Validation of minimally invasive measurement of myocardial perfusion using electron beam computed tomography and application in human volunteers

[Possibilities of Stress Computed Tomography Myocardial Perfusion Imaging in the Diagnosis of Ischemic Heart Disease]

Computed tomography angiography (CT-angiography, CTA) allows noninvasive visualization of coronary arteries (CA). This method is highly sensitive in detecting coronary atherosclerosis. However, standard CTA does not allow evaluation of the hemodynamic significance of found CA stenoses, which requires additional functional tests for detection of myocardial ischemia. This review focuses on possibilities of clinical use, limitations, technical aspects, and prospects of a combination of CT-angiography and CT myocardial perfusion imaging in diagnostics of ischemic heart disease.

Functional Information in Coronary Artery Disease: The Case of Computed Tomography Myocardial Perfusion

PURPOSE OF REVIEW

To review methodological and logistical aspects of CT myocardial perfusion, current clinical evidence and possible future directions, with specific focus on use in patients with coronary artery disease (CAD).

RECENT FINDINGS

CT myocardial perfusion imaging may be performed as an add-on to standard coronary CT angiography (CCTA), to identify regions of myocardial hypoperfusion, at rest and during adenosine stress. The principle of measurement is well-validated in animal experimental models, and CT myocardial perfusion imaging has a high degree of concordance with already clinically available perfusion imaging methods. Combining CCTA and CT myocardial perfusion imaging increases the diagnostic accuracy to identify patients with CAD associated with ischemia. In patients suspected of CAD, CCTA frequently detects coronary atherosclerotic lesions, in which revascularization could be clinically beneficial. CT myocardial perfusion imaging may be helpful to identify coronary lesions associated with myocardial ischemia, and thus potentially suitable for coronary intervention.

Infarct characterization using CT

Myocardial infarction (MI) is a major cause of death and disability worldwide. The incidence is not expected to diminish, despite better prevention, diagnosis and treatment, because of the ageing population in industrialized countries and unhealthy lifestyles in developing countries. Nowadays it is highly requested an imaging tool able to evaluate MI and viability. Technology improvements determined an expansion of clinical indications from coronary plaque evaluation to functional applications (perfusion, ischemia and viability after MI) integrating additional phases and information in the mainstream examination. Cardiac computed tomography (CCT) and cardiac MR (CMR) employ different contrast media, but may characterize MI with overlapping imaging findings due to the similar kinetics and tissue distribution of gadolinium and iodinated contrast media. CCT may detect first-pass perfusion defects, dynamic perfusion after pharmacological stress, and delayed enhancement (DE) of non-viable territories.

Myocardial blood flow quantification for evaluation of coronary artery disease by positron emission tomography, cardiac magnetic resonance imaging, and computed tomography

The noninvasive detection of the presence and functional significance of coronary artery stenosis is important in the diagnosis, risk assessment, and management of patients with known or suspected coronary artery disease. Quantitative assessment of myocardial perfusion can provide an objective and reproducible estimate of myocardial ischemia and risk prediction. Positron emission tomography, cardiac magnetic resonance, and cardiac computed tomography perfusion are modalities capable of measuring myocardial blood flow and coronary flow reserve. In this review, we will discuss the technical aspects of quantitative myocardial perfusion imaging with positron emission tomography, cardiac magnetic resonance imaging, and computed tomography, and its emerging clinical applications.

Dual-energy CT of the heart

OBJECTIVE

Interest in dual-energy CT (DECT) for evaluating the myocardial blood supply, as an addition to coronary artery assessment, is increasing. Although it is still in the early clinical phase, assessment of myocardial ischemia and infarction by DECT constitutes a promising step toward comprehensive evaluation of coronary artery disease with a single noninvasive modality.

CONCLUSION

Compared with dynamic CT approaches, DECT has advantages regarding radiation dose and clinical applicability. In this review, the literature on DECT of the heart is discussed.

CT of coronary heart disease: Part 1, CT of myocardial infarction, ischemia, and viability

OBJECTIVE

This article reviews the CT-based approaches aimed at the assessment of myocardial infarction, ischemia, and viability described in the recent literature.

CONCLUSION

Rapid advances in CT technology not only have improved visualization of coronary arteries but also increasingly enable noncoronary myocardial applications, including analysis of wall motion and the state of the myocardial blood supply. These advancements hold promise for eventually accomplishing the goal of comprehensively evaluating coronary heart disease with a single noninvasive modality.

