Acceleration apportionment: A method of improved 2D SENSE acceleration applied to 3D contrast-enhanced MR angiography

Parallel imaging using two-dimensional (2D) techniques was initially described a decade ago (1) and, in recent years, has been used in various applications of three-dimensional Fourier transformation acquisition, perhaps most notably in 3D contrast-enhanced MR angiography (CE-MRA) (2–8). Just as for one-dimensional (1D) acceleration, 2D acceleration is implemented by increasing the distance between samples in k-space, but in two phase-encode directions rather than one. Specific to SENSE acceleration, a model-based image-space-based parallel imaging framework, two other specific characteristics of the 1D implementation can be extended to the 2D case. First, regions within the field of view (FOV) which are known to have zero magnetization, typically the air outside the object under study, can be forced to have zero signal in the SENSE-unfolded image and thereby reduce signal-to-noise ratio (SNR) loss in the unfolding process (9). Second, because acceleration is implemented by simply adjusting the incremental spacings along the kY and kZ directions, the accelerations along these directions, defined here as RY and RZ, respectively, can assume non-integer values. The net SENSE acceleration R is the product: R = RY × RZ. The above aspects of SENSE acceleration provide more flexibility in selecting the desired acceleration factors for the 2D vs. the 1D case. The fraction of the volume of a 3D FOV actually filled by an object depends on the anatomy under study, but is often no more than about 60% and even considerably smaller for some anatomic regions. It is generally not obvious how the zero magnetization regions in such a case can be exploited by appropriate choice of RY and RZ. Also, for a given target overall acceleration R, there are many pairs (RY, RZ), or “R-pairs,” for which R ≈ RY × RZ, but again it is generally not clear beforehand which specific R-pair is best to use. This uncertainty in selecting (RY, RZ) is further compounded in applications such as contemporary accelerated 3D CEMRA where acceleration factors of R = 6–8 or higher are routinely used (10–14). That is to say, the region over RY−RZ “acceleration” space in which high quality images can be generated is much broader than if, say, practical overall acceleration factors were limited to R < 4. Previously, the Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration (CAIPIRINHA) technique (15,16) has been presented as a means to improve acceleration performance for a given R by making aliasing more benign through adjustment of the 2D SENSE under-sampling kernel to shift the directions of aliasing. CAI-PIRINHA has been further generalized (17). In contrast, the purpose of this work is to develop Acceleration Apportionment in which the acceleration values (RY, RZ) themselves are altered rather than the undersampling lattice pattern. Specifically, we illustrate the potential gains available when RY and RZ are allowed to vary for a given R, show how the choice of an optimum R-pair depends upon the subject, and describe an algorithm for selecting patient-specific RY and RZ based on a quality metric calculated from the subject-specific 3D g-factor map. We also describe an implementation for determining the optimum R-pair within one minute after acquisition of the coil sensitivity images, allowing incorporation into the subsequent accelerated scan. The feasibility of the method is demonstrated in CE-MRA studies of the lower extremities.