Keyhole-3D Phase Contrast Magnetic Resonance Angiography: A Time-Resolved Reconstruction Method Dong-Hoon Lee, 1,2 Cheol-Pyo Hong, 2 Man-Woo Lee, 3 Bong-Soo Han 1 * 1 Department of Radiological Science, College of Health Science, Yonsei University, Wonju, Korea 2 Center for Medical Metrology, Division of Convergence Technology, Korea Research Institute of Standards and Science (KRISS), Daejeon, Korea 3 Department of Research and Development, Health and Medical Equipment, Samsung Electronics Co., LTD., Suwon, Korea Received 11 September 2013; accepted 1 October 2013 ABSTRACT: In this study, we studied the keyhole imaging technique to 3D Phase-contrast magnetic resonance angiography (PC MRA) to improve its temporal resolution. Previously, our research group has already studied the 2D PC MRA combined with keyhole technique, and evaluated the applicability. For keyhole-3D PC MRA, the keyhole factor was used from 12.5% to 50% of the full k-space. With keyhole factors above 50%, the images were similar to the original image and the vessels in the brain were well observed. We believe the keyhole- 3D PC MRA will give some advantages for improving the temporal resolution of MR systems. V C 2014 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 24, 1–7, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/ima.22072 Key words: 3D phase contrast magnetic resonance angiography (3D PCMRA); keyhole; zero-padding I. INTRODUCTION Phase-contrast magnetic resonance angiography (PC MRA) is a typi- cal MRA technique, along with time-of-flight (TOF) MRA and contrast-enhanced (CE) MRA. However, PC MRA is less commonly used to produce angiographic images because of its relatively long scan time compared with other MRA techniques (Dumoulin et al., 1991; Miyazaki and Lee, 2008). In particular, three-dimensional (3D) PC MRA image acquisition takes longer than two-dimensional (2D) PC MRA because it is necessary to acquire more image slices in the image slab. On the other hand, 3D PC MRA has a special advantage for venous observation, as the veins are slower moving than other vessels in the body, and this technique can be used to con- trol the area values of the velocity encoding gradients. Due to this advantage, therefore, a wide variety of techniques have recently been proposed to improve the temporal resolution of 3D PC MRA (Markl et al., 2003; Pai, 2007; Rolf et al., 2009; Velikina et al., 2010; Chang et al., 2011). As a part of the pulse sequence and k-space trajectory methods, steady-state free precession (SSFP) sequence, parallel imaging, and non-Cartesian k-space trajectory are typical examples of techniques for PC MRA in which the temporal resolution has been improved (Markl et al., 2003; Pai, 2007; Rolf et al., 2009; Veli- kina et al., 2010; Chang et al., 2011). Parallel to the aforementioned fast-imaging methods, alternative approaches have also been developed to achieve fast imaging; these are performed at the stage of post-processing, without expensive spe- cific hardware devices and performances. From this point of view, many researchers have also reported the achievement of improved acquisition efficiency over conventional phase encoding, including through the use of wavelets, singular value decomposition (SVD), and the keyhole imaging method (Weaver et al., 1992; van Vaals et al., 1993; Hu, 1994; Zientara et al., 1994; Hanson et al., 1997; Chen et al., 2007; Lustig et al., 2007; Taschner et al., 2008; Beranek- Chiu et al., 2009; Hadizadeh et al., 2011). In particular, keyhole imaging is a typical simple method that uses conventional phase encoding and the Fourier transform reconstruction technique on almost MRI system. Due to these advantages, many researchers have applied the keyhole imaging technique to functional MRI (fMRI), CE MRA, perfusion and diffusion MRI, interventional MRI, and MR temperature imaging to improve the temporal resolution of images (van Valls et al., 1993; Hu, 1994; Gao et al., 1996; Oesterle et al. 2000; Hliand et al., 2001; Shankaranarayanan et al., 2001; Taschner et al., 2008; Beranek-Chiu et al., 2009; Sun et al., 2010; Han and Mun, 2011). In keyhole imaging, only the central region of the k- space is acquired during the dynamic part of the acquisition to improve temporal resolution (van Valls et al., 1993; Hu, 1994; Beranek-Chiu et al., 2009). This technique requires a full k-space imaging that is acquired only once, at the start or end of the time course. Most of the intensity and contrast information in an image Correspondence to: Bong-Soo Han; e-mail:bshan@yonsei.ac.kr V C 2014 Wiley Periodicals, Inc.