A fast calibration method for 3-D tracking of ultrasound images using a spatial localizer

Development and validation of a gel wax phantom to evaluate geometric accuracy and measurement of a hyperechoic target diameter in diagnostic ultrasound imaging

Diagnostic ultrasound (US) scanners are generally evaluated using proprietary quality assurance (QA) phantoms, but their prohibitively high cost may prevent organizations to perform the necessary tests. This study aimed to develop a low-cost gel wax phantom with targets to determine the lateral and axial resolution and diameter of a hyperechoic target in an US scanner. The acoustic property (AP) of gel wax, which includes the speed of sound (cus), acoustic impedance (Z), and attenuation coefficient (µ), were determined for multiple transducers operating at 2.25, 5, 10, 15, and 30 MHz. These results were compared to the AP of soft tissue. Two polytetrafluoroethylene (PTFE) rectangular frames with holes separated by 5, 10, and 20 mm were constructed. Nylon filaments and stainless-steel disc (SS disc) (diameter = 16.8 mm) were threaded through the frames and suitably placed in gel wax to obtain orthogonal targets in the phantom. The target dimensions obtained from computerized tomography (CT) and US images of the phantom were compared for phantom validation. The average cus=1431.4 m/s, mass density ρ = 0.87 g/cm3, Z = 1.24 MRayls, and µ ranged from 0.7 to 0.98 dB/cm/MHz for gel wax at 22 °C. The US image measurement exhibited a maximum error in determining the diameter of the SS disc, resulting in a value of 18 mm instead of its actual value of 16.8 mm. The phantom volume decreased by 1.8% in 62 weeks. The present phantom is affordable, stable, customizable, and can be used to evaluate diagnostic US scanners across multiple centers.

Coplanarity Constrained Ultrasound Probe Calibration Based on N-Wire Phantom

OBJECTIVE

N-wire phantom-based ultrasound probe calibration has been used widely in many freehand tracked ultrasound imaging systems. The calibration matrix is obtained by registering the coplanar point cloud in ultrasound space and non-coplanar point cloud in tracking sensor space based on the least squares method. This method is sensitive to outliers and loses the coplanar information of the fiducial points. In this article, we describe a coplanarity-constrained calibration algorithm focusing on these issues.

METHODS

We verified that the out-of-plane error along the oblique wire in the N-wire phantom followed a normal distribution and used it to remove the experimental outliers and fit the plane with the Levenberg-Marquardt algorithm. Then, we projected the points to the plane along the oblique wire. Coplanarity-constrained point cloud registration was used to calculate the transformation matrix.

RESULTS

Compared with the other two commonly used methods, our method had the best calibration precision and achieved 25% and 36% improvement of the mean calibration accuracy than the closed-form solution and in-plane error method respectively at depth 16. Experiments at different depths revealed that our algorithm had better performance in our setup.

CONCLUSION

Our proposed coplanarity-constrained calibration algorithm achieved significant improvement in both precision and accuracy compared with existing algorithms with the same N-wire phantom. It is expected that calibration accuracy will improve when the algorithm is applied to all other N-wire phantom-based calibration procedures.

Calibration method for a breast intervention robot based on four-dimensional ultrasound image guidance

In breast interventional ultrasound therapy, it is difficult to directly diagnose the location of a tumor in 2-D ultrasound images. To assist surgeons in treatment more intuitively, a four-dimensional ultrasound image-guided breast intervention robot is proposed. The calibration approach of the ultrasonic image for the robot is one of the main contents of the research. This method is based on the establishment of a complete coordinate system conversion model, and it uses the ORB (oriented FAST and rotated BRIEF) feature extraction method to obtain and record the real-time image marker pixel positions, calculate the unknown parameters of the coordinate system conversion matrix, and establish a complete calibration system. This article demonstrates the feasibility of the calibration approach through experiments in our developed US-guided robotic system. Additional experimental and parametrical comparisons of the proposed method with state-of-the-art methods were conducted to thoroughly evaluate the outperformance of the proposed method.

Remote Ultrasound Scan Procedures with Medical Robots: Towards New Perspectives between Medicine and Engineering

Background

This review explores state-of-the-art teleoperated robots for medical ultrasound scan procedures, providing a comprehensive look including the recent trends arising from the COVID-19 pandemic.

Methods

Physicians' experience is included to indicate the importance of their role in the design of improved medical robots. From this perspective, novel classes of equipment for remote diagnostics based on medical robotics are discussed in terms of innovative engineering technologies.

Results

Relevant literature is reviewed under the system engineering point of view, organizing the discussion on the basis of the main technological focus of each contribution.

Conclusions

This contribution is aimed at stimulating new research to obtain faster results on teleoperated robotics for ultrasound diagnostics in response to the high demand raised by the ongoing pandemic.

A Novel Design of N-Fiducial Phantom for Automatic Ultrasound Calibration

Background:

Freehand ultrasound (US) is a technique used to acquire three-dimensional (3D) US images using a tracked 2D US probe. Calibrating the probe with a proper calibration phantom improves the precision of the technique and allows several applications in computer-assisted surgery. N-fiducial phantom is widely used due to the robustness of precise fabrication and convenience of use. In principle, the design supports single-frame calibration by providing at least three noncollinear points in 3D space at once. Due to this requirement, most designs contain multiple N-fiducials in unpatterned and noncollinear arrangements. The unpatterned multiple N-fiducials appearing as scattered dots in the US image are difficult to extract, and the extracted data are usually contaminated with noise. In practice, the extraction mostly relied on manual interventions, and calibration with N-fiducial phantom has not yet achieved high accuracy with single or few frame calibrations due to noise contamination.

Aims:

In this article, we propose a novel design of the N-fiducial US calibration phantom to enable automatic feature extraction with comparable accuracy to multiple frame calibration.

Materials and Methods:

Along with the design, the Random Sample Consensus (RANSAC) algorithm was used for feature extraction with both 2D and 3D models estimation. The RANSAC feature extraction algorithm was equipped with a closed-form calibration method to achieve automatic calibration.

