Background
Intravascular ultrasonography (IVUS) is an invasive imaging procedure that provides intravascular images of the coronary arteries and other blood vessels. Intravascular ultrasonography has played a critical role in enhancing the understanding of coronary atherosclerosis pathophysiology and has facilitated the refinement of diagnostic and therapeutic strategies for various vascular pathologies. Intravascular ultrasonography has become increasingly important in both clinical and research applications, [1] and it has played an integral role in the evolution of interventional cardiology. [2, 3] Intravascular ultrasonography in interventional cardiology is an adjunctive procedure to coronary angiogram; as such, any contraindication to coronary angiography applies to intravascular ultrasonography as well.
In general, risks and discomforts involved in IVUS include those associated with all catheterization procedures. Major complications, including dissection or vessel closure, are rare (<0.5%). The most frequently reported complication is transient coronary spasm (occurring in 1-3% of examinations), which responds to intracoronary glyceryl trinitrate.
(See the image below.)
A still intravascular ultrasonography image demonstrating a normal coronary artery.
An expert consensus committee commissioned by the American College of Cardiology, in collaboration with the European Society of Cardiology, has provided a framework for standardization of nomenclature, methods of measurement, and reporting of intravascular ultrasonography results. [4]
Indications
Preinterventional intravascular ultrasonography imaging allows the assessment of plaque distribution, ostial involvement, lumen and vessel area and diameters, extent of calcification, and the presence of thrombi or dissections. [5, 6, 7] It can alter strategy and the decision to use a particular device.
Two major trials, the Strategy for Intracoronary Ultrasound-Guided PTCA and Stenting (SIPS) trial and the Balloon Equivalent to Stent (BEST) study evaluated the potential benefit and demerits between ultrasound-guided balloon angioplasty and routine stenting. [8, 9] In the SIPS trial, approximately 50% of patients in each group received a stent at the time of the index procedure. Acute gain was greater in the IVUS-guided group than in the angiography-guided group, but angiographic 6-month follow-up revealed no difference in the primary endpoint of minimum lumen diameter. Although no difference was noted in the secondary endpoint of short-term target lumen revascularization, long-term clinical follow-up showed a significant decrease in clinically driven target lumen revascularization in the ultrasound group compared with the angiography group. In the BEST trial, at 6 months, 20 of 119 patients in the aggressive balloon angioplasty group and 21 of 116 patients in the routine stent implantation group had restenosis, along with no statistical difference in minimal luminal diameter or lumen cross-sectional area, thus fulfilling the prespecified criteria for noninferiority.
Although intravascular ultrasound-guided balloon angioplasty is a noninferior alternative to routine stenting, this approach is certainly more time consuming and requires meticulous attention to detail and expertise in intravascular ultrasonography image acquisition and interpretation. These studies have made apparent that the crossover rate is high, with more than 50% of patients requiring adjunctive stent implantation. In routine clinical practice, stent implantation has gained preference over intravascular ultrasound-guided balloon angioplasty. [10]
The Multicenter Ultrasound Stenting in Coronaries (MUSIC) study established the safety and feasibility of intravascular ultrasound-guided stent implantation. [11] Fitzgerald et al evaluated whether routine ultrasound guidance of stent implantation improved clinical outcome as compared to angiographic guidance alone in the Can Routine Ultrasound Influence Stent Expansion (CRUISE) trial. [12] Although no clinical outcome benefits were demonstrated with routine use of intravascular ultrasonography, a more effective stent expansion was noted when compared with angiographic guidance alone .
Casella et al conducted a meta-analysis of studies done on this topic and demonstrated that intravascular ultrasound-guided stent implantation has a neutral effect on long-term death and nonfatal myocardial infarction compared with an angiographic optimization. [13] However, it was noted that intravascular ultrasound-guided stenting significantly lowers 6-month angiographic restenosis and target vessel revascularizations.
In their appraisal of intravascular ultrasonography and its application in routine angioplasty, Oxford et al have noted that intravascular ultrasonography is better than contrast angiography in key procedural variables, such as measurement of postdeployment stent dimensions, confirming complete stent apposition, and excluding edge dissections that may predispose to both early and late complications, including in-stent restenosis. [14] At present, no guidelines exist on the routine use of intravascular ultrasound-guided angioplasty, but the interventionist should weigh the risks and benefits of this procedure before its application. The clinical usefulness of ultrasound guidance in stent deployment maintains its value. Particularly in small vessels, bifurcation stenting, ostial lesions, long segments, and in left main stenting, ultrasound can provide beneficial guidance. [15, 16]
Intravascular ultrasound plays a vital role in characterization of plaque structures and planning of debulking (atherectomy) procedures by differentiating between superficial (intimal) and deep calcium deposits . [17] Intravascular ultrasonography has emerged as superior to routine angiography at guiding selective plaque removal. [18] Rotational atherectomy is preferred over directional atherectomy in superficial calcification. [19]
Technical Considerations
Intravascular ultrasonography consists of a miniature ultrasound-mounted catheter that is connected to an electronics console to reconstruct the images transmitted by sound waves. The ultrasound signal is produced by passing an electrical current through the piezoelectric (pressure-electric) crystalline material of the transducer that expands and contracts when electrically excited.
After reflection from tissue, part of the ultrasound energy returns to the transducer. The received signal is converted to electrical energy and sent to an external signal processing system for amplification, filtering, scan conversion, user-controlled modification, and graphic presentation. The ultrasound beam upon reflection remains fairly parallel for a distance (near field) and then begins to diverge (far field). The quality of ultrasound images is greater in the near field because the beam is narrower and more parallel, the resolution is greater, and the characteristic backscatter (reflection of ultrasound energy) from a given tissue is more accurate.Therefore, larger transducers with lower frequencies are used for examination of large vessels because they create a deeper near field.
(See the video below.)
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A still intravascular ultrasonography image demonstrating a normal coronary artery.
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An intravascular ultrasonography run from a segment of normal coronary artery.
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An intravascular ultrasonography video highlighting the appearance of a branch from the main stem coronary artery. Note the coronary artery branch appearing at the 1'o clock position of the screen.
