TLD based method to estimate bowtie filter shape in PET/CT

Background: In CT systems, the machine utilizes a bowtie filter to shape the X -ray beam and remove lower energy photons. The shape of this bowtie filter is complicated and its geometry is often not available in detail. These renders the CT dose index [CTDI] to have different values in measurement versus Monte Carlo simulation studies and other analytical calculations especially in dosimetry of internal organs. In existing literature, the bowtie filter shape is extracted by using expensive sensors

Materials and Methods: In the present work, the shape of the bowtie filter of the Biograph 6 PET/CT was derived by using Thermolumenecence dosimeters [TLDs]. Subsequently, to evaluate the accuracy of the body bowtie filter shape as generated by TLDs, Monte Carlo simulation of CT was performed. 16 X-ray sources in various angles were used within the Monte Carlo code [MCNP-4C] to simulate the CT section of the PET -CT Biograph 6 system and to calculate dose

Results: The relative difference between simulated and measured CTDI value for the PET/CT Siemens Biograph 6 at 80, 110 and 130 kVp were 4.2, 2.9 and 2.3%, respectively

Conclusion: In this study, we demonstrate that it is possible to calculate the bowtie filter shape by using an inexpensive TLD method. The results showed that it is possible to determine the shape of the bowtie filter in PET/CT using TLDs with acceptable accuracy?

[Prediction of maxillary canine impaction with panoramic radiographs]

The incidence of maxillary canine impaction is between 1% and 3% populations. Determining whether impaction will occur and timing the treatment modalities that are affected by impacted canine [s] are paramount for a successful out come, panoramic radiograph to identify canine impaction; not only is economical but treatment result will be more desirable. The aim of this investigation was to develop a reliable method for predictors of maxillary canine impaction on a panoramic radiograph. Sixty - four panoramic radiographs of mixed dentition patients were inspected to belong to two groups of patient with impacted and not impacted canines. 88 were found with no impaction and 40 with impacted canines. For predicting canine impaction panoramic radiographs were inspected for two indices: 1- The erupted lateral tooth were divided to fourfold zone from 4 to 1 sectors in mesiodistal dimension; therefore identified the unerupted permanent canines cusp tip location related to the erupted permanent lateral. 2-The internal angle between the long axis of the unerupted canine and a horizontal line between superior points of condyles was measured. The data were analyzed by descriptive statistics and logistic regression. All of not impacted teeth were found in sector I and II. [69.3% in sector I and 30.7% were placed in sector II]. 17.5% of impacted teeth were in sector I, 42.5% in sector II, 27.5% in sector Ill and 12.5% were placed in sector IV. Furthermore the mean angle for non impacted teeth was 74.72' and for impacted teeth was 67.12'. The location of canine cusp tip in panoramic radiograph is the most important factor in predicting probable impaction, but often, the angle of canine is not worth