Seth T. Dahlberg

Intracoronary Doppler assessment of moderate coronary artery disease: comparison with 201Tl imaging and coronary angiography. FACTS Study Group.

BACKGROUND Coronary angiography may not reliably predict whether a stenosis causes exercise-induced ischemia. Intracoronary Doppler ultrasound may enhance diagnostic accuracy by providing a physiological assessment of stenosis severity. The goal of this study was to compare intracoronary Doppler ultrasound with both 201Tl imaging and coronary angiography. METHODS AND RESULTS Fifty-five patients with 67 stenotic coronary arteries underwent coronary angiography with intracoronary Doppler ultrasound and had exercise 201Tl testing within a 1-week period. Coronary flow reserve was measured, and analyses were performed by independent core laboratories. The mean stenosis was 59+/-12%; 51 of 67 stenoses were intermediate in severity (40% to 70%). A coronary flow reserve < 1.7 predicted the presence of a stress 201Tl defect in 56 of 67 stenoses (agreement=84%; kappa=0.67; 95% CI=0.48 to 0.86). In the patients who achieved 75% of their predicted maximum heart rate, the Doppler and 201Tl imaging data agreed in 46 of 52 stenoses (agreement=88%; kappa=0.77; 95%CI=0.57 to 0.97). Scatter was evident when angiography was compared with coronary flow reserve (r=.43), and the angiogram did not reliably predict the results of the 201Tl stress test (kappa=0.21; agreement=57% to 63%). CONCLUSIONS Doppler-derived coronary flow reserve accurately predicts the presence of exercise-induced ischemia on stress 201Tl imaging, and coronary angiography does not reliably assess the physiological significance of an intermediate coronary stenosis.

A Monte Carlo investigation of artifacts caused by liver uptake in single-photon emission computed tomography perfusion imaging with technetium 99m-labeled agents

BackgroundSignificant hepatobiliary accumulation of technetium 99m-labeled cardiac perfusion agents has been shown to cause alterations in the apparent localization of the agents in the cardiac walls. A Monte Carlo study was conducted to investigate the hypothesis that the cardiac count changes are due to the inconsistencies in the projection data input to reconstruction, and that correction of the causes of these inconsistencies before reconstruction, or including knowledge of the physics underlying them in the reconstruction algorithm, would virtually eliminate these artifacts.Methods and ResultsThe SIMIND Monte Carlo package was used to simulate 64×64 pixel projection images at 128 angles of the three-dimensional mathematical cardiac-torso (MCAT) phantom. Simulations were made of (1) a point source in the liver, (2) cardiac activity only, and (3) hepatic activity only. The planar projections and reconstructed point spread functions (PSFs) of the point source in the liver were investigated to study the nature of the inconsistencies introduced into the projections by imaging, and how these affect the distribution of counts in the reconstructed slices. Bull’s eye polar maps of the counts at the center of the left ventricular wall of filtered back-projection (FBP) and maximum-likelihood expectation-maximization (MLEM) reconstructions of projections with solely cardiac activity, and with cardiac activity plus hepatic activity scaled to have twice the cardiac concentration, were compared to determine the magnitude and location of apparent changes in cardiac activity when hepatic activity is present. Separate simulations were made to allow the investigation of stationary spatial resolution, distance-dependent spatial resolution, attenuation, and scatter. The point source projections showed significant inconsistencies as a function of projection angle with the largest effect being caused by attenuation. When consistent projections were simulated, no significant impact of hepatic activity on cardiac counts was noted with FBP, or 100 iterations of MLEM. With inconsistent projections, reconstruction of 180 degrees resulted in greater apparent cardiac count losses than did 360 degrees reconstruction for both FBP and MLEM. The incorporation of attenuation correction in MLEM reconstruction reduced the changes in cardiac counts to that seen in simulations in which attenuation was not included, but resulted in increased apparent localization of activity in the posterior wall of the left ventricle when scatter was present in the simulated images.ConclusionsThe apparent alterations in cardiac counts when significant hepatic localization is present is due to the inconsistency of the projections inherent in imaging. Prior correction of these, or accounting for them in the reconstruction algorithm, will virtually eliminate them as causes of artifactual changes in localization. Attenuation correction and scatter correction are both required to overcome the major sources of apparent count changes in the heart associated with hepatic uptake.

