Transcript of 4D Flow MRI
- 1. 4D Flow MRI BENJAMIN CULPEPPER
- 2. What is 4D flow? MRI techniques carry valuable tools for:
Diagnosing cardiac and vascular diseases Measuring disease severity
Assessing patient response to medical and surgical therapy.
Provides morphological information Provides functional information
on cardiac perfusion, myocardial viability, and blood flow. Figure
1: a 3D rendering of the heart with blood flow depicted through
streamlines of varying color. The different colors determine areas
of high/low velocity. [4]
- 3. What is 4D flow? Further development of phase contrast (PC)
techniques has resulted in the acquisition of a time-resolved
(CINE), 3D PC-MRI. This includes: Three-directional velocity
encoding. This is known as 4D flow MRI. 4D flow MRI can provide
information on a temporal and spatial evolution of 3D blood flow
that includes full volumetric coverage of any cardiac region of
interest. MRI_gadolinium -enhanced MR angiogarphy with 3D
reconstruction. [1]
- 4. What is 4D flow? Images from 2D CINE PC-MRI examination of
right renal artery. [1] A Particular advantage over 2D CINE PC-MRI
is related to the possibility for retrospective selection of
territories at any location inside the 3D data volume to perform
post-hoc quantification of blood flow parameters like: 1. Total
flow 2. Peak velocity 3. Regurgitant fraction 3D CINE PC-MRI of a
looped cardiac cycle. [1]
- 5. A number of 4D flow MRI studies have attempted to assess and
corroborate blood flow parameters: 1. Peak pressure gradient 2.
Peak and mean velocities 3. Net flow over the cardiac cycle 4.
Vessel area 4D flow MRI can be used to derive hemodynamic measures
like: 1. Wall shear stress 2. Pressure difference 3. Pulse wave
velocity 4. Turbulent kinetic energy for improved characterization
of cardiovascular disease. Summary of time-averaged wall shear
stress (WSS) results for all subjects. Mean WSS color maps (left
panel) for two representative age- and sex-matched (top) normal
subjects and (bottom) PAH patients. Mean WSS was lower in the
proximal arteries of the PAH patients than in those of the normal
subjects. Mean WSS averaged over the area of 10-mm circumferential
strips taken at the LPA and RPA (proximal) and distal locations was
significantly different between the two populations (right panel).
[5]
- 6. MRI flow measurements provide information of blood supply of
various vessels and tissues as well as cerebro spinal fluid
movement. In addition to PC and CINE sequences, flow can be
measured and visualized with time of flight angiography; also,
contrast enhanced MRI methods may be implemented. The different
flow types encountered in 4D flow are: 1) Stagnant flow 2) Laminar
Flow 3) Vortex Flow 4) Turbulent Flow Figure: Evolution of vortex
structures during pulsatile cycle. [1]
- 7. Flow voids Is the occurrence of a low signal in regions of
flow. [1] Ghost images These are caused by pulsatile flow of the
vessel extending across the image in the phase encoding direction.
This image is a subtraction of two T1 weighted pre- and post
contrast images. The motion artifact appears as ghosting and
blurring caused by fluid and bowel motion. [1] Flow related
dephasing Occurs when spin isochromats are moving with different
velocities in an external gradiaent field so that they acquire
different pahses. Figure: Flow dephasing from turbulent flow around
tumor. [1]
- 8. Flow Artifacts: The inconsistency of the signal resulting
from pulsatile flow can lead to artifacts in the image. Spin Phase
Effect, Flow: are vascular ghosts (ghosting artifact), and
anomalous intensities in images. Reason: movement of bodily fluids.
Help: flow compensation, presaturation, triggering. Image Guidance:
reduction in artifacts in reducing phase shifts with flow
compensation, suppression of the blood signal. Radio Frequency
Overflow, Data Clipping: it is a non-uniform image. Reason: signal
too intense. Help: Manually decrease of the receiver gain. The
received radio frequency signal is too strong, resulting in a
washed out image. Image Guidance: Auto-prescanning usually adjusts
the amplification at the receiver. Figure: Ghosting from abdominal
fat, oriented in the phase encoding direction. [2]
- 9. Flow Artifacts: The inconsistency of the signal resulting
from pulsatile flow can lead to artifacts in the image. Cerebro
Spinal Fluid Pulsation Artifact: may be described by ghosting.
