Angiographic Projections Made Simple: An Easy Guide to Understanding Oblique Views
Every year, we get a new group of cardiology fellows-in-training who are unfamiliar with angiograms and the angulations needed to best display the coronary arteries. Almost as frequently are the visitors to the cath lab or new trainees in the cath lab who are in need of an orientation to the angiogram, and its specific angulations and abnormalities.
For the past several years, whenever I give an introductory lecture on the cath lab and angiography, I use a simple method to show how the heart and the arteries move when changing from anterior-posterior (AP) projection to the left and right anterior oblique views (LAO, RAO) with cranial and caudal angulation. The method simply uses the fingers of the left hand to represent the coronary arteries. The entire demonstration can be seen on the DVD “Cath Lab Essentials” (www.cathlabessentials.com). I will take you through the easy steps of this angiography method.
Before beginning, a common starting point for naming the angles is needed (see Table 1 and Figure 1). For all cath labs, the x-ray source is under the table and the image intensifier is directly above the patient. The body surface of the patient that faces the image intensifier (or flat panel) determines the specific view. This relationship holds true whether the patient is supine, standing, or rotated (Figure 1). To obtain an oblique (angulated from the perpendicular) view, the image intensifier is moved toward the patient’s right or left shoulder. In the days before moving C-arms, angulated views were obtained by rotating the patient with his right or left shoulder propped upward (moved anteriorly) by a pillow or wedge. Thus, moving the left shoulder forward produces the LAO view and moving the right shoulder forward produces the RAO view. For the demonstrations of how the arteries change position with different angulations, the hand is kept in position, and either the right or left shoulder is rotated toward the observer (image intensifier) to duplicate the LAO and RAO views.
The cardiac silhouette in LAO and RAO
The heart is the size of one’s fist (Figure 2). It is shaped like an ice cream cone, with the tip toward the sternum. In Figure 2, my open hand is positioned as it would be seen in an AP projection. When the left shoulder is moved forward (LAO projection) and the hand is seen more on end, that is the heart made shorter and rounder in the LAO (Figure 3). When the right shoulder is moved forward (RAO projection), the hand is seen in a profile that is made longer with the tip extending to the left chest wall. These 2 movements of the hand in the LAO and RAO will remind you how the heart should look in each projection, whether seen on a plain chest x-ray, or on fluoroscopy or ventriculography during cardiac cath (Figure 3).
In the LAO view, the LV segments, moving from 12 clockwise are the high lateral, lateral, apical, septal, and base (Figure 4). In the RAO view, from the 1 o’clock position, the LV segments are the anterior base, anterior, apical, inferior, and inferior base. The lateral wall cannot be seen in the RAO projection.
The left coronary arteries – LAO views
By placing the left hand fingers over the clenched right fist, the index finger becomes the left anterior descending artery (LAD) and runs over the knuckles, which represent the anterior interventricular groove (Figure 5). The middle finger is spread, lying on the finger joints, and represents the circumflex artery (CFX). The thumb runs horizontal to the wrist joint and represents the initial course of the right coronary artery (RCA) arising from the right sinus of Valsalva. Later, we will see that the posterior descending artery (PDA) of the RCA can be shown by the index finger placed along the bottom of the fist to demonstrate the course of the PDA in the oblique views.
In cranial angulation, the image intensifier moves toward the head of the patient and produces a downward tilt of the LAO view, exaggerating the left main segment but keeping the relationship between LAD and CFX almost the same (Figure 6a).
To show the effect of caudal angulation, think of the image intensifier moving toward the foot of the patient. This position views the coronary arteries from underneath, tipping the LAO view upward and producing a branching appearance some call the “spider” view. Figure 6b and similar images provide a CTA reconstruction of the coronary arteries, with the lower panels showing a subtracted image duplicating what would be seen on traditional coronary angiography in the cath lab. The lower right panel of Figure 6b shows the LAO, caudal angulation, and is called the ‘spider’ view for obvious reasons.
The left coronary arteries – RAO projections
When demonstrating how the coronary arteries move in relationship to one another, it is now easy to visualize that in the LAO projection, the LAD (index finger) is on the right side and the CFX (middle finger) is on the left side. If you rotate over to RAO, then the position of the fingers (LAD/CFX) changes orientation such that the LAD is now on the left, and the CFX is in the middle or more rightward than in the LAO view (Figure 7a).
The RAO with caudal angulation (Figure 7a, top left) tips the CFX downward, separating it more from the LAD. For the RAO with cranial angulation (Figure 7a, top right), the CFX is tipped upward, foreshortened, and overlapped with the LAD. Cranial views are best used to see the LAD and diagonals, while caudal views are best to see the CFX and LM segments. Figure 7b shows the angiograms of the RAO, caudal and cranial angulations. Note that the colored dots (red for LAD, yellow for CFX, green for LM, and blue for RCA) correspond to the same arteries for all figures.
For review, in the RAO, the branches of the LAD are the septal branches running downward from the LAD to the bottom of the heart, and the diagonal branches running almost parallel to the LAD. In the LAO projection, the diagonals run leftward to the margin of the heart.
For branches arising from the CFX, there are only obtuse marginal arteries. In the RAO, they originate from the CFX (traveling in the AV groove over the coronary sinus) and run toward the tip of the heart. In the LAO, the obtuse marginal branches run to the edge of the heart (the margin).
The posterior descending artery from the RCA
The posterior descending (PDA) portion of the RCA runs along the inferior interventricular groove. The PDA along the bottom of the heart can be represented by the index finger (Figure 8a). In the LAO, cranial angulation, the PDA runs along the bottom of the heart and is tipped downward to better visualize the length without foreshortening (Figure 8a). In the RAO without cranial or caudal angulation, the PDA is seen lengthwise, running from the base to apex of the heart (Figure 8a, right panels). Figure 8b shows the CTA and contrast-filled angiograms of the RCA in the LAO, cranial, and RAO projections.
Using the finger model, it should be easy to visualize and understand the coronary and ventriculographic images with the different oblique views, both with and without cranial and caudal angulations. I hope this will be as helpful to your “angiographic IQ” as it was to mine.