with respect to a manually distorted gold standard, and apply it to a set of human in vivo scans to show a clear increase in spacial specificity. RBR further automates the process of non-linear distortion correction. This is an important step towards routine human laminar fMRI for large field of view acquisitions. We provide the code for the RBR algorithm, as well as a variety of functions to better investigate registration performance in a public GitHub repository, https://github.com/TimVanMourik/OpenFmriAnalysis, under the GPL 3.0 license.

Spin echo based cardiac diffusion imaging at 7T: An ex vivo study of the porcine heart at 7T and 3T.

Purpose of this work was to assess feasibility of cardiac diffusion tensor imaging (cDTI) at 7 T in a set of healthy, unfixed, porcine hearts using various parallel imaging acceleration factors and to compare SNR and derived cDTI metrics to a reference measured at 3 T. Magnetic resonance imaging was performed on 7T and 3T whole body systems using a spin echo diffusion encoding sequence with echo planar imaging readout. Five reference (b = 0 s/mm2) images and 30 diffusion directions (b = 700 s/mm2) were acquired at both 7 T and 3 T using a GRAPPA acceleration factor R = 1. Scans at 7 T were repeated using R = 2, R = 3, and R = 4. SNR evaluation was based on 30 reference (b = 0 s/mm2) images of 30 slices of the left ventricle and cardiac DTI metrics were compared within AHA segmentation. The number of hearts scanned at 7 T and 3 T was n = 11. No statistically significant differences were found for evaluated helix angle, secondary eigenvector angle, fractional anisotropy and apparent diffusion coefficient at the different field strengths, given sufficiently high SNR and geometrically undistorted images. R≥3 was needed to reduce susceptibility induced geometric distortions to an acceptable amount. On average SNR in myocardium of the left ventricle was increased from 29±3 to 44±6 in the reference image (b = 0 s/mm2) when switching from 3 T to 7 T. Our study demonstrates that high resolution, ex vivo cDTI is feasible at 7 T using commercial hardware.

Variable slice thickness (VAST) EPI for the reduction of susceptibility artifacts in whole-brain GE-EPI at 7 Tesla.

OBJECTIVE: A new technique for 2D gradient-recalled echo echo-planar imaging (GE-EPI) termed 'variable slice thickness' (VAST) is proposed, which reduces signal losses caused by through-slice susceptibility artifacts, while keeping the volume repetition time (TR) manageable. The slice thickness is varied across the brain, with thinner slices being used in the inferior brain regions where signal voids are most severe. MATERIALS AND METHODS: Various axial slice thickness schemes with identical whole-brain coverage were compared to regular EPI, which may either suffer from unfeasibly long TR if appropriately thin slices are used throughout, or signal loss if no counter-measures are taken. Evaluation is based on time-course signal-to-noise (tSNR) maps from resting state data and a statistical group-level region of interest (ROI) analysis on breath-hold fMRI measurements. RESULTS: The inferior brain region signal voids with static B0 inhomogeneities could be markedly reduced with VAST GE-EPI in contrast to regular GE-EPI. ROI-averaged event-related signal changes showed 48% increase in VAST compared to GE-EPI with regular "thick" slices. tSNR measurements proved the comparable signal robustness of VAST in comparison to regular GE-EPI with thin slices. CONCLUSION: A novel acquisition strategy for functional 2D GE-EPI at ultrahigh magnetic field is presented to reduce susceptibility-induced signal voids and keep TR sufficiently short for whole-brain coverage.

Diffusional kurtosis imaging (DKI) incorporation into an intravoxel incoherent motion (IVIM) MR model to measure cerebral hypoperfusion induced by hyperventilation challenge in healthy subjects.

