Velocity-encoded Phase-contrast Magnetic Resonance Research Articles

Impact of Prosthetic Valve Regurgitation Assessed by Echocardiography and Magnetic Resonance Imaging on Long-Term Clinical Outcomes after TAVR

Background: Prosthetic valve regurgitation (PVR) impairs early and mid-term outcomes after transcatheter aortic valve replacement (TAVR). We explored the impact of PVR assessed by transthoracic echocardiography (TTE) and magnetic resonance imaging-regurgitation fraction (MRI-RF) on long-term clinical outcomes. Methods: PVR was assessed by TTE applying the Valve Academic Research Consortium criteria and MRI-RF (from velocity-encoded phase contrast magnetic resonance sequence) in 424 patients. Results: MRI-RF correlated modestly with the echocardiographic grades of PVR (Spearman’s rank correlation coefficient = 0.32, p < 0.001). Using an MRI-RF ≥20% to define ≥ moderate PVR, echocardiography and MRI-RF agreed on PVR classification in 412 patients (97.2%; kappa statistic = 0.56, p < 0.001). Five-year mortality or reintervention was higher in patients with echocardiographic ≥ moderate PVR (83.3% vs. 45.0%, log rank p value = 0.002; HR [95% CI]: 3.18 [1.48–6.84]) as well as in patients with MRI-RF ≥20% (79.3% vs. 43.2%, log rank p value <0.001; HR [95% CI]: 2.68 [1.53–4.70]), while the outcomes of patients with echocardiographic mild PVR were not significantly different from those with none-trace PVR. In the two latter groups (echocardiographic < moderate PVR), MRI-RF ≥20% was associated with a significantly higher 5-year mortality or reintervention as compared with MRI-RF <20% (79.5% vs. 42.2%, log rank p value = 0.023; HR [95% CI]: 2.26 [1.10–4.65]). Conclusions: Greater than mild PVR as defined by TTE or MRI-RF is associated with impaired long-term clinical outcomes after TAVR. MRI-RF can be used to further risk-stratify patients with echocardiographic less-than-moderate PVR.

Multivariable Technique for the Evaluation of the Trans-stenotic Pressure Gradient.

This study establishes a reliable image-based multivariable technique for measuring the trans-stenotic pressure gradient. A self-made in vitro steady flow model based on adjustable velocities and stenotic properties were used as the experimental subject. The pre-stenotic flow velocity, severity, and length of the stenosis were used as the input variables. Based on equations used to fit the plots of the physically measured pressure gradient values versus each input variable, a multivariable formula for the pressure gradient measurement could then be derived. The flow model was scanned using velocity-encoded phase-contrast magnetic resonance imaging (PC-MRI) to validate the derived formula while simultaneously measuring the trans-stenotic pressure gradient. The correlation between the physically-measured pressure gradient values and the pressure gradient values calculated using the new formula were subsequently analyzed. The results of linear regression analysis using the physically measured pressure gradient values for the new method were compared to values obtained using the simplified Bernoulli equation (R2, 0.991, and 0.975, respectively). In a paired t-test, no statistically significant difference was found between the new method and the physical measurements. The derived multivariable technique was found to reliably measure the trans-stenotic pressure gradient, with better performance than a traditional procedure based on the simplified Bernoulli equation.

The Role of Cardiovascular Magnetic Resonance in Congenital Heart Disease

The increasing prevalence of congenital heart disease (CHD) can be attributed to major improvements in diagnosis and treatment. Although echocardiography is the most commonly used imaging modality for diagnosis and follow up of subjects with CHD, the evolution of both cardiovascular magnetic resonance (MR) imaging and computed tomography (CT) does offer new ways to visualize the heart and the great vessels. The development of cardiovascular MR techniques such as spinecho and gradient-echo imaging, velocity-encoded phase contrast MR and gadolinium-enhanced MR angiography allow comprehensive assessment of cardiac anatomy and function. This provides information about the long-term sequelae of the underlying complex anatomy, hemodynamic assessment of residual post-operative lesions and complications of surgery. As much of the functional data in CHD patients is usually acquired with invasive X-ray angiography, non-invasive alternatives such as cardiovascular MR and CT are desirable. This review evaluates the role of both these modalities in the management of subjects with CHD, particularly detailing recent developments in imaging techniques as they relate to the various CHD diagnoses we commonly encounter in our practice.

