**2. Coronary CT-angiography**

#### **2.1. Indication**

Despite its many advantages, cCTA is only one out of many clinically approved methods to examine coronary arteries. Although there are notable technical developments in evaluating functional parameters as well [6–8], the current indication is predominantly the investigation of anatomical and morphological vessel characteristics. Especially in the exclusion of coronary artery disease (CAD), cCTA plays a decisive role [9–11]. Patients presenting with symptoms of CAD and low-to-intermediate risk patients undergo rapid evaluation of their coronary arteries. To estimate the suitable method for the individual patient, pre-test riskstratification calculation plays a key role. For this purpose, Diamond-Forrester (**Table 1**) [12] and Genders (**Table 2**) [13] are well-established charts to obtain a pre-test probability of CAD based on age, sex, and chest pain constellation. However, further established cardiovascular


**Table 1.** Pre-test likelihood of CAD in symptomatic patients according to age and sex.


**Table 2.** Updated pre-test likelihood of CAD in symptomatic patients according to age and sex.

risk factors such as smoking, dyslipidemia, hypertension, diabetes, and family history of cardiac diseases should be considered in the risk stratification as well. Depending on the individual risk constellation, cCTA may be the suitable modality in low-to-intermediate risk patients, as for high-risk patients, invasive coronary angiography remains still the gold standard, as recommended by the ESC/AHA/ACC guidelines [9, 10]. Due to the three-dimensional visualization that can be constructed by cCTA, it can also be even used in planning and evaluating coronary artery bypass grafts (CABG) and detecting in-stent restenosis (ISR).

#### *2.1.1. Suspected coronary artery disease*

electron beam computed tomography (EBCT), which already had scan times lower than 100 ms [1]. Clinical relevance of the coronary CT angiography (cCTA) increased distinctly with the introduction of multi detector CT (MDCT) in the late 1990s—initially with four parallel detectors, the launch of the 64-slice MDCT generation enabled cCTA to become established in routine clinical practice [2, 3]. Nowadays, there are systems with up to 320-slices in clinical use, providing even lower scan times and a very high spatial resolution. Another landmark development was the introduction of the dual-source CT (DSCT) technology. DSCT contains of two tubes and detectors arranged in a 90° angle, also resulting in a higher temporal resolution due to the halved rotation time. The dual-energy CT (DECT) scans allow two different tube voltages, resulting in a significant lower radiation exposure for the patient [4]. As spatial and temporal resolution achieved remarkable dimensions, recent technologic improvement emphasized particularly the reduction of radiation dose on the one hand (see Section 3.1) [5], and the expansion of cCTA on additionally functional and morphological aspects, e.g., plaque characterization, myocardial perfusion imaging, or even CT-based fractional flow

Despite its many advantages, cCTA is only one out of many clinically approved methods to examine coronary arteries. Although there are notable technical developments in evaluating functional parameters as well [6–8], the current indication is predominantly the investigation of anatomical and morphological vessel characteristics. Especially in the exclusion of coronary artery disease (CAD), cCTA plays a decisive role [9–11]. Patients presenting with symptoms of CAD and low-to-intermediate risk patients undergo rapid evaluation of their coronary arteries. To estimate the suitable method for the individual patient, pre-test riskstratification calculation plays a key role. For this purpose, Diamond-Forrester (**Table 1**) [12] and Genders (**Table 2**) [13] are well-established charts to obtain a pre-test probability of CAD based on age, sex, and chest pain constellation. However, further established cardiovascular

**Non-anginal chest pain Atypical angina Typical angina Age Men Women Men Women Men Women 30–39** 5.2 ± 0.8 0.8 ± 0.3 21.8 ± 2.4 4.2 ± 1.3 69.7 ± 3.2 25.8 ± 6.6 **40–49** 14.1 ± 1.3 2.8 ± 0.7 46.1 ± 1.8 13.3 ± 2.9 87.3 ± 1.0 55.2 ± 6.5 **50–59** 21.5 ± 1.7 8.4 ± 1.2 58.9 ± 1.5 32.4 ± 3.0 92.0 ± 0.6 79.4 ± 2.4 **60–69** 28.1 ± 1.9 18.6 ± 1.9 67.1 ± 1.3 54.4 ± 2.4 94.3 ± 0.4 90.6 ± 1.0

Each value represents the percentage ± 1 standard deviation. Adapted from Diamond et al. [12].

**Table 1.** Pre-test likelihood of CAD in symptomatic patients according to age and sex.

reserve (CT-FFR).

