Type:
Educational Exhibit
Keywords:
Radioprotection / Radiation dose, Radiation physics, Radiographers, CT-Angiography, CT, Dosimetry, Radiation safety, Dosimetric comparison
Authors:
M. Kato1, K. Chida2, T. Moritake3, F. Sasaki4, T. Oomura4, N. Takahashi5, H. Oosaka4, H. Toyoshima4, T. Kinoshita4; 1Akita, Akita/JP, 2Sendai/JP, 3Kitakyusyu/JP, 4Akita/JP, 5Akita, Japan/JP
DOI:
10.1594/ecr2018/C-0406
Findings and procedure details
Development of the actual measurement system of ESD in CCTA
We devised a direct measurement system using many arrayed radiophotoluminescence glass dosimeters (RPLDs) to accurately measure the ESD and the dose distribution map of CCTA (Fig.3).
The RPLDs were placed in 84 pockets sewn into the dosimetry gown.
RPLDs are usually equipped with energy compensation filters made of tin.
However,
compensation filters are radiopaque materials.
Thus,
we chose the GD-302M (12 mm in length and 1.5 mm in diameter) with no energy compensation filter as the RPLD.
Therefore,
for more accurate measurements in a clinical setting,
correction is required using the energy calibration factor corresponding to the tube voltage used.
The tube voltage for CCTA was fixed at 120 kV and the calibration factor was 0.342 (Fig.4).
Materials and Methods
ESD and modulation transfer function (MTF) were measured by the normal CCTA protocol using a CT dosimetry gown and chest phantom.
A 64-slice DSCT (SOMATOM Definition,
Siemens Medical Solutions) was used in this study.
CCTA examination protocol (Fig.5)
- TopogramTopogram
- Chest plain CT
- Coronary calcium score
- Test injection of the contrast medium
- ECG-gated Coronary CT
The scan parameters on the 64-slice DSCT were: tube voltage 120 kV,
tube current 340 mAs/rot,
collimation 64 × 0.6 mm and rotation time 330 ms.
Results
Irradiation field and bed height of the CCTA
The ESD was measured using a dosimetry gown worn on the chest phantom.
After the radiation exposure,
we created the dose distribution map of the ESD,
based on the value of the GD-302M.
Simultaneously,
we confirmed the irradiation fields with the dosimetry film (GAFCHROMIC XR-RV3; International Specialty Products) (Fig.6).
The CCTA irradiation field suggested that the maximum ESD field was the test injection site of the contrast medium.
Similarly,
a dose distribution map of the ESD using the GD-302M dosimetry gown shows that the maximum ESD field was the test injection site of the contrast medium.
The average bed height of the CCTA was 145 mm below the CT gantry center.
The mammary ESD was 120 mGy and the dorsal ESD was 80 mGy (Fig.7).
The bed height of the CCTA at our facility was approximately 15 mm below the normal CT.
Method for reducing the mammary entrance surface dose
Figure 8 shows the dose distribution map of CCTA at the normal CT bed height which is 15 mm higher than the average bed height of the CCTA.
At the normal CT bed height,
both the mammary and dorsal surface doses were 100 mGy.
The mammary ESD was reduced by 20 mGy.
Discussion
The reason behind the high mammary surface dose at the average CCTA is assumed to be due to the influence of the central high dose range on the mammary area,
as a result of the bow-tie filter when the heart was centered on the gantry (Fig.9).
Image quality (Modulation transfer function)
Figure 10 shows the evaluation of the influence of the bed height by MTF on CCTA images.
The figure showed no difference in MTF from the CT gantry center up to 20 mm.