4D radial acquisition contrast-enhanced MR angiography and intracranial arteriovenous malformations: quickly approaching digital subtraction angiography

Most Promising Approaches to Improve Brain AVM Management: ARISE I Consensus Recommendations

Brain arteriovenous malformations (bAVMs) are complex, and rare arteriovenous shunts that present with a wide range of signs and symptoms, with intracerebral hemorrhage being the most severe. Despite prior societal position statements, there is no consensus on the management of these lesions. ARISE (Aneurysm/bAVM/cSDH Roundtable Discussion With Industry and Stroke Experts) was convened to discuss evidence-based approaches and enhance our understanding of these complex lesions. ARISE identified the need to develop scales to predict the risk of rupture of bAVMs, and the use of common data elements to perform prospective registries and clinical studies. Additionally, the group underscored the need for comprehensive patient management with specialized centers with expertise in cranial and spinal microsurgery, neurological endovascular surgery, and stereotactic radiosurgery. The collection of prospective multicenter data and gross specimens was deemed essential for improving bAVM characterization, genetic evaluation, and phenotyping. Finally, bAVMs should be managed within a multidisciplinary framework, with clinical studies and research conducted collaboratively across multiple centers, harnessing the collective expertise and centralization of resources.

Non-Contrast Magnetic Resonance Angiography: Techniques, Principles, and Applications

Several non-contrast magnetic resonance angiography (MRA) techniques have been developed, providing an attractive alternative to contrast-enhanced MRA and a radiation-free alternative to computed tomography (CT) CT angiography. This review describes the physical principles, limitations, and clinical applications of bright-blood (BB) non-contrast MRA techniques. The principles of BB MRA techniques can be broadly divided into (a) flow-independent MRA, (b) blood-inflow-based MRA, (c) cardiac phase dependent, flow-based MRA, (d) velocity sensitive MRA, and (e) arterial spin-labeling MRA. The review also includes emerging multi-contrast MRA techniques that provide simultaneous BB and black-blood images for combined luminal and vessel wall evaluation.

Intracranial 3D and 4D MR Angiography Using Arterial Spin Labeling: Technical Considerations

In the 1980’s some of the earliest studies of arterial spin labeling (ASL) MRI have demonstrated its ability to generate MR angiography (MRA) images. Thanks to many technical improvements, ASL has been successfully moving its position from the realm of research into the clinical area, albeit more known as perfusion imaging than as MRA. For MRA imaging, other techniques such as time-of-flight, phase contrast MRA and contrast-enhanced (CE) MRA are more popular choices for clinical applications. In the last decade, however, ASL-MRA has been experiencing a remarkable revival, especially because of its non-invasive nature, i.e. the fact that it does not rely on the use of contrast agent. Very importantly, there are additional benefits of using ASL for MRA. For example, its higher flexibility to achieve both high spatial and temporal resolution than CE dynamic MRA, and the capability of vessel specific visualization, in which the vascular tree arising from a selected artery can be exclusively visualized. In this article, the implementation and recent developments of ASL-based MRA are discussed; not only focusing on the basic sequences based upon pulsed ASL or pseudo-continuous ASL, but also including more recent labeling approaches, such as vessel-selective labeling, velocity-selective ASL, vessel-encoded ASL and time-encoded ASL. Although these ASL techniques have been already utilized in perfusion imaging and their usefulness has been suggested by many studies, some additional considerations should be made when employing them for MRA, since there is something more than the difference of the spatial resolution of the readout sequence. Moreover, extensive discussion is included on what readout sequence to use, especially by highlighting how to achieve high spatial resolution while keeping scan-time reasonable such that the ASL-MRA sequence can easily be included into a clinical examination.

Eye Movements in Medical Image Perception: A Selective Review of Past, Present and Future

The eye movements of experts, reading medical images, have been studied for many years. Unlike topics such as face perception, medical image perception research needs to cope with substantial, qualitative changes in the stimuli under study due to dramatic advances in medical imaging technology. For example, little is known about how radiologists search through 3D volumes of image data because they simply did not exist when earlier eye tracking studies were performed. Moreover, improvements in the affordability and portability of modern eye trackers make other, new studies practical. Here, we review some uses of eye movements in the study of medical image perception with an emphasis on newer work. We ask how basic research on scene perception relates to studies of medical 'scenes' and we discuss how tracking experts' eyes may provide useful insights for medical education and screening efficiency.

Time-of-Arrival Parametric Maps and Virtual Bolus Images Derived From Contrast-Enhanced Time-Resolved Radial Magnetic Resonance Angiography Improve the Display of Brain Arteriovenous Malformation Vascular Anatomy

OBJECTIVES

Time-of-arrival (TOA) maps can be derived from high-resolution 4-dimensional (4D) contrast-enhanced magnetic resonance angiography (MRA) data sets to provide a quantitative description of contrast material arrival time in each voxel. This information can further be processed to create a compressed time evolution curve that virtually shortens the contrast bolus (virtual bolus [VB]). The purpose of this project was to determine whether TOA-enhanced 4D MRA and/or VB imaging improve the display of contrast kinetics in patients with vascular disease.

