CT vs. VolumeScope: image quality and dose comparison V.N. Vasiliev *a, A.F. Gamaliy **b, M.Yu. Zaytsev b, K.V. Zaytseva ***b a Russian Sci. Center of Roentgenology & Radiology, 86, Profsoyuznaya, Moscow, 117997, Russia b Institute for Roentgen Optics, PMZR, 10, 1 st Volokolamsky Pr., Moscow, 123060, Russia ABSTRACT A number of examinations were performed on the VolumeScope Compton scatter tomographic device and a CT scanner (Siemens Somatom Plus 4) using an anthropomorphic tissue-equivalent heterogeneous phantom simulating a human body. Three main parts of the phantom were under examination: head, thorax and pelvis. An absorbed dose in the phantom was measured by LiF thermoluminescent detectors. The CT scanner demonstrated better space resolution in cross-plane and lower statistical noise. On the other hand, absorbed dose due to CT scanning is about 20 times higher at the same points of measurement. Examples of reconstructed images are presented. Keywords: VolumeScope, absorbed dose, digital filter, image quality, anthropomorphic phantom, 3D visualization. 1. INTRODUCTION The VolumeScope (Fig. 1) is an X-ray scanner based on Compton scatter detection from the point of measurement 1. After some mathematical reconstruction of the measured data 2, the device allows to determine an electron density distribution of the object under examination and represent it as a 3D grid of volume elements, voxels. The resulting distributions can be displayed as gray-scale sections of the examined object or as 3D surfaces of internal structures for further analysis. Figure 1. General view of the VolumeScope. To evaluate the image quality, a number of measurements were performed on the VolumeScope and on the CT scanner Siemens Somatom Plus 4 installed in the Russian Oncological Center. As the VolumeScope is a laboratory prototype * E-mail: vnvasil@orc.ru ** E-mail: gam@iroptic.ru *** E-mail: kst@iroptic.ru Proc. of SPIE Vol. 5943 248
device, the purpose was to recognize its most significant problems and limitations in comparison with a commercial medical instrument for further improvement. Another task was to determine an absorbed dose inside the patient under examination as one of important characteristics of any X-ray diagnostic equipment. 2. METHODS The measurements were performed with the ATOM anthropomorphic phantom 3 (CIRS Inc., USA) simulating a human body. The phantom is made of tissue-equivalent materials and includes skeleton, lungs and brain models. All photon interaction cross-sections of the tissue substitutes are close to the cross sections of corresponding biological tissues within 1% down to the photon energy 30 kev. The phantom is dissected for 2.5 cm slices and has a grid of 5 mm holes inside to place thermoluminescent detectors for absorbed dose measurements. When not used, the holes are filled with cylindrical tissue-equivalent inserts. Three typical regions of the phantom were investigated: head, thorax and pelvis. Properties of the phantom materials are presented in Table 1. Physical properties of phantom materials Table 1 Tissue substitutes Physical density, Electron density, g/cm 3 10 23 per cm 3 Soft tissue 1.05 3.434 Spinal Cord 1.07 3.448 Spinal Disks 1.15 3.694 Lung 0.21 0.681 Average Bone 1.60 5.03 Brain 1.07 3.47 Before examining the anthropomorphic phantom, a preliminary measurement with a homogeneous water sample was performed for the estimation of a statistical noise level. A set of Plastic Water 4 plates and a liquid water sample were used with the CT scanner and the VolumeScope and the noise distribution parameters were determined by accompanying software, the DICOM viewer efilmlite 1.5.2 and the VolumeScope viewer respectively. The VolumeScope X-ray tube operated with the voltage 150 kv and anode current 4.5 ma. A primary X-ray beam of 2 mm in diameter was filtered by 1 mm Al and 0.5 mm Cu. The scanning speed was 90 mm/s and the voxel size was 2 mm along each coordinate axis. As scanning volume of the VolumeScope was limited by 200 x 200 x 70 mm due to the collimator design, front and back sides of the phantom were scanned separately. The CT scanner tube