Science, art, and artifacts: how important is quantification for the practicing physician interpreting myocardial perfusion studies?

Head-to-head comparison of diagnostic accuracy of stress-only myocardial perfusion imaging with conventional and cadmium-zinc telluride single-photon emission computed tomography in women with suspected coronary artery disease

BACKGROUND

Breast attenuation may impact the diagnostic accuracy of stress myocardial perfusion imaging (MPI) with single-photon emission computed tomography (SPECT). We compared the performance of conventional (C)-SPECT and cadmium-zinc-telluride (CZT)-SPECT systems in women with low-intermediate likelihood of coronary artery disease (CAD).

METHODS AND RESULTS

A total of 109 consecutive women underwent stress-optional rest MPI by both C-SPECT and CZT-SPECT. In the overall study population, a weak albeit significant correlation between total perfusion defect (TPD) measured by C-SPECT and CZT-SPECT was observed (r = 0.38, P < .001) and at Bland-Altman analysis the mean difference in TPD (C-SPECT minus CZT-SPECT) was 2.40% (P < .001). Overall concordance of semi-quantitative diagnostic performance between C-SPECT and CZT-SPECT was observed in 52 (48%) women with a κ value of 0.09. Normalcy rate was significantly higher using CZT-SPECT compared to C-SPECT (P < .001). Machine learning analysis performed through the implementation of J48 algorithm proved that CZT-SPECT has higher sensitivity, specificity, and accuracy than C-SPECT.

CONCLUSIONS

In women with low-intermediate likelihood of CAD, there is a poor concordance of diagnostic performance between C-SPECT and CZT-SPECT, and CZT-SPECT allows better normalcy rate detection compared to C-SPECT.

Clinical decision support systems in myocardial perfusion imaging

Diagnostic imaging is becoming more complicated, physicians are also required to master an ever-expanding knowledge base and take into account an ever increasing amount of patient-specific clinical information while the time available to master this knowledge base, assemble the relevant clinical data, and apply it to specific tasks is steadily shrinking. Compounding these problems, there is an ever increasing number of aging "Baby Boomers" who are becoming patients coupled with a declining number of cardiac diagnosticians experienced in interpreting these studies. Hence, it is crucial that decision support tools be developed and implemented to assist physicians in interpreting studies at a faster rate and at the highest level of up-to-date expertise. Such tools will minimize subjectivity and intra- and inter-observer variation in image interpretation, help achieve a standardized high level of performance, and reduce healthcare costs. Presently, there are many decision support systems and approaches being developed and implemented to provide greater automation and to further objectify and standardize analysis, display, integration, interpretation, and reporting of myocardial perfusion SPECT and PET studies. This review focuses on these systems and approaches.

Validation of automated quantitation of myocardial perfusion and fatty acid metabolism abnormalities on SPECT images

BACKGROUND

Myocardial perfusion and fatty acid imaging have played important roles in the risk stratification of patients with coronary artery disease (CAD). However, visual image assessment requires considerable experience and training. Therefore, an automated program has been developed that can quantify perfusion and fatty acid uptake on myocardial single emission computed tomography (SPECT). The present study aimed to validate the automated quantitative program.

METHODS AND RESULTS

A total of 50 patients were studied with known or suspected CAD who underwent stress ²⁰¹Thallium (²⁰¹Tl) and resting ¹²³I-labelled β-methyl iodophenyl pentadecanoic acid (BMIPP) SPECT. The SPECT images were quantified in 17 segments visually and using our Heart Score View software. Values were compared with those in a normal Japanese database and calculated summed stress (SSS), summed rest (SRS), summed difference (SDS), and summed BMIPP scores for each modality. Summed scores obtained using standard visual analysis and Heart Score View significantly correlated (²⁰¹Tl: SSS: r=0.934; SRS: r=0.827; SDS: r=0.743 summed BMIPP score: r=0.913) (each P<0.001) and Bland-Altman analysis revealed good agreement between the 2 approaches.

CONCLUSIONS

Correlations between scores determined using Heart Score View software and standard visual interpretation were linear for both perfusion and fatty acid images. Thus, our new automated program might be useful for the risk stratification of patients with CAD in the clinical setting.

Automatic and visual reproducibility of perfusion and function measures for myocardial perfusion SPECT

BACKGROUND

We define the repeatability coefficients (RC) of key quantitative and visual perfusion and function parameters that can be derived by the QGS/QPS automated software and by expert visual observer from gated myocardial perfusion SPECT (MPS) scans.

