A practical introduction to the hemodynamic analysis of the

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Radiología. 2014;56(6):485---495
www.elsevier.es/rx
UPDATE IN RADIOLOGY
A practical introduction to the hemodynamic analysis
of the cardiovascular system with 4D Flow MRI夽
J.A. Pineda Zapata a,∗ , J.A. Delgado de Bedout a , S. Rascovsky Ramírez a ,
C. Bustamante a , S. Mesa b , V.D. Calvo Betancur a
a
b
Grupo de Investigación, Instituto de Alta Tecnologiá Médica (IATM), Medellín, Antioquia, Colombia
Universidad CES, Medellín, Antioquia, Colombia
Received 15 November 2013; accepted 14 August 2014
KEYWORDS
4D Flow;
Cardiovascular
magnetic resonance
imaging;
Phase contrast
angiography;
Hemodynamics;
Blood flow
PALABRAS CLAVE
4D Flow;
Resonancia
magnética
cardiovascular;
Angiografía con
contraste de fase;
Hemodinámica;
Flujo sanguíneo
Abstract The 4D Flow MRI technique provides a three-dimensional representation of blood
flow over time, making it possible to evaluate the hemodynamics of the cardiovascular system
both qualitatively and quantitatively. In this article, we describe the application of the 4D Flow
technique in a 3 T scanner; in addition to the technical parameters, we discuss the advantages
and limitations of the technique and its possible clinical applications. We used 4D Flow MRI to
study different body areas (chest, abdomen, neck, and head) in 10 volunteers. We obtained
3D representations of the patterns of flow and quantitative hemodynamic measurements. The
technique makes it possible to evaluate the pattern of blood flow in large and midsize vessels
without the need for exogenous contrast agents.
© 2013 SERAM. Published by Elsevier España, S.L.U. All rights reserved.
Introducción práctica al análisis hemodinámico del sistema cardiovascular mediante
la técnica «4D Flow»
Resumen La técnica de resonancia magnética 4D Flow permite evaluar cualitativa y cuantitativamente la hemodinámica del sistema cardiovascular representando en tres dimensiones
los patrones de flujo sanguíneo en el tiempo y cuantificando variables hemodinámicas. En este
trabajo describimos la técnica 4D Flow en un equipo de resonancia de 3 T y adicionalmente se
exponen, además de los parámetros técnicos, las ventajas, las limitaciones y las posibles aplicaciones clínicas. Para esto estudiamos a diez voluntarios con la técnica 4D Flow en diferentes
áreas corporales (tórax, abdomen, cuello y cráneo) con la que obtuvimos representaciones
夽 Please cite this article as: Pineda Zapata JA, Delgado de Bedout JA, Rascovsky Ramírez S, Bustamante C, Mesa S, Calvo Betancur VD.
Introducción práctica al análisis hemodinámico del sistema cardiovascular mediante la técnica «4D Flow». Radiología. 2014;56:485---495.
∗ Corresponding author.
E-mail address: [email protected] (J.A. Pineda Zapata).
2173-5107/© 2013 SERAM. Published by Elsevier España, S.L.U. All rights reserved.
486
J.A. Pineda Zapata et al.
tridimensionales de los patrones del flujo y medidas cuantitativas hemodinámicas. La técnica
permite evaluar los patrones de flujo sanguíneo en vasos grandes y medianos sin la necesidad
de contrastes exógenos.
© 2013 SERAM. Publicado por Elsevier España, S.L.U. Todos los derechos reservados.