METHODS

High-resolution whole-brain contrast-enhanced 4D MRA examinations with 1.2-second temporal reconstruction were acquired by using radial acquisition and highly constrained projection reconstruction (radial 4D contrast-enhanced HYPRFlow, abbreviated as HFMRA in this article) in 10 patients (8 patients with arteriovenous malformations [AVM], 1 patient with an arteriovenous fistula, and 1 patient with a high-grade intracranial stenosis). The TOA for each voxel was defined as the time point when the signal intensity reached 20% of its maximum. In the first method, TOA maps were generated, color-encoded, and then multiplied with the time-resolved contrast-enhanced MRA images at each time frame to form new 4D MRA images (TOA-enhanced HFMRA), which contains the contrast arrival times with defined color encoding. In the second method, each time frame was weighted by a Gaussian distribution in the time domain to form a virtual 4D bolus map. This 4D bolus map was then color-coded and multiplied with the HFMRA images to form a digital subtraction angiography (DSA)-like VB, where at each time frame, only vessels with certain TOA values within the defined bolus length appear. HFMRA, TOA maps, and VB images were scored qualitatively with regard to delineation of arteries, veins, and nidus, as well as artifacts. Furthermore, diagnostic confidence and arteriovenous overlap were evaluated and compared between techniques. A comparison with DSA was performed where DSA served as the reference standard in terms of number of arterial feeders, draining veins, and Spetzler-Martin score of AVMs. In addition, TOA maps were evaluated quantitatively.

RESULTS

Overall, diagnostic confidence score of TOA was significantly higher compared with that of HFMRA (P = 0.03). Virtual bolus showed significantly higher scores for overall diagnostic confidence (P = 0.02) and reduced arteriovenous overlap (0.01) compared with HFMRA. Furthermore, VB-reduced arteriovenous overlap scores were significantly higher compared with TOA (P = 0.04). Agreement regarding AVM draining veins was lower between DSA and HFMRA (κ = 0.3) compared with TOA and VB (κ = 0.56). Agreement regarding Spetzler-Martin score was lower between DSA and HFMRA (κ = 0.56) compared with TOA and VB (κ = 0.74).

CONCLUSIONS

TOA-enhanced HFMRA provides serial images and time of arrival maps in one inclusive display. In this study, TOA mapping combined with Virtual Bolus imaging improved diagnostic confidence in AVM patients and facilitated arteriovenous separation. The VB method further reduced overlap of arterial and venous structures.

Cerebrovascular Imaging: Which Test is Best?

Optimal diagnosis and characterization of cerebrovascular disease requires selection of the appropriate imaging exam for each clinical situation. In this review, we focus on intracranial arterial disease and discuss the techniques in current clinical use for imaging the blood vessel lumen and blood vessel wall, and for mapping cerebral hemodynamic impairment at the tissue level. We then discuss specific strategies for imaging intracranial aneurysms, arteriovenous malformations, dural arterial venous fistulas, and arterial steno-occlusive disease.

4D DSA for Dynamic Visualization of Cerebral Vasculature: A Single-Center Experience in 26 Cases

BACKGROUND AND PURPOSE

4D DSA allows acquisition of time-resolved 3D reconstructions of cerebral vessels by using C-arm conebeam CT systems. The aim of our study was to evaluate this new method by qualitative and quantitative means.

MATERIALS AND METHODS

2D and 4D DSA datasets were acquired in patients presenting with AVMs, dural arteriovenous fistulas, and cerebral aneurysms. 4D DSA was compared with 2D DSA in a consensus reading of qualitative and quantitative parameters of AVMs (eg, location, feeder, associated aneurysms, nidus size, drainage, Martin-Spetzler Score), dural arteriovenous fistulas (eg, fistulous point, main feeder, diameter of the main feeder, drainage), and cerebral aneurysms (location, neck configuration, aneurysmal size). Identifiability of perforators and diameters of the injection vessel (ICA, vertebral artery) were analyzed in 2D and 4D DSA. Correlation coefficients and a paired t test were calculated for quantitative parameters. The effective patient dose of the 4D DSA protocol was evaluated with an anthropomorphic phantom.

RESULTS

In 26 patients, datasets were acquired successfully (AVM = 10, cerebral aneurysm = 10, dural arteriovenous fistula = 6). Qualitative and quantitative evaluations of 4D DSA in AVMs (nidus size: r = 0.99, P = .001), dural arteriovenous fistulas (diameter of the main feeder: r = 0.954, P = .03), and cerebral aneurysms (aneurysmal size: r = 1, P = .001) revealed nearly complete accordance with 2D DSA. Perforators were comparably visualized with 4D DSA. Measurement of the diameter of the injection vessel in 4D DSA was equivalent to that in 2D DSA (P = .039). The effective patient dose of 4D DSA was 1.2 mSv.

CONCLUSIONS

4D DSA is feasible for imaging of AVMs, dural arteriovenous fistulas, and cerebral aneurysms. 4D DSA offers reliable visualization of the cerebral vasculature and may improve the understanding and treatment of AVMs and dural arteriovenous fistulas. The number of 2D DSA acquisitions required for an examination may be reduced through 4D DSA.

Intracranial Arteriovenous Shunting: Detection with Arterial Spin-Labeling and Susceptibility-Weighted Imaging Combined

BACKGROUND AND PURPOSE

Arterial spin-labeling and susceptibility-weighted imaging are 2 MR imaging techniques that do not require gadolinium. The study aimed to assess the accuracy of arterial spin-labeling and SWI combined for detecting intracranial arteriovenous shunting in comparison with conventional MR imaging.

MATERIALS AND METHODS

Ninety-two consecutive patients with a known (n = 24) or suspected arteriovenous shunting (n = 68) underwent digital subtraction angiography and brain MR imaging, including arterial spin-labeling/SWI and conventional angiographic MR imaging (3D TOF, 4D time-resolved, and 3D contrast-enhanced MRA). Arterial spin-labeling/SWI and conventional MR imaging were reviewed separately in a randomized order by 2 blinded radiologists who judged the presence or absence of arteriovenous shunting. The accuracy of arterial spin-labeling/SWI for the detection of arteriovenous shunting was calculated by using the area under receiver operating curve with DSA as reference standard. κ coefficients were computed to determine interobserver and intermodality agreement.

