Pressure Gradient Measurement in the Coronary Artery Using Phase Contrast (PC)-MRI: Initial Patient Results Towards Noninvasive Quantification of Fractional Flow Reserve
Zixin Deng1,2, Sangeun Lee3, Zhaoyang Fan1, Christopher Nguyen1, Iksung Cho3, Qi Yang1, Xiaoming Bi4, Byoung-Wook Choi5, Jung-Sun Kim3, Daniel Berman1, Hyuk-Jae Chang3, and Debiao Li1

1Biomedical Imaging Research Institute, Cedars-Sinai Medical Center, Los Angeles, CA, United States, 2Bioengineering, University of California, Los Angeles, Los Angeles, CA, United States, 3Cardiology, Severance Hospital, Yonsei Univeristy College of Medicine, Seoul, Korea, Republic of, 4R&D, Siemens Healthcare, Los Angeles, CA, United States, 5Radiology, Severance Hospital, Yonsei Univeristy College of Medicine, Seoul, Korea, Republic of


Fractional flow reserve is an invasive diagnostic tool to evaluate the functional significance of a coronary stenosis by quantifyin­g the pressure gradient (ΔP) across the stenosis. We proposed a non-invasive technique to derive ΔP using Phase-contrast (PC)-MRI in conjunction with the Navier-Stokes equations (ΔPMR). Excellent correlation was observed between derived ΔPMR and measure ΔP from a pressure transducer in a small caliber phantom model. A significant increase in ΔPMR was seen in the patient group vs. healthy controls. Preliminary results suggested that noninvasive quantification of ΔPMR in coronary arteries is feasible.


Fractional flow reserve (FFR) is an invasive procedure evaluating the functional significance of an intermediate coronary stenosis in patients with coronary artery disease (CAD)1. Quantification of pressure gradient (ΔP) across a particular stenosis is the key to the determination of FFR, where FFR<0.80 is considered a functionally significant stenosis. Noninvasive ΔP measurement (ΔPMR) using phase-contrast (PC)-MRI in conjunction with Navier-Stokes (NS) equations has been attempted in large to medium sized vessels2-4. Our previous work has shown the feasibility of deriving ΔPMR in small caliber phantom models and healthy coronary arteries5-6. To ensure the accuracy of the proposed method, this study aimed to investigate the reproducibility of ΔPMR in stenotic phantom models at various diameters and its correlation with measured ΔP values via a pressure transducer (ΔPPT). The feasibility of the method was investigated in diseased coronary arteries and compared to healthy controls.


Phantom studies: 11 phantom models (0%-85% area stenosis, reference diameter=4.8mm) were individually connected to a flow pump (gadolinium-doped water, constant volume velocity=250mL/min) while 2D cross-sectional PC-MRI images were acquired. Contiguous slices (10-20) were consecutively collected across each narrowing (fig.1a/b). Imaging parameters were: FA=15o; in-plane resolution=~0.55x0.55mm2; slice thickness=3.2mm; Venc=z(40-260cm/s) and x,y(40-80cm/s), depending on the degree of narrowing. Repeat scans were performed in 7/11 phantom models. Immediately following the PC-MRI scans, pressure was measured using an arterial catheter connected to a pressure transducer before and after the maximum narrowing.

Human studies: 11 healthy controls (47.3±14.6 years) and 9 patients (67.3±7.3 years, four with known invasive FFR) were studied. Patient inclusion criteria: known/suspected CAD, ≥1 coronary lesion (proximal stenosis ≥30%) detected by CTA and/or invasive coronary angiography (ICA). Coronary PC-MRI acquisitions were ECG-triggered (mid-diastole) and navigator-gated (end-expiration)5-6. Fat-suppression pre-pulses were applied to avoid chemical shift effects and increase vessel contrast7-8. Contiguous slices (4-10) were consecutively collected across the proximal coronary segment (healthy controls) or stenotic lesion (patients). Imaging parameters were: Venc=35-65cm/s in all 3 directions, cardiac phase=2(~70ms/phase), in-plane resolution=0.5-0.6x0.5-0.6mm2, slice thickness=3.2mm and TA=2-4min/slice.

