Blood must be completely flushed from within the lumen during OCT imaging to prevent artifacts resulting from OCT light scattered by red blood cells. When blood is present, lumen artifacts can appear in the OCT image as signal-rich areas instead of a transparent lumen completely free of OCT signal (Fig. 5.2). Scattering of light from within the lumen by blood can cause shadowing that will decrease the intensity of the OCT signal from the arterial wall and can cause difficulty in interpretation of arterial structure. [1–6] Scattering by blood can also introduce significant changes to stent strut appearance including merry-go-round, blooming, and ghost strut artifacts (Fig. 5.2b), which are discussed in Sect. 5.4. If OCT signal is observed in the vessel lumen, modifying the flush parameters and repeating the imaging procedure can correct this artifact. At times there is concern that multiple power injections will increase trauma to the coronary wall and complications to the patient, or that the increased contrast volume will result in renal insufficiency. However, literature demonstrating the safety of power injection of coronary arteries mitigate some of these concerns. For trained cardiologists, hand injection is also a suitable alternative to clear blood during intravascular OCT imaging [7]. When concerns regarding contrast induced renal damage exist Dextran-40 can be used to clear the blood instead of contrast [8, 9].

Gas bubbles can form inside fluid-filled catheters. These bubbles are highly scattering due to the large refractive index difference between the gas and surrounding fluid and will also cause the appearance of a shadow that will decrease the signal intensity of the arterial wall (Fig. 5.4) [1–4, 6]. They are recognized by a distinct region of brightness inside the catheter and diminished signal in the part of the OCT image in the path of the disrupted region of the catheter. These gas bubbles can form inside the catheter if the operator does not correctly pre-flush the imaging catheter before placement into the patient for imaging. Gas bubbles can also appear in the lumen of the vessel in cadaver heart studies, causing the appearance of a bright outline of the bubble and a shadow in the OCT image behind the bubble.

Metal stents block incident OCT light and thus cause shadows behind each strut. A substantial fraction of the arterial cross-section can be disrupted by these shadows, which can then significantly complicate the process of characterizing the underlying tissue structure. Dropout of light occurs behind metallic struts, where struts typically appear as a single leading edge feature that casts a single expanding broad shadow over the arterial wall beginning with the width of the stent strut. This light dropout can often be used as a strategy to recognize the presence of older stents in regions of complex neoimtima and new plaque growth. In contrast to metallic stents, biodegradable stent strut shadows are normally observed along the two lateral sides of the strut, leaving a much greater volume of the native tissue structure (e.g., plaque, intima, media) interpretable for light interrogation.

Attenuation of light by macrophage accumulations can cause shadows in OCT images that appear falsely as an underlying lipid pool or necrotic core. Thus a relatively normal artery with superficial infiltration of macrophages can even appear as a thin-capped fibroatheroma (Fig. 5.5), causing a non-vulnerable plaque to incorrectly appear vulnerable to rupture. Shadowing is not due to the macrophage itself, but rather is caused by either (1) large pools of lipid-filled macrophages (foam cells) that cause light to attenuate similarly to a lipid pool due to the low index of refraction (IR) of lipid or (2) its association with compact structures with high IR such as micro-calcification and cholesterol crystals [10]. Increased scattering and shadowing by these structures is anticipated by classical scattering theory (e.g., Mie), which describes the way light scatters from objects. In addition to ingesting lipid, macrophages can engulf microcalcifications [11] and cholesterol crystals [12]. Small cholesterol crystals and microcalcifications, both of which have high IRs compared with surrounding tissue structures can cause shadowing. In general, macrophages do not necessarily have optical properties that would cause a shadow, unless they are a large cluster of lipid-rich foam cells or have engulfed a micro-calcification or plaque component that has an IR substantially higher than that of lipid.

When OCT light is incident on an object that causes a large backscattered signal the OCT detector can saturate. The saturation effect introduces signal energy in much of the OCT signal detection bandwidth so that a bright line appears for that particular A-scan in the OCT image (Fig. 5.6). Polar-to-rectangular conversion causes the saturation artifact to bleed laterally into adjacent A-scans [1–4]. Radial extent of the bright line depends on the artifactual frequencies introduced and can extend beyond the strongly backscattering object deep into the arterial wall and extend to the edge of the OCT image. At larger radial distances from the catheter, polar-to-rectangular conversion results in broadening of the artifact. Objects that can cause a saturation artifact include stent struts, the guide wire, and sometimes components in the arterial wall or tissue surface such as microcalcifications and cholesterol crystals.

The catheter location within the vessel lumen (center or off-center) and the vessel diameter can affect the OCT image appearance. Additionally, motion of the imaging catheter, due to rotation and pullback speed, can alter the OCT image appearance as well.