Introduction
During the last years we have seen important innovations
in magnetic resonance images (MRI), especially in the field
of cardiovascular imaging; this means that today not only
the anatomical study but also the functional study of heart
and vessels is possible. The very first maps of in vivo vascular velocities were reported back in the early 1980s1 and
since then angioresonance sequences like phase contrast
angiography (PCA)---consisting of 2D retrospective acquisitions with the cardiac cycle (2D cine PCA) are available
in most MRI equipments. This sequence is essential for the
functional evaluation of the cardiovascular system and it can
be acquired during one apnea only.2
The time-resolved three-dimensional flow-sensitive MRI
with three-directional velocity encoding (4D Flow) modality
consists of MRI acquired though a phase contrast angiography
that acquires blood flow data in three (3) spatial directions
(3D) during the whole cardiac cycle3 for the qualitative and
quantitative evaluation of vascular hemodynamics.4 In this
case acquisition lasts more (5---20 min) and it is necessary
to synchronize it with breathing. With 4D Flow data we can
both see and analyze quantitatively blood flow patterns
in heart and large vessels through 3D cine representations during the whole cardiac cycle by means of pathlines,
streamlines, vector graphics and 3D velocities. It also allows
us to perform hemodynamic measurements of mid, peak
and minimum velocities, ejection volumes, vessel wall shear
forces and pressure gradients.5 Given it is a 3D modality
among the multiples advantages of 4D Flow the possibility
of quantifying the flow in any interesting views and vessels
by postprocessing images is included when the patient is
no longer in the resonance equipment without the need
for any more sequences in other views as it happens with
the 2D cine PCA. Also 4D Flow is a modality that does not
use any ionizing radiation or contrast media.6,7 Its clinical
potential is enormous because it allows us to perform the
anatomic and hemodynamic evaluation of the patient’s vessels with cardiac or cardiovascular diseases like: congenital
heart diseases, heart failure, arteriovenous malformations,
aneurysms, fistulas or stenoses in blood vessels, among
others.
In this review we will describe the 4D Flow modality in ten
(10) volunteers through a 3T MRI and we will be discussing
the advantages, limitations and possible clinical alterations.
Principles of the 2D cine PCA
The 2D cine PCA is an MRI modality synchronized with the
electrocardiogram (ECG) that delivers phase contrast images
velocity-sensitive. 2D cine PCA images are acquired in different stages of the cardiac cycle (retrospective acquisition)
and they can be seen dynamically. Their main goal is to
evaluate hemodynamic parameters like flows, volumes, and
velocities.
Flow is quantified by measuring spin-echo transverse
magnetization in two (2) different time frames after applying one bipolar pulsed magnetic field gradient with a positive
«lobule» followed by a negative one of the same magnitude
but different direction. The coding of blood velocity in any
spatial directions is based on the phase difference between
the moving spin transverse magentization vector and static
spins.8,9 This is possible due to the fact that the static spins
do not accumulate phase difference because when they
experience the positive gradient they rotate to a certain
angle and when they experience the negative gradient they
rotate to the same angle but in a different direction. This
means that the net phase displacement is zero. However,
moving spins accumulate phase difference due to the fact
that the intensity experienced by the positive and negative
gradients is not the same due to movement and change of
position (Fig. 1). The phase difference angle is directly proportional to both the velocity and movement of the spins.10
It is important to highlight that the maximum velocity detected is determined by the Velocity Encoding
parameter---expressed in cm/s. Blood flow velocities beyond
this parameter are coded erroneously, so aliasing9 occurs.
With the data acquired magnitude and phase images are
built (Fig. 2) through the complex difference and the phase
difference of the transverse magnetization vectors.11
4D Flow
The 2D cine PCA modality can be extended to the 3D spatial
acquisition in an effort to code the blood flow in the three (3)
directions of Cartesian coordinate system in different phases
of the cardiac cycle. Because we need three (3) directions
for flow coding we need four (4) sequences---three (3) that
are sensitive to velocity and one (1) reference sequence the
total acquisition time is longer than with 2D modality. This
modality is known as 4D Flow since it is named after four (4)
dimensions---three (3) spatial dimensions plus a 4th temporal
dimension.