0488: Assessment of aortic regurgitation severity: a cardiac magnetic resonance and echocardiographic comparison study

Assessment of chronic aortic regurgitation (AR) severity remains challenging. While transthoracic echocardiography (TTE) is the most widely used method, velocity-encoded phase-contrast magnetic resonance (PCMR) imaging in the ascending aorta for flow quantification is considered the gold standard method to calculate the regurgitant volume (Reg Vol). The aim of our study was to compare the AR severity by TTE and PCMR in a large serie of consecutive patients with aortic valvular heart disease (VHD). between 2007 and 2014, 283 patients underwent clinically indicated TTE and PCMR (Philips ACHIEVA 1.5 Tesla) within 30 days. Multiparametric approach was used by TTE (vena contracta, pisa method and semi-quantitative method) to grade AR severity. Mean age was 81±9 years, 53% were male. LVEF was not significantly different between TTE and CMR (55±13% and 53±15%); mean trans aortic gradient was >40mmHg in 53% patients. AR was respectively graded 0, 1, 2, 3 and 4 in 51%, 19%, 13%; 8% and 8% by TTE which corresponded to a Reg Vol of 5.3±3.9mL, 16±3, 26±4, 37±5, and 57±9mL respectively as obtained by PCMR. There was no significant overlap between different Reg vol by PCMR and the 5 grade obtained by TTE; The relation between AR- Reg vol and grade by TTE was not significantly affected by the mean trans-aortic gradient (i.e >or <40mmHg). TTE as used in routine practice, allows a good discrimination and quantification of chronic AR when compared to that performed by CMR. However, PCMR can be an excellent alternative to TTE in patients who undergo TAVR in whom AR assessment by TTE is often challenging.

Age-related greater Achilles tendon compliance is not associated with larger plantar flexor muscle fascicle strains in senior women

The aim of the present study was to test the hypothesis that the age-associated decrease of tendon stiffness would necessitate greater muscle fascicle strains to produce similar levels of force during isometric contraction. Greater fascicle strains could force sarcomeres to operate in less advantageous regions of their force-length and force-velocity relationships, thus impairing the capacity to generate strong and explosive contractions. To test this hypothesis, sagittal-plane dynamic velocity-encoded phase-contrast magnetic resonance images of the gastrocnemius medialis (GM) muscle and Achilles tendon (AT) were acquired in six young (YW; 26.1 ± 2.3 yr) and six senior (SW; 76.7 ± 8.3 yr) women during submaximal isometric contraction (35% maximum voluntary isometric contraction) of the plantar flexor muscles. Multiple GM fascicle lengths were continuously determined by automatically tracking regions of interest coinciding with the end points of muscle fascicles evenly distributed along the muscle's proximo-distal length. AT stiffness and Young's modulus were measured as the slopes of the tendon's force-elongation and stress-strain curves, respectively. Despite significantly lower AT stiffness at older age (YW: 120.2 ± 52.3 N/mm vs. SW: 53.9 ± 44.4 N/mm, P = 0.040), contraction-induced changes in GM fascicle lengths were similar in both age groups at equal levels of absolute muscular force (4-5% fascicle shortening in both groups), and even significantly larger in YW (YW: 11-12% vs. SW: 6-8% fascicle shortening) at equal percentage of maximum voluntary contraction. These results suggest that factors other than AT stiffness, such as age-associated changes in muscle composition or fascicle slack, might serve as compensatory adaptations, limiting the degree of fascicle strains upon contraction.

