Wednesday Case of the Day
Physics
Author:
David M. Gauntt, Ph.D.
University of Alabama at Birmingham Medical Center
History: The image on the left is a C-spine reformat
that was added to a Neck CTA protocol. There is
significant loss of detail in the thoracic spine
compared to the C-spine. In a dedicated CSP
protocol the image quality is uniform throughout.
What difference between the protocols is most likely
to account for the difference?
A.
B.
C.
D.
E.
Different tube voltages
Different mA modulation
Different filter kernels
Different acquisition resolution
Different contrast media protocol (with vs. without)
CSP reformat in CTA Neck protocol
Answer:
B. Different tube current modulation
The CTA neck protocol (image on left) uses a manual mA mode with a fixed high tube
current. Higher attenuation through the shoulders decreases the photon flux and
increases the image noise.
The C-Spine protocol (image on right) uses an AEC mode that varies the tube current
as a function of the table position. The tube current is increased through the shoulders
to hold the noise constant.
Diagnosis:
This problem was encountered when the C-spine reformats were
added to a CTA Neck protocol. The CTA Neck protocol is
performed with a manual mA setting; the tube current was
uniform throughout the acquisition. The tube current was set to
ensure that there was adequate photon signal in the neck.
However, there is much more tissue in the beam in the upper
thorax than in the neck and head, leading to photon starvation in
the thoracic spine.
The C-Spine protocol is performed with CT Automatic Exposure
Control (CT AEC) turned on. The scanner uses the information
in the topogram (scout) views to adjust the tube current
throughout the acquisition. The tube current is automatically
increased as the beam passes through the shoulders,
compensating for the reduced x-ray transmission, and keeping
the noise in the image approximately constant.
The problem in the CTA Neck reformats was corrected by turning
on CT AEC, allowing the scanner to increase tube current in the
upper thorax. To ensure that the change did not increase noise
in the CTA images, the minimum tube current limit was set equal
to the original mA setting.