Thrombus Visualization Using Low-Pressure Contrast Injection via a Small-Bore Aspiration Catheter in M2 Thrombectomy: A Technical Note

Abstract

Objective

Accurate localization of thrombi in middle cerebral artery (M2) segment occlusions remains technically challenging but is essential for successful mechanical thrombectomy. We report a novel visualization technique that uses the aspiration catheter system itself to delineate the thrombus, thereby minimizing the risks associated with conventional microcatheter-based contrast injections.

Case Presentation

An 81-year-old woman presented with acute ischemic stroke caused by an occlusion of the left M2 inferior trunk. Aspiration thrombectomy was performed using the described visualization technique. After positioning the aspiration catheter proximal to the thrombus, the inner microcatheter and guidewire were withdrawn, and contrast medium was gently injected at low pressure through the intermediate catheter. This approach produced a stagnant contrast column that clearly outlined the proximal surface of the clot as a meniscus sign, confirming direct catheter–thrombus contact. Complete reperfusion (thrombolysis in cerebral infarctions grade 3) was achieved after 2 aspiration passes, with no procedural complications observed.

Conclusion

This novel technique offers a simple, safe, and effective method for direct thrombus visualization in M2 occlusions. By utilizing the intermediate catheter for controlled, low-pressure contrast injection, it may reduce the risk of distal embolization and simplifies the procedural workflow, potentially enhancing the rate of 1st-pass recanalization success. This technical note introduces the concept and provides preliminary evidence supporting further investigation and validation in larger patient cohorts.

Introduction

Mechanical thrombectomy is an established treatment for acute ischemic stroke caused by large-vessel occlusion, leading to significantly improved functional outcomes. Its application has been extended to medium vessel occlusions (MeVOs), including the M2 segment of the middle cerebral artery. However, whether the benefits demonstrated in large-vessel occlusions apply equally to MeVOs remains uncertain, and the role of thrombectomy in M2 occlusions is still debated.¹⁾

One of the main reasons for lower success rates and prolonged procedure times in M2 occlusions is the difficulty in accurately localizing the thrombus—a critical step for the direct aspiration 1st-pass technique (ADAPT).²⁾ Angiography performed from the guiding catheter often fails to reveal the “meniscus sign” or “claw sign,” angiographic findings characterized by meniscoid, tram-track, or edge-like contrast opacification along the proximal face of the thrombus.³⁾ These signs are considered predictors of successful recanalization⁴⁾; however, in distal occlusions, antegrade flow from the guiding catheter is often insufficient to opacify the space between the vessel wall and the clot face (Fig. 1). This limitation may cause the aspiration catheter to wedge against the vessel wall instead of engaging the thrombus directly. To overcome this challenge, microcatheter-based contrast injection techniques have been proposed to better delineate the thrombus face.⁵⁾

Fig. 1. Schematic illustration of the misjudgment of thrombus location in M2 occlusions. When contrast is injected from a proximal GC, it opacifies the patent M2 branch. In the adjacent occluded branch, the contrast stops at an apparent cutoff point (arrow) because antegrade flow is insufficient to outline the true thrombus.

GC, guiding catheter

However, the microcatheter-based approach introduces additional procedural complexity and carries an inherent risk of thrombus dislodgement due to high-pressure contrast injection.⁵⁾ To address these limitations, we developed a novel technique in which contrast is gently injected at low pressure through the intermediate catheter to create stagnation at the proximal end of the thrombus. This approach enables clearer visualization of the clot face and confirms direct catheter–thrombus contact. Here, we report our initial clinical experience using this method.

Case Presentation

An 81-year-old woman with a history of hypertension and atrial fibrillation presented with right hemiplegia, global aphasia, and left conjugate eye deviation. Her National Institutes of Health Stroke Scale (NIHSS) score was 27. MRI confirmed an occlusion of the left M2 inferior trunk, with a diffusion-weighted imaging–Alberta Stroke Program Early CT Score of 3 (Fig. 2). Intravenous tissue plasminogen activator was contraindicated because of a recent gastrointestinal bleed, and the patient proceeded directly to mechanical thrombectomy.

