Photograph of the mechanical apparatus showing the (a) four motors and (b) encoders which drive the linkage. The needle positioning device mounted below the ultrasound transducer is supporting the needle through a pair of spherical couplings at a compound angle relative to the long axis of the ultrasound transducer. The transducer is connected to a cradle, which in turn is attached to a motorized assembly to rotate the transducer about its long axis for 3D ultrasound acquisition.
A partial rear perspective view of the device illustrating the different components of the linkage mechanism. The linkage is pinned to a common shaft which serves as the coordinate reference frame for the needle positioning device. This mechanism is an overconstrained linkage where 13 linkage components are configured in a manner where the needle is confined to four DOF about two points in space. The mechanism consists of two pinned parallelograms supporting a needle guide through a pair of spherical couplings. The needle guide is pinned to the forward and connected to the rear spherical coupling via a telescoping slide.
The device is decoupled through two remote pivot points created from the spherical couplings pinned to each parallelogram. Since the needle guide axis is also aligned with each stationary point, any movement from either parallelogram will result in the needle guide axis to pivot about the stationary point of the opposing spherical linkage, thus resulting in no linear displacement of the intersection point. The physician can manually align the needle axis in two simple steps. (a) First, the physician aligns the forward RCM by moving the forward parallelogram (the physician would manipulate the apparatus from *). (b) Then, by moving the rear arms (from the point *), the physician can target a point of interest within the patients’ anatomy by angulating the needle through the forward RCM to the target. The counterweights support the weight of the needle guide and prevent the linkage from drifting when the brakes are not applied.
Illustration of the 3D needle guidance interface to facilitate the systematic targeting of each needle. (a) shows the targeting interface, illustrating the location of the needle track. The image displays the current path of the needle in yellow, and the planned needle path in pink by displaying two points on the needle path. The cross illustrates the piercing point between the needle axis and patient’s skin projected onto the transverse image plane and the circle represents the intersection of the needle axis to the transverse image plane showing the location of the intended target. To align the needle to the planned trajectory in a therapy procedure, the physician would first align the needle piercing point over the patient’s skin by moving the front parallelogram linkage as illustrated in (b). (c) Then, by manipulating the rear parallelogram, the physician would then adjust the needle trajectory about the RCM until the needle is aligned to the target when the yellow circle is superimposed onto the pink circle. Since the front RCM remains stationary while the physician manipulates the needle, the alignment of the pink and yellow crosses does not change.
The device was coupled to a dividing head, which in turn sat on a granite surface plate and served as the reference plane for the calibration procedure. The dividing head was used to measure both the horizontal (-axis) and vertical position (-axis) of the tooling ball relative to the surface plate by indexing the chuck (and attached robot) by ±90°. The height gauge was used to determine the height of the tooling balls which were aligned to the RCM of the spherical couplings.
(a) Photograph of the experimental setup used to align the ultrasound image (b) to the coordinate reference frame of the needle positioning device. To determine the relationship between the coordinate systems, a multilayered string phantom was constructed and mounted to the needled positioning device to constrain the string intersections to a known location.
Graph showing the image lag (in degrees) as a function of the scan velocity at three different U.S. frame rates (8, 16, and 32 Hz). Image lag is defined as the angular misalignment of the image about the -axis of the machine coordinate system. The image lag is proportional to the motor speed and inversely to the frame rate. The error bars in the graph represent the standard deviation in the measurements.
Illustration showing the error metrics used to evaluate the users ability to guide a needle to a 3D target and record the location of the needle within 3D image. The mean targeting error is the mean distance between each identified bead location in the 3D CT image located at the end of the each air track and associated target location , which is a virtual point in the ultrasound coordinate frame represented by the open circle. The mean target error is the mean distance between the each bead location in the 3D U.S. image located at the end of each air track and the corresponding target location . The NGE is the mean total error associated with the system’s ability to guide the needle path to predefined targets. is the angle between the needle trajectory identified in the CT image and the planned needle path when projected on to a plane perpendicular to the line , which is the shortest distance between the two lines. NLE is defined as the minimum distance between the true needle axis from CT and the recorded 3D TRUS needle axis . is the angle between the needle trajectories when projected on a plane perpendicular to NLE.
3D images of an agar phantom used in the mock seed implant experiment illustrating the needle at various oblique trajectories from 0° (top left) to 15° (bottom right).
Measurement results of the calibration errors for both the forward and rear parallelogram arms of the mechatronic device. The location of the tooling ball was measured five times to determine the mean position (highlighted in bold) at 3 cm increments in and over the entire area of a 6 cm square grid typically used for brachytherapy [consisting of at least seven rows (A–G) and columns (1–7)]. All other values not highlighted in bold contain only one measurement.
Results illustrating the maximum tooling ball displacement (mm) for various rear RCM locations relative to the forward tooling ball. The maximum displacement was determined from a total of 72 different oblique trajectories where the rear tooling ball was displaced from nine different parallel trajectories in 3 cm increments over a 6 cm square grid, except for the values in bold where only one data point was available for the given configuration (i.e., forward and rear tooling balls are in opposite corners of the 6 cm grid pattern used in this experiment).
Measurement results of string separations from the 3D geometric reconstruction experiment illustrating the mean distance between the strings and the measurement error , where and standard deviation (STD).
3D system accuracy for the parallel needle trajectories based on the error analysis described in Sec. IV B. Mean targeting errors and [Eqs. (2) and (1)], NGE [Eq. (4)], and [Eq. (5)] all evaluate the seed placement and needle targeting accuracy. The localization metrics NLE and [see Eqs. (6) and (7)] indicate the errors in recording the 3D location of the needle trajectories. All of the values are reported as except for the TRE between CT and U.S. and is quantified by Eq. (3).
3D mechatronic system needle guidance results for the oblique needle trajectories at varying angles from 5° to 15°. Mean targeting errors and [Eqs. (2) and (1)], NGE [Eq. (3)], and [Eq. (4)] all determine the seed placement and needle guidance accuracy. The localization metrics NLE and [see Eqs. (5) and (6)] indicates the errors in recording the 3D location of the needle trajectories. All of the values are reported as except for the TRE between CT and U.S. and is quantified by Eq. (3).
Article metrics loading...
Full text loading...