3-D Abdominal CT angiogram acquired on a 16 detector scanner in a patient with abdominal aortic aneurysm (AAA)

Abdominal aortic aneurysm (AAA) is a relatively common and often fatal condition that primarily affects older patients. With an aging population, the incidence and prevalence of AAA is certain to rise [1]. It is important to distinguish which patients are at risk for the development of AAA and what is  the appropriate evaluation once a patient has been diagnosed with an aneurysm. In the current project, we chose to visualize an abdominal aortic aneurysm in 3D in order to have a clear view of the aneurysm’s stage and its severity.

An abdominal aortic aneurysm is a permanent and irreversible localised (in the abdomen) dilatation of the aortic artery. It is the most common form of aortic aneurysm. The major complication of abdominal aortic aneurysms is rupture, which is life-threatening, as large amounts of blood spill into the abdominal cavity, and can lead to death within minutes [2]. AAAs are generally asymptomatic until very large or ruptured. They are often found when patient undergoes tests or procedures for other reasons. Some patients have vague symptoms, such as back pain or abdominal pain.  The gravity of an AAA depends on its size. The maximum diameter of the infrarenal aorta in healthy individuals is 2 cm. By convention, an infrarenal aorta 3 cm in diameter or larger is considered aneurysmal. If the outer diameter exceeds 5 cm, the aneurysm is considered to be dangerous and can lead at any moment to AAA rupture [3][4].

                  Figure 1 : The location of an ΑΑΑ .

To create our visualization of an AAA, we have used a database composed of Dicom files. These files have been obtained by an Abdominal CT angiogram acquired on a 16 detector scanner in a patient with abdominal aortic aneurysm. DICOM (Digital Imaging and Communications in Medicine) is a standard for handling, storing, printing, and transmitting information in medical imaging. It includes a file format definition. DICOM files can be exchanged between two entities that are capable of receiving image and patient data in DICOM format [5].

To show the abdominal aortic aneurysm (AAA), we have chosen to represent the abdominal aorta in 3D. To make the visualization, we have used DeVIDE and MeshLab. First, we have created the visualization on DeVIDE by using many modules that we will describe in the same order as we implemented them: 

Figure 2: Function used in DeVIDE and their link.

• DICOMReader reads the 288 DICOM images that we have chosen 
• Slice3dVWR gives a 3D view of the images. We can turn the image on itself to see it from different angles. We can see the entire images one after the other to see the evolution.
• doubleThreshold puts a color on every object whose intensity is between the minimal and maximal chosen values. We have chosen Lower threshold = 250 and Upper threshold = 380. Then it puts in green the aorta on all images. We had to be precise when choosing the threshold values because we have to see the aorta on every picture but the less “not aorta” areas in green, the better.
• seedConnect helps us to highlight (by coloring it in green) only the aorta. It deletes the areas which were in green although they were not aorta.
•  Closing is used to clean up the shape by doing an erosion and then a dilatation on the complete dataset
•   Contour extracts isosurface from volume data : we then see the aorta in 3D (iso value = 0.5)
•  wsMeshSmooth smooths the data      
•  stlWRT writes STL format data in order to be able to use the created visualization on a program like Meshlab.

Figure 3: Final visualization of the aorta obtained with DeVIDE.

In a second time, we have used MeshLab to improve the visualization that we have imported from DeVIDE. We have added colors to make the aneurysm clearer. And we have used the measuring tool to measure the diameter of the aorta at two different locations to compare the healthy part and the part concerned by the aneurysm. However, we had an issue about measuring the aneurysm because we can’t measure inside of it. We then have measured outside to have an approximated idea of the diameter and of the ratio diameter normal aorta on diameter aneurysm.

Figure 4: Visualization in colour obtained with MeshLab

(the yellow part represents the ΑΑΑ ).

Figure 5: Measuring of the diameter of the aorta on MeshLab.

Figure 6: Diameter measuring of the aorta where the aneurysm takes place.

We can already observe clearly  in Figure 4 the ΑΑΑ  (shown in yellow) of the patient. The use of colours makes it clear: we notice that the yellow part of the aorta is much bigger than the red part.

The figure 5 shows how we have measured the diameter of the aorta in a part not concerned by the aneurysm. We verify that its diameter accords the expected value: 22 mm. As we said before, an aneurysm is usually defined as an outer aortic diameter over 30 mm. If the outer diameter exceeds 50 mm, the aneurysm is considered to be large and can lead at any moment to AAA rupture, which is very dangerous and can be fatal. We see in figure 6 that the aneurysm of the patient measures approximately 48 mm. It’s dangerous because it is not so far from 50 mm. The patient is in danger of an AAA rupture.

In regard to the appropriate treatment, the patient has a big abdominal aortic aneurysm so he requires urgent surgical attention. He should be medically optimized before repair, including institution of beta blockade. Surgery is recommended when the aneurysm is large enough (>5 cm in diameter) that the risk of surgery (1% to 6%) is less than the risk of rupture. In open surgery, the surgeon opens the abdomen and stitches in a replacement section of artery; in endovascular surgery the surgeon feeds the replacement section through the patient's artery and replaces it from inside. In this case, it might be a little too soon for the patient to consider surgery, but he has to do often follow-up check and consider referral to a vascular specialist [6].

To conclude, we can consider we have made a useful visualization because we can directly see on it if the patient suffers of an AAA. Then, measured the size of the AAA , so as to inform about the severity of the situation. Besides, thanks to this visualization, the surgeon knows the shape of the aneurysm before practicing the surgery, which is a great advantage. The measurements that have been done in this visualization should be improved by using a DeVIDE module so as to obtain more precise measurements.

Besides, it is generally accepted that this 3D visualization is viewer friendly. The patient can have an optical view of the disease he/she is already suffering from. This illustration helps him to take into consideration the severity of the pathologic abnormality and follow the physician’s advice.

As a future work, it could be effective for the medical doctors to have access to a software that can measure the dimensions of a normal aorta and of an aneurysm simultaneously. This data could be saved in a table in the patient’s file and used to study the long-time evolution of the aneurysm.

References :

[1]    G. Uphurch, T. Schaub. Abdominal Aortic Aneurysm. American Family Physician. April 1, 2006, Vol. 73, 7, pp. 1198-1204.

[2]    Mayo Clinic. Abdominal aortic aneurysm. [Online] [Cited: December 12, 2012.] http://www.mayoclinic.com/health/abdominal-aortic-aneurysm/DS01194/DSECTION=complications

[3]    Jonston KW, Rutherford RB, Tilson MD, Shah DM, Holler L., Stanley JC. Suggested standards for reporting on arterial aneurysms. J Vase Surg. 1991, Vol. 13, pp. 452-458.

[4]    N. Sakalihasan, R. Limet,. Abdominal aortic aneurysm. The Lancet. 2005, Vol. 365, 9470, pp. 1577–1589.

[5]    http://www.osirix-viewer.com/datasets/  database use dis called PANORAMIX

[6]    Kent KC, Zwolak RM, Jaff MR, Hollenbeck ST, Thompson RW, Schermer-horn M. Screening for abdominal aortic aneurysm. J Vasc Surg. 2004, Vol. 39, pp. 267-269.