Matthias Günther

Recommended implementation of arterial spin-labeled Perfusion mri for clinical applications: A consensus of the ISMRM Perfusion Study group and the European consortium for ASL in dementia

This review provides a summary statement of recommended implementations of arterial spin labeling (ASL) for clinical applications. It is a consensus of the ISMRM Perfusion Study Group and the European ASL in Dementia consortium, both of whom met to reach this consensus in October 2012 in Amsterdam. Although ASL continues to undergo rapid technical development, we believe that current ASL methods are robust and ready to provide useful clinical information, and that a consensus statement on recommended implementations will help the clinical community to adopt a standardized approach. In this review, we describe the major considerations and trade‐offs in implementing an ASL protocol and provide specific recommendations for a standard approach. Our conclusion is that as an optimal default implementation, we recommend pseudo‐continuous labeling, background suppression, a segmented three‐dimensional readout without vascular crushing gradients, and calculation and presentation of both label/control difference images and cerebral blood flow in absolute units using a simplified model. Magn Reson Med 73:102–116, 2015.

Single-shot 3D imaging techniques improve arterial spin labeling perfusion measurements

Arterial spin labeling (ASL) can be used to measure perfusion without the use of contrast agents. Due to the small volume fraction of blood vessels compared to tissue in the human brain (typ. 3–5%) ASL techniques have an intrinsically low signal‐to‐noise ratio (SNR). In this publication, evidence is presented that the SNR can be improved by using arterial spin labeling in combination with single‐shot 3D readout techniques. Specifically, a single‐shot 3D‐GRASE sequence is presented, which yields a 2.8‐fold increase in SNR compared to 2D EPI at the same nominal resolution. Up to 18 slices can be acquired in 2 min with an SNR of 10 or more for gray matter perfusion. A method is proposed to increase the reliability of perfusion quantification using QUIPSS II derivates by acquiring low‐resolution maps of the bolus arrival time, which allows differentiation between lack of perfusion and delayed arrival of the labeled blood. For arterial spin labeling, single‐shot 3D imaging techniques are optimal in terms of efficiency and might prove beneficial to improve reliability of perfusion quantitation in a clinical setup. Magn Reson Med 54:491–498, 2005.

Arterial spin labeling in combination with a look‐locker sampling strategy: Inflow turbo‐sampling EPI‐FAIR (ITS‐FAIR)

Arterial spin labeling (ASL) permits quantification of tissue perfusion without the use of MR contrast agents. With standard ASL techniques such as flow‐sensitive alternating inversion recovery (FAIR) the signal from arterial blood is measured at a fixed inversion delay after magnetic labeling. As no image information is sampled during this delay, FAIR measurements are inefficient and time‐consuming. In this work the FAIR preparation was combined with a Look‐Locker acquisition to sample not one but a series of images after each labeling pulse. This new method allows monitoring of the temporal dynamics of blood inflow. To quantify perfusion, a theoretical model for the signal dynamics during the Look‐Locker readout was developed and applied. Also, the imaging parameters of the new ITS‐FAIR technique were optimized using an expression for the variance of the calculated perfusion. For the given scanner hardware the parameters were: temporal resolution 100 ms, 23 images, flip‐angle 25.4°. In a normal volunteer experiment with these parameters an average perfusion value of 48.2 ± 12.1 ml/100 g/min was measured in the brain. With the ability to obtain ITS‐FAIR time series with high temporal resolution arterial transit times in the range of −138 − 1054 ms were measured, where nonphysical negative values were found in voxels containing large vessels. Magn Reson Med 46:974–984, 2001.

Assessment of irradiated brain metastases by means of arterial spin-labeling and dynamic susceptibility-weighted contrast-enhanced perfusion MRI: Initial results

