Imaging of blood flow through the heart and associated veins can improve diagnosis and treatment of cardiac diseases. In particular, estimation of myocardial blood flow or blood flow through heart muscular tissue can be useful as described below.
By one approach, nuclear based medicine can be used to produce useful medical images. In such an approach, radioactive elements are introduced into the bloodstream such that when the radioactive elements experience a radioactive decay, the byproducts of that decay (often the reaction with particles called positrons) can be sensed to produce an image of the area where the radioactive elements are placed. An example approach to this kind of imaging is called positron emission tomography (PET). Several radioactive elements, called positron emitting tracers, are available for these studies with the most common being 82Rb and 13N-Ammonia.
Currently, PET is a primary method for determining non invasive coronary flow reserve. Coronary flow reserve can be defined as a ratio of a maximum hyperemic flow to baseline flow. In normal patients this ratio can typically range between 3-5, which is a essentially a measure of the function of coronary circulation and is particularly useful in the detection of early abnormalities due to coronary artery disease. Because the coronary flow reserve determination is a ratio, it is unaffected by a uniform reduction in both baseline and maximal flow.
Unfortunately, coronary flow reserve does not reflect true vasodilation capacity. A reduction in coronary flow reserve could be caused either by increased flow in the baseline state or by reduced maximum hyperemic flow. Factors that increase myocardial oxygen demand, for example hypertension, increased left ventricular wall stress, increases in inotropic state, and tachycardia, can lead to an increased basal flow. Differentiating between this case and the reduced maximal hyperemic flow due to significant coronary stenosis is difficult without absolute myocardial blood flow measurements. Measurements of hyperemic blood flow in absolute units provide a more direct estimate of vasodilation capacity. Accordingly, only by accurate determination of absolute myocardial blood flow can the existence of uniform diffuse disease be determined.
Since the early 1990's there have been validated techniques for estimating absolute myocardial blood flow. Nevertheless, absolute myocardail flow estimation has not been adopted for routine use in a clinic setting because of technical limitations. These limitations can include lack of technical expertise in a clinical setting, time taken to perform the calculations, and the lack of widely available commercial products to perform the calculations and display the results. On the other hand, numerous reports indicate the effect on absolute myocardial blood flow of various interventions or conditions. Yet, calculating absolute blood flow for clinical studies remains rare. The result is that diagnostic decisions are usually based on relative myocardial blood flow or relative changes in myocardial blood flow between rest and stress, often aided by a software tool that compares images to a normal database.
There are at least three different kinetic models that have been used to understand the distribution over time of flow tracers in myocardial tissue. These works include spillover correction because of a finite resolution of the scanner and because the myocardium is moving during the scan. In one known approach, factor analysis was used to obtain spillover independent time activity curves of the right ventricle (RV) and left ventricle (LV) and myocardial blood tissue. By using curves generated from factor analysis, the spillover component in the model can be eliminated in theory; however, factor analysis does not correct for the under measurement due to the partial volume effect. Correction for this would require the use of a contrast recovery coefficient. Methods for addressing the non-uniqueness problem of kinetic modeling have been proposed. Also, kinetic modeling directly from sinograms from a dynamic sequence has been suggested.
In the following, let aij denote the activity in voxel i of frame j. In factor analysis, it is assumed that the activity is a linear combination of K primary factor curves, where the summation coefficients are
                              a          ij                =                              ∑                          k              =              1                        K                    ⁢                                          ⁢                                    c              ik                        ⁢                                          f                kj                            .                                                          (        1        )            
The primary factor curves for this application are the right ventricular blood pool, the left ventricular blood pool, and the myocardial tissue curve. The mathematical task is to find both the factors and coefficients so that the linear combination of factor curves for every pixel in the image matches the measured curve as close as possible. This problem is constrained by requiring that the tissue curves and the linear coefficients are all positive.
Ammonia or Rubidium uptake is generally analyzed with a two or three compartment model. The models all have a blood compartment in contact with an extracellular free distribution compartment, which is in turn in contact with a metabolically trapped compartment. These models are much easier to calculate if it is assumed that the clearance from the metabolically trapped compartment is zero (or near zero) over the duration of the experiment. As a result, for accurate myocardial blood flow modeling, several authors recommend collecting and analyzing only two minutes of data. When using smooth data generated by averaging all pixels within a large range of interest, this is a reasonable approach.
While there have been significant advances in the art, further advances are possible. For example, it is desirable to have a myocardial blood flow analysis with greater acuracy than is presently known in the art.