Evaluation of the precision of magnetic resonance phase velocity mapping for blood flow measurements.

Evaluating the in vivo accuracy of magnetic resonance phase velocity mapping (PVM) is not straightforward because of the absence of a validated clinical flow quantification technique. The aim of this study was to evaluate PVM by investigating its precision, both in vitro and in vivo, in a 1.5 Tesla scanner. In the former case, steady and pulsatile flow experiments were conducted using an aortic model under a variety of flow conditions (steady: 0.1-5.5 L/min; pulsatile: 10-75 mL/cycle). In the latter case, PVM measurements were taken in the ascending aorta of ten subjects, seven of which had aortic regurgitation. Each velocity measurement was taken twice, with the slice perpendicular to the long axis of the aorta. Comparison between the measured and true flow rates and volumes confirmed the high accuracy of PVM in measuring flow in vitro (p > 0.85). The in vitro precision of PVM was found to be very high(steady: y = 1.00x + 0.02, r = 0.999; pulsatile: y = 0.98x + 0.72, r = 0.997; x: measurement #1, y: measurement #2) and this was confirmed by Bland-Altman analysis. Of great clinical significance was the high level of the in vivo precision (y = 1.01x - 0.04, r = 0.993), confirmed statistically (p = 1.00). In conclusion, PVM provides repeatable blood flow measurements. The high in vitro accuracy and precision, combined with the high in vivo precision, are key factors for the establishment of PVM as the "gold-standard" to quantify blood flow.

In vivo flow dynamics of the total cavopulmonary connection from three-dimensional multislice magnetic resonance imaging.

BACKGROUND: The total cavopulmonary connection (TCPC) design continues to be refined on the basis of flow analysis at the connection site. These refinements are of importance for myocardial energy conservation in the univentricular supported circulation. In vivo magnetic resonance phase contrast imaging provides semiquantitative flow visualization information. The purpose of this study was to understand the in vivo TCPC flow characteristics obtained by magnetic resonance phase contrast imaging and compare the results with our previous in vitro TCPC flow experiments in an effort to further refine TCPC surgical design. METHODS: Twelve patients with TCPC underwent sedated three-dimensional, multislice magnetic resonance phase contrast imaging. Seven patients had intraatrial lateral tunnel TCPC and 5 had extracardiac TCPC. RESULTS: In all patients in both groups a disordered flow pattern was observed in the inferior caval portion of the TCPC. Flow at the TCPC site appeared to be determined by connection geometry, being streamlined at the superior vena cava-pulmonary junction when the superior vena cava was offset and flared toward the left pulmonary artery. Without caval offset, intense swirling and dominance of superior vena caval flow was observed. In TCPC with bilateral superior vena cavae, the flow patterns observed included secondary vortices, a central stagnation point, and influx of the superior vena cava flow into the inferior caval conduit. A comparative analysis of in vivo flow and our previous in vitro flow data from glass model prototypes of TCPC demonstrated significant similarities in flow disturbances. Three-dimensional magnetic resonance phase contrast imaging in multiple coronal planes enabled a comprehensive semiquantitative flow analysis. The data are presented in traditional instantaneous images and in animated format for interactive display of the flow dynamics. CONCLUSIONS: Flow in the inferior caval portion of the TCPC is disordered, and the TCPC geometry determines flow characteristics.

Fluid mechanic assessment of the total cavopulmonary connection using magnetic resonance phase velocity mapping and digital particle image velocimetry.

The total cavopulmonary connection (TCPC) is currently the most promising modification of the Fontan surgical repair for single ventricle congenital heart disease. The TCPC involves a surgical connection of the superior and inferior vena cavae directly to the left and right pulmonary arteries, bypassing the right heart. In the univentricular system, the ventricle experiences a workload which may be reduced by optimizing the cavae-to-pulmonary anastomosis. The hypothesis of this study was that the energetic efficiency of the connection is a consequence of the fluid dynamics which develop as a function of connection geometry. Magnetic resonance phase velocity mapping (MRPVM) and digital particle image velocimetry (DPIV) were used to evaluate the flow patterns in vitro in three prototype glass models of the TCPC: flared zero offset, flared 14 mm offset, and straight 21 mm offset. The flow field velocity along the symmetry plane of each model was chosen to elucidate the fluid mechanics of the connection as a function of the connection geometry and pulmonary artery flow split. The steady flow experiments were conducted at a physiologic cardiac output (4 L/min) over three left/right pulmonary flow splits (70/30, 50/50, and 30/70) while keeping the superior/inferior vena cavae flow ratio constant at 40/60. MRPVM, a noninvasive clinical technique for measuring flow field velocities, was compared to DPIV, an established in vitro fluid mechanic technique. A comparison between the results from both techniques showed agreement of large scale flow features, despite some discrepancies in the detailed flow fields. The absence of caval offset in the flared zero offset model resulted in significant caval flow collision at the connection site. In contrast, offsetting the cavae reduced the flow interaction and caused a vortex-like low velocity region between the caval inlets as well as flow disturbance in the pulmonary artery with the least total flow. A positive correlation was also found between the direct caval flow collision and increased power losses. MRPVM was able to elucidate these important fluid flow features, which may be important in future modifications in TCPC surgical designs. Using MRPVM, two- and three-directional velocity fields in the TCPC could be quantified. Because of this, MRPVM has the potential to provide accurate velocity information clinically and, thus, to become the in vivo tool for TCPC patient physiological/functional assessment.

