Sunday Case of the Day
Physics
Author: William F. Sensakovic, PhD, DABR
Florida Hospital, Orlando, FL
History: During review of pediatric protocols a highly attenuating object
(234HU) extending through all CT sections was noted in the scan of a
3 week old patient. The protocol used 100kVp and z-axis tube current
modulation.
Figure 2. Example of object (arrows) in CT sections
of the thorax and abdomen. (Object thickness varied
from 1-4cm)
Figure 1. Object is visible on patient
scout radiograph. (arrows)
How does the presence of the object impact patient dose?
A.Increases by 20% - 50%
B.Increases by 3% - 5%
C.No Change
D.Decreases by 3% - 5%
E.Decreases by 20% - 50%
Findings:
Discussions with CT technologists and pediatric nursing revealed that
the object was a warming mattress used to prevent hypothermia in
neonates
1.A
solid water phantom was scanned with and without the warming mattress in the CT scanners of
three different manufacturers. (Figs. 3&4) Tube current selection, tube current modulation, and tube
voltage selection were also varied when available for each CT scanner.
Figure 3: Phantom with warming mattress
Figure 4: Phantom without warming mattress
Findings:
(continued)
2.Noise
(standard deviation) was increased in regions of interest placed at the posterior level of the
warming mattress. This occurred even when tube current modulation and tube voltage selection were
implemented. (Figs. 5&6)
3.Mean
HU was decreased in regions of interest placed at the anteroposterior level of the warming
mattress. This occurred even when tube current modulation and tube voltage selection were
implemented. (Figs. 5&6)
Figure 5: No warmer, 100kVp, Automated
tube current selection implemented
Mean: 8.0HU
Standard deviation: 13.3HU
Figure 6: Same parameters with warmer
causing increased noise and beam hardening
Mean: 0.2HU
Standard deviation: 29.3HU
Diagnosis:
A. Increases 20% - 50%
Discussion:
Automated techniques to select and modulate mAs and automated
techniques to select kVp are generally implemented to reduce patient
dose. The CT scanner determines the mAs/kVp to apply by analyzing the
pre-scan planning radiograph (Fig. 7) and/or the radiation detected from
previous sections in the current CT scan.
At diagnostic x-ray energies, the radiation is roughly
halved every 3.5cm of soft tissue the beam travels
through. The more attenuation the CT scanner expects
in the path of the radiation beam, the higher the mAs
and/or kVp are set. The pediatric warming mattress is
several centimeters thick and has an attenuation
coefficient between water and bone (as is apparent
from its HU value). The CT scanners tested increased
the tube current 20% - 50% to compensate for the
increased attenuation of the warming mattress.
Figure 7: Pre-scan planning radiographs
without (top) and with (bottom) pediatric
warming mattress. Increased attenuation
from the warming mattress causes the CT
scanner to increase patient dose.
Despite the increased mAs generated by the machine
when the pediatric warming mattress is used,
increased image noise and decreased mean HU (beam
hardening artifact ) are still observed at the posterior
level of the warming mattress. (Fig. 6)
Discussion:
Although it is theoretically possible that the presence of the warming
mattress could cause the machine to select a higher kVp when
automated tube voltage selection is implemented, this was not observed
in phantom experiments.
Some scans using angular tube current modulation demonstrated
relatively small increases in patient dose, noise, and beam hardening
artifact compared to scans using tube current selection alone. This was
vendor specific and differences were likely due to different algorithms
used to control modulation.
Figure 8: Phantom images with pediatric warming mattress in place. Left: Image with angular dose modulation implemented.
Right: Image with automatic tube current selection, but not angular modulation
References/Bibliography:
Mannudeep KK, et al. Techniques and Applications of Automatic Tube Current
Modulation for CT. Radiology 233(3): 649, 2004.
Raman SP, et al. CT Dose Reduction Applications: Available Tools on the Latest
Generation of CT Scanners. J Am Coll Radiol 10: 37, 2013.
Siegel MJ, et al. Automated low-kilovoltage selection in pediatric computed
tomography angiography: phantom study evaluating effects on
radiation dose and image quality. Invest Radiol 48(8): 584, 2013.