Steven B. Gillispie

Medical versus Early Surgical Therapy in Patients with Triple-Vessel Disease and Mild Angina Pectoris: A CASS Registry Study of Survival

Results of coronary artery bypass grafting were evaluated in 856 nonrandomized patients in the Coronary Artery Surgery Study (CASS) registry with mild angina (Canadian Cardiovascular Society Classes I and II) and three-vessel disease, defined as 70% or more stenosis in the proximal or middle segment of the three major coronary arteries. There were 413 patients with medical therapy and 443 with early operation. Patients with delayed operation were kept in the medical group for analysis. Six-year survival adjusted for left ventricular (LV) function and number of proximal stenoses was 67% for medical and 84% for surgical patients (p less than 0.0001). Patients with normal LV function had equal survival with medicine or surgical intervention. Those with mild or moderate LV dysfunction (CASS LV wall motion score 6 to 9 and 10 to 15, respectively) and at least one proximal stenosis (the dominant right coronary artery) had increased probability of being alive at six years with surgical treatment. In patients with severe LV impairment (LV score higher than 15) and in those whose only proximal stenosis of 70% or more (in three-vessel disease) was located in the left anterior descending coronary artery, increased survival with surgical treatment could not be demonstrated. This is a nonrandomized observational study with the limitations of such studies: the need to adjust for differences in baseline traits between medical and surgical groups and the possibility of an unrecognized imbalance in baseline characteristics. In a Cox analysis of variables influencing outcome, early surgical treatment was an independent predictor of survival with 43% the risk of medical treatment (95% confidence range: 29 to 62%). Adjustment by propensity analysis to reduce selection bias from known differences in baseline variables did not alter results.

Improving the efficiency of emission tomography simulations using variance reduction techniques

The development for public distribution of a general-purpose code, SIMSET, for the modeling of positron-emission and single-photon-emission tomographs (PET and SPECT) is discussed. An important part of the SIMSET project is the development of methods for efficient photon simulation which can handle heterogeneous distributions of activity and attenuation. For both positron and single-photon tomographs, the solid angle of acceptance of the detector array is small, due to collimation and to the size of the detector array itself. This results in significant computational inefficiencies with conventional Monte Carlo simulation, because only a few percent of the photons generated and tracked will actually be detected. A similar kind of problem arises in radiation shielding calculations, where the flux through the shield, although significant, may correspond to only a tiny fraction of the initial emitted photons. To improve the efficiency problem of simulations, some techniques from the radiation shielding literature have been adapted to increase the yield of detected photons. >

The size distribution for Markov equivalence classes of acyclic digraph models

Bayesian networks, equivalently graphical Markov models determined by acyclic digraphs or ADGs (also called directed acyclic graphs or dags), have proved to be both effective and efficient for representing complex multivariate dependence structures in terms of local relations. However, model search and selection is potentially complicated by the many-to-one correspondence between ADGs and the statistical models that they represent. If the ADGs/models ratio is large, search procedures based on unique graphical representations of equivalence classes of ADGs could provide substantial computational efficiency. Hitherto, the value of the ADGs/models ratio has been calculated only for graphs with n = 5 or fewer vertices. In the present study, a computer program was written to enumerate the equivalence classes of ADG models and study the distributions of class sizes and number of edges for graphs up to n = 10 vertices. The ratio of ADGs to numbers of classes appears to approach an asymptote of about 3.7. Distributions of the classes according to number of edges and class size were produced which also appear to be approaching asymptotic limits. Imposing a bound on the maximum number of parents to any vertex causes little change if the bound is sufficiently large, with four being a possible minimum. The program also includes a new variation of orderly algorithm for generating undirected graphs.

Time to first new myocardial infarction in patients with mild angina and three-vessel disease comparing medicine and early surgery: a CASS registry study of survival