Fig. 2. Pre-procedural MRI. (A) DWI on admission shows a high-intensity area with a DWI-Alberta Stroke Program Early CT Score of 3. (B) FLAIR image demonstrates a vascular hyperintensity in the left M2 inferior trunk (arrow). (C) MRA confirms occlusion of the left M2 inferior trunk (arrow). (D) T2*-weighted image reveals a susceptibility vessel sign in the left M2 inferior trunk (arrow).

DWI, diffusion-weighted imaging

Under local anesthesia, a right femoral approach was used, and an 8-Fr Branchor XF guiding catheter (Asahi Intecc, Aichi, Japan) was advanced into the left internal carotid artery (ICA). Internal carotid angiography confirmed an M2 inferior trunk occlusion (Fig. 3). A coaxial system consisting of an Esperance71 aspiration catheter (Wallaby Medical, Laguna Hills, CA, USA), an Esperance3+ intermediate catheter (Wallaby Medical), a Passerelle21 2.2/2.8F microcatheter (Medicos Hirata, Osaka, Japan), and a Synchro SELECT soft 0.014-inch guidewire (Stryker, Kalamazoo, MI, USA) was navigated to the occlusion site.

Fig. 3. Pre-procedural DSA. (A) Anteroposterior view showing occlusion of the left M2 inferior trunk (arrow). (B) Lateral view demonstrating the occlusion at the same location (arrow).

After positioning, the microcatheter and guidewire were removed, leaving the Esperance3+ intermediate catheter just proximal to the occlusion within the Esperance71. Using a 10-mL syringe, 1–2 mL of 50:50 diluted contrast was manually injected at low pressure over approximately 2–3 s through the hub of the Esperance3+. This gentle injection produced a stagnant contrast column that outlined the proximal surface of the clot as a meniscus sign. Guided by this visualization, the Esperance3+ was advanced into direct contact with the thrombus (Fig. 4). Aspiration was then initiated using a Dominant Flex pump (Penumbra, Alameda, CA, USA). Firm thrombus engagement was confirmed by the immediate cessation of blood return into the pump canister. The 1st-pass fragmented the clot, and a 2nd pass using the same technique achieved complete retrieval. Final angiography confirmed thrombolysis in cerebral infarction (TICI) grade 3 reperfusion (Fig. 5).

Fig. 4. Intraprocedural images demonstrating the low-pressure contrast injection technique. (A) Lateral DSA view during a slow, low-pressure contrast injection through the Esperance3+ intermediate catheter. A clear meniscus sign (arrow) is visualized at the proximal face of the thrombus. The arrowhead indicates the tip of the Esperance3+. (B) Roadmap image showing the aspiration catheter advanced to contact the proximal thrombus, guided by the visualization in (A). The arrowhead marks the tip of the inner Esperance3+ for reference. (C) Unsubtracted lateral fluoroscopic image demonstrating the column of stagnated contrast medium (arrow) trapped at the proximal end of the thrombus, clearly delineating its location.

Esperance3+, Wallaby Medical, Laguna Hills, CA, USA

Fig. 5. Post-procedural DSA. (A) Anteroposterior view. (B) Lateral view. Both views confirm successful reperfusion of the M2 inferior trunk, achieving a thrombolysis in cerebral infarction score of 3.

Post-procedural non-contrast head CT revealed no evidence of hemorrhage. The patient demonstrated rapid neurological improvement, with near-complete recovery of right-sided motor function. At discharge, she had residual motor aphasia and mild cognitive deficits and was transferred to a rehabilitation facility.

Discussion

This method was developed to address the challenges associated with M2 thrombectomy. In our initial experience at our institution, this technique was applied to 4 patients, including the present case. A summary of the cases is presented in Table 1, and representative images of low-pressure contrast injection in all 4 patients are shown in Fig. 6. All procedures resulted in successful recanalization (TICI 2c or greater) without hemorrhagic complications. Unlike microcatheter contrast injection, which carries the risk of distal clot migration due to high-pressure injection and may add operator-dependent procedural steps,⁶⁾ this technique uses a larger-bore intermediate catheter for gentle low-pressure injection, thereby simplifying the workflow.

