Abdominal Aortic Aneurysm Volume Measurement

The Journal for Vascular Ultrasound 27(1):20–25, 2003

Abdominal Aortic Aneurysm Volume Measurement with Three-Dimensional Ultrasound— A Feasibility Study Khalil F. Dajani, PhD; Sergio Salles-Cunha, PhD; Hugh G. Beebe, MD ABSTRACT Introduction.—Endoluminal stent grafts are replacing conventional abdominal aortic aneurysm (AAA) surgery in an increasing number of patients in an attempt to minimize morbidity and mortality. Long-term follow-up of endograft-treated AAA demands image-based surveillance to detect endoleak, graft migration, and morphology change in the excluded AAA. AAA diameter is a traditional but simplistic measurement that has inherent flaws and has been shown to be insensitive to changes in AAA sac morphology. Volume measurement, performed by CT data acquisition and computerized postprocessing, has been proposed as the most sensitive index of successful AAA stent graft exclusion. We evaluated two ultrasound (US) volume measuring techniques for AAA volume determination: Virtual Organ Computer-aided AnaLysis (VOCAL) and Multi-Plane Area Summation (MPAS). Methods.—US images of an endograft-treated AAA were obtained with a commercially available three-dimensional (3-D) scanner. A fast rotating motor inside the probe allowed registration of multiple two-dimensional (2-D) images in real time. Data from these images were assembled in a 3-D dataset. With VOCAL, the 3-D AAA boundaries were identified, and volume was calculated. The operator traced AAA boundaries in six virtual planes selected by the software. With MPAS, aneurysm boundaries were traced in 2-D virtual images perpendicular to the longitudinal axis of the aneurysm obtained every 1 mm. AAA area was calculated and multiplied by this 1-mm step to obtain incremental volumes that were summed to obtain the AAA volume. VOCAL and MPAS volumes were calculated 10 times each for one AAA scan. Results.—Average AAA volumes were 86.7 ± 3.7 cm3 with VOCAL and 87.6 ± 3.1 cm3 with MPAS. These averages were not statistically significantly different by t test (p = 0.54). Standard deviation (SD) to average ratio was 4.3% for VOCAL and 3.5% for MPAS. Conclusion.—Volume of an endograft-treated AAA was successfully measured multiple times with two 3-D US techniques. Volumes obtained were comparable, demonstrating feasible reproducibility.

Introduction Abdominal aortic aneurysm (AAA) is a common and serious condition representing one of the leading causes of death in Western countries.1 Ultrasound (US) is commonly used for AAA size evaluation to aid in clinical decisions regarding treatment. Computed tomography (CT) has largely replaced angiography as a supplemental AAA diagnostic imaging modality.2 ,3 Recently, less-invasive endovascular graft (EVG) procedures have been developed to replace open surgical repair.2 Multiple, precise measurements of arterial and aneurysm geometry are required for pretreatment

Presented at the 25th Anniversary Annual Conference of the Society of Vascular Technology, New Orleans, LA, July 31–August 4, 2002. From the Jobst Vascular Center, The Toledo Hospital, Toledo, Ohio. Corresponding author Sergio Salles-Cunha, PhD, Jobst Vascular Center, 2109 Hughes Drive, Suite 400, Toledo, OH 43606.

planning of endovascular repair,4 and, follow-up after EVG requires serial image-based evaluation to assess treatment outcome. Because EVG treatment of this life-threatening disease is a newly introduced technology and has been observed to have late failures of various kinds,5 continuous follow-up to prove durable AAA exclusion is required. AAA volume measurement has been proposed as the most sensitive means of evaluating EVG during follow-up.6,7 Volume measurement of the excluded AAA, and the EVG attachment zones above and below the aneurysm are known to be highly relevant in predicting long-term success of EVG. Thinslice CT scanning together with postprocessing by computerized three-dimensional (3-D) modeling is an accurate method of obtaining AAA volume and dimensional data after EVG that is widely used in routine clinical practice and clinical trials. This method, however, is more expensive in the data acquisition process, involves the use of radiocontrast injection, and incurs a cumulative radiation exposure risk to the

