Feasibility and optimization of the combined injection of contrast medium and normal saline with a single-syringe CT injection system

Purpose: To optimize the technique of injecting contrast medium followed by saline flush with a single-syringe CT injection system. Materials and Methods: A CT injection system with a single syringe was filled with 100 ml of contrast medium (Iopromide 300 or Iomeprol 400) and 50 ml of normal saline solution. The following variables were evaluated: filling sequence, filling rate, syringe position during filling, ejection rate and delay before ejection. In consecutive portions of 10 ml, the whole volume was continuously ejected into test tubes that were subjected to CT densitometry on these test tubes. Each experiment was performed three times. Results: Filling the syringe first with saline solution and subsequently with contrast medium resulted in no substantial mixing, whereas filling in the reverse order caused noticeable mixing of both solutions. Manual filling at a rate ≤1 ml/s gase the best results. At faster filling rates (4 ml/s, 10 ml/s), substantial mixing of contrast agent and saline solution was observed. The separation of the two liquids was optimal when the syringe nozzle was pointed downward. Ejection rate (3 ml/s, 4 ml/s) and time delay between filling and ejection (0 min, 10 min) did not influence the distribution of density. The maximum iodine concentration using this approach was 235 mg/ml for Iopromide 300 and 327 mg/ml for Iomeprol 400. This indicates a reduction of the maximum iodine concentration of up to 28% even when using the optimal technique for controlling mixing of both liquids. Conclusion: Manual filling of the CT injection syringe with saline solution followed by contrast media with the syringe nozzle pointed downward provides the best separation between contrast medium and normal saline solution but reduces the maximum iodine concentration by up to 28%. For examinations requiring a high iodine concentration, such as CT-angiography, a iodine concentration of 400 mg/ml should be selected for the combined injection of contrast medium and normal saline solution with a single-syringe CT injection system. (orig.)