Flow MRI Research Articles

Impact of beta-blocker therapy on thoracic aorta 3D wall shear stress in patients with bicuspid aortic valve

Background Beta-blockers are the recommended medical treatment for slowing ascending aorta (AAo) dilation in patients with bicuspid aortic valve (BAV). Wall shear stress (WSS) has been shown to promote endothelial cell dysfunction [1] and AAo WSS may play a role in aortic aneurysm growth. Time-resolved 3D phase contrast (4D flow) MRI allows for the quantification of 3D WSS regionally in the thoracic aorta. The aim of this study was to assess changes in thoracic aorta WSS associated with b-blocker therapy in BAV patients.

K‐t GRAPPA accelerated four‐dimensional flow MRI in the aorta: Effect on scan time, image quality, and quantification of flow and wall shear stress

The purpose of this study was to evaluate the utility of k-t parallel imaging for accelerating aortic four-dimensional (4D)-flow MRI. The aim was to systematically investigate the impact of different acceleration factors and number of coil elements on acquisition time, image quality and quantification of hemodynamic parameters. k-t accelerated 4D-flow MRI (spatial/temporal resolution = 2.1 × 2.5 × 2.5 mm/40.0 ms) was acquired in 10 healthy volunteers with acceleration factors R = 3, 5, and 8 using 12- and 32-channel receiver coils. Results were compared with conventional parallel imaging (GRAPPA [generalized autocalibrating partial parallel acquisition], R = 2). Data analysis included radiological grading of three-dimensional blood flow visualization quality as well as quantification of blood flow, velocities and wall shear stress (WSS). k-t GRAPPA significantly reduced scan time by 28%, 54%, and 68%, for R = 3, 5, and 8, respectively, while maintaining image quality as demonstrated by overall similar image quality grading. Significant differences in peak WSS (diff12ch = -5.9%, diff32ch = 18.5%) and mean WSS (diff32ch = 13.9%) were found at the descending aorta for both receiver coils for R = 5 (PWSS < 0.04). Peak velocity differed for R=8 at the aortic root (-7.4%) and descending aorta (-12%) with PpeakVelo < 0.03. k-t GRAPPA acceleration with a 12- or 32-channel receiver coil and an acceleration of 3 or 5 can compete with a standard GRAPPA R = 2 acceleration.

A sensitivity analysis on 3D velocity reconstruction from multiple registered echo Doppler views

We present a new method for reconstructing a 3D+t velocity field from multiple 3D+t colour Doppler images. Our technique reconstructs 3D velocity vectors from registered multiple standard 3D colour Doppler views, each of which contains a 1D projection of the blood velocity. Reconstruction is based on a scalable patch-wise Least Mean Squares approach, and a continuous velocity field is achieved by using a B-spline grid. We carry out a sensitivity analysis of clinically relevant parameters which affect the accuracy of the reconstruction, including the impact of noise, view angles and registration errors, using simulated data. A realistic simulation framework is achieved by a novel noise model to represent variations in colour Doppler images based on multiscale additive Gaussian noise. Simulations show that, if the Target Registration Error <2.5mm, view angles are >20° and the standard deviation of noise in the input data is <10 cm/s, the reconstructed velocity field presents visually plausible flow patterns and mean error in flow rate is approximately 10% compared to 2D+t Flow MRI. These results are verified by reconstructing 3D velocity on three healthy volunteers. The technique is applied to reconstruct 3D flow on three paediatric patients showing promising results for clinical application.

Anatomical, blood flow, oxygenation and functional MRI of the retina

Anatomical, blood flow, oxygenation and functional MRI of the retina

Recent advances in Flow MRI

The past five years have seen exciting new developments in Flow MRI. Two-dimensional images are now routinely acquired in 100–200ms and, in some cases, acquisition times of 5–10ms are possible. This has been achieved not only by advances in the implementation of existing pulse sequences but also in data acquisition strategies, such as Compressed Sensing and Bayesian approaches, and technical advices in parallel imaging and signal enhancement methods. In particular, the short imaging timescales that are now achieved offer significant opportunities in the study of transient flow phenomena.

