A. Novel Motion-Enabled Cardiac Phantom
A polyvinyl alcohol (PVA) based cardiac phantom was molded from a real human heart and has similar mechanical properties to soft-tissue [3
]. The apex from the cardiac phantom was then removed and replaced with a detachable “targeting slab” (). This “targeting slab” is easily removed and then re-attached to the apex of the heart phantom. Within this slab were four circular holes where removable targets can be placed (). The purpose of this configuration was so that injections and corresponding analysis could be conducted efficiently, allowing for a large number of iterations for each experiment. The phantom consists of replaceable parts including targets, to ensure consistent physical integrity of the model for numerous experiments.
Heart phantom with attached targeting slab. A catheter enters from the ports on the right, and injects into the targeting slab.
Detached slab showing the removable targets. Injections of dye are made into the targets, and afterwards accuracy measurements are made. Targets are easily replaced so that numerous injection experiments can be made rapidly.
The cardiac phantom was mounted onto a pneumatically controlled, closed-loop motion actuator. A RAPU microcontroller (Remote Advanced Playback Unit, Brookshire software) was used to control a linear pneumatic actuator cylinder. The position signal from the RAPU controller and the position error signal from the output of a string potentiometer attached to the pneumatic cylinder are fed into a differential amplifier. The signal from this amplifier controls a differential pneumatic valve, which in turn controls the position of the pneumatic cylinder. This closed-loop feedback ensures precise and repeatable movement of the phantom. A schematic of the control system is shown in . The actuator cylinder is attached to the phantom and moves the phantom back and forth over a ramp, which allowed for the imitation of the coupled superior-inferior and posterior-anterior cardio-respiratory motion present in most humans. Respiratory motion-like signals were created to control the linear position of the phantom as a function of time. Beating-heart motion is not included in this version of the phantom, due to the fact that most interventionalists only inject at end-diastole. A future version of the phantom will include this capability.
A schematic of the phantom motion control system. User-generated positions signals are downloaded into the RAPU microcontroller, and the closed-loop system moves the phantom
B. EM Tracked Catheter
A prototype steerable EM tracked injection catheter was fabricated from an off the shelf steerable catheter sheath (‘Channel’ Model, Bard Medical), nitinol hypotube (Johnson-Matthey Medical), and a 5DOF EM tracking sensor (NDI) (). The EM sensor was fixed in the lumen tip of the catheter, while the hypotube was able to advance freely through the lumen in order to penetrate the PVA. The EM sensor has a 1mm offset from the hypotube, introducing a source of error into the system.
Diagram and photo of the EM tracked injection catheter. The EM sensor is fixed in place at the tip the catheter, while the sharpened hypotube is able to move freely within the lumen.
C. MR Acquisitions
Axial MR scans of the phantom were acquired on a 1.5T system (Signa HDxt, GE Healthcare, Milwaukee, WI) with the following parameters: 8-channel cardiac coil; 3D balanced SSFP (bSSFP/FIESTA) pulse sequence; TR/TE/Flip = 3.6ms / 1.5ms / 55 deg.; FOV/Matrix = 35cm/384×384; Num. Slices/Sl. Thickness = 256/1mm. After the images were acquired, we digitally altered the MR volumes so that a small point appeared in the center of each target () using a circular Hough transform on each slice. This allowed the system operator to aim for the exact center of the circle rather than an interpretation of the center. The MR images are acquired once pre-operatively, and the same image is used for registration in all subsequent targeting experiments. This is acceptable due to the fact that the PVA structure is rigid over a long period of time (years) if properly maintained.
Digitally Altered MR image of the phantom, with enhanced signal at the center of each target.
Registration is performed in the cathlab at the time of the injection experiment. MR images are registered to the EM coordinate space by matching the location of fiducials in the MR series to location of fiducials in EM space. The fiducial locations are segmented in the MR images, and an EM tracked sensor is touched to the corresponding fiducials. The resulting set of points clouds are registered by finding the rigid transformation matrix that minimizes error between the point locations segmented from MR and the transformed point locations measured in the EM space. The associated error is known as the “fiducial registration error” (FRE).
Fiducials consisted of 5 indentations on the exterior of the targeting slab. This was done so that any incidental movement between the PVA phantom and its holding frame would not affect the resulting MR to EM registration. The geometric configuration of the fiducials was similar in configuration to what is typical of a clinical fiducial scenario, except the spatial distribution in the axial direction was smaller, and the distance between the fiducials, targets, and the centroid of the fiducial configuration was smaller, which affects the expected targeting error and is explained later in the paper.
E. Accuracy Measurements
The most clinically relevant measure of accuracy is the distance between the actual injection and the center of the target, what we refer to as the “combined error” (CE). The CE is a vector sum of two errors: the “system error”, also known as the “target registration error”, (TRE), and the “operator error” (OE). The TRE is the distance between the actual injection location and the catheter tip location reported by the fusion system at the time of the procedure. The OE is the distance between the catheter tip and the target center as displayed by the fusion system at the time of injection, and is a function of the operator’s ability to guide the catheter to a specific location. In this study, it was difficult to measure the OE at the exact time of injection when respiratory motion was present, so we only report CE for respiratory motion experiments.
Accuracy measurements were carried out by taking a high resolution photograph of the PVA “slab” and a ruler following a set of injections. This was used to measure the distance between the target center and the actual injection location to obtain the CE. To measure TRE and OE, a screenshot of the injection was registered to the photograph by matching landmarks (5 small circles shown in and ). By doing this, the intra-procedural location of the catheter tip at time of injection could be compared to the actual injection location, as well as the center of the target (). The registration error for this technique was very low, 0.19 mm, compared to the typical system error in this study.
Photograph of targeting slab showing injections, which is registred to an intra-procedure image of the target at the time of injection. The target is zoomed in.
CE was characterized as a function of respiratory motion amplitude and registration error. Respiratory motion amplitude corresponds to the amount of translation the phantom makes during a single cycle. Targeting experiments were done at translations of 0, 5, and 20 mm, which cover most of the range found in humans. The respiratory rate was 15 cycles per minute for all experiments. Injections were performed at “end expiration” which corresponds to the respiratory phase of fiducial based registration initialization. To determine end expiration, the US volume was registered to the MR volume at the beginning of the experiment at the end expiration position, and end expiration was determined qualitatively as the moment that the US volume seemed to be best registered with MR. This is clinically comparable to pre-procedural registration during a breath hold.