Real-time Phase Contrast Research Articles

4D real-time phase-contrast flow MRI with sparse sampling.

Conventional phase-contrast (PC) MRI relies on electrocardiogram (ECG)-synchronized cine acquisition and respiration control. It often results in relatively low data acquisition efficiency, and is unable to assess blood flow variabilities. Real-time imaging is a promising technique to overcome these limitations; however, it results in a challenging image reconstruction problem with highly-undersampled (k; t)-space data. This paper presents a novel model-based imaging method, which integrates low-rank modeling with parallel imaging, to enable 4D real-time PC MRI without ECG gating and respiration control. The proposed method achieves an isotropic spatial resolution of 2.4 mm and temporal resolution of 35.2 ms, with three directional flow encodings. Moreover, it is able to resolve beat-by-beat flow variations, which cannot be achieved by the conventional cine-based approach. The proposed method was evaluated with in vivo experiments with one healthy subject and one arrhythmic patient. For the first time, we demonstrate the feasibility of 4D real-time PC MRI.

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.

Real-time magnetic resonance imaging of deep venous flow during muscular exercise-preliminary experience.

The accurate assessment of peripheral venous flow is important for the early diagnosis and treatment of disorders such as deep-vein thrombosis (DVT) which is a major cause of post-thrombotic syndrome or even death due to pulmonary embolism. The aim of this work is to quantitatively determine blood flow in deep veins during rest and muscular exercise using a novel real-time magnetic resonance imaging (MRI) method for velocity-encoded phase-contrast (PC) MRI at high spatiotemporal resolution. Real-time PC MRI of eight healthy volunteers and one patient was performed at 3 Tesla (Prisma fit, Siemens, Erlangen, Germany) using a flexible 16-channel receive coil (Variety, NORAS, Hoechberg, Germany). Acquisitions were based on a highly undersampled radial FLASH sequence with image reconstruction by regularized nonlinear inversion at 0.5×0.5×6 mm3 spatial resolution and 100 ms temporal resolution. Flow was assessed in two cross-sections of the lower leg at the level of the calf muscle and knee using a protocol of 10 s rest, 20 s flexion and extension of the foot, and 10 s rest. Quantitative analyses included through-plane flow in the right posterior tibial, right peroneal and popliteal vein (PC maps) as well as signal intensity changes due to flow and muscle movements (corresponding magnitude images). Real-time PC MRI successfully monitored the dynamics of venous flow at high spatiotemporal resolution and clearly demonstrated increased flow in deep veins in response to flexion and extension of the foot. In normal subjects, the maximum velocity (averaged across vessel lumen) during exercise was 9.4±5.7 cm·s-1 for the right peroneal vein, 8.5±4.6 cm·s-1 for the right posterior tibial vein and 17.8±5.8 cm·s-1 for the popliteal vein. The integrated flow volume per exercise (20 s) was 1.9, 1.6 and 50 mL (mean across subjects) for right peroneal, right posterior tibial and popliteal vein, respectively. A patient with DVT presented with peak flow velocities of only about 2 cm·s-1 during exercise and less than 1 cm·s-1 during rest. Real-time PC MRI emerges as a new tool for quantifying the dynamics of muscle-induced flow in deep veins. The method provides both signal intensity changes and velocity information for the assessment of blood flow and muscle movements. It now warrants extended clinical trials to patients with suspected thrombosis.

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.

A semi-automatic method for quantification of respiratory variation in early mitral inflow velocity using real time phase contrast cardiac magnetic resonance - normal values and clinical feasibility

Background Echocardiography has shown that patients with constrictive pericarditis or hemodynamically significant pericardial effusion show an increased respiratory variation in early transmitral inflow velocity. The methodology for quantification of respiratory variation in early transmitral inflow velocity using real time phase contrast (RT-PC) cardiovascular magnetic resonance (CMR) images is currently cumbersome and manual. The aim of this study was to develop a method for quantifying the respiratory variation in early transmitral inflow velocity using semiautomatic analysis of RT-PC CMR images.

Thermally driven smoothening of molecular thin films: Structural transitions in n-alkane layers studied in real-time.

