The influence of modulation transfer function shape on computed tomographic image quality.

The influence of modulation transfer function (MTF) shape on computed tomographic (CT) image quality was studied by computer simulation of a recently proposed x-ray detector with a nonuniform, shaped response to radiation. The shaped detector, while maintaining a high value of limiting spatial resolution, was shown to depress MTF and signal-to-noise ratio values at moderate spatial frequencies. This led to a significant loss of sharpness on noise-free images. Despite a superior radiation dose capture efficiency in the shaped detector, the MTF suppression was shown to produce inferior visibility of 1.0-mm objects in an image with typical CT noise. The significance for clinical imaging of the MTF suppression was demonstrated using a scan of a human head. The implication is that better methods than those currently used are needed for evaluating CT image quality.