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

Automatic spatial calibration of freehand ultrasound probe with a multilayer N-wire phantom

The classic N-wire phantom has been widely used in the calibration of freehand ultrasound probes. One of the main challenges of the phantom is accurately identifying N-fiducials in ultrasound images, especially with multiple N-wire structures. In this study, a method using a multilayer N-wire phantom for the automatic spatial calibration of ultrasound images is proposed. All dots in the ultrasound image are segmented, scored, and classified according to the unique geometric features of the multilayer N-wire phantom. A recognition method for identifying N-fiducials from the dots is proposed for calibrating the spatial transformation of the ultrasound probe. At depths of 9, 11, 13, and 15 cm, the reconstruction error of 50 points is 1.24 ± 0.16, 1.09 ± 0.06, 0.95 ± 0.08, 1.02 ± 0.05 mm, respectively. The reconstruction mockup test shows that the distance accuracy is 1.11 ± 0.82 mm at a depth of 15 cm.

A novel ultrasound probe calibration method for multimodal image guidance of needle placement in cervical cancer brachytherapy

PURPOSE

Interstitial needles placement is a critical component of combined intracavitary/interstitial (IC/IS) brachytherapy (BT). To ensure precise placement of interstitial needles, we proposed a novel ultrasonic (US) probe calibration method to accurately register the US image in the magnetic resonance imaging (MRI) image and provide multimodal image guidance for needle placement.

METHODS

A wire-based calibration phantom combined with the stylus was developed for the calibration of US probe. The calibration phantom helps to quickly align the imaging plane of the US probe with the fiducial points to obtain US images of these points. The coordinates of fiducial points in US images were located automatically by feature extraction algorithms and were further corrected by the proposed correction method. Ingenious structures were designed on both sides of the calibration phantom to accurately obtain the coordinates of the fiducial points relative to the tracking device. Marker validation and pelvic phantom study were performed to evaluate the accuracy of the proposed calibration method.

RESULTS

In the marker validation, the US probe calibration method with corrected transformation achieves a registration accuracy of 0.694 ± 0.014 mm, and the uncorrected one is 0.746 ± 0.018 mm. In the pelvic phantom study, the needle tip difference was 1.096 ± 0.225 mm and trajectory difference was 1.416 ± 0.284 degrees.

CONCLUSION

The proposed US probe calibration method is helpful to achieve more accurate multimodality image guidance for needle placement.

A Novel Ultrasound Probe Spatial Calibration Method Using a Combined Phantom and Stylus

Intra-operative ultrasound (US) is a popular imaging modality for its non-radiative and real-time advantages. However, it is still challenging to perform an interventional procedure under two-dimensional (2-D) US image guidance. Accordingly, the trend has been to perform three-dimensional (3-D) US image guidance by equipping the US probe with a spatial position tracking device, which requires accurate probe calibration for determining the spatial position between the B-scan image and the tracked probe. In this report, we propose a novel probe spatial calibration method by developing a calibration phantom combined with the tracking stylus. The calibration phantom is custom-designed to simplify the alignment between the stylus tip and the B-scan image plane. The spatial position of the stylus tip is tracked in real time, and its 2-D image pixel location is extracted and collected simultaneously. Gaussian distribution is used to model the spatial position of the stylus tip and the iterative closest point-based optimization algorithm is used to estimate the spatial transformation that matches these two point sets. Once the probe is calibrated, its trajectory and the B-scan image are collected and used for the volume reconstruction in our freehand 3-D US imaging system. Experimental results demonstrate that the probe calibration approach results in less than 1-mm mean point reconstruction accuracy. It requires less than 5 min for an inexperienced user to complete the probe calibration procedure with minimal training. The mockup test shows that the 3-D images are geometrically correct with 0.28°-angle accuracy and 0.40-mm distance accuracy.

Efficient image based method using water-filled balloons for improving probe spatial calibration in 3D freehand ultrasonography

The ultrasound (US) probe spatial calibration is a key prerequisite for enabling the use of the 3D freehand US technique. Several methods have been proposed for achieving an accurate and precise calibration, although these methods still require specialised equipment. This equipment is often not available in research or clinical facilities. Therefore, the present investigation aimed to propose an efficient US probe calibration method that is accessible in terms of cost, easy to apply and capable of achieving results suitable for clinical applications. The data acquisition was carried out by performing two perpendicular US sweeps over water filled balloon phantoms. The data analysis was carried out by computing the similarity measures between 2D images from the first sweep and the corresponding images of the 3D reconstruction of the second sweep. These measures were maximized by using the Nelder-Mead algorithm, to find the optimal solution for the calibration parameters. The calibration results were evaluated in terms of accuracy and precision by comparing known phantom geometries with those extracted from the US images. The accuracy and the precision after applying the calibration method were improved. By using the parameters obtained from the plane phantom method as initialization of the calibration parameters, the accuracy and the precision in the best scenario was 0.4 mm and 1.5 mm, respectively. These results were in line with the methods requiring specialised equipment. However, the applied method was unable to consistently produce this level of accuracy and precision. The calibration parameters were also tested in a musculoskeletal application, revealing sufficient matching of the relevant anatomical features when multiple US sweeps are combined in a 3D reconstruction. To improve the current results and increase the reproducibility of this research, the developed software is made available.

Spatial calibration of a 2D/3D ultrasound using a tracked needle

Purpose

Spatial calibration between a 2D/3D ultrasound and a pose tracking system requires a complex and time-consuming procedure. Simplifying this procedure without compromising the calibration accuracy is still a challenging problem.

Method

We propose a new calibration method for both 2D and 3D ultrasound probes that involves scanning an arbitrary region of a tracked needle in different poses. This approach is easier to perform than most alternative methods that require a precise alignment between US scans and a calibration phantom.

Results

Our calibration method provides an average accuracy of 2.49 mm for a 2D US probe with 107 mm scanning depth, and an average accuracy of 2.39 mm for a 3D US with 107 mm scanning depth.

Conclusion

Our method proposes a unified calibration framework for 2D and 3D probes using the same phantom object, work-flow, and algorithm. Our method significantly improves the accuracy of needle-based methods for 2D US probes as well as extends its use for 3D US probes.

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.