Real-time Phase Contrast Magnetic Resonance Research Articles

Immediate impact of yogic breathing on pulsatile cerebrospinal fluid dynamics

Cerebrospinal fluid (CSF), a clear fluid bathing the central nervous system (CNS), undergoes pulsatile movements. Together with interstitial fluid, CSF plays a critical role for the removal of waste products from the brain, and maintenance of the CNS health. As such, understanding the mechanisms driving CSF movement is of high scientific and clinical impact. Since pulsatile CSF dynamics is sensitive and synchronous to respiratory movements, we are interested in identifying potential integrative therapies such as yogic breathing to regulate CSF dynamics, which has not been reported before. Here, we investigated the pre-intervention baseline data from our ongoing randomized controlled trial, and examined the impact of four yogic breathing patterns: (i) slow, (ii) deep abdominal, (iii) deep diaphragmatic, and (iv) deep chest breathing with the last three together forming a yogic breathing called three-part breath. We utilized our previously established non-invasive real-time phase contrast magnetic resonance imaging approach using a 3T MRI instrument, computed and tested differences in single voxel CSF velocities (instantaneous, respiratory, cardiac 1st and 2nd harmonics) at the level of foramen magnum during spontaneous versus yogic breathing. In examinations of 18 healthy participants (eight females, ten males; mean age 34.9 ± 14 (SD) years; age range: 18–61 years), we observed immediate increase in cranially-directed velocities of instantaneous-CSF 16–28% and respiratory-CSF 60–118% during four breathing patterns compared to spontaneous breathing, with the greatest changes during deep abdominal breathing (28%, p = 0.0008, and 118%, p = 0.0001, respectively). Cardiac pulsation was the primary source of pulsatile CSF motion except during deep abdominal breathing, when there was a comparable contribution of respiratory and cardiac 1st harmonic power [0.59 ± 0.78], suggesting respiration can be the primary regulator of CSF depending on the individual differences in breathing techniques. Further work is needed to investigate the impact of sustained training yogic breathing on pulsatile CSF dynamics for CNS health.

Respiratory cerebrospinal fluid flow is driven by the thoracic and lumbar spinal pressures.

Respiration plays a key role in the circulation of cerebrospinal fluid (CSF) around the central nervous system. During inspiration increased venous return from the cranium is believed to draw CSF rostrally. However, this mechanism does not explain why CSF has also been observed to move caudally during inspiration. We show that during inspiration decreased intrathoracic pressure draws venous blood from the cranium and lumbar spine towards the thorax. We also show that the abdominal pressure was associated with rostral CSF displacement. However, a caudal shift of cervical CSF was seen with low abdominal pressure and comparably negative intrathoracic pressures. These results suggest that the effects of epidural blood flow within the spinal canal need to be considered, as well as the cranial blood volume balance, to understand respiratory-related CSF flow. These results may prove useful for the treatment of CSF obstructive pathology and understanding the behaviour of intrathecal drug injections. It is accepted that during inspiration, cerebrospinal fluid (CSF) flows rostrally to compensate for decreased cranial blood volume, caused by venous drainage due to negative intrathoracic pressure. However, this mechanism does not explain observations of caudal CSF displacement during inspiration. Determining the drivers of respiratory CSF flow is crucial for understanding the pathophysiology of CSF flow disorders. To quantify the influence of respiration on CSF flow, real-time phase-contrast magnetic resonance imaging (MRI) was used to record CSF and blood flow, while healthy subjects (5:5 M:F, 25-50 years) performed either a brief expiratory or inspiratory effort between breaths. Transverse images were taken perpendicular to the spinal canal in the middle of the C3 and L2 vertebrae. The same manoeuvres were then performed after a nasogastric pressure catheter was used to measure the intrathoracic and abdominal pressures. During expiratory-type manoeuvres that elevated abdominal and intrathoracic pressures, epidural blood flow into the spinal canal increased and CSF was displaced rostrally. With inspiratory manoeuvres, the negative intrathoracic pressure drew venous blood from C3 and L2 towards the thoracic spinal canal, and cervical CSF was displaced both rostrally and caudally, despite the increased venous drainage. Regression analysis showed that rostral displacement of CSF at both C3 (adjusted R2 =0.53; P<0.001) and L2 (adjusted R2 =0.38; P<0.001) were associated with the abdominal pressure. However, with low abdominal pressure and comparably negative intrathoracic pressure, cervical CSF flowed caudally. These findings suggest that changes in both the cranial and spinal pressures need to be considered to understand respiratory CSF flow.

