Coronary anatomy and segment nomenclature (SCCT 2026)

In practice. Each segment is reviewed in long-axis and cross-sectional planes, and plaque type and stenosis grade are recorded for each one. The report uses the anatomic name, such as mid LAD, not the segment number.

Pitfall. Writing "segment 7 stenosis" in the report forces the referring physician to look up the diagram. The guideline recommends stating the anatomic location.

Other sources. The angiographic SYNTAX scheme also derives from the AHA model, but it uses 16 segments with subsegments, such as 9a or 12b, and numbers some branches differently: the ramus intermedius is 12, not 17. (Sianos 2005, SYNTAX, Table 1 and Fig. 1, pp. 220–221)

In practice. It is reviewed in long-axis and cross-sectional planes from the ostium to the carina of the bifurcation. The guideline notes that a normal reference segment is hard to find in the left main. In that case, stenosis can be graded using normal reference diameters.

Pitfall. Recording a stenosis at the LM bifurcation in the proximal LAD. In the LM, a 50–99% stenosis is CAD-RADS 4B; in the proximal LAD, a 50–69% stenosis is CAD-RADS 3.

Other sources. The interventional literature often divides the LM into ostial, shaft and distal (bifurcation) portions. SCCT 2026 treats it as a single segment and recommends stating the location in anatomic terms, not by number alone. (Dąbrowski 2022, section 2.3.5; SCCT 2026, p. 4)

In practice. On axial and oblique images of the aortic root, count the ostia in each sinus and follow each artery from its ostium. Without a left main there is no segment 5, so plaque at the ostium of the LAD or LCx is assigned to its proximal segment. SCCT 2026 recommends reporting anatomical variants (Table 6, p. 20). Ganga 2021 discusses it as a variant of little clinical importance but notes that it should be known before coronary bypass or aortic root surgery. At catheterization, if only one of the two ostia is engaged, the other artery may appear occluded or absent (Singh 2026).

Pitfall. Calling an R-III single coronary artery, in which the LAD and LCx arise separately from the proximal RCA and the left sinus has no ostium, an absent left main. One published case uses that name in its title and then classifies the anatomy as Lipton R-III (Barman 2024).

Other sources. In 7,694 adult CCTA studies, 0.41% had no left main (Ganga 2021), and in 3,181, 0.16% (Karatoprak 2026). Ganga 2021 classifies it among anomalies of origin and course, in a table adapted from Angelini. The CAD-RADS 2.0 text does not mention it when describing modifier E. (Ganga 2021, Table 1 and Results; Karatoprak 2026, abstract; CAD-RADS 2.0, section 3.3.6)

Source: Ganga 2021, Table 1, and Singh 2026 (absent left main); SCCT 2026, section j, p. 12, and Barman 2024, Discussion (anomalous LCx); Al Umairi 2019, Discussion, and Elbadawi 2018, Table 1 (single coronary artery); tabs verified on the map on 2026-10-09.

In practice. On the curved reconstruction of the LAD, find the first large septal branch, where the proximal segment ends. Then estimate the midpoint of the remaining course to the apex; that point separates the mid from the distal LAD.

Other sources. SCCT 2014 ended the proximal LAD at the first large septal branch or at D1, whichever arose first. The 2024 ASCI-PT consensus applied that definition: when a D1 larger than 1.5 mm arises very close to the LAD ostium, it classified the segment beyond D1 as the mid LAD (moderate agreement, 64.7%). This glossary follows SCCT 2026, which uses only the septal branch. (SCCT 2014, Appendix 1, p. 358; Kim 2024, ASCI-PT, Issue 2)

In practice. Review every diagonal of 1.5 mm or more and pick the two largest. Of these, the more proximal one is D1. The guideline also asks that the number and size of branch vessels be described.