We use thermal annealing to improve smoothness and to increase the lateral size of crystalline islands of n-tetratetracontane (TTC, C44H90) films. With in situ x-ray diffraction, we find an optimum temperature range leading to improved texture and crystallinity while avoiding an irreversible phase transition that reduces crystallinity again. We employ real-time optical phase contrast microscopy with sub-nm height resolution to track the diffusion of TTC across monomolecular step edges which causes the unusual smoothing of a molecular thin film during annealing. We show that the lateral island sizes increase by more than one order of magnitude from 0.5 μm to 10 μm. This desirable behavior of 2d-Ostwald ripening and smoothing is in contrast to many other organic molecular films where annealing leads to dewetting, roughening, and a pronounced 3d morphology. We rationalize the smoothing behavior with the highly anisotropic attachment energies and low surface energies for TTC. The results are technically relevant for the use of TTC as passivation layer and as gate dielectric in organic field effect transistors.

Aortic Pulse Wave Velocities Using Real-Time Phase-Contrast MRI

Purpose: To evaluate aortic pulse wave velocities obtained by real-time phase-contrast (PC) MRI in comparison to cine PC MRI. Methods: Real-time PC MRI of eight healthy volunteers employed highly undersampled radial FLASH sequences and phase-sensitive image reconstructions by regularized nonlinear inversion (NLINV) at 40 ms temporal resolution and 1.3 mm in-plane resolution. Pulse wave velocities were analyzed for combinations of 2, 3 and 4 locations of aortic flow using time-to-upslope and cross-correlation methods. Results: For the time-to-upslope analysis mean pulse wave velocities ranged from 3.5 to 3.9 m s-1 for real-time PC MRI and from 3.5 to 3.8 m s-1 for cine PC MRI. A cross-correlation analysis of the same data resulted in 2.9 to 3.3 m s-1 and 3.3 to 3.7 m s-1, respectively. Conclusion: Real-time PC MRI determined aortic pulse wave velocities from single cardiac cycles in close correspondence to values obtained by cine PC MRI.

Validation of segmented and real-time EPI phase contrast flow quantification against segmented gradient echo sequences

Background MR based Flow Quantification is an established method for measuring cardio-pulmonary haemodynamics, but is challenging to perform in situations where it may be advantageous to asses real-world physiological behavior, for example in exercise. The availability of a new real time flow quantification sequence presents the opportunity to measure velocity and flow without these challenges. This study is a preliminary validation of the method as a precursor to use of the sequence in subjects undergoing assessment of induced haemodynamic change by exercise and preload alteration. Methods

Real-time brightfield, darkfield, and phase contrast imaging in a light-emitting diode array microscope.

We demonstrate a single-camera imaging system that can simultaneously acquire brightfield, darkfield, and phase contrast images in real time. Our method uses computational illumination via a programmable light-emitting diode (LED) array at the source plane, providing flexible patterning of illumination angles. Brightfield, darkfield, and differential phase contrast images are obtained by changing the LED patterns, without any moving parts. Previous work with LED array illumination was only valid for static samples because the hardware speed was not fast enough to meet real-time acquisition and processing requirements. Here, we time multiplex patterns for each of the three contrast modes in order to image dynamic biological processes in all three contrast modes simultaneously. We demonstrate multicontrast operation at the maximum frame rate of our camera (50 Hz with 2160 × 2560 pixels).

Assessment of cardiac time intervals using high temporal resolution real‐time spiral phase contrast with UNFOLDed‐SENSE

