Personalization of CM Injection Protocols in Coronary Computed Tomographic Angiography

Cardiac diseases still are the leading cause of death in the western world. Besides the high mortality of the cardiac disease itself, recent studies have shown an increasing evidence of the prognostic value of cardiac diseases, particularly coronary artery disease (CAD), in a broad variety of other diseases. Especially in patients with oncological pathologies undergoing radiotherapy or lung resection surgery [Ambrogi et al. 2003; Kanzaki et al. 2017; Khakoo et al. 2008; Basacaraju et al. 2002; Fajardo et al. 1972]. Cardiac computed tomography angiography (CCTA) is one of the standard non-invasive imaging techniques allowing imaging of the heart and coronary arteries with a high temporal and spatial resolution. The high sensitivity and negative predictive value (NPV) of CCTA make it a valuable tool in the assessment of CAD. Especially to rule out CAD in patients with low to intermediate risk for CAD [Hendel et al. 2006; Goldstein et al. 2011; Budoff et al. 2008; Meijboom et al. 2008; Roffi et al. 2016].

The aim of CCTA is to achieve a high diagnostic accuracy, which depends on both optimal intravascular enhancement (in Hounsfield Units (HU); minimal 325 HU) and contrast-to-noise ratio (CNR). Optimal intravascular enhancement and CNR depend on multiple factors, such as scan technique (e.g. tube voltage (kV) and tube potential), parameters of the administered contrast material (CM) (e.g. concentration, flow rate) and patient related factors (e.g. body weight (BW), heart rate or cardiac output [CO]) [Bae et al. 2004; Awai et al. 2004]. Dedicated CT protocols are necessary to image the heart and the coronary arteries. These protocols require a correlation to the patient's electrocardiogram (ECG) in order to minimize cardiac motion and deliver a sharp image of the heart and coronary arteries. Depending on the heart rate of the patient, the scan can either be a prospectively ECG-triggered high-pitch or adaptive sequence, or retrospectively ECG-gated helical scan.

Patients with cardiac diseases often have multiple risk factors for developing contrast induced nephropathy (CIN), e.g. diabetes mellitus (DM), advanced age, hypertension, cardiovascular disease and chronic kidney disease. Although the relationship between CTA and CIN has recently come to discussion (AMACING trial; Nijssen et al. 2017), it is still desirable to minimise the CM volume used in these patients. Multiple strategies have been studied to reduce CM volume in CTA imaging [Kok et al 2016; Kok et al. 2015; Kok et al. 2016; Kok et al. 2016, Mihl et al. 2016; Seehofnerova et al. 2015; Hendriks et al. 2016]. For instance, lowering tube voltage to achieve a higher attenuation, the latter explained by the fact that mean photon energy of lower tube voltage settings in the x-ray beam moves closer to the K-edge of iodine (33.2 keV).

The personalisation of injection protocols to the individual patient is gaining more attention in CT imaging, and the goal is to individualise the injection protocols to a level, where the patient only receives the minimal amount of CM needed to make a diagnostic scan with a diagnostic image quality. Besides the techniques mentioned above, other techniques are possible, e.g. adjustment according to patients CO, lean body weight (LBW), and BW.

Both blood volume and CO increase with an increasing BW. When CO increases, the distribution of CM is also increased. On one hand this results in a fastened test bolus arrival, on the other hand in a decreased and shortened intravascular attenuation profile (in comparison to a decreased CO) [Bae KT 2010]. Therefore, both timing and CM volume can be adjusted to CO in order to achieve a similar intravascular attenuation profile.