Performance evaluation of the first model of 4D CT-scanner

Performance evaluation of the first model of 4D CT-scanner

International Congress Series 1256 (2003) 19 – 25 Performance evaluation of the first model of 4D CT-scanner Masahiro Endo a,*, Sinichiro Mori a, Tak...

401KB Sizes 0 Downloads 31 Views

International Congress Series 1256 (2003) 19 – 25

Performance evaluation of the first model of 4D CT-scanner Masahiro Endo a,*, Sinichiro Mori a, Takanori Tsunoo a, Susumu Kandatsu a, Shuzi Tanada a, Hiroshi Aradate b, Yasuo Saito b, Hiroaki Miyazaki b, Kazumasa Satoh c, Satoshi Matsusita c, Masahiro Kusakabe d a

Research Centre of Charged Particle Therapy, National Institute of Radiological Sciences, 9-1, Anagawa 4-Chome, Inage, Chiba 263-8555, Japan b Toshiba Corp. Medical System Company, Otawara, 324-8550, Japan c Sony Corp. Frontier Science Laboratories, Tokyo 141-0001, Japan d Fukui University Faculty of Engineering, Fukui 910-8508, Japan Received 21 March 2003; received in revised form 21 March 2003; accepted 21 March 2003

Abstract Four-dimensional computed tomography (4D CT) is a dynamic volume imaging system of moving organs with an image quality comparable to conventional CT. With 4D CT, one could carry out not only new diagnoses but also provide new interventional therapy by real-time observation of its procedure. In order to realize 4D CT, we have developed a novel 2D detector on the basis of the present CT technology, and mounted it on the gantry frame of the state of the art CT-scanner. We have evaluated its performances with standard stationary phantoms and scanned normal volunteers. In the present report, we describe the results of such performance evaluations. D 2003 Published by Elsevier Science B.V. Keywords: Four-dimensional computed tomography (4D CT); Dynamic volume imaging; Phantom study

1. Introduction Since the advent of computed tomography (CT) in 1973, dynamic imaging of moving organs in a living person has been one of the biggest dreams in this field [1]. The concept * Corresponding author. Tel.: +81-43-206-3178; fax: +81-43-206-3246. E-mail address: [email protected] (M. Endo). 0531-5131/03 D 2003 Published by Elsevier Science B.V. doi:10.1016/S0531-5131(03)00332-7

20

M. Endo et al. / International Congress Series 1256 (2003) 19–25

is simply called as 4D CT because it takes three-dimensional (3D) image with additional dimension of time. With 4D CT, one could carry out not only new diagnoses but also provide new interventional therapy by real-time observation of its procedures. Because volume data (3D data) can be acquired by cone-beam CT using a rotation of the cone-beam [2,3], continuous rotation of the cone-beam allows dynamic volume data (4D data) to be acquired. We have developed a prototype of four-dimensional (4D) CTscanner by mounting a discrete 2D detector on the gantry frame of the state-of-the-art CTscanner (Toshiba Aquillion) [4– 6]. It can take dynamic volume data of 10 cm long in an axial direction for 14 s with a rotation speed of 1.0 second per rotation. We have evaluated its performances with standard stationary phantoms, normal volunteers. In the present paper, we report the results of such performance evaluations.

2. Method 2.1. Scanner system The newly developed prototype scanner employs a wide-area 2D detector designed on the basis of the present CT technology. The number of detector elements is 912 channels (in an azimuthal direction)  256 segments (in an axial direction); element size is approximately 1  1 mm and the detector element consists of a pair of scintillator and photodiode. The scanner can take dynamic volume data of 10 cm long in an axial direction for 14 s with a rotation speed of 1.0 second per rotation. Data sampling rate is 900 views (frames) per second, and the dynamic range of A/D converter is 16 bit. A FDK (Feldkamp – Davis –Kress) algorithm [7] is used for reconstruction after preprocessing of raw data. It takes about 6 min to reconstruct volume data of 512  512  256 voxels by parallel use of 128 microprocessors. Fig. 1 shows a photograph of the scanner.

Fig. 1. Photograph of 4D CT-scanner.