A comparative study of MRI and ultrasonic cardiography in morphology and blood flow of cavopulmonary anastomosis for patients with bidirectional Glenn shunt

Objective To investigate the difference of morphology and blood flow of cavopulmonary anastomosis by MRI and that by ultrasonic cardiography (UCG) in patients with bidirectional Glenn shunt (BGS).Methods Phase-contrast MRI (PC-MRI) and contrast enhanced MRI (CE-MRI) were performed for superior vena cava ( SVC ) and inferior vena cava (IVC) in 22 patients with BGS on 3.0 T MR scanner.PC-MRI was used for measuring blood flow and CE-MRI for illustrating morphology.The width,peak flow velocity and gradient pressure of cavopulmonary anastomosis were calculated by using Report Card software.The similar data of UCG was collected.The parameters by MRI and that by UCG were compared statistically by t test and Pearson correlation.Results Based on the MRI data,the blood flow of SVC [ ( 1.002 ±0.208) L/min ] was significantly lower than that of IVC [ ( 1.794 ± 0.392 ) L/min ] ( t =- 15.148,P ＜0.01 ),while the regurgitation fraction of SVC [ (26.54 ± 12.82)％ ] was significantly higher than that ofIVC [ ( 17.44 ± 10.17)％ ] (t =11.060,P ＜0.01 ).The morphology displayed with MRI angiography couldnot be detected with UCG.The width of cavopulmonary anastomosis measured by MRI [ (12.46 ±3.43 ) mm ] was significantly higher than that of UCG[ ( 11.04 ± 2.63 ) mm] ( t =4.048,P ＜ 0.01 ),while the peak flow velocity of cavopulmonary anastomosis measured by MRI [ (47.77 ± 10.44) cm/s] was significantly lower than that of UCG [ (52.19 + 9.63) cm/s] ( t =- 2.237,P ＜ 0.05 ).No significant difference was found in gradient pressure of cavopulmonary anastomosis between the values by MRI [(0.95+0.42) mm Hg(1 mm Hg =0.133 kPa)] and that by UCG [(1.12+0.38)mm Hg] (t=2.010,P ＞ 0.05).The width,peak flow velocity and gradient pressure of cavopulmonary anastomosis by MRI were closely correlated with those by UCG (r =0.858,0.489,0.427,all P＜ 0.05 ).Conclusions A good correlation is found in the width,peak flow velocity,and gradient pressure of the cavopulmonary anastomosis obtained by 3.0 T MRI and UCG.MRI is more useful tool to display the width and abnormal morphology of cavopulmonary anastomosis than UCG. Key words: Magnetic resonance imaging; Heart defects, congenital; Comparative study

Comparison of vascularity measures from PET and MRI for improved breast tumor characterization and therapeutic response evaluation

589 Background: Dynamic contrast-enhanced MRI (DCE-MRI) and positron emission tomography (PET) are two imaging modalities previously shown to be predictive of response to neoadjuvant therapy in breast cancer. As the two techniques are functionally very different, the extent of their agreement or discordance for tumor characterization is not well understood. The purpose of this study was therefore to investigate the association between quantitative DCE-MRI enhancement kinetics and 15O-water PET/18F-FDG PET blood flow and metabolic measures of breast tumors. Methods: We retrospectively evaluated exams for 20 patients with locally advanced breast cancer (LABC) who underwent both 15O-water/18F-FDG PET and DCE-MRI. PET measures included 15O-water blood flow, 18F-FDG transport rate constant from blood to tissue (K1), and FDG metabolic rate. DCE-MRI measures included functional tumor volume, initial peak enhancement (PE), and a delayed signal enhancement ratio (SER) assessing washout. Pearson’s correlations and multivariate stepwise regression analyses were performed; p&lt;0.05 was considered significant. Results: PET 15O-water blood flow correlated significantly with DCE-MRI peak SER (r=0.8) and tumor volume (r=0.7). PET K1 measures correlated significantly with DCE-MRI peak PE (r=0.7), peak SER (r=0.6) and tumor volume (r=0.6). By multivariate analysis, SER was the strongest independent MRI correlate of PET blood flow, and PE the strongest MRI correlate for K1. No significant correlations were observed between 18F-FDG PET metabolic rate and the DCE-MRI kinetic parameters. Also, while blood flow and K1 were strongly correlated (r=0.9, p=0.0001), blood flow and metabolic rate were not (r=0.4, p=0.12). Conclusions: The associations between PET and MRI measures may lead to a better understanding of angiogenesis, vascular permeability, and glucose transport in LABC. A lack of correlation of metabolic rate with blood flow and DCE MRI kinetics suggests that 18F-FDG PET provides complimentary metabolic information independent of vascular factors. The combination of MRI and PET may be helpful in understanding tumor pharmacodynamics in response to novel therapy. No significant financial relationships to disclose.

