Adaptation and estimation of dose in CT examinations of obese patients

Congress:

ECR 2012

Poster Number:

C-0788

Type:

Scientific Paper

Keywords:

Radiation physics, Radioprotection / Radiation dose, CT, Dosimetry, Radiation safety, Physics

Authors:

T. Johnson, R. P. Marcus, K. Neumair, M. F. Reiser, F. Bamberg; Munich/DE

DOI:

10.1594/ecr2012/C-0788

DOI-Link:

https://dx.doi.org/10.1594/ecr2012/C-0788

Fig. 1: Alderson phantom equipped with TLDs.

Fig. 2: Alderson phantom with the additional 10 cm layer of fat tissue.

Fig. 3: CT image obtained with set-up I.

Fig. 4: CT image obtained with set-up II.

Purpose

In CT examinations of normal weight subjects, standard conversion factors can be used to estimate radiation exposure. In obese patients, a large share of the dose is absorbed in subcutaneous fat, so that the standard factors yield false high values for radiation doses. The actual biologically relevant exposure of the organs is certainly lower, but the quantification is difficult, i.e. it is difficult to estimate the dose increase due to limited transmission and scattered radiation on the one hand and “shielding” of the organs due...

Methods and Materials

An Alderson phantom (RSD Radiology Support Devices) was equipped with 52 thermoluminiscent detectors evenly distributed through the abdomen. The phantom was scanned on aSiemens Definition Flash CTin three different set-ups: (I) The original phantom using automatic voltage adaptation and tube current modulation. (Fig. 1) (II) The phantom embedded in a circumferential layer of pork fat 10 cm thick with automatically increased voltage and current. (Fig.2 ) (III) Original phantom using the same CT settings as for the “obese” phantom. Dose was calculated using organ-specific weighing...

Results

Subjective image quality was deemed similar in set-ups (I) and (II) (cf. Fig. 3&4). However, the image noise was considerably higher in setting II compared with the initial examination (image noise 12 HU vs 8 HU), as well as the measured dose with a difference of 57%(7.9 mSv vs. 2.8 mSv; Fig. 4&5). Dose measurements using same radiation parameters with and without added fat (4.9 mSv vs. 7.9 mSv) yielded a corrected conversion factor of 0.0092 mSv/mGy*cm for the “obese” phantom.

Conclusion

Present CT technology allows automatic voltage and tube current modulation to adapt the radiation dose in order to achieve images of comparable quality in obese as normal subjects, avoiding overexposure and ensuring diagnostic value. Large patients require a higher dose to achioeve a sufficient image quality, but the standard conversion factors yield false-high equivalent dose values and should not be used to extimate exposure of obese patients. For our “obese phantom” set-up simulating a patient measuring 175 cm and weighing 120 kg, the corrected conversion...

References

Challenges for computed tomography of overweight patients. Bamberg F, Marcus R, Petersilka M, Nikolaou K, Becker CR, Reiser MF, Johnson T. Radiologe. 2011 May;51(5):366-71. German.