2-CT Cardiac axis

In step 2, a line is drawn from the cardiac apex to the midpoint of the mitral valve, using true sagittal reformatted images exclusively. With the apex marked, the mitral valve can be identified by scrolling through the sagittal images from the patient’s left towards the midline. Because of its orientation in the sagittal plane, the mitral valve will be seen on several consecutive images (Figure 2).

Five consecutive 1.25mm thick sagittal images though the mitral valve (white arrow). The valve plane is seen on several images.

Ideally, the center of the mitral valve will be located on a sagittal slice in which the left ventricle appears crescentic. However, for an individual patient, the left ventricle may not be symmetrically crescentic on the best slice. With the cross hairs fixed on the apex, one of the lines is rotated until it crosses the center of the mitral valve (Figures 3 and ​and4). 

The chosen sagittal image with one line (red) of the cross hair positioned through the center of the mitral valve. Note that when the sagittal images are scrolled to visualize the mitral valve, the cross hair remains in the same plane as the apex (as defined in Step 1) and typically is seen within the right ventricular cavity.

Illustration of Step 2. The long black line from the cardiac apex through the mitral valve defines the plane of a 4-chamber view. The volume sketch in the upper left-hand corner shows the plane of the main illustration in relation to the surface of the heart.

The plane defined by this line is the 4-chamber view routinely used in echocardiography and cardiac magnetic resonance to evaluate the left ventricle (Figure 5).

Fig. 5

Four-chamber view. This view is used to evaluate left ventricular wall motion abnormalities. The anatomic plane of the 4-chamber view is shown in the top left hand corner. The larger illustration (middle) shows the orientation of the cardiac chambers. When CT images are viewed in this plane as a cine loop, left ventricular wall motion can be assessed. The corresponding patient image (right) shows the cross hairs in the center of the mitral valve; this is the basis of Step 3 in which the short axis views are created.

 In step 3, the orientation of the short axis “stack” of images is defined by centering the cross-hair in the mitral valve (Figure 5) and bisecting the left ventricle. The short axis stack is the set of images orthogonal to this line (Figure 6); these images are used to evaluate left ventricular volumes.

Fig. 6

Short-axis view at the base of the heart. Scrolling the images in this plane will give the short axis stack to be used to calculate left ventricular volumes. The left hand image shows the orientation of the cross hairs at the end of Step 3, namely after the creation of the short axis stack. As noted in the description of Step 4, a 4-chamber view aligned along the plane defined by the red line will not demonstrate the maximum diameter of the right ventricle. In the right hand image (Step 4) the red line has been turned so that the 4-chamber plane it defines includes the full extent of the right ventricle.

 As noted earlier, steps 1 through 3 are based solely on left heart landmarks, and these maneuvers will yield reliable, reproducible imaging of the left ventricle. There is a growing potential role for CT in the assessment of the right ventricle after pulmonary embolism. The parameter most widely studied is the ratio of the right over left ventricular diameter ratio measured on the 4-chamber view. As noted in the discussion, a potential pitfall in this method is variability in the right ventricular size on the 4-chamber view. For patients diagnosed with pulmonary embolism, step 4 represents a systematic approach for determining the maximum diameter of the right ventricle.

On the workstation, the user identifies the entire curvature of the right ventricular free wall by scrolling through (from mitral valve to apex) the short axis images. It is important to view the entire set of short axis images, because the image chosen for Step 4 should be the image with the largest right ventricular size (as measured from the center of the left ventricle). On this image, with the cross-hairs in the center of the left ventricle, the line should be “turned” to intersect the point of the free wall of the right ventricle with maximum curvature (Figures 6 and ​and7).7). This point is the location on the right ventricle free wall that is farthest from the center of the left ventricle. The line that is “turned” defines the 4-chamber view that demonstrates the maximum diameter of the right ventricle (Figure 8).

 

Fig. 7

Illustration of the “turn” used to optimally visualize the right ventricle. A dashed line was the result of Step 3, the creation of the short axis stack. The dashed line defined a plane through the right ventricle, but this plane does not fully represent the size of the right ventricle. This can cause an underestimation of the right ventricular diameter on 4-chamber views. In Step 4, the dashed line is “turned” to the solid line, with the center point (cross-hairs) rotating through the center of the left ventricle. The plane defined by the solid line characterizes the full diameter of the right ventricle. The upper left-hand corner shows the plane illustrated with respect to the surface of the heart.

 

Fig. 8

4-chamber view after Step 4 (see text for details) that demonstrates the full extent of the right ventricular chamber size.