Aortic Surveillance for the Non-Radiologist—Understanding Imaging and Measurement Techniques

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This video aims to provide the basics of obtaining accurate aortic dimensions. Aortic surveillance is critical for the early detection and lifelong monitoring of aortic disease, enabling appropriately timed treatments that can prevent life-threatening complications.  

It is important for all members of the multidisciplinary team to understand imaging techniques and how to accurately interpret imaging findings to effectively manage aortic conditions.  

Precise surveillance improves patient outcomes by guiding surgical decisions and ensuring that treatments are tailored to each patient's needs. 

Imaging Modalities 

Cardiac-gated computed tomography angiography (CTA) is the most common imaging modality for visualizing the aorta. This method provides detailed three-dimensional images, which are crucial for both diagnosing and monitoring the progression of aortic disease. 

Magnetic resonance angiography (MRA) is another great option, offering high-resolution images without ionizing radiation. Guidelines for managing aortic pathologies are strongly influenced by the maximum measurements of the aorta. 
 
Aortic Anatomy and Measurement Principles 

The aorta resembles a cylindrical tube that varies in size along its length and follows a complex, often tortuous path through the body. Additionally, the aorta may not present a perfectly circular cross-section, particularly when diseased. Such irregularities add complexity to measuring its true diameter. It is essential that measurements are taken perpendicular to the cylindrical axis of the aorta. Proper alignment of the measurement plane helps avoid distortions from oblique angles, ensuring that the aorta’s actual size is accurately captured.  

To demonstrate this concept, a piece of tubing with a 1 cm diameter has been used to illustrate the impact of an oblique cut. After the angled cut, the diameter appeared to be 1.5 cm, resulting in a 50 percent increase. 

Landing Zones and Clinical Relevance 

The most clinically significant area of the aorta is often where it has the largest diameter, as this determines the urgency and approach to intervention. With the growing use of endovascular stenting, understanding the 11 aortic landing zones is essential for planning and placing stent grafts. Throughout this video, anatomical terms and landing zones may be used interchangeably for clarity. 

Screen Layout and Orientation 
The video starts by introducing the layout of the screen. The top left displayed the sagittal slice, while the bottom left showed the coronal slice. The right side presented the axial slice. These anatomical slices were positioned in orthogonal planes, meaning they were positioned at 90-degree angles to each other. Once the planes were manipulated, they no longer represented true sagittal, coronal, or axial slices; however, for convenience, these terms were continued to be used. The bone window was employed for this demonstration, but other window settings may be more helpful depending on individual preferences or the specifics of the scan. 

Aortic Root Alignment and Measurement 

The process began by dragging the sagittal slider to the center of the root. Next, the coronal slider was adjusted to align with the root, followed by the axial slider. Each slider corresponded to one of the other screens, allowing all three planes to align simultaneously. Once the approximate center of the aortic valve was positioned, each slider was fine-tuned to ensure precise alignment. The goal was to place the crosshairs at the center of the aortic valve and orient the views in the direction of blood flow. 

On the axial screen, a clear trileaflet valve was visible. As scrolling occurred up and down, each cusp became visible. The key was to adjust the coronal and sagittal slices until all three cusps disappeared at the same time on the axial slice. The point where all three cusps disappeared defined the aortic annulus. With the annulus identified, the aortic root was measured. By moving up and down on the axial slice, the widest part of the root was located. It is important to avoid including the coronary ostia in this measurement. To measure accurately, the aortic wall was included. The authors prefer to measure from cusp-to-cusp, as this yielded the largest diameter; however, many radiologists also included the commissure-to-commissure measurement in their reports for completeness. 

Measuring the Ascending Aorta 

Next, the focus shifted to the ascending aorta, which required a slightly different approach. When no obvious dilation is present, as observed in this scan,  the right pulmonary artery (RPA) is used as a reference point to ensure reproducibility in future studies. The process began by dragging the axial slider on the sagittal slice until it aligned with the center of the RPA. Once aligned, the crosshairs were positioned at the center of the aorta at that level. 

