IVUS Tales: From Research to its Clinical application in ...
Transcript of IVUS Tales: From Research to its Clinical application in ...
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IVUS Tales: From IVUS Tales: From Research to its Research to its
Clinical application Clinical application in Contemporary in Contemporary
Interventions.Interventions.Presenter: Islam BoladPresenter: Islam Bolad
Attending: Jose DiezAttending: Jose Diez
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•Visual interpretation of coronary angiography exhibits intraobserver and interobserver variability (<50%)
• Angiography & postmortem histology.
• QCA
• Glagov phenomenon
Coronary Angiography
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Am J Cardiol 2002;89(suppl):24B-31B
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IVUS
• Vessel wall vs. lumen.
• Internal electronic distance scale
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IVUS Technology
• Real time high resolution imaging.
• 2D tomographic assessments of vessels Also longitudinal and 3D computer asssited reconstruction.
• Allows assessment of total vessel lumen and plaque dimension in vivo.
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Two main IVUS systems are currently in use:
1- A mechanical system that contains a flexible imaging cable which rotates a single transducer at its tip inside an echo-lucent distal sheath.
2- An electronic solid state catheter system with multiple imaging elements at its distal tip, providing cross sectional imaging by sequentially activating the imaging elements in a circular way.
• 1 is usually smaller than 2.
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• IVUS catheters max. diameter 2.6-3 Fr (0.89-1mm)
• Motorized pull back of transducer (0.25-1mm/sec, usually 0.5mm/sec)
• Volumetric measurement.
• Imaging frequencies increased- improved qualitative assessment of atherosclerotic plaques. - Soft, low echogenecity - Fibrous, high echogenicity - Calcified, high echogenicity with acoustic shadowing/ reverberations.
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•Recently, more advanced IVUS plaque characterization has been introduced.
1- Analysis of the backscatter IVUS radiofrequency data provided a color coded mapping based on the different backscatter signals among the tissue types (virtual histology).
- Allows examination of the different plaque components in more details (fibrous, fibro-lipidic, calcium, lipid core)
Nair et al Circulation. 2002 Oct 22;106(17):2200-6.
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2- Intravascular elastography. IVUS radiofrequency acquired at different levels of intravascular pressure can measure tissue strain reflecting the mechanical properties of the vessel wall.
- Help identify vulnerable plaque prior to rupture.
• Both techniques require further validation.
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Technical Aspects
• Transducers with US frequencies ranging between 20-50 MHz are used (usually 30MHz).
• High frequencies provide excellent theoretical resolution, as US wavelength which determines the maximum resolution is inversely proportional to frequency.
• AT 30MHz, the wavelength is ≈50µm, which permits an axial resolution of 100µm. Lateral resolution ≈250µm.
Metz JA et al.
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Resolutions
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Perivascular Landmarks.
• A well defined imaging protocol is vital for proper IVUS interpretation in the coronary tree.
• Slow pullback from distal to proximal vessel.
• Perivascular markings are important reference for axial position and tomographic orientation within the artery.
• Important for reproducibility of examination within same segment.
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LAD
• Diagonals
• Anterior Interventricular Vein. - Left of proximal and mid LAD in 85%. D1 & D2 emerge from LAD on same side of vein. - Right of LAD in 15% and crosses it near bifurcation of the LCx. - In 30%, the AIV branches into 2 beyond D2.
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In its distal portion, the anterior interventricular vein (AIV) may branch into two vessels accompanying the LAD on both sides
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The elliptic shape of the AIV can be appreciated at 3 o'clock in this cross section from the mid LAD
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LAD/LCx bifurcation, LCx, GCV triangle. Triangle of Brocq & Mouchet
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LCx
• Distally, Cx is accompanied by posterior LV vein
•Proximally, Cx is accompained and crossed by great cardiac vein.
• GCV & posterior LV vein form coronary sinus, best visualized from distal RCA.
• GCV runs superior to Cx , just inferior to LA appendage.
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The great cardiac vein is seen best from the proximal Cx as a large, almost clearstructure filled with fine blood speckle.
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RCA
• Translational effect (like CX) as it is an AV groove artery.
• The marginal veins (in contrast to LAD) cross over artery in an arcing pattern.
