doi:10.1016/j.jcmg.2010.09.007 2010;3;1166-1167 J. Am. Coll. Cardiol. Img.

Robert J. Lederman, and Anthony Z. Faranesh Getting Closer for High-Resolution Vascular MRI

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E D I T O R I A L C O M M E N T

Getting Closer for High-Resolution Vascular MRI*

Robert J. Lederman, MD, Anthony Z. Faranesh, PHD

Bethesda, Maryland

Since the 1980s, investigators have tried to enhancevascular magnetic resonance imaging (MRI) andspectroscopy of deep structures by positioning MRIreceiver coils (antennae) inside the body, closer tothe tissue of interest (1,2). Unfortunately, intravas-cular MRI has been largely disappointing becauseof relatively poor sensitivity profiles of the smallcatheter antennae and because of motion artifacts.

See page 1158

In this issue of iJACC, the team headed by PaulBottomley at Johns Hopkins University has ele-gantly engineered a combination of advances toaddress some of these limitations (3). They reportan “endoscopic” imaging probe that in many waysresembles intravascular ultrasound, achieving a fieldof view of about a centimeter, a spatial resolution�100 �m, and a frame rate of 2/s or more. Theresult should prove useful in assessing small struc-tures and atheromata, and in performing MRI-guided interventional procedures.

To appreciate Bottomley’s advance requires anappreciation of the challenge. Proton magneticresonance creates terrific images from minute radiosignals. Magnetic resonance images are not ac-quired directly as a photograph or X-ray pixelmatrix, but instead as a frequency spectrum resem-bling an audio equalizer display. Higher-resolution

*Editorials published in JACC: Cardiovascular Imaging reflect the views ofthe authors and do not necessarily represent the views of JACC: Cardio-vascular Imaging or the American College of Cardiology.

From the Translational Medicine Branch, Division of Intramural Re-search, National Heart, Lung, and Blood Institute, National Institutes ofHealth, Bethesda, Maryland. This work was supported by the Division ofIntramural Research, National Heart, Lung, and Blood Institute, Na-tional Institutes of Health (1ZIAHL005062). NHLBI and Siemens havea Collaborative Research and Development Agreement that includestechnical development for interventional cardiovascular magnetic reso-

nance imaging. The authors report that they have no relationships todisclose.

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pictures require sampling a wider range of spatialfrequencies, which is time consuming. Unfortu-nately, motion during these slow acquisitions blursthe pictures. Moreover, imaging small things likeblood vessels inside a large body in MRI is espe-cially difficult because the Fourier technique re-quires taking time to image the entire body and notjust the small region of interest. Attempts to speedup this process by reducing the amount of informa-tion gathered (“undersampling”) suffer from ambig-uous encoding of position on spins (“foldover arti-fact”). Obvious noninvasive approaches to suppressfoldover artifact for this high-resolution applica-tion—such as blackening (“saturating”) structuresoutside the region of interest, or selectively excitingonly the “inner volume” of interest (4)—have beendisappointing in moving structures inside the torso.

Intravascular MRI probes can act like a flashlightto detect only nearby tissues. Bottomley’s team hadnoticed earlier (5) that the field of view for intra-vascular catheters is exponentially larger at 3.0-Tthan at 1.5-T, in a millimeter-to-centimeter rangethat is convenient for his application of atheroscle-rosis imaging. What makes this work compelling isthe combination of four critical elements. First, theydesigned a custom MRI catheter probe that canimage a relatively thin “sensitive disk” analogous tothe field of view of intravascular ultrasound. Toreduce the possibility of heating (a real consider-ation during rapid imaging at 3.0-T using conduc-tive MRI catheters), they operate their MRI cath-eter probe in “transmit and receive mode.” Thisallows the radio power used to excite proton spinsto be reduced, from the kilowatt range used instandard body-coil excitation, to a subwatt rangeless likely to heat bystander tissues. Second, fromthis catheter, they deliver a special adiabatic radio-frequency excitation-pulse waveform that creates auniform, localized region of excitation. Third, they

