After completion of MS/PhD from Mechanical Engineering, I've been with Division of Vascular Surgery at Stanford since 2011. I am motivated to apply Engineering-based techniques to patient anatomy or medical device. Also, I am actively teaching and training undergrad students, medical school students and trainees.
Adjunct Professor, Surgery - Vascular Surgery
- Quantification of motion of the thoracic aorta after ascending aortic repair of type-A dissection INTERNATIONAL JOURNAL OF COMPUTER ASSISTED RADIOLOGY AND SURGERY 2017; 12 (5): 811-819
Dynamic Geometric Analysis of the Renal Arteries and Aorta following Complex Endovascular Aneurysm Repair.
Annals of vascular surgery
Aneurysm regression and target vessel patency during early and mid-term follow-up may be related to the effect of stent-graft configuration on the anatomy. We quantified geometry and remodeling of the renal arteries and aneurysm following fenestrated (F-) or snorkel/chimney (Sn-) endovascular aneurysm repair (EVAR).Twenty-nine patients (mean age, 76.8 ± 7.8 years) treated with F- or Sn-EVAR underwent computed tomography angiography at preop, postop, and follow-up. Three-dimensional geometric models of the aorta and renal arteries were constructed. Renal branch angle was defined relative to the plane orthogonal to the aorta. End-stent angle was defined as the angulation between the stent and native distal artery. Aortic volumes were computed for the whole aorta, lumen, and their difference (excluded lumen). Renal patency, reintervention, early mortality, postoperative renal impairment, and endoleak were reviewed.From preop to postop, F-renal branches angled upward, Sn-renal branches angled downward (P < 0.05), and Sn-renals exhibited increased end-stent angulation (12 ± 15°, P < 0.05). From postop to follow-up, branch angles did not change for either F- or Sn-renals, whereas F-renals exhibited increased end-stent angulation (5 ± 10°, P < 0.05). From preop to postop, whole aortic and excluded lumen volumes increased by 5 ± 14% and 74 ± 81%, whereas lumen volume decreased (39 ± 27%, P < 0.05). From postop to follow-up, whole aortic and excluded lumen volumes decreased similarly (P < 0.05), leaving the lumen volume unchanged. At median follow-up of 764 days (range, 7-1,653), primary renal stent patency was 94.1% and renal impairment occurred in 2 patients (6.7%).Although F- and Sn-EVAR resulted in significant, and opposite, changes to renal branch angle, only Sn-EVAR resulted in significant end-stent angulation increase. Longitudinal geometric analysis suggests that these anatomic alterations are primarily generated early as a consequence of the procedure itself and, although persistent, they show no evidence of continued significant change during the subsequent postoperative follow-up period.
View details for DOI 10.1016/j.avsg.2016.12.005
View details for PubMedID 28390918
Quantification of motion of the thoracic aorta after ascending aortic repair of type-A dissection.
International journal of computer assisted radiology and surgery
To quantify cardiac and respiratory deformations of the thoracic aorta after ascending aortic graft repair.Eight patients were scanned with cardiac-resolved computed tomography angiography during inspiratory/expiratory breath-holds. Aortic centerlines and lumen were extracted to compute the arclength, curvature, angulation, and cross-section shape.From systole to diastole, the angle of graft [Formula: see text] arch increased by 2.4[Formula: see text] ± 1.8[Formula: see text] (P < 0.01) and the angle of arch [Formula: see text] descending aorta decreased by 2.4[Formula: see text] ± 2.6[Formula: see text] (P < 0.05), while the effective diameter of the proximal arch decreased by 2.4 ± 1.9% (P < 0.01), a greater change than those of the graft or distal arch (P < 0.05). From inspiration to expiration, the angle of graft [Formula: see text] arch increased by 2.8[Formula: see text] ± 2.6[Formula: see text] (P < 0.02) with the peak curvature increase (P < 0.05). Shorter graft length was correlated with greater cardiac-induced graft [Formula: see text] arch angulation, and longer graft length was correlated with greater respiratory-induced arch [Formula: see text] descending aorta angulation (R [Formula: see text] 0.50).The thoracic aorta changed curvature and angulation with cardiac and respiratory influences, driven by aortic root and arch motion. The thoracic aortic geometry and deformation are correlated with the ascending aortic graft length.
