Time-resolved MR Research Articles

Diagnostic accuracy of 4D-MRA for the detection and localization of spinal dural arteriovenous fistulas: A retrospective 10-year cohort study.

The goal of this study was to assess the diagnostic accuracy of spinal time-resolved contrast-enhanced MR angiography (4D-MRA) for the detection and localization of spinal dural arteriovenous fistulas (SDAVF) in our institution. Single center retrospective cohort study of patients with the clinical suspicion of a SDAVF. Patients were included who had undergone spinal 4D-MRA in the period January 2010-February 2021. A subgroup was identified, who had subsequent digital subtraction angiography (DSA), or clinical/radiological follow-up. Diagnostic performance measures of 4D-MRA were calculated (sensitivity, positive predictive value, specificity, and negative predictive value) with DSA serving as the reference standard. Overall, 120 patients with the clinical suspicion of having a SDAVF and who underwent spinal 4D-MRA were identified. A subgroup of 30 patients had both 4D-MRA and DSA, or follow-up. In this group, the sensitivity of 4D-MRA for the detection of a SDAVF was 100% and specificity was 91% (positive predictive value of 95% and a negative predictive value of 100%). Sidedness was correctly identified on 4D-MRA in 74% of the cases and the level of the SDAVF in 68%. The results indicate that 4D-MRA has a high sensitivity and specificity for the detection and localization of a SDAVF, which is in line with previous studies published in literature. Furthermore, 4D-MRA can serve to guide DSA and shorten the procedural time, which reduces the risk of angiography related complications, and decreases patient discomfort.

Time-resolved MR fingerprinting for T2* signal extraction: MR fingerprinting meets echo planar time-resolved imaging.

This study leverages the echo planar time-resolved imaging (EPTI) concept in MR fingerprinting (MRF) framework for a new time-resolved MRF (TRMRF) approach, and explores its capability for fast simultaneous quantification of multiple MR parameters including T1, T2, T2*, proton density, off resonance, and B1 +. The proposed TRMRF method uses the concept of EPTI to track the signal change along the EPI echo train for T2* weighting with a k-t Poisson-based sampling order designed for acquisition. A two-dimensional decomposition algorithm was designed for the image reconstruction, enabling fast and precise subspace modeling. The accuracy of proposed method was evaluated by a T1/T2 phantom. The feasibility was demonstrated through 5 healthy volunteer brain studies. In the phantom studies, T1, T2, and T2* maps of TRMRF correlated strongly with gold-standard methods. The concordance correlation coefficients are 0.9999, 0.9984 and 0.9978, and R2s are 0.9998, 0.9971, and 0.9983. In the in vivo studies, quantitative maps were acquired with 5 healthy volunteers. TRMRF was demonstrated to have comparable results with spiral MRF and gradient-echo EPTI. TRMRF scans using 16, 10, and 6 s per slice were also evaluated to demonstrate the capability of shorter scan times. A new approach is proposed to exploit the advantage of EPTI in the MRF framework. We demonstrate in phantom and in vivo experiments that T1, T2, T2*, proton density, off resonance, and B1 + can be simultaneously quantified within 6 s/slice by TRMRF.

Diagnostic Accuracy of Non-Contrast-Enhanced Time-Resolved MR Angiography to Assess Angioarchitectural Classification Features of Brain Arteriovenous Malformations.

