Michael Helle

Superselective pseudocontinuous arterial spin labeling.

A new technique for the imaging of flow territories of individual extra‐ and intracranial arteries is presented. The method is based on balanced pseudocontinuous arterial spin labeling but employs additional time‐varying gradients in between the radiofrequency pulses of the long labeling train. The direction of the additional gradient vector is perpendicular to the selected artery and its azimuthal angle is switched after every radiofrequency pulse. The phases of the radiofrequency pulses are adopted to cancel out the phase accrual of the spins at the center of the target vessel due to the extra applied gradients. This results in efficient inversion at the targeted position, whereas elsewhere time‐varying phase changes will result in marginal inversion efficiency. By changing the moment of the added gradients, the size of the labeling focus can be adjusted. Influence of the temporal order of the additional gradients on the labeling efficiency and on the selectivity was investigated by simulations and experimental measurements. In a volunteer study, the acquisition of high signal‐to‐noise ratio flow territory images of small branches of the anterior cerebral artery distal to the circle of Willis was demonstrated. This shows the methods flexibility for dealing with complicated arterial geometries and its ability to superselectively label small intracranial arteries. Magn Reson Med, 2010.

Maladaptive Aortic Properties in Children After Palliation of Hypoplastic Left Heart Syndrome Assessed by Cardiovascular Magnetic Resonance Imaging

Background— The status of the reconstructed aorta in hypoplastic left heart syndrome is considered an important determinant of long-term prognosis. Therefore, we assessed the anatomy, elastic properties, and viability of the aorta and right ventricular function in patients with hypoplastic left heart syndrome by cardiovascular magnetic resonance imaging. Methods and Results— Cardiovascular magnetic resonance imaging was performed in 40 patients with hypoplastic left heart syndrome (age, 6.0±2.2 years) and 13 control subjects (age, 6.6±2.2 years). Aortic dimensions and distensibility were calculated at different locations of the aorta using gradient-echo cine imaging at 3.0 T. Additionally, pulse-wave velocity, right ventricular ejection fraction, and aortic late gadolinium enhancement for viability assessment were measured. Compared with control subjects, patients with hypoplastic left heart syndrome had increased axial diameters of the aortic root (36.0±5.5 versus 24.1±2.7 mm/m2; P<0.01), ascending aorta (32.0±5.0 versus 21.3±1.5 mm/m2; P<0.01), and transverse aortic arch (22.7±5.2 versus 18.7±2.5 mm/m2; P<0.01). Wall distensibility was reduced in the ascending aorta (4.1±2.4 versus 13.5±7.2 10−3 mm Hg−1; P<0.01) and transverse aortic arch (5.4±3.6 versus 10.3±3.5 10−3 mm Hg−1; P<0.01). Pulse-wave velocity trended higher in patients (P=0.06). Reduced distensibility in the ascending aorta correlated with the amount of late gadolinium enhancement in a volume that included the aortic root and the ascending aorta (r=−0.72, P<0.01), and both parameters correlated with decreased right ventricular ejection fraction. Conclusions— Adverse aortic properties post palliation of hypoplastic left heart syndrome manifest themselves by aortic dilatation, decreased distensibility, and increased volume of nonviable aortic wall tissue. The negative association between aortic late gadolinium enhancement and right ventricular ejection fraction suggests unfavorable aortic-ventricular coupling. The potential impact of these findings on long-term right ventricular function should be evaluated in future studies.

Joint blood and cerebrospinal fluid suppression for intracranial vessel wall MRI

To develop and evaluate a joint blood and cerebrospinal fluid (CSF) suppression technique for improved intracranial vessel wall MR imaging.

Intraoperative dynamic susceptibility contrast weighted magnetic resonance imaging (iDSC-MRI) - Technical considerations and feasibility.

