# Calibrated Hydraulic Resistance Adjuncts for Carbon Dioxide Angiography Optimization
## 4 Discussion  
It is well known that the image quality of CO\({}_{2}\) angiography is limited by the low contrast of the gas, and the operators try to inject as much gas as possible in the shortest time to fulfill the target vascular cavities. The hydraulic resistance of the injection line has been seen as the main limitation to obtaining a rapid filling of large vascular cavities, and some tricks have been suggested to overcome it. Why do we suggest increasing the injection line resistance by inserting an adjunct tube in series? By using new automatic gas injectors [13,25] with settable injection pressure, it has been demonstrated that in small diameter vessels (leg arteries), using standard catheters, the best imaging is obtained with low injection pressure, slightly higher than the maximum arterial value [2]. Unfortunately, this setting has the drawback of arterial pressure pulsatility, which hardly modulates the gas injection in an undesirable way. In fact, the gas input flow results are high when the blood flow is low, producing a nonregular vascular gas filling with gas-filled vascular sectors followed by gas-empty ones. A solution to compensate for the effect of the physiological pulsatility is to increase the injection pressure, increasing the hydraulic resistance of the line to maintain the gas inflow at the desired value. With this solution, the gas bubble sequence changes towards a more regular distribution and possibly better imaging. This solution is convenient only for small vessels where a low gas inflow is required. The gas injection flow must be very high for large cavities and vessels with high blood flow, and no resistance adjunct is permitted. This separation of radiological targets with different procedural approaches is not easily understood by clinicians, especially if unusual physics concepts are involved. For this reason, we simulated the clinical context and produced \"visible\" evidence of the expected benefits. Apart from the methods used to build the resistance adjuncts and to measure the resistance values for CO\({}_{2}\) gas fluid, this approach permits putting the CO\({}_{2}\) angiographic procedure in a more rigorous scientific frame, where the operator can move quantitatively to the desired results. For CO\({}_{2}\) angiography, described as a highly operator-dependent procedure, the possibility to control each step represents a revolution. This revolution can be possible only thanks to the availability of settable pressure and volume automatic injectors and a more precise biomechanical description. This paper focused our analysis on medium-sized arterial vessels (iliac/femoral arteries). Still, the same process can be moved to smaller arterial vessels (pedidial arteries) and some venous applications (arterirovenous artificial fistula, transcatheter chemoembolization, etc.) with a significant possibility to improve the diagnostic and therapeutic results by optimizing the gas injection process. Regarding the results of this paper, the most important one is the correspondence between what was obtained and visually observed and what was calculated and desired. It was a pleasure to observe how the different resistance adjuncts modify the bubbles' shapes and sequence and how the different sequences may produce useful diagnostic images, with contrast gas saving, lower local pressure increases (mean increases of 7 mmHg vs. 13 mmHg for systolic and 10 mmHg vs. 25 mmHg for diastolic pressures), and the same time interval to produce a complete image of the considered district (time differences: 0 s for P\({}_{\text{inj}}\) = 206 mmHg; 0.2 s for P\({}_{\text{inj}}\) = 234 mmHg; and 0.6 s for P\({}_{\text{inj}}\) = 288 mmHg). The injections without Radj produced significant reflux (>24 cm) and bigger bubbles compared to what happened with the adjunct resistances (reflux of about 6 cm). Moreover, the delays between injection start and gas output with saline-filled catheters are comparable (6 s and 7 s). This last evidence seems incomprehensible: higher line resistance, same gas output delay!