View details for PubMedID 27882488
Three-Dimensional Modeling Analysis of Visceral Arteries and Kidneys during Respiration.
Annals of vascular surgery
2016; 34: 250-260
Visceral arteries are commonly involved in endovascular repair of complex abdominal aortic aneurysms (AAAs). To improve repair techniques and reduce long-term complications involving visceral arteries, it is crucial to understand in vivo arterial geometry and the deformations due to visceral organ movement with respiration. This study quantifies deformation of the celiac, superior mesenteric (SMA), and renal arteries during respiration and correlates the deformations with diaphragmatic excursion.Sixteen patients with small AAAs underwent magnetic resonance angiography during inspiratory and expiratory breathholds. From geometric models of the aorta and visceral arteries, vessel length, branch angle, curvature, and positions were computed, along with degree of diaphragmatic excursion as indicated by kidney translation.From inspiration to expiration, the celiac artery exhibited axial shortening of 4.8 ± 6.4% (P < 0.001) and a mean curvature increase of 0.03 ± 0.02 mm(-1), greater than other visceral arteries (P < 0.01). With expiration, the SMA, left and right renal arteries (LRA and RRA) angled upward by -9.8 ± 6.4°, -6.4 ± 6.4°, and -5.2 ± 5.0°, respectively (P < 0.005). All vessels translated superiorly (P < 0.0005) and posteriorly (P < 0.01), and the SMA translated rightward additionally (P < 0.005). The left and right kidneys translated by 22 ± 9 mm and 21 ± 9 mm, mostly superiorly (P < 0.001). Translations of all visceral arteries were moderately correlated to the right kidney (R > 0.50). Correlation of the LRA with the left kidney was greater than that of the RRA with the right kidney.The celiac artery exhibited less branch angle change, and greater axial and curvature deformations than the other visceral arteries, due to the vicinity to the liver and influence of the median arcuate ligament. Correlation between visceral arteries and kidney translations revealed that diaphragmatic excursion affects vessel mobility. Weaker correlation of the RRA to the right kidney indicates mechanical shielding from the inferior vena cava.
View details for DOI 10.1016/j.avsg.2016.04.004
View details for PubMedID 27116907
View details for PubMedCentralID PMC4930742
- Comparative geometric analysis of renal artery anatomy before and after fenestrated or snorkel/chimney endovascular aneurysm repair JOURNAL OF VASCULAR SURGERY 2016; 63 (4): 922-929
Comparative geometric analysis of renal artery anatomy before and after fenestrated or snorkel/chimney endovascular aneurysm repair.
