Rapid online reconstruction of non-Cartesian magnetic resonance images using commodity graphics cards

In Magnetic Resonance Imaging, energy of electromagnetic waves is used to excite protons placed in a static magnetic field. This generates a signal, which is further spatially encoded with linear magnetic field gradients. The signal exists in frequency domain called k-space. Traditionally, the signal is sampled in lines stored on a Cartesian grid. Next, Fast Fourier Transform is applied to generate images. However, the consecutive manner (line-by-line) of this strategy makes it very slow. Faster sampling strategies exist, but acquisitions with them require a more complex image reconstruction process. There is an obvious trade-off between acquisition time and complexity of image reconstruction. Real-time assessment protocols for day-to-day clinical work demand both data acquisition with rapid sampling trajectories and fast, robust image reconstructions. Computational solutions in form of parallel architectures can be used to aid image reconstruction, which has been proven to significantly speed-up reconstruction process. Regrettably, this is often done in off-line mode, where the data need to be downloaded from the scanner and reconstructed elsewhere. This process hinders the clinical workflow substantially. This work describes challenges entailed with translation of advanced imaging protocols into the clinical environment; (i) use of the advanced sequences is limited by their reconstruction time, and (ii) fast implementations exist but they still run in off-line mode. These were addressed and resolved with development of a novel online, heterogeneous image reconstruction system for Magnetic Resonance Imaging. The external platform was designed to support fast implementation of advanced reconstruction algorithms. An external computer equipped with a Graphic Processing Unit card was integrated into the scanner’s image reconstruction pipeline. This allowed direct access to high performance parallel hardware on which the rapid data reconstruction can be realised. Also, the automation of data transmission and reconstruction execution has preserved the non-interrupted assessment workflow

Perturbed spiral real-time phase-contrast MR with compressive sensing reconstruction for assessment of flow in children

PURPOSE: we implemented a golden‐angle spiral phase contrast sequence. A commonly used uniform density spiral and a new ‘perturbed’ spiral that produces more incoherent aliases were assessed. The aim was to ascertain whether greater incoherence enabled more accurate Compressive Sensing reconstruction and superior measurement of flow and velocity. METHODS: A range of ‘perturbed’ spiral trajectories based on a uniform spiral trajectory were formulated. The trajectory that produced the most noise‐like aliases was selected for further testing. For in‐silico and in‐vivo experiments, data was reconstructed using total Variation L1 regularisation in the spatial and temporal domains. In‐silico, the reconstruction accuracy of the ‘perturbed’ golden spiral was compared to uniform density golden‐angle spiral. For the in‐vivo experiment, stroke volume and peak mean velocity were measured in 20 children using ‘perturbed’ and uniform density golden‐angle spiral sequences. These were compared to a reference standard gated Cartesian sequence. RESULTS: In‐silico, the perturbed spiral acquisition produced more accurate reconstructions with less temporal blurring (NRMSE ranging from 0.03 to 0.05) than the uniform density acquisition (NRMSE ranging from 0.06 to 0.12). This translated in more accurate results in‐vivo with no significant bias in the peak mean velocity (bias: −0.1, limits: −4.4 to 4.1 cm/s; P = 0.98) or stroke volume (bias: −1.8, limits: −9.4 to 5.8 ml, P = 0.19). CONCLUSION: We showed that a ‘perturbed’ golden‐angle spiral approach is better suited to Compressive Sensing reconstruction due to more incoherent aliases. This enabled accurate real‐time measurement of flow and peak velocity in children

Adiposity is associated with blunted cardiovascular, neuroendocrine and cognitive responses to acute mental stress

