# Calibrated Hydraulic Resistance Adjuncts for Carbon Dioxide Angiography Optimization
## 1 Introduction  
Carbon dioxide angiography [1] is experiencing significant growth thanks to the placing on the market of injectors capable of controlling all injection parameters and radiological equipment with dedicated protocols [2, 3]. Many of the clinical innovations were born in laboratories and operating rooms. An accurate bench-based evaluation of what happens in biomechanical terms for optimizing procedures has rarely been carried out in-depth, and some aspects can be improved [4, 5, 6]. Nowadays, automatic constant pressure CO\({}_{2}\) injectors permit the setting of the optimal gas input flow (mL/s) in a vascular cavity to obtain good radiological imaging with acceptable patient discomfort [2, 7, 8, 9, 10]. CO\({}_{2}\) angiography differs from traditional angiography with an iodinated contrast medium for three main reasons: (1) The iodinated contrast medium mixes with the blood, while the CO\({}_{2}\) displaces and replaces it. (2) Gas bubbles behave differently depending on the vessel's size and blood flow. (3) CO\({}_{2}\) injection catheters have very low hydraulic resistance, reducing the possibility of controlling gas injection by acting on pressure, which has an impact on imaging quality. To optimize CO\({}_{2}\) imaging, it is essential to know the behavior of the injected gas bubbles within the bloodstream and, therefore, to change the injection mode depending on the site where the CO\({}_{2}\) is being injected. We can consider three cases: huge cavities, medium-sized arteries and veins, and small diameter vessels (<6 mm). In all biomechanical conditions, to have a good radiological image, the easiest way is to fill the blood vessel with gas as much as possible so that CO\({}_{2}\) replaces the blood in the portion of interest. However, the blood moves with pulsatile pressure and flow, complicating the procedure. As far as large arteries are concerned [11; 12; 13], it is necessary to inject a large amount of gas to fill the cavity as much as possible, and this is carried out using high injection pressures, generating pain and discomfort for the patient [14; 15; 16]. For arteries and veins with a diameter greater than 6 mm, imaging optimization occurs mainly by optimizing the gas injection flow according to the blood flow in the vessel [5]. In the case of small blood vessels (diameter < 6 mm), the gas bubbles fill their entire lumen. If the injection gas flow is lower than blood flow, the vessel fills partially and presents areas with gas alternating with regions of blood flowing along the duct. In this case, if the subtraction angiography is not performed correctly, there is a risk of a suboptimal visualization of the area of interest. Suppose large quantities of gas are injected, and gas flow overcomes the blood flow. In that case, reflux can push the blood backward to the large arteries with a local increase in pressure at the injection site and possible pain for the patient (with consequent movements and artifacts on final images). The optimal solution is to control the injection flow better, setting it slightly lower than blood flow. However, this is very difficult to achieve because, with catheters with low hydraulic resistance, minor variations in pressure due to vascular pulsatility are sufficient to modify what happens inside the blood vessel.