The technical basis of optical coherence imaging
OCT (optical coherence tomography imaging) is an invasive endocoronary imaging method characterized by a fine axial (10-20 µm) and lateral (20-90 µm) resolution allowing a very efficient analysis of the arterial surface as well as the first millimeters of a wall.
The technique has improved progressively over the years until it spread in hemodynamics laboratories in its current form (frequency-domain OCT / FD-OCT).
1 How is the image constructed?
The general principle of OCT
Optical coherence tomography imaging is based on propagation, absorption, and reflection and a low coherence lightwave, whose length is close to that of infrared photons (1250 -1350 nm) on structures surrounding. This choice of wavelength makes it possible to overcome the refraction between the vascular walls, in connection with the interaction with the components of the blood, but in return limits the depth of penetration of the afferent beam.
The beam emitted by a laser-type light-emitting diode is divided into two beams with the same physical properties (energy, wavelength). A mirror reflects the first reference beam to an internal sensor before any propagation. The second beam will, via a mirror placed at 45 degrees, be directed towards the structure to be analyzed. This second beam will propagate in the tissues, undergoing a process of dispersion and reflection (on the interface regions) before returning to its source, with a delay and a given signal intensity (optical echo). The comparison between the beam reflected on a given line (A-line / axis of the optical beam), and the reference beam is performed according to the principle of interferometry, making it possible to amplify or attenuate the signal. The intensity of the reflected signal can then be coded in a given direction (amplitude of the signal according to the distance to the source) and converted into a light pixel.
In the context of frequency-domain OCT, the light source consists of a laser emitting on different wavelengths.
And the result of interferometry is an optical frequency type signal which after application of the Fourier transform, can be converted into an exploitable signal (amplitude, frequency, and distance to the source). This technique exploited through the FD-OCT and OFDI (Terumo) systems allows faster signal acquisition over a longer portion of the artery, bypassing the balloon occlusion of the vessel used during image acquisition in TD-OCT (time-domain OCT).
A classic cross-sectional view of OCT, therefore, corresponds to the analysis of the light beam reflected by the vascular structures and of the sum of the optical echoes in all directions (360 °) around the catheter.
Beam absorption & dispersion
The absorption and dispersion (backscattering) of the light beam through the different physical media crossed before and after its reflection is the base of the construction of the image. The intensity of the signal collected depends on several parameters:
1. The distance between the structure analyzed and the emission source. The intensity of the signal obtained at a given point is lower the greater this distance, due to the properties of photons with a wavelength of 1300nm which have only a small tissue penetrance (2 to 5 mm in the vascular wall)
2. The nature of the target analyzed. Indeed, each biological tissue has specific dispersion and absorption coefficients.
Thus, fibrosis creates little absorption and dispersion of photons during its crossing, therefore little loss of signal intensity, allowing a more in-depth analysis of the tissue.
On the contrary, the lipid pools create a strong attenuation of the signal by absorption and dispersion of the photons, with an analysis of the tissue limited in depth and explaining the appearance of the edges of the zone.
The calcification zones create a strong attenuation of the signal in their most superficial part (hence the appearance of hypo signal), but only afterward create a limited absorption and dispersion of the beam, allowing a more in-depth analysis of the tissues and a well-defined aspect of the edges.
Similarly, the white thrombus, poor in red blood cells, absorbs very little photons, unlike red thrombus, which contains several red blood cells and creates a significant absorption of these same photons.
The reflection of the photon beam at an interface zone depends on the refractive indices between the two media, the angle of the incident beam, and the polarization. The signal will be all the more reflected (and therefore will give a sharper image) that the difference between the two refractive indices will be great and that the beam will arrive perpendicularly on the target. Thus, a stent mesh or the angioplasty guide (metal / vascular wall interface) will create an almost exclusive reflection of the beam explaining the absence of a signal behind it. On the contrary, in case of peripheral incidence of the beam on the structure, the reflection will be less, and the image sometimes less clear.
2 Image acquisitions
The OCT technique by frequency domain allows rapid acquisition (20 to 40 mm / s) of an image on a portion of artery of 52 to 75 mm (FD-OCT) or up to 150 mm, in minimal time.
The probe must be rid of its red cells. This can be achieved by injecting contrast medium or even viscous macromolecular solution to obtain the best possible image. The volume to be injected depends on the size of the artery, 15 to 20 cc for the left coronary artery, and 15 cc for the right coronary artery. The flow of the contrast medium should be between 3 to 3.5 ml /s. An automatic injector is particularly useful for carrying out the examination but may be replaced by a 20 cc lure-lock (screwed) syringe in the event of manual injection.
Before carrying out the acquisition, certain points should be checked in the form of a checklist to obtain the optimal image quality and avoid artifacts:
1. Have calibrated the probe (z-offset)
2. Have carefully purged the light of the fiber with the contrast agent to avoid the accumulation of red blood cells
3. Test the quality of the flush by injecting a few cc of contrast product, the image of the vessel obtained must then be clear, without the presence of volutes of red blood cells. If it is not, then check the intubation of the guiding catheter in the vessel and possibly increase the injection rate.
Author: Vicki Lezama