RESULTS

Of the 92 patients, DSA showed arteriovenous shunting in 63 (arteriovenous malformation in 53 and dural arteriovenous fistula in 10). Interobserver agreement was excellent (κ =0.83-0.95). In 5 patients, arterial spin-labeling/SWI correctly detected arteriovenous shunting, while the conventional angiographic MR imaging did not. Compared with conventional MR imaging, arterial spin-labeling/SWI was significantly more sensitive (0.98 versus 0.90, P = .04) and equally specific (0.97) and showed significantly higher agreement with DSA (κ = 0.95 versus 0.84, P = .01) and higher area under the receiver operating curve (0.97 versus 0.93, P = .02).

CONCLUSIONS

Our study showed that the combined use of arterial spin-labeling and SWI may be an alternative to contrast-enhanced MRA for the detection of intracranial arteriovenous shunting.

Development of De Novo Arteriovenous Malformation Following Ischemic Stroke: Case Report and Review of Current Literature

BACKGROUND

Arteriovenous malformations (AVMs) are hypothesized to be static, congenital lesions developing as early as 4 weeks of fetal life. New literature has shown that AVMs may represent dynamic and reactive vascular lesions arising from cerebral infarction, inflammation, or trauma. A literature search reveals 17 previously reported cases of new AVM formation after previous negative imaging studies. This reactive development or "second hit" theory suggests that at a molecular level, growth factors may play a vital role in aberrant angiogenesis and maturation of an arteriovenous fistula into an AVM.

CASE DESCRIPTION

A 52-year-old female presented with a ruptured left frontal AVM demonstrated by computed tomography angiography and digital subtraction angiography. The patient had suffered an acute ischemic stroke in the similar cerebral vascular territory 8 years prior due to left internal carotid artery occlusion. Detailed neuroimaging at that time failed to reveal any vascular malformation, suggesting that the AVM might have developed in response to initial vascular insult.

CONCLUSIONS

We believe that there might exist a subset of AVMs that display dynamic characteristics and could potentially appear, grow, or resolve spontaneously without intervention, especially in the presence of local growth factors and molecular signaling cascades. When combined with a previous cerebral insult such as stroke, trauma, or inflammation, de novo AVM formation may represent a "second hit" with abnormal angiogenesis and vessel formation.

Utility of noncontrast-enhanced time-resolved four-dimensional MR angiography with a vessel-selective technique for intracranial arteriovenous malformations

PURPOSE

To evaluate the utility of a vessel-selective four-dimensional (4D) magnetic resonance angiography (MRA) technique for the evaluation of intracranial arteriovenous malformations (AVMs).

MATERIALS AND METHODS

Twelve AVM patients were evaluated retrospectively. Time-of-flight (TOF) MRA, nonvessel-selective 4D-MRA (NS-4D-MRA), and vessel-selective 4D-MRA (VS-4D-MRA) were performed using a 3T MR unit in all patients, and used to identify feeding arteries and draining veins and measure nidus size. The diagnostic accuracy of the three techniques was compared using digital subtraction angiography (DSA). If a multifeeder was observed, the percentage of blood flow of each feeding artery to the entire nidus was evaluated and compared to the DSA findings using the "error value," defined as the degree of overestimation of the blood flow. All imaging findings were assessed by two neuroradiologists.

RESULTS

In both raters, the detectability of feeding arteries by VS-4D-MRA (12 and 11 patients) was significantly higher than those of TOF-MRA (7 and 6 patients) and NS-4D-MRA (8 and 7 patients) (P < 0.016). The detectability of drainer veins by TOF-MRA (10 and 10 patients) was significantly higher than that of VS-4D-MRA (7 and 6 patients). In the percentage of the blood flow of each feed artery to the entire nidus, the DSA findings (error value; 27.1 ± 5.7) indicated overestimations of the blood flow compared to the VS-4D-MRA (error value; 7.1 ± 3.9) (P < 0.001).

CONCLUSION

VS-4D-MRA was shown to be a useful technique for the evaluation of intracranial AVMs, especially for detecting feed arteries and estimating details of the nidus structure. J. MAGN. RESON. IMAGING 2016;44:834-845.

Recent advances in 3D time-resolved contrast-enhanced MR angiography

Contrast-enhanced magnetic resonance angiography (CE-MRA) was first introduced for clinical studies approximately 20 years ago. Early work provided 3-4 mm spatial resolution with acquisition times in the 30-second range. Since that time there has been continuing effort to provide improved spatial resolution with reduced acquisition time, allowing high resolution 3D time-resolved studies. The purpose of this work is to describe how this has been accomplished. Specific technical enablers have been: improved gradients allowing reduced repetition times, improved k-space sampling and reconstruction methods, parallel acquisition, particularly in two directions, and improved and higher count receiver coil arrays. These have collectively made high-resolution time-resolved studies readily available for many anatomic regions. Depending on the application, ∼1 mm isotropic resolution is now possible with frame times of several seconds. Clinical applications of time-resolved CE-MRA are briefly reviewed.

Fast contrast-enhanced 4D MRA and 4D flow MRI using constrained reconstruction (HYPRFlow): potential applications for brain arteriovenous malformations

BACKGROUND AND PURPOSE

HYPRFlow is a novel imaging strategy that provides fast, high-resolution contrast-enhanced time-resolved images and measurement of the velocity of the entire cerebrovascular system. Our hypothesis was that the images obtained with this strategy are of adequate diagnostic image quality to delineate the major components of AVMs.

MATERIALS AND METHODS

HYPRFlow and 3D TOF scans were obtained in 21 patients with AVMs with correlative DSA examinations in 14 patients. The examinations were scored for image quality and graded by using the Spetzler-Martin criteria. Mean arterial transit time and overlap integrals were calculated from the dynamic image data. Volume flow rates in normal arteries and AVM feeding arteries were measured from the phase contrast data.