OBJECTIVES: The objectives were to investigate the diffusional kurtosis imaging (DKI) incorporation into the intravoxel incoherent motion (IVIM) model for measurements of cerebral hypoperfusion in healthy subjects. MATERIALS AND METHODS: Eight healthy subjects underwent a hyperventilation challenge with a 4-min diffusion weighted imaging protocol, using 8 b values chosen with the Cramer-Rao Lower Bound optimization approach. Four regions of interest in gray matter (GM) were analyzed with the DKI-IVIM model and the bi-exponential IVIM model, for normoventilation and hyperventilation conditions. RESULTS: A significant reduction in the perfusion fraction (f) and in the product fD* of the perfusion fraction with the pseudodiffusion coefficient (D*) was found with the DKI-IVIM model, during the hyperventilation challenge. In the cerebellum GM, the percentage changes were f: -43.7 ± 40.1, p = 0.011 and fD*: -50.6 ± 32.1, p = 0.011; in thalamus GM, f: -47.7 ± 34.7, p = 0.012 and fD*: -47.2 ± 48.7, p = 0.040. In comparison, using the bi-exponential IVIM model, only a significant decrease in the parameter fD* was observed for the same regions of interest. In frontal-GM and posterior-GM, the reduction in f and fD* did not reach statistical significance, either with DKI-IVIM or the bi-exponential IVIM model. CONCLUSION: When compared to the bi-exponential IVIM model, the DKI-IVIM model displays a higher sensitivity to detect changes in perfusion induced by the hyperventilation condition.

The role of diffusion-weighted MRI (DWI) in the differentiation of benign from malignant skeletal lesions of the pelvis.

PURPOSE: To evaluate the utility of Diffusion-weighted MRI in the differentiation of benign from malignant skeletal lesions of the pelvis. MATERIALS AND METHODS: In this retrospective study 33 patients with indeterminate skeletal lesions of the pelvis were evaluated with DWI. Minimum, mean, maximum ADC-values of the skeletal lesions were measured followed by qualitative assessment of DWI. All patients underwent histological confirmation using CT-guided biopsy or surgical resection. The histology of the skeletal lesions was correlated with the findings on DWI. RESULTS: There were 13 malignant lesions and 20 benign lesions. The mean, minimum and maximum ADC values (×10-6mm2/s) for benign skeletal lesions was higher than the mean ADC-values for malignant lesions (1422.2 vs 1263.7; 780.4 vs 771.8; 1969.6 vs 1676.8 respectively). These differences were however not statistically significant (P-values=0.29; 0.94; 0.149 respectively). The sensitivity, specificity, positive predictive value and negative predictive value for qualitative assessment of Diffusion-weighted MRI in the differentiation of benign from malignant skeletal lesions were: 53.9%, 85%, 70%, 73.9% respectively. Qualitative assessment of DWI (restricted diffusion versus non-restricted diffusion) allowed differentiation of benign from malignant skeletal lesions (P-value=0.0259). CONCLUSIONS: Qualitative assessment of DWI may aid in the differentiation of benign skeletal lesions from malignant skeletal lesions of the pelvis. Although DWI has a low sensitivity in the distinction of the two disease entities, it may be a useful adjunct due to its relatively high specificity. This is of particular importance in lymphoma where biopsy may only show chronic inflammatory cells and hence may be false negative.

Pilot Assessment of Brain Metabolism in Perinatally HIV-Infected Youths Using Accelerated 5D Echo Planar J-Resolved Spectroscopic Imaging.