Results:

The accuracy, precision, and shape reconstruction errors of the calibration acquired from the experiment were significantly matched with the previous literature reports.

Conclusions:

The results showed that our proposed method has a high efficiency to perform automatic feature extraction compared to conventional extraction performed by humans.

Freehand 3-D Ultrasound Imaging: A Systematic Review

Two-dimensional ultrasound (US) imaging has been successfully used in clinical applications as a low-cost, portable and non-invasive image modality for more than three decades. Recent advances in computer science and technology illustrate the promise of the 3-D US modality as a medical imaging technique that is comparable to other prevalent modalities and that overcomes certain drawbacks of 2-D US. This systematic review covers freehand 3-D US imaging between 1970 and 2017, highlighting the current trends in research fields, the research methods, the main limitations, the leading researchers, standard assessment criteria and clinical applications. Freehand 3-D US systems are more prevalent in the academic environment, whereas in clinical applications and industrial research, most studies have focused on 3-D US transducers and improvement of hardware performance. This topic is still an interesting active area for researchers, and there remain many unsolved problems to be addressed.

3D Catheter Shape Determination for Endovascular Navigation Using a Two-Step Particle Filter and Ultrasound Scanning

In endovascular catheter interventions, the determination of the three-dimensional (3D) catheter shape can increase navigation information and help reduce trauma. This study describes a shape determination method for a flexible interventional catheter using ultrasound scanning and a two-step particle filter without X-ray fluoroscopy. First, we propose a multi-feature, multi-template particle filter algorithm for accurate catheter tracking from ultrasound images. Second, we model the mechanical behavior of the catheter and apply a particle filter shape optimization algorithm to refine the results from the first step. Finally, the acquired catheter's 3D shapes are displayed together with the preoperative 3D images of the cardiac structures to provide intuitive endovascular navigation. We validated our method using ultrasound scanning of the straight and curved catheters in a water tank, and the shape determination errors were 1.44 ± 0.38 mm and 1.95 ± 0.46 mm, respectively. Further, endovascular catheter shape determination was validated in a catheter intervention experiment with a heart phantom. The error of the acquired endovascular catheter shape was 2.23 ± 0.87 mm. These results demonstrate that our two-step method is both accurate and effective. Using ultrasound scanning for shape determination of a flexible catheter will be helpful in endovascular interventions, reducing exposure to radiation and providing rich navigation information.

Secondary Stroke Prevention: Improving Diagnosis and Management with Newer Technologies

Treatment of hypertension, diabetes, high cholesterol, smoking cessation, and healthy lifestyle have all contributed to the decline in the incidence of vascular disease over the last several decades. Patients who suffer an acute stroke are at a high risk for recurrence. Introduction of newer technologies and their wider use allows for better identification of patients in whom the risk of recurrence following an acute stroke may be very high. Traditionally, the major focus for diagnosis and management has focused on patient history, examination, imaging for carotid stenosis/occlusion, and detection of AF and paroxysmal AF (PAF) with 24-48 h cardiac monitoring. This review focuses on the usefulness of three newer investigative tools that are becoming widely available and lead to better prevention. Continuous ambulatory blood pressure measurements for 24 h or longer and 3D Doppler measures of the carotid arteries provide key useful information on the state of vascular health and enhance our ability to monitor the response to preventive therapies. Furthermore, the detection of PAF can be significantly improved with prolonged cardiac monitoring for 3 weeks or longer, enabling the initiation of appropriate prevention therapy. This review will focus on the potential impact and importance of these emerging technologies on the prevention of recurrent stroke in high-risk patients.

User-friendly freehand ultrasound calibration using Lego bricks and automatic registration

PURPOSE

As an inexpensive, noninvasive, and portable clinical imaging modality, ultrasound (US) has been widely employed in many interventional procedures for monitoring potential tissue deformation, surgical tool placement, and locating surgical targets. The application requires the spatial mapping between 2D US images and 3D coordinates of the patient. Although positions of the devices (i.e., ultrasound transducer) and the patient can be easily recorded by a motion tracking system, the spatial relationship between the US image and the tracker attached to the US transducer needs to be estimated through an US calibration procedure. Previously, various calibration techniques have been proposed, where a spatial transformation is computed to match the coordinates of corresponding features in a physical phantom and those seen in the US scans. However, most of these methods are difficult to use for novel users.

METHODS

We proposed an ultrasound calibration method by constructing a phantom from simple Lego bricks and applying an automated multi-slice 2D-3D registration scheme without volumetric reconstruction. The method was validated for its calibration accuracy and reproducibility.

RESULTS

Our method yields a calibration accuracy of [Formula: see text] mm and a calibration reproducibility of 1.29 mm.

CONCLUSION

We have proposed a robust, inexpensive, and easy-to-use ultrasound calibration method.

A Robotic Ultrasound Scanner for Automatic Vessel Tracking and Three-Dimensional Reconstruction of B-Mode Images

Locating and evaluating the length and severity of a stenosis is very important for planning adequate treatment of peripheral arterial disease (PAD). Conventional ultrasound (US) examination cannot provide maps of entire lower limb arteries in 3-D. We propose a prototype 3D-US robotic system with B-mode images, which is nonionizing, noninvasive, and is able to track and reconstruct a continuous segment of the lower limb arterial tree between the groin and the knee. From an initialized cross-sectional view of the vessel, automatic tracking was conducted followed by 3D-US reconstructions evaluated using Hausdorff distance, cross-sectional area, and stenosis severity in comparison with 3-D reconstructions with computed tomography angiography (CTA). A mean Hausdorff distance of 0.97 ± 0.46 mm was found in vitro for 3D-US compared with 3D-CTA vessel representations. To evaluate the stenosis severity in vitro, 3D-US reconstructions gave errors of 3%-6% when compared with designed dimensions of the phantom, which are comparable to 3D-CTA reconstructions, with 4%-13% errors. The in vivo system's feasibility to reconstruct a normal femoral artery segment of a volunteer was also investigated. These results encourage further ergonomic developments to increase the robot's capacity to represent lower limb vessels in the clinical context.

