Dysfunctional but viable myocardium - ischemic heart disease assessed by magnetic resonance imaging and single photon emission computed tomography

The assessment of ischemic heart disease (IHD) often focuses on the detection of dysfunctional but viable myocardium which may improve in function following revascularization. Dysfunctional but viable myocardium is identified by distinct characteristics with regards to function, perfusion and viability. Therefore, in Paper I we developed a method for quantitative polar representation of left ventricular myocardial function, perfusion and viability using single photon emission computed tomography (SPECT) and cardiac magnetic resonance (CMR). Polar representation of these parameters was feasible, and the quantitative method agreed with visual assessment. Paper II showed that wall thickening decreases with increasing infarct transmurality. However, the variation in wall thickening was large, and importantly, influenced more so by the function of adjacent myocardium than by infarct transmurality. This underscores the difficulty of using resting function alone to accurately assess myocardial infarction in revascularized IHD. In Paper III we assessed the relationship between left ventricular ejection fraction (LVEF) and infarct size and found that LVEF cannot be used to estimate infarct size, and vice versa. However, the study showed that LVEF can be used to estimate a maximum predicted infarct size, and that infarct size can be used to estimate a maximum predicted LVEF. These results emphasize the importance of direct infarct imaging by CMR when attempting to estimate the size of infarction in patients with IHD. Paper IV was designed to assess the time course of recovery of myocardial perfusion and function after revascularization. The recovery of perfusion was found to occur in the first month, while the recovery of function was delayed in segments with non-transmural infarction. In summary, the presented studies emphasize the importance of direct infarct imaging by CMR for the accurate identification of infarction in the assessment of dysfunctional myocardium. Neither regional nor global myocardial function have a close correlation to infarction, but the presence of non-transmural infarction is a marker for delayed recovery of function following revascularization

Off-policy evaluation beyond overlap: partial identification through smoothness

Off-policy evaluation (OPE) is the problem of estimating the value of a target policy using historical data collected under a different logging policy. OPE methods typically assume overlap between the target and logging policy, enabling solutions based on importance weighting and/or imputation. In this work, we approach OPE without assuming either overlap or a well-specified model by considering a strategy based on partial identification under non-parametric assumptions on the conditional mean function, focusing especially on Lipschitz smoothness. Under such smoothness assumptions, we formulate a pair of linear programs whose optimal values upper and lower bound the contributions of the no-overlap region to the off-policy value. We show that these linear programs have a concise closed form solution that can be computed efficiently and that their solutions converge, under the Lipschitz assumption, to the sharp partial identification bounds on the off-policy value. Furthermore, we show that the rate of convergence is minimax optimal, up to log factors. We deploy our methods on two semi-synthetic examples, and obtain informative and valid bounds that are tighter than those possible without smoothness assumptions

Pulmonary intravascular blood volume changes through the cardiac cycle in healthy volunteers studied by cardiovascular magnetic resonance measurements of arterial and venous flow

BACKGROUND: This study aims to present a novel method for using cardiovascular magnetic resonance (CMR) to non-invasively quantify the variation in pulmonary blood volume throughout the cardiac cycle in humans. METHODS: 10 healthy volunteers (7 males, 3 female, age range 21-32 years) were studied. The blood flow in the pulmonary artery and all pulmonary veins was quantified during free breathing using phase contrast velocity encoded CMR. The difference in flow between the pulmonary artery and the pulmonary veins was integrated to calculate the change in pulmonary blood volume throughout the cardiac cycle. RESULTS: The stroke volumes in the pulmonary artery and the sum of the pulmonary veins were (mean +/- SEM) 103 +/- 6 ml and 95 +/- 6 ml, respectively. The pulmonary blood volume variation (PBVV) was 48 +/- 5 ml, and the PBVV expressed as percent of the pulmonary artery stroke volume was 46 +/- 3%. The maximum increase in pulmonary blood volume occurred 310 +/- 12 ms after the R-wave from the ECG (32 +/- 2% of the cardiac cycle). PBVV did not correlate to change in cross-sectional area in the pulmonary artery (R2 = 0.03, p = 0.66). CONCLUSION: It is feasible to non-invasively quantify the change in pulmonary blood volume during the cardiac cycle in humans using CMR. The average pulmonary blood volume variation in healthy volunteers was approximately 50 ml and this was approximately 50% of the stroke volume. Further studies are needed to assess the utility of the pulmonary blood volume variation as a measure for identifying cardiac and pulmonary vascular disease

