Kspace Options

Another important concept concerning imaging parameters is the k-space acquisition order (Fig. 8) [7,8,12-14,48-51]. As with all MR acquisitions, the resulting image contrast is dominated by data within the center of k-space (also called low spatial frequency data) (Fig. 9). Traditional k-space acquisition schemes are linear in that k-space data is acquired sequentially one line at a time from top to bottom (Fig. 10). The central k-space data are thus typically acquired during the middle of any given scanning period.

With the development of real-time timing methods for CE MRA, it is important for the central k-space data to be acquired early during the imaging period, as this ensures image acquisition when contrast is detected within the target vessel. Reliance on linear acquisition schemes in which central k-space data is centered within the imaging period (e.g. about 15 seconds into a 30 second acquisition) would necessitate placement of the signal monitoring volume or MR fluoroscopic image

Elliptical Space

Fig. 8. Diagram illustrates the proper alignment of preferential arterial-phase enhancement for a variety of k-space schemes used for CE-MRA. The critical issue for all the schemes is that central k-space data (i.e. low spatial frequency data) be acquired during the plateau phase of arterial enhancement. In the conventional sequential k-space scheme, the central k-space data is acquired during the middle of the data acquisition period. In both the conventional centric and elliptical centric acquisition schemes, the central k-space data is obtained at the beginning of imaging. Note that with conventional centric acquisitions, k-space is only centric in ky and that the high spatial frequency encodings in kz are also acquired during each linear pass and thus the central k-space encodings in ky and kz are more efficiently gathered (i.e. acquired more quickly) in the elliptical centric acquisition scheme (see Fig. 2). Partial Fourier imaging with reverse sequential acquisition ordering can also provide a compact acquisition of low spatial frequency data during the beginning of image acquisition. Note that low spatial frequency data is best obtained during the plateau period of arterial enhancement. Acquisition of central k-space data prematurely during the rapid rise in arterial signal (large hollow arrow) can result in significant "ringing artifacts" (see Fig. 12) (Reprinted and adapted with permission from [7])

Fig. 8. Diagram illustrates the proper alignment of preferential arterial-phase enhancement for a variety of k-space schemes used for CE-MRA. The critical issue for all the schemes is that central k-space data (i.e. low spatial frequency data) be acquired during the plateau phase of arterial enhancement. In the conventional sequential k-space scheme, the central k-space data is acquired during the middle of the data acquisition period. In both the conventional centric and elliptical centric acquisition schemes, the central k-space data is obtained at the beginning of imaging. Note that with conventional centric acquisitions, k-space is only centric in ky and that the high spatial frequency encodings in kz are also acquired during each linear pass and thus the central k-space encodings in ky and kz are more efficiently gathered (i.e. acquired more quickly) in the elliptical centric acquisition scheme (see Fig. 2). Partial Fourier imaging with reverse sequential acquisition ordering can also provide a compact acquisition of low spatial frequency data during the beginning of image acquisition. Note that low spatial frequency data is best obtained during the plateau period of arterial enhancement. Acquisition of central k-space data prematurely during the rapid rise in arterial signal (large hollow arrow) can result in significant "ringing artifacts" (see Fig. 12) (Reprinted and adapted with permission from [7])

Angiography Space

Fig. 9a-c. The influence of high and low frequency data on image appearance. Contrast is dominated by data within the center of k-space, also called low spatial frequency data (a). Reconstruction of an image using only these low frequency data results in an image that has high contrast, but in which details are missing. Conversely, if only high spatial frequency phase encoding steps at the edges of k-space are utilized (b) the image has low contrast and high detail visibility. Only if central and peripheral data are combined for reconstruction will the image (c) have both high contrast and high spatial resolution [Image courtesy of Dr. G. Schneider]

Fig. 9a-c. The influence of high and low frequency data on image appearance. Contrast is dominated by data within the center of k-space, also called low spatial frequency data (a). Reconstruction of an image using only these low frequency data results in an image that has high contrast, but in which details are missing. Conversely, if only high spatial frequency phase encoding steps at the edges of k-space are utilized (b) the image has low contrast and high detail visibility. Only if central and peripheral data are combined for reconstruction will the image (c) have both high contrast and high spatial resolution [Image courtesy of Dr. G. Schneider]

proximal (or "upstream") to the desired vascular bed. In addition to increasing the uncertainty for timing, this also introduces logistical problems related to proper localization as the center of the FOV for the monitoring volume and the 3D CE MRA may not be similar. Contrast agent arrival and imaging are best synchronized if the acquisition of central k-space data is more intimately timed to the actual bolus arrival within the target vessels. For this purpose, centric and more recently elliptical centric phase ordering schemes (Fig. 11) have been introduced for CE MRA. In centric phase ordering, the central lines of k-space are preferentially acquired early during the imaging period. With elliptical centric phase ordering the most central points (i.e. radial distance from center) are acquired first, resulting in a shortened and more compact imaging period for the acquisition of central k-space data (Fig. 8).

