Affiliations: Department of Radiology, Boston University Medical
Center, Boston, MA, USA | Global CT Engineering, General Electric Healthcare,
Waukesha, WI, USA | Department of Radiology, Emory University, Atlanta,
GA, USA
Note: [] Corresponding author: Baojun Li, Ph.D., Department of Radiology,
Boston University Medical Center, 80 E Concord Street, Boston, MA 02118, USA.
Tel.: +1 617 638 7186; Fax: +1 617 638 7509; E-mail: Baojunli@bu.edu
Abstract: Future generations of CT systems would need a mean to cover an
entire organ in a single rotation. A way to accomplish this is to physically
increase detector size to provide, e.g., 120∼160 mm z (head-foot) coverage
at iso-center. The x-ray cone angle of such a system is usually 3∼4 times
of that of a 64-slice (40 mm) system, which leads to more severe cone beam
artifacts in cardiac scans. In addition, the extreme x-ray take-off angles for
such a system cause severe heel effect, which would require an increase in
anode target angle to compensate for it. One shortcoming of larger target angle
is that tube output likely decreases because of shorter thermal length. This
would result in an increase of image noise. Our goal is to understand from a
physics and math point of view, what is the theoretical entitlement of
artifacts, resolution, and noise impact of such a system. The image artifacts
are assessed through computer simulation of a helical body phantom and visual
comparison of reconstructed images between a 140 mm system and a 64-slice
system. The IQ impact from target angle increase is studied analytically and
experimentally by first finding the proper range of target angles that give the
acceptable heel effect, then estimating the impact on peak power (flux) and z
resolution using an empirical model of heel effect for given target angle and
analytical models of z resolution and tube current loading factor for given
target thermal length. The results show that, for a 140 mm system, 24.5% of
imaging volume exhibits more severe cone beam artifacts than a 64-slice system,
which also poses a patient dose concern. In addition, this system may suffer
from a 36% peak power (flux) loss, which is equivalent to about 20% image noise
increase. Therefore, a wide coverage CT system using a single x-ray source is
likely to face some severe challenges in IQ and clinical accuracy.
