Technical Corner
Adjustment of acquisition arc in cardiac malposition during myocardial perfusion SPECT imaging: computer simulation based on deterministic modeling

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Abstract

Objectives

To simulate cardiac malpositions, leftward and rightward shift and dextrocardia, and also to compare distribution of activity of septal and lateral walls of left ventricle acquired in standard acquisition arc and after relevant adjustment.

Methods

In this study, digital phantoms with cardiac malpositions are designed and procedure of acquisition of scan in standard arc (from right anterior oblique to left posterior oblique) and adjusted acquisition arc is simulated. The three situations of malposition including leftward and rightward shift and dextrocardia are considered. For all types, acquisition is conducted in standard and then adjusted arcs (from anterior to posterior and also from right to left for leftward and rightward shifts, respectively, and for dextrocardia, from left anterior oblique to right posterior oblique). All obtained projections are reconstructed using the algorithm of filtered back projection. During forward projection to obtain sinograms, radiation attenuation is also modeled by incorporation of a simplified transmission map to emission map. The resulting tomographic slices of the LV (septum, apex, and lateral wall) are presented visually and are compared by plotting intensity profiles of the walls. Finally, normalized error images are also computed. All the computations are performed in MATLAB software package.

Results

In transverse slice, septum and lateral wall are attenuated progressively from apex, which is closer to the camera, to the base in similar fashion. In tomographic slices of standard acquisition arc, the septum shows remarkably higher activity compared to lateral wall. However, after adjustment, both seems equally intense and progressively being attenuated from apex to base, similar to that found in phantom with normally positioned heart. Likewise, for the phantom with rightward shift, when the scanning was done in standard arc, the septum is more intense than the lateral wall. And similarly, adjustment of the arc renders both walls equally intense. In dextrocardia, level of attenuation of basal parts of septum and lateral wall is higher in 360° arc compared to adjusted 180° arc.

Conclusion

Adjustment of acquisition arc exerts perceptible changes in distribution of activity over LV walls which are more compatible with normally positioned heart.

Introduction

Myocardial perfusion SPECT imaging plays a key role in the diagnosis of ischemia in patients with known or suspected coronary arterial disease. The basis of the interpretation of the scan is visual comparison of the distribution of activity in the corresponding walls from stress to rest phases. Any changes in the respective intensity of uptake as visualized by different hues of color or shades of gray are regarded as either true or artefactual perfusion defect. Apart from the commonly occurred attenuation effect of soft tissue components around the heart, noise, and extra-cardiac activity, the relative distribution or uptake map of the left ventricular walls is influenced by other physical factors, including heart-to-camera distance. Therefore, dominance of anterior to inferior and lateral to septal walls is a common pattern in non-attenuation-corrected SPECT images. Sometimes it causes confusing dilemma in distinguishing true defects from artifacts.1,2

Moreover, another item that exerts an important effect but has been less investigated is the position and orientation of the heart in the thorax. For most patient with normally positioned hearts, the left ventricle is positioned in the left hemithorax oriented to the left anterior aspect of the chest. In such situations, for image acquisition, the camera is set to rotate around the patient’s chest spanning a 180-degree arc from right anterior oblique (RAO) to left posterior oblique (LPO) aspects. In patients with dextrocardia, it is well known that modifying acquisition arc from left posterior oblique to right anterior oblique prevents occurrence of perfusion defects of lateral and septal walls.3, 4, 5 However, similar appropriate adjustment of the acquisition arc in patients with other cardiac malposition, for example, shift to the right or shift to the left of any causes either as congenital anomaly or an acquired form, is yet to be investigated.

In this study, it is aimed to simulate such situations in three major forms of cardiac malposition, including leftward and rightward shift and dextrocardia. And also to compare the distribution of activity of the septal and lateral walls acquired in standard (from RAO to LAO) and adjusted (from anterior to posterior in leftward shift and from right to left sides of chest in rightward shift of the heart and from LAO to RPO in dextrocardia) arcs.

Section snippets

Methods and materials

  • 1

    A patient study As a case example, in a patient who was referred for a stress rest myocardial perfusion SPECT, the imaging procedure was conducted based on standard routines. More or less 45 minutes after stress test with dipyridamole, the post stress imaging was acquired by the parameters as followed: 180° arc spanning from RAO to LPO, 32 projections, 30 seconds per projection, 64 × 64 matrix, and circular orbit with a radius of 25 cm. Upon viewing cinematic images as also transverse slices

Results

As is demonstrated in Figures 7 and 8, using FBP, tomographic slices are reconstructed from sinograms. Each pair demonstrates a sinogram with the resulting tomographic slice. In Figure 7, the upper row belongs to the phantom with normally positioned heart and its related sinogram (A). In the transverse slice, in which apex, septum, and lateral wall are visualized, the septum and lateral walls are attenuated progressively from apex, which is closer to the camera, to the base in similar fashion.

Discussion

Proper image acquisition effectively guarantees the final image quality. It is generally recommended that the parameters involved in image acquisition be used consistently by the practitioners according to standardized guidelines.1,2 However, occasionally, some modifications, either minor or major, may be required based on the specification of the patient under scanning. The span of arc, 180° or 360°, has been widely investigated.11,12 So, in standard guidelines, the 180-degree scanning is

Conclusion

Adjustment of acquisition arc during myocardial perfusion SPECT imaging exerts perceptible changes in the pattern of distribution of activity over LV myocardial walls. More predictability of the relative intensity of myocardial walls achieved by such adjustments leads to less confusion in the interpretation of perfusion defects.

New knowledge gained

Cardiac malpositions, including leftward and rightward shift and also dextrocardia, exert significant impacts on the relative distribution of activity across myocardial walls on myocardial perfusion SPECT when scanning in standard acquisition arc from RAO to LPO. In these circumstances, according to the position and orientation of the left ventricle, some modifications in acquisition arc are necessary. Although well known in dextrocardia, in which the arc is set to 180- (from LAO to RPO) or

Disclosures

The author, Mohsen Qutbi, has no conflict of interest relevant to this work to disclose.

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