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Most recently, time-resolved 2D MRA after injection of a contrast agent bolus for various applications has been proposed. Similar to conventional digital subtraction angiography (DSA), 2D MR DSA offers the ability to observe the dilution of the bolus in the vascular system during the passage with a temporal resolution considerably below 1 sec. The purpose of this paper is to present strategies to improve the inherent low signal-to-noise ratio of 2D angiograms while retaining some temporal resolution. This can be achieved by applying algorithms for time series analysis as used in functional MRI. The significantly improved image quality is demonstrated on examples from clinical studies from bronchial MRA as well as cardiovascular MRA. In addition to the increased signal-to-noise ratio, correlation analysis leads to suppression of background signals and to a better discrimination of overlapping vessels. Further improvements in the temporal discrimination of vessels is afforded by the use of consecutive multiple contrast agent boli as demonstrated by numerical simulations and experiments. Magn Reson Med 43:303–309, 2000. © 2000 Wiley-Liss, Inc.
In the last three years, the development of gradient systems for ultra fast imaging has made time-resolved projection angiography after administration of a bolus of contrast agent (c.a.) feasible (1, 2). Similar to conventional x-ray DSA, a thick slab or a non-slice selective projection through the volume is imaged consecutively with a temporal resolution below 1 sec, using an ultra-rapid T1-weighted gradient echo sequence. This offers the possibility to directly observe the dynamics of c.a. propagation. Due to the short image acquisition times, projection angiography is very insensitive to any kind of motion. This allows vascular imaging of the thorax without breathhold and even without synchronization of data acquisition to the ECG-signal. Moreover, the method avoids the requirements for exact bolus timing as in 3D MRA in which venous overlap obscure the delineation of the arteries when sampling of central k-space lines does not hit the peak concentration of the contrast agent in the arteries (3). Background signal that mainly arises from fatty tissue due to short T1 can be eliminated by (complex) subtraction of an image acquired before the c.a.-injection. Additional depth information can be obtained by acquiring images alternatively under different projection angles.
Several clinical application of 2D MR DSA have already been reported, demonstrating the diagnostic feasibility of the method especially for vessel segments of the vascular system with short arterial transit and strong venous return like the carotid arteries, intracranial vessels or the pulmonary vessels (4–8).
The major drawback of acquiring the 2D samples with a high temporal resolution is the low signal-to-noise ratio (SNR) of the projection images. The SNR can, of course, be improved by averaging over n consecutive projection images, which, however, will lead to a loss of functional information in the time resolved scan. The purpose of this paper is to propose and discuss the use of correlation analysis (9) as a post-processing strategy to improve the SNR in projection angiographic images while maintaining the functional information about the propagation of the c.a. bolus.
