4D flow MRI is a technique that uses phase contrast MRI to acquire 3D velocity data throughout the cardiac cycle, providing a time-resolved 3D velocity field. This allows for comprehensive visualization and quantification of blood flow in the heart and surrounding vessels. After acquisition, the 4D flow MRI data undergoes preprocessing and corrections before 3D visualization of blood flow patterns like streamlines or pathlines over the cardiac cycle. Whole heart 4D flow MRI has been used to assess congenital heart disease patients after surgery and has identified abnormal flow patterns in the pulmonary arteries of patients with repaired tetralogy of Fallot.

1. 1
4D Flow MRI
Benjamin Culpepper
December 2, 2014
Phase contrast (PC) MRI has seen broad clinical acceptance for the visualization and
quantitative evaluation of blood flow in the heart, aorta and large vessels. Further development
of phase contrast techniques has resulted in the acquisition of a time-resolved (CINE), 3D PC-
MRI with three-directional velocity encoding. This is what we define, or refer to, as 4D Flow MRI,
and concisely answers the initial question of, “What is 4D flow MRI?” Using this definition I will –
in my presentation – move to the description of 4D flow MRI in the heart for obvious reasons.
Once we appropriately define 4D flow MRI for investigating and properly imaging blood flow, I
detail the various flows that are exhibited throughout blood vessels: stagnant flow, laminar flow,
vortex flow, and turbulent flow. Due to the girth of the presentation, we do not spend lengthy
moments discussing them any further than necessary.
As one might assume, motion of material being imaged – particularly of flowing blood –
can result in many possible effects in the produced images. Effects that can appear are: flow
voids due to fast moving blood, ghost images of the vessel extending across the image in the
phase encoding direction, and flow related dephasing occurring when spin isochromats are
moving with different velocities in an external gradient field so that they acquire different phases.
The inconsistency of the signal resulting from pulsatile flow can lead to artifacts in the image.
Artifacts that are expanded upon include: spin phase effect, flow artifacts, radio frequency
overflow, data clipping artifacts, and cerebro spinal fluid pulsation artifacts.
Here we digress to a pre-4D flow situation where we examine standard 2D PC-MRI, PC-
MRI and velocity encoding sensitivity, and then 4D flow MRI. The first two topics are used to
build-up and transition into the realm of 4D flow MRI.
Standard 2D PC-MRI (also termed as ‘flow-sensitive MRI’ or ‘MR velocity mapping’)
takes advantage of the direct relationship between blood flow velocity and the phase of the MR
signal that is acquired during an MRI measurement. In short, we acquire two acquisitions with
different velocity-dependent signal phases to encode and measure blood flow velocity along a
single direction, remove “all” background phase effects via subtraction of the above phase
images, and, finally, we obtain a resulting phase difference image directly related to the blood
flow, which can be used to visualize and quantify blood flow. A clinical application is briefly
described along with the technique in how data acquisition occurs for various patients. Typical
measurement parameters for 2D CINE PC-MRI:
1) Spatial resolution, 1.5 – 2.5 mm
2) Temporal resolution, 30 – 60 ms
3) Slice thickness, 5 – 8 mm

2. 2
For cardiac gating, this method synchronizes the heartbeat with the beginning of the
repetition time (TR), whereas the R wave is used as the trigger. Cardiac gating times the
acquisition of MR data to physiological motion in order to minimize motion artifacts. ECG gating
techniques are useful whenever data acquisition is too slow to occur during a short fraction of
the cardiac cycle. If a series of images using cardiac gating or real-time echo planar imaging
EPI are acquired over the entire cardiac cycle, pixel-wise velocity and vascular flow can be
obtained. In simple cardiac gating, a single image line is acquired in each cardiac cycle. Lines
for multiple images can then be acquired successively in consecutive gate intervals. By using
the standard multiple slice imaging and a spin echo pulse sequence, a number of slices at
different anatomical levels is obtained.
PC-MRI and velocity encoding sensitivity is considered immediately. An important PC-
MRI parameter is the maximum flow velocity that can be acquired. When the underlying velocity
exceeds the acquisition setting for velocity encoding, then velocity aliasing can occur, which is
Figure:Standard2D CINE PC-MRI
withone-directional through-plane
(Z) velocityencoding. [4]
Figure:Thisis supposedtobe a
loopingvideo(.gif file)thatdepictsa
cardiac infarct4 chamberview
includingthe leftventricularoutflow
tract. [3]

3. 3
typically visible as a sudden change from high to low velocity within a region of flow. Note that
the velocity encoding (Venc) can be increased and the acquisition is repeated to avoid aliasing. It
is important, also, that velocity noise is directly related to the maximum flow velocity. Therefore,
selecting a high Venc may alleviate the issue of velocity aliasing but will also increase the level of
velocity noise in flow velocity images. To capture the best image quality, the chosen Venc should
represent the physiological velocity of the vessel of interest and be adapted to the measurement
of interest and present hemodynamic conditions.
Typical settings for Venc are:
1) 150 – 200 cm/s in the thoracic aorta.
2) 250 – 400 cm/s in the aorta with aortic stenosis or coarctation.
3) 100 – 150 cm/s for intra-cardiac flow.
4) 50 – 80 cm/s in large vessels of the venous system.
In 4D flow MRI, velocity is encoded along all three spatial directions throughout the
cardiac cycle, thus providing a time resolved 3D velocity field. Three-directional velocity
measurements can be efficiently achieved by interleaved four-point velocity encoding. After
completion of the 4D flow acquisition, four time-resolved 3D datasets are generated. Due to this
large amount of data that has to be collected, efficient data acquisition is necessary to achieve
practical scan times for 4D flow MRI in clinical applications. From a hardware point-of-view, the
availability of high performance gradients has reduced both the echo and repetition times and,
thereby, total scan time. The introductions of phased-array coils, multi-receiver channels, and
parallel imaging technology have also been applied to PC-MRI, primarily to reduce the scan
time. Other methodological improvements include the use of advanced accelerated imaging
approaches such as:
1) Radial under-sampling,
2) Kt-BLAST,
3) Kt-SENSE,
4) Kt-GRAPPA,
5) Or compressed sensing.
Once we have acquired all 4D flow MRI information, the next step in visualizing the data
is to go through preprocessing and corrections analysis. The potential sources of error that
Figure:(A-C) 2D CINEPC-MRI with
aliasingina patientwithbicuspid
aortic valve diseaseandaortic
coarctation.The patientunderwent
standardMRA as well as2D CINE PC-
MRI forthe quantificationof
ascendingaortaand post-coarctation
flow velocity. [4]

