Perfusion Measurement with Arterial Spin Labeling at 9.4 T
Local and global variations in brain perfusion can occur in a number of diseases like stroke, vascular malfunctions and inflammation processes. The main physiological process which leads to changes in cerebral blood flow (CBF) is neural activity. The most common method to map neural activity evoked by a specific task is blood oxygen level dependent (BOLD) fMRI. The interpretation of BOLD signals, however, is complicated by their dependence on several additional parameters like blood volume and oxygen consumption. Furthermore BOLD fMRI has a low spatial specificity and some methods are strongly influenced by draining vessels.
Variations in CBF can be measured independently with several invasive methods like 15O positron emission tomography (PET) and dynamic susceptibility contrast (DSC) MRI. However, the only non-invasive and rapidly repeatable method to map variations in cerebral blood flow caused by neural activity is arterial spin labeling (ASL) MRI. ASL-MRI has a high temporal and spatial resolution and offers the possibility for quantitative measurements. Furthermore, arterial spin labeling can be combined with BOLD fMRI in calibrated experiments to measure changes in the metabolic rate of oxygen. The major drawback of this method is that the change in signal intensity due to perfusion is only in the range of few percent. Therefore, the SNR in ASL-MRI is poor and signal averaging and/or a lower spatial resolution has to be used to achieve reasonable SNR in the resulting CBF maps.
Ultra high-field MRI systems provide the capability for higher spatial and temporal resolution due the linear relationship between SNR and the strength of the magnetic field. In addition, perfusion measurements with ASL-MRI may benefit from longer longitudinal relaxation times at higher field strengths. The limiting factors of ASL at 9.4T are the high power demands of the arterial spin labeling pulses and the resulting high SAR values as well as inhomogeneous transmit fields and shorter transverse relaxation times.
Implementation of Pulsed ASL at 9.4 T
Quantitative ASL - comparison between 9.4 T and 3 T
The pulsed arterial spin labeling technique FAIR has a comparatively low power deposition and is virtually unaffected by magnetization transfer effects. For this reason, a FAIR sequence was implemented using an optimized adiabatic inversion pulse and an in-plane presaturation method with reduced sensitivity to transmit field inhomogeneities. Quantitative measurements were performed at 3 T and 9.4 T to determine the actual performance of ASL at ultra-high field. Corrected for the different voxel size, the 9.4 T showed a 2.3 higher SNR compared to 3 T.
Acquisition parameters: Preparation TR 4 s, TE 17/18 ms (9.4 T/ 3T), 400-2800 ms (stepwidth 400 ms), voxel size 2x2x3 mm³ (9.4 T), 3x3x3 mm³ (3 T).
Functional arterial spin labeling signal can be contaminated with BOLD related signal changes which makes its interpretation difficult. This is especially the case at ultra-high field when a gradient echo based EPI readout with cartesian k-space sampling is used due to the shorter T2* relaxation times. As an alternative with reduced T2* and thus BOLD weighting, a balanced SSFP (bSSFP) readout was implemented and evaluated in a functional experiment where the subjects were asked to perform a simple finger tapping task. A clear increase in perfusion signal during the task can be seen for both readouts.
Acquisiton parameters: Preparation TR 5 s, TI 1700 ms, voxel size 2x2x2.5 mm³, 60 tag-control pairs.
Obtained activation maps of the ASL sequences and a reference BOLD measurement co-registered on an anatomical GRE.
Implementation of Dual-Coil Continuous ASL at 9.4 T
Continuous labeling techniques typically permit a higher signal to noise ratio than pulsed labeling techniques. However, a flow driven adiabatic inversion can only be achieved using either SAR intensive approaches where a high number of RF pulses are applied in a thin slab located in the inferior region of the head (so called pseudo continuous ASL) or by using a dedicated labeling coil in the neck of the subject (also called dual-coil continuous ASL). However, beside the additional labeling coil the latter approach requires an external RF amplifier which is capable of transmitting long RF pulses (duration >1 s) and an additional safety circuit to avoid risks for the subject due to RF heating. The picture shows one of the labeling coils for 9.4 T.
since Nov. 2010 PhD Student at the Magnetic Resonance Center of the Max-Planck Institute for Biological Cybernetics, Tübingen, Germany.
PhD project: Magnetic Resonance Perfusion Measurement with Arterial Spin Labeling at Ultra-High Field.
2005-2010 Medical engineering studies with an emphasis on medical equipment technology at the University of Applied Sciences, Ulm, Germany.
Degree: Diplom-Ingenieur (Fachhochschule)
Diploma Thesis (October 2009 March 2010) at the Max-Planck Institute for Biological Cybernetics, Tübingen, Germany.
Topic: Non-invasive Temperature Measurement at Ultra-high Fields.
Practical training semester (March 2008 August 2008) at the Department of Radiation Sciences, University Umeå, Sweden.
Project: Development and evaluation of a SPM based Matlab tool to compensate patient movements in dynamic contrast enhanced MRI (DCE-MRI) time series.