Comments on "Geometric Deconvolution: A Meta-Algorithm for Limited View Computed Tomography"

A recent paper1 on improving computed tomography images suffering from artifacts due to a missing sector of projection angles is criticized. It is shown that the proposed deconvolution approach is not applicable, since the transfer function to be inverted is zero in the whole sector of interest.     

Quantitative and Qualitative Evaluation of Geometric Deconvolution of Distortion in Limited-View Computed Tomography

Images reconstructed using a set of a few projections spanning a narrow angular range suffer from a systematic geometric distortion due to the point spread function of the reconstruction process. This distortion can be removed by deconvolving a complementary set of projections calculated from the initial reconstruction. Homomorphic deconvolution and inverse filtering techniques were used for this purpose. A second reconstruction is computed from the union of both sets of projections. Although the distortion is removed, the results are noisy due to problems associated with inverse filtering. Two-dimensional (2-D) filtering of the second reconstruction is performed to reduce the noise, while preserving the reduction in geometric distortion. Results obtained using several 2-D filters were compared visually and by point noise content, distortion, and projection error measures. Quantitative measures of geometric distortion removal for test images are also described.     

Reply to "Comments on 'isotope effects in MNOS Transistors'"

The authors arguments in "Comments on 'Isotope Effects in MNOS Transistors'" mentioned above do not logically support his hypothesis and in fact agree with the hypothesis of the original letter.     

Reply to "Comments on 'structure-enhanced MOSFET degradation due to hot-electron injection'"

The equal-substrate-current stressing condition used to compare hot-electron-induced MOSFET degradation in different device structures is discussed. The error introduced by using this stressing condition is quantitatively assessed and shown to be too small to affect the interpretation of the experimental data. The post-stress transconductance degradation characterisitics in LDD MOSFET's are also discussed.     

Reply to "Comments on 'a theory of the Hooge parameters of solid state devices'"

We clarify here our use of the Hooge parameter and indicate what we mean with verifying Handel's quantum theory of the1/fnoise.     

Comments on "A fast analog to Gray code converter"

The successive-approximation method which encodes an analog voltage into Gray Code by using the absolute values of successive residues is very old, and references are given to some of the earliest publications. A different algorithm is described which is only a slight modification of the successive approximation method for natural binary encoding.     

A Method for Stop-Action Imaging of the Heart Using Gated Computed Tomography

A new method for the reconstruction of limited angle projection data in rotary fan-beam X-ray computed tomography (CT) is presented. Missing views resulting from ECG-gated cardiac CT are estimated, and the standard fan-beam reconstruction algorithm is used to convolve and backproject both measured and estimated views. The estimation of the missing views takes place in three stages: first, the projection data is augmented by incorporating into each missing view the line integrals that do not pass through the heart, and which otherwise would be considered missing due to ECG-gating; second, line integrals corresponding to source positions in the range 180°±fan angle away from missing view angles are reflected; third, those line integrals that remain missing are estimated by interpolation. This method has been applied to ECG-gated cardiac imaging in dogs without requiring extensive interpolation; end-systolic and end-diastolic images were generated with short-interval gating (?cycle) and total scan time (breath holding period) of 12 s. An important advantage of this method over other proposed limited angle reconstruction techniques is that it uses the existing fan-beam convolution-backprojection algorithm for image reconstruction.     

Iterative Reconstruction?Reprojection: An Algorithm for Limited Data Cardiac-Computed Tomography

Cardiac X-ray computed tomography (CT) has been limited due to scanning times which are considerably longer (1 s) than required to resolve the beating heart (0.1 s). The otherwise attractive convolution-backprojection algorithm is not suited for CT image reconstruction from measurements comprising an incomplete set of projection data. In this paper, an iterative reconstruction-reprojection (IRR) algorithm is proposed for limited projection data CT image reconstruction. At each iteration, the missing views are estimated based on reprojection, which is a software substitute for the scanning process. The standard fan-beam convolution-backprojection algorithm is then used for image reconstruction. The proposed IRR algorithm enables the use of convolution-backprojection in limited angle of view and in limited field of view CT cases. The potential of this method for cardiac CT reconstruction is demonstrated using computer simulated data.