Panorama Ultrasound for Navigation and Guidance of Epidural Anesthesia

Despite the common use of epidural anesthesia in obstetrics and surgery, the procedure can be challenging, especially for obese patients. We propose the use of an ultrasound guidance system employing a transducer-mounted camera to create 3-D panorama ultrasound volumes of the spine, thereby allowing identification of vertebrae and selection of puncture site, needle trajectory and depth of insertion. The camera achieves absolute position estimation of the transducer with respect to the patient using a specialized marker strip attached to the skin surface. The guidance system is validated first on a phantom against a commercial optical tracking system and then in vivo by comparing panorama images from human subjects against independent measurements by an experienced sonographer. The results for measuring depth to the epidural space, intervertebral spacing and registration of interspinous gaps to the skin prove the potential of the system for improving guidance of epidural anesthesia. The tracking and visualization are implemented in real time using the 3D Slicer software package.

A closed-form differential formulation for ultrasound spatial calibration: single wall phantom

Calibration is essential in freehand 3-D ultrasound to find the spatial transformation from the image coordinates to the sensor coordinate system. Ease of use, simplicity, precision and accuracy are among the most important factors in ultrasound calibration, especially when aiming to make calibration more reliable for day-to-day clinical use. We introduce a new mathematical framework for the simple and popular single-wall calibration phantom with a plane equation pre-determination step and the use of differential measurements to obtain accurate measurements. The proposed method provides a novel solution for ultrasound calibration that is accurate and easy to perform. This method is applicable to both radiofrequency (RF) and B-mode data, and both linear and curvilinear transducers. For a linear L14-5 transducer, the point reconstruction accuracy (PRA) of reconstructing 370 points is 0.73 ± 0.23 mm using 100 RF images, whereas the triple N-wire PRA is 0.67 ± 0.20 mm using 100 B-mode images. For a curvilinear C5-2 transducer, the PRA using the proposed method is 0.86 ± 0.28 mm on 400 points using 100 RF images, whereas N-wire calibration gives a PRA of 0.80 ± 0.46 mm using 100 B-mode images. Therefore, the accuracy of the proposed variation of the single-wall method using RF data is practically similar to the N-wire method while offering a simpler phantom with no need for accurate design and construction.

Review of ultrasound image guidance in external beam radiotherapy: I. Treatment planning and inter-fraction motion management

In modern radiotherapy, verification of the treatment to ensure the target receives the prescribed dose and normal tissues are optimally spared has become essential. Several forms of image guidance are available for this purpose. The most commonly used forms of image guidance are based on kilovolt or megavolt x-ray imaging. Image guidance can also be performed with non-harmful ultrasound (US) waves. This increasingly used technique has the potential to offer both anatomical and functional information.This review presents an overview of the historical and current use of two-dimensional and three-dimensional US imaging for treatment verification in radiotherapy. The US technology and the implementation in the radiotherapy workflow are described. The use of US guidance in the treatment planning process is discussed. The role of US technology in inter-fraction motion monitoring and management is explained, and clinical studies of applications in areas such as the pelvis, abdomen and breast are reviewed. A companion review paper (O'Shea et al 2015 Phys. Med. Biol. submitted) will extensively discuss the use of US imaging for intra-fraction motion quantification and novel applications of US technology to RT.

A closed-form differential formulation for ultrasound spatial calibration: multi-wedge phantom

Calibration is essential in freehand 3-D ultrasound to find the spatial transformation from the image coordinates to the sensor coordinate system. Calibration accuracy has significant impact on image-guided interventions. We introduce a new mathematical framework that uses differential measurements to achieve high calibration accuracy. Accurate measurements of axial differences in ultrasound images of a multi-wedge phantom are used to calculate the calibration matrix with a closed-form solution. The multi-wedge phantom has been designed based on the proposed differential framework and can be mass-produced inexpensively using a 3-D printer. The proposed method enables easy, fast and highly accurate ultrasound calibration, which is essential for most current ultrasound-guided applications and also widens the range of new applications. The precision of the method using only a single image of the phantom is comparable to that of the standard N-wire method using 50 images. The method can also directly take advantage of the fine sampling rate of radiofrequency ultrasound data to achieve very high calibration accuracy. With 100 radiofrequency ultrasound images, the method achieves a point reconstruction error of 0.09 ± 0.39 mm.

Calibration for position tracking of swept motor 3-D ultrasound

Tracking the position and orientation of a 3-D ultrasound transducer has many clinical applications. Tracking requires calibration to find the transformation between the tracking sensor and the ultrasound coordinates. Typically the set of image slice data are scan converted to a Cartesian volume using assumed motor geometry and a single transformation to the sensor. We propose, instead, the calibration of individual slices using a 2-D calibration technique. A best fit to a subset of slices is performed to decrease data collection time compared with that for calibration of all slices, and to reduce the influence of random errors in individual calibrations. We compare our technique with four scan conversion-based techniques: 2-D N-wire on the center slice, N-wire using a 3-D volume, N-wire using a 3-D volume including the edge points and a new closed-form planar method using a 3-D volume. The proposed multi-slice technique produced the smallest point reconstruction error (0.82 mm using a tracked stylus).

An accurate calibration method of ultrasound images by center positions of a metal ball

This paper provides a novel method for three-dimensional tracking of ultrasound images. One of the issues to determine the position of a ultrasound image plane is the thickness of the image plane. The proposed methodology address the issue by the calibration phantom using a fiducial sphere with the diameter of 5.5 mm because comet-trail artifact can be observed in the image plane through the center of the sphere. Meanwhile, to measure the sphere center accurately by a tracking device, a pointer tool with the same sphere at the tip is also proposed. To validate the feasibility of the method, simulation and phantom tests were conducted. From the results of the phantom test, the accuracy of the calibration was 0.65, 0.40, and 0.42 mm in 10, 50, 100 points calibration. The results demonstrate that the proposed method has a great potential for accurate US probe calibration.

