CARAT Tutorials: Coronary Segment and Branch Variants


The coronary tree assembly strategy of CARAT is a stepwise selection of coronary branches and sizes from template options presented under the tabs on the right half of the CARAT window. Branches selected from the template tabs are those that follow a ‘usual’ pathway or configuration and have no sub-branches.  The secondary branch selection process described in this tutorial is intended to capture more branch detail when clinically important sub-branching patterns and pathway variants require documentation.

Secondary branch options and variants are revealed by ‘right-clicking’ on the branch or segment in need of alteration. An alternate selection is then made by ‘left-clicking’ on one of the options presented. (In order to see this secondary menu, a Menu Item cannot be active. Active Menu Items are shown in the space at the lower left corner of the CARAT window. If a Menu Item is present in this space, it must first be cleared by ‘left clicking’ on any open space on the assembly palate – left half of CARAT window.)

This tutorial describes branching options and variants for the following:

RV and Acute Marginal Branches

By default, a single mid-RCA RV branch plus an AM branch are included in the basic tree construction. However, there is clinical value in capturing several important variants of these branches.

It may be of value to show that the RV branch is particularly “Large”, “Small” or “Absent”.  Also two varieties of “Dual” RV branches are offered. The most common is a proximal and mid-RCA pairing; however, a mid and distal pairing is useful to recognize and label. (This distal – posterior - RV free wall branch variant is often mistaken for the PDA with implication to revascularization planning.) RV branch variants are shown below.

Figure 1 -  RV Branch Variants

PDA Variants

The initial step in characterizing inferior and inferoseptal arteries begins with selection of the configuration of branches that supply the entire inter-ventricular groove. After dominance is decided upon, the related IV Groove tab is selected (see Figure 2).

Figure 2 – IV Groove Configurations

The top three choices reflect the classsic balanced configurations of LAD and PDA (Types I, II and III). PDA variants for the large, normal and small PDA branches are shown in Figures 3, 4 and 5.

The two IV groove variants shown by the middle icons of Figure 2 represent the following scenarios: left, a Type II LAD is paired with a small PDA; and right, a Type I LAD is paired with a small PDA. In these cases the resulting ‘gap’ in the AV groove is supplied by an  anomalous adjacent branch. (These options are shown in Figures 6 and 7.)

The final two variants represent uncommon anomalies in which a Type I or II apical LAD originates from the proximal RCA.

(a) Normal PDA (Type II LAD) Variants

The diagram below indicates the variants available for a ‘normal’ sized PDA.  A normal PDA spans the basal, mid and apical thirds of the inferior IV groove.

Figure 3 – Normal PDA Variants

 (b) Small PDA (Type III LAD) Variants

The diagram below indicates the variants available for a ‘small’ sized PDA.  A small PDA spans the basal and mid thirds of the inferior IV groove with unbranched, branched and dual variants shown in Figure 4. In the lower right of Figure 4 is a special variant in which the PDA supplies only the basal portion of the AV groove accommodating an exceptionally large Type III LAD (also see Figure 8).

Figure 4 – Small PDA Variants

(c) Large PDA (with small Type 1 LAD) Variants

The diagram below indicates the variants available for a ‘large’ sized PDA that spans the entire inferior IV groove AND wraps around the apex to cover a variable amount of the apical anterior IV groove.

The first variant (left) includes a prominent branch supplying the apical inferior LV segment while the second contains a double (‘serpent’s tongue’) configuration at the apex. Although clearly not large PDA variants, the two options on the right allow for operational short-cuts during the assembly process.  Specifically, these options allow transitioning from a Type I  IV Groove tab to one of the IV groove “gap” configurations shown in Figures 6 and 7.

Figure 5 – Large PDA Variants

(d) Apical inferior IV groove ‘gap’

As indicated in the left central icon in Figure 2, certain IV groove configurations require branches other than the LAD and PDA to complete the interventricular groove pathway.  ‘Right-click’ on the branch that is felt to supply the distal PDA “gap” territory. From the branching options list, select the configuration that applies. The four branches that can supply the distal PDA territory are, from left to right, the RV branch, acute marginal, right posterolateral and obtuse marginal.

