DOI: 10.1148/radiol.2452061706
(Radiology 2007;245:357-361.)
© RSNA, 2007
MR Angiography versus CT Angiography in the Evaluation of Neurovascular Disease1
Brian C. Bowen, MD, PhD
1 From the Department of Radiology, University of Miami Miller School of Medicine, 520 Gondoliere Ave, Miami, FL 33143. Received October 2, 2006; accepted November 1; final version accepted December 15.
Correspondence: Address correspondence to the author (e-mail: bbowen{at}med.miami.edu).
Editor's note: Please see my January 2007 From the Editor communication titled "Radiology 2007: The Year Ahead" for information regarding this new feature—Controversies—in Radiology.
—Anthony V. Proto, MD, Editor
At the 2005 annual meeting of the Radiological Society of North America, eight topics were included in two separate focus sessions titled Controversies in Neuroradiology. Among these was an exchange of viewpoints concerning the relative merits of magnetic resonance (MR) angiography versus computed tomographic (CT) angiography in neurovascular disease. The proponents of each were Brian Bowen, MD, PhD (for MR angiography) and Charles Truwit, MD (for CT angiography). Although disagreements may persist about which technique to use in different clinical situations, the readers of these two contributions will obtain a sense of the advantages and disadvantages of each technique.
—Robert M. Quencer, MD
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INTRODUCTION
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MR angiography is not a single technique but rather a variety of techniques that complement each other. For most clinical applications, two major types of techniques are used: (a) time-of-flight (TOF) MR angiography, which is implemented as a two-dimensional or three-dimensional (3D) acquisition, and (b) dynamic 3D contrast material–enhanced MR angiography. In TOF techniques, vascular contrast is due to the inflow of unsaturated spins. Arteries and veins are differentiated by the differences in residual intravascular longitudinal magnetization, often aided by the application of spatial presaturation pulses. Unlike CT angiography and contrast-enhanced MR angiography, TOF MR angiography requires no carefully timed contrast agent injection and is repeatable. Acquisition data are rapidly processed online by using widely accepted standard algorithms, which generate maximum intensity projections (MIPs), multiplanar reconstructions, and other displays from source images. In 3D contrast-enhanced MR angiographic techniques, vascular contrast depends primarily on the T1 shortening of intravascular spins by an injected gadolinium-based contrast agent and the use of echo times on the order of a millisecond to overcome phase dispersion and saturation effects that sometimes limit the TOF MR angiographic techniques. Temporal resolution of vascular filling differentiates the major cervicocerebral arteries and veins. Thus, the dynamic contrast-enhanced MR angiographic techniques are analogous to the CT angiographic technique and can display vascular anatomy without degradation by flow effects, yielding angiograms of the neurovasculature. In addition, MR angiography is performed in conjunction with conventional MR imaging, which is unmatched by CT with respect to tissue characterization in its depiction of diffusion-perfusion mismatch in acute ischemic stroke, vulnerable plaque, and evolving blood products in hemorrhagic lesions.
In advocating the use of MR angiography, I will emphasize three challenging areas of noninvasive neurovascular imaging: carotid and vertebral artery disease in the neck and skull base region, intracranial aneurysms, and the specialized area of spinal vascular lesions. For evaluation of the carotid and vertebral arteries in the neck and skull base region, MR angiography has been extensively validated. In patients with atherosclerotic disease of the carotid bifurcation, TOF techniques yield results similar to results with 3D contrast-enhanced MR angiography for mild-to-moderate stenosis (Fig 1), thus obviating time spent on contrast-enhanced MR angiography setup and processing of images. For high-grade stenosis, which can cause intravascular flow gaps on TOF MIP images, the addition of contrast-enhanced MR angiography to the imaging protocol provides sensitivity and specificity similar to that of CT angiography in determining the severity of stenosis (relative to intraarterial digital subtraction angiography as the reference standard) (1). In patients with dissection, MR angiography is complemented by MR imaging with fat saturation sequences that aid in detection and characterization of dissecting hematomas, associated pseudoaneurysms, and the length and caliber of the residual patent lumen, especially at the skull base (Fig 2). The improved recognition of these features afforded by MR angiography performed together with MR imaging affects treatment decisions, such as the deployment of stents and other endovascular devices.