Electrocardiographic textbooks based on template hearts warped using ultrasonic images

Advances in technology make the application of sophisticated approaches to assessing electrical condition of the heart practical. Estimates of cardiac electrical features inferred from body-surface electrocardiographic (ECG) maps are now routinely found in a clinical setting, but errors in those inverse solutions are especially sensitive to the accuracy of heart model geometry and placement within the torso. The use of a template heart model allows for accurate generation of individualized heart models and also permits effective comparison of inferred electrical features among multiple subjects. A collection of features mapped onto a common template forms a textbook of anatomically specific ECG variability. Our template warping process to individualize heart models based on a template heart uses ultrasonic images of the heart from a conventional, phased-array system. We chose ultrasound because it is nonionizing, less expensive, and more convenient than MR or CT imaging. To find the orientation and position in the torso model of each image, we calibrated the ultrasound probe by imaging a custom phantom consisting of multiple N-fiducials and computing a transformation between ultrasound coordinates and measurements of the torso surface. The template heart was warped using a mapping of corresponding landmarks identified on both the template and the ultrasonic images. Accuracy of the method is limited by patient movement, tracking error, and image analysis. We tested our approach on one normal control and one obese diabetic patient using the mixed-boundary-value inverse method and compared results from both on the template heart. We believe that our novel textbook approach using anatomically specific heart and torso models will facilitate the identification of electrophysiological biomarkers of cardiac dysfunction. Because the necessary data can be acquired and analyzed within about 30 min, this framework has the potential for becoming a routine clinical procedure.

Automated intraoperative calibration for prostate cancer brachytherapy

PURPOSE

Prostate cancer brachytherapy relies on an accurate spatial registration between the implant needles and the TRUS image, called "calibration". The authors propose a new device and a fast, automatic method to calibrate the brachytherapy system in the operating room, with instant error feedback.

METHODS

A device was CAD-designed and precision-engineered, which mechanically couples a calibration phantom with an exact replica of the standard brachytherapy template. From real-time TRUS images acquired from the calibration device and processed by the calibration system, the coordinate transformation between the brachytherapy template and the TRUS images was computed automatically. The system instantly generated a report of the target reconstruction accuracy based on the current calibration outcome.

RESULTS

Four types of validation tests were conducted. First, 50 independent, real-time calibration trials yielded an average of 0.57 ± 0.13 mm line reconstruction error (LRE) relative to ground truth. Second, the averaged LRE was 0.37 ± 0.25 mm relative to ground truth in tests with six different commercial TRUS scanners operating at similar imaging settings. Furthermore, testing with five different commercial stepper systems yielded an average of 0.29 ± 0.16 mm LRE relative to ground truth. Finally, the system achieved an average of 0.56 ± 0.27 mm target registration error (TRE) relative to ground truth in needle insertion tests through the template in a water tank.

CONCLUSIONS

The proposed automatic, intraoperative calibration system for prostate cancer brachytherapy has achieved high accuracy, precision, and robustness.

Three-dimensional ultrasound scanning

The past two decades have witnessed developments of new imaging techniques that provide three-dimensional images about the interior of the human body in a manner never before available. Ultrasound (US) imaging is an important cost-effective technique used routinely in the management of a number of diseases. However, two-dimensional viewing of three-dimensional anatomy, using conventional two-dimensional US, limits our ability to quantify and visualize the anatomy and guide therapy, because multiple two-dimensional images must be integrated mentally. This practice is inefficient, and may lead to variability and incorrect diagnoses. Investigators and companies have addressed these limitations by developing three-dimensional US techniques. Thus, in this paper, we review the various techniques that are in current use in three-dimensional US imaging systems, with a particular emphasis placed on the geometric accuracy of the generation of three-dimensional images. The principles involved in three-dimensional US imaging are then illustrated with a diagnostic and an interventional application: (i) three-dimensional carotid US imaging for quantification and monitoring of carotid atherosclerosis and (ii) three-dimensional US-guided prostate biopsy.

Improvement of freehand ultrasound calibration accuracy using the elevation beamwidth profile

This article presents a novel approach that incorporates an ultrasound slice-thickness profile into a filtered, weighted-least-square framework to improve the reconstruction accuracy of a real-time freehand calibration system. An important part of the system is a slice-thickness calibration device that aids in the extraction of the slice thickness across a wide range of imaging depths. Extensive experiments were conducted on a 10,000-image dataset to evaluate the effects of the framework on the calibration accuracy. The results showed that three-dimensional (3-D) reconstruction errors were significantly reduced in every experiment (p < 0.001). Real-time testing showed that the proposed method worked effectively with a small number of input images, suggesting great potential for intraoperative use where only a limited number of data may be available. This new framework can enable efficient quality control of calibration accuracy in real-time operating-room use.

Alignment and calibration of dual ultrasound transducers using a wedge phantom

We present a novel method of aligning two orthogonal ultrasound transducers into a coincident scan plane. A wedge phantom design provides visual feedback to the user to facilitate alignment. Calibration provides the transformation from one transducer to the other as well as a measure of the residual error in alignment. Mean alignment error is shown to be under 1° in the rotation axes and 1 mm in translation after repeated manual alignments. The repeatability of wedge based calibration has similar results compared with N-fiducial based calibration. The accuracy of the calibration for mapping points from one transducer to the other is found to have a mean error of 1.6 mm. The dual transducer system is well suited to imaging anatomy such as the breast and may be used for spatial compounding for improving B-mode images and motion estimation compounding for improving elastography results.

Three-dimensional ultrasound image-guided robotic system for accurate microwave coagulation of malignant liver tumours

BACKGROUND

The further application of conventional ultrasound (US) image-guided microwave (MW) ablation of liver cancer is often limited by two-dimensional (2D) imaging, inaccurate needle placement and the resulting skill requirement. The three-dimensional (3D) image-guided robotic-assisted system provides an appealing alternative option, enabling the physician to perform consistent, accurate therapy with improved treatment effectiveness.

