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Show EDITORIAL Future Directions in Imaging of Neck and Brain Vessels John Huston, III, MD Contained in this issue are three articles regarding non- invasive imaging of the neck and brain with computed tomography angiography ( CTA) and magnetic resonance angiography ( MRA). It is remarkable how important these technology- driven techniques have become in the daily care of our patients. Technical advances during the past decade have propelled CTA and MRA into a central role in the evaluation and treatment of patients with cervical and cerebrovascular disease. These technological innovations show no sign of slowing. For example, a newly installed 64- slice CT scanner at the Mayo Clinic offers the possibility of isotropic 0.4 x 0.4 x 0.4- mm resolution and time- resolved capabilities. Multi- planar reconstructions of high resolution data sets can produce coronal and sagittal images of the orbits and temporal bones with a single axial acquisition, cutting the typical radiation dose in half. These imagers, which will soon be widely available, can perform CTA from the aortic arch to the circle of Willis in approximately 10 seconds ( Paul Lindell, MD, personal communication, August 2004). Coupled with cardiac gating, these exams can demonstrate four- dimensional images ( three- dimensional [ 3D] images displayed at sequential time points), including pulsatility studies of the aortic arch and intracranial aneurysms. The utility of MRA has increased with the introduction of parallel imaging techniques and the proliferation of clinical 3.0T imaging systems ( 1). Parallel imaging offers faster image acquisition, higher spatial resolution, or a combination of the two. MR units with a 3.0T magnet have twice the signal compared with standard 1.5T units, which allows improved spectroscopy as well as functional and diffusion tensor imaging. This improving technology will allow MR to move from a technique that makes pictures of anatomy to a technique that quantifies brain physiology. Although it is easy to become mesmerized by this forward march of technology, it is critical to heed the lessons of the enclosed articles. Tsai et al ( 2) and Gandhi ( 3) stress the importance of reviewing source images. Many clinicians rely only on the maximum intensity projection ( MIP) images created from the source images. The MIP's ability to make images from multiple directions and present the data in a useful 3D manner results in a user- friendly format. However, the seductive similarity to conventional angiographic representation of the vasculature hides the degradation of image information that occurs during the MIP process. As these authors state, it is essential to review the individual source images to accurately assess for the presence of a dural fistula. Relying on MIP images alone leads to an overestimation of the degree of carotid stenosis ( 4). Review of source images is also a critical element for the detection and evaluation of intracranial aneurysms. Vascular irregularities clearly visible on the source images can be less conspicuous or even completely hidden by overlapping vessels on the MIP images. The development of versatile and powerful 3D workstations has made it possible to rapidly review the source and reformatted images. These new and faster workstations can quickly segment or remove tissue elements such as bone and muscle. Department of Radiology, Mayo Clinic, Rochester, Minnesota. Address correspondence to John Huston, III, MD, Department of Radiology, Mayo Clinic, 200 First Street SW, Rochester, MN 55905; E- mail: jhuston@ mayo. edu J Neuro- Ophthalmol, Vol. 24, No. 4, 2004 283 JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 Editorial Shah et al ( 5) stress the importance of using multiple sequences to extract the maximum imaging information in the evaluation of cervicocranial arterial dissections. Relying on an individual sequence can lead to an erroneous result or a missed diagnosis. Gandhi ( 3) points out that this is also true in the identification of dural sinus thrombosis. Correlation of standard Tl and T2 MR imaging, two-dimensional ( 2D) time- of- flight MRA coronal source images, and contrast- enhanced venography gives clinicians the greatest chance of distinguishing anatomic variants from acute or chronic thrombus within the dural sinuses. This concept is important with imaging of carotid atherosclerotic disease as well ( 6). Frequently, 2D time- of- flight MRA is used as a scout for a contrast- enhanced MRA. The 2D time- of- flight sequence typically has a signal void when stenosis is 70% or greater. Occasionally, a focal high grade, weblike stenosis cannot be resolved with the contrast-enhanced technique. When a discordance occurs between a signal void on the 2D MRA and no significant stenosis on the contrast- enhanced MRA, it is essential to carefully correlate the results with an ultrasound or proceed to digital subtraction angiography. Gandhi ( 3) has presented a widely accepted listing of the appropriate tests for various clinical indications ( see his Table 1). However, whether CTA or MRA is performed remains dependent on local expertise, equipment capabilities and availability, and the preferences of the individual practice. It is important to remember that catheter digital subtraction angiography remains the imaging test of final recourse and will identify dural fistulas and dissections not yet resolvable with the best MRA or CTA techniques. However, as the capabilities of MRA and CTA continue to evolve at such a remarkable pace, we need to incorporate the lessons regarding CTA and MRA contained in this issue to provide improved care for our patients. REFERENCES 1. Gibbs GF, Huston J, Bernstein MA, et al. Improved image quality of intracranial aneurysms: 3.0- T versus 1.5- T time- of- flight MR angiography. AJNR Am JNeuroradiol 2004; 25: 84- 7. 2. Tsai YF, Chen LK, Su CT, et al. The utility of source images of 3D time- of- flight magnetic resonance angiography in the diagnosis of indirect carotid- cavernous sinus fistulas. J Neurooophthalmol 2004; 24: 285- 9. 3. Gandhi D. Computed tomography and magnetic resonance angiography in cervicocranial vascular disease. J Neurooophthalmol 2004; 24: 306- 14. 4. Huston J, Fain SB, Wald JT, et al. Carotid artery: elliptic centric contrast- enhanced MR angiography compared with conventional angiography. Radiology 2001; 218: 138^ 13. 5. Shah GV, Quint DJ, Trobe JD. Magnetic resonance imaging of suspected cervicocranial arterial dissections. JNeuroophthalmol 2004; 24: 315- 8. 6. DeMarco JK, Huston J, Bernstein MA. Evaluation of classic 2D time- of- flight MR angiography in the depiction of severe carotid stenosis. AJR Am J Roentgenol 2004; 183: 787- 93. 284 © 2004 Lippincott Williams & Wilkins |
