Table 2.

Technical characteristic of IVUS and OCT and their clinical significance.

Technical characteristic	IVUS	${\mathrm{OCT}}^2$	Clinical significance
Source of image	Ultrasound waves (Grey technology)	Low-coherence light (Orange technology)	Because light is attenuated by red blood cells, OCT requires blood clearance, which is achieved usually after contrast injection [16]
Sheath compatibility	5-Fr or ${\mathrm{larger}}^1$	5- or 6-Fr or ${\mathrm{larger}}^2$	Although imaging with OCT can be performed with the use of a 5-Fr sheath and guide catheter, that can produce images with lower quality, due to the need of blood clearance [17, 18]. Thus a 6-Fr or larger guide catheter is generally preferred, whereas imaging with a 5-Fr guide catheter with the use of IVUS does not affect the images to this extent
Catheter design	Minimum guide catheter: ${\mathrm{5F}}^1$	Minimum guide catheter: ${\mathrm{6F}}^2$	OCT catheter should be placed deeper in the target vessel and acquires images from the point of the mid to the proximal marker, whereas IVUS catheter can acquire images from its tip [19, 20]. This can put limitations to the use of OCT for lesions in the medial or distal part of a vessel
	Boston Scientific™ OPTICROSS HD: Three radiopaque markers: (i) distal marker, 5 mm from the tip of the catheter, with the transducer 20 mm from the tip of the catheter (ii) two proximal markers	Abbot™ Dragonfly OPTIS: Three radiopaque markers: (i) distal marker, at the tip of the catheter (ii) mid marker (position of the optical lens), 27 mm from the tip (iii) proximal marker, located 50 mm from the mid marker
	Working length: 135 mm	Working length: 135 mm
	Volcano Therapeutics™ Eagle Eye Platinum ST: Three radiopaque markers with 10 mm spacing and transducer 2.5 mm from the tip of the catheter	Fastview™ for OFDI: Three radiopaque markers and sensor position from tip 24 mm
	Working length: 150 mm	Working length: 150 mm
Tissue penetration (mm)	4–8	1–3	Ultrasound waves can penetrate deeper into tissue than light, so IVUS is better for the examination of all vessel walls, as well as vessel remodeling, something which could be problematic with OCT in cases of large luminal diameter or increased plaque burden [21]
Axial Resolution ($\mu$m)	100–150	10–20	Light enables more detailed visualization than ultrasound waves, so OCT achieves better visualization of vessel anatomy, intracoronary structures (i.e., plaque characteristics) and more reproducible images [21, 22]
Lateral Resolution ($\mu$m)	150–300	20–90
Aqcuisition speed (mm/sec)	0.5	18–20	OCT images can be acquired in less time than with IVUS, which may play a crucial role, especially in complex clinical settings [18, 23]
Blood clearance	$\times$	$\mathrm{✓}$	IVUS does not require contrast, so it could be the preferable method in renal impairment [16]
3D-reconstruction	limited	$\mathrm{✓}$	OCT has better resolution and can produce better 3D images, something exceptionally useful in complex clinical settings [24, 25, 26]

¹The data are referred to the IVUS system of Boston Scientific™ OPTICROSS HD (which incorporates a mechanical catheter) and Volcano Therapeutics™ Eagle Eye Platinum ST (which incorporates a digital catheter).

²The data are referred to the OCT system of Abbot™ Dragonfly OPTIS and OFDI system Fastview™.

IVUS, Intravascular Ultrasound; OCT, Optical Coherence Tomography.