Rationale and Objectives:To assess if preradiation and early follow-up measurements of relative regional cerebral blood flow (rrCBF) can predict treatment outcome in patients with cerebral metastases and to evaluate rrCBF changes in tumor and normal tissue after stereotactic radiosurgery using arterial spin-labeling (ASL) and first-pass dynamic susceptibility-weighted contrast-enhanced (DSC) perfusion MRI. Methods:In 25 patients with a total of 28 brain metastases, DSC MRI and ASL perfusion MRI using the Q2TIPS sequence were performed with a 1.5-T unit. Measurements were performed prior to and at 6 weeks, 12 weeks, and 24 weeks after stereotactic radiosurgery. Follow-up examinations were completely available in 25 patients for Q2TIPS and 17 patients with 18 metastases for DSC MRI. The rrCBF of the metastases and the normal brain tissue was determined by a region-of-interest analysis. rrCBF values were correlated with the treatment outcome that was classified according to tumor volume changes at 6 months. Results:The alteration of the rrCBF at the 6-week follow-up was highly predictive for treatment outcome. A decrease of the rrCBF value predicted tumor response correctly in all metastases for Q2TIPS and in 13 of 16 metastases for DSC MRI. The pretherapeutic rrCBF was not able to predict treatment outcome. The rrCBF values in normal brain tissue affected by radiation doses less than 0.5 Gy remained unchanged after therapy. Conclusion:These preliminary results suggest that ASL and DSC MRI techniques determining rrCBF changes in brain metastases after stereotactic radiosurgery allow the prediction of treatment outcome.

The Value of Arterial Spin-Labeled Perfusion Imaging in Acute Ischemic Stroke: Comparison With Dynamic Susceptibility Contrast-Enhanced MRI

Background and Purpose— The purpose of this study was to evaluate the potential clinical value of arterial spin-labeled (ASL) perfusion MRI in acute ischemic stroke (AIS) through comparison with dynamic susceptibility contrast (DSC) enhanced perfusion MRI. Methods— Pseudocontinuous ASL with 3-dimensional background-suppressed gradient and spin echo readout was applied with DSC perfusion MRI on 26 patients with AIS. ASL cerebral blood flow and multiparametric DSC perfusion maps were rated for image quality and lesion severity/conspicuity. Mean ASL cerebral blood flow and DSC perfusion values were obtained in main vascular territories. Kendall coefficient of concordance was calculated to evaluate the reliability of ratings. Spearman correlation coefficients were calculated to compare ratings and quantitative perfusion values between ASL and DSC perfusion maps. Results— ASL cerebral blood flow and DSC perfusion maps provided largely consistent results in delineating hypoperfused brain regions in AIS. Hyperemic lesions, which also appeared frequently in the AIS cases studied, were more conspicuous on ASL cerebral blood flow than on DSC cerebral blood flow, mean transit time and time to the maximum of the tissue residual function maps. Conclusions— As a rapid, noninvasive, and quantitative technique, ASL has clinical use in detecting blood flow abnormalities in patients with AIS.

Comparison of arterial spin-labeling techniques and dynamic susceptibility-weighted contrast-enhanced MRI in perfusion imaging of normal brain tissue

Objectives:To evaluate relative cerebral blood flow (rCBF) in normal brain tissue using arterial spin-labeling (ASL) methods and first-pass dynamic susceptibility-weighted contrast-enhanced (DSC) magnetic resonance imaging (MRI). Methods:Sixty-two patients with brain metastases were examined on a 1.5 T-system up to 6 times during routine follow-up after stereotactic radiosurgery. Perfusion values in normal gray and white matter were measured using the ASL techniques ITS-FAIR in 38 patients, Q2TIPS in 62 patients, and the first-pass DSC echo-planar (EPI) MRI after bolus administration of gadopentetate dimeglumine in 42 patients. Precision of the ASL sequences was tested in follow-up examinations in 10 healthy volunteers. Results:Perfusion values in normal brain tissue obtained by all sequences correlated well by calculating Pearson’s correlation coefficients (P < 0.0001) and remained unchanged after stereotactic radiosurgery as shown by analysis of variance (P > 0.05). Conclusion:Both ASL and DSC EPI MRI yield highly comparable perfusion values in normal brain tissue.

High-intensity focused ultrasound: Principles, therapy guidance, simulations and applications

In the past two decades, high-intensity focused ultrasound (HIFU) in combination with diagnostic ultrasound (USgFUS) or magnetic resonance imaging (MRgFUS) opened new ways of therapeutic access to a multitude of pathologic conditions. The therapeutic potential of HIFU lies in the fact that it enables the localized deposition of high-energy doses deep within the human body without harming the surrounding tissue. The addition of diagnostic ultrasound or in particular MRI with HIFU allows for planning, control and direct monitoring of the treatment process. The clinical and preclinical applications of HIFU range from the thermal treatment of benign and malign lesions, targeted drug delivery, to the treatment of thrombi (sonothrombolysis). Especially the therapy of prostate cancer under US-guidance and the ablation of benign uterine fibroids under MRI monitoring are now therapy options available to a larger number of patients. The main challenges for an abdominal application of HIFU are posed by partial or full occlusion of the target site by bones or air filled structures (e.g. colon), as well as organ motion. In non-trivial cases, the implementation of computer based modeling, simulation and optimization is desirable. This article describes the principles of HIFU, ultrasound and MRI therapy guidance, therapy planning and simulation, and gives an overview of the current and potential future applications.