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Figure 1a: Oblique MIP images show similar appearance of mild stenosis (arrowhead) of the left internal carotid artery. (a) Three-dimensional contrast-enhanced and (b) 3D TOF MR angiographic images. Note the greater coverage of the carotid arteries afforded by coronal contrast-enhanced MR angiography (repetition time msec/echo time msec, 3.4/1.3; flip angle, 30°; section or source image thickness, 1 mm; field of view, 300 x 250 mm; matrix, 384 x 240; effective voxel size, 0.8 x 1 x 1 mm; and acquisition time, 14 seconds), as compared with transverse TOF MR angiography (19/3.4; flip angle, 20°; section thickness, 0.8 mm; field of view, 200 x 175 mm; matrix, 256 x 224; effective voxel size, 0.8 x 0.8 x 0.8 mm; and acquisition time, 7.2 minutes), enabling evaluation of the carotid siphon and common carotid origin for tandem lesions.
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Figure 1b: Oblique MIP images show similar appearance of mild stenosis (arrowhead) of the left internal carotid artery. (a) Three-dimensional contrast-enhanced and (b) 3D TOF MR angiographic images. Note the greater coverage of the carotid arteries afforded by coronal contrast-enhanced MR angiography (repetition time msec/echo time msec, 3.4/1.3; flip angle, 30°; section or source image thickness, 1 mm; field of view, 300 x 250 mm; matrix, 384 x 240; effective voxel size, 0.8 x 1 x 1 mm; and acquisition time, 14 seconds), as compared with transverse TOF MR angiography (19/3.4; flip angle, 20°; section thickness, 0.8 mm; field of view, 200 x 175 mm; matrix, 256 x 224; effective voxel size, 0.8 x 0.8 x 0.8 mm; and acquisition time, 7.2 minutes), enabling evaluation of the carotid siphon and common carotid origin for tandem lesions.
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Figure 2a: Right internal carotid artery dissection with intramural hematoma causing severe narrowing of the residual patent lumen, as well as pseudoaneurysm at the skull base. (a) Transverse source 3D TOF MR image (19/3.4; flip angle, 20°) and (b) transverse fat-suppressed T1-weighted MR image (532/14; section thickness, 5 mm; intersection gap, 0.5 mm; field of view, 210 x 184 mm; matrix, 210 x 168) at level of C1-C2 show the narrowed cervical carotid artery lumen with flow and the thickened wall (arrow) with unsuppressed hyperintensity, consistent with subacute hematoma. (c) Transverse source 3D TOF MR image at level of carotid canal entrance shows outpouching of pseudoaneurysm (arrow). (d) Oblique MIP image displays the length of the narrowed lumen (distance between arrows), beginning at the midcervical carotid atery (small arrow) and ending at the carotid canal and the pseudoaneurysm (long arrow).
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Figure 2b: Right internal carotid artery dissection with intramural hematoma causing severe narrowing of the residual patent lumen, as well as pseudoaneurysm at the skull base. (a) Transverse source 3D TOF MR image (19/3.4; flip angle, 20°) and (b) transverse fat-suppressed T1-weighted MR image (532/14; section thickness, 5 mm; intersection gap, 0.5 mm; field of view, 210 x 184 mm; matrix, 210 x 168) at level of C1-C2 show the narrowed cervical carotid artery lumen with flow and the thickened wall (arrow) with unsuppressed hyperintensity, consistent with subacute hematoma. (c) Transverse source 3D TOF MR image at level of carotid canal entrance shows outpouching of pseudoaneurysm (arrow). (d) Oblique MIP image displays the length of the narrowed lumen (distance between arrows), beginning at the midcervical carotid atery (small arrow) and ending at the carotid canal and the pseudoaneurysm (long arrow).