Assessment of cardiac computed tomography-myocardial perfusion imaging - promise and challenges -

Cardiac computed tomography (CT) has evolved rapidly over the last decade into a reliable imaging modality for the non-invasive assessment of coronary artery disease. With the advancement in multi-detector CT technology, there has developed an increasing body of evidence that suggests that the role of cardiac CT can be extended to include functional assessment of the myocardium not only at rest but also during stress. Simultaneous anatomical and functional assessment approaches will have a number of advantages such as evaluation of the transmural extent of myocardial perfusion defects (including small subendocardial perfusion defects), reduced risk associated with multiple sources of radiation, and short image acquisition time. Although initial results hold some promise, CT myocardial perfusion imaging is a modality in the early stages of development and further work and studies are required to define, validate, and optimize this technique. This review will provide an overview of this novel perfusion imaging method, its underlying principles, evolution, limitations and future directions.

Comparison of four mathematical models to analyze indicator-dilution curves in the coronary circulation

While several models have proven to result in accurate estimations when measuring cardiac output using indicator dilution, the mono-exponential model has primarily been chosen for deriving coronary blood/plasma volume. In this study, we compared four models to derive coronary plasma volume using indicator dilution; the mono-exponential, power-law, gamma-variate, and local density random walk (LDRW) model. In anesthetized goats (N = 14), we determined the distribution volume of high molecular weight (2,000 kDa) dextrans. A bolus injection (1.0 ml, 0.65 mg/ml) was given intracoronary and coronary venous blood samples were taken every 0.5–1.0 s; outflow curves were analyzed using the four aforementioned models. Measurements were done at baseline and during adenosine infusion. Absolute coronary plasma volume estimates varied by ~25% between models, while the relative volume increase during adenosine infusion was similar for all models. The gamma-variate, LDRW, and mono-exponential model resulted in volumes corresponding with literature, whereas the power-model seemed to overestimate the coronary plasma volume. The gamma-variate and LDRW model appear to be suitable alternative models to the mono-exponential model to analyze coronary indicator-dilution curves, particularly since these models are minimally influenced by outliers and do not depend on data of the descending slope of the curve only.

Regulation of the human coronary microcirculation

Atherosclerosis of conduit epicardial arteries is the principal culprit behind the complications of coronary heart disease, but a growing body of literature indicates that the coronary microcirculation also contributes substantially to the pathophysiology of cardiovascular disease. An understanding of mechanisms regulating microvascular function in humans is an essential foundation for understanding the role in disease, especially since these regulatory mechanisms vary substantially across species and vascular beds. In fact all subjects whose coronary tissue was used in the studies described have medical conditions that warrant cardiac surgery, thus relevance to the normal human must be inferential and is based on tissue from subjects without known arteriosclerotic disease. This review will focus on recent advances in the physiological and pathological mechanisms of coronary microcirculatory control, describing a robust plasticity in maintaining endothelial control over dilation, including mechanisms that are most relevant to the human heart. This article is part of a Special Issue entitled "Coronary Blood Flow".

Relationship between surface area of nonperfused myocardium and extravascular extraction of contrast agent following coronary microembolization

Myocardial microvascular permeability and coronary sinus concentration of muscle metabolites have been shown to increase after myocardial ischemia due to epicardial coronary artery occlusion and reperfusion. However, their association with coronary microembolization is not well defined. This study tested the hypothesis that acute coronary microembolization increases microvascular permeability in the porcine heart. The left anterior descending perfusion territories of 34 anesthetized pigs (32 ± 3 kg) were embolized with equal volumes of microspheres of one of three diameters (10, 30, or 100 μm) and at three different doses for each size. Electron beam computed tomography (EBCT) was used to assess in vivo, microvascular extraction of a nonionic contrast agent (an index of microvascular permeability) before and after microembolization with microspheres at baseline and during adenosine infusion. A high-resolution three-dimensional microcomputed tomography (micro-CT) scanner was subsequently used to obtain ex vivo, the volume and corresponding surface area of the embolized myocardial islands within the perfusion territories of the microembolized coronary artery. EBCT-derived microvascular extraction of contrast agent increased within minutes after coronary microembolization (P < 0.001 vs. baseline and vs. control values). The increase in coronary microvascular permeability was highly correlated to the micro-CT-derived total surface area of the nonperfused myocardium (r = 0.83, P < 0.001). In conclusion, myocardial extravascular accumulation of contrast agent is markedly increased after coronary microembolization and its magnitude is in proportion to the surface area of the interface between the nonperfused and perfused territories.

Quantitative and qualitative analysis and interpretation of CT perfusion imaging

Coronary artery disease (CAD) remains the leading cause of death in the United States. Rest and stress myocardial perfusion imaging has an important role in the non-invasive risk stratification of patients with CAD. However, diagnostic accuracies have been limited, which has led to the development of several myocardial perfusion imaging techniques. Among them, myocardial computed tomography perfusion imaging (CTP) is especially interesting as it has the unique capability of providing anatomic- as well as coronary stenosis-related functional data when combined with computed tomography angiography (CTA). The primary aim of this article is to review the qualitative, semi-quantitative, and quantitative analysis approaches to CTP imaging. In doing so, we will describe the image data required for each analysis and discuss the advantages and disadvantages of each approach.

