0. IVUS Principle and Image interpretation
Liu Jian, M.D.
Peking University People’s Hospital

1. Ultrasound Principles
Two types of imaging systems
Mechanical (rotating transducer)
Electronic array
Rotating Element
Drive Shaft
Multi-Element Array

2. Basic principles (I)
Medical ultrasound images are produced by passing an electrical current through a piezoelectric crystal (usually a ceramic) that expands and contracts to produce sound waves when electrically excited
After reflection from tissue, these same sound waves return to the crystal (transducer) to create an electrical impulse that is then converted into the image

3. Basic principles (II)
The beam remains fairly parallel for a distance (near field) and then begins to diverge (far field)
The quality of ultrasound images is better in the near field because the beam is more parallel and the resolution greater
The length of the near field is expressed by the equation L = r2 / , where L is the length of the near field, r is the radius of the transducer, and is the wavelength
Therefore, larger transducers with lower frequencies are used for examination of large vessels because they create a deeper near field

4. Ultrasound Principles
High frequency sound waves echo off vessel walls and are sent back to system
System electronics process the signal

5. Basic principles (III)
Image quality can be described by two important factors
Spatial resolution (空间分辨率)
Ability to discriminate small adjacent objects within the image
For a 30-40MHz IVUS transducer the typical resolution is microns axially and microns laterally
Contrast resolution（对比度分辨率）

6. Ultrasound Principles
A=Axial Resolution
L=Lateral Resolution

7. Basic principles (IV) Contrast resolution or dynamic range（动态范围）
= the distribution of the grayscale of the reflected signal
Low dynamic range image appears “black and white” with only a few “in-between” grayscale levels
High dynamic range image has more shades of grey, are often “softer,” and have more preserved subtleties in the image presentation

8. Ultrasound Principles
Low Dynamic Range
High Dynamic Range

9. Basic principles (V)
Ultrasound will bounce off of some vascular structures and pass through others
A structure’s acoustic impedance（声阻抗） (density) determines if ultrasound will bounce off or travel through the structure
If sound bounces off the structure, and returns to the transducer, it will appear ‘white’ on the screen
Very dense material, like calcium, will reflect all the ultrasound (appear very white) and not allow any ultrasound to pass through (producing a black acoustic shadow beyond the calcium)

10. Artifacts Nearfield artifacts Ringdown（环晕）
Blood speckle is more intense with higher frequency transducers and slows blood flow limiting the ability to differentiate lumen from tissue (especially “soft” plaque, neointima, and thrombus)
Flushing contrast or saline through the guiding catheter may clear the lumen and help to identify tissue borders
Motion artifacts
NURD (Non-Uniform Rotation Distortion )不均匀旋转伪像
Axial catheter motion with cardiac cycle

11. Artifacts
NURD
Blood speckle
Flush

12. Artifacts (II) Mistaking guiding catheter for aorto-ostial stenosis
Position artifacts
Catheter obliquity
Catheter eccentricity
Vessel curvature

13. Mistaking guiding catheter for aorto-ostial stenosis
Side Lobes

14. IVUS images interpretation

15. Plaque ID and Characteristics
Appearance
Intimal disease
Plaque is dense and will appear ‘white’
Media
Made of homogeneous smooth muscle cells and does not reflect ultrasound (appears dark)
Adventitia
Has ‘sheets’ of collagen that reflect a lot of ultrasound (appears white)

16. Plaque ID and Characteristics
Normal
Diseased

17. Calcium Bright echoes (brighter than the adventitia)
Obstructs the penetration of ultrasound (acoustic shadowing)
Only the leading edge is detected and thickness cannot be determined
Results in reverberations—the oscillation of ultrasound between transducer and calcium causing repeating ‘arcs’

18. Calcium Calcium is quantified by measuring the “arc” it encompasses
Superficial
Deep
Deep
80°
Calcium is classified by its location within the plaque
Superficial calcium is closer to the lumen than to the adventitia
Deep calcium is closer to the adventitia than to the lumen

19. Fibrotic plaque
As bright as or brighter than the adventitia (hyperechoic)
Majority of atherosclerotic lesions are fibrotic
Very dense, fibrous plaques may cause so much acoustic shadowing that they could be misclassified as calcified

20. Soft plaque Not as bright as the adventitia (hypoechoic)
“Soft” refers to the low echogenicity, generally due to high lipid content in a mostly cellular lesion
Reduced echodensity may also be due to
Necrotic zone within plaque
Intramural hemorrhage
Thrombus

21. Vulnerable plaque
Fibrous Cap
Lipid Core

22. Mixed plaque

23. Thrombus
Intraluminal mass, often with a layered, lobulated or pedunculated appearance
Relatively echolucent or variable grayscale with speckling or scintillation
Blood flow in “microchannels” may also be apparent within some thrombi
Stagnant blood flow can simulate a thrombus
Injection of contrast or saline may disperse the stagnant flow, clear the lumen (appears black), and allow differentiation of stasis from thrombosis
No one feature is pathognomonic for thrombus and the diagnosis of thrombus by IVUS should be considered presumptive

24. Thrombus 1
4 mm

25. Thrombus

26. Thrombus in Both Main & Branch

27. Plaque Rupture
Fibrous cap
End of rupture
MLA

28. True Aneurysm

29. Pseudo Aneurysm

30. Complex Plaque

31. Rupture with Thrombus
Proximal

32. Filling Defect : Calcium Nodule
2.0
6.0mm

33. Inside the muscle bridge, no plaque at all!
Diastolic
Systolic

34. “Spontaneous Dissection”
55y.o. Female, AMI
Proximal
“Spontaneous Dissection”

35. Normal Artery, Hematoma
42y.o. Male, Unstable AP B C A D B A C D
Normal Artery, Hematoma
Maehara et al. AJC 2002

36. Thank you for your attention!