IVUS Principle and Image interpretationIVUS Principle and Image interpretation

Liu Jian, M.D.

Peking University People’s Hospital

Ultrasound PrinciplesUltrasound Principles

Rotating Element

Drive Shaft

Multi-Element Array

• Two types of imaging systemsTwo types of imaging systems Mechanical (rotating transducer)Mechanical (rotating transducer) Electronic arrayElectronic array

Basic principles (I)Basic principles (I)

Medical ultrasound images are produced by passing Medical ultrasound images are produced by passing

an electrical current through a piezoelectric crystal an electrical current through a piezoelectric crystal

(usually a ceramic) that expands and contracts to (usually a ceramic) that expands and contracts to

produce sound waves when electrically excitedproduce sound waves when electrically excited

After reflection from tissue, these same sound waves After reflection from tissue, these same sound waves

return to the crystal (transducer) to create an return to the crystal (transducer) to create an

electrical impulse that is then converted into the electrical impulse that is then converted into the

imageimage

The beam remains fairly parallel for a distance The beam remains fairly parallel for a distance (near field) and then begins to diverge (far (near field) and then begins to diverge (far field)field)• The quality of ultrasound images is better in the The quality of ultrasound images is better in the

near field because the beam is more parallel and near field because the beam is more parallel and the resolution greaterthe resolution greater

• The length of the near field is expressed by the The length of the near field is expressed by the equation L = r2 / , where L is the length of the near equation L = r2 / , where L is the length of the near field, r is the radius of the transducer, and is the field, r is the radius of the transducer, and is the wavelengthwavelength

• Therefore, larger transducers with lower Therefore, larger transducers with lower frequencies are used for examination of large frequencies are used for examination of large vessels because they create a deeper near field vessels because they create a deeper near field

Basic principles (II)Basic principles (II)

Ultrasound PrinciplesUltrasound Principles

High frequency sound waves echo off vessel walls and are sent back to system

System electronics process the signal

Image quality can be described by two Image quality can be described by two important factorsimportant factors• Spatial resolution (Spatial resolution ( 空间分辨率空间分辨率 ))

– Ability to discriminate small adjacent objects within the Ability to discriminate small adjacent objects within the imageimage

– For a 30-40MHz IVUS transducer the typical resolution is For a 30-40MHz IVUS transducer the typical resolution is 80-100 microns axially and 200-250 microns laterally 80-100 microns axially and 200-250 microns laterally

• Contrast resolutionContrast resolution （对比度分辨率）（对比度分辨率）

Basic principles (III)Basic principles (III)

Ultrasound PrinciplesUltrasound Principles

A

L

A=Axial ResolutionL=Lateral Resolution

Contrast resolution or dynamic rangeContrast resolution or dynamic range （动态范围）（动态范围）

= the distribution of the grayscale of the reflected signal = the distribution of the grayscale of the reflected signal

• Low dynamic range image appears “black and Low dynamic range image appears “black and

white” with only a few “in-between” grayscale levelswhite” with only a few “in-between” grayscale levels

• High dynamic range image has more shades of grey, High dynamic range image has more shades of grey,

are often “softer,” and have more preserved are often “softer,” and have more preserved

subtleties in the image presentationsubtleties in the image presentation

Basic principles (IV)Basic principles (IV)

Low Dynamic Range High Dynamic Range

Ultrasound PrinciplesUltrasound Principles

Basic principles (V)Basic principles (V)

Ultrasound will bounce off of some vascular Ultrasound will bounce off of some vascular structures and pass through othersstructures and pass through others• A structure’s acoustic impedanceA structure’s acoustic impedance （声阻抗）（声阻抗）

(density) determines if ultrasound will bounce off (density) determines if ultrasound will bounce off or travel through the structureor travel through the structure

• If sound bounces off the structure, and returns to If sound bounces off the structure, and returns to the transducer, it will appear ‘white’ on the screenthe transducer, it will appear ‘white’ on the screen

• Very dense material, like calcium, will reflect all the Very dense material, like calcium, will reflect all the ultrasound (appear very white) and not allow any ultrasound (appear very white) and not allow any ultrasound to pass through (producing a black ultrasound to pass through (producing a black acoustic shadow beyond the calcium)acoustic shadow beyond the calcium)

