Table of contents
1. K-space key hole technology - data acquisition
For Cartesian k-space, a phase-encoded row is sometimes called a k-space view . In general,The K-space data of each frame image is completely independent, the view sharing mentioned here is a reconstruction method , in order to reconstruct two or more frames of different images, some K-space view data are used repeatedly.i.e. some k-space views are shared across multiple datasets.
Imaging dynamic or time-varying processes, such as cardiac motion , FMRI task activation , post -contrast bolus injection enhancement, joint motion, or catheter tracking, is required for clinical research. In such studies, high spatial resolution or large FOVs of coverage are also often required. Therefore, imaging dynamic processes usually involvestime resolution、spatial resolutionandspace coveragean appropriate compromise between . The compromise method is partial K-space refresh. The keyhole method is to frequently refresh the data in the center of K-space, which sacrifices spatial resolution. This is based on the assumption that dynamic information is band-limited in K-space (that is, image changes have low spatial resolution). But time resolution is guaranteed.
View sharing is not to interpolate K-space data, but to copy selected views from one storage location to another for reuse. For this reason, we call the additional image reconstructed with view sharing the intermediate image .
Examples of shared acquisitions during reconstruction include keyhole , BRISK , TRICKS techniques , real-time imaging with partial K-space data refresh , and segmented K-space cardiac acquisition (FASTCARD) , etc.View sharing is also used to reconstruct the intermediate image of the cardiac triggered phase contrast examination。
In the original keyhole method , a small number of views, called keyhole views , are symmetrical about the central row of K-space (K=0), collected repeatedly, and provide time-series images with low spatial resolution to monitor dynamic processes. refer tohigh spatial frequency of the data, combined with each keyhole dataset to generate a complete K-space dataset for reconstructing a series of images .
Keyhole acquisition was originally introduced to improve the temporal resolution of contrast-enhanced imaging, based on the assumption that most of the information in the contrast bolus is contained at low spatial frequencies . Although the reconstructed image hasfull spatial resolutionThe performance of the dynamic change data is actually low spatial resolution keyhole acquisition and reconstruction, and the high frequency information is used as an anatomical position reference without transmitting any dynamic information .
(a) 8 keyhole views
(b) 32 reference views
For 256 matrices, the data of 8 phase encoding rows in K space is one view data
2. BRISK technology
The block regional interpolation scheme for K- space (BRISK) is an extension based on the keyhole idea .Keyhole is based on the assumption that dynamic information is a low-k spatial frequency, so high-frequency data is not refreshed. This assumption is sometimes not true and needs to be improved. When tracking most of the dynamic process, the high-frequency information of BRISK is also refreshed, but the refresh frequency of the K-space edge is lower than that of the central data .
BRISK initiates 2D multiphase cardiac scans for unsegmented K-space acquisition to reduce acquisition time. In the traditional non-segmented scanning, each slice of K-space is used to collect Ncp times per heartbeat cycle (the number of phases of the heart, that is, the number of time periods between RR waves). Therefore, for collecting a full K-space dataset at each slice, the number of heartbeats required is equal to the number of K-space rows .
BRISK can be combined with segmented K-space acquisition, sometimes called ultra-fast (turbo)-BRISK .The original BRISK uses Fourier interpolation to fill in the missing K-space of each phase. Later, linear interpolation was used to increase the reconstruction speed .
Views close to the central segment of K-space are sampled in each phase (blocks 8, 9). In other views, the sampling frequency gradually decreases from inside to outside, and the segments 1-5 and 12-16 near the edge are only sampled once per heartbeat cycle. One column represents the full K-space data set of a phase . Here it can be called a section or a block asa view, taking the rectangle 256 as an example, there are 16 rows of data in one view or one section .
The figure showsBRISK's K-space sampling mechanism, in this example K-space is divided into 16 segments. The heartbeat cycle is divided into 20 phases. Shaded blocks represent sampled views, white squares represent missing views that need to be filled by interpolation . Views near the center of K-space (blocks 8 and 9) are sampled in each phase, and views far from the center of K-space are sampled at low frequencies. Specifically, 28 near the edge of K space (blocks 1-5 and 12-16) are only sampled once per heartbeat cycle; blocks 6 and 11 are sampled 5 times; blocks 7 and 10 are sampled 10 times. In this way, the scanning time of each layer is 1/4 of the traditional scanning time (the number of shaded blocks in the figure is 1/4 of the total number of blocks).
In addition to cardiac imaging ,Ultrafast BRISK is also used for quantitative phase-contrast (velocity-encoded) imaging of flow in large arteries。
3. TRICKS technology
Time resolved imaging of contrast kinetics ( time resolved imaging of contrast kinetics ,TRICKS) is a variant of the general idea of keyhole acquisition and BRISK .TRICKS are commonly used to improve the temporal resolution of 3D contrast-enhanced scans. The Kx-Ky plane is divided into equal areas, sampled cyclically in time. Ky and Kz are the two phase encoding directions in 3D rectilinear sampled K-space. Similar to BRISK, it is the most frequent sampling in the central region of K-space .