• Usually, small amount of fluid near the crux.
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Veins are associated with RV marginal branches and are characterized by an arching pattern around the RCA.
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The Endovascular Anatomy. The arterial wall.
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Different echogenic qualities is due to the relative amount of collagen (1000x reflectance than smooth muscles) and elastin
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Blood
• Speckled pattern that is constantly changing with systolic and diastolic blood flow alterations ( > echogenic in systole).
• In real-time imaging, lumen/ intima has distinct appearance; in still frames blood speckle can have a pattern similar to plaque.
• Blood stagnation proximal to a stenosis may have a similar effect. Saline flush can clear the lumen temporarily.
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Calcific Plaque
• Calcific plaque is the simplest tissue type to identify
• Bright reflection of intense signal attenuation.
• “Ghost Arcsor” reverbrations.
• Calcification is seen in 60-80% of target lesions using IVUS compared to 30-40% by angiography.
• 180 degrees of vascular circumference must be calcified before it can be visualized by angiography.
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• Sometimes, shadowing with no bright reflections occurs in calcified lesions.
• IVUS imaging of calcium is angle dependent, and the calcific plaque itself is imaged only when the beam is perpendicular
• Acosutic shadowing can occur in the absence of calcium in the presence of dense fibrous tissue.
• Therefore it is correct to refer to lesions with shadowing on IVUS as fibrocalcific. This distinction does not have major clinical implication.
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Fibrous Plaque
• Plaques with echogenicity that is < bright than than calcium, but higher than that from muscle or fat tissue.
• In general, brightness of fibrous tissue is similar to that of adventitia.
• No reverebrations.
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Fatty Plaque
• Radiolucent, and has a soft grey-scale appearance on IVUS.
• Radiolucent areas within fibrous plaques reflect accumulation of lipid.
• Shadowing from a heavily fibrotic plaque can be mistaken for lipid.
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Plaque distribution and remodeling
• Significant plaque burden (30-40%) & normal arteries by angiography.
• Positive remodeling.
•Concentric / focal remodeling.
•Positive remodeling is exhausted when 50% of the lumen is occupied by plaque, and further growth results in lumen encroachment.
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RCA: IVUS from segments 5mm apart without vessel branching between them.A- Small vessel with some element of focal calcification.B- Dramatic vessel remodeling in a fibrofatty lesion.
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• Negative remodeling / de-remodeling.
• Commonly seen as part of restenosis process following PCI.
• Vessel scarring & shrinkage may in some caces contribute significantly to late lumen loss after PTCA
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Endovascular Entities. Thrombus.
One of the most difficult tissue types to identify by IVUS
• Sparkling pattern on real time IVUS imaging.
• Presence of microchannels, echodensity < 50% of the surrounding adventitia and deep Ca are clues to the correct diagnosis of thrombus.
• Sometimes, lobular or cauliflower-like appearance.
•Identification of thrombus after stenting may sometimes be vital.
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Thrombus after stent deployment.
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Post PTCA and Reopro
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False Lumen
• Recognition of 3 layered appearance (true lumen), observation of slower and more echogenic blood reflectance (commonly in false lumen) and identification of branches taking off from true lumen provide clues to discriminate the 2 lumina.
• Contrast material injection can sometimes be helpful because the echogenic patterns from contrast hung-up and takes longer to evacuate a false lumen.
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Aneurysms
• Useful in discriminating between true and false aneurysms.
• Histologically, presence of media differentiates true from false aneurysms.
• In true aneurysms, the media is thinned and expanded but fully encompasses the perimeter of the aneurysm.
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Black Holes
•Initially described following brachytherapy.
•Thought to represent tissue is acellular and necrotic and lacks connective tissue elements1.
• Kay et al2 showed that it is tissue rich in proteoglycans while poor in mature collagen &elastin
•Now seen with DES.
1- Circulation. 2001 Feb 6;103(5):778. 2- Int J Cardiovasc Intervent. 2003;5(3):137-42.
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Quantitative Coronary Ultrasound (QCU)
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Evaluating Intermediate Coronary Lesions.
• Abizaid et al compared various IVUS parameters with CFR.
• Linear relation between CFR and minimum LCSA.
