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Rationale and Objectives Catheter-directed intraarterial (IA) injections of gadolinium contrast agents may be used during endovascular interventions with magnetic resonance (MR) imaging guidance. Injection protocols require further validation. Using a flow phantom and swine, the authors aimed to ( a ) measure the optimal arterial gadolinium concentration ([Gd]) required for MR angiography and ( b ) validate a proposed IA injection protocol for gadolinium-enhanced MR angiography. Materials and Methods For in vitro experiments, the authors placed a catheter in the aorta of an aorto-renal-iliac flow phantom. Injected [Gd], injection rates, and aortic blood flow rates were varied independently for 36 separate IA gadolinium injections. The authors performed 2D and 3D MR angiography with a fast spoiled gradient-recalled echo sequence. For subsequent in vivo experiments, they selectively placed catheters within the aorta, renal artery, or common iliac artery of three pigs. Injection rate and injected [Gd] were varied. The authors performed 32 separate IA gadolinium injections for 2D MR angiography. Signal-to-noise ratios (SNRs) were compared for the various combinations of injection rate and injected [Gd]. Results In vitro, an arterial [Gd] of 2%–4% produced an optimal SNR for 2D MR angiography, and 3%–5% was best for 3D MR angiography. In swine, an arterial [Gd] of 1%–4% produced an optimal SNR. In the phantom and swine experiments, SNR was maintained at higher injection rates by inversely varying the injected [Gd]. Conclusion Dilute arterial [Gd] is required for optimal IA gadolinium-enhanced MR angiography. To maintain an optimal SNR, injection rates and injected [Gd] should be varied inversely. The postulated injection protocol was validated.