Flair

The Big Heart Disease Lie

Most Effective Heart Disease Treatment

Get Instant Access

T1-nori

Figure 5 Hyperintense vessel on FLAIR and MRA in acute stroke. The occluded middle cerebral artery is seen on MRA (arrow head, top left) 6 hr following stroke. Hyperintense vessels are noted on FLAIR imaging (arrow, bottom left). The T2 and T1 noncontrast images are normal [adapted with permission from R. Bakshi and L. Ketonen, Brain MRI in clinical neurology. In Baker's Clinical Neurology (R. J. Joynt and R. C. Griggs, Eds.). Copyright Lippincott, Williams & Wilkins, 2001].

techniques have been developed to image vascular anatomy and stenosis due to plague, to measure flow velocity by Doppler shift analysis, and to quantitate flow. Neurosonology techniques are noninvasive, safe, reliable, economical, and informative.

a. B-Mode Imaging of the Carotid and Vertebral

Arteries Cross sections and longitudinal sections depicting arterial anatomy are readily obtained using B-mode (brightness-mode) ultrasound. Such images depict the thickness of the vessel wall (which correlates with stroke risk in the carotid circulation and in the aorta) and the presence and thickness of plaque. Intraplaque hemorrhages (which increase stroke risk) can be identified with an accuracy of 90%, sensitivity of 96%, and specificity of 88%. Ulceration is difficult to identify with accuracy. Homogeneity can be assessed as well. Heterogeneous plaques are associated with increased stroke risk because they are more likely than homogenous plaques to contain hemorrhage. The characterization of plaque heterogeneity and typology is ongoing, and further research will be required to determine the clinical significance of findings such as ulceration.

b. Doppler Ultrasound of the Carotid and Vertebral

Arteries Ultrasound echoes returning from a target, such as blood flowing through a vessel, experience a change in their frequency in direct proportion to blood flow velocity. These Doppler shifts are analyzed using fast Fourier transformation to generate a blood flow velocity spectrum. Mean flow velocity, peak systolic velocity, end diastolic velocity, and a variety of other important indices may be derived. Faster flow velocities generally indicate greater stenosis, unless critical stenosis is present. These principles are used to grade the degree of carotid artery stenosis as A (normal), B (1-15% stenosis), C (16-49% stenosis),

Figure 6 MRV and FLAIR imaging shows transverse sinus occlusion. The top image (arrow) reveals an area of hyperintensity on FLAIR imaging. The hyperintensity corresponds to a thrombus within the right transverse sinus. The bottom image is a magnetic resonance venogram showing lack of venous blood flow through the occluded transverse sinus, manifested as an absence of normal (bright) flow signal. The opposite transverse sinus (shown on the right) has the normal appearance of bright signal, indicating venous blood flow (graphic provided by Rohit Bakshi, M.D.).

Figure 6 MRV and FLAIR imaging shows transverse sinus occlusion. The top image (arrow) reveals an area of hyperintensity on FLAIR imaging. The hyperintensity corresponds to a thrombus within the right transverse sinus. The bottom image is a magnetic resonance venogram showing lack of venous blood flow through the occluded transverse sinus, manifested as an absence of normal (bright) flow signal. The opposite transverse sinus (shown on the right) has the normal appearance of bright signal, indicating venous blood flow (graphic provided by Rohit Bakshi, M.D.).

D (50-79% stenosis), D+ (80-99% stenosis), or occlusion. Because the degree of stenosis closely relates to stroke risk, noninvasive vascular testing with Doppler ultrasound is extremely valuable in the management of cerebrovascular disease. The combined use of B-mode and Doppler spectrum analysis is called duplex sonography and is currently the preferred tool for extracranial vascular screening in patients with stroke. Testing of the vertebral arteries is limited by the vertebrae but may still provide important diagnostic information (e.g., reversal of flow in the left vertebral artery suggests the subclavian steal syndrome). The direction of flow is easily determined using Doppler ultrasound.

c. Transcranial Doppler Ultrasound of Intracranial

Arteries TCD provides a noninvasive method for determining intracranial hemodynamics. The direction of flow within the major intracranial vessels can be determined to rule out significant extracranial stenosis (e.g., subclavian steal) or intracranial disease. Normal ranges for flow velocity have been determined for most large intracranial vessels, and flow velocity changes also may confirm intracranial stenosis. For example, TCD is utilized to detect vasospasm following subarachnoid hemorrhage. TCD can also detect emboli, which appear as high-intensity signals within the Doppler spectrum. Embolus detection is useful for identifying intracardiac shunts, such as patent foramen ovale, and for the detection of microemboli resulting from cardiac surgery. TCD is used for intermittent assessment of vascular patency following thrombolysis, to assess vascular reactivity during provocative tests such as carbon dioxide inhalation or acetazolamide administration, and to continually monitor intracranial flow during surgical procedures. Flow changes have recently been related to cognitive activity.