Other sources. SCCT 2014 and SYNTAX name as D1 the first diagonal to arise, not the first of the two largest. In SYNTAX, D2 arises from the distal LAD or at its junction with the mid LAD. (SCCT 2014, Appendix 1, p. 358; Sianos 2005, SYNTAX, Fig. 1, p. 221)

In practice. If the LAD appears to end high in the anterior interventricular groove, look for the long LAD on curved reformats and volume-rendered images: next to the groove, on the left or right ventricular side, within the septum or, if it arises from the RCA, in front of the right ventricular outflow tract. According to SCCT 2026, the septal branches arise from the short LAD and the diagonals from the long LAD, but in Table 1 of Moulton 2013 this varies by type, so it is worth stating which LAD each diagonal arises from. Table 1 of SCCT 2026 numbers a single LAD and does not say how to divide segments 6, 7 and 8 between the two arteries. The guideline recommends locating lesions with anatomical terms and not by segment number alone (p. 4); here, that means naming the short or long LAD and its portion.

Pitfall. Reporting a mid LAD occlusion without having looked for the long LAD, when the artery that ends high in the groove is the short LAD. SCCT 2026 warns of this error, especially when the long LAD arises from the RCA.

Other sources. SCCT 2026 mentions other variants and cites a type X (Celik 2015), and the expanded table of Maggialetti 2023 reaches type XIII, with two long LADs. Published figures vary widely: 0.07% in 7,694 CCTA studies (Ganga 2021) and 5.96% in 2,080 (Maggialetti 2023), where a quarter of the cases were type XIII. (SCCT 2026, p. 12 and reference 93; Maggialetti 2023, Table 1 and Results; Ganga 2021, Results and Discussion)

Source: SCCT 2026, text, nonatherosclerotic coronary artery abnormalities (section j), p. 12 (the four forms); Moulton 2013, Table 1, and Maggialetti 2023, Table 1 (Spindola-Franco 1983 numbering; the original could not be consulted).

In practice. Before reporting a ramus intermedius, follow the branch back to its origin and confirm that the left main trifurcates. Without a trifurcation, the 2024 ASCI-PT consensus classifies an LCx branch larger than 1.5 mm that arises near the bifurcation as OM1 (76.4% agreement). If the branch arises from the LAD and traverses the anterolateral wall, it is a diagonal.

Other sources. Some left main PCI studies define a trifurcation by the distance between branches: Tamburino 2009 required two side branches of 2.25 mm or more whose origins were less than 3 mm apart. Neither the SCCT CCTA guidelines nor the 2024 ASCI-PT consensus uses a distance; SCCT only requires that the left main trifurcate. (Tamburino 2009, Methods; Kim 2024, ASCI-PT, Issue 3)

In practice. The only boundary is the origin of OM1, found on the curved reconstruction of the LCx. What lies before it is the proximal LCx, and what continues within the atrioventricular groove is the distal LCx.

Pitfall. Writing “mid LCx”: SCCT 2026 does not define that segment. Everything beyond the origin of OM1 within the atrioventricular groove is the distal LCx.

Other sources. SCCT 2014 called segment 13 the "mid and distal LCx". In the SYNTAX scheme, the distal LCx begins after the origin of the last obtuse marginal, not OM1. This glossary follows SCCT 2026. (SCCT 2014, Appendix 1, p. 358; Sianos 2005, SYNTAX, Fig. 1, p. 221)

In practice. Follow them from the LCx to the lateral wall and number them by order of origin, unlike the diagonals, which are chosen by size. Find OM1 first, because its origin sets the boundary between the proximal and distal LCx.

Other sources. In the SYNTAX scheme, the obtuse marginals are segments 12a and 12b, and 14 is the left posterolateral branch. This glossary follows SCCT 2026, where OM2 is 14 and the left posterolateral branch is 18. (Sianos 2005, SYNTAX, Table 1 and Fig. 1, pp. 220–221)

In practice. The landmark is the acute margin of the right ventricle. The midpoint of the course to that point separates the proximal from the mid RCA and may coincide with a right ventricular branch. If there is no R-PDA, the distal RCA ends at the interventricular groove.

