Aortic dilatation is a widely accepted risk factor for AD and rupture. Natural complications are rare in moderate size. According to retrospective studies, annual risk of dissection and rupture is 0.08% at diameters of 45 mm, 0.22% of 50 mm, 0.58% of 55 mm in ascending AA, and there is a substantial increase when the diameter exceeds 60 mm to 6.9% yearly [10]. In descending AA, a substantial increase occurs at diameters over 70 mm [11]. For the purpose of preventing aneurysm expansion beyond the critical point, the guidelines recommend prophylatic surgical treatments at 55 mm and 55 to 60 mm for ascending and descending AA respectively [5, 12]. However, patients with connective tissue disease have a significantly high risk of a poor prognosis. For example, patients with Marfan syndrome need lower surgical threshold; preventive surgery is indicated $\ge$50 mm, or $\ge$45 mm when accompanying risk factors, such as AA growth $>$3 mm/year, massive aortic valvular regurgitation or family history of AD [5, 6]. Meanwhile, the bicuspid aortic valve (BAV) is also considered a risk factor to justify surgical intervention at diameters $\ge$50 mm [13].

Despite the evidence supporting the relationship between aortic dilatation and unfavorable outcomes, a large proportion of catastrophic acute aortic events occur below the criteria for surgical intervention. Retrospective studies have demonstrated that nearly 70% of patients with type A dissection and 80% of patients with type B dissection have aortic diameters $\ge$55 mm when AD occurs [14, 15]. However, this reflects the late stage of pathology, in which the aortic wall is already severely damaged [16]. Aortic diameter is an insufficient parameter for risk stratification and predicting catastrophic events, in spite of the diameter could predict dissection and rupture at a demography level.

The length of human aorta has been proved increase with age, like aortic diameters (Fig. 1A, Ref. [17, 18]) [17, 19]. Compared with the youth, the length of ascending aorta increases nearly twofold at 80 years of age, which is a great change in length compared with diameter, increasing approximately 12% for length and 3% for diameter per decade [20]. Several lines of evidence suggest that the pressure on the vessel wall increases over ten-fold, as the shape of the ascending aortic changes form straight to curved with normal aortic diameter, blood pressure and cardiac output [21].

Fig. 1.

Aorta anatomic parameters of AA. (A) 3D modelled CT images of the aorta of a female aged 24 years (a) and a female aged 85 years (b). Reproduced with permission from BMJ Publishing Group Ltd. [17]. (B) 3D wall stress distributions of the two abdominal aortic aneurysm models: (a) unaltered model, (b) no-calcification model. Reproduced with permission from Elsevier [18].

Studies have shown a significant increase in volume, while the diameter remains stable in AAs patients [22, 23]. Meanwhile, volume in patients who had surgical treatment are significantly different from that in patients who did not have surgery [24], and this observation could be used in the long-term follow-up of AAs [25]. Although volume measurement can be obtained by modern imaging [26], the value isuncertain and needs further validation.

Calcification is the main characteristic of atherosclerotic cardiovascular disease, and intimal calcification of atherosclerotic plaques strongly correlates with a high risk of cardiovascular events [27]. Fragmentation of intimal calcifications on multi-detector CT is often considered one of the symptoms of unstable AAs (Fig. 1B) [28]. The existing calcification decreases the biomechanical stability of AAs and augments the peak wall shear stress (WSS) of the aorta [18]. For AD, intimal tears are frequently located near or exactly on calcifications in the aorta [29]. However, aortic calcification is not considered as an independent risk factor according to current studies and statistics [30]. Thus, further studies need to be developed on aortic anatomic parameters as predictors and independent risk factors for AAs.

In recent years, hemodynamic assessments have attracted great attention owing to the progress of imaging technology, such as time-resolved three-dimensional phase contrast cardiovascular magnetic resonance (4D-flow CMR). According to the guidelines, CMR and echocardiography are recommended to regularly monitor the aorta [6, 31]. Several studies of 4D-flow CMR have proven the value of this technique as a potential tool to evaluate the WSS of aortic hemodynamics with 3D flow patterns [32, 33]. In addition, certain regions of the aorta with increased WSS accompany the stiffness of the aortic wall and accelerate AAs growth [34]. WSS also influences endothelial cell function and triggers pathways that promote vessel wall remodeling and aortic dilatation [35].

