View Ordering in MRA

I know there are different view ordering options for MRA, such as linear and elliptical centric. What do these mean, and when is each used?  

Contrast-enhanced MR angiography (CE-MRA) is typically performed in three-dimensional (3D) mode, generating a rectangular (Cartesian) array of data. This scheme uses a single frequency-encoding direction (x) and two phase-encoding directions (y,z). To reduce imaging time fewer phase-encoding steps are often acquired in the slice/slab direction (z), giving rise to slightly anisotropic voxels and an asymmetric 3D k-space matrix.

Gradient echo sequence for a 3D CE-MRA

Various strategies exist for filling this 3D data array. Because frequency-encoded data is acquired automatically with each echo, corresponding data points along each kx line are obtained for each (ky, kz) choice. Several possibilities exist for the order and direction in which the two phase-encoding gradients (y,z) are played out.

In linear (sequential) ordering, the slice-select gradient is first turned on to a fixed value (kz), and the in-plane phase-encode gradient is stepped through its full range of possible values (from −ky to +ky). The 3D k-space data is thus acquired in layer by layer (a,b,c,d,e,...) as shown below. These layers may be acquired in any order (top-to-bottom, bottom-to-top, middle-to-outside, outside-to-middle, or randomly). 

In radial ordering, each kx line is acquired while the y- and z-gradients are changed incrementally in a coordinated fashion. The "layers" are now concentric cylinders of increasing size from the center of k-space outward. Because the number of phase-encoding steps in the ky and kz directions are usually not equal, the cylindrical segments form ellipses (ovals) rather than circles. The resultant pattern is called elliptic centric filling. As with linear ordering, the region at the center of k-space is typically acquired first and the periphery later, but a reverse filling pattern (from the periphery inward) is sometimes used. 

Linear view ordering of 3D-kspace. Linear centric acquires data in sequence a-b-c-d-e. Reverse linear centric acquires data in order e-d-c-b-a.

Radial (elliptical) view ordering of 3D-k-space. Elliptical centric acquires data in order a-b-c-d-e; reverse elliptical centric in order e-d-c-b-a.

Elliptical-centric ordering is now the preferred k-space sampling method used for most CE-MRA examinations. Linear methods may still be used on older equipment and for MRA of the distal extremities where arrival of the contrast bolus may be prolonged or its exact timing difficult to predict.  

A vendor-specific nomenclature exists. GE and Siemens use a more generic terminology: "elliptic-centric" and "elliptical scanning" respectively. The other major vendors use acronyms: Philips CENTRA ("Contrast-ENhanced Timing Robust Angiography"), Hitachi PEAKS ("PEak Arterial K-Space filling"), and Toshiba DRKS ("Differential Rate K-space Sampling")

"Recessed" elliptical-centric ordering with k-space center placed asymmetrically in data window corresponding to peak arrival of contrast bolus.

In all forms of centric ordering the lowest spatial frequency points (near the center of k-space) are sampled first. These central points contribute most to overall image contrast, which is timed to correspond to the arrival of the arterial bolus. In addition to accentuating arterial signal, centric order also minimizes venous signal because appearance of contrast in the venous system occurs during the time when high spatial frequencies (edges/details) are being sampled. Some vendors allow the center of k-space data collection to be moved asymmetrically within the data sampling window. This variation has been called recessed elliptical centric ordering.

Advanced Discussion (show/hide)»

One modification of all centric methods concerns the actual order data points are acquired within a given layer or ellipse. Most manufacturers use a structured method of data sampling beginning at the center of k-space, proceeding outward in a spiral or zig-zag direction. Here it is desirable to have the middle point of k-space) coincide with the peak arterial bolus. However, some theoretical benefits exist for randomizing data collection within the center of k-space. By sampling the central region of k-space in a random (stochastic) fashion over the full arterial window, the first view does not have to coincide exactly with the bolus peak. This may reduce artifacts occurring from slight mistiming of maximum bolus arrival. Philips' CENTRA and Siemens' TWIST methods utilize this randomized sampling technique.

Elliptic-centric with outward spiral trajectory.

Elliptic-centric with random (stochastic) sampling.

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References
     McNeal GR, Natsuaki Y, Kroeker R, et al. Improved workflow and performance for contrast-enhanced MR angiography sequences. MAGNETOM Flash 2009;2:95-98. (Sales brochure including discussion of Siemens time-to-peak view ordering adjustment)
     Watts R, Wang Y, Prince MR. Method and apparatus for anatomically tailored k-space sampling and recessed elliptical view ordering for bolus-enhanced 3D MR angiography. US Patent # 7,003,343 B2, filed Feb 21, 2006.
     Willinek WA, Gieseke J, Conrad R, et al. Randomly segmented central k-space ordering in high-spatial-resolution contrast-enhanced MR angiography of the supraaortic arteries: initial experience. Radiology 2002; 225:583-588. (Description of Philips CENTRA method).
     Wilman AH, Riederer SJ, King BF, et al. Fluoroscopically triggered contrast-enhanced three-dimensional MR angiography with elliptical centric view order: application to the renal arteries. Radiology 1997; 205:137-146.

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Related Questions
     How do you compute the arrival of contrast in a vessel to know when to start the MR acquisition?  

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