Flow patterns in ascending aortic aneurysms: Determining the role of hypertension using phase contrast magnetic resonance and computational fluid dynamics.

Thoracic aortic aneurysm (TAA) is a local dilation of the thoracic aorta. Although universally used, aneurysm diameter alone is a poor predictor of major complications such as rupture. There is a need for better biomarkers for risk assessment that also reflect the aberrant flow patterns found in TAAs. Furthermore, hypertension is often present in TAA patients and may play a role in progression of aneurysm. The exact relation between TAAs and hypertension is poorly understood. This study aims to create a numerical model of hypertension in the aorta by using computational fluid dynamics. First, a normotensive state was simulated in which flow and resistance were kept unaltered. Second, a hypertensive state was modeled in which blood inflow was increased by 30%. Third, a hypertensive state was modeled in which the proximal and peripheral resistances and capacitance parameters from the three-element Windkessel boundary condition were adjusted to mimic an increase in resistance of the rest of the cardiovascular system. One patient with degenerative TAA and one healthy control were successfully simulated at hypertensive states and were extensively analyzed. Furthermore, three additional TAA patients and controls were simulated to validate our method. Hemodynamic variables such as wall shear stress, oscillatory shear index, endothelial cell activation potential (ECAP), vorticity and helicity were studied to gain more insight on the effects of hypertension on flow patterns in TAAs. By comparing a TAA patient and a control at normotensive state at peak-systole, helicity and vorticity were found to be lower in the TAA patient throughout the entire domain. No major changes in flow and flow derived quantities were observed for the TAA patient and control when resistance was increased. When flow rate was increased, regions with high ECAP values were found to reduce in TAA patients in the aneurysm region which could reduce the risk of thrombogenesis. Thus, it may be important to assess cardiac output in patients with TAA.

Ascending aorta curvature and flow displacement are associated with accelerated aortic growth at long-term follow-up: A MRI study in Marfan and thoracic aortic aneurysm patients.

BACKGROUND: Aortic aneurysm formation is associated with increased risk of aortic dissection. Current diagnostic strategies are focused on diameter growth, the predictive value of aortic morphology and function remains underinvestigated. We aimed to assess the long-term prognostic value of ascending aorta (AA) curvature radius, regional pulse wave velocity (PWV) and flow displacement (FD) on aortic dilatation/elongation and evaluated adverse outcomes (proximal aortic surgery, dissection/rupture, death) in Marfan and non-syndromic thoracic aortic aneurysm (NTAA) patients. METHODS: Long-term magnetic resonance imaging (MRI) and clinical follow-up of two previous studies consisting of 21 Marfan and 40 NTAA patients were collected. Baseline regional PWV, AA curvature radius and normalized FD were assessed as well as diameter and length growth rate at follow-up. Multivariate linear regression was performed to evaluate whether baseline predictors were associated with aortic growth.=. RESULTS: Of the 61 patients, 49 patients were included with MRI follow-up (n = 44) and/or adverse aortic events (n = 7). Six had undergone aortic surgery, no dissection/rupture occurred and one patient died during follow-up. During 8.0 [7.3-10.7] years of follow-up, AA growth rate was 0.40 ± 0.31 mm/year. After correction for confounders, AA curvature radius (p = 0.01), but not FD or PWV, was a predictor of AA dilatation. Only FD was associated with AA elongation (p = 0.01). CONCLUSION: In Marfan and non-syndromic thoracic aortic aneurysm patients, ascending aorta curvature radius and flow displacement are associated with accelerated aortic growth at long-term follow-up. These markers may aid in the risk stratification of ascending aorta elongation and aneurysm formation.

Determining the optimal interval for imaging surveillance of ascending aortic aneurysms.