To develop a real-time phase contrast MR sequence with high enough temporal resolution to assess cardiac time intervals. The sequence utilized spiral trajectories with an acquisition strategy that allowed a combination of temporal encoding (Unaliasing by fourier-encoding the overlaps using the temporal dimension; UNFOLD) and parallel imaging (Sensitivity encoding; SENSE) to be used (UNFOLDed-SENSE). An in silico experiment was performed to determine the optimum UNFOLD filter. In vitro experiments were carried out to validate the accuracy of time intervals calculation and peak mean velocity quantification. In addition, 15 healthy volunteers were imaged with the new sequence, and cardiac time intervals were compared to reference standard Doppler echocardiography measures. For comparison, in silico, in vitro, and in vivo experiments were also carried out using sliding window reconstructions. The in vitro experiments demonstrated good agreement between real-time spiral UNFOLDed-SENSE phase contrast MR and the reference standard measurements of velocity and time intervals. The protocol was successfully performed in all volunteers. Subsequent measurement of time intervals produced values in keeping with literature values and good agreement with the gold standard echocardiography. Importantly, the proposed UNFOLDed-SENSE sequence outperformed the sliding window reconstructions. Cardiac time intervals can be successfully assessed with UNFOLDed-SENSE real-time spiral phase contrast. Real-time MR assessment of cardiac time intervals may be beneficial in assessment of patients with cardiac conditions such as diastolic dysfunction.

Assessment of cardiac time intervals using high temporal resolution real-time spiral phase contrast with UNFOLD-SENSE

Assessment of cardiac time intervals using high temporal resolution real-time spiral phase contrast with UNFOLD-SENSE

Observation of the dynamic germination of single bacterial spores using rapid Raman imaging

The dynamics of bacterial spore germination were successfully observed using a fast Raman imaging system, in combination with real-time phase contrast microscopy. By using a multifocus scan scheme, the spontaneous Raman-scattering imaging acquisition speed was increased to ~30 s per frame while maintaining diffraction-limited resolution, which allowed monitoring of the dynamics of spore germination on a time scale of tens of seconds to a few minutes. This allowed simultaneous gathering of rich spatial distribution information on different cellular components including time-lapse molecular images of Ca-dipicolinic acid, protein, and nucleic acid during germination of single bacterial spores for the periods of 30 to 60 min.

Real-time cardiac phase contrast MRI blood flow including Valsalva and Mueller maneuver. Initial experiences

Background A high resolution real-time phase-contrast MRI flow technique was used to measure flow dynamics in the ascending aorta as well as superior vena cava. MRI technique based on undersampled radial fast low-angle shot acquisitions with phase-sensitive image reconstructions by regularized nonlinear inversion. Normal flow values as well as flow measurements during physiologic stress tests like Valsalva (increased intrathoracic pressure) and Mueller (decreased intrathoracic pressure, reverse of Valsalva maneuver) were obtained in healthy volunteers.

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.

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.

Simultaneous Right and Left Heart Real-Time, Free-Breathing CMR Flow Quantification Identifies Constrictive Physiology

The purpose of this study was to evaluate the ability of a novel cardiac magnetic resonance (CMR) real-time phase contrast (RT-PC) flow measurement technique to reveal the discordant respirophasic changes in mitral and tricuspid valve in flow indicative of the abnormal hemodynamics seen in constrictive pericarditis (CP). Definitive diagnosis of CP requires identification of constrictive hemodynamics with or without pericardial thickening. CMR to date has primarily provided morphological assessment of the pericardium. Sixteen patients (age 57 ± 13 years) undergoing CMR to assess known or suspected CP and 10 controls underwent RT-PC that acquired simultaneous mitral valve and tricuspid valve inflow velocities over 10 s of unrestricted breathing. The diagnosis of CP was confirmed via clinical history, diagnostic imaging, cardiac catheterization, intraoperative findings, and histopathology. Ten patients had CP, all with increased pericardial thickness (6.2 ± 1.0 mm). RT-PC imaging demonstrated discordant respirophasic changes in atrioventricular valve inflow velocities in all CP patients, with mean ± SD mitral valve and tricuspid valve inflow velocity variation of 46 ± 20% and 60 ± 15%, respectively, compared with 16 ± 8% and 24 ± 11% in patients without CP (p < 0.004 vs. patients with CP for both) and 17 ± 5% and 31 ± 13% in controls (p < 0.001 vs. patients with CP for both). There was no difference in atrioventricular valve inflow velocity variation between patients without CP compared with controls (p > 0.3 for both). Respiratory variation exceeding 25% across the mitral valve yielded a sensitivity of 100%, a specificity of 100%, and an area under the receiver-operating characteristic curve of 1.0 to detect CP physiology. Using a cutoff of 45%, variation of transtricuspid valve velocity had a sensitivity of 90%, a specificity of 88%, and an area under the receiver-operating characteristic curve of 0.98. Accentuated and discordant respirophasic changes in mitral valve and tricuspid valve inflow velocities characteristic of CP can be identified noninvasively with RT-PC CMR. When incorporated into existing CMR protocols for imaging pericardial morphology, RT-PC CMR provides important hemodynamic evidence with which to make a definite diagnosis of CP.