Using a Novel In Vitro Fontan Model and Condition-Specific Real-Time MRI Data to Examine Hemodynamic Effects of Respiration and Exercise.

Several studies exist modeling the Fontan connection to understand its hemodynamic ties to patient outcomes (Chopski in: Experimental and Computational Assessment of Mechanical Circulatory Assistance of a Patient-Specific Fontan Vessel Configuration. Dissertation, 2013; Khiabani et al. in J Biomech 45:2376-2381, 2012; Taylor and Figueroa in Annu Rev Biomed 11:109-134, 2009; Vukicevic et al. in ASAIO J 59:253-260, 2013). The most patient-accurate of these studies include flexible, patient-specific total cavopulmonary connections. This study improves Fontan hemodynamic modeling by validating Fontan model flexibility against a patient-specific bulk compliance value, and employing real-time phase contrast magnetic resonance flow data. The improved model was employed to acquire velocity field information under breath-held, free-breathing, and exercise conditions to investigate the effect of these conditions on clinically important Fontan hemodynamic metrics including power loss and viscous dissipation rate. The velocity data, obtained by stereoscopic particle image velocimetry, was visualized for qualitative three-dimensional flow field comparisons between the conditions. Key hemodynamic metrics were calculated from the velocity data and used to quantitatively compare the flow conditions. The data shows a multi-factorial and extremely patient-specific nature to Fontan hemodynamics.

Quantifying the influence of respiration and cardiac pulsations on cerebrospinal fluid dynamics using real-time phase-contrast MRI.

To validate a real-time phase contrast magnetic resonance imaging (RT-PCMRI) sequence in a controlled phantom model, and to quantify the relative contributions of respiration and cardiac pulsations on cerebrospinal fluid (CSF) velocity at the level of the foramen magnum (FM). To validate the 3T MRI techniques, in vitro studies used a realistic model of the spinal subarachnoid space driven by pulsatile flow waveforms mimicking the respiratory and cardiac components of CSF flow. Subsequently, CSF flow was measured continuously during 1-minute RT-PCMRI acquisitions at the FM while healthy subjects (N = 20) performed natural breathing, deep breathing, breath-holding, and coughing. Conventional cardiac-gated PCMRI was obtained for comparison. A frequency domain power ratio analysis determined the relative contribution of respiration versus cardiac ([r/c]) components of CSF velocity. In vitro studies demonstrating the accuracy of RT-PCMRI within 5% of input values showed that conventional PCMRI measures only the cardiac component of CSF velocity (0.42 ± 0.02 cm/s), averages out respiratory effects, and underestimates the magnitude of CSF velocity (0.96 ± 0.07 cm/s). In vivo RT-PCMRI measurements indicated the ratio of respiratory to cardiac velocity pulsations averaged over all subjects as [r/c = 0.14 ± 0.27] and [r/c = 0.40 ± 0.47] for natural and deep breathing, respectively. During coughing, the peak CSF velocity increased by a factor of 2.27 ± 1.40. RT-PCMRI can noninvasively measure instantaneous CSF velocity driven by cardiac pulsations, respiration, and coughing in real time. A comparable contribution of respiration and cardiac pulsations on CSF velocity was found during deep breathing but not during natural breathing. 1 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:431-439.

