Peter Schmidlin

Improved iterative image reconstruction using variable projection binning and abbreviated convolution

Noise propagation in iterative reconstruction can be reduced by exact data projection. This can be done by area-weighted projection using the convolution method. Large arrays have to be convolved in order to achieve satisfactory image quality. Two procedures are described which improve the convolution method used so far. Variable binning helps to reduce the size of the convolution arrays without loss of image quality. Computation time is further reduced by abbreviated convolution. The effects of the procedures are illustrated by means of phantom measurements.

Iterative reconstruction of PET images using a high-overrelaxation single-projection algorithm

An iterative image reconstruction procedure is described which is able to calculate high-precision images within eight iterative steps. High initial overrelaxation parameters are used which drop down towards about one during continuing iteration. The parameters are determined pragmatically, postulating-maximum gain of image quality during a sequence of iterative steps. Results with simulated and measured data show that parameters derived from one data set may be widely used for other data sets. The acceleration of iterative reconstruction achieved by the estimation of optimum overrelaxation parameters is important for large data sets, especially for fully 3D reconstruction.

Subsets and overrelaxation in iterative image reconstruction

A number of iterative image reconstruction algorithms were integrated into one formula characterizing each algorithm by only two parameters: overrelaxation and number of subsets. From the formula it follows that the ordered-subsets iteration (OS-EM) is equivalent to iteration with overrelaxation, where the OS level corresponds to the overrelaxation parameter. Algorithms represented by the formula were studied with respect to speed of convergence and image characteristics. In particular, OS-EM was compared with a single-projection iteration procedure using an optimized sequence of overrelaxation parameters (HOSP) which combines rapid convergence with reduced storage requirements. As a result, OS-EM with a constant number of subsets either needed more iteration steps than HOSP or provoked additional noise, depending on the number of subsets used during iteration. OS-EM can be improved by using decreasing OS levels, imitating the decreasing overrelaxation parameters used for HOSP. The resulting OS-EM may be slightly more rapid than HOSP, due to the increasing number of projections used simultaneously.

Renal perfusion and mean vascular transit time

A method was developed to determine the mean vascular (arterio-venous) transit time (MVTT) of renal transplants. The renal transit times were calculated from perfusion curves obtained when grafts were examined with 99mTc-DTPA. About 400 examinations were performed during a 6-month period. As a result, influences of clinical complications on transit times were demonstrated. It was found that the individual MVTT is not a suitable parameter for assessment of renal grafts since individual transit times vary widely.

Effects of Distorted PET Projection Data on the Reconstructed Image Using Different Reconstruction Algorithms

In a simulation study, the effects of multiplicative and additive distortions of the projection data on the reconstructed image quality and quantitation capability were investigated. Three different reconstruction algorithms were compared: (i) filtered back-projection (FBP), (ii) an iterative method with multiplicative correction term (ML-EM), and (iii) an iterative method with additive correction term (AR-AC). Distortions were uniformly applied to all projections with projection angles ranging from 72° through 108°. Multiplicative distortion factors ranged from 0.5 to 1.4, additive distortions were −30% to +30%. The reconstructed images and activity concentrations were not affected by the multiplicative distortions if ML-EM was applied, whereas additive distortions were best tolerated when FBP or AR-AC were used for image reconstruction.

Estimating the effective renal plasma flow from the hippurate renogram.

A method for estimating the effective renal plasma flow (ERPF) from renograms is presented. Renograms obtained in 91 patients were used to test the method. The clearance values derived from renography were compared with those obtained with infusion clearance. It was shown that the ERPF was able to be estimated from the renogram with a standard error of ±94 ml min-1.

Prognostic Value of Post-Transplant Hippurate Scintigraphy

131Iodine hippurate sequential scintigrams obtained after transplantation contain prognostic information about the long-term development of a graft. Results of 131 131iodine hippurate sequential scintigrams made within 48 hours after graft implantation were compared to the graft status 2 years after transplantation. Hippurate uptake and transport were rated on a 4-step scale. Disturbed hippurate uptake and abnormal tracer transit were associated with increased frequency of early graft destruction and with a high frequency of chronic rejection. The chi-square test demonstrated a significant relationship between results of the initial hippurate scintigrams and long-term graft development.

Discriminant Analysis of Exercise Renograms

A quantitative analysis of renogram pattern observed in hypertensive patients during standing and exercise was performed. The discriminating power of different renogram variables was evaluated.