Aquilion ONE / ViSION Edition CT Scanner Realizing 3D Dynamic Observation with Low-Dose Scanning

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1 Aquilion ONE / ViSION Edition CT Scanner Realizing 3D Dynamic Observation with Low-Dose Scanning KAZAMA Masahiro SAITO Yasuo Computed tomography (CT) scanners have been continuously advancing as essential diagnostic imaging equipment for the diagnosis and treatment of a variety of diseases, including the three major disease classes of cerebrovascular disease, cardiovascular disease, and cancer. Through the development of helical CT scanners and multislice CT scanners, Toshiba Medical Systems Corporation (TMSC) has developed the Aquilion ONE, a CT scanner with a scanning range of up to 160 mm per rotation that can obtain three-dimensional (3D) images of the brain, heart, and other organs in a single rotation. We have now developed the Aquilion ONE / ViSION Edition, a next-generation 320-row multislice CT scanner incorporating the latest technologies that achieves a shorter scanning time and significant reduction in dose compared with conventional products. This product with its low-dose scanning technology will contribute to the practical realization of new diagnosis and treatment modalities employing four-dimensional (4D) data based on 3D dynamic observations through continuous rotations. 1. Introduction Computed tomography (CT) scanners were developed and applied in clinical practice in the first half of the 1970s. Since its introduction over 40 years ago, CT has evolved into an essential diagnostic imaging method supporting a variety of clinical applications, from screening to detailed examinations. During the initial stage of its introduction, improvements were made to CT so that a single plane could be visualized with more detail in a shorter time. By around 1990 when helical CT scanners were developed and clinically employed, permitting scanning to be performed continuously while the patient is moved through the scanner, CT had become an imaging method that permits the human body to be visualized three-dimensionally (1). In the second half of the 1990s, multislice CT was introduced, enabling simultaneous acquisition of multiple slices. This CT technique dramatically expanded the usefulness of helical scanning (2). In succeeding years, Toshiba Medical Systems Corporation (TMSC) made efforts to develop a new CT scanner that would enable the target organ to be scanned in a single gantry rotation without the necessity for helical scanning, and enable visualization of a moving organ with continuous scanning. After initially developing a prototype 256-row multislice CT scanner (3), TMSC eventually developed the 320-row multislice CT scanner Aquilion ONE in This CT scanner supports continuous scanning at 0.35 s/rotation and makes it possible to observe the dynamics of the target organ three-dimensionally. Since the Aquilion Figure 1. Aquilion ONE / ViSION Edition. This is a CT scanner that permits dynamic 3D imaging to be performed for a 160-mm-wide area at a low exposure dose. ONE realizes a new CT examination method, rather than the conventional method that focused on the helical scan technique, it is referred to as area detector CT (ADCT) or dynamic volume CT. We have now developed the Aquilion ONE / ViSION Edition, a next-generation CT scanner that evolved from the Aquilion ONE. Since this scanner features a shorter scan time, significant reduction in exposure dose, improved operability, and energy-saving technology, it is both patient- and user-friendly (Figure 1). In particular, realization of low-dose scanning has expanded the scope of dynamic 3D imaging by continuousrotation scanning, and target regions for functional diagnosis based on dynamic scanning have expanded accordingly (4). 12