DSC-MRI was applied intraoperatively during brain tumor removal. Immediately after presumed complete tumor resection an MRI including a dynamic susceptibility contrast T2-weighted EPI sequence was performed in 6 patients while the skull was still open using a flexible two-channel coil system at an intraoperative 1.5-Tesla MR scanner. After an initial baseline period of this iDSC-MRI sequence a bolus of contrast agent was administered intravenously. Maps of relative regional blood flow (rCBF), blood volume (rCBV) and the mean transit time (MTT) were calculated. These maps were compared to preoperatively acquired DSC-MRI data. The extent of the resection was compared with the postoperative MRI performed 24 h after the operation. In five patients complete tumor removal was already achieved at the time of iDSC-MRI and no areas of elevated perfusion values adjacent to the resection cavity were found. Complete removal was again documented on the postoperatively performed MRI. In one case there was residual tumor that showed both contrast enhancement and identical perfusion ratios as in the preoperatively acquired data. Removal of the remaining tumor was performed. iDSC-MRI is technically feasible as there are no significant susceptibility artifacts. DSC-MRI has been used to distinguish different tumor entities preoperatively and recurrent disease from radiation necrosis. Despite brain shift and thus invalidated preoperative image data or contrast leakage caused by intraoperative manipulation, iDCS-MRI furthermore reliably detects residual tumor intraoperatively at a timepoint where further resection is still possible and thus enables the neurosurgeon to complete the resection during the same procedure.

Superselective arterial spin labeling applied for flow territory mapping in various cerebrovascular diseases.

In three example patients suffering from internal carotid artery occlusion, intracranial steno‐occlusive disease, and symptomatic arteriovenous malformation (AVM), a new method named superselective pseudo‐continuous arterial spin labeling (pCASL) was used in addition to clinical routine measurements. The capabilities of this method are demonstrated to gain important information in diagnosis, risk analysis, and treatment monitoring that are neither accessible by digital subtraction angiography nor by existing selective arterial spin labeling methods and thus to propose future applications in clinical routine. In all cases superselective pCASL enabled the assessment of tissue viability and of territorial brain perfusion at different levels starting from major brain feeding vessels to collateral circulation at the level of the Circle of Willis to even distal branching arteries. This made it possible to estimate the contribution of an extracranial‐intracranial bypass to the brain perfusion; to depict individual arteries to important functional brain areas; to identify en‐passant feeding vessels of an AVM and to track possible changes in their perfusion territories after intervention. J. Magn. Reson. Imaging 2013;38:496–503.

Validation of planning-free vessel-encoded pseudo-continuous arterial spin labeling MR imaging as territorial-ASL strategy by comparison to super-selective p-CASL MRI.

Vessel‐encoded (VE) pseudo‐continuous arterial spin labeling (p‐CASL) is a territorial ASL (T‐ASL) technique to identify the perfusion territories of cerebral arteries. The aim of this study was to validate the output of three Vessel‐encoded p‐CASL image processing methods, k‐means clustering with and without subsequent linear analysis and a Bayesian framework, by comparison with the perfusion maps acquired with super‐selective p‐CASL.

Superselective pseudo-continuous arterial spin labeling angiography

PURPOSE To evaluate the utility of a novel non-contrast enhanced, vessel-selective magnetic resonance angiography (MRA) approach based on superselective pseudo-continuous arterial spin labeling (ASL) for the morphologic assessment of intracranial arteries when compared to a clinically used time-of-flight (TOF) MRA. MATERIALS AND METHODS Three sets of selective ASL angiographies (right and left internal carotid artery, basilar artery) as well as one TOF data set were obtained from each of the five volunteers included in this study on a clinical 1.5T system. The depiction of arterial segments as well as their delineation was evaluated and independently analyzed by two radiologists. Additionally, the ASL angiography approach was performed in two patients suffering from arterio-venous malformations (AVM) in order to illustrate potential applications in a clinical setting. RESULTS In both angiography techniques, intracranial arteries and their segments (distal branches up to A5 segments of the anterior cerebral arteries, M8 segments of the middle cerebral arteries, and P5 segments of the posterior cerebral arteries) were continuously depicted with excellent inter-reader agreement (κ>0.81). In AVM patients, reconstructed images of the TOF angiography presented similar information about the size and shape of the AVM as did superselective ASL angiography. In addition, the acquired ASL angiograms of selected vessels allowed assessing the blood supply of individually labeled arteries to the AVM which could also be confirmed by digital subtraction angiography. CONCLUSION Superselective ASL angiography makes it possible to visualize arterial trees of selected vessels, thereby, providing information about the macrovascular blood supply and flow territories of intracranial arteries. Similar image quality is achieved when compared to clinically used TOF angiography with respect to the identification and delineation of arterial segments. Initial application of superselective ASL angiography in two patients with AVMs demonstrates the ability to gather additional important information about feeding vessels and blood supply.

