Fig. 2.1
Post-acoustic shadows due to severe calcified lesions, metal stent struts, and guidewires. (a) Heavy calcified lesions mask the plaque underneath between the 9 and 3 o’clock positions (arrowheads). (b) Multiple postshadows (arrows) by stent struts combined with calcification (2–7 o’clock, arrowhead) can be observed. (c) Dual postshadows are present due to dual guidewires (arrows) with calcified plaque (arrowhead)
2.1.2 Ring-Down Artifacts
A luminous ring of false images surrounding the transducer or the catheter of IVUS, which presents as several layers around the catheter that compromise evaluation of the area adjacent to the catheter (Fig. 2.2). Often it is called near-field artifacts when using other medical ultrasound devices. Digital subtraction of a reference mask can suppress ring-down artifact; however, it also limits the ability to distinguish extremely near tissue from the surface of the catheter [2].
Fig. 2.2
Ring-down or near-field artifacts . (a) An extremely displayed ring-down artifact can be noted due to ultrasound element defect (arrow). (b) Presented here is ring-down artifact of around 10 mm diameter with several bright layers at the center of IVUS image (arrow). (c) Reduced ring-down artifact is noted (arrow)
2.1.3 Nonuniform Rotational Distortion (NURD)
NURD is the unique motion artifact that can only be observed in IVUS system. It results from hindered constant rotational as well as fullback velocity of the transducer due to nonuniform friction of the coronary artery lumen [1, 3]. It can be present especially in bending or tortuous vessels, sometimes being influenced by small guided catheter lumens, the kinking of coronary wires in the same catheter, instability of catheter engagement, and the hub or drive machine itself. One of the other problems is the transducer can move up to 5 mm longitudinally according to systolic and diastolic movement of the heart. This movement can also cause significant motion artifacts. NURD is critically limited for the quantitative analysis of IVUS use (Fig. 2.3).
Fig. 2.3
Nonuniform rotational distortion (NURD) . (a) NURD artifact is present between 5 and 7 o’clock with distortion of the underlying plaque (arrowhead). (b) Multiple NURD artifacts occurred in a small-sized vessel (less than 2.5 mm diameter, arrowheads)
2.1.4 Side Lobes
Outside of main and high energy ultrasound beams, there are low energy beams called side lobes [1]. If there is a strong echo reflector such as calcium or stent strut, where the side lobe beams should pass, it will reflect these low energy echoes back to the transducer. Hence, false images of circumferential sweep will present adjacent to the calcium or stents. These false images mimic dissection flaps and may compromise precise evaluation of true lumen border (Fig. 2.4). One tip to overcome side lobe artifact is to reduce gain setting.
Fig. 2.4
Side lobes . (a) False images of circumferential lines are showing between 7 and 8 o’clock (arrowheads). (b) Slices of circumferential lines are present between 6 and 9 o’clock positions being confused with the dissection flap of the intima (arrowheads)
2.1.5 Reverberations
Reverberations are the production of repetitive false echo images due to reflections between two interfaces with a high acoustic impedance mismatch [2]. When the ultrasound beam bumps into strong reflectors such as calcium, metal stents, guide wires, and guiding catheters, it may be repeatedly reflected back and forth before returning to the transducer. These repeated reflections are displayed as multiple equidistantly spaced circumferential layers on IVUS (Fig. 2.5).