The axial slice remained fixed while the sagittal and coronal planes were adjusted to be perpendicular to the direction of blood flow. If calcification was present at the widest part of the aorta, it was included in the measurement, as it formed part of the aortic wall. Always measure the widest diameter and its perpendicular counterpart. 

Aortic Arch: Candy Cane View 

The focus then shifted to the aortic arch. The first step was to obtain what is called the “candy cane view.” This involved sliding up on the axial view until both the ascending and descending aorta were visible. The crosshairs were then positioned in the center of either the ascending or descending aorta. The sagittal and coronal sliders were rotated so that the sagittal plane transected both the ascending and descending aorta, providing a clear sagittal-plane view of the entire arch. In this case, an aneurysm near the level of the left subclavian artery was identified.

At this level, the aorta traveled left, posteriorly, and caudally, making alignment more challenging. Adjustments were made one plane at a time while referencing the other windows to ensure accurate alignment. It is important to note that the cross-section at this level may not be perfectly circular. Differences in grayscale along the aortic wall helped define its boundaries. 

Mid-Descending Aorta 

The focus then moved to the mid-descending aorta. Similar to the ascending aorta, when no clear dilation was present, a consistent anatomical landmark was used. In this case, the pulmonary bifurcation was selected as a reference, located on the coronal slice. On the axial slice, the crosshairs were positioned in the center of the aorta while avoiding any adjustments to the axial position up or down.

From that point, the sagittal and coronal slices were adjusted until a perfect cross-section of the aorta was obtained on the axial slice. The measurement was performed, ensuring that only the aortic wall was included. 

Zone 5 and Thoracic-Abdominal Transition 

Next, the measurement of the distal border of zone 5 was conducted, marking the transition from the thoracic to the abdominal aorta. This was most easily identified by locating the celiac trunk on the sagittal slice. The authors typically do this by moving the sagittal slider on the coronal view until the celiac trunk becomes visible. Once located, the slices were aligned to be perfectly perpendicular to blood flow, as previously demonstrated. With the alignment set, the measurement proceeded. It is worth noting that distinguishing between the diaphragm and the aortic wall at this level can be difficult. Switching between window presets can help improve contrast and visualization of these structures. 

Post Type A Dissection 

This scan was performed following the initial repair of a Type A aortic dissection. On the sagittal slice, the residual dissection was observed beginning just distal to the left subclavian artery. However, the authors want to draw attention to a second ring located proximal to the subclavian artery and visible on the coronal slice. This was not contrast or residual dissection; rather, it was a felt strip that some surgeons incorporated into their anastomosis. It was important to avoid including this in the measurements of the aortic wall during post-repair surveillance. 

Post TEVAR: Remodeling and Thrombosis 

In this next example, a patient who had undergone thoracic endovascular aortic repair (TEVAR) as part of their Type A repair was examined. The first step was to identify the maximal aortic transverse diameter, followed by assessing the false lumen for signs of thrombosis—a dynamic process that serves as an important marker of aortic remodeling and healing. When measuring a dissected aorta, the true lumen was used as the alignment reference. At the level of maximal diameter, the false lumen appeared dark, homogeneous, and fully thrombosed. However, as the examination moved distally around the T8 level, contrast was observed within the false lumen, indicating partial thrombosis. This continued further down, extending to at least the iliac arteries. 

Infrarenal AAA

In this final segment, the focus shifted to an infrarenal abdominal aortic aneurysm (AAA). It was noted that distinguishing the aorta from surrounding loops of bowel could be challenging, especially when there was intraluminal thrombus. Different window presets were switched to enhance contrast and better differentiate the aorta, thrombus, and surrounding structures. 

Conclusion 

In conclusion, aortic imaging is essential for surgical practice, as accurate measurements directly inform clinical decision-making. Additional resources are needed to help trainees understand imaging interpretation. While management strategies for aortic disease remain a topic of debate, sharing and standardizing serial aortic measurements can advance research, improve patient care, and lead to better outcomes. 

References

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