Use of Optical Imaging as an Alternative to Fluoroscopy

Rahul Sheth, MD Massachusetts General Hospital, Boston, MA
Published November 25, 2014

Optical imaging is a high-resolution, real-time, ionizing radiation-free imaging discipline that is poised to make a significant impact in diagnostic and interventional imaging over the next decade [1].

Recent advances in both optical imaging systems and molecularly targeted optical imaging probes have provided extensive preclinical demonstrations of optical imaging’s microscopic resolution and molecularly specific imaging. Optical imaging as a field encompasses a vast array of imaging modalities; of particular relevance to radiologists are minimally invasive, catheter-based systems that can provide endoluminal imaging of hollow viscera or blood vessels.

It is anticipated that optical imaging’s introduction into clinical diagnostic and interventional imaging will occur through fusion imaging systems that combine fluoroscopic images with optical imaging data. Optical imaging will most likely serve to complement fluoroscopy’s anatomic information with real-time molecular data that highlight cellular perturbations responsible for the development of disease. These data will not only allow for more accurate diagnostics but also better inform interventionalists regarding optimal treatment strategies. Moreover, as optical imaging can provide real-time image guidance, this technology may reduce the reliance on fluoroscopy for endoluminal procedures.

The following examples illustrate optical imaging technologies that may play a role in procedures that are currently performed under fluoroscopic guidance.

Endoscopic Imaging of Fluorescent Compounds

Optical imaging of fluorescent compounds represents a burgeoning imaging modality with expanding applications across a range of clinical specialties. A key advantage to working within the optical regime is the abundance of extant clinically approved endoscopes and laparoscopes that can be readily adapted to perform fluorescence imaging. As such, fluorescence endoscopy, in which conventional endoscopes have been modified to additionally provide a fluorescence imaging channel, has enjoyed tremendous growth in the past two decades.

A wide array of exogenously administered fluorescent imaging compounds have been developed in the preclinical setting, some of which are “non-specific” and accumulate in target lesions via non-targeted processes, such as perfusion or phagocytosis. Others, however, are true “molecular beacons” of disease that localize to areas of abnormality with exquisite sensitivity and specificity [2].

A number of fluorescent compounds are available for clinical use today [3], with many more in the clinical trial pipeline. One important clinically approved fluorescent imaging agent is indocyanine green (ICG). Approved for clinical use since 1959, ICG has a short half-life (~4 minutes) and minimal toxicity. It is exclusively taken up by the liver and excreted in the biliary system. ICG fluoresces in the near infrared (NIR) and localizes with high target to background ratios (TBR) in hepatocellular carcinoma (HCC) and liver metastases [4,5]. This localization can be measured through a fiberoptic catheter [6], an ability that may assist in improving the accuracy of microcatheter positioning during fluoroscopically-guided transarterial interventional oncology procedures.

With the development of molecularly-targeted imaging agents, optical imaging offers the potential for molecularly-guided interventions. For example, fluorescent compounds that highlight specific enzyme activity can be used to identify the level of vascular inflammation occurring within the walls of abdominal-aortic aneurysms. This information can be acquired via an endovascular approach and provide real-time, molecularly-specific guidance regarding the risk of rupture and necessity for fluoroscopically-guided endovascular repair [7].

Optical Coherence Tomography 

Optical coherence tomography (OCT) is a circumferential optical imaging technology based on the reflection of near-infrared wavelength light. This technology has been incorporated into the tip of a catheter and can be used in conjunction with standard interventional tools. OCT imaging complements traditional fluoroscopic information by providing endoluminal details regarding the structure of the arterial wall with micron resolution.

OCT has already made significant inroads in coronary angiography. For example, high-resolution images of atherosclerotic plaques or in-stent restenosis can be acquired with OCT [8]. These data can be used to quantify the volume of neointimal hyperplasia or stent endothelialization [9,10].

OCT can also accurately calculate cap-to-core ratios for atherosclerotic plaque, quantitative information that can then be used to gauge the appropriateness of subsequent angioplasty and/or stenting. Initial clinical results suggest that the addition of OCT to standard coronary angiography improve outcomes in cardiac-related mortality and myocardial infarction [11].

Photoacoustic Imaging

Photoacoustic imaging is a fusion-imaging technology that combines optical imaging with ultrasound. By blending these two modalities, photoacoustic imaging can acquire optical imaging data from targets that are buried within soft tissue, with spatial resolution on the order of microns. Laser light is used to illuminate tissue from a modified handheld ultrasound transducer. When the laser light is absorbed by a molecule, heat is generated; this tiny change in temperature generates an ultrasound wave, which is then measured by the same modified ultrasound transducer. Thus, in photoacoustic imaging, light is used to generate sound waves from which images are generated [1].

Photoacoustic imaging can provide high-resolution imaging of microvasculature, including blood-oxygen saturation and blood-volume measurements. This technology may play a role in transarterial-embolization procedures, by providing a more accurate surrogate measure of tissue ischemia and diminish the need for the conventional but indirect fluoroscopic end-point of stasis of blood flow within the artery supplying the target lesion [12].

Endoscope-based photoacoustic imaging systems have been developed in the preclinical setting [13,14]. These devices provide simultaneous endoluminal ultrasound and photoacoustic imaging. Clinical translation of this technology has the potential to revolutionize visualization of hollow viscera and blood vessels, advancing beyond the macroscopic "luminography" provided by fluoroscopy to a microscopic, endoluminal and molecular perspective.

For example, during a percutaneous cholangiography procedure in a patient with a suspected malignant stricture, while conventional fluoroscopy may be able to identify the location of the stricture, photoacoustic endoscopy could provide high-resolution ultrasound-guided anatomic data as well as molecularly specific, optically-guided functional data. This complementary information could increase yield of brush biopsies or even provide an “in vivo histology” determination of malignancy.

References

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