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Arterial Spin Labeling in Combination With a Look-Locker Sampling Strategy: Inflow Turbo-Sampling EPI-FAIR (ITS-FAIR) Matthias Gu ¨ nther, *Michael Bock, and Lothar R. Schad Arterial spin labeling (ASL) permits quantification of tissue per-fusion without the use of MR contrast agents. With standard ASL techniques such as... |
the labeled blood from the capillary exchange site. Fur-thermore, a large number of signal averages (about 20 ormore) are necessary to achieve a sufficient SNR in thedifference images. Thus, sampling just three different TIvalues with a single-shot imaging technique such as echoplanar imaging (EPI) results in a total me... |
R1,eff5R12ln~cosa! TI2. [4] The LL acquisition has been combined with an imaging technique where a single raw data line is sampled perinversion preparation (TOMROP (27)). If the conventionalreadout is replaced by a fast, single-shot EPI acquisition(20,21), T 1parameter maps can be acquired in about 3 sec with a spatial... |
fitting is the Marquardt-Levenberg algorithm (28), where the squared deviation of the model function Mfrom the measured data is minimized iteratively. In each step of theiteration a correction for every fit parameter is computedusing the so-called curvature matrix, C, which is of the form (29): ~C! ij5O k N1 sk2S]Mk ]pi ... |
preparation to dephase flowing spins (4). Unfortunately, Eq. [12] requires the nondiffusion-weighted signal from avoxel containing only blood to compute M a,0for absolute perfusion quantification. Therefore, a single nondiffusion-weighted EPI image without FAIR preparation is acquiredprior to the Look-Locker image series... |
This might be due to the finite length of the labeled blood bolus, which is not considered in our theoretical model. An extended perfusion model can describe the signal be-havior more accurately (for derivation, see Appendix A)but it is numerically unstable at low SNR, since it requiresan additional fit parameter. From t... |
suppress signal from moving spins. This suppression method is also used for ITS-FAIR. However, the actualvalue of the time lag is of further importance since thelongitudinal magnetization is already diminished by thesuccessive readout excitations of the LL pulsing scheme. The latter is not considered in the present mod... |
rates a limited extent of the labeled blood bolus can be formulated which matches the experimental data moreaccurately. Unfortunately, the nonlinear fitting process be-comes numerically unstable in some cases of low SNR ifthe additional fit parameter is introduced. By using the simple perfusion model together with a weig... |
Performance Evaluation The ITS-FAIR technique offers several adjustable measure-ment parameters which affect the quality of the least-squares fit used to quantify perfusion. These parametersinclude the flip angle a, the number of Look-Locker images N LL, the time spacing scheme (here, a constant spacing with a separation... |
Extensions Perfusion quantification with a Look-Locker readout is not limited to the FAIR inversion scheme but is also applica-ble to other techniques such as STAR (2). Furthermore,multislice measurements are possible; however, the tem-poral resolution is lowered by the number of slices ac-quired. This diminishes the ad... |
The signal behavior of the FAIR technique can be de-scribed by the general kinetic model of Buxton et al. (18). Considering a 90° readout pulse the measured signal attime TI is proportional to: S FAIR~t!,sin 90 8z22z Ma,0zf d RFAIR 3H00,t#t e2R1azt~12ed RFAIRz~t2t!! t,t#td e2R1aztd~12ed RFAIRz~td2t!!ze2R1appz~t2td!t. t... |
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