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Neuroimaging
Original research
Vascular geometry of the extracranial carotid arteries: an analysis of length, diameter, and tortuosity
  1. Farooq A Choudhry1,
  2. John T Grantham1,
  3. Ansaar T Rai2,
  4. Jeffery P Hogg3
  1. 1Department of Radiology, West Virginia University, Morgantown, West Virginia, USA
  2. 2Department of Interventional Neuroradiology, West Virginia University, Morgantown, West Virginia, USA
  3. 3Department of Radiology, West Virginia University Healthcare, Morgantown, West Virginia, USA
  1. Correspondence to Dr Ansaar T Rai, Department of Interventional Neuroradiology, West Virginia University, Room 2278, HSCS, P.O. Box 9235, Morgantown, WV 26506, USA; ansaar.rai{at}gmail.com

Abstract

Background Stable access is essential for successful intracranial interventions. Quantifying variations in extracranial carotid arteries may help in the selection and development of access catheters. This study describes the vascular dimensions from the aortic arch to the skull base.

Methods CT angiography analysis was performed on 100 patients. The lengths, diameters, and tortuosity of the common carotid artery (CCA) and internal carotid artery (ICA) were measured from the aortic arch to the skull base.

Results The mean±SD length of the carotid artery from the aortic arch to the skull base was 22.2±2.2 cm for the right side and 20.8±1.9 cm for the left side (p<0.0001). The length of the right CCA was 13.6±1.2 cm and the length of the left CCA was 12.4±1.4 cm (p<0.0001). The length of the right ICA was 8.6±1.4 cm compared with 8.4±1.4 cm for the left ICA (p=0.3). The ICA length in men and women was 8.9±1.3 cm and 8.2±1.3 cm, respectively (p=0.0001), and the CCA length in men and women was 13.6±1.5 cm and 12.3±1.6 cm, respectively (p<0.0001). The lengths of the CCA and ICA in patients aged ≥60 years were 13.3±1.7 cm and 8.9±1.5 cm, respectively compared with 12.8±1.7 cm and 8.2±1.1 cm, respectively, for patients aged <60 years (p=0.04 for CCA, p=0.0002 for ICA). Tortuosity of the CCA and ICA was 1.2±0.2 and 1.3±0.1, respectively, in patients aged ≥60 years compared with 1.1±0.1 for both the ICA and CCA in patients aged <60 years (p<0.0001 for both). There was a consistent ratio of CCA/ICA length of 1.6±0.3 on the right and 1.5±0.3 on the left (p<0.0001). The arterial diameters did not show any significant difference.

Conclusions The distance from the aortic arch to the skull base is longer on the right than on the left side. Both the CCA and ICA are longer in men and in patients aged ≥60 years. The tortuosity of both segments significantly increases with age.

  • Artery
  • Device
  • CT Angiography
  • Angiography
  • Statistics

http://dx.doi.org/10.1136/neurintsurg-2015-011671

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    Introduction

    The cervical carotid vasculature serves as the major gateway for interventions involving the intracranial vasculature such as treatment of aneurysms, stroke, vascular malformations, and tumors. Knowledge of vascular geometry and any predictable age-related changes can be helpful in planning these catheter-driven procedures. Previous reports of vascular dimensions were based on cadaveric studies1 ,2 and, thus, do not reflect in vivo physiology that is now available with current neuroimaging. Similar analyses using advanced imaging techniques have been reported on the intracranial internal carotid and vertebrobasilar circulations.3 ,4 The current study aims to quantify the variations in the carotid vasculature from the aortic arch to the skull base, thus complementing the previous intracranial analyses.

    Materials and methods

    Sample size

    A sample was calculated to represent men and women as well as right and left carotid vessels. The age cut-off was set at 40 years. We reviewed the literature regarding previous measurements of cervical and intracranial carotid vessels2 ,3 in order to obtain a sufficient sample to account for the inherent variations in the population. We obtained a sample size of 100 patients (50 men and 50 women) that accounted for the following subgroups: male/right, male/left, female/right, and female/left. These 100 patients formed the cohort on which the common carotid artery (CCA) and internal carotid artery (ICA) measurements were obtained.