Several studies in patients with BAV have shown that aberrant valve opening can lead to disorganized outflow patterns, which are related to aortic morphology and result in markedly altered regional WSS. In particular, the WSS patterns were significantly different in BAV patients compared with individuals with tricuspid aortic valve (TAV). Outflow patterns vary depending on the types of BAV [36, 37]. TAA formation is one of the frequent complications in BAV patients [38]. Nearly 80% of BAV patients exhibit aortic dilatation with a high risk of AD and rupture [38, 39]. In addition, children and young adults with Marfan syndrome had abnormal WSS, and altered aortic flow patterns are mostly located in the proximal aorta, where the segments are at high risk of aortic dilatation and complications. The two parameters correlated with regional size could be potential markers of risk assessment [40].

In patients with AAs, immunohistochemical analysis and enzyme-linked immunosorbent assay have revealed an inflammatory activity and infiltration in the aneurysm vessel wall [7, 41]. In addition, a large number of CD3+ and CD68+ cells was found in the medial layer of thoracic AA [42]. It has also been demonstrated in positron emission tomography (PET)/computed tomography (CT) that patients who have a progressive course or clinical symptoms of AD exhibiting higher inflammatory cell activity in the aorta than patients who are clinically stable or asymptomatic [43]. Recent data demonstrate that inflammation contributes greatly to aortic wall remodeling, even in the absence of genetic diseases [41, 44]. Basic research and clinical studies have shown that inflammatory cells are associated with medial layer degeneration. These cells are also detected in the wall of the vasa vasorum in the adventitia and at the edges of the ruptured media of dissection [42].

Macrophages are one type of the inflammatory cells that initially infiltrate the aorta. This suggests that macrophages play a major role in ECM degradation processes in the aortic wall of AAs patients regardless of their genetic predisposition [45, 46]. Macrophages and their products, such as collagenases, elastase and cytokines, facilitate inflammatory cell recruitment, increase cytokine stimulation and protease production, and promote neovascularization and lymphocyte differentiation [45]. Recent studies have demonstrated that two types of macrophages, M1 and M2, have distinct functions. M1 macrophages sustaining are accumulate at the site of arterial injury as highly proinflammatory macrophage subse. M2 macrophages found in human AAs samples are mostly associated with inflammation resolution and promote the dissection healing process [47, 48].

Lymphocytes are another inflammatory cell type associated with the inflammatory mechanism of AAs. In lymphocytes, T helper 1 lymphocytes are predominantly related to plaque formation with a proliferative pattern [49]. Meanwhile, T helper 2 lymphocytes, which make up a majority of lymphocytes in aortic lesions, are associated with atherosclerotic development of AAs [50], and secrete interleukins [51]. Recent research suggests that infiltration of T lymphocytes, especially T helper 2 lymphocytes, contribute to modulating the immune response and VSMC apoptosis by activating death-promoting pathways [51].

Recent studies have shown that surface receptor integrin might be a new biomarker in AAs. The integrin CD11b/CD18, also known as macrophage-1 Ag (Mac-1), plays an important role in immune-inflammatory responses as pathogen-associated molecular patterns and damage-associated molecular patterns recognition receptor [52, 53]. Mac-1, which has been reported as a biomarker of inflammatory cells in infarcted myocardium and atherosclerotic plaques [54], predominantly induced cellular immune responses in macrophages [55].