This study aims to assess the diagnostic accuracy of non-contrast-enhanced 4D MR angiography (NCE-4D-MRA) compared to contrast-enhanced 4D MR angiography (CE-4D-MRA) for the detection and angioarchitectural characterisation of brain arteriovenous malformations (bAVMs). Utilising a retrospective design, we examined 54 MRA pairs from 43 patients with bAVMs, using digital subtraction angiography (DSA) as the reference standard. Both NCE-4D-MRA and CE-4D-MRA were performed using a 3-T MR imaging system. The primary objectives were to evaluate the diagnostic performance of NCE-4D-MRA against CE-4D-MRA and DSA and to assess concordance between imaging modalities in grading bAVMs according to four main scales: Spetzler-Martin, Buffalo, AVM embocure score (AVMES), and R2eDAVM. Our results demonstrated that NCE-4D-MRA had a higher accuracy and specificity compared to CE-4D-MRA (0.85 vs. 0.83 and 95% vs. 85%, respectively) and similar agreement, with DSA detecting shunts in bAVMs or residuals. Concordance in grading bAVMs was substantial between NCE-4D-MRA and DSA, particularly for the Spetzler-Martin and Buffalo scales, with CE-4D-MRA showing slightly higher kappa values for interobserver agreement. The study highlights the potential of NCE-4D-MRA as a diagnostic tool for bAVMs, offering comparable accuracy to CE-4D-MRA while avoiding the risks associated with gadolinium-based contrast agents. The safety profile of imaging techniques is a significant concern in the long-term follow up of bAVMs, and further prospective research should focus on NCE-4D-MRA protocol improvement for clinical use.

Retrospective reconstruction of four-dimensional magnetic resonance from interleaved cine imaging – A comparative study with four-dimensional computed tomography in the lung

Background and purposeImaging of respiration-induced anatomical changes is essential to ensure high accuracy in radiotherapy of lung cancer. We expanded here on methods for retrospective reconstruction of time-resolved volumetric magnetic resonance (4DMR) of the thoracic region and benchmarked the results against 4D computed tomography (4DCT). Materials and methodMR data of six lung cancer patients were collected by interleaving cine-navigator images with 2D data frame images, acquired across the thorax. The data frame images have been stacked in volumes based on a similarity metric that considers the anatomical deformation of lungs, while addressing ambiguities in respiratory phase detection and interpolation of missing data. The resulting images were validated against cine-navigator images and compared to paired 4DCTs in terms of amplitude and period of motion, assessing differences in internal target volume (ITV) margin definition. Results4DMR-based motion amplitude was on average within 1.8 mm of that measured in the corresponding 2D cine-navigator images. In our dataset, the 4DCT motion and the 4DMR median amplitude were always within 3.8 mm. The median period was generally close to CT references, although deviations up to 24 % have been observed. These changes were reflected in the ITV, which was generally larger for MRI than for 4DCT (up to 39.7 %). ConclusionsThe proposed algorithm for retrospective reconstruction of time-resolved volumetric MR provided quality anatomical images with high temporal resolution for motion modelling and treatment planning. The potential for imaging organ motion variability makes 4DMR a valuable complement to standard 4DCT imaging.

Automatic detection of aortic valve pathology using 3-chamber cine MRI

Abstract Funding Acknowledgements Type of funding sources: None. Introduction The assessment of aortic valve pathology using magnetic resonance imaging (MRI) typically relies on blood velocity estimates acquired using phase contrast (PC) MRI. However, abnormalities in blood flow through the aortic valve often manifest by the dephasing of blood signal in gated balanced steady-state free precession (bSSFP) scans (cine MRI) [1]. This suggests cine MRI may be used to identify patients with blood flow abnormalities who would benefit from additional detailed PC-MRI scans. Nevertheless, there are currently no tools to automatically identify abnormalities in blood flow through the aortic root in cine MRI which can be used to improve diagnosis and optimise clinical CMR protocols. Aim To determine whether a classification neural network (NN) can automatically detect aortic valve pathology from 3-chamber cine MRI, without the need for dedicated additional imaging or manual annotation. Data We train and test our approach on a retrospective clinical dataset from three UK hospitals, using single-slice bSSFP 3-chamber cine MRI (cardiac frames: 31 ± 15, spatial resolution: 1.17–1.56 x 1.17–1.56 x 8 mm3) from n = 577 patients. Using information from clinical assessments, each image was labelled as: aortic regurgitation (AR) (13%), aortic stenosis (AS) (11%), mixed valve disease (MVD) (9%), or showing no relevant pathology (67%). 322 randomly selected images were used for training, 81 for validation and 174 for testing. Methods We trained a 3D classification NN (DenseNet201 model [2] with 4 dense blocks which include 3D (i.e., spatiotemporal) convolutional layers; Adam optimisation and a cross-entropy loss function) to classify the time-resolved cine MR images. We used binary labels: "pathology" (corresponding to AR, AS or MVD) or "no pathology". As a pre-processing step, we automatically identified the heart in all cine MRI frames using an in-house algorithm that detected motion in the image across the cardiac cycle, and cropped the images to remove non-cardiac anatomical structures. After the classification, we used gradient-weighted class activation mapping (Grad-CAM) [3] to gain confidence that the classification was most dependent on voxels close to the aortic root. Fig 1 shows an overview of the image processing pipeline. Results Our classification model accurately predicts aortic valve pathology using 3-chamber cine MRI: Fig 2 shows the confusion matrix and the evaluation metrics of the study. The Grad-CAM heatmaps showed the aortic region contributed the most to the classification, as seen in Fig 1. Conclusion Convolutional NNs can identify aortic valve pathology automatically from 3-chamber cine MRI, without the need for manual annotations. This can be used to improve diagnosis and optimise clinical CMR protocols, namely as a triage tool to identify the patients most likely to benefit from additional PC-MRI sequences.