METHODS

Our robotic system is constructed by integrating an imaging module, a needle-driven robot, a MW thermal field simulation module, and surgical navigation software in a practical and user-friendly manner. The robot executes precise needle placement based on the 3D model reconstructed from freehand-tracked 2D B-scans. A qualitative slice guidance method for fine registration is introduced to reduce the placement error caused by target motion. By incorporating the 3D MW specific absorption rate (SAR) model into the heat transfer equation, the MW thermal field simulation module determines the MW power level and the coagulation time for improved ablation therapy. Two types of wrists are developed for the robot: a 'remote centre of motion' (RCM) wrist and a non-RCM wrist, which is preferred in real applications.

RESULTS

The needle placement accuracies were < 3 mm for both wrists in the mechanical phantom experiment. The target accuracy for the robot with the RCM wrist was improved to 1.6 +/- 1.0 mm when real-time 2D US feedback was used in the artificial-tissue phantom experiment. By using the slice guidance method, the robot with the non-RCM wrist achieved accuracy of 1.8 +/- 0.9 mm in the ex vivo experiment; even target motion was introduced. In the thermal field experiment, a 5.6% relative mean error was observed between the experimental coagulated neurosis volume and the simulation result.

CONCLUSION

The proposed robotic system holds promise to enhance the clinical performance of percutaneous MW ablation of malignant liver tumours.

Ultrasound guided robotic biopsy using augmented reality and human-robot cooperative control

Ultrasound-guided biopsy is a proficient mininvasive approach for tumors staging but requires very long training and particular manual and 3D space perception abilities of the physician, for the planning of the needle trajectory and the execution of the procedure. In order to simplify this difficult task, we have developed an integrated system that provides the clinician two types of assistance: an augmented reality visualization allows accurate and easy planning of needle trajectory and target reaching verification; a robot arm with a six-degree-of-freedom force sensor allows the precise positioning of the needle holder and allows the clinician to adjust the planned trajectory (cooperative control) to overcome needle deflection and target motion. Preliminary tests have been executed on an ultrasound phantom showing high precision of the system in static conditions and the utility and usability of the cooperative control in simulated no-rigid conditions.

A real-time freehand ultrasound calibration system with automatic accuracy feedback and control

This article describes a fully automatic, real-time, freehand ultrasound calibration system. The system was designed to be simple and sterilizable, intended for operating-room usage. The calibration system employed an automatic-error-retrieval and accuracy-control mechanism based on a set of ground-truth data. Extensive validations were conducted on a data set of 10,000 images in 50 independent calibration trials to thoroughly investigate the accuracy, robustness, and performance of the calibration system. On average, the calibration accuracy (measured in three-dimensional reconstruction error against a known ground truth) of all 50 trials was 0.66 mm. In addition, the calibration errors converged to submillimeter in 98% of all trials within 12.5 s on average. Overall, the calibration system was able to consistently, efficiently and robustly achieve high calibration accuracy with real-time performance.

Reducing registration error in cross-beam vector doppler imaging with position sensor

Various vector Doppler methods have been proposed in the last several decades to overcome the Doppler angle dependency in both conventional spectral Doppler and color Doppler by measuring both the speed and direction of blood flow. However, they have not been adopted for routine use because most of them require specialized hardware, which is not available in commercial ultrasound systems. An alternative approach (cross-beam method) that uses color Doppler images obtained from different steered beam angles is more feasible, but there is error in registering multiple color Doppler images because they are not acquired simultaneously. To alleviate this problem, we have evaluated a cross-beam vector Doppler system that registers spatially with a position sensor two color Doppler images from two different angles and temporally with ECG synchronization. The registration error was reduced to an average of 0.92 mm from 2.49 mm in 9 human subjects. Vector Doppler carotid artery images of a healthy subject and a patient with atherosclerotic plaques are also presented.

Comparison of freehand 3-D ultrasound calibration techniques using a stylus

In a freehand 3-D ultrasound system, a probe calibration is required to find the rigid body transformation from the corner of the B-scan to the electrical center of the position sensor. The most intuitive way to perform such a calibration is by locating fiducial points in the scan plane directly with a stylus. The main problem of this approach is the difficulty in aligning the tip of the stylus with the scan plane. The thick beamwidth makes the tip of the stylus visible in the B-scan, even if the tip is not exactly at the elevational center of the scan plane. We present a novel stylus and phantom that simplify the alignment process for more accurate probe calibration. We also compare our calibration techniques with a range of styli. We show that our stylus and cone phantom are both simple in design and can achieve a point reconstruction accuracy of 2.2 mm and 1.8 mm, respectively, an improvement from 3.2 mm and 3.6 mm with the sharp and spherical stylus. The performance of our cone stylus and phantom lie between the state-of-the-art Z-phantom and Cambridge phantom, where accuracies of 2.5 mm and 1.7 mm are achieved.

