  Four-dimensional (4D) wide coverage computed tomography (CT) is an effective
imaging modality for measuring the mechanical function of the myocardium.
However, repeated CT measurement across several heartbeats is still a concern.
A projection-domain noise emulation method is presented to generate accurate
low-dose (mA modulated) 4D cardiac CT scans from high-dose scans, enabling
protocol optimization to deliver sufficient image quality for functional
cardiac analysis while using a dose level that is as low as reasonably
achievable. Given a targeted low-dose mA modulation curve, the proposed noise
emulation method injects both quantum and electronic noise of proper magnitude
and correlation to the high-dose data in projection domain. A spatially varying
detector gain term as well as its calibration method were proposed to further
improve the noise emulation accuracy. To determine the low dose threshold, a
projection domain image quality (IQ) metric was proposed that is based on the
number of projection rays that do not fall under the non-linear region of the
detector. Experiments were performed to validate the noise emulation method
with both phantom and clinical data. For both phantom and clinical data, the
low-dose emulated images exhibited similar noise magnitude, artifacts, and
texture to that of the real low-dose images. The proposed channel-dependent
detector gain term resulted in additional increase in emulation accuracy. Using
the proposed IQ metric, recommended kVp and mA settings were calculated for low
dose 4D Cardiac CT acquisitions for patients of different sizes. In conclusion,
a detailed method to estimate system-dependent parameters for a raw-data based
low dose emulation framework was described. The proposed low-dose emulation
method can be used to prospectively select patient-specific minimal-dose
protocols for functional cardiac CT.
