Richard A. Levy

A Histogram-Based Algorithm for Semiautomated Three-Dimensional Craniofacial Modeling

RATIONALE AND OBJECTIVES I conducted a study designed to facilitate thresholding and reduce volume averaging in three-dimensional (3D) computed tomography (CT) craniofacial modeling. METHODS Three-dimensional CT reconstructions of orbits from two cadavers and seven clinical cases were generated from paired axial and coronal data. A histogram-based algorithm that was based on preliminary phantom and craniofacial specimen trials was applied to orbital data to identify volume averaged regions of thin bone to be used in conjunction with standard bone thresholds. Region-of-interest measurement of the orbital floors on the original two-dimensional slice data assessed algorithm performance. RESULTS In five of the nine cases (55%), configuration of the superimposed histograms predicted regions of volume averaging. In only one case did such a region localize to the orbital floor. In the remaining four cases, the air-mucosa interface deep to the orbital floor was identified by the histogram method. CONCLUSION Operator-dependent editing remains superior to this histogram-based algorithm in reducing volume averaging in 3D craniofacial modeling.

CT-Generated Porous Hydroxyapatite Orbital Floor Prosthesis as a Prototype Bioimplant

Hydroxyapatite bioceramic was used for the manufacture of an orbital floor prosthesis from spiral CT data acquired transaxially at 1-mm beam collimation, pitch of 1, and 0.2-mm reconstruction intervals. CT data were converted to vector file format for subsequent prosthesis manufacture on a stereo-lithography machine. The orbital floor prosthesis was engrafted onto an acrylic model of the orbit as a qualitative indication of its overall accuracy. High anatomic accuracy was achieved, as determined by visual inspection. Cross-hatching of the vector file data allowed a porous internal architecture of the prosthesis. Refinements in chemical structure of the hydroxyapatite bioceramic are expected to enhance mechanical properties.

Preliminary Experience witb Selective Laser Sintigrapbic (SLS) Models of tbe Human Temporal Bone

Materials and Methoqs Thin section CT of a left human cadaveric temporal bone was performed using multiple scan planes (axial, coronal, sagittal) at 1.5 mm slice thickness, 0.25 mm pixel size with an edge enhancement 2D algorithm. CT data was converted to toggle point format based upon a threshold value of 200 obtained from prior experimentation with a CT phantom (unpublished data). Selective Laser Sintering of polycarbonate powder was performed at a beam thickness of 0.060 inches, 100 scan lines/inch, layer thickness of 0.010 inches, and layer repeat factor of 4. The polycarbonate models were then scanned in the axial, coronal and sagittal planes and compared with the original CT data. Anatomic dissection of the models was performed for further verification of the imaging findings.