Anatomical imaging for radiotherapy

The goal of radiation therapy is to achieve maximal therapeutic benefit expressed in terms of a high probability of local control of disease with minimal side effects. Physically this often equates to the delivery of a high dose of radiation to the tumour or target region whilst maintaining an acceptably low dose to other tissues, particularly those adjacent to the target. Techniques such as intensity modulated radiotherapy (IMRT), stereotactic radiosurgery and computer planned brachytherapy provide the means to calculate the radiation dose delivery to achieve the desired dose distribution. Imaging is an essential tool in all state of the art planning and delivery techniques: (i) to enable planning of the desired treatment, (ii) to verify the treatment is delivered as planned and (iii) to follow-up treatment outcome to monitor that the treatment has had the desired effect. Clinical imaging techniques can be loosely classified into anatomic methods which measure the basic physical characteristics of tissue such as their density and biological imaging techniques which measure functional characteristics such as metabolism. In this review we consider anatomical imaging techniques. Biological imaging is considered in another article. Anatomical imaging is generally used for goals (i) and (ii) above. Computed tomography (CT) has been the mainstay of anatomical treatment planning for many years, enabling some delineation of soft tissue as well as radiation attenuation estimation for dose prediction. Magnetic resonance imaging is fast becoming widespread alongside CT, enabling superior soft-tissue visualization. Traditionally scanning for treatment planning has relied on the use of a single snapshot scan. Recent years have seen the development of techniques such as 4D CT and adaptive radiotherapy (ART). In 4D CT raw data are encoded with phase information and reconstructed to yield a set of scans detailing motion through the breathing, or cardiac, cycle. In ART a set of scans is taken on different days. Both allow planning to account for variability intrinsic to the patient. Treatment verification has been carried out using a variety of technologies including: MV portal imaging, kV portal/fluoroscopy, MVCT, conebeam kVCT, ultrasound and optical surface imaging. The various methods have their pros and cons. The four x-ray methods involve an extra radiation dose to normal tissue. The portal methods may not generally be used to visualize soft tissue, consequently they are often used in conjunction with implanted fiducial markers. The two CT-based methods allow measurement of inter-fraction variation only. Ultrasound allows soft-tissue measurement with zero dose but requires skilled interpretation, and there is evidence of systematic differences between ultrasound and other data sources, perhaps due to the effects of the probe pressure. Optical imaging also involves zero dose but requires good correlation between the target and the external measurement and thus is often used in conjunction with an x-ray method. The use of anatomical imaging in radiotherapy allows treatment uncertainties to be determined. These include errors between the mean position at treatment and that at planning (the systematic error) and the day-to-day variation in treatment set-up (the random error). Positional variations may also be categorized in terms of inter- and intra-fraction errors. Various empirical treatment margin formulae and intervention approaches exist to determine the optimum strategies for treatment in the presence of these known errors. Other methods exist to try to minimize error margins drastically including the currently available breath-hold techniques and the tracking methods which are largely in development. This paper will review anatomical imaging techniques in radiotherapy and how they are used to boost the therapeutic benefit of the treatment.

Real-time freehand 3D ultrasound calibration

Z-fiducial phantoms allow three-dimensional ultrasound probe calibration with a single B-scan. One of the main difficulties in using this phantom is the need for reliable segmentation of the wires in the ultrasound images, which necessitates manual intervention. In this article, we have shown how we can solve this problem by mounting a thin rubber membrane on top of the phantom. The membrane is segmented automatically and the wires can be easily located as they are at known positions relative to the membrane. This enables us to segment the wires automatically at the full PAL frame rate of 25 Hz, to produce calibrations in real-time, while achieving accuracies similar to those reported in the literature. We have also devised a technique to improve the estimation of the elevational offset (calibration parameter) by capturing a few images of the planar membrane. If spatial calibration is known, fully automatic wire segmentation allows the fiducials to be tracked in real-time. This also enables temporal calibration to be performed in real-time as the probe is moved away from the phantom. We have evaluated the performance of our phantom by calibrating a probe at 8 cm and 15 cm depth. The precision of the calibrations are 0.7 mm and 1.2 mm, respectively. The point reconstruction accuracies of fiducial points provided by the same Z-phantom are slightly below 1.5 mm. The point reconstruction accuracies obtained by scanning the end of a wire tip are 2.5 mm and 3.0 mm. These results match the accuracies achieved in the literature. It takes approximately 2 min to set up the experiment, submerge the phantom in the water bath, locate the phantom in space with a pointer and capture six images of the planar membrane. After this, spatial calibration can be performed in less than a second. Temporal calibration can be completed in approximately 3 s.

Automated 3D freehand ultrasound calibration with real-time accuracy control

3D ultrasound (US) is an emerging new imaging technology that appeals to more and more applications in intraoperative guidance of computer-assisted surgery. In a freehand US imaging system, US probe calibration is typically required to construct a 3D image of the patient's anatomy from a set of 2D US images. Most of the current calibration techniques concern primarily with the precision and accuracy. However, for computer-assisted surgeries that may require a calibration task inside the operating room (OR), many other important aspects have to be considered besides accuracy. In this paper, we propose a novel system for automated calibration that is optimized for the OR usage with real-time feedback and control of the calibration accuracy. We have also designed a novel N-wire phantom, with greatly reduced complexity to facilitate mass production without compromising the accuracy and robustness.

Quantitative evaluation of three calibration methods for 3-D freehand ultrasound

In this paper, three different calibration methods for three-dimensional (3-D) freehand ultrasound (US) are evaluated. Calibration is the process of estimating the rigid transformation from US image coordinates to the coordinate system of the tracking sensor mounted onto the probe. Calibration accuracy has an important impact on quantitative studies. Geometrical precision can also be crucial in many interventions and surgery. The proposed evaluation framework relies on a single point phantom and a 3-D US phantom which mimics the US characteristics of human liver. Four quality measures are used: 3-D point localization criterion, distance and volume measurements, and shape based criterion. Results show that during the acquisition procedure, volumetric measurements and shapes of the reconstructed object depend on probe motion used, particularly fan motions for which errors are larger. It is also shown that accurate calibration is essential to obtain reliable quantitative information.

A system for ultrasound-guided computer-assisted orthopaedic surgery

Current computer-assisted orthopedic surgery (CAOS) systems typically use preoperative computed tomography (CT) and intraoperative fluoroscopy as their imaging modalities. Because these imaging tools use X-rays, both patients and surgeons are exposed to ionizing radiation that may cause long-term health damage. To register the patient with the preoperative surgical plan, these techniques require tracking of the targeted anatomy by invasively mounting a tracking device on the patient, which results in extra pain and may prolong recovery time. The mounting procedure also leads to a major difficulty of using these approaches to track small bones or mobile fractures. Furthermore, it is practically impossible to mount a heavy tracking device on a small bone, which thus restricts the use of CAOS techniques. This article presents a novel CAOS method that employs 2D ultrasound (US) as the imaging modality. Medical US is non-ionizing and real-time, and our proposed method does not require any invasive mounting procedures. Experiments have shown that the proposed registration technique has sub-millimetric accuracy in localizing the best match between the intraoperative and preoperative images, demonstrating great potential for orthopedic applications. This method has some significant advantages over previously reported US-guided CAOS techniques: it requires no segmentation and employs only a few US images to accurately and robustly localize the patient. Preliminary laboratory results on both a radius-bone phantom and human subjects are presented.

