Angular disparity in ETACT scintimammography

Optimization of TACT imaging protocols for in situ visualization of cochlear electrode arrays in cat temporal bones

OBJECTIVE

To explore the effect of the number of two-dimensional (2D) images and x-ray projection angles on the resolution of reconstructed three-dimensional (3D) volumes of intracochlear electrode arrays in cadaveric cat temporal bones using Tuned Aperture Computed Tomography (TACT).

DESIGN

Multiple 2D radiographs (basis images (BI)) of implanted cadaveric cat temporal bones were acquired using a range of projection angles, and imported into the TACT workbench. 3D volumes were reconstructed using varying numbers of BIs. Contrast resolution in the image was determined by comparing the contrast ratio (using maximum and minimum grayscale values) in specified anatomic areas of interest.

RESULTS

Systematically increasing the number of BIs used in the reconstruction process resulted in a systematic increase in contrast resolution. Likewise, increasing the range of effective projection angles, as also the number of such angles used in the TACT computation also increased the contrast resolution of the resulting images.

CONCLUSIONS

Precise determination of the location of cochlear implant electrodes in situ is critical to understanding the factors influencing efficacy of electrical stimulation of the deaf ear. Renderings generated with the TACT algorithm produce 3D images permitting visualization of implant electrode features and anatomic details with resolution sufficient to accurately localize electrode contacts within scala tympani. The quality of resulting images, evaluated as a function of image contrast, improved with a larger number of BIs in the reconstruction. Wider projection angles also improved image detail in addition to generating thinner slices. Any loss in contrast was compensated for by the number of BIs. TACT can thus be optimized to provide useful data to help characterize the location of intracochlear electrode arrays.

Determination of projection geometry from quantitative assessment of the distortion of spherical references in single-view projection radiography

A method is introduced, inferring the three-dimensional (3-D) location from the 2-D radiographic shadow of an opaque spherical reference body of known radius by considering its elliptical distortion, the 2-D shadow location and a known source-to-receptor distance. Three noncollinear spheres fixed to a rigid object constitute all possible degrees of freedom, i.e., the entire 3-D imaging geometry. The method may be used (a) to determine the 3-D imaging geometry from a single 2-D view and (b) to correct for foreshortening of object distances coplanar with the plane defined by the sphere triplet. Apart from the mathematical background the article describes a small feasibility experiment, performed with four different sphere diameters and a commercial dental ccd-receptor system (pixel length: 0.0195 mm). The mouse-cursor based image evaluation revealed an average underestimation of the critical depth- (x-) coordinate decreasing with increasing radius (-30.3% for r=0.5 mm to 2.8% for r=2.5 mm). Intraobserver reliability (the standard deviation between three single cursor-based assessments) ranged between 0% and 8% of the actual true depth. The main source of the input error is associated with the assessment of the amount of elliptical distortion, where subpixel accuracy is demanded. Consequently, software-based automated image evaluation is required using available methods for pattern recognition and point-spread correction. Provided sufficient accuracy, the method provides an important tool for foreshortening correction, depth assessment, motion analysis, and 3-D reconstruction from two or more 2-D views.

Application of TACT to the evaluation of total joint arthroplasty

Three-dimensional imaging may be valuable in the evaluation of total joint arthroplasty. The partial-volume effect and streak artifacts from the metallic components of these devices limit the applicability of computed tomography (CT) to this application. Tuned-aperture computed tomography (TACT) is a tomographic approach that has been successfully used in other medical applications. In TACT, the acquisition geometry is inferred from the localization of fiducial markers in a series of projection images. The flexibility and robustness of TACT, as well as the fact that through an appropriate choice of reconstruction algorithms it can suppress streak artifacts, potentially makes it an appropriate approach for evaluating total joint arthroplasty. A simple computer-simulated model of a total knee replacement (TKR) was generated and used to evaluate the accuracy and artifacts associated with three-dimensional (3-D) renderings produced using TACT. A knee specimen from a cadaver that had received a TKR was used to investigate further the potential of TACT for this application. In both tests, TACT provided high-quality 3-D representations of the object. Both simple back-projection and minimum pixel back-projection were used to reconstruct the data. Minimum pixel back-projection provided high-contrast images that appeared to be relatively free of tomosynthetic artifacts. In summary, the potential of TACT in the evaluation of total joint arthroplasty was demonstrated. Future investigations will study TACT's ability to quantify the spatial relationship between the metallic components of these devices as well as TACT's ability to identify bony changes of diagnostic consequence.