METHODS

Standard QGS/QPS algorithms have been applied to derive quantitative perfusion and function parameters in 200 99mTc-tetrofosmin rest/stress MPS scans, obtained in 100 consecutive patients who underwent 2 separate gated rest/stress scans on the same camera. Variables included stress, rest, and ischemic total perfusion deficit (TPD), ejection fraction, motion, and thickening. Visual perfusion/motion scores were derived by an expert reader using randomized scan order and normalized to % myocardium.

RESULTS

Quantitative and visual parameters were highly reproducible with smaller RC for some quantitative measures as compared to visual measures (P < .0001). RC for quantitative measures were 3.3% for stress TPD, 1.8% for rest TPD, and 3.2% for ischemic TPD and for visual scoring 4.8% for stress, 3.8% for rest, and 4.3% for ischemic (P ≤ .002). The results in each vessel territory showed that in the right coronary artery (RCA) territory the quantitative approach had improved reproducibility as compared to visual reading. Visual thickening scoring was more reproducible than motion scoring (P < .0001).

CONCLUSIONS

This study demonstrates that standard perfusion and function parameters derived from MPS by visual or quantitative analysis are highly reproducible with some advantages to the quantitative approach.

Prognostic performance of quantitative PET tools for stratification of patients with ischemic cardiomyopathy undergoing myocardial viability assessment

OBJECTIVES

This study was performed to determine the prognostic performance of quantitative PET tools in the stratification of patients with ischemic cardiomyopathy undergoing myocardial viability assessment.

METHODS

We applied four different quantitative tools to 104 consecutive patients with coronary artery disease and previous myocardial infarction who had undergone rest Rb/gated F-fluorodeoxyglucose (FDG) PET, to assess myocardial viability for potential revascularization. One of these tools was based on the FDG study alone and the other three tools assessed the extent of match/mismatch defects using FDG in comparison with a perfusion reference database. The four quantitative tools used in this research to define viability were (i) FDG alone, which calculates the percentage of left ventricular myocardium (LVM) that is above the 50% of the maximum LVM FDG counts, (ii) low flow match/mismatch, which determines the area with a 5% increase in normalized FDG counts in relation to defined resting perfusion defects as compared with a reference database, (iii) all regions match/mismatch, which computes the area with a 10% increase in normalized FDG counts in relation to the left ventricle resting perfusion distribution, and (iv) percentage max FDG match/mismatch, which defines the area with FDG uptake greater than 60% of the maximum LVM FDG counts within defined perfusion defects as determined by the reference database. The primary endpoint for this analysis was cardiac death.

RESULTS

During the follow-up period (22+/-14 months), 19 patients (18%) died; in 17 of these the cause of death was cardiac. Using univariate analysis, none of the methods were predictive of cardiac death. Receiver operating characteristic analysis defined the optimal thresholds for the extent of myocardial viability for the four tools in the prediction of cardiac death: FDG alone=20%, low flow match/mismatch=15%, all regions match/mismatch=35%, and percentage max FDG match/mismatch=20%. A censored survival analysis using a Kaplan-Meier method showed a statistically significant difference between patients with cardiac death and those with no cardiac death using only the low flow match/mismatch (hazard ratio=0.29, P=0.01) and percentage max FDG match/mismatch criteria (hazard ratio=0.23, P=0.005) tools.

CONCLUSION

The low flow match/mismatch and percentage max FDG match/mismatch quantitative PET tools are useful for prognostic stratification of patients with ischemic cardiomyopathy undergoing myocardial viability assessment.

Quantitative assessment of myocardial perfusion abnormality on SPECT myocardial perfusion imaging is more reproducible than expert visual analysis

BACKGROUND

Current guidelines of Food and Drug Administration for the evaluation of SPECT myocardial perfusion imaging (MPI) in clinical trials recommend independent visual interpretation by multiple experts. Few studies have addressed whether quantitative SPECT MPI assessment would be more reproducible for this application.

METHODS AND RESULTS

We studied 31 patients (age 68 +/- 13, 25 male) with abnormal stress MPI who underwent repeat exercise (n = 11) or adenosine (n = 20) MPI within 9-22 months (mean 14.9 +/- 3.8 months) and had no interval revascularization or myocardial infarction and no change in symptoms, stress type, rest or stress ECG, or clinical response to stress on the second study. Visual interpretation per FDA Guidance used 17-segment, 5-point scoring by two independent expert readers with overread of discordance by a third expert, and percent myocardium abnormal was derived from normalized summed scores. The quantitative magnitude of perfusion abnormality was assessed by the total perfusion deficit (TPD), expressing stress, rest, and ischemic perfusion abnormality. High linear correlations were observed between visual and quantitative size of stress, rest, and ischemic defects (R = 0.94, 0.92, 0.84). Correlations of two tests were higher by quantitative than by visual methods for stress (R = 0.97 vs R = 0.91, P = 0.03) and rest defects (R = 0.94 vs R = 0.82, P = 0.03), respectively, and statistically similar for ischemic defects (R = 0.84 vs R = 0.70, P = ns).

