Quantification of coronary artery bypass graft flow by magnetic resonance phase velocity mapping

Analysis of Computational Fluid Dynamics and Particle Image Velocimetry Models of Distal-End Side-to-Side and End-to-Side Anastomoses for Coronary Artery Bypass Grafting in a Pulsatile Flow

BACKGROUND

Intimal hyperplasia (IH) is a major cause of graft failure. Hemodynamic factors such as stagnation and disturbed blood flow are involved in IH formation. The aim of this study is to perform a comparative analysis of distal-end side-to-side (deSTS) and end-to-side (ETS) anastomoses using computational fluid dynamics (CFD) after validating the results via particle image velocimetry (PIV).Methods and Results:We investigated the characteristics of our target flow fields using CFD under steady and pulsatile flows. CFD via PIV under steady flow in a 10-times-actual-size model was validated. The CFD analysis revealed a recirculation zone in the heel region in the deSTS and ETS anastomoses and at the distal end of the graft, and just distal to the toe of the host artery in the deSTS anastomoses. The recirculation zone sizes changed with the phase shift. We found regions of low wall shear stress and high oscillating shear index in the same areas. The PIV and CFD results were similar.

CONCLUSIONS

It was demonstrated that the hemodynamic characteristics of CFD and PIV is the difference between the deSTS and ETS anastomoses; that is, the deSTS flow peripheral to the distal end of the graft, at the distal end and just distal to the toe of the host artery is involved in the IH formation.

CABG MODELS FLOW SIMULATION STUDY ON THE EFFECTS OF VALVE REMNANTS IN THE VENOUS GRAFT

Venous valves and sinuses are frequently observed in vein grafts in the coronary artery bypass grafts (CABG). However, from the biomedical engineering viewpoint, vein grafts are always assumed as smooth tubes in the existing simulations, and no effort has been made to investigate the effects of jaggedness of the graft inner wall due to the valve cusps remnants and valve sinus (in case of valve-stripped saphenous vein (SV) grafts) on the blood flow patterns and hemodynamic parameters (HPs). In this paper, the effects of the inner surface irregularities of a vein graft on the blood flow is investigated in the graft as well as in the distal anastomotic region, with a more realistic geometry of valve-stripped SV, by means of numerical simulation of pulsatile, Newtonian blood flow. The simulation results demonstrate that the valve remnants and sinuses cause disturbances in the flow field within the graft (due to vortices formation within the valve sinuses) and undesirable distribution of HPs, which can result in early atherosclerotic lesion development in the graft.

Compliant model of a coupled sequential coronary arterial bypass graft: effects of vessel wall elasticity and non-Newtonian rheology on blood flow regime and hemodynamic parameters distribution

We have recently developed a novel design for coronary arterial bypass surgical grafting, consisting of coupled sequential side-to-side and end-to-side anastomoses. This design has been shown to have beneficial blood flow patterns and wall shear stress distributions which may improve the patency of the CABG, as compared to the conventional end-to-side anastomosis. In our preliminary computational simulation of blood flow of this coupled sequential anastomoses design, the graft and the artery were adopted to be rigid vessels and the blood was assumed to be a Newtonian fluid. Therefore, the present study has been carried out in order to (i) investigate the effects of wall compliance and non-Newtonian rheology on the local flow field and hemodynamic parameters distribution, and (ii) verify the advantages of the CABG coupled sequential anastomoses design over the conventional end-to-side configuration in a more realistic bio-mechanical condition. For this purpose, a two-way fluid-structure interaction analysis has been carried out. A finite volume method is applied to solve the three-dimensional, time-dependent, laminar flow of the incompressible, non-Newtonian fluid; the vessel wall is modeled as a linearly elastic, geometrically non-linear shell structure. In an iteratively coupled approach the transient shell equations and the governing fluid equations are solved numerically. The simulation results indicate a diameter variation ratio of up to 4% and 5% in the graft and the coronary artery, respectively. The velocity patterns and qualitative distribution of wall shear stress parameters in the distensible model do not change significantly compared to the rigid-wall model, despite quite large side-wall deformations in the anastomotic regions. However, less flow separation and reversed flow is observed in the distensible models. The wall compliance reduces the time-averaged wall shear stress up to 32% (on the heel of the conventional end-to-side model) and somewhat increases the oscillatory nature of the flow. It is found that the effects of wall compliance and non-Newtonian rheology are not independent, and they interact with each other. In spite of the modest influence of wall compliance and non-Newtonian rheology on the hemodynamic parameters distribution, the inclusion of these properties has unveiled further advantages of the coupled sequential anastomoses model over the conventional end-to-side anastomosis which had not been revealed in the previous study with the rigid-wall and Newtonian fluid models. Hence, the inclusion of wall compliance and non-Newtonian rheology in flow simulation of blood vessels can be essential in quantitative and comparative investigations.

