Session Item

Stereotactic radiation surgery is one of the therapeutic modalities currently available to treat cerebral arteriovenous malformations. Conventionally, digital subtraction angiography using orthogonal x-rays are used to identify the target volume for SRS treatment, but often 2D images are not compatible with the treatment planning systems (TPS). The purpose of this study was to evaluate the image quality of a C-arm cone beam computed tomography (CBCT) comparing it with a CT simulator image.

CBCT images were acquired using an Allura Xper FD20 (Philips) angiograph, and compared with those obtained from an Aquilion LBTSX-201 A (Toshiba) CT simulator.

The spatial distortion of CBCT was evaluated by analyzing the images of a Lucy (Standard Imaging) phantom displaced from the center of rotation with the following configuration: a radiopaque grid (Fig. 1a), and four equispaced peripheral cylinders with radiopaque markers inside.

Fig. 1. CBCT axial slices of Lucy phantom: a) distortion grid b) irregular air volumes.

 The same phantom was also imaged using an insert with irregular air cavities (Fig. 1b) of known volume to quantify geometric accuracy. These images were analyzed using the automatic segmetation tool of RayStation (RaySearchLab) TPS.

Images of centered CATPHAN 600 phantom were also acquired, along with AutoQALite (QA Benchmark) analysis  software, to assess basic image quality parameters.

For the CBCT, the distance observed between the four radiopaque markers differed less than 0,1 mm from their real value. Neither did the grid analysis show appreciable distortion. The mean value (µ) and standard deviation (σ)  of the grid size were 0,34 + 0,24 mm and 0,36 + 0,36 mm for horizontal and vertical direction, respectively. It should be noted that the image in the upper area of the grid was diffuse, probably due to the off-axis location of the phantom and the conical geometry of the beam.

The volumes obtained by automatic segmentation differed between 1,6 and 5,6% from the nominal values provided by the manufacturer.

The results of the comparison between the CBCT angiography and the CT simulator, based on the analysis of the CATPHAN phantom images are shown in Fig. 2.

Fig. 2. CBCT vs CT-simulator results comparison with CATPHAN phantom.

The limitation given by the CBCT pixel size (0,98 mm) prevented meeting the usual tolerance for head CT examinations of 6 lp/cm at MTF 2%.

Although the low-contrast resolution of the CBCT was worse, both 3D imaging modalities met the usual tolerances: 3% and 0,8% for 3,5 and 8 mm diameter objects, respectively.

The results obtained from the quality assurance of the CBCT angiography showed acceptable values, although its spatial resolution is at the limit of what is recommended for its use in delimiting volumes for stereotactic treatments.