Before CAS procedure each patient must be evaluated accurately with different diagnostic image modalities in order to get all the information regarding Fig.1
- arch morphology
- common carotid arteries origin
- carotid arteries morphology
- site of the lesion
- tandem lesions
Ultrasound-Color-Doppler (USCD) is the modality of choice for  patients’ screening.
It is a non-invasive non-expensive, no time consuming method that can be performed in an out-patient condition.
USCD permits an accurate evaluation of the carotid stenosis (degree of the stenosis, plaque evaluation and blood flow evaluation).
However no information about the CCA origin and the arch morphology can be obtained.
The most critical aspect of this exam is its high dependence on the operator’s skill.
In fact if the practitioner isn’t well trained in the study of carotid pathology, resultant information may be discordant.
Examination and recording of pathologic findings on gray scale and Color Doppler sonography are followed by spectral Doppler hemodynamic analysis.
Despite technological progress in sonographic equipment and improvements in operator experience, there is still no uniformity in practice and interpretation between different centers and even between operators in the same laboratory.
Therefore, standardized protocols are highly recommended for reproducibility and reliability of information provided by carotid sonography.
Standard protocols include carotid examination with a high-resolution linear array transducer (7 MHz or broad spectrum 5-12 MHz).
Three modalities must be used: B-mode gray scale imaging, color flow Doppler, both on transverse and longitudinal planes, and spectral Doppler velocities on longitudinal planes.
B-Mode US is the best method for measuring intima-media thickness (IMT) and for detecting atherosclerotic plaques.
Mean IMT is calculated in the CCA between 2 interfaces: blood-intima and media-adventitia.
IMT measurement can be performed either manually or by computer software.
In the manual method, multiple measurements are made by placing electronic cursors at multiple sites and averaging values.
In the computerized method, 3 measurements are made on the far wall at anterior, lateral, and posterior projections and averaged.
Normal IMT is less than 0.8 mm. Intima-media thickness increases with age; a thickness of 0.8 to 1.0 mm is considered indeterminate.
A thickness of 1.1 mm or greater is actually a more accepted abnormal value. Fig.2
Evaluation of plaque location and characteristics is an important part of the standard carotid duplex sonographic examination.
Plaque definition both in the carotid bifurcation and along the ICA, includes size with its corresponding hemodynamic alteration, surface and echogenicity. Fig.3
According to hemodynamics, the plaque is classified on a scale from H1 to H5: H1 being mild, less than 50% diameter reduction; H2, moderate, 50% to 69% diameter stenosis; H3, severe, 70% near occlusion; H4, 90% critical, near occlusion; and H5, occluding, total occlusion.
Plaque echogenicity is classified P1, homogeneous and P2, heterogenous; S1 to S3 indicate the surface characteristics, from smooth (S1) to irregular with a surface defect of less than 2 mm (S2) and ulcerated with a defect greater than 2 mm (S3).
Ulcerated lesions are associated with intraplaque hemorrhage; 50% to 70% of patients with this type of plaque have hemispheric symptoms.
The estimation of the plaque diameter percentage reduction is considered a primary parameter together with ICA peak systolic velocity (PSV) for the diagnosis of the degree of ICA stenosis:

- PSV <125cm/sec - normal artery - stenosis <50%
- PSV 125-230 cm/sec - 50-70% stenosis
- PSV >230 cm/sec - stenosis >70%
- non valuable - pre-occlusion

Color Doppler sonography provides a "road map" for the identification of ICA origin and course and is extremely useful in the presence of tortuous arteries and high bifurcations and to distinguish between severe stenosis and occlusion.
Tortuosity of the ICA due to kinking and coiling secondary to atherosclerosis may deliver a high PSV in the ICA. In such cases, the absence of plaques as a source of increased velocities must be confirmed on gray scale and color flow imaging. Heavily calcified plaques may act as a barrier to the ultrasound waves and may provoke posterior acoustic shadowing. This shadow makes the involved segment inaccessible to gray scale or Doppler examination. In the presence of a circumferential calcified plaque, flow patterns immediately distal to the lesion should be observed: if flow remains regular or moderately disturbed, appreciable stenosis may be excluded. Substantial distal stenosis, beyond the reach of the ultrasound probe, may be suggested by indirect signs such as increased resistance within the ICA proximal to the lesion.
A potential role for sonographic contrast agents, either to improve conspicuity of the lumen or to improve pulsed wave Doppler signals, has been described.

CT Angiography: Spiral CT Angiography (CTA) with axial and 3D images allows to appraise the morphology of the aortic arch, the common carotid artery and the carotid bifurcation.
Also the degree of the stenosis, the presence of calcification at the level of the stenosis and tandem lesions can be assessed.
Images are evaluated on the axial plane and after 3D reconstruction, using different algorithms (MIP, SSD, Volume Rendering).
Technique: Acquisition from the aortic arch up to the internal circle is done in the arterial phase after injection of 140ml non-ionic contrast media at 4ml/sec.
It will be useful to perform a "bolus care": the acquisition starting when 130Hu are measured at the level of the aortic arch with 4 seconds delay added.

MR-Angiography (MRA) should be performed in those patients with a poor renal function to avoid the use of Iodinated Contrast Media. MRA permits to evaluate the vessels anatomy and the plaque morphology as well as the aortic arch morphology, the origin of the carotid arteries and the stenosis.
However this technique is not able to show calcifications and tends to overestimate the degree of the stenosis.
MRA seems to be a valid non-invasive method to analyze the plaque components in order to determine the stability of the lesion.
Technique: images are acquired using a standardized MR angiography protocol after injection of non-iodinate contrast media. Images are evaluated after 3D reconstruction with MIP technique.

Diffusion-Weighted Magnetic Resonance (DW-MRI): MR performed using diffusion weighted images (TR 3100; TE 93; Fov 230; slice Th 5.0mm) is able to locate new ischemic lesions and is of great help to detect the presence of new lesions after CAS.
It should be performed just before the procedure (within 3 hours) and 24 hours after CAS.

Digital Subtraction Angiography (DSA): Must be avoided in the pre-treatment evaluation.
In fact all information about carotid morphology and plaque characteristics can be obtained using non invasive image modalities (CTA, MRA, USCD). Fig.4
Moreover we have to consider that diagnostic angiography of the carotid arteries is correlated with 0,5% of stroke and 2,5% of complications procedure related.
Technique: From a conventional femoral approach a 4-5 Fr. Pig-tail catheter is advanced into the ascending aorta.
Images are acquired in the lateral anterior projection (LAO) 40° in order to open the aortic arch and have a better view of the vessels origin.
30 cc of non-ionic contrast media must be injected via an electronic injector at a flow rate of 20cc/sec and images should be acquired at a frame rate of 2 frame/sec.
Then a selective injection of each common carotid artery is suggested. Injection of non-ionic contrast media is to be performed with manual injection to avoid debris from the carotid plaque consequent to a too high power injection.
Images are acquired in different projections (LAO and RAO) to visualize the carotid bifurcation correctly. Fig.5
KEY NOTE: As DSA can give only a bidimensional visualization and the artery lumen can’t be seen totally, a contrast media injection in two opposite projections is recommended.

References