Comparison of arterial and venous coronary artery bypass flow measurements using 3-T magnetic resonance phase contrast imaging

OBJECTIVE

Comparison of arterial and venous coronary artery bypass flow measurements using 3-T magnetic resonance (MR) phase contrast in correlation with intraoperative Doppler flow measurements.

METHODS

Fifty-six coronary bypasses (right coronary artery n=18, left internal mammary artery to left anterior descending artery n=16, marginal artery n=7, circumflex artery n=7, diagonal artery n=6, left anterior descending artery n=1, and right internal mammary artery to right coronary artery n=1) were studied in 27 asymptomatic patients. In this prospective study, each bypass was studied intra-operatively using Doppler flow measurement. Within one week post surgery, patients were studied using a 3-T MR scanner (Magnetom Verio, Siemens, Erlangen, Germany) using velocity encoded phase-contrast flow measurements.

RESULTS

Intraoperative Doppler flow measurements demonstrated regular flow patterns in all vascular territories supplied. All bypasses were patent on MRI and flow measurement results were as follows: median flow 60ml/min (interquartile range (IQR): 37.5-78.5ml/min). For comparison, the corresponding median intraoperative flow was 58ml/min (IQR: 41-80ml/min) (p<0.001; R=0.44). Linear regression analysis demonstrated a significant correlation for venous bypasses (p=0.0002; R=0.48), but not for arterial bypasses (p=0.09; R=0.24).

CONCLUSION

This study demonstrated that MR flow measurements of venous bypass grafts agreed more with Doppler than arterial bypass grafts. However, bypass patency was confirmed for all patients. In the future, this technique may be used for non invasive coronary bypass graft follow-up.

Perioperative monitoring of flow and patency in native and grafted internal mammary arteries using a combined MR protocol

The objective of this study was to evaluate graft flow (f) and patency (p) in patients with internal mammary artery (f,p) and venous (p only) grafts using a combined MR protocol with phase-contrast technique and MR angiography. 42 patients with 42 left internal mammary artery (LIMA) and 63 venous grafts were examined pre and 6 months post coronary artery bypass graft (CABG) surgery. Phase-contrast flow measurements were applied to the IMA. Post-operatively, a contrast enhanced MR angiogram was performed to assess bypass patency. LIMA/venous grafts were occluded in 3/42 and 13/63, respectively. Flow in LIMA decreased from 19.4+/-10.4 ml min(-1) m(-2) pre-operatively to 13.4+/-9.7 ml min(-1) m(-2) post-operatively (p<0.002). In contrast, flow in the native right IMA increased from 17.6+/-8.7 ml min(-1) m(-2) pre-operatively to 24.8+/-9.0 ml min(-1) m(-2) post-operatively (p<0.001). MRI allows a combined assessment of bypass patency and flow. This study protocol may be applicable to perioperative follow-up studies in patients after CABG surgery.

Assessment of minimally invasive direct coronary artery bypass grafting of the left internal thoracic artery by means of magnetic resonance imaging

OBJECTIVES

We sought to evaluate graft patency, flow, and flow reserve in patients with minimally invasive direct coronary artery bypass surgery of internal thoracic artery grafts by a combined magnetic resonance protocol with a phase-contrast technique and magnetic resonance angiography.

METHODS

At 1.5 T (Magnetom Sonata, Siemens), 30 symptomatic patients with 30 left internal thoracic artery grafts were examined 6 years after minimally invasive surgical intervention. Navigator-gated magnetic resonance angiography and contrast-enhanced FLASH-3D magnetic resonance angiography (0.2 mmol gadopentate-diethylene triamine pentetic acid [Gd-DTPA]/kg body weight) was used to assess bypass patency. Phase-contrast flow measurements with retrospective gating were performed in the internal thoracic artery grafts at rest and after stress induction with dipyridamole (0.57 mg/kg body weight). Graft patency was evaluated by means of multidetector computed tomography (Sensation 16, Siemens).