Journal of vascular surgery
2016; 63 (4): 922-929
The durability of stent grafts may be related to how procedures and devices alter native anatomy. We aimed to quantify and compare renal artery geometry before and after fenestrated (F-) or snorkel/chimney (Sn-) endovascular aneurysm repair (EVAR).Forty patients (75 ± 6 years) underwent computed tomographic angiography before and after F-EVAR (n = 21) or Sn-EVAR (n = 19), with a total of 72 renal artery stents. Renal artery geometry was quantified using three-dimensional model-based centerline extraction. The stented length was computed from the vessel origin to the stent end. The branch angle was computed relative to the orthogonal configuration with respect to the aorta. The end-stent angle was computed relative to the distal native renal artery. Peak curvature was defined as the inverse of the radius of the circumscribed circle at the highest curvature within the proximal portion from the origin to the stent end and the distal portion from the stent end to the first renal artery bifurcation.Sn-renals had greater stented length compared to F-renals (P < .05). From the pre- to the postoperative period, the origins of the Sn-left renal artery and right renal artery (RRA) angled increasingly downward by 21 ± 19° and 13 ± 17°, respectively (P < .005). The F-left renal artery and RRA angled upward by 25 ± 15° and 14 ± 15°, respectively (P < .005). From the pre- to the postoperative period, the end-stent angle of the Sn-RRA increased by 17 ± 12° (P < .00001), with greater magnitude change compared to the F-RRA (P < .0005). Peak curvature increased in distal Sn-RRAs by .02 ± .03 mm(-1) (P < .05). Acute renal failure occurred in 12.5% of patients, although none required dialysis following either F- and Sn-EVAR. Renal stent patency was 97.2% at mean follow-up of 13.7 months. Three type IA endoleaks were identified, prompting one secondary procedure, with the remainder resolving at 6-month follow-up. One renal artery reintervention was performed due to a compressed left renal stent in an asymptomatic patient.Stented renal arteries were angled more inferiorly after Sn-EVAR and more superiorly after F-EVAR due to stent configuration. Sn-EVAR induced significantly greater angle change at the stent end and curvature change distal to the stent compared to F-EVAR, although no difference in patency was noted in this small series with relatively short follow-up. Sn-RRAs exhibited greater end-stent angle change from the pre- to the postoperative period as compared to the F-RRA. These differences may exert differential effects on long-term renal artery patency, integrity, and renal function following complex EVAR for juxta- or pararenal abdominal aortic aneurysms.
View details for DOI 10.1016/j.jvs.2015.10.091
View details for PubMedID 26755068
Geometry and respiratory-induced deformation of abdominal branch vessels and stents after complex endovascular aneurysm repair
JOURNAL OF VASCULAR SURGERY
2015; 61 (4): 875-884
This study quantified the geometry and respiration-induced deformation of abdominal branch vessels and stents after fenestrated (F-) and snorkel (Sn-) endovascular aneurysm repair (EVAR).Twenty patients (80% male; mean age, 75.2 ± 7.4 years; mean aneurysm diameter, 6.2 ± 1.8 cm) underwent computed tomography angiography during inspiratory and expiratory breath hold protocols after F-EVAR (n = 11) or Sn-EVAR (n = 9). Centerlines for the aorta and visceral vessels were extracted from three-dimensional models. Branch angles were computed relative to the orthogonal plane at the branch ostia, and end-stent angles of the left renal artery (LRA) and right renal artery (RRA) were computed relative to the distal stent orientation. The radius of peak curvature was defined by the circumscribed circle at the highest curvature.Sn-renal branches were more downward-angled than F-renal branches (P < .04). At the distal ends of the RRA stents, Sn-RRAs were angled greater than F-RRAs (P < .03) and had a smaller radius of peak curvature (P < .03). With expiration, the end-stent angle of Sn-LRAs increased by 4° ± 4° (P < .02) and exhibited a significant reduction of radius of curvature (P < .04). The unstented celiac arteries were more downward-angled (P < .02, inspiration), with a smaller radius of curvature (P < .00001), than the unstented superior mesenteric arteries. With expiration, the celiac arteries angled upwards by 9° ± 9° (P < .0005), which was greater than the superior mesenteric arteries (P < .03). At a median postoperative follow-up of 12.6 months (range, 1.0-37.1 months), branch vessel patency was 100%, serum creatinine levels remained stable, and one reintervention was required for a type III endoleak at the main body-LRA stent interface.Sn-renals were angled more inferiorly at the branch and more angulated at the stent end than F-renals due to stent placement strategies. Sn-LRAs exhibited a significant change in end-stent angle and curvature during respiration, a finding that may compromise long-term durability for parallel stent graft configurations. Further investigation is warranted to better optimize anatomic, patient, and branch vessel stent selection between fenestrated and snorkel strategies and their relationship to long-term patency.