BACKGROUND: Cardiovascular guidelines recommend (bi-)annual computed tomography (CT) or magnetic resonance imaging (MRI) for surveillance of the diameter of thoracic aortic aneurysms (TAAs). However, no previous study has demonstrated the necessity for this approach. The current study aims to provide patient-specific intervals for imaging follow-up of non-syndromic TAAs. METHODS: A total of 332 patients with non-syndromic ascending aortic aneurysms were followed over a median period of 6.7 years. Diameters were assessed using all available imaging techniques (echocardiography, CT and MRI). Growth rates were calculated from the differences between the first and last examinations. The diagnostic accuracy of follow-up protocols was calculated as the percentage of subjects requiring pre-emptive surgery in whom timely identification would have occurred. RESULTS: The mean growth rate in our population was 0.2 ± 0.4 mm/year. The highest recorded growth rate was 2.0 mm/year, while 40.6% of patients showed no diameter expansion during follow-up. Females exhibited significantly higher growth rates than men (0.3 ± 0.5 vs 0.2 ± 0.4 mm/year, p = 0.007). Conversely, a bicuspid aortic valve was not associated with more rapid aortic growth. The optimal imaging protocol comprises triennial imaging of aneurysms 40-49 mm in diameter and yearly imaging of those measuring 50-54 mm. This strategy is as accurate as annual follow-up, but reduces the number of imaging examinations by 29.9%. CONCLUSIONS: In our population of patients with non-syndromic TAAs, we found aneurysm growth rates to be lower than those previously reported. Yearly imaging does not lead to changes in the management of small aneurysms. Thus, lower imaging frequencies might be a good alternative approach.

Validation of aortic valve 4D flow analysis and myocardial deformation by cardiovascular magnetic resonance in patients after the arterial switch operation.

BACKGROUND: Aortic regurgitation (AR) and subclinical left ventricular (LV) dysfunction expressed by myocardial deformation imaging are common in patients with transposition of the great arteries after the arterial switch operation (ASO). Echocardiographic evaluation is often hampered by reduced acoustic window settings. Cardiovascular magnetic resonance (CMR) imaging provides a robust alternative as it allows for comprehensive assessment of degree of AR and LV function. The purpose of this study is to validate CMR based 4-dimensional flow quantification (4D flow) for degree of AR and feature tracking strain measurements for LV deformation assessment in ASO patients. METHODS: A total of 81 ASO patients (median 20.6 years, IQR 13.5-28.4) underwent CMR for 4D and 2D flow analysis. CMR global longitudinal strain (GLS) feature tracking was compared to echocardiographic (echo) speckle tracking. Agreements between and within tests were expressed as intra-class correlation coefficients (ICC). RESULTS: Eleven ASO patients (13.6%) showed AR > 5% by 4D flow, with good correlation to 2D flow assessment (ICC = 0.85). 4D flow stroke volume of the aortic valve demonstrated good agreement to 2D stroke volume over the mitral valve (internal validation, ICC = 0.85) and multi-slice planimetric LV stroke volume (external validation, ICC = 0.95). 2D flow stroke volume showed slightly less, though still good agreement with 4D flow (ICC = 0.78) and planimetric LV stroke volume (ICC = 0.87). GLS by CMR was normal (- 18.8 ± 4.4%) and demonstrated good agreement with GLS and segmental analysis by echocardiographic speckle tracking (GLS = - 17.3 ± 3.1%, ICC of 0.80). CONCLUSIONS: Aortic 4D flow and CMR feature tracking GLS analysis demonstrate good to excellent agreement with 2D flow assessment and echocardiographic speckle tracking, respectively, and can therefore reliably be used for an integrated and comprehensive CMR analysis of aortic valve competence and LV deformation analysis in ASO patients.

Coupling between MRI-assessed regional aortic pulse wave velocity and diameters in patients with thoracic aortic aneurysm: a feasibility study.

AIMS: Thoracic aortic aneurysm (TAA) is potentially life-threatening and requires close follow-up to prevent aortic dissection. Aortic stiffness and size are considered to be coupled. Regional aortic stiffness in patients with TAA is unknown. We aimed to evaluate coupling between regional pulse wave velocity (PWV), a marker of vascular stiffness, and aortic diameter in TAA patients. METHODS: In 40 TAA patients (59 ± 13 years, 28 male), regional aortic diameters and regional PWV were assessed by 1.5 T MRI. The incidence of increased diameter and PWV were determined for five aortic segments (S1, ascending aorta; S2, aortic arch; S3, thoracic descending aorta; S4, suprarenal and S5, infrarenal abdominal aorta). In addition, coupling between regional PWV testing and aortic dilatation was evaluated and specificity and sensitivity were assessed. RESULTS: Aortic diameter was 44 ± 5 mm for the aortic root and 39 ± 5 mm for the ascending aorta. PWV was increased in 36 (19 %) aortic segments. Aortic diameter was increased in 28 (14 %) segments. Specificity of regional PWV testing for the prediction of increased regional diameter was ≥ 84 % in the descending thoracic to abdominal aorta and ≥ 68 % in the ascending aorta and aortic arch. CONCLUSION: Normal regional PWV is related to absence of increased diameter, with high specificity in the descending thoracic to abdominal aorta and moderate results in the ascending aorta and aortic arch.

