219 229 doi:10.1038/ncpcardio1123 received 5 october 2007 accepted 13 december 2007 published online: 26 february 2008 correspondence department of medicine, division of cardiology, medizinische klinik und poliklinikcampus innenstadt, ludwig maximilians universit t, ziemssenstrae 1, 80336 munich, germany a tiny ultrasound wand is attached to the top of a thin tube called a catheter. This ultrasound catheter is inserted into an artery in your groin area and moved up to the heart. It is different from conventional duplex ultrasound, which is done from the outside of your body by placing the transducer on the skin.

A computer measures how the sound waves reflect off blood vessels, and changes the sound waves into pictures. Ivus gives the health care provider a look at your coronary arteries from the inside out. Angioplasty gives a general look at the coronary arteries, but it cannot show the walls of the arteries. Ivus images show the artery walls and can reveal cholesterol and fat deposits plaques.

Ivus is commonly done to make sure a stent is correctly placed during angioplasty. doi: 10.1186/1476 7120 9 2 received: 27 october 2010 accepted: 30 january 2011 published: 30 january 2011 intravascular ultrasound ivus is an invasive modality which provides cross sectional images of a coronary artery. In these images both the lumen and outer vessel wall can be identified and accurate estimations of their dimensions and of the plaque burden can be obtained. In addition, further processing of the ivus backscatter signal helps in the characterization of the type of the plaque and thus it has been used to study the natural history of the atherosclerotic evolution. On the other hand its indigenous limitations do not allow ivus to assess accurately stent struts coverage, existence of thrombus or exact site of plaque rupture and to identify some of the features associated with increased plaque vulnerability. In order this information to be obtained, other modalities such as optical coherence tomography, angioscopy, near infrared spectroscopy and intravascular magnetic resonance imaging have either been utilized or are under evaluation.

The aim of this review article is to present the current utilities of ivus in research and to discuss its advantages and disadvantages over the other imaging techniques. Utilization of conventional coronary angiography has certain limitations in the prognosis of coronary atherosclerosis, as the risk of experiencing a coronary event does not only depend upon the severity and extent of a lesion, but also on the size, histology and biological activity of the plaque. Some of these limitations can be addressed by intravascular ultrasound ivus a modality which provides two dimensional 2 d cross sectional arterial images.

In these images the lumen, outer vessel wall, plaque and stent can be identified and accurate measurements can be obtained. The fact that it provides reliable results in real time and it is widely available has rendered it a useful tool in clinical practice and research. Thus, though it is an expensive and time consuming procedure and caries a small risk of complications mainly spasm but also the risk of embolism, thrombus formation and dissection , ivus is often used to assess the severity of intermediate lesions, to guide treatment in high risk patients and complex lesions and to examine the final outcome after a percutaneous coronary intervention pci 1. Apart from its clinical applications, ivus has also been proven a useful tool in research in the study of plaque evolution and in the evaluation of new interventional or pharmacological treatments. Recent developments in ivus processing and especially the analysis of intravascular ultrasound radiofrequency ivus rf backscatter signal have provided further information regarding the composition and mechanical properties of the plaque and enhanced the role of ivus in the study of atherosclerosis 3 . However, ivus still has indigenous limitations such as the noise and the low axial resolution, which do not allow detailed visualisation of certain lumen and plaque characteristics. For these limitations to be addressed, alternative invasive modalities with different strengths and weaknesses have been developed such as angioscopy, optical coherence tomography oct , near infrared spectroscopy nirs and intravascular magnetic resonance imaging iv mri.

The aim of this review article is to discuss the advantages and disadvantages of ivus over the other imaging techniques and highlight its value in research. The superiority of ivus imaging over angiography were obvious from its initial steps 4. To facilitate its application in research a number of tools were developed that provided fast and reliable ivus processing and quantitative analysis 6 – 8 .

This allowed the use of ivus in numerous studies which helped us to explain the mechanisms of atherosclerotic process and affected the evolution in interventional cardiology. In the early stages of pcis ivus was used to elucidate the mechanisms of action of balloon angioplasty arterial expansion and plaque fracture and understand the causes of restenosis vessel wall negative remodelling and intima proliferation 9. These data suggested the use of stents as it was believed that they would reduce the risk of restenosis.

