cardiovascular molecular imaging1 · be used to rule out myocardial ischemia and is associated with...

Cardiovascular MolecularImaging1

Joseph C. Wu, MD, PhDFrank M. Bengel, MDSanjiv S. Gambhir, MD, PhD

The goal of this review is to highlight how molecular imag-ing will impact the management and improved understand-ing of the major cardiovascular diseases that have substan-tial clinical impact and research interest. These topicsinclude atherosclerosis, myocardial ischemia, myocardialviability, heart failure, gene therapy, and stem cell trans-plantation. Traditional methods of evaluation for thesediseases will be presented first, followed by methods thatincorporate conventional and molecular imaging ap-proaches.

� RSNA, 2007

1 From the Department of Medicine, Division of Cardiology(J.C.W.), Department of Radiology, Molecular ImagingProgram at Stanford (J.C.W., S.S.G.), and Bio-X Program(S.S.G.), Stanford University, 300 Pasteur Dr, EdwardsBldg R354, Stanford, CA 94305-5344; and Department ofNuclear Medicine, Johns Hopkins University, Baltimore,Md (F.M.B.). Received January 25, 2006; revision re-quested March 24; revision received April 7; final versionaccepted June 1. Address correspondence to J.C.W.(e-mail: [email protected]).

� RSNA, 2007

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OLECULARIM

AGINGSERIES

Radiology: Volume 244: Number 2—August 2007 337

Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights.

Molecular imaging is a rapidly ad-vancing biomedical researchand clinical discipline. Com-

pared with traditional in vitro tissue cul-ture and in vivo animal studies, molecu-lar imaging allows noninvasive, quanti-tative, and repetitive imaging oftargeted biological processes at both thecellular and subcellular levels within aliving organism (1). This kind of imagingprovides an extremely powerful tech-nique with numerous applications, suchas the monitoring of endogenous tran-scriptional regulation, analysis of genetransfer, tracking of tumor cell survival,screening for transgenic animal pheno-types, and expediting of drug discovery(2). Although the primary focus tradi-tionally has been in the field of cancerbiology, molecular imaging approacheshave now been extended to several car-diovascular-related applications.

The goal of this review is to highlighthow molecular imaging will impact themanagement and improved understand-ing of the major cardiovascular diseasesthat have substantial clinical impact andresearch interest. These topics includeatherosclerosis, myocardial ischemia,myocardial viability, heart failure (HF),gene therapy, and stem cell transplanta-tion. Traditional methods of evaluationfor these diseases will be presentedfirst, followed by methods that incorpo-rate conventional and molecular imag-ing approaches. Conventional cardio-vascular imaging focuses on obtaininganatomic, physiologic, or metabolic in-formation and includes modalities suchas echocardiography, magnetic reso-nance (MR) imaging, computed tomog-raphy (CT), single photon emissioncomputed tomography (SPECT), andpositron emission tomography (PET).

Although echocardiography, MRimaging, and CT typically are used toevaluate anatomic structures of theheart, they have limited capabilities inthe assessment or visualization of phys-iologic and metabolic processes. Con-versely, SPECT and PET can be used toevaluate physiologic and metaboliccharacteristics but are limited in theircapability for visualization of anatomicstructures. By using standard anatomicimaging modalities combined with mo-

lecular imaging technologies such asSPECT and PET, we now have the po-tential to detect disease processes at theanatomic, physiologic, metabolic, andmolecular levels and, thereby, allow(a) earlier detection of diseases, (b) ob-jective monitoring of therapies, and(c) better prognostication of diseaseprogression (3).

Atherosclerosis

Atherosclerosis is a dynamic multifacto-rial disease of the arterial wall. Al-though it generally involves the entirevascular system, cardiovascular mani-festations are found most frequentlyand ultimately result in clinically overtcoronary artery disease (CAD). A vari-ety of factors contribute to developmentand progression of atherosclerosis.Dysfunction of the endothelium, whichmaintains vascular homeostasis by reg-ulating vascular tone, smooth musclecell proliferation, and thrombogenicity,is thought to be the earliest step in thedevelopment of CAD. The endothelialdysfunction results in the imbalance ofvascular regulatory mechanisms tocause damage to the arterial wall (4).Inflammation, macrophage infiltration,lipid deposition, calcification, extracel-lular matrix digestion, oxidative stress,cell apoptosis, and thrombosis areamong further molecular mechanismsthat contribute to plaque developmentand progression (5) (Fig 1a, 1b).

