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

Abstract

Objectives Time-of-arrival (TOA) maps can be derived from high-resolution 4D contrast-enhanced magnetic-resonance-angiography (MRA) datasets to provide a quantitative description of contrast material arrival time in each voxel. This information can further be processed in order to create a compressed time-evolution curve that virtually shortens the contrast bolus (Virtual Bolus [VB]). The purpose of this project is to determine whether TOA-enhanced 4D MRA and/or Virtual Bolus Imaging improve the display of contrast kinetics in patients with vascular disease. Methods High-resolution whole brain contrast enhanced 4D MRA exams with 1.2 second temporal reconstruction were acquired by using radial acquisition and highly constrained projection reconstruction (radial 4D CE HYPRFlow, is abbreviated to HFMRA in this manuscript) in 10 patients (8 patients with vascular malformations (AVM), one patient with an arteriovenous fistula (AVF) and one 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 virtual bolus, where at each time frame, only vessels with certain TOA values within the defined bolus length appear. HFMRA, TOA maps and virtual bolus 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 to HFMRA (p=0.03). Virtual Bolus (VB) showed significantly higher scores for overall diagnostic confidence (p=0.02) and reduced arteriovenous overlap (0.01) compared to HFMRA. Furthermore, Virtual Bolus reduced arteriovenous overlap significantly compared to TOA (p=0.04). Agreement regarding AVM draining veins was lower between DSA and HFMRA (k=0.3) compared to TOA and VB (k=0.56). Agreement regarding Spetzler-Martin score was lower between DSA and HFMRA (k=0.56) compared to TOA and VB (k=0.74). Conclusion 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 sep...

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