Ernesta M. Meintjes

Tracking Myocardial Motion From Cine DENSE Images Using Spatiotemporal Phase Unwrapping and Temporal Fitting

Displacement encoding with stimulated echoes (DENSE) encodes myocardial tissue displacement into the phase of the MR image. Cine DENSE allows for rapid quantification of myocardial displacement at multiple cardiac phases through the majority of the cardiac cycle. For practical sensitivities to motion, relatively high displacement encoding frequencies are used and phase wrapping typically occurs. In order to obtain absolute measures of displacement, a two-dimensional (2-D) quality-guided phase unwrapping algorithm was adapted to unwrap both spatially and temporally. Both a fully automated algorithm and a faster semi-automated algorithm are proposed. A method for computing the 2-D trajectories of discrete points in the myocardium as they move through the cardiac cycle is introduced. The error in individual displacement measurements is reduced by fitting a time series to sequential displacement measurements along each trajectory. This improvement is in turn reflected in strain maps, which are derived directly from the trajectories. These methods were validated both in vivo and on a rotating phantom. Further measurements were made to optimize the displacement encoding frequency and to estimate the baseline strain noise both on the phantom and in vivo. The fully automated phase unwrapping algorithm was successful for 767 out of 800 images (95.9%), and the semi-automated algorithm was successful for 786 out of 800 images (98.3%). The accuracy of the tracking algorithm for typical cardiac displacements on a rotating phantom is 0.24plusmn0.15mm. The optimal displacement encoding frequency is in the region of 0.1 cycles/mm, and, for 2 scans of 17-s duration, the strain noise after temporal fitting was estimated to be 2.5plusmn3.0% at end-diastole, 3.1plusmn3.1% at end-systole, and 5.3plusmn5.0% in mid-diastole. The improvement in intra-myocardial strain measurements due to temporal fitting is apparent in strain histograms, and also in identifying regions of dysfunctional myocardium in studies of patients with infarcts

Volumetric navigators for prospective motion correction and selective reacquisition in neuroanatomical MRI.

We introduce a novel method of prospectively compensating for subject motion in neuroanatomical imaging. Short three‐dimensional echo‐planar imaging volumetric navigators are embedded in a long three‐dimensional sequence, and the resulting image volumes are registered to provide an estimate of the subjects location in the scanner at a cost of less than 500 ms, ∼ 1% change in contrast, and ∼3% change in intensity. This time fits well into the existing gaps in sequences routinely used for neuroimaging, thus giving a motion‐corrected sequence with no extra time required. We also demonstrate motion‐driven selective reacquisition of k‐space to further compensate for subject motion. We perform multiple validation experiments to evaluate accuracy, navigator impact on tissue intensity/contrast, and the improvement in final output. The complete system operates without adding additional hardware to the scanner and requires no external calibration, making it suitable for high‐throughput environments. Magn Reson Med, 2012.

The neural substrates of mindfulness: An fMRI investigation

“Mindfulness” is a capacity for heightened present-moment awareness that we all possess to a greater or lesser extent. Enhancing this capacity through training has been shown to alleviate stress and promote physical and mental well-being. As a consequence, interest in mindfulness is growing and so is the need to better understand it. This study employed functional magnetic resonance imaging (fMRI) to identify the brain regions involved in state mindfulness and to shed light on its mechanisms of action. Significant signal decreases were observed during mindfulness meditation in midline cortical structures associated with interoception, including bilateral anterior insula, left ventral anterior cingulate cortex, right medial prefrontal cortex, and bilateral precuneus. Significant signal increase was noted in the right posterior cingulate cortex. These findings lend support to the theory that mindfulness achieves its positive outcomes through a process of disidentification.

Motion Artifact Removal for Functional Near Infrared Spectroscopy: A Comparison of Methods

Near infrared spectroscopy (NIRS) is rapidly gaining popularity for functional brain imaging. It is well suited to studies of patients or children; however, in these populations particularly, motion artifacts can present a problem. Here, we propose the use of imaging channels with negligible distance between light source and detector to detect subject motion, without the need for an additional motion sensor. Datasets containing deliberate motion artifacts were obtained from three subjects. Motion artifacts could be detected in the signal from the co-located channels with a minimum sensitivity of 0.75 and specificity of 0.98. Five techniques for removing motion artifact from the functional signals were compared, namely two-input recursive least squares (RLS) adaptive filtering, wavelet-based filtering, independent component analysis (ICA), and two-channel and multiple-channel regression. In most datasets, the median change in SNR across all channels was the greatest using ICA or multiple-channel regression. RLS adaptive filtering produced the smallest increase in SNR. Where sharp spikes were present, wavelet filtering produced the largest SNR increase. ICA and multiple-channel regression are promising ways to reduce motion artifact in functional NIRS without requiring time-consuming manual techniques.

