Works-in-Progress package Version 1.0 For the Installation and User s Guide NUMARIS/4VA21B January 22, 2003 Section of Medical Physics, University Hospital Freiburg, Germany Contact: Klaus Scheffler PhD, Hugstetterstr. 55, D 79106 Freiburg, email: klaus.scheffler@unibas.ch
Table of Contents 1 Basic principles... 3 1.1 Implemented ging techniques... 3 1.2 Sequence structure and timing... 5 1.3 References... 6 2 Software Installation Procedure... 7 2.1 Installation... 7 2.2 Restart the System... 7 3 Sequences and Protocols... 8 3.1 Slice and inversion slab positioning... 8 3.2 Other parameters... 9 3.3 Calculation of ASL images... 11 Page 2 of 11
1 Basic principles Pulsed arterial spin labeling techniques (PASL) such as PICORE, TILT, STAR, and FAIR, are frequently used to measure regional cerebral blood flow and perfusion (1-4). These techniques avoid the application of a contrast agent and have the potential to give quantitative perfusion values. Most applications of PASL techniques are based on a combination of spin labeling and image acquisition with echo planar imaging (EPI). Due to susceptibility artifacts the application of EPI-based labeling techniques is limited. Alternatively, single-shot TSE methods such as STAR-HASTE may cause blurring along phase-encoding direction due to T2 relaxation, and the temporal resolution compared to EPI is reduced (5). The ASL TrueFISP technique is based on the combination of a steady state free precession (TrueFISP, balanced FFE, FIESTA) imaging sequence and pulsed arterial spin labeling (6-8). 1.1 Implemented ging techniques Four different pulsed arterial spin labeling techniques, PICORE, TILT, STAR, and FAIR, are implemented together with a TrueFISP readout. Orientation and thickness of the inversion slabs and imaging slices can be chosen arbitrarily. Inplane saturation before inversion can be switched ON or OFF. The number of dummy repetitions before averaging can be chosen between 0 and 6. For averaging an inner or outer loop can be chosen: for inner loop averaging (short term) all ged images will be averaged followed by averaging of the non-ged images; for outer loop averaging (long term) pairs of ged and non-ged images will be averaged (see 1.2 and 3.2). The number of contrasts is always 2 (ged and non-ged). PICORE: Selective inversion below imaging plane versus off-resonance inversion. selective inversion 10 khz offresonance inversion Page 3 of 11
TILT: Selective inversion (90 +90 ) versus selective nothing (90-90 ) below imaging plane. selective inversion 90 + 90 90-90 STAR: Selective inversion below imaging plane versus selective inversion above imaging plane. selective inversion selective inversion FAIR: Selective inversion within imaging slice versus global inversion. global inversion selective inversion Note: Tagging (inversion) slabs shoud not be too close to the imaging slice in order to prevent partial saturation of the imaging plane. A distance between ging slab and image plane of 10% of the slab thickness is recommended. Page 4 of 11
1.2 Sequence structure and timing Imaging is performed with a conventional α/2-prepared 2D TrueFISP sequence. RF and echo spacing is equal to 2TE. Selective and non-selective inversion pulses are based on an optimized adiabatic full passage pulse (included in external file hypersec.pls). The Tag Delay is the time between end of pulse and start of imaging. TR is the time between consecutive s/s: in-plane sat (optional) imaging slice(s) or Tag Delay TR Averaging mode can be inner (short term) our outer (long term) averaging: outer averaging inner averaging inner averaging Page 5 of 11
1.3 References (1)Wong E et al. NMR in Biomed. 10:237; 1997. (2)Golay X et al. JMRI 9 :454 ; 1999. (3)Edelmann RR et al Radiology 192 :512 ; 1994. (4)Kim S-G. MRM 34 : 293 ; 1995. (5)Chen Q et al. MRM 38 : 404 ; 1997 (6)Scheffler K. Proc. MR Angiography 2001, p. 119. (7)Martirosian P et al. Proc. ISMRM 2001, p. 1560. (8) Scheffler K. ISMRM 2002, p. 628. Page 6 of 11
2 Software Installation Procedure 2.1 Installation The ASL TrueFISP installation CD contains the following files: 1. The Word document WIP-ASL_trufi_VA21B.doc 2. A folder named protocol that contains 2.1. A file named WIP_EXPO.edx 2.2. A folder named WIP_EXPO.edxdir To install the WIP package please: 1. Place the CD in the drive tray. 2. Open the Exam Explorer. 3. Open a program region and a subprogram region. 4. Click on this subprogram region and select Object/Import. 5. Select the CD drive in the pop-up window, double-click on the folder protocol and select the file WIP_EXPO.edx. To install the external RF pulse please: 6. Place the CD in the drive tray. 7. Open the Windows NT Explorer (with -escape). 8. Select the CD drive 9. Go to folder protocol/wip_expo.edxdir/customerseq. 10. Copy the file hypersec.pls to C/MedCom/MriSiteData/measurement/. 11. Close the Windows NT Explorer and eject CD. 2.2 Restart the System In order for the changes to take effect (external RF pulse), you will need to restart System. Page 7 of 11
3 Sequences and Protocols 3.1 Slice and inversion slab positioning The positioning of the slice(s) and the ging slab(s) is free and independent. This offers the possibility of dedicated ASL measurements, for examples in the kidneys. For PICORE, FAIR, and TILT there is only one ging slab, STAR requires two ging slabs. Tagging slabs can be accessed and modified via the UI on the Geometry/Saturation card. In the PICORE, FAIR, and TILT mode the user must switch on one region as showed below: For STAR the user have to select two regions. Page 8 of 11
Changing from STAR to PICORE, FAIR, or TILT one region will be automatically deleted. Examples for slice and region positioning are shown below for brain and kidney measurements: PICORE or TILT FAIR STAR PICORE or TILT For the ASL measurement on the kidneys the region is in saggital orientation in order to the blood in the descending aorta. The imaging slice is coronal or transvers. 3.2 Other parameters The four different ging techniques can be chosen via the Contrast card by selecting the Tagging button: Page 9 of 11
The most important parameters are the Tag Delay (time between inversion and start of image acquisition) and TR (time between successive inversion pulses). For multislice imaging the Tag Delay is still the time between and start of image acquisition. Images will be acquired sequentially (descending or ascending) and will thus have different Tag Delays. Typically a Tag Delay of 1000ms 1300ms and a TR of 2000ms 2500ms, and a flip angle of 60-70 is recommended: The signal-to-noise ration of the ASL images is proportional to the square root of the number of averages. For brain studies about 20 and for the kidney about 4 averages are recommended. As depicted in 1.2 the user can chose between inner (short term) and outer (long term) averaging. Long term averaging is recommended. Page 10 of 11
3.3 Calculation of ASL images The sequence produces pairs of ged and non-ged images separately. Perfusion images have thus to be calculated by subtraction using the Dynamic Analysis software on the scanner: For PICORE, and TILT image 1 have to be subtracted from image 2: 2-1. For FAIR image 2 has to be subtracted from image 1: 2-1. For STAR subtraction depends on the positioning of the two inversion slabs. Page 11 of 11