Classification of Fontan Hemodynamics by Respiration Using Real-Time Phase-Contrast Magnetic Resonance

Objective: Outcome of Fontan patients is often complicated by impaired hemodynamics caused by increased pulmonary vascular resistance and/or ventricular stiffness. The influence of respiration and vasodilative agents on vessel flow might be of value to investigate their quality of hemodynamics. Cardiac magnetic resonance (CMR) is highly accurate and can measure flow in realtime by generation of respiratory-dependent stroke volumes.

Respiratory Effects on Fontan Circulation During Rest and Exercise Using Real-Time Cardiac Magnetic Resonance Imaging

It is known that respiration modulates cavopulmonary flows, but little data compare mean flows under breath-holding and free-breathing conditions to isolate the respiratory effects and effects of exercise on the respiratory modulation. Real-time phase-contrast magnetic resonance combined with a novel method to track respiration on the same image acquisition was used to investigate respiratory effects on Fontan caval and aortic flows under breath-holding, free-breathing, and exercise conditions. Respiratory phasicity indices that were based on beat-averaged flow were used to quantify the respiratory effect. Flow during inspiration was substantially higher than expiration under the free-breathing and exercise conditions for both inferior vena cava (inspiration/expiration: 1.6 ± 0.5 and 1.8 ± 0.5, respectively) and superior vena cava (inspiration/expiration: 1.9 ± 0.6 and 2.6 ± 2.0, respectively). Changes from rest to exercise in the respiratory phasicity index for these vessels further showed the impact of respiration. Total systemic venous flow showed no significant statistical difference between the breath-holding and free-breathing conditions. In addition, no substantial difference was found between the descending aorta and inferior vena cava mean flows under either resting or exercise conditions. This study demonstrated that inferior vena cava and superior vena cava flow time variance is dominated by respiratory effects, which can be detected by the respiratory phasicity index. However, the minimal respiration influence on net flow validates the routine use of breath-holding techniques to measure mean flows in Fontan patients. Moreover, the mean flows in the inferior vena cava and descending aorta are interchangeable.

Peak flow velocities in the ascending aorta-real-time phase-contrast magnetic resonance imaging vs. cine magnetic resonance imaging and echocardiography.

This prospective study of eight healthy volunteers evaluates peak flow velocities (PFV) in the ascending aorta using real-time phase-contrast magnetic resonance imaging (MRI) in comparison to cine phase-contrast MRI and echocardiography. Flow measurements by echocardiography and cine phase-contrast MRI with breath-holding were performed according to clinical standards. Real-time phase-contrast MRI at 40 ms temporal resolution and 1.3 mm in-plane resolution was based on highly undersampled radial fast low-angle shot (FLASH) sequences with image reconstruction by regularized nonlinear inversion (NLINV). Evaluations focused on the determination of PFV. Linear regressions and Bland-Altman plots were used for comparisons of methods. When averaged across subjects, real-time phase-contrast MRI resulted in PFV of 120±20 cm s(-1) (mean ± SD) in comparison to 122±16 cm s(-1) for cine MRI and 124±20 cm s(-1) for echocardiography. The maximum deviations between real-time phase-contrast MRI and echocardiography ranged from -20 to +14 cm s(-1) (cine MRI: -10 to +12 cm s(-1)). Thus, in general, real-time phase-contrast MRI of cardiac outflow revealed quantitative agreement with cine MRI and echocardiography. The advantages of real-time MRI are measurements during free breathing and access to individual cardiac cycles.