Impact of respiration on stroke volumes in paediatric controls and Fontan patients assessed by MR real-time phase velocity mapping

Background Routine blood volume quantification is generally performed by using two-dimensional velocity-encoded phase-contrast magnetic resonance imaging (PC-MRI). Due to their long acquisition times averaging of flow information occur leading to underestimation of peak velocities and preventing the examination of respirationrelated stroke volume (SV) variations. Methods Non-electrocardiographic triggered real-time PC-MRI using EPI combined with half-Fourier technique (TR/ TEeff/excitation angle = 12-14 ms/3.3 ms/40°, SENSEfactor = 4, temporal resolution = 24-28 ms) was applied to study respiration-driven SV fluctuations in the ascending aorta (AAo) and both caval veins (SVC, IVC) under natural and forced breathing in 34 healthy children and 10 pediatric Fontan patients. Data were collected during a 12-14 s time interval. The subject’s physiological data (ECG & respiration) were recorded simultaneously for matching with flow data. Virtual stroke volumes were generated dividing the respiration curve in four segments: expiration, breathing in, inspiration and breathing out. Spirometric measurements were performed to validate respiratory volumes. Statistical differences were analyzed by using ANOVA, paired Student t-test and Bland-Altman statistics. Results Stroke volume variability in healthy subjects was around 6% for AAo and >15% for SVC and IVC while breathing naturally. Under forced breathing variability increased to 9% and >34%, respectively. In Fontan patients a two to three-fold augmentation was observed in both respiratory maneuvers. Whereas in volunteers aortic SV was elevated during expiration (6%) and decreased during inspiration (-6%) in relation to mean respiratory SV, highest blood flow was detected during breathing in SVC (10%) and IVC (20%) and lowest blood flow during breathing out (-12%, -15%). Differences were increased under forced breathing. All differences were statistically significant. Regarding patients SV variability was drastically increased and had to be related to the patient’s individual quality of Fontan circulation. Intraobserver and interobserver varability of the four separated respiration-dependent SV was below 5%. Conclusions Due to its non-averaging character real-time PC-MRI allows a physiological assessment of respiratory-related SV fluctuations in healthy subjects as well as in Fontan patients and demonstrates its capability for detection of short-term effects in clinical routine work. It can be expected that real-time PC-MRI has the potential to classify the quality of Fontan hemodynamics. Funding None.

Cardiac magnetic resonance: is phonocardiogram gating reliable in velocity-encoded phase contrast imaging?

To assess the diagnostic accuracy of phonocardiogram (PCG) gated velocity-encoded phase contrast magnetic resonance imaging (MRI). Flow quantification above the aortic valve was performed in 68 patients by acquiring a retrospectively PCG- and a retrospectively ECG-gated velocity-encoded GE-sequence at 1.5 T. Peak velocity (PV), average velocity (AV), forward volume (FV), reverse volume (RV), net forward volume (NFV), as well as the regurgitant fraction (RF) were assessed for both datasets, as well as for the PCG-gated datasets after compensation for the PCG trigger delay. PCG-gated image acquisition was feasible in 64 patients, ECG-gated in all patients. PCG-gated flow quantification overestimated PV (Δ 3.8 ± 14.1 cm/s; P = 0.037) and underestimated FV (Δ -4.9 ± 15.7 ml; P = 0.015) and NFV (Δ -4.5 ± 16.5 ml; P = 0.033) compared with ECG-gated imaging. After compensation for the PCG trigger delay, differences were only observed for PV (Δ 3.8 ± 14.1 cm/s; P = 0.037). Wide limits of agreement between PCG- and ECG-gated flow quantification were observed for all variables (PV: -23.9 to 31.4 cm/s; AV: -4.5 to 3.9 cm/s; FV: -35.6 to 25.9 ml; RV: -8.0 to 7.2 ml; NFV: -36.8 to 27.8 ml; RF: -10.4 to 10.2 %). The present study demonstrates that PCG gating in its current form is not reliable enough for flow quantification based on velocity-encoded phase contrast gradient echo (GE) sequences. Phonocardiogram gating is an alternative to ECG-gating in cardiac MRI. Phonocardiogram gating shows only limited reliability for velocity-encoded cardiac MRI. Further refinements of the post-processing algorithm are necessary.

