Robust myocardial T2 and T2 * mapping at 3T using image-based shimming.

PURPOSE: Intramyocardial hemorrhage and area at risk are both prognostic markers in acute myocardial infarction (AMI). Myocardial T2 and T2 * mapping have been used to detect such tissue changes at 1.5T but these techniques are challenging at 3.0T due to additional susceptibility variation. We studied T2 and T2 * myocardial mapping techniques at 3.0T on a system employing B1 shimming and compared two different methods of B0 shimming. MATERIALS AND METHODS: Fifteen volunteers and six AMI patients were scanned on a 3T system. Volume and image-based (IB) B0 shimming techniques were implemented. Single breath-hold, multiecho gradient, and spin echo sequences were employed from which T2 * and T2 maps were calculated. RESULTS: In volunteers, there was no significant difference in mean values obtained with volume or IB shimming for T2 mapping (39.1 ± 6.0 msec vs. 39.4 ± 6.1 msec; P > 0.05) or for T2 * mapping (24.2 ± 6.7 msec vs. 24.1 ± 5.2 msec; P > 0.05). There were no significant regional differences in mean T2 values between septal, anterior, and posterior segments with either shimming technique (all P > 0.05); but there were significant regional differences in mean T2 * values using volume shimming (27.8 ± 5.2 msec vs. 28.4 ± 5.8 msec vs. 15.9 ± 8.3 msec; P < 0.05)-but not with IB shimming (25.7 ± 5.4 msec vs. 25.3 ± 5.9 msec vs. 18.7 ± 4.6 msec; P > 0.05). CONCLUSION: At 3.0T, cardiac T2 mapping is robust. Although T2 * mapping is prone to more regional heterogeneity this can be reduced by using IB instead of conventional volume B0 shimming.

3.0T, time-resolved, 3D flow-sensitive MR in the thoracic aorta: Impact of k-t BLAST acceleration using 8- versus 32-channel coil arrays.

PURPOSE: To evaluate the performance of 4D flow MR in the thoracic aorta with 8- and 32-channel coil arrays using k-t BLAST and SENSE acceleration techniques and compare this to a conventional 2D SENSE approach. MATERIALS AND METHODS: Fifteen healthy subjects and eight patients underwent magnetic resonance imaging (MRI) at 3.0T using: 1) 2D SENSE phase contrast velocity mapping as the reference standard and 2) 4D-flow pulse sequences accelerated with SENSE and k-t BLAST, using both 8- and 32-channel coil arrays. Data processing was performed using GT Flow. Image quality of the magnitude images and pathline visualization were graded and mean scan times, flow, peak velocity, stroke volume, and image quality were compared between techniques. RESULTS: Mean scan times were significantly lower for 4D-flow sequences accelerated with k-t BLAST compared to SENSE (5.5 vs. 25.2 min; P < 0.01). 4D k-t BLAST acquisition had greater magnitude and pathline image quality than 4D SENSE acquisition for both 32-channel and 8-channel data (P < 0.001); both 4D SENSE and 4D k-t BLAST acquisitions had significantly greater image quality when 32 channels were utilized compared to 8 (P < 0.05). On Bland-Altman analysis, all 4D flow pulse sequences showed significant agreement with the 2D SENSE reference for peak velocity measurement (P > 0.05); the lowest bias being observed with the 4D 32 channel k-t BLAST sequence. There were no significant differences in measured flow, peak velocity, or stroke volume with any of the four investigated 4D acquisition techniques compared to reference technique values (P > 0.05). In patients, there were no significant differences in flow, peak velocity, or stroke volume measurements between 32-channel 4D k-t BLAST and the reference acquisition. CONCLUSION: 4D flow MR using k-t BLAST and 32 channel coils allows a reduction in total scan time while improving overall image quality compared to a standard 2D SENSE and 4D SENSE acquisitions. The use of 32 channels rather than 8 channels with the 4D k-t BLAST was also preferable in terms of image quality.

Reproducibility of myocardial strain and left ventricular twist measured using complementary spatial modulation of magnetization.

