Walsh-ordered hadamard time-encoded pseudocontinuous ASL (WH pCASL).

PURPOSE: Walsh ordering of Hadamard encoding-matrices and an additional averaging strategy are proposed for Hadamard-encoded pseudocontinuous arterial spin labeling (H-pCASL). In contrast to conventional H-pCASL the proposed method generates more perfusion-weighted images which are accessible already during a running experiment and even from incomplete sets of encoded images. THEORY: Walsh-ordered Hadamard matrices consist of fully decodable Hadamard submatrices. At any time during a measurement these submatrices may yield perfusion-weighted images, even at runtime and with incomplete datasets. The usage of an additional so-called mirrored matrix for averaging leads to more decodable subboli. METHODS: Perfusion-weighted images (five healthy volunteers) are generated using both a Walsh-ordered and a corresponding mirrored Hadamard matrix. To test their correctness they are compared with equivalent images from conventional multi postlabeling-delay (PLD) pCASL-measurements. RESULTS: All predicted perfusion-weighted images could be generated and correlated very well with multi-PLD images. Already small subsets of encoded images, acquired early during the measurement, yielded perfusion-weighted images. The mirrored matrix generates more perfusion-weighted images without time penalty. CONCLUSION: Early access to perfusion-weighted images despite incomplete datasets allows dynamic adaptation of parameters and increases robustness against artifacts. Thus, the approach may be well suited for clinical applications, where pathologies demand rapid parameter adaptation and increase the chance of artifacts. Magn Reson Med 76:1814-1824, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

An integrated model-based software for FUS in moving abdominal organs.

Focused ultrasound surgery (FUS) is a non-invasive method for tissue ablation that has the potential for complete and controlled local tumour destruction with minimal side effects. The treatment of abdominal organs such as the liver, however, requires particular technological support in order to enable a safe, efficient and effective treatment. As FUS is applied from outside the patient's body, suitable imaging methods, such as magnetic resonance imaging or diagnostic ultrasound, are needed to guide and track the procedure. To facilitate an efficient FUS procedure in the liver, the organ motion during breathing and the partial occlusion by the rib cage need to be taken into account in real time, demanding a continuous patient-specific adaptation of the treatment configuration. Modelling the patient's respiratory motion and combining this with tracking data improves the accuracy of motion predictions. Modelling and simulation of the FUS effects within the body allows the use of treatment planning and has the potential to be used within therapy to increase knowledge about the patient status. This article describes integrated model-based software for patient-specific modelling and prediction for FUS treatments of moving abdominal organs.

Contraction assessment of abdominal muscles using automated segmentation designed for wearable ultrasound applications.

PURPOSE: Wearable ultrasound devices can be used to continuously monitor muscle activity. One possible application is to provide real-time feedback during physiotherapy, to show a patient whether an exercise is performed correctly. Algorithms which automatically analyze the data can be of importance to overcome the need for manual assessment and annotations and speed up evaluations especially when considering real-time video sequences. They even could be used to present feedback in an understandable manner to patients in a home-use scenario. The following work investigates three deep learning based segmentation approaches for abdominal muscles in ultrasound videos during a segmental stabilizing exercise. The segmentations are used to automatically classify the contraction state of the muscles. METHODS: The first approach employs a simple 2D network, while the remaining two integrate the time information from the videos either via additional tracking or directly into the network architecture. The contraction state is determined by comparing measures such as muscle thickness and center of mass between rest and exercise. A retrospective analysis is conducted but also a real-time scenario is simulated, where classification is performed during exercise. RESULTS: Using the proposed segmentation algorithms, 71% of the muscle states are classified correctly in the retrospective analysis in comparison to 90% accuracy with manual reference segmentation. For the real-time approach the majority of given feedback during exercise is correct when the retrospective analysis had come to the correct result, too. CONCLUSION: Both retrospective and real-time analysis prove to be feasible. While no substantial differences between the algorithms were observed regarding classification, the networks incorporating the time information showed temporally more consistent segmentations. Limitations of the approaches as well as reasons for failing cases in segmentation, classification and real-time assessment are discussed and requirements regarding image quality and hardware design are derived.