2 This report describes features of the Aquilion ONE / ViSION Edition, the latest technologies available in this system, and prospects for the future. 2. Features of the Aquilion ONE / ViSION Edition A CT scanner is a diagnostic imaging device with an X-ray tube and an X-ray detector located at diametrically opposite positions, with the scan target between them. It continuously rotates around the scan target to acquire 360 of projection data and images the 3D distribution of the X-ray attenuation coefficients inside the scan target. The Aquilion ONE is provided with an X-ray tube and an X-ray detector that cover a wide 160-mm range in the axial direction and permits a 3D image of entire organs such as the brain or the heart to be acquired in a single rotation. By performing continuous-rotation scanning, dynamic scanning of the target organ is possible. The new-generation CT scanner Aquilion ONE / ViSION Edition has the following features. 2.1 Shorter scan time By improving the ability of the CT scanner main unit to withstand centrifugal forces, the shortest scan time has been improved from 0.35 s/rotation to s/ rotation (more than 20%). This has made it possible to obtain images with less blurring, even for patients with high heart rates, facilitating low-dose scanning in a single cardiac cycle. 2.2 Significant reduction of exposure dose The scanner incorporates a new X-ray detector with 1.4 times higher sensitivity than a conventional detector, and the adaptive iterative dose reduction 3D (AIDR 3D) function, an image reconstruction technology that reduces image noise by up to 50% (corresponding to reduction of up to 75% for dose conversion). These features have made it possible to significantly reduce exposure dose in dynamic 3D imaging (which requires continuous-rotation scanning), which has contributed to an acceleration of clinical research into dynamic assessment. 2.3 Improved operability A large gantry bore (opening) of 780 mm has been achieved by using compact units inside the scanner main unit. This has made it possible to improve access to the patient during examination and easier for the patient to positively move joints etc. during examination, thus facilitating dynamic scanning. 2.4 Reduction of metal artifacts At clinical sites, various types of metals are used in treatment etc. The Aquilion ONE / ViSION Edition is provided with single energy metal artifact reduction (SEMAR) which accurately reduces artifacts generated from such metals. This has made it possible to assess the structures around such metal objects, which was previously difficult. 2.5 Reduced power consumption Standby power consumption accounts for a large part of total system power consumption. We have therefore employed an optimized image reconstruction unit and introduced sleep mode as an energy-saving measure for the gantry, achieving an approximately 40% reduction in power consumption. In addition, shorter scan times and low-dose scan conditions using AIDR 3D have led to reduced power consumption during scanning, which makes the Aquilion ONE / ViSION Edition a CT scanner that benefits both hospital management and the global environment. 3. Dose reduction technologies that support dynamic 3D imaging Because dynamic 3D imaging, which was a concept for development of the Aquilion ONE, is based on continuous-rotation scanning of the same region, it was essential to reduce the exposure dose. Although dynamic 3D imaging was made available when the Aquilion ONE was developed and released in 2007, subjects were limited because the use of the technique increases the total exposure dose in a study. In order to expand the field of dynamic 3D imaging and promote wider acceptance of this technique as a new functional study method, it was necessary to reduce the exposure dose to the same level as for a conventional CT study, even though continuous-rotation scanning is required. In order to significantly reduce the exposure dose in dynamic 3D imaging, it was necessary to make major improvements to fundamental items such as X-ray detector sensitivity and image reconstruction technologies to reduce noise. The high-sensitivity large-area X-ray detector and the AIDR 3D function employed in the Aquilion ONE / ViSION Edition are described below. 3.1 High-sensitivity large-area X-ray detector In order to acquire 160-mm-wide high-definition projection data with a slice thickness of 0.5 mm in a single gantry rotation, the Aquilion ONE requires a large-area X-ray detector with detector elements of approximately precisely arranged pixels. For the Aquilion ONE / ViSION Edition, we have developed a new X-ray detector with significantly improved sensitivity in order to ensure high image quality, even at low-dose scanning (Figure 2). Image noise significantly depends on the signal-tonoise (SN) ratio of the X-ray detector, and in order to 13

3 Acquired projection data Scanner model Anatomical model-based noise reduction Image update AIDR 3D image Projection data noise reduction Statistical model Blending % Figure 2. High-sensitivity large-area X-ray detector. This is a new X-ray detector with a sensitivity 1.4 times higher than the conventional detector, ensuring high image quality even in lowdose scanning. Figure 3. Outline of AIDR 3D technology. Noise can be reduced efficiently while maintaining image resolution by applying a scanner model, a statistical noise model, and an anatomical model. improve the SN ratio, it is necessary to improve the efficiencies of the X-ray detector (geometric efficiency, absorption efficiency, emission efficiency, and condensing efficiency), thus enhancing its sensitivity. By employing an inter-element separator that optimizes the reflection factor while minimizing the dead space for the incident X-rays, and improving the scintillator manufacturing process to produce a scintillator that can efficiently convert X-rays into light and direct it to the optical sensor, the geometric efficiency and condensing efficiency have been increased in a wellbalanced manner. In addition, emission efficiency has been increased by improving the raw materials used in the scintillator. With these improvements, we have succeeded in developing a high-sensitivity X-ray detector with 1.4 times higher sensitivity than that of a conventional X-ray detector. This corresponds to exposure dose reduction of up to 30% when low-dose scanning is performed. 3.2 AIDR 3D Noise reduction processing in image reconstruction has been a challenge since the beginning of CT scanner development, and various methods have been applied. However, the conventional noise reduction processing methods basically involve a tradeoff with resolution. Specifically, if noise reduction is given priority, image resolution is significantly reduced, and vice versa. AIDR 3D is an image reconstruction technology that employs an iterative method to reduce image noise, taking into account the characteristics of the scan target (the human body). With this technology, it is possible to significantly reduce noise in the target region where uniformity is required while maintaining the required level of resolution. Noise elements are extracted from the acquired projection data based on the scanner model and statistical model. Noise elements are also extracted from the image data based on a 3D anatomical model (target region and tissue structure). By repeating extraction and elimination of noise elements, image noise can be reduced effectively while maintaining the required level of 3D spatial resolution (Figure 3). AIDR 3D reduces image noise by up to 50%. This corresponds to 75% dose reduction when converted to the exposure dose required to acquire an image with the same level of noise (the exposure dose can be reduced to 1/4 that of the conventional method). The reconstruction time is almost the same as that for the conventional method. In-scan dose setting is also possible, which is another feature of this function. Specifically, AIDR 3D is a noise reduction processing technology that can be used while maintaining throughput comparable to that for conventional methods (5). 4. 4D applications to realize dynamic 3D imaging Diagnosis using dynamic 3D data acquired by the Aquilion ONE / ViSION Edition, which permits continuous-rotation scanning to be performed at lower exposure dose, is being accelerated in clinical practice. To support this, we have been supplying a range of 4D application software packages. Typical recent applications are described below D cerebral artery morphological analysis application for the cerebral blood vessels To support assessment of the risk of rupture of a cerebral aneurysm, it is necessary to visualize the aneurysm and measure it with high precision. The 4D cerebral artery morphological analysis application is provided with functions for automatically extracting a cerebral aneurysm and measuring the longaxis diameter, volume, aspect ratio, and volume-to-ostium ratio (Figure 4). Currently, clinical research is in progress. It is expected that a change in the wall of the aneurysm during pulsation can be visualized by performing electrocardiogram (ECG)-gated analysis of the image data acquired by continuous-rotation scanning and varies over time, thus making it possible to identify a portion with a high risk of rupture. 14