Live-view 4D GRASP MRI: A framework for robust real-time respiratory motion tracking with a sub-second imaging latency.

To propose a framework called live-view golden-angle radial sparse parallel (GRASP) MRI for low-latency and high-fidelity real-time volumetric MRI. Live-view GRASP MRI has two stages. The first one is called an off-view stage and the second one is called a live-view stage. In the off-view stage, 3D k-space data and 2D navigators are acquired alternatively using a new navi-stack-of-stars sampling scheme. A 4D motion database is then generated that contains time-resolved MR images at a sub-second temporal resolution, and each image is linked to a 2D navigator. In the live-view stage, only 2D navigators are acquired. At each time point, a live-view 2D navigator is matched to all the off-view 2D navigators. A 3D image that is linked to the best-matched off-view 2D navigator is then selected for this time point. This framework places the typical acquisition and reconstruction burden of MRI in the off-view stage, enabling low-latency real-time 3D imaging in the live-view stage. The accuracy of live-view GRASP MRI and the robustness of 2D navigators for characterizing respiratory variations and/or body movements were assessed. Live-view GRASP MRI can efficiently generate real-time volumetric images that match well with the ground-truth references, with an imaging latency below 500 ms. Compared to 1D navigators, 2D navigators enable more reliable characterization of respiratory variations and/or body movements that may occur throughout the two imaging stages. Live-view GRASP MRI represents a novel, accurate, and robust framework for real-time volumetric imaging, which can potentially be applied for motion adaptive radiotherapy on MRI-Linac.

High diagnostic performance of time-resolved MR angiography in spinal arteriovenous shunts

Background and purposeIn comparison with the limited efficacy of conventional MR imaging and the invasiveness of catheter-based digital subtraction angiography (DSA), time-resolved MR angiography (TR-MRA) has been proposed as a promising examination for early diagnosis of spinal arteriovenous shunts (SAVSs). This paper aims to investigate the diagnostic performance of TR-MRA with scan parameters optimized for SAVSs evaluation in a large number of patients. MethodsOne hundred patients with suspected SAVSs were enrolled. Each patient underwent preoperative TR-MRA with optimized scan parameters followed by DSA. The presence or absence of SAVS, the types and the angioarchitecture of SAVSs in the TR-MRA images were diagnostically analyzed. ResultsAmong the final 97 patients, 80 cases (82.5 %) were diagnosed and classified by TR-MRA as spinal cord arteriovenous shunts (SCAVSs; n = 22), spinal dural arteriovenous shunts (SDAVSs; n = 48), and spinal extradural arteriovenous shunts (SEDAVSs; n = 10). The agreement for classifying SAVSs between TR-MRA and DSA was excellent (κ = 0.91). The sensitivity, specificity, PPV, NPV, and accuracy of TR-MRA for the diagnosis of SAVSs were 100 % (95 % CI, 94.3–100.0 %), 76.5 % (95 % CI, 49.8–92.2 %), 95.2 % (95 % CI, 87.6–98.5 %), 100 % (95 % CI, 71.7–100.0 %), and 95.9 % (95 % CI, 89.9–98.4 %). The accuracy rates of TR-MRA for the detection of feeding arteries were 75.9 %, 91.7 %, and 80.0 % for SCAVSs, SDAVSs, and SEDAVSs respectively. ConclusionTime-resolved MR angiography showed excellent diagnostic performance for SAVSs screening. Additionally, this method can classify SAVSs and identify feeding arteries in SDAVSs with high diagnostic accuracy.

