Portable Optically Tracked Ultrasound System for Scoliosis Measurement
Monitoring spinal curvature in adolescent kyphoscoliosis requires regular radiographic examinations; however, the applied ionizing radiation increases the risk of cancer. Ultrasound imaging is favorable over X-ray because it does not emit ionizing radiation. It has been shown in the past that tracked ultrasound can be used to localize vertebral transverse processes as landmarks along the spine to measure curvature angles. Tests have been performed with spine phantoms, but scanning protocol, tracking system, data acquisition and processing time has not been considered in human subjects yet. In this paper, a portable optically tracked ultrasound system for scoliosis measurement is presented. It provides a simple way to acquire data in the clinical environment with the aim of comparing results to current X-ray-based measurement. The workflow of the procedure was tested on volunteers. The customized open-source software is shared with the community as part of our effort to make a clinically practical system.
KeywordsAdolescent idiopathic kyphoscoliosis Scoliosis Kyphosis Tracked sonography Tracked ultrasound snapshot
1.1 Adolescent Idiopathic Scoliosis
The most frequent form of spinal deformities is adolescent idiopathic scoliosis. Studies conducted in different countries found its prevalence between 1 and 5 %, therefore, it is considered a common disease in children [1, 2, 3]. The etiology of this disease has not yet been fully discovered, however, genetic factors influence both the incidence and the severity of scoliosis . It is most commonly diagnosed in early adolescence (9–12 years of age) and progresses until the spine reaches full development, around the age of 18–20 years.
Symptoms of adolescent idiopathic scoliosis include lateral curvature of the spine in straight standing position, kyphosis, and permanent vertebral rotation around the axis of the spine. The severity of scoliosis is characterized by the angle of curvature between vertebrae above and below the curvature (Cobb angle). Therapeutic protocols are based on Cobb angle: (i) curvatures less than \(20^\circ \) require X-ray monitoring every 3–6 months, depending on the rate of progression; (ii) scoliosis between \(20^\circ \)–\(40^\circ \) degrees is treated with bracing; and (iii) scoliosis over 40\(^\circ \), or if chest deformation causes breathing difficulties, is treated surgically, permanently fusing the vertebrae in a straight position. Scoliosis monitoring continues after surgical treatment as well using regular spine imaging. About 10 % of adolescent scoliosis cases progress to a state when they require therapy.
Although screening children in the early adolescent age for scoliosis would be necessary for optimal treatment and optimal use of clinical resources, currently affordable screening methods are inaccurate . Therefore, patients may be diagnosed when scoliosis progressed in an advanced stage.
Once diagnosis is confirmed, it is important to frequently monitor all cases to make sure progression is detected early and appropriate therapy is started. However, frequent X-ray imaging in children can lead to increased risk of cancer. Girls who undergo regular spine X-ray, have a nearly twofold risk of breast cancer as adults [6, 7]. Another study found that repeated X-ray exams in childhood significantly contribute to leukemia and prostate cancer . Further studies may be needed to establish accurate estimates of the risks of X-ray, but a safe scoliosis monitoring method would improve the health of this young patient population.
1.2 Scoliosis Monitoring Techniques
Radiation-free scoliosis monitoring methods have been investigated in the past, but none of them have been successful in replacing X-ray in the clinical practice. The optimal method needs to be: free of ionizing radiation, accessible to the patient population, and accurate for therapeutic decision making.
Magnetic resonance imaging (MRI) is a safe and accurate alternative to X-ray. Open MRI machines permit scanning from a standing patient position, making these images suitable for scoliosis angle measurement. Unfortunately, MRI is less accessible than X-ray due to its high cost and the patient has to stand motionless for several minutes while the scanner captures the entire spinal column, which further limits its use for routine monitoring in an adolescent population .
Inspection will yield visual signs of scoliosis on the back of the patient. Visual signs of spinal curvatures have been measured using computerized topographic scans of the skin surface using laser scanners and stereo camera technology . However, surface scans are not informative enough to support therapeutic decisions .
