Theme: Radiation Protection & Radiobiolgy
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Measurements of peak surface dose using radiochromic films during helical computed tomography scan of the sinus with pitch factor of less than one.
Authors: -Salman Z. Altimyat
Dr. Khaled Soliman
Institutions: Prince Sultan Military Medical City-
 
Summary of Work

During computed tomography (CT) helical scanning mode the patient surface dose distribution is assumed to be non-uniform, therefore point dose measurement methods may lead to imprecise estimation of the radiation dose received by the patient skin in particular. We have used XRQA2 radiochromic films to measure peak surface dose (PSD). in helical CT scanning with pitch factor of less than one like in this study there is x-ray beam overlap and the volume of tissue is irradiated more than once per scan. Such acquisition allows better image quality but higher patient dose. CTDI(vol)  is dependent on acquisition parameters, as well as calibration phantom diameter, and can result in a significant overestimate or underestimate of the actual dose delivered to the patient because of discrepancies in body size versus calibration phantom size. The PSD is expected to be higher than the ESD in helical CT scanning mode due to overlapping of axial radiation fields at the patient entrance surface. In an effort to verify the clinical dose levels during CT examination of the sinus, we are proposing to measure the ESD using XRQA2 films.

The films were placed under the patient head rest in order to sample the entrance surface dose in-vivo. We have performed in-vivo film irradiation on 23 patients in this study in order to verify the suitability of the method clinically. We have calculated the ratio of: PSD/CTDI (vol) and ESD/ CTDI (vol).  

 

Summary of Results

The measured average ESD in the sinus exam was 11.7±1.0 mGy, the PSD was 15.7±1.7 mGy and the scanner calculated CTDI (vol) was 13.3±0.1mGy. The calculated ratios of ESD/ CTDI (vol) and PSD/CTDI (vol) were 0.88 and 1.18 respectively.

 These results seem to indicate that the CTDI (vol) underestimates the PSD by 18%. On the other hand the CTDI (vol) was found to be a conservative estimator of the average ESD in terms of radiation protection. 

Conclusion

The use of radiochromic film as In-vivo dosimeter does not interfere with the clinical radiological exam and does not produce any image artifacts. The method can be used to study other CT examinations specially the ones with large beam width and high pitch factor. The method allows measurement of the peak skin dose, examination of the CT dose profile and the radiation dose distribution. Using the measured ESD will allow more accurate comparison between scanners and imaging protocols and may serve as dose reference level (DRL) metric in CT imaging. The measured ESD can also be used as an estimator of the patient eye dose in CT examination of the sinus.

References

Lian, C. P. L., Wong, J. H. D., Young, A., Cutajar, D., Petasecca, M., Lerch, M. L. F. & Rosenfeld, A. B. (2013). Measurement of multislice

computed tomography dose profile with the Dose Magnifying Glass and the MOSkin radiation dosimeter. Radiation

Measurements, 55 51-55.

Summary of Work

picture number (1) (a) Sinus Exam radiation beam profile captured by the film. (b) XR-QA2 film slice placed under the patient head and analysed after digitization using MATLAB routine. The beam profile is expressed here as reflective density pixel values. We notice the sinusoidal shape of the profile which is a characteristic of the helical mode of acquisition.

picture number (2)

Helical scan mode at 20 mm nominal beam collimation, pitch 0.875. FWHM (DMG)= 70.0 mm, FWHM (film)=74.6 mm, % difference in FWHM ( film-DMG)=6.0 %. MOSkin point dose measurement at beam isocentre =34.8 mGy

Summary of Results
Conclusion
References
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Date: 2018-01-31 08:23:20
i couldn't upload picture
Date: 2018-01-30 15:44:39
difficult to use