Counterfactual Evaluation of Peer-Review Assignment Policies

Peer review assignment algorithms aim to match research papers to suitable expert reviewers, working to maximize the quality of the resulting reviews. A key challenge in designing effective assignment policies is evaluating how changes to the assignment algorithm map to changes in review quality. In this work, we leverage recently proposed policies that introduce randomness in peer-review assignment--in order to mitigate fraud--as a valuable opportunity to evaluate counterfactual assignment policies. Specifically, we exploit how such randomized assignments provide a positive probability of observing the reviews of many assignment policies of interest. To address challenges in applying standard off-policy evaluation methods, such as violations of positivity, we introduce novel methods for partial identification based on monotonicity and Lipschitz smoothness assumptions for the mapping between reviewer-paper covariates and outcomes. We apply our methods to peer-review data from two computer science venues: the TPDP'21 workshop (95 papers and 35 reviewers) and the AAAI'22 conference (8,450 papers and 3,145 reviewers). We consider estimates of (i) the effect on review quality when changing weights in the assignment algorithm, e.g., weighting reviewers' bids vs. textual similarity (between the review's past papers and the submission), and (ii) the "cost of randomization", capturing the difference in expected quality between the perturbed and unperturbed optimal match. We find that placing higher weight on text similarity results in higher review quality and that introducing randomization in the reviewer-paper assignment only marginally reduces the review quality. Our methods for partial identification may be of independent interest, while our off-policy approach can likely find use evaluating a broad class of algorithmic matching systems

Extracellular volume fraction mapping in the myocardium, part 2: initial clinical experience

BACKGROUND: Diffuse myocardial fibrosis, and to a lesser extent global myocardial edema, are important processes in heart disease which are difficult to assess or quantify with cardiovascular magnetic resonance (CMR) using conventional late gadolinium enhancement (LGE) or T1-mapping. Measurement of the myocardial extracellular volume fraction (ECV) circumvents factors that confound T1-weighted images or T1-maps. We hypothesized that quantitative assessment of myocardial ECV would be clinically useful for detecting both focal and diffuse myocardial abnormalities in a variety of common and uncommon heart diseases. METHODS: A total of 156 subjects were imaged including 62 with normal findings, 33 patients with chronic myocardial infarction (MI), 33 with hypertrophic cardiomyopathy (HCM), 15 with non-ischemic dilated cardiomyopathy (DCM), 7 with acute myocarditis, 4 with cardiac amyloidosis, and 2 with systemic capillary leak syndrome (SCLS). Motion corrected ECV maps were generated automatically from T1-maps acquired pre- and post-contrast calibrated by blood hematocrit. Abnormally-elevated ECV was defined as >2SD from the mean ECV in individuals with normal findings. In HCM the size of regions of LGE was quantified as the region >2 SD from remote. RESULTS: Mean ECV of 62 normal individuals was 25.4 ± 2.5% (m ± SD), normal range 20.4%-30.4%. Mean ECV within the core of chronic myocardial infarctions (without MVO) (N = 33) measured 68.5 ± 8.6% (p < 0.001 vs normal). In HCM, the extent of abnormally elevated ECV correlated to the extent of LGE (r = 0.72, p < 0.001) but had a systematically greater extent by ECV (mean difference 19 ± 7% of slice). Abnormally elevated ECV was identified in 4 of 16 patients with non-ischemic DCM (38.1 ± 1.9% (p < 0.001 vs normal) and LGE in the same slice appeared “normal” in 2 of these 4 patients. Mean ECV values in other disease entities ranged 32-60% for cardiac amyloidosis (N = 4), 40-41% for systemic capillary leak syndrome (N = 2), and 39-56% within abnormal regions affected by myocarditis (N = 7). CONCLUSIONS: ECV mapping appears promising to complement LGE imaging in cases of more homogenously diffuse disease. The ability to display ECV maps in units that are physiologically intuitive and may be interpreted on an absolute scale offers the potential for detection of diffuse disease and measurement of the extent and severity of abnormal regions