MRI and CT in the diagnosis of coronary artery disease: indications and applications

In recent years, technical advances and improvements in cardiac computed tomography (CT) and cardiac magnetic resonance imaging (MRI) have provoked increasing interest in the potential clinical role of these techniques in the non-invasive work-up of patients with suspected coronary artery disease (CAD) and correct patient selection for these emerging imaging techniques. In the primary detection or exclusion of significant CAD, e.g. in the patient with unspecific thoracic complaints, and also in patients with known CAD or advanced stages of CAD, both CT and MRI yield specific advantages. In this review, the major aspects of non-invasive MR and CT imaging in the diagnosis of CAD will be discussed. The first part describes the clinical value of contrast-enhanced non-invasive CT coronary angiography (CTCA), including the diagnostic accuracy of CTCA for the exclusion or detection of significant CAD with coronary artery stenoses that may require angioplastic intervention, as well as potentially valuable information on the coronary artery vessel wall. In the second section, the potential of CT for the imaging of myocardial viability and perfusion will be highlighted. In the third and final part, the range of applications of cardiac MRI in CAD patients will be outlined.

Comparison of dual-energy computed tomography of the heart with single photon emission computed tomography for assessment of coronary artery stenosis and of the myocardial blood supply

To evaluate the performance of dual-energy computed tomography (CT) for integrative imaging of the coronary artery morphology and the myocardial blood supply, 36 patients (15 women, mean age 57 +/- 11 years) with equivocal or incongruous single photon emission CT (SPECT) results were investigated by a single-contrast medium-enhanced, retrospectively electrocardiographic-gated dual-energy CT (DECT) scan with simultaneous acquisition of high and low x-ray spectra. Thirteen patients subsequently underwent invasive coronary angiography (ICA). The DECT data were used to reconstruct anatomic coronary CT angiographic images and to map the myocardial iodine distribution within the left ventricular myocardium. Two independent observers analyzed all DECT studies for stenosis and myocardial iodine defects. A segmental comparison was performed between the stress/rest SPECT perfusion defects and DECT iodine defects and between the ICA and coronary CT angiographic findings for stenosis. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were estimated, along with the kappa statistics. Overall, DECT had 92% sensitivity and 93% specificity, with 93% accuracy for detecting any type of myocardial perfusion defect seen on SPECT. Contrast defects at DECT correctly identified 85 (96%) of 89 fixed and 60 (88%) of 68 reversible myocardial perfusion defects. The interobserver agreement was very good (weighted kappa = 0.87). Compared with ICA, coronary CT angiography had 90% sensitivity, 94% specificity, and 93% accuracy for the detection of >50% stenosis. In conclusion, our initial experience suggests that DECT, as a single examination, might be promising for the integrative analysis of the coronary artery morphology and the myocardial blood supply and is in good agreement with ICA and SPECT.

A technical solution to avoid partial scan artifacts in cardiac MDCT

Quantitative evaluation of cardiac image data obtained using multidetector row computed tomography (CT) is compromised by partial scan reconstructions, which improve the temporal resolution but significantly increase image-to-image CT number variations for a fixed region of interest compared to full reconstruction images. The feasibility of a new approach to solve this problem is assessed. An anthropomorphic cardiac phantom and an anesthetized pig were scanned on a dual-source CT scanner using both full and partial scan acquisition modes under different conditions. Additional scans were conducted with the electrocardiogram (ECG) signal being in synchrony with the gantry rotation. In the animal study, a simple x-ray detector was used to generate a signal once per gantry rotation. This signal was then used to pace the pig's heart. Phantom studies demonstrated that partial scan artifacts are strongly dependent on the rotational symmetry of angular projections, which is determined by the object shape and composition and its position with respect to the isocenter. The degree of partial scan artifacts also depends on the location of the region of interest with respect to highly attenuating materials (bones, iodine, etc.) within the object. Single-source partial scan images (165 ms temporal resolution) were significantly less affected by partial scan artifacts compared to dual-source partial scan images (82 ms temporal resolution). When the ECG signal was in synchrony with the gantry rotation, the same cardiac phase always corresponded to the same positions of the x-ray tube(s) and, hence, the same scattering and beam hardening geometry. As a result, the range of image-to-image CT number variations for partial scan reconstruction images acquired in synchronized mode was decreased to that achieved using full reconstruction image data. The success of the new approach, which synchronizes the ECG signal with the position of the x-ray tube(s), was demonstrated both in the phantom and animal experiments.

