**6. Multidetector computed tomography and multidetector computed tomography angiography for the diagnosis of intracranial atherosclerotic disease (ICAS)**

### **6.1 Nonenhanced multidetector computed tomography (MDCT)**

The study of the prevalence, and of the risk factors for intracranial internal carotid artery calcification (ICAC), as a marker of intracranial atherosclerosis was determined by Bos and coworkers. They assessed a white population (2495 persons) from the population-based Rotterdam Study with a no enhanced multidetector (16-slice or 64-slice) computed tomography (MDCT) of the intracranial ICAs. A calcified plaque had >130 Hounsfield units. They concluded that ICAC was highly prevalent and occurred in over 80% of older, white persons [66].

### **6.2 Multidetector computed tomography angiography (MCTA)**

A comprehensive high-resolution intracranial vessel assessment is possible by the introducing of multislice CT (MSCT: multiple row scanning: 4, 16, 64, up to 320 rows, associated with an increased rotational speed: 0.5 s/rotation). MSCT represents the first-line imaging modality in stroke patients [67]. CTA calculates the degree of stenosis using the published method for the Warfarin-Aspirin Symptomatic Intracranial Disease Study (WASID) (vide supra) [2].

According to Arenillas, CTA detects the degree of stenosis for each of 15 large intracranial arterial segments assessed: bilateral supraclinoid ICAs, A1-ACA, M1-ACM, M2-ACM, P1-PCA, proximal, mid, and distal BA, and intracranial VA. CTA identifies and characterizes the ICAS of different histopathological nature (including partially recanalized emboli), being unable to directly assess plaque instability. It allows the examination of ICAS progression and in-stent restenosis [3, 68]. The stenotic lesions are considered to be atherosclerotic in nature, if no cases with subarachnoid hemorrhage or intracerebral hemorrhage are detected by CT head (in consequence, vasospasm is unlikely the cause of these ICAS). Arterial segments are excluded from the analyses of stenosis, if they are identified to be congenitally hypoplastic or seen only through collaterals or cross-filling [69].

The prevalence, distribution, calcification, and the risk factors predisposing ICAS in a white stroke population were investigated by Homburg and coworkers. All patients underwent MDCT of the brain and MDCTA (with a 16-slice MDCT scanner or a 64-slice MDCT scanner with a standardized protocol) of the extracranial and intracranial arteries in a single session [70]. They concluded that the majority of ICAS was observed in the posterior circulation. ICAS in the proximal intracranial arteries was mainly classified, but in distal arteries, it was frequently nonclassified, indicating a different pathophysiology of atherosclerotic disease in the two segments. The absence of calcification on CT of the brain does not exclude the presence of ICAS in the distal arteries. Association of nonclassified ICAS and ESR may indicate a prominent role for inflammatory factors in intracranial arteries disease (ICAD) [71].

#### **6.3 CTA versus DSA**

CTA was compared with DSA for the detection and measurement of stenosis/ occlusions in large intracranial arteries by Nguyen and coworkers [69]. They reported high sensitivity and a high PPV for CTA for the detection of occlusion and stenosis of greater than 50%. CTA has relatively fewer risks, costs less, is more readily available, and appears to have the same accuracy as DSA, to identify the exact site of arterial occlusion in acute ischemic stroke. Maximum intensity projections and volume rendering can help to quickly identify the occlusion. On the other hand, CTA does not appear to be as reliable as DSA for determining the presence of stenosis in small arteries distal to the first 1 cm of the artery [72].

#### **6.4 CTA versus MRA**

CTA has several advantages compared with MRA: better anatomic visualization of the circle of Willis and of the state of the arteries [73] and quite accurate in the evaluation of stenosis, since the latter tends to overestimate high-grade stenosis attributable to turbulent flow; CTA is more accurate for identifying occlusion (sensitivity, 100%; specificity, 99.4–100%) than for measuring the degree of stenosis [68]. CTA is minimally invasive, performed quickly, modest cost, scanner availability 24/7, operator-independent, less susceptible to motion artifacts than MRA, and less dependent on hemodynamic effects compared with MRA [68, 69, 74, 75].

