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Arterial MR Imaging Phase-Contrast Flow Measurement: Improvements with Varying Velocity Sensitivity during Cardiac Cycle¹

¹ From the MR Center, Institute of Experimental Clinical Research, Aarhus University Hospital, Skejby Sygehus, Brendstrupgaardsvej, DK-8200 Aarhus N, Denmark. Received May 19, 2003; revision requested August 6; revision received October 13; accepted December 9. Supported by Danish Medical Research Council grant 9902664, Danish Heart Foundation grant 97–2-1–5-22549, and the Kirsten Antonius Foundation. Address correspondence to S.R. (e-mail: steffen@mr.au.dk).

View larger version (24K): [in a new window]   Figure 1. Schematic of automatic variable PC procedure. A, Approximate peak velocities throughout heart cycle of artery of interest are measured with fast low-spatial-resolution imaging. B, Measured peak velocities are interpolated with cubic-spline algorithm to allow different heart phase intervals in prescan and final scan. C, Peak velocity curves are filtered with a multiplication factor, an addition value, and a minimum value. D, Resulting values are used as Venc values in final variable PC scan.

View larger version (14K): [in a new window]   Figure 2. Typical flow curve in common carotid artery. Measurements were analyzed at five time points: 1 = end diastole immediately before R wave of electrocardiogram, 2 = peak systole, 3 = early diastole before dicrotic notch peak, 4 = dicrotic notch peak, and 5 = late diastole 200 msec after dicrotic notch peak.

View larger version (55K): [in a new window]   Figure 3a. Transverse velocity images obtained through the neck in a typical study include (a) fixed Venc and variable PC scans (b) before and (c) after unwrapping of phase aliasing. In a, signal intensity of common carotid artery (bottom vessel in each image in a-c) varies throughout cardiac cycle, while that of jugular vein (top vessel in each image in a-c) is almost constant. In b and c, however, signal intensity of carotid artery was almost constant, while that of jugular vein varied. Notice the high spatial resolution (0.4 x 0.4 mm).

View larger version (38K): [in a new window]   Figure 3b. Transverse velocity images obtained through the neck in a typical study include (a) fixed Venc and variable PC scans (b) before and (c) after unwrapping of phase aliasing. In a, signal intensity of common carotid artery (bottom vessel in each image in a-c) varies throughout cardiac cycle, while that of jugular vein (top vessel in each image in a-c) is almost constant. In b and c, however, signal intensity of carotid artery was almost constant, while that of jugular vein varied. Notice the high spatial resolution (0.4 x 0.4 mm).

View larger version (37K): [in a new window]   Figure 3c. Transverse velocity images obtained through the neck in a typical study include (a) fixed Venc and variable PC scans (b) before and (c) after unwrapping of phase aliasing. In a, signal intensity of common carotid artery (bottom vessel in each image in a-c) varies throughout cardiac cycle, while that of jugular vein (top vessel in each image in a-c) is almost constant. In b and c, however, signal intensity of carotid artery was almost constant, while that of jugular vein varied. Notice the high spatial resolution (0.4 x 0.4 mm).

View larger version (16K): [in a new window]   Figure 4. Line graph depicts measured VNRs of variable PC (VARPC) and fixed Venc images compared with Venc ratios of the two image sets. Data were averaged for eight high-spatial-resolution studies. Error bars indicate SDs. Gain in VNR with variable PC imaging is highest in cardiac phases with low velocities (eg, time point 3), with no gain in systole (time point 2). Measured VNR gain (experimental) matched theoretic value well.

View larger version (15K): [in a new window]   Figure 5. Line graph depicts root mean square errors (RMSE) for VNR obtained with three-dimensional paraboloid fitting for scans without (Non VARPC) and those with (VARPC) variable PC. Data were averaged for eight high-spatial-resolution studies. Error bars indicate SDs. With variable PC imaging, errors were significantly (P < .05) reduced, except at systole.

View larger version (21K): [in a new window]   Figure 6. Line graph shows relative VNR data for a fast exercise scan, with ratio between measured VNR for variable PC (VARPC) and fixed Venc scans compared with Venc ratio for the two scans. With variable PC, VNR more than tripled in part of cardiac cycle. Theoretic and measured (experimental) VNR gains matched reasonably well, except at one point where measured VNR gain was inaccurate because of very low velocities.