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Figure 2c: Right internal carotid artery dissection with intramural hematoma causing severe narrowing of the residual patent lumen, as well as pseudoaneurysm at the skull base. (a) Transverse source 3D TOF MR image (19/3.4; flip angle, 20°) and (b) transverse fat-suppressed T1-weighted MR image (532/14; section thickness, 5 mm; intersection gap, 0.5 mm; field of view, 210 x 184 mm; matrix, 210 x 168) at level of C1-C2 show the narrowed cervical carotid artery lumen with flow and the thickened wall (arrow) with unsuppressed hyperintensity, consistent with subacute hematoma. (c) Transverse source 3D TOF MR image at level of carotid canal entrance shows outpouching of pseudoaneurysm (arrow). (d) Oblique MIP image displays the length of the narrowed lumen (distance between arrows), beginning at the midcervical carotid atery (small arrow) and ending at the carotid canal and the pseudoaneurysm (long arrow).
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Figure 2d: Right internal carotid artery dissection with intramural hematoma causing severe narrowing of the residual patent lumen, as well as pseudoaneurysm at the skull base. (a) Transverse source 3D TOF MR image (19/3.4; flip angle, 20°) and (b) transverse fat-suppressed T1-weighted MR image (532/14; section thickness, 5 mm; intersection gap, 0.5 mm; field of view, 210 x 184 mm; matrix, 210 x 168) at level of C1-C2 show the narrowed cervical carotid artery lumen with flow and the thickened wall (arrow) with unsuppressed hyperintensity, consistent with subacute hematoma. (c) Transverse source 3D TOF MR image at level of carotid canal entrance shows outpouching of pseudoaneurysm (arrow). (d) Oblique MIP image displays the length of the narrowed lumen (distance between arrows), beginning at the midcervical carotid atery (small arrow) and ending at the carotid canal and the pseudoaneurysm (long arrow).
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In general, 3D TOF MR angiography (Fig 3) and dynamic 3D contrast-enhanced MR angiography have sensitivity and specificity similar to those of CT angiography for detection of intracerebral aneurysms that are 5 mm or larger in diameter and have lower sensitivity for detection of aneurysms that are smaller than 5 mm (2,3). The question that has yet to be answered, however, is whether the lower sensitivity of MR angiography for detection of aneurysms smaller than 5 mm results in increased morbidity and mortality for patients who undergo MR angiographic rather than CT angiographic screening for unruptured aneurysms. The results of the International Study of Unruptured Intracranial Aneurysms, or ISUIA (4), trial suggest that performance of MR angiography would result in similar treatment, because the conclusion of the trial was that incidental aneurysms that are smaller than 10 mm should not be treated (exceptions may be made for individuals with daughter aneurysm formation, individuals with a family history of subarachnoid hemorrhage [SAH], and individuals who are young). The clinical setting in which MR angiography clearly outperforms CT angiography is in the assessment of aneurysms that were treated with an endovascular coil (5). CT angiography is nondiagnostic for the presence of a residual lumen and the size and location of the remnant neck because of streak artifacts from the nidus of the coils (Fig 4). TOF MR angiography, however, depicts the residual lumen and neck with a sensitivity of 71%–91% and a specificity of 89%–100% (6). Performance of dynamic 3D contrast-enhanced MR angiography further improves detection, as shown for the remnants of anterior communicating artery aneurysms treated with coils, and a sensitivity of 100% and a specificity of 93% are achieved (5).
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Figure 3: Targeted coronal MIP image from 3D TOF MR angiographic image (19/3.4; flip angle, 20°) shows readily detected and characterized paraclinoid aneurysms measuring 5 x 7 mm (long arrow) and 3 mm in diameter (short arrow).
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Figure 4a: Right ophthalmic artery aneurysm following coil embolization. (a) CT angiographic source image was nondiagnostic for residual lumen because of streak artifacts. (b) Transverse source 3D TOF MR angiographic image (19/3.4; flip angle, 20°) at level of aneurysm dome reveals central and eccentric hypointensity (arrow) due to packed coils and peripheral hyperintensiy due to flow-related enhancement in residual lumen. (c) Transverse source 3D TOF MR angiographic image at level of aneurysm neck also shows evidence of flow through patent neck remnant (arrow). (d) Coronal MIP image demonstrates continuity of flow into neck (short arrow) and dome (long arrow) remnants of aneurysm treated with coil.