4. 4
might require corrections include: eddy currents, Maxwell terms, and gradient field nonlinearity,
and it is important to apply appropriate correction strategies to compensate for these potential
errors before further processing of the data for 3D visualization or flow quantification. Maxwell
terms and gradient field nonlinearity can be corrected during image reconstruction, but eddy
current correction has to be integrated into the data analysis workflow (the tactic considering
eddy currents will be discussed in the presentation). Following applied corrections we may
proceed to the 3D blood flow visualization, which will include a graphical depiction of 3D
streamlines to identify specific systolic flow features such as outflow jets or helix flow. For
visualization of the temporal evolution of 3D blood flow over one or more heartbeats, time-
resolved pathlines are the visualization method of choice. These pathlines are best viewed and
displayed dynamically to fully appreciate the dynamic information and changes in blood flow
over the cardiac cycle.
Throughout this report (and – as shall occur – in the presentation) we have defined 4D
flow MRI, discussed what artifacts might arise from visualization of uncorrected data, discussed
2D PC-MRI and PC-MRI and velocity encoding sensitivity so to build a foundation that led us to
the discussion of 4D flow MRI, preprocessing, corrections, visualization, and quantification. Now
we shift our attention to that of a clinical application through congenital heart disease (CHD).
When complex CHD is suspected, imaging evaluations provide clinicians with key
diagnostic and surgical planning information. However, some patients develop serious
complications and regular imaging evaluations are critical to their follow-up care. Whole heart
4D flow MRI techniques allow for a non-invasive comprehensive assessment of cardiovascular
hemodynamics in the heart and surrounding great vessels. For this technique, the FOV (field of
view) is adjusted to contain the heart and surrounding large vessels to obtain flow data for the
entire region in one imaging protocol. The main advantages of whole heart imaging are that it
facilitates the systematic assessment of blood flow in multiple vessels and enables the
retrospective analysis of any region of interest within the imaging FOV. 4D flow MRI also has
the potential to predict or detect complications of CHD earlier in the disease course, which could
impact outcomes through improved risk stratification and disease management in these
patients. 4D flow whole heart MRI with 3D visualization and quantitative flow analysis has been
performed in patients after Tetralogy of Fallot (TOF) repair and marked variations in flow
characteristics were observed. Findings included retrograde flow and vortex formation in the
pulmonary trunk (PT) and pulmonary arteries (PA) as well as higher right/left pulmonary artery
blood flow ratios, flow velocity and WSS in the PT than healthy patients. These results indicate
Figure:Data acquisitionandanalysis
workflow for4D flow MRI.This figure
representsaverygeneral procedure
for acquiringimage data,
preprocessingthatimage data,and
thenconstructinga visual graphic(of
2D or – inthiscase – 3D). [5]

5. 5
the feasibility of the comprehensive evaluation of 3D hemodynamics by 4D flow MRI for the
post-surgical assessment of patients with TOF.
Figure:17 year-oldfemale with
Tetralogyof Fallotrepairedwith
transannularpatchat 2 years of age.
Particle trace visualizationduringaright
ventriculardiastolictime frame
demonstratespulmonaryregurgitation
(closedarrow).The majorityof the flow
fromthe rightatrium(RA) intothe RV is
directedabnormallytowardthe RV apex
(curveddashedarrow) withasmaller
vortex justbeyondthe tricuspidvalve
(openarrow).Color-codingwas
achievedwithrespecttothe absolute
acquiredvelocities.SVC=superiorvena
cava; IVC= inferiorvenacava;MPA =
mainpulmonaryartery;RPA = right
pulmonaryartery. [6]

6. 6
References:
[1] “Flow.” Magnetic Resonance – Technology Information Portal. Softways 2003. n.d. Web. 4
December 2014.
[2] “Flow Artifact.” Magnetic Resonance – Technology Information Portal. Softways 2003. n.d.
Web. 4 December 2014.
[3] “Cardiac Gating.” Magnetic Resonance – Technology Information Portal. Softways 2003. n.d.
Web. 4 December 2014.
[4] Stankovic, Zoran. Allen, Bradley D. Garcia, Julio. Jarvis, Kelly B. Markl, Michael. “4D flow
Imaging with MRI.” The Cardiovascular Diagnosis & Therapy. 21 October 2013. Web. 1
December 2014.
[5] Choe, Yeon Hyeon. Kang, I-Seok. Park, Seung Woo. Lee, Heung Jae. “MR Imaging of
Congenital Heart Disease in Adolescents and Adults.” US National Library of Medicine. National
Institutes of Health. Korean Society of Radiology. 30 September 2001. Web. 1 December 2014.
[6] Geiger, J. Arnold, R. Frydrychowicz, A. Stiller, B. Langer, M. Markl, M. “Whole Heart Flow
Sensitive 4D MRI in Congenital Heart Disease.” n.p. n.d. Web. 1 December 2014.