Perspectives of in vivo magnetic resonance imaging of cell dynamics in the brain

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10.2217/14796708.3.4.423 © 2008 Future Medicine Ltd ISSN 1479-6708

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Perspectives of in vivo magnetic resonance imaging of cell dynamics in the brainTracy D Farr† & Mathias Hoehn†Author for correspondenceMax-Planck Institute for Neurological Research, In vivo-NMR Laboratory, Gleueler Strasse 50, Cologne, 50931, GermanyTel.: +49 221 472 6339;Fax: +49 221 472 6337;[email protected]

Keywords: cell labeling, cell migration, imaging reporters, molecular imaging, MRI, responsive contrast agents

MRI is an established diagnostic tool, but it also has great attraction for use in experimental research, particularly in neuroscience and neurology. In vivo imaging of specific cell populations in the brain is particularly attractive for furthering understanding of cell behavior in animal models of neurological disease and injury. Approaches towards this end typically make use of iron oxide nanoparticles as MRI contrast agents. These contrast agents can be taken up by peripheral inflammatory cells, by endogenous CNS cell populations, or by in vitro cell cultures for transplantation experiments. Molecular imaging of functional cell status, using MRI in combination with molecular biology, is a rapidly expanding field with great promise. The present review summarizes the current status of cellular MRI in the brain in the context of ischemia models, and relevant issues and approaches that aim to improve translation of cell therapy strategies into the clinic.

MRI offers excellent soft tissue contrast, highspatial and temporal resolution and 3D imag-ing capability. Improvements to hardware anddevelopments of new imaging sequences havecontinued to expand the sensitivity and reduceacquisition times. This makes MRI the mostversatile modality in which to develop strategiesfor cellular and molecular imaging.

Cellular MRI is a rapidly expanding fieldwith much emphasis on cell-based therapy forneurological disease and injury. A number ofstrategies have been used to monitor specificcell populations in the brain, and the majorityof these employ iron oxide nanoparticles as ameans to label cells for in vivo detection againstthe host tissue background. Approaches rangefrom the exploitation of the phagocytic capac-ity of peripheral inflammatory cells followinginjury to the endocytotic capability of endog-enous brain stem cell populations. By far themost widely studied approach is in vitro label-ing of different stem cell populations with ironoxide nanoparticles for transplantation experi-ments. Molecular MRI is an emerging field thataims to provide information regarding func-tionality or activation state using biologicallycoupled MRI contrast.

In this review we describe the status regard-ing the use of MRI to detect cells in the brainand to observe their migration and behavior,primarily in the context of ischemia models.We also discuss developments of innovativelabeling strategies, as well as the potential con-tribution of molecular imaging to the field ofneuroscience and neurology.

Imaging cellular inflammation in the brainBy nature, macrophages are phagocytotic and willendocytose substances such as MRI contrastagents. Of the MRI contrast agents, the mostcommon choice are iron oxide nanoparticles,which are administered intravenously in order toimage movement of iron-containing macrophagesfrom the blood into the brain parenchyma.

This phenomenon has been observed exten-sively in rodent stroke models [1], and in braininflammation produced by multiple sclerosis,which will not be discussed in this article but isreviewed in [2]. Dextran-coated, ultrasmall,superparamagnetic iron oxide (USPIO) nano-particles (e.g., ferumoxtran-10 [Sinerem®],10–40 nm diameter, Guerbet, Paris, France)have a much longer blood half-life (∼5–6 h inrats at a dose of 300 µmol Fe/kg [3]) when com-pared with dextran-coated superparamagneticiron oxide (SPIO) nanoparticles (e.g., Ferumox-ide [Endorem®], 60–150 nm diameter, Guerbet,Paris, France) [4]. The decrease in blood availabil-ity is a result of active removal of iron oxidenanoparticles by the immune cells of the reticu-loendothelial system (lymph nodes, spleen andKupffer liver cells) [5].