Non-gated fetal MRI of umbilical blood flow in an acardiac twin

Currently, the standard method of diagnosis of twin reversed arterial perfusion (TRAP) sequence is ultrasound imaging. The use of MRI for flow visualization may be a useful adjunct to US imaging for assessing the presence of retrograde blood flow in the acardiac fetus and/or umbilical artery. The technical challenge in fetal MRI flow imaging, however, is that fetal electrocardiogram (ECG) monitoring required for flow imaging is currently unavailable in the MRI scanner. A non-gated MRI flow imaging technique that requires no ECG monitoring was developed using the t-test to detect blood flow in 20 slices of phase-contrast MRI images randomly scanned at the same location over multiple cardiac cycles. A feasibility study was performed in a 24-week acardiac twin that showed no umbilical flow sonographically. Non-gated MRI flow images clearly indicated the presence of blood flow in the umbilical artery to the acardiac twin; however, there was no blood flow beyond the abdomen. This study leads us to conjecture that non-gated MRI flow imaging is sensitive in detecting low-range blood flow velocity and can be an adjunct to Doppler US imaging.

MRI of slow flow in artificial duct in swine.

Our aim was to evaluate whether it is possible to visualize slow flow within a small catheter placed inside a living animal. We used a flow-sensitive, single-shot turbo spin-echo (SS-TSE) MRI sequence, developed in house, based on diffusion-weighted (DW) techniques. Four anesthetized pigs were used as models. A plastic catheter was surgically placed within the common bile duct (CBD). To mimic flow, the catheter was filled with Ringer's acetate and connected to a pump. b factors (s/m(2)) of 0, 6, and 12, with flow velocities raging from 0 to 1.32 cm/s, were used. A total of 375 images were obtained and examined. After correction for bowel movement artifacts, all images displayed the catheter on zero flow. With a flow of 0.66 cm/s or higher, no images displayed the catheter with a b factor of 6 or 12. On the slower flow velocities, it was variable whether the catheter was visible or not, but at b=6 and flow 0.17 cm/s all catheters were viewable. This method made it possible to perform a semiquantitative evaluation of flow velocities in vivo, dividing flow into three groups.

Imaging techniques to identify morbidly adherent placenta praevia: a prospective study

The objective of this study was to assess the sensitivity and specificity of Gray scale ultrasonography, magnetic resonance (MRI) and colour flow Doppler imaging in the diagnosis of morbidly adherent placenta praevia. We collected prospectively 30 cases of placenta praevia diagnosed by Gray scale ultrasound, colour flow Doppler and MRI. Delivery by elective caesarean section was carried out at the 38th week, before the onset of labour, unless obstetric complications supervened. Data regarding maternal outcome and newborn status were collected. Three patients had a morbid adherence of the placenta at delivery, two of whom required caesarean hysterectomy, and the other an internal iliac artery ligation. One patient was delivered at 33 weeks' gestation due to uncontrollable bleeding and the remainder were delivered at 38 weeks. All mothers and infants were well at the time of discharge. Colour flow Doppler was shown to be more specific in the diagnosis of the morbidly adherent placenta praevia than MRI. Doppler had a negative predictive value of 95%.