Reason: Inconsistencies in phase and amplitude. Help: flow
compensation, cardiac triggering. Image Guidance: Flow compensation
should be used to reduce these artifacts. This applies an
additional gradient to eliminate phase differences for both
stationary and moving spins at the echo time. Data Clipping: These
artifacts give rise to an image to appear washed-out, non-uniform.
The overall intensity loss as well as the extensive signal is
reconstructed outside of the object. This effect is called clipping
because on a plot of signal amplitude vs time, it appears as if the
top and bottom of the echo has been clipped off with scissors.
- 10. Standard 2D PC-MRI: PC-MRI takes advantage of the direct
relationship between blood flow velocity and the phase of the MR
signal that is acquired during an MRI measurement. Signal
intensities in resulting phase difference images are directly
related to blood flow, allowing us to visualize and quantify blood
flow. 2D MRI of blood flow. Cardiac mitral valve proplapse in
vertical long axis view. [4] 3D PC-MRI image composed of a series
of 2D slices for blood flow intensities. [6]
- 11. 2D PC data is acquired over multiple cardiac cycles using
ECG gated CINE imaging to measure pulsatile blood flow. In clinical
applications, the 2D imaging slice is typically positioned normal
to the vessel lumen. Data acquisition is of single-direction
velocity measurement performed during a 10-20 second breath holding
period. 2D CINE PC-MRI yield a series of anatomical and flow
velocity images. Typical parameters: 1. Spatial resolution, 1.5 2.5
, 2. Temporal resolution, 30 60 ms, 3. Slice thickness, 5 8 mm.
Figure: Standard 2D CINE PC-MRI with one- directional through-plane
velocity encoding. [4]
- 12. Cardiac Gating This first 4 MRI CINE imaging slices are
carried out in conventional short axis orientation (two chamber
view) from a apical to a midventricular slice of the heart. [3]
Synchronizes heartbeat with beginning of repetition time (TR),
whereat the r wave is used as the trigger. ECG gating techniques
are useful whenever data acquisition is too slow to occur during a
short fraction of the cardiac cycle. Image blurring occurs for
imaging times above approx. 50 ms in systole, while imaging during
diastole is of the order 200 300 ms. Cardiac infarct 4 chamber view
including the left ventricular outflow tract. [3]
- 13. PC-MRI and Velocity Encoding Sensitivity: Important PC-MRI
parameter is the maximum flow velocity. When the underlying
velocity exceeds the acquisition setting for Venc, the velocity
aliasing can occur which is typically visible as a sudden change
from high to low velocity within a region of flow. velocity noise
is directly related to the maximum flow velocity. selecting a high
Venc may alleviate the issue of velocity aliasing but will also
increase the level of velocity noise in flow velocity images.
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. 2D CINE PC-MRI with aliasing
in a patient with bicuspid aortic valve disease and aortic
coarctation. Patient underwent standard MRA along with 2D CINE
PC-MRI for quantification of ascending aorta and post-coarctation
flow velocity. [4]
- 14. 4D Flow MRI: Velocity is encoded along all three spatial
dimensions throughout the cardiac cycle, providing a time-resolved
3D velocity field. Three-directional velocity measurements can be
achieved by interleaved four-point velocity encoding. After
completion of the 4D flow acquisition, four time-resolved (CINE) 3D
datasets are generated. Data acquisition and analysis workflow for
4D flow MRI. [4]
- 15. 4D Flow MRI: Efficient data acquisition is necessary to
achieve practical scan times for 4D flow MRI in clinical
application. From a hardware point of view, the availability of
high performance gradients has reduced both the echo and repetition
times (TE and TR, respectively) and, thereby, total scan time.
Introductions of phased-array coils, multi-receiver channels, and
parallel imaging technology have been able to reduce scan time.
Other methodological improvement approaches: 1. Radial
under-sampling 2. Kt-BLAST. 3. Kt-SENSE. 4. Kt-GRAPPA. 5. Or
compressed sensing. Radial data sampling combined with
under-sampling is being increasingly used for 4D flow MRI. PC-VIPR
can reduce occurrence of motion artifacts, enabling self-gating due
to intrinsic properties of radial data acquisition strategies.