Two categories--patients alive and free from new myocardial infarction (MI) and time to first new MI (nonfatal and fatal)--were compared in medical and early surgical groups in the Coronary Artery Surgery Study (CASS) registry with Class I or II angina and three-vessel disease in a six-year follow-up. There were 413 in the medical group and 443 in the early surgical group. A broad definition of MI using ECG and clinical criteria on hospital discharge and follow-up was used to include as many new MIs as possible, including perioperative MIs. Stratification was by left ventricular wall motion score and number of proximal segment stenoses and by quintile of propensity score to reduce selection bias in therapy groups. Adjusted by propensity analysis, 79% of medical and 88% of surgical patients (p = .005) were free from new MI; death without diagnosis of new MI was censored. Similarly adjusted, 57% of medical and 76% of surgical patients (p less than .0001) were alive and free from new MI at six years. For patients with previous MI, surgery offered the probability of protection from new MI: with multiple prior MIs, 66% of medical and 88% of surgical patients were free from new MI at six years (p = .0019). This is a nonrandomized, observational study with the limitations of such studies: the need to adjust for differences in baseline traits in medical and surgical groups and the unknown effects of unobserved variables. Fifty-one variables, including therapy, were tested by Cox model with time to new MI as the end point. Early surgery was the strongest independent predictor of freedom from new MI (p = .002) with a relative risk of 51% compared with medical therapy (95% confidence limits of 33 to 78%). In patients with multiple prior MIs, the new MI risk with early surgery was 24% of that for medicine, with an upper 95% confidence point of 64%.

Monte Carlo simulation of BaF/sub 2/ detectors used in time-of-flight positron emission tomography

To better understand the performance of the SP3000 time-of-flight PET (positron emission tomography), machine, the authors performed Monte Carlo calculations of photon interactions in a BaF/sub 2/ detector module and calculations of crystal optics for BaF/sub 2/ UV scintillation light. Evaluated were photon detection sensitivities, the amount of photon spillover, crystal crosstalk, interring miscodes, and true-to-random coincidence ratios as a function of the lower-level energy discriminator setting and crystal/photomultiplier tube energy resolution. UV light collection efficiencies for the timing tube and decode tube were calculated assuming different crystal geometries, reflective materials, and reflectances. For a typical SP3000 detector module the detection efficiency for single 511-keV photons varied between 20 and 60% depending on the angle of incidence. >

The effects of object size, attenuation, scatter, and random coincidences on signal to noise ratio in simulations of time-of-flight positron emission tomography

Time-of-flight (TOF) positron emission tomography (PET) can improve signal-to-noise ratio (SNR) relative to conventional PET by improving coincidence localization and reducing random coincidences. Previous work showed that good TOF resolution can dramatically improve SNR in the absence of attenuation. This paper examines the effect of attenuation, scatter and randoms coincidences on SNR in TOF. The SimSET program was modified to generate random events and used to simulate multiple realizations of cylindrical phantoms, adding attenuation, scatter and randoms one-by-one. Improving TOF resolution generally improved SNR, but the improvement over conventional tomography was not well predicted by a widely used model.

Design and Implementation of a Block Detector Simulation in SimSET

SimSET (a Simulation System for Emission Tomography) is being modified to more accurately simulate typical PET block detector tomographs. A new detector module that models the detector system as a collection of rectangular boxes is being added to the software. The new model is sufficiently general to allow many more imaginative or speculative detector systems to be simulated.

Approximating the Conway–Maxwell–Poisson distribution normalization constant

By adding a second parameter, Conway and Maxwell created a new distribution for situations where data deviate from the standard Poisson distribution. This new distribution contains a normalization constant expressed as an infinite sum whose summation has no known closed-form expression. Shmueli et al. produced an approximation for this sum but proved that it was valid only for integer values of the second parameter, although they conjectured it was also valid for non-integers. Here we prove their conjecture to be true and discuss for what range of parameters the approximation can be accurately applied.

An apple II-based slide-projector laboratory

We describe the software and hardware of a laboratory running under UCSD Pascal on an Apple II computer. The system includes: two random-access projectors and two standard projectors, all equipped with tachistoscopic shutters; two filter wheels; a voice key; a tone generator; and eight response boxes. The laboratory can be used for any experiment in which visual stimuli are to be presented and in which precise display times, reaction times, and luminances are required. It is particularly well suited to picture-perception and picture-memory experiments.

Parametric design study of a long axial field-of-view PET scanner using a block-detector tomograph simulation of a cylindrical phantom

We are conducting a two-phase study, which aims to select design parameters of a long-axis positron emission tomography (PET) scanner that are a compromise between detection performance and cost. In this first phase, we examine the effects of axial length, detector thickness and collimator geometry on the noise equivalent count rate per axial length (λNEC) and noise equivalent count rate per slice (dNEC). We use these metrics as approximate, but quickly computed, indicators of a PET-scanners performance at a detection task. From this first phase, we select a subset of scanner designs for which we can conduct a detailed study of tumor detectability and quantitation accuracy in whole-body PET imaging.