RESULTS

Internal thoracic artery grafts were occluded in 5 of 30 patients. In 6 patients the anastomosis to the left anterior descending artery was highly stenotic (>70 % ) at multidetector computed tomography. In patients with regular grafts (multidetector computed tomography), a significant improvement of graft flow ( P < .001) and diastolic/systolic peak velocity ratio ( P < .001) after stress induction was detected. Magnetic resonance angiography combined with flow reserve measurements could differentiate between occluded-stenotic and regular minimally invasive direct coronary artery bypass grafts.

CONCLUSIONS

Magnetic resonance imaging allows a combined assessment of bypass patency and flow with flow reserve in patients after the minimally invasive direct coronary artery bypass operation. The protocol of this study might be applicable for the evaluation of graft status in symptomatic patients after revascularization.

Human Saphenous Vein Coronary Artery Bypass Graft Morphology, Geometry and Hemodynamics

Coronary artery bypass graft (CABG) failure has been linked to graft hemodynamics, in particular wall shear stress. This study characterizes the morphology, geometry and wall shear stress patterns in human CABGs. The intimal thickness (IT) in 49 human saphenous vein CABGs was measured by digital light microscopy. The geometry of six saphenous vein CABGs was replicated by post-mortem infusion of Batson's #17 anatomical corrosion casting compound. Graft hemodynamics were evaluated in two flow models, fabricated from the casts, under steady (Re = 110) and pulsatile flow (Re = 110, alpha = 2) conditions. Saphenous vein CABGs in situ for more than 2 months had, on average, the greatest IT on the hood and suture sites of the distal anastomosis. Floor thickening was highly variable and significantly less than IT at the hood, suture site and graft body. All casts showed an indentation along the floor and 5/6 casts displayed a sharp local curvature on the hood. In both flow models, a large increase in wall shear rate occurred on the hood, just proximal to the toe. The local geometry of the hood created this large spatial gradient in wall shear stress which is a likely factor in hood intimal hyperplasia.

MR imaging in ischemic heart disease

This article reviews the current MR imaging literature with respect to ischemic heart disease and focuses on the clinical practicalities of cardiac MR imaging today.

Assessment of coronary artery bypass graft disease using cardiovascular magnetic resonance determination of flow reserve

OBJECTIVES

The purpose of this study was to assess the value of cardiovascular magnetic resonance (CMR)-determined graft flow and flow reserve in differentiating significant from non-significant vein graft disease.

BACKGROUND

In patients after coronary artery bypass grafting (CABG), non-invasive testing may be helpful in the detection of recurrent graft disease.

METHODS

Randomly selected patients (n = 21) scheduled for X-ray angiography because of recurrent chest complaints after CABG were included for evaluation of vein grafts (n = 40) by CMR. Three-dimensional contrast-enhanced CMR angiography was performed and followed by flow measurements at rest and during hyperemia in patent grafts only. Flow reserve was calculated when resting flow exceeded 20 ml/min. Analysis was based on four categories defined by X-ray angiography: occluded grafts (n = 3), grafts with stenosis >50% (n = 19), grafts with stenosis <50% with diseased graft run-off (n = 8), and grafts with stenosis <50% and normal run-off (n = 10).

RESULTS

The CMR angiography demonstrated occlusion of three grafts. In nine of the 37 patent grafts, basal blood flow was <20 ml/min, all demonstrating significant stenosis at X-ray angiography. In grafts with resting flow >20 ml/min (n = 28), flow reserve significantly differed between grafts without stenosis and grafts with significant stenosis or with diseased run-off (2.5 +/- 0.7 vs. 1.8 +/- 0.9, p = 0.04). An algorithm combining basal volume flow <20 ml/min and graft flow reserve <2 had a sensitivity and specificity of 78% and 80% respectively for detecting grafts with significant stenosis or diseased run-off.

CONCLUSIONS

This feasibility study showed that quantification of flow and flow reserve by CMR may serve as a non-invasive adjunct to differentiate between vein grafts without stenosis and grafts with significant stenosis or diseased run-off.