**97**

*Diagnosis of Symptomatic Intracranial Atherosclerotic Disease*

Its disadvantages, besides radiation risk exposure, are patient movement, contrast risk reactions (allergy to iodine contrast agents), different contraindications (nephron-toxicity-serum creatinine levels >1.2 mg/dL, etc.), and difficult evaluation of arteries within bone canals (particularly of the carotid siphon) due to bone artifacts. However, intracranial, with the proper examination and postprocessing techniques, is possible to use CTA to assess the petrous and cavernous portions of the ICA. Multiplanar reformation (MPR) can display 2D images in various planes without any loss of information. Maximum intensity projection (MIP) enhances high attenuation contrast tissues, including bone, wall calcification, or blood vessels. Since calcifications can interfere with the evaluation of the degree of stenosis, bone elimination is done during post-

Bash et al. [76] retrospectively examined 28 subjects with ischemic stroke or TIA comparing CTA and MRA using DSA as the gold standard, among intracranial arteries with stenosis >30% (anterior circulation vessels 42 versus 58% posterior circulation arteries). They concluded that CTA demonstrated a higher sensitivity, specificity, and PPV than those of MRA for the evaluation of stenotic and occluded intracranial vessel segments. CTA has a high interoperator reliability for the quantitation of stenotic lesions when expert readers are used. Helical CTA is superior to DSA in the demonstration of arterial patency in posterior circulation arteries when very low- or balanced flow states are present due to a severe stenosis [76]. In the era of the mechanical thrombectomy with stent-retriever, when faster puncture time to endovascular therapy became very important, CTA became essential due to its shorter scan time and the evaluations of collaterals on multiphase imaging, which

Roubec and coworkers compared ICAS in 67 patients with stroke using three different methods: TCCS, CTA, and DSA in a common clinical practice. They found substantial agreement between CTA and DSA, and moderate agreement between

The frequency and clinical course of patients with acute ischemic stroke or TIA who had intracranial nonocclusive thrombus (iNOT) on CTA of the circle of Willis were assessed by Puez and coworkers [78]. Before CTA, a noncontrast CT (NCCT) was accomplished in all cases. iNOT has been described first on DSA or MRA [79, 80]. Criteria to diagnose iNOT rather than occlusive thrombus or atherosclerotic stenosis were: (1) residual lumen present and eccentric; (2) nontapering thrombus; (3) smooth and well-defined thrombus margins; and (4) absence of vessel wall calcification [78]. Puez concluded that iNOT was relatively uncommon. Probably, iNOT may be more frequently diagnosed when performing early CTA in such patients. The majority of patients had a good clinical outcome. Clinical deterioration was associated with unchanged or enlarged iNOT in repeated vascular studies, whereas diminished or resolved iNOT was associated with a benign clinical course. Particularly, in patients with minor symptoms, iNOT may indicate increased risk for clinical deterioration. Puez's study supported the importance of urgent vascular

TCCS and DSA as well as CTA and TCCS, for the evaluation of ICAS [77].

can contribute to faster recanalization and better evolution [68].

**6.7 Intracranial nonocclusive thrombosis (iNOT)**

*DOI: http://dx.doi.org/10.5772/intechopen.90250*

**6.5 CTA versus 3D-TOF-MRA and DSA**

**6.6 CTA versus DSA versus TCCS**

imaging in these patients [78].

processing [68, 69].

*Diagnosis of Symptomatic Intracranial Atherosclerotic Disease DOI: http://dx.doi.org/10.5772/intechopen.90250*

*New Insight into Cerebrovascular Diseases - An Updated Comprehensive Review*

**6.2 Multidetector computed tomography angiography (MCTA)**

Symptomatic Intracranial Disease Study (WASID) (vide supra) [2].