Quantification of paravalvular leaks associated with TAVI implants using 4D MRI in an aortic root phantom made possible by the use of 3D printing.

Transcatheter aortic valve implantation (TAVI) has become an alternative to surgical replacement of the aortic valve elderly patients. However, TAVI patients may suffer from paravalvular leaks (PVL). Detecting and grading is usually done by echocardiography, but is limited by resolution, 2D visualization and operator dependency. 4D flow magnetic resonance imaging (MRI) is a promising alternative, which did not reach clinical application in TAVI patients. The aim of this study was applying 3D printing technologies in order to evaluate flow patterns and hemodynamics of PVLs following TAVI, exploiting 4D flow MRI and standard ultrasound. An MR-compatible, anatomically left ventricle, aortic root, and ascending aorta model was fabricated by combining 3D-printed parts and various soft silicone materials to match physiological characteristics. An Abbott Portico™ valve was used in continuous antegrade flow (12-22 l/min), retrograde flow with varying transvalvular pressures (60-110 mmHg), and physiological pulsatile hemodynamics (aortic pressure: 120/80 mmHg, cardiac output: 5 l/min) Time-resolved MR measurements were performed above and below the TAVI stent and compared with color Doppler ultrasound measurements in exactly the same setup. The continuous antegrade flow measurements from MRI largely agreed with the flowmeter measurements, and a maximum error of only 7% was observed. In the retrograde configuration, visualization of the paravalvular leaks was possible from the MR measurements, but flow was overestimated by up to 33%. The 4D MRI measurement in the pulsatile setup revealed a single main PVL, which was also confirmed by the color Doppler measurements, and velocities were similar (2.0 m/s vs. 1.7 m/s). 4D MRI techniques were used to qualitatively assess flow in a patient-specific, MR-compatible and flexible model, which only became possible through the use of 3D printing techniques. Flow patterns in the ascending aorta, identification and quantification of PVLs was possible and the location and extent of PVLs were confirmed by ultrasound measurements. The 4D MRI flow technique allowed evaluation of flow patterns in the ascending aorta and the left ventricle below the TAVI stent with good results in identifying PVLs, demonstrating its capabilities over ultrasound by providing the ability to visualize the paravalvular jets in three dimensions at however, additional expenditure of time and money.

Highly time-resolved 4D MR angiography using golden-angle radial sparse parallel (GRASP) MRI

Current dynamic MRA techniques are limited by temporal resolution and signal-to-noise penalties. GRASP, a fast and flexible MRI technique combining compressed-sensing, parallel imaging, and golden-angle radial sampling, acquires volumetric data continuously and can be reconstructed post hoc for user-defined applications. We describe a custom pipeline to retrospectively reconstruct ultrahigh temporal resolution, dynamic MRA from GRASP imaging obtained in the course of routine practice. GRASP scans were reconstructed using a custom implementation of the GRASP algorithm and post-processed with MeVisLab (MeVis Medical Solutions AG, Germany). Twenty consecutive examinations were scored by three neuroradiologists for angiographic quality of specific vascular segments and imaging artifacts using a 4-point scale. Unsubtracted images, baseline-subtracted images, and a temporal gradient dataset were available in 2D and 3D reconstructions. Distinct arterial and capillary phases were identified in all reconstructions, with a median of 2 frames (IQR1-3 and 2–3, respectively). Median rating for vascular segments was 3 (excellent) in all reconstructions and for nearly all segments, with excellent intraclass correlation (range 0.91–1.00). No cases were degraded by artifacts. GRASP-MRI obtained in routine practice can be seamlessly repurposed to produce high quality 4D MRA with 1–2-s resolved isotropic cerebrovascular angiography. Further exploration into diagnostic accuracy in disease-specific applications is warranted.