Morphological and functional carotid vessel wall properties in relation to cerebral white matter lesions in myocardial infarction patients.

OBJECTIVE: Atherosclerotic large vessel disease is potentially involved in the pathogenesis of cerebral small vessel disease related to occurrence of white matter lesions (WMLs) in the brain. We aimed to assess morphological and functional carotid vessel wall properties in relation to WML using magnetic resonance imaging (MRI) in myocardial infarction (MI) patients. MATERIALS AND METHODS: A total of 20 MI patients (90 % male, 61 ± 11 years) underwent carotid artery and brain MRI. Carotid vessel wall thickness (VWT) was assessed, by detecting lumen and outer wall contours. Carotid pulse wave velocity (PWV), a measure of elasticity, was determined using the transit-time method. Patients were divided according to the median VWT into two groups. Brain MRI allowed for the WML score. RESULTS: Mean VWT was 1.41 ± 0.29 mm and mean carotid PWV was 7.0 ± 2.2 m/s. A significant correlation (Pearson r = 0.45, p = 0.046) between VWT and PWV was observed. Furthermore, in the group of high VWT, the median WML score was higher as compared with the group with lower VWT (4.0 vs 3.0, p = 0.035). CONCLUSIONS: Carotid artery morphological and functional alterations are correlated in MI patients. Patients with high VWT showed a higher amount of periventricular WMLs. These findings support the hypothesis that atherosclerotic large vessel disease is potentially involved in the pathogenesis of cerebral small vessel disease.

The role of branch vessels in aortic type B dissection: an in vitro study.

OBJECTIVES: In acute type B aortic dissection (ABAD) a patent false lumen portends a poor outcome. Patent branch vessels originating from the false lumen in a type B aortic dissection are assumed to contribute to persistent blood flow and patent false lumen. Therefore, the morphologic changes of the false lumen generated by different outflow rates in an in vitro model were investigated. METHODS: An artificial dissection was created in two ex vivo porcine aortas. A thin cannula was placed in the false lumen, simulating a branch vessel originating from it. The aorta was positioned in a validated in vitro circulatory system with physiological pulsatile flow (1,500-2,700 mL/minute) and pressure characteristics (130/70 mm Hg). The cannula was attached to a small silicone tube with an adjustable valve mechanism. Three different valve settings were used for creating outflow from the false lumen (fully closed, opened at 50%, and fully opened at 100%). Measurements of lumen areas and flow rates were assessed with time-resolved magnetic resonance imaging. In order to study reproducibility, the experiment was performed twice in two different porcine aortas with a similar morphology. RESULTS: Increasing antegrade outflow through the branch vessel of the false lumen resulted in a significant (p < .01) increase of the mean false lumen area at the proximal and distal location in both models. The distal false lumen expanded up to 107% in the case of high outflow via the false lumen through the branch vessel. CONCLUSIONS: Increasing antegrade outflow through a branch vessel originating from the false lumen when no distal re-entry tear is present results in an expansion of the cross sectional false lumen area.

High field carotid vessel wall imaging: a study on reproducibility.

