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, and it has played an integral role in the evolution of interventional cardiology. Intravascular ultrasonography in interventional cardiology is an adjunctive procedure to coronary angiography; as such, any contraindication to coronary angiography applies to IVUS as well. [1]
Intravascular ultrasonography, a diagnostic tool that relies on sound waves to produce precise images of the vessel being evaluated, was originally introduced for peripheral artery imaging, but it was quickly adapted for coronary interventions. IVUS can be used for vessel measurement, preprocedural and postprocedural planning, treatment optimization, and detection of thrombus, dissection, or calcium severity. Angiography remains the standard imaging modality, but studies have shown that IVUS can provide more accurate imaging detail. [2, 3]
IVUS can characterize lesion morphology, quantify plaque burden, guide stent sizing, assess stent expansion, identify procedural complications, and evaluate stent failure with stent thrombosis or in-stent restenosis. IVUS can distinguish between calcified plaque, lipid, and neointimal proliferation and can identify calcified plaque that may not be possible with angiography alone. IVUS overcomes many of the limitations associated with angiography, since angiography produces a 2-dimensional image of a 3-dimensional structure using x-rays. IVUS assesses the vessel internally rather than externally. [4, 5, 1, 6, 7, 8, 9]
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.
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 IVUS results. [10]
(See the image below.)
A still intravascular ultrasonography image demonstrating a normal coronary artery.
Guidelines
The following recommendations were made by the American College of Cardiology (ACC) regarding the use of IVUS for percutaneous coronary intervention (PCI) [11] :
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IVUS is a reasonable option to assess angiographically indeterminant left main coronary artery disease (CAD).
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IVUS and coronary angiography are within reason 4-6 wk and 1 yr after cardiac transplantation to rule out donor CAD, detect rapidly progressive cardiac allograft vasculopathy, and provide prognostic information.
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IVUS is a reasonable option to determine the mechanism of stent restenosis.
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IVUS may be reasonable in assessing non–left main coronary arteries possessing angiographically intermediate coronary stenoses (ie, 50-70% diameter stenosis).
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IVUS may be considered for the guidance of coronary stent implantation, especially in cases of left main coronary artery (LMCA) stenting.
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IVUS may be reasonable for the determination of the mechanism of stent thrombosis.
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IVUS for routine lesion assessment is not a recommendation if revascularization with PCI or coronary artery bypass graft (CABG) is not being contemplated.
The European Society of Cardiology (ESC) recommended the following for IVUS [12] :
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To assess the severity and optimize the treatment of unprotected left main coronary lesions.
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To detect stent-related mechanical problems leading to restenosis.
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To optimize stent implantation.
Indications
Preinterventional IVUS allows assessment of plaque distribution, ostial involvement, lumen and vessel area and diameters, extent of calcification, and the presence of thrombi or dissections. [13, 14, 15] It can alter strategy and the decision to use a particular device.
Intravascular ultrasonography can be highly useful for percutaneous coronary intervention (PCI). PCI results can be optimized with the use of interactive IVUS during the procedure. [16] However, the use of IVUS during PCI remains low, and additional prospective randomized, controlled trials are needed. [17]
Two major trials—the Strategy for Intracoronary Ultrasound-Guided PTCA and Stenting (SIPS) trial and the Balloon Equivalent to Stent (BEST) study—evaluated potential benefits and limitations of ultrasound-guided balloon angioplasty and routine stenting. [18, 19] 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 as 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 IVUS-guided balloon angioplasty is a noninferior alternative to routine stenting, this approach is certainly more time consuming and requires meticulous attention to detail, along with expertise in IVUS image acquisition and interpretation. Researchers 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 IVUS-guided balloon angioplasty. [20]
The Multicenter Ultrasound Stenting in Coronaries (MUSIC) study established the safety and feasibility of IVUS-guided stent implantation. [21] Fitzgerald and associates evaluated whether routine ultrasound guidance of stent implantation improved clinical outcomes as compared with angiographic guidance alone in the Can Routine Ultrasound Influence Stent Expansion (CRUISE) trial. [22] Although no clinical outcome benefits were demonstrated with routine use of IVUS, more effective stent expansion was noted when compared with angiographic guidance alone.
Casella et al conducted a meta-analysis of studies on this topic and found that IVUS-guided stent implantation has a neutral effect on long-term death and nonfatal myocardial infarction compared with angiographic optimization. [23] However, study authors noted that IVUS-guided stenting significantly lowers 6-month angiographic restenosis and target vessel revascularization.
In their appraisal of IVUS and its application in routine angioplasty, Oxford and coworkers noted that IVUS is better than contrast angiography in key procedural variables, such as measuring postdeployment stent dimensions, confirming complete stent apposition, and excluding edge dissections that may predispose to both early and late complications, including in-stent restenosis. [24] The clinical usefulness of ultrasound guidance in stent deployment maintains its value. Particularly for small vessels, bifurcation stenting, ostial lesions, long segments, and left main stenting, ultrasound can provide beneficial guidance. [25, 26]
Intravascular ultrasound plays a vital role in characterizing plaque structures and in planning debulking (atherectomy) procedures by differentiating between superficial (intimal) and deep calcium deposits. [27] Intravascular ultrasonography has emerged as superior to routine angiography for guiding selective plaque removal. [28] Rotational atherectomy is preferred over directional atherectomy in cases of superficial calcification. [29]
Interventional radiology applications of IVUS continue to expand, complementing intraprocedural angiography and helping to guide endovascular interventions. Vascular conditions can be visualized from the planar appearance of an opacified vascular lumen; perivascular targets are visualized on the basis of fluoroscopic landmarks. Common applications of IVUS include deep venous thrombosis, May-Thurner syndrome, nutcracker syndrome, transjugular intrahepatic portosystemic shunts, aortic interventions, peripheral arterial disease, and endovascular or perivascular biopsy. [30]
Technical Considerations
Intravascular ultrasonography consists of a miniature ultrasound-mounted catheter that is connected to an electronics console to reconstruct 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, a portion of ultrasound energy returns to the transducer. The signal received is converted to electrical energy and is 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, resolution is greater, and the characteristic backscatter (reflection of ultrasound energy) from a given tissue is more accurate. Thus, 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.
