Jihye Jang1,2, Yansong Zhao1, Jouke Smink3, Andrew J Powell2, and Mehdi H Moghari2
1Philips Healthcare, Gainesville, FL, United States, 2Department of Pediatrics, Harvard Medical School, Boston, MA, United States, 3Philips Healthcare, Best, Netherlands
Synopsis
Phase-contrast
(PC) cine MRI is used for the clinical assessment of blood flow in various
cardiovascular diseases. One of the challenges of PC is a trade-off between
velocity aliasing artifacts and velocity-to-noise ratio (VNR). To improve VNR
without velocity aliasing, we implemented a novel dual-venc dual-echo 2D cine PC
sequence where both high and low-venc data are acquired within a single TR and
velocity is measured using data from both venc images. In 10 patients, the
dual-venc PC sequence demonstrated similar blood flow measurements with significantly
better VNR compared to the standard single-venc PC sequence.
Introduction
Time-resolved
cine phase-contrast (PC) MRI is widely used for the assessment of blood flow in
various cardiovascular diseases.1,2 In a PC acquisition, the
velocity sensitivity (venc) is typically pre-defined to a value higher than the
expected maximum velocity to avoid aliasing. Vessels with a wide velocity range
suffer from a lower velocity-to-noise ratio (VNR), particularly in the slower
velocity ranges. To improve VNR without aliasing artifacts, we sought to
develop a dual-venc dual-echo 2D cine PC sequence to acquire high and low-venc images
simultaneously and use data from both vencs for the velocity measurement.Methods
A
novel retrospective ECG-gated 2D cine dual-venc dual-echo PC sequence was developed
on a 1.5T Achieva-dStream Philips scanner. To minimize the acquisition time
associated with the additional venc, the second venc was acquired in a second
echo within a single TR3 and flyback gradients were employed to
maintain the same gradient polarity and minimize phase errors (Figure 1).
Velocity was measured by unwrapping the velocity of the low-venc and using the high-venc
phase image as an unwrapping threshold (Figure 2). To minimize the impact of
the unwrapping border effects, a bilateral filter for edge-preserving smoothing4
was applied after the unwrapping of the phase image. Magnitude images from both
echoes were averaged to improve the signal-to-noise ratio.
The
standard single-venc PC and the dual-venc dual-echo PC data were acquired in 10
patients referred for clinical cardiac MRI exams (4 female, median age 21
(14-41) years old). All patients provided written consent before participating
in the study. Single-venc was acquired with venc of 200 – 350 cm/s, and
dual-venc was acquired with a high-venc identical to the single-venc sequence and
a low-venc of 50 cm/s. TR/TE for single-venc was 4.8 ms/2.9 ms and for
dual-venc was 7.9 ms/(2.9, 6.0) ms. This results in a scan time of 1:13 min vs
2.01 min for single-venc vs. dual-venc at a heart rate of 60 bpm. Other imaging
parameters were identical between the single-venc and dual-venc sequences:
spatial resolution = 1.56 × 1.56 mm, slice thickness = 6 mm, field-of-view =
275 x 275 mm, flip angle = 12°, acquired heart phases = 21 reconstructed to 30,
compressed SENSE = 2, and number of signals averages = 3.
Net blood
flow and peak velocity were measured in the single and dual-venc PC sequences acquired
to measure flow in the main pulmonary artery (MPA) and in the ascending aorta
(AAo). The VNR was measured as the ratio of the mean velocity to the standard
deviation5 of velocity during diastole in the MPA and AAo, and
compared between single and dual-venc PC MRI. The measured blood flow and VNR
were statistically compared using a non-parametric Wilcoxon signed-rank test
and p<0.05 was considered statistically significant. The intraclass
correlation coefficient was calculated to measure the agreement.Results
The dual-venc dual-echo PC sequence demonstrates similar blood flow measurements in
both the MPA and AAo and had improved VNR compared to the standard single-venc
PC sequence (Figure 3). Table 1 summarizes the blood flow measurements in MPA
and AAo. The dual-venc sequence demonstrated comparable net blood flow to the
standard single-venc PC sequence (MPA: 112 ± 52 ml vs. 110 ± 55 ml, p-value=0. 39;
and AAo: 93 ± 26 ml vs. 96 ± 32 ml, p-value=0.72) and peak velocity (MPA: 143 ±
78 cm/s vs. 140 ± 80 cm/s, p-value=0.26; and AAo: 135 ± 38 cm/s vs. 134 ± 36
cm/s, p-value=0.59). The dual-venc sequence demonstrated higher VNR compared to
the standard single-venc PC sequence (MPA: 0.78 ± 0.94 vs. 1.49 ± 1.00,
p-value=0.01; and AAo: 0.64 ± 1.11 vs. 0.97 ± 1.10; p-value=0.02).Discussion
Our
novel retrospectively ECG-gated 2D cine dual-venc dual-echo PC sequence
demonstrates comparable blood flow measurements in the MPA and AAo and improved
VNR. The application of this technique in vessels with a wide velocity range
such as the AAo in aortic regurgitation and in a 4D flow sequence for the simultaneous
assessment of the arterial and venous blood flow will be investigated in future
studies.Conclusion
Dual-venc
dual-echo 2D cine PC sequence demonstrates higher VNR and comparable blood flow
measurements in the MPA and AAo. Future work will focus on incorporating this technique
for the 4D flow sequence and using this technique in routine clinical practice.Acknowledgements
No acknowledgement found.References
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