PURPOSE: To measure cerebral metabolite levels in perinatally HIV-infected youths and healthy controls using the accelerated five dimensional (5D) echo planar J-resolved spectroscopic imaging (EP-JRESI) sequence, which is capable of obtaining two dimensional (2D) J-resolved spectra from three spatial dimensions (3D). MATERIALS AND METHODS: After acquisition and reconstruction of the 5D EP-JRESI data, T1-weighted MRIs were used to classify brain regions of interest for HIV patients and healthy controls: right frontal white (FW), medial frontal gray (FG), right basal ganglia (BG), right occipital white (OW), and medial occipital gray (OG). From these locations, respective J-resolved and TE-averaged spectra were extracted and fit using two different quantitation methods. The J-resolved spectra were fit using prior knowledge fitting (ProFit) while the TE-averaged spectra were fit using the advanced method for accurate robust and efficient spectral fitting (AMARES). RESULTS: Quantitation of the 5D EP-JRESI data using the ProFit algorithm yielded significant metabolic differences in two spatial locations of the perinatally HIV-infected youths compared to controls: elevated NAA/(Cr+Ch) in the FW and elevated Asp/(Cr+Ch) in the BG. Using the TE-averaged data quantified by AMARES, an increase of Glu/(Cr+Ch) was shown in the FW region. A strong negative correlation (r < -0.6) was shown between tCh/(Cr+Ch) quantified using ProFit in the FW and CD4 counts. Also, strong positive correlations (r > 0.6) were shown between Asp/(Cr+Ch) and CD4 counts in the FG and BG. CONCLUSION: The complimentary results using ProFit fitting of J-resolved spectra and AMARES fitting of TE-averaged spectra, which are a subset of the 5D EP-JRESI acquisition, demonstrate an abnormal energy metabolism in the brains of perinatally HIV-infected youths. This may be a result of the HIV pathology and long-term combinational anti-retroviral therapy (cART). Further studies of larger perinatally HIV-infected cohorts are necessary to confirm these findings.

Early MRI in neonatal hypoxic-ischaemic encephalopathy treated with hypothermia: Prognostic role at 2-year follow-up.

UNLABELLED: The prognostic role of early MRI (≤ 6 days of life) is still uncertain in hypoxic-ischaemic encephalopathy (HIE) treated with hypothermia. OBJECTIVE: To compare the prognostic value of early (≤ 6 days) and late MRIs (≥ 7 days) in predicting adverse outcome at 2 years old in asphyxiated term neonates treated with hypothermia. METHODS: This retrospective study included all asphyxiated neonates eligible for hypothermia treatment between November 2009 and July 2012. Two MRI scans were performed at a median age of day 4 (early MRI) and day 11 (late MRI). Two radiologists analysed independently each MRI. Imaging was classified as normal/subnormal or abnormal, using a visual analysis. Apparent diffusion coefficient (ADC) values were measured within predefined areas and posterior limb of internal capsule (PLIC) signal intensity was analysed. Neurodevelopmental outcome was assessed at 18-41 months (median age 24 months) as favourable or adverse. RESULTS: Of the 38 neonates followed up, 8 had an adverse outcome, all related to abnormal MRIs. Twenty-nine neonates had both MRIs sequentially. Both early and late MRIs yielded 100% sensitivity for adverse outcome by using the visual analysis. Early MRI had a higher specificity than late MRI (96.3% versus 89.3%). ADC measurements did not provide further information than visual analysis. PLIC signal abnormalities were a good predictor of adverse outcome on both MRIs. CONCLUSION: Early MRI (≤ 6 days) was a good predictor of neurodevelopmental outcome at 2 years old. It could reliably guide intensive care decisions after the end of hypothermia treatment.

Efficient high-resolution RF pulse design applied to simultaneous multi-slice excitation.

RF pulse design via optimal control is typically based on gradient and quasi-Newton approaches and therefore suffers from slow convergence. We present a flexible and highly efficient method that uses exact second-order information within a globally convergent trust-region CG-Newton method to yield an improved convergence rate. The approach is applied to the design of RF pulses for single- and simultaneous multi-slice (SMS) excitation and validated using phantom and in vivo experiments on a 3T scanner using a modified gradient echo sequence.

MR elastography of the liver at 3.0 T in diagnosing liver fibrosis grades; preliminary clinical experience.

