Thick and thin: Bridging the gap to a better understanding of apical thinning
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On the other hand, for decades, post-mortem studies have established the remarkable geometry of the LV apex.7 In a gross and histologic series of 60 hearts free of local disease at necropsy, Bradfield et al. found that the mean wall thickness of the LV apex at its thinnest point was 1.3 ± 0.7 mm. In fact, the apical thin point measured ≤ 2 mm in 97% and ≤ 1 mm in 67% of the hearts in this series. This anatomic finding has been confirmed in the modern non-invasive imaging era with multidetector CT.8,9 Ferencik et al. found that the mean thickness of the LV apex at its thinnest point was 2.3 ± 1.2 mm from the axial plane or 1.7 ± 0.7 mm if measured from the long-axis plane. A thickness of ≤ 3 mm was observed in 77% and 100% of the hearts from the axial and long-axis plane, respectively. These are measurements that, independent of the study, are below the spatial resolution of the reconstructed axial image of PET (3.9-5.8 mm) and SPECT (~ 10 mm at 10 cm) systems, and thus would be subject to varying degrees of partial volume averaging, an inherent limitation to cross-sectional imaging in general but particularly relevant to MPI.10
To date, this effect had not been studied in the same patient population across imaging modalities. In the present issue,11 Steffen et al report the important results of a single-center retrospective analysis of 57 patients without coronary artery disease who underwent MPI with 13N-Ammonia PET as well as prospectively gated contrast-enhanced coronary CT angiography (CTA). The authors compared the base-to-apex changes in myocardial wall thickness derived from CTA with a number of PET markers of myocardial perfusion, including normalized percent tracer uptake (summed, end-diastolic, end-systolic), MBF, and k2 washout rate. They noticed that 93% of patients had evidence of reduced relative apical tracer uptake (on static/non-gated images) and were considered, thus, as having apical thinning. Interestingly, while they also found a clear base-to-apex gradient in wall thickness and in most PET parameters (except for k2 washout rate), they failed to observe a significant correlation between apical myocardial thickness and perfusion at the apical segments, hence, suggesting that partial volume average may not the be sole cause of apical thinning on MPI. The study is important from a mechanistic perspective, as it provides new insights into this common imaging phenomenon. However, one must be careful with the interpretation of these results. The fact that no correlation was observed between wall thickness and perfusion (limited to the apical segments) is not a sufficient proof to disregard partial volume as the driving force explaining apical thinning, especially since the range of variation of the analyses within the boundaries of the apical segments may not have been sufficient to identify a correlation coefficient. On the contrary, the study offers enough evidence to further support the critical role of partial volume in contributing to the development of apical thinning as suggested by the following findings: (1) there was a significant gradient in wall thickness from base (9.0 cm ± 1.6), apex (4.6 cm ± 0.7), and to the thinnest point of the apex (2.3 cm ± 0.8); (2) this gradient was also evidenced in all of the PET parameters known to be affected by partial volume effect including normalized percent tracer uptake and retention-based quantitative MBF; (3) apical tracer uptake improved significantly at end-systole (when wall thickening is the greatest); and (4) myocardial k2 washout, a quantitative PET parameter that is less likely to be affected by partial volume effect,12 failed to show a base-to-apex gradient.
Given the potential diagnostic challenges imposed on nuclear cardiologists by the high prevalence of apical thinning during myocardial perfusion imaging, future studies should focus on the investigation and development of more accurate methods for partial volume correction on MPI SPECT and PET systems. This will translate into better image interpretability and thus improved diagnostic performance.
The authors have no conflict of interest to disclose.
- 11.Steffen DA, Giannopoulos AA, Grossmann M, Messerli M, Schwyzer M, Grani C, et al. Apical thinning: Relations between myocardial wall thickness and apical left ventricular tracer uptake as assessed with positron emission tomography myocardial perfusion imaging. J Nucl Cardiol 2018. https://doi.org/10.1007/s12350-018-1397-2.CrossRefPubMedGoogle Scholar