Pitfall. The RCA is not divided into thirds of length. Table 1 sets its boundaries by the acute margin of the right ventricle and the origin of the R-PDA.

Other sources. The 28-segment CASS model, optional in SCCT 2014, numbers the proximal, mid and distal RCA the same way (1, 2 and 3), but segment 5 is the right posterior atrioventricular segment (RPAS) and the left main is 11. (SCCT 2014, section 5.3 and Table 5, CASS model)

In practice. It is found at the acute margin of the right ventricle. As with other segments, plaque in it is described by its anatomic name. If the branch measures less than 1.5 mm and the plaque is clearly seen, describe it and note the small caliber.

Other sources. Anatomy atlases and textbooks call the acute marginal the right marginal branch of the right coronary artery (Latin ramus marginalis dexter). It is the same artery; this glossary uses the SCCT 2026 name. (IMAIOS e-Anatomy)

In practice. Follow each branch back to its origin on the curved reconstruction, because the vessel of origin defines its name. For the posterior descending artery, the origin also determines dominance. In a codominant circulation with both an R-PDA and an L-PDA, each is assessed as its own segment.

Other sources. In the SYNTAX score, the left posterolateral branch is segment 14 and the right posterolateral branches (16) are 16a, 16b and 16c. In SCCT 2026, 14 is OM2 and the L-PL is 18. (Sianos 2005, SYNTAX, Fig. 1; SCCT 2026, Table 1)

In practice. Follow the posterior descending artery along the posterior interventricular groove to its vessel of origin. Establish dominance before assigning lesions in the posterior branches. Table 6 gives a strong recommendation to state it in the findings.

Other sources. Some texts call the circulation balanced or codominant when the posterior descending artery arises from the RCA and the posterolateral branches from the LCx. The SYNTAX score allows only right or left dominance. This glossary follows SCCT 2026. (Halpern 2011, p. 52; Sianos 2005, SYNTAX)

In practice. A focal lesion at a junction is recorded only in the preceding segment. A plaque that continues along the next segment is recorded in both: each segment adds one point to the SIS and carries the grade of its own maximal stenosis, because stenosis is assessed per segment. The guideline does not state how far a plaque can extend and still count as a junction lesion; this glossary counts it as one when it does not extend beyond the carina or the branch ostium. Near bifurcations, SCCT 2026 recommends location descriptors that are more intuitive than segment numbers alone and allows attributes such as “ostial” and “bifurcation”.

Pitfall. Duplicating a focal junction lesion in both segments, which inflates the SIS for no reason, or recording the grade of the maximal stenosis in every segment a long plaque occupies. If a plaque runs from the LM into the proximal LAD and the 50–69% stenosis is in the LAD, recording that grade in the LM as well creates a left main stenosis that by itself is CAD-RADS 4B.

Other sources. The 2024 ASCI-PT consensus, with unanimous agreement of its panel, classifies lesions within the bifurcation toward the LAD or LCx as part of the LM, which gives the same result as the preceding-segment rule. In the cath lab, bifurcation lesions are usually described with the Medina classification: three digits, 1 or 0, according to whether the proximal main vessel, the distal main vessel and the side branch are involved. It does not replace assignment to the preceding segment. (Kim 2024, ASCI-PT, Issue 1 and Fig. 1; Medina 2006)

In practice. Diameter is estimated in the short axis. When a segment of 1.5 mm or more cannot be interpreted, name it and explain why. In a small vessel with clearly visible plaque, describe the plaque and note the small caliber.

Other sources. CAD-RADS 1.0 and 2.0 grade stenosis in vessels 1.5 mm or more, without regard to scan quality. In the SYNTAX score, vessels smaller than 1.5 mm are not considered candidates for PCI or surgery. (CAD-RADS 1.0, Tables 2 and 3; CAD-RADS 2.0, Tables 4 and 5; Sianos 2005, SYNTAX, p. 223)