RESULTS

HYPRFlow was equivalent to 3D-TOF in delineating normal arterial anatomy, arterial feeders, and nidus size and was concordant with DSA for AVM grading and venous drainage in 13 of the 14 examinations. Mean arterial transit time on the AVM side was 0.49 seconds, and on the normal contralateral side, 2.53 seconds with P < .001. Across all 21 subjects, the mean arterial volume flow rate in the M1 segment ipsilateral to the AVM was 4.07 ± 3.04 mL/s; on the contralateral M1 segment, it was 2.09 ± 0.64 mL/s. The mean volume flow rate in the largest feeding artery to the AVM was 3.86 ± 2.74 mL/s.

CONCLUSIONS

HYPRFlow provides an alternative approach to the MRA evaluation of AVMs, with the advantages of increased coverage, 0.75-second temporal resolution, 0.68-mm isotropic spatial resolution, and quantitative measurement of flow in 6 minutes.

Advances and innovations in brain arteriovenous malformation surgery

Arteriovenous malformations (AVMs) of the brain are very complex and intriguing pathologies. Since their initial description by Luschka and Virchow in the middle of the 19th century, multiple advances and innovations have revolutionized their management and surgical treatment. Here, we review the historical landmarks in the surgical treatment of AVMs and then illustrate the most recent and futuristic technologies aiming to improve outcomes in AVM surgeries. In particular, we examine potential advances in patient selection, imaging, surgical technique, neuroanesthesia, and postoperative neuro-rehabilitation and quantitative assessments. Finally, we illustrate how concurrent advances in radiosurgery and endovascular techniques might present new opportunities to treat AVMs more safely from a surgical perspective.

Imaging of acute stroke prior to treatment: current practice and evolving techniques

Standard imaging in acute stroke is undertaken with the aim of diagnosing the underlying cause and excluding stroke mimics. In the presence of ischaemic stroke, imaging is also needed to assess patient suitability for treatment with intravenous thrombolysis. Non-contrast CT is predominantly used, but MRI can also exclude any contraindications to thrombolysis treatment. Advanced stroke imaging such as CT and MR angiography and perfusion imaging are increasingly used in an acute setting. In this review, we discuss the evidence for the application of these advanced techniques in the imaging of acute stroke.

MR selective flow-tracking cartography: a postprocessing procedure applied to four-dimensional flow MR imaging for complete characterization of cranial dural arteriovenous fistulas

PURPOSE

To assess the feasibility of a selective flow-tracking cartographic procedure applied to four-dimensional (4D) flow imaging and to demonstrate its usefulness in the characterization of dural arteriovenous fistulas (DAVFs).

MATERIALS AND METHODS

Institutional review board approval was obtained, and all patients provided written informed consent. Eight patients (nine DAVFs) underwent 3.0-T magnetic resonance (MR) imaging and digital subtraction angiography (DSA). Imaging examinations were performed within 24 hours of each other. 4D flow MR imaging was performed by using a 4D radial phase-contrast vastly undersampled isotropic projection reconstruction pulse sequence with an isotropic spatial resolution of 0.86 mm (5 minutes 35 seconds). Two radiologists independently reviewed images from MR flow-tracking cartography and reported the location of arterial feeder vessels and the venous drainage type and classified DAVFs according to the risk of rupture (Cognard classification). These results were compared with those at DSA. Quadratic weighted κ statistics with their 95% confidence intervals (CIs) were used to test intermodality agreement in the identification of arterial feeder vessels, draining veins, and Cognard classification.

RESULTS

Interreader agreement for shunt location on MR images was perfect (κ = 1), with good-to-excellent interreader agreement for arterial feeder vessel identification (κ = 0.97; 95% CI = 0.92, 1.0), and matched in all cases with shunt location defined at DSA. There was good-to-excellent agreement between MR cartography and DSA in the definition of the main feeding arteries (κ = 0.92; 95% CI = 0.83, 1.0), presence of retrograde flow in dural sinuses (κ = 1), presence of retrograde cortical venous drainage (κ = 1), presence of venous ectasia (κ = 1), and final Cognard classification of DAVFs (κ = 1, standard error = 0.35).

CONCLUSION

MR selective flow-tracking cartography enabled the noninvasive characterization of cranial DAVFs.

RAZER: a pulse sequence for whole-brain bolus tracking at high frame rates

PURPOSE

To introduce a pulse sequence that obtains whole-brain perfusion measurements at 1.7 mm isotropic voxel resolution by dynamic susceptibility contrast MRI bolus tracking despite using a temporal resolution of 10.3 s: RAdial kZ-blipped 3D GRE-echo-planar imaging (GRE-EPI) for whole-brain pERfusion (RAZER).

METHODS

In RAZER, in-plane radial and through-plane 3D GRE-EPI Cartesian sampling was used to produce a 3D stack-of-stars k-space. In vivo scans on one healthy volunteer and one patient with Moyamoya disease were performed using RAZER and a typical 2D GRE-EPI pulse sequence as a reference standard. Agreement in perfusion metrics was reported using linear regression analysis and Bland-Altman plots.

RESULTS

Sliding window reconstruction recovered dynamic information lost in the large temporal acquisition window of RAZER. Inline phase correction scans corrected N/2 ghosting artifacts and view-dependent phase variations. Whole-brain images of cerebral blood volume, cerebral blood flow, and mean transit time were calculated with RAZER at 1.7 mm isotropic voxel resolution and good reference standard agreement in both subjects when sliding window reconstruction was used (r(2) > 0.7, mean bias in mean transit time measurements < 0.5 s).

CONCLUSIONS

Despite using a temporal resolution of 10.3 s, in vivo data indicates that RAZER is able to obtain whole-brain perfusion measurements at 1.7 mm isotropic voxel resolution and good reference standard agreement.