Real‐time flow MRI of the aorta at a resolution of 40 msec

To evaluate a novel real-time phase-contrast magnetic resonance imaging (MRI) technique for the assessment of through-plane flow in the ascending aorta. Real-time MRI was based on a radial fast low-angle shot (FLASH) sequence with about 30-fold undersampling and image reconstruction by regularized nonlinear inversion. Phase-contrast maps were obtained from two (interleaved or sequential) acquisitions with and without a bipolar velocity-encoding gradient. Blood flow in the ascending aorta was studied in 10 healthy volunteers at 3 T by both real-time MRI (15 sec during free breathing) and electrocardiogram (ECG)-synchronized cine MRI (with and without breath holding). Flow velocities and stroke volumes were evaluated using standard postprocessing software. The total acquisition time for a pair of phase-contrast images was 40.0 msec (TR/TE = 2.86/1.93 msec, 10° flip angle, 7 spokes per image) for a nominal in-plane resolution of 1.3 mm and a section thickness of 6 mm. Quantitative evaluations of spatially averaged flow velocities and stroke volumes were comparable for real-time and cine methods when real-time MRI data were averaged across heartbeats. For individual heartbeats real-time phase-contrast MRI resulted in higher peak velocities for values above 120 cm s(-1). Real-time phase-contrast MRI of blood flow in the human aorta yields functional parameters for individual heartbeats. When averaged across heartbeats real-time flow velocities and stroke volumes are comparable to values obtained by conventional cine MRI.

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

To demonstrate the feasibility of real-time phase contrast magnetic resonance (PCMR) assessment of continuous cardiac output with a heterogeneous (CPU/GPU) system for online image reconstruction. Twenty healthy volunteers underwent aortic flow examination during exercise using a real-time spiral PCMR sequence. Acquired data were reconstructed in online fashion using an iterative sensitivity encoding (SENSE) algorithm implemented on an external computer equipped with a GPU card. Importantly, data were sent back to the scanner console for viewing. A multithreaded CPU implementation of the real-time PCMR reconstruction was used as a reference point for the online GPU reconstruction assessment and validation. A semiautomated segmentation and registration algorithm was applied for flow data analysis. There was good agreement between the GPU and CPU reconstruction (-0.4 ± 0.8 mL). There was a significant speed-up compared to the CPU reconstruction (15×). This translated into the flow data being available on the scanner console ≈9 seconds after acquisition finished. This compares to an estimated time using the CPU implementation of 83 minutes. Our heterogeneous image reconstruction system provides a base for translation of complex MRI algorithms into clinical workflow. We demonstrated its feasibility using real-time PCMR assessment of continuous cardiac output as an example.

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

To demonstrate the feasibility of real-time phase contrast magnetic resonance (PCMR) assessment of continuous cardiac output with a heterogeneous (CPU/GPU) system for online image reconstruction.Twenty healthy volunteers underwent aortic flow examination during exercise using a real-time spiral PCMR sequence. Acquired data were reconstructed in online fashion using an iterative sensitivity encoding (SENSE) algorithm implemented on an external computer equipped with a GPU card. Importantly, data were sent back to the scanner console for viewing. A multithreaded CPU implementation of the real-time PCMR reconstruction was used as a reference point for the online GPU reconstruction assessment and validation. A semiautomated segmentation and registration algorithm was applied for flow data analysis.There was good agreement between the GPU and CPU reconstruction (-0.4 ± 0.8 mL). There was a significant speed-up compared to the CPU reconstruction (15×). This translated into the flow data being available on the scanner console ≈9 seconds after acquisition finished. This compares to an estimated time using the CPU implementation of 83 minutes.Our heterogeneous image reconstruction system provides a base for translation of complex MRI algorithms into clinical workflow. We demonstrated its feasibility using real-time PCMR assessment of continuous cardiac output as an example.

Diastolic function imaging: a comparison of real-time phase contrast magnetic resonance (CMR) imaging with segmented phase contrast CMR and Doppler echocardiography