T2 Hypointense Schwannoma Masquerading As A Vascular Lesion: A Case Report.

Spinal schwannomas are rare nerve root tumors that typically produce subtle symptoms from nerve root compression. These tumors are known to be heterogeneously hyperintense on T2-weighted MRI and avidly enhance with contrast. However, here we describe an L5 spinal schwannoma in a 66-year-old woman that was hypointense on T2-weighted imaging with other radiographic findings more closely aligning with a vascular lesion. The neuroradiologic characteristics on MRI, time resolved MR angiography, and catheter digital subtraction angiography are presented. The patient underwent a full workup for possible dural arteriovenous fistula and thrombosed venous varix before the nerve sheath tumor was discovered intraoperatively and confirmed as a schwannoma on histopathology.

Dynamic monitoring of vital functions and tissue re-organization in Saturnia pavonia (Lepidoptera, Saturniidae) during final metamorphosis by non-invasive MRI

Magnetic resonance imaging (MRI) is the key whole-body imaging technology for observing processes within a living object providing excellent resolution and contrast between soft tissues. In the present work, we exploited the non-destructive properties of MRI to track longitudinally the dynamic changes that take place in developing pupae of the Emperor Moth (Saturnia pavonia) during the last days before eclosion. While in diapause pupae, body fluid was almost homogeneously distributed over the internal compartments, as soon as wings, legs, flight muscles and the head region were fully developed, a significant redistribution of water levels occurred between thoracic and abdominal regions. During the last two days before eclosion, the developing moths transferred substantial amounts of liquid into the gut and the labial gland, and in case of females, into developing eggs. Concomitantly, the volume of the air sacs increased drastically and their expansion/compression became clearly visible in time-resolved MR images. Furthermore, besides ventilation of the tracheal system, air sacs are likely to serve as volume reservoir for liquid transfer during development of the moths inside their pupal case. In parallel, we were able to monitor noninvasively lipid consumption, cardiac activity and haemolymph circulation during final metamorphosis.

Feasibility of Single-Shot Whole Thoracic Time-Resolved MR Angiography to Evaluate Patients with Multiple Pulmonary Arteriovenous Malformations.

ObjectiveTo evaluate the feasibility of single-shot whole thoracic time-resolved MR angiography (TR-MRA) to identify the feeding arteries of pulmonary arteriovenous malformations (PAVMs) and reperfusion of the lesion after embolization in patients with multiple PAVMs.Materials and MethodsNine patients (8 females and 1 male; age range, 23–65 years) with a total of 62 PAVMs who underwent percutaneous embolization for multiple PAVMs and were subsequently followed up using TR-MRA and CT obtained within 6 months from each other were retrospectively reviewed. All imaging analyses were performed by two independent readers blinded to clinical information. The visibility of the feeding arteries on maximum intensity projection (MIP) reconstruction and multiplanar reconstruction (MPR) TR-MRA images was evaluated by comparing them to CT as a reference. The accuracy of TR-MRA for diagnosing reperfusion of the PAVM after embolization was assessed in a subgroup with angiographic confirmation. The reliability between the readers in interpreting the TR-MRA results was analyzed using kappa (κ) statistics.ResultsFeeding arteries were visible on the original MIP images of TR-MRA in 82.3% (51/62) and 85.5% (53/62) of readers 1 and 2, respectively. Using the MPR, the rates increased to 93.5% (58/62) and 95.2% (59/62), respectively (κ = 0.760 and 0.792, respectively). Factors for invisibility were the course of feeding arteries in the anteroposterior plane, proximity to large enhancing vessels, adjacency to the chest wall, pulsation of the heart, and small feeding arteries. Thirty-seven PAVMs in five patients had angiographic confirmation of reperfusion status after embolization (32 occlusions and 5 reperfusions). TR-MRA showed 100% (5/5) sensitivity and 100% (32/32, including three cases in which the feeding arteries were not visible on TR-MRA) specificity for both readers.ConclusionSingle-shot whole thoracic TR-MRA with MPR showed good visibility of the feeding arteries of PAVMs and high accuracy in diagnosing reperfusion after embolization. Single-shot whole thoracic TR-MRA may be a feasible method for the follow-up of patients with multiple PAVMs.

