**5. Magnetic resonance angiography (MRA) and high-resolution magnetic resonance imaging (HR-MRI) for the diagnosis of intracranial atherosclerotic disease (ICAD)**

ICAD includes two major features: (a) atherosis caused by lipid deposits in the intima of the arteries and inflammation; and (b) sclerosis, as a result of endothelial dysfunction, leading to arterial stiffness [49].

Strokes associated with ICAD occur in association with four major stroke mechanisms: in situ thrombotic occlusion; branch occlusion; artery-to-artery embolism; and hemodynamic insufficiency [50, 51]. Unstable intracranial plaques can suddenly lead to thrombotic occlusion. Using transcranial duplex monitoring, artery-to-artery embolism can be discovered, which commonly causes multiple cortico-subcortical infarcts. Branch occlusive disease (BOD) is one of the main stroke mechanisms of ICAD, which can be characterized by a milder degree of stenosis [19] and comma-shaped infarcts extending to the basal surface of the parent artery [52].

The first two mechanisms are the consequences of plaque rupture, which reveals the thrombogenic core to clotting factors, resulting a thrombus that occludes the artery locally or embolizes distally [50].

The third mechanism, specific to ICAD, is the growth of plaque over the ostia of penetrating arteries resulting in occlusion, which was described by Caplan [53] as branch atheromatous disease. Lastly, high-grade narrowing or occlusion of the lumen may lead to the fourth mechanism: hypoperfusion of the distal brain territory, particularly in cases with inadequate collateral flow [31, 50].

MRA, CTA, DSA, TCD, and TCCS can detect ICAS of different histopathological nature (including partially recanalized emboli) and can assess ICAS progression and in-stent restenosis. Unfortunately, these methods are unable to directly exam plaque instability, with the exception of microembolic signals (MES) detection by TCD as a surrogate marker [3].

Arenillas suggested that for the detection of intracranial plaque morphology, the imaging techniques used include high-resolution MRI (HR-MRI), high magnetic field (3T) preferable, and intravascular ultrasound. This new concept allows the detection and characterization of nonstenotic intracranial atheroma, establishing its role in stroke of undetermined origin, and may have the power to confirm the atherosclerotic nature of ICAS [3].

#### **5.1 Magnetic resonance angiography (MRA)**

Degnana noted that 3D time-of-flight (TOF)-MRA is a imaging technique that noninvasively explores the intracranial arteries. It takes the advantage of the contrast between nonsaturated spins in the blood entering the imaging plane and the stationary adjacent tissue, which remains saturated. 3D TOF-MRA allows the visualization of any variation in blood flow. It provides detailed information about the lumen status of the intracranial vessels (**Figure 5**) [54].

The following arterial segments are assessed: bilateral intracranial ICA, ACA-A1/ A2, MCA-M1/M2, PCA-P1/P2, BA, and VA. According to the severity of stenosis, there are four groups in which patients are classified: <50% or no stenosis, 50–69% stenosis, 70–99% stenosis, and occlusion groups. Focal flow void found on MRA with distal filling is considered as severe stenosis (70–99%) [55].

Other MR sequences, such as T2-/T1-weighted imaging, fluid-attenuated inversion recovery sequences, and diffusion-weighted imaging (DWI), are also performed on a conventional MRI on a 3.0 or 1.5T MR scanner [56].

Degnana asserted that MRA offers good equivalency with DSA for the detection of >50% stenosis with the reported sensitivity, specificity, and accuracy of 92, 91, and 91%, respectively [54].

Higher field strength scanners may carry additional benefits in improving signal intensity-to-noise ratio and background suppression; other sequences, such as novel sensitivity encoding (SENSE) TOF-MRA protocols, also have substantially abbreviated acquisition times [57].

**93**

CTA [51, 54].

**Figure 5.**

*stenosis in this particular case [54].*

[54, 60].

*Diagnosis of Symptomatic Intracranial Atherosclerotic Disease*

MRA has some advantages: noninvasive, no radiation, low cost, and widely available, but for the detection of ICAS requires confirmation by another imaging modality [51, 54]. The main disadvantage is that it overestimates stenosis. MRA can render ambiguous or erroneous results. The inability to distinguish between high-

*Comprehensive imaging of a patient with recent stroke depicting left MCA stenosis. A−C, DSA (A) confirms stenosis, but contrast – enhanced MRA (B) and volume-reduced TOF MRA (C) overestimate the degree of* 

In addition, MRA has magnet contraindications: limited to no use in morbidly obese and claustrophobic patients and those with implanted metallic objects [51]. Although CTA performs better overall compared with MRA for the purpose of determining the degree of stenosis (CTA can better visualize high-grade stenoses than MRA since the latter tends to overestimate the degree of stenosis), MRA is better than CTA at evaluating the petrous and cavernous ICA as bony artifacts affect

Degnana noted that MRA can be superior in the evaluation of MCA stenosis compared with DSA but still only provides information about vessel patency alone. MRA can be an effective screening technique for patients with suspected MCA syndromes to detect stenosis within the intracranial vessels and to indicate the need

It provides better anatomic visualization, particularly in the regions of changing blood flow directions; however, the visualization of smaller arteries remains limited

Prabhakaran noted that QMRA, utilizing phase-contrast techniques, quantifies anterograde blood flow at distal site of stenosis; it exploits the phase shift in the signal of flowing blood, which is proportional to flow velocity, to quantify flow rate

The conventional imaging such as DSA, MRA, CTA, and TCD fall short in characterizing the presence of no occlusive atherosclerotic disease, because it focuses on the vessel lumen estimate luminal stenosis by measuring blood flow velocity [2, 54].

grade stenosis and occlusion is another major disadvantage [58].

for HR-MRI to accurately image the stenotic region [54, 59].

