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The IAC Standards and Guidelines |
Click here for a printer-friendly PDF of the Vascular
Testing Standards |
Carotid Intima-Media Thickness (IMT)
IAC Vascular Testing Executive Summary
IMT: Common Carotid
Artery vs. Other Segments
Carotid IMT measurements are commonly obtained
from the common carotid artery (CCA), as this vessel offers the easiest
standardization due to its location, tubular shape, and parallel walls in most
patients. In the Atherosclerosis Risk in Communities (ARIC) study involving
carotid ultrasound examinations in 13,824 individuals, IMT measurements were
obtainable from the CCA in 91.4%, from the bifurcation in 77.3%, and from the
internal carotid artery (
In addition, use of the CCA IMT has correlated
well with prevalent cardiovascular disease and/or outcome. In the
Cardiovascular Health Study (CHS), the combination of CCA and ICA IMT resulted
in similar relative risks for subsequent myocardial infarction or stroke than
did CCA or ICA IMT alone (1.36 vs. 1.27 and 1.30, respectively, for 1 SD
increase)2. Based on the ease of imaging and the general correlation
with cardiovascular disease and clinical events, use of the CCA is generally
advised to measure the IMT.
Some advocate evaluation of a broader/more
widespread selection of arterial segments to provide a more stable and robust
prediction of risk. Therefore IMT measurements must be obtained from the far
wall of the distal 1-2 cm of the CCA, and may also be obtained from the near
wall of the CCA segment, as well as the near and far wall of the bifurcation,
and the proximal 1 cm of the
B.
IMT: Far Wall vs. Near Wall
The IMT may be measured from the near (closest
to the transducer) and/or the far wall. Although measurement
reproducibility of the near and far walls has been reported to be comparable3,
measurement yield of the near wall is lower 4 and accuracy may be
less than that of the far wall due to technical considerations. Therefore,
measurement is best obtained from the far wall of the CCA, and is commonly
taken from the distal 1-2 cm of the distal CCA, proximal to the flow divider5.
C. IMT: B-mode vs. M-mode Measurement
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IMT has most commonly
been measured from B-mode images. Alternatively, B-mode guided M-mode images
of the distal CCA may be obtained. Whatever the method, because of the very
small diameter of the intima-media layer, wall thicknesses should be measured
using computer assistance with electronic calipers or semi-automated
edge-detection algorithms6. |
D.
IMT: Timing of Measurement
Variations in IMT and lumen diameter must be
anticipated, and therefore, electrocardiographic-gating and/or determination of
minimal (end-diastolic) and maximal (peak-systolic) diameters are important
components of IMT measurements. With systolic expansion of lumen diameter,
obligatory thinning of IMT will occur through conservation of mass (although
some degree of longitudinal stretch will occur)7. Therefore,
measurements must be obtained at the identical timing of the cardiac cycle
(preferably at end-diastole) within a particular facility so as to avoid these
physiologic changes.
E.
Definition of Abnormal IMT
IMT increases with age and, on average, is
larger in men than women8. In addition, modest racial differences in
IMT have been reported.9 Thus, a single
threshold value for abnormality (e.g., 1 mm) may result in systematic under-detection
of abnormality in younger individuals and over-detection in older individuals.
Therefore, a standardized table of IMT measurements accounting for age, gender,
and race must be used to determine the true value of single IMT measurements. The
extent to which carotid intimal-medial thickening is a manifestation of early
or diffuse atherosclerosis, as opposed to smooth muscle hypertrophy and/or
hyperplasia induced by pressure overload and/or age-related sclerosis, remains
uncertain.
Internal diameter of the vessel lumen (usually
the CCA) can be measured at a single point in time from B-mode images, and
determination of minimum and maximum lumen diameters is mandatory for
assessment of vascular mechanics.10
F.
Non-Obstructive Plaque
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Plaque
characterization or dimensions should not be incorporated into IMT
measurements, and must be reported separately in those cases where plaque is
present.11
Plaque is defined as:
i. Focal structure
encroaching in the lumen >0.5 mm OR
ii. 50% of the surrounding
intima-media thickness OR
iii. Plaque thickness >1.5
mm.12
Process
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Carotid IMT
measurements should be performed by technologists with training and
experience in vascular ultrasound testing. IMT measurements are
obtained with the patient in the supine position with the neck slightly
hyperextended and the head rotated to the opposite side. High frequency
ultrasound probes are used, with a frequency of >7 MHz. Measurements are
obtained in the distal CCA, 1-2 cm from the flow divider, in the far wall,
using automated edge detection software. Measurements should be obtained from
both vessels. Plaque should be reported separately from the IMT measurements.
IMT measurements should ideally be reported using tables that account for
age, race, and gender. Facilities must submit internally validated diagnostic
criteria based on their experience and published literature. In addition,
facilities must develop patient education tools that will assist in educating
patients on the meaning of the carotid IMT and the importance of risk factor
intervention to modify cardiovascular risk. |
Finally, all facilities must provide details of
their internal quality assurance programs to support the performance of carotid
IMT measurements.
Summary
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CIMT has been
effectively used as a marker of atherosclerosis in many patient populations,
and has also been used as a primary endpoint demonstrating therapeutic efficacy
with different pharmacologic therapies. Studies using CIMT to make
treatment decisions based on a single IMT measurement, with documentation of
the outcome for specific interventions, for individual patients, are lacking.
The IAC does not advocate use of carotid IMT as a screening method for
atherosclerotic risk until further peer-reviewed literature is available. If
providers choose to perform CIMT testing, rigorous methodological protocols
should be strictly followed. |