Does the application of gadolinium-DTPA have an impact on magnetic resonance phase contrast velocity measurements? Results from an in vitro study

INTRODUCTION/OBJECTIVE

To evaluate the potential influence of various concentrations of gadolinium (Gd)-DTPA on magnetic resonance phase contrast (MR PC) velocimetry.

MATERIAL AND METHODS

Imaging was done with a 1.0 T scanner using a standard Flash 2D sequence and a circular polarized extremity coil. In a validated flow phantom with a defined 75% area stenosis different concentrations of Gd-DTPA, diluted in a 10:1 water-yogurt mixture, MR PC measurements were correlated with a Doppler guide wire as gold standard.

RESULTS

MR PC measurements correlated well with the Doppler derived data (r=0.99; P<0.01; maximum pre-stenotic velocity: 21.6+/-0.5 cm/s; maximum intra-stenotic velocity: 81.7+/-0.6 cm/s). Following Gd-DTPA administration no significant (P>0.05; Student's t-test) flow measurement changes were noted (maximum pre-stenotic velocity: 21.3+/-1.3 cm/s; maximum intra-stenotic velocity: 84.0+/-3.6 cm/s). However, delineation of the perfused lumen was enhanced after the application of Gd-DTPA.

DISCUSSIONS AND CONCLUSION

The application of Gd-DTPA does not affect MR PC velocimetry. However, the application of contrast media allowed a more accurate vessel segmentation. MR PC measurements can be reliably carried out after application of Gd-DTPA.

Contrast-enhanced magnetic resonance angiography of mammary artery grafts after minimally invasive coronary bypass surgery

BACKGROUND

The aim of the study was to investigate the application of the contrast-enhanced magnetic resonance angiography (CE-MRA) for the visualization of left internal mammary artery (LIMA) bypass.

METHODS

A total of 30 patients with LIMA bypass (22 men, 8 women, 35 to 77 years) received a CE-MRA 4 to 20 days after surgery. The non-ECG-triggered CE-MRA was performed during expiration using a body array coil at a 1.5 Tesla scanner (Magnetom-Vision). A three-dimensional gradient-echo sequence with slice interpolation technique was applied. For the three-dimensional visualization, single coronal slices were postprocessed with maximal intensity projection. Of 30 patients 22 agreed to a comparative coronary angiography.

RESULTS

Five bypasses were identified up to the end-to-side anastomosis. A total of 80% of the bypass course was detectable in 13 patients and 60% in 11 patients. In two LIMA bypasses only 30% of the proximal part could be viewed; one was found by conventional coronary angiography to be occluded. The other conventional coronary angiography showed the LIMA bypass to be patent.

CONCLUSIONS

The complete course of the LIMA bypass to the left anterior descending coronary artery can be visualized by the MRA technique. The most reliable imaging of the distal anastomosis can be realized by reducing the negative influence of the beating heart.

Morphologic and functional evaluation of coronary artery bypass conduits

There is clear evidence in the literature that conventional spin-echo and gradient-echo magnetic resonance imaging (MRI) is capable of assessing patency of coronary artery vein grafts. With more recently introduced breath-hold two-dimensional (2D) and contrast-enhanced 3D techniques, the predictive accuracy has further improved, with sensitivities and specificities in the 90% range. Limitations arise with regard to assessing obstructive disease and evaluating distal segments of sequential grafts, due to insufficient spatial resolution, low signal-to-noise ratio, and cardiac motion. Imaging of arterial grafts is complicated by the metallic clip artifacts. Adding information on graft flow patterns and flow reserve using velocity-encoded cine MRI may help to reduce some of the problems. Clinically, these functional measurements may become of use in non-invasive monitoring of gradually increasing graft narrowing. However, apart from a few exceptions, most patients undergo evaluation of their grafts because they are considered for a re-intervention by angioplasty or coronary artery bypass graft surgery. In these cases information on the status of the native coronary arteries is required. A broader clinical use of MRI in the evaluation of patients with coronary artery bypass grafts may therefore only be expected with further improvement in MR techniques for coronary angiography. J. Magn. Reson. Imaging 1999;10:734-740.

