L E V Y Vol. 2, No. 7, July 1995

My findings indicate that the histogram-based algorithm cannot be applied reliably in vivo. Rather, certain special- ized in vitro situations, specifically those in which the tar- get structure is interfaced with air (or other high-contrast tissue) in a bicompartmental model, provided initial prom- ising results. This situation was further borne out in vivo such that the only orbital floor identified by the algorithm was in an enucleated orbit (cadaver 1, Table 1).

A survey of other thresholding algorithms yielded no approach similar to that evaluated in this article. Perhaps histogram concavity analysis [6] or exponential hull [7] thresholding algorithms would have become more rele- vant had the thresholding algorithm proposed in this research proved applicable using craniofaciat CT data sets. Efforts to predict and facilitate the detection of volume averaging in 3D CT craniofacial modeling should continue.

A C K N O W L E D G M E N T S

I wish to thank John Pallister, Jian Gong, and Dongli Yang for generating the histograms; Alphonse Burdi

for providing the craniofacial specimens; Hazem Maher Aly for preparing the craniofacial specimens; Dan Dyson for technical assistance and advice in gen- erating the 3D reconstructions; Bob Combs for prepar- ing the photographs; and Jane B. Mitchell for typing the manuscript.

R E F E R E N C E S

1. Plewes DB, Dean PB. Detectability of spherical objects by computed tomography. Radiology 1979; 133:785-786.

2. Goodenough D, Weaver K, Davis D, LaFalce S. Volume averaging limita- tions of computed tomography. A JR 1982;138:313-316.

3. Hemmy DC, Tessier PL. CT of dry skulls with craniofacial deformities: accu- racy of th tee-dimensional reconstruction. Radiology 1985; 157:113-116.

4. Levy RA. Semiautomated three-dimensional craniofacial imaging and modeling. Invest Radio11994;29:150-155.

5. Levy RA. A histogram-based algorithm for semiautomated three-dimen- sional imaging and modeling (abstr). Proceedings of the 32nd Annual Meeting of the American Society of Neuroradiology 1994:186-187.

6. Rosenfield A, de la Torre P. Histogram concavity analysis as an aid in threshold selection. IEEE Trans Systems, Man and Cyber 1983;SMC- 13:231-235.

7. Whatmough RJ. Automatic threshold selection from a histogram using the "exponential hull." Graph Models Image Proc 1991 ;53:592-600.

A n n o u n c e m e n t

The 20th Annual Nuclear Cardiology Symposium and Workshop will be held September 27-29, 1995, at the Grand Hotel in Milwaukee, WI. It is sponsored by the Cardiovascular Disease Section, University of Wisconsin Med- ical School, Milwaukee Clinical Campus; Milwaukee Heart Institute of Sinai Samaritan Medical Center; and Continu- ing Medical Education, University of Wisconsin Medical School. AMA Category 1 credit, AART Category A, and Illinois & Iowa Nuclear Safety credit will be awarded.

For more information, contact Sarah Aslakson, Continuing Medical Education, 2715 Marshall Ct., Madison, W1 53705; (608) 263-2856, fax (608) 262-8421.

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