2003 ABDOMINAL AORTIC ANEURYSM VOLUME MEASUREMENT WITH THREE-DIMENSIONAL ULTRASOUND

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patient. An ultrasound technique would be highly desirable for continuous and serial monitoring of AAA volume. At present, US volume-measuring techniques are available primarily for nonvascular applications in fetal, gynecologic, and obstetrics scanning.8,9 Two US volume-measuring techniques are Virtual Organ Computer-aided AnaLysis (VOCAL) and Multi-Plane Area Summation (MPAS). We tested the feasibility and reproducibility of these two techniques to measure AAA volume in a patient with an EVG.

planes were adjusted accordingly. This manipulation included parallel shifting or rotation of planes. AAA volume was measured by use of two algorithms: VOCAL and MPAS. VOCAL algorithm measured volume on the basis of surface geometry. The AAA surface was reconstructed from AAA contours obtained at different imaging planes. These imaging planes were recreated at different angles of rotation around a preselected axis.

Methods

The rotation angle for each image plane was selectable at angles of 6°, 9°, 15°, and 30° with 30, 20, 12, and 6 planes, respectively. For example, we selected a rotation of 30° angle and six image planes for AAA contour outline were generated. The 2-D contours were defined automatically and manually. The first AAA contour was manual. In the following imaging planes, either manual or automatic contour tracing can be selected. As shown in Figure 3, the top left image was selected as reference to define axis of rotation. Two green arrows were placed where the axis met the top and bottom of the AAA boundary. The first manual contour outlined the AAA in the first image plane, and the outlined contour was colored with white dots. The program provided the second image plane with an automatic contour of the AAA. The operator could adjust this automatic contour by replacing selected yellow dots while moving the trackball (Figure 4). Three-dimensional surface reconstruction was performed based on these six plane contours, and volume within the 3-D surface was calculated. Manipulation

With informed consent, an AAA patient treated with EVG volunteered for this study, which was approved by the institutional review board. Standard preparation before abdominal US testing was accomplished. AAA ultrasonography was performed on the patient resting supine using 3-D US imaging system (Kretztechnik VOLUSON 730; GE Medical Systems, Milwaukee, WI) with an RAB4-8(P) abdominal probe. The transducer was a broadband electronic curved array with a frequency range of 4–8 MHz. The probe included a servomotor that rotates the crystal array, switching the angle of the US beams from 30° to 70° (in 2° steps) around a long axis (Figure 1). To obtain a 3-D image, multiple sectional images were created as the motor rotated the crystal bar inside the probe. From the 3-D image, orthogonal planes were displayed simultaneously (Figure 2). Any one plane could be manipulated into any desired orientation so as to optimally display relevant anatomy. Once one plane was selected for manipulation, the other two orthogonal

Imaging Protocol

Figure 1 RAB4-8(P) abdominal probe (GE Medical Systems) of 4–8 MHz (right). The probe included a servomotor that rotates the crystal array with a switchable angle of the ultrasound beams from 30° to 70° (in 2°-steps) around a long axis (left).

Figure 2 Simultaneous display of orthogonal planes (axial, sagittal, and coronal) from 3-D data of AAA patient treated with EVG.

Figure 3 Top left image was selected as a reference to define axis of rotation. Green arrows were placed where the axis met the top and bottom of the AAA boundary. The first manual contour outlined the AAA in the first image plane.


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Figure 4 Top left, VOCAL program provided image planes with an automatic contour of the AAA. Replacing selected yellow dots while moving the trackball performed contour adjustment. Three-dimensional surface reconstruction of AAA was generated with the calculated volume.

of the 3-D reconstructed image was performed for visualization of the graft inside (Figure 5). MPAS algorithm calculated volume based on AAA area measurements obtained in parallel imaging planes. The planes were selected along a longitudinal axis represented by the white dot placed in the center of the region of interest (Figure 6). These transverse planes were obtained every 1 mm along the longitudinal axis. After the first manual selection, MPAS provided an automatic contour of the AAA area in the next plane as shown in Figure 6 at the top right plane. The operator could accept or alter the AAA boundary contours. The area of each contour was automatically calculated. VOCAL and MPAS volumes were calculated 10 times each for one AAA US scan. Means of VOCAL and MPAS volume measurements were compared using a t test. Results AAA average volume with VOCAL was 86.7 ± 3.7 cm3 and with MPAS was 87.6 ± 3.1 cm3 (Table 1). Mean values were not statistically significantly different by t test (p = 0.54). Standard deviation (SD) to average ratio was 4.3% for VOCAL and 3.5% for MPAS.