Unpaired Training of Deep Learning tMRA for Flexible Spatio-Temporal Resolution.

Time-resolved MR angiography (tMRA) has been widely used for dynamic contrast enhanced MRI (DCE-MRI) due to its highly accelerated acquisition. In tMRA, the periphery of the k -space data are sparsely sampled so that neighbouring frames can be merged to construct one temporal frame. However, this view-sharing scheme fundamentally limits the temporal resolution, and it is not possible to change the view-sharing number to achieve different spatio-temporal resolution trade-offs. Although many deep learning approaches have been recently proposed for MR reconstruction from sparse samples, the existing approaches usually require matched fully sampled k -space reference data for supervised training, which is not suitable for tMRA due to the lack of high spatio-temporal resolution ground-truth images. To address this problem, here we propose a novel unpaired training scheme for deep learning using optimal transport driven cycle-consistent generative adversarial network (cycleGAN). In contrast to the conventional cycleGAN with two pairs of generator and discriminator, the new architecture requires just a single pair of generator and discriminator, which makes the training much simpler but still improves the performance. Reconstruction results using in vivo tMRA and simulation data set confirm that the proposed method can immediately generate high quality reconstruction results at various choices of view-sharing numbers, allowing us to exploit better trade-off between spatial and temporal resolution in time-resolved MR angiography.

Intramesenteric dynamic contrast pediatric MR lymphangiography: initial experience and comparison with intranodal and intrahepatic MR lymphangiography

To report on our initial experience with intramesenteric (IM) dynamic contrast magnetic resonance lymphangiography (DCMRL) for evaluation of the lymphatics in patients with concern for mesenteric lymphatic flow disorders and to compare IM-DCMRL with intrahepatic (IH) and intranodal (IN) DCMRL. This is a retrospective review of imaging findings in 15 consecutive patients who presented with protein losing enteropathy (PLE) and/or ascites undergoing IM-DCMRL, IH-DCMRL, and IN-DCMRL. The IM-DCMRL technique involves the injection of a gadolinium contrast agent into the mesenteric lymphatic ducts or lymph nodes followed by imaging of the abdomen and chest with dynamic time-resolved MR lymphangiography. IM-DCMRL was successfully performed in 14/15 (93%) of the patients. When comparing IN-DCMRL with IM-DCMRL, there was a significant difference in the visualization of dermal backflow (p = 0.014), duodenal perfusion (p = 0.003), duodenal leak (p = 0.014), and peritoneal leak (p = 0.003). IM-DCMRL demonstrated peritoneal leak in 7 patients in contrast to IH-DCMRL which demonstrated leak in 4 patients and IN-DCMRL which did not demonstrate any peritoneal leaks. Duodenal leaks were seen by IH-DCMRL in 9 patients versus 5 with IM-DCMRL and none with IN-DCMRL. In one patient with congenital PLE, the three modalities showed different disconnected flow patterns with duodenal leak only seen by IM-DCMRL. There were no short-term complications from the procedures. IM-DCMRL is a feasible imaging technique for evaluation of the mesenteric lymphatics. In certain mesenteric lymphatic flow abnormalities, such as PLE and ascites, this imaging may be helpful for diagnosis and the planning of interventions and warrants further studies. • Intramesenteric dynamic contrast magnetic resonance lymphangiography (IM-DCMRL) is a new imaging technique to evaluate mesenteric lymphatic flow disorders such as ascites. • IM-DCMRL is able to image lymphatic leaks in patients with ascites and protein losing enteropathy not seen with intranodal (IN-DCMRL) imaging.

Utility of dynamic MRA in the evaluation of male erectile dysfunction.