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Figure 4b: Right ophthalmic artery aneurysm following coil embolization. (a) CT angiographic source image was nondiagnostic for residual lumen because of streak artifacts. (b) Transverse source 3D TOF MR angiographic image (19/3.4; flip angle, 20°) at level of aneurysm dome reveals central and eccentric hypointensity (arrow) due to packed coils and peripheral hyperintensiy due to flow-related enhancement in residual lumen. (c) Transverse source 3D TOF MR angiographic image at level of aneurysm neck also shows evidence of flow through patent neck remnant (arrow). (d) Coronal MIP image demonstrates continuity of flow into neck (short arrow) and dome (long arrow) remnants of aneurysm treated with coil.
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Figure 4c: Right ophthalmic artery aneurysm following coil embolization. (a) CT angiographic source image was nondiagnostic for residual lumen because of streak artifacts. (b) Transverse source 3D TOF MR angiographic image (19/3.4; flip angle, 20°) at level of aneurysm dome reveals central and eccentric hypointensity (arrow) due to packed coils and peripheral hyperintensiy due to flow-related enhancement in residual lumen. (c) Transverse source 3D TOF MR angiographic image at level of aneurysm neck also shows evidence of flow through patent neck remnant (arrow). (d) Coronal MIP image demonstrates continuity of flow into neck (short arrow) and dome (long arrow) remnants of aneurysm treated with coil.
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Figure 4d: Right ophthalmic artery aneurysm following coil embolization. (a) CT angiographic source image was nondiagnostic for residual lumen because of streak artifacts. (b) Transverse source 3D TOF MR angiographic image (19/3.4; flip angle, 20°) at level of aneurysm dome reveals central and eccentric hypointensity (arrow) due to packed coils and peripheral hyperintensiy due to flow-related enhancement in residual lumen. (c) Transverse source 3D TOF MR angiographic image at level of aneurysm neck also shows evidence of flow through patent neck remnant (arrow). (d) Coronal MIP image demonstrates continuity of flow into neck (short arrow) and dome (long arrow) remnants of aneurysm treated with coil.
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Although noninvasive imaging of spinal vessels and vascular lesions is a specialized area of neurovascular imaging, it showcases the value of multifaceted MR angiography combined with MR imaging in obtaining diagnostic information that is not available from CT angiography. Performance of MR angiography and MR imaging together has been shown to help in the prediction of the level of spinal dural arteriovenous fistula to within one vertebral segment above or one vertebral segment below the actual level in at least 73% of studies with true-positive findings, and these results are significantly better (P = .016) than those with MR imaging alone (Fig 5) (7). Findings in studies have also indicated that patients undergoing spinal angiography following MR angiography require half the fluoroscopy time and volume of iodinated contrast material if the level and side of the fistula have been identified at screening MR angiography (8). MR angiography and CT angiography have similar sensitivity values in screening for the level of the artery of Adamkiewicz (9).
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Figure 5a: Spinal dural arteriovenous fistula. (a) T2-weighted fast spin-echo sagittal MR image (3800/122; section thickness, 3.5 mm; intersection gap, 0.5 mm; number of signals acquired, one; field of view, 300 x 300 mm; matrix, 512 x 256) of thoracic spine shows hyperintensity of spinal cord (at and above arrows) and of the vertebrae above T8, as well as serpentine intradural flow voids (arrowheads) in a patient with a history of radiation therapy for lung carcinoma and progressive myelopathy. 10 = T10 vertebral body. (b) Coronal MIP image from a sagittal 3D steady-state contrast-enhanced MR angiographic image (32/4.8; flip angle, 15°; section or source image thickness, 0.8 mm; field of view, 300 x 300 mm; matrix, 448 x 367; effective voxel size, 0.7 x 0.8 x 0.8 mm; and acquisition time, 7 minutes) of the same thoracic region reveals an enlarged, tortuous vessel (arrow) extending from approximately the right T11 (11) foramen to abnormal vessels on the cord surface. Findings are typical for dural arteriovenous fistula. (c) Subsequent angiography confirmed the diagnosis. Anteroposterior view shows the enlarged vessel (arrow) to be the medullary vein draining from the fistula in the T11 (11) foramen to the coronal venous plexus on the cord surface.