A comprehensive high-resolution intracranial vessel assessment is possible by the introducing of multislice CT (MSCT: multiple row scanning: 4, 16, 64, up to 320 rows, associated with an increased rotational speed: 0.5 s/rotation). MSCT represents the first-line imaging modality in stroke patients [67]. CTA calculates the degree of stenosis using the published method for the Warfarin-Aspirin

According to Arenillas, CTA detects the degree of stenosis for each of 15 large intracranial arterial segments assessed: bilateral supraclinoid ICAs, A1-ACA, M1-ACM, M2-ACM, P1-PCA, proximal, mid, and distal BA, and intracranial VA. CTA identifies and characterizes the ICAS of different histopathological nature (including partially recanalized emboli), being unable to directly assess plaque instability. It allows the examination of ICAS progression and in-stent restenosis [3, 68]. The stenotic lesions are considered to be atherosclerotic in nature, if no cases with subarachnoid hemorrhage or intracerebral hemorrhage are detected by CT head (in consequence, vasospasm is unlikely the cause of these ICAS). Arterial segments are excluded from the analyses of stenosis, if they are identified to be congenitally hypoplastic or seen only through collaterals or cross-filling [69]. The prevalence, distribution, calcification, and the risk factors predisposing ICAS in a white stroke population were investigated by Homburg and coworkers. All patients underwent MDCT of the brain and MDCTA (with a 16-slice MDCT scanner or a 64-slice MDCT scanner with a standardized protocol) of the extracranial and intracranial arteries in a single session [70]. They concluded that the majority of ICAS was observed in the posterior circulation. ICAS in the proximal intracranial arteries was mainly classified, but in distal arteries, it was frequently nonclassified, indicating a different pathophysiology of atherosclerotic disease in the two segments. The absence of calcification on CT of the brain does not exclude the presence of ICAS in the distal arteries. Association of nonclassified ICAS and ESR may indicate a prominent role for inflammatory factors in intracranial arteries disease (ICAD) [71].

CTA was compared with DSA for the detection and measurement of stenosis/ occlusions in large intracranial arteries by Nguyen and coworkers [69]. They reported high sensitivity and a high PPV for CTA for the detection of occlusion and stenosis of greater than 50%. CTA has relatively fewer risks, costs less, is more readily available, and appears to have the same accuracy as DSA, to identify the exact site of arterial occlusion in acute ischemic stroke. Maximum intensity projections and volume rendering can help to quickly identify the occlusion. On the other hand, CTA does not appear to be as reliable as DSA for determining the presence of

CTA has several advantages compared with MRA: better anatomic visualization of the circle of Willis and of the state of the arteries [73] and quite accurate in the evaluation of stenosis, since the latter tends to overestimate high-grade stenosis attributable to turbulent flow; CTA is more accurate for identifying occlusion (sensitivity, 100%; specificity, 99.4–100%) than for measuring the degree of stenosis [68]. CTA is minimally invasive, performed quickly, modest cost, scanner availability 24/7, operator-independent, less susceptible to motion artifacts than MRA, and less dependent on hemodynamic effects compared with MRA [68, 69, 74, 75].

stenosis in small arteries distal to the first 1 cm of the artery [72].

**96**

**6.3 CTA versus DSA**

**6.4 CTA versus MRA**

Its disadvantages, besides radiation risk exposure, are patient movement, contrast risk reactions (allergy to iodine contrast agents), different contraindications (nephron-toxicity-serum creatinine levels >1.2 mg/dL, etc.), and difficult evaluation of arteries within bone canals (particularly of the carotid siphon) due to bone artifacts. However, intracranial, with the proper examination and postprocessing techniques, is possible to use CTA to assess the petrous and cavernous portions of the ICA. Multiplanar reformation (MPR) can display 2D images in various planes without any loss of information. Maximum intensity projection (MIP) enhances high attenuation contrast tissues, including bone, wall calcification, or blood vessels. Since calcifications can interfere with the evaluation of the degree of stenosis, bone elimination is done during postprocessing [68, 69].