Toward designing the optimal total cavopulmonary connection: an in vitro study.

BACKGROUND: Understanding the total cavopulmonary connection (TCPC) hemodynamics may lead to improved surgical procedures which result in a more efficient modified circulation. Reduced energy loss will translate to less work for the single ventricle and although univentricular physiology is complex, this improvement could contribute to improved postoperative outcomes. Therefore to conserve energy, one surgical goal is optimization of the TCPC geometry. In line with this goal, this study investigated whether addition of caval curvature or flaring at the connection conserves energy. METHODS: TCPC models were made varying the curvature of the caval inlet or by flaring the anastomosis. Steady flow pressure measurements were made to calculate the power loss attributed to each connection design over a range of pulmonary flow splits (70:30 to 30:70). Particle flow visualization was performed for each design and was qualitatively compared to the power losses. RESULTS: Results indicate that curving the cavae toward one pulmonary artery is advantageous only when the flow rate from that cavae matches the flow to the pulmonary artery. Under other pulmonary flow split conditions, the losses in the curved models are significant. In contrast, fully flaring the anastomosis reduced losses over the range of pulmonary flow splits. Power losses were 56% greater for the curving as compared to flaring. Fully flaring without caval offset reduced losses 45% when compared to previous models without flaring. If flaring on all sides was implemented with caval offset, power losses reduced 68% compared to the same nonflared model. CONCLUSIONS: The results indicate that preferentially curving the cavae is only optimal under specific pulmonary flow conditions and may not be efficient in all clinical cases. Flaring of the anastomosis has great potential to conserve energy and should be considered in future TCPC procedures.

Quantification of mitral regurgitation with MR phase-velocity mapping using a control volume method.

Reliable diagnosis and quantification of mitral regurgitation are important for patient management and for optimizing the time for surgery. Previous methods have often provided suboptimal results. The aim of this in vitro study was to evaluate MR phase-velocity mapping in quantifying the mitral regurgitant volume (MRV) using a control volume (CV) method. A number of contiguous slices were acquired with all three velocity components measured. A CV was then selected, encompassing the regurgitant orifice. Mass conservation dictates that the net inflow into the CV should be equal to the regurgitant flow. Results showed that a CV, the boundary voxels of which excluded the region of flow acceleration and aliasing at the orifice, provided accurate measurements of the regurgitant flow. A smaller CV provided erroneous results because of flow acceleration and velocity aliasing close to the orifice. A large CV generally provided inaccurate results because of reduced velocity sensitivity far from the orifice. Aortic outflow, orifice shape, and valve geometry did not affect the accuracy of the CV measurements. The CV method is a promising approach to the problem of quantification of the MRV.

Fluid dynamic studies for the year 2000.

The authors recommend changes to the paradigm employed in the current ISO Heart Valve Standard (ISO 5840) so that future patients who receive a heart valve prosthesis are assured of a device that will function with minimal complications for at least 25 years. Based on valve failures of the past decade, it is clear that current standards are inadequate because present-day Standards and Regulatory Agencies operate in a manner which inhibits innovation and creativity. Thus, engineers and scientists in this field react to problems, rather than proact. As we approach the new millennium, the authors consider it time to rethink the ground rules.

Quantification of the aortic regurgitant volume with magnetic resonance phase velocity mapping: a clinical investigation of the importance of imaging slice location.