Real-time motion and B0 corrected single voxel spectroscopy using volumetric navigators.

In population groups where head pose cannot be assumed to be constant during a magnetic resonance spectroscopy examination or in difficult‐to‐shim regions of the brain, real‐time volume of interest, frequency, and shim optimization may be necessary. We investigate the effect of pose change on the B0 homogeneity of a (2 cm)3 volume and observe typical first‐order shim changes of 1 μT/m per 1° rotation (chin down to up) in four different volumes of interest in a single volunteer. An echo planar imaging volume navigator was constructed to measure and apply in real‐time within each pulse repetition time: volume of interest positioning, frequency adjustment, and first‐order shim adjustment. This volume navigator is demonstrated in six healthy volunteers and achieved a mean linewidth of 4.4 Hz, similar to that obtained by manual shim adjustment of 4.9 Hz. Furthermore, this linewidth is maintained by the volume navigator at 4.9 Hz in the presence of pose change. By comparison, a mean linewidth of 7.5 Hz was observed, when no correction was applied. Magn Reson Med, 2011.

Impaired delay and trace eyeblink conditioning in school-age children with fetal alcohol syndrome.

BACKGROUND Classical eyeblink conditioning (EBC) involves contingent temporal pairing of a conditioned stimulus (e.g., tone) with an unconditioned stimulus (e.g., air puff). Impairment of EBC has been demonstrated in studies of alcohol-exposed animals and in children exposed prenatally at heavy levels. METHODS Fetal alcohol syndrome (FAS) was diagnosed by expert dysmorphologists in a large sample of Cape Coloured, South African children. Delay EBC was examined in a new sample of 63 children at 11.3 years, and trace conditioning in 32 of the same children at 12.8 years. At each age, 2 sessions of 50 trials each were administered on the same day; 2 more sessions the next day, for children not meeting criterion for conditioning. RESULTS Six of 34 (17.6%) children born to heavy drinkers were diagnosed with FAS, 28 were heavily exposed nonsyndromal (HE), and 29 were nonexposed controls. Only 33.3% with FAS and 42.9% of HE met criterion for delay conditioning, compared with 79.3% of controls. The more difficult trace conditioning task was also highly sensitive to fetal alcohol exposure. Only 16.7% of the FAS and 21.4% of HE met criterion for trace conditioning, compared with 66.7% of controls. The magnitude of the effect of diagnostic group on trace conditioning was not greater than the effect on short delay conditioning, findings consistent with recent rat studies. Longer latency to onset and peak eyeblink CR in exposed children indicated poor timing and failure to blink in anticipation of the puff. Extended training resulted in some but not all of the children reaching criterion. CONCLUSIONS These data showing alcohol-related delay and trace conditioning deficits extend our earlier findings of impaired EBC in 5-year-olds to school-age. Alcohol-related impairment in the cerebellar circuitry required for both forms of conditioning may be sufficient to account for the deficit in both tasks. Extended training was beneficial for some exposed children. EBC provides a well-characterized model system for assessment of degree of cerebellar-related learning and memory dysfunction in fetal alcohol exposed children.

An FMRI study of number processing in children with fetal alcohol syndrome.

BACKGROUND Number processing deficits are frequently seen in children exposed to alcohol in utero. METHODS Functional magnetic resonance imaging was used to examine the neural correlates of number processing in 15 right-handed, 8- to 12-year-old children diagnosed with fetal alcohol syndrome (FAS) or partial FAS (PFAS) and 18 right-handed, age- and gender-matched controls from the Cape Coloured (mixed ancestry) community in Cape Town, South Africa, using Proximity Judgment and Exact Addition tasks. RESULTS Control children activated the expected fronto-parietal network during both tasks, including the anterior horizontal intraparietal sulcus (HIPS), left posterior HIPS, left precentral sulcus, and posterior medial frontal cortex. By contrast, on the Proximity Judgment task, the exposed children recruited additional parietal pathways involving the right and left angular gyrus and posterior cingulate/precuneus, which may entail verbally mediated recitation of numbers and/or subtraction to assess relative numerical distances. During Exact Addition, the exposed children exhibited more diffuse and widespread activations, including the cerebellar vermis and cortex, which have been found to be activated in adults engaged in particularly challenging number processing problems. CONCLUSIONS The data suggest that, whereas control children rely primarily on the fronto-parietal network identified in previous studies to mediate number processing, children with FAS/PFAS recruit a broader range of brain regions to perform these relatively simple number processing tasks. Our results are consistent with structural neuroimaging findings indicating that the parietal lobe is relatively more affected by prenatal alcohol exposure and provide the first evidence for brain activation abnormalities during number processing in children with FAS/PFAS, effects that persist even after controlling statistically for group differences in total intracranial volume and IQ.

