Inflammation is increasingly being recognized as contributing to the underlying pathophysiology of cerebral aneurysms (CAs) and brain arteriovenous malformations (bAVMs).1 This has been well studied in human and animals.2,3 Macrophages infiltration seems to play the predominant role by increasing the activity of matrix metalloproteinase (MCP-2 and MCP-9), as well as other markers of inflammation, leading to degradation of the extracellular matrix and weakening of the aneurysm wall. Hence, it has been hypothesized that inflammation is involved in formation as well as progression toward aneurysms rupture. There is currently no noninvasive means established to detect inflammation in intracranial aneurysms or arteriovenous malformation (AVM). We aim to present a review article on magnetic resonance imaging (MRI) with ferumoxytol that makes use of this biochemical phenomenon and hence might be utilized in the future to further understand the behavior of neurovascular diseases. Such study could potentially help assist the physician in his judgment for intervening, as aneurysms or bAVMs that look macroscopically the same may behave differently.
AN OVERVIEW OF SUPERPARAMAGNETIC PARTICLES
Over the last 3 decades, the utility of superparamagnetic iron oxides in a variety of biomedical applications such as MRI contrast enhancement has been rapidly growing.4–9 Superparamagnetic iron oxide nanostructure is based on inorganic core of magnetite (Fe3 O4), or maghemite (gFe2 O3) molecules encased in polysaccharide, synthetic polymers, or monomer coatings.6,10,11 Coating plays an important role in determining circulation time and half-life, accessibility to tissues, opsonization, and rate of cellular uptake.5 On the basis of hydrodynamic diameter, Superparamagnetic particles are categorized into the following: very small superparamagnetic iron oxide particles (VSPIOs) (less than 10 nm), ultrasmall superparamagnetic iron oxide particles (USPIOs) (10 to 50 nm), and standard superparamagnetic iron oxide particles (SPIOs) (50 to 180 nm). In the presence of an applied field, SPIOs exhibit strong magnetization; however, upon removal of the field, they retain no permanent magnetization. This is of particular importance when biomedical application considered as magnetic dipole-induced aggregation of particles within the vessels would have the potential to cause embolization.12 Ferumoxytol (Feraheme; AMAG Pharmaceuticals Inc, Cambridge, MA) is a USPIO nanoparticle, approved by the Food and Drug Administration in 2009 as a treatment for iron deficiency anemia in patients with chronic renal failure.13 Remarkably, off-label use of ferumoxytol in variable MRI studies has been well established in the literature.4,5,8,9 This is explained by the inflammatory properties exhibited by ferumoxytol, as it leaks across permeable vessels and is taken up by perivascular macrophages hours to day(s) after administration.7,14 The nondextran semisynthetic carbohydrate-coated ferumoxytol has a hydrodynamic diameter of 30 nm.14 This modified carbohydrate coat makes ferumoxytol better tolerated for bolus injections. Significantly, this coat has a great influence on the immunologic response to USPIOs, making them of minimal impact on cellular viability and functionality.7,15,16 Because of the relatively larger molecular size (compared with the 1 nm gadolinium chelate), USPIOs extravasate much more slowly than standard, thus considered a true blood pool agent.17 Moreover, USPIO exhibits no renal elimination enhancing safety profile in patients with renal dysfunction, and eliminating the risk for contrast-induced nephropathy and nephrogenic systemic fibrosis.14 Unlike rapidly extravagasing gadolinium-based contrast agents, ferumoxytol is beneficial at early time points (seconds to minutes) in characterization of vasculature by magnetic resonance angiography (MRA) due to slow extravasation properties (blood pool).14 Furthermore, ferumyxytol particles do not enter circulating phagocytic cells.14 Such properties make ferumoxytol of particular importance in malignant gliomas assessment.18 In addition, ferumoxytol-enhanced MRI has been used in endoleak monitoring in patients with aortic stent grafts, depiction of deep vein thrombosis, tumor progression, and cancer staging.19–21 Ferumoxytol, as a contrast-enhancing agent, has not been found to have significant toxicities.18,22,23 Being iron-based agent, expected side effects include hypersensitivity, hypotension, nausea, vomiting, diarrhea, constipation, and peripheral edema.17 Finally, it is gaining recognition for its utility in MRI and is being increasingly used for both its prolonged intravascular imaging characteristics and its utility as an inflammatory marker when imaged in a delayed fashion (as it is cleared by reticuloendothelial system macrophages).18 Ferumoxytol appears hypointense on T2_GE sequences and can appear hyperintense on T1 pulse-gated sequences. The drug can be visualized intravascularly for up to 72 hours but begins to clear within 24 hours and can be visualized intracellularly (secondary to macrophage-uptake) within 24 hours. Prior studies have indicated that peak visualization occurs at 24 to 28 hours.18