Figure  6 -  ‘Remote’ supply of distal PDA

(e) IV groove ‘gap’ at apex (also apical inferior region plus the apex)

As indicated in the right central icon of Figure 2, a supply ‘gap’ can exist in the apex or apex plus apical inferior section of the IV groove. Figure 7 indicates one adjacent branch (acute marginal) and one more remote branch (anterolateral obtuse marginal) that can fill this gap.

Figure  7 -  ‘Remote’ supply of distal PDA + Apex


(f) Type III LAD variants involving inferior IV groove

A Type III LAD supplies the non-cavitary apex and most of the apical inferior segment of the IV groove. In some cases the LAD is even longer, supplying the mid and even the basal portions of the inferior IV groove. Creating these configurations requires an adjustment to the distal LAD and to the small PDA as shown in Figure 8. (The menus in the upper left and upper right of Figure 8 are obtained by ‘left-clicking’ on the distal type III LAD and small PDA, respectively.)

Figure  8 -  Type III LAD Variations


Posterolateral branch variants

Although the original CASS investigators provided for 3 PL branches in their coronary model, we have limited the right dominant, left dominant and co-dominant basic PL configurations to two PL branches. The goal, in so doing, is to align the PL branch designations to the LV segment rows in the “17-segment model” (specifically inferior and Inferolateral). If more PL branches are desired, they can be accommodated within the branching variants described below. (For example, if three PL branches of equivalent size are seen, this can be simulated by selecting a two-branch option with subsequent selection of a ‘dual’ variant configuration for one of the two initial branches.)

(a) Single PL branch

Figure 9 indicates branch variations for size 1, 2 and 3 posterolateral branches. As indicated in the Tutorial on The Template Selection Process, it is assumed that a single branch in this region makes equivalent contributions to the inferior as well as the Inferolateral LV myocardial segments (a size 1 branch supplies the combined equivalent of one LV segment, a size 2 branch the equivalent of two segments and a size 3 branch supplies the majority of three segments).

Figure 9 – PL variants by vessel size

(b) Two PL branches

The Posterolateral coronary artery configuration tabs (RD-PL, LD-PL and CoD-PL) portray PL branch combinations available for right, left and co-dominant arterial systems. When a two-branch option is selected, the rPL branch is assigned to the inferior LV segments while the lPL branch is assigned to the Inferolateral LV segments. The branch variants for rPL and lPL branches are similar in configuration to the variants shown in Figure 9.

Obtuse Marginal Branches

The CASS reporting system also allowed for three branches in the obtuse marginal region. Similar to the posterolateral region, we reduced the branch number allowance to 2 in order to align with the 2 obtuse margin LV region rows , the anterolateral and inferolateral segments. (See heading below entitled Posterolateral and marginal arteries supply inferolateral LV segments. Two two distinct branch types supply the basal and mid inferolateral and apical lateral LV segments, the left posterolateral branch and the inferolateral marginal branch.)

(a) Anterolateral marginal

The size 1 AL variants shown in Figure 10 supply the basal anterolateral LV segment. The dual variant provides a means to convey the existence of more than 2 marginal branches, the extra branch contributing the ALM segments. A thin variant has been found useful (for many other branches as well) to convey the existence of a branch of significant length but a small diameter to rendering it unsuitable for intervention.

Figure 10 – AL Marginal Size 1 Variants

Figure 11 – AL Marginal Size 2 Variants

Figure 12 – AL Marginal Size 3 Variants

The AL Marginal Size 3 variant list includes three unique branch configurations, termed transverse variants. The ALM normally follows a curvilinear path, initially parallel to the AV groove and finally following an axial course over the basal AL, mid AL and apical lateral LV segments. In a transverse variant, the ALM pathway has a final transverse rather than an axial direction terminating in the mid inferolateral segment.

(b) Inferolateral Marginal

Figure 13 – IL Marginal Size 1 Variants

Figure 14 – IL Marginal Size 2 Variants

Figure 15 – IL Marginal Size 3 Variants

The nature of most variant configurations presented for all branches are self-evident, but two shaped deserve special discussion. The first is the double branch, a relatively common branch configuration that has three important and relatively equal terminating sub-branches. The final variant is the asymmetric variant with two important branches, but one that is clearly larger than then other. Somewhat arbitrarily, we have specified that the region supplied by this variant will be divided in a 2/3 – 1/3 proportion between the two branches. Also, the asymmetric variant can be converted into two branches of unequal size, if needed to describe a branch arrangement not provided for in the basic branch templates (see tutorial on vessel editing).