Analysis of complex cardiovascular flow with three‐component acceleration‐encoded MRI

Functional information regarding cardiac performance, pressure gradients, and local flow derangement are available from blood acceleration fields. Thus, this study examines a 2D and 3D phase contrast sequence optimized to efficiently encode three-directional, time-resolved acceleration in vitro and in vivo. Stenosis phantom acceleration measurements were compared to acceleration derived from standard velocity encoded phase contrast-magnetic resonance imaging (i.e., "velocity-derived acceleration"). For in vivo analysis, three-directional 2D acceleration maps were compared to velocity-derived acceleration using regions proximal and distal to the aortic valve in six healthy volunteers at 1.5 and 3.0 T (voxel size = 1.4 × 2.1 × 8 mm, temporal resolution = 16-20 ms). In addition, a 4D acceleration sequence was evaluated for feasibility in a healthy volunteer and postrepair biscuspid aortic valve patient with an ascending aortic aneurysm. The phantom magnetic resonance acceleration measurements were more accurate (nonturbulent root mean square error = 2.2 vs. 5.1 m/s(2) for phase contrast-magnetic resonance imaging) and 10 times less noisy (nonturbulent σ = 0.9 vs. 13.6 m/s(2) for phase contrast-magnetic resonance imaging) than velocity-derived acceleration. Acceleration mapping of the left ventricular outflow tract and aortic arch exhibited signal voids colocated with complex flow events such as vortex formation and high order motion. 4D acceleration data, visualized in combination with the velocity data, may provide new insight into complex flow phenomena.

Study of lower extremity muscle function in stroke patients by velocity-encoded phase-contrast magnetic resonance imaging

To compare the difference of muscle dynamic characteristics for the ankle dorsiflexors and plantarflexors between stroke patients at the chronic stage and healthy controls so as to provide a new method of assessing the in vivo muscle function in patients with hemiplegia. From May 2008 to May 2009, 26 stroke patients and 21 age-and gender-matched normal controls were recruited. All subjects were positioned on a scanner table and requested to perform the voluntary movement of ankle flexion-extension. The velocity encoded phase contrast magnetic resonance imaging (VE-PC MRI) provided the images of tibialis anterior muscle (TA), medial head of gastrocnemius muscle (MG) and soleus muscle (SOL) during a movement cycle. By measuring the calf muscle contraction velocity, the balance function was assessed by Berg balance scale (BBS). The correlation between scores of BBS and the mean maximum velocity were compared and analyzed. The peak velocity of TA (1 - 8 phase, 8.900 - 21.120 mm/s vs 12.99 - 34.50 mm/s), MG (12-19phase, 13.60 - 13.28 mm/s vs 25.85 - 18.38 mm/s) and SOL (12 - 16 phase, 18.63 - 33.62 mm/s vs 27.68 - 47.22 mm/s) was lower in the affected side than that in the controls during ankle extension (P < 0.05); During ankle dorsiflexion, the co-contraction index of SOL/TA (2 - 9 phase, 0.81 - 0.82 vs 0.27 - 0.44) and the co-contraction index of GM/TA (2 - 9 phase, 0.73 - 0.58 vs 0.10 - 0.11) was markedly higher in the affected side than the controls. The patient score of BBS was negatively correlated with the mean velocity of TA (r = -0.69, P = 0.001) and GM (r = -0.47, P = 0.01) in the affected side. There was correlation between TA (r = -0.60, P = 0.001) and GM (r = -0.49, P = 0.01) in the unaffected side. During the movement of active ankle flexion-extension, the velocities of TA, SOL and MG are lower in the affected side. The co-contraction index is markedly higher in the affected side during ankle dorsiflexion. This in turn leads to a decline of balance function in patients. VE-PC MRI can provide quantitative in vivo measurements of lower extremity muscle function in stroke patients.