Magnetic Resonance Imaging Assessment of Cerebral Arteriovenous Malformation Obliteration After Stereotactic Radiosurgery

BACKGROUND:

Stereotactic radiosurgery is ideal for treating small cerebral arteriovenous malformations (AVMs) that are surgically inaccessible. However, given the inherent delay of AVM obliteration and the potential for radiosurgical failure, detailed evaluation of the neurovascular architecture is necessary to monitor persistence of residual flow. Modern imaging systems such as magnetic resonance imaging (MRI) and angiography allow clinicians to assess transnidus flow after radiosurgical intervention.

OBJECTIVE:

To determine the accuracy of an MRI diagnosis of complete thrombosis and to identify variables that affect the precision of MRI assessment.

METHODS:

One hundred twenty patients were reviewed after receiving radiosurgery at the University of Florida from 1990 to 2010. Each patient had an MRI demonstrating AVM obliteration and an angiogram either confirming or denying AVM thrombosis.

RESULTS:

MRI correctly predicted complete AVM obliteration in 82% of patients. There was a significant correlation between AVM volume and MRI accuracy in 2 separate models. In the first model, logistic regression analysis revealed a significant linear relationship between the natural log of AVM volume and MRI accuracy. The second model showed significant evidence of a cutoff point in MRI accuracy near an AVM volume of 2.80 cm3, above which MRI agreement with angiography is 90% and below which MRI agreement falls off sharply to remain constant at 70%.

CONCLUSION:

MRI is a useful diagnostic system for assessing AVM obliteration, but its accuracy is inherently linked to the nidus volume it is measuring. These results suggest that MRI may be able to take on an increasingly independent role in the evaluation of AVM regression.

Advanced neuroimaging to guide acute stroke therapy

Traditionally non-contrast CT has been considered the first choice imaging modality for acute stroke. Acute ischemic stroke patients presenting to the hospital within 3-hours from symptom onset and without any visible hemorrhages or large lesions on CT images are considered optimum reperfusion therapy candidates. However, non-contrast CT alone has been unable to identify best reperfusion therapy candidates outside this window. New advanced imaging techniques are now being used successfully for this purpose. Non-invasive CT or MR angiography images can be obtained during initial imaging evaluation for identification and characterization of vascular lesions, including occlusions, aneurysms, and malformations. Either CT-based perfusion imaging or MRI-based diffusion and perfusion imaging performed immediately upon arrival of a patient to the hospital helps estimate the extent of fixed core and penumbra in ischemic lesions. Patients having occlusive lesions with small fixed cores and large penumbra are preferred reperfusion therapy candidates.

Intracranial 4D flow MRI: toward individualized assessment of arteriovenous malformation hemodynamics and treatment-induced changes

BACKGROUND AND PURPOSE

Arteriovenous malformations are an important etiology of hemorrhagic stroke. However, current imaging modalities and risk do not provide insights into individual AVM hemodynamics and its role in pathophysiology. The aims of this study are to determine whether intracranial 4D flow MR imaging can provide insights into arteriovenous malformation hemodynamics independent of the Spetzler-Martin grade and to report the changes in flow observed during staged embolization.

MATERIALS AND METHODS

Intracranial 3D blood flow was assessed in 20 patients with AVM (age = 39 ± 15 years, Spetzler-Martin grade ranging from 1-4) with the use of 4D flow MR imaging (temporal resolution = 45 ms, spatial resolution = [1.2-1.6mm](3)). AVM hemodynamics were visualized by means of time-integrated 3D pathlines depicting the AVM arterial feeding and venous draining patterns over the cardiac cycle. Analysis included the grading of feeding and draining velocities on a 3-point scale (0 = low <25 cm/s, 1 = medium <50 cm/s, 2 = high >50 cm/s). For 4 of 20 patients undergoing 4D flow MR imaging follow-up after staged embolization, peak velocities were quantified in arterial feeders, draining veins, the sagittal sinus, and contralateral arteries.

RESULTS

In 50% of the cases with Spetzler-Martin grade >2, heterogeneous flow (velocity grade differences >1) was found across arteries and veins. Velocities in draining veins increased from Spetzler-Martin grade = 1 (grading = 0.5 ± 0.6) to Spetzler-Martin grade ≥3 (1.1 ± 0.6), whereas arterial velocities were similar (1.7 ± 0.6 versus 1.5 ± 0.6). In the postembolization subgroup of 4 patients, 4D flow MR imaging demonstrated successively more compact AVM and redistribution of velocities. Changes in arterial and venous velocities during treatment were highly different among individuals.

CONCLUSIONS

Spetzler-Martin grade does not reflect differences in 3D AVM arterial and venous hemodynamics, and an individual assessment of AVM hemodynamics may be needed for improved lesion characterization. Four-dimensional flow MR imaging may have the potential to monitor and guide embolization treatment planning.

Volumetric analysis of intracranial arteriovenous malformations contoured for CyberKnife radiosurgery with 3-dimensional rotational angiography vs computed tomography/magnetic resonance imaging

BACKGROUND

Accurate target delineation has significant impact on brain arteriovenous malformation (AVM) obliteration, treatment success, and potential complications of stereotactic radiosurgery.

OBJECTIVE

We compare the nidal contouring of AVMs using fused images of contrasted computed tomography (CT) and magnetic resonance imaging (MRI) with matched images of 3-dimensional (3-D) cerebral angiography for CyberKnife radiosurgery (CKRS) treatment planning.

METHODS

Between May 2009 and April 2012, 3-D cerebral angiography was integrated into CKRS target planning for 30 consecutive patients. The AVM nidal target volumes were delineated using fused CT and MRI scans vs fused CT, MRI, and 3-D cerebral angiography for each patient.