Posterolateral and marginal arteries supply inferolateral LV segments

Two distinct coronary artery configurations can be associated with the basal and mid inferolateral LV segments (see Figure 16 below): (1) Inferolateral marginal branch that originates from the anterior AV groove circumflex and follows a curvilinear pathway before following the long axis of the LV; (2) Posterolateral branch with an almost perpendicular connection to the posterior AV groove portion of the RCA or circumflex. Figure 16 depicts a configuration in which a distal obtuse (inferolateral) marginal branch and a left posterolateral branch both supply the basal and inferior inferolateral LV segments. (Note the regions on the polar plot jeopardized by a severe mid circumflex lesion.)

Figure 16 – Dual IL Supply

Ramus Intermedius (RI)

The ramus intermedius designation is associated with a unique pathway that is between a diagonal and a marginal in its distribution with less variability in its location. A Size 1 RI supplies the basal anterior segment, a Size 2 supplied the basal anterior and mid anterolateral segments and a Size 3 extends its distribution to the apical lateral segment.

Figure 17 – Ramus Intermedius Size 1 Variants

Figure 18 – Ramus Intermedius Size 2 Variants

Figure 19 – Ramus Intermedius Size 3 Variants

Diagonal Branches

Most diagonal branches follow a predictable oblique pathway reflective of the orientation of the underlying superficial myocardial fibers. The diagonal branches are categorized according to their LV myocardial segment of origin – basal, mid or apical.

(a) Basal diagonal (variably called D1 or proximal diagonal)

Figure 20 – Basal (proximal) Diagonal Size 1 Variants

A size 1 branch supplies the majority of one LV segment. Because of the origin and trajectory of the basal diagonal, this arterial distribution is divided between the basal and mid anterior segments. A size 2 basal diagonal continues the size 1 pathway to supply all of the mid anterior and half of the mid anterolateral segments, a total of 2 LV segments. Finally, a size 3 basal diagonal continues the size 2 pathway to include most of the apical lateral LV segment.

As with other coronary branches, diagonal variants include proximal and distal branching, asymmetric branching, double branches, dual arteries and thin shapes. In addition two important variants in the usual oblique vessel pathway are seen. The first is the transverse variant that follows a divergent lateral pathway usually terminating in the mid anterolateral or inferolateral LV segment, depending upon branch size.

The second unique variant is the axial configuration in which the direction of the branch runs parallel to the LAD and interventricular groove. This variant can be unbranched with flow to the anterior LV segments only or it may give discrete side branches the mid anterolateral or apical lateral segments or to both.

Figure 21 – Basal (proximal) Diagonal Size 2 Variants

Figure 22 – Basal (proximal) Diagonal Size 3 Variants

(b) Mid diagonal

Figure 23 – Mid Diagonal Size 1 Variants

The mid diagonal has a size 1 distribution to the mid anterior LV segment, a size 2 distribution that extends to the apical anterior segment and a size 3 variety that continues laterally over the apical lateral LV segment. As with the larger basal diagonal, the mid diagonal can have transverse and axial variants as shown in Figure 24 and 25.

Figure 24 – Mid Diagonal Size 2 Variants

Figure 25 – Mid Diagonal Size 3 Variants

(c) Distal (apical) diagonal

The configurations of distal diagonal variants are shown in Figures 26 and 27. The size 1 variant distribution is limited to the apical anterior segments and the less common size 2 branch extends this distribution area to the apical lateral LV segment.

Figure 26 – Distal (Apical) Diagonal Size 1 Variants

Figure 27 – Distal (Apical) Diagonal Size 2 Variants

Other vessel segments with variant designations

(a) Mid LAD

‘Left clicking’ on the mid LAD segment also activates the secondary segment variant list. Two additional variant options are given, one with a muscle bridge and a second option with smaller calibre vessel.

(b) Apical LAD

In addition to the long Type III variants described in Figure 8, activation of the distal LAD variant list will reveal an apical muscle bridge variant as well as a variation with a prominent side branch to the apical lateral LV segment.

(c) Left main

Options are available for a short (<1 cm) or long (>2 cm) variant

(d) Conus artery

Options available include a large branch and a variant with a separate origin from the right coronary cusp.

(e) Mid AV groove circumflex

Activating the variant list for this segment allows adjustments to the vessel calibre and the addition of a prominent atrial branch.

Key Points