Applications Guide for Interleaved

Transcription

1 Applications Guide for Interleaved rephase/dephase MRAV Authors: Yongquan Ye, Ph.D. Dongmei Wu, MS. Tested MAGNETOM Systems : 7TZ, TRIO a Tim System, Verio MR B15A (N4_VB15A_LATEST_ ) MR B17A (N4_VB17A_LATEST_ _P8)

2 Important Note: This document provides a description of techniques developed by MR facility, Wayne State Univ (WSU), Detroit, MI, USA. This work-in-progress software has been tested internally on our Verio, Tim Trio and Siemens-Bruker ClinScan 7T systems. However, each user should be aware of the fact that incorrect use of this software may produce unknown results. The sequences contained in this software package do not exceed the FDA safety performance parameter guidelines for MRI exams. Specifically, there is no change to patient risk as compared to routine operation of the MAGNETOM with regard to: static magnetic field; the time rate of change of the gradient magnetic fields; the rate at which RF power is deposited into the body (SAR); or the acoustic noise created by the MAGNETOM. The software has been tested internally but not yet in a clinical environment. For routine applications, its functionality may not be complete, and use of this software will remain investigational. In general, the clinical user will, in its sole responsibility, decide on the use of this application package or on subsequent therapeutic or diagnostic techniques and shall apply such techniques in its sole responsibility. WSU and the authors will not take responsibility for the correct application of, or consequences arising from use of, this applications package. The software in this package may change in the future, or may not be available in future software versions. In case of any questions that are related to the use of this package please contact one of the authors listed.

3 Table of Contents Overview... 4 Sequence Description... 5 Sequence Installation... 9 Setting up Scanning Protocol... 9 Image Examples... 10

4 Overview This package contains an extended variant of the 2D/3D GRE (Gradient Refocused Echo) sequence, which offers: 1) Simultaneous acquisition of TOF-MRA and SWI, i.e. MRAV; 2) Simultaneous acquisition of flow rephrased (FR) and flow dephased (FD) image for enhanced MRA calculation; and 3) TR-interleaved mode for both 1) and 2). By acquiring all data with a single scan, the motion induced image misregistration can be effectively removed. The focus of this package is to provide a means for simultaneous acquisition of MRAV images, including TOF-MRA and SWI as its basic function. Also since normal TOF-MRA has limited vessel-tissue contrast, we have developed and implemented a non-linear subtraction postprocessing method to selectively enhance artery-tissue contrast by additionally acquiring the FD data. The final output of the sequence, depending on the selected imaging mode, may include the following images: 1) Two short TE flow compensated images, both magnitude and phase; 2) Long TE flow compensated images, including magnitude, filtered and unfiltered phase images, SWI images and SWI mip images; 3) Flow dephased dark blood images; 4) and finally from our processing: enhanced TOF-MRA images.

5 Sequence Description The interleaved rephase/dephase MRAV sequencee is derived from the product a_gre sequence. Specifically, an interleaved mode has been created to enable TR-interleaved acquisition of double echoes images with different flow rephase/dephase combinations, i.e. both echoes are flow rephased in TR1 while the second echo is flow dephased in TR2. The diagram of the sequencee is shown in Fig.1. Fig.1 Diagram of the Interleaved Rephase/Dep hase MRAV sequence. FR denotes flow rephase (i.e. flow compensation), and FD denotes flow dephase. In TR1, the two echoes are acquired with flow rephase gradients, and the TR2, the first echo is flow rephase while the second echo is flow dephase by a pair of bipolar gradients with low VENC value. TR1 and TR2 can be acquired in the same scan in an interleaved manner, or can be acquired separately, depending on the sequence mode selected. Sequence mode The sequence is implemented with 3 scanningg modes: Interleaved, MRAV and enhanced MRA. The selection of the sequence mode can be found on the special card, as shown in Fig.2. Interleaved: This mode obtains both MRAV and enhanced MRA results simultaneou usly, by acquiring thee echoes in TR 1 and TR 2 in an