    Patient selection

    Consecutive patients who had undergone CT angiography (CTA) of the head and neck since 1 January 2013 were screened. Patients younger than 40 years were excluded based on our experience that the majority of patients undergoing neurovascular interventions are above this age. Patients with any significant vascular abnormality visible on CT were excluded. These included severe atherosclerotic disease or stenosis, arterial dissections or other traumatic vascular injuries, prior carotid endarterectomy, or non-conventional branching of the aortic arch. Conventional branching was defined as the right CCA arising from the brachiocephalic trunk and the left CCA arising directly from the arch. Additionally, examinations with excessive quantum mottling artifact, poor contrast bolus opacification, or excessive metallic streak artifact from spinal fusion or other implanted hardware were also excluded. Based on these criteria, we included 50 consecutive men and 50 women aged ≥40 years. For each of the male and female subgroups, right and left measurements were obtained. We further divided the two groups based on age (40–60 and ≥60 years).

    Imaging technique

    All CTA studies were performed using either an Aquilion-64 or an Aquilion-1 CT scanner (Toshiba America Medical Systems, Tustin, California, USA). Our protocol prescribes a weight-based dose of 50 mL if the patient weighs <200 lbs and 70 mL if ≥200 lbs when using the Aquilion-64 CT scanner. All patients scanned on the Aquilion-1 are given a 50 mL dose, made possible by faster table translation and a greater number of detectors. Contrast media used at our facility include Optiray 350 (Covidien, Hazelwood, Missouri, USA) and Isovue 370 (Bracco, Monroe Township, New Jersey, USA). All studies use a large-bore antecubital venous access with 4 mL/s power injection. A tube current of 140 kV is used with automated dose reduction technique. The initial localizer is obtained at the level of the carina with a region of interest drawn over the aortic arch, and the study is automatically initiated when the aorta is opacified. The field of view is set per patient but all studies use 512×512 matrix size with 1.0×1.0×0.5 image thickness and reconstruction interval. Multiplanar reconstruction is performed on a Vitrea workstation (Vital Images, Minnetonka, Minnesota, USA). All source data are stored on a server from which it can be accessed via a picture archival and imaging system, as well as the workstation.

    Image processing and analysis

    In order to obtain these measurements, the source CTA data for the 100 studies were imported into a workstation running the Vitrea Core software V.6.6.2 (Vital Images). Using the software’s vascular package, which includes a ‘vessel probe’ tool, a three-dimensional model of the cervical carotid vasculature with automated removal of bone and soft tissues was generated. An automated center line was obtained along the long axis of the blood vessel and was manually corrected when required. The vessel of interest was segmented out from the rest of the model. In addition to the three-dimensional model, a curved planar reformation (CPR) of the target blood vessel along its center line was also obtained (figure 1). The cross-sectional area was automatically calculated throughout the vessels and the values were obtained at the desired positions. The diameter was then calculated using πR2.⇓

    Figure 1
    Figure 1

    (A) ‘Runoff’ view displaying a three-dimensional maximum intensity projection of the selected vessel, which in this case is the right common carotid artery (CCA) and the internal carotid artery (ICA) from the aortic arch to the skull base. The center line is depicted in green and the measured CCA length in blue, with its origin and terminus marked by rectangles. (B) Curved planar reformat view with the vessel conformed to straight line (1) shows the vessel length and tortuosity. The corresponding luminal (red line) and wall (yellow line) diameter variations are depicted along the vessel length (2). A transverse view can be obtained at any point along the measured segment depicting cross-sectional luminal and mural diameters (3). (C) Three-dimensional multiplanar reformatted views in sagittal (top), coronal (middle), and axial planes (bottom). The extent of the CCA and ICA as selected by the software's ‘vessel probe’ tool is denoted in red.

    Figure 2
    Figure 2

    Diagrammatic representation of the vascular measurements from the aortic arch to the skull base. The lengths of the common carotid arteries (from the aortic arch to the common carotid artery bifurcation) are represented by the solid black lines and the internal carotid arteries by the dashed black lines. The diameters at the origin and termination of the vessels are represented by solid ovals and the diameters at the mid points by gray ovals.

    A tortuosity index (TI) was measured on the CPR image as a ratio of the curved length of the blood vessel to the straight line distance between the two endpoints. Thus, a higher TI represents a more tortuous vessel. For example, a curved length of 3 cm and a straight line distance of 2 cm between two points gives a TI of 1.5, meaning the curved length is 1.5 times as long as a perfectly straight vessel.3 ,4 Two reviewers independently performed the measurement which were then correlated. After confirming good correlation between the two observers, the mean of the two measurements was obtained to perform the final analyses.