In the past, invasive coronary an-giography has been the only diagnosticprocedure that could be used to identifycoronary atherosclerosis. The reduc-tion of the vessel lumen caused by ste-nosis is used as an indicator of the pres-ence of plaques and allows assessmentof the extent and severity of occlusiveCAD. Advances in CT and MR imagingtechnology have led to the developmentof algorithms for noninvasive contrastmaterial–enhanced angiography by us-ing these techniques (6). These ap-proaches are characterized by high neg-ative predictive values, but their diag-nostic accuracy, especially for theassessment of smaller distal parts of thecoronary arterial tree, is still limited atpresent.

Although angiography still serves asa key test in the management of symp-tomatic CAD, several clinical observa-tions have emphasized the need for amore detailed analysis of the structureand biology of atherosclerotic plaques.First, epidemiologic observations haveshown that a large proportion of peoplewho suffer a sudden cardiac event(acute ischemic syndrome or suddencardiac death) have no prior symptoms(7). Second, it has been found thatacute coronary syndromes often resultfrom plaque rupture at sites with no oronly modest luminal narrowing at an-giography (8) (Fig 1a). Vascular remod-eling, which consists of atherosclerosis-associated morphologic and biologicchanges of the vessel wall without sub-stantial stenosis, often has occurred atsuch sites (9). Therefore, there is con-siderable demand for diagnostic proce-dures that go beyond assessment of thevessel lumen to identify rupture-pronevulnerable plaques as the most frequentcause of sudden cardiac events (5).

As a consequence, several methodshave been developed in recent years toprovide detailed information about ves-sel wall and plaque morphology. Intra-vascular ultrasonography (US) is an in-vasive technique that allows assessmentof vessel wall thickness and structure(10). In addition, optical coherence to-mography has been introduced as an-other invasive technique that providesimages of vessel wall morphology at al-most histologic quality (11). Finally, MRimaging approaches also have been de-veloped, and these approaches allownoninvasive characterization of the ves-

Published online before print10.1148/radiol.2442060136

Radiology 2007; 244:337–355

Abbreviations:CAD � coronary artery diseaseFDG � fluorine 18 fluorodeoxyglucoseFHBG � 9-(4-[18F]fluoro-3-hydroxymethylbutyl)guanineHF � heart failureICD � implantable cardioverter defibrillatorMI � myocardial infarctionMIBG � m-iodobenzylguanidineVEGF � vascular endothelial growth factor

Authors stated no financial relationship to disclose.

MOLECULAR IMAGING SERIES: Cardiovascular Molecular Imaging Wu et al

338 Radiology: Volume 244: Number 2—August 2007

sel wall (12). Some studies have shownthat CT angiography allows additionalassessment of plaque attenuation (13).The goal of these techniques is to iden-tify vascular remodeling and describeplaques with regard to specific criteriaof vulnerability, such as a thin fibrouscap and a large lipid core.

At least as important as its morphol-ogy, however, is the biology of a plaque(Fig 1b). Inflammation is a key featureof active, rupture-prone plaques, whichcan be identified invasively by usingthermography (14). Plaque inflamma-tion also may be identified noninvasivelyby using nuclear imaging with fluorine18 (18F) fluorodeoxyglucose (FDG) orother markers of inflammatory activity(15). Further molecular features of un-stable plaques, which have been tar-geted by specific probes, are macro-phage infiltration (16,17), proliferatingsmooth muscle cells (18), matrix metal-loproteinase activation (19), apoptosisof macrophages and smooth musclecells (20,21), oxidative stress (22), andproangiogenetic factors (23) (Table 1).

