MRI: Principles and Applications, Friday, 8.30 9.20 am Pulse sequences for non CE MRA S. I. Gonçalves, PhD Radiology Department University Hospital Coimbra Autumn Semester, 2011 1
Magnetic resonance angiography (MRA) In conventional MRI, one tries to visualize organ parenchyma. In this sense, blood vessels and the effects of blood flow are often confounders and a source of artifacts; For that reason, cardiac gating and blood flow compensation are commonly used strategies to overcome these problems; However, imaging blood vessels has an interest on its own, and that is the goal of Magnetic Resonance Angiography (MRA); 2
MRA methods RARE Dark Blood IR MRA pulse sequences RARE Rapid acquisition with refocused echoes (FSE) IR Inversion Recovery CE Contrast enhanced TOF Time of flight PC MRI Phase Contrast MRI SSFP Steady state free precession Bright Blood CE MRA Non CE MRA TOF PC MRI SSFP 3
MRA methods RARE Dark Blood IR MRA pulse sequences RARE Rapid acquisition with refocused echoes (FSE) IR Inversion Recovery CE Contrast enhanced TOF Time of flight PC MRI Phase Contrast MRI SSFP Steady state free precession Bright Blood CE MRA Non CE MRA TOF PC MRI SSFP 4
Introduction MRA can be accomplished by enhancing the signal from blood through the inflow of fresh (unsaturated) spins into the slice of interest via the use of an exogenous T 1 reducing contrast agent CE MRA; Or It can be accomplished by looking carefully into blood flow effects and take advantage of those to retrieve information about vessel morphology (e.g. measurement of blood vessel lumen) but also about flux velocities: Time Of Flight (TOF) or Phase Contrast (PC ) MRI; 5
Introduction CE MRA is the traditional method to acquire MRA images; It has short acquisition times and it is characterized by good SNR; Improved anatomical coverage; Decreased susceptibility artifacts due to blood flow and pulsatility; There have been reports linking gadolinium usage to nephrogenic systemic fibrosis in patients with kidney disease or insufficiencies (http://www.nephrogenicsystemicfibrosis.org/nsf/); Current CE MRA using conventional Gd contrast agents are limited to the need of acquiring images very fast during the first pass of the contrast in the vessels of interest; 6 Cost;
MRA methods RARE Dark Blood IR MRA pulse sequences RARE Rapid acquisition with refocused echoes (FSE) IR Inversion Recovery CE Contrast enhanced TOF Time of flight PC MRI Phase Contrast MRI SSFP Steady state free precession Bright Blood CE MRA Non CE MRA TOF PC MRI SSFP 7
Time Of Flight (TOF) In a conventional analysis of imaging sequences, tissues (spins) are assumed to be stationary; With the exception of a few single shot methods, the magnetization must be excited several times (at least once per TR), in order to acquire an image; This leads to magnetization saturation and signal loss due to incomplete T 1 relaxation; 8
Time Of Flight (TOF) Stationary tissue RF excitation Measured signal 9
Time Of Flight (TOF) Stationary tissue RF excitation Measured signal 10
Time Of Flight (TOF) Stationary tissue RF excitation Measured signal Saturation 11
Time Of Flight (TOF) Blood flow (moving spins) RF excitation v Imaging volume Measured signal 12
Time Of Flight (TOF) It is the most commonly used non CE MRA technique, especially for intracranial and peripheral applications; Rapid series of slice selective RF excitation pulses that saturate (null) background signal; Signal from flowing blood results from the constant entrance of fresh magnetization into the imaging plane Flow Related Enhancement (FRE) (Axel, 1984); FRE increases with blood flow velocity; 13
Time Of Flight (TOF) 3D TOF MRA 14
Time Of Flight (TOF) Carotid arteries Carotid arteries Vertebral arteries Native image MIP reconstruction 15 2D TOF MRA Hartung et al., 2011