CONCLUSIONS

In stable patients having serial SPECT MPI, quantification is more reproducible than visual for magnitude of perfusion abnormality, suggesting its superiority for use in randomized clinical trials and monitoring the effects of therapy in an individual patient.

Detecting changes in serial myocardial perfusion SPECT: a simulation study

BACKGROUND

New algorithms were evaluated for their efficacy in detecting and quantifying serial changes in myocardial perfusion from single photon emission computed tomography (SPECT).

METHODS AND RESULTS

We generated 72 simulations with various left ventricular positions, sizes, count rates, and perfusion defect severities using the nonuniform rational B-splines (NURBs)-based CArdiac Torso (NCAT) phantom. Images were automatically aligned by use of both full linear and rigid transformations and quantified for perfusion by use of the CEqual program. Changes within a given perfusion defect were compared by use of a Student t test before and after registration. Registration approaches were compared by use of receiver operating characteristic analysis. Changes of 5% were not detected well in single patients with or without alignment. Changes of 10% and 15% could be detected with false-positive rates of 15% and 10%, respectively, in single studies if alignment was performed before perfusion analysis. Alignment also reduced the number of studies necessary to demonstrate a significant perfusion change (P < .05) in groups of patients by about half.

CONCLUSION

Comparison of mean uptake by t values in SPECT perfusion defects can be used to detect 10% and greater differences in serial perfusion studies of single patients. Image alignment is necessary to optimize automatic detection of perfusion changes in both single patients and groups of patients.

Direct imaging of exercise-induced myocardial ischemia with fluorine-18-labeled deoxyglucose and Tc-99m-sestamibi in coronary artery disease

BACKGROUND

Scintigraphic myocardial perfusion imaging is the most widely used noninvasive modality for the detection of coronary artery disease (CAD). A technique for direct imaging of exercise-induced myocardial ischemia is highly desirable and preferable over perfusion imaging but is presently unavailable. We evaluated the feasibility and diagnostic accuracy of direct imaging of exercise-induced myocardial ischemia with fluorine-18-2-deoxyglucose (18FDG).

METHODS AND RESULTS

Twenty-six patients with known or suspected CAD and no prior myocardial infarction underwent simultaneous myocardial perfusion and ischemia imaging after the intravenous injection of Tc-99m-sestamibi (99mTc-sestamibi) and 18FDG at peak exercise. Rest perfusion imaging was carried out separately. All patients underwent coronary angiography. Exercise 18FDG myocardial images were compared with exercise-rest 99mTc-sestamibi images and coronary angiography. Of 22 patients with > or =50% narrowing of > or =1 coronary arteries, 18 had perfusion abnormalities (sensitivity 82%) whereas 20 had abnormal myocardial 18FDG uptake (sensitivity 91%, P=NS). Perfusion abnormalities were seen in myocardial segments corresponding to 25 vascular territories of a total of 51 vessels with > or =50% luminal narrowing in 22 patients (sensitivity 49%), whereas increased 18FDG uptake was seen in 34 vascular territories (sensitivity 67%, P=0.008). 18FDG images were of high quality and easy to interpret but required simultaneous perfusion images for localizing abnormal myocardial 18FDG uptake.

CONCLUSIONS

Exercise-induced myocardial ischemia can be imaged directly with 18FDG. Combined exercise 18FDG-99mTc-sestamibi imaging provides a better assessment of exercise-induced myocardial ischemia compared with exercise-rest perfusion imaging. Direct ischemia imaging eliminates some of the limitations of presently used myocardial perfusion imaging. Large-scale clinical studies are warranted.

Magnetic resonance first-pass myocardial perfusion imaging: clinical validation and future applications

Clinical studies suggest that magnetic resonance first-pass (MRFP) perfusion imaging is comparable to current diagnostic tests that are used clinically for the assessment of myocardial perfusion. In addition, magnetic resonance imaging (MRI) perfusion imaging is a noninvasive method for determining myocardial blood flow. The spatial resolution (in-plane spatial resolution < 3 mm) is sufficient to differentiate between subendocardial perfusion and subepicardial perfusion. The measurement can be repeated regularly without any adverse effects for the patient. MRI perfusion measurements can be combined with the evaluation of global function and regional wall thickening. Currently, there is no other imaging technique that offers similar advantages. The MRI perfusion measurements can be carried out during baseline conditions and during maximal hyperemia induced with either adenosine or dipyridamole. The ratio of the measured myocardial blood flows provides an estimate of the absolute and relative myocardial perfusion reserve. The perfusion reserve determined with MRFP imaging is a quantitative measure for the assessment of the collateral-dependent myocardial flow. Based on the available data using MRFP perfusion imaging, the current clinical first-line perfusion imaging tests are going to be challenged in the near future. J. Magn. Reson. Imaging 1999;10:676-685.