Administering contrast agents 24 h prior toimaging ensures most of the free particles haveeither been removed from the system or phago-cytosed by blood-borne macrophages. Repeatedadministration of carboxydextran-coated SPIOswith an ionic charge (ferucarbotran [Resovist®],mean size 62 nm, Schering, Berlin, Germany)was shown to result in hypointense contrast in

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T2-weighted images [6], detectable in the bound-ary of a photothrombotic lesion approximately5–6 days postinsult. In general, iron oxide nano-particles exhibit a high R2 relaxivity and short-ened T2 relaxation time, which results in ahypointensity in T2-weighted images. Iron oxidenanoparticles also produce small magnetic fielddistortions, referred to as susceptibility effects,which results in a stronger hypointensity inT2*-weighted imaging. This hypointensity washistologically correlated with iron-containingmacrophage accumulation (Prussian blue stain-ing for iron; ED-1 staining for macrophages) [6].A similar experiment confirmed these findings at5 days postinsult using USPIOs (Sinerem) andT2*-weighted imaging (Figure 1, left column) [7].However, while photothrombosis produces areproducible, cortical insult, there are a numberof problems associated with this model thatrequire consideration. It is characterized by anextensive and rapid BBB breakdown, thus, theobserved MRI contrast could be due to wash-inof free contrast agent into the brain and notinflammatory-produced labeled macrophages. Inaddition, extravasation of erythrocytes followingvascular endothelial damage in this model,including erythrocytes that have taken up theiron contrast in the periphery, may contribute toa false-positive contrast in T2- and T2*-weightedimaging. Indeed, some degree of hypointensityhas been reported in animals treated with salineafter photothrombosis [7]. Any source of extra-vasated iron is also subject to incorporation byresident microglia.

In two studies using a permanent middle cere-bral artery occlusion (MCAO) model, USPIOs(Sinerem) were administered 5–6 h after strokeinduction, and a hypointense signal was observedin T2- or T2*-weighted images in the lesionboundary within the first 24 h [8,9]. However,such results contradict the known time course ofblood-borne macrophage infiltration (starting3–4 days post-MCAO) [10]. Therefore, theseresults must be interpreted with caution. The sur-gical technique of electrocoagulation of the MCAapplied in the studies cited above, involves imme-diate disruption of the BBB and cerebral spinalfluid, an additional route for free contrast accu-mulation. Indeed, BBB breakdown was apparentat 6 h post-MCAO in gadolinium (Gd)-enhancedT1-weighted images [9]. Free contrast agent hasbeen shown to become trapped in developingthrombi when it is administered near to the timeof the insult [11]. Finally, extravasation of freecontrast agent, erythrocytes and erythrocytes

containing contrast could also contribute to anonspecific signal. Interestingly, there was noobservable hypointensity in T2-weighted imagesfollowing the same protocol with a transientmodel of MCAO produced with the intraluminalfilament technique [12]. In the same study, aT1-weighted hyperintensity, produced by ironoxide nanoparticle shortening of T1 relaxationtime, was observed in the ischemic striatum at48–72 h post-MCAO, which was interpreted asiron-containing macrophage infiltration.

This strategy of labeling macrophages in thebloodstream has been applied in a clinicalPhase II pilot trial. USPIOs (Sinerem) wereinfused 1 week after symptom onset in tenpatients with infarcts in the MCA territory [13].A hypointense signal around the infarct wasapparent in T2/T2*-weighted images, whichdeclined the following day, and a hyperintensesignal was apparent in T1-weighted images,which increased the following day. The decreasein T2/T2* was interpreted to be the result ofincreased contrast availability in the blood,whereas an increase in T1 could represent con-trast accumulation in the parenchyma. Theseresults are supported by a similar study, in whichadditional Gd-enhanced imaging provided noevidence of BBB breakdown [14].