Time Of Flight (TOF) TOF can be 2D or 3D; Assumed to have velocity compensation; Susceptible to signal voids in regions with slow flow, tortuous vessels or in plane vessels; 3D TOF most commonly used for intra cranial vasculature (where in plane flow is more frequent); 2D more frequently used for carotid evaluation; Variable flip angles and MOTSA techniques to overcome signal saturation; 2D TOF MRA Pelvis, thighs, calves Bilateral occlusion of superficial femoral arteries, flow to runoff arteries (calves) through collateral vessels in the thighs, arising from the profunda femoris arteries. Hartung et al., 2011 16
Phase Contrast (PC )MRI PC MRI generates an image by applying an extra velocity encoding gradient; The resulting signal is proportional to flow velocity, as in TOF but also to the strength of the velocity encoding gradient (which is controlled by the parameter Venc); Contrary to TOF, the application of velocity encoding gradients allows the retrieval of information over blood flow velocities; Thus, PC MRI provides anatomical images of the vessels as well as information about the hemodynamics of blood flow; 17
Phase Contrast (PC )MRI RF slice phase readout signal Bipolar velocity encoding Echo 1, 1 TR 18
Phase Contrast (PC )MRI RF slice phase readout signal Echo 2, 2 Bipolar velocity encoding 19 TR
Phase Contrast (PC )MRI Velocity information: 1 2 Phase Image from which velocities can be derived PC MRI can be used to identify the direction and velocity of flow; Parameter Venc gives the maximum velocity that can be measured without aliasing; Better background suppression than TOF; Limited by longer image acquisition times and higher sensitivity to changes in velocity and magnitude of blood flow during cardiac cycle; Sensitive to types of flow which can be important for diagnosis (e.g. signal voids in regions of turbulent flow in the vicinity of a stenosis; 4D PC MRI is the new development of PC MRI where time resolved images are acquired with the add of acceleration techniques such as parallel imaging, 3D radial undersampling trajectories, etc; 20
Phase Contrast (PC )MRI Signal void indicates stenosis with important consequences in the hemodynamics of blood flow 3D CE MRA 3D PC MRI Digital subtraction angiography 21 Patient with renal artery stenosis Hartung et al., 2011
Phase Contrast (PC )MRI Phase image of aortic arch and major pulmonary vessels Color coded sagittal phase images of the thoracic aorta with flow quantification 22 https://www.medical.siemens.com/webapp/wcs/stores/servlet/
Steady State Free Precession (SSFP) MRA Popular because b SSFP sequences are weighted on T 2 /T 1 which provides an inherent bright blood contrast that have little dependence on blood inflow; Both arteries and veins have bright signal which makes these sequences very suited for thoracic MRA applications (evaluation of large vessels (arteries and veins) in congenital heart disease); Fast image acquisition in 3D; Very good SNR; Wide application in coronary MRA; 23
SSFP MRA Patient with a saccular aortic arch aneurysm 24 Hartung et al., 2011
SSFP MRA Segmental renal artery branches Accessory renal artery Accessory renal artery Main renal artery Main renal artery SSFP MRA CE MRA 25 Hartung et al., 2011
SSFP MRA IR SSFP MRA CE MRA Digital subtraction angiography Nishimura et al., 1987 Renal artery stenosis Iliac artery stenosis 26 Patient with renal transplant artery stenosis Hartung et al., 2011
A couple of articles for further reading Hartung et al., Journal of Cardiovascular Magnetic Resonance 2011, 13:19 http://www.jcmr online.com/content/13/1/19 Chribiri et al., Progress in Cardiovascular Diseases 54 (2011) 240 252 http://www.onlinepcd.com Wilson and Maki, Magn Reson Imaging Clin N Am. 2009 Feb;17(1):13 27 Gough, J Vasc Surg. 2011 Oct;54(4):1215 8. Epub 2011 Aug 25 Bongartz et al., Eur J Radiol. 2008 May;66(2):213 9. 27