View details for DOI 10.1016/j.jvs.2014.11.075
View details for Web of Science ID 000351776100005
View details for PubMedID 25601499
- A longitudinal comparison of hemodynamics and intraluminal thrombus deposition in abdominal aortic aneurysms AMERICAN JOURNAL OF PHYSIOLOGY-HEART AND CIRCULATORY PHYSIOLOGY 2014; 307 (12): H1786-H1795
Aortic Arch Vessel Geometries and Deformations in Patients with Thoracic Aortic Aneurysms and Dissections
JOURNAL OF VASCULAR AND INTERVENTIONAL RADIOLOGY
2014; 25 (12): 1903-1911
To quantify aortic arch geometry and in vivo cardiac-induced and respiratory-induced arch translations and arch branch angulations using three-dimensional geometric modeling techniques.Scanning with electrocardiogram-gated computed tomography angiography during inspiratory and expiratory breath holds was performed in 15 patients (age, 64 y ± 14) with thoracic aortic aneurysms or dissections. From the lumen models, centerlines of the thoracic aorta, brachiocephalic artery, left common carotid artery, and left subclavian artery and their branching ostia positions were quantified. Three-dimensional translation of vessel ostia, branching angles, and their changes secondary to cardiac pulsation and respiration were computed.During expiration, all ostia translated rightward from systole to diastole (P < .035). Regardless of cardiac phase, all ostia translated posteriorly and superiorly from inspiration to expiration (P < .05). Respiration induced greater posterior and superior translations than cardiac pulsation (P < .03). The left common carotid artery branch angled significantly more toward the aortic arch compared with the brachiocephalic artery and left subclavian artery (P < .03). No significant changes in branching angle were found from systole to diastole or inspiration to expiration.In patients with thoracic aortic aneurysms or dissections, the thoracic aortic arch translated significantly secondary to inspiration and expiration and to a lesser extent secondary to cardiac pulsation. Insignificant branching angle changes suggest that the aortic arch and its branch origins move predominantly in unison.
View details for DOI 10.1016/j.jvir.2014.06.012
View details for Web of Science ID 000345676700011
View details for PubMedID 25066591
Respiratory-Induced 3D Deformations of the Renal Arteries Quantified With Geometric Modeling During Inspiration and Expiration Breath-Holds of Magnetic Resonance Angiography
JOURNAL OF MAGNETIC RESONANCE IMAGING
2013; 38 (6): 1325-1332
PURPOSE: To quantify renal artery deformation due to respiration using magnetic resonance (MR) image-based geometric analysis. MATERIALS AND METHODS: Five males were imaged with contrast-enhanced MR angiography during inspiratory and expiratory breath-holds. From 3D models of the abdominal aorta, left and right renal arteries (LRA and RRA), we quantified branching angle, curvature, peak curve angle, axial length, and locations of branch points. RESULTS: With expiration, maximum curvature changes were 0.054 ± 0.025 mm(-1) (P < 0.01), and curve angle at the most proximal curvature peak increased by 8.0 ± 4.5° (P < 0.05) in the LRA. Changes in maximum curvature and curve angles were not significant in the RRA. The first renal bifurcation point translated superiorly and posteriorly by 9.7 ± 3.6 mm (P < 0.005) and 3.5 ± 2.1 mm (P < 0.05), respectively, in the LRA, and 10.8 ± 6.1 mm (P < 0.05) and 3.6 ± 2.5 mm (P < 0.05), respectively, in the RRA. Changes in branching angle, axial length, and renal ostia locations were not significant. CONCLUSION: The LRA and RRA deformed and translated significantly. Greater deformation of the LRA as compared to the RRA may be due to asymmetric anatomy and mechanical support by the inferior vena cava. The presented methodology can extend to quantification of deformation of diseased and stented arteries to help renal artery implant development. J. Magn. Reson. Imaging 2013;. © 2013 Wiley Periodicals, Inc.