Congenital Heart Disease and Multi-modality Imaging

The increasing prevalence of adult congenital heart disease (CHD) can be attributed to major improvements in diagnosis and treatment of children with CHD. Although, echocardiography is the most commonly used imaging modality for diagnosis and follow up of subjects with CHD, the evolution of both cardiovascular magnetic resonance (MR) imaging and computed tomography (CT) does offer new ways to visualise the heart and great vessels. Cardiovascular MR techniques such as spin-echo and gradient-echo imaging, velocity-encoded phase-contrast MR and gadolinium-enhanced MR angiography allow comprehensive assessment of cardiac anatomy and function. This provides information about the long-term sequelae of the underlying anatomy, haemodynamic assessment of residual post-operative lesions and complications of surgery. Similarly, the development of spiral and subsequently multi-detector CT enables the acquisition of data during a single breath-hold and during the first pass of a contrast bolus, so that images can be reconstructed in any two-dimensional plane or in three-dimensions. As much of the functional data in CHD patients was traditionally acquired with invasive X-ray angiography, non-invasive alternatives such as cardiovascular MR and CT are desirable. This review evaluates the role of imaging modalities in the management of subjects with CHD, particularly detailing recent developments in imaging techniques as they relate to the various CHD diagnoses commonly encountered in practice.

In vivo intramuscular fascicle-aponeuroses dynamics of the human medial gastrocnemius during plantarflexion and dorsiflexion of the foot

Velocity-encoded phase-contrast magnetic resonance (MR) imaging techniques and a computer-controlled MR-compatible foot pedal device were used to investigate the medial gastrocnemius muscle and aponeurosis deformations during passive and active eccentric movements of the plantarflexors. Intrafascicular strain, measured as the ratio of strain in the fascicle segment at its insertion to strain at its origin, was nonuniform along the proximodistal axis of the muscle (P < 0.01), progressively increasing from the proximal to distal direction. The high intrafascicular strain regions appeared to correlate with the muscle regions that are likely to encounter high stress concentrations, i.e., the regions where the muscle physiological cross section decreases close to the tendons. The architectural gear ratio, i.e., the mechanical amplification ratio of fascicle length displacement to that of tendon/aponeuroses in a pennate muscle, also exhibited significant regional differences, with the highest ratios in the proximal region of the muscle accompanied by a higher initial pennation angle and a larger range of fascicular rotation about the origin. Values close to unity in the distal region of the muscle suggest that the aponeurosis separation may decrease in this region. Fascicle length and pennation angle changes were significantly influenced by force generation in the muscle, probably due to a shortening of the loaded muscle fibers relative to a passive condition. Overall, our data illustrate significant proximodistal intramuscular heterogeneity as supported by a regionally variable end-to-end strain ratio of fascicles and angle changes in the medial gastrocnemius muscle during passive and active ankle movements. These observations emphasize the need to reassess current conceptual models of muscle-tendon mechanics.

1065 Three-directional myocardial motion in patients with univentricular hearts using velocity-encoded phase contrast MRI

Introduction Patients with univentricular hearts rely on a single functioning ventricle to provide blood flow to both the systemic and pulmonary circuits following completion of the Fontan circulation. Tracking the function of the remaining ventricle in these patients is of interest as their long-term survival continues to improve and the ability of this single ventricle to sustain an increased load or serve an atypical role, as in the case of systemic right ventricles, is continually tested. Velocity-encoded phase contrast magnetic resonance imaging (PC MRI) may provide an efficient technique with high spatial resolution to attain threecomponent myocardial tissue velocities in addition to flow information throughout the cardiac cycle which could be useful in the monitoring of these patients.