Most of these biologic imaging tech-niques are at present still limited to ex-perimental settings, but the increasingnumber of upcoming approaches and ofresearch groups active in the field willhelp for rapid future clinical establish-ment. Hybrid imaging systems such asPET/CT and SPECT/CT cameras areanticipated to contribute substantiallyto a breakthrough in clinical identifica-tion of vulnerable plaques, becausethese systems allow combined assess-ment of morphology and biology of vas-cular structures at sufficiently high spa-tial resolution (Fig 1c).

Myocardial Ischemia

As mentioned previously, atherosclero-sis ultimately progresses to clinicallyovert CAD. In the treatment of clinicalCAD, a paradigm change has occurredin recent years. It is increasingly em-phasized that the decision for invasivework-up and intervention cannot bebased on symptoms and morphologicdetection of coronary stenoses alone.Functional tests allow accurate identifi-cation of the presence, extent, and se-

verity of myocardial ischemia, which isclosely correlated to patient outcome.At present, detection of myocardial is-chemia thus serves as a gatekeeper tocoronary angiography and guides theclinician to choose interventional or

medical therapy on the basis of individ-ual cardiovascular risk (Fig 2).

The clinical usefulness of myocardialperfusion scintigraphy for diagnosticand prognostic evaluation of patientswith suspected or known CAD is sup-

Figure 1

Figure 1: From atherosclerotic plaque biology toward molecular plaque imaging. (a) Atherosclerosis andvascular structure. Image shows development of expansive vascular remodeling, which may result in sub-stantially increased vulnerability without luminal narrowing. Ultimately, changes lead to acute plaque ruptureor to chronic stenosis with luminal narrowing (right). (b) Criteria of plaque vulnerability as targets for imaging.Image depicts morphologic and biologic features of vulnerable plaques, which are suitable targets for imagingapproaches. (c) Future perspective in regard to multimodality imaging of plaque morphology and biology.Image highlights the potential of hybrid imaging technologies, which may allow noninvasive fusion of mor-phology from angiography with biology from nuclear imaging of plaque-targeted molecular probes (nuclearand fusion images are simulated).



ported by a large body of evidence. Anormal myocardial perfusion scan canbe used to rule out myocardial ischemiaand is associated with a low cardiovas-cular event rate of less than 1% per year(24). Cardiac risk, and thus the benefitfrom invasive therapeutic strategies, in-creases in relation to the severity ofmyocardial ischemia and perfusion ab-normalities (25,26). Noninvasive ische-mia-guided strategies for diagnostic andtherapeutic work-up of patients withCAD were shown to be cost effective ingeneral (27). In addition, the impor-tance of perfusion imaging for decisionmaking has been demonstrated in spe-cific situations, such as in acute coro-nary syndromes (28), after myocardialinfarction (MI) (29), prior to noncar-diac surgery, or in high-risk subgroupssuch as diabetic patients (30).

Nonnuclear imaging techniques fordetection of myocardial ischemia arebecoming increasingly available. Thesetechniques have the advantage of no ra-diation exposure, but their diagnosticand prognostic usefulness is still lesswell established compared with per-fusion scintigraphy. Echocardiographycan be applied for detection of wall mo-tion abnormalities during physical exer-cise (31) or dobutamine stimulation(32). Furthermore, contrast-enhancedechocardiography has been establishedfor measuring perfusion with echogenicmicrobubbles (33). A promising ap-proach to detection of ischemia is MRimaging. This technique is thought to beless dependent on patient conditionsand observer experience. MR imagingcan be effectively used for detection of

ischemia-associated wall motion abnor-malities during dobutamine stimulation(34) or for direct assessment of myocar-dial perfusion after a bolus injection of agadolinium-based contrast agent at restand during pharmacologic vasodilation(35). Although these nonnuclear meth-ods are likely to grow in the future, it isthought that nuclear imaging will bepushed by the hybrid PET/CT andSPECT/CT cameras, which allow fur-ther refinement of functional cardiac as-sessment with measurement of myocar-dial perfusion, geometry, function, andcoronary morphology within a singleimaging session.

