Fully Automated Scan Technique Optimisation of Scan Timing in Chest CT

Computed Tomography Angiography (CTA) is a non-invasive imaging tool widely used for various indications. Contrast media (CM) is used to enhance the intravascular lumen and organ parenchyma, depending on the indication. Recent technical advances in CT scan techniques allow for a very fast scan acquisition with substantially increased image quality in terms of temporal and spatial resolution. These faster scan times account for a significant reduction in radiation dose, which is desirable in light of the "As Low As Reasonably Achievable" (ALARA) principle. Another advantage of the newer 'high-end' scanners is the use of lower tube voltages and lower CM volumes, since many studies have shown that CM volumes can be reduced with usage of lower tube voltages.

However, with faster scan acquisition, challenges arise with regard to CM bolus timing. The risk of outrunning the CM bolus in these fast acquisitions is higher, subsequently leading to a decreased or even non-diagnostic enhancement (in Hounsfield Units (HU)). In addition, decreased CM volumes due to usage of lower tube voltages also add to the risk of outrunning the bolus. When reducing the CM bolus, the injection time decreases and the window of peak enhancement is shorter and more narrow. Also, when injecting these smaller CM volumes at higher flow rates, although the peak enhancement is increased, the window of peak enhancement decreases more rapidly. Thus, when administered with the same flow rate, the peak of the enhancement curve will be lower, narrower and faster compared to larger CM volumes. This, in combination with the faster scan acquisition makes the timing of the start of the scan (scan start delay) highly important, since scanning at the peak enhancement is necessary to achieve a diagnostic image quality.

To determine scan delay, two techniques frequently used in daily clinical routine are the 'test bolus' and 'bolus tracking' technique. With the first, a smaller CM bolus is administered before the actual scan, and the time to peak of the intravascular enhancement is determined with help of dedicated software (DynEva, Siemens Healthineers, Forchheim, Germany). When using the 'bolus tracking' technique, no additional CM volume is administered. A region of interest (ROI) is placed in a large artery of interest (e.g. ascending or descending aorta), and a threshold enhancement is set prior to the scan (e.g. 100 HU). Repetitive low dose scans are acquired at the same level and the arrival of the CM bolus is followed. Once the threshold is reached, the scanner automatically starts the scan. Between reaching the threshold and the actual start of the scan, a manual post-tracking delay is set prior to scanning. This delay is necessary for both the table movement of the scanner to the start of the scan and the breath hold command. The problem is that this manual post-tracking delay is set prior to the scan, without information of the patient's cardiovascular dynamics (e.g. cardiac output). Since cardiac output can vary greatly inter- and intra-patient, this fixed post-tracking delay may not be appropriate for all patients. Scanning with a sub-optimal post tracking delay could potentially result in suboptimal arterial enhancement and insufficient diagnostic quality.

With new bolus tracking auto-delay software (Fully Automated Scan Technique, FAST, Siemens Healthineers) the incidence of scans made at a suboptimal attenuation could be reduced. This software is similar to the 'bolus tracking' technique, the difference is that the manual post-tracking delay is calculated automatically by the software. During the low-dose repetitive scans at the level of the ROI, the attenuation in the ROI is used to predict the optimal enhancement curve. The software takes the injection protocol, tube voltage and patient parameters into account. A previously acquired database of numerous enhancement curves is consulted to predict a best fitted enhancement curve of the individual patient. The software then calculates the optimal post-tracking scan delay to scan at the peak enhancement. Thus, the optimal individual scan delay and enhancement based on the patients physiology can be achieved, and the risk of non-diagnostic scans should decrease. Therefore, this study aims to evaluate the performance of the FAST software in patients receiving standard chest CT with regard to the number of non-diagnostic scans (< 300 HU) and compare this with standard care (manual set pre-scan delay).