PURPOSE: Currently, a multi-contrast protocol, including a combination of five MR-sequences is used as reference standard for morphologic imaging and quantitative measurements of the carotid artery vessel wall. The purpose of this study is to investigate the scan-rescan reproducibility together with intra- and inter-observer reproducibility of each of the five MR-sequences. METHODS: Twenty healthy volunteers (55% male, mean age=26 years) underwent repeated MR-examinations (3T-Philips-MRI) of the left carotid artery vessel wall with five sequences; T1-TFE, T2-TSE, PD-TSE, T1-TSE and 3D TOF. A standard phased-array coil with two flexible elements of 14cm×17cm was used to obtain nine transverse imaging sections of the left carotid artery with identical in-plane resolution (0.46mm×0.46mm). Reproducibility analysis was performed in 3 slices of the common carotid artery for all sequences. RESULTS: For, scan-rescan reproducibility, intra class correlation coefficients (ICC) were excellent for all sequences and ranged from 0.79 to 0.95. The intra-observer ICC ranged from 0.89 to 0.98 and the inter-observer ICC ranged from 0.84 to 0.96, for both lumen and vessel wall assessment. CONCLUSIONS: By high field MR imaging, vessel wall and lumen area of the carotid artery can be assessed with excellent scan-rescan, intra- and inter-observer reproducibility for all five sequences.

The effect of hypertension on aortic pulse wave velocity in type-1 diabetes mellitus patients: assessment with MRI.

To investigate in type-1 diabetes mellitus (DM1) patients the role of hypertension and of DM1 itself on aortic stiffness by using magnetic resonance imaging (MRI). Consecutive patients from the diabetes and hypertension outpatient clinic and healthy volunteers were included in our study. Subjects were divided into four groups: 32 healthy volunteers (mean age: 54.5 ± 6.8 years), 20 DM1 patients (mean age: 48.3 ± 5.9 years), 31 hypertensive patients (mean age: 59.9 ± 7.2 years) and 28 patients with both DM1 and hypertension (mean age: 50.1 ± 6.2 years). Aortic stiffness was measured by means of pulse wave velocity (PWV) using velocity-encoded MRI. Analysis of variance (ANOVA), uni- and multivariable regression models and the Bonferroni-test for multiple testing, were used for statistical analyses. Mean aortic PWV was 5.7 ± 1.2 m/s in healthy volunteers, 5.9 ± 1.2 m/s in DM1 patients without hypertension, 7.3 ± 1.2 m/s in hypertensive patients and 7.3 ± 1.3 m/s in patients with both DM1 and hypertension. Compared to healthy control subjects, aortic PWV was significantly higher in patients with hypertension (P < 0.001) and in patients with both DM1 and hypertension (P < 0.001), whereas aortic PWV was not increased in patients having DM1 alone. Furthermore, aortic PWV was significantly higher in DM1 patients with hypertension than in patients with DM1 alone (P = 0.002). These findings remained after adjustment for confounding factors. Hypertension has a predominant contributive effect on aortic stiffness in DM1 patients whereas the direct diabetic effect on aortic stiffness is small.

Recovery of right and left ventricular function after acute pulmonary embolism.

AIM: To evaluate recovery of cardiac function after acute pulmonary embolism (PE). MATERIALS AND METHODS: Routine breath-held computed tomography (CT)-pulmonary angiography was performed in patients with suspected PE to confirm or exclude the diagnosis of PE at initial presentation. Electrocardiogram (ECG)-triggered cardiac CT was performed to assess biventricular function. After 6 months, cardiac magnetic resonance imaging (MRI) was performed. In total, 15 consecutive patients with PE and 10 without were studied. A significant change in ventricular volume was defined as a >15% change in end-diastolic or -systolic volumes (EDV, ESV), and significant ventricular function improvement as a >5% increase in ejection fraction (EF) as based on reported cut-off values. RESULTS: Right and left ventricular (RV and LV) EDV and ESV changed non-significantly (<1.3%) in the patients without PE, indicating good comparability of those values measured by CT and MRI. PE patients with baseline normal RV function (RVEF ≥ 47%) revealed a >5% improvement in the RVEF (+5.4 ± 3.1%) due to a decrease in the RVESV. Patients with baseline abnormal RV function showed a >5% improvement in the RVEF (+14 ± 15%) due to decreases in both the RVESV and RVEDV. Furthermore, the LVEDV increased in this latter patient group. CONCLUSIONS: The present study demonstrated an improvement in RV function in the majority of patients with PE, independent of baseline RV function. The degree of RV and LV recovery was dependent on the severity of baseline RV dysfunction.

Aortic stiffness is associated with cardiac function and cerebral small vessel disease in patients with type 1 diabetes mellitus: assessment by magnetic resonance imaging.