Inflammatory responses accelerate metabolic processes by increasing the consumption of glucose. PET imaging with a radioactive tracer, 18-fluoro-2-deoxyglucose (FDG) can detect the high expression area, which has been shown to be correlated with the presence of macrophages and related to the risk of AD progression (Fig. 2A, Ref. [43, 56, 57, 58, 59]) [43, 60]. A study with PET/CT identified that translocator protein (TSPO) as a diagnostic technique in cardiovascular disease [61]. Also, TSPO expressions can be utilized to assess populations of macrophage during pathological progression [62]. Mac-1 is also a specific receptor of superparamagnetic iron oxide nanoparticles (SPIONs), which accumulate into macrophages by endocytosis and can be detected by MRI [63]. In another study, Mac-1 was used as the central biomarker in a mouse model of atherosclerosis, and a nuclear imaging probe, ${}^{99m}$Tc-MAG3-anti-CD11b, was developed to assess inflammatory status with single photon emission computed tomography/computed tomography (SPECT/CT). Histological anatomy and section staining verified the formation of atherosclerotic plaques and Mac-1 high expression in the areas displayed by SPECT/CT (Fig. 2B) [56]. In our experiment, anti-CD11b-TCO/Tz-PEG11-HYNIC-99mTc, a pre-targeting imaging molecular probe has been established and which allowed the SPECT/CT image detection of CD11b infiltration in progressive AA [64] (Table 1, Ref. [43, 56, 57, 58, 59, 60, 61, 63, 65, 66]).

Fig. 2.

Application of different molecular targets in different imaging techniques. (A) The FDG-uptake in the dissected aortic wall shown in CT, PET, PET/CT and 2 months, 2 years, 3 years follow-up with CT. Reproduced with permission from BMJ Publishing Group Ltd. [43]. (B) ${}^{99m}$Tc-MAG3-anti-CD11b SPECT/CT images with pathological and immunohistochemical confirmation. Reproduced under CC-BY 4.0 license from Springer Nature [56]. (C) (a,b) ${}^{99m}$Tc-RYM1 imaging of carotid aneurysm, carotid arteries ex vivo photography (a) and autoradiography (b) without (left) and with (right) pre-injection of excess of MMP inhibitor RYM; (c,d) ${}^{99m}$Tc-RYM1 SPECT/CT images of abdominal aortic aneurysm animals model, (c) low remodeling group, (d) aneurysm group, Arrows: areas of maximal tracer uptake in aorta. This research was originally published in JNM. Toczek J et al. [57] Preclinical Evaluation of RYM1, a Matrix Metalloproteinase-Targeted Tracer for Imaging Aneurysm. J Nucl Med. 2017; 58: 1318–1323. © SNMMI . (D) Representative MRI images of mice after injection with CG and CDR, following BAPN administration for 0, 2, and 4 weeks. Mice were examined by BL after MR imaging. The red arrows for the CG or CDR groups indicate the same position. CG: DOTA-Gd, CDR: Col-IV-DOTA-RhB, BLI: bioluminescence imaging. Reproduced under CC-BY 4.0 license from Ivyspring International Publisher [58]. (E) In vivo imaging of MMP activation in aneurysm. left (L): carotid arteries aneurysmal; right (R): control. This research was originally published in JNM. Razavian M et al. [59] Molecular imaging of matrix metalloproteinase activation to predict murine aneurysm expansion in vivo. J Nucl Med. 2010; 51: 1107–1115. © SNMMI .

Inflammation may lead to increased C-reactive protein (CRP) levels. One study investigated the relationship between serum CRP-to-albumin ratio (CAR) and progression in patients with AAA, and the results indicated that increased serum CAR was significantly associated with larger diameter [67]. However, changes in inflammatory factors in blood are subject to a variety of diseases and lack specificity for AAs. The imaging technology combined with inflammatory markers is more intuitive and effective.

MMPs are endopeptidases secreted by several matrix-resident and inflammatory cells of the vessel wall and belong to the metzincins superfamily, which contain a wide spectrum of zinc-dependent elastases and collagenases [68]. Specific endogenous inhibitors of MMPs, tissue inhibitors of metalloproteinases (TIMPs), and MMPs are indispensable for natural physiological ECM remodeling, but the dysregulated interaction between two endopeptidases results in ECM degradation [69, 70]. MMPs not only lead to ECM degradation but also act on non-matrix substrates [71]. Matthew and colleagues [72] have demonstrated that MMPs mediate proteolysis by regulating immune-inflammation responses, promoting the migration of inflammatory cells and modulating non-matrix molecules. Moreover, MMPs play a vital role in the recruitment of inflammatory cells, migration of VSMCs into the vessel intima and promotion of neovascularization [73].

