Object ID: 00000018WIA3088C770GYZ
Topic ID: id_40021975 Version: 1.2
Date: May 10, 2022 3:27:46 AM

K-Space

Just as the spatial positions can be mapped into frequencies, temporal information can be mapped into k-information. Hence, MR is said to acquire data in k-space. A Fourier transformation converts k-space data to image data. Because an image is at least a 2D space, we speak of k-space which is a plane for 2D MR imaging and a 3D space for 3D MR imaging.

Typically in MR imaging, data points in k-space are sampled line by line. However many other strategies, have been devised to sample data points in k-space. Use the k-Space User CV to select from five types of k-space filling: sequential, centric, elliptic centric, reverse centric, and reverse elliptic centric.

Do not use SmartPrep automatic triggering with Reverse Elliptical-Centric or Reverse-Centric. Instead you need to perform a test bolus and calculate the delay. Here are some suggested formulas.

When using Reverse-Centric: Injection delay = Contrast transit time + 2 seconds - 90% of the scan duration.
When using Reverse Elliptical-Centric: Injection delay = Contrast transit time - 1/2 scan duration.
- If the result is a positive number, use a scan delay.
- If the result is a negative number, use an injection delay.

Sequential ordering

Use to acquire the contrast sensitive information along the Ky axis at the middle of the scan, filling contrast sensitive data equally along the Kz (slice) axis throughout the total scan time.

Centric ordering

Use to acquire the contrast sensitive data along the Ky axis at the beginning of the scan. Along the Kz (slice) axis, data is recorded throughout the total scan.
With centric ordering, the center lines are filled in first and should be used when your concern is capturing peak arterial contrast enhancement.
It allows a relatively long acquisition to achieve the image contrast associated with the moment when central k-space data are acquired.

Elliptic Centric ordering

Use to acquire the contrast sensitive information in both the Ky and the Kz (slice) axes simultaneously.
Data is collected in a much shorter time than in sequential or centric ordering.
It is used to increase the contrast between IV contrast and background tissue. It is recommended for use with Fluoro Trigger, especially for carotid applications.
An Elliptical-Centric selection of 2 for delay acquires the center of k-space three seconds after the initiation of the 3D acquisition This setting is only applicable in 3D sequences and is independent of SmartPrep setting. It is available with 3D Vascular TOF GRE/SPGR scans.
It is more robust relative to bolus timing mistakes in comparison to standard Elliptical-Centric.
Scan can start at the beginning of contrast arrival capturing more arterial phase with less venous contribution.
The center lines are acquired during peak contrast enhancement.

Reverse-Centric

It can be useful when performing run-off CEMRA exams.
This choice allows you to start the scan acquisition at the first station even before you begin the contrast injection. Since the contrast is in the vascular system for a shorter period of time as the table advances to the middle and lower stations, this can reduce the appearance of venous contamination by the time you reach the lower stations.
Do not use with SmartPrep or Fluoro Trigger.

Reverse Elliptical-Centric

Use for top and middle stations of a CEMRA exam. Elliptical-Centric is often used for lower stations such as feet.

Table 1. K-space filling techniques
k-space selection	Filling options
Reverse Elliptical-Centric	Fills the center lines and slices of k-space during the last 1/9 of the total scan time
Elliptical-Centric	Fills the center lines and slices of k-space during the first 1/9 of the total scan time
Centric	Fills the center lines of k-space for all slices during the first 1/3 of the total scan time
Reverse Centric	Fills the center lines of k-space for all slices during the last 1/3 of the total scan time