CT coronary angiography: quantitative assessment of myocardial perfusion using test bolus data-initial experience

The aim of this study is to quantify myocardial perfusion during coronary CT angiography using data from a modified timing test-bolus acquisition. Institutional review board approval and informed consent were obtained. Nineteen patients with suspected coronary artery disease underwent combined coronary CT angiography and cardiac (82)Rubidium-PET perfusion. Prior to the CT angiogram a retrospectively ECG-gated dynamic test bolus was obtained following 25 mls of IV contrast medium injected at 5 ml/s. Images were acquired every 1.5 s for 30 s using 4 x 1.25-mm slices at 120 kV, 35 mAs. Regions of interest were drawn to delineate the myocardium and aorta on the resulting transaxial images. Time density curves were created and perfusion calculated using two simple approaches: maximum-slope method and peak method. In patients with normal PET myocardial perfusion, the mean (SD) resting myocardial perfusion estimated by CT using the maximum-slope method was 0.89 (+/-0.27) ml/min/g and 0.93 (+/-0.21) ml/min/g at end-systole and end-diastole, respectively, and 0.69 (+/-0.11) ml/min/g and 0.79 (+/-0.19) at end-systole and end-diastole, respectively, for the peak method. Thus quantification of myocardial perfusion from a routine coronary CT angiography test bolus is possible. CT-derived myocardial perfusion values are consistent with published values derived from other techniques.

Quantification of myocardial perfusion using dynamic 64-detector computed tomography

OBJECTIVES

The purpose of this study was to determine the ability of dynamic 64 slice multidetector computed tomography (d-MDCT) to provide an accurate measurement of myocardial blood flow (MBF) during first-pass d-MDCT using semiquantitative and quantitative analysis methods.

MATERIALS AND METHODS

Six dogs with a moderate to severe left-anterior descending artery stenosis underwent adenosine (0.14 mL . kg-1 . min-1) stress d-MDCT imaging according to the following imaging protocol: iopamidol 10 mL/s for 3 seconds, 8 mm x 4 collimation, 400 milliseconds gantry rotation time, 120 kV, and 60 mAs. Images were reconstructed at 1-second intervals. Regions of interest were drawn in the LAD and remote territories, and time-attenuation curves were constructed. Myocardial perfusion was analyzed using a model-based deconvolution method and 2 upslope methods and compared with the microsphere MBF measurements.

RESULTS

The myocardial upslope-to-LV-upslope and myocardial upslope-to-LV-max ratio strongly correlated with MBF (R2 = 0.92, P < 0.0001 and R2 = 0.87, P < 0.0001, respectively). Absolute MBF derived by model-based deconvolution analysis modestly overestimated MBF compared with microsphere MBF (3.0 +/- 2.5 mL . g-1 . min-1 vs. 2.6 +/- 2.7 mL . g-1 . min-1, respectively). Overall, MDCT-derived MBF strongly correlated with microspheres (R = 0.91, P < 0.0001, mean difference: 0.45 mL . g-1 . min-1, P = NS).

CONCLUSIONS

d-MDCT MBF measurements using upslope and model-based deconvolution methods correlate well with microsphere MBF. These methods may become clinically applicable in conjunction with coronary angiography and next generation MDCT scanners with larger detector arrays and full cardiac coverage.

Evaluation of porcine myocardial microvascular permeability and fractional vascular volume using 64-slice helical computed tomography (CT)

OBJECTIVES

Myocardial microvascular permeability-surface area product (MPSP) and fractional vascular volume (FVV), indices of endothelial function and microvascular perfusion, can be noninvasively evaluated by electron beam computed tomography (EBCT), but it remains unknown whether comparable assessments can be obtained with 64-slice multidetector CT (CT-64).

METHODS

We studied 12 pigs with both EBCT and CT-64 in randomized order 1 week apart, before and during IV adenosine infusion. Myocardial attenuation changes in the cardiac wall were assessed after a central-venous injection of iopamidol. Time-attenuation curves were analyzed using both indicator-dilution and Patlak models to calculate MPSP and FVV.

RESULTS

CT-64 and EBCT assessments of basal MPSP obtained by the Patlak method were similar (0.37 +/- 0.03 vs. 0.37 +/- 0.04 mL/min/g), as was its response to adenosine, and correlated significantly (r = 0.87). Patlak FVV was also similar between CT-64 and EBCT at baseline (0.08 +/- 0.02 vs. 0.07 +/- 0.02 mL blood/mL) and during adenosine, and correlated well (r = 0.93). MPSP and FVV estimated by the indicator-dilution method were not significantly correlated.