RESULTS

The mean volume of the AVM nidus contoured with the addition of 3-D cerebral angiography to the CT/MRI fusion (9.09 cm(3), 95% confidence interval: 5.39 cm(3)-12.8 cm(3)) was statistically smaller than the mean volume contoured with CT/MRI fused scans alone (14.1 cm(3), 95% confidence interval: 9.16 cm(3)-19.1 cm(3)), with a mean volume difference of δ = 5.01 cm(3) (P = .001). Diffuse AVM nidus was associated with larger mean volume differences compared with a compact nidus (δ = 6.51 vs 2.11 cm(3), P = .02). The mean volume difference was not statistically associated with the patient's sex (male δ = 5.61, female δ = 5.06, P = .84), previous hemorrhage status (yes δ = 5.69, no δ = 5.23, P = .86), or previous embolization status (yes δ = 6.80, no δ = 5.95, P = .11).

CONCLUSION

For brain AVMs treated with CKRS, the addition of 3-D cerebral angiography to CT/MRI fusions for diagnostic accuracy results in a statistically significant reduction in contoured nidal volume compared with standard CT/MRI fusion-based contouring.

Neuroimaging in acute stroke: choosing the right patient for neurointervention

Although the non-contrast computed tomography head continues as the sole mandatory imaging technique before intravenous thrombolysis, the increased availability of advanced infarct/penumbral imaging techniques and confidence in their use have led many to adopt them into routine practice--most particularly before intra-arterial interventions. Computed tomography versus magnetic resonance-based routes to imaging the cerebral vasculature, cell death, and parenchymal perfusion have differing advantages in terms of speed, availability, exposures to contrast and radiation, sensitivity, and resolution. Continued refinement and future developments, such as the ability to quantitate perfusion, promise to lead to tailored treatment protocols that respect the individual variations in anatomy, physiology, and pathology. This should lead both to an extension of treatment to patients currently excluded by rigid time windows and the avoidance of futile therapies and their associated morbidities.

Developmental venous anomaly of the internal auditory canal in a child with unilateral sensorineural hearing loss--a rare association

Developmental venous anomalies (DVAs) are extremely unusual in the cerebellopontine angle region and internal auditory canal, and constitute a rare etiology of unilateral sensorineural hearing loss (SNHL) in children. We report, to the best of our knowledge, one of the first documented cases of DVA as a probable cause of unilateral SNHL in a child. Our emphasis is on the radiological features.

Sensitivity of CT angiography, T2-weighted MRI, and magnetic resonance angiography in detecting cerebral arteriovenous malformations and associated aneurysms

An increasing number of patients with arteriovenous malformations (AVM) are being screened with expedient, safe, and non-invasive methods such as computed tomography angiography (CTA), MRI, and magnetic resonance angiography (MRA). Only a paucity of small series have examined the sensitivity of these modalities in detecting AVMs. In this study, we reviewed 125 patients with AVMs, demonstrating overall sensitivities of 90% for CTA, 89% for T2-weighted MRI, and 74% for MRA. The greater sensitivity of CTA in detecting AVMs compared to MRA did meet statistical significance (p(CTA, MRA)=0.036). For unruptured AVMs, the sensitivities were 96%, 97% and 71% (p(CTA, MRA)=0.043) and for ruptured AVMs, they were 87%, 83% and 77%, respectively. The sensitivity was 100% for both CTA and T2-weighted MRI for AVMs larger than 3 cm. Importantly, associated aneurysms were best detected via CTA, with a sensitivity of 88%, as compared to 29% for MRI and 27% for MRA (p(CTA, MRA)=1.2×10(-6)), with the greatest distinction occurring within the subgroup of intranidal aneurysms with a sensitivity of 83% for CTA compared to 0% for both MRI and MRA (p(CTA, MRA)=0.005).

Combined renal MRA and perfusion with a single dose of contrast

Both anatomical and functional scans are often performed when diagnosing renovascular diseases, which in many cases require two separate contrast injections. With nephrogenic systemic fibrosis being associated with gadolinium, minimizing contrast injection dosage is desirable. In this study, a technique which performs time-resolved renal magnetic resonance angiography (MRA) and perfusion with a single scan and single dose of contrast has been evaluated in six healthy volunteers. A previously developed three-dimensional MRA technique called Contrast-enhanced Angiography with Multi-Echo and Radial k-space (CAMERA) has been used to acquire images, and perfusion analysis was performed using deconvolution methods. Time-resolved MRA, as well as renal blood flow, renal volume of distribution and mean transit time maps, were acquired.

Vascular disorders--magnetic resonance angiography: brain vessels

Magnetic resonance angiography (MRA) of the brain obtained at 3 T imaging has made a significant clinical impact. MRA benefits from acquisition at higher magnetic field strength because of higher available signal-to-noise ratio and improved relative background suppression due to magnetic field strength-related T1 lengthening. Parallel imaging techniques are ideally suited for high-field MRA. Many of the developments that have made 3 T MRA of the brain successful can be regarded as enabling technologies that are essential for further development of 7 T MRA, which brings additional challenges.

Imaging of cerebral arteriovenous malformations and dural arteriovenous fistulas

Imaging plays a major role in the identification, grading, and treatment of cerebral arteriovenous malformations and cerebral dural arteriovenous fistulas. Digital subtraction angiography is the gold standard in the diagnosis and characterization of these vascular malformations, but advances in both magnetic resonance imaging and computed tomography, including advanced imaging techniques, have provided new tools for further characterizing these lesions as well as the surrounding brain structures that may be affected. This article discusses the role of conventional as well as advanced imaging modalities that are providing novel ways to characterize these vascular malformations.

Magnetization spoiling in radial FLASH contrast-enhanced MR digital subtraction angiography

PURPOSE

To increase the in-plane spatial resolution and image update rates of 2D magnetic resonance (MR) digital subtraction angiography (DSA) pulse sequences to 0.57 × 0.57 mm and 6 frames/sec, respectively, for intracranial vascular disease applications by developing a radial FLASH protocol and to characterize a new artifact, not previously described in the literature, which arises in the presence of such pulse sequences.