IMAGING FOR CEREBRAL ANEURYSMS
It is clear by now that inflammation plays a central role in the pathogenesis of CAs. The presumed model is the following: an inflammatory process is initiated by a hemodynamic insult and leads to matrix MMPs–mediated degradation of the extracellular matrix and apoptosis of smooth muscle cells (SMCs), which are the predominant matrix-synthesizing cells of the vascular wall. These processes act in concert to weaken the arterial wall progressively, resulting in dilatation, aneurysm formation, and ultimately rupture. The data supporting a major role for inflammation in CA pathogenesis are strong and derive from both experimental and human studies. The 2 main constituents of the inflammatory response and the associated degenerative response are macrophages and SMCs.24
Hasan et al25 conducted a pilot study that aimed to visualize macrophages in the human CA wall using ferumoxytol-enhanced MRI in view of establishing a protocol for safety, feasibility, and instituting optimal parameters. Nineteen unruptured aneurysms in 11 patients were imaged using T2_GE-MRI sequence. Two protocols were used. Protocol A was an infusion of 2.5 mg/kg of ferumoxytol and imaging at day 0 and 1. Protocol B was an infusion of 5 mg/kg of ferumoxytol and imaging at day 0 and 3. All MRI was completed on a Siemens 3T TIM Trio system, by Siemens AG, (Malven, PA). Patients completed a baseline MRI consisting of time-of-flight (TOF) angiography and T2_GE sequences. The TOF angiographic sequence was collected using a 3D multislab technique with the following parameters: TE_3.6 ms, TR_20 ms, field-of-view_200_200_200 mm, matrix_384_384_20, bandwidth_165 Hz/pixel. Four slabs were collected with a 20% overlap. The T2_ weighted sequence was collected using a 2D gradient-echo sequence with the following parameters: TE_20 ms, TR_500 ms, flip angle_20, FOV_220_220, matrix_512_384, slice thickness/gap_3.0/0.3 mm, bandwidth_260 Hz/pixel. Protocol A involved imaging at 2 timepoints 24 hours apart. Protocol B involved 2 stages. Stage 1 involved imaging at 5 timepoints: preinfusion, immediately postinfusion, and 24, 72, and 120 hours postinfusion. This was modified to imaging at 3 timepoints for stage 2 of protocol B: preinfusion, immediately postinfusion, and 72 hours postinfusion. Fifty percent (5/10) of aneurysms in protocol A showed ferumoxytol-associated signal changes in aneurysm walls compared with 78% (7/9) of aneurysms in protocol B. Aneurysm tissue harvested from patients infused with ferumoxytol stained positive for both CD68, demonstrating macrophage infiltration, and Prussian blue, demonstrating uptake of iron particles. Tissue harvested from controls stained positive for CD68 but not Prussian blue. This study helped establishing that infusion dosing of 5 mg/kg of ferumoxytol and imaging at 72 hours postinjection using T2* GE MRI demonstrates an optimal dose and timing parameters for macrophages imaging within aneurysm wall. The finding of early uptake at 24 h intrigued the investigators and they decided to further evaluate this phenomenon. Hence, another pilot study was conducted,26 in which 30 unruptured aneurysms were imaged with MRI 24 hours after infusion of ferumoxytol. Eighteen aneurysms were also imaged 72 hours after infusion of ferumoxytol. Aneurysm dome tissue was collected from 4 patients with early MRI signal changes, 5 patients with late signal changes, and 5 other patients with ruptured aneurysms. The tissue was immunostained for expression of cyclooxygenase-1, cyclooxygenase-2, microsomal prostaglandin E2 synthase-1, and macrophages. In 23% (7/30) of aneurysms, there was pronounced early uptake of ferumoxytol. Four aneurysms were clipped. The remaining 3 aneurysms were managed conservatively; all 3 ruptured within 6 months (Fig. 1). In 53% (16 of 30) of aneurysms, there was a pronounced uptake of ferumoxytol at 72 hours. Eight