CONCLUSIONS

CT-64 assessments of myocardial MPSP and FVV may not be reliable when using indicator-dilution analysis, likely due to its sensitivity to scan duration. However, CT-64 assessments obtained using the Patlak model are feasible.

Assessment of Myocardial Microvascular Function: New Opportunities in Fast Computed Tomography

It has been increasingly recognized that the initial site of cardiac damage in several forms of cardiovascular disease resides in the microcirculation. Noninvasive or minimally invasive evaluation of myocardial microvascular functional attributes, such as myocardial perfusion or microvascular permeability, could be an invaluable tool in the clinical practice. Advances in the field of computed tomography over the past three decades culminated in the advent of fast scanners, which show promise to provide both fine cardiac anatomic detail and quantification of the function of the myocardial microcirculation. This review describes the approach and utility of measurements of myocardial microvascular function obtained with state-of-the-art cardiac computed tomography.

Multidetector computed tomography myocardial perfusion imaging during adenosine stress

OBJECTIVES

The purpose of this study is to validate the accuracy of multidetector computed tomography (MDCT) to measure differences in regional myocardial perfusion during adenosine stress in a canine model of left anterior descending (LAD) artery stenosis, during first-pass, contrast-enhanced helical MDCT.

BACKGROUND

Myocardial perfusion imaging by MDCT may have significant implications in the diagnosis and treatment of coronary artery disease.

METHODS

Eight dogs were prepared with a LAD stenosis, and contrast-enhanced MDCT imaging was performed 5 min into adenosine infusion (0.14 to 0.21 mg/kg/min). Images were analyzed using a semiautomated approach to define the regional signal density (SD) ratio (myocardial SD/left ventricular blood pool SD) in stenosed and remote territories, and then compared with microsphere myocardial blood flow (MBF) measurements.

RESULTS

Mean MBF in stenosed versus remote territories was 1.37 +/- 0.46 ml/g/min and 1.29 +/- 0.48 ml/g/min at baseline (p = NS) and 2.54 +/- 0.93 ml/g/min and 8.94 +/- 5.74 ml/g/min during adenosine infusion, respectively (p < 0.05). Myocardial SD was 92.3 +/- 39.5 HU in stenosed versus 180.4 +/- 41.9 HU in remote territories (p < 0.001). There was a significant linear association of the SD ratio with MBF in the stenosed territory (R = 0.98, p = 0.001) and between regional myocardial SD ratio and MBF <8 ml/g/min, slope = 0.035, SE = 0.007, p < 0.0001. Overall, there was a significant non-linear relationship over the range of flows studied (LR chi-square [2 degrees of freedom] = 31.8, p < 0.0001).

CONCLUSIONS

Adenosine-augmented MDCT myocardial perfusion imaging provides semiquantitative measurements of myocardial perfusion during first-pass MDCT imaging in a canine model of LAD stenosis.

Statistical assessment of regional time-density measurement of myocardial perfusion

RATIONALE AND OBJECTIVES

The measurement of time-density relationships of the myocardium in studies of magnetic resonance perfusion images is a clinical technique used in assessing myocardial perfusion. This article presents a new technique, allowing regional time-density measurement and display of myocardial perfusion with improved accuracy compared with traditional manual trace techniques. Moreover, a method using statistical methods to discriminate relative decreased perfusion regions that differ significantly from the normally perfused myocardial tissue is introduced.

MATERIALS AND METHODS

Human datasets were obtained using a 1.5 T Signa Echospeed system (GE Medical Systems, Milwaukee, WI). The perfusion sequence was a 2D cardiac-gated fast gradient echo sequence with echo train readout, generating an in-plane pixel size of 1.46 mm2. Seven 10-mm-thick contiguous short axis tomographic slice images were obtained during a prolonged single breathhold. Data was collected at 30 time phases per slice image level during passage of 20 cc gadolinium contrast injected at a rate of 4-5 cc/sec into an antecubital vein.

RESULTS

Dilution properties can be determined and displayed as color-encoded regions superimposed on the myocardial slice according to the area of interest. Time-density curves throughout the perfusion study can be generated. Moreover, displays of normal and decreased perfusion areas can be used as statistically enhanced diagnosis guides.

CONCLUSION

This measurement, display, and diagnosis technique adds diagnostically important information to previous measurement and visualization techniques, providing enhanced detection and quantitative evaluation of regional deficits in myocardial contractility and perfusion, providing improved reliability and reproducibility of clinical diagnoses from MR-perfusion data.