Vascular reactivity is typically assessed by combining TCD with hyperventilation or carbon dioxide inhalation. The former can reduce middle cerebral artery flow velocity within 15 seconds, and lower it by 35%. In contrast, carbon dioxide inhalation can increase flow velocity by as much as 52.5%. A 1000-mg dose of acetazolamide intravenously can begin to increase flow velocity within 3 min, reaching a peak flow velocity 35% higher than baseline within 10 min. Patients with cerebrovascular risk factors and reduced ability to increase flow velocity in response to carbon dioxide inhalation (i.e., reduced ''cerebral perfusion reserve'') are at higher risk for stroke than individuals with normal reserve. By lowering blood pressure in a controlled fashion, the ability of the cerebral circulation to maintain CBF during periods of hypotension can be measured using TCD. The blood pressure threshold leading to a sudden and severe decrease in flow velocity is defined as the lower limit of cerebral autoregulation. Future research will determine whether determination of the lower limit of cerebral autoregulation will have clinical applicability in preventing strokes attributable to hypotension.

Figure 7 Evolutionary changes of acute and chronic hemorrhage. The top images depict an acute (3 days) cerebellar hemorrhage. Deoxyhemoglobin in the core of the hemorrhage is isointense to slightly hypointense on T1 (left) and proton density (right) images, whereas it is clearly hypointense on T2-weighted imaging (top center). Met-hemoglobin is hyperintense on T1 and proton density imaging but hypointense on T2. Thus, the combination of these techniques differentiates these two hemoglobin degradation forms. The bottom images depict bilateral, chronic (> 4 weeks), basal ganglia hemorrhages (straight arrows). A large, late subacute hemorrhage (curved arrow) is shown also. The chronic hemorrhages are hypointense due to a rim of hemosiderin and ferritin. The bright signal within the late subacute hemorrhage (approximately 4 weeks) is due to extracellular met-hemoglobin accumulation. Thus, the age of these hemorrhages can be readily determined using a combination of MR techniques [adapted with permission from R. Bakshi and L. Ketonen, Brain MRI in clinical neurology. In Baker's Clinical Neurology (R. J. Joynt and R. C. Griggs, Eds.). Copyright Lippincott, Williams & Wilkins, 2001].

Figure 7 Evolutionary changes of acute and chronic hemorrhage. The top images depict an acute (3 days) cerebellar hemorrhage. Deoxyhemoglobin in the core of the hemorrhage is isointense to slightly hypointense on T1 (left) and proton density (right) images, whereas it is clearly hypointense on T2-weighted imaging (top center). Met-hemoglobin is hyperintense on T1 and proton density imaging but hypointense on T2. Thus, the combination of these techniques differentiates these two hemoglobin degradation forms. The bottom images depict bilateral, chronic (> 4 weeks), basal ganglia hemorrhages (straight arrows). A large, late subacute hemorrhage (curved arrow) is shown also. The chronic hemorrhages are hypointense due to a rim of hemosiderin and ferritin. The bright signal within the late subacute hemorrhage (approximately 4 weeks) is due to extracellular met-hemoglobin accumulation. Thus, the age of these hemorrhages can be readily determined using a combination of MR techniques [adapted with permission from R. Bakshi and L. Ketonen, Brain MRI in clinical neurology. In Baker's Clinical Neurology (R. J. Joynt and R. C. Griggs, Eds.). Copyright Lippincott, Williams & Wilkins, 2001].

d. New Ultrasound Techniques (Power Doppler, Color Velocity Flow Imaging, Three-Dimensional Doppler Imaging, etc.) A variety of new neurosonol-ogy techniques are being developed. Power Doppler measures the intensity of the returning echoes rather than the frequency shifts, and it may be useful for low-flow states. Color velocity flow imaging, or volume flow, computes quantitative blood flow values from mean flow velocity over a fixed period of time through a vessel of known size. Three-dimensional Doppler imaging has been used to noninvasively map aneur-ysms and other vascular abnormalities. It seems likely that there will be many important clinical applications of these techniques in the future.

Was this article helpful?

0 0
Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Post a comment