4D Flow images allow us to quantitatively evaluate
the regional and global dynamics of blood flow through
hemodynamic measurements like the average, maximum
and minimum velocities, the average beat flow, regurgitant
and progressive flows, ejection volumes and mathematical
approximations to pressure gradients and shear forces
in the vessel walls. It also allows us to analyze quantitatively the temporal evolution of the complex patterns
Hemodynamic analysis of the cardiovascular system with 4D Flow MRI
Time 1
487
Time 2
Bipolar
gradient
Magnetic
field
*
*
Flow
Spin
mismatch
Static
spin
Moving
spin
Static
spin
Moving
spin
Figure 1 Velocity coding principle in 2D cine PCA images. In time 1 a positive gradient is applied resulting in a phase displacement
that is the same both for the static (circle without asterisk) and mobile spin (circle with asterisk). In time 2 a gradient of the same
magnitude but different opposed to the 1st one is applied. It is now that the mobile spin has been displaced from its original
position while perceiving a different gradient magnitude from the static spin. After the application of the bipolar gradient the
result is that static spin phase between time 1 and time 2 is zero; however, the mobile spin rotates different angles in two (2) times
and consequently accumulates a phase proportional to the velocity and distance gone by.
of blood flow through streamlines, pathlines, 3D velocity
charts, and vectors; representations that describe blood
trajectory across the cardiovascular system.12---14
At the beginning this modality was known thanks to its
capacity to give qualitative information through the 3D representation of cardiovascular flow. However, recently with
4D Flow some methods have been developed for the quantitative analysis of blood flow. Also the new technological
advancements in both hardware and software allow us to
reduce the sequence time of acquisition and apply it in the
clinical setting.13
Technique of acquisition
For the gathering of time-based volumetric data combined
with velocity coding data in the three (3) spatial directions we use k-space segmentation methods synchronized
Figure 2 2D cine PCA axial images at the level of the pulmonary artery bifurcation. (A) Magnitude image. Blood vessel anatomic
information. (B) Phase image. Blood velocity is coded through the intensity of pixels---bright color meaning flow coming out of the
image and dark color meaning flow coming in.
488
TECG
Bipolar gradient
in the x-axis
Reference
Flow coding
in x
Bipolar gradient
in the y-axis
Bipolar gradient
in the z-axis
Flow coding
in y
Flow coding
in z
Ky
Nsec
Kz
Kx
Magnitude image
Phase images
Figure 3 Image acquisition scheme through the 4D Flow MRI modality. For each phase (t) of the cardiac cycle (synchronized
with the ECG signal) four (4) different sets of 3D data are collected---one (1) reference set of data and three (3) sets of data to code
flow. After data acquisition the phase image reconstruction is performed with the existing phase difference between the reference
data and the phase codification data for every direction. The magnitude image is reconstructed through a complex difference. This
figure from left to right shows both the magnitude image and velocity images in the x-, y-, z-axes, respectively.
with the ECG signal---for a total description the reader can
use the quote.15 The Cartesian segmentation method uses
the k-space with its three (3) perpendicular axes (kx, ky,
kz) and divides it into a 3D matrix (Nx, Ny, Nz). Then during each beat of the heart it acquires a set of single-cut
k-space lines (Nseg) for all phases of the cardiac cycle.4,16 If
Ny represents the lines all across the phase image (ky-axis)
and Nz represents the cuts (kz-axis) the acquisition of the
whole 3D volume will last for a certain number of beats equal
to NyNz/Nseg and a time equal to Tadq = (NyNz/Nseg)TECG ,
where Tadq is the duration of all acquisition and TECG is
the duration of a single beat. For every line of k-space
four (4) different data need to be gathered: one reference
acquisition and three (3) velocity codifications resulting
from the application of bipolar gradients in the x, y, z
directions.4,16 After the gathering of data the 3D images
are reconstructed---including one (1) anatomic or magnitude image and three (3) phase images representing blood
velocity in the three (3) axes of the coordinate system
(Fig. 3).
Image acquisition
We studied 10 volunteers (9 healthy ones and one with a
transplanted kidney) using a Philips Ingenia 3T kit (Philips
Healthcare, Eindhoven, The Netherlands) with 4D Flow
in some of the following body areas: chest, abdomen, neck
and skull. Acquisition parameters can be seen in Table 1. We
proceeding with free breathing, electrocardiographic synchronization and a chest, head, or neck cardiac coil based
on the examination area. Also in certain volunteers 2D cine
PCA with parameters of temporal resolution similar to those
of 4D Flow were acquired.