From the experimental studies, it becomesclear that caution is required when interpretingsignal change as macrophage infiltration intobrain tissue (Figure 1, right column). Further experi-ments are needed to determine how many of theobserved effects are macrophage specific. Ironoxide nanoparticle uptake of both mouse andhuman macrophages in vitro has been found tobe extremely low for both USPIOs (Sinerem; lessthan 1%) and SPIOs (Endorem; between 1–3%)[4]. Assuming comparable uptake in vivo, it isunlikely that sufficient numbers of iron-labeledmacrophages would reach the brain. A recentstudy characterized the uptake of the four mostcommon iron oxide nanoparticles (Sinerem,Endorem, Resovist and ionic carboxydextrancoated SHU 555 C [Resovist S®], mean size21 nm, Schering, Berlin, Germany) by humanmacrophages in vitro [15]. Again, SPIO uptakewas better than USPIO uptake. While the car-boxydextran coating on the Resovist compoundsimproved uptake substantially, the doses of ironprovided were extremely large compared withthe plasma half-life of the compounds in vivo.

An alternative to systemic contrast adminis-tration may be molecular imaging probes, whichtarget a specific cellular component or event.

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Figure 1. Imaging the inflammatory response in rat models of photothrombosis and middle cerebral artery occlusion.

Imaging the inflammatory response in rat models of photothrombosis (left column) and middle cerebral artery occlusion (MCAO; right column). Left: ultrasmall, superparamagnetic iron oxides (Sinerem®) were administered intravenously (300 µmol Fe/kg) at 5 days postphotothrombosis and a hypointense signal was observed in the lesion 24 h later (top row) using T2*-weighted MRI (3D-gradient echo sequence). Prussian blue staining (second row) revealed extensive iron accumulation in the lesion site, and ED-1 immunohistochemistry (third row) revealed the presence of macrophages (figures modified and reproduced with permission from [7]). Right: the superparamagnetic iron oxide Endorem® was administered intravenously (300 μmol Fe/kg) 3 days post-MCAO and some hypointense signal was observed in the injured striatum 24 h later (top row) using T2*-weighted imaging (multigradient echo sequence, image 9 ms). Prussian blue staining revealed accumulation of iron in this region (second row, arrow). However, there was also evidence of erythrocyte accumulation in addition to the Prussian blue-positive macrophages, indicating localized hemorrhage. While hemorrhage could have been detected by imaging prior to contrast administration, there was no evidence of hypointense signal that could be attributed to iron-containing macrophages. ED-1 immunohistochemistry (third row) revealed extensive macrophage accumulation throughout the injured tissue.Images courtesy of Farr TD and Hoehn M [Unpublished Data].

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Recently, microparticles of iron oxide (MPIO)(700–1500 nm diameter), containing more ironthan USPIOs or SPIOs, were conjugated to anantibody against vascular endothelial cell adhe-sion molecules (VCAMs) [16]. Local braininflammation was produced by IL-1β injectionand the VCAM–MPIOs were subsequentlyinjected intravenously; a strong hypointensity inT2*-weighted images was detected in theinflamed hemisphere.

Imaging stem cells in the brainThe brain continuously generates endogenousstem cells, and the ability to observe their fatenoninvasively is highly attractive. The subven-tricular zone (SVZ) contains neural progenitorcells that travel along the rostral migratorystream (RMS) for differentiation into granularcells at the olfactory bulb. Recently, MPIOs cou-pled with green fluorescence were infused intothe ventricles of a rat brain and 3D T2* imagingrevealed a hypointense signal change along theRMS 5 weeks later [17]. Although this method oflabeling endogenous cells for in vivo monitoringholds great promise for future applications, itmust be cautioned that two other groups experi-mented with iron oxide nanoparticles and foundonly nonspecific free contrast after stereotacticinjection into the SVZ [18,19]. In conclusion, thestrategy of in situ labeling of cells still requiresfurther validation to better characterize theexperimental conditions for reliable, specific celllabeling.