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Figure 5b: Spinal dural arteriovenous fistula. (a) T2-weighted fast spin-echo sagittal MR image (3800/122; section thickness, 3.5 mm; intersection gap, 0.5 mm; number of signals acquired, one; field of view, 300 x 300 mm; matrix, 512 x 256) of thoracic spine shows hyperintensity of spinal cord (at and above arrows) and of the vertebrae above T8, as well as serpentine intradural flow voids (arrowheads) in a patient with a history of radiation therapy for lung carcinoma and progressive myelopathy. 10 = T10 vertebral body. (b) Coronal MIP image from a sagittal 3D steady-state contrast-enhanced MR angiographic image (32/4.8; flip angle, 15°; section or source image thickness, 0.8 mm; field of view, 300 x 300 mm; matrix, 448 x 367; effective voxel size, 0.7 x 0.8 x 0.8 mm; and acquisition time, 7 minutes) of the same thoracic region reveals an enlarged, tortuous vessel (arrow) extending from approximately the right T11 (11) foramen to abnormal vessels on the cord surface. Findings are typical for dural arteriovenous fistula. (c) Subsequent angiography confirmed the diagnosis. Anteroposterior view shows the enlarged vessel (arrow) to be the medullary vein draining from the fistula in the T11 (11) foramen to the coronal venous plexus on the cord surface.
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Figure 5c: Spinal dural arteriovenous fistula. (a) T2-weighted fast spin-echo sagittal MR image (3800/122; section thickness, 3.5 mm; intersection gap, 0.5 mm; number of signals acquired, one; field of view, 300 x 300 mm; matrix, 512 x 256) of thoracic spine shows hyperintensity of spinal cord (at and above arrows) and of the vertebrae above T8, as well as serpentine intradural flow voids (arrowheads) in a patient with a history of radiation therapy for lung carcinoma and progressive myelopathy. 10 = T10 vertebral body. (b) Coronal MIP image from a sagittal 3D steady-state contrast-enhanced MR angiographic image (32/4.8; flip angle, 15°; section or source image thickness, 0.8 mm; field of view, 300 x 300 mm; matrix, 448 x 367; effective voxel size, 0.7 x 0.8 x 0.8 mm; and acquisition time, 7 minutes) of the same thoracic region reveals an enlarged, tortuous vessel (arrow) extending from approximately the right T11 (11) foramen to abnormal vessels on the cord surface. Findings are typical for dural arteriovenous fistula. (c) Subsequent angiography confirmed the diagnosis. Anteroposterior view shows the enlarged vessel (arrow) to be the medullary vein draining from the fistula in the T11 (11) foramen to the coronal venous plexus on the cord surface.