exploit the narrow sensitivity profile of their local

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Lederman and Faranesh

Editorial Comment

1167

RI catheter probe to reduce the amount ofnformation (number of phase encodes), and there-ore the time needed, to create rapid high-esolution images in that narrow field of view. Thisealizes the promise of noninvasive alternatives toeduce foldover artifact described in the previousext, and may reduce the motion disturbance thatas limited previous attempts at local intravascularRI. Fourth, they integrate simple image control

nd post-processing steps to create a realistic work-ow: intermittent “snap-to” determinations of theosition of the moving catheter to ensure the MRIradients are properly angled for good imaging;ross-correlation of the images to create picturesrom the probe point-of-view (which physiciansxpect); and correction for extra brightness of spinslosest to the probe. As they show in vitro, in situ,nd in vivo, the resulting pictures are marvelous andast.

However, 3.0-T has disadvantages comparedith lower-field systems. Balanced steady-state-

ree-precession (SSFP) MRI, the workhorse pulseequence for interventional MRI, remains poor at.0-T compared with 1.5-T. This usually forces.0-T interventionists to operate real-time MRIsing less-efficient gradient echo techniques thateduce the signal-to-noise ratio benefit of 3.0-T, yettill suffer from the increased specific absorption

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RI catheter probe remains specifically vulnerableo heating during MRI (of the rest of the body)sing the body-coil for excitation (2), but here, too,ottomley has recently reported a promising new

billabong” shield approach (6) to mitigate heating.Much work remains to miniaturize the reported

ntravascular MRI probe from the 9-F prototypeere, and to optimize MRI scanner protocols thatrovide high-resolution MRI “endoscopy” imagesn anatomic context and as part of a clinicallyelevant workflow. Unfortunately, the coil sensitiv-ty can be expected to degrade with miniaturization.

With the reported tool in hand, one couldnvision enhanced standalone or contrast-enhancedmaging of atherosclerosis, or lesion planning andssessment during wholly MRI-guided interven-ional procedures. However, the chief limitation tonterventional cardiovascular MRI remains theommercial unavailability of safe and conspicuous

RI catheter devices. We hope this fascinatingeport brings us one step closer.

eprint requests and correspondence: Dr. Robert J. Led-rman, Translational Medicine Branch, Division of In-ramural Research, National Heart, Lung, and Bloodnstitute, National Institutes of Health, Building 10,oom 2c713 MSC1538, Bethesda, Maryland 20892-

ate (SAR) and heating at 3.0-T. The conductive 1538. E-mail: lederman@nih.gov.

6

Ka

E F E R E N C E S

. Kantor HL, Briggs RW, Balaban RS.In vivo 31P nuclear magnetic resonancemeasurements in canine heart using acatheter-coil. Circ Res 1984;55:261–6.

. Martin AJ, Baek B, Acevedo-BoltonG, Higashida RT, Comstock J, SalonerDA. MR imaging during endovascularprocedures: an evaluation of the poten-tial for catheter heating. Magn Reson

3. Sathyanarayana S, Schär M, Kraitch-man DL, Bottomley PA. Towards real-time intravascular endoscopic magneticresonance imaging. J Am Coll CardiolImg 2010;3:1158–65.

4. Feinberg DA, Hoenninger JC, CrooksLE, Kaufman L, Watts JC, ArakawaM. Inner volume MR imaging: techni-cal concepts and their application. Ra-diology 1985;156:743–7.

5. El-Sharkawy AM, Qian D, BottomleyPA. The performance of interventional

ae at higher mag- M

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netic field strengths. Med Phys 2008;35:1995–2006.

. Bottomley PA, Kumar A, EdelsteinWA, Allen JM, Karmarkar PV. De-signing passive MRI-safe implantableconducting leads with electrodes. MedPhys 2010;37:3828–43.

ey Words: angiography ytherosclerosis y interventional

RI.

y on May 8, 2012

doi:10.1016/j.jcmg.2010.09.007 2010;3;1166-1167 J. Am. Coll. Cardiol. Img.

Robert J. Lederman, and Anthony Z. Faranesh Getting Closer for High-Resolution Vascular MRI

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