In the original TRICKS, the Ky-Kz plane is only divided in the Ky direction, and later it is developed that the Ky-Kz plane is divided by concentric ellipses . It is usually divided into 4 areas, marked as A, B, C, and D, and the A area corresponds to the center of K space . Before contrast agent injection, K-space is fully sampled, that is, all views in all 4 areas are sampled,The resulting image is used as a subtraction mask. Next, the 4 regions are collected in the order of ABACADABACAD... until the end of the scan.
(a) Segmentation along the first phase encoding axis Ky ,Kx and Kz code for frequency and level respectively;
(b) bisect the two-dimensional area (KyKz) with concentric ellipses ,Ky and Kz are phase and slice codes respectively
4. Live Imaging and Sliding Window Reconstruction
In real-time imaging , sliding window reconstruction is commonly used. Assuming that the time to collect a view is TR ,Wait until N*TR to complete the acquisition of the second set of views and build a completely new image. With sliding window reconstruction , a refreshed image can be built after every TR interval . Unless the image changes, all images are the same regardless of noise . The purpose of this type of acquisition is to detect changes in an object, such as motion . Certain images reconstructed with view sharing have abrupt transitions if there is constant linear motion .
Unlike a resting organ, the heart is always beating periodically. It may be advisable to divide the heartbeat cycle into Ncp time periods , and each period represents a state of the heart beating (cardiac phase) . To complete the image acquisition of one heartbeat cycle on a heart level, it is necessary to collect Ncp K-plane data, which requires many heartbeat cycles to complete
Therefore, each heart phase needs to be imaged, and a K-plane data needs to be collected for each heart phase。
5. Acquisition of ECG-triggered movies (CINE)
Although each row of data on the K-plane of a cardiac phase is collected in different heartbeat cycles , due to the synchronous effect of R-wave triggering, each row of data is approximately collected at the same time point in the cardiac cycle (ie, cine imaging ).
If there is arrhythmia , the time information of ECG can be used for interpolation before image reconstruction , which can help align all row data sets on the K-plane to the same time point. This technique is called cine imaging .
In order to reduce the total imaging time of CINE, it can be collected in segments, that is, multiple K-space line ECG ( Electrocardiogram ) can be collected at one R wave trigger in each cardiac phase , (also known as EKG, an abbreviation from the German “Elektro-kardiographie”) is An experiment (test) used to record the timing and intensity of the electrical signal sequence that triggers the heartbeat. The ECG records the electrical pulse sequence that triggers the heartbeat .
(a) Movie (CINE) method, (b) Segmented K-space method, 4 views per segment
6. Segmented heart acquisition and view sharing
forReduced overall scan time, it is necessary to reduce the number of ECG triggers required to complete the entire K-space data acquisition . this methodSegment the RR interval into several phases corresponding to K-space segments, and collect multiple K-space lines in each phase.
The number of K-space lines collected in each segment is called the number of views per segment (vps) . Although segmentation shortens the total scan time by a factor of vps, at the expense of temporal resolution the same factor is reduced. Because the data of each line in a heart phase is not collected at the same time point, it may cause image blur .
View sharing is commonly used to reconstruct segmented ECG-triggered acquisitions, and up to (vps-1) intermediate phases between any two phases can be reconstructed by reconstruction with a sliding window . Reconstructed mesophases improve performance or effective temporal resolution by providing a smoother changing movie display. However,The real time resolution is the time to collect a K-space segment: vps*TR.
View sharing does not improve the true temporal resolution because it does not add any new k-space data, but reuses existing data one or more times .
In segmented k-space acquisition, views corresponding to the center of k-space are usually arranged in the same view. For example, assuming each section has 2 views, a total of 8 views, the sections may be arranged as (2,3), (4,5), (6,7) and (8,1). If there is arrhythmia, placing the K-space center in the same view can reduce artifacts. Multiphasic ECG-triggered imaging can be acquired in several ways. One is called segmented cardiac acquisition or FASTCARD acquisition .
Although the above discussion focuses on single-level acquisition , it is possible to extend to multi-level. In order to observe the overall shape of the heart, it is feasible to trigger multi-level interleaved acquisition of ECG .
Perspective sharing is used to reconstruct additional intermediate mental phases . In the first RR interval k-space lines 1-4 are acquired repeatedly; in the second RR interval k-space lines 5-8 are acquired, and so on, until the data matrix is completely filled.View sharing reconstruction between any two phases allows up to (vps-1) intermediate phases to be reconstructed。