Assessment of ischemia by usingmeasurement of perfusion and functionwill remain the method for stratificationof patients with known CAD. Identifica-tion of ischemia-associated molecularalterations, however, may find a role incertain situations that are not yet possi-ble through conventional assessment ofperfusion and/or contractile function.Specific identification of myocardiumthat has previously been exposed to is-chemia, but is now normally perfused,would be of considerable interest. Anischemic memory marker would allowidentification of myocardium at risk inpatients with acute coronary syndromesor extensive CAD. It may also provideinsights into the clinical role of ischemicpreconditioning, a cellular mechanismby which short episodes of ischemia andreperfusion result in an improved toler-ance of prolonged ischemic episodes(36).

Annexin V is a protein that can beradiolabeled and that binds to phospha-

tidylserine, a molecule expressed on thecell surface during the early phase ofapoptosis (programmed cell death). Re-cent studies have shown that annexin Vuptake occurs not only in irreversiblydamaged infarcted myocardium (37)but also temporarily in myocardium af-ter reversible severe ischemia (38,39).These observations suggest that earlyphases of apoptosis are reversible andthat annexin V may be used as a molec-ular marker to identify areas that havepreviously suffered severe ischemia buthave not yet transformed to scar. An-other approach for molecular imaging ofischemic memory is the application ofthe radiolabeled fatty acid analogue io-dine 123 (123I) �-methyl-p-iodophenyl-pentadecanoic acid. After episodes ofischemia, which result in a metabolicshift from fatty acids to glucose as thepreferred substrate for energy produc-tion, regional uptake of this tracerseems to be reduced for a longer periodof time (40).

Coronary Artery Disease

Another area of interest for targetedmolecular imaging is to identify patientswith CAD who have a high likelihood forischemia-induced remodeling, a phe-nomenon that results in transition toHF. Early identification of such patientswould allow prevention of the develop-ment of remodeling-associated ventric-ular dilatation and reduction of contrac-tile function. Alterations of the cardiacsympathetic nervous system seem to playa critical role in left ventricular remodel-ing. Integrity of presynaptic sympatheticnerve endings can be identified by usingradiolabeled catecholamines such as123I–m-iodobenzylguanidine (MIBG). Re-searchers in several studies have identi-fied innervation defects larger than per-fusion defects in patients with CAD andafter MI; these findings suggest thatsympathetic neurons are more sensitiveto ischemia than are myocytes (41,42).

An imbalance of autonomic signaltransduction may contribute to the mal-adaptive process of postischemic re-modeling. This idea has been suggestedby investigators who observed acceler-

Table 1

Visualization of Biologic Features of Vulnerable Plaques

Molecular Mechanism Probe Imaging Method

Plaque inflammation Thermography, FDG Invasive, nuclearMacrophage

infiltrationSuperparamagnetic iron oxide–labeled macrophages,

radiolabeled monocyte chemoattractant protein 1MR imaging, nuclear

Apoptosis Radiolabeled annexin V NuclearMatrix degradation Radiolabeled matrix metalloproteinase inhibitors NuclearAngiogenesis Labeled integrin ligands Nuclear, MR imagingThrombosis Labeled fibrin-binding peptides Nuclear, MR imagingSmooth muscle cell

proliferationRadiolabeled Z2D3 antibodies Nuclear



ated ventricular dilatation in patientswith larger innervation defects thatwere indicated by reduced myocardialMIBG uptake (43). In addition to pre-synaptic innervation, postsynaptic re-ceptor density can be evaluated by usingPET and radiolabeled adrenergic recep-tor antagonists. Findings in a studyshowed that reduction of �-adrenergicdensity early after MI can be used topredict ventricular volumes at 6 monthsafter the event (44); such results sug-gest that receptor-targeted imaging maybe another approach to identify candi-dates for remodeling. Other moleculartargets in the ischemic myocardium in-clude integrins, a group of adhesionmolecules that play a central role in an-giogenesis. These can be identified withspecific �v�3-integrin–targeted radio-tracers in vivo (45) and may allow as-sessment of postischemic recovery andangiogenesis therapy in the future (46).Finally, myocyte apoptosis is thought tobe another relevant feature of remodel-ing that may be targeted by noninvasivemolecular imaging although no detailedtrials have been performed up to now.