4 (a) A series of images showing chronological change in the cerebral aneurysm (a) 3D image and several cross-sectional images that allow dynamic observation (b) Graph showing the relationship between the cardiac phase and the VOR (change in the dome-to-neck ratio of the aneurysm over time) Data courtesy of Fujita Health University Hospital Figure 4. Example of temporal changes of cerebral aneurysm obtained by 4D cerebral artery morphological analysis. By visualizing and measuring the change in a cerebral aneurysm over time, it is possible to identify a portion with a high risk of rupture D airways analysis application for the chest (bronchi) For the diagnosis of tracheobronchomalacia and chronic obstructive pulmonary disease (COPD), it is useful to observe the 3D image of the bronchi in multiple phases during the respiratory cycle. The 4D airways analysis application is provided with a function for tracking movement of the bronchi during the respiratory cycle with high precision and automatically measuring the luminal diameter and cross-sectional area of the bronchi (Figure 5). It is expected to improve the diagnostic accuracy for tracheobronchomalacia and COPD by quantitatively visualizing the change in shape of the bronchi during respiration, and clinical research using this function is under way D orthopedic analysis application Among patients who feel pain when they move joints etc. during daily activities, there are many who do not show clear morphological abnormalities. In this case, it may be possible to identify complex mechanical abnormalities causing symptoms in each patient by visualizing continuous movement. The 4D orthopedic analysis application is provided with visualization and measurement functions to precisely view an abnormality in a joint. In order to intuitively understand an abnormality in the complicated movement of multiple bones such as a joint, a display method in which a specific bone is fixed is more useful than a display method in which all bones move. (b) Graph showing changes in the specified measurement data during the respiratory cycle (c) List of the positions (ROI) and types (distance, area, volume, etc.) of measurement data Data courtesy of Ohara General Hospital Figure 5. Example of trachea data for one breathing cycle obtained by 4D airways analysis. It is possible to measure the change in the luminal diameter and cross-sectional area of the bronchi during the respiratory cycle. Because all bones move, it is difficult to recognize an abnormality. (a) Conventional display method It is easier to view the movements of the surrounding bones. (b) Display method in which the specified bone is fixed in one space Data courtesy of CHU Central Hospital (Nancy, France) Figure 6. Example of 4D data of joint obtained by 4D orthopedic analysis. An abnormality in movement can be detected more easily by fixing and displaying the bone specified in advance. 15

5 The 4D orthopedic analysis application is provided with a function in which, rather than fixing the viewpoint and direction in space for 3D display of 4D joint movement data, the coordinate axes are dynamically adjusted in advance. They are set to a specified target bone so that it remains stationary during observation. This function makes it easier to intuitively identify an abnormality in complex movement of several bones (Figure 6). The 4D orthopedic analysis application is also provided with a function that automatically measures distance and angle for 4D data of several bones to quantitatively display changes over time, allowing abnormalities in movement to be assessed using a quantitative index. 5. Conclusion The Aquilion ONE development project started more than 20 years ago with the goal of freely visualizing motion in the human body. The newly developed Aquilion ONE / ViSION Edition has made it possible to perform dynamic 3D imaging at a lower exposure dose, and the future vision established at the beginning of Aquilion ONE development is becoming reality. At TMSC, we have been making efforts to develop CT scanners that provide faster, more detailed, and widerrange scanning at lower exposure dose. We will continue to bring innovation to the basic performance of CT scanners in order to further shorten scan times, improve resolution, expand the scan range, dramatically reduce noise, and reduce the exposure dose. Through such advances, we aim to contribute widely to healthcare. References (1) Kimura, K. et al., eds Basic Principles and Clinical Applications of Helical Scan: Applications of Continuous- Rotation CT. Iryokagakusha (in Japanese). (2) Saito, Y Multislice X-ray CT Scanner. Medical Review 66: 1 8. (3) Endo, M. et al Development and performance evaluation of the first model of 4-D CT-scanner. IEEE Trans. Nuclear Science 50 (5): (4) Katada, K Evolution of Area-Detector CT. INNERVI- SION 27 (6): (in Japanese). (5) Sugihara, N Aquilion ONE and AIDR 3D Technologies. New Medicine in Japan 38 (12): 92 (in Japanese). KAZAMA Masahiro Senior Specialist, CT Systems Development Department, CT Systems Division, Toshiba Medical Systems Corporation. He is engaged in the development of X-ray CT systems. SAITO Yasuo Senior Specialist, CT Systems Development Department, CT Systems Division, Toshiba Medical Systems Corporation. He is engaged in the development of X-ray CT systems. 16