Purpose:To improve the signal-to-noise ratio (SNR) of breath-held coronary magnetic resonance angiography (CMRA) without increasing the number or duration of breath holds. Materials and Methods:In this BACSPIN (Breathing AutoCorrection with SPiral INterleaves) technique, a single breath-held electrocardiogram (ECG)-gated multi-slice interleaved-spiral data set is acquired, followed by repeated imaging of the same slices during free breathing. Each spiral interleaf from the breath-held data set is used as a standard for comparison with corresponding acquisitions at the same interleaf angle during free breathing. The most closely matched acquisitions are incorporated into a multislice, multi-average data set with increasing SNR over time. In-plane translations of the coronary artery can be measured and compensated for each accepted acquisition before combination with the other acquisitions.Results: CMRA was performed on six volunteers, with improved SNR and minimal motional blurring. In some cases, breath holding could be dispensed with completely and the average respiratory position used as a reference.Conclusion: BACSPIN provides a promising method for CMRA with improved SNR and limited breath-holding requirements.
Key words: spiral; pulse sequence design; trajectory; logarithmic; cardiac Spiral acquisitions are used in dynamic imaging applications, such as cardiac imaging (1), because they traverse k-space more efficiently than the simpler, raster methods. The oscillating gradient spiral waveforms null the gradient moments responsible for flow artifacts. An important objective in designing a spiral trajectory for cardiac imaging is to improve the resolution and/or contrast of the coronary arteries constrained by both the cardiac motion and instrument limitations.Fast imaging of the heart is limited by both the gradient amplitude and slew rate. The first optimization of constant pitch (Archimedian) spirals was applied to a related problem of 2D selective excitation (2). The idea was to traverse the spiral angle at the two-thirds power of time for constant slew rate and the one-half power of time for constant gradient amplitude. However, there was a singularity near the origin of k-space, which was solved by a Runge Kutta numerical integration of the trajectory differential equations (3). A graphical method provides a means to calculate variable pitch spiral acquisitions (4). Most of the information on gross characteristics is located at low spatial frequencies, while the high frequencies are important in delineating the finer features of an object. The motion artifacts were reduced in dynamic imaging by oversampling low k-space spatial frequencies and by decreasing the density of spiral arms with increasing radius (5,6). Recently, a linear decrease in sampling density with radius reduced the aliasing artifacts compared to a spiral of uniform pitch (7). The aliasing ring of the Archimedian spiral is a result of the uniform arm spacing creating a peak in the point spread function. Varying the arm spacing (7) attenuates this peak. A 3D "stack of spirals" trajectory designed for cardiac imaging combines a projection reconstruction method with an Archimedian spiral (7).Our method uses a critical sampling density within a circle in k-space and then gradually increases the arm spacing to obtain higher resolution in the same imaging time. Inside the circle the trajectory is Archimedian, while outside the trajectory is logarithmic. The logarithmic spiral is found in nature in the chambered nautilus. Near the origin of a logarithmic spiral, the curvature is tight and not suitable for an effective k-space trajectory. Hence, near the origin we use a constant pitch spiral to give a critical sampling density. The logarithmic spiral arm spacing increases with radius and may be smoothly joined to the Archimedian spiral. In designing the trajectory, we approximate the spiral trajectory of constant slew rate by an analytical expression. An advantage of the logarithmic trajectory design is its constant arm spacing in the central region using an analytic implementation. A previous variable pitch trajectory gives a linear decrease in density (7). An approximate analytical expression was previously derived for an Archimedian spiral (8). However...
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