Aortic Aneurysm
Quantification of Adipose Tissue Around the Aortic Aneurysm ― At the Dawn of a New Era ―
2021 年 85 巻 12 号 p. 2181-2183
詳細
2021 年 85 巻 12 号 p. 2181-2183
The relationship between adipose tissue and atherosclerotic disease has been heavily researched. Visceral fat tissue was originally targeted for the evaluation of this link, but the focus has shifted to epicardial and then pericardial fat.1–4 Recently, measurement and evaluation of perivascular adipose tissue (PVAT) of the coronary arteries have attracted considerable attention.5 PVAT mechanically protects vessels and generates many types of metabolic substances, such as adipokines, chemokines and hormone-like factors (Figure 1), which may influence the vasculature. Antonopoulos et al used computed tomography (CT) to measure the perivascular fat attenuation index (FAI) to demonstrate the relationship between perivascular inflammation and the FAI.6 These authors revealed a good correlation between 18F-fluoro-deoxy-glucose positron emission tomography (18F-FDG PET) uptake and the FAI, showing that measurement of the FAI appears to be a good indicator of perivascular inflammation. Recently, Kwiecinski et al demonstrated that an increased density of pericoronary adipose tissue was associated with focal 18F-NaF PET uptake. The ability of the perivascular FAI to predict the clinical outcome has been suggested by a retrospective, multicenter study with a large number of patients.7
Function and dysfunction of perivascular adipose tissue (from Chang et al3).
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The research focus regarding the measurement of PVAT has expanded from the coronary arteries to include the abdominal aorta. In 2009, Schlett et al proposed a method for measuring periaortic adipose tissue on CT images acquired with multidetector row CT (MDCT).8 In their method, the threshold was set between −195 and −45 Hounsfield units (HU) to define adipose tissue by excluding nonfat tissue around the aorta. Dias-Neto et al applied this technique to patients with abdominal aortic aneurysms (AAAs) and demonstrated higher PVAT density around the aneurysm sac than around the healthy neck; they also found that intraindividual PVAT differences presented the highest correlation with aortic volume.9 In their study, the threshold was between −195 and −30 HU.
In this issue of the Journal, Yamaguchi et al apply this technique to 77 patients with AAAs and reveal that the periaortic adipose tissue attenuation index (PAAI) independently predicted aneurysmal progression (≥5 mm annually) using a threshold between −190 and −20 HU, as well as the baseline aneurysmal diameter.10
There are 2 substantial differences in measuring the perivascular adipose tissue of the coronary arteries and of the abdominal aorta. First, the aorta is wider in diameter and longer in the craniocaudal direction than the coronary arteries. Second, the abdominal aorta has adjacent tissues, including the vertebrae and the kidneys, containing the contrast material. The latter potentially affects the CT value, which is known as the beam hardening effect, as the authors mention in the study limitations section. Beam hardening is the phenomenon that occurs when an X-ray beam composed of polychromatic energies passes through an object, and the photon with the lowest energy reduces.11 As a result, the CT values increase after passing through objects with high density, including bone. The effect on measuring the PAAI is unknown. In addition, technical factors, including the tube voltage of the scanner, may also affect the CT values.12 Currently, there are many factors that may affect quantification of the CT values, and furthermore, the threshold range defined as adipose tissue varies with each study.
The authors of this study being discussed propose 4 measurement areas: the combination of 2 locations (infrarenal and supra bifurcation with 100 mm length) and 2 circular diameters (5 mm and 10 mm). The PAAI-renal-5 mm method provided the best results compared with the other methods used in this study. This is the best way to establish the correlation between the measured PAAI and aneurysmal growth; however, optimization of the measurement method needs to be improved for use in further studies and clinical practice.
In addition, rapid improvements have recently occurred regarding CT scanner technology, including ultrahigh-resolution CT (up to 1,024 × 1,024 matrices), dual-energy CT (DECT) and artificial intelligence (AI)-based noise reduction.13 DECT, also known as spectral CT, is a new technique that uses 2 separate X-ray photons with different energy spectra, allowing the distinction of materials with different attenuation properties at different energies (Figure 2). DECT reduces the beam hardening effect, and in the cardiovascular system, it enables calcium-free angiography, iodine perfusion imaging and plaque characterization.14 In addition, fat quantification has been investigated in the liver and bone marrow.15 Such technical developments will also enhance studies related to evaluation or quantification of perivascular adipose tissue.
Example of fat quantification using dual-energy CT.
Inflammation of perivascular adipose tissue is important to deriving the pathogenic effects on the vessel wall. Currently, 18F-FDG PET has been investigated as a possible indicator of inflammation of the vessel wall; however, it should be an additional examination of the patient.16 Conversely, CT fat evaluation can be performed using routine enhanced CT images collected in clinical practice, as the authors of the current study have shown. This study is an important milestone in the implementation of quantification of perivascular adipose tissue in research and clinical practice.
