This paper describes the development of a detailed 3D geometric model of an acoustical guitar. Modeling an instrument is a sophisticated task considering the individual parts and their complex shapes. The geometry of the parts visible from the outside can be measured using appropriate tools, but it is very difficult to measure the details of the internal parts like bracing, heels, and other features by hand through the sound hole. Otherwise, it would be necessary to disassemble the guitar to measure the precise position and dimensions of the parts inside it. Reassembling the guitar could result in improper functioning. To avoid damaging the instrument by disassembling or taking inaccurate measurements through the sound hole, a computer tomography (CT) scan of the guitar body was performed. Using this method, cross-sectional images of the guitar body in all the three dimensions were extracted with 1 mm spacing between adjacent images. In total, approximately 2000 images were generated and used in developing the geometric model of the guitar. The 3D model will be further used to develop a vibro- acoustic simulation model of the guitar.