Image processing
Obtained data were extracted in RAR-REC format and
postprocessing was performed using the GTFlow software
(Gyrotools LLC, Zurich, Switzerland) that allowed us to estimate each point, blood velocity in the three (3) spatial
Table 1
489
Acquisition parameters for the 4D Flow modality based on the area of study.
Field of view (mm)
Voxel size (mm)
Images
RT (ms)/ET (ms)
Venc (cm/s)
SENSE (factor)
Angle of inclination (◦ )
Phases
Thorax
Abdomen
Neck and skull
350 × 350 × 120
2.5 × 2.5 × 5
48---56
4/2.1
200---150
2
6
16
350 × 350 × 120
2.5 × 2.5 × 5
40---50
4/2.1
100
2
6
20
270 × 270 × 70
1.5 × 1.5 × 1.5
40---60
4/2.1
110---160
2
6
16
ET, echo time; RT, repetition time; Venc, velocity encoding.
directions and in all phases of cardiac cycle. Postprocess
is detailed in the three (3) simplified steps shown in Fig. 4.
Additionally the following hemodynamic variables were
estimated in different vessels: ejected volume, average
velocity, maximum and minimum velocity17 by picking out
2D regions of interest from one (1) automatic edge detection
algorithm.
Methods of representation
4D Flow data are commonly viewed as streamlines, pathlines, vector charts and 3D meshes that allow us to represent
3D blood flow patters in time.18 In Fig. 5 the different
kinds of 3D representation that can be obtained through this
modality can be seen.
1. Streamlines. Streamlines are curves that connect
elements (particles) of a fluid in space with the characteristic that the curve is tangential to the velocity vector
of several particles in a given time. This is why streamlines give us a general view of the flow pattern described
by a fluid element in the cardiac cycle.19
2. Pathlines. Pathlines indicate the trajectory followed by
one particle of fluid. This representation can be regarded
as the pathway followed by one fluid element within the
flow at a given time. Pathlines are used to analyze
the temporal evolution of blood flow patterns during the
cardiac cycle. They can be coded with colors representing flow velocity too.20
3. Vector charts and meshes. For every particle vectors
charts associate one vector with the magnitude and
direction of flow velocity at that point so that for every
point the evolution of the flow velocity pattern can
be observed through time. You can also draw regions
of interest in any vessels in order to analyze the behavior of blood flow through any of the aforementioned
methods. You can also build 3D meshes to get profile
information of blood flow velocity on one vessel or region
if interest.
Artifacts and difficulties
Phase contrast images are susceptible to errors induced
by eddy currents and other heterogeneities of the magnetic field21 that have a tendency to increase as the
studied region moves away from the magnetic isocenter. Aliasing can occur since the velocity encoding (Venc)
sampling is lower than blood velocity. It is important to
correct data when these artifacts show up in order to
guarantee the vision and quantification of flow.4,7,9 To prevent these artifacts from happening due to aliasing the
use of one 2D cine PCA sequence is recommended and
based on the estimated peak blood velocity the picking of the most adequate value for Venc in the 4D Flow
Figure 4 4D Flow image postprocessing. Firstly the vascular structures in the magnitude image were segmented by choosing an
appropriate intensity threshold and secondly a 3D isosurface was created based on the maximum velocity data included in the
segmentation. Finally the vessel isosurface was used to generate a certain number of seeds (particles) inside the isosurface that
were used to estimate the streamlines, the pathlines and the 3D vector charts and meshes.
490
A
B
Velocity (cm/s)
Velocity (cm/s)
100.0
100.0
83.3
83.3
66.7
66.7
50.0
50.0
33.3
33.3
16.7
16.7
0.0
0.0
C
D
Velocity (cm/s)
Velocity (cm/s)
100.0
100.0
83.3
83.3
66.7
66.7
50.0
50.0
33.3
33.3
16.7
16.7
0.0
0.0
Figure 5 3D view methods of the aorta. 4D Flow images. (A) The streamlines give a 3D perspective of the movement of blood
at the vascular structures. (B) The pathlines give us information on the trajectory, velocity, direction, and evolution of blood flow
patters in every phase of cardiac cycle. (C) Vector charts show the magnitude of velocity and direction for every particle inside the
flow in a 2D view. At the image both the aorta and pulmonary trunk are shown. (D) Representation of the profile of blood velocity
in the ascending and descending aorta through two (2) 3D meshes.
