Synopsis
Contrast-enhanced MR angiography is an adaptable imaging technique that can be tailored to the clinical question posed. CE-MRA relies on subtracted and unsubtracted techniques applied to single station, multi station, and time-resolved CE-MRA. CE-MR Angiography is considered a reference standard for arterial evalation. More recent developments rely on imaging in the steady state with ECG-gating, applying acceleration schema to shorten imaging time without compromising the spatial resolution.
Introduction
Contrast-enhanced Magnetic Resonance Angiography (CE-MRA) is a flexible technique for vascular imaging throughout the body. CE-MRA relies on the paramagnetic effect of gadolinium contrast media to shorten the T1 values of the contrast-enhanced blood. Coupled with mask subtraction this results in excellent conspicuity of the vascular territory of interest. CE-MRA can be performed with electrocardiographic and respiratory gating to freeze motion of blood vessels in the thorax. Time-resolved CE-MRA enables catheter angiogram like images of the blood vessels. This talk will review the technical basics of CE-MRA as well as briefly touch on different CE-MRA techniques.Technical Basics of Contrast-Enhanced MRA
The signal generated from CE-MRA imaging techniques is directly proportional to field strength; 3T magnets have twice the signal to noise ratio (SNR) of 1.5T systems. Gradient echo pulse sequences used for CE-MRA are robust at higher field strengths, and do not suffer from attendant field inhomogeneties and flow-related artifacts. This greater SNR can be translated into higher spatial resolution and/or imaging acceleration (parallel imaging). Parallel imaging introduces a SNR penalty based on the degree of acceleration and the coil geometry factor. The relationship of native SNR to accelerated SNR is represented by: SNR accel = SNR native /(g* sqrt R), where g = the coil geometry factor, and R = the acceleration factor (1). For example, SNR at 3T with an acceleration factor of 3 is approximately 28% greater than an unaccelerated acquisition at 1.5T (2). Coil selection is an important factor. CE-MRA is commonly performed with a combination of surface coils and integrated table coils; the additional coil elements enable improved SNR (3). CE-MRA is a time-sensitive acquisition where it is necessary to capture the essential imaging (k-space) data to generate contrast in the image during the optimal phase of vascular enhancement. K-space is divided into central and peripheral regions – central regions primarily generate the contrast in an image, whereas the peripheral lines provide edge-definition (image sharpness). Bolus-tracking or timing bolus strategies are employed to ensure that the CE-MRA acquisition is initiated to coincide with peak contrast enhancement in the vascular territory of interest. As the total bolus duration of CE-MRA is short, the central k-space acquisition must be completed in seconds; the peripheral k-space acquisition takes longer. Contrast Media
Many different Gadolinium-based contrast media are available, with unique molecular structures. Gadolinium-based contrast media are divided into linear and macrocyclic agents based on their chemical structure. Macrocyclic agents have a higher stability than linear agents. Gadolinium, a paramagnetic element, effects both the T1 and T2 relaxation of protons in the magnetic field, with a non-linear relationship between the [Gadolinium] and signal. Iodinated contrast on the other hand demonstrates a linear relationship with Hounsfield unit density. Consequently, the total volume and rate of injection of gadolinium-based contrast media is less than that for iodinated contrast at computed tomography (0.1-2mL/kg for Gadolinium-based contrast vs >1mL/kg for iodinated contrast). Recently a blood pool gadolinium-based contrast agent (Gadofoveset trisodium, Ablavar, Lantheus Medical Imaging) was removed from the market; after administration 80% of the agent is reversibly bound to albumin resulting in a distribution ½ life of approximately 30 minutes enabling both first pass and steady state CE-MRA applications (4,5). Ferumoxytol (Feraheme) is an ultra-small superparamagnetic iron agent that is approved for treatment of Fe-deficiency anemia and has shown promise as a blood-pool contrast agent for MRI (6).Techniques
CE-MRA can be acquired as a mask-subtracted acquisition with multiple phases (7). This is the most common CE-MRA technique, with a single station acquisition to capture early arterial, late arterial, and venous phase imaging. Two or more stations can be combined to be captured with a single contrast bolus – this is typical for CE-MRA of the thoracoabdominal aorta or lower extremity vasculature. Time-resolved CE-MRA is an optimized acceleration technique taking advantage of view sharing, with more frequent updating of central k-space compared to the periphery of k-space (viewsharing) (8). Coupled with parallel imaging and an appropriate spatial resolution for the desired temporal resolution, this technique is able to show the arteriovenous transit of contrast and is useful in shunts, fistulae, and arteriovenous malformations. This technique is also commonly used for tibial artery imaging and capturing optimal left atrium / pulmonary vein opacification. Modifications of this technique can CE-MRA can also be performed using the DIXON fat-water separation method (9). This technique is advantageous in that it obviates the need for mask subtraction, which can introduce subtraction errors when the vascular anatomy is positioned differently between acquisitions. More recently accelerated pseudo-steady-state acquisition techniques have been introduced (5). When combined with ECG- and respiratory navigator gating, these 3-dimensional acquisitions can be obtained with isotropic spatial resolution using either balanced steady-state free precession (bSSFP) or gradient recalled echo-based acquisitions. bSSFP acquisitions are optimized through the use of T2-prepration and/or inversion recovery pulses to reduce the conspicuity of fat in the imaging volume. Time-resolved, 3D CE-MRA acquisitions methods have been recently introduced (10); when coupled with respiratory gating these enable imaging of the vasculature and heart (11). These are typically acquired using a blood pool contrast agent (Ferumoxytol) extending the CE-MRA paradigm to the simultaneous visualization of the arteries and veins with high spatial resolution in the steady state of contrast enhancement.Novel Acceleration Methods
In addition to the aforementioned parallel imaging acceleration strategies, undersampling methodology is now being applied to CE-MRA. This can take the form of undersampled cartesian or radial acquisitions with deep learning methodology applied to facilitate the reconstruction. This methodology has been applied to accelerate the acquisition of steady-state pulmonary vein CE-MRA in the steady state (12).Conclusion
CE-MRA is a flexible imaging technique which performs well across a range of field strengths and has benefited from imaging acceleration (parallel imaging), coil design, and undersampling methods. CE-MRA uses a non-nephrotoxic contrast medium. The increasing evidence base supporting the safety of an iron-based blood pool contrast agent shows promise for a non-Gadolinium based contrast medium for CE-MRA.Acknowledgements
No acknowledgement found.References
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