Arterial spin labeling MRI study of age and gender effects on brain perfusion hemodynamics.

Normal aging is associated with diminished brain perfusion measured as cerebral blood flow (CBF), but previously it is difficult to accurately measure various aspects of perfusion hemodynamics including: bolus arrival times and delays through small arterioles, expressed as arterial-arteriole transit time. To study hemodynamics in greater detail, volumetric arterial spin labeling MRI with variable postlabeling delays was used together with a distributed, dual-compartment tracer model. The main goal was to determine how CBF and other perfusion hemodynamics vary with aging. Twenty cognitive normal female and 15 male subjects (age: 23-84 years old) were studied at 4 T. Arterial spin labeling measurements were performed in the posterior cingulate cortex, precuneus, and whole brain gray matter. CBF declined with advancing age (P < 0.001). Separately from variations in bolus arrival times, arterial-arteriole transit time increased with advancing age (P < 0.01). Finally, women had overall higher CBF values (P < 0.01) and shorter arterial-arteriole transit time (P < 0.01) than men, regardless of age. The findings imply that CBF and blood transit times are compromised in aging, and these changes together with differences between genders should be taken into account when studying brain perfusion.

Arterial spin labelling: final steps to make it a clinical reality.

Since its inception in the early 1990s, arterial spin labelling (ASL) has been developed in numerous variants. Tremendous improvements in both pulse sequences and hardware have allowed improved image quality and reduced scan time, to the point where the technique might be ready for clinical application. However, although now routinely used in research centres and universities, its application in daily clinical routine remains restricted to a few centres. Its popularity in magnetic resonance imaging (MRI) research centres and among physicists may have contributed to the lack of enthusiasm from the clinical community, as the variety of possible sequences available might deter even the most die-hard technology-driven neuroradiologist from using this technique. Therefore, a joint action directed towards the harmonisation of all the existing ASL pulse sequences and the development of a reliable and common post-processing pipeline to provide guidance for the major MRI manufacturers was recently endeavoured and dubbed the 'ASL Network' ( http://www.asl-network.org ). This network was established to provide a communication platform among physicists, engineers and clinicians who are keen to see this technique finally make it to daily clinical practice. Finally, a networking European Grant from the COST Office was secured to help fund some of the activities of the ASL Network, in particular as they pertain to the application of ASL in dementia ( http://www.cost.eu/domains_actions/bmbs/Actions/BM1103 ). Here, both the ASL Network and the COST Action are described, with their respective goals and aspirations.

Renal disease: value of functional magnetic resonance imaging with flow and perfusion measurements.

PURPOSE: To differentiate healthy kidneys from diseased kidneys, we propose a combined magnetic resonance (MR) examination that includes measurements of renal arterial blood flow and parenchymal perfusion. MATERIALS AND METHODS: A total of 130 kidneys (patients/healthy volunteers: 83/47) were examined using renal artery MR flow measurements and renal parenchymal perfusion measurements, as well as contrast-enhanced MR angiography. Cine phase-contrast-flow measurements were performed using an ECG-gated fast low angle shot pulse sequence; perfusion was measured with an arterial spin labeling flow-sensitive alternating inversion recovery technique. Contrast-enhanced MR angiography was performed with a fast 3D gradient echo sequence in a single breath hold. For evaluation, kidneys were divided into groups based on nephrologic diagnosis of the patient. Recursive partitioning and Wilcoxon rank-sum tests were used to separate the different groups. RESULTS: Significant differences in mean renal artery flow and parenchymal perfusion were found in kidneys with renal artery stenosis as well as parenchymal disease as compared with healthy kidneys. Using a classification tree derived from the recursive partitioning, a specificity of 99% and sensitivity of 69% with a positive/negative predictive value of 97%/84% was achieved for the separation of healthy kidneys from kidneys with vascular, parenchymal or combined disease. The overall accuracy was 88%. CONCLUSION: The combination of cine PC flow measurements and MR perfusion measurements offers a comprehensive assessment of both renovascular and renoparenchymal disease and provide a noninvasive approach to differentiate between these kidneys and normal kidneys.

