Intrinsic signal amplification in the application of 2D SENSE parallel imaging to 3D contrast‐enhanced elliptical centric MRA and MRV

Abstract

The relative signal-to-noise ratio (SNR) provided by 2D sensitivity encoding (SENSE) when applied to 3D contrast-enhanced MR angiography (CE-MRA) is studied. If an elliptical centric phase-encoding order is used to map the waning magnetization of the contrast bolus to k-space, the application of SENSE will reduce the degree of k-space signal modulation, providing a signal amplification A over corresponding nonaccelerated acquisitions. This offsets the SNR loss in R-accelerated SENSE due to ͱR and the geometry (g) factor. The theoretical bound on A is R and is reduced from this depending on the properties of the bolus profile and the duration over which it is imaged. In this work a signal amplification of 1.14 -1. There has been considerable interest in the last several years in the application of parallel imaging techniques to MR image formation. These include image-based methods, such as sensitivity encoding (SENSE) (1), and k-spacebased techniques, such as simultaneous acquisition of spatial harmonics (SMASH) (2) and generalized autocalibrating partially parallel acquisitions (GRAPPA) (3). It is well accepted that any reduction in acquisition time provided by these methods is in general accompanied by a reduction of signal-to-noise ratio (SNR). The theoretical limitations of the SNR of SENSE have been extensively studied (4,5), going back to the original description of the technique (1). Compared to a reference acquisition with equal spatial resolution, the SNR of a SENSE image acquired with acceleration factor R is reduced in proportion to the product of ͱR and the "geometry (g)-factor," where the latter is a measure of the noise amplification intrinsic to inverting the algebraic equations of the SENSE reconstruction matrices. Because the g-factor is spatially variable, so is the noise level in the SENSE image.The original analysis of SNR in SENSE presumed that the magnetization level of interest was constant during the acquisition. However, parallel imaging can also be applied when this is not the case, one notable example being 3D contrast-enhanced MR angiography (CE-MRA) (6 -13). In this case the magnetization level is time-varying, as caused by the dynamic nature of the contrast bolus. The purpose of this work is to provide a technical analysis of the SNR in SENSE as applied to CE-MRA acquired with the elliptical centric (EC) phase-encoding order (14,15). It is hypothesized that the acceleration provided by the SENSE acquisition may intrinsically provide an amplification of the signal that can partially compensate for the intrinsic loss of SNR due to the factor g ͱR. The significance of this is that it may allow the clinical acceptance of highly accelerated CE-MRA acquisitions that might otherwise seem implausible due to low SNR.The SNR gain and possible improved spatial resolution arise when the SENSE acceleration causes waning magnetization to have less decay across k-space than in the unaccelerated case. This effect has been studied previously in detail in 1D accelerated imaging for SMASH with multiple...

Peripheral Vasculature: High-Temporal- and High-Spatial-Resolution Three-dimensional Contrast-enhanced MR Angiography

“…This is attributable to the following three specific elements of the imaging technique: (a) the improved performance of 2D versus onedimensional parallel imaging, (b) the use of a peripheral vascular coil array that accounts for the disproportionate anterior-to-posterior to left-to-right FOV, and (c) the intrinsic signal amplification that can be attained with accelerated imaging of transient MR signals (48).…”

“…The 3D contrast-enhanced MR angiographic examinations we performed also benefited from the signal enhancement effect associated with accelerated imaging of the passage of the contrast material bolus (48). Briefly stated, the SENSE acceleration focuses the peak contrast signal over a greater extent of the k-space than nonaccelerated imaging.…”

Time-of-Arrival Mapping at Three-dimensional Time-resolved Contrast-enhanced MR Angiography

“…The MR angiograms acquired in this study were typical of those generated in multiple previous studies and had adequate SNR for radiologic interpretation (19,29). We believe this was attributable to (a) receiver coil arrays that were matched to the anatomic region being studied (24), (b) our use of two-dimensional rather than one-dimensional acceleration techniques to preserve the SNR (14), and (c) some intrinsic preservation of the SNR at accelerated contrastenhanced MR angiography (30). In the future, it may be desirable to extend the superior-to-inferior coverage of the custom-built leg coils.…”

Three-Station Three-dimensional Bolus-Chase MR Angiography with Real-time Fluoroscopic Tracking

“…Despite the accompanying intrinsic reduction in SNR due to the high acceleration factors, good image quality was still acquired as evidenced by the fine detail seen in the angiograms and the good assessment scores for both vessel signal and sharpness. We attribute this to the high performance of the circumferentially-placed coil arrays (23) at each station as well as the intrinsic SNR retention when acceleration is applied to CE-MRA (24, 25). …”