M. Endo et al. / International Congress Series 1256 (2003) 19–25

21

2.2. Phantom experiments The image characteristics such as noise, uniformity and spatial resolution were evaluated with stationary phantoms in a single rotation scan. The evaluation results compared those of a state-of-the-art multi-detector (MD) CT-scanner routinely used at our institute (SOMATOM Volume Zoom Plus4). Scan conditions for both CT were chosen as same as possible. The noise and uniformity were evaluated with a water-filled phantom of 200 mm in diameter and 250 mm in height. The phantom wall was made of 10-mm-thick lucite. The phantom was scanned with 4D CT and MD CT using the same X-ray conditions. For 4D CT, voxel size was a 0.5-mm cube, while for MD CT, pixel size and slice thickness were chosen to be 0.5 mm at a single-slice scan. From transverse images, means and standard deviations of CT-number were calculated in nine regions of interests (ROIs) with a diameter of 10 mm aligned at every 20 mm along the diameter of the phantom. For 4D CT, calculations were made at midplane (z = 0), z = 20 mm and z = 40 mm. The spatial resolution was evaluated with a specially designed rod phantom consisting of five sets of high contrast rods [8]. Each set consists of three rods with the same diameter and material. The minimum diameter of rods was 0.5 mm. The rod phantom was inserted to the center of a lucite cylinder of with a diameter of 200 mm and the same height in two directions, perpendicular or parallel to the transverse plane. For 4D CT, the rod phantom was set at three different positions, at midplane (z = 0), z = 20 mm and z = 40 mm. For MD CT, a single-slice scan and a helical scan were made to take transverse image and longitudinal image, respectively. Exposure dose to an object was measured with an extension of the standard measurement method of CT dose index (CTDI) for 4D CT. A lucite cylinder with a diameter of 320 mm was used in this measurement. The length of the phantom was 300 mm, which was

Fig. 2. Noise along x-axis.

22

M. Endo et al. / International Congress Series 1256 (2003) 19–25

twice the standard one because the longitudinal field of view (collimator aperture) was 128 mm along the rotation axis in the present 4D CT-scanner. Because the effective length of the standard chamber was 100 mm, exposure dose was measured at three contiguous positions separated by intervals of 100 mm. CTDI was obtained by the sum of ion chamber output at three positions divided by nominal slice thickness. CTDI for MD CT was measured with the standard method to 10-mm collimator width. 2.3. Volunteer study Several volunteers were scanned to explore clinical potentials. While most were scanned in a 3D mode in which an object was scanned with a sep-and-shoot manner, two were scanned during intended motions in a 4D mode in which scanning was with multiple rotation. Total exposure dose for each volunteer was limited to 600 mAs that corresponds to exposures of 5 mSv for head scan and 10 mSv for body scan.

3. Results and discussion 3.1. Phantom study The magnitude of noise was obtained from the standard deviations of CT-number in the nine ROIs in a transverse section. Fig. 2 shows the relationships between standard deviation and the position along x-axis for 4D CT and MD CT. As approaching to the center of the phantom, the magnitude of noise became a little higher in the both scanners. This may be attributed to ring artifacts, those would become more significant when approaching to the center. Because average standard deviations of nine ROIs were 17.7 HU at the midplane (z = 0 mm), 16.7 HU at z = 20 mm, 16.9 HU at z = 40 mm for 4D CT

Fig. 3. Uniformity along x-axis.

M. Endo et al. / International Congress Series 1256 (2003) 19–25

23

Fig. 4. Results of spatial resolution measurement: (a) cross-section of the phantom, (b) MD CT (transverse), (c) MD CT (longitudinal), (d) 4D CT (transverse at midplane), (e) 4D CT (transverse at z = 20 mm), (f) 4D CT (transverse at z = 40 mm), (g) 4D CT (longitudinal).

and 11.2 HU for MD CT, the magnitude of noise was independent of longitudinal positions (z-coordinates) for 4D CT. The magnitude of noise was slightly higher in 4D CT. Those causes were now under examinations. For uniformity measurement, we calculated averages of CT-number in ROIs in the transverse section. Fig. 3 shows relationships between the average and the position along x-axis for 4D CT and MD CT. From the figure, CT-numbers are little higher than 0 for 4D CT, which might be attributed to incomplete calibration of CT-number. The standard deviations of the average were 4.9 HU at the midplane (z = 0 mm), 3.8 HU at z = 20 mm, 2.9 HU at z = 40 mm and 2.5 HU for MD CT. Slight elevation of these values for 4D CT especially in the midplane might be attributed to ring artifacts at the center ROIs. Fig. 4 shows the results of spatial resolution measurement. Fig. 4a and b shows the results of MD CT for a transverse section (Fig. 4a) and a longitudinal section (Fig. 4b). While down to 0.5 mm rods are separable in the transverse section, 1.6-mm rods are not clearly separated in the longitudinal section. Fig. 4c –f shows the results of 4D CT. Fig. 4c –e corresponds to transverse sections at the midplane (z = 0 mm), z = 20 mm and z = 40 mm, respectively, and Fig. 4f shows the result for a longitudinal section. For 4D CT, down Table 1 Result of exposure dose measurement (in mGy/100 mAs)

4D CT MD CT

CTDIcenter

CTDIperiphery

CTDIw

10.2 11.1

16.7 26.6

14.5 21.4

24

M. Endo et al. / International Congress Series 1256 (2003) 19–25

Fig. 5. Transverse images of volunteers (left: head, right: liver).

to 0.5 mm rods are separable in the transverse sections independent of z-coordinates and also separable in the longitudinal section. The present result shows that for 4D CT, isotropic resolving power of less than 0.5 mm was achieved at a wide range of zcoordinates, which seems difficult for MD CT in a short scan time. With regards to exposure dose, CTDIperiphery and CTDIcenter were measured for the both CT. The results are listed in Table 1. CTDIw is calculated by the following equation: CTDIw ¼

1 2 CTDIcenter þ CTDIperiphery 3 3

From the table, CTDIs are smaller for 4D CT than for MD CT and its reasons are now under examinations. 3.2. Volunteer study Figs. 5 and 6 show examples of volunteer results. Fig. 5 shows transverse images of volunteers. These images were obtained from 3D data of volunteers. Voxel sizes were 0.5 mm for head and 0.6 mm for liver, respectively. Because slice thickness was the same as the voxel size for each case, image noises were more significant than ordinary images, which are usually taken with thicker slices. These noises may be decreased by z-direction

Fig. 6. Dynamic 3D rendered images (4D image) of a volunteer during intended motion of his jaws.

M. Endo et al. / International Congress Series 1256 (2003) 19–25

25

smoothing. Fig. 6 shows dynamic 3D rendered images (4D image) of a volunteer during intended motion of his jaws. These images were examples of 30 3D images reconstructed with a 0.1-s interval. Clinical studies as well as animal experiments are planned to explore the possibilities of 4D CT.

References [1] R.A. Robb, The spatial reconstructor: an X-ray video-fluoroscopic CT scanner for dynamic volume imaging of moving organs, IEEE Trans. Med. Imag. 1 (1982) 22 – 33. [2] D. Saint-Felix, Y. Trousset, C. Picard, C. Ponchut, R. Romeas, A. Rougee, In vivo evaluation of a new system for computerized angiography, Phys. Med. Biol. 39 (1994) 585 – 595. [3] M. Endo, K. Yoshida, N. Kamagata, K. Satoh, T. Okazaki, Y. Hattori, et al., Development of a 3D CT-scanner using a cone beam and video-fluoroscopic system, Radiat. Med. 16 (1998) 7 – 12. [4] M. Endo, T. Tsunoo, S. Kandatsu, S. Tanada, H. Aradate, Y. Saito, 4-dimensional computed tomography (4D CT)—its concepts and preliminary development, M7-6, 2001, IEEE NSS Conf. Record. [5] M. Endo, T. Tsunoo, S. Kandatsu, S. Tanada, H. Aradate, Y. Saito, 4-dimensional computed tomography (4D CT)—its concepts and preliminary development, Radiat. Med. 21 (2003) 17 – 21. [6] M. Endo, T. Tsunoo, S. Kandatsu, S. Tanada, H. Aradate, Y. Saito, M. Kusakabe, K. Satoh, S. Matsusita, Development and performance evaluation of the first model of 4D CT-scanner, Proc. of CARS, 2002, pp. 372 – 376. [7] L.A. Feldkamp, L.C. Davis, J.W. Kress, Practical cone-beam algorithm, J. Opt. Soc. Am. A1 (1984) 612 – 619. [8] M. Endo, K. Nishizawa, K. Iwai, M. Matsumoto, K. Yoshida, K. Satoh, et al., Image characteristics and effective dose estimation of a cone beam CT using a video-fluoroscopic system, IEEE Trans. Nucl. Sci. 46 (1999) 686 – 690.