Mechanical behavior of the quadriceps femoris muscle tendon unit during low-load contractions

We examined the relationships between morphology and muscle-tendon dynamics of the quadriceps femoris muscle of 11 men using velocity-encoded phase-contrast magnetic resonance imaging (MRI). Thigh muscle electromyography and joint range of motion were first measured outside the MRI scanner during knee extension-flexion tasks that were performed at a rate of 40 times/min with elastic bands providing peak resistance of 5.2 kp (SD 0.4) to the extension. The same movement was repeated inside the MRI scanner bore where tissue velocities and muscle morphology were recorded. The average displacement in the proximal and distal halves of the rectus femoris and vastus intermedius aponeuroses was different (P = 0.049), reflecting shortening (1.6%), but the tensile strain along the length of the aponeuroses was uniform. The aponeurosis behavior varied among individuals, and these individual patterns were best explained by the differences in relative cross-sectional area of rectus femoris to vastus muscles (r = 0.71, P = 0.014). During dynamic contraction, considerable deformation of muscles in the axial plane caused an anatomic measure such as muscle thickness to change differently (decrease or increase) in different sites of measurement. For example, when analyzed from the axial images, the vastus lateralis thickness did not change (P = 0.946) in the frontal plane through femur but increased in a 45 degrees oblique plane between the frontal and sagittal planes (P = 0.004). The present observations of the heterogeneity and individual behavior emphasize the fact that single-point measurements do not always reflect the overall behavior of muscle-tendon unit.

The Paradoxical Flow Hypothesis of the Carotid Artery: Supporting Evidence from Phase-contrast Magnetic Resonance Imaging

Narrowing of the poststenotic internal carotid artery (ICA) has been found to be associated with reduced risk of ipsilateral stroke. A paradoxical mechanism has been hypothesized to explain this finding: narrowing of the distal-normal (reference) ICA is associated with low blood flow rates (Q) in the stenotic ICA, and lower Q causes lower risk of ipsilateral stroke, perhaps by an associated reduction in mechanical stress on the atherosclerotic plaque. The purpose of this study was to confirm that the reference ICA diameter (RICAD) is indeed predictive of Q, a finding that would indirectly support the hypothesis of a relationship between lower Q and lower risk of ipsilateral stroke. Magnetic resonance imaging from 38 patients was included in the study. The study included 17 stenotic carotid arteries and 59 normal carotid arteries. All patients underwent contrast-enhanced magnetic resonance angiography from which measurements were obtained of the RICAD and the internal-common carotid diameter ratio. Patients underwent cardiac-gated, velocity-encoded phase-contrast magnetic resonance imaging for measurement of Q. Mean flow rates differed between stenotic (4.3 +/- 1.7 mL/s) and normal (5.4 +/- 1.7 mL/s) arteries (P = .02). RICAD was found to be a predictor of Q for stenotic arteries (P = .009) and for all arteries (P = .025) but not for the group of normal arteries (P = .162). Right-left differences in RICAD were highly predictive of right-left differences in Q in the subgroup of individuals with normal arteries (P < .001) and in the group of all participants (P < .001). Internal-common carotid diameter ratio was not found to be a statistically significant predictor of Q in the subgroup of stenotic arteries (P = .156). This study demonstrated that, as hypothesized, RICAD is correlated with Q.