Quantitative SPECT techniques

Quantitative imaging involves first, a set of measurements that characterize an image. There are several variations of technique, but the basic measurements that are used for single photon emission computed tomography (SPECT) perfusion images are reasonably standardized. Quantification currently provides only relative tracer activity within the myocardial regions defined by an individual SPECT acquisition. Absolute quantification is still a work in progress. Quantitative comparison of absolute changes in tracer uptake comparing a stress and rest study or preintervention and postintervention study would be useful and could be done, but most commercial systems do not maintain the data normalization that is necessary for this. Measurements of regional and global function are now possible with electrocardiography (ECG) gating, and this provides clinically useful adjunctive data. Techniques for measuring ventricular function are evolving and promise to provide clinically useful accuracy. The computer can classify images as normal or abnormal by comparison with a normal database. The criteria for this classification involve more than just checking the normal limits. The images should be analyzed to measure how far they deviate from normal, and this information can be used in conjunction with pretest likelihood to indicate the level of statistical certainty that an individual patient has a true positive or true negative test. The interface between the computer and the clinician interpreter is an important part of the process. Especially when both perfusion and function are being determined, the ability of the interpreter to correctly assimilate the data is essential to the use of the quantitative process. As we become more facile with performing and recording objective measurements, the significance of the measurements in terms of risk evaluation, viability assessment, and outcome should be continually enhanced.

Technetium-99m labeled myocardial perfusion imaging agents

99mTc labeled myocardial perfusion tracers have significantly advanced the field of noninvasive diagnostic evaluation and risk stratification of patients with known or suspected coronary artery disease by providing comprehensive information about myocardial perfusion and function from a single study. Of various currently available invasive and noninvasive test modalities, myocardial perfusion imaging provides the most powerful prognostic information that is incremental to the information obtained from invasive evaluation. Future research should focus on the development of perfusion tracers that linearly track myocardial blood flow over a wide range and have minimal splanchnic uptake. Availability of an effective attenuation and scatter correction program would further eliminate some of the current limitations of this technique.

Quantitative comparison of single-isotope and dual-isotope stress-rest single-photon emission computed tomographic imaging for reversibility of defects

BACKGROUND

Dual-isotope rest/stress single-photon emission computed tomographic (SPECT) imaging is a time-saving imaging protocol. However, the stress radiotracer, technetium 99m-labeled sestamibi, and the rest radiotracer, thallium 201, have different physical properties and myocardial kinetics. In patients with abnormal resting myocardial perfusion, these differences may affect quantification of rest defect size and defect reversibility. The purpose of the study was to compare myocardial perfusion defect reversibility quantitatively by single-isotope (rest/stress sestamibi) and dual-isotope (rest thallium/stress sestamibi) SPECT.

METHODS AND RESULTS

Thirty patients with prior myocardial infarction underwent rest/stress sestamibi SPECT imaging and rest thallium SPECT imaging. Defects were quantified according to circumferential count profiles with a normal sestamibi database. The images of a subgroup of 21 patients were processed with radiotracer-specific normal databases. Defect size and defect reversibility were compared quantitatively for single-isotope and dual-isotope SPECT. Rest sestamibi defect size was significantly larger than rest thallium defect size (19 +/- 15 vs 14 +/- 16; p = 0.007). Defect reversibility was larger with thallium than with sestamibi (10 +/- 9 vs 6 +/- 6; p = 0.002). With radiotracer-specific normal databases, mean rest sestamibi and thallium defect sizes in 21 patients were not different (23 +/- 19 vs 21 +/- 17; difference not significant). With radiotracer-specific normal databases, mean defect reversibility was not different with either sestamibi or thallium (6 +/- 6 vs 8 +/- 9; difference not significant), although correlation among individual patients was only fair (r2 = 0.48).

CONCLUSION

In patients with prior myocardial infarction, stress-induced defect reversibility is quantitatively larger with dual-isotope imaging than with single-isotope imaging. Quantitative processing of dual-isotope images requires radiotracer-specific normal databases. Because of different characteristics of sestamibi and thallium, assessment of defect reversibility on dual-isotope images should be made with caution. Only relatively large defect reversibility can be assumed to represent true stress-induced myocardial ischemia.