Comparison of 1- and 2-day protocols for myocardial SPECT: a Monte Carlo study

Nuclear cardiology for a cardiothoracic surgeon

Cardiac surgeons are commonly faced with issues regarding the balance between the potential risk and the potential benefit of a surgical procedure. Nuclear cardiology procedures such as single-photon emission computed tomography and positron emission tomography provide the surgeon with objective information that augments standard clinical and angiographic assessments related to the diagnosis, prognosis, and potential benefit from any intervention. Myocardial perfusion is imaged with the use of radiopharmaceuticals that accumulate rapidly in the myocardium in proportion to the myocardial blood flow. Radionuclide lung imaging most commonly involves the demonstration of pulmonary perfusion using technetium-99 m macro aggregate albumin (Tc-99 m MAA), as well as the assessment of ventilation using inspired inert gas, usually xenon, or Tc-99 m-labelled aerosols. Nuclear cardiology is extensively used as a part of the work-up of ischemic heart disease and cardiac failure in deciding the optimal therapeutic strategy with its ability to predict the severity of the disease. It has also proved extremely useful in the management of congenital heart disease and the diagnosis of pulmonary embolism, among many other applications. Myocardial perfusion imaging is a basic adjunct to the noninvasive assessment of patients with stable angina, baseline electrocardiogram (ECG) abnormalities, post-revascularisation assessment, and heart failure. This review article covers a summary of basic concepts of nuclear cardiology about what a cardiac surgeon should be aware of. To many, it is just a perfusion test, but the versatility, reliability, and future of the technology are without a doubt.

Subendocardial versus transmural ischaemia in myocardial perfusion SPECT--a Monte Carlo study

Myocardial perfusion imaging with single-photon emission computed tomography (SPECT) is useful for the evaluation of patients with known or suspected coronary artery disease. Parameters of interest are the reduction in the blood perfusion (severity) and the lesion volume (extent). The aim of this study was to evaluate these parameters, as calculated by automatic quantification software, for different cases of subendocardial and transmural myocardial lesions.

METHODS

A computer phantom was used to simulate 32 male patients with different defect locations and activity uptakes, which were based on clinical patient studies. The Monte Carlo program SIMIND was used to simulate realistic SPECT projections which were reconstructed to give short-axis images, analysed by the AutoQUANT program using the same procedure as for a real patient.

RESULTS

The results showed a disparity between the quantification of transmural and subendocardial lesions with the same lesion activity uptake reduction and this could be confirmed by visual interpretation. Neither the parameters given by the quantification program nor visual interpretation could distinguish between the transmural lesions and the subendocardial lesions with activity uptake reduction twice as high as in the corresponding transmural lesions.

CONCLUSION

Transmural lesions and the corresponding subendocardial lesions with the same activity uptake reduction could be separated by the quantification software for SPECT imaging and visual analysis. The subendocardial lesions with activity uptake reduction twice as high as in the corresponding transmural lesions could not be differentiated neither by the quantification software nor by visual interpretation. Thus these lesions will get the same scoring when analysed by the AutoQUANT program.

A model that accounts for the interdependence of extent and severity in the automatic evaluation of myocardial defects

BACKGROUND

The extent and severity are two important parameters when describing a regional defect in myocardial single-photon emission computed tomography (SPECT) perfusion imaging. Intuitively, these two parameters should be independent of each other, but we have shown in a previous study that there is an interdependence. This interdependence has been investigated in two commercially available analysis programs (AutoQUANT and 4D-MSPECT) using Monte Carlo-simulated SPECT data.

METHODS

An anthropomorphic digital computer phantom and a Monte Carlo program were used to generate SPECT data. Several defects of different volumes and lesion activity uptake reductions were simulated and evaluated. Comparison of the measures of extent and severity with their corresponding known values led to the development of a correction model based on least-squares parameter estimation. This model was then applied to a test group consisting of 10 different computer-simulated patients.

RESULTS

Our results showed that the interdependence was reduced significantly for most of our test cases after applying the correction method. The application of the correction model to the test group demonstrated that the model was robust with respect to different patient geometries. A further test with projections that simulated a perfect SPECT system revealed that the interdependence between the extent and severity was not caused by the limited spatial resolution of the SPECT system, but rather the inherent design of the algorithms of the analysis programs.

CONCLUSIONS

A model has been developed to take into account the interdependence of the extent and severity.

Interdependence between measures of extent and severity of myocardial perfusion defects provided by automatic quantification programs

AIM

To evaluate the accuracy of the values of lesion extent and severity provided by the two automatic quantification programs AutoQUANT and 4D-MSPECT using myocardial perfusion images generated by Monte Carlo simulation of a digital phantom. The combination between a realistic computer phantom and an accurate scintillation camera simulation tool allows the generation of realistic single-photon emission computed tomography (SPECT) images similar to those obtained in clinical patient studies.

METHODS

The NCAT phantom and the SIMIND Monte Carlo program were used to simulate myocardial perfusion studies. Perfusion defects with sizes ranging from 5 to 17% of the left ventricular wall volume and reductions in tracer uptake of 20, 60 and 100% were simulated in three vascular territories.

RESULTS

The values of the extent provided by the programs were dependent on the reduction in tracer uptake, i.e. the severity. Similarly, the measures of severity were dependent on the size of the lesions. The severity provided by AutoQUANT for different defects was not dependent on the location, whereas 4D-MSPECT presented different values depending on the location in the left ventricle. The measures of extent and severity of the defects with the same true extent and activity uptake reduction provided by the two programs were different.

CONCLUSIONS

The NCAT phantom and the SIMIND Monte Carlo program were shown to be useful in simulating clinical myocardial SPECT studies. The quantification programs gave values of lesion extent that were dependent on the magnitude of the severity. Users should therefore consider this dependence when interpreting results from these programs.