Contrast Enhancement in Coronary MRA

Using MRI, the contrast between the coronary blood-pool and the surrounding tissue can be manipulated using the in-flow effect [29] or by the application of MR pre-pulses. Non-exogenous contrast enhancement between the coronary arteries and the surrounding tissue has been obtained by the use of fat-saturation pre-pulses [29], magnetization transfer contrast pre-pulses (MTC) [30] or more recently T2 preparatory pulses (T2Prep) [31, 32] which take advantage of natural T2 differences between blood and the surrounding myocardium. With these techniques, the coronary lumen appears bright while the surrounding myocardium appears with reduced signal intensity. An alternative to bright-blood visualization of the coronary arteries is black-blood coronary MRA, in which the coronary lumen appears signal attenuated while the surrounding tissue displays with high signal intensity [33].

With the use of MR contrast agents, the T1 relaxation of blood can be shortened, allowing for increased contrast-to-noise ratio (CNR) for coronary MRA [19, 21]. The contrast agents currently available for coronary MRA are the traditional extracellular gadolinium-based contrast agents. However, because extracellular agents quickly extravasate into the extravascular space, their use requires rapid first-pass imaging, thereby necessitating breath-holding [8]. First-pass coronary MRA with extravascular contrast agents is also limited by the need for repeated contrast injections when more than one slab is imaged. With each subsequent injection, the CNR will be lower, as the signal from the extracellular space continuously increases following initial contrast administration.

An attempt to overcome the inherent limitations of extracellular contrast agents has seen the development of newer intravascular agents (the so-called blood-pool agents) based either on gadolinium (e.g. B22956 and MS-325) or iron oxide (e.g. AMI 227 and NC100150) [19-21, 34, 35]. The use of intravascular agents has the advantage of allowing image acquisition over longer time periods after intravenous administration of the contrast agent. Thus, non-breath-hold schemes can be employed, and repeated scans have similar CNR values thereby obviating the need for repeated injections [21].Figure 1 displays a left coronary arterial system with high contrast acquired with B-22956 (Bracco Imaging S.p.A., Milan, Italy) and a previously described free-breathing navigator-gated and corrected 3D inversion technique [21]. Using this specific intravascular contrast agent, a substantial (50%) enhancement of the SNR was accompanied by a 160% improvement in CNR when compared to a standard non-contrast enhanced technique [22,36]. Simultaneously, a 20% improvement in vessel sharpness suggested superior vessel

Fig. 1. Left (a) and right (b) coronary arterial systems acquired with the intravascular contrast agent B-22956 (Bracco Imaging SpA, Milan, Italy) and a free-breathing navigator gated and corrected 3D inversion technique [21]. The images were acquired as part of an international collaboration: IBT ETH Zurich, Switzerland, German Heart Center, Berlin, Germany, Bracco Imaging SpA, Italy, Beth Israel Deaconess Medical Center, MA, USA, Philips Medical Systems, Best, The Netherlands

Fig. 1. Left (a) and right (b) coronary arterial systems acquired with the intravascular contrast agent B-22956 (Bracco Imaging SpA, Milan, Italy) and a free-breathing navigator gated and corrected 3D inversion technique [21]. The images were acquired as part of an international collaboration: IBT ETH Zurich, Switzerland, German Heart Center, Berlin, Germany, Bracco Imaging SpA, Italy, Beth Israel Deaconess Medical Center, MA, USA, Philips Medical Systems, Best, The Netherlands delineation post contrast [22]. These findings are visually supported by the images displayed in Fig. 1. Similar results were found in a parallel volunteer study using the contrast agent SH L 643A (Schering, Berlin, Germany) [37].

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