Acute myocardial infarction: contrast-enhanced multi-detector row CT in a porcine model

PURPOSE

To assess the role of contrast material-enhanced retrospectively electrocardiographically (ECG) gated multi-detector row computed tomography (CT) in the detection of acute myocardial infarction in a porcine model of total coronary occlusion.

MATERIALS AND METHODS

Seven Yorkshire farm pigs were studied with contrast-enhanced retrospectively ECG-gated multi-detector row CT 3 hours after total occlusion of the distal left anterior descending artery (n = 5) or the second diagonal branch (n = 2). Reformatted short-axis end-systolic and end-diastolic CT data sets were assessed for myocardial perfusion deficits, coronary occlusion, and abnormal myocardial wall motion. Perfusion deficits were compared with microsphere-determined blood flow and triphenyltetrazolium chloride (TTC)-stained tissue samples for infarct assessment by using Bland-Altman analysis and analysis of variance.

RESULTS

Myocardial perfusion deficits, occlusion of the left anterior descending artery or second diagonal branch, and akinesis of the infarcted segment were identified in all five animals that completed the study. One animal died, and one data set had nondiagnostic image quality. The CT end-diastolic (mean, 16.1% +/- 4.8 [SD]; range, 8.6%-22.2%) and end-systolic (mean, 17.0% +/- 6.4; range, 8.7%-26.8%) volume of perfusion deficit was similar to that of infarcted tissue at TTC staining (mean, 13.6% +/- 6.0; range, 7.8%-30.9%). Infarcted myocardium at CT demonstrated a 76.1% reduction in microsphere-determined blood flow and a significant reduction of myocardial CT attenuation compared with normal myocardium (P <.01). Myocardial wall motion analysis demonstrated absence of systolic wall thickening in infarcted myocardium.

CONCLUSION

Multi-detector row CT with retrospective ECG gating permits the detection and further characterization of acute myocardial infarction in a porcine model of complete coronary occlusion.

Long-term antioxidant intervention improves myocardial microvascular function in experimental hypertension

Hypertension increases oxidative stress, which can impair myocardial microvascular function and integrity. However, it is yet unclear whether long-term antioxidant intervention in early hypertension would preserve myocardial perfusion and vascular permeability responses to challenge. Pigs were studied after 12 weeks of renovascular hypertension without (n=8) or with daily supplementation of antioxidants (100 IU/kg vitamin E and 1 g vitamin C, n=6), and compared with normal controls (n=7). Myocardial perfusion and microvascular permeability were measured in vivo by electron beam computed tomography before and after 2 cardiac challenges (intravenous adenosine and dobutamine). Basal left ventricular muscle mass was also obtained. Mean arterial pressure was significantly increased in both groups of hypertensive animals (without and with antioxidants, 123+/-9 and 126+/-4 mm Hg, respectively, versus normal, 101+/-4 mm Hg; both P<0.05), but muscle mass was not different among the groups. The impaired myocardial perfusion response to adenosine observed in hypertensives (normal, +51+/-14%; P<0.05 versus baseline; hypertension, +14+/-15%; P=0.3 versus baseline) was preserved in hypertensive pigs that received antioxidants (+44+/-15%; P=0.01 compared with baseline). Long-term antioxidant intervention also preserved subendocardial microvascular permeability responses in hypertension. On the other hand, antioxidant intervention had little effect on the hypertension-induced myocardial vascular dysfunction observed in response to dobutamine. This study demonstrates that the impaired myocardial perfusion and permeability responses to increased cardiac demand in early hypertension are significantly improved by long-term antioxidant intervention. These results support the involvement of oxidative stress in myocardial vascular dysfunction in hypertension and suggest a role for antioxidant strategies to preserve the myocardial microvasculature.

Transitions: noninvasive coronary angiography using electron beam computed tomography: technique, clinical application, future prospective

Electron beam computed tomography has been available clinically for 20 years. It is the only computed tomography scanner specifically developed for cardiac imaging. Over the past decade, with improvements in methodology and computer software, electron beam computed tomography has been shown to provide an excellent method to perform noninvasive coronary angiography. This article looks at the historical aspects of electron beam computed tomography and comments on how to perform and interpret electron beam angiography studies. The expanding development of noninvasive coronary and peripheral angiography methods using computed tomography will have a significant influence on cardiovascular specialists and their practices.

Imaging of myocardial microvasculature using fast computed tomography and three-dimensional microscopic computed tomography

In the past, much attention in imaging research was focused on the macroscopic morphology and patency of arteries. Only recently, research and clinical interest have shifted to the microcirculation and its impact on the long-term prognosis in patients with vascular diseases. This focus and newly developed therapeutic strategies require high-resolution imaging modalities, which do not focus exclusively on the macroscopic aspects of the arterial tree. Provided here is a comprehensive perspective of using computed tomography technology to image and quantify the function and morphology of myocardial and vascular adventitial microvessels in normal and disease states.

Clinical use of coronary calcium scanning with computed tomography

EBT has undergone rigorous testing for reliability and validity of CAC measurements, and has been proven to be useful in identifying individuals with, or at risk for, coronary heart disease. Although MDCT is a promising tool for coronary calcium scoring, more studies are needed that compare EBT and MDCT scans in the same patients, especially with calcium scores of less than 100. Further radiation dose reduction strategies for MDCT are currently being evaluated. MDCT studies of progression, reproducibility, and outcomes are needed to fully evaluate its potential to measure and serially follow atherosclerosis compared to EBT. Studies examining the benefit of serial coronary calcium scoring to non-invasively assess the progression or regression of coronary calcium are currently underway. EBT is a method that can be used to estimate the overall coronary atherosclerotic plaque burden. It can be used to diagnose the presence and determine the extent of coronary atherosclerosis; furthermore, the calcium score information can be used to assess the likelihood of advanced obstructive disease and to provide prognostic information. Finally, serial CAC measurements by EBT have the potential to determine the efficacy of therapeutic interventions by demonstrating progression, stabilization, or regression of coronary atherosclerotic disease during therapy.

Endothelin type A receptor antagonism restores myocardial perfusion response to adenosine in experimental hypercholesterolemia

Experimental hypercholesterolemia is characterized by increased endothelin-1 (ET-1) activity and is associated with an attenuated myocardial perfusion response and an inappropriate increase in coronary microvascular permeability during episodes of increased myocardial demand. This study was designed to determine the effect of chronic selective ET type A (ET(A)) receptor antagonism on coronary vascular response to simulated cardiac stress in experimental hypercholesterolemia. Twenty-one pigs were randomized to three groups: normal diet (N), high-cholesterol diet (HC), and HC diet plus ABT-627, a selective ET(A) receptor antagonist, (HC+ABT-627). After 12 weeks, cardiac electron beam computed tomography (EBCT) was performed before and during intravenous infusion of adenosine, and myocardial perfusion (ml/min per g) and coronary microvascular permeability index (arbitrary units) were calculated. Basal myocardial perfusion was similar in all groups (N: 0.91+/-0.10; HC: 0.95+/-0.08; HC+ABT-627: 1.03+/-0.09; P=0.64). Adenosine infusion led to a significant increase in myocardial perfusion in the N (1.32+/-0.15; P<0.001) but not in the HC (0.95+/-0.07) group. However, in the HC+ABT-627 group, adenosine also significantly increased myocardial perfusion (1.33+/-0.12; P=0.001). Basal permeability index did not differ between the groups (N: 1.56+/-0.13; HC: 1.34+/-0.19; HC+ABT-627: 1.62+/-0.10; P=0.38). Adenosine infusion significantly increased permeability index in HC pigs (2.29+/-0.22; P<0.001) but not in N (1.71+/-0.21) and HC+ABT-627 (1.82+/-0.08) pigs. We conclude that chronic selective ET(A) receptor antagonism preserves myocardial perfusion response and coronary microvascular integrity during episodes of increased myocardial demand in experimental hypercholesterolemia, indicating an important role for the endogenous endothelin system in this disorder.

Simvastatin preserves myocardial perfusion and coronary microvascular permeability in experimental hypercholesterolemia independent of lipid lowering

OBJECTIVES; This study was designed to assess the lipid-independent effects of simvastatin on myocardial perfusion (MP) and coronary microvascular permeability index (PI) at baseline and during episodes of increased cardiac demand in experimental hypercholesterolemia.

BACKGROUND

Simvastatin preserves coronary endothelial function in experimental hypercholesterolemia independent of its lipid-lowering effect. However, the functional significance of this observation is unknown.

METHODS

Pigs were randomized to three groups: normal diet (N), high-cholesterol diet (HC) and HC diet plus simvastatin (HC+S) for 12 weeks. Subsequently, cardiac electron beam computed tomography was performed before and during intravenous infusion of adenosine and dobutamine, and MP and PI were calculated.

RESULTS

Total and low density lipoprotein cholesterol levels were similarly and significantly increased in HC and HC+S animals compared with N. Basal MP was similar in all groups. Myocardial perfusion significantly increased in response to either adenosine or dobutamine in N and HC+S animals. Dobutamine also significantly increased MP in HC animals. However, the changes of MP in response to either drug were significantly lower in the HC group compared with the other two groups (p < 0.01 for adenosine and p < 0.05 for dobutamine vs. N and HC+S). Basal PI was similar in all groups and was not altered by either drug in N and HC+S animals. In contrast, PI significantly increased in HC pigs during infusion of either adenosine (p < 0.001) or dobutamine (p < 0.05).

