**3. Technical development**

#### **3.1. Radiation dose**

*2.1.2. Coronary artery stent*

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

5 Occluded

200 Interventional Cardiology

According to SCCT guidelines.

*2.1.3. Coronary artery bypass graft*

According to SCCT guidelines.

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

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,

3 Moderate Plaque with moderate stenosis that may be of hemodynamic significance

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.

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.

0 Normal Absence of plaque and no luminal stenosis

1 Minimal Plaque with <25% stenosis

 Mild 25–49% stenosis Moderate 50–69% stenosis Severe 70–99% stenosis Occluded 100% stenosis

while stent patency was assumed when the contrast enhancement matched [15].

0 Normal Absence of plaque and no luminal stenosis 1 Minimal Plaque with negligible impact on lumen 2 Mild Plaque with mild narrowing of the lumen

4 Severe Plaque with probable flow limiting disease

Since its introduction into clinical use, a constantly mentioned point of criticism of cCTA is the radiation the patient is exposed to. While referring to this topic, one should distinguish the terms "radiation exposure," which describes the radiation emitted by the X-ray source, and "radiation dose," which indicates the amount of radiation absorbed by the patient [25]. The early concerns were not unjustified, as the novel scanners with 16 or 64 slices showed radiation doses above 10 mSv, even up to 21 mSv [26, 27], and radiation resulting of CT examinations make up a large share of the populations radiation exposure [28]. But subsequently, a substantial reduction of the applied radiation doses was achieved by different approaches: cCTA images are usually acquired using retrospective ECG gating, which requires a lower pitch and a longer duration, resulting in higher doses than prospective ECG triggering. Dose reduction is acceived using ECG gating or implementation of suitable ECG-triggering protocols. The first option is realized through ECG-dependent tube current modulation. The best image quality is obtained in the late-diastolic phase of the heart cycle; therefore, the tube current can be decreased in the remaining phase, resulting in a radiation dose lowered up to 50% [29, 30]. Under certain circumstances, it is possible to perform cCTA by prospective ECG triggering and sequential scanning. Patients with a low and stable heart rhythm and without an indication for functional testing are qualified for this technique in line with SCCT guidelines [31]. This attempt could reduce the radiation dose to 70–80% [29, 32]. Both options are optimal if either a scanner with 256 or more slices or a DSCT is used. Furthermore, use of DSCT enables further decrease due to its higher pitch rates at higher heart rates, since multisegment reconstruction is not necessary [33, 34]. Additional reduction is accomplished by a tube voltage of 100 kV or even 80 kV instead of the usual 120 kV, which can be performed depending on the patient's body mass [30, 31]. The image postprocessing technique of iterative reconstruction (chapter 3.2) also contributes to reduction of radiation dose. With all these measures taken into consideration, cCTA reached radiation doses lower than 4 mSv, therefore being in the range of the average yearly background radiation dose, in certain conditions even in submillisievert range [35, 36].

#### **3.2. Image reconstruction**

Nowadays, two methods of image reconstruction are in use, analytical filtered back projection (FBP) and iterative reconstruction (IR). The initially used technique was indeed the more complex IR [37], but soon its use was limited by the computational power of erstwhile processors. The method was displaced by FBP, which still is the most widely used technique nowadays.

In FBP, the measured intensity is described as an integral function, and the reconstruction data is obtained through solution of the resulting equations, which is called back projection. Additionally, a filter component compensates low-pass signals. If a higher spatial resolution is required, the filter can be adjusted accordingly. However, this adaptation of the filter causes a higher image noise, since image sharpness and image noise are proportional [38].

IR seeks to solve this problem, and since nowadays, not only CT hardware but also software underwent enormous advances, complex computational operations are more and more available. Iterative reconstruction accomplishes the back projection through the comparison of two components; a simulated first image estimation on the one hand and the actual measured projection on the other hand. Both images are automatically compared and, in case of discrepancy, the estimation is altered and another comparison is made until a default condition is achieved [38]. The underlying complex mathematical algorithms are propriety of the respective companies. Not only was IR able to break the correlation between image noise and spatial resolution, but it does so while simultaneously reducing the applied radiation dose up to 40–70%, while maintaining or even increasing subjective image quality and diagnostic accuracy [39–42].
