Field Strength. Regional Perfusion Imaging (RPI) matches cerebral arteries to flow territories

Transcription

1 Field Strength Changing how the world looks at MR. Regional Perfusion Imaging (RPI) matches cerebral arteries to flow territories Research groups in Utrecht, Baltimore and Singapore collaborate on this ASL technique for brain perfusion studies. This article is part of Field Strength Issue 26 September 2005

2 Regional Perfusion Imaging research attempts to match arteries to flow territories ASL-based research technique may afford clearer picture of selected artery s brain perfusion role Regional Perfusion Imaging (RPI), an investigational variant of the arterial spin labeling (ASL) MR perfusion research technique, is designed to provide selective mapping of the flow territories of the right and left internal carotid arteries, in addition to the basilar and vertebral arteries. The RPI development effort is a joint collaboration between Dr. Jeroen Hendrikse at University Medical Center Utrecht (UMCU, Utrecht, The Netherlands), Dr. Xavier Golay at the National Neuroscience Institute (NNI, Singapore) and Philips scientists. RPI may become the first non-invasive technique to match up major cerebral arteries and collateral vessels with the regions they perfuse. Knowing the relationship between collaterals and regional perfusion could inform therapeutic decisions for stroke and other vascular conditions. Jeroen Hendrikse, M.D., Ph.D. Xavier Golay, Ph.D. Arterial spin labeling or ASL is a research technique developed to help quantify cerebral perfusion. In ASL, arterial blood upstream of its expected perfusion territory is magnetically labeled by inverting the spins, and then allowed to flow into the downstream tissues, where the labeled blood changes the tissue s magnetization. Subsequent imaging without labeling the blood is performed, followed by subtraction methods in an effort to provide clues about the perfusion status of downstream areas. ASL, however, labels all feeding arteries in a given territory, making this research technique nonselective, says Jeroen Hendrikse, M.D., Ph.D., a radiology fellow and scientist at UMCU, which is using its Achieva 1.5T system for regional perfusion imaging (RPI) clinical research. RPI is designed to isolate a vessel of interest first and then conduct the perfusion study to yield data that might help us determine that artery s contribution to the perfusion of downstream tissues, and whether the vessel s ability to supply the tissues is compromised. Additionally, we may find that collateral vessels are augmenting or even entirely replacing the tissue perfusion duties of a major artery. RPI methodology The key to RPI s selectivity is the ability to implement an inversion of the spins of arterial water that are included in a specified slab, which can be angulated freely in relation to the imaging slices. This allows selective labeling of the left and right carotid arteries, as well as the vertebral and basilar arteries. Efficient workflow could mean quick exam RPI studies in clinical research have taken just 20 minutes, comprising just five minutes for phase contrast surveys, MRA, and planning of labeling volumes and 15 minutes for RPI. RPI is designed to provide standard perfusion and cerebral blood flow data, but the novel information may be flow territory information, he observes. We may now be able to non-invasively determine the contribution of each artery separately in addition to the role of collateral vessels. Hypothetically, in a patient with cerebral ischemia caused by a vascular occlusion, for example, clinicians using RPI data might be able to deduce the degree of a perfusion territory s vulnerability based on SENSE 3D Phase Contrast MRA demonstrating the complex distribution of the cerebral blood flow in the cerebral vasculature. Issue 26 - September 2005 Field Strength 21

3 EC-IC Bypass 1 2 R L Left BA Example of planning the selective labeling slab for an extracranial to intracranial bypass and selective labeling of the contralateral on the basis of a 2D phase contrast survey and a time-of-flight MRA of the circle of Willis. Anatomy Slice 1 Slice 2 Slice 3 Slice 4 Slice 5 Internal carotid artery occlusion Coronal survey PC and RPI images of the flow territory of the left internal carotid artery (; arrow 2) and basilar artery (BA; arrow 1) in a patient with atherosclerotic occlusion of the right. Left to right collateral flow (small arrowheads) and posterior to anterior collateral flow is observed (large arrowheads). The application of two consecutive 90º RF pulses in the labeling slab accomplishes inversion of inflowing spins. In the control scan, the phase of the second 90º pulse is shifted by 180º. Three 90º saturation pulses, followed by strong dephasing gradients (shaded) are applied after RPI labeling to remove residual static magnetization. After a labeling delay (inversion time/ti) of 1200 ms to enable the labeled blood to change tissue magnetization in the perfusion territory five slices are acquired in descending slice order. 22 Field Strength Issue 26 - September 2005

4 EC-IC Bypass Right BA Right EC-IC Bypass Anatomy Slice 1 Slice 2 Slice 3 Slice 4 Slice 5 High flow bypass surgery Conventional DSA of the right internal carotid artery () and an extracranial-intracranial (EC-IC) high flow bypass between the external carotid artery and the intracranial internal carotid artery (arrow 1). In this patient with a giant aneurysm of the left, therapeutic balloon occlusion (arrow 2) of the diseased artery was performed. RPI images demonstrate the postoperative flow territories of the extracranial-intracranial bypass, right and basilar artery (BA). Net Forum Visit the MRI NetForum Community for extended information. the contribution of collateral vessels. If a large stenosed vessel is supplying a large territory, the decision to intervene is more urgent if collateralization is inadequate. Similarly, RPI might have the potential to inform a decision to sacrifice an artery with a large aneurysm in an internal carotid artery if the study shows sufficient collateralization of the artery s perfusion territory. An RPI study could conceivably even help us identify the biggest vascular contributor to an arteriovenous malformation, and whether it s worth the risk to embolize the vessel, he adds. Recent RPI work at NNI in Singapore involves the Achieva 3.0T. The RPI research results using Achieva 3.0T are even better, says Dr. Golay, who conducted initial RPI work at his former position at Johns Hopkins University. The SNR is doubled and the T1 of blood is longer, so there s a big increase in signal, allowing RPI experimental scan time to be decreased to less than 10 min. You see more distal vessels because the T1 decay is slower and you can still see the perfusion signal from these inflowing spins even after two to three seconds. References: 1. X. Golay, J. Hendrikse,T.C.C. Lim Perfusion Imaging Using Arterial Spin Labeling Top Magn Reson Imag, 15(1), (2004) 2. J. Hendrikse, J. van der Grond, H. Lu, P.C.M. van Zijl, X. Golay Flow Territory Mapping of the Cerebral Arteries with Regional Perfusion MRI (RPI) Stroke, 35(4), (2004) 3. X. Golay, E.T. Petersen, F. Hui PULsed STAR labeling of Arterial Regions (PULSAR): a Robust Regional Perfusion Technique for High Field Imaging Magn Reson Med, 53(1), (2005) Issue 26 - September 2005 Field Strength 23

5 Philips Medical Systems is part of Royal Philips Electronics Interested? Would you like to know more about our imaginative products? Please do not hesitate to contact us. We would be happy to provide specific information about our products and services, or put you on our mailing list for news about new product developments, upcoming events, for our clinical journal Medica Mundi, or our MR journal Field Strength. We would be glad to hear from you. On the web Via Asia Tel: Europe, Middle East, Africa Tel: Latin America Tel: North America Tel: Koninklijke Philips Electronics N.V All rights are reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright holder. Philips Medical Systems Nederland B.V. reserves the right to make changes in specifications and/or to discontinue any product at any time without notice or obligation and will not be liable for any consequences resulting from the use of this publication. This reprint comes from Field Strength issue 26: / /781 (USA) By fax By postal service Philips Medical Systems Global Information Center P.O. Box BD Eindhoven The Netherlands