Ultrasound does not have a large enough field of view to directly visualize spinal curvatures. There have been attempts to use indirect ways of measuring spinal curvatures with ultrasound. A correlation between vertebral rotation and scoliosis angles in untreated patients was discovered  but this correlation is unreliable and completely lost in patients receiving therapy .
One of the recently emerging imaging modalities is tracked ultrasound: a combination of conventional ultrasound and position tracking technology. With tracked ultrasound it is possible to create a 3D reconstruction from 2D ultrasound images. Position tracking of the ultrasound transducer allows to display the whole spine region in 3D. Experimental results have confirmed that landmarks on reconstructed image volumes could be used to monitor spine curvature angles [14, 15]. Tracked ultrasound appears to be the only alternative to X-ray that fulfills all three requirements: safety, accessibility and accuracy.
A new method for scoliosis measurement has been developed previously, and tested on phantom models . Tracked ultrasound (TUS) has been successfully used in medical imaging and image-guided interventions. In particular, it has been tested in spinal injection navigation  and vertebra localization for spine surgery .
Our goal is to develop a clinically practical system for scoliosis monitoring. As part of this translational effort we build upon of the proof of concept phantom system proposed by  and present a portable TUS system based on a new ultrasound machine and optical tracking. We streamline the workflow of the procedure so it can be easily replicated by other researchers and operated without deep technical background. The workflow is implemented as an open-source module of the 3D Slicer1 application, and can be conveniently installed from the 3D Slicer extension manager (app store). We evaluated the usability of the developed TUS system.
2 Materials and Methods
2.1 System Design
The TUS system has three main components. An optical tracker (Micron Tracker, Claron Technologies Inc., Toronto, Ontario, Canada), a portable ultrasound scanner with USB connection (General Purpose 99–5914 Probe, Interson Corporation, Pleasanton, CA, USA), and a laptop computer (Fig. 1).
Contrary to  we use an optical tracker instead of an electromagnetic tracker. Optical trackers can use wireless markers, are generally more accurate and are not affected by metallic or electronic objects in the environment. Sensors used as reference markers are made of paper, which is very cheap and also makes feasible the tracking of multiple reference markers. This can be used for patient motion detection and to be more robust to markers occlusions.
Hardware interfaces are implemented in the PLUS software2 . PLUS implements an abstraction layer over tracker and imaging devices, and provides calibration and synchronization methods for tracked ultrasound. It transmits tracked images to 3D Slicer through the OpenIGTLink protocol .
2.2 System Setup
The hardware described in previous section was integrated using the PLUS library. Configuration files to connect different acquisition systems are provided in PLUS website. In particular, sample configuration files used to connect the Interson Probe and the Micron Tracker can be found in the PLUS website.
As with any TUS system, it requires a calibrated probe. If US imaging parameters are not changed, the calibration can be done once and saved in the configuration file. We found that 10 cm depth is adequate for this application.
When the patient comes in, at least one reference marker is attached to his back. This reference marker is used because the patient may move during the acquisition. The assumption is that the relative position between the transverse processes and the reference marker is the same during the whole scanning.
The tracker should be facing a wall where a reference marker must be attached. During the acquisition, the patient will be asked to place in front of the wall. The reason why a marker is attached to the wall is to be able to project the transverse processes positions to the wall. This makes the angle measurements compatible with the traditional procedure done on frontal X-ray radiographies.
2.3 Slicelet Implementation for Spinal Curvature Measurement
We designed and implemented a custom workflow and user interface, called a slicelet, based on the 3D Slicer application platform and its SlicerIGT extension.3 Slicelets are custom user interfaces programmed for the 3D Slicer application that typically support a single clinical workflow without the complexity of the full 3D Slicer user interface. Our slicelet helps in the localization of vertebral transverse processes as landmarks along the spine to measure curvature angles. We build upon  and added new features and interfaces to streamline the procedure, reducing the lengthy analysis time.