OBJECTIVE: To evaluate, with the use of magnetic resonance imaging (MRI), whether aortic pulse wave velocity (PWV) is associated with cardiac left ventricular (LV) function and mass as well as with cerebral small vessel disease in patients with type 1 diabetes mellitus (DM). MATERIALS AND METHODS: We included 86 consecutive type 1 DM patients (49 male, mean age 46.9 +/- 11.7 years) in a prospective, cross-sectional study. Exclusion criteria included aortic/heart disease and general MRI contra-indications. MRI of the aorta, heart and brain was performed for assessment of aortic PWV, as a marker of aortic stiffness, systolic LV function and mass, as well as for the presence of cerebral white matter hyperintensities (WMHs), microbleeds and lacunar infarcts. Multivariate linear or logistic regression was performed to analyse the association between aortic PWV and outcome parameters, with covariates defined as age, gender, mean arterial pressure, heart rate, BMI, smoking, DM duration and hypertension. RESULTS: Mean aortic PWV was 7.1 +/- 2.5 m/s. Aortic PWV was independently associated with LV ejection fraction (ss = -0.406, P = 0.006), LV stroke volume (ss = -0.407, P = 0.001), LV cardiac output (ss = -0.458, P = 0.001), and with cerebral WMHs (P < 0.05). There were no independent associations between aortic stiffness and LV mass, cerebral microbleeds or lacunar infarcts. CONCLUSION: Aortic stiffness is independently associated with systolic LV function and cerebral WMHs in patients with type 1 DM.

Aortic elasticity and size are associated with aortic regurgitation and left ventricular dysfunction in tetralogy of Fallot after pulmonary valve replacement.

BACKGROUND: Aortic wall pathology and concomitant aortic dilatation have been described in tetralogy of Fallot (TOF) patients, which may negatively affect aortic valve and left ventricular systolic function. OBJECTIVE: To assess aortic dimensions, aortic elasticity, aortic valve competence and biventricular function in repaired TOF patients after pulmonary valve replacement (PVR) using magnetic resonance imaging (MRI). METHODS: MRI was performed in 16 patients with TOF after PVR (10 male; mean age 31 years (SD 15)) and 16 age and gender-matched healthy subjects. RESULTS: TOF patients showed aortic root dilatation (mean difference 7.8-8.8 mm, p<0.01 at all four predefined levels) and reduced aortic elasticity (pulse wave velocity in aortic arch 5.5 m/s (1.2) vs 4.6 m/s (0.9), p = 0.04; aortic root distensibility 1.4/10(-3) mm Hg (1.7) vs 5.7/10(-3) mm Hg (3.6), p<0.01). Minor degrees of aortic regurgitation (AR) (AR fraction 6% (8) vs 1% (1), p<0.01) and reduced left ventricular ejection fraction (LVEF) were present (51% (8) vs 58% (6), p = 0.01), whereas right ventricular ejection fraction (RVEF) was within normal limits (47% (8) vs 52% (7), p = 0.06). The degree of AR fraction was associated with dilatation of the aortic root (r = 0.39-0.49, p<0.05) and reduced aortic root distensibility (r = 0.44, p = 0.02), whereas reduced LVEF was correlated with degree of AR and RVEF (r = 0.41, p = 0.02 and r = 0.49, p<0.01, respectively). CONCLUSIONS: Aortic root dilatation and reduced aortic elasticity are frequently present in patients with TOF, in addition to minor degrees of AR and reduced left ventricular systolic function. Aortic wall pathology in repaired TOF patients may therefore represent a separate mechanism leading to left ventricular dysfunction, as part of a multifactorial process of left ventricular dysfunction.

Right ventricular hypertrophy and diastolic dysfunction in arterial switch patients without pulmonary artery stenosis.