However, during the initial use of bare metal stents bms high rate of acute and sub acute stent thrombosis as well as restenosis were noted. Ivus imaging identified as predictors of sub acute stent thrombosis the incomplete stent strut apposition, the asymmetrical stent expansion and the residual lumen narrowing and also showed that restenosis mainly occurs within the first 6 months post stent implantation 11 . To overcome these problems post stent dilation with larger balloons and higher pressures was recommended while research was driven towards the creation of advanced stent platforms and the development of drug eluting stents des 12 . Coronary angiography is unable to provide detailed information regarding plaque burden, as initially atherosclerosis may not cause luminal narrowing accommodating the evolving plaque in the vessel wall which expands outward positive remodelling. On the other hand ivus permits complete vessel visualisation and accurate assessment of the atherosclerotic burden and thus it appears more sensitive than quantitative coronary angiography qca in detecting the progression of atherosclerosis 13 . The fact that high plaque burden is related to a higher risk of cardiovascular events has allowed ivus measurements to be used as surrogate endpoints, instead of clinical endpoints, in trials that investigated the effect of several pharmacological treatments on plaque progression 14.

In this way ivus imaging appeared a cost effective technique as it permitted studies to be conducted with a smaller number of patients and completed in shorter time interval. Hence, today it is known that aggressive lipid lowering therapy with high doses of atorvastatin or rosuvastatin induces plaque regression and that pioglitazone has a favourable effect on coronary atherosclerosis 16 – 20 . Similarly, the camelot study used ivus to show that amlodipine reduces plaque burden while the perspective study demonstrated that perindopril does not affect the progression of the atheroma 21. In addition, serial ivus examinations were implemented to study the effect of new drugs such as the reconstituted hdl csl 1 , the dalapladip a lipoprotein associated phospholipase a₂ inhibitor and the pactimibe a non selective inhibitor of acyl coenzyme a:cholesterol acyltransferase on plaque development.

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In these studies it was found that all the new treatments had a neutral effect on plaque burden though dalapladip appeared to reduce the lipid core expansion 23 – 25 . The ability of ivus to display both the plaque and the whole vessel wall provided us with an insight into the mechanisms, and the prognostic value of vascular remodelling. Ivus has been used to show that negative remodelling defined as a shrinkage of the vessel wall at the lesion site is more common in elderly patients and stable plaques, whilst on the other hand it appears that plaques with positive remodelling defined as vessel wall expansion at the lesion site contain more lipid rich components, and are associated with an increased risk for acute coronary events 26.

Compared to other invasive imaging techniques ivus is superior in quantifying changes in plaque volume and measuring vessel wall dimensions and thus is the preferable modality for assessing the effect of pharmacological treatment on coronary atherosclerosis. Angioscopy cannot be used to measure the plaque as it provides only imaging of the luminal surface and gives no data on the vessel wall. On the other hand, oct although allows imaging of the atheroma, and more accurate computation of plaque volume in case of calcium deposits, since it lacks the shadowing artefacts, often cannot portray the whole vessel as it has poor tissue penetration figure 1 . Spectral radar oct, use of a parallel ultrasound beam, image processing techniques etc.

However, further development is necessary before these techniques can be used in vivo for the evaluation of vascular pathology 28 . oct imaging may allow visualisation of the vessel wall behind a calcified plaque a but on the other hand often fails to fully visualise the arterial wall because of its poor penetration b . The limitation of ivus to identify the type and the extent of the plaque behind the calcium arrow c has been successfully addressed by ivus rf analysis d. It is well known that ivus has limited capability in assessing plaque composition and detecting the features associated with plaque's vulnerability. To address these drawbacks analysis of ivus radiofrequency backscatter signal has been proposed. This signal processing approach was validated using histopathologic findings as gold standard and it was found that it can identify the type of the plaque with high sensitivity, specificity and predictive accuracy 29. This ability, the comprehensive images and the quantitative measurements that it provides allowed the broad use of ivus rf in the study of plaque development.