ABSTRACT
Objective:To investigate the effect of different scanning centers on eye lens dose, image quality, and the dose reduction rate when using the organ dose modulation (ODM) technique in head CT.Methods:The porus acusticus externus of the head phantom was considered the scanning isocenter. The ODM was initiated and the spiral scans were performed at the scanning centers with the height of porus acusticus externus and its upper and lower 2, 4, and 6 cm, respectively. The scanning range was from the top of the head to the base of the head. Three thermoluminescent dosimeters (TLD) were placed on the surface of two eyes at each scan and the average measurement value was regarded as the radiation dose to the eye lens. The volume CT dose index (CTDI vol) and dose length product (DLP) were recorded. The scans were repeated with no ODM and the dose reduction rates at each scanning center were calculated. The regions of interest (ROI) in each group of images with ODM were drawn and the noise (SD), signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) were evaluated. Results:Compared with the isocenter, the maximum change rates of CTDI vol and DLP in each scanning center were 2.46% and 2.43%, respectively. The eye lens dose increased as the scanning centre moving upwars (i.e. the bed dropping) by 39.02% at the position of 6 cm above the isocenter and decreased by 35.91% at the position of 6 cm below the isocenter. With the seven groups of scanning centers, the reduction rates of CTDI vol and DLP caused by ODM were 7.95%-8.61%, 7.91%-8.61% respectively, and the difference in the reduction rate of each dose value was not statistically significant( P>0.05). The reduction rate for eye lens dose ranged from 18.09% to 26.14%, with the highest reduction rate at the position of 4 cm above the isocentre and the second rate at the isocentre (24.73%). The difference in the rate of reduction at each scanning center was statistically significant( t=0.13, P<0.05). As the scanning center moved up, the SD of the eye region decreased and the SNR increased, and the highest CNR at the isocentre was 239.79. The SD and SNR of the brain parenchyma region were 6.85-7.96 and 3.08-4.19 respectively, and the highest CNR at the isocentre was 244.79. Conclusions:When ODM technique is used in head CT, the scan centre has a significant effect on the eye lens dose and image quality. Meanwhile, the reduction rate of the eye lens dose caused by ODM is also affected. Therefore, porus acusticus externus is recommended as the scanning center in head CT.
ABSTRACT
Objective:To compare the dose and image quality of selected photon shield (SPS) technique, organ-based tube current modulation (OBTCM) technique and the combination of these two techniques for reducing the organ dose in head CT examination for infants.Methods:Two anthropomorphic head phantoms (CIRS 1-yr-old and 5-yr-old) were scanned by using Reference mode, Reference + OBTCM mode, SPS mode and SPS + OBTCM mode, respectively. Radiation doses to the lens of the eye, the anterior of the brain, the posterior of the brain, noise level and CNR of orbit and brain in different phantoms were measured and compared by using different scanning modes.Results:Compared with Reference mode, the doses to the lens of the eye in 1-yr-old and 5-yr-old phantom decreased by (21.89 ± 0.01)% and (28.33 ± 0.34)%, respectively. In SPS mode, the reduction in doses to the lens of the eye in 1-yr-old and 5-yr-old phantom were (71.38 ± 1.30)% and (53.72 ± 2.42)%, respectively. In SPS + OBTCM mode, the reduction was (71.12 ± 2.54)% and (55.73 ± 1.90)%, respectively. There was significant difference in the noise level of orbit and brain in different phantoms under various scanning modes ( F=5.67-85.47, P< 0.05). The noise level in OBTCM mode compared with reference mode increased slightly (<1.45 HU) in various phantoms. SPS and SPS + OBTCM mode resulted in a small noise increase (<2.58 HU). There was no significant difference in CNR of different phantoms under various scanning modes ( P>0.05). Conclusions:SPS and SPS + OBTCM mode can significantly reduce the radiation dose of lens and the whole image plane in the head CT scan for infants, with maintaining the image quality.