ArtifactsArtifacts

• Nearfield artifactsNearfield artifacts RingdownRingdown（环晕）（环晕） Blood speckle is more intense with higher frequency Blood speckle is more intense with higher frequency

transducers and slows blood flow limiting the ability to transducers and slows blood flow limiting the ability to differentiate lumen from tissue (especially “soft” plaque, differentiate lumen from tissue (especially “soft” plaque, neointima, and thrombus) neointima, and thrombus)

• Flushing contrast or saline through the guiding Flushing contrast or saline through the guiding catheter may clear the lumen and help to identify catheter may clear the lumen and help to identify tissue borderstissue borders

• Motion artifactsMotion artifacts NURD (Non-Uniform Rotation Distortion )NURD (Non-Uniform Rotation Distortion ) 不均匀旋转伪像不均匀旋转伪像 Axial catheter motion with cardiac cycleAxial catheter motion with cardiac cycle

ArtifactsArtifacts

Flush

• NURDNURD

• Blood Blood specklespeckle

Artifacts (II)Artifacts (II)

Mistaking guiding catheter for aorto-ostial Mistaking guiding catheter for aorto-ostial

stenosisstenosis

Position artifactsPosition artifacts

Catheter obliquityCatheter obliquity

Catheter eccentricityCatheter eccentricity

Vessel curvatureVessel curvature

Mistaking guiding catheter for aorto-ostial stenosisMistaking guiding catheter for aorto-ostial stenosis

Side Lobes

Guiding catheterGuiding Catheter

IVUS images interpretationIVUS images interpretation

Plaque ID and CharacteristicsPlaque ID and Characteristics

• AppearanceAppearance Intimal diseaseIntimal disease

• Plaque is dense and will appear Plaque is dense and will appear ‘white’‘white’

MediaMedia

• Made of homogeneous smooth Made of homogeneous smooth muscle cells and does not reflect muscle cells and does not reflect ultrasound (appears dark)ultrasound (appears dark)

Adventitia Adventitia

• Has ‘sheets’ of collagen that Has ‘sheets’ of collagen that reflect a lot of ultrasound reflect a lot of ultrasound (appears white)(appears white)

Normal Diseased

Plaque ID and CharacteristicsPlaque ID and Characteristics

CalciumCalcium

Bright echoes (brighter than the adventitia)Bright echoes (brighter than the adventitia) Obstructs the penetration of ultrasound Obstructs the penetration of ultrasound

(acoustic shadowing)(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’

CalciumCalcium

Calcium is Calcium is quantifiedquantified by measuring the by measuring the “arc” it encompasses“arc” it encompasses

Superficial

DeepDeep80°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

Fibrotic plaqueFibrotic plaque

As bright as or brighter than the adventitia As bright as or brighter than the adventitia (hyperechoic)(hyperechoic)

Majority of atherosclerotic lesions are fibrotic Very dense, fibrous plaques may cause so much

acoustic shadowing that they could be misclassified as calcified

Soft plaqueSoft plaque

Not as bright as the adventitia (hypoechoic)Not as bright as the adventitia (hypoechoic)

““Soft” refers to the low echogenicity, generally due Soft” refers to the low echogenicity, generally due

to high lipid content in a mostly cellular lesionto high lipid content in a mostly cellular lesion

Reduced echodensity may also be due to• Necrotic zone within plaque• Intramural hemorrhage• Thrombus

Vulnerable plaqueVulnerable plaque

Fibrous Cap Lipid Core

ThrombusThrombus

Thrombus in Both Main & BranchThrombus in Both Main & Branch

MLA

End of rupture

Fibrous cap

Plaque RupturePlaque Rupture

True Aneurysm True Aneurysm

Pseudo Aneurysm Pseudo Aneurysm

Complex Plaque Complex Plaque

Proximal

Rupture with ThrombusRupture with Thrombus

0 2.0 6.0mm

Filling Defect : Calcium Nodule Filling Defect : Calcium Nodule

““Muscle Bridge”Muscle Bridge”

DiastolicDiastolic SystolicSystolic

Inside the muscle bridge, Inside the muscle bridge, no plaque at all!no plaque at all!

Proximal

5555y.o. Female, AMIy.o. Female, AMI

“Spontaneous Dissection”

A B CD

BA C D

42y.o. Male, Unstable AP42y.o. Male, Unstable AP

Normal Artery, HematomaMaehara et al. AJC 2002

Thank you for your attention!