MATERIALS AND METHODS

The pulse sequence was optimized and artifacts were characterized using simulation and phantom studies. With Institutional Review Board (IRB) approval, the pulse sequence was used to acquire time-resolved images from healthy human volunteers and patients with x-ray DSA-confirmed intracranial vascular disease.

RESULTS

Artifacts were shown to derive from inhomogeneous spoiling due to the nature of radial waveforms. Gradient spoiling strategies were proposed to eliminate the observed artifact by balancing gradient moments across TR intervals. The resulting radial 2D MR DSA sequence (2.6 sec temporal footprint, 6 frames/sec with sliding window factor 16, 0.57 × 0.57 mm in-plane) demonstrated small vessel detail and corroborated x-ray DSA findings in intracranial vascular imaging studies.

CONCLUSION

Appropriate gradient spoiling in radial 2D MR DSA pulse sequences improves intracranial vascular depiction by eliminating circular banding artifacts. The proposed pulse sequence may provide a useful addition to clinically applied 2D MR DSA scans.

Noninvasive evaluation of cerebral arteriovenous malformations by 4D-MRA for preoperative planning and postoperative follow-up in 56 patients: comparison with DSA and intraoperative findings

BACKGROUND AND PURPOSE

4D-MRA is a promising technique in the diagnosis and follow-up of cAVMs. The purpose of this study was to compare 4D-MRA in the pre- and postoperative evaluation of cAVMs with DSA or intraoperative findings as the standards of reference regarding qualitative and quantitative parameters.

MATERIALS AND METHODS

Fifty-six consecutive patients with cAVMs (30 women) underwent both 4D-MRA and DSA. Preoperative 4D-MRA was excluded from analysis in 1 patient (movement artifacts). Twenty-five patients underwent surgery on cAVMs and underwent both imaging modalities pre- and postoperatively. 4D-MRA was performed with either 0.5-mol/L gadolinium-diethylene-triamine pentaacetic acid (group 1: voxel size, 1.1 × 1.1 × 1.4 mm(3); 608 ms/dynamic frame; 19 patients) or 1.0-mol/L gadobutrol (group 2: voxel size, 1.1 × 1.1 × 1.1 mm(3); 572 ms/dynamic frame; additional alternating view sharing; 37 patients). Two readers independently reviewed 4D-MRA and DSA regarding the Spetzler-Martin classification, arterial feeders, and postoperative residual filling. Vessel sharpness, vessel diameter, and VBC of 4D-MRA were quantified.

RESULTS

Preoperative Spetzler-Martin classification 4D-MRA and DSA ratings matched in 55/55 patients (Spetzler-Martin grades: I, 12; II, 22; III, 15; IV, 5; V, 1), and 93/100 arterial feeders were correctly identified by preoperative 4D-MRA (7 additional arterial feeders identified by DSA only: group 1, 3/19; group 2, 4/36). Postoperative 4D-MRA and DSA matched in 25/25 patients (residual filling, 1/25). Vessel sharpness and diameters did not differ substantially between the 2 groups. VBC was significantly higher in group 2 (P < .005).

CONCLUSIONS

4D-MRA is a reliable tool that allows predicting Spetzler-Martin classification and postoperative residual filling; it hence allows substituting DSA in the pre- and postoperative evaluation of patients with cerebral AVMs.

The predictive value of 3D time-of-flight MR angiography in assessment of brain arteriovenous malformation obliteration after radiosurgery

BACKGROUND AND PURPOSE

The purpose of radiosurgery of bAVMs is complete angiographic obliteration of its nidus. We assessed the diagnostic accuracy of 1.5T T2-weighted MR imaging and TOF-MRA images for detecting nidus obliteration after radiosurgery.

MATERIALS AND METHODS

The pre- and postradiosurgery MR images and DSA images from 120 patients who were radiosurgically treated for a bAVM were re-evaluated by 2 observers for patency of the nidus (preradiosurgery) and obliteration (postradiosurgery: final follow-up MR imaging), by using a 3-point scale of confidence. Consensus reading of the DSA after radiosurgery was considered the criterion standard for obliteration. Sensitivity, specificity, PPVs, and NPVs, and overall diagnostic performance by using ROC were determined.

RESULTS

Mean bAVM volume during radiosurgery was 3.4 mL (95% CI, 2.6-4.3 mL). Sixty-six patients (55%) had undergone previous endovascular embolization. The mean intervals between radiosurgery and follow-up MR imaging and for DSA, respectively, were 35.6 months (95% CI, 32.3-38.9 months) and 42.1 months (95% CI, 40.3-44.0 months). With ROC, an area under curve of 0.81-0.83 was found. PPVs of final follow-up MR-imaging for definitive obliteration varied between 0.89 [corrected] and 0.95. NPV was 0.52 [corrected] . An average false-positive rate, meaning overestimation of nidus obliteration of 0.10 [corrected] and an average false-negative rate, meaning underestimation of nidus obliteration of 0.42 [corrected] were found.

CONCLUSIONS

MRA is insufficient to diagnose obliteration in the follow-up of bAVMs after radiosurgery. A remaining nidus diameter <10 mm seems to be the major limiting factor for reliable assessment of obliteration. We highly recommend follow-up DSA for definitive diagnosis of complete obliteration.

Postcontrast susceptibility-weighted imaging: a novel technique for the detection of arteriovenous shunting in vascular malformations of the brain

BACKGROUND AND PURPOSE

The purpose of this study was to determine the utility of postcontrast susceptibility-weighted MRI (PCSWI) in the evaluation of vascular malformations of the brain (BVM).

