Volume Measurement of Intracranial Aneurysms from 3D Rotational Angiography: Improvement of Accuracy by Gradient Edge Detection

Phantoms

For the purpose of this study, 13 aneurysm phantoms of different shapes and sizes were made. Seven phantoms were detachable latex microballoons (Goldvalve, Nycomed, Paris, France) that were filled with a contrast agent (Omnipaque 300, Nycomed, Paris, France). The volume of the contrast agent inside the balloons was calculated by weighing the balloon before and after filling. Subtracting both magnitudes resulted in the weight of the contrast agent, and dividing this weight by the density provided the volume of the contrast agent. The other 6 phantoms were constructed as single- or double-truncated spheric aneurysms attached to a cylindric vessel in Perspex (Fig 3A). A computerized drill was used with a perfect spheric tip with known dimensions and known displacement during drilling. Volumes of the phantoms were calculated with simple geometry formulas. The effective diameters (diameter of a sphere with the same volume) of the phantoms ranged between 2.9 and 12.5 mm.

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Fig 3.

A, Four aneurysm phantoms on a parent artery in different shapes and sizes in Perspex.

B, Radiograph of aneurysm phantoms filled with contrast material in a human skull placed in a water-filled bucket. Injection lines filled with contrast material simulate blood vessels.

The phantoms were placed inside a human skull in a water-filled plastic bucket. A flexible 76-cm injection line filled with contrast agent was placed inside the skull adjacent to the phantoms to mimic surrounding vessels (Fig 3B). 3D rotational imaging with a 17-cm image intensifier was performed on a biplane angiographic system (Integris V3000, Philips Medical Systems, Best, the Netherlands). A magnification of 33% of the initial 3D reconstruction volume was made in a matrix of 256³. Voxels were cubic with sizes between 0.2 and 0.5 mm.

Manual Volume Calculation

The volumes of the 13 phantoms were calculated by manual threshold setting as described in the introduction. In the magnification of the 3D reconstruction, one of us (M.J.S.) selected a threshold setting that subjectively provided the image that was thought to represent the phantom and surrounding phantom vessels best. The Perspex phantoms were subsequently segmented from the parent vessel. The volume of the phantoms was calculated by using the machine software (3DRA workstation, Philips Medical Systems, Best, the Netherlands).

Gradient Edge Detection

In the gradient edge detection method, the edge of a blood vessel or aneurysm is defined as the isosurface of voxels for which the gradient magnitude is maximal (1). The gradient of a given voxel in the volume represents the magnitude of variation of the value of this voxel and the values of its surrounding voxels. Voxels with low gradient values correspond to homogeneous locations in the volume. Voxels with high gradient values correspond to locations where the voxel values in the vicinity of this location differ largely, such as at the interfaces between contrast-filled blood vessels with high voxel values and the surrounding structures with low voxel values. The computer generates gradient values for every voxel in the 3D dataset. The isosurface of voxels with maximum gradients in the volume represents the contours of the blood vessels. In homogeneous parts of the volume, some gradient differences are present, caused by image noise. These small gradient differences are easily filtered out, leaving the voxels with the maximal gradients in the 3D dataset (Fig 2C, -D). These voxels with the maximal gradients have fixed locations in the dataset that are independent of the threshold setting. This means that with every threshold setting, the same contours of the vessels and aneurysm are delineated by the gradients and volume calculations are constant and reproducible.