aneurysms were surgically clipped, and 8 were managed conservatively; none ruptured or increased in size after 6 months. Expression of cyclooxygenase-2, microsomal prostaglandin E2 synthase-1, and macrophages was similar in unruptured aneurysms with an early uptake of ferumoxytol and ruptured aneurysms. Expression of these inflammatory molecules was significantly higher in aneurysms with early uptake of ferumoxytol versus aneurysms with late uptake. This study strongly suggests that uptake of ferumoxytol in aneurysm walls within the first 24 hours strongly suggests aneurysm instability and probability of rupture within 6 months, and may warrant urgent intervention. Furthermore, this study in fact changed the concept that inflammation is a postrupture phenomenon that was the previously dogma. It also showed a direct association between rupture and inflammation, regardless of the aneurysm size, which has been the single most important factor in decision making in patient selection for intervention. More so, if the findings are validated in a larger study, ferumoxytol-enhanced MRI could be applied in clinical practice to differentiate unstable aneurysms that require early intervention from stable aneurysms in which observation may be safe, specifically in patients that often pose a therapeutic dilemma (elderly patients or patient with <7 mm aneurysm).
IMAGING FOR ARTERIOVENOUS MALFORMATION
Recently, attention has been turned to the inflammation role in the growth, progression, and rupture of AVMs, which is not yet not clearly understood (Fig. 2). BAVMs display an increased expression of inflammatory cytokines and proteases, compared with the normal brain,27,28 which induces the overexpression of cell adhesion molecules in bAVM endothelial cells, resulting in enhanced recruitment of leukocytes.29 The increased leukocyte-derived release of MMP-9 is known to degrade structural elements of the AVM walls and leading to rupture.30 In pro-angiogenic environments, such as the one that exists in the bAVM nidus,31 macrophages may have a major contribution to MMP-9 activity.32 Inflammation is also involved in altering the bAVM angioarchitecture via the upregulation of angiogenic factors that affect endothelial cell proliferation, migration, and apoptosis. In bAVM tissue, macrophages and other inflammatory cells27,28,33 are seen in the vascular walls and intervening stroma, even with no history of rupture and without pre-resection embolization or radiotherapy. Given a strong potential link between inflammation and the risk of bAVM hemorrhage, a noninvasive means to detect inflammation could be of value for identifying bAVM at risk for hemorrhage. In this light, the senior author Hasan et al34 attempted a feasibility study of imaging macrophages within the bAVM nidus using ferumoxytol-enhanced MRI. This study included 4 patients with already diagnosed bAVMs using iron-sensitive imaging (ISI; T2*-GE-MRI sequence). Patients were imaged at baseline and at either 1 day (n = 2) or 5 days (n = 2) after infusion of 5 mg/kg of ferumoxytol. Residual intravascular ferumoxytol obscured evaluation for uptake in bAVM vascular walls and stroma at the 1-day time point. The 2 cases imaged at 5 days showed less intravascular tracer but had signal loss in the nidal region consistent with ferumoxytol localization. One case underwent surgical resection and was found to have prominent vascular wall CD68 staining. Thus, ferumoxytol-enhanced MRI for assessing bAVM inflammatory cell burden appears feasible and has the potential to be developed as a biomarker to study lesional inflammatory events. Understanding the prevalence and time course of inflammation, as assessed by macrophage infiltration, could provide a means to better understand the biology of the disease and may provide information on the risk of spontaneous rupture. Further, macrophage burden could present a potential target for therapeutic manipulation. Several useful points come from this preliminary pilot study that can help guide future studies. Because there appears to be some unpredictable difference among patients in how fast intravascular tracer is cleared, it may be useful to scan patients immediately after ferumoxytol injection, to test how the intravascular compartment appears without the anticipated parenchymal or vascular wall signal loss that will be seen at the delayed imaging time-point. For this purpose, probably a small portion of the total injected ferumoxytol could be given first for this purpose to avoid extensive blooming artifacts (Fig. 2).