OBJECTIVES: To clarify the usefulness of 3.0-T MR elastography (MRE) in diagnosing the histological grades of liver fibrosis using preliminary clinical data. MATERIALS AND METHODS: Between November 2012 and March 2014, MRE was applied to all patients who underwent liver MR study at a 3.0-T clinical unit. Among them, those who had pathological evaluation of liver tissue within 3 months from MR examinations were retrospectively recruited, and the liver stiffness measured by MRE was correlated with histological results. Institutional review board approved this study, waiving informed consent. RESULTS: There were 70 patients who met the inclusion criteria. Liver stiffness showed significant correlation with the pathological grades of liver fibrosis (rho = 0.89, p < 0.0001, Spearman's rank correlation). Areas under the receiver operating characteristic curve were 0.93, 0.95, 0.99 and 0.95 for fibrosis score greater than or equal to F1, F2, F3 and F4, with cut-off values of 3.13, 3.85, 4.28 and 5.38 kPa, respectively. Multivariate analysis suggested that grades of necroinflammation also affected liver stiffness, but to a significantly lesser degree as compared to fibrosis. CONCLUSIONS: 3.0-T clinical MRE was suggested to be sufficiently useful in assessing the grades of liver fibrosis. KEY POINTS: MR elastography may help clinicians assess patients with chronic liver diseases. Usefulness of 3.0-T MR elastography has rarely been reported. Measured liver stiffness correlated well with the histological grades of liver fibrosis. Measured liver stiffness was also affected by necroinflammation, but to a lesser degree. 3.0-T MRE could be a non-invasive alternative to liver biopsy.

Optimization of Scan Parameters to Reduce Acquisition Time for Diffusion Kurtosis Imaging at 1.5T.

PURPOSE: To shorten acquisition of diffusion kurtosis imaging (DKI) in 1.5-tesla magnetic resonance (MR) imaging, we investigated the effects of the number of b-values, diffusion direction, and number of signal averages (NSA) on the accuracy of DKI metrics. METHODS: We obtained 2 image datasets with 30 gradient directions, 6 b-values up to 2500 s/mm(2), and 2 signal averages from 5 healthy volunteers and generated DKI metrics, i.e., mean, axial, and radial kurtosis (MK, K∥, and K⊥) maps, from various combinations of the datasets. These maps were estimated by using the intraclass correlation coefficient (ICC) with those from the full datasets. RESULTS: The MK and K⊥ maps generated from the datasets including only the b-value of 2500 s/mm(2) showed excellent agreement (ICC, 0.96 to 0.99). Under the same acquisition time and diffusion directions, agreement was better of MK, K∥, and K⊥ maps obtained with 3 b-values (0, 1000, and 2500 s/mm(2)) and 4 signal averages than maps obtained with any other combination of numbers of b-value and varied NSA. Good agreement (ICC > 0.6) required at least 20 diffusion directions in all the metrics. CONCLUSION: MK and K⊥ maps with ICC greater than 0.95 can be obtained at 1.5T within 10 min (b-value = 0, 1000, and 2500 s/mm(2); 20 diffusion directions; 4 signal averages; slice thickness, 6 mm with no interslice gap; number of slices, 12).

[Importance of diffusion weighted magnetic resonance imaging at differentiation of cholesteatoma and granulation tissue in patients with chronic suppurative otitis media]. / Kronik süpüratif otitis mediali hastalarda kolesteatom ve granülasyon dokusu ayriminda difüzyon agirlikli manyetik rezonans görüntülemenin önemi.

OBJECTIVES: This study aims to report the significance of echo-planar diffusion-weighted imaging (EP-DWI) in preoperative magnetic resonance imaging of patients with surgically corrected cholesteatoma and granulation tissue according to DWI and apparent diffusion coefficient (ADC) values. PATIENTS AND METHODS: Ninety-one patients (52 males, 39 females; mean age 40.7±15.8 years; range 3 to 77 years) who admitted to radiology clinic of our hospital between December 2009 and May 2011 with a pre-diagnosis of chronic otitis media with primary acquired cholesteatoma and assessed preoperatively in our clinic by ear magnetic resonance imaging and DWI were included in the study. Diffusion-weighted imaging results were compared with operative findings and pathology results. Patients were retrospectively evaluated using picture archiving and communication system. Diffusion-weighted imaging and ADC images were examined qualitatively and quantitatively. RESULTS: Ninety-one patients suspected of cholesteatoma were operated. According to the results of operations, 50 patients had cholesteatoma and 41 patients had granulation tissue. The mean DWI values of patients with cholesteatoma were significantly higher than patients with granulation tissue (p<0.05). The mean ADC values of patients with cholesteatoma were significantly lower than patients with granulation tissue (p<0.05). The sensitivity and specificity of EP-DWI in detection of cholesteatoma were 97.6% and 92.0%, respectively. CONCLUSION: Echo-planar diffusion-weighted imaging by using ADC and DWI is a valuable tool with high sensitivity and specificity rates in detecting cholesteatoma particularly bigger than 5 mm and in differentiating them from other pathologies.