Although MMPs are a spectrum of enzymes that have similar functions, each MMP has specific functions and is expressed at different levels in different tissues during inflammation [69]. Particularly, in patients with AAs, MMP-2/9/12 expression have been shown significantly higher in serum and aortic tissue, regardless of the genetic predisposition [74, 75, 76]. Distinct from other MMPs, MMP-12 is exclusively secreted by macrophages and could regulate the degradation of elastin and type IV collagen [77]. The enzyme can induce the activation of MMP-2 and MMP-3, which both activate MMP-7, MMP-8, and MMP-13. This amplifying cascade effect results in excessive degradation of ECM [78].

MMPs have three complexes of zinc ions in the catalytic center, and they are exposed only when MMPs are activated. Thus, the structural changes associated with MMP activation can be exploited for molecular diagnosis. In the early stage, antibody probes specific to the activation-specific epitopes had low specificity for MMPs. Importantly, each MMP has specific substrates. Therefore, substrate specificity can be exploited to enhance the selectivity to target MMPs [79]. Inhibitor-based probes are another technology to identify MMPs and obtain good results [80, 81]. Research with PET/CT reported an MMP inhibitor, RYM, and demonstrated that 99mTc-RYM1 probes are more abundant in AAA (Fig. 2C) [57]. In the SPECT technique, the probes were designed with special tracers of MMPs, such as RP782 and RP805. 99mTc-RP805 signal was significantly higher in the inflammatory area (Fig. 2E) [59, 65]. Serum MMPs levels, specifically MMP-2 and MMP-9, were shown to be associated with AAs. Studies on serum MMP levels are based on patients who have been clinically diagnosed, which may be significant for risk assessment, but inadequate to predict AAs [82, 83]. Thus, MMPs could be a possible biomarker to visualize inflammation by molecular imaging for early diagnosis and may serve as a potential target for treatment (Table 1).

Recent evidence has shown that apoptosis is another vital process in the pathogenesis of AAs [84, 85]. VSMC is the main effector cell constituting the aorta tunica media and play an essential role in maintaining vascular wall structure and function [86]. Several studies have demonstrated that the inflammatory cells induce apoptosis and MMP synthesis [42]. Moreover, endothelial injury mediates the up-regulated expression of long non-coding RNA H19 and Toll like Receptors (TLRs). H19 mediates the expression levels of the transcription factor HIF1$\alpha$, which has been shown to activate apoptosis of VSMCs. The TLR4-related signaling pathways increase the expression of the MMP-9 level, consequently stimulating the VSMCs dysfunction. VSMC apoptosis is considered to weaken the aortic structural integrity and participates in the pathogenesis of AAs [87, 88]. Additionally, SPECT with a 99mTc-duramycin probe was effective in evaluating apoptosis in AAs [66] (Table 1).

Histological studies have shown that collagen is an important component of the ECM, which strengthens the aortic wall structure and affects cell proliferation and adhesion by binding to integrins [89]. Moreover, collagen regulates the local activity of cytokines [90]. Col IV is one of component of the subendothelial basement membrane of the tunica intima in the aortic wall. It has been demonstrated that medial degeneration leads to long-term chronic stimulation, which results in intimal tearing of AD [8, 91]. Studies have also shown that Col IV is exposed initially at sites of endothelial injury [92]. Meanwhile, the activation of MMP-2/9 degrades basement membrane proteins expression in SMCs, including Col IV [75, 93].

Col IV belongs to the subendothelial basement membrane and is exposed on the aortic intima of early phase AAs. Recent research has demonstrated the MRI and fluorescence imaging with a Col IV-targeted probe, Col-IV-DOTA-Gd-RhB, to visualize aortic lesions, which can effectively predict AD and rupture of AAs (Fig. 2D) [58] (Table 1).