The subgranular zone in the hippocampaldentate gyrus also generates endogenous stemcells that differentiate into functional hippo-campal granular cells. A recent study usedmagnetic resonance spectroscopy (MRS) toidentify a metabolic biomarker (likely a combi-nation of lipids, saturated and monounsaturatedfatty acids) that was unique to first cultured,and then endogenous, rodent and humanhippocampal neural progenitor cells in vivo [20].Despite its low sensitivity, it is the first approachto image an endogenous population of stemcells without additional contrast application.This study will, however, need furtherindependent investigations to either confirm orchallenge the present findings.

Transplantation of exogenous stem cells intomodels of neurodegenerative disease hasgenerated extensive interest in recent years. Assuch, much emphasis has been placed ondeveloping noninvasive means of observing cellgrafts in vivo. Green fluorescent protein

(GFP)-positive murine embryonic stem cells,labeled with USPIOs (Sinerem) were trans-planted into the intact hemisphere of rats at2 weeks post-MCAO [21]. In this study, hypo-intense signal on T2*-weighted imaging showedstem cell migration, observed along the corpuscallosum, into the injured tissue (Figure 2), andimmunohistochemistry for GFP was produced21 days later, confirming the arrival of cells inthe lesion periphery [21] and corresponding dif-ferentiation into neurons and glia in the targetarea [22]. In another study, SVZ progenitor cellslabeled with superparamagnetic particles wereintracisternally transplanted into rats at 48 hpost-MCAO, and visible signal change wasobserved in the injured striatum 4 days afterimplantation, which persisted up to 5 weeks [23].Neural stem cells labeled with Gd-rhodaminedextran were implanted into rats that had under-gone MCAO 3 months prior, and were shown tomigrate into the injured hemisphere by 7 dayspostimplantation with T1- and T2-weightedimages [24]. Mesenchymal and embryonic stemcells labeled with SPIOs (Endorem) migratedtowards a cortical photothrombotic insult inrats [25]. SPIO (Endorem)-labeled, adipose-derived stem cells were imaged following trans-plantation adjacent to an MCAO-induced strokein the mouse [26], while SPIO (Endorem)-labeledneural progenitor cells, transplanted into theintact rat brain, were reported to migratetowards an ischemic injury produced after cellimplantation [27]. A review of cell labeling andMRI tracking strategies can be found in [28–33].The first report to visualize iron oxide nanoparti-cle-labeled stem cells in the brain of a patientwith brain trauma (cells were cultured from thepatient following an emergency procedure) hasrecently been published [34].

These studies convincingly demonstrate thatit is possible, using MRI, to visualize migrationof labeled stem cells towards the site of cerebralinjury. However, there are some limitations tothis labeling strategy that require consideration.Iron oxide nanoparticle labels produce strongsusceptibility effects, which makes visualizationeasy, however voids, vessels or hemorrhagictransformation produce the same contrast,making unambiguous interpretation of the sig-nal source sometimes difficult. The acquisitionof images prior to contrast administrationwould also show susceptibility effects that arealready present in the tissue prior to transplan-tation. A strategy to overcome signal ambiguityis the modulation of the inhalation gas

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Figure 2. Migration of murine embryonic stem cells in the rat brain after middle cerebral artery occlusion in T2*-weighted MRI.

Migration of murine embryonic stem (ES) cells in the rat brain after middle cerebral artery occlusion in T2*-weighted MRI. ES cells were implanted into the striatum (yellow circle) and the cortex (outside image section) just above the corpus callosum in the hemisphere contralateral to the focal cerebral ischemia 2 weeks after stroke induction [21]. At 20 days postimplantation (left column), cells are accumulating at the site of the subventricular zone at the horn of the lateral ventricle of the normal hemisphere (red circle). The extent of cell accumulation around the ventricle is depicted in the enlarged multislice presentation (below) from the 3D data set (spatially isotropic resolution of 78 µm) in consecutive sections, with arrows pointing at the dominant areas of cell accumulation. 3 days later (right column), in the same animal, the extent of cells lining the ventricle is clearly reduced, in correspondence with observations that cells have migrated towards the ischemic target zone [21,22]. At the same time, a large cluster of cells is detected on the choroid plexus of the ischemic hemisphere (blue circle).Images courtesy of Hoehn M, Wiedermann D, Küstermann E [Unpublished Data].

