High temporal and high spatial resolution MR angiography (4D-MRA)

Abstract

!In the first decade of the twenty-first century, whole-body magnetic resonance scanners with high field strengths (and thus potentially better signal-to-noise ratios) were developed. At the same time, parallel imaging and "echo-sharing" techniques were refined to allow for increasingly high spatial and temporal resolution in dynamic magnetic resonance angiography ("time-resolved" = TR-MRA). This technological progress facilitated tracking the passage of intra-venously administered contrast agent boluses as well as the acquisition of volume data sets at high image refresh rates ("4D-MRA"). This opened doors for many new applications in non-invasive vascular imaging, including simultaneous anatomic and functional analysis of many vascular pathologies including arterio-venous malformations. Different methods were established to acquire 4D-MRA using various strategies to acquire k-space trajectories over time in order to optimize imaging according to clinical needs. These include "keyhole"-based techniques (e. g. 4D-TRAK), TRICKS -both with and without projection -and HYPR-reconstruction, TREAT, and TWIST. Some of these techniques were first introduced in the 1980 s and 1990 s, were later enhanced and modified, and finally implemented in the products of major vendors. In the last decade, a large number of studies on the clinical applications of TR-MRA was published. This manuscript provides an overview of the development of TR-MRA methods and the 4D-MRA techniques as they are currently used in the diagnosis, treatment and follow-up of vascular diseases in various parts of the body. Key statements▶ 4D-MRA, which differs according to manufacturer, generates high temporal and spatial resolution MRA volume data sets.

Dixon-based fat-free MR-angiography compared to first pass and steady-state high-resolution MR-angiography using a blood pool contrast agent

“…With the introduction of blood pool contrast agents (BPCA), which offer a prolonged time window of high vessel-to-background contrast, a marked increase in spatial resolution was realized [5][6][7]. Thereby, arteriovenous separation, which would otherwise require timeresolved imaging techniques [8], could be realized.…”

Cross-sectional imaging of aortic infections

“…Magnetic resonance imaging (MRI) is not our first-line cross-sectional modality for the assessment of suspected aortic valve infection, but it can be a useful tool in providing functional haemodynamic information which can inform treatment decisions. Contrast-enhanced MR angiography (CE-MRA) with gadolinium-based contrast agents provides excellent anatomical detail of the aorta with excellent soft tissue contrast, and without ionizing radiation [6]. Double inversion recovery (DIR) sequences null the blood pool, providing excellent visualisation of the aortic wall distinct from the aortic lumen [5].…”

Determining End Points for Critical Limb Ischemia Interventions

“…Time-resolved imaging of the tibial vessels allows evaluation of flow into a segment of tissue over time, simulating conventional angiography and allowing readers to select the time point at which each segment is optimally opacified. 27 This technique increases sensitivity, specificity and accuracy for PAD in all segments of the lower extremity arteries, but most importantly below the knee, making it an effective method for evaluating patients with CLI. 28 Nonenhanced MRA techniques including time of flight, quiescentinterval single shot, and flow-spoiled fresh blood imaging have all been used to varying degrees of success for patients with allergies to gadolinium or renal insufficiency.…”