An investigation of the estimation of ejection fractions and cardiac volumes by a quantitative gated SPECT software package in simulated gated SPECT images.

BackgroundThe purpose of this investigation was to determine the accuracy of the estimation of ejection fractions (EFs) and left ventricular volumes from a commercially available soft-ware package (Quantitative Gated SPECT[QGS]) as a function of different true EFs, count level in the acquisitions, severity and location of perfusion defects, increasing hepatic, activity, and modified wall motion.Methods and ResultsThe dynamic mathematic cardiac-torso digital phantom was used to create three-dimensional source and attenuation maps representing the distribution of a technetium-99m-labeled cardiac perfusion agent in the chest. Three hearts with varying end-systolic volumes were used to investigate different EFs. Perfusion defects were created as localized uptake within selected portions of the cardiac walls, scaled to the desired fraction of the normal wall uptake, and subtracted from the normal distribution. The hepatic uptake was increased up to five times of the normal heart uptake to investigate the influence of a “hot” liver. Alteration a lateral wall motion was also investigated. A three-dimensional projector that included the influence of distance-dependent spatial resolution and nonuniform attenuation was then used to create projection images. The projections were scaled to the desired acquisition count level, and Poisson noise was added. Automatic determination of EF slightly overestimated the true EF for normal count levels by 3% to 7% of the true EF and underestimated the true EF by up to 9% for very low count levels for 180-degree reconstructions. The accuracy for determining the volumes was not as high as for the EFs (an average error of 12% was observed). The calculated EFs were relatively accurate for perfusion defects of 50% or less. When perfusion defects exceeded 50%, extracardiac counts were included in the heart contours causing larger underestimations of EF. With removal of the extracardiac counts, the EFs increased. With a hepatic uptake of two or more times the heart uptake, no meaningful EF could be obtained. Either drawing a single region of interest for every slice or use of the manual mode with constrain option could remarkably improve the estimation. The accuracy of the calculation of EF and volumes for the heart with stationary wall was fairly high but decreased significantly when coupled with perfusion defects.ConclusionIt is concluded that the QGS program evaluates the functional parameter of EF accurately. The biggest limitations occurred in determining the appropriate cardiac contour if areas with very high extracardiac counts were present in the heart slices, and when a greater than 50% decrease occurred in uptake for perfusion defects.

Influence of image quality on the accuracy of real time three-dimensional echocardiography to measure left ventricular volumes in unselected patients: a comparison with gated-SPECT imaging.

Background: Patient selection, often restricted to those with ideal image quality, and timing of studies in relation to reference methods may limit clinical applicability of cardiac volumes derived from 3D echocardiography. Methods: To test the influence of image quality on LV volumes by real time 3DE (RT3DE), we compared results obtained by RT3DE to those from gated‐SPECT imaging in 64 consecutive patients referred for clinically indicated nuclear perfusion imaging. To minimize hemodynamic effects, RT3DE was performed immediately following G‐SPECT. LV volumes by RT3DE were calculated using at least three orthogonal plane pairs. Image quality was rated as good if 75–100% of the endocardial border was visualized, fair if 60–74% was visualized, and poor if 50–60% was visualized. Results: Image quality was good in 25 (39%), fair in 20 (31%), and poor in 13 (20%) patients. Six patients (9%) were excluded for uninterpretable echo images. For the entire cohort, EDV and ESV agreed closely (all P = NS). When stratified by image quality, the EDV and ESV of those with good and fair image quality agreed closely with minimal bias (average 1 ± 9 mL and 2 ± 7 mL, respectively). Poor image was associated with less strong agreement and much greater bias for EDV and ESV (7 ± 25 mL and 7 ± 20 mL, respectively). Conclusions: When applied to patients studied in routine clinical practice, LV volumes by RT3DE compare favorably to G‐SPECT. RT3DE results are more reliable when >60% of endocardium is visualized.

Gender Difference in the Risk Factors for Sudden Cardiac Death

Sudden cardiac death is most often caused by ventricular dysrhythmias. Although women have an incidence of sudden death lower than that of men, 34% of coronary deaths in women are sudden deaths. The atherogenic risk factors do not predict which women are at the highest risk of sudden cardiac death. Left ventricular hypertrophy is a strong risk factor for sudden cardiac death in women. Asymptomatic ventricular dysrhythmias, a risk factor for death after myocardial infarction in men, do not increase the risk of death in women. This gender difference in the risk of sudden cardiac death deserves further attention, since it affects the evaluation of interventions designed to reduce the rate of sudden cardiac death.

Estimation of attenuation maps from scatter and photopeak window single photon-emission computed tomographic images of technetium 99m-labeled sestamibi

BackgroundIn single photon-emission computed tomographic imaging of the chest, nonuniform attenuation correction requires use of a patient-specific attenuation map. The aim of this study was to determine whether an estimate of the regions of the lungs and nonpulmonary tissues of the chest could be obtained by segmenting the photopeak and Compton scatter window images in a phantom and in patients to estimate patient-specific attenuation maps.Methods and ResultsThe photopeak and scatter window slices from 16 consecutive 99mTc-labeled sestamibi perfusion studies were segmented interactively. In these studies, visually reasonable regions could be obtained by estimating a “cold” lung region from scatter window data with additional anatomic information of the myocardium region, the backbone and sternum locations, the liver, and the rib cage from the photopeak window data. In an anthropomorphic torso phantom study and a patient study, comparison was made between the attenuation maps based on segmentation of the emission images and transmission imaging with a slant-hole collimator. It was determined that good agreement in the estimation of the body regions can be achieved with segmentation of the emission images in both the phantom and patient data. Attenuation correction using the maximum-likelihood expectation maximization method was performed on the phantom and the patient data. In both studies, attenuation correction with the segmented attenuation map improved uniformity of the inferior wall region in comparison with the other walls.ConclusionsThe estimation of patient-specific attenuation maps by segmenting the scatter and photopeak window slices of 99mTc-labeled sestamibi studies may be a way of reducing the loss of specificity due to attenuation artifacts. The potential limitations on the accuracy of correction inherent in the method due to the estimation of the regions and assignment of the attenuation coefficients need to be determined further, and the method needs to be further automated before it can be considered for routine clinical use.

Left ventricular cavity size determined by preoperative dipyridamole thallium scintigraphy as a predictor of late cardiac events in vascular surgery patients

We hypothesized that left ventricular (LV) cavity size measured on dipyridamole thallium scintigraphy identifies patients at risk for late nonfatal myocardial infarction and cardiovascular death. Accordingly, we retrospectively evaluated the predictive value of clinical and scintigraphic variables, including transendocardial LV cavity measurement performed on formatted images, in 335 vascular surgery patients. A nonhomogeneous perfusion pattern and enlarged LV cavity size were the most significant predictors of late events, and the interaction between these two variables was more predictive than was either variable alone. Life-table analysis demonstrated that patients with normal perfusion patterns had the lowest incidence of late events regardless of cavity size (p < 0.0005). Conversely, patients with a nonhomogeneous perfusion pattern and the largest LV cavity measurements were at the highest risk for late cardiac events (p < 0.0001). Therefore, this study demonstrated that a measurement of LV scintigraphic cavity size can provide important risk stratification for late cardiac events.