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composition, which has recently demonstratedsuccessful separation of hemodynamic and tis-sue contributions to the hypointense signal [35].Another strategy is the generation of positivecontrast from the iron oxide nanoparticle label.This has been accomplished using radio fre-quency (rf ) to excite and refocus the off-reso-nance water surrounding the labeled cells [36],an inversion recovery with ON-resonant watersuppression sequence [37], or a gradient echoacquisition for superparamagnetic particleswith positive contrast [38]. Nevertheless, dilu-tion of the iron oxide nanoparticle label overtime by continued cell proliferation must beassessed [39], as well as viability of the cells afterincorporation of the label [40], and in the case ofstem cells, differentiation and survival.

An alternative cell-labeling strategy employsviral vectors to incorporate reporter genes into thegenome of the cell population; the cell will thusbe able to produce its own contrast and the labelwill not be lost over time. Ferritin is an endog-enous iron storage protein; its upregulation willcause cells to sequester iron inside the crystallineferrihydrite core. An adenoviral vector with a fer-ritin reporter was injected into the mouse brain,and signal change was observed up to 5 weekswith MRI [41]. In another study, ferritin and thetransferrin receptor were stably coexpressed in amurine neural stem cell line and contrast wasvisualized with T2* imaging followingtransplantation into the mouse brain [42].

Tracking stem cells that have not been forcedto incorporate or produce iron-based contrastagents is of course preferable. A recent studyused MRS to distinguish metabolite profiles ofin vitro mouse embryonic stem cells anddifferentiated neural and glial precursor cells [43].However, future detectability in vivo followingtransplantation with this technique poses achallenge, as well as limits regarding spatialresolution. One exciting candidate for trackingcells without contrast agents are chemical-exchange saturation transfer (CEST) agents.CEST contrast relies on nonmetallic agents thatcan be induced to exchange protons with waterfollowing rf transmission, thus, providing a labelactivated only during rf saturation. Recently, anonmetallic and biodegradable lysine-richprotein, which produces reversible contrastbased on the energy transfer between the pro-tein’s amide and surrounding water protons,was transfected into a rat glioma cell line andimaged following transplantation into themouse brain [44].

Molecular imaging of stem cell dynamics in the brainLocalization of stem cell populations in vivo usingMRI has been a significant step to observe cellsafter implantation into the brain. However, evenmore fascinating would be to address specific bio-logical processes by noninvasive imaging of cellfate, for example, a change in functional status.Traditionally, single photon emission computedtomography, positron emission tomography andoptical imaging have been used to provide infor-mation regarding receptor activation or cellmetabolism. Recently, differentiation of endog-enous neural progenitor cells in vivo was observedwith optical imaging [45]. However, much interestis focused on the development of molecular imag-ing probes for use with MRI, in order to exploitits high spatial resolution and 3D capabilities.

There are two different strategies that deserveattention. The first is responsive contrast agents,which exploit the expression, or upregulation, ofselected enzymes to produce contrast agents inMRI. Here, modulation of contrast is due to thechange of enzyme activity in the targeted cells.This approach was first demonstrated in a larvaemodel where incorporated Gd3+ with the chela-tor diethylenetriamine penta-acetic acid was acti-vated only when the β-galactosidase enzyme wasexpressed [46]. Contrast activation, executed by acell status-specific enzyme, was recentlyreported; the contrast agent was activated by anenzyme that is expressed only by mature den-dritic cells and not their progenitors [47]. Thesecond strategy relies on molecular biology. Plac-ing an MRI reporter gene under the control of aspecific promoter offers the potential to select acell population, produce a conditional label, orproduce a label that is associated with a specificevent or tissue type. This technology is still in itsinfancy. Conditional ferritin expression, underthe control of tetracycline, has been produced intumor cells [48], but proof of principle was alsorecently shown in neural progenitor cells [28,49].A summary of the labelling strategies discussedin this review is presented in Table 1.