Background CMR measurement of mitral inflow velocities for the assessment of diastolic function is often infeasible in patients with dyspnea - patients who may benefit the most - due to their inability to breath-hold. Although real-time phase contrast (RT-PC) imaging may overcome this limitation, it has not been systematically evaluated. The objective of this study was to assess the accuracy of RT-PC for the measurement of mitral inflow velocities against segmented PC CMR and Doppler echocardiography. Methods 37 healthy volunteers (aged 28 ± 10 years, 20 males) had echo and CMR studies within a week. Early (E) and late (A) mitral inflow velocities were measured by echo, segmented, and RT-PC CMR (Figure). The E and A velocities were obtained by averaging data from 2 heart beats by RT-PC and 3 heart beats by echo. RT-PC parameters were: TR/TE = 14.0ms/2.3ms, water excitation flip angle=25○ ,10mm slice, 90 x128 matrix, EPI factor=15, TSENSE rate=3, and VENC=150cm/s. Shared velocity encoding was used to achieve an effective temporal resolution of 28ms, but true temporal resolution was 56ms. Retro-gated segmented PC acquisition parameters: TR/TE = 4.5/1.9ms, 10mm slice, 100 x 192 matrix, TSENSE rate=3, VENC=150cm/s, true temporal resolution 36ms. E and A velocities, and E/A ratios between RT-PC and segmented PC CMR or Doppler echocardiography were compared using paired t-tests. Agreement between the techniques was assessed using concordance correlation coefficients and Bland-Altman analysis.

Shared velocity encoding: A method to improve the temporal resolution of phase‐contrast velocity measurements

Phase-contrast magnetic resonance imaging (PC-MRI) is used routinely to measure fluid and tissue velocity with a variety of clinical applications. Phase-contrast magnetic resonance imaging methods require acquisition of additional data to enable phase difference reconstruction, making real-time imaging problematic. Shared Velocity Encoding (SVE), a method devised to improve the effective temporal resolution of phase-contrast magnetic resonance imaging, was implemented in a real-time pulse sequence with segmented echo planar readout. The effect of SVE on peak velocity measurement was investigated in computer simulation, and peak velocities and total flow were measured in a flow phantom and in volunteers and compared with a conventional ECG-triggered, segmented k-space phase-contrast sequence as a reference standard. Computer simulation showed a 36% reduction in peak velocity error from 8.8 to 5.6% with SVE. A similar reduction of 40% in peak velocity error was shown in a pulsatile flow phantom. In the phantom and volunteers, volume flow did not differ significantly when measured with or without SVE. Peak velocity measurements made in the volunteers using SVE showed a higher concordance correlation (0.96) with the reference standard than non-SVE (0.87). The improvement in effective temporal resolution with SVE reconstruction has a positive impact on the precision and accuracy of real-time phase-contrast magnetic resonance imaging peak velocity measurements.

Split‐acquisition real‐time CINE phase‐contrast MR flow measurements

The temporal and spatial resolution of real-time phase-contrast magnetic resonance (PCMR) is restricted by the need to acquire two interleaved phase images. In this article, we propose a split-acquisition real-time CINE PCMR technique, where the acquisition of flow-encoded and flow-compensated data is divided into separate blocks. By comparing magnitude images, automatic matching of data in cardio-respiratory space allows subtraction of background phase offsets. Thus, the data is acquired in real-time but with phase correction originating from a different heart beat. This effectively doubles the frame rate, allowing either higher temporal or spatial resolution. Two split-acquisition sequences were tested: one with high-temporal resolution and one with high-spatial resolution. Both sequences showed excellent agreement in stroke volumes in 20 adults when validated against cardiac-gated PCMR and interleaved real-time PCMR (cardiac gated: 95.2 ± 20.0 mL, interleaved real-time: 96.2 ± 20.7 mL, high-temporal resolution: 95.6 ± 20.1 mL, high-spatial resolution: 95.5 ± 20.4 mL). In six children, the high-spatial resolution sequence provided more accurate flow measurements than interleaved real-time PCMR, when compared with cardiac-gated PCMR (cardiac gated: 20.6 ± 7.6 mL, interleaved real-time: 24.3 ± 9.2 mL, high-spatial resolution: 20.8 ± 7.8 mL), due to the increased spatial resolution. The matching technique is shown to be accurate (truth: 94.6 ± 21.8, split-acquisition: 95.0 ± 21.9 mL) and quantitative image quality (signal-to-noise ratio, velocity-to-noise ratio and edge sharpness) is acceptable.