METHODS

We retrospectively evaluated PCSWI and digital subtraction angiography data from 16 consecutive patients with known or suspected BVM, which had been entered into a prospectively maintained database during a 1-year period. There had been no intervening treatment or change in patients' symptoms between the PCSWI and digital subtraction angiography studies. The use of PCSWI in the detection of arteriovenous shunting was compared with that of routine noncontrast susceptibility-weighted imaging, time-of-flight MR angiography, and contrast-enhanced MR angiography using digital subtraction angiography results as the reference standard. The presence of arteriovenous shunting in PCSWI or susceptibility-weighted imaging sequences was defined by the presence of abnormal signal hyperintensity in the venous structures adjacent to the BVM.

RESULTS

A total of 17 BVMs were identified by digital subtraction angiography (9 newly diagnosed arteriovenous malformations, 3 dural arteriovenous fistulas, 4 treated arteriovenous malformations with residual arteriovenous shunting, and 1 complex developmental venous anomaly). PCSWI was 100% sensitive and 100% specific with 100% positive predictive value and 100% negative predictive value for the detection of arteriovenous shunting in these BVMs. The PCSWI/susceptibility-weighted imaging signal intensity ratio in the most prominent early draining venous structure was 1.2±0.32.

CONCLUSIONS

PCSWI appears to be superior to susceptibility-weighted imaging, time-of-flight MR angiography, and contrast-enhanced MR angiography in detecting arteriovenous shunting in BVMs and may be useful in the initial diagnosis and follow-up of patients with BVMs.

Time-resolved magnetic resonance angiography: evaluation of intrapulmonary circulation parameters in pulmonary arterial hypertension

PURPOSE

To determine whether pulmonary arterial and venous transit times measured by time-resolved magnetic resonance angiography (MRA) can be used as a diagnostic tool for pulmonary arterial hypertension (PAH).

MATERIALS AND METHODS

Twelve patients with confirmed PAH and 10 healthy volunteers were scanned with Institutional Review Board (IRB) approval. Time-resolved MRA and 2D phase contrast flow images of the pulmonary vasculature were acquired. Pulmonary arterial and venous transit times (PaTT and PvTT) and pulmonary valve flow (PVF) were obtained. Pulmonary arterial and pulmonary venous blood volumes (PaBV and PvBV) were calculated as the product of flow and transit time.

RESULTS

Patients with PAH showed statistically significant increases in PaTT and PvTT (P < 0.0004, P < 0.05, respectively) compared to controls. PaBV (165.2 ± 92.0 mL) was significantly higher in PAH subjects than controls (97.0 ± 47.1 mL) (P < 0.04), whereas PvBV (127.9 ± 148.9 mL) of PAH subjects had no significant increase from those of healthy controls (142.5 ± 104.1 mL) (P < 0.38).

CONCLUSION

Pulmonary arterial transit times measured using time-resolved MRA can be used as a simple, noninvasive metric for detection of altered hemodynamics in PAH.

Accuracy of susceptibility-weighted imaging for the detection of arteriovenous shunting in vascular malformations of the brain

BACKGROUND AND PURPOSE

to determine the accuracy of susceptibility-weighted MRI (SWI) for the detection of arteriovenous shunting (AVS) in vascular malformations of the brain (BVM).

METHODS

we retrospectively identified 60 patients who had been evaluated for known or suspected BVM by both SWI and digital subtraction angiography, without intervening treatment, during a 3-year period. SWI images were retrospectively assessed by 2 independent reviewers for the presence of AVS as determined by the presence of signal hyperintensity within a venous structure in the vicinity of the BVM. Discrepancies were resolved by consensus among a panel of 3 neuroradiologists. Accuracy parameters of SWI for the detection of AVS were calculated using digital subtraction angiography as the reference standard.

RESULTS

a total of 80 BVM were identified in the 60 patients included in our study. Of the 29 BVM with AVS on digital subtraction angiography, 14 were untreated arteriovenous malformations, 10 were previously treated arteriovenous malformations, and 5 were untreated dural arteriovenous fistulas. Overall, SWI was 93% sensitive and 98% specific for the detection of AVS in BVM, with excellent interobserver agreement (κ=0.94). In the 14 previously treated arteriovenous malformations, SWI was 100% sensitive and specific for the detection of AVS. In the 28 BVM associated with intracerebral hemorrhage, SWI was 100% sensitive and 96% specific for the detection of AVS.

CONCLUSIONS

SWI is accurate for the detection of arteriovenous shunting in vascular malformations of the brain and, for some patients, SWI may offer a noninvasive alternative to angiography in screening for or follow-up of treated BVM.

Anomalous venous blood flow and iron deposition in multiple sclerosis

Multiple sclerosis (MS) is primarily an autoimmune disorder of unknown origin. This review focuses iron overload and oxidative stress as surrounding cause that leads to immunomodulation in chronic MS. Iron overload has been demonstrated in MS lesions, as a feature common with other neurodegenerative disorders. However, the recent description of chronic cerebrospinal venous insufficiency (CCSVI) associated to MS, with significant anomalies in cerebral venous outflow hemodynamics, permit to propose a parallel with chronic venous disorders (CVDs) in the mechanism of iron deposition. Abnormal cerebral venous reflux is peculiar to MS, and was not found in a miscellaneous of patients affected by other neurodegenerative disorders characterized by iron stores, such as Parkinson's, Alzheimer's, amyotrophic lateral sclerosis. Several recently published studies support the hypothesis that MS progresses along the venous vasculature. The peculiarity of CCSVI-related cerebral venous blood flow disturbances, together with the histology of the perivenous spaces and recent findings from advanced magnetic resonance imaging techniques, support the hypothesis that iron deposits in MS are a consequence of altered cerebral venous return and chronic insufficient venous drainage.