OBJECTIVE: To assess pulmonary flow dynamics and right ventricular (RV) function in patients without significant anatomical narrowing of the pulmonary arteries late after the arterial switch operation (ASO) by using magnetic resonance imaging (MRI). METHODS: 17 patients (mean (SD), 16.5 (3.6) years after ASO) and 17 matched healthy subjects were included. MRI was used to assess flow across the pulmonary trunk, RV systolic and diastolic function, and RV mass. RESULTS: Increased peak flow velocity (>1.5 m/s) was found across the pulmonary trunk in 14 of 17 patients. Increased RV mass was found in ASO patients: 14.9 (3.4) vs 10.0 (2.6) g/m2 in normal subjects (p<0.01). Delayed RV relaxation was found after ASO: mean tricuspid valve E/A peak flow velocity ratio = 1.60 (0.96) vs 1.92 (0.61) in normal subjects (p = 0.03), and E-deceleration gradients = -1.69 (0.73) vs -2.66 (0.96) (p<0.01). After ASO, RV mass correlated with pulmonary trunk peak flow velocity (r = 0.49, p<0.01) and tricuspid valve E-deceleration gradients (r = 0.35, p = 0.04). RV systolic function was well preserved in patients (ejection fraction = 53 (7)% vs 52 (8)% in normal subjects, p = 0.72). CONCLUSIONS: Increased peak flow velocity in the pulmonary trunk was often observed late after ASO, even in the absence of significant pulmonary artery stenosis. Haemodynamic consequences were RV hypertrophy and RV relaxation abnormalities as early markers of disease, while systolic RV function was well preserved.

Accuracy of short-axis cardiac MRI automatically derived from scout acquisitions in free-breathing and breath-holding modes.

To qualitatively assess the accuracy of automated cardiovascular magnetic resonance planning procedures devised from scout acquisitions in free-breathing and breath-holding modes, to quantitatively evaluate the accuracy of the derived left ventricular volumes, mass and function and compare these parameters with the ones obtained from the manually planned acquisitions. Ten healthy volunteers underwent cardiovascular MR (CMR) acquisitions for ventricular function assessment. Short-axis data sets of the left ventricle (LV) were manually planned and generated twice in an automatic fashion. Automated planning parameters were derived from gated scout acquisitions in free-breathing and breath-holding modes. End-diastolic volume (EDV), end-systolic volume (ESV), ejection fraction (EF), and left ventricular mass (LVM) were measured. The agreement between the manual and automatic planning methods, as well as the variability of the aforementioned measurements were assessed. The differences between two automated planning methods were also compared. The mean differences between the manual and automated CMR planning derived from gated scouts in free-breathing mode were 8.05 ml (EDV), 1.84 ml (ESV), 0.69% (EF), and 4.72 g (LVM). The comparison between manual and automated CMR planning derived from gated scouts in breath-holding mode yielded the following differences: 4.22 ml (EDV), 0.34 ml (ESV), 0.3% (EF), and -0.72 mg (LVM). The variability coefficients were 3.72 and 3.66 (EDV), 5.6 and 8.19 (ESV), 3.46 and 4.31 (EF), 6.49 and 5.20 (LVM) for the automated CMR planning methods derived from scouts in free-breathing and breath-holding modes, respectively. Automated CMR planning methods can provide accurate measurements of LV dimensions in normal subjects, and therefore may be utilized in the clinical environment to provide a cost-effective solution for functional assessment of the human cardiovascular system.

Mitral valve regurgitation: accurate blood flow quantification with MRI.

BACKGROUND: The quantification of transvalvular blood flow through the mitral valve (MV) and regurgitant flow in particular is difficult with echocardiography, which is the method of choice to diagnose patients selected for valve repair or replacement. With magnetic resonance imaging, information on the intraventricular blood flow can be obtained. Several scanning techniques have attempted to assess the regurgitant flow. These techniques either do not directly assess the complete flow through the MV, or they do not measure the flow at the location of the valve. AIM: To investigate the accuracy of a novel method using three-directional velocity-encoded MRI to acquire the transvalvular blood flow directly from the intraventricular blood flow field, also representing the regurgitant flow during systole. METHODS: Ten volunteers without cardiac valvular disease were recruited. The transvalvular MV flow volume was measured with three-directional velocity-encoded MRI (3-dir MV flow). RESULTS: The transvalvular flow measurements correlate very well with the flow measured in the aorta (rp=0.92, p<0.01). The small differences (mean -5±7 ml) are insignificant (p=0.06) and demonstrate the high accuracy of the new method. Intra- and inter-observer studies showed non-significant mean differences of 0.9±5.1 ml and 1.3±5.6 ml, respectively, thereby proving the high reproducibility. CONCLUSION: Three-directional velocity-encoded MRI is a patient-friendly and easy-to-use method suitable for quantifying the regurgitant MV flow in clinical practice.