To assess the efficacy of time-resolved MR angiography (MRA) in evaluating penile vasculature in patients with clinically suspected vascular anomalies contributing to their erectile dysfunction correlating with penile doppler ultrasound (PDUS) findings and clinical outcomes after surgical intervention. Men (n = 26) with signs of early vascular shunting on PDUS underwent time-resolved, contrast-enhanced (0.1mMol/kg gadobutrol at 1ml/s followed by saline flush) 3-dimensional spoiled gradient echo T1-weighted MRA sequence performed over 3min with 4.6s frame rate after intracavernosal injection of an erectogenic agent. Additional T1- and T2-weighted sequences were performed for anatomic co-localization and tissue characterization. MRA images were evaluated for early filling of draining veins as well as arteriovenous malformations and fistulas and correlated with findings at surgery. 29 MRA examinations on 26 patients (mean age 39years) demonstrated abnormal early venous drainage (n = 22) as well as diminutive/delayed cavernosal enhancement (n = 3), incomplete tumescence (n = 2), and combined arterial inflow/venous outflow disease (n = 1). The MRA had a concordance of 85.2% at determining the presence, or lack thereof of a shunt/AVM when compared to PDUS. Time-resolved MRA allows for both temporal and spatial resolution with visualization of both arterial and venous abnormalities which may be suggested with a screening PDUS examination. This technique allows us to provide detailed anatomic information prior to any surgical intervention.

Mapping cerebral perfusion from time-resolved contrast-enhanced MR angiographic data

A common method to acquire both perfusion and angiographic information is to have separate MRI scans for each information. In this study, we propose to achieve the goal by deriving perfusion parameters, specifically cerebral blood volume (CBV) and Tmax, from time-resolved contrast-enhanced magnetic resonance angiography (CE-MRA). Both CE-MRA and DSC-MRI were performed on seven subjects with a diagnosed ischemic stroke. Concentration functions from CE-MRA were modeled using a modified gamma-variate function to appreciate the full first-pass transition of the tracer bolus. Perfusion parameters were calculated using concentration function derived from each imaging method and were compared to each other both visually and quantitatively by means of correlation studies. CBV and Tmax maps generally showed good agreement between the two methods. This study proved the concept of using time-resolved CE-MRA as both vascular imaging and tissue perfusion mapping while using a single injection of contrast agent, potentially reducing cost and improving patient safety and comfort.

A Novel Collateral Imaging Method Derived from Time-Resolved Dynamic Contrast-Enhanced MR Angiography in Acute Ischemic Stroke: A Pilot Study.

Assessment of the collateral status has been emphasized for appropriate treatment decisions in patients with acute ischemic stroke. The purpose of this study was to introduce a multiphase MRA collateral imaging method (collateral map) derived from time-resolved dynamic contrast-enhanced MRA and to verify the value of the multiphase MRA collateral map in acute ischemic stroke by comparing it with the multiphase collateral imaging method (MRP collateral map) derived from dynamic susceptibility contrast-enhanced MR perfusion. From a prospectively maintained registry of acute ischemic stroke, MR imaging data of patients with acute ischemic stroke caused by steno-occlusive lesions of the unilateral ICA and/or the M1 segment of the MCA were analyzed. We generated collateral maps using dynamic signals from dynamic contrast-enhanced MRA and DSC-MRP using a Matlab-based in-house program and graded the collateral scores of the multiphase MRA collateral map and the MRP collateral map independently. Interobserver reliabilities and intermethod agreement between both collateral maps for collateral grading were tested. Seventy-one paired multiphase MRA and MRP collateral maps from 67 patients were analyzed. The interobserver reliabilities for collateral grading using multiphase MRA or MRP collateral maps were excellent (weighted κ = 0.964 and 0.956, respectively). The agreement between both collateral maps was also excellent (weighted κ = 0.884; 95% confidence interval, 0.819-0.949). We demonstrated that the dynamic signals of dynamic contrast-enhanced MRA could be used to generate multiphase collateral images and showed the possibility of the multiphase MRA collateral map as a useful collateral imaging method in acute ischemic stroke.

