1. 1. 4D Flow MRI BENJAMIN CULPEPPER
2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 19. Data Analysis 3D Blood Flow Visualization [5]
20. 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. 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. 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. 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. 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. 25. End