CHICAGO, Nov. 27—Radiation dosage rates delivered during the acquisition of diagnostic imaging exams follow the principle of as low as reasonably achievable (ALARA). A pair of studies from the Children’s Hospital of Capital Medical University in Beijing, China, presented at the 93rd annual meeting of the Radiological Society of North America (RSNA), demonstrated the capability to reduce radiation dose for pediatric abdominal and chest CT studies while still acquiring acceptable clinical images.
Yun Peng, MD, and colleagues selected 59 children with leukemia (35 boys, 24 girls, between 6 months to 3-1/2 years old) who underwent 64-slice CT abdominal scans utilizing a standard protocol of 150mAs.
The researchers selected two anatomical levels (the portal hepatic and left renal vein) and added different levels of noise to the images to simulate dose reductions. They used a noise-addition research tool developed by GE Healthcare, based on the firm’s z-axis automA automatic exposure control feature, to simulate dose reductions of 20 percent, 30 percent, 40 percent, 50 percent and 70 percent, respectively, from the facility’s standard protocol.
Image noise was measured to verify that the correct amount of noise had been added. A total of 708 image data sets with 5 levels of lower dose scans were simulated for each of the two anatomic levels for the 59 patients.
“Two experienced pediatric radiologists independently assessed image quality including confidence in detection lesion, soft-tissue contrast, sharpness of different structures, and artifacts on a 5-point scale with 5 being the best,” the researchers wrote. “Scores greater than or equal to 3 were considered clinically acceptable.”
According to the scientists, the average noise for original images was 4.7. For the simulated images, the researchers noted that as the noise (and dose reduction potential) increased the image quality score decreased. At a 50 percent dose reduction, the noise index was 6.6 at a 5mm slice thickness, and the image quality score was 3.2, considered clinically acceptable, the researchers reported.
The team wrote that although their simulation indicated that a pediatric abdominal CT scan could be clinically acceptable at a 50 percent dose reduction, further testing was warranted with clinical cases that require repeated scans to confirm the finding.
Jinjin Zeng, MD, and colleagues put the ALARA principle to practice in their investigation of dose reduction potentials for children with suspected severe respiratory infection with automatic tube current (mA) modulation on 64-slice CT.
Their study encompassed 26 children (17 boys, 9 girls, with mean age of 3.2 years) with suspected severe respiratory infection who underwent a chest scan with a GE Healthcare 64-slice LightSpeed VCT before and after treatment. The team reported that the first scan was performed with fixed mAs of 150 or 200 that was age dependent, which is the facility’s routine scan protocol.
For the subsequent scan after treatment, the researchers evaluated the impact of reduced mAs on image quality for future dose reduction. They used the automatic tube current modulation (ATCM) feature on the LightSpeed system for the patient’s scan after treatment with noise indices ranging from 8 to 10 at 5mm slice thickness. The CT Dose Index Volume (CTDIvol) was recorded and image noises were measured for each scan with and without mAs modulation.
“Two experienced pediatric radiologists blindly evaluated the image quality including image noise, artifact, sharpness and visualization of small airways, pulmonary vascular, mediastinum structure and lesion detection confidence using scores of 1 to 5 with 5 being the highest,” the researchers wrote. “Scores greater or equal to 3 were considered clinically acceptable.”
The scientists reported that CTDIvol for scans with a fixed mAs was 6.85mGy (6.39 – 9.96), but the image noise had a large variation due to the patients’ size variations.
“With ATCM and age-dependent noise index selection, noise variations were greatly reduced, and the average was about 2.59mGy (1.46-4.51), which was about 60% lower than that with fixed mAs,” the team wrote.
The group found that the average image quality score with ATCM was 4.1 and all the examinations were rated as diagnostic. All diagnosed pathologies correlated well with clinical outcomes, they noted.
“Automatic tube current modulation on 64-slice CT can be used to produce consistent image quality