Fig. 15.1
Representative images of pre-procedural OCT images with poor preparation. Visualization of the distal portion of the MLA was not achieved because contrast agent could not pass the MLA site (indicated by a rectangle composed of white dashed lines). After balloon dilatation (2.0 mm balloon), visualization of the distal portion was much improved. Distal reference area (cross-sectional image) became clear after balloon dilatation. (a) Baseline OCT image and (b) post-balloon OCT image. OCT optical coherence tomography, MLA minimal lumen area
Several checkpoints were summarized in Table 15.1 to obtain high-quality OCT images.
Table 15.1
Checklist for obtaining good pre-interventional OCT image
Proper guiding catheter position | The guiding catheter must be deep seated in coaxially maintained position |
Adequate vessel preparation | 1. Intracoronary nitrate must be used prior of OCT examination to avoid coronary spasm and to obtain accurate vessel size 2. In case of severe stenosis, small-caliber balloon can be needed to visualize the distal portion |
Imaging catheter position | Be sure to include the area of interest because the indicator is located within proximal and distal part of imaging catheter |
Imaging catheter status | Make sure that there is no blood or air in the imaging catheter before pullback |
Adequate synchronization of OCT pullback after flushing | It is important to minimize the use of contrast by keeping in mind the synchronization of OCT pullback after flushing. Both the operator and the assistants must match their feet |
15.3 Artifacts
As with other intravascular imaging methods, OCT also shows several types of artifacts requiring interpretation. Here are some typical examples of artifact (Fig. 15.2).
Fig. 15.2
Frequently observed OCT artifact images. (a) Incomplete imaging catheter preparation makes signal attenuation. (b) Incomplete blood clearance of coronary artery makes attenuated image. (c) Seam-line artifact is caused by rapid artery or wire movement during single-frame formation. (d) Deformation of imaging catheter can make mirror artifact. (e) Fold-over artifact . This artifact is the result of “phase wrapping” or “aliasing” along the Fourier transform when the structural signal is selected outside the system’s field of view. (f) Tangential signal drop can be confused with plaque disruption or thin-cap fibroatheroma
The most common avoidable artifact is the signal attenuation caused by the suboptimal purge of blood in the imaging catheter (Fig. 15.2a). Even if enough preparation was done prior to examination, during the passage of the guiding catheter and lesion, some blood may enter the imaging catheter. So, rechecking of standby imaging before pullback is necessary. This artifact can be prevented with additional purge.
Incomplete blood clearing also frequently causes artifacts (Fig. 15.2b). Incomplete guiding catheter engagement, too large vessel size, significantly angulated vessel, and inadequate contrast agent filling can affect this artifact. The stagnation of blood can be confused with thrombus as seen from a single cut.
The “sew-up artifact (seam-line artifact)” is misalignment on the lumen surface due to fast imaging wire movement during the construction of a single frame (Fig. 15.2c).
Nonuniform rotational distortion can lead to shape distortion and mirror artifact (Fig. 15.2d).
“Fold-over artifact” is caused by the inherent property of the FD-OCT. This artifact is the result of “phase wrapping” or “aliasing” along the Fourier transform when the structural signal is selected outside the system’s field of view (Fig. 15.2e).
Tangential signal drop due to the location of the eccentric catheter adjacent to the lumen wall may result in misinterpretation of the lesion as thin-cap fibroatheroma (TCFA) or cap disruption (Fig. 15.2f).
Most of artifacts can be distinguished from true lesions by observing continuous changes in the serial cuts.
15.4 The Role of OCT in Pre-procedural Assessment
OCT provides detailed information on vessel walls and microstructure due to its superior resolution compared to CAG and IVUS. Especially, OCT may be more helpful in the following cases.
15.4.1 Role of OCT in Ambiguous Lesions
OCT provides a lot of information about plaque extension and characteristics that have not been fully evaluated in conventional CAG due to its excellent resolution.
This allows accurate diagnosis of suspicious findings that could not be confirmed and quantitative analysis of intracoronary thrombus.
Ambiguous angiographic visualization of lesion is not infrequent in real practice, and OCT provides us the correct answer, especially when mixed with intermediate lesions, short lesions, thrombus, or calcification [9].
Kubo and colleagues conducted a comparison study using OCT, IVUS, and angioscopy in 30 consecutive patients with acute myocardial infarction (AMI) to assess the ability of each imaging method to detect the specific characteristics of culprit lesion. OCT was superior in detecting plaque rupture, plaque erosion, and thrombus, respectively [5].
Recent OCT studies have revealed three major mechanisms in acute coronary syndrome (ACS): PR, PE, and calcified nodule [10, 11] (Fig. 15.3).
Fig. 15.3
Culprit lesion OCT findings of acute coronary syndrome. (a) Plaque rupture, (b) plaque erosion, (c) calcified nodule
In case of haziness on CAG without significant stenosis, there are cases of thrombus, dissection, heavy calcification, and ruptured plaque when examined through OCT [9].
If no evident lesion was seen in ACS presenting vasospastic angina, plaque disruption or thrombus was identified in OCT on a significant number of cases [12, 13] (Figs. 15.4, 15.5 and 15.6).
Fig. 15.4
Representative case of unstable angina without evident coronary disease. Coronary angiogram only revealed minimal stenosis and OCT revealed presence of recanalized thrombus at proximal left anterior descending artery
Fig. 15.5
Representative case of ambiguous coronary lesion which confirmed by OCT. After stent deployment at mid-RCA, linear slit-like lesion was observed. OCT confirmed clear image of edge dissection with thrombus
Fig. 15.6
Another case with ambiguous coronary lesion which confirmed by OCT. In coronary angiography, there was round filling defect on right coronary artery (in circle with white dashed line). OCT clearly revealed presence of thrombus without evidence of plaque disruption, lipid plaque, or calcified nodule. This is a representative image of probable plaque erosion
The diagnosis of spontaneous coronary artery dissection (SCAD) is not always apparent on coronary angiography, and OCT has been greatly helpful to the diagnosis of this unfamilial disease entity (Fig. 15.7).
Fig. 15.7
Representative case of spontaneous coronary artery dissection . The 60-year-old woman received mitral valve replacement 10 years before and was on anticoagulation. Clinical presentation was acute ST-segment elevation myocardial infarction. The angiography showed intermediate stenosis in distal portion of left anterior descending artery. OCT revealed presence of hematoma in coronary artery without evidence of atherosclerosis. This patient was conservatively treated due to patent coronary perfusion, and she was well recovered
Abovementioned findings were not previously identified as CAG alone, which led to breakthroughs in the diagnosis by introduction of OCT.
15.4.2 Role of OCT in Lesion Severity Assessment
In addition, OCT can be helpful to determine functionally significant lesion in intermediate stenosis lesion.
Although the gold standard method to identify functional significance of the coronary lesion is fractional flow reserve (FFR) in intermediate angiographic stenotic lesion, minimal lumen area (MLA) can be used as surrogate marker for functional significance. In comparison with IVUS, OCT showed slightly superior in identifying hemodynamically severe coronary stenosis (especially in vessel diameter less than 3 mm) [14].
Recently, a dedicated, semiautomated contour detection system (OPTIS™, St. Jude, MN, USA) is used for measurements. A contour detection algorithm that automatically traces lumen boundaries of the longitudinal (L)-mode view was implemented, and it allowed us to automatically detect MLA position within few seconds (Fig. 15.6). In a comparison study regarding automatically detected and manually detected lumen analysis, there was excellent correlation with two methods [15].
Through this method, we can easily identify the location and severity of the minimal lumen area (Figs. 15.8 and 15.9).
