Recent Advances in Optical Coherence Tomography for the Diagnoses of Lung Disorders

Randy Hou; Tho Le; Septimiu D Murgu; Zhongping Chen; Matt Brenner

Disclosures

Expert Rev Resp Med. 2011;5(5):711-724. 

In This Article

Abstract and Introduction

Abstract

There have been many advances in the field of diagnostic and therapeutic pulmonary medicine in the past several years, with major progress in the field of imaging. Optical coherence tomography (OCT) is a high-resolution (micron level) imaging modality currently being advanced with the potential to image airway wall structures in real time and at higher resolution than previously possible. OCT has the potential to increase the sensitivity and specificity of biopsies, create 3D images of the airway to guide diagnostics, and may have a future role in diverse areas such as the evaluation and treatment of patients with obstructive sleep apnea, tracheal stenosis, airway remodeling and inhalation injury. OCT has recently been investigated to monitor airway compliance in chronic obstructive pulmonary disease and asthma patients as well as differentiate causes of pulmonary hypertension. In future clinical and research applications, OCT will likely be combined with other endoscopic based modalities such as ultrasound, spectroscopy, confocal, and/or photoacoustic tomography to determine functional and biomolecular properties. This article discusses the current uses of OCT, its potential applications, as it relates to specific pulmonary diseases, and the future directions for OCT.

Introduction

The world of pulmonary imaging has advanced dramatically over the past decade. Flexible bronchoscopy as a minimally invasive technique allows examination of the trachea, bronchi and subsegmental bronchi to the level of fourth to fifth order but new technologies are necessary because conventional white light bronchoscopy (WLB) is of limited use for detecting mucosal changes that might be just a few cells thick or below the tissue surface.[1] The future of flexible bronchoscopy is in optical diagnostics that go beyond gross visualization. In fact, rates for diagnosing even visible lesions in patients with endobronchial tumors using WLB have been reported as between 75–98% with an overall specificity of 64–96% and pathologic diagnostic error rates of 1.5–3%. For central tumors (i.e., those visible during bronchoscopy), diagnostic yield increases with increasing numbers of biopsy samples, with the highest success reported at around four biopsies.[2]

Selection of biopsy site is prone to sampling error and biopsies may be associated with complications including bleeding with pneumothorax or persistent air-leak. Therefore, improved guidance for airway biopsy would increase yield and decrease risks associated with the large number of biopsies currently necessary for diagnosis. When we apply these imaging modalities in practice we need to keep in mind their potential and predicted role. In general, systems with good depth of penetration have lower resolution and cover larger areas, while systems with shallow depth of penetration can have higher resolution and cover smaller areas (Figure 1). For example, the resolution of clinical ultrasound is typically 0.1–1 mm and depends on the sound wave frequency (3–40 MHz) used for imaging. Optical coherence tomography (OCT) resolution is 1–15 µm and imaging depth is 2–4 mm, while with confocal endomicroscopy (CFM), the depth of focus is on the order of 50 µm and the lateral resolution of <1–3 µm. New optical diagnostic technologies are being developed and they continue to improve for surface, and deeper tissue diagnostics (i.e., visualizing the normal airway layered microstructures or their disruption in various disease processes). Since the depth of penetration and resolution varies greatly among different optical and acoustic techniques, the future of pulmonary imaging will likely consist of a multimodality approach using a combination of technologies able to visualize deeper structures such as cartilage and potential invasion (e.g., high-frequency endobronchial ultrasound [EBUS]), complemented with higher-resolution technologies that reveal alterations in the microstructures (i.e., CFM and OCT).

Figure 1.

Technologies with greater depth of penetration have lower resolution and cover larger interrogation regions, while systems with higher resolution have shallower depth of penetration and cover small regions. For example, the resolution of clinical ultrasound is typically 0.1–1 mm and depends on the sound wave frequency (3–40 MHz). The resolution of optical computed tomography is 1–15 µm and imaging depth is 2–3 mm. Photoacoustic tomography is a hybrid imaging technique that converts optical illumination into acoustic waves to produce high-resolution images with a depth range of 3–30 mm; for example, a 50 MHz ultrasound transducer provides 15-µm axial and 45-µm lateral resolution with approximately 3-mm imaging depth.

Optical coherence tomography is an optical signal acquisition and processing method based on measurement of the reflected light from tissue optical interfaces, and uses the principles of optical interferometry. This optical technology has been shown to be capable of imaging the surface and sub-surface airway wall layers at near microscopic resolution, with good correlation with histology specimens.

Optical coherence tomography fulfills a niche in pulmonary imaging not previously available: real time, minimally invasive imaging of subsurface mucosa at virtually histologic-level resolution.[3] Advantages of OCT over conventional computed tomography (CT) are the ultrahigh resolution capabilities and no exposure to ionizing radiation. There are no contraindications for patients who have metallic implants or patients who cannot tolerate maintaining a supine position for a significant time, as currently limited with MRI. OCT can penetrate approximately three-times deeper into tissues than CFM. In addition, OCT is less susceptible to motion artifact from movements such as cardiac pulsation and respirations compared with CFM. On the other hand, high-frequency EBUS, is able to visualize the airway wall layers in various benign and malignant central airway processes but at much lower resolution levels than OCT. In addition, unlike sound waves, light waves do not require a liquid-based coupling medium (i.e., can be used in noncontact modes) and thus are well suited for use inside the air-filled lumen of the tracheobronchial tree.[4]

Optical coherence tomography technology and clinical applications have advanced significantly over the past decade. The purpose of this article, in the perspective of the overall role of OCT in airway imaging presented above, is to provide a brief background in OCT technique, a synopsis of the current areas of research and clinical application, and an extended look into the future of OCT for clinical application in pulmonary medicine.

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