In vivo visualization of the PICA perfusion territory with super-selective pseudo-continuous arterial spin labeling MRI

In this work a method is described to discern the perfusion territories in the cerebellum that are exclusively supplied by either or both vertebral arteries. In normal vascular anatomy the posterior inferior cerebellar artery (PICA) is supplied exclusively by its ipsilateral vertebral artery. The perfusion territories of the vertebral arteries were determined in 14 healthy subjects by means of a super-selective pseudo-continuous ASL sequence on a 3T MRI scanner. Data is presented to show the feasibility of determining the PICA perfusion territory. In 10 subjects it was possible to accurately determine both PICA perfusion territories. In two subjects it was possible to determine the perfusion territory of one PICA. Examples in which it was not possible to accurately determine the PICA territory are also given. Additionally, the high variability of the extent of the PICA territory is illustrated using a statistical map. The posterior surface of the cerebellum is entirely supplied by the PICA in six subjects. The most posterior part of the superior surface is supplied by the PICA in eight subjects, and the inferior half of the anterior surface in six subjects. The inferior part of the vermis is supplied by the PICA in all subjects. Two subjects were found with interhemispheric blood flow to both tonsils from one PICA without contribution from the contralateral PICA. With the method as presented, clinicians may in the future accurately classify cerebellar infarcts according to affected perfusion territories, which might be helpful in the decision whether a stenosis should be considered symptomatic.

Selective multivessel labeling approach for perfusion territory imaging in pseudo‐continuous arterial spin labeling

Recently, a new method for perfusion territory imaging named superselective pseudo‐continuous arterial spin labeling was introduced. The method uses additional time‐varying gradients to create a circular labeling spot that can be adjusted in size and thus adapted to individual arteries. In this study, the additional gradients are adjusted in such a way that an elliptical labeling spot is formed, which can be applied to label the blood in multiple vessels simultaneously in conjunction with an increased labeling efficiency compared with the original superselective approach. When compared with other selective multivessel strategies, the proposed technique allows for an improved and flexible adaption of the labeling focus to different anatomical variations of the arteries in the neck so that a total of five perfusion territories from the data acquired in three measurements can be recalculated in a reduced scan time. These include not only the perfusion territories of the cerebrum but also the perfusion territories in the cerebellum fed by individual vertebral arteries. Magn Reson Med, 2012.

Generation of brain pseudo-CTs using an undersampled, single-acquisition UTE-mDixon pulse sequence and unsupervised clustering.

PURPOSE MR-based pseudo-CT has an important role in MR-based radiation therapy planning and PET attenuation correction. The purpose of this study is to establish a clinically feasible approach, including image acquisition, correction, and CT formation, for pseudo-CT generation of the brain using a single-acquisition, undersampled ultrashort echo time (UTE)-mDixon pulse sequence. METHODS Nine patients were recruited for this study. For each patient, a 190-s, undersampled, single acquisition UTE-mDixon sequence of the brain was acquired (TE = 0.1, 1.5, and 2.8 ms). A novel method of retrospective trajectory correction of the free induction decay (FID) signal was performed based on point-spread functions of three external MR markers. Two-point Dixon images were reconstructed using the first and second echo data (TE = 1.5 and 2.8 ms). R2(∗) images (1/T2(∗)) were then estimated and were used to provide bone information. Three image features, i.e., Dixon-fat, Dixon-water, and R2(∗), were used for unsupervised clustering. Five tissue clusters, i.e., air, brain, fat, fluid, and bone, were estimated using the fuzzy c-means (FCM) algorithm. A two-step, automatic tissue-assignment approach was proposed and designed according to the prior information of the given feature space. Pseudo-CTs were generated by a voxelwise linear combination of the membership functions of the FCM. A low-dose CT was acquired for each patient and was used as the gold standard for comparison. RESULTS The contrast and sharpness of the FID images were improved after trajectory correction was applied. The mean of the estimated trajectory delay was 0.774 μs (max: 1.350 μs; min: 0.180 μs). The FCM-estimated centroids of different tissue types showed a distinguishable pattern for different tissues, and significant differences were found between the centroid locations of different tissue types. Pseudo-CT can provide additional skull detail and has low bias and absolute error of estimated CT numbers of voxels (-22 ± 29 HU and 130 ± 16 HU) when compared to low-dose CT. CONCLUSIONS The MR features generated by the proposed acquisition, correction, and processing methods may provide representative clustering information and could thus be used for clinical pseudo-CT generation.