In order to acquire images of peripheral artery without the need of background saturation RF pulses and an external ECG trigger.

The sequence diagram of the velocity sensitive GRE¹ is shown in Fig. 1. Velocity-sensitive GRE sequence is composed of two RF pulses of α and –α degrees and velocity encoding (VENC) gradient between the two RF pulses. In this sequence, the signal intensity is determined by the phase change of magnetization vector between the first and second RF pulses, where the phase change (Φ) can be defined as follows:

\[\phi=\gamma G(T/2)^{2}v+\phi_{0}\tag1\]

where γ is the gyromagnetic ratio, G and T are the magnitude and duration of VENC gradient, respectively, v is the flow velocity and Φ₀ is the phase change caused by field inhomogeneity. By eq. (1), the magnitude of an acquired image is proportional to the velocity of blood flow for a given VENC gradient. Based on the velocity-sensitive GRE sequence, the golden angle radial acquisition scheme was used to acquire highly under-sampled images of each cardiac phase. Images of each cardiac phase is reconstructed by using the sliding window reconstruction scheme² (Fig. 2) to improve temporal resolution. As shown in Fig. 2, N_total is the total number of radial spokes, N_view is the number of radial spokes in each reconstruction window and N_dist is the distance between adjacent reconstruction windows. The signal intensity in artery is varying according to the time-varying blood velocity in artery, but those in vein and background are constant. By using the time varying signals in artery, acquired radial views are classified into high and low signal intensity groups and two images are reconstructed for high (I_high) and low (I_low) signal intensity groups in artery, respectively (Fig. 3). By the way, the signal intensities in background and vein are same in two images, but that of artery is different by time-varying blood velocity. Thus, angiogram is generated by subtracting two images (Fig. 3).

In-vivo experiments were performed at a 3.0T MRI system (Siemens Magnetom Verio, Erlagen, Germany) with a matrix coil to verify the proposed angiography. The angiogram was acquired using the following parameters; field-of-view (FOV) = 380 × 380 mm², slice thickness = 3 mm, flip angle = 40°, and TR/TE = 8.5/6 ms. 200 slices were acquired to span the peripheral arteries. N_total, N_view and N_dist were 400, 40 and 2, respectively. Thus, total acquisition time was 11min 20 sec. As shown in Fig. 4, the proposed method well reconstructed peripheral angiography without saturation pulses. In this paper, a new MRA technique using velocity sensitive spoiled-GRE was proposed, which could produce peripheral angiogram without saturation RF pules and ECG triggering.