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ADVANTAGES OF MR ANGIOGRAPHY
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1. MR angiography is complemented by conventional MR imaging for tissue characterization
2. Minimal image degradation with newer vascular treatment devices: coils, clips, and stents
3. No ionizing radiation, no high doses of iodinated contrast agent, and no venous access required for TOF MR angiography (or MR venography)
4. Rapid standardized postprocessing algorithms with a short learning curve for implementation of views, recognition of tissue contrast, and recognition of vascular anatomy
5. Assessment of atherosclerotic disease at the carotid bifurcation, as well as at the carotid siphon (tandem lesion), in a single study
6. Internal carotid artery lesions, such as dissection and/or aneurysm, extending to or originating at the skull base
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DISADVANTAGES OF MR ANGIOGRAPHY
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1. Uncertainty in relationship of vessels to osseous anatomy and/or aerated structures
2. Loss of flow-related intravascular signal intensity or inhomogeneous signal intensity at TOF imaging of vascular stenoses or aneurysms
3. Longer acquisition times than for CT angiography for similar anatomic coverage and spatial resolution (MR angiographic parameters improved by using 3-T MR imaging and parallel imaging)
4. Reduced availability of MR imagers in acute care setting (emergency department, trauma center), as well as the need for precautions in the vicinity of strong magnetic fields
5. Several contraindications to implanted electrical and electromagnetic devices (pacemakers) and ferromagnetic materials
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SUMMARY
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For the past decade, a major focus of neuroimaging has been the development of noninvasive methods for detecting vascular disease. To resolve the issue of which method, MR angiography or CT angiography, is appropriate for a given neurovascular application in a specified clinical practice setting, researchers in future clinical investigations must compare the results of both procedures, preferably in the same patients and under commonly available technical conditions, in regard to a number of parameters that are evaluated by reviewers experienced in MR angiography and CT angiography. The outcomes of such investigations can then be used as the foundation for higher levels of evidence in justifying the performance of these methods within the guidelines of evidence-based medicine.
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FOOTNOTES
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Author stated no financial relationship to disclose.
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References
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- Villablanca JP, Jahan R, Hooshi P, et al. Detection and characterization of very small cerebral aneurysms by using 2D and 3D helical CT angiography. AJNR Am J Neuroradiol 2002;23:1187–1198.[Abstract/Free Full Text]
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- Leclerc X, Navez JF, Gauvrit JY, et al. Aneurysms of the anterior communicating artery treated with Guglielmi detachable coils: follow-up with contrast-enhanced MR angiography. AJNR Am J Neuroradiol 2002;23:1121–1127.[Abstract/Free Full Text]
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- Luetmer PH, Lane JI, Gilbertson JR, et al. Preangiographic evaluation of spinal dural arteriovenous fistulas with elliptic centric contrast-enhanced MR angiography and effect on radiation dose and volume of iodinated contrast material. AJNR Am J Neuroradiol 2005;26:711–718.[Abstract/Free Full Text]
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Rebuttal of Dr Bowen's Comments in Advocating MR Angiography
Charles L. Truwit, MD
Dr Bowen has offered his comments in favor of MR angiography. However, CT angiography allows faster imaging and can be offered at any time of the day or night. This feature is not to be taken lightly when evaluating patients with acute stroke and SAH.
In regard to the work-up for acute stroke, the fundamental arguments in favor of MR angiography are the accompanying diffusion-weighted MR imaging and the reported capability of MR imaging to reveal intracranial hemorrhage, specifically SAH. The first argument contends that the patient is already in the MR imager, and it costs little in time to add performance of MR angiography of the carotid artery and circle of Willis. I offer the counter argument that MR imaging is not readily available in the middle of the night. In my hospital, MR imaging is available only by attending physician to attending physician request, essentially eliminating middle-of-the-night MR imaging that was being performed for referring physician convenience. We insist that a resident awaken his or her attending physician to call the attending radiologist directly. If the referring physician contends that MR imaging needs to be performed, we will do it without hesitation.
In regard to the work-up for SAH, I agree that with the use of strict guidelines, and in the best of circumstances, we can reliably detect SAH on turbo fluid-attenuated inversion-recovery images. In clinical practice, however, my own experience leads me to believe that most of us are not as adept at detecting SAH on MR images as we are at detecting it on CT scans. Artifact within the basal cisterns on the turbo fluid-attenuated inversion-recovery images is commonplace and can be a source of confusion.
In short, although MR angiography offers advantages, in my opinion, these advantages are offset by the advantages and ease of performance (day or night) of CT angiography for the diagnosis and evaluation of neurovascular diseases, especially for the work-up of patients with acute stroke and SAH and for presurgical planning.
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T. J. Kaufmann and D. F. Kallmes
Diagnostic Cerebral Angiography: Archaic and Complication-Prone or Here to Stay for Another 80 Years?
Am. J. Roentgenol.,
June 1, 2008;
190(6):
1435 - 1437.
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TOP
INTRODUCTION
ADVANTAGES OF MR ANGIOGRAPHY