Rapid, easy and reliable calibration for freehand 3D ultrasound

This paper presents improvements to the plane-based technique for calibrating freehand 3D ultrasound systems. The improvements are designed to make it easier for inexperienced users to perform plane-based calibration and to know that they have got a reliable result. In particular, we enable the calibration to be performed using water at room temperature while producing a result that is valid for average soft tissue and we show how it is possible to provide feedback on the reliability of the calibration using a metric based on the curvature of the calibration criterion function. We present comprehensive results showing that these innovations improve the precision of the calibration and offer useful feedback to the user.

Self-calibrating 3D-ultrasound-based bone registration for minimally invasive orthopedic surgery

Intraoperative freehand three-dimensional (3-D) ultrasound (3D-US) has been proposed as a noninvasive method for registering bones to a preoperative computed tomography image or computer-generated bone model during computer-aided orthopedic surgery (CAOS). In this technique, an US probe is tracked by a 3-D position sensor and acts as a percutaneous device for localizing the bone surface. However, variations in the acoustic properties of soft tissue, such as the average speed of sound, can introduce significant errors in the bone depth estimated from US images, which limits registration accuracy. We describe a new self-calibrating approach to US-based bone registration that addresses this problem, and demonstrate its application within a standard registration scheme. Using realistic US image data acquired from 6 femurs and 3 pelves of intact human cadavers, and accurate Gold Standard registration transformations calculated using bone-implanted fiducial markers, we show that self-calibrating registration is significantly more accurate than a standard method, yielding an average root mean squared target registration error of 1.6 mm. We conclude that self-calibrating registration results in significant improvements in registration accuracy for CAOS applications over conventional approaches where calibration parameters of the 3D-US system remain fixed to values determined using a preoperative phantom-based calibration.

Computer-assisted 3D ultrasound-guided neurosurgery: technological contributions, including multimodal registration and advanced display, demonstrating future perspectives

BACKGROUND

Navigation systems are now frequently being used for guiding surgical procedures. Existing neuronavigation systems suffer from the lack of updated images when tissue changes during surgery as well as from user-friendly displays of all essential images for accurate and safe surgery guidance.

METHODS

We have developed various new technologies for improved neuronavigation. Using intraoperative 3D ultrasound (US) imaging, we have developed various registration algorithms for using and updating a complete multimodal and multivolume 3D map for navigation.

RESULTS

We experienced that advanced multimodal visualization makes it easy to interpret information from several image volumes and modalities simultaneously. Using high quality intraoperative 3D ultrasound, essential preoperative information could be corrected due to brain shift. fMRI and other important preoperative data could then be used together with intraoperative ultrasound imaging for more accurate, safer and improved guidance of therapy.

CONCLUSIONS

We claim that new features, as demonstrated in the present paper, using intraoperative 3D ultrasound in combination with advanced registration and display algorithms will represent important contributions towards more accurate, safer and more optimized future patient treatment.

Comparison of calibration methods for spatial tracking of a 3-D ultrasound probe

Real-time 3-D ultrasound (US) is a new-generation US system that uses a dedicated probe to create volume data sets instead of standard 2-D cross-sectional images. For applications in image-guided surgery and radiation therapy, a position tracker is added to the probe so that the volumes can be located in space. Calibration plays a critical role in determining the overall accuracy of an US volume-tracking system. In this paper, three calibration methods are developed specifically for 3-D probes. The three methods are based on a IXI-shaped wire phantom, a cube phantom and a stylus. The performance of each method was evaluated in terms of calibration reproducibility, point accuracy and reconstruction accuracy by distance measurement. The mean errors in the reproducibility tests were 1.50 mm (IXI-wire), 1.16 mm (cube) and 5.13 mm (stylus). The root mean square errors of the point accuracy measure were 2.15 mm (IXI-wire), 4.91 mm (cube) and 2.36 mm (stylus). The root mean square errors of the reconstruction accuracy by distance measure were 1.52 mm (IXI-wire), 1.59 mm (cube) and 1.85 mm (stylus). Overall, the IXI-wire phantom achieved the best results.

A phantom with reduced complexity for spatial 3-D ultrasound calibration

The design of a new phantom for 3-D ultrasound calibration is presented. The phantom provides a viable alternative to existing phantoms that are significantly more complex and require high precision fabrication. The phantom, referred to as a "plane-of-wires" phantom, consists of two wires mounted at the same fixed height above the bottom of a water tank. Data collection for calibration involved rotating and translating the phantom so that the wires remained in a single plane parallel to the tank bottom. The mean reconstruction accuracy of the plane-of-wires calibration is 0.66 mm at a mean depth of 12.3 mm, with a precision of 1.23 mm at the same mean depth. The calibration was used to determine the volume of a cube with known volume with an error of 2.51%. The calibration performance achieved is comparable with that of existing approaches.

A review of calibration techniques for freehand 3-D ultrasound systems

Three-dimensional (3-D) ultrasound (US) is an emerging new technology with numerous clinical applications. Ultrasound probe calibration is an obligatory step to build 3-D volumes from 2-D images acquired in a freehand US system. The role of calibration is to find the mathematical transformation that converts the 2-D coordinates of pixels in the US image into 3-D coordinates in the frame of reference of a position sensor attached to the US probe. This article is a comprehensive review of what has been published in the field of US probe calibration for 3-D US. The article covers the topics of tracking technologies, US image acquisition, phantom design, speed of sound issues, feature extraction, least-squares minimization, temporal calibration, calibration evaluation techniques and phantom comparisons. The calibration phantoms and methods have also been classified in tables to give a better overview of the existing methods.