PURPOSE: To establish the reproducibility of complementary spatial modulation of magnetization (CSPAMM) tagged cardiovascular MR (CMR) data in normal volunteers. MATERIALS AND METHODS: Twelve healthy volunteers underwent CMR studies on two separate occasions using an identical CSPAMM pulse sequence with images acquired in three short axis slices. Data were analyzed by two independent observers using harmonic phase analysis (HARP). Lagrangian circumferential and radial strain, rotation, and left ventricular twist were calculated. RESULTS: The intraobserver reproducibility of circumferential strain (CoV [coefficient of variation] 1.5-4.3%) and LV twist (CoV 1.2-4.4%) was better than radial strain (CoV 10.6-14.8%). For interobserver reproducibility, circumferential strain (CoV 3.5-6.2%) and LV twist (CoV 3.5-7.2%) were more reproducible than radial strain (CoV 11.8-21.8%). Interstudy reproducibility of circumferential strain (CoV 3.7-5.5%) and LV twist (CoV 9.8-12.2%) were good but radial strain (CoV 13.8-23.4%) but showed poorer interstudy reproducibility. Sample size calculations suggested 20 or fewer subjects are needed to detect a 10% change in circumferential strain (power 90%; α error 0.05), whereas for twist, 66 subjects would be required. CONCLUSION: In normal volunteers, the intraobserver, interobserver, and interstudy reproducibility of circumferential strain and LV twist measured from CSPAMM tagged CMR data are good, but are less so for radial strain.

Three-dimensional balanced steady state free precession myocardial perfusion cardiovascular magnetic resonance at 3T using dual-source parallel RF transmission: initial experience.

BACKGROUND: The purpose of this study was to establish the feasibility of three-dimensional (3D) balanced steady-state-free-precession (bSSFP) myocardial perfusion cardiovascular magnetic resonance (CMR) at 3T using local RF shimming with dual-source RF transmission, and to compare it with spoiled gradient echo (TGRE) acquisition. METHODS: Dynamic contrast-enhanced 3D bSSFP perfusion imaging was performed on a 3T MRI scanner equipped with dual-source RF transmission technology. Images were reconstructed using k-space and time broad-use linear acquisition speed-up technique (k-t BLAST) and compartment based principle component analysis (k-t PCA). RESULTS: In phantoms and volunteers, local RF shimming with dual source RF transmission significantly improved B1 field homogeneity compared with single source transmission (P=0.01). 3D bSSFP showed improved signal-to-noise, contrast-to-noise and signal homogeneity compared with 3D TGRE (29.8 vs 26.9, P=0.045; 23.2 vs 21.6, P=0.049; 14.9% vs 12.4%, p=0.002, respectively). Image quality was similar between bSSFP and TGRE but there were more dark rim artefacts with bSSFP. k-t PCA reconstruction reduced artefacts for both sequences compared with k-t BLAST. In a subset of five patients, both methods correctly identified those with coronary artery disease. CONCLUSION: Three-dimensional bSSFP myocardial perfusion CMR using local RF shimming with dual source parallel RF transmission at 3T is feasible and improves signal characteristics compared with TGRE. Image artefact remains an important limitation of bSSFP imaging at 3T but can be reduced with k-t PCA.

Susceptibility-weighted cardiovascular magnetic resonance in comparison to T2 and T2 star imaging for detection of intramyocardial hemorrhage following acute myocardial infarction at 3 Tesla.

BACKGROUND: Intramyocardial hemorrhage (IMH) identified by cardiovascular magnetic resonance (CMR) is an established prognostic marker following acute myocardial infarction (AMI). Detection of IMH by T2-weighted or T2 star CMR can be limited by long breath hold times and sensitivity to artefacts, especially at 3T. We compared the image quality and diagnostic ability of susceptibility-weighted magnetic resonance imaging (SW MRI) with T2-weighted and T2 star CMR to detect IMH at 3T. METHODS: Forty-nine patients (42 males; mean age 58 years, range 35-76) underwent 3T cardiovascular magnetic resonance (CMR) 2 days following re-perfused AMI. T2-weighted, T2 star and SW MRI images were obtained. Signal and contrast measurements were compared between the three methods and diagnostic accuracy of SW MRI was assessed against T2w images by 2 independent, blinded observers. Image quality was rated on a 4-point scale from 1 (unusable) to 4 (excellent). RESULTS: Of 49 patients, IMH was detected in 20 (41%) by SW MRI, 21 (43%) by T2-weighted and 17 (34%) by T2 star imaging (p = ns). Compared to T2-weighted imaging, SW MRI had sensitivity of 93% and specificity of 86%. SW MRI had similar inter-observer reliability to T2-weighted imaging (κ = 0.90 and κ = 0.88 respectively); both had higher reliability than T2 star (κ = 0.53). Breath hold times were shorter for SW MRI (4 seconds vs. 16 seconds) with improved image quality rating (3.8 ± 0.4, 3.3 ± 1.0, 2.8 ± 1.1 respectively; p < 0.01). CONCLUSIONS: SW MRI is an accurate and reproducible way to detect IMH at 3T. The technique offers considerably shorter breath hold times than T2-weighted and T2 star imaging, and higher image quality scores.

A Novel Functional Magnetic Resonance Imaging Paradigm for the Preoperative Assessment of Auditory Perception in a Musician Undergoing Temporal Lobe Surgery.