Automated segmentation and analysis of vascular structures in magnetic resonance angiographic images.

The accurate assessment of the presence and extent of vascular disease, and planning of vascular interventions based on MRA requires the determination of vessel dimensions. The current standard is based on measuring vessel diameters on maximum intensity projections (MIPs) using calipers. In order to increase the accuracy and reproducibility of the method, automated analysis of the 3D MR data is required. A novel method for automatically determining the trajectory of the vessel of interest, the luminal boundaries, and subsequent the vessel dimensions is presented. The automated segmentation in 3D uses deformable models, combined with knowledge of the acquisition protocol. The trajectory determination was tested on 20 in vivo studies of the abdomen and legs. In 93% the detected trajectory followed the vessel. The luminal boundary detection was validated on contrast-enhanced (CE) MRA images of five stenotic phantoms. The results from the automated analysis correlated very well with the true diameters of the phantoms used in the in vitro study (r = 0.999, P < 0.001). MRA and x-ray angiography (XA) of the phantoms also correlated well (r = 0.895, P < 0.001). The average unsigned difference between the MRA and XA measurements was 0.08 +/- 0.05 mm. In conclusion, the automated approach allows the accurate assessment of vessel dimensions in MRA images.

Validation of the coupling of magnetic resonance imaging velocity measurements with computational fluid dynamics in a U bend.

Magnetic resonance imaging (MRI) can be used in vivo in combination with computational fluid dynamics (CFD) to derive velocity profiles in space and time and accordingly, pressure drop and wall shear stress distribution in natural or artificial vessel segments. These hemodynamic data are difficult or impossible to acquire directly in vivo. Therefore, research has been performed combining MRI and CFD for flow simulations in flow phantoms, such as bends or anastomoses, and even in human vessels such as the aorta, the carotid, and the abdominal bifurcation. There is, however, no unanimity concerning the use of MRI velocity measurements as input for the inflow boundary condition of a CFD simulation. In this study, different input possibilities for the inflow boundary conditions are compared. MRI measurements of steady and pulsatile flow were performed on a U bend phantom, representing the aorta geometry. PAMFLOW (ESI Software, Krimpen aan den Ussel, The Netherlands), an industrial CFD software package, was used to solve the Navier-Stokes equations for incompressible flow. Three main parameters were found to influence the choice of an inflow boundary condition type. First, the flow rate through a vessel should be exact, since it proves to be a determining factor for the accuracy of the velocity profile. The other decisive parameters are the physiology of the flow profile and the required computer processing unit time. Our comparative study indicates that the best way to handle an inflow boundary condition is to use the velocities measured by MRI at the inflow plane as being fixed velocities. However, before using these MRI velocity data, they first should be corrected for the partial volume effect by filtering and second scaled in order to obtain the correct flow rate. This implies that a reliable flow rate measurement absolutely is needed for CFD calculations based on MRI velocity measurements.

Vessel diameter measurements in gadolinium contrast-enhanced three-dimensional MRA of peripheral arteries.