Inflammation plays a central role in the pathogenesis of CAs and bAVMs. Knowledge of these mechanisms has allowed the conception of promising molecular-based imaging studies identifying rupture-prone CAs and preliminary studies are further characterizing bAVMs. Pharmacological therapy targeting the inflammatory reaction is also being investigated as a potential therapeutic strategy for CAs. Future experimental studies and clinical trials will be needed to further deepen our knowledge of this complex disease and to provide patients with efficient and innocuous therapies.24 Perhaps MRI could not only be used for diagnosis but also to monitor treatment response.
Cerebral AVMs entail a significant risk of intracerebral hemorrhage owing to the direct shunting of arterial blood into the venous vasculature without the dissipation of the arterial blood pressure. The mechanisms involved in the growth, progression and rupture of AVMs are not clearly understood, but a number of studies point to inflammation as a major contributor to their pathogenesis. The upregulation of proinflammatory cytokines induces the overexpression of cell adhesion molecules in AVM endothelial cells, resulting in enhanced recruitment of leukocytes. The increased leukocyte-derived release of metalloproteinase-9 is known to damage AVM walls and leads to rupture. Inflammation is also involved in altering the AVM angioarchitecture via the upregulation of angiogenic factors that affect endothelial cell proliferation, migration, and apoptosis. The effects of inflammation on AVM pathogenesis are potentiated by certain single-nucleotide polymorphisms in the genes of proinflammatory cytokines, increasing their protein levels in the AVM tissue. Furthermore, studies on metalloproteinase-9 inhibitors and on the involvement of Notch signaling in AVMs provide promising data for a potential basis for pharmacological treatment of AVMs. Potential therapeutic targets and areas requiring further investigation are highlighted.29
MRI was completed on a Philips 1.5T Intera Power MRI System (Amsterdam, Netherland). Patients completed a baseline MRI consisting of TOF angiography, T1-weighted black blood imaging, and a T2* GE sequence. The TOF angiographic sequence was collected using a 3D multi-slab technique with the following parameters: TE = 6.9 ms, TR = 23 ms, field-of-view = 200 mm × 200 mm × 50 mm, matrix = 256 × 256 × 50, Bandwidth = 165 Hz/pixel. Two slabs were collected with a 20% overlap. The T1-weighted sequence was sequential single-slice 2D Fast Spin Echo sequences with double inversion recovery for blood suppression with the following parameters: TE = 23 ms, TR = 680 ms, and an echo train length of 7. The T2*weighted sequences were collected using a 2D gradient-echo sequence with the following parameters: TR/TE = 680/23 ms, flip angle = 20, FOV = 240 × 240, matrix = 256x256, slice thickness/gap = 2.4/1.0 mm, Bandwidth = 260 Hz/pixel, NEX = 3. At the other institution, MRI was completed on a Siemens 3T TIM Trio system, by Siemens AG, (Malven, PA). Patients completed a baseline MRI consisting of TOF angiography and T2*GE sequences. The TOF angiographic sequence was collected using a 3D multi-slabtechnique with the following parameters: TE = 3.6 ms, TR = 20 ms, field-of-view = 200 × 200 × 200 mm, matrix = 384 × 384 × 20, Bandwidth = 165 Hz/pixel. Four slabs were collected with a 20% overlap. The T2* weighted sequence was collected using a 2D gradient-echo sequence with the following parameters: TE = 20 ms, TR = 500 ms, flip angle = 20, FOV = 220 × 220, matrix = 512 × 384, slice thickness/gap = 3.0/0.3 mm, Bandwidth = 260 Hz/pixel.
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