Partial K-space Research Articles

Influence of Contrast Media Dose in Elliptical Ordered MR Angiography Image Quality of the Carotid Arteries: Preliminary Results

The contrast timing is crucial in fast imaging magnetic resonance (MR), particularly in the angiographic techniques. In MR sequences the central lines of k space own the image contrast information (1,2). Hence, the synchronization of the acquisition of central views of k-space during the maximum concentration of contrast media (CM) into the examined vessel is mandatory to obtain images of higher quality (2–7). The lack of adequate timing with premature acquisition yields to poor examination with no visualization of the vessels. On the opposite, the continuous infusion or a delayed acquisition cause the enhancement of veins and soft tissues that may obscure the examined structures (5, 8–11). In fact, although fast imaging sequences increase the image quality and lessen the contrast media injected volume, a shorter acquisition time allows a smaller error in contrast timing (6, 10). Additional factors occurs with high flow vessels such as renal and carotid arteries in which the window of selective enhancement is very narrow (4, 10–13). The delay of the scan is not accurately predicted by patient’s physiological parameters (3, 7, 12). Several techniques have been developed to ensure a proper timing (14). The first one is based on a test bolus administered prior to angiographic acquisition during continuous monitoring scans at the level of the region of interest. The scan delay is estimated from the measurement of the transit time of the test bolus from the site of injection to the examined vessel (3, 7, 12, 15). The second technique is based on a series of consecutive short angiographic sequences expecting that the vessels of interest fill at least during the central period of one of them (16, 17). The third one consists on partial k-space updating with increased time resolution through more frequent sampling of the k-space’s centre (5, 14, 18). All these techniques suffer from several drawbacks because of the inverse relationship between the spatial resolution and the acquisition time. Moreover they are less accurate in high flow vessels. Magnetic resonance angiography (MRA) with centric ordered acquisition is a relatively new technique which has been already investigated by several Authors (6, 10, 13, 19– 22). Basically in this technique the central views of k-space are acquired at the very beginning of the acquisition. The triggering is automatic or manual as the vessel enhancement is detected through an automatic volume sampling with a threshold or through a monitoring “fluoroscopic” slice. The asymmetric structure of this sequence and the advantages on contrast timing lead to a different use of the contrast media. The aim of this study is to assess the influence of the contrast media dose in MRA with elliptical ordering of the carotid arteries and to identify the parameters affecting this technique.

Reducing flow artifacts in echo-planar imaging.

Echo-planar imaging (EPI) is very susceptible to flow artifacts. Two ways to improve its flow properties are presented. First, "partial flyback" is proposed to reduce artifacts arising from flow in the readout direction. Near the center of k-space, only the even echoes of the EPI echo-train are used. Partial flyback is shown to improve the readout-flow properties at the expense of a slight worsening of the phase-encode flow and off-resonance properties. We recommend that the flyback region acquire 95% of the energy in k-space. Second, "inside-out" EPI is used to reduce artifacts arising from flow in the phase-encode direction. Data collection begins at the center of k-space, with separate interleaves to acquire the top and bottom, halves of k-space. Partial flyback is combined with partial-Fourier EPI and inside-out EPI. Partial-flyback inside-out EPI has worse off-resonance properties than partial-flyback partial-Fourier EPI but demonstrates better flow properties and does not require partial k-space reconstruction.