MR angiography with three-dimensional MR digital subtraction angiography

We have developed a time-resolved, contrast-enhanced, volume-imaging technique for magnetic resonance (MR) angiography, known as three-dimensional (3D) MR digital subtraction angiography (DSA). This technique greatly improves MR angiogram quality because it combines the injection of a contrast agent with the ability to image the temporal passage of this agent and, thereby, obviates the need for timing scans or other complicated synchronization schemes. Three-dimensional MR DSA also represents a potential improvement in the sense that, relative to DSA and computed tomography (CT) angiography, the contrast agent is less toxic. Additionally, unlike CT angiography, images may be acquired during the passage of the contrast agent. Therefore, 3D MR DSA shows the sequential passage of contrast through the arterial and venous system, followed by uptake in various organs. Unlike conventional DSA, 3D MR DSA imaging acquires full volume datasets, which allows subsequent reprojection and reformatting. Because images are obtained at approximately 2-6 s time intervals using a temporal aperture on the order of several seconds, motion (such as respiration) causes only a temporary disruption of image quality, similar to that observed in MR fluoroscopy. These temporal characteristics also make the proposed sequence insensitive to variations in the shape and timing of the contrast-pass curve. Although the individual time-resolved images will have somewhat decreased signal-to-noise ratio (SNR) relative to nontime-resolved scans collected in the same acquisition time, the SNR improvement due to the gadolinium appears to accommodate this trade-off. Additionally, if motion between successive images is small, then the full suite of temporal processing schemes, previously investigated in connection with DSA and time-resolved two-dimensional (2D) MR, such as mask mode subtraction, simple matched filtering and Eigen filtering, can be used to obtain composite images. These derived images generally have an increased SNR or negligible venous signal if an arterial-phase image is not obtained in the early time-resolved images. In summary, 3D MR DSA will significantly advance MR angiography because of the following intrinsic advantages: (1) improved signal-to-noise, (2) scan orientation may be chosen independently of the direction of blood flow, (3) uniform vascular signal, even from regions of complex flow, (4) minimization of motion artifacts, (5) greatly reduced sensitivity to variation in the shape and timing of the contrast bolus, (6) ability to be reformatted or reprojected, and (7) ability to apply a variety of temporal postprocessing techniques.