자기공명 영상기법을 이용한 인체 가자미근 건막의 기계학적 특성 연구

Muscle force produced by muscle fibers is transmitted to bones via tendinous structures(aponeuroses and tendon), resulting in joint(s) movement. As force-transmitting elements, mechanical behavior of aponeuroses and tendon are closely related with the function of muscle-tendon complex. The purpose of this study was to determine strain characteristics of aponeuroses for in-vivo human soleus muscle during submaximal voluntary contractions using an advanced medical imaging technique, velocity-encoded phase-contrast magnetic resonance imaging (VE-PC MRI). VE-PC MRI of the soleus muscle-tendon complex was acquired during submaximal isometric plantarflexion contraction-relaxation cycle (n = 7), using 3.0T Trio MRI scanner(Siemens AG, Malvern, MA). From the VE-PC MRI containing the tissue velocity in superior-inferior direction, twenty regions of interest(20 ROI; 10 on the anterior aponeurosis and 10 on the posterior aponeurosis) were tracked. During the isometric plantarflexion contraction-relaxation cycle, velocity and displacement profiles were different between the anterior and posterior aponeuroses, indicating heterogeneous strain behavior along the length of the leg. The anterior aponeurosis elongated while the posterior aponeurosis shortened during the initial phase of the contraction. Moreover, strain behavior of the posterior aponeurosis was different from that of the Achilles tendon. Possible explanation for the observed variations in strain behavior of aponeuroses was investigated with morphological assessment of the soleus muscle and it was found that the intramuscular tendinous structures significantly vary among subjects. In conclusion, the heterogeneous mechanical behavior of the soleus aponeuroses and the Achilles tendon suggests that the complexity of skeletal muscle-tendon complex should be taken into consideration when modeling the complex for better understanding of its functions.

Investigating myocardial motion by MRI using tissue phase mapping

Velocity-encoded phase contrast magnetic resonance imaging (MRI) provides a tool to quantify regional myocardial wall motion of the entire heart. It allows the acquisition of three-directional velocity vector fields with high spatial resolution that reflect the temporal evolution of myocardial velocities over the cardiac cycle. In contrast to other imaging modalities such as echocardiography left ventricular performance can be assessed without limited anatomical or functional coverage. Compared to other techniques that quantify local myocardial contractility (e.g. implanted ultrasonic crystals) by means of regional displacement, phase contrast MRI provides information about local and global left ventricular velocities (i.e. motion) by utilizing the intrinsic motion sensitivity of MRI. The resultant motion components of contraction, expansion, rotation, lengthening, and shortening of the left ventricle are described in high spatial and temporal detail. Phase contrast measurements were performed in 12 healthy volunteers with a respiratory-gated technique in order to achieve a high temporal resolution of 13.8ms to demonstrate the detailed assessment of global and regional myocardial motion. Data revealed details in left ventricular motion patterns that were previously not seen in phase contrast measurements and are only known from echocardiography. For all volunteers, characteristic myocardial motion patterns and locally different radial (i.e. contraction and expansion), rotational (i.e. twisting and untwisting) and longitudinal (i.e. lengthening and shortening) motion components could be detected with high accuracy. The phase contrast MRI technique for high temporal resolution velocity mapping is therefore very promising for the investigation and better understanding of the myocardial motion in normal subjects and patients with disturbed left ventricular performance and may validate further testing of different models of cardiac structure.

The effect of magnesium sulfate on large cerebral artery blood flow in preeclampsia

Objectives. To determine the effect of a 6 gram intravenous bolus of magnesium sulfate on maternal cerebral blood flow in women with preeclampsia.Study Desiqn. Velocity-encoded phase-contrast magnetic resonance imaging studies were performed on twelve preeclamptic women prior to and immediately after infusion of a 6 gram magnesium sulfate loading dose. Cerebral blood flow was determined at the bilateral proximal middle and posterior cerebral arteries. Study participants returned 6 weeks postpartum for a non-pregnant measurement of cerebral blood flow. The Wilcoxon paired-sample test was used with statistical significance defined as p<0.05.Results. There was no significant difference in cerebral vessel diameter nor blood flow for any of the examined arteries between the pre- and post magnesium sulfate therapy states.Conclusions. The absence of a significant difference in cerebral blood flow of the middle and posterior cerebral arteries before and after infusion of a 6 gram loading dose of magnesium sulfate in women with preeclampsia could suggest the absence of vasoconstriction of the large cerebral arteries in preeclampsia and question the role of magnesium sulfate as a vasodilator of these arteries.