One additional approach to the early acquisition of central k-space information is to use a partial Fourier acquisition scheme with reverse sequential ordering (Fig. 10) [7,8,54]. Partial Fourier imaging exploits the fact that one half of k-space closely mirrors the other half, and thus the entire k-space can be extrapolated from a little more than half the k-space data set. By acquiring the partial Fourier ordering in a reverse sequential manner, the central k-space data can be acquired early during the imaging period. Partial Fourier

Sequential Acquisition Order

overscan region for low spatial frequency estimation overscan region for low spatial frequency estimation

Reversed Sequential ^anial-Fourioi A(;qi;isi;itin OifiD: c a

k

n n n n ^ \

1 —

Mia^mmr^mmm

o a a a & 5 • COO 6 -rn - O" *> y^- 7

r overscan region for low spatial frequency estimation

-•-O-O-

Conventional (Sequential)

Partial fourier Acquisition Greer b

Fig. 10a-c. Conventional k-space acquisition schemes are sequential or linear (a). Consecutive lines of k-space data are acquired with the central lines during the middle of the acquisition period. Partial Fourier schemes enable faster image acquisition but at the expense of diminished signal to noise. Arterial signal, however, is uncommonly a concern on CE-MRA if imaging is properly timed for arterial phase imaging. Partial Fourier acquisitions also provide the added flexibility to place central k-space sampling either during the end (b, conventional sequential) or beginning (c, reverse sequential) of the imaging period. The darker dots represent the low spatial frequency views (center of k-space) in both the slice (kz) and view (ky) encoding directions. The black arrows represent the views acquired at the beginning; and the gray arrows, those at the end of the imaging period. Please note that not all k-space lines are drawn and only representative k-space data is shown (Reprinted and adapted with permission from [7])

Centric Space

Centric Acquisition Order a

Centric Acquisition Order a

Elliptic Centric Space

Fig. 11a, b. Centric k-space acquisition schemes acquire central or low spatial frequency k-space data (represented by the central black dots in the k-space diagram) early in the imaging period. Conventional centric acquisitions (a) obtain data in a sequential or linear alternating fashion above and below ky=0. Elliptical centric acquisitions (b) acquire data from the center out to the next most radial point-data acquisition is thus centrically acquired in both ky and kz. This results in the more efficient (i.e. compact) acquisition of low spatial frequency data by elliptical centric acquisitions (Reprinted and adapted with permission from [7])

imaging has the benefit of significantly decreasing acquisition time, but this comes at the expense of SNR.

Note that the number of phase encoding steps (y-resolution) can be partially sampled in several other ways, all of which reduce overall scan time, but affect SNR and spatial resolution to varying degrees [54]. Partial Fourier imaging (e.g. 0.5 acquisition or NEX; also known as "half scan") enables an asymmetric reduction of phase encoding steps. Although this approach reduces imaging time at the expense of SNR, it has the benefit of preserving spatial resolution. On the other hand, the homodyne reconstruction algorithm used for such acquisitions assumes a constant phase across the image volume, which may not be true in the presence of flow, and may lead to artifacts.

Time can also be saved by reducing the high spatial frequency phase encoding steps at the edges of k-space (i.e. number of phase encoding steps). This reduction, also known as symmetric reduction or scan %, results in lower spatial resolution in the phase encoding direction, but has the benefit of increasing the SNR. A general rule of thumb is to decrease the number of phase encodings until the spatial resolution in the y (phase) dimension is approximately equal to the z (slice or partition) thickness, thereby "balancing" the reso lution in slice and phase. The individual options for k-space reduction vary depending on vendor, but most are capable of asymmetric (partial Fourier, 0.5 NEX or half scan) k-space sampling, which is particularly well suited for CE MRA. Properly timed, CE MRA will generally yield sufficient arterial SNR for successful arterial visualization using partial Fourier acquisition schemes.

An additional factor worthy of consideration with respect to k-space is the coordination with the contrast agent bolus itself. Ideally, the central k-space views should be timed with the more stable (i.e. early plateau) phase of the contrast bolus -hence the short delay after contrast detection to allow the contrast agent concentration to reach its peak. The acquisition of central k-space data during the rapid rise phase of Gd concentration (Fig. 8) will result in a "ringing artifact" (Fig. 12), which is recognized by the presence of alternating bright and dark lines that parallel the early enhancing vessel [55].

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment