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Variability of flow parameters when subjected to changes of MR acquisitions parameters in 4D flow MRI using a realistic thoracic aortic phantom

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Variability of flow parameters when subjected to changes of MR acquisitions parameters in 4D flow MRI using a
realistic thoracic aortic phantom.
Cristian Montalba , Jesus Urbina , Julio Sotelo , Marcelo Andia , Cristian Tejos , Pablo Irrarazaval , Israel Valverde , and Sergio Uribe
Biomedical Imaging Center, Pontificia Universidad Católica de Chile, Santiago, Chile, School of Medicine, Pontificia Universidad Católica de Chile, Santiago, Chile, Electrical
Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile, Radiology Department, Pontificia Universidad Católica de Chile, Santiago, Chile, Institute of
Biomedicine of Seville, Universidad De Sevilla, Seville, Spain, Cardiology Unit, Hospital Virgen del Rocio, Universidad de Sevilla, Seville, Spain
Synopsis
4D flow is a MRI technique characterized by long scanning times. Because of that, it is difficult to study
the variability of flow parameters when subjected to changes of the MR parameters. The purpose of this
work is to study the variability of different flow parameters due to changes of spatial and temporal
resolutions in 4D flow acquisitions through controlled experiments using a realistic normal adult thoracic
aortic phantom. We conclude that changing the spatial and temporal resolutions in the 4D flow imaging
greatly affects different flow parameters with induced errors of up to 23.9%.
Purpose
4D flow is a MRI technique characterized by long scanning times, which leads to that the acquisition of these data
sets is usually confined to a small volume with restricted spatial and temporal resolution. Recently, there has been
a consensus regarding the minimum requirements of the MR parameters for the acquisition of 4D flow data (1).
However, due to the long scanning times, it is difficult to study the variability of the results obtained when subjected
to changes of the MR parameters without having other variables influencing the results. The purpose of this work
is to study the variability of different flow parameters due to changes of spatial and temporal resolutions in 4D flow
acquisitions through controlled experiments using a realistic normal adult thoracic aortic phantom.
Methods
The experiments were performed using a whole-body 1.5T MR scanner (Philips Achieva, Best, Netherlands) with a
4-element phased-array body coil. The phantom model (T-S-N-005, Elastrat Sarl, Geneva, Switzerland) is used
with a pulsatile MR-compatible flow pump (Simutec, London, Ontario, Canada), with the same features as
explained by Urbina et al (2). We are able to control the pump in terms of flow waveform, heart rate and cardiac
output, which allows us to study six different hemodynamic conditions: heart rates of 68 and 88 bpm, each one
with different maximum flow rate of 200, 230 and 260 mL/s. For each hemodynamic condition, two 2D phase
contrast (PC) sequences were acquired in the ascending and descending aorta (Figure 1). Also, nine 4D flow data
were acquired in the entire phantom with different combinations of spatial and temporal resolutions (Table 1).
Furthermore, two acquisitions with medium (2.0x2.0x2.0mm3) and low spatial resolution (1.5x1.5x1.5mm3) and
with 40ms of temporal resolution were re-acquired for reproducibility analysis. Each phantom condition was
scanned at different days and the data acquisitions lasted about 5 hours each session.
We compared the peak flow, mean velocity, maximum velocity between all 4D flow data with the 2D
PC sequence. The analyses were performed using a commercial software GT Flow 2.2.15
(Gyrotools LLC, Zurich, Switzerland).
Results
Figure 2 summarizes the results of peak flow, mean velocity and maximum velocity in each hemodynamic
condition comparing all 4D flow data with 2D PC data at the ascending (Ao1) and descending aorta (Ao5). We
observed a high variability of the results obtained when subjected to changes of the studied MR parameters,
particularly sensitive to changes of the temporal resolution. Considering the 2D PC sequence as gold standard,
the mean, minimum and maximum peak flow errors are showed in Table 2. We observed minimum errors when
the data is acquired with high temporal resolutions and greater errors when the data was acquired with low
temporal resolutions. Result of the reproducibility experiment, including the mean difference (bias) and the
standard deviations of difference between the first and second measurement are summarized in Table 3.
Discussion
We observed a high variability of the results obtained and a greater accuracy of the flow parameters when using
high temporal resolution compared to 2D PC values. From Figure 2 we observed that the temporal resolutions
1 1,2 1,3 1,4 1,3 1,3 5,6 1,4
1 2 3
4 5
6
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greatly affected the results obtained. However, for a given temporal resolution the results showed small variations
for the different spatial resolutions. These findings can be appreciated for any hemodynamic condition studied.
Low spatial resolution images obtained result more similar than high spatial resolutions. Result of the
reproducibility studied showed an excellent agreement between the two measurements for the medium and low
resolutions in Ao1 and in Ao5, showing a maximum bias of 3.2% for both spatial resolution studied.
Conclusion
Changing the spatial and temporal resolutions in 4D flow imaging greatly affects different flow parameters that
induced errors of up to 23.9%, being the 4D flow sequence particularly sensitive to changes of the temporal
resolution.
Acknowledgements
Grant Sponsor: Anillo ACT1416, Grant Sponsor: Fondo Nacional de Desarrollo Científico y Tecnológico
(FONDECYT), Ministerio de Educación, Chile. Grant Number: FONDECYT #1141036.
References
1.- Dyverfeldt et al. (2015). 4D flow cardiovascular magnetic resonance consensus statement. J Cardiovasc Magn
Reson. 2015 Aug 10; 17(1):72.
2. - Jesus Urbina et al. A realistic MR compatible thoracic aortic phantom to study coarctations using
catheterization and cine PC-MRI sequences. ISMRM 2014.
Figures
Figure 1: Different segments of the aorta that were analyzed in 2D PC and 4D flow images. The planes corresponded to the Ascending (Ao1) and the descending (Ao5) aorta, both located at the level of the coronary arteries.
Table 1: Summary of the different spatial and temporal resolutions of the acquired 2D and 4D flow images.
Figure 2: Flow related measurements from 2D and 4D flow images obtained under different hemodynamic conditions. A, B and C shows the measurements of peak flow, mean velocity and maximum velocity respectively in Ao1. D,
E and F show the same flow parameters in Ao5.
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Table 2: Summary of the results of mean, minimum and maximum error in the 4D flow acquisitions for different spatial and temporal resolutions between all hemodynamic conditions. A) results in Ao1, B) results in Ao5.
Table 3: Summary of the results of bias and standard deviation % for the reproducibility in 4D flow images. A. results in Ao1, B. results in Ao5.
Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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Article
Pulsatile blood flow through the cavities of the heart and great vessels is time-varying and multidirectional. Access to all regions, phases and directions of cardiovascular flows has formerly been limited. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has enabled more comprehensive access to such flows, with typical spatial resolution of 1.5×1.5×1.5 - 3×3×3 mm(3), typical temporal resolution of 30-40 ms, and acquisition times in the order of 5 to 25 min. This consensus paper is the work of physicists, physicians and biomedical engineers, active in the development and implementation of 4D Flow CMR, who have repeatedly met to share experience and ideas. The paper aims to assist understanding of acquisition and analysis methods, and their potential clinical applications with a focus on the heart and greater vessels. We describe that 4D Flow CMR can be clinically advantageous because placement of a single acquisition volume is straightforward and enables flow through any plane across it to be calculated retrospectively and with good accuracy. We also specify research and development goals that have yet to be satisfactorily achieved. Derived flow parameters, generally needing further development or validation for clinical use, include measurements of wall shear stress, pressure difference, turbulent kinetic energy, and intracardiac flow components. The dependence of measurement accuracy on acquisition parameters is considered, as are the uses of different visualization strategies for appropriate representation of time-varying multidirectional flow fields. Finally, we offer suggestions for more consistent, user-friendly implementation of 4D Flow CMR acquisition and data handling with a view to multicenter studies and more widespread adoption of the approach in routine clinical investigations.