MRSI via fully-refocused spatiotemporal encoding with polychromatic spectral pulses.

A novel method for the rapid acquisition of quality multi-slice 2D images targeting a small number of spectroscopic resonances, is introduced and illustrated. The method exploits the robustness derived from recently proposed spatiotemporal encoding (SPEN) methods, when operating in the so-called "fully refocused" mode. Fully-refocused SPEN provides high-fidelity single-shot images thanks to its refocusing of all offset-derived effects throughout the course of the acquisition. This refocusing, however, prevents exploiting such robustness for spectroscopic imaging. We propose here a solution to this limitation, based on the use of polychromatic refocusing pulses. It is shown that if used to address a series of a priori known resonance positions, these pulses can lead to quality spectroscopic images in a small number of scans - generally equal or slightly larger than the number of targeted peaks. Such strategy is explored in combination with both fully-refocused SPEN and echo-planar-imaging (EPI) acquisitions. The expected SPEN advantages were observed in both phantom-based models, and in in vivo results of fat and water separation in mice at 7 T.

Focal liver lesions detection: Comparison of respiratory-triggering, triggering and tracking navigator and tracking-only navigator in diffusion-weighted imaging.

PURPOSE: To compare low b value (10s/mm(2)) spin-echo echo-planar (SE-EP) diffusion-weighted imaging (DWI) acquired with respiratory-triggering (RT), triggering and tracking navigator (TT), tracking only navigator (TRON) techniques for image quality and focal liver lesions (FLL) detection in non-cirrhotic patients. MATERIAL AND METHODS: This bi-centric study was approved by the institutional review boards; informed consent was obtained. Eighty-three patients were prospectively included and SE-EP-DWI with RT, TT and TRON techniques were performed. DWI sequences were randomized and independently analyzed by two readers. The qualitative evaluation was based on a 3-point score for axial artifacts (motion, ghost, susceptibility artifacts and distortion) and stair-step artifacts. Sensitivity of FLL detection was calculated for all lesions together and after lesion size stratification (≤ 10 mm, >10-20mm and >20mm). The standard of reference consisted of a retrospective reading of the conventional MRI, the three DWI sequences and by follow-up (12 months): a total of 409 FLL were detected. Data between sequences was compared with non-parametric tests. Cohen's kappa coefficient was used for inter-observer agreement. RESULTS: Image quality was comparable for RT and TT. TRON showed statistically significantly more axial artifacts for the two readers (p<0.05). Stair-step artifacts were not statistically significantly different between DWI sequences. Overall sensitivities for RT, TT, TRON were 85%, 86%, 82% and 86%, 89% 83%, respectively, for readers 1 and 2. The inter-observer agreement was very good. CONCLUSION: Image quality was better for RT and TT compared to TRON. Overall sensitivities for FLL detection were comparable between techniques and readers.

Differentiation of squamous cell carcinoma and inverted papilloma using non-invasive MR perfusion imaging.

