The foundation layer of quantitative cardiac PET/MRI: Attenuation correction. Again

Abstract

It is a well-known fact that attenuation correction is the prerequisite of quantification in PET. It is, however, also the (necessarily related, but even more obvious) requirement that a structure-especially one with a significant extent as the heart represents-with homogeneous tracer uptake is depicted precisely like this: homogeneous. This property is one of the confounding factors why cardiac PET is superior to cardiac SPECT even using static imaging. 1 Although the fraction of attenuation-corrected SPECT examinations is increasing, the overwhelming majority of SPECT scans is performed without this procedure. From a plain technical perspective, the necessary information is relatively simply derived from any technique, which assesses the radio-density of the imaged body. Typically, a CT scan is used for this purpose. However, as the energy of the x-ray photons is anywhere between 80 and 140 keV, only a modest extrapolation to the energy of the photons from the radioactive decay (70 to 159 keV) is needed. For PET/CT, it is more challenging: as the energy of the annihilation photons is 511 keV, this extrapolation is a non-trivial task. Fortunately, a relatively simple algorithm based on experimental data can be used with sufficient accuracy, although one should remember that this is still an extrapolation. 2 But PET/MRI further complicates the situation. An extrapolation is not possible anymore as the MRI signal per se does not contain information about the radio-density for photons of any energy whatsoever. Consequently, attenuation correction was considered-and is still in some settings-an obstacle, which can be exacerbated if we consider scatter as it relies on an attenuation map. When we started the development of an algorithm which is now implemented on the vast majority of all commercially available PET/MRI systems, 3 an important element was our knowledge of how attenuation maps from the days of standalone PET scanners looked like ( Figure 1A). The use of rotating Germanium rod sources was timeconsuming and produced-depending on the age of the sources-images of a quality which required a substantial amount of post-processing to reveal the basic structures of the body-typically answering the question: 4 body or air. Still, data from those scanners provided the solid platform of PET's value in research and clinical use in cardiology, neurology, and oncology. A value which was confirmed by PET/CT systems-but never challenged-although the patient throughput increased almost an order of magnitude due to the rapid CT scan. What happened, however, was a change in visual perception ( Figure 1C). As CT images show so much detail especially in high-contrast objects such as bones (and only a few people ever looked at conventional transmission images which never showed bones unless scanning ex vivo specimen for hours, (Figure 1A), there was the anticipation that an MR-based attenuation map should perfectly resemble a CT scan. Fortunately, this is not the case-at least in the thorax, which is of interest here (the hea...

Hybrid PET- and MR-driven attenuation correction for enhanced 18F-NaF and 18F-FDG quantification in cardiovascular PET/MR imaging

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Impact of improved attenuation correction on 18F-FDG PET/MR hybrid imaging of the heart

“…Technical challenges remain applying PET/MR to the cardiovascular system [8] [9] [10] [11] [12]. Attenuation correction (AC) of PET data is an essential step in obtaining accurate and quantitative PET images [13] [14].…”