- 16. Data Analysis: Preprocessing and Corrections There exist
multiple sources of phase offset errors that can degrade image
quality. Most commonly encountered errors: 1. Eddy currents 2.
Maxwell terms 3. Gradient field nonlinearity Appropriate correction
strategies must be included to compensate for all errors. Eddy
current correction cannot easily be automated and has to be
integrated into the data analysis workflow. (Middle) Data
preprocessing corrects for errors due to noise, aliasing and eddy
currents and calculates the 3D PC-MRA. [4]
- 17. Data Analysis: Preprocessing and Corrections Black areas
with bright spots and an overall bad image quality are
characteristic for eddy currents. [2] The image distortion is
visible over the whole slice. [2]
- 18. Data Analysis 3D Blood Flow Visualization Two examples of
systolic 3D streamline representation of 4D flow MRI data in
patients with bicuspid aortic valve. [4] 4D flow MRI in a 3.5
year-old pediatric patient with bicuspid aortic valve and aortic
coarctation at the distal arch/proximal descending aorta junction.
[4]
- 19. Data Analysis 3D Blood Flow Visualization [5]
- 20. Clinical Application: CHD 17 year-old female with Tetralogy
of Fallot repaired with transannular patch at 2 years of age.
Particle trace visualization during a right ventricular diastolic
time frame demonstrates pulmonary regurgitation (closed arrow). The
majority of the flow from the right atrium (RA) into the RV is
directed abnormally toward the RV apex (curved dashed arrow) with a
smaller vortex just beyond the tricuspid valve (open arrow).
Color-coding was achieved with respect to the absolute acquired
velocities. SVC = superior vena cava; IVC = inferior vena cava; MPA
= main pulmonary artery; RPA = right pulmonary artery. [6] When
complex CHD is suspected, imaging evaluations provide clinicians
with key diagnostic and surgical planning information. Some
patients develop serious complications and regular imaging
evaluations are critical to their follow-up care.
- 21. Clinical Application: CHD Estimation of the severity of
pulmonary regurgitation after repair of tetralogy of Fallot using
velocity-encoded cine MRI. The curve on the right plots pulmonary
arterial flow against ECG- trigger delay time. The area between the
baseline and the curve below the baseline represents regurgitant
volume. [5] Primary imaging modality for early evaluation with
complex CHD is ultrasound; specifically, transthoracic and
transesophageal echocardiography. 4D flow MRI techniques allow for
a non-invasive comprehensive assessment of cardiovascular
hemodynamics. For this, FOV is adjusted to contain heart and
surrounding vessels. Main advantages, it facilitates the systematic
assessment of blood flow in multiple vessels. 4D flow MRI has the
potential to predict or detect complications of CHD earlier in the
diseases course. Tubular hypoplasia of the aortic arch and
coarctation in a 17-year-old female. Maximal intensity projection
image obtained by contrast- enhanced MR angiography clearly shows
arch hypoplasia and coarctation (arrow). This patient also suffered
intracerebral hemorrhage, probably associated with coarctation.
[5]
- 22. Clinical Application: CHD Patients with Fontan circulation
have been evaluated for flow and mixing characteristics.
Time-resolved pathlines were generated to illustrate the spatial
distribution and dynamics of blood flow during the cardiac cycle.
Fontan hemodynamics can thus be substantially different between
patients despite similar Fontan geometry. Additionally, these
findings indicate that some patients have uneven distribution of
hepatic-rich venous return from the lower body to the left and
right lungs. In healthy volunteers with Fontan circulation, one
group found agreement between flow shunting measurements based on
4D flow MRI pathline counting and 4D flow MRI net forward flow
measurements.
- 23. Clinical Application: CHD Clinical applications not
discussed for this presentation: 1. Thoracic Aorta 2. Hepatic and
Portal Venous Flow 3. Advanced 4D flow Hemodynamic Markers 4.
Intracranial Hemodynamics 5. Carotid Arteries 6. Whole Heart 7.
Pulmonary Arteries 8. Renal Arteries 9. Peripheral Vessels and
Peripheral Arterial Occlusive Disease
- 24. 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.
- 25. End