## 1 Introduction  
Embolization is a mainstay of all interventional radiology practices with procedures encompassing the arterial and venous circulatory systems and lymphatic system [1]. The ubiquitous nature and widespread need for embolic procedures has led to great innovations in available embolic devices. Currently, there are three major types of embolic devices available: liquid, particles, and metallic devices (coils or plugs). Often, operators choose to use different embolic agents in order to achieve the desired occlusive result. These decisions are based on the anatomy, pathology, and accessibility of the lesion, as well as operator ability and preference [2]. Metallic embolic devices such as coils, Amplatzer vascular plugs (AVPs), microvascular plugs (MVPs), and Caterpillars have been used in a wide range of embolization procedures [1, 3, 4, 5, 6, 7, 8, 9, 10]. When selective embolization is required, metallic devices such as coils and plugs are often used due to these devices being controllable to achieve precise deployment and rapid occlusion of the target vessel [5]. Coils are the oldest and most commonly used metallic embolic materials. Coils come in multiple configurations and are widely available and relatively inexpensive [7]. However, multiple coils are often needed to achieve complete occlusion, necessitating a relatively long-segment landing zone, increased costs, procedure time, and radiation dose [7; 9; 11]. In multiple studies of pulmonary arteriovenous malformations (PAVMs), the use of coils alone was not sufficient and caused a higher rate of both PAVM persistence, re-perfusion, and recanalization of the treated vessel compared to an AVP, often requiring repeat interventions [8; 9; 10; 12]. Several risk factors for recalization of a PAVM when using coils are proximal coil placement, oversizing, and using too few coils. This suggests that the choice of embolic device can be a significant factor in determining overall procedure success [11]. The AVP was first developed in order to address many of the issues with coils. The AVP was designed to reduce the number of required embolic devices; a single AVP can replace multiple coils, decreasing procedure and radiation time, as well as associated costs, while achieving better control in high-flow vessels [1; 5]. However, the device is stiff and requires a minimum catheter size of 4 Fr for deployment [2]. This significantly limits its utility in distal and small vessels. In addition, the time to complete occlusion for both coils and AVPs can be a limiting factor. Multiple studies reported occlusion times ranging from 1 to 20 min, with thrombosis occurring gradually over time [6; 11]. The MVP consists of a detachable and re-sheathable soft nitinol skeleton with a PTFE coating. It is deployed through a microcatheter. This allows for precise and near-immediate embolization of extremely tortuous or small vessels and addresses some of the shortcomings of an AVP without sacrificing functionality [7]. Using an MVP limits both the number of \"judgement calls\" in deciding the size, number, and packing density of the coils [11]. However, the MVP itself does have several disadvantages as it requires precise sizing, a straight landing zone, and suboptimal radio-opacities [2]. Under-sizing of the device is particularly dangerous as it can lead to partial occlusion of the vessel, as well as increased risk of device migration [5]. Compared to coils or other plugs, the Caterpillar Arterial Embolization (Caterpillar) device is a newer metallic embolic device, composed of two sets of opposing nitinol fibers for stability and thrombosis with a proximal polyurethane and polyethylene occlusion membrane for rapid occlusion [1]. Initial experience with the Caterpillar device suggests an advantage in terms of improved flexibility, deployment accuracy, and rapid occlusion [1].