BACKGROUND: Presurgical evaluation for temporal lobe epilepsy routinely assesses speech and memory lateralization and anatomic localization of the motor and visual areas but not baseline musical processing. This is paramount in a musician. Although validated tools exist to assess musical ability, there are no reported functional magnetic resonance imaging (fMRI) paradigms to assess musical processing. We examined the utility of a novel fMRI paradigm in an 18-year-old left-handed pianist who underwent surgery for a left temporal low-grade ganglioglioma. METHODS: Preoperative evaluation consisted of neuropsychological evaluation, T1-weighted and T2-weighted magnetic resonance imaging, and fMRI. Auditory blood oxygen level-dependent fMRI was performed using a dedicated auditory scanning sequence. Three separate auditory investigations were conducted: listening to, humming, and thinking about a musical piece. RESULTS: All auditory fMRI paradigms activated the primary auditory cortex with varying degrees of auditory lateralization. Thinking about the piece additionally activated the primary visual cortices (bilaterally) and right dorsolateral prefrontal cortex. Humming demonstrated left-sided predominance of auditory cortex activation with activity observed in close proximity to the tumor. CONCLUSIONS: This study demonstrated an fMRI paradigm for evaluating musical processing that could form part of preoperative assessment for patients undergoing temporal lobe surgery for epilepsy.

Increased functional magnetic resonance imaging activity during nonconscious perception in the attentional blink.

Presenting two targets for identification within a few hundred milliseconds reliably yields reduced performance on the second relative to the first. This attentional blink phenomenon has been extensively studied behaviourally but, until recently, has eluded direct investigation by functional magnetic resonance imaging. The few published imaging studies agree that the attentional blink recruits a fronto-temporo-parietal 'attentional' network. What remains controversial is the specific role played by the 'object processing' ventro-occipital regions of the brain. Two studies used different tasks and stimulus onset asynchronies leaving the cause for this divergence unknown. The present study resolves this discrepancy by examining the different methodologies. Our results suggest that task difficulty and masking must be factored in to any conclusions drawn about the brain activity during the attentional blink paradigm.

Activation of SI is modulated by attention: a random effects fMRI study using mechanical stimuli.

Animal experiments on tactile attention suggest a modulation of sensory processing on the level of sensory representations but correspondent neuroimaging data in humans is inconclusive. The present experiment used mechanical stimuli to study tactile processing while varying the focus of attention. Activations were contrasted between attend and ignore conditions, both of which employed identical stimulation characteristics and an active task. Random effects analysis revealed significant attention effects in area SI (primary somatosensory cortex) in that the blood oxygenation level-dependent response was greater for attended than for ignored stimuli. Modulations were further found in the secondary somatosensory cortex and the middle temporal gyrus. These findings suggest that stimulus processing at the level of primary representations in area SI is modulated by attention.

Pre-surgical mapping of eloquent cortex for paediatric epilepsy surgery candidates: Evidence from a review of advanced functional neuroimaging.

PURPOSE: A review of all published evidence for mapping eloquent (motor, language and memory) cortex using advanced functional neuroimaging (functional magnetic resonance imaging [fMRI] and magnetoencephalography [MEG]) for paediatric epilepsy surgery candidates has not been conducted previously. Research in this area has predominantly been in adult populations and applicability of these techniques to paediatric populations is less established. METHODS: A review was performed using an advanced systematic search and retrieval of all published papers examining the use of functional neuroimaging for paediatric epilepsy surgery candidates. RESULTS: Of the 2724 papers retrieved, 34 met the inclusion criteria. Total paediatric participants identified were 353 with an age range of 5 months-19 years. Sample sizes and comparisons with alternative investigations to validate techniques are small and variable paradigms are used. Sensitivity 0.72 (95% CI 0.52-0.86) and specificity 0.60 (95% CI 0.35-0.92) values with a Positive Predictive Value of 74% (95% CI 61-87) and a Negative Predictive Value of 65% (95% CI 52-78) for fMRI language lateralisation with validation, were obtained. Retrieved studies indicate evidence that both fMRI and MEG are able to provide information lateralising and localising motor and language functions. CONCLUSIONS: A striking finding of the review is the paucity of studies (n=34) focusing on the paediatric epilepsy surgery population. For children, it remains unclear which language and memory paradigms produce optimal activation and how these should be quantified in a statistically robust manner. Consensus needs to be achieved for statistical analyses and the uniformity and yield of language, motor and memory paradigms. Larger scale studies are required to produce patient series data which clinicians may refer to interpret results objectively. If functional imaging techniques are to be the viable alternative for pre-surgical mapping of eloquent cortex for children, paradigms and analyses demonstrating concordance with independent measures must be developed.