The effects of mindfulness-based cognitive therapy in patients with bipolar disorder: A controlled functional MRI investigation

BACKGROUND Preliminary research findings have shown that mindfulness-based cognitive therapy improves anxiety and depressive symptoms in bipolar disorder. In this study, we further investigated the effects of MBCT in bipolar disorder, in a controlled fMRI study. METHOD Twenty three patients with bipolar disorder underwent neuropsychological testing and functional MRI. Sixteen of these patients were tested before and after an eight-week MBCT intervention, and seven were wait listed for training and tested at the same intervals. The results were compared with 10 healthy controls. RESULTS Prior to MBCT, bipolar patients reported significantly higher levels of anxiety and symptoms of stress, scored significantly lower on a test of working memory, and showed significant BOLD signal decrease in the medial PFC during a mindfulness task, compared to healthy controls. Following MBCT, there were significant improvements in the bipolar treatment group, in measures of mindfulness, anxiety and emotion regulation, and in tests of working memory, spatial memory and verbal fluency compared to the bipolar wait list group. BOLD signal increases were noted in the medial PFC and posterior parietal lobe, in a repeat mindfulness task. A region of interest analysis revealed strong correlation between signal changes in medial PFC and increases in mindfulness. LIMITATIONS The small control group is a limitation in the study. CONCLUSION These data suggest that MBCT improves mindfulness and emotion regulation and reduces anxiety in bipolar disorder, corresponding to increased activations in the medial PFC, a region associated with cognitive flexibility and previously proposed as a key area of pathophysiology in the disorder.

Motion-guided segmentation for cine DENSE MRI.

Defining myocardial contours is often the most time-consuming portion of dynamic cardiac MRI image analysis. Displacement encoding with stimulated echoes (DENSE) is a quantitative MRI technique that encodes tissue displacement into the phase of the complex MRI images. Cine DENSE provides a time series of these images, thus facilitating the non-invasive study of myocardial kinematics. Epicardial and endocardial contours need to be defined at each frame on cine DENSE images for the quantification of regional displacement and strain as a function of time. This work presents a reliable and effective two-dimensional semi-automated segmentation technique that uses the encoded motion to project a manually-defined region of interest through time. Contours can then easily be extracted for each cardiac phase. This method boasts several advantages, including, (1) parameters are based on practical physiological limits, (2) contours are calculated for the first few cardiac phases, where it is difficult to visually distinguish blood from myocardium, and (3) the method is independent of the shape of the tissue delineated and can be applied to short- or long-axis views, and on arbitrary regions of interest. Motion-guided contours were compared to manual contours for six conventional and six slice-followed mid-ventricular short-axis cine DENSE datasets. Using an area measure of segmentation error, the accuracy of the segmentation algorithm was shown to be similar to inter-observer variability. In addition, a radial segmentation error metric was introduced for short-axis data. The average radial epicardial segmentation error was 0.36+/-0.08 and 0.40+/-0.10 pixels for slice-followed and conventional cine DENSE, respectively, and the average radial endocardial segmentation error was 0.46+/-0.12 and 0.46+/-0.16 pixels for slice following and conventional cine DENSE, respectively. Motion-guided segmentation employs the displacement-encoded phase shifts intrinsic to DENSE MRI to accurately propagate a single set of pre-defined contours throughout the remaining cardiac phases.

Facial dysmorphism across the fetal alcohol spectrum.

OBJECTIVE: Classic facial characteristics of fetal alcohol syndrome (FAS) are shortened palpebral fissures, smooth philtrum, and thin upper vermillion. We aim to help pediatricians detect facial dysmorphism across the fetal alcohol spectrum, especially among nonsyndromal heavily exposed (HE) individuals without classic facial characteristics. METHODS: Of 192 Cape Coloured children recruited, 69 were born to women who reported abstaining from alcohol during pregnancy. According to multifaceted criteria, the remainder were allocated clinically to the FAS (n = 22), partial FAS (n = 26) or nonsyndromal HE (n = 75) categories. We used dense surface modeling and signature analyses of 3-dimensional facial photographs to determine agreement between clinical categorization and classifications induced from face shape alone, to visualize facial differences, and to consider predictive links between face shape and neurobehavior. RESULTS: Face classification achieved significant agreement with clinical categories for discrimination of nonexposed from FAS alone (face: 0.97–1.00; profile: 0.92) or with the addition of partial FAS (face: 0.90; profile: 0.92). Visualizations of face signatures delineated dysmorphism across the fetal alcohol spectrum and in half of the nonsyndromal HE category face signature graphs detected facial characteristics consistent with prenatal alcohol exposure. This subgroup performed less well on IQ and learning tests than did nonsyndromal subjects without classic facial characteristics. CONCLUSIONS: Heat maps and morphing visualizations of face signatures may help clinicians detect facial dysmorphism across the fetal alcohol spectrum. Face signature graphs show potential for identifying nonsyndromal heavily exposed children who lack the classic facial phenotype but have cognitive impairment.