Table 1. Summary of cases utilizing the low-pressure small-bore aspiration catheter injection technique.

Case	Age/sex	Occlusion site	Proximal diameter (mm)	Configuration	Number of passes	TICI score	Complication
1	76/F	Right M2 superior trunk	1.3	Mildly curved	1	3	None
2	81/F	Left M2 inferior trunk	1.8	Straight	2	2c	None
3	52/M	Left M2 inferior trunk	1.4	Straight	1	2c	None
4	76/F	Left M2 inferior trunk	1.5	Straight	2	3	None

Proximal diameter was measured immediately proximal to the thrombus.

Configuration refers to the occluded M2 segment.

F, female; M, male; TICI, thrombectomy achieved complete reperfusion

Fig. 6. Representative low-pressure contrast injections in the other 3 patients. (A–C) Lateral angiographic images obtained during slow, low-pressure injection of 1–2 mL of 50% diluted contrast medium through a small-bore aspiration catheter positioned proximal to the thrombus in cases 1, 3, and 4, respectively (see Table 1). In each panel, a meniscus-like contrast interface (arrows) delineates the proximal surface of the M2 thrombus. Arrowheads indicate the tip of the small-bore aspiration catheter. The present case (case 2) is illustrated separately in Fig. 4.

Although injecting contrast through a microcatheter already in place is straightforward, microcatheter-based thrombus visualization usually requires additional steps such as reintroducing the microwire, advancing it to or beyond the presumed clot position, and then withdrawing the microcatheter before aspiration. These additional manipulations increase the number of procedural steps, whereas our technique delineates the thrombus via the intermediate catheter already in place.

Accurate thrombus localization is crucial in MeVO, but it remains difficult. Proximal angiography often overestimates clot length, particularly in distal M2 occlusions, compared with M1.⁶⁾ Although microcatheter injection can provide more precise localization, a forceful injection through a small-lumen catheter may dislodge the thrombus and push it distally.^5,7) This method addresses these issues by exploiting the larger lumen of the intermediate catheter, enabling gentle low-pressure injection that is less likely to dislodge the clot. It also eliminates additional microcatheter manipulations and exchanges, thus reducing procedure time. The feasibility of this direct approach has been enhanced by the recent development of smaller-bore aspiration catheters with improved trackability, which enable more reliable distal access and stable positioning at the clot face.

According to Poiseuille’s law, the hydraulic resistance of a catheter segment depends strongly on its inner radius (to the fourth power) and, to a lesser extent, on its length. In the present series, navigation was performed with a Passerelle21 microcatheter (Medicos Hirata; inner diameter 0.53 mm, working length 175 cm), and microcatheter-based contrast injections, when used, would typically be delivered through this 0.53-mm lumen. In contrast, our technique uses an Esperance3+ intermediate catheter (Wallaby Medical; distal inner diameter 1.04 mm, working length 145 cm). Because the Esperance3+ has almost twice the inner diameter while having a comparable working length, its hydraulic resistance is estimated to be only about 6% of that of the Passerelle21. In other words, to achieve the same flow rate, the pressure gradient required through the Esperance3+ should be approximately 15–20 times lower than that needed through the microcatheter. In our protocol, we inject only 1–2 mL of 50% contrast over 2–3 s (approximately 0.3–1.0 mL/s) by gentle hand injection through the Esperance3+, so the actual intraluminal pressure rise is expected to remain far below the maximum injection pressure specified by the microcatheter manufacturer and substantially lower than that during typical high-pressure microcatheter angiography. Together with the lower viscosity of diluted contrast, this Poiseuille-based comparison qualitatively supports our assumption that low-pressure distal access catheter injection may be less likely to disturb the thrombus or injure the vessel wall than high-pressure microcatheter-based injections. This Poiseuille-based estimate is intended as a simplified steady-flow approximation and does not account for pulsatile cerebral blood flow or vessel compliance. In addition, we did not perform a head-to-head comparison between the 2 injection techniques.