6 interleaved manner. Selecting this relative to the other two modes. mode will double the scanning time MRAV: This mode obtains only the TR 1 data without carrying out TR 2 scans, and offers flow rephrased images with both short and long TE, respectively. The short TE imagess can be used as normal TOF MRA image, and the long TE images (both magnitude and phase) can be used for SWI calculation. Enhanced MRA: This mode obtains only the TR 2 data without carrying out TR 1 scans, and offers flow rephrased images with short TE (TE 1 ) and flow dephased images with long TE (TE 2 ). Both images are processed using a non-linear subtraction method to enhanced artery-tissue contrast while minimizing vein-tissue contrast in the final images, thus giving an enhanced MRA image. Fig. 2 Sequencee mode selection, asymmetric echo setting, VENC gradient setting and phase high pass filter size. Asymmetric readout When asymmetric echo acquisition is set to Allowed in Sequence->Part1, one can select the asymmetric factor and asymmetric echo type in the Special card. The schemes of both asymmetric echo type are shown inn Fig. 3. Original AsymEcho: This asymmetric echo modee is the same as what people have normally done and fills K x partially with the echo center at the K x center. The AsymFactor Bef indicates the number of K x points (in terms of the percentagee of the whole K x length) acquired beforee the echo center. Unacquired K x points are zero filled. This

7 is the default mode that can be used in most cases. POCS partial Fourier reconstruction will bee performed along RO direction if this mode is activated. Extented AsymEcho: This asymmetric echo modee fills the whole K x with shifted echo center. The AsymFactor Bef is also defined as the number of K x points (in terms of the percentage of the whole K x length) acquired before the echo center. The echo center no longer coincides with the K x center. This mode is designed for extremely high resolution imaging that t exceeds the resolution limit of the scanner. Fig.3 Schemes of original asymmetric echoo and extended asymmetric echo acquisition. VENC gradients (for flow dephasing) We used velocity encoding (VENC) gradients to achieve flow dephasing. Very low VENC values, e.g. ~ 1cm/s, should be used for best flow dephasing performance. According to VENC = /(2 Gτ 2 ), where G is the bipolar gradient amplitude and the duration of echo gradient lobe, one needs a high G 2 product for a low VENC value. We have set up the default values of G = 24mT/m and = 4ms for a VENC value of 1.46cm/s. One can freely adjust both values via the Special card (Fig.2). The VENC gradients is implemented with the sequence mode of both Interleaved and Enhanced MRA.

8 HP filter size This parameter sets the phase high pass filter size along RO direction for SWI processing. The filter size along PE direction is calculated as the product of this RO filter size and N ky /N kx. VFA Variable flip angle. This is a testing parameter and is currently not used. Use the default No option.

9 Sequence Installation 1).dll and.i86 sequence files > C:\MedCom\MriCustomer\Seq\ 2) IceProgramMRAV.evp > C:\MedCom\MriCustomer\IceConfigurators\ 3) MRAVConfigurator.dll +.evp > C:\MedCom\bin\ 4) libmravconfigurator.so > C:\MedCom\MCIR\Med\lib\ 5) Ctrl+Esc > run > type in ideacmdtool > select PAS unload 6) If the sequence cannot run due to Image reconstruction error, restart image reconstruction via System >Control >Image Reconstruction System card > Restart Image Calculation. The restart only takes a few seconds, and the sequence would have recognized the ICE program. Setting up Scanning Protocol Since the sequence is still under development, minor conflicts between protocol parameters may exist which will lead to certain parameters became unadjustable. This part will explain some tricks, which is actually simple and easy, to set up the protocol as desired without running into problems. 1) The contrast, i.e. echo number in each TR, is fixed to 2 for this sequence. The default sequence mode is 3D. 2) Turn on the flow compensation option for both echoes and set the high resolution as the first step. 3) Set the desired slice thickness. 4) Set the desired FOV and RO bandwidth. 5) Set the desired TE after steps 2-4. If using MRAV mode, use a smaller VENC Grad Duration in sequence->special to achieve a shorter TE for the 2 nd echo. 6) Enable SWI process, i.e. check the SWI option in contrast -> common 7) Set the desired HP filter size via sequence -> special. 8) Set all other parameters as desired.

10 Image Examples With the SWI process enabled, the sequence will some or alll of the following images depending on the sequencee mode: 1) original magnitude images, 2) HP filtered and unfiltered phase images, 3) single slice and mip of the SWI images, and 4) single slice enhanced MRA images. If the SWI process is not enabled, or 2D scanning mode is used, then only original magnitude and unfiltered phase images will be generated. Below are the examples of these images. Magnitude Magnitude (Echo11 & Echo 21, FR) (Echo12, FR) Magnitude (Echo22, FD) Magnitude MIP (Echo11, post process) SWI mip (Echo12, FR) SWI (Echo12, FR) Enhanced MRA Enhanced MRA MIP (post process) Original phase (Echo12, FR) HP filtered phase (Echo12, FR)