ConclusionCellular MRI is a rapidly growing field with highpotential. However, the novelty of theapproaches employed, and the excitement thathas been generated, have led, in some cases, tothe development of high expectations or evenpremature acceptance without sufficient valida-tion. Systemic contrast administration formacrophage labeling as a means to image the

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inflammatory response needs further detailedcritical assessment, which includes considerationof the recognized inflammation timecourse,before it may become an established technique.Concerning the tracking of implanted cells,exciting results have become feasible, justifyingcontinued enthusiasm. But the need for ongoingbasic validation is required to assure result specif-icity with noninvasive MRI. Aspects such as pas-sive endocytosis by endogenous stem cellpopulations, cell proliferation resulting in labeldilution or long-term transplant observations,must be assessed every time a new experimentalmodel is studied. Molecular imaging strategiesaim to advance the field by producing condi-tional expression of cell-specific markers toextend imaging from cell localization to cellfunction. This represents a highly attractiveextension of cell imaging through a tight combi-nation of contrast agent chemistry and molecu-lar biology. This is a very young branch, and careshould be taken to ensure normal cell viabilityand homeostasis following any geneticmanipulation. However, early significantadvances have created high expectations forfuture developments, and improvements insensitivity and specificity.

Future perspectiveThe progress of cell tracking will continue toexpand into more areas of experimental neurol-ogy. Beyond mere cell localization, ‘functionalcell imaging’ of specific status will develop

rapidly over the next 5 to 10 years. Real inno-vations in molecular MRI will be realizedthrough an increasingly tight connection withtwo important disciplines: chemistry in orderto develop the new generation of smart contrastagents, in particular responsive and CESTagents, and molecular biology generating thecellular equivalent. Innovative and excitingexperiments will increasingly rely on transgenicanimals and cells, or the exploitation ofmolecular biological strategies to custom-tailorimaging reporters of gene expression or enzymeactivity. As such, the next decade willexperience a strong boost in multimodal imag-ing strategies that take advantage of comple-mentary information contents. In this context,MRI techniques, both cellular and molecular,may safely be expected to be a major player inthe future.

Financial & competing interests disclosure The authors have received support by grants from theEU-FP6 programs: DiMI (LSHB-CT-2005–512146),EMIL (LSHC-CT-2004–503569) and StemStroke(LSHB-CT-2006–037526), as well as from the BMBFproject ‘Stem cell based regeneration after stroke’(01GN0509). The authors have no other relevant affilia-tions or financial involvement with any organization orentity with a financial interest in or financial conflict withthe subject matter or materials discussed in the manuscriptapart from those disclosed.

No writing assistance was utilized in the production ofthis manuscript.

Table 1. Methods for producing MRI contrast for cellular and molecular imaging.

Agent Description

Iron oxides• Ferumoxtran-10 (Sinerem®)• SHU 555C (Resovist S®)• Ferumoxide (Endorem®)• Ferucarbotran (Resovist®)

• Dextran-coated USPIOs (10–40 nm)• Carboxydextran-coated USPIOs (21 nm)• Dextran-coated SPIOs (60–150 nm)• Carboxydextran-coated SPIOs (62 nm)

Gd-based• Gd-rhodamine dextran

• Responsive Gd

• Gd3+ covalently linked to dextran with red fluorescence (rhodamine)• Gd-DTPA linked to aliphatic chains by ester bonds (500–1000 nm) is later cleaved by lipases

Ferritin • Gene expression of this iron-storage protein with a ferrihydrite core gene can be induced with viral vectors

Nonmetal-based agents• CEST • Lysine-rich protein capable of exchanging

protons with the surrounding water

CEST: Chemical-exchange saturation transfer; DTPA: Diethylenetriamine penta-acetic acid; Gd: Gadolinum; SPIO: Superparamagnetic iron oxide; USPIO: Ultrasmall superparamagnetic iron oxide.

