Perfusion imaging by a flow‐sensitive alternating inversion recovery (Fair) technique: Application to functional brain imaging

We summarize here current methods for the quantification of CBF using pulsed arterial spin labeling (ASL) methods. Several technical issues related to CBF quantitation are described briefly, including transit delay, signal from larger arteries, radio frequency (RF) slice profiles, magnetization transfer, tagging efficiency, and tagging geometry. Many pulsed tagging schemes have been devised, which differ in the type of tag or control pulses, and which have various advantages and disadvantages for quantitation. Several other modifications are also available that can be implemented as modules in an ASL pulse sequence, such as varying the wash-in time to estimate the transit delay. Velocity-selective ASL (VS-ASL) uses a new type of pulse labeling in which inflowing arterial spins are tagged based on their velocity rather than their spatial location. In principle, this technique may allow ASL measurement of cerebral blood flow (CBF) that is insensitive to transit delays. IN PULSED arterial spin labeling (ASL) (1-3), a single radio frequency (RF) pulse or a train of RF pulses is applied to modify (or tag) the longitudinal magnetization of arterial blood proximal to the tissues of interest. The RF pulse or train of pulses must be short in duration compared to the T 1 of blood. After a period of time TI after the initiation of the RF tagging pulse, an image (the tag image) is acquired of the region of interest (ROI), which represents a mixture of the preexisting magnetization in the ROI and the magnetization of the tagged blood that has flowed into the ROI. In typical implementations, a second image (the control image) is acquired without modulation of the magnetization of inflowing arterial blood. The difference between tag and control images reflects the amount of tagged blood that has flowed into the ROI during the time interval TI and is closely related to cerebral blood flow (CBF).A simplified expression for the signal difference ⌬S between tag and control images in pulsed ASL is given bywhere ␣ is the tagging efficiency, M 0B is the MRI signal from a voxel full of arterial blood, is the temporal width of the bolus of blood that reaches the ROI, and T 1B is the T 1 of blood. In this expression, 2␣M 0B is the initial magnetization difference between tagged and control blood, the product CBF is the amount of tagged blood that flows into the ROI, and the exponential factor reflects T 1 relaxation of the tag. A feature of ASL that distinguishes it from other tracer techniques is the short lifetime of the tracer. The T 1 of blood is approximately 1300 msec at 1.5 T and is the time constant for the decay of the tag. This sets the required time scale for the ASL experiment and determines many of the important properties of ASL. The ASL experiment is a race between the decay of the tag and the delivery of tagged blood to the ROI. QUANTITATION ISSUESSeveral potential sources of systematic errors can affect the quantitation of CBF and are summarized here. Many of the pulsed ASL techniques listed in the next section are aime...

Section: Tagging Schemesmentioning

confidence: 99%

Quantifying CBF with pulsed ASL: Technical and pulse sequence factors

Wong

2005

“…Since ASL spin preparation does not rely on susceptibility changes, the ASL scheme can be used for perfusion quantification in brain areas of high susceptibility changes. A commonly applied ASL preparation is the flow-sensitive alternating inversion recovery (FAIR) (8,9) scheme. The most extensively used data acquisition technique for ASL is EPI (8 -10), which is extremely sensitive to susceptibility artifacts.…”

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confidence: 99%

FAIR‐TrueFISP imaging of cerebral perfusion in areas of high magnetic susceptibility differences at 1.5 and 3 Tesla

Boss

Martirosian

Klose

et al. 2007

Purpose:To estimate cerebral blood perfusion in areas of strong magnetic susceptibility changes with high spatial and temporal resolution using a flow-sensitive alternating inversion recovery (FAIR) arterial spin labeling (ASL) method. Materials and Methods:We implemented an ASL method that is capable of imaging perfusion in areas of high magnetic susceptibility changes by combining a FAIR spin preparation with a true fast imaging in steady precession (TrueFISP) data acquisition strategy. A TrueFISP readout sequence was applied especially in regions with magnetic field inhomogeneities and compared with corresponding FAIR measurements obtained with a standard echo-planar imaging (EPI) readout. Quantitative perfusion images were obtained at 1.5 Tesla (1.5T) from eight healthy volunteers (24 -42 years old) and one patient (23 years old). FAIRTrueFISP perfusion images were compared with FAIR echoplanar images. T1 maps, which are necessary for quantitative perfusion estimation, were obtained with inversion recovery (IR) TrueFISP and IR EPI. Additionally, high-resolution perfusion measurements were performed on four volunteers at 3T. Results:The two ASL perfusion imaging modalities yielded comparable diagnostic image quality in brain areas with low susceptibility differences at 1.5T. Cerebral perfusion of gray matter (GM) areas was 105.7 Ϯ 5.2 mL/100 g/minute for FAIR-TrueFISP and 88.8 Ϯ 14.6 mL/100 g/minute for FAIR-EPI at 1.5T, and 70.4 Ϯ 7.1 mL/100 g/minute for FAIR-TrueFISP and 63.5 Ϯ 6.9 mL/100 g/minute for FAIR-EPI at 3.0T. Higher perfusion values at 1.5T were due to more pronounced partial-volume effects from fast moving spins at lower spatial resolution. The FAIR-TrueFISP sequence showed no significant distortions and markedly reduced signal void artifacts in areas of high susceptibility changes (e.g., near brain-bone transitions and close to metallic clips) compared to FAIR-EPI. At 3T, highly resolved FAIR-TrueFISP perfusion images were acquired with an in-plane resolution of up to 1.3 mm.Conclusion: FAIR-TrueFISP allows for assessment of cerebral perfusion in areas of critically high susceptibility changes with conventional ASL methods.

“…The proof of principle for obtaining pulmonary perfusion using MRI was demonstrated in 1985 (1). In the years since this pioneer work, two basic perfusion imaging methods have been established: The arterial spin labeling (ASL) techniques (2,3), which use blood as an endogenous contrast-agent, and imaging techniques in conjunction with the application of contrast-altering substances (eg, Gd-DTPA) (4,5). However, both strategies suffer from certain disadvantages.…”

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confidence: 99%

Assessment of pulmonary perfusion in a single shot using SEEPAGE

Fischer¹,

Pracht²,

Arnold³

et al. 2007

Purpose: To present a single-shot perfusion imaging sequence that does not require contrast agents or a subtraction of a tag and a control image to create the perfusionweighted contrast. The proposed method is based on SEEPAGE. Materials and Methods:Experiments with healthy volunteers were performed to qualitatively and quantitatively obtain pulmonary perfusion values in coronal as well as sagittal orientation. In addition, a first experiment with a lung cancer patient was performed to explore the potentials of SEEPAGE in a clinical application. Results:All experiments clearly showed a perfusionweighted contrast, providing clinical quality images with high spatial resolution. The quantified perfusion rates were consistent in the different imaging orientations and covered the interval of 1.00 -4.00 mL/min/mL. In addition, the gravitational dependence of pulmonary perfusion, the influence of adiabatic pulse duration on signal intensity, and the tracer saturation effect were examined. In the patient examination the presented technique provided additional information of the lung deficiency compared to a conventional anatomical image.Conclusion: SEEPAGE has proved to be a robust and reproducible technique for obtaining perfusion-weighted images in a single measurement and for quantifying pulmonary perfusion using an additional reference scan. Furthermore, the proposed method shows promise for future clinical application.