Increased cerebral blood flow in preeclampsia with magnetic resonance imaging

The purpose of this study was to compare third trimester and nonpregnant cerebral blood flow of women with preeclampsia to normotensive control subjects with the use of magnetic resonance imaging techniques. Nine normotensive pregnant women and 12 untreated women with preeclampsia underwent velocity-encoded phase contrast magnetic resonance imaging of the bilateral middle and posterior cerebral arteries in the third trimester and at 6 to 8 weeks after delivery. The Student t test was used for comparison, with a probability value of <.05 considered significant. Third-trimester large cerebral artery blood flow was significantly higher in preeclampsia. Mean vessel diameter was unchanged, except for the left posterior cerebral artery. There was no difference in mean vessel diameter or cerebral blood flow between the 2 groups while the women were not pregnant. Cerebral blood flow is increased significantly in preeclampsia. We hypothesize that increased cerebral blood flow ultimately could lead to eclampsia through hyperperfusion and the development of vasogenic edema.

Maternal cerebral blood flow changes in pregnancy

Objective This study was undertaken to determine blood flow changes in the large cerebral arteries during normal pregnancy. Study design Ten healthy pregnant volunteers underwent velocity-encoded phase contrast magnetic resonance imaging at 4 time intervals: 14 to 16, 28 to 32, and 36 to 38 weeks' gestation, and at 6 to 8 weeks' postpartum. Analysis consisted of serial paired Student t tests, with P<.05 considered significant. Results By using postpartum values for comparison, cerebral blood flow decreased by 14 to 16 weeks in the middle cerebral artery ( P<.001), but was not significantly changed in the posterior cerebral artery. Significant decreases occurred in both the middle ( P<.0001) and posterior ( P = .002) cerebral arteries in late pregnancy. Conclusion An approximately 20% reduction in large artery cerebral blood flow occurs during normal pregnancy, secondary to changes in velocity, whereas the area of these vessels remains unchanged. These findings may represent generalized vasodilatation of downstream resistance arterioles, assuming constant blood flow at the tissue level.

Myocardial velocity gradient imaging by phase contrast MRI with application to regional function in myocardial ischemia.

Velocity-encoded phase contrast magnetic resonance imaging (MRI) has the potential to quantify regional myocardial contractile function with a sensitivity to motion comparable to implanted ultrasonic crystals. An MRI sequence and post-processing algorithm were developed to measure myocardial velocity gradients on a 1.5 T MRI scanner. These methods were validated on a rotating phantom and applied to dogs before (n = 11) and during prolonged coronary occlusion (n = 5). In phantom validation studies, the average absolute error corresponded to motion equivalent to 0.03 +/- 0.04 mm (mean +/- SD) during the repetition time of the experiment. Rigid body corrections during post-processing significantly simplified the interpretation of myocardial velocity vectors. In vivo, rigid body motion contributes substantially to the recorded myocardial velocities in systole and diastole and can give the false impression of regional wall motion abnormalities. After rigid body correction, normal systolic and diastolic velocity vectors in short-axis views of the left ventricle were primarily directed toward the center of the left ventricle. Transmural radial strain rate was 2.0 +/- 0.6 sec-1 during systole and -3.6 +/- 1.1 sec-1 during early diastole in normal canine hearts. Ischemic myocardium was easily discriminated from normal left ventricle by velocity-encoded phase contrast MRI both qualitatively and quantitatively (P < 0.01 in systole and P < 0.05 in early diastole). Although the myocardial velocity images have a spatial resolution on the order of a millimeter, the velocity encoding describes the mechanical consequences of focal myocardial ischemia with sensitivity to submillimeter displacement of the pixels. The three-dimensional nature of velocity-encoded MRI is particularly well suited to the study of the complex motion of the heart in vivo.