acquisition always bearing in mind that a high Venc eliminates artifacts due to aliasing but it also limits the low
velocity-flow sensitivity.22,23 When it comes to artifacts
caused by respiratory motion there are several compensation modalities by using sensors (navigators) capable of
detecting the patient’s respiration through the diaphragmatic motion while synchronizing such respiration with 4D
Flow acquisition.4,7,24,25
The main setback of 4D Flow is the duration of 4D
Flow acquisition sequences---time especially increases when
more spatial and temporal resolution is needed or simply when the acquired volume is increased. To solve this
problem there are speed-up methods including parallel
images, sampling strategies exploring the correlations in the
space---time domain and radial acquisitions of k-space.26---28
These modalities can produce overlapping artifacts or
reduce the temporal resolution of images. Some of these
strategies are currently under investigations and validation.
The difficulty of clinically applying 4D Flow is due
to both the time of sequence acquisition and its
complexity. However, both scientific investigation and the
interest of the industry on MRIs predict a bright future for
this modality.
Applications of the 4D Flow modality
The 4D Flow modality allows us to assess blood flow
both quantitatively and qualitatively in any vessels of
interest and without any means of contrast. It allows
us to assess the heart large vessels, its ventricles and
valves in systole and diastole in one acquisition only. Also
thanks to 3D acquisition it allows us to select within
the acquired volume 2D views through any angles. Similarly and based on the area of interest abdominal, neck,
and skull vessels can be assessed among other vascular
structures.29---31
B
A
491
C
Velocity (cm/s)
Velocity (cm/s)
100.0
100.0
98.0
80.0
80.0
81.7
60.0
60.0
Velocity (cm/s)
65.3
49.0
40.0
40.0
20.0
20.0
0.0
0.0
32.7
16.3
528.00 ms
0.0
Figure 6 4D Flow images in large vessels and heart. (A) Segmentation of aorta and representation of blood flow through pathlines.
(B) View of blood trajectory in the aortic arch and the pulmonary artery in one healthy volunteer. (C) Heart ventricular filling in
diastole.
To better illustrate the capacity of quantitative analysis
this modality has the videos that can be seen in files 4D Flow
1 and 4D Flow 2.
1. Aorta and heart (Fig. 6). One of the various potential
clinical applications of 4D Flow is the study of flow in
the ascending aorta. Hope et al.31 evaluated one subgroup of patients with bivalve aortas and found within
this group an eccentric abnormal blood flow associated
with major shear forces in the vessel walls as well as
a greater risk of aneurysms. Consequently the risk of
aneurysm in patients with bivalve aorta can be stratified by quantifying the shear forces obtained through 4D
Flow, which allows more objectivity both in the diagnosis
and possible interventions in the ascending aorta of these
patients. Also possibly though 4D Flow we will be able to
quantify both the abnormal flow and the hemodynamic
load; define the risk of vascular diseases like aneurysms
or dissections before developing symptoms.31
The 4D Flow modality will possibly be useful in numerous clinical situations associated with congenital thoracic
cardiovascular anomalies like persistent arterial ductus,
conotruncal anomalies like the Fallot tetralogy, partial or total anomalous pulmonary venous returns and
in patients with left-to-right shunting. Also with this
modality we can identify patients at risk of pulmonary
hypertension and further development of Eisenmenge
syndrome.32 Similarly the MRI is the postoperative chosen
modality since it allows us to assess both anatomic and
functionally the cardiac cavities and the integrity and
functionality of surgical corrections like interatrial baffles, outflow septostomies and Fontan conduits, among
others. This is why 4D Flow images have a greater clinical value because they give us information of both
intra and extra cardiac blood flow patterns in these
situations.17,33
2. Abdomen.The Doppler ultrasound has been the less
invasive modality for the study of blood flows and the
quantification of hemodynamic parameters in the vena
cava, the portal system and vascular diseases (Fig. 7).
