Signal density of left ventricular myocardial segments and impact of beam hardening artifact: implications for myocardial perfusion assessment by multidetector CT coronary angiography

Abstract

Since myocardial perfusion by MDCT is based on the myocardial signal density (SD), it is pivotal to determine the normal values of myocardial SD and to identify potential mechanisms of misinterpretation of perfusion defects. In routine MDCT acquisitions, we commonly visualize a considerable SD drop at the posterobasal wall resembling perfusion defects, being attributed to beam hardening artifacts. Consecutive asymptomatic patients without history of coronary artery disease (CAD) and low probability of CAD who were referred for MDCT evaluation at our institution due to inconclusive or discordant functional tests constituted the study population. Perfusion defects were defined as a myocardial segment having a SD two standard deviations below the average myocardial SD for the 16 left ventricular American Heart Association (AHA) segments. Thirty six asymptomatic patients constituted the study population. Myocardial SD was evaluated in 576 American Heart Association (AHA) segments and 36 posterobasal segments. The mean myocardial SD at the posterobasal segment was 53.5 +/- 35.1 HU, whereas the mean myocardial SD at the basal, mid and apical myocardium was 97.4 +/- 17.3, with significant differences (p < 0.001) between posterobasal and all AHA segments. Posterobasal "perfusion defects" were identified in 26 (72%) patients. The only variable associated to the presence of posterobasal SD deficit was the heart rate (61.8 +/- 6.2 bpm vs. 56.3 +/- 8.1 bpm, p = 0.04), whereas body mass index, blood SD of the left and right ventricles, contrast-to-noise ratio, and the extent of atherosclerosis were not related to the presence of "perfusion defects". In an asymptomatic population with no history of coronary artery disease, a myocardial signal density deficit mimicking a perfusion defect is a common finding in the posterobasal wall and is not related to body mass index or scan quality.

Beam-Hardening Correction for Virtual Monochromatic Imaging of Myocardial Perfusion via Fast-Switching Dual-kVp 64-Slice Computed Tomography

“…11 The myocardial CT value of the area affected by BH (49.9 HU) was comparable to the results reported by Rodriguez-Granillo et al (54 HU). 4 These CT values were also similar to those for the non-perfused area in our study (52-53 HU), indicating that the CT value affected by BH mimics a perfusion defect.…”

“…9 Both the 120-kVp and dual-kVp image data sets were reconstructed into a 5.0-mm thickness with a 20-cm field-of-view on a dedicated workstation (Advantage Workstation VolumeShare 4; GE Healthcare), according to a previously described method. 4 The dual-kVp data were reconstructed into a VMI at 69 keV, which is comparable to 120-kVp images in terms of the CT value. 10 The CT value and image noise at the anterior, posterobasal and inferior walls of the LV were measured 5 times using a 5.0-mm 2 region-of-interest.…”

“…2, 3 In conventional single-kVp CT images, however, a considerable decrease in the myocardial CT value is often seen at the posterobasal wall, mimicking perfusion defects, and such decreases have been attributed to beam-hardening (BH) artifacts caused by high iodine concentrations in the left ventricular (LV) chamber and descending aorta. 4 Therefore, a rigorous BH correction algorithm for coronary CT imaging is essential for more accurate myocardial perfusion assessments. 5, 6 Virtual monochromatic images (VMI) generated using dualkVp projection-based material decomposition technology are a known solution to reducing BH artifacts in imaging of the head.…”

CT-Based Myocardial Perfusion Imaging-Practical Considerations: Acquisition, Image Analysis, Interpretation, and Challenges

“…This can occur in the context of bone or contrast-enhanced LV cavity or aorta, all of which can cause the appearance of a perfusion defect due to the resulting hypoattenuation. The areas most likely to be affected by this particular artifact are the basal inferior wall [51] that is affected by the contrastfilled descending aorta and LV cavity and less frequently, the anterior wall, which may be affected by the overlying ribs as well as the contrast-filled LV cavity. Successful resolution of this important issue has been effectively achieved by algorithms that specifically correct for myocardial beam hardening [31].…”

Quantitative and qualitative analysis and interpretation of CT perfusion imaging

“…41 Beam-hardening correction algorithms specific to the myocardium have been shown to improve the measurement of myocardial perfusion and will be essential for the accurate assessment of myocardial perfusion. 44,45 In addition, different reconstruction kernels can alter the accurate reflection of iodine concentrations in the myocardium. One aspect, in particular, is edge enhancement.…”