Considering the heightened sensitivity of young patients to radiation's ill effects, and the approximately 7 million CT scans of U.S. children annually, radiologists are working overtime to minimize the dose for every scan.
Bismuth breast shields are one way to reduce the dose to a child's sensitive upper thorax region during scans, and the shields have been growing in popularity in recent years thanks to their efficacy, low cost, and ease of use in patients of all ages.
A recent study explores new territory with respect to the shields, asking what would happen if they were used in tandem with dose modulation software. Radiologists might consider this option counterintuitive, inasmuch as automated tube currents can increase in response to the shields.
Fortunately, Dr. Courtney Coursey, Dr. Donald Frush, and colleagues from Duke University Medical Center in Durham, NC, and the University of Arkansas Children's Hospital in Little Rock conducted their tests on a pediatric phantom. And interestingly, the dose was lower when the bismuth shields (F & L Medical Products, Vandergrift, PA) and z-axis dose modulation were used together.
"The combined use of automatic tube current modulation and a shielding device might be expected to reduce radiation dose more than either technique alone," the authors wrote in the American Journal of Roentgenology. "However, it may also be that automatic tube current modulation technology that modulates in the z-axis on the basis of the topogram (scout image) may compensate for the presence of the shield by increasing the tube current and thus increasing the dose during the diagnostic scan when the shield is present while the scout image is obtained" (AJR, January 2008, Vol. 190:1, pp. W54 -W61).
To the authors' knowledge, the combination of bismuth shielding and automatic tube current modulation had not been reported.
The study aimed to determine the combined effect of the two on radiation dose, and measure image quality in terms of noise from data with the use of breast shields along with z-axis automatic tube current modulation in children.
The group performed age-based chest CT on a 16-slice scanner (LightSpeed, GE Healthcare, Chalfont St. Giles, U.K.) using a phantom (Atom Pediatric 5-Year-Old Phantom, model 705-D, CIRS, Norfolk VA).
Scanner settings included 120 kVp, 0.5-second rotation time, 1.375 pitch, 16 x 1.25-mm effective collimation, 5-mm slice thickness, 2.5-mm reconstructions, and a small field-of-view. Images were reconstructed using a standard algorithm.
Two scans were obtained in each of four sequences. The first was without a shield at a fixed tube current of 65 mAs, and the second with a two-ply (1.7 g/cm²) bismuth shield. The third exposure used automatic tube current modulation (65 mAs maximum, 10 mAs minimum, noise index of 12.0 HU standard deviation [SD]) using a scout image that was obtained after placing the shield. The fourth exposure was made using automatic tube current modulation with a scout image obtained before placement of the shield, the authors wrote.
The researchers used metal oxide semiconductor field effect transistor (MOSFET) technology (Best Medical Canada, Ottowa, Ontario) to measure the radiation dose in 20 organ locations, facilitated by corresponding holes in the phantom that accommodated the MOSFET detectors. Console dose-length product was used to measure the effective dose, and the SD of Hounsfield units in the identical regions of interest, the authors noted.
The results showed that the bismuth breast shield with fixed mAs reduced the dose to the breast by 26%, from 0.38 cGy to 0.28 cGy. However, shielding in addition to automatic tube current modulation cut the breast dose by 52%, they stated.
In addition, organ doses were lowest when the shield was placed after obtaining the scout radiograph, at a cost of a slight increase in noise, the authors wrote.
When the shield was placed after obtaining the scout image, the mean image noise in the range of the shields increased from 11.4 HU to 13.1 HU in the superior mediastinum, and from 10.0 HU to 12.8 HU in the heart (p < 0.01). The noise increase remained near the target noise index of 12.0 HU SD, however.
The use of the automated tube current modulation reduced the effective dose 35% (total effective dose of 1.4 mSv) when the shields were applied after the scout image, and by 20% (total effective dose of 1.6 mSv) when the shield was present during the scout image acquisition.
"Comparing the examinations with automatic tube current modulation when the shield was placed before the scout was obtained versus when the shield was placed after, measured doses were higher (average dose increase, 48%) at 10 of 14 locations (71.4%) under the shield, unchanged at three locations (21.4%) under the shield, and lower (average dose decrease, 27.8%) at one location (7.1%) under the shield when the shield was present in the scout image," they wrote.
A comparison of the highest dose acquisition (using fixed tube current, age-based chest CT protocol, and no shield) with the lowest-dose scan (using automated tube current modulation and bismuth shield with shield placed after obtaining the scout image), showed that doses dropped by an average of 51.3% in the 16 locations under the bismuth shield.
Placing the shields before obtaining the scout image enhanced the dose savings, yielding a slight increase in noise that remained within the target range, the authors wrote.
"Bismuth breast shields have been shown to reduce breast radiation dose by 29% in children, and by 27% to 52% in adults, depending on the thickness of the shield," they wrote. The current study results were in line with these previous findings.
There was a 4% decrease in dose at four locations outside the shield and a 5% increase in dose at one location outside the shield, but this was not unexpected in a detection system that can yield as much as 10% uncertainty in readings, the team wrote.
Study limitations included the use of a single scanner model and (z-axis) modulation system, the authors wrote; other systems might perform differently.
"Automatic tube current modulation in the x, y-axis has been shown to reduce mAs by 31% to 39% in the setting of pediatric chest CT, and to reduce mean effective mAs by 16.9% in the setting of adult chest CT," they wrote. "Note, however, that tube current reductions are surrogates for the actual patient dose."
Dose reduction is greatest when the breast shield is placed after the scout radiograph is obtained, the authors concluded.
By Eric Barnes
AuntMinnie.com staff writer
February 26, 2008
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