A mechanical instrument for 3D ultrasound probe calibration

We present a novel technique for 3D ultrasound probe calibration. The principle of operation is that the beam is aligned with a set of coplanar wires strung across a rigid frame. The probe and frame are mounted on a precision-manufactured mechanical instrument which allows adjustment and measurement of their relative pose. Semi-automatic image processing facilitates alignment of the beam and wires to within a tolerance of around 0.2 mm, despite the considerable beam thickness. The calibration process requires just a single view and relatively little user expertise. In a series of experiments with different ultrasound probes, we demonstrate the technique's high accuracy and precision. The latter is partly due to the elimination of the position sensor, a significant source of measurement noise, from the end-user calibration process.

A review of calibration techniques for freehand 3-D ultrasound systems

Three-dimensional (3-D) ultrasound (US) is an emerging new technology with numerous clinical applications. Ultrasound probe calibration is an obligatory step to build 3-D volumes from 2-D images acquired in a freehand US system. The role of calibration is to find the mathematical transformation that converts the 2-D coordinates of pixels in the US image into 3-D coordinates in the frame of reference of a position sensor attached to the US probe. This article is a comprehensive review of what has been published in the field of US probe calibration for 3-D US. The article covers the topics of tracking technologies, US image acquisition, phantom design, speed of sound issues, feature extraction, least-squares minimization, temporal calibration, calibration evaluation techniques and phantom comparisons. The calibration phantoms and methods have also been classified in tables to give a better overview of the existing methods.

Redundant system of passive markers for ultrasound scanhead tracking

Scanhead tracking by opto-electronic (OE) systems allows high accuracy in three-dimensional (3-D) freehand ultrasound imaging. In this paper, a new set of methods is proposed and compared with the standard approach [Gram-Schmidt method (GS)]. Three redundancy-based algorithms are introduced to compensate for possible loss of markers during data acquisition: regression plane (RP), multiple Gram-Schmidt (MGS), and center of mass least square (CMLS). When combined with the ultrasound instrument, the root-mean-squared (RMS) uncertainty in locating target points, over a working volume of 420 mm x 490 mm x 100 mm, improved by 7% and 24% using MGS and CMLS method respectively, compared to GS. A lower improvement was obtained with RP methods (5%), using the best marker configuration. In conclusion, CMLS method provides a robust and accurate procedure for 3-D freehand ultrasound scanhead tracking, able to manage possible loss of markers, with interesting perspectives for image fusion and body referenced 3-D ultrasound.

Real-Time Quality Control of Tracked Ultrasound

The overwhelming majority of intra-operative hazard situations in tracked ultrasound (US) systems are attributed to failure of registration between tracking and imaging coordinate frames. We introduce a novel methodology for eal-time in-vivo quality control of tracked US systems, in order to capture registration failures during the clinical procedure. In effect, we dynamically recalibrate the tracked US system for rotation, scale factor, and in-plane position offset up to a scale factor. We detect any unexpected change in these parameters through capturing discrepancies in the resulting calibration matrix, thereby assuring quality (accuracy and consistency) of the tracked system. No phantom is used for the recalibration. We perform the task of quality control in the background, transparently to the clinical user while the subject is being scanned. We present the concept, mathematical formulation, and experimental evaluation in-vitro. This new method can play an important role in guaranteeing accurate, consistent, and reliable performance of tracked ultrasound.

A Novel Phantom-Less Spatial and Temporal Ultrasound Calibration Method

This paper introduces a novel method for ultrasound calibration for both spatial and temporal parameters. The main advantage of this method is that it does not require a phantom, which is usually expensive to fabricate. Furthermore, the method does not require extensive image processing. For spatial calibration, we solve an optimization problem established by a set of equations that relate the orientations of a line (i.e., calibration pointer) to the intersection points appearing in the ultrasound image. The line orientation is provided through calibration of both ends of the calibration pointer. Temporal calibration is achieved by processing of the captured pointer orientations and the corresponding image positions of intersection along with the timing information. The effectiveness of the unified method for both spatial and temporal calibration is apparent from the quality of the 3D reconstructions of a known object.

Surface extraction with a three-dimensional freehand ultrasound system

This paper presents a system for acquiring three-dimensional ultrasound data and extracting surfaces of the examined structures. The acquisition is performed freehand with a PC-based two dimensional ultrasound machine and an optical tracker. The extraction of surfaces from ultrasound data are normally inhibited by speckle, shadowing and gaps in the acquired data. A new method is developed that extracts a surface directly from the irregularly spaced, noisy freehand ultrasound data. The freehand data are first interpolated with radial basis functions and then a mesh is formed along an isosurface of the functional interpolation. The ability of radial basis functions to smooth speckle and interpolate across gaps is demonstrated on a series of experiments with phantoms and human tissue in a water bath. The geometry of the extracted surface matches the external measurements with an average difference ranging from 0.8 to 2.9 mm. These differences are within the range of errors from calibration, resolution and landmark localization. The experiments also show the ability to create continuous and realistic surfaces from scans that require multiple sweeps over a structure.

An efficient calibration method for freehand 3-D ultrasound imaging systems

A phantom has been developed to quickly calibrate a freehand 3-D ultrasound (US) imaging system. Calibration defines the spatial relationship between the US image plane and an external tracking device attached to the scanhead. The phantom consists of a planar array of strings and beads, and a set of out-of-plane strings that guide the user to proper scanhead orientation for imaging. When an US image plane is coincident with the plane defined by the strings, the calibration parameters are calculated by matching of homologous points in the image and phantom. The resulting precision and accuracy of the 3-D imaging system are similar to those achieved with a more complex calibration procedure. The 3-D reconstruction performance of the calibrated system is demonstrated with a magnetic tracking system, but the method could be applied to other tracking devices.