    Target measurements

    The vascular dimensions were analyzed by a senior medical student and a senior radiology resident with oversight from two experienced neuroradiologists. The CCA and ICA constituted the targeted vessels, with the goal of obtaining the following parameters (figure 2):

    1. CCA

      1. Length: The left CCA length was measured from its origin at the aortic arch to the CCA bifurcation. The right CCA length was measured from the innominate artery origin at the aortic arch to the right CCA bifurcation. This is the length we used in our comparative analyses. However, we also measured the more conventional right CCA length from the innominate artery bifurcation to the CCA bifurcation.

      2. TI of the CCA.

      3. The diameter at the point of origin from the aortic arch.

      4. The diameter at the midpoint: this is the midpoint between the aortic arch and the CCA bifurcation.

      5. The diameter of the bifurcation (which was the same as the ICA origin).

    2. ICA

      1. Length (ICA origin to the skull base).

      2. TI of the ICA.

      3. The diameter at the midpoint: this is the midpoint between the ICA origin and the skull base.

      4. The diameter at the skull base.

    The CCA origin was considered to be the first axial slice demonstrating a circumferential CCA lumen arising from the arch; for the left side it was the CCA origin and for the right side the innominate artery origin was taken to represent the CCA. The CCA length was then measured from the aortic arch to the CCA bifurcation. This convention of measuring the right CCA length from the aortic arch (versus the innominate artery) was used as the total CCA length from the aortic arch to the bifurcation is more useful for endovascular procedures than the inherently shorter length obtained if measured from the innominate artery origin. However, in order to comply with the traditional nomenclature, we also measured the CCA length from the innominate artery bifurcation and report it separately. The CCA terminus was defined by the first axial slice demonstrating a circumferential ICA lumen. The distal extent of the cervical ICA at the skull base was defined by the first axial slice where the petrous bone approximated the vessel lumen.

    Statistical analysis

    The data were analyzed using JMP statistical software V.11 (SAS Institute, Cary, North Carolina, USA). Mean values and SDs for all the measurements were generated along with the respective 95% CIs. We performed a bivariate analysis of age with the vessel parameters to determine any association of vessel size with age and used the Student t test to compare the means based on gender and side.

    Results

    The mean±SD age of the entire cohort was 57±10 years. There was excellent correlation between the two reviewers: CCA length (R2=0.98), CCA tortuosity (R2=0.97), CCA diameter at midpoint (R2=0.85), ICA length (R2=0.94), and ICA tortuosity (R2=0.91). Pooled data from two analysts were then averaged and used for descriptive statistics as well as comparative analyses. The mean length of the carotid artery from the aortic arch to the skull base (CCA+ICA) was 22.2±2.2 cm for the right side and 20.8±1.9 cm for the left side (p<0.0001). The mean length of the right CCA from the innominate artery bifurcation to the CCA bifurcation was 10.9±1.9 cm. The separate values for the CCA (aortic arch to common carotid bifurcation) and the ICA based on the right and left (table 1) demonstrate a longer CCA on the right side, but there was no difference in the ICA lengths. The right CCA origin diameter was significantly larger, although this is because of an inherently larger innominate artery which we purposely chose as the point of origin for the right side. The measurements for men and women separated by right and left side (table 2) showed generally longer lengths and larger caliber vessels in men. A comparison of the dimensions by age (table 3) showed that tortuosity and length for both the CCA and ICA increased with age whereas vessel diameter showed no significant change. The CCA/ICA length ratio was 1.6±0.3 on the right side and 1.5±0.3 on the left (p=0.02). Regardless of differences in length among men and women or old and young patients (≥60 vs <60 years), this ratio remained relatively constant for each side.

    View this table:
    Table 1

    Mean±SD values by laterality (n=100)

    View this table:
    Table 2

    Mean±SD values by gender (n=100)