198 Interventional Cardiology

**2.1. Indication**

**2. Coronary CT-angiography**

cCTA is excellent in visualizing coronary morphology and has emerged to an appropriate method of ruling out obstructive CAD. But by cCTA alone, the pathophysiological relevance of a detected CAD remains often unclear. Despite the remarkable advancements regarding functional parameters as for example perfusion imaging achieved by new DECT approaches, many conventional cCTAs show a rather moderate specificity regarding the functional assessment of cCTA measured stenosis. The methodical approach, as proposed by the SCCT guidelines for the interpretation and reporting of cCTA, consists of a systematic inspection of each coronary segment in multiple planes, the contemplation of image quality and artifacts and finally the evaluation of the respective lesions in regard of morphology, composition, and stenosis severity. A modified version of the well-established 1975 American Heart Association (AHA) model is used to refer to the certain segments [14]. Coronary abnormalities, plaque description or insufficient interpretability due to artifacts should be mentioned. Following this, a qualitative assessment for each segment is obtained and should be reported according to **Table 3**. Subsequently, a quantitative assessment of the stenosis severity is performed; the findings should be reported according to **Table 4**.

It has to be mentioned that these classifications are founded on morphological features only and, based on these findings, conclusions about functional or ischemic insufficiencies are not to be inferred.


**Table 3.** Descriptors of qualitative stenosis severity.

#### *2.1.2. Coronary artery stent*

Due to the limited spatial resolution of the first electronic beam CT, it was initially not possible to visualize of the stented lumen and an indirect approach was applied to assess the stent patency. For this reason, contrast density was measured distally to the stent and compared with the density pattern proximal to the stented segment, in the aorta or the left ventricle, while stent patency was assumed when the contrast enhancement matched [15].

With the introduction of 64-slice scanners, a high negative predictive value could be reached for the evaluation of in-stent restenosis, while the positive predictive value is still rather worse as demonstrated by meta-analysis [16, 17]. However, there are specific technical limitations such as blooming caused by metal artifacts resulting in an underestimation of the stent lumen.

#### *2.1.3. Coronary artery bypass graft*

The value of cCTA in the assessment of coronary artery bypass graft (CABG) and native coronary arteries after bypass graft surgery continues to grow with advances in CT technology [18, 19]. The improvement of spatial resolution allows the cardiovascular radiologist and cardiac surgeon to evaluate the patency of CAGB in a rapid and noninvasive manner [20]. The major advantage of cCTA over invasive angiography is the ability to simultaneously evaluate for alternate postoperative complications like malposition, kinking, or pericardial effusion.


**Table 4.** Descriptors of quantitative stenosis severity.

#### **2.2. Benefits and limitation**

The main benefit of cCTA is its noninvasive character. Although invasive coronary angiography (ICA) is an approved and secure procedure, it still involves the possibility of serious complications such as bleeding, stroke, or coronary dissection [6]. In comparison, the risks of cCTA, such as extravasation or allergic reaction to the contrast agent are less severe and common. As previously mentioned, cCTA is able to rule out CAD with excellent sensitivity and negative predictive value, both up to 99% in several studies [9, 21, 22]. Therefore, a preceding cCTA can reduce the share of unnecessarily performed ICA [11]. On the other hand, currently, the moderate specificity of cCTA causes a following ICA to validate the findings [9]. Recent developments seek to solve this issue. Further limitations result from technical conditions of computed tomography:

Although the temporal resolution has achieved levels below 80 ms, it is still necessary for the patient to maintain a heart frequency under 70 beats per minute to obtain a sufficient image quality. This might be accomplished using beta-blockers, but not all patients are suitable for auxiliary agents. Regarding patients who are unable to follow breathing orders, but especially patients with cardiac arrhythmias, prospectively electrocardiogram (ECG)-triggered images are prone to artifacts. New approaches in ECG triggering seek to react flexibly to arrhythmia but have to be implemented in the clinical routine. Retrospectively ECG-gated image acquisition is less interference-prone, but is along going with higher radiation doses. ECG-dependent dose reduction is required. Furthermore, a high coronary calcification or iatrogenic metallic material may lead to so-called blooming or streak artifacts, which tend to over-estimate the severity of stenoses [23, 24]. A better temporal resolution, acquired e.g., by using DSCT allows reduction of blooming artifacts. Radiation dose represents another important disbenefit of cCTA, which is explained later in detail.
