Original Research Preferential Arterial Imaging Using Gated Thick- Slice Gadolinium-Enhanced Phase-Contrast Acquisition in Peripheral MRA Thomas K.F. Foo, 1 * Vincent B. Ho, 2 Maureen N. Hood, 2 Sandra L. Hess, 3 and Peter L. Choyke 3 Purpose: To investigate the feasibility of preferential arte- rial imaging using gadolinium-enhanced thick-slice phase- contrast imaging. Methods: Six healthy volunteers were studied using a peripheral-gated segmented k-space CINE phase-contrast pulse sequence using four views per RR interval with flow encoding in the superior-inferior direc- tion. Images at the level of the popiteal trifurcation were acquired postcontrast with different section thicknesses (4 – 8 cm) and VENC values (20 –150 cm/sec), and phase- difference processing. Results: The post-gadolinium con- trast-enhanced thick-slice phase-contrast acquisitions demonstrated the ability to visualize the tibio-peroneal (trifurcation) arteries, especially in systole. With MR con- trast agents, the signal from blood is raised significantly above that of stationary tissue from T 1 shortening such that the partial volume artifact is reduced in thick-slice acquisitions. Furthermore, by selecting the VENC value as a function of the cardiac cycle, the noise floor can be raised to selectively suppress flow values less than that of the noise threshold, allowing better accentuation of arte- rial structures at systole. Conclusions: Thick-slice phase- contrast acquisition with phase-difference processing has been observed to reduce partial volume artifacts when an MR contrast agent substantially increases signal in the vasculature over that of normal background tissue. Prefer- ential arterial images can be obtained by either increasing the VENC value to selectively suppress signal from slow flow in the veins or by subtracting the diastolic phase image from the peak systolic phase image. J. Magn. Reson. Imaging 2001;13:714 –721. © 2001 Wiley-Liss, Inc. Index terms: pulse sequences; MR angiography; phase con- trast; peripheral angiography; angiography methods; contrast agents; pulse sequence WITH THE INCREASING USE of contrast-enhanced magnetic resonance angiography (MRA), venous signal contamination of arterial images is a common concern. Synchronizing the acquisition with the arrival of the contrast bolus using automatic (1) or fluoroscopically- triggered (2) bolus detection, or a timing bolus (3) ad- dresses the problem of arterial-venous separation by triggering the acquisition while the contrast material is still exclusively in the arterial enhancement phase. However, once contrast has already been administered, subsequent acquisitions will be degraded by venous enhancement from recirculating contrast media. Ve- nous contamination is even more pronounced following the administration of intravascular contrast agents (4 – 7). The reduced T 1 relaxation time of blood (from 1200 msec to 50 –200 msec, depending on the administered contrast concentration) diminishes the possibility of using spatial saturation radiofrequency (RF) pulses to eliminate either arterial or venous signals for direc- tional flow discrimination. A different approach to MR phase contrast imaging that efficiently discriminates between arteries and veins in the steady state is described. This is accom- plished by combining contrast-enhanced MRA and a cardiac-gated phase-contrast acquisition. In systole, where arterial flow is maximal, the velocity-encoding value (VENC) (where the VENC value is the maximum velocity that can be represented by the phase contrast aquisition without aliasing) is adjusted such that the venous flow velocities are below that of the noise threshold level in the phase (velocity) images. The phase-velocity images will preferentially illustrate the arteries. Slower venous flow signals will be suppressed in the phase-velocity image, but signal from both arte- rial and venous structures will remain in the magnitude image. Cardiac-gated phase-contrast studies have been pre- viously used to reduce pulsatile flow artifacts in imag- ing the peripheral vasculature (8 –10) using single VENC encoding values. Although variable VENC encod- ing according to the cardiac cycle has been demon- strated, it has been used primarily to provide improved image signal-to-noise ratio (SNR) in phase-contrast im- 1 Applied Science Laboratory, GE Medical Systems, Milwaukee, Wiscon- sin. 2 Department of Radiology, Uniformed Services University of the Health Sciences, Bethesda, Maryland. 3 Diagnostic Radiology Department, National Institutes of Health, Be- thesda, Maryland. *Address reprint requests to: T.K.F.F., Applied Science Laboratory, GE Medical Systems, Room 110-MRI, Johns Hopkins Hospital, 600 N. Wolfe St, Baltimore, MD 21287. E-mail: thomas.foo@med.ge.com Received June 6, 2000; Accepted November 7, 2000. JOURNAL OF MAGNETIC RESONANCE IMAGING 13:714 –721 (2001) © 2001 Wiley-Liss, Inc. 714