Pathophysiological responses after acute myocardial infarction include edema, hemorrhage, and microvascular obstruction along with cellular damage. The in vivo evolution of these processes simultaneously throughout infarct healing has not been well characterized. The purpose of our study was to quantitatively monitor the time course of these mechanisms by MRI in a porcine model of myocardial infarction. Ten pigs underwent MRI before coronary occlusion with subgroups studied at day 2 and weeks 1, 2, 4, and 6 post‐infarction. Tissue characterization was performed using quantitative T2 and T2* maps to identify edema and hemorrhage, respectively. Contrast‐enhanced MRI was used for infarct/ microvascular obstruction delineation. Inflammation was reflected by T2 fluctuations, however at day 2, edema and hemorrhage had counter‐acting effects on T2. Hemorrhage (all forms) and mineralization (calcium) could be identified by T2* in the presence of edema. Simultaneous resolution of microvascular obstruction and T2* abnormality suggested that the two phenomenon were closely associated during the healing process. Our study demonstrates that quantitative T2 and T2* mapping techniques allow regional, longitudinal, and cross‐subject comparisons and give insights into histological and tissue remodeling processes. Such in vivo characterization will be important in grading severity and evaluating treatment strategies for myocardial infarction, potentially improving clinical outcomes. Magn Reson Med, 2011.

We conducted a systematic review of recent literature to understand the current challenges in the use of optical see-through head-mounted displays (OST-HMDs) for augmented reality (AR) assisted surgery. Using Google Scholar, 57 relevant articles from 1 January 2021 through 18 March 2022 were identified. Selected articles were then categorized based on a taxonomy that described the required components of an effective AR-based navigation system: data, processing, overlay, view, and validation. Our findings indicated a focus on orthopedic ($n=20$) and maxillofacial surgeries ($n=8$). For preoperative input data, computed tomography (CT) ($n=34$), and surface rendered models ($n=39$) were most commonly used to represent image information. Virtual content was commonly directly superimposed with the target site ($n=47$); this was achieved by surface tracking of fiducials ($n=30$), external tracking ($n=16$), or manual placement ($n=11$). Microsoft HoloLens devices ($n=24$ in 2021, $n=7$ in 2022) were the most frequently used OST-HMDs; gestures and/or voice ($n=32$) served as the preferred interaction paradigm. Though promising system accuracy in the order of 2–5 mm has been demonstrated in phantom models, several human factors and technical challenges—perception, ease of use, context, interaction, and occlusion—remain to be addressed prior to widespread adoption of OST-HMD led surgical navigation.

The type and extent of myocardial infarction encountered clinically is primarily determined by the severity of the initial ischemic insult. The purpose of the study was to differentiate longitudinal fluctuations in remodeling mechanisms in porcine myocardium following different ischemic insult durations. Animals (N = 8) were subjected to coronary balloon occlusion for either 90 or 45 min, followed by reperfusion. Imaging was performed on a 3 T MRI scanner between day‐2 and week‐6 postinfarction with edema quantified by T2, hemorrhage by T2*, vasodilatory function by blood‐oxygenation‐level‐dependent T2 alterations and infarction/microvascular obstruction by contrast‐enhanced imaging. The 90‐min model produced large transmural infarcts with hemorrhage and microvascular obstruction, while the 45 min produced small nontransmural and nonhemorrhagic infarction. In the 90‐min group, elevation of end‐diastolic‐volume, reduced cardiac function, persistence of edema, and prolonged vasodilatory dysfunction were all indicative of adverse remodeling; in contrast, the 45‐min group showed no signs of adverse remodeling. The 45‐ and 90‐min porcine models seem to be ideal for representing the low‐ and high‐risk patient groups, respectively, commonly encountered in the clinic. Such in vivo characterization will be a key in predicting functional recovery and may potentially allow evaluation of novel therapies targeted to alleviate ischemic injury and prevent microvascular obstruction/hemorrhage. Magn Reson Med, 70:1095–1105, 2013.

Myocardial hemorrhage is a frequent complication following reperfusion in acute myocardial infarction and is predictive of adverse outcomes. However, it remains unsettled whether hemorrhage is simply a marker of a severe initial ischemic insult or directly contributes to downstream myocardial damage. Our objective was to evaluate the contribution of hemorrhage towards inflammation, microvascular obstruction and infarct size in a novel porcine model of hemorrhagic myocardial infarction using cardiovascular magnetic resonance (CMR). Myocardial hemorrhage was induced via direct intracoronary injection of collagenase in a novel porcine model of ischemic injury. Animals (N = 27) were subjected to coronary balloon occlusion followed by reperfusion and divided into three groups (N = 9/group): 8 min ischemia with collagenase (+HEM); 45 min infarction with saline (I-HEM); and 45 min infarction with collagenase (I+HEM). Comprehensive CMR was performed on a 3 T scanner at baseline and 24 h post-intervention. Cardiac function was quantified by cine imaging, edema/inflammation by T2 mapping, hemorrhage by T2* mapping and infarct/microvascular obstruction size by gadolinium enhancement. Animals were subsequently sacrificed and explanted hearts underwent histopathological assessment for ischemic damage and inflammation. At 24 h, the +HEM group induced only hemorrhage, the I-HEM group resulted in a non-hemorrhagic infarction, and the I+HEM group resulted in infarction and hemorrhage. Notably, the I+HEM group demonstrated greater hemorrhage and edema, larger infarct size and higher incidence of microvascular obstruction. Interestingly, hemorrhage alone (+HEM) also resulted in an observable inflammatory response, similar to that arising from a mild ischemic insult (I-HEM). CMR findings were in good agreement with histological staining patterns. Hemorrhage is not simply a bystander, but an active modulator of tissue response, including inflammation and microvascular and myocardial damage beyond the initial ischemic insult. A mechanistic understanding of the pathophysiology of reperfusion hemorrhage will potentially aid better management of high-risk patients who are prone to adverse long-term outcomes.

By aligning virtual augmentations with real objects, optical see-through head-mounted display (OST-HMD)-based augmented reality (AR) can enhance user-task performance. Our goal was to compare the perceptual accuracy of several visualization paradigms involving an adjacent monitor, or the Microsoft HoloLens 2 OST-HMD, in a targeted task, as well as to assess the feasibility of displaying imaging-derived virtual models aligned with the injured porcine heart. With 10 participants, we performed a user study to quantify and compare the accuracy, speed, and subjective workload of each paradigm in the completion of a point-and-trace task that simulated surgical targeting. To demonstrate the clinical potential of our system, we assessed its use for the visualization of magnetic resonance imaging (MRI)-based anatomical models, aligned with the surgically exposed heart in a motion-arrested open-chest porcine model. Using the HoloLens 2 with alignment of the ground truth target and our display calibration method, users were able to achieve submillimeter accuracy (0.98 mm) and required 1.42 min for calibration in the point-and-trace task. In the porcine study, we observed good spatial agreement between the MRI-models and target surgical site. The use of an OST-HMD led to improved perceptual accuracy and task-completion times in a simulated targeting task.