• They defined minimum LCSA as ≤4mm2 and demonstrated concordance of 89% with CFR (abnormal CFR <2).
Am J Cardiol. 1998 Aug 15;82(4):423-8.
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• Nishioka et al compared IVUS parameter with nuclear perfusion imaging.
• They found that minimum LCSA ≤4mm2 had sensitivity of 88% and specificity of 90% for predicting reversible perfusion defect.
• Other IVUS parameters (eg % area stenosis) performed less well.
J Am Coll Cardiol. 1999 Jun;33(7):1870-8
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• Takagi et al compared IVUS parameters with FFR for determining functional significance of moderate lesions.
• Strong correlation between minimum LCSA and FFR
•Using cutoff of ≤3mm2 to define abnormal minimum LCSA and < 0.75 to define abnormal FFR, the investigators found IVUS had a sensitivity of 83% and specificity of 92% for detecting ischemia producing lesions based on FFR.
Circulation. 1999 Jul 20;100(3):250-5.
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• Briguori et al compared IVUS with FFR only in patients with intermediate lesions.
• IVUS minimum LCSA was significantly related to FFR (r=0.41, p<0.004).
• The sensitivity and specificity of minimum IVUS LCSA of ≤4mm2 for predicting FFR ≤0.75 were 92% and 56%.
Am J Cardiol. 2001 Jan 15;87(2):136-41.
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What about the LMS?
• Jasti et al examined 55 patients with an angiographically ambiguous LMCS, a pressure guidewire was used to calculate FFR, and IVUS parameters were calculated after automatic pullback.
• IVUS MLD = 3.8±0.61 mm, MLA = 7.65±2.9 mm2, cross- sectional narrowing (CSN) = 59±13%, , and area stenosis (AS) = 47±19%.
Circulation. 2004;110:2831-2836
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• Regression analysis demonstrated strong correlations between FFR and MLD as well as between FFR and MLA.
• Compared with FFR as the "gold standard," an MLD of 2.8 mm had the highest sensitivity and specificity (93% and 98%, respectively) for determining the significance of an LMCS, followed by an MLA of 5.9 mm2 (93% and 95%, respectively).
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• Fassa et al performed IVUS on 121 patients with angiographically normal LMSs to determine the lower range of normal minimum lumen area (MLA), defined as the mean - 2 SD.
• They also conducted IVUS studies on 214 patients with angiographically indeterminate LMS lesions, and deferral of revascularization was recommended when the MLA was larger than this predetermined value.
J Am Coll Cardiol. 2005 Jan 18;45(2):204-11
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• In the normal LMSs group, LCSA was 16.25±4.3 mm2.
The lower N value (mean MLA-2SD) was 7.65mm2.
• 7.5mm2 was used as the lower range of normal.
• The majority of patients < 7.5 underwent revasc.
• Follow-up (mean 3.3±2.0 yrs) showed no significant difference in MACE between patients with an MLA <7.5 mm2 who underwent revascularization and those with an MLA ≥7.5 mm2 deferred for revascularization (p = 0.28).
• Based on outcome, the best cut-off MLA by ROC was 9.6 mm2.
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Limitations of IVUS
Ring-down artifact.
• Caused by transducer oscillation filling the area immediately adjacent to the catheter with noise, making this area unavailable for imaging.
• Seen as bright halo of variable thickness surrounding the catheter.
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NURD
• Occurs when the rotating transducer inside the US catheter is exposed to frictional forces (eg catheter bending, hemostatic valve too tight)
• Portions of the images are stretched or compacted
• Catheter manipulation eliminates artifact.
• Can be a problem in calcific arteries.
• NURD does not occur in solid state design (advantage over mechanical design).
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Image Quantification Errors
• Catheter positioning.
• Catheter angulation, especially in large arteries. Can alter vessel geometry.
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Ghost Images
• Occurs when structures of high echogenicity are imaged (eg Calcium, stent struts)
• Appear of the side of the transducer that is opposite the bright structure being imaged.
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Summary
• Gold standard for vessel visualization.
• Led to new insights into the pathophysiology of coronary plaque accumulation.
• Advances in technology will certainly revolutionalize this imaging modality.