View details for DOI 10.1002/jmri.24101
View details for Web of Science ID 000327756800003
View details for PubMedID 23553967
Respiration-induced Deformations of the Superior Mesenteric and Renal Arteries in Patients with Abdominal Aortic Aneurysms.
Journal of vascular and interventional radiology
2013; 24 (7): 1035-1042
To quantify respiration-induced deformations of the superior mesenteric artery (SMA), left renal artery (LRA), and right renal artery (RRA) in patients with small abdominal aortic aneurysms (AAAs).Sixteen men with AAAs (age 73 y ± 7) were imaged with contrast-enhanced magnetic resonance angiography during inspiratory and expiratory breath-holds. Centerline paths of the aorta and visceral arteries were acquired by geometric modeling and segmentation techniques. Vessel translations and changes in branching angle and curvature resulting from respiration were computed from centerline paths.With expiration, the SMA, LRA, and RRA bifurcation points translated superiorly by 12.4mm ± 9.5, 14.5mm ± 8.8, and 12.7mm ± 6.4 (P < .001), and posteriorly by 2.2mm ± 2.7, 4.9mm ± 4.2, and 5.6mm ± 3.9 (P < .05), respectively, and the SMA translated rightward by 3.9mm ± 4.9 (P < .01). With expiration, the SMA, LRA, and RRA angled upward by 9.7° ± 6.4, 7.5° ± 7.8, and 4.9° ± 5.3, respectively (P < .005). With expiration, mean curvature increased by 0.02mm(-1) ± 0.01, 0.01mm(-1) ± 0.01, and 0.01mm(-1) ± 0.01 in the SMA, LRA, and RRA, respectively (P < .05). For inspiration and expiration, RRA curvature was greater than in other vessels (P < .025).With expiration, the SMA, LRA, and RRA translated superiorly and posteriorly as a result of diaphragmatic motion, inducing upward angling of vessel branches and increased curvature. In addition, the SMA exhibited rightward translation with expiration. The RRA was significantly more tortuous, but deformed less than the other vessels during respiration.
View details for DOI 10.1016/j.jvir.2013.04.006
View details for PubMedID 23796090
View details for PubMedCentralID PMC3694359
Hemodynamic Changes Quantified in Abdominal Aortic Aneurysms with Increasing Exercise Intensity Using MR Exercise Imaging and Image-Based Computational Fluid Dynamics
ANNALS OF BIOMEDICAL ENGINEERING
2011; 39 (8): 2186-2202
Abdominal aortic aneurysm (AAA) is a vascular disease resulting in a permanent, localized enlargement of the abdominal aorta. We previously hypothesized that the progression of AAA may be slowed by altering the hemodynamics in the abdominal aorta through exercise [Dalman, R. L., M. M. Tedesco, J. Myers, and C. A. Taylor. Ann. N.Y. Acad. Sci. 1085:92-109, 2006]. To quantify the effect of exercise intensity on hemodynamic conditions in 10 AAA subjects at rest and during mild and moderate intensities of lower-limb exercise (defined as 33 ± 10% and 63 ± 18% increase above resting heart rate, respectively), we used magnetic resonance imaging and computational fluid dynamics techniques. Subject-specific models were constructed from magnetic resonance angiography data and physiologic boundary conditions were derived from measurements made during dynamic exercise. We measured the abdominal aortic blood flow at rest and during exercise, and quantified mean wall shear stress (MWSS), oscillatory shear index (OSI), and particle residence time (PRT). We observed that an increase in the level of activity correlated with an increase of MWSS and a decrease of OSI at three locations in the abdominal aorta, and these changes were most significant below the renal arteries. As the level of activity increased, PRT in the aneurysm was significantly decreased: 50% of particles were cleared out of AAAs within 1.36 ± 0.43, 0.34 ± 0.10, and 0.22 ± 0.06 s at rest, mild exercise, and moderate exercise levels, respectively. Most of the reduction of PRT occurred from rest to the mild exercise level, suggesting that mild exercise may be sufficient to reduce flow stasis in AAAs.
