  Purpose: To compare two methods that use x-ray spectral information to image
externally administered contrast agents: K-edge subtraction and basis-function
decomposition (the A-space method), Methods: The K-edge method uses narrow band
x-ray spectra with energies infinitesimally below and above the contrast
material K-edge energy. The A-space method uses a broad spectrum x-ray tube
source and measures the transmitted spectrum with photon counting detectors
with pulse height analysis. The methods are compared by their signal to noise
ratio (SNR) divided by the patient dose for an imaging task to decide whether
contrast material is present in a soft tissue background. The performance with
iodine or gadolinium containing contrast material is evaluated as a function of
object thickness and the x-ray tube voltage of the A-space method. Results: For
a tube voltages above 60 kV and soft tissue thicknesses from 5 to 25 g/cm^2,
the A-space method has a larger SNR per dose than the K-edge subtraction method
for either iodine or gadolinium containing contrast agent. Conclusion: Even
with the unrealistic spectra assumed for the K-edge method, the A-space method
has a substantially larger SNR per patient dose.