CMR real-time, free-breathing, phase contrast flow quantification: a novel approach to assess ventricular coupling in constrictive pericarditis

Background/objective Constrictive pericarditis (CP) is an important cause of heart failure; however, with accurate diagnosis and directed treatment it is potentially curable. Cardiac magnetic resonance imaging (CMR) has played a diagnostic role, primarily by allowing assessment of pericardial morphology but with limited depiction of physiological changes. We sought to examine the feasibility of a novel CMR approach that enables real-time phase contrast (RT-PC) assessment of discordant respirophasic changes in trans-mitral and tricuspid flow velocity the signature findings in CP due to enhanced ventricular interdependence. Method Patients referred to the CMR lab pre-pericardectomy or for assessment of suspected CP were included. Following routine CMR examination for CP, transmitral (MV) and tricuspid valve (TV) flow velocities were simultaneously obtained by through-plane RT-PC imaging during unrestricted respiration using a slice position to include both valves (Figure 1) with the following parameters: TR/TE=13.7ms/2.5ms, water excitation pulse with flip angle=25, 10mm slice thickness, 160x120 matrix, EPI factor=15, TSENSE rate=2, slice thickness=10mm, and VENC=150cm/s. Shared velocity encoding was used to achieve an effective temporal

Detailed assessment of the hemodynamic response to psychosocial stress using real‐time MRI

To demonstrate that combining the Montreal Imaging Stress Task (MIST) with real-time cardiac magnetic resonance imaging (MRI) allows detailed assessment of the cardiovascular mental stress response. 22 healthy volunteers (1:1 M:F, 26-64 years) underwent MRI during rest and the MIST. Real-time spiral phase contrast MR, accelerated with sensitivity encoding (SENSE) was used to assess stroke volume (SV), and radial k-t SENSE was used to assess ventricular volumes. Simultaneous heart rate (HR) and blood pressure (BP) measures allowed calculation of cardiac output (CO), systemic vascular resistance (SVR), and arterial compliance (TAC). Endocrine responses were assessed using salivary cortisol. In response to stress, BP increased due to increased CO and reduced TAC but not increased SVR, which fell. HR, not SV, determined CO increases. Greater BP responses occurred in men due to greater CO increases and relatively higher SVR. Older participants had greater BP responses due to greater falls in TAC. Greater cortisol response was correlated with greater falls in TAC but resting cortisol and TAC were not related. This new approach allows detailed, accurate assessment of stress physiology. Preliminary findings suggest stress exposes relationships, not seen at rest, of cardiovascular function with age, sex, and endocrine function.

Automatic segmentation propagation of the aorta in real‐time phase contrast MRI using nonrigid registration

To assess the use of a nonrigid registration technique for semi-automatic segmentation of the aorta from real-time velocity mapping MRI. Real-time phase contrast images were acquired to measure flow and stroke volumes in 10 subjects, during free breathing, at rest, and during exercise. A nonrigid registration algorithm was developed to propagate a manually drawn region of interest in the aorta from one frame to all other frames of the real-time sequence (148 images). Thus the technique provided a semi-automatic segmentation over the whole sequence of images. The accuracy was assessed by comparison with manual segmentations in terms of Dice overlap measures and stroke volumes (SV). Semi-automatic segmentations were comparable to manual ones (Dice score of 0.89 ± 0.04). Inter-observer reproducibility was similar for manual and semi-automatic segmentations (Dice score of 0.90 ± 0.04 in both cases, the difference was not significant). SV measurements also showed good agreement between manual and semi-automatic segmentations (correlation coefficient r > 0.94), and the differences were not statistically significant. Although real-time phase contrast images have compromised image quality, a fast and robust segmentation of the aorta was possible using the registration-based technique.