    View this table:
    Table 3

    Mean±SD values by age

    Discussion

    Knowledge of vascular dimensions and morphology is important in neurovascular procedures. In contrast to cadaveric studies,2 ,5 CTA may reflect more accurate vascular measurements due to a functioning cardiovascular system. Ultrasound evaluations for vessel size and morphology6 are limited by the anatomic coverage achievable. Our objective in using CTA and advanced image processing was to establish an updated and current database regarding mean values for length, diameter, and tortuosity of the extracranial carotid circulation. We have previously reported on similar dimensions for the intracranial circulation3 ,4 and, by extending these data to the aortic arch, we now have a quantified road map of the cerebrovascular system pertinent to endovascular procedures. Previous evaluations of extracranial vascular geometry have also been performed using ultrasonography,7 ,8 CTA,9 and both CTA and MR angiography.10 These are primarily studies of cross-sectional areas and diameters, either comparing different modalities or focusing on carotid bifurcation and ICA origin segments with atherosclerotic disease in mind. In contrast, the current study focused on quantifying the carotid anatomy from the aortic arch to the skull base as a platform for intracranial procedures. Ultrasonography studies are inherently operator-dependent, may not reflect the diameter along the true axis of a vessel, and the level of the measurement may vary from patient to patient. Current imaging processing and vascular analysis software allows for accurate and reproducible measurements along the long axis of the vessel making them more relevant to patient care.

    Ultrasonography studies have shown that the CCA and ICA diameter is smaller in women.6 ,7 ,11 Most of these measurements have been obtained in the distal CCA prior to the carotid bulb and in the distal ICA. Although we found a gender-based difference at the CCA and ICA origins, we found no difference in the midpoint diameters or distal ICA between men and women, contrary to prior work.7 Cadaveric studies have reported the ICA origin diameter to be closer to 5 mm2, which is smaller than our measurements and smaller than previous studies using ultrasound.6 In terms of length, cadaveric studies have shown the left CCA to be longer than the right; we found reported values for CCA length were often shorter than our findings.2 In addition to the traditional definition of the right CCA arising at the level of the innominate artery bifurcation, we report a non-conventional measurement of the right CCA length from the aortic arch to the common carotid bifurcation. The rationale for extending the right CCA to the aortic arch was to give a true representation of the total vascular length required to be traversed in endovascular procedures from the aortic arch onwards. This is also the reason why the right CCA in our study measures longer than the left CCA, whereas the opposite has been previously reported.1 ,2 ,7

    The ICA lengths in our study were similar to previous reports.1 ,2 ,7 An interesting observation in our database is that of a fairly constant ratio between CCA and ICA lengths regardless of gender or age. This CCA/ICA ratio was 1.6±0.3 on the right and 1.5±0.3 on the left, again keeping in mind that we measured the right CCA from the aortic arch. An increase in arterial caliber with aging has been reported for large arteries in general12 ,13 and intracranial arteries in particular.3 ,4 An increase in pulse pressure due to decreasing compliance and elasticity as large arteries stiffen with age is also documented.14 However, we did not find any significant difference in arterial caliber of the CCA or ICA in patients younger or older than 60 years. One possible explanation is that our sample did not include patients aged <40 years. The impact of aging on arterial caliber may be more evident if studied across a wider age spread than ours. Previous studies have reported on differences in vessel tortuosity with aging.15 However, we quantify and report the increase in arterial tortuosity for both the CCA and ICA in patients aged ≥60 years.

    Limitations of the study

    The main limitation of our study is that our sample excluded patients with major vascular pathology. In clinical practice, extremes of vessel tortuosity may be encountered in neurovascular procedures that are not represented in our population. The exclusion of any major vascular pathology as described in the methodology was to develop a baseline for vascular dimensions in the general population aged ≥40 years. We also used a non-conventional right CCA origin from the aortic arch as opposed to the innominate bifurcation, and any comparison of right CCA length in our report with others should take this difference into account.

    Conclusion

    The mean length of the carotid artery from the aortic arch to the skull base was 22.2±2.2 cm for the right side and 20.8±1.9 cm for the left side (p<0.0001). The tortuosity and length of both the CCA and ICA increased with age, and men generally had longer and larger caliber arterial segments. There was a consistent ratio of CCA/ICA length of 1.6±0.3 on the right and 1.5±0.3 on the left regardless of age or gender. Quantification and knowledge of the extracranial carotid vasculature is important in planning stable access for endovascular procedures in the anterior circulations.

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    Footnotes

    • Contributors ATR contributed to study design, statistical analysis, and manuscript preparation. FAC contributed to data collection, image analysis, and manuscript preparation. JTG contributed to image analysis. JPH contributed to study design, image analysis, and manuscript preparation.

    • Competing interests None.

    • Ethics approval Ethics approval was obtained from the IRB.

    • Provenance and peer review Not commissioned; externally peer reviewed.

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