BACKGROUND AND AIMS OF THE STUDY: Current techniques for assessment of aortic regurgitation (AR) are mainly qualitative. Magnetic resonance phase velocity mapping (PVM) provides accurate measurements of arterial blood blow. In AR, the aortic regurgitant volume (ARV) can be quantified with a single imaging slice measurement in the ascending aorta. The aim was to use PVM to: (i) quantify the regurgitant volume in patients with AR using an in vitro validated technique; and (ii) confirm in vivo our previous in vitro findings of the importance of measurement location. METHODS: Four healthy volunteers and 19 patients with AR, varying from mild to severe, were examined in a 1.5 Tesla MRI scanner. In 13 patients, the slice was placed: (i) between the aortic valve and the coronary ostia; (ii) at the sinotubular junction (SJ); and (iii) 2 cm above the SJ. In six patients, one measurement was taken as close as technically possible to the aortic valve. PVM measurements of the ARV were compared with angiographic/echocardiographic AR grading. RESULTS: No ARV was measured in healthy subjects. In patients, PVM results correlated well with angiographic/echocardiographic data. Repeatability of the PVM results was excellent and interobserver variability very small. The measured ARV decreased as the slice distance from the aortic valve increased, due to aortic compliance, in agreement to previous in vitro results. Close to the valve, acceleration did not affect the accuracy of velocity measurements. CONCLUSIONS: PVM has great potential to measure AR in a purely quantitative manner. Measurement location is important and results suggest that the closer the measurement to the valve the more accurate the ARV quantification.

The importance of slice location on the accuracy of aortic regurgitation measurements with magnetic resonance phase velocity mapping.

Although several methods have been used clinically to evaluate the severity of aortic regurgitation, there is no purely quantitative approach for aortic regurgitant volume (ARV) measurements. Magnetic resonance phase velocity mapping can be used to quantify the ARV, with a single imaging slice in the ascending aorta, from through-slice velocity measurements. To investigate the accuracy of this technique, in vitro experiments were performed with a compliant model of the ascending aorta. Our goals were to study the effects of slice location on the reliability of the ARV measurements and to determine the location that provides the most accurate results. It was found that when the slice was placed between the aortic valve and the coronary ostia, the measurements were most accurate. Beyond the coronary ostia, aortic compliance and coronary flow negatively affected the accuracy of the measurements, introducing significant errors. This study shows that slice location is important in quantifying the ARV accurately. The higher accuracy achieved with the slice placed between the aortic valve and the coronary ostia suggests that this slice location should be considered and thoroughly examined as the preferred measurement site clinically.

Slice location dependence of aortic regurgitation measurements with MR phase velocity mapping.

Although several methods have been used clinically to assess aortic regurgitation (AR), there is no "gold standard" for regurgitant volume measurement. Magnetic resonance phase velocity mapping (PVM) can be used for noninvasive blood flow measurements. To evaluate the accuracy of PVM in quantifying AR with a single imaging slice in the ascending aorta, in vitro experiments were performed by using a compliant aortic model. Attention was focused on determining the slice location that provided the best results. The most accurate measurements were taken between the aortic valve annulus and the coronary ostia where the measured (Y) and actual (X) flow rate had close agreement (Y = 0.954 x + 0.126, r2 = 0.995, standard deviation of error = 0.139 L/min). Beyond the coronary ostia, coronary flow and aortic compliance negatively affected the accuracy of the measurements. In vivo measurements taken on patients with AR showed the same tendency with the in vitro results. In making decisions regarding patient treatment, diagnostic accuracy is very important. The results from this study suggest that higher accuracy is achieved by placing the slice between the aortic valve and the coronary ostia and that this is the region where attention should be focused for further clinical investigation.

The influence of acoustic impedance mismatch on post-stenotic pulsed-Doppler ultrasound measurements in a coronary artery model.

Acoustic impedance mismatch at the fluid-wall interface was shown to affect the spectra from an intravascular Doppler device in an in vitro model with a diameter typical of human coronary arteries. Measurements were obtained first under Poiseuille flow conditions with impedance mismatches of 0%, 7% and 12%, and then under stenosed conditions for the 0% and 7% mismatch cases. For the zero mismatch case, the Doppler spectra could be readily interpreted in terms of fluid mechanical phenomena. When mismatch was present, the spectra from Poiseuille flow exhibited multiple peaks which could not be directly related to the velocity profile. Also, the spectra from stenosed flow with a mismatch of 7% were similar to those from the zero mismatch case but did not exhibit the specific flow-related features as clearly. These results indicate that the impedance mismatch alters the acoustic environment inside the model and that this causes artifact in the Doppler spectra.