Discussion

Analogous to intravascular ultrasound (IVUS), OCT is a minimally invasive catheter based imaging modality that provides a cross sectional view of the artery being interrogated. Images generated utilizing OCT are based on the emission of near infrared light which enables 10–25 times higher in vivo resolution imaging of the arterial wall, of the order of 10–20 μm, about twice the size of a red blood cell. Clinically this translates to tissue penetration of 1–3 mm compared with the 4–8 mm achieved by IVUS.1 ,2 The spatial resolution yielded from OCT exceeds what is possible with routine imaging methods such as from catheter directed angiography or even non-invasive CT angiography or MR angiography.

Intravascular OCT utilizes a single fiberoptic wire to emit light at a bandwidth in the near infrared spectrum with central wavelengths of approximately 1300 nm. The images are then generated when back scattered or reflected light from the target tissue produces a measurable signal intensity and echo time delay.3 The single fiberoptic image wire is designed to handle both light emission as well as record the reflection or backscatter of light. Two main technologies are utilized to obtain these OCT images, frequency domain and time domain OCT systems, with the latter using a moving reference mirror to obtain images at varying tissue depth. By simultaneously scanning through the entire bandwidth of near infrared wavelengths and in turn analyzing thousands of data points or echo time delays, frequency domain OCT imaging technology is significantly faster than analyzing the tissue point by point as in time domain OCT.4

The single fiberoptic image wire used to acquire the OCT images rotates inside an over the wire catheter with an external lumen of 0.019 inch and is designed to be compatible with a standard 0.014 inch microwire and 6 F guide catheter for easy adoption. During image acquisition, blood in the field of view can cause significant signal attenuation and artifact whereby necessitating clearing of blood from the lumen of the artery. Initial experience with earlier generation OCT catheters within the coronary arteries suggests this can be resolved by proximal balloon occlusion with subsequent infusion of saline or Ringer's lactate at a rate of 0.5–1.0 ml/s. It is recommended that a power injector be used to maintain a constant rate; furthermore, multiple pullbacks can be made in order to completely evaluate the complete extent of the lesion at a rate of approximately 0.5–3.0 mm/s. Balloon occlusion can affect image clarity primarily by inadequate clearing of blood from the field of view. Other limiting factors include lesions exceeding 30 mm in length.5 Non-occlusive techniques with newer generation catheters have recently been developed and studied where simultaneous infusion of viscous iso-osmolar contrast agents and pull back of the image wire/catheter system are employed so as to reduce imaging time and increase clearing of blood from the lumen.

Morphological characteristics of atherosclerosis by OCT have been well studied in the coronary arteries. Precise high resolution visualization and quantification of the thickness of the fibrous cap and calcium are possible and well documented. However, it should be noted that light absorbed by lipid pools, which is sharply demarcated from the overlying signal rich fibrous cap, acts as a diffuse signal poor region making the extent difficult to quantify. Plaque rupture and/or ulceration can be detected by OCT as an area of contiguity between the lipid pool and the fibrous cap at a rate similar to that of pathologic studies5 (figure 3A). Kume et al suggested possible discrimination of red and white thrombus with the aid of OCT, as confirmed by histopathologic correlation6 (figure 3B, 3C).

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Figure 3

Cross sectional optical coherence tomography (OCT) images of diseased coronary arteries. (A) Large ruptured plaque is noted with demonstration of contiguity between the overhanging thin fibrous cap (curved arrow) and the lipid pool (arrow). (B) Red thrombus (curved arrow) causes marked signal attenuation owing to the presence of red blood cell components. (C) Contrary to this, white thrombus (curved arrow) is platelet rich and exhibits lower signal attenuation than that of red thrombus. Also on this image, superiorly located, is a sharply demarcated curvilinear low attenuating calcified plaque (arrow). (D) OCT imaging clearly demonstrates good apposition of the stent struts in relation to the diffusely thickened arterial wall.

Similar to IVUS, but at a higher spatial resolution, information yielded from the OCT cross sectional view of a diseased artery cannot only aid in lesion assessment prior to intervention but furthermore in proper stent sizing prior to deployment. Likewise, discrepancies between degrees of stenosis by different imaging modalities can be resolved.7 ,8 One may underestimate the size of a diseased artery when using only conventional angiographic techniques as this measures only luminal diameter. Conversely, because of the accurate measurements of both the luminal stenosis as well as the true diameter of the artery, one can choose the correct balloon or stent size when utilizing OCT. In addition, the precise length of the diseased stenotic area can be assessed so that a proper stent length may be determined ahead of time. In doing so, this can reduce the need for overlapping stents which are known to increase the risk of thrombosis.9 After selecting a proper stent, in conjunction with OCT, one can confirm optimal stent placement and stent wall apposition, as well as ensuring full expansion of the stent, thereby reducing complications such as the incidence of in-stent thrombosis. Once the stent has been implanted, OCT can clearly demonstrate the stent struts in relation to both the arterial wall and plaque (figure 3D). When compared with IVUS, OCT demonstrates higher sensitivity in assessing strut apposition.10 ,11 Furthermore, if there is any area that requires further attention post stent deployment, a larger balloon can be directed to the area of concern so as to optimally expand the stent.

The surgical approach to the vertebral arteries is difficult and therefore in cases of stenosis, such as in our case, intravascular stenting is a very attractive option; albeit, a disproportionately high in-stent restenosis rate has been observed within the vertebral arteries which has been attributed to a wide variety of factors. One of the causes of in-stent thrombosis or restenosis in neurovascular stenting may be poor stent apposition, such as when the stent may not have been fully expanded to the width of the artery. A direct correlation between rates of in-stent restenosis with the length of an atherosclerotic lesion was seen in one series of 88 patients with 90 lesions by Lin et al.10 Another key contributor to in-stent restenosis is intimal hyperplasia, with robust evidence demonstrating lower rates of restenosis associated with drug eluting stents compared with bare metal stents.12 ,13

Other potential cerebrovascular applications that may be amenable to and benefit from intravascular high resolution OCT cross sectional images are varied.14 Mathews et al recently reported on the clinical feasibility of neuroendovascular OCT within non-diseased arteries of the intracranial circulation in three human cadaveric patients and two pigs with corresponding ex vivo tissue samples for histologic comparison.15 Studies such as Stenting and Angioplasty with Protection in Patients at High Risk for Endarterectomy (SAPPHIRE) as well as the Carotid Revascularization Endarterectomy versus Stenting Trial (CREST) have increased the use of stents within stenotic carotid arteries with relatively low complication and restenosis rates.16 ,17 Poor stent approximation has been noted as a cause for restenosis; however, with OCT guidance improved stent placement as well as better patient risk stratification may be possible. Intracranial artery stenosis, unruptured aneurysms and even arteriovenous malformations are areas in which future applications of OCT imaging technology may prove to be a powerful diagnostic modality in the neurointerventionalist's armamentarium.