Phase and amplitude correction for multi-echo water-fat separation with bipolar acquisitions

Autor: Ann Shimakawa, Jean H. Brittain, Scott B. Reeder, Charles A. McKenzie, Wenmiao Lu, R. Scott Hinks, Huanzhou Yu
Rok vydání: 2010
Předmět:
Zdroj: Paediatrics Publications
ISSN: 1522-2586
1053-1807
DOI: 10.1002/jmri.22111
Popis: Chemical shift based multi-echo water–fat separation techniques (1–3) have seen a recent resurgence of interest for clinical use. These methods acquire multiple images at different echo times so that chemical shift induced phase shifts can be used to separate water and fat signals. Traditionally, each echo is collected in a separate sequence repetition (“single-echo per TR”), allowing flexible imaging prescriptions at the cost of doubling or tripling the minimum scan times. More recent implementations collect all echoes in one TR (4–7), significantly shortening the required scan times. In these “multi-echo per TR” sequences, there are two approaches to collect the echoes. The “unipolar” sequences (with “fly-back” gradients) acquire all echoes using the same gradient polarity (7), ensuring phase consistency among the echoes. The second approach, using “bipolar” acquisitions (also referred to as “non–fly-back” sequences), collects echoes during both positive and negative gradient polarities (4–6). The bipolar approach offers many advantages as compared to the unipolar sequence. Removing the “fly-back” gradients between the echoes greatly improves the SNR efficiency of the bipolar sequence (5). The minimum TR and scan time are also significantly shortened. In addition, the minimum echo spacing (echo time increment) is reduced, effectively increasing the spectral bandwidth in which water–fat can be unambiguously determined. Thus, a more robust water–fat separation may be achievable (8,9). Alternatively, a higher resolution in the read-out direction can be achieved if the echo spacing remains unchanged. However, the bipolar acquisition also brings unique challenges. First, the chemical shift now appears in opposite directions at echoes with different gradient polarities; therefore, k-space water–fat separation methods are required to correct for the chemical shift artifact (5,10). More importantly, the bipolar sequences must account for phase errors that result from eddy currents and other system nonidealities. While these phase errors also exist in unipolar acquisitions, they effectively add a constant phase on all the echoes, so the relative phases between the echoes remain unchanged. For bipolar acquisitions, the phase errors are modulated in opposite directions spatially for positive and negative gradient polarities, disrupting the inter-echo phase consistency that is critical for water–fat separation. The phase errors associated with the switching of the gradient polarities have been studied extensively for echo planar imaging acquisitions. Gradient delays and eddy currents caused by the rapidly changing gradient fields may induce phase errors in all spatial directions (11–13). The linear phase error in the read-out direction is the dominant component seen in the bipolar multi-echo acquisitions, which can be estimated and effectively corrected by detecting a shift of the k-space peak (5) or collecting reference scans without phase encoding (4). The latter approach is capable of correcting for a nonlinear phase error. The remaining phase errors in other spatial directions may be caused by eddy currents in other conducting structures in the system, e.g., the radiofrequency (RF) shield (14), concomitant gradients, and cross term eddy currents. Furthermore, for obliquely oriented imaging planes, the anisotropic gradient delays result in a dominant linear phase error that may not be aligned with the read-out direction, introducing relatively substantial linear phase errors in phase encoding or slice directions (15). The inconsistency between the echoes collected with bipolar gradients is not limited to the phase errors (16). The receiver chain filters in general have a non-flat frequency response, effectively introducing an asymmetric amplitude modulation on the read-out lines (16–18), but in opposite spatial directions for echoes collected with positive and negative gradient polarities. This amplitude modulation is solely in the read-out direction. In this work, we introduce a method to estimate and correct for the two-dimensional (2D) phase errors and amplitude modulation before separating water and fat, achieved by collecting additional phase encoded k-space data with the reversed gradient polarity. We demonstrate that the high order phase and amplitude inconsistency can be effectively removed, which leads to more uniform water–fat separation.
Databáze: OpenAIRE
Externí odkaz: