Intravascular Ultrasound (IVUS) is a technique providing cross-sectional, high-resolution tomographic images of the arterial wall. This technique yields qualitative and quantitative assessment of the extent and severity of arterial atherosclerotic diseases.

By detecting luminal boundaries, external elastic membrane (EEM) boundaries and stent boundaries IVUS is able to quantify luminal area, plaque thickness, area obstruction etc. thus evaluating the results of devices and pharmaceutical treatments¹. The Cardialysis IVUS Core Laboratory has extensive experience in analysing ‘stent studies’ as well as ‘plaque progression/regression’ studies^2-4.

Assessment of Incomplete Stent Apposition
Recently, Incomplete stent apposition (ISA) has gained a lot of attention in the field of interventional cardiology. A potential relation between ISA and (late) stent thrombosis in (drug-eluting) stents is being investigated extensively in contemporary clinical trials. In the Cardialysis Core lab, ISA will be analyzed at baseline as well as at follow-up in order to assess whether ISA has been resolved (present after stenting but not at follow-up); is persistent (present both after stenting and follow-up); or has been late-acquired (not present at baseline but present at follow-up). ISA can be scored using both as a visual (qualitative) assessment and as a quantitative analysis⁵.

Virtual Histology IVUS
Virtual Histology is an intravascular ultrasound derived colour coded plaque characterization technique. It is based on the analysis and classification of the radiofrequency component of the backscattered ultrasound signal. As compared with standard IVUS, this imaging modality has the potential for more detailed assessment of different plaque components. In preliminary in vitro studies, four plaque types (fibrous tissue, fibro-fatty, necrotic core and dense calcium) as determined by histology, could be correlated with a specific spectrum of the radiofrequency signal⁶. This technique, given its ability to identify necrotic-rich plaques, would be of great value in identifying potentially vulnerable plaque^{7, 8}.

Palpography Intravascular
Palpography is an imaging technique based on IVUS that assesses the local elasticity of the vessel wall and plaque. The underlying principle is that soft material will deform more readily than hard material when force is applied to the tissue. The naturally occurring pulsatile arterial pressure provides the force. The relative deformability of coronary plaque components can be estimated by measuring the relative displacements of radiofrequency (RF) signals, recorded during IVUS acquisition, at 2 different pressure levels. Post-processing of RF signals generates data regarding deformation of the tissue and allows the construction of a “strain” image in which “harder” (low strain) and “softer” regions (high strain) can be identified^{9, 10}.

NIR Spectroscopy
Another imaging modality able to characterize coronary atherosclerosis (i.e. lipid core) invasively is NIR spectroscopy¹¹. To this aim, the 3.2F FDA approved NIRS catheter is used. This catheter is compatible with a conventional 0.014” guidewire, contains a rotating (240Hz) NIRS light source at its tip, and is pulled back by a motor drive unit at 0.5mm/s¹². A newer catheter has been introduced. The 3.2F rapid exchange Apollo catheter combines a 40 MHz real time IVUS catheter with a standard NIRS catheter. In Europe, this imaging modality is being used in IBIS 3 trial which is a study that will be able to assess the effects of rosuvastatin on the content of necrotic core (IVUS-VH) and lipid-containing regions (NIR spectroscopy) at 52 weeks. These trials are being conducted by Cardialysis.

Literature References
1. Serruys PW, Ormiston JA, Onuma Y, Regar E, Gonzalo N, Garcia-Garcia HM, et al. A bioabsorbable everolimus-eluting coronary stent system (ABSORB): 2-year outcomes and results from multiple imaging methods. Lancet 2009;373(9667):897-910.
2. Serruys PW, Morice MC, Kappetein AP, Colombo A, Holmes DR, Mack MJ, et al. Percutaneous coronary intervention versus coronary-artery bypass grafting for severe coronary artery disease. N Engl J Med 2009;360(10):961-72.
3. Serruys PW, Garcia-Garcia HM, Buszman P, Erne P, Verheye S, Aschermann M, et al. Effects of the direct lipoprotein-associated phospholipase A(2) inhibitor darapladib on human coronary atherosclerotic plaque. Circulation 2008;118(11):1172-82.
4. Gerstein HC, Ratner RE, Cannon CP, Serruys PW, Garcia-Garcia HM, van Es GA, et al. Effect of Rosiglitazone on Progression of Coronary Atherosclerosis in Patients With Type 2 Diabetes Mellitus and Coronary Artery Disease. The Assessment on the Prevention of Progression by Rosiglitazone on Atherosclerosis in Diabetes Patients With Cardiovascular History Trial. Circulation.
5. Tanabe K, Serruys PW, Degertekin M, Grube E, Guagliumi G, Urbaszek W, et al. Incomplete stent apposition after implantation of paclitaxel-eluting stents or bare metal stents: insights from the randomized TAXUS II trial. Circulation 2005;111(7):900-5.
6. García-García HM MG, Lerman A, Vince DG, Margolis MP, van Es GA, Morel MA, Nair A, Virmani R,, Burke AP SG, Serruys PW. Tissue characterisation using intravascular radiofrequency data analysis: recommendations for acquisition, analysis, interpretation and reporting. Eurointervention 2009;5:177.
7. Garcia-Garcia HM, Goedhart D, Schuurbiers JC, Kukreja N, Tanimoto S, Daemen J, et al. Virtual histology and remodeling index allow in vivo identification of allegedly high risk coronary plaques in patients with acute coronary syndromes: a three vessel intravascular ultrasound radiofrequency data analysis. Eurointervention 2006;2:338-344.
8. Garcia-Garcia HM, Gonzalo N, Regar E, Serruys PW. Virtual histology and optical coherence tomography: from research to a broad clinical application. Heart 2009;95(16):1362-74.
9. Schaar JA, De Korte CL, Mastik F, Strijder C, Pasterkamp G, Boersma E, et al. Characterizing vulnerable plaque features with intravascular elastography. Circulation 2003;108(21):2636-41.
10. Schaar JA, Regar E, Mastik F, McFadden EP, Saia F, Disco C, et al. Incidence of high-strain patterns in human coronary arteries: assessment with three-dimensional intravascular palpography and correlation with clinical presentation. Circulation 2004;109(22):2716-9.
11. Gardner CM TH, Hull EL, Lisauskas JB, Sum ST, Meese TM, Jiang C, Madden SP, Caplan JD, Burke AP, Virmani R, Goldstein J, Muller JE. Detection of Lipid Core Coronary Plaques in Autopsy Specimens with a Novel Catheter based Near-infrared Spectroscopy System. Am Coll Cardiol Img 2008;1:638-48.
12. Gardner CM, Tan H, Hull EL, Lisauskas JB, Sum ST, Meese TM, et al. Detection of lipid core coronary plaques in autopsy specimens with a novel catheter-based near-infrared spectroscopy system. JACC Cardiovasc Imaging 2008;1(5):638-48.
13. Waxman S, Dixon SR, L'Allier P, Moses JW, Petersen JL, Cutlip D, et al. In vivo validation of a catheter-based near-infrared spectroscopy system for detection of lipid core coronary plaques: initial results of the SPECTACL study. JACC Cardiovasc Imaging 2009;2(7):858-68.