In practice, this method differs from the standard ADAPT approach, in which operators often judge thrombus contact based on the loss of blood returning to the pump system. Such feedback is ambiguous because arterial collapse from suction can mimic thrombus engagement.⁸⁾ Our technique provides direct, real-time angiographic visualization of the catheter–thrombus interface before aspiration. This pre-aspiration confirmation reduces ambiguity, allowing operators to distinguish true clot appositions from vessel wall wedging, potentially improving procedural precision and safety.

Anatomically, our technique was applied to M2 trunks with reference diameters of 1.3–1.8 mm and relatively simple configurations. Three of the 4 cases involved straight inferior trunks, whereas 1 case involved a superior trunk in which the thrombus was located just beyond a gentle curve. Even in this curved segment, the small-bore aspiration catheter could be advanced to the clot face without excessive resistance, and full reperfusion was achieved by simple push aspiration. In all 4 cases, to ensure sufficient pushability of the small-bore aspiration catheter, the outer large-bore aspiration catheter was advanced as distally as safely feasible—typically to the distal M1 segment or, when anatomy permitted, the proximal M2 trunk—to provide stable support for inner catheter advancement. In contrast, we deliberately avoided using this technique in vessels where lesion crossing with a microwire or microcatheter was required to advance the aspiration catheter, or where the thrombus was situated beyond a sharply tortuous segment. In such anatomies, we consider ADAPT to be less suitable and preferentially employ a stent retriever–assisted strategy. Based on these experiences, we currently regard the technique as best suited for relatively straight or mildly curved M2 trunks of small but not extreme caliber.

However, the clinical role of thrombectomy in patients with MeVO remains unclear. Recent randomized trials, including the ESCAPE-MeVO, DISTAL, and DISCOUNT trials, have not demonstrated superiority over medical management.^1,9–11) These neutral outcomes may reflect technical challenges, such as lower reperfusion rates compared to large vessel occlusions, highlighting the difficulty of intervening in smaller distal vessels. Therefore, procedural innovations are essential. By enabling direct thrombus localization, this method may increase the 1st-pass effect, reduce retrieval attempts, and shorten procedure duration. These findings suggest that neutral trial results may partly reflect current technical limitations rather than a fundamental lack of benefit, making the development of safer and more efficient techniques critical for improving outcomes and identifying the patients most likely to benefit.

This study has several limitations that must be noted. First, the success reported herein was facilitated by the relatively straight vessel anatomy. This raises the possibility of a selection bias in our small series, and the applicability of this technique to more tortuous anatomies requires further investigation. As a refinement of contact aspiration, this technique has limitations, particularly in tortuous or sharply angulated vessels where catheter–thrombus apposition is difficult.¹²⁾ Second, although designed to minimize clot disturbance, a particularly friable thrombus can theoretically be dislodged by a contrast puff. Third, our technique has not yet been validated in an in vitro model. We did not obtain direct measurements of injection-related pressure or flow changes, and the range of vessel diameters in which this technique can be safely and effectively applied has not been determined experimentally. Experimental in vitro studies using vascular models will be necessary to confirm these assumptions. Finally, our study was limited to 4 patients, and the findings should be regarded as preliminary. Although we propose this technique to overcome the procedural hurdles highlighted by recent neutral MeVO trials, we also acknowledge the alternative hypothesis that these trial outcomes may reflect fundamental biological limitations in certain patient populations. Larger studies are required to validate the safety and efficacy across broader clinical and anatomical scenarios.

Conclusion

Low-pressure contrast injection through an intermediate catheter is a simple and intuitive method for M2 thrombectomy. In our preliminary experience, it provided clear thrombus visualization and streamlined workflow and may have reduced the risk of distal embolization compared with microcatheter-based methods. Despite these limitations, this approach addresses the critical challenge of thrombus localization and warrants further investigation to establish its role in improving the outcomes of patients with MeVO.

Ethics Statement

This case series was conducted in accordance with the Declaration of Helsinki and institutional guidelines. Institutional review board approval was not required under our institutional policy for case reports/case series using anonymized clinical data. Written informed consent for participation and publication was obtained from all patients or their legally authorized representatives.

Disclosure Statement

The authors declare that they have no conflicts of interest.