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.