MRI of Dynamic Cord Deformation and CSF Flow: Effects of Tethering and Compression, and Visualization with Vector Fields

The purpose of this study is to evaluate the spatial-temporal characteristics of spinal cord motion and CSF flow in normal subjects and in cases involving cord compression or tethering and cystic diseases using MRI. MR examinations were conducted in healthy subjects as well as in patients with spinal cord compression, spinal stenosis, myelopathy, cord atrophy, syringomyelia and cystic diseases. Gradient and spin echo phase contrast sequences were used to determine pulsatile motion of the CSF and spinal cord. A new method of displaying the spatial distribution of velocities in the spinal cord and CSF as a vector field superimposed over the corresponding anatomical images was implemented to directly visualize the two dimensional spatial-temporal characteristics of cord motion and CSF flow.

In-Vitro Comparative Study by MRI and DPIV of Flow Through Normal and Thrombosed Bileaflet Aortic Valve: Velocity and Vorticity Mapping and Shear Stress Analysis

In-Vitro Comparative Study by MRI and DPIV of Flow Through Normal and Thrombosed Bileaflet Aortic Valve: Velocity and Vorticity Mapping and Shear Stress Analysis

Dynamic MRI of cerebrospinal fluid flow in endoscopic percutaneous ventriculostomy

A bstract Endoscopic percutaneous ventriculostomy (EPV) is a new technique to restore the flow of the cerebrospinal fluid (CSF) tothe basal cistern in patientswith non-communicating hydrocephalus(NCH). Cardiac gated MRI techniqueshave been applied to demonstrate normal and abnormal CSF flow in humans. The aim of this study was to evaluate the ability of a new multislice spin echo dynamic MRI technique and amplitude and phase reconstruction to demonstrate the impairment of CSF flow through the aqueduct preoperatively and to assess the patency of the ventriculostomy after surgery. Thirteen patients with NCH were studied with dynamic MRI before and after EPV using multiple sagittal multislice gated SE acquisitions to demonstrate the CSF flow. Nine patients were clinically improved by EPS, two remained unchanged and two deteriorated. The permeability of the ventriculostomy was confirmed in all patients. We think that MRI flow studiesare useful todemonstrate the site of impairment of CSF flow and itsaetiology preoperatively, and to assess the permeability of the ventriculostomy after surgery in a non invasive way.

A practical approach to MRI of coronary artery bypass graft patency and flow.

Direct visualization of coronary artery bypass grafts can be obtained non-invasively by magnetic resonance imaging. Several studies demonstrated a high sensitivity and somewhat lower specificity for detection of vein-graft patency, using the conventional spin-echo and gradient-echo techniques. In addition, the true functional status can be assessed by determining the flowrate within the graft using phase velocity mapping. Important limitations of the previously applied techniques include the inability to accurately evaluate the different segments of jump grafts and the presence of graft stenoses. Further improvement is to be expected from the recent introduction of breath-hold imaging sequences and the forthcoming introduction of bloodpool-avid contrast agents.

Dipole-tracing of abnormal slow brain potentials after cerebral stroke — EEG, PET, MRI correlations

A patient with major neurological deficits 5 years after a left cerebral infarction underwent correlative EEG, MRI and PET studies of cerebral blood flow and oxygen metabolism. The EEG showed abnormal slow electroencephalographic activity in the frontopolar region. The intracranial location of the slow electrical activity was estimated, as an equivalent current dipole, by using a newly developed dipole tracing (DT) method. The DT analysis showed that the dipole equivalent of the slow wave is approximately located at the frontal part of the left cingulate gyrus, away from the margins of the infarction and enlarged left lateral ventricle demonstrated by MRI, and in a region with intact oxygen consumption rate. The genesis of the slow wave is discussed.