The slicelet currently consists of the steps depicted in Fig. 2: communication with acquisition devices, tracked ultrasound snapshots and continuous tracked ultrasound video acquisition, localization of vertebral transverse processes in the acquired images and computation of angles between vertebrae.
2.3.1 Communication with Acquisition Devices
Images and tracker positions are provided by the PLUS server. Once the PLUS server is running, the status of the communication can be seen in the slicelet as shown in Fig. 3. A tool viewer, with a convenient color code, shows markers and tools visibility. This tool is very useful when working with optical trackers.
2.3.2 Tracked US Images Acquisition
The scoliosis monitoring slicelet can be used to measure vertebral angles using both: tracked ultrasound snapshots and video sequences. We decided to offer both possibilities because transverse process localization is easier with snapshots, while time occupied during patient scan is lower when video sequences are acquired. In both cases, data can be saved to be inspected afterwards and can also be used to build a database.
2.3.3 Transverse Processes Localization
3 Curvature Angle Computation
The workflow described in previous section has been implemented; it provides all of the required capabilities for scoliosis monitoring using tracked ultrasound snapshots and video sequences.
As an example, Fig. 6 shows the tracked ultrasound snapshots acquired during a procedure. Red points correspond to the transverse processes locations in the ultrasound images. The red lines connect two transverse process that belong to the same vertebra.
The software can be installed from the extension manager of the latest version of the 3D Slicer application. The extension manager is a plug-in mechanism that allows researchers and developers to add extensions to the core 3D application. Those extensions can be installed by the user after installing the main 3D application without any programming required. The name of the slicelet is Scoliosis Monitoring and is part of the Scoliosis extension. All software used in this study is open-source, freely available for research or commercial use without any restrictions. This allows incremental research, and focus work on unsolved problems like transverse process automatic segmentation or spine volume reconstruction.
The whole procedure can be tested with various ultrasound and tracker systems supported by PLUS. This includes the most popular tracking systems (Polaris, Micron, Aurora, Ascension) and the Interson USB and Ultrasonix ultrasound machine. Besides, Epiphan or Imaging Control frame grabber can be used to acquire the images of any ultrasound machines with video output.
The workflow was implemented with the portable USB ultrasound probe, which shows sufficient image quality for this application. The size and weight of the acquisition systems that were used allows to pack and move the system, which will make this a portable solution for scoliosis measurement in future clinical applications.
Although some previous works have shown the feasibility of tracked ultrasound to localize vertebral processes to measure spinal curvature, the problem is still open. We believe we are sharing a valuable tool that can be very useful to show physicians the potential of the methods and also to identify the main challenges we need to tackle in order to bring scoliosis monitoring with ultrasound into routine clinical use.
The portable optically tracked ultrasound system can save ultrasound video sequences along with tracking data. This enables the acquisition of experimental studies from volunteers and patients looking forward to building the essential dataset for the research in the field. Currently, vertebral transverse processes are marked manually by the physician and the data is used to compute the curvature angle. This expert segmentation can also be saved in the application and this valuable information will enable testing future automatic segmentation algorithms.
The tool is integrated into the Slicer platform and the Slicer-IGT extension. This Slicer platform can be freely used by the research community and it enables an easy manipulation of the tracked ultrasound studies and the integration with other modalities such as X-ray, CT or MRI. Future developments in the field can also be rapidly integrated into Slicer as extensions (its plug-in mechanism) and shared through the Slicer app-store.
Our next step in this ongoing project will be to study the correlation between the radiography and sonographic curvature measuring methods in real patients. Another future research line will be related to the segmentation of the transverse processes. In some patients it is hard for the physician to correctly identify some transverse processes. We plan to explore the feasibility of using tracked ultrasound volume reconstructions of the spine in conjunctions with models of the spine. A nice 3D model of the spine might give the physician some spatial information that could greatly facilitate the transverse process localization procedure.
This work was supported in part by the Canada-Latin America Research Exchange Grant (LACREG) program. Comision Sectorial de Investigacion Cientıfica (CSIC, Universidad de la Republica, Uruguay) supported the internship of G. Carbajal at Perklab.
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