EEM and lumen measurement
Lumen cross-sectional area
The area bounded by the luminal border
Minimum lumen diameter
The shortest diameter through the center point of the lumen
Maximum lumen diameter
The longest diameter through the center point of the lumen
Lumen eccentricity
[(Maximum lumen diameter – minimum lumen diameter)/maximum lumen diameter]
Lumen area stenosis
(Reference lumen CSA – minimum lumen CSA)/reference lumen CSA
External elastic membrane cross-sectional area
EEM is an interface at the border between the media and the adventitia. Synonyms: vessel area, total vessel area
Vessel volume
Vessel area measurements can be added to calculate volumes (Simpson’s rule)
Lumen volume
Lumen area measurements can be added to calculate volumes (Simpson’s rule)
Plaque measurement
Plaque + media CSA (atheroma area)
EEM CSA-lumen CSA
Maximum plaque + media(or atheroma) thickness
The largest distance from the intimal leading edge to the EEM along any line passing through the center of the lumen
Minimum plaque + media(or atheroma) thickness
The shortest distance from the intimal leading edge to the EEM along any line passing through the luminal center of mass
Plaque + media (or atheroma) eccentricity
(Maximum plaque plus media thickness – minimum plaque plus media thickness)/maximum plaque plus media thickness
Plaque (or atheroma) burden
Plaque + media CSA/EEM CSA
Plaque volume
Plaque + media CSA measurements can be added to calculate volumes (Simpson’s rule)
Calcium measuremen t
Superficial/deep calcium
The leading edge of the acoustic shadowing appears within the most shallow/the deepest 50% of the plaque plus media thickness
Arc
Measured in degrees by using an electronic protractor centered on the lumen
Semiquantitation
Absent or subtracting one, two, three, or four quadrants
3.1 Vessel Wall Identification
Due to reflection of ultrasound waves at tissue interface, generally, there are two such interfaces in the normal coronary artery [1]. It is important to recognize the leading edge of boundaries when measuring the IVUS image quantitatively [2]. Outside the lumen, the second layer is the media, and the third and outer layer consists of the adventitia and periadventitial tissues [3–8]. Interpretation of IVUS image begins with recognition of these two interfaces: blood/intimal (lumen) and medial/adventitial interface. A relative echo-translucency of media compared with intima and adventitia gives rise to a three-layered appearance of coronary wall (bright, dark, bright) in muscular arteries such as the coronary arteries. As intimal layer reflects ultrasound more strongly than media, there is a spillover effect, called blooming, resulting in slight overestimation of intimal layer and correspondingly underestimation of medial layer. However, the medial-adventitial border is accurately identified as a step-up in echoreflectivity occurring at this border with no blooming effect. In diseased arteries, media may not appear as a distinct layer around the vessel. The adventitia and periadventitial structures are similar in echodensity so that a distinct outer-adventitial border cannot be defined (Fig. 3.1).
Fig. 3.1
Quantitative measurements of IVUS. Panels a and b show the reference segment. Panels c and d represent the target lesion. The EEM and lumen areas are traced (b). The minimum and maximum lumen diameters are shown inside the lumen. In panel d, the minimum and maximum plaque plus media thickness is also assessed using double-headed arrows (blue for minimum and yellow for maximum). The EEM and lumen areas are demonstrated, and the arc of calcification (dotted line) is shown. Panel e represents the measurement of lesion length. EEM external elastic membrane
3.2 Lumen Measurements
The most important parameter of quantitative analysis is the measurement of coronary lumen. Lumen measurements are performed using the interface between the lumen and the leading edge of the intima. The leading edge of the innermost echogenic layer should be used as the lumen boundary. The intimal leading edge can be easily identified because the intima has thickened enough to be resolved as a separate layer and has sufficiently different acoustic impedance from the lumen in normal segments. The vessel wall has a single-layer appearance because the intima cannot be resolved as a discrete layer with a thin, inner echolucent band corresponding to the intima and media, particularly in younger normal subjects (e.g., posttransplantation). The thickness of this layer will be <160 μm and will be a negligible error to the measurement.
3.3 EEM Measurements
The third and outer layer consists of the adventitia and periadventitial tissues. But external elastic membrane area is the outer layer of the vessel in IVUS measurement because the border of adventitia and periadventitial tissue is not distinct. A discrete interface at the border between the media and the adventitia is usually present within IVUS images due to the relative echo-translucency of media. The term of this measurement is EEM CSA, rather than alternative terms such as “vessel area”. The measurement of EEM border should be avoided at the sites where large side branches originate or with extensive calcification and acoustic shadowing. If acoustic shadowing involves a relatively small arc (<90°), planimetry of the circumference can be performed by estimation from the closest identifiable EEM borders, although the accuracy and reproducibility will be reduced. If calcification is more extensive than 90° of arc, EEM measurements should not be reported. Normal arteries are circular, but a noncircular configuration is often due to the atherosclerotic remodeling.
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