Four-dimensional phase contrast MRI with accelerated dual velocity encoding

Abstract

Purpose: To validate a novel approach for accelerated four-dimensional phase contrast MR imaging (4D PC-MRI) with an extended range of velocity sensitivity.

Materials and methods: 4D PC-MRI data were acquired with a radially undersampled trajectory (PC-VIPR). A dual V(enc) (dV(enc) ) processing algorithm was implemented to investigate the potential for scan time savings while providing an improved velocity-to-noise ratio. Flow and velocity measurements were compared with a flow pump, conventional 2D PC MR, and single V(enc) 4D PC-MRI in the chest of 10 volunteers.

Results: Phantom measurements showed excellent agreement between accelerated dV(enc) 4D PC-MRI and the pump flow rate (R(2) ≥ 0.97) with a three-fold increase in measured velocity-to-noise ratio (VNR) and a 5% increase in scan time. In volunteers, reasonable agreement was found when combining 100% of data acquired with V(enc) = 80 cm/s and 25% of the high V(enc) data, providing the VNR of a 80 cm/s acquisition with a wider velocity range of 160 cm/s at the expense of a 25% longer scan.

Conclusion: Accelerated dual V(enc) 4D PC-MRI was demonstrated in vitro and in vivo. This acquisition scheme is well suited for vascular territories with wide ranges of flow velocities such as congenital heart disease, the hepatic vasculature, and others.

Figure 1

Schematic of acquisition and dual V_enc reconstruction. The parameters shown are those that were used in volunteers. Three, 12–13 minute PC VIPR data sets are acquired with three V_encs: one high V_enc (160 cm/s) and two low V_encs (80, 40 cm/s). Dual V_enc images are reconstructed with a low V_enc data set and different percentages of the high V_enc data set. The undersampled high V_enc reconstruction mimics acquiring less high V_enc data.

Figure 2

Slice locations analyzed in the comparison study. Volume rendering of the phase contrast angiogram derived from a PC VIPR exam was not part of the analysis. Arterial (red) and venous (blue) system have been segmented to facilitate orientation. Velocity and flow measurements were made in the ascending and descending aorta (AAO and DAO), main pulmonary artery (MPA), and superior and inferior vena cava (SVC and IVC). The left and right atrium (LA, RA) and ventricles (LV, RV) are labeled for reference.

Figure 3

Flow rates measured with high V_enc PC VIPR and dual V_enc PC VIPR in a flow phantom programmed to deliver constant flow. Scatter was added around the set pump flow rates to allow for a better appreciation of the data. The dashed line marks the identity line. Overall, PC VIPR methods somewhat underestimated the flow rate but in most cases, flow rates measured agreed well with the programmed flow rates.

Figure 4

Relative gain in VNR efficiency of dual V_enc in comparison to a single/high V_enc PC VIPR scan. A marked increase in VNR efficiency can be achieved by decreasing undersampling expressed by relative scan times. With a 5% increase in scan time, the VNR efficiency can be increased from 26.7% to 77% (factor of 2.88) using a 4:1 ratio of high-to-low V_enc scan during the reconstruction. When only 2% of the high V_enc data is used to correct the low V_enc scan, the VNR plummets due to phase unwrapping errors.

Figure 5

Summary of the results from Bland-Altman analysis comparing average (A) flow volume and (B) peak velocity measurements with 95% confidence intervals in high V_enc PC VIPR and dual V_enc PC VIPR with 2D PC. Flow volume [ml] was measured in the AAO, DAO, MPA, SVC, and IVC. Peak velocity was only measured in the arterial system (AAO, DAO, and MPA) because peak venous velocities are often difficult to define. The flow volumes and peak velocities measured with PC VIPR techniques are lower than those measured by 2D PC in most cases. This is most likely a result of the temporal filtering used in the PC VIPR reconstruction.

Figure 6

Measured average Q_p/Q_s ratios with error bars indicating ±1 standard deviation. The expected value of 1.03 ± 0.03 for normal volunteers is indicated by the solid line with the grey box representing 1 SD (21). Most techniques slightly underestimate Q_p/Q_s on average; however, all average values are within 10% of the expected Q_p/Q_s. For all techniques, the expected value of Q_p/Q_s is within ±1 standard deviation.

Figure 7

Example high, low and dual V_enc PC VIPR images. The inlay in each image shows the descending aorta. Black arrows point to locations of uncorrected velocity aliasing. Each column of images is labeled with the relative scan time, increasing from left to right (100% for a single V_enc PC VIPR acquisition). The dual V_enc PC VIPR images have the same dynamic range as high V_enc PC VIPR but with increased VNR. Phase errors decrease in dual V_enc PC VIPR when the scan time is increased.

Figure 8

Representative flow waveforms acquired in (a) ascending aorta (AAO) and (b) superior vena cava (SVC) with 2D PC, balanced encoded PC VIPR and two dual V_enc acquisitions. The ascending aorta waveforms agree extremely well across all techniques. There is more variation between the waveforms acquired in the SVC, most likely due to slower blood flows in the venous system. Note, the 2D PC waveforms cover a smaller time interval because the 2D PC acquisition is prospectively gated and the PC VIPR acquisitions are retrospectively gated.