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Executive summary

Introduction

• MRI is a highly versatile imaging modality in which to develop cellular imaging strategies because of its excellent soft tissue contrast, high spatial and temporal resolution and 3D capability.

• Cellular imaging has a wide range of applications relevant to CNS diseases.

Imaging cellular inflammation in the brain

• Systemic administration of iron oxide nanoparticles for uptake by peripheral macrophages has been employed to image brain inflammation in response to disease or injury; this review discusses the application in models of experimental stroke.

• Certain features of the ischemia models can produce noninflammatory signal changes that may have been mistaken for labeled macrophages: erythrocyte accumulation, contrast entrapment in vessels, extravasation or free contrast accumulation in the brain due to a disrupted BBB or cerebral spinal fluid.

• Results should therefore be interpreted with caution and compared with the known timecourse of blood-borne macrophage infiltration into ischemic tissue.

• Promising alternatives to systemic contrast infusion include molecular imaging probes that target specific receptors, cell types or events.

Imaging stem cells in the brain

• Imaging endogenous stem cell populations using MRI is possible.• Signal change along the rostral migratory stream has been observed following intraventricular

administration of iron oxide nanoparticles for uptake by subventricular zone progenitor cells. However, it remains to be seen how much nonspecific, free contrast diffusion, mimicks migration.

• Magnetic resonance spectroscopy has been used to characterize metabolites that are specific to subgranular zone cells.

• Imaging of transplanted stem cell populations is possible if the cells are labeled for MRI detection. This is typically accomplished with iron oxide nanoparticles.

• Migration of transplanted, iron oxide nanoparticle labeled stem cells towards brain injury, particularly in experimental stroke models, has been visualized with MRI, and confirmed histologically, by a number of different groups.

• Iron oxide nanoparticle labels produce high susceptibility contrast in images. The label may belost over time through cell division and biodegradation of the dextran coating and iron. Labeled cells can also be removed in the brain by phagocytic activity of activated microglia and infiltrating macrophages.

• Viral vectors can be used to incorporate MRI reporter genes into the genome of stem cells. The endogenous iron-storage protein ferritin has generated interest as a potentially viable reporter.

• Nonmetallic chemical exchange saturation transfer (CEST) agents may also represent an effective and reversible stem cell labeling strategy.

Molecular imaging of cell dynamics in the brain

• Reporter genes under control of a promoter of interest can provide functional information on cell status.

• This technology has been applied to tumor cells; and preliminary data exist for neural progenitor cells.• Use of responsive contrast agents opens new doors to image functional cell status through enzyme

activity reporters.

Future perspective

• Cell tracking experiments will become an established procedure.• Improvements will focus on cell-labeling strategies together with a strong emphasis on molecular

biology, including: driving cells to produce their own contrast, development of responsive contrast agents or contrast that does not depend on metallic sources, such as CEST agents.

• The primary focus of future research into cellular MRI will be to expand our knowledge through the employment of molecular imaging probes. Combining MRI with other noninvasive imaging techniques will result in multimodal experimental designs to exploit complementary information provided by the different techniques.

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Affiliations• Tracy D Farr, PhD

Max-Planck Institute for Neurological Research, In vivo-NMR Laboratory, Gleueler Strasse 50, Cologne, 50931, GermanyTel.: +49 221 472 6339;Fax: +49 221 472 6337;[email protected]

• Mathias Hoehn, PhD

Max-Planck Institute for Neurological Research, In vivo-NMR Laboratory, Gleueler Strasse 50, Cologne, 50931, GermanyTel.: +49 221 472 6315;Fax: +49 221 472 6337;[email protected]

Perspectives of in vivo magnetic resonance imaging of cell dynamics in the brain

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