Reduction of flow- and eddy-currents-induced image artifacts in coronary magnetic resonance angiography using a linear centric-encoding SSFP sequence - PubMed (original) (raw)
Reduction of flow- and eddy-currents-induced image artifacts in coronary magnetic resonance angiography using a linear centric-encoding SSFP sequence
Xiaoming Bi et al. Magn Reson Imaging. 2007 Oct.
Abstract
Coronary magnetic resonance angiography (MRA) acquired using steady-state free precession (SSFP) sequences tends to suffer from image artifacts caused by local magnetic field inhomogeneities. Flow- and gradient-switching-induced eddy currents are important sources of such phase errors, especially under off-resonant conditions. In this study, we propose to reduce these image artifacts by using a linear centric-encoding (LCE) scheme in the phase-encoding (PE) direction. Abrupt change in gradients, including magnitude and polarity between consecutive radiofrequency cycles, is minimized using the LCE scheme. Results from numeric simulations and phantom studies demonstrated that signal oscillation can be markedly reduced using LCE as compared to conventional alternating centric-encoding (ACE) scheme. The image quality of coronary arteries was improved at both 1.5 and 3.0 T using LCE compared to those acquired using ACE PE scheme (1.5 T: ACE/LCE=2.2+/-0.8/3.0+/-0.6, P=.02; 3.0 T: ACE/LCE=2.1+/-1.1/3.0+/-0.8, P=.01). In conclusion, flow- and eddy-currents-induced imaging artifacts in coronary MRA using SSFP sequence can be markedly reduced with LCE acquisition of PE lines.
Figures
Figure 1
Schematic of the k-space trajectory of the conventional ACE (a) and proposed LCE (b) PE orders. K-space data of one imaging slice is filled by interleaving two segments in this example. The trajectory of ACE scheme moves from central to outer lines in the PE direction and alternates between positive and negative regions in each segment as illustrated by two segments (1st: solid line, 2nd: dashed line) in figure (a). Note the sign as well as the magnitude of the PE gradient (Gy) alternates from one RF cycle to another (Fig.c). Using LCE PE order, the k-space trajectory travels uni-directionally from central to outer lines in each segment as illustrate in (b). The magnitude of Gy is slightly increased from RF cycle to another. Polarity of gradient pairs remains unchanged in the whole readout train as shown in (d).
Figure 2
Schematic of a trapezoidal gradient G(t), gradient induced eddy current Geddy, and accumulated phase φeddy from eddy current in a typical Cartesian SSFP sequence. Eddy currents decay exponentially with multiple time constant τn, thus gradient-switching induced currents from ramp-up and ramp-down gradients do not cancel each other. The accumulated phase is proportional to the amplitude of the trapezoidal gradient ΔG with fixed ramping time Tr and duration Td, as shown in Equation 3.
Figure 3
Simulated transverse magnetization in one readout train with three different phase error steps (Δφ = 0°, 0.3°, and 1°). 20 linear ramp-ups are applied before data readout. Using conventional centric-encoding (a) PE order, signal oscillation increases with phase error step, as indicated by increased signal fluctuation from 0° (dashed line) to 0.3° (solid line), and 1° (dash-dotted line) in the figure. In comparison, linear-encoding order (b) shows improved signal stability with phase errors. Signal fluctuation is relatively small even with phase error step as high as 1°. Note the signal stability is related to local frequency offset. As illustrated in (c), strong signal oscillation is expected with a 30 Hz frequency offset and 0.3° phase error step using centric-encoding scheme (thin line). Linear-encoding shows much better tolerance to off-resonance in contrast. Signal fluctuation is very small indicated by thick line in (c). Parameters for simulation include: 20 linear ramp-ups, TR/TE = 3.2/1.6 msec, flip angle = 70°, 124 phase-encoding lines, 31 lines per heart beat, T1/T2 = 1200/250 msec.
Figure 4
Schematic of the sequence used for phantom study. A new block was inserted between original linear ramp-ups and data readout. The sequence was selected to run in one of the three modes: mode 1: no PE gradient (Gy); mode 2: five Gy pairs in ACE order; mode 3: five Gy pairs in LCE order. All PE gradients were turned off during data readout. Constant Gx and Gz were played out in linear ramp-ups, inserted block, and data readout. Difference of measured signal magnitude among these three modes was originated from Gy in the inserted block.
Figure 5
Signal magnitude measured from a static oil phantom at 1.5 T (a) and 3.0 T (b). The PE gradient (Gy) pairs were selected to run in one of three modes. Signal magnitude from the 1st readout line to the 64th line decreases smoothly in mode 1 (no Gy pairs) at both field strengths. Strong signal oscillations are observed in mode 2 (paired ACE Gy gradients) especially for those readout lines close to the inserted Gy pairs. Moderate oscillation is observed in mode 3 (LCE Gy gradients). Note the fluctuations are significantly reduced from mode 2 to mode 3. The signal magnitudes of mode 3 and mode 1 are similar and they are higher than that of the mode 2 especially at 3.0 T.
Figure 6
Phantom images acquired with the magnetization prepared SSFP sequence at 1.5 T (a, b) and 3.0 T (c, d). Imaging artifacts as indicated by arrows in images (a) and (c) acquired with the ACE PE scheme were effectively reduced by using LCE PE order **(**images b and d).
Figure 7
Images acquired from a healthy volunteer at 1.5 T using conventional linear-encoding (a, d), alternating-centric-encoding (b, e), and the proposed linear-centric-encoding (c, f). Homogenous signal intensity is achieved using linear-encoding. However, fat suppression is suboptimal as indicated by white arrow in images (a) and (d). By collecting low k-space lines at time points close to the fat saturation pulse, fat signal is effectively suppressed using centric-encoding. Flow and eddy current induced artifacts using ACE acquisition (black arrows in images b and e) are markedly reduced with LCE PE order. The right coronary artery is sharply depicted with LCE acquisition as shown in image (f).
Figure 8
Coronary artery images acquired with the magnetization prepared SSFP sequence at 1.5 T (a, b) and 3.0 T (c, d). Imaging artifacts as indicated by solid arrows in images (a) and (c) acquired with the ACE PE scheme were effectively reduced by using LCE PE order **(**dashed arrows in images b and d). The distal portion of RCA and proximal portion of LAD were sharply depicted with LCE PE scheme.
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