Acute effects of alcohol on brain perfusion monitored with arterial spin labeling magnetic resonance imaging in young adults.

While a number of studies have established that moderate doses of alcohol increase brain perfusion, the time course of such an increase as a function of breath alcohol concentration (BrAC) has not yet been investigated, and studies differ about regional effects. Using arterial spin labeling (ASL) magnetic resonance imaging, we investigated (1) the time course of the perfusion increase during a 15-minute linear increase of BrAC up to 0.6 g/kg followed by a steady exposure of 100 minutes, (2) the regional distribution, (3) a potential gender effect, and (4) the temporal stability of perfusion effects. In 48 young adults who participated in the Dresden longitudinal study on alcohol effects in young adults, we observed (1) a 7% increase of global perfusion as compared with placebo and that perfusion and BrAC are tightly coupled in time, (2) that the increase reaches significance in most regions of the brain, (3) that the effect is stronger in women than in men, and (4) that an acute tolerance effect is not observable on the time scale of 2 hours. Larger studies are needed to investigate the origin and the consequences of the effect, as well as the correlates of inter-subject variations.

Imaging mesial temporal lobe activation during scene encoding: comparison of fMRI using BOLD and arterial spin labeling.

Memory encoding is a critical brain function subserved by the hippocampus (HP) and mesial temporal lobe (mTL) structures. Visualization of mTL memory activation with BOLD fMRI is complicated by the presence of static susceptibility gradients in this region. Arterial spin labeled (ASL) perfusion fMRI offers an alternative approach not dependent on susceptibility contrast that instead suffers from lower intrinsic signal-to-noise ratio. An improved ASL perfusion fMRI approach combining pseudo-continuous ASL and a T(2)*-insensitive sequence (GRASE) with background suppression was compared to BOLD fMRI at 3 T during a scene encoding task known to activate the HP. Overall, an approximate sixfold sensitivity increase of ASL fMRI was achieved, with improved coverage in the anterior mTL, while suppression of the static tissue enhanced the stability of the ASL series by a factor of 2.4. Perfusion fMRI using this approach with 4 mm isotropic resolution yielded better localized and stronger group activation maps than BOLD fMRI at a standard resolution of 3 mm isotropic voxels. Increasing the resolution for BOLD to 2.5 mm isotropic produced stronger mTL and hippocampal activation in the group and individual subjects than the ASL technique, due to superior temporal resolution and reduced partial volume effects. Future improvements in ASL spatial and temporal resolution would allow the benefits of both approaches to be combined to further enhance the sensitivity for detecting mTL activation during memory encoding.

Continuous arterial spin labeling perfusion measurements using single shot 3D GRASE at 3 T.

Single shot 3D GRASE is less sensitive to field inhomogeneity and susceptibility effects than gradient echo based fast imaging sequences while preserving the acquisition speed. In this study, a continuous arterial spin labeling (CASL) pulse was added prior to the single shot 3D GRASE readout and quantitative perfusion measurements were carried out at 3 T, at rest and during functional activation. The sequence performance was evaluated by comparison with a CASL sequence with EPI readout. It is shown that perfusion measurements using CASL GRASE can be performed safely on humans at 3 T without exceeding the current RF power deposition limits. The maps of resting cerebral blood flow generated from the GRASE images are comparable to those obtained with the 2D EPI readout, albeit with better coverage in the orbitofrontal cortex. The sequence proved effective for functional imaging, yielding time series of images with improved temporal SNR with respect to EPI and group activation maps with increased significance levels. The method was further improved using parallel imaging techniques to provide increased spatial resolution and better separation of the gray-white matter cerebral blood flow maps.