OBJECTIVES: To investigate the diagnostic value of tumour blood flow (TBF) obtained with pseudocontinuous arterial spin labelling for the differentiation of squamous cell carcinoma (SCC) and inverted papilloma (IP) in the nasal or sinonasal cavity. METHODS: We retrospectively analysed the cases of 33 patients with SCC and 8 patients with IP in the nasal or sinonasal cavity. Pseudocontinuous arterial spin labelling scanning was performed for all patients using a 3.0-T MR unit. Quantitative TBF values were measured by two neuroradiologists by respectively delineating the whole-tumour regions of interest, and the mean of them was determined as TBF value in each patient. Additionally, the presence of imaging findings of convoluted cerebriform pattern (CCP) on MR T2 weighted images was determined in all patients. As a subgroup analysis, patients with IP were divided into aggressive and non-aggressive IPs depending on their progression range. First, an intraclass correlation coefficient (ICC) of TBF values between two neuroradiologists was determined. Next, a statistical comparison of the TBF value by a Mann-Whitney U test between the patients with SCC and IP was performed. Additionally, the comparison by an ANOVA with a post hoc test of Tukey's method among the SCC, non-aggressive IP and aggressive IP groups was also performed. If significance was observed, the diagnostic accuracy to differentiate SCCs from IPs was calculated. Diagnostic accuracy by CCP findings alone and by the combination of CCP findings and TBF were also assessed. RESULTS: The ICC of TBF values between two neuroradiologists was 0.82. The mean TBF values in the patients with SCC, all patients with IP, those with aggressive IP and those with non-aggressive IP were 141.2 ± 33.1, 77.8 ± 31.5, 109.4 ± 16.7 and 58.8 ± 19.9 ml 100 g⁻¹ min⁻¹, respectively. A significant difference was observed between SCC and IP (p < 0.001), SCC and non-aggressive IP (p < 0.01) and non-aggressive IP and aggressive IP (p < 0.01). The diagnostic accuracy values obtained with receiver operating characteristic curve analysis for the differentiation of SCC from IP and for SCC from non-aggressive IP were 0.90 and 0.92, respectively. The diagnostic accuracy was elevated (0.95 from 0.88) by adding the TBF value to CCP findings. CONCLUSIONS: The pseudocontinuous arterial spin labelling technique can be a useful non-invasive diagnostic tool to differentiate SCC from IP in nasal or sinonasal cavity.

Enhanced identification of BOLD-like components with multi-echo simultaneous multi-slice (MESMS) fMRI and multi-echo ICA.

The recent introduction of simultaneous multi-slice (SMS) acquisitions has enabled the acquisition of blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) data with significantly higher temporal sampling rates. In a parallel development, the use of multi-echo fMRI acquisitions in conjunction with a multi-echo independent component analysis (ME-ICA) approach has been introduced as a means to automatically distinguish functionally-related BOLD signal components from signal artifacts, with significant gains in sensitivity, statistical power, and specificity. In this work, we examine the gains that can be achieved with a combined approach in which data obtained with a multi-echo simultaneous multi-slice (MESMS) acquisition are analyzed with ME-ICA. We find that ME-ICA identifies significantly more BOLD-like components in the MESMS data as compared to data acquired with a conventional multi-echo single-slice acquisition. We demonstrate that the improved performance of MESMS derives from both an increase in the number of temporal samples and the enhanced ability to filter out high-frequency artifacts.

Combined acquisition technique (CAT) for neuroimaging of multiple sclerosis at low specific absorption rates (SAR).

