Reduced field of view single‐shot spiral perfusion imaging

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Excerpt

In the United States alone, over 8.2 million people suffer from angina pectoris, resulting in the performance of nearly 10 million stress tests annually to evaluate for the presence of coronary artery disease (CAD) 1. Adenosine stress perfusion cardiac magnetic resonance (CMR) has emerged as a technique with high diagnostic and prognostic use in the evaluation of CAD 2. Currently available CMR perfusion imaging techniques still suffer from limitations, including dark rim artifacts 6 resulting from cardiac motion and Gibbs ringing at high‐contrast interfaces and limited spatial coverage of the ventricle, both of which are related to temporal footprint of the acquisition and the limited time available for data acquisition in each heartbeat. Recent 3D techniques have addressed the issue of spatial coverage of the ventricle and have demonstrated clinical use in multicenter studies 7. However, these techniques have a relatively long temporal footprint, which limits in‐plane spatial resolution 10.
We have shown that spiral pulse sequences demonstrate reduced motion‐induced dark rim artifacts and can accurately detect obstructive CAD as defined by quantitative coronary angiography 11. Given the high acquisition efficiency of spiral techniques, we recently demonstrated whole‐heart coverage with a three‐interleaved spiral pulse sequence capable of imaging eight slices with a 2‐mm in‐plane resolution with a temporal resolution footprint of 35 ms per slice. 14. However, the need for multiple interleaves necessitates the use of a flip angle of around 30 °, and each interleave traverses the center of k‐space at a separate time relative to the saturation pulse. Given the high acquisition efficiency of spiral trajectories, we sought to determine the feasibility of acquiring a complete perfusion image following a single RF excitation. This single‐shot excitation approach acquires data with a very short temporal footprint due to the high sampling efficiency, but also requires highly accelerated spiral trajectories with an associated signal‐to‐noise ratio (SNR) penalty which can be mitigated using a 90‐degree excitation pulse.
Although the heart only occupies a small region of the chest, an imaging field of view (FOV) that encompasses the whole chest is required to avoid spatial aliasing that results from violation of the Nyquist sampling rate. For spiral imaging, because the readout direction is continually changing, a traditional anti‐aliasing filter—which can be used to restrict the FOV in the RO direction—cannot be used, and the full extent of the object must be supported in all directions to avoid aliasing. By using an outer‐volume suppression (OVS) strategy to achieve a reduced FOV (rFOV), k‐space can be sampled more coarsely resulting in improved sampling efficiency 15. Multiple studies have demonstrated successful application of OVS for cardiac imaging 16; however, OVS has not yet been applied to T1‐weighted gadolinium (Gd)‐enhanced myocardium first‐pass perfusion imaging.
The goal of this study was to develop a saturation recovery (SR) and OVS strategy suitable for a single‐shot rFOV spiral perfusion sequence capable of achieving full heart coverage with high spatial resolution and a short temporal footprint. Given the high acquisition efficiency of spiral pulse sequences, we sought to develop a technique that can image a slice with a single 90 ° excitation and a single spiral readout to achieve an unprecedented temporal footprint of <10 ms per image.

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