In this study, the possibilities for quantification of vessel diameters of peripheral arteries in gadolinium contrast-enhanced magnetic resonance angiography (Gd CE MRA) were evaluated. Absolute vessel diameter measurements were assessed objectively and semi-automatically in maximum intensity projections (MIPs) of contrast-enhanced T1-weighted 3D spoiled gradient-echo datasets, studied with digital subtraction techniques. In vivo, the complete peripheral arterial bed of six patients was studied, from the aorto-iliac bifurcation down to the distal run-off. By measuring the signal intensity (SI) over the lumen of a vessel in the MIP, an SI-plot was obtained. Next, the vessel boundaries were determined using a threshold algorithm; from these boundary points individual diameter values could be obtained along the trajectory of the vessel. In an in vitro study, an optimal threshold value of 30% of the range of SI-values between the background and the maximal SI in the vessel was obtained for accurate diameter measurement in Gd CE MRA (i.e., full-width 30%-maximum). Furthermore, the relationship between the accuracy of these measurements and the scan resolution was investigated. Accuracy was found to be acceptable (i.e., less than 10% over/underestimation) for vessel sizes covering at least 3 pixels. In six patients, diameters were measured in MIPs of the total datasets (i.e., D(T)) as well as in selective MIPs of the clipped datasets (i.e., D(S)) (n = 209). D(T) and D(S) were statistically significantly correlated (p < 0.01) with a Pearson correlation coefficient rP = 0.98. Measurements in the total MIPs yielded statistically significant (p < 0.01) smaller diameter values compared with measurements in selective MIPs, with a mean difference of 0.15 mm. Diameter values from the selective MIPs of the aorto-iliac arteries were also compared with diameter values measured at corresponding anatomic positions in X-ray angiograms of these patients (i.e., D(x)) (n = 70). D(X) and D(S) were statistically significantly correlated (p < 0.01) with a Pearson correlation coefficient rP = 0.92. Diameters measured in the selective MIPs were smaller than those measured in the X-ray angiograms (mean difference 0.49 mm) and this difference was statistically significant (p < 0.01). In conclusion, diameter values can be evaluated accurately in MIPs of vessels with at least 3 pixels in diameter, using the full-width 30%-maximum criterion.

Gadolinium contrast-enhanced three-dimensional MRA of peripheral arteries with multiple bolus injection: scan optimization in vitro and in vivo.

In this study, a scanning protocol was developed to image the arterial bed of the pelvis and both legs along their entire length in patients with peripheral arterial disease, using standard hard- and software. Three adjacent stations are acquired consecutively, with some small overlap; per station; one Gadolinium contrast bolus is administered. The scanning protocol was optimized in an in vitro phantom study. The optimal flip angle was found to be 50 degrees. Also, the optimal scan delay was chosen to be equal to the arrival time of the contrast bolus thereby minimizing artifacts. Three contrast bolus injections showed sufficient enhancement of the vessels after image subtraction. Finally, stenosis quantification by manual caliper was performed by five observers in the MRA images and correlated with the percent diameter reduction determined by quantitative angiography from corresponding X-ray images. The results of the MRA measurements were reproducible and intra- and inter-observer variabilities were statistically non-significant (p = 0.54 and p = 0.12, respectively). Stenosis quantification performed by four observers showed a good correlation with the X-ray derived values (rp > 0.90, p < 0.02); the results from one observer were not significantly correlated. Five patients with proven peripheral disease were investigated with this new MRA scanning protocol. The images were of good quality which allowed adequate clinical evaluation; the original diagnoses obtained from X-ray examinations, were confirmed with MRA. In conclusion, peripheral arterial disease can be evaluated adequately with this MR scanning protocol.

Scan optimization of gadolinium contrast-enhanced three-dimensional MRA of peripheral arteries with multiple bolus injections and in vitro validation of stenosis quantification.

In this study, a T1-weighted three-dimensional (3D) spoiled gradient-echo scanning protocol was developed to image the complete arterial system of the pelvis and both legs along their entire length in patients with peripheral arterial disease. Three adjacent stations were to be acquired consecutively, with some overlap, to image the entire area of interest; per station one gadolinium (Gd) contrast bolus would be administered. In an in vitro phantom study, the scanning protocol was optimized. The optimal flip angle was found to be 50 degrees. Also, the optimal scan delay was chosen to be equal to the arrival time of the contrast bolus, thereby minimizing artifacts. Three contrast bolus injections showed sufficient enhancement of the vessels after image subtraction. Finally, stenosis quantification by manual caliper was performed by five observers in the magnetic resonance angiography (MRA) images and correlated with the percent diameter reduction determined by quantitative angiography from corresponding X-ray images. The MRA measurements were reproducible, and intra- and interobserver variabilities were statistically non-significant (p=0.54 and p=0.12, respectively). Stenosis quantification performed by four observers showed a good correlation with the X-ray-derived values (rp > 0.90, p < 0.02); the results from one observer were not significantly correlated. Five patients with proven peripheral disease were investigated with this new MRA scanning protocol, using standard hardware and software. The images were of good quality, which allowed adequate clinical evaluation; the original diagnoses obtained from X-ray examinations, were confirmed with MRA. In conclusion, peripheral arterial disease can be evaluated adequately with this magnetic resonance scanning protocol.