CONCLUSIONS

These findings demonstrate that chronic administration of simvastatin preserves myocardial perfusion response and coronary microvascular integrity during cardiac stress in experimental hypercholesterolemia independent of lipid lowering.

Current results and new developments of coronary angiography with use of contrast-enhanced computed tomography of the heart

Electron beam computed tomography (EBCT) is the reference standard for x-ray-based tomographic imaging of the heart because of its high temporal resolution, but it is available in only a few centers. Quantification of coronary calcium is the most widely recognized use of EBCT for cardiac imaging. This technique requires no contrast media and provides an accurate assessment of overall plaque burden in the coronary tree; however, it does not directly identify or localize coronary stenoses. Multislice spiral (helical) CT (MSCT) is a new technology that provides images of the beating heart in diagnostic quality under many circumstances and may facilitate the broader application of cardiac and coronary CT. Currently, for imaging of the heart, much more experience exists with EBCT than with MSCT. Contrast-enhanced CT coronary angiography (CTCA) can be done with EBCT or MSCT to obtain images of the major branches of the coronary tree and to define luminal narrowing. Studies at experienced centers performed with small numbers of patients show that sensitivity, specificity, and negative predictive value are good with CTCA in the assessment of obstructive coronary artery disease, but CTCA remains an investigational technique for these applications. Computed tomographic coronary angiography can be clinically useful for assessing coronary artery bypass graft patency and congenital coronary abnormalities.

The endothelium: dysfunction and beyond

Since its anatomic discovery in the 19th century, the endothelium was considered to fulfill no other purpose than that of a physical barrier between blood and tissue, until Furchgott and colleagues defined endothelium-dependent vasoreactivity in the late 1970s. Henceforth, a functional paradigm defined the balance between endothelium-derived relaxing factors and endothelium-derived contracting factors as the hallmark of endothelial cell integrity. As a consequence, any reflection of a deviation from this state was defined as endothelial dysfunction, most notably the impairment of vasorelaxation in response to pharmacologic stimuli such as acetylcholine or nonpharmacologic stimuli such as shear stress and cold pressor. Within the coronary artery tree these alterations have been recognized before the development of obstructive coronary artery disease, affecting the microcirculation before the epicardial conduit vessel. Furthermore, recent clinical trials outlined the prognostic significance of these changes, whereby impairment of increase in coronary blood flow in response to endothelium-dependent stimuli seemed to be of utmost importance. Thus it is intriguing to speculate on the evolving role of myocardial perfusion imaging in combination with pharmacologic and nonpharmacologic stimuli for the noninvasive assessment of coronary endothelial dysfunction in patients at risk for future adverse events. At a minimum, this review aims to put endothelial dysfunction into an imaging perspective beyond the scope of the conventional approach.

Hypercholesterolemia impairs myocardial perfusion and permeability: role of oxidative stress and endogenous scavenging activity

OBJECTIVES

We intended to study the effect of hypercholesterolemia (HC) on myocardial perfusion and permeability response to increased cardiac demand.

BACKGROUND

Hypercholesterolemia is associated with increased incidence of cardiac events and characterized by impaired coronary vascular function, possibly mediated partly through increased pro-oxidative conditions in plasma and tissue. However, it is yet unclear whether HC is also associated with impaired myocardial perfusion and vascular permeability responses in vivo.

METHODS

For 12 weeks pigs were fed a normal, HC or HC diet supplemented daily with antioxidants (HC + AO, 100 IU/kg vitamin E and 1 g vitamin C). Myocardial perfusion and vascular permeability were measured in vivo using electron beam computed tomography before and after cardiac challenge with intravenous adenosine. Plasma and tissue oxidative status was determined ex vivo.

RESULTS

Plasma cholesterol increased in all cholesterol-fed pigs but was associated with increased markers of oxidative stress only in HC pigs. Myocardial perfusion increased in response to adenosine in normal and HC + AO (+37 +/- 13% and +58 +/- 22%, respectively, p < 0.05 vs. baseline) but not in HC, whereas vascular permeability index increased only in HC pigs (+ 92 +/- 25%, p = 0.002). In HC animals, tissue endogenous oxygen radical scavengers and antioxidant vitamins were depleted and LDL oxidizability enhanced, but both were normalized in HC + AO pigs. Myocardial perfusion response was directly, and permeability inversely, associated with plasma and tissue vitamin concentrations.

CONCLUSIONS

This study demonstrates that experimental HC is associated with blunted myocardial perfusion and increased vascular permeability responses in vivo to increased cardiac demand, which may be partly mediated by a shift in oxidative status.