PURPOSE: To compare a novel combined acquisition technique (CAT) of turbo-spin-echo (TSE) and echo-planar-imaging (EPI) with conventional TSE. CAT reduces the electromagnetic energy load transmitted for spin excitation. This radiofrequency (RF) burden is limited by the specific absorption rate (SAR) for patient safety. SAR limits restrict high-field MRI applications, in particular. MATERIAL AND METHODS: The study was approved by the local Medical Ethics Committee. Written informed consent was obtained from all participants. T2- and PD-weighted brain images of n = 40 Multiple Sclerosis (MS) patients were acquired by CAT and TSE at 3 Tesla. Lesions were recorded by two blinded, board-certificated neuroradiologists. Diagnostic equivalence of CAT and TSE to detect MS lesions was evaluated along with their SAR, sound pressure level (SPL) and sensations of acoustic noise, heating, vibration and peripheral nerve stimulation. RESULTS: Every MS lesion revealed on TSE was detected by CAT according to both raters (Cohen's kappa of within-rater/across-CAT/TSE lesion detection κCAT = 1.00, at an inter-rater lesion detection agreement of κLES = 0.82). CAT reduced the SAR burden significantly compared to TSE (p<0.001). Mean SAR differences between TSE and CAT were 29.0 (± 5.7) % for the T2-contrast and 32.7 (± 21.9) % for the PD-contrast (expressed as percentages of the effective SAR limit of 3.2 W/kg for head examinations). Average SPL of CAT was no louder than during TSE. Sensations of CAT- vs. TSE-induced heating, noise and scanning vibrations did not differ. CONCLUSION: T2-/PD-CAT is diagnostically equivalent to TSE for MS lesion detection yet substantially reduces the RF exposure. Such SAR reduction facilitates high-field MRI applications at 3 Tesla or above and corresponding protocol standardizations but CAT can also be used to scan faster, at higher resolution or with more slices. According to our data, CAT is no more uncomfortable than TSE scanning.

Transfer function analysis of respiratory and cardiac pulsations in human brain observed on dynamic magnetic resonance images.

Magnetic resonance (MR) imaging provides a noninvasive, in vivo imaging technique for studying respiratory and cardiac pulsations in human brains, because these pulsations can be recorded as flow-related enhancement on dynamic MR images. By applying independent component analysis to dynamic MR images, respiratory and cardiac pulsations were observed. Using the signal-time curves of these pulsations as reference functions, the magnitude and phase of the transfer function were calculated on a pixel-by-pixel basis. The calculated magnitude and phase represented the amplitude change and temporal delay at each pixel as compared with the reference functions. In the transfer function analysis, near constant phases were found at the respiratory and cardiac frequency bands, indicating the existence of phase delay relative to the reference functions. In analyzing the dynamic MR images using the transfer function analysis, we found the following: (1) a good delineation of temporal delay of these pulsations can be achieved; (2) respiratory pulsation exists in the ventricular and cortical cerebrospinal fluid; (3) cardiac pulsation exists in the ventricular cerebrospinal fluid and intracranial vessels; and (4) a 180-degree phase delay or inverted amplitude is observed on phase images.

Parallel imaging acceleration of EPIK for reduced image distortions in fMRI.

EPI with Keyhole (EPIK) is a hybrid imaging technique used to improve the performance of EPI in dynamic MRI applications. The method had been previously validated at 1.5 T with both phantom and in vivo images; EPIK was able to provide a higher temporal resolution and less image distortions than single-shot EPI. The data presented here demonstrate that the performance of EPIK can be further improved by accelerating it with the parallel imaging. For this work, this combination was tested at 3 T. After initial evaluation using phantom images, use of the method in functional MRI was verified with visual fMRI measurements as well as MRI simulation results. The results showed that accelerated EPIK had increased temporal resolution with favorable robustness against susceptibility artifacts when compared with EPI or non-accelerated EPIK.

Phase informed model for motion and susceptibility.

Field inhomogeneities caused by variations in magnetic susceptibility throughout the head lead to geometric distortions, mainly in the phase-encode direction of echo-planar images (EPI). The magnitude and spatial characteristics of the distortions depend on the orientation of the head in the magnetic field and will therefore vary with head movement. A new method is presented, based on a phase informed model for motion and susceptibility (PIMMS), which estimates the change in geometric distortion associated with head motion. This method fits a model of the head motion parameters and scanner hardware characteristics to EPI phase time series. The resulting maps of the model fit parameters are used to correct for susceptibility artifacts in the magnitude images. Results are shown for EPI-based fMRI time-series acquired at 3T, demonstrating that compared with conventional rigid body realignment, PIMMS removes residual variance associated with motion-related distortion effects. Furthermore, PIMMS can lead to a reduction in false negatives compared with the widely accepted approach which uses standard rigid body realignment and includes the head motion parameters in the statistical model. The PIMMS method can be used with any standard EPI sequence for which accurate phase information is available.