Although modern multislice CT systems can quickly scan a wide range in fine detail, the high exposure dose and long examination time of CT studies are still important issues. CT is therefore generally considered to be unsuitable for screening. This is particularly true for pediatric patients, who are especially sensitive to radiation, and plain radiography is therefore frequently selected for examination of the temporal bone. We have developed an ultra-low-dose CT imaging method using a 320-row area detector CT (ADCT) scanner that may be suitable for screening for diseases of the ear even in pediatric patients.
Temporal bone specimens were scanned with an ADCT scanner (Aquilion ONE
<TM>, Toshiba Medical Systems, Otawara, Japan) using various scan conditions. Image quality was evaluated, and the optimal scan conditions providing clinically acceptable images at the lowest possible exposure dose were identified. A human-body phantom was then scanned with ADCT using these scan conditions. Plain radiography using the Schu..ller method was also performed for the same phantom to acquire images of the temporal bones bilaterally. The CT and radiographic exposure doses were compared to assess the suitability of our ADCT method for screening.
The gantry of the ADCT scanner includes an X-ray source unit and a detector (area detector) which is several times larger than that used in a conventional CT system. Three-dimensional data can be acquired over a wide range in a single high-speed rotation, making it possible to quickly obtain images in the same temporal phase.
As the results, the optimal scan conditions were found to be tube voltage 100 kV, tube current 10 mA, and exposure time 1.5 s/rot. by 1 rotation. The CTDIVOL (standard dose) was 1.7 mGy, which is 0.7% of the usual value of 243.4 mGy. The maximum skin dose to the head in plain radiography of the temporal bones bilaterally was 2.12 mGy on average. The maximum skin dose to the external ear in CT scanning with the above conditions was 1.59 mGy on average, which was lower than that for the Schüller method. However, the dose in CT scanning was slightly higher at the front and rear.
With our proposed CT imaging method, images are acquired in a very short time, three-dimensional image data is obtained, and the exposure dose is equal to or lower than that of plain radiography. This method is therefore expected to be a useful new tool for morphological screening examinations.
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