Navigator techniques for coronary MRA at 7 T

Coronary MRAs of the right coronary artery (RCA) were obtained in seven healthy volunteers (23 ± 3 years) during free breathing on a commercial Philips Achieva 7 T whole body MR scanner. The scans were performed using a custom-built quadrature transmit/receive surface coil (two overlapping 13 cm diameter elements). A 3D gradient echo sequence (TR = 4.3 ms; TE = 1.38 ms; α = 15°; acquisition window = 107.5 ms; voxel size = 0.82 × 0.86 × 2mm3; gating window = 3 mm; adiabatic fat saturation; saturation band over anterior chest wall) with a non-spatially selective navigator and a 7 T-optimized 2D-selective navigator was used. Optimizations of the 2D-selective navigator included localization at the lung-heart interface to account for limited transmit/receive surface coil coverage, reduced duration of RF pulse (4.38 ms) to minimize T2* effects and an RF excitation angle of 25°. The non-selective navigator replaces the pencil beam excitation in the pulse sequence (Figure 1). Signal for the non-selective navigator was spatially constrained by the sensitivity profile of transmit/receive surface coil and by a saturation band (Figure 2). An RF excitation angle of 15° was used for the non-selective navigator. SNR, CNR, coronary vessel length, diameter and sharpness, acquisition time and navigator efficiency were compared using the Wilcoxon matched-pair test.

All scans were successfully completed with both navigator approaches. An RCA acquired with both techniques in the same volunteer is shown in Figure 3. Quantitative results are listed in Table 1. Although there was a tendency for improved image quality using the optimized 2D-selective navigator, no statistically significant difference was found between the two approaches.

	2D-selective pencil beam navigator	Non-selective navigator
SNR blood	30.1 ± 17.0	24.8 ± 14.5
RCA vessel length (cm)	7.47 ± 2.84	6.49 ± 1.29
RCA vessel sharpness (%)	58 ± 9	53 ± 5
RCA vessel diameter (mm)	2.86 ± 0.25	2.88 ± 0.26
CNR	25.3 ± 15.2	20.8 ± 14.2
Acquisition time (sec)	560 ± 210	508 ± 210
Navigator efficiency (%)	52 ± 22	51 ± 12

All p = n.s.

RCA: right coronary artery, SNR: signal (blood pool in aorta, near the offspring of the RCA) to noise ratio, CNR: Contrast (between blood pool and myocardium) to noise ratio.