View details for DOI 10.1007/s10439-011-0313-6
View details for Web of Science ID 000292268900008
View details for PubMedID 21509633
View details for PubMedCentralID PMC3362397
Quantification of Particle Residence Time in Abdominal Aortic Aneurysms Using Magnetic Resonance Imaging and Computational Fluid Dynamics
ANNALS OF BIOMEDICAL ENGINEERING
2011; 39 (2): 864-883
Hemodynamic conditions are hypothesized to affect the initiation, growth, and rupture of abdominal aortic aneurysms (AAAs), a vascular disease characterized by progressive wall degradation and enlargement of the abdominal aorta. This study aims to use magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) to quantify flow stagnation and recirculation in eight AAAs by computing particle residence time (PRT). Specifically, we used gadolinium-enhanced MR angiography to obtain images of the vessel lumens, which were used to generate subject-specific models. We also used phase-contrast MRI to measure blood flow at supraceliac and infrarenal locations to prescribe physiologic boundary conditions. CFD was used to simulate pulsatile flow, and PRT, particle residence index, and particle half-life of PRT in the aneurysms were computed. We observed significant regional differences of PRT in the aneurysms with localized patterns that differed depending on aneurysm geometry and infrarenal flow. A bulbous aneurysm with the lowest mean infrarenal flow demonstrated the slowest particle clearance. In addition, improvements in particle clearance were observed with increase of mean infrarenal flow. We postulate that augmentation of mean infrarenal flow during exercise may reduce chronic flow stasis that may influence mural thrombus burden, degradation of the vessel wall, and aneurysm growth.
View details for DOI 10.1007/s10439-010-0202-4
View details for Web of Science ID 000287213300022
View details for PubMedID 21103933
View details for PubMedCentralID PMC3066149
Quantifying In Vivo Hemodynamic Response to Exercise in Patients With Intermittent Claudication and Abdominal Aortic Aneurysms Using Cine Phase-Contrast MRI
JOURNAL OF MAGNETIC RESONANCE IMAGING
2010; 31 (2): 425-429
To evaluate rest and exercise hemodynamics in patients with abdominal aortic aneurysms (AAA) and peripheral occlusive disease (claudicants) using phase-contrast MRI.Blood velocities were acquired by means of cardiac-gated cine phase-contrast in a 0.5 Tesla (T) open MRI. Volumetric flow was calculated at the supraceliac (SC), infrarenal (IR), and mid-aneurysm (MA) levels during rest and upright cycling exercise using an MR-compatible exercise cycle.Mean blood flow increased during exercise (AAA: 130%, Claudicants: 136% of resting heart rate) at the SC and IR levels for AAA participants (2.6 +/- 0.6 versus 5.8 +/- 1.6 L/min, P < 0.001 and 0.8 +/- 0.4 versus 5.1 +/- 1.7 L/min, P < 0.001) and claudicants (2.3 +/- 0.5 versus 4.5 +/- 0.9 L/min, P < 0.005 and 0.8 +/- 0.2 versus 3.3 +/- 0.9 L/min, P < 0.005). AAA participants had a significant decrease in renal and digestive blood flow from rest to exercise (1.8 +/- 0.7 to 0.7 +/- 0.6 L/min, P < 0.01). The decrease in renal and digestive blood flow during exercise correlated with daily activity level for claudicants (R = 0.81).Abdominal aortic hemodynamic changes due to lower extremity exercise can be quantified in patients with AAA and claudication using PC-MRI. The redistribution of blood flow during exercise was significant and different between the two disease states.
View details for DOI 10.1002/jmri.22055
View details for Web of Science ID 000274117200019
View details for PubMedID 20099356
View details for PubMedCentralID PMC2963312
Quantification of three-dimensional motion of the renal arteries using image-based modeling techniques
ASME Summer Bioengineering Conference
AMER SOC MECHANICAL ENGINEERS. 2007: 715–716
View details for Web of Science ID 000252105700358