Data Analysis: Eddy-current correction was done offline followed by NS calculations5,6,9 to obtain ΔPMR. Reproducibility of the velocity values from PC-MRI and the derived ΔPMR of the phantom studies were assessed using intra-class correlation coefficient (ICC) and Bland-Altman plot. Correlation of ΔPMR and ΔPPT was assessed via linear regression analysis.


Phantom studies: Bland-Altman plots of peak velocities and ΔPMR are shown in fig.2a. For velocity measurements, excellent correlation was seen in the through plane peak velocities (Vz, ICC=0.90) and lower in Vx (ICC=0.57) and Vy (ICC=0.58). For ΔPMRs, overall ICC=0.87; When observed individually, higher correlation was seen at smaller stenosis degrees and weaker as stenosis increased (fig.1c). This could be due to the increased velocity in larger stenoses, causing minor turbulence distal of the narrowing, thus, inconsistent velocity and ΔPMR between the two scans. Furthermore, ΔPMR and ΔPPT were highly correlated (fig.2b). We also observed that as %area stenosis increased, ΔPMR also increased (fig.2c).

Human studies: A significant (p<0.01) increase in ΔPMR was seen in the patient group (5.26±3.99mmHg) vs. healthy controls (0.70±0.57mmHg) (fig.3a). CTA/ICA reports in all patients showed a range of stenoses from 30%-50% at the left main or proximal left anterior descending coronary artery (pLAD). ICA/FFR was performed in 4/9 patients where 3/4 had a functionally non-significant lesion (FFR=0.93±0.70), corroborating with the proposed method (ΔPMR≈3.0±1.70mmHg, low pressure drop). In one of the four patients who underwent ICA/FFR, a diffused, 50% lumen narrowing at the pLAD was observed (fig3b-c) with FFR=0.56, suggesting a functionally significant lesion. The same patient showed a ΔPMR of ~14mmHg, likewise suggesting a functionally significant lesion (relatively high pressure drop).


Preliminary results suggest that noninvasive quantification of ΔPMR in coronary arteries is feasible. In phantom studies, excellent correlation was found between the derived ΔPMR and measured ΔPPT. In human studies, patients with 30-50% stenoses were found to have a higher ΔPMR than healthy volunteers. In patients who underwent invasive FFR, high FFR (low pressure drop) and low FFR (high pressure drop) both corroborated the ΔPMR results. More patient studies with invasive FFR comparison are underway to further investigate the sensitivity of the approach in differentiating between a functionally non-significant and significant lesion. In addition, further technical improvements in terms of spatial, temporal resolutions and reduction of noise are also being developed to further improve the accuracy of the ΔPMR calculations.


Our preliminary studies demonstrated the feasibility of using PC-MRI to measure pressure gradient across coronary lesions. This approach has the potential to serve as a gatekeeper for unnecessary invasive catheterization procedures in patients with coronary artery disease.


No acknowledgement found.


1.Pijls et al. NEJM1996; 2.Bock et al. MRM2011; 3.Lum et al. RY2007; 4.Bley et al. RY2011; 5.Deng et al. ISMRM2014; 6.Deng et al. SCMR2015; 7.Middione et al, MRM2013; 8.Keegan et al. JCMR2015; 9.Yang et al. MRM1996;


Figure 1. a. Stenotic phantom model examples (% area stenosis at the maximum narrowing). b. 2D PC-MRI images in the through-plane direction (velocity maps, cm/s) for 71% area stenosis phantom model. c. Intra-class correlation coefficients (ICC) of the peak velocities and ΔPMRs for the example phantom models.

Figure 2. a. Bland-Altman plots of peak velocities at all cross-sectional slice from repeat PC-MRI scans and the derived ΔP of the phantom models. Mean (bias) and 95% confidence interval limits are displayed. b. Pressure measurement comparison between ΔPMR and ΔPPT. Excellent correlation (R2=0.938) was observed between the two techniques. c. % area stenosis versus ΔPMR measurement. An exponential increase in ΔPMR was observed as % area stenosis increases.

Figure 3. a. ΔP comparison for healthy controls and patients. The high standard deviation in the patient group is due to the range of the stenotic level among the patient group. b. Patient: Invasive coronary angiography (ICA) and magnetic resonance angiography (MRA). c. Patient: 6 cross-sectional slices obtained from PC-MRI over the stenotic lesion at the proximal left anterior descending artery. top row: flow compensated, bottom row: phase contrast (in the z-direction).

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)