However, it can be greatly affected by factors such
as obesity, intestinal air or edema. This is why the
MR-angiography is more and more important---because it
does an excellent anatomical evaluation. Markl et al.4
published their results showing the potentiality of the
4D Flow modality both for the quantification and characterization of blood flow of the different intra-abdominal
vascular structures and in some pathological situations
like portal hypertension.34
In patients with renal transplant the MRIs can be very
useful because they give us important information like
perfusion defects, tumors, post-transplant proliferative
conditions and collections, among others. Through the
4D Flow modality we will be able to evaluate the size,
orientation and relation to other abdominal organs and
integrity of the intra-renal vascular structures. It is also
possible to evaluate the anastomoses of the renal artery
to the iliac artery and of the renal vein to the iliac vein
while watching and quantifying blood flow through these
anastomoses (Fig. 7).4
3. Carotid and cerebral arteries. The digital subtraction
angiography is the chosen method to evaluate neck and
intra-cranial arteries but it is also an invasive method
requiring iodinated contrast media. Thus techniques
like CT-angiography (with iodinated contrast too though)
and MR-angiography are more and more important for
the study of carotid and cerebral arteries. The carotid
Doppler is a very valuable low cost-tool thanks to its
great accuracy for the study of carotid bifurcations
yet, it is limited to small fields, it depends on the
operator and is sensitive to a poor acoustic window.
The 4D Flow modality has all the characteristics of the
aforementioned methods while giving us anatomic and
hemodynamic information (Fig. 8). It allows us to assess
the vascular structures of the neck and the distribution
of the carotid walls shear forces, find carotid stenoses
and quantify blood velocity in the common carotid
artery and the carotid bifurcation.35 It is also useful to
evaluate and quantify both flow and velocity patterns
in aneurysms and intra- and extra-cranial arteriovenous
malformations.36---38 Studies performed in 7 T ultra-high
magnetic field resonance imaging devices have shown
that smaller caliber-vessels can adequately be assessed
(e.g. the posterior cerebral artery) and their pulsatility
492
A
B
C
Velocity (cm/s)
Velocity (cm/s)
100.0
100.0
80.0
80.0
60.0
60.0
40.0
40.0
20.0
20.0
0.0
0.0
D
F
E
Velocity (cm/s)
Velocity (cm/s)
100.0
100.0
80.0
80.0
60.0
60.0
40.0
40.0
20.0
20.0
292.40 ms
0.0
0.0
Figure 7 4D Flow images in abdominal vessels and in one patient with a renal transplant. (A) Healthy volunteer. Abdominal
aorta, main efferent branches and main intra-abdominal venous structures. (B) Detail from the origin of the celiac trunk, the
superior mesenteric artery and the portal vein. (C) Saggital image with anatomic reference of the abdominal aorta and some of
its main branches. (D) Patients with a renal transplant. Coronal view where 3D streamlines of the vascular structures irrigating the
transplanted kidney can be seen. (E) Detail of the arterial (caudal vessel) and venous flow (celiac vessel) and blood velocity coded
based on a scale of colors. (F) Pathlines of former case.
A
C
B
Velocity (cm/s)
Velocity (cm/s)
Velocity (cm/s)
40.0
40.0
100.0
33.3
33.3
80.0
26.7
26.7
20.0
20.0
13.3
13.3
6.7
6.7
0.0
0.0
60.0
40.0
20.0
0.0
Figure 8 Blood flow patterns at the main cerebral arteries and neck. (A) 4D Flow of the carotid and vertebrobasilar system of one
healthy volunteer performed in 3T. Color patterns are coded according to velocity. (B) View through pathlines at the bifurcation of
the right carotid and vertebral arteries. (C) Velocity patterns at the right carotid vertebral arteries showing greater peak velocity
in the carotid artery. Note the parabolic profiles so characteristic of laminar flow in both arteries.
