Breathheld autocalibrated phase-contrast imaging

Quantifying velocity and blood flow is of great importance in the detection of cardiovascular disease. Shunt ratio, which is the ratio of pulmonary artery flow to aortic flow, is an important parameter that assesses the need for closing shunt lesions (1-5). Another parameter is the amount of blood regurgitated into a cardiac chamber divided by the stroke output, or regurgitant fraction (6). Measurements of blood flow in the aorta and pulmonary trunk help characterize these parameters. Phase-contrast magnetic-resonance imaging (PC-MRI) is a noninvasive technique sensitive to blood flow in the great arteries (7-10). PC-MRI is particularly suited for quantification and has sensitivity that is adjustable to flow velocity. A primary disadvantage of PC-MRI is the lengthy scan time needed to acquire the many flow measurements at adequate temporal and spatial resolution. Sensitivity encoding (SENSE) (11) is an important parallel imaging technique that significantly reduces acquisition time. Combining SENSE with PC-MRI can either shorten scan time or increase temporal resolution of segmented k-space techniques. Shorter scan time improves patient compliance in breathhold imaging. PC-MRI combined with SENSE has been demonstrated and reported in the literature (12,13). In PC-MRI the full field of view is not needed for flow quantification, as outer tissue such as the chest wall is irrelevant to the flow measurement. Some spatial aliasing is commonly used in order to maximize the spatial resolution and thus reduce partial volume effects. However, the spatial aliasing may interfere with a SENSE reconstruction. It has been reported that SENSE may be unable to correctly unalias the data due to coil sensitivity maps that do not match that of the aliased tissue in the full field of view image (14,15). The reconstructed images will have artifacts that may compromise the accuracy of flow measurements. One solution to avoid such an artifact is to prescribe a field of view large enough to avoid aliasing. However, this typically requires reduced spatial resolution, causing a loss of accuracy in the flow measurements due to partial volume effects. On the other hand, spatial resolution can be maintained at the cost of a larger imaging matrix, but only at the cost of increased scan time. Also, in certain anatomies, prescribing a slice that avoids aliasing may be difficult. Autocalibrating sequences like modified SENSE (mSENSE) (16), generalized encoding matrix (GEM) (17), vdSENSE (18), and generalized autocalibrating partially parallel acquisitions (GRAPPA) (19) can avoid the spatial aliasing of a SENSE reconstruction. In image-based schemes, like mSENSE and GEM, the coil sensitivity maps are calculated from lower-resolution images taken from the central k-space profiles. These lower-resolution images possess the same spatial extent, and consequently the same aliasing behavior, of the undersampled image. In k-space schemes like GRAPPA each line of k-space is not affected by the neighboring lines and is independent of the actual field of view. In this study autocalibrating sequences are applied to PC-MRI for both aortic and pulmonary flow and peak velocity measurements. Different techniques in estimating coil sensitivity maps for PC-MRI application are investigated. The data are reconstructed using the mSENSE and GRAPPA methods and then compared to the results with SENSE. Through Bland–Altman statistical analysis, autocalibrating sequences are shown to have mean differences from conventional PC-MRI that are comparable to SENSE mean differences from conventional PC-MRI in flow and peak velocity measurements. When aliasing is present, autocalibrating sequences are able to remove aliasing and provide accurate measurements.