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited - Copyright @ 2012 Jones et al.Obesity and mental stress are potent risk factors for cardiovascular disease but their relationship with each other is unclear. Resilience to stress may differ according to adiposity. Early studies that addressed this are difficult to interpret due to conflicting findings and limited methods. Recent advances in assessment of cardiovascular stress responses and of fat distribution allow accurate assessment of associations between adiposity and stress responsiveness. We measured responses to the Montreal Imaging Stress Task in healthy men (N=43) and women (N=45) with a wide range of BMIs. Heart rate (HR) and blood pressure (BP) measures were used with novel magnetic resonance measures of stroke volume (SV), cardiac output (CO), total peripheral resistance (TPR) and arterial compliance to assess cardiovascular responses. Salivary cortisol and the number and speed of answers to mathematics problems in the task were used to assess neuroendocrine and cognitive responses, respectively. Visceral and subcutaneous fat was measured using T2*-IDEAL. Greater BMI was associated with generalised blunting of cardiovascular (HR:β=−0.50 bpm.unit−1, P=0.009; SV:β=−0.33 mL.unit−1, P=0.01; CO:β=−61 mL.min−1.unit−1, P=0.002; systolic BP:β=−0.41 mmHg.unit−1, P=0.01; TPR:β=0.11 WU.unit−1, P=0.02), cognitive (correct answers: r=−0.28, P=0.01; time to answer: r=0.26, P=0.02) and endocrine responses (cortisol: r=−0.25, P=0.04) to stress. These associations were largely determined by visceral adiposity except for those related to cognitive performance, which were determined by both visceral and subcutaneous adiposity. Our findings suggest that adiposity is associated with centrally reduced stress responsiveness. Although this may mitigate some long-term health risks of stress responsiveness, reduced performance under stress may be a more immediate negative consequence.This work is funded by the UK National Institute of Health Research (NIHR), Siemens Medical Systems, British Heart Foundation (BHF), NIHR Senior Research Fellowship & The Fondation Leducq, BHF Intermediate Fellowship

Deep artifact suppression for spiral real-time phase contrast cardiac magnetic resonance imaging in congenital heart disease

PURPOSE: Real-time spiral phase contrast MR (PCMR) enables rapid free-breathing assessment of flow. Target spatial and temporal resolutions require high acceleration rates often leading to long reconstruction times. Here we propose a deep artifact suppression framework for fast and accurate flow quantification. METHODS: U-Nets were trained for deep artifact suppression using 520 breath-hold gated spiral PCMR aortic datasets collected in congenital heart disease patients. Two spiral trajectories (uniform and perturbed) and two losses (Mean Absolute Error -MAE- and average structural similarity index measurement -SSIM-) were compared in synthetic data in terms of MAE, peak SNR (PSNR) and SSIM. Perturbed spiral PCMR was prospectively acquired in 20 patients. Stroke Volume (SV), peak mean velocity and edge sharpness measurements were compared to Compressed Sensing (CS) and Cartesian reference. RESULTS: In synthetic data, perturbed spiral consistently outperformed uniform spiral for the different image metrics. U-Net MAE showed better MAE and PSNR while U-Net SSIM showed higher SSIM based metrics. In-vivo, there were no significant differences in SV between any of the real-time reconstructions and the reference standard Cartesian data. However, U-Net SSIM had better image sharpness and lower biases for peak velocity when compared to U-Net MAE. Reconstruction of 96 frames took ~59 s for CS and 3.9 s for U-Nets. CONCLUSION: Deep artifact suppression of complex valued images using an SSIM based loss was successfully demonstrated in a cohort of congenital heart disease patients for fast and accurate flow quantification

Real time flow with fast GPU reconstruction for continuous assessment of cardiac output

A novel approach for continuous cardiac output quantification during an exercise was developed and implemented on a heterogeneous image reconstruction system. Combination of spiral real-time PCMR sequence with parallel imaging allowed on high-temporal acquisition. Application of a GPU for image processing resulted in almost instantaneous reconstruction. An external computer equipped with the GPU was networked using CORBA technology. This let on seamless processing from a clinician point of view. The implementation was tested and validated against our multi -core CP

Magnetic Resonance-Augmented Cardiopulmonary Exercise Testing Comprehensively Assessing Exercise Intolerance in Children with Cardiovascular Disease

BACKGROUND: Conventional cardiopulmonary exercise testing can objectively measure exercise intolerance but cannot provide comprehensive evaluation of physiology. This requires additional assessment of cardiac output and arteriovenous oxygen content difference. We developed magnetic resonance (MR)–augmented cardiopulmonary exercise testing to achieve this goal and assessed children with right heart disease. METHODS AND RESULTS: Healthy controls (n=10) and children with pulmonary arterial hypertension (PAH; n=10) and repaired tetralogy of Fallot (n=10) underwent MR-augmented cardiopulmonary exercise testing. All exercises were performed on an MR-compatible ergometer, and oxygen uptake was continuously acquired using a modified metabolic cart. Simultaneous cardiac output was measured using a real-time MR flow sequence and combined with oxygen uptake to calculate arteriovenous oxygen content difference. Peak oxygen uptake was significantly lower in the PAH group (12.6±1.31 mL/kg per minute; P=0.01) and trended toward lower in the tetralogy of Fallot group (13.5±1.29 mL/kg per minute; P=0.06) compared with controls (16.7±1.37 mL/kg per minute). Although tetralogy of Fallot patients had the largest increase in cardiac output, they had lower resting (3±1.2 L/min per m2) and peak (5.3±1.2 L/min per m2) values compared with controls (resting 4.3±1.2 L/min per m2 and peak 6.6±1.2 L/min per m2) and PAH patients (resting 4.5±1.1 L/min per m2 and peak 5.9±1.1 L/min per m2). Both the PAH and tetralogy of Fallot patients had blunted exercise–induced increases in arteriovenous oxygen content difference. However, only the PAH patients had significantly reduced peak values (6.9±1.3 mlO2/100 mL) compared with controls (8.4±1.4 mlO2/100 mL; P=0.005). CONCLUSIONS: MR-augmented cardiopulmonary exercise testing is feasible in both healthy children and children with cardiac disease. Using this novel technique, we have demonstrated abnormal exercise patterns in oxygen uptake, cardiac output, and arteriovenous oxygen content difference

Real-time assessment of right and left ventricular volumes and function in children using high spatiotemporal resolution spiral bSSFP with compressed sensing

Background: Real-time cardiovascular magnetic resonance (CMR) assessment of ventricular volumes and function enables data acquisition during free-breathing. The requirement for high spatiotemporal resolution in children necessitates the use of highly accelerated imaging techniques. Methods: A novel real-time balanced steady state free precession (bSSFP) spiral sequence reconstructed using Compressed Sensing (CS) was prospectively validated against the breath-hold clinical standard for assessment of ventricular volumes in 60 children with congenital heart disease. Qualitative image scoring, quantitative image quality, as well as evaluation of biventricular volumes was performed. Standard BH and real-time measures were compared using the paired t-test and agreement for volumetric measures were evaluated using Bland Altman analysis. Results: Acquisition time for the entire short axis stack (~ 13 slices) using the spiral real-time technique was ~ 20 s, compared to ~ 348 s for the standard breath hold technique. Qualitative scores reflected more residual aliasing artefact (p < 0.001) and lower edge definition (p < 0.001) in spiral real-time images than standard breath hold images, with lower quantitative edge sharpness and estimates of image contrast (p < 0.001). There was a small but statistically significant (p < 0.05) overestimation of left ventricular (LV) end-systolic volume (1.0 ± 3.5 mL), and underestimation of LV end-diastolic volume (− 1.7 ± 4.6 mL), LV stroke volume (− 2.6 ± 4.8 mL) and LV ejection fraction (− 1.5 ± 3.0%) using the real-time technique. We also observed a small underestimation of right ventricular stroke volume (− 1.8 ± 4.9 mL) and ejection fraction (− 1.4 ± 3.7%) using the real-time imaging technique. No difference in inter-observer or intra-observer variability were observed between the BH and real-time sequences. Conclusions: Real-time bSSFP imaging using spiral trajectories combined with a compressed sensing reconstruction showed good agreement for quantification of biventricular metrics in children with heart disease, despite slightly lower image quality. This technique holds the potential for free breathing data acquisition, with significantly shorter scan times in children