Venous Malformations: Phleboliths Correlate With the Presence of Arteriovenous Microshunts

The objective of this study was to identify differences between venous malformations (VMs) with arteriovenous (AV) microshunts and VMs without AV microshunts. In this retrospective study, 83 patients with untreated VMs of the extremities underwent a prospectively defined MRI protocol performed at 3 T. MR images were reviewed to determine localization and extent of each VM and tissue involvement. The VM volume and the lesion volume relative to body surface were measured. The number of phleboliths within the lesions was determined. Using time-resolved MR angiography, the hemodynamic subtype (VM with AV microshunts versus VM without AV microshunts) was classified, and the interval between the onset of arterial enhancement and lesion enhancement (tonset) was measured. Clinical data were reviewed to determine patient demographic characteristics, the onset of symptoms, the frequency of thrombophlebitis, and the pain score. Of the 83 patients, 56 (67%) presented with AV microshunts. The tonset (p ≤ 0.001), absolute and relative lesion volume (p = 0.001), and number of phleboliths (p = 0.038) differed significantly between these two subgroups. With use of a multivariable regression model, the strongest indicator for VM with AV microshunts was the presence of more than two phleboliths (odds ratio, 2.80; p = 0.007). No differences were noted regarding patient demographic characteristics and history or lesion morphologic features or anatomy. Occurrence of AV microshunts in untreated VM is a frequent phenomenon. This hemodynamic subtype is significantly associated with phleboliths and large lesions. The subtypes did not differ in terms of history and clinical course.

Autocalibrated multiband CAIPIRINHA with through-time encoding: Proof of principle and application to cardiac tissue phase mapping.

In conventional multiband (MB) CAIPIRINHA, additional reference scans are acquired to allow the separation of the excited slices. In this study, an acquisition-reconstruction technique that makes use of the MB data to calculate these reference data is presented. The method was integrated into a 2D time-resolved phase-contrast MR sequence used to assess velocities of the myocardium. The RF phases of the MB pulse are cycled through time so that consecutive cardiac phases can be grouped to form reference scans at a lower temporal resolution. These reference data are subsequently used to separate the original slices at the original, high temporal resolution using slice/split-slice GRAPPA algorithms. Slice separation performances are evaluated and compared with conventional methods at 3T, and 3 different strategies for the calibration of the kernels are proposed and compared. Finally, 6 subjects were scanned to assess velocities of the myocardium. Because the acquisition of external references is not needed, no additional breath-holds are required and the full MB acceleration could be exploited. Because the reference and MB data have the same resolution and phase structure, better slice separation was achieved when comparing the proposed technique to conventional workflows. Finally, time-resolved velocities of the myocardial tissue were successfully quantified from MB data, showing good agreement with single-band measurements. Our built-in reference method allows the full exploitation of the MB acceleration and it limits the number of breath-holds.

I093] Mr-guided radiotherapy at the LMU in Munich: Preliminary studies

Purpose The clinical introduction of an MR-guided radiotherapy unit offers great potential to improve future patient treatment but is at the same time associated with several challenges from the perspective of medical physics. Before the final implementation of an MR-Linac in our clinical environment several preliminary studies are performed to prepare the introduction of the new system into the routine patient care as well as covering medical physics related research aspects. Methods One focus of the projects is a comparison of the adaptive workflow based on daily MR and daily cone beam CT imaging. In order to mimic the MR-guided radiotherapy workflow an MR-compatible flat table top is equipped with immobilization devices for offline MR imaging in treatment position. A flexible, modular phantom is developed based on lego bricks, enabling fast distortion phantom prototyping and consequently distortion measurements. The effect of magnetic fields on dose measurements with thermoluminescent dosimeters is evaluated. Image registration methods are applied to time-resolved 4D MR and 4D CT images of patients as well as to breathing phantoms evaluating how to include 4D data into the online adaptive treatment workflow. Results The preliminary results of the projects mentioned above will be presented and discussed. This includes a comparison of CT- and MR-based internal target volume concepts as a surrogate for reproducibility of breathing motion of the patient, the first results of the 4D phantom studies and the TLD measurements in a magnetic field. Conclusions Several studies are performed at the university hospital of the LMU in Munich to pave the way for a smooth introduction of the MR-Linac into clinical routine and into research in MR-guided radiotherapy.