Time-Resolved Contrast-Enhanced 3D MR Angiography Frank R. Korosec, Richard Frayne, Thomas M. Grist, Charles A. Mistretta zy An MR angiographic technique, referred zyxwvuts to as 3D TRICKS (30 time-resolved imaging of contrast kinetics) has been devel- oped. This technique combines and extends to 3D imaging several previously published elements. These elements in- clude an increased sampling rate for lower spatial frequen- cies, temporal interpolationof k-space views, and zero-filling in the slice-encoding dimension. When appropriately com- bined, these elements permit reconstruction of a series of 3D image sets having an effective temporal frame rate of one volume every 2-6 s. Acquiring a temporal series of images offers advantages over the current contrast-enhanced 3D MRA techniques in that it i) increases the likelihood that an arterial-only 3D image set will be obtained, ii) permits the passage of the contrast agent to be observed, and iii) allows temporal-processing techniques to be applied to yield addi- tional information, or improve image quality. Key words: MR angiography; time-resolved MR imaging; con- trast-enhanced zyxwvutsrq MI? imaging; rapid 3D MR imaging. INTRODUCTION Recently a number of groups, including our own, have had encouraging results using intravenous contrast agent in conjunction with MRA, particularly in the abdomen and the lower extremities zyxwvutsr (1-11). In these techniques, contrast between blood and stationary tissues is achieved by injecting a TI-shortening contrast agent into the blood stream. When imaged by using a short zyxwvutsr TR, short TE gradient-echo sequence, the blood appears very bright and the stationary tissues appear dark. Vascular images acquired by using contrast-enhanced 3D MRA tech- niques have an inherently high signal-to-noise ratio, and thus are affected very little by many of the flow-related artifacts that reduce the sensitivity and specificity of current MRA techniques (12). Because of the high signal- to-noise ratio and the relative lack of flow-related arti- facts in these images, they have an appearance similar to those obtained by using digital subtraction angiography (DSA). Current contrast-enhanced 3D MR angiographic tech- niques yield excellent images of the arteries if the center of k-space is acquired during peak concentration of the contrast agent in the arteries. Obtaining high quality im- ages, however, requires appropriate timing of the injec- MRM 36345351 (1996) zyxwvutsrqpo From the Departments of Radiology and Medical Physics, University of Wisconsin - Madison, Madison, Wisconsin. Address correspondence to: Frank Korosec, Ph.D., UW Hospital and Clin- ics, Department of Radiology, E3/311, 600 Highland Avenue, Madison, WI Received March 8, 1996; revised May 8, 1996; accepted May 24, 1996. This work was supported in part by NIH grants #R01 HL 52747 and R01 HL 51370, and a grant from the Whittaker Foundation (F.R.K.); by a Heart and Stroke Foundation of Canada Fellowship (R.F.); and in part by NIH grant #KO8 HL 02848 (T.M.G.). 53792-3252. 0740-3194/96 $3.00 Copyright zyxwvutsrqpon 0 1996 by Williams & Wilkins All rights of reproduction in any form reserved. tion of the contrast agent relative to the start of image acquisition. If the center of k-space is acquired too early, maximum signal in the arteries will not be achieved, and if the center of k-space is acquired too late, the veins will be enhanced, causing the arteries to be obscured. An- other drawback of current contrast-enhanced 3D meth- ods is that they permit acquisition of only a single high quality scan, because image quality on subsequent scans is compromised by the residual contrast agent remaining from the first injection, which causes veins and station- ary tissues to be enhanced. Also, current contrast-en- hanced techniques do not reveal any information regard- ing the passage of the contrast agent. We have developed a time-resolved, contrast-en- hanced, 3D MR angiographic technique (3D TRICKS - 3D time-resolved imaging of contrast kinetics] that re- peatedly acquires images from a volume during the passage of a contrast agent. With the appropriate choice of parameters, acquisition of multiple 3D image sets can be completed in a single breath-hold. Because this technique images the passage of the contrast agent, it increases the probability that a 3D image set showing only arteries will be obtained. The technique also pro- vides information on the bolus transit time, which is an indicator of blood flow rate. In addition, it permits the uptake of the contrast agent by the organs to be observed. The rate of uptake has the potential to yield information regarding the physiological effects of the pathology. Also, because a series of time-resolved im- ages is acquired, many of the postprocessing methods used with DSA and 2D time-resolved MR such as mask mode subtraction, simple matched filtering, and Eigen filtering (13, 14), can be applied to enhance the infor- mation content or quality of the images. Several previously published techniques that im- prove the effective temporal resolution of MR images are employed in 3D TRICKS. These include not sam- pling all of the high spatial frequencies for every time frame, sharing data among time frames, and temporally interpolating between acquired blocks of data to deter- mine values for uncollected data. “Keyhole” imaging (15) is an example of a method that provides improved temporal resolution by acquiring the high spatial fre- quency information less frequently than the low spa- tial frequency information. The keyhole technique has been generalized in the BRISK (block regional interpo- lation scheme for k-space) algorithm (16), which schedules the acquisition of k-space based on the tem- poral characteristics of the object being imaged. The BRISK technique was illustrated in the context of 2D gated cardiac imaging and used Fourier interpolation to fill in missing views. Sharing of views among mul- tiple time frames and employing a sliding window during reconstruction is a strategy that has been used in MR fluoroscopy (17) and has recently been applied in connection with rapid 2D spiral scanning (18, 19). 345