Visualizing regional myocardial oxygenation changes with statistically optimal colormaps
© Tsaftaris et al; licensee BioMed Central Ltd. 2009
Published: 28 January 2009
Blood-oxygen-level dependent (BOLD) MRI may be used for detecting myocardial oxygenation (MO) changes secondary to coronary artery stenosis. Under pharmacological stress, areas in the myocardium supplied by a stenotic coronary artery are hypointense relative to healthy regions. Visualizing these changes requires manual windowing. In this paper a method for automatic visualization of myocardial signal changes reflecting the regional variations in oxygenation is presented, using images obtained from a canine study under controlled conditions. The objective of this study is to overcome the rather subjective step of windowing by establishing an optimal colormap that permits visualization of statistical changes in signal intensities between healthy and pathological cases.
To facilitate the evaluation of myocardial BOLD images by automating the detection of regional abnormalities in MO under pharmacological stress in the presence of coronary artery stenosis.
Breath-held and ECG-gated short-axis cardiac phase-resolved 2D SSFP-based myocardial BOLD images were acquired in two dogs under pharmacological stress with and without left-circumflex coronary artery stenosis in a Siemens 1.5 T scanner. Scan parameters: voxel size = 1.2 × 1.2 × 6 mm3; flip angle = 90°; TR/TE = 5.7/2.9 ms; 20 cardiac phases.
Image processing algorithms
In order to have the greatest myocardial surface available for analysis, only the end systolic images of the two image series (adenosine without stenosis (AD) and adenosine with severe stenosis (SS)) were identified. A rectangular region of interest was chosen around the heart and a segmentation algorithm was utilized to isolate and segment the myocardium. The myocardial AD intensities (ADv) were collected and a student-T distribution was fitted by maximum likelihood estimation of its parameters (mean (m), variance (s), and degrees-of-freedom). An optimal pixel colormap was created by assigning red hues to signal values below the threshold m-s and yellow hues to larger values. The color-coded myocardial segments for both images were overlaid over the grayscale original images. All operations were performed in MATLAB.
Discussion and conclusion
As Figure 1 demonstrates, without proper windowing the appreciation of MO differences is cumbersome. It takes several minutes for a reader to window the images. Moreover, windowing is subjective and has large intra- and inter-observer variability. Figure 2 illustrates that student-T distribution closely approximates the intensity distribution of the myocardium. The motivation for choosing the m-s as the midpoint for the colormap is to highlight intensities below this value, which are expected to have low probability of occurrence in a normal healthy case. It is expected that regions supplied by stenotic arteries will be hypointense, and will thus shift the intensity distribution towards the left. Hence, in cases where stenosis is present, the occurrences of intensities lower than m-s are greater, and therefore the amount of red-colored pixels are greater. Note that the contiguous red-colored region corresponds to the left-circumflex territory (Fig. 3). There are also mild signal changes in the AD images as well, that may be due to imaging artifacts, normal physiology changes, and/or presence of an inadvertent stenosis. Nevertheless, the presented method allows the observer to rapidly assess the presence of disease, as well as, potentially identify the section of the coronary tree that is stenotic on the basis of BOLD MRI. The proposed method forms an initial step in the development of improved visualization capabilities for myocardial BOLD MRI. The method could be fully automated method if combined with end-systolic image identification and left ventricle localization algorithms.
This article is published under license to BioMed Central Ltd.