Spatial orientation of cross-sectional images of coronary arteries: point of view in intracoronary imaging

Spatial orientation of microscopic sections

When a section is cut for microscopic examination, the section may be laid down on the glass slide on one of two sides. Photomicrographs of a section lying on different sides will be mirror images of each other. So, although they are photomicrographs of the same section, i.e. similar position and rotation, the intraplaque localizations of plaque components will differ (Figure 1).

With strict adherence to procedures throughout the preparation of arterial sections for microscopic examination, the same known spatial orientation of sections can be obtained. Thereby, with proper attention to orientation in the preparation of tissue section for microscopy, one can always decide and know the imagined point of view, i.e., whether one should imagine looking at a coronary artery cross-section standing proximal (at the coronary ostium) or distal to the cross section (Figure 1).

Spatial orientation of intravascular images

An intravascular imaging catheter looks from the arterial lumen and outward on the arterial wall. But the displayed images created with intravascular imaging represent cross-sections that have one of two possible spatial orientations, just like the microscopic section. Throughout the pull back, the observer should imagine herself/himself standing either proximal or distal to the cross-section when looking at the cross-sectional images (Figure 1). The spatial orientation and thereby the imagined point of view is decided by the reconstruction algorithm. When making comparisons of intravascular and microscopic images, one has to know the spatial orientation of the intravascular images in addition to the images from microscopy and make sure they are the same before any comparison is meaningful. To know the spatial orientation, one has to either know the reconstruction algorithm or determine the spatial orientation experimentally.

Revealing point of view

To determine the spatial orientation experimentally, we made an asymmetric phantom using a short piece of tubing with two wire points not directly across from each other on the cross-section of the tube. One wire alone constituted one wirepoint and two wires close together constituted the other. We then decided which end was to be considered the proximal and investigated the tube with two different IVUS systems and one OCT system inserting the IVUS and OCT catheters from the imagined proximal end (Figure 2).

The IVUS systems were Volcano In-Vision Gold system with an Eagle Eye Gold catheter and Boston Scientific iLab system with an Atlantis SR Pro catheter. The IVUS investigations were performed with the phantom submerged in water. Manual pull back recordings were used. The OCT system was Light Lab C7XR with a Lightlab C7 Dragonfly Imaging Catheter.

The spatial orientation (point of view)

On the IVUS and OCT images, all three wires were clearly visible making the two asymmetrically placed wire points clearly distinguishable. Owing to the asymmety of the phantom, it was evident that the intravascular images were displayed to the observer, so that the observer should imagine herself/himself looking at the cross-sectional images standing proximal to the displayed cross-sectional image of the artery (Figure 3).

In studies comparing cross sectional images from these imaging systems to images from microscopy, investigators should then ensure that their cross sectional images from microscopy display the proximal view (same spatial orientation). Otherwise, comparison will not be meaningful.

When does knowledge about spatial orientation not make a difference?

Clinically, treatments, such as angioplasty or stenting, are applied equally to the entire arterial circumference. Therefore, for making the clinical decision whether to apply a treatment or not, the spatial orientation of the cross-sectional image does not make a difference.

In clinical research, area measurements on intravascular images are used as study end points [5]. Measurements on areas, e.g. of a plaque or necrotic core from reconstructed images, yield the same results regardless of the spatial orientation and therefore the thin-cap fibroatheroma (TCFA)[6, 7] can be correctly identified from both points of view (Figure 1).

However, meaningful use of an intravascular imaging modality, both clinically and in clinical research, provides that the intravascular imaging modality has been properly validated for identification and quantification of the plaque components that are of interest clinically and in clinical research. I.e., the use of these imaging modalities clinically and in clinical research is dependent upon reliable validation of the imaging systems. It is in this validation that spatial orientation is important.

When does knowledge about spatial orientation make a difference?

For validation of imaging modalities focusing on the identification and quantification of plaque components, intraplaque localization is used and the spatial orientation of the cross-sectional images becomes critical (Figure 4). When the images from intravascular imaging and microscopy have the same spatial orientation, comparison makes sense. But if the images from intravascular imaging and microscopy have different spatial orientations, the compared images are not superimposable and direct comparison makes no sense. The spatial orientation can and must be known before any comparison of the images because knowledge about spatial orientation is crucial for correct interpretation of cross-sectional images and validation of technologies creating such images (Figure 4).

These considerations apply to the comparison of asymmetric cross-sectional images in general, irrespective of imaging modalities. In this study, other imaging modalities such as computed tomography, magnetic resonance imaging, near infrared spectroscopy etc., were not investigated; however the considerations about spatial orientation also apply whenever images from these modalities are compared to images from other imaging modalities, e.g. microscopy.

Intravascular imaging catheter rotation

Intravascular imaging catheters can rotate along their longitudinal axis during pull back. Therefore, spatial orientation cannot be determined based on localization of two structures present on different images from the same pull back. Neither can spatial orientation be determined based on the circumferential localization of a single landmark, such as a sidebranch, as catheter rotation randomly determines the localization of this landmark. Determining spatial orientation experimentally requires asymmetry in one image.