As the first biomarker identified in 1993, microRNAs (miRNAs) had been reported to contribute to both physiological and pathologic processes of cardiovascular diseases [94]. Overexpression of miRNAs, such as miR-29, -195, -21 and -143/145. miR-29 and miR-195 lead to the degradation of ECM and decrease miR-21 [95]. MiR-21 is associated with transforming growth factor $\beta$ (TGF-$\beta$) signaling pathway dysfunction [96, 97]. MiR-1 43/145 promotes a phenotypic switch of VSMC and induces degeneration of the medial layer [98]. It has been demonstrated that miRNAs are strikingly stable in human plasma/serum [99]. In patients with AAs and AD, miRNAs show over 10- to 40-fold increases in plasma [100].

Additionally, TGF-$\beta$ signaling molecules and TGF-$\beta$ receptors (TGFBR) play a multitude of roles in various physiological processes. TGF-$\beta$ signals activate Smad2/3 through TGFBR to induce the phosphorylation of Smad2/3 proteins. Moreover, TGF-$\beta$ signals can be mediated by non-Smad pathways, which can transduce signals from Ang II and be mediated by mitogen-activated protein kinases [101]. Furthermore, the TGF-$\beta$ and Ang II pathways can modulate vascular tone and the SMC phenotype by interacting with each other [9]. Mutations in the TGF-$\beta$ pathway are associated with Marfan syndrome and Loeys–Dietz syndrome [102].

Although molecularly targeted medicines show specificity and effectiveness against specific target molecules, systemic delivery may activate or inhibit the molecules that are essential for normal homeostasis. Meanwhile, the effectiveness of molecularly targeted medicines is limited because of its poorly water-soluble and requirement for parenteral administration, such as rapamycin and MMP inhibitors [129, 130]. Therefore, an appropriate delivery system is required to reduce side effects of molecularly targeted drugs while maintaining the efficacies.

The ever-increasing use of NPs in biomedicine reflects the great advances in novel imaging and drug delivery systems. In addition, NPs can be easily functionalized and applied in a variety of diseases [131]. Exosomes are the smallest membrane-delimited extracellular vesicles released by cells [132]. Exosome mainly function as regulators of cell-to-cell communication at short or long distances [133]. It has been used as a biomarker to monitor the progression of diseases [132, 134]. Exosomes have unique endogenous features and biological properties, such as stability in body fluids, immune tolerance and the ability to carry RNAs, DNAs, and proteins naturally [135, 136]. In addition, exosomes range from 50 to 150 nm in size, which can escape macrophage phagocytosis and promote permeation across biological barriers [137]. Because of these advantages, exosomes can be a suitable candidate for drug delivery as NPs. However, the low production quantity and difficulty of isolation, drug loading and delivery efficiency limit the application of exosomes in clinical practice [138, 139]. To address this problem, several studies have focused on synthetic NPs, which are easier to prepare and more efficient in drug encapsulation and delivery (up to nearly 90%) [139, 140]. However, synthetic NPs may induce a stronger immune response than exosomes [131].

The delivery system based on NPs is primarily used for molecularly targeted drugs, targeted antibodies or peptides and may be coated with molecules to protect the potency of the drugs or to enhance their effect [108, 129]. A study reported a TP-Gd/miRNA-Col IV NP delivery system, which is a multifunctional nucleic acid delivery nanosystem [109]. This nanosystem can be visualized by MRI and deliver nucleic acid and targeted peptide Col IV simultaneously (Fig. 3C). In addition, this nanosystem can efficiently deliver miR-145 to stabilize the vascular structure. In the paper, nanosystem was successfully prepared and used for predicting and monitoring AD [109]. As previously mentioned, the NPs loaded with BB-94 successfully released BB-94 in the lesion region, which inhibited local MMP activity and suppressed aortic dilation at small doses [115]. Poly (lactic-co-glycolic acid) has been used in nanotechnology applications, such as targeted and controlled drug delivery [141]. As an application of poly (lactic-co-glycolic acid), a doxycycline-loaded poly(lactic-co-glycolic acid) NP delivery system has been developed. It has been demonstrated that elastin have the potential to bind and inhibit MMPs to prevent the progression of AAs [112].