Evaluation of coronary artery bypass grafts by magnetic resonance imaging

Magnetic resonance (MR) angiography and flow mapping have the potential to become a major noninvasive diagnostic tool for the assessment of coronary artery bypass graft morphology and function. Several MR sequences, such as conventional non-respiratory compensated methods, and phase contrast cine flow sequences have been reported for the evaluation of bypass graft patency. However the visualization of different graft segments and the detection of graft stenosis remains difficult. Recent advances in MR coronary angiography and flow mapping are volume coronary angiongraphy with targeted scans, navigator gated angiography, contrast-enhanced angiography, and breath-hold or navigator gated flow sequences. Future approaches, such as navigator gated fast MR techniques resulting in high-resolution angiography in combination with breath-hold MR flow mapping with high temporal resolution, might allow a comprehensive evaluation of bypass graft stenosis and function. This review article will address the major issues concerning the MR evaluation of bypass grafts.

Noninvasive coronary artery imaging in the diagnosis and management of patients with ischemic heart disease

The inherent limitations of x-ray coronary angiography have led to the development for both noninvasive and minimally invasive techniques for imaging the coronary arteries to assist in the diagnosis and management of patients with ischemic heart disease. Significant advances in transesophageal echocardiography, electron beam computed tomography, and magnetic resonance imaging now permit imaging of the proximal to mid-coronary arteries. Moreover, results of initial studies demonstrate the promise of these methods to detect coronary artery stenoses. In addition, each of these methods provides biochemical or physiologic data about the stenoses that are not obtainable through x-ray angiography. Quantification of coronary calcification via electron beam computed tomography has shown promise as a surrogate marker of coronary atherosclerosis. Transesophageal echocardiography and magnetic resonance imaging appear useful in evaluating the physiologic significance of angiographically detectable coronary artery stenoses via assessment of coronary blood flow. However, it should be noted that significant improvements in technology or acquisition parameters must occur before these techniques can be used on a routine clinical basis for coronary artery imaging. The relative merits and ultimate clinical potential of each of these techniques are discussed in this article.

Evaluation of coronary bypass flow with color-Doppler and magnetic resonance imaging techniques: comparison with intraoperative flow measurements

OBJECTIVES

After coronary artery bypass surgery, patency and flow assessment is based on invasive methods such as angiography and intravascular ultrasound or flow wire techniques. The aim of the study was to compare intraoperative transit time flow measurements of coronary bypass grafts with early postoperative color-Doppler and MR-imaging assessment.

METHODS

In 22 patients (62+/-8.5 years) undergoing elective coronary bypass surgery the flow was measured in all internal mammary artery grafts (IMA) and saphenous vein grafts using the transit time flow technique. Postoperatively (days 5-7) all patients had a color-Doppler IMA graft assessment followed by a MR-angiography and flow measurement (navigator echo phase contrast technique with and without contrast bolus application) to determine patency and graft flow.

RESULTS

Data are expressed as the mean +/- SD). (1) In all patients the left IMA graft to the left anterior descending coronary artery (LAD) could be identified and flow could be assessed with both color-Doppler and MRI. Venous grafts could only be visualized by MRI. The use of an intravenous contrast bolus enhanced the visualization of coronary artery bypass grafts. (2) The mean IMA to LAD flow was 33+/-17 ml/min intraoperatively by transit time and postoperatively 36+/-25 ml/min by MR respectively 66+/-54 ml/min by color-Doppler technique. (3) The systolic/diastolic flow ratio was 0.44+/-0.12 intraoperatively and 0.43+/-0.17 postoperatively by MR respectively 0.67+/-1.0 by color-Doppler. (4) A statistically significant correlation could be demonstrated between intraoperative transit time and postoperative MR flow measurements (r = 0.57; P < 0.04), whereas the correlations to color-Doppler flow were poor. Postoperatively MR and color-Doppler showed a good correlation of systolic/diastolic flow ratio (r = 0.88; P < 0.008).

CONCLUSIONS

The color-Doppler method during echocardiography and MR-imaging are useful non-invasive techniques to visualize postoperative IMA grafts for patency assessment. The quantification of IMA flow is still difficult with either technique, but MR flow measurements showed the best correlation to the intraoperatively measured transit time flow. The MR technique is the most promising non-invasive method for postoperative evaluation of coronary bypass grafts, since it allows visualization and reliable flow quantification.