Myocardial kinetics of radiolabeled perfusion agents: Basis for perfusion imaging

The myocardial deposition of radiolabeled perfusion agents permits the noninvasive assessment of regional coronary blood flow. The design of imaging protocols and the optimal interpretation of clinical perfusion studies are based on an understanding of the kinetics of blood-tissue exchange for these compounds. Thallium 201 and the technetium 99m-labeled compounds sestamibi, teboroxime, and tetrofosmin show differing myocardial extraction and retention. This review focuses on studies that used cell culture, isolated heart, and intact animal models that form the basis of our current understanding of the myocardial kinetics of these imaging agents.

Evaluation of scatter compensation strategies and their impact on human detection performance in Tc-99m myocardial perfusion imaging

Human-observer receiver-operating characteristic (ROC) results with clinical patient studies indicate that ordered-subset expectation-maximization (OSEM) reconstructions with a combination of corrections for attenuation, scatter, and distance-dependent resolution (DDR) significantly improves perfusion defect detection accuracy over filtered back-projection (FBP) images with no compensation. A recent Monte Carlo study has also shown that for scatter correction (SC) in particular, selection of an appropriate strategy can impact cardiac uniformity. The spatial domain based effective source scatter estimation (ESSE) technique was shown to provide more robust performance in improving cardiac uniformity than the triple-energy window (TEW) method. In this work, we investigate if further improvements in observer performance could be obtained by using the ESSE method of SC instead of the currently employed TEW SC, in combination with attenuation correction (AC) and resolution compensation (RC). We evaluated the effectiveness of the ESSE method to provide additional improvements in image quality objectively by using human-observer ROC studies on clinically acquired patient acquisitions. Results indicate that both ESSE and TEW SC in combination with AC and RC provide significantly higher detection accuracy than FBP with no compensation for the overall detection of coronary artery disease (CAD) as well as in localizing perfusion defects in the left anterior descending (LAD) and left circumflex (LCx) territories. Comparing, the two implementations of SC evaluated in this study, we note that the ESSE method resulted in larger aggregate areas under the ROC curve (A/sub z/) in each case. However, a statistically significant improvement over TEW correction was only observed in the LAD territory. This indicates that SC implemented with the ESSE and TEW methods were close in terms of their improvement in detection accuracy for perfusion defects in the clinical images of this investigation, with the ESSE method arguably being slightly better. However, the clinical implementation of ESSE will be hampered by its longer computing time.

Focal myocardial injury following blunt chest trauma: A comparison of indium-111 antimyosin scintigraphy with other noninvasive methods

The diagnosis of myocardial contusion is often difficult, as traditional methods such as serial electrocardiograms, cardiac enzyme (creatine kinase [CK-MB]) analysis, and echocardiography lack sensitivity and specificity. Recent reports have shown that 111In labelled antimyosin scanning has high sensitivity for detecting cardiac injury. However, no prior studies have been reported for antimyosin imaging with patients suspected of sustaining a cardiac contusion. Accordingly, 17 patients with severe multisystem trauma (intrathoracic vascular injury in eight patients, pneumothorax and pulmonary contusion in 13) underwent antimyosin scintigraphy, echocardiography, 12-lead electrocardiograms, and CK-MB determinations. Arrhythmias were noted in seven patients, four of whom died. All patients has elevated CK levels but CK-MB isoenzyme was greater than 4% in only three. Abnormal ST segments were noted in nine subjects, only one of whom had CK-MB elevation. Echocardiography revealed pericardial effusions in four patients but was technically suboptimal in 53% of the studies. Blinded interpretation of the antimyosin scans revealed only one with focal myocardial uptake; this same patient had the only discrete wall motion abnormality on the echocardiogram and also had ST depression with ectopy but normal CK-MB. Thus in patients with suspected myocardial contusion, echocardiography is frequently limited technically and the electrocardiogram and CK analysis appear to lack diagnostic accuracy. In contrast, monoclonal antimyosin imaging may be performed in patients with trauma without limitation and yields results that are concordant with echocardiograms. In patients with suspected myocardial contusion, focal antimyosin uptake is uncommon despite severe thoracic injury, which suggests that extensive myocardial necrosis is not the primary method of injury.