### **6.5 CTA versus 3D-TOF-MRA and DSA**

Bash et al. [76] retrospectively examined 28 subjects with ischemic stroke or TIA comparing CTA and MRA using DSA as the gold standard, among intracranial arteries with stenosis >30% (anterior circulation vessels 42 versus 58% posterior circulation arteries). They concluded that CTA demonstrated a higher sensitivity, specificity, and PPV than those of MRA for the evaluation of stenotic and occluded intracranial vessel segments. CTA has a high interoperator reliability for the quantitation of stenotic lesions when expert readers are used. Helical CTA is superior to DSA in the demonstration of arterial patency in posterior circulation arteries when very low- or balanced flow states are present due to a severe stenosis [76]. In the era of the mechanical thrombectomy with stent-retriever, when faster puncture time to endovascular therapy became very important, CTA became essential due to its shorter scan time and the evaluations of collaterals on multiphase imaging, which can contribute to faster recanalization and better evolution [68].

#### **6.6 CTA versus DSA versus TCCS**

Roubec and coworkers compared ICAS in 67 patients with stroke using three different methods: TCCS, CTA, and DSA in a common clinical practice. They found substantial agreement between CTA and DSA, and moderate agreement between TCCS and DSA as well as CTA and TCCS, for the evaluation of ICAS [77].

#### **6.7 Intracranial nonocclusive thrombosis (iNOT)**

The frequency and clinical course of patients with acute ischemic stroke or TIA who had intracranial nonocclusive thrombus (iNOT) on CTA of the circle of Willis were assessed by Puez and coworkers [78]. Before CTA, a noncontrast CT (NCCT) was accomplished in all cases. iNOT has been described first on DSA or MRA [79, 80]. Criteria to diagnose iNOT rather than occlusive thrombus or atherosclerotic stenosis were: (1) residual lumen present and eccentric; (2) nontapering thrombus; (3) smooth and well-defined thrombus margins; and (4) absence of vessel wall calcification [78]. Puez concluded that iNOT was relatively uncommon. Probably, iNOT may be more frequently diagnosed when performing early CTA in such patients. The majority of patients had a good clinical outcome. Clinical deterioration was associated with unchanged or enlarged iNOT in repeated vascular studies, whereas diminished or resolved iNOT was associated with a benign clinical course. Particularly, in patients with minor symptoms, iNOT may indicate increased risk for clinical deterioration. Puez's study supported the importance of urgent vascular imaging in these patients [78].

Clot length can be examined by thin-sliced noncontrast CT and CTA. A better visualization of collateral circulation (which is an important prognostic factor for favorable outcome) can be realized with multiphase CTA [81, 82].

## **7. Conclusions**

This chapter focuses on key findings and recent approaches in diagnosis of intracranial arterial atherosclerotic stenosis (ICAS), with an emphasis on novel procedures to define the underlying mechanisms of stroke in intracranial atherosclerotic disease (ICAD). The importance of ICAS as a principal cause of ischemic stroke in Caucasians is undervalued as compared to that of extracranial atherosclerotic stenosis (ECAS) and nonvalvular atrial fibrillation (NVAF). On the other hand, intracranial arterial calcifications, stenosis, and occlusions represent the most frequent disturbance observed in intracranial arteries [83].

Intracranial arterial stenosis is caused by an atherosclerotic plaque in more than 90% of cases (ICAS). Intracranial atherosclerotic stroke differs from extracranial atherosclerotic stroke in many aspects, including risk factors and stroke patterns. Unlike in patients with ECAS or NVAF, stroke correlated to ICAD occurs in association with various stroke mechanisms such as in situ thrombotic occlusion, arteryto-artery embolism, branch occlusion, and hemodynamic insufficiency [83].