Multi-delay multi-parametric arterial spin-labeled perfusion MRI in acute ischemic stroke — Comparison with dynamic susceptibility contrast enhanced perfusion imaging

The purpose of the present study was to present a multi-delay multi-parametric pseudo-continuous arterial spin labeling (pCASL) protocol with background suppressed 3D GRASE (gradient and spin echo) readout for perfusion imaging in acute ischemic stroke. PCASL data at 4 post-labeling delay times (PLD = 1.5, 2, 2.5, 3 s) were acquired within 4.5 min in 24 patients (mean age 79.7 ± 11.4 years; 11 men) with acute middle cerebral artery (MCA) stroke who also underwent dynamic susceptibility contrast (DSC) enhanced perfusion imaging. Arterial transit times (ATT) were estimated through the calculation of weighted delays across the 4 PLDs, which were included in the calculation of cerebral blood flow (CBF) and arterial cerebral blood volume (CBV). Mean perfusion parameters derived using pCASL and DSC were measured within MCA territories and infarct regions identified on diffusion weighted MRI. The results showed highly significant correlations between pCASL and DSC CBF measurements (r > = 0.70, p < = 0.0001) and moderately significant correlations between pCASL and DSC CBV measurements (r > = 0.45, p < = 0.027) in both MCA territories and infarct regions. ASL ATT showed correlations with DSC time to the maximum of tissue residual function (Tmax)(r = 0.66, p = 0.0005) and mean transit time (MTT)(r = 0.59, p = 0.0023) in leptomeningeal MCA territories. The present study demonstrated the feasibility for noninvasive multi-parametric perfusion imaging using ASL for acute stroke imaging.

Whole-Body MR Imaging in the German National Cohort: Rationale, Design, and Technical Background

PURPOSE To detail the rationale, design, and future perspective of implementing whole-body magnetic resonance (MR) imaging in the German National Cohort, a large multicentric population-based study. MATERIALS AND METHODS All institutional review boards approved the study, and informed consent is obtained before study enrollment. Participants are enrolled from a random sample of the general population at five dedicated imaging sites among 18 recruitment centers. MR imaging facilities are equipped with identical 3.0-T imager technology and use uniform MR protocols. Imager-specific hardware and software settings remained constant over the study period. On-site and centralized measures of image quality enable monitoring of completeness of the acquisitions and quality of each of the MR sequences. Certified radiologists read all MR imaging studies for presence of incidental findings according to predefined algorithms. RESULTS Over a 4-year period, six participants per day are examined at each center, totaling a final imaging cohort of approximately 30 000 participants. The MR imaging protocol is identical for each site and comprises a set of 12 native series to cover neurologic, cardiovascular, thoracoabdominal, and musculoskeletal imaging phenotypes totaling approximately 1 hour of imaging time. A dedicated analysis platform as part of a central imaging core incorporates a thin client-based integrative and modular data handling platform to enable multicentric off-site image reading for incidental findings. Scientific analysis will be pursued on a per-project hypothesis-driven basis. CONCLUSION Population-based whole-body MR imaging as part of the German National Cohort will serve to compile a comprehensive image repository, will provide insight into physiologic variants and subclinical disease burden, and has the potential to enable identification of novel imaging biomarkers of risk.

Ultrasound-guided MRI: Preliminary results using a motion phantom

In principal, both ultrasound (US) imaging and MRI can be performed simultaneously within the magnetic field of the scanner due to their different physical natures (i.e., sound waves vs. electromagnetic radiation). One potential application is to use US information for dynamic spatial organ localization to improve MR imaging, similar to MR navigator echoes. In this work, preliminary results are presented for US‐guided MRI using position and orientation information extracted from US data to update the image slice position of an SSFP sequence in real time. Effective prospective motion compensation is shown for a phantom whose sinusoidal displacement can be fully corrected. Possible applications of simultaneous US‐MRI acquisitions include real‐time US‐guided cardiac MRI or interventional procedures. Magn Reson Med 52:27–32, 2004.