70.0
A
Average velocity (cm/s)
cm/s
C
60.0
493
Vavg XYZ
50.0
50
40.0
40
30.0
30
20.0
10.0
20
0.0
10
–10.0
0
Velocity (cm/s)
100.0
80.0
60.0
Average velocity (cm/s)
B
40.0
20.0
100 200 300 400 500 600 700 800 900
Time (ms)
1
70.0
cm/s
60.0
50
3
5
7
9
11
13
15
Vavg XYZ
50.0
40
40.0
30.0
30
20.0
20
10.0
0.0
10
–10.0
0.0
0
100
200 300 400 500
Time (ms)
600 700 800 900
1
3
5
7
9
11
13
15
Figure 9 4D Flow and 2D PCA. (A) Quantification of the average velocity of flow in the ascending aorta through the 2D cine PCA
modality. To the left we can see flow direction for every vessel and to the right the chart of velocity values in the cardiac cycle.
(B) The same quantification was performed with 4D Flow. Note the similarity of values of the average velocity of blood measured
through 2D cine PCA and 4D Flow modalities (B). (C) Selection of the ascending aorta and the pulmonary artery in one single 4D
Flow study. The charts of average velocity of aorta (up) and pulmonary artery blood (down) are shown here as well. In charts A and
B y-axis is blood velocity (cm/s) while x-axis is time (s). However, in chart C x-axis represents the cardiac cycle divided into sixteen
(16) different phases.
index be measured which can be applied in processes
like trigeminal neuralgia.39
4D Flow and 2D cine PCA
Today 2D cine PCA is widely used for the study of blood flow
in cardiovascular MRIs. However, its main setback is that
is uses 2D cut views that need to be selected during the
examination and require a highly skilled operator managing
the resonance equipment. In contrast in the 4D Flow modality the whole heart is 3D volume-studied, so the planning
of the sequence is greatly simplified without contribution
from the operator (Fig. 9). However, the greater advantage
of the 4D Flow modality lies in its capacity to perform multiplanar image analyses in the postprocessing since it allows
the quantification of blood flow in any arbitrary views and
angles without the need for acquiring new images. Other
than measuring flow and velocity it also allows us to estimate hemodynamic biomarkers such as shear stress in the
blood vessel walls, pressure gradients and pulse wave velocity without the use of any venous contrast media.40,41
Some studies have confirmed that the 4D Flow measurements of flow and velocity strongly correlate to that
of 2D cine PCA.42,43 On the other hand these modalities
have a tendency to underestimate 20---25% of the maximum
velocity or average systolic peak observed through Doppler
ultrasound.35 However, we need to understand that in the
MRIs velocity is always averaged since both multiple cardiac beats and a greater voxel size are needed to be able
to obtain 3D images which in turn translates into a lower
temporal and spatial resolution than that of Doppler ultrasound. When the resolution of the 4D Flow sequence is very
high only those voxels displaying the fastest velocity will
show values more proximal to that of Doppler ultrasound.
Conclusion
In this review the 4D Flow modality is shown to be valid,
non-invasive and allowing us to perform quantitative and
qualitative analyses of blood flow in large and mediumsize vessels without the need for exogenous contrasts. This
modality has a great clinical potential since it gives us
complete morphologic and anatomical information on the
cardiovascular system by estimating hemodynamic biomarkers and visualizing blood flow patterns.
Conflict of interests
The authors reported no conflicts of interests.
Ethical responsibilities
Protection of people and animals. Authors confirm that no
experiments have been performed on human beings or animals.
Data confidentiality. Authors confirm that the protocols of
their centers have been followed on matters concerning the
publishing of data from patients. They also confirm that all
patients included in this study have been given enough information and handed over their written informed consent for
their participation in this study.
Right to privacy and informed consent. Authors confirm
that in this report there are no personal data of patients.
494
Authors/collaborators
Manager of the integrity of the study: JAPZ, JADB, SRR.
Original idea of the study: JAPZ, JADB, SRR.
Study design: JAPZ, VDCB, CBA, SMV.
Data mining: JAPZ, CBA.
Data analysis and interpretation: JAPZ, VDCB, CBA, SMV.
Statistical analysis: VDCB.
Reference search: CBA, SMV.
Writing: JAPZ, VDCB, SMV, CBA, JADB, SRR.
Manuscript critical review with intellectually relevant
contributions: JADB, SRR.
10. Final version approval: JAPZ, VDCB, SMV, CBA, JADB,
SRR.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.rxeng.2014.08.001.
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