**5.2 Contrast-enhanced (CE) MRA**

**5.3 Quantitative MRA (QMRA)**

in medium and large vessels [51].

**5.4 High-resolution MRI (HR-MRI)**

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

**Figure 5.**

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

tory, particularly in cases with inadequate collateral flow [31, 50].

ent artery [52].

artery locally or embolizes distally [50].

TCD as a surrogate marker [3].

atherosclerotic nature of ICAS [3].

and 91%, respectively [54].

ated acquisition times [57].

**5.1 Magnetic resonance angiography (MRA)**

the lumen status of the intracranial vessels (**Figure 5**) [54].

with distal filling is considered as severe stenosis (70–99%) [55].

performed on a conventional MRI on a 3.0 or 1.5T MR scanner [56].

Strokes associated with ICAD occur in association with four major stroke mechanisms: in situ thrombotic occlusion; branch occlusion; artery-to-artery embolism; and hemodynamic insufficiency [50, 51]. Unstable intracranial plaques can suddenly lead to thrombotic occlusion. Using transcranial duplex monitoring, artery-to-artery embolism can be discovered, which commonly causes multiple cortico-subcortical infarcts. Branch occlusive disease (BOD) is one of the main stroke mechanisms of ICAD, which can be characterized by a milder degree of stenosis [19] and comma-shaped infarcts extending to the basal surface of the par-

The first two mechanisms are the consequences of plaque rupture, which reveals the thrombogenic core to clotting factors, resulting a thrombus that occludes the

The third mechanism, specific to ICAD, is the growth of plaque over the ostia of penetrating arteries resulting in occlusion, which was described by Caplan [53] as branch atheromatous disease. Lastly, high-grade narrowing or occlusion of the lumen may lead to the fourth mechanism: hypoperfusion of the distal brain terri-

MRA, CTA, DSA, TCD, and TCCS can detect ICAS of different histopathological nature (including partially recanalized emboli) and can assess ICAS progression and in-stent restenosis. Unfortunately, these methods are unable to directly exam plaque instability, with the exception of microembolic signals (MES) detection by

Arenillas suggested that for the detection of intracranial plaque morphology, the imaging techniques used include high-resolution MRI (HR-MRI), high magnetic field (3T) preferable, and intravascular ultrasound. This new concept allows the detection and characterization of nonstenotic intracranial atheroma, establishing its role in stroke of undetermined origin, and may have the power to confirm the

Degnana noted that 3D time-of-flight (TOF)-MRA is a imaging technique that noninvasively explores the intracranial arteries. It takes the advantage of the contrast between nonsaturated spins in the blood entering the imaging plane and the stationary adjacent tissue, which remains saturated. 3D TOF-MRA allows the visualization of any variation in blood flow. It provides detailed information about

The following arterial segments are assessed: bilateral intracranial ICA, ACA-A1/ A2, MCA-M1/M2, PCA-P1/P2, BA, and VA. According to the severity of stenosis, there are four groups in which patients are classified: <50% or no stenosis, 50–69% stenosis, 70–99% stenosis, and occlusion groups. Focal flow void found on MRA

Degnana asserted that MRA offers good equivalency with DSA for the detection of >50% stenosis with the reported sensitivity, specificity, and accuracy of 92, 91,

Higher field strength scanners may carry additional benefits in improving signal intensity-to-noise ratio and background suppression; other sequences, such as novel sensitivity encoding (SENSE) TOF-MRA protocols, also have substantially abbrevi-

Other MR sequences, such as T2-/T1-weighted imaging, fluid-attenuated inversion recovery sequences, and diffusion-weighted imaging (DWI), are also

**92**

*Comprehensive imaging of a patient with recent stroke depicting left MCA stenosis. A−C, DSA (A) confirms stenosis, but contrast – enhanced MRA (B) and volume-reduced TOF MRA (C) overestimate the degree of stenosis in this particular case [54].*

MRA has some advantages: noninvasive, no radiation, low cost, and widely available, but for the detection of ICAS requires confirmation by another imaging modality [51, 54]. The main disadvantage is that it overestimates stenosis. MRA can render ambiguous or erroneous results. The inability to distinguish between highgrade stenosis and occlusion is another major disadvantage [58].

In addition, MRA has magnet contraindications: limited to no use in morbidly obese and claustrophobic patients and those with implanted metallic objects [51]. Although CTA performs better overall compared with MRA for the purpose of determining the degree of stenosis (CTA can better visualize high-grade stenoses than MRA since the latter tends to overestimate the degree of stenosis), MRA is better than CTA at evaluating the petrous and cavernous ICA as bony artifacts affect CTA [51, 54].

Degnana noted that MRA can be superior in the evaluation of MCA stenosis compared with DSA but still only provides information about vessel patency alone. MRA can be an effective screening technique for patients with suspected MCA syndromes to detect stenosis within the intracranial vessels and to indicate the need for HR-MRI to accurately image the stenotic region [54, 59].