Discussion Although clinical efficacy of EVG has been assessed using AAA diameter measurement,10 ,11 there is growing evidence that volume measurement is a more appropriate method.6,7 We demonstrated that AAA volume measurements using 3-D US were feasible and reproducible. The mean volume obtained, 87.7 cm3 , was comparable to expected ranges of AAA volumes described in the literature.12 ,13 Accurate geometric measurements and long-term monitoring are essential components of AAA management. CT angiography and 2-D US are commonly used for conventional preoperative and postoperative aortic imaging, but each has limitations.1 4 Diameter measurement errors in the proximal infrarenal aorta are common when using axial CT data. An aorta that lies curved in the CT plane appears elliptical. To compensate for such artifact, physicians commonly use the smallest diameter and assume the aorta is round.1 5 However, the actual shape of the aorta is not always round.16 Diameter measurements made with conventional 2-D US are sensitive to image plane orientation. The orientation and placement of the imaging planes change from visit to visit, which contributes to measurement variability in studies over time. The feasibility of 3-D CT to assess AAA dimensions postoperatively has been established.7,1 4,1 6– 18 Three-

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JVU 27(1)

DAJANI ET AL.

Figure 5 Three-dimensional reconstruction of AAA data with visualization of the graft inside.

dimensional US offers a lower cost, noninvasive alternative to the commonly used fine collimation CT scanning method. The commercially-available VOCAL and MPAS 3-D US techniques have been used successfully in some diagnostic applications8 ,9 but to date have not been used for vascular applications. We demonstrated the feasibility of using these algorithms to measure AAA volume in a patient treated with EVG. Limitations of the US approach to post-EVG surveillance include the well-known difficulty of abdominal US in obese or poorly prepared patients. There are

also body habitus limitations on abdominal US when attempting to visualize the renal artery origins and the infrarenal aortic segment. Although image quality was excellent in this case, there is a requirement for US technologist skill that may require significant experience to achieve. The feasibility of this limited study needs to be demonstrated in future studies of a larger series of patients and compared with 3-D postprocessing of CT scanning for accuracy and cost. The variability of multiple scans by different observers must be determined.

Table 1 10 AAA Volume (cm) Measurements with VOCAL and MPAS Programs Measurement

VOCAL MPAS VOCAL MPAS

1

2

3

4

5

6

7

8

9

10

82.56 85.41

89.32 88.6

83.81 81.76

86.21 87.45

90.81 91.63

88.12 91.63

88.12 89.53

85.03 91.05

92.97 84.26

81.39 89.32

Average

Standard Deviation

Variance

86.75 87.64

3.68 3.09

13.56 9.55

Measurements with VOCAL and MPAS programs were performed at 30°.


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Figure 6 Top left, MPAS program provided selected planes along a longitudinal axis represented by white dot placed in the center of the graft. After the first manual selection, automatic contour of AAA area was displayed in top right plane.

Conclusion We demonstrated that AAA volume measurements were feasible with US. We obtained an acceptable intraobserver variability in 10 measurements for a single scan. Three-dimensional US may become a feasible and economical diagnostic procedure for monitoring EVG-treated AAA volume. References 1. Rutherford RB. Arterial aneurysms. In: Rutherford RB, ed. Vascular Surgery, Volume II. 3rd ed. Philadelphia: WB Saunders; 1989: 906–1003. 2. Beebe HG, Jackson T, Pigott JP. Aortic aneurysm morphology for planning endovascular aortic grafts: limitations of conventional imaging methods. J Endovasc Surg. 1995;2:139–148. 3. Resch T, Ivancev K, Lindh M, et al. Abdominal aortic aneurysm morphology in candidates for endovascular repair evaluated with spiral computed tomography and digital subtraction angiography. J Endovasc Surg. 1999;6:227–232. 4. Lederle FA, Wilson SE, Johnson GR, et al. Variability in measurement of abdominal aortic aneurysms. Abdominal Aortic Aneurysm Detection and Management Veterans Administration Cooperative Study Group. J Vasc Surg. 1995;21:945–952. 5. Beebe HG. Late failures of devices used for endovascular treatment of abdominal aortic aneurysm: What have we learned and what is the task for the future? In: Gloviczki P, ed. Perspectives in Vascular Surgery and Endovascular Therapy. New York: Thieme Medical Publishers; 2001:29–49. 6. Fillinger MF. New imaging techniques in endovascular surgery. Surg Clin North Am. 1999;79:451–475. 7. Fillinger MF. Postoperative imaging after endovascular AAA

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