voltage was 120 kv, the anode current and voxel size are shown in Table 2. The CT scanner operation parameters Table 2 Phantom Anode current, ma Voxel size, mm PW Cube 150 0.967 x 0.967 x 5 Head 185 0.455 x 0.455 x 8 Thorax 110 0.807 x 0.807 x 8 Pelvis 170 0.807 x 0.807 x 8 3. RESULTS A standard deviation of the statistical noise was calculated for numerous regions of interest (ROIs) in the Plastic Water sample by the DICOM viewer efilmlite. Typical value was about 25 Hounsfield units i.e. about 2.5% of full signal range. The anthropomorphic phantom was examined with greater voxel volume (pelvis, thorax) or higher anode current (head) and demonstrated lesser noise standard deviation obtained in homogeneous ROIs typically 1-1.5%. The statistical noise standard deviation calculated by the VolumeScope viewer for liquid water sample was equal to 3.6%. To reduce the noise, we used a 2D low-pass digital filter with cutoff frequency 0.3 cycles/mm. This filter effectively suppressed the noise but saved high frequency details of the image. Using the filter, the standard deviation of Proc. of SPIE Vol. 5943 249
the noise in the water sample decreased to 1.6% and was approximately equal to the CT values. Currently, this filter is a standard tool of the data reconstruction VolumeScope software. Nevertheless, all 2D images below are presented without any postprocessing. Typical sections of the phantom head are shown in Fig. 2. The images demonstrate all internal bone structures, skull, spine and cartilage. The measured relative electron density of bones is in reasonable agreement with the data tabulated by the phantom manufacturer. A comparison of cross-plane sections of the phantom head obtained by the VolumeScope and the CT scanner is presented in Fig 3. The VolumeScope images demonstrated a significant blurring of boundaries and higher statistical noise in comparison with the CT data. This is resulted from better spatial resolution of the CT scanner in cross plane. On the other hand, some blurring of skull surface is visible on the CT image (Fig. 3f) due to significant voxel size and density averaging in the longitudial direction. The VolumeScope results demonstrate some electron density underestimation under the bone structure concerned with additional photoabsorption in bone due to its higher atomic number. a) b) c) d) Figure 2. Typical sections of the phantom head and neck (VolumeScope data): a frontal face section, b frontal section along the spine, c cross-plane neck section, d frontal section of the skull top. Proc. of SPIE Vol. 5943 250
a) b) c) d) e) f) Figure 3. Comparison of the VolumeScope (left) and the CT scanner (right) data for the phantom head cross-plane sections The VolumeScope program allows to reconstruct surfaces of internal structures inside the object of examination. External and skull surfaces of the phantom face region are presented as 3D objects in Fig. 4 on the base of VolumeScope data. As the figure demonstrates, most significant parts of the skeleton are successfully reconstructed. The objects in front of the eye-sockets and in the middle of forehead are bone-equivalent cylindrical inserts of 5 mm in diameter and 25 mm in length used for dosimetry. Proc. of SPIE Vol. 5943 251
Figure 4. External and skull surfaces of the phantom face reconstructed on the base of VolumeScope data. Examples of front and back parts of the thorax are presented in Fig. 5. Lungs, sternum, spine, spinal cartilage, ribs and clavicles are visible. The sternum and spine can be separated from the surrounding tissues and reconstructed as 3D objects (Fig. 6). a) b) c) d) Figure 5. The VolumeScope (left) and the CT (right) images of front (up) and back (bottom) parts of the thorax. Proc. of SPIE Vol. 5943 252
Figure 6. Sternum (with clavicles) and spine reconstructed as 3D surfaces from the VolumeScope data. 4. DOSIMETRY An absorbed dose inside the patient under examination was measured by thermoluminescent detectors LiF TLD-100 of size 3.2 x 3.2 x 0.9 mm and a TLD reader model 3500 (Harshaw, USA). The detectors were calibrated in a Secondary Standard Dosimetry Laboratory, total uncertainty of the detector response was 6.6% at the 95% confidence level. 29, 49 and 39 detectors were placed inside the head, thorax and pelvis parts of the phantom during the CT examination. As scanned volume on the VolumeScope was limited by 200 x 200 x 70 mm, the number of detectors in this case was 19, 30 and 28 in the head, thorax and pelvis respectively. TLD positions inside the phantom as well as details of the detector response correction are presented in Ref. 5. Minimum, maximum and mean absorbed doses are presented in Table 3. Absorbed dose per examination in the anthropomorphic phantom Table 3 CT scanner VolumeScope Min Max Mean Min Max Mean Head 22.7 50.7 37.3 0.82 3.05 1.43 Thorax 8.0 23.1 15.1 0.60 1.20 0.90 Pelvis 11.0 26.2 16.7 0.62 1.08 0.81 As the table demonstrates, the patient dose on the VolumeScope is about 20 times less than on the CT scanner. The CT dose data are in good agreement with the results by Giacco et al. 6 measured inside pediatric phantoms with similar CT scanner, Siemens Somatom Plus 4, tube voltage 120 kv and current 170 ma and obtained the CTDI value 30.9 mgy. 5. CONCLUSIONS The performed measurements demonstrated that a 3D electron density distribution of biological objects can be obtained by the VolumeScope within the volume 200 x 200 x 70 mm with voxel size 2 mm. The statistical noise level is higher than in the CT data, nevertheless, it can be reduced to the CT level by simple 2D nonadaptive digital filter. There are good possibilities for further noise reduction by using more advanced adaptive 3D filters designed on the base signal and noise power spectrum densities analysis. Equal voxel size along each coordinate axis allows a convenient 3D surface generation of internal structures for further visual analysis. Proc. of SPIE Vol. 5943 253
A significant advantage of the VolumeScope is low absorbed dose in the patient, 1-3 mgy. This is approximately 20 times less than on the CT scanner. Limited scanning volume equal to 200 x 200 x 70 mm and long scanning time are most significant disadvantages of the VolumeScope and must be improved. Some boundaries blurring was observed on all VolumeScope images and resulted from limited spatial resolution 7. This problem can be solved by improvement of the collimator design and deconvolution of the measured data with the point spread function as a kernel. Both approaches are in work. Some reconstruction artifacts were found in the VolumeScope data that appeared as density underestimation in deep slices and are most pronounced under bone structures. These effects are resulted from additional photoabsorption in bone due to its higher atomic number and can be reduced by the reconstruction algorithm improvement. REFERENCES 1. M.A. Kumakhov, A.F. Gamaliy, V.N. Vasiliev, M.Yu. Zaytsev, K.V. Zaytseva, A.A.Markelov, and Yu.V. Ozerov, Scattered X-rays in medical diagnostics, in 2-nd International conference on X-ray and neutron capillary optics. 22-26 September 2004, Zvenigorod, Proc. SPIE 5943, pp. 210-218, 2005. 2. V.N. Vasiliev and K.V.Zaytseva, An algorithm and program for data processing from Compton scatter imaging device, in 2-nd International conference on X-ray and neutron capillary optics. 22-26 September 2004, Zvenigorod, Proc. SPIE 5943, pp. 232-241, 2005. 3. Atom Adult Male Phantom, Model 701-D, Handling Instructions, CIRS Inc. 4. Plastic Water. CIRS Catalog, p.83-84, 2002. 5. V.N. Vasiliev and M.Yu. Zaytsev, An absorbed dose due to examination on the VolumeScope X-ray device, in 2- nd International conference on X-ray and neutron capillary optics. 22-26 September 2004, Zvenigorod, Proc. 5943, pp. 242-247, 2005. 6. G. Giacco, V. Cannata, C. Furetta, F.Santopietro, and G. Fariello, On the use of pediatric phantoms in the dose evaliation during computed tomography (CT) thorax examination, Med. Phys. 28, pp. 199-204, 2001. 7. V.N. Vasiliev and M.Yu. Zaytsev, Frequency characteristics of images measured on the VolumeScope scanning device, in 2-nd International conference on X-ray and neutron capillary optics. 22-26 September 2004, Zvenigorod, Proc. SPIE 5943, pp. 228-231, 2005. Proc. of SPIE Vol. 5943 254