Myocardial Viability

It has been extensively documented thatpatients with poor left ventricular func-tion and advanced multivessel CADshow improved clinical outcome aftersurgical revascularization (47–50). Thesepatients may benefit most from revascu-larization, but the decision to proceedwith interventional therapy is not an in-consequential one. Patients with severeventricular dysfunction undergo coro-nary artery bypass graft surgery or per-cutaneous coronary intervention with aconsiderable risk of procedure-relatedmorbidity and mortality (50). Hence,accurate methods to identify patientswho will benefit most are required tojustify the potential risks.

There has been increasing clinicalawareness that contractile dysfunctionin patients with CAD does not necessar-ily reflect the presence of scar tissue(51). In investigations of contractile re-serve in the catheterization laboratoryin the 1970s, reversibility of left ventric-

ular dysfunction was described (52,53).Since then, several noninvasive imagingtechniques have been developed toidentify tissue viability in dysfunctionalmyocardium and to thereby determinewhich patients with ventricular dysfunc-tion are the most appropriate candi-dates for revascularization (54).

Two major pathogenetic mecha-nisms, which coexist in most clinical sit-uations, contribute to development ofdysfunctional but still viable myocar-dium. First, “myocardial stunning” de-scribes a state of persistent postische-mic contractile dysfunction despite res-toration of near-normal blood flow. Inaddition to “acute stunning” that occursas a consequence of a single episode ofischemia, the definition of myocardial

stunning has been extended to chronicleft ventricular dysfunction associatedwith repetitive episodes of ischemia,which is then referred to as “repetitivestunning” (55). Molecular mechanismsfor stunning include intracellular cal-cium overload caused by ischemia,which results in desensitization and lysisof myofilaments (56), and generation ofoxygen-derived free radicals, which in-hibit ionic pumps and mitochondrialfunction, at reperfusion (57).

Second, “myocardial hibernation”has been described as a consequence ofreduced resting perfusion caused by se-vere coronary stenosis that leads to anadaptive downregulation of contractilefunction, which is reversible after rees-tablishment of normal perfusion (51,58).

Figure 2

Figure 2: Imaging in myocardial ischemia. (a) Imaging markers in CAD. (b) Examples of morphologic,functional, and molecular images. LAD � left anterior descending artery, LCA � left coronary artery, LCX �left circumflex artery, RCA � right coronary artery.



Uncoupling of contractile work andmyocardial blood flow is thought to bepart of this adaptation. Since approxi-mately 60% of oxygen consumption islinked to contractile performance, en-ergy can be saved and the tolerance toischemia can be increased at the ex-pense of regional dysfunction. It hasbeen shown that this adaptive process isassociated with molecular alterations,such as dedifferentiation of myocytes,decreased expression of contractile pro-teins, accumulation of glycogen, loss ofsarcoplasmic reticulum, small mito-chondria, and increasing interstitial fi-brosis (59–61).

The prediction of functional recov-ery by using detection of myocardial vi-ability has emerged as a clinical applica-tion for molecular imaging. Reversibleleft ventricular dysfunction is generallyassociated with maintained or even in-creased tissue uptake of the glucosemetabolic marker FDG (62–64). Hiber-nating and repetitively stunned myocar-dia are characterized by a perfusion-metabolism mismatch pattern with re-duced regional uptake of a perfusiontracer but preserved uptake of FDGmetabolic tracer. Scar tissue, on theother hand, is characterized by amatched reduction of perfusion and

FDG uptake (Fig 3). Studies of myocar-dial metabolism by using FDG and PEThave provided important informationfor a better understanding of thepathophysiologic interrelations amongmyocardial blood flow, substrate me-tabolism, and contractile function in is-chemically compromised myocardium.These studies have also played a funda-mental role in the clinical emergence ofnoninvasive viability imaging and are re-garded as an imaging reference stan-dard (65,66). An extensive body of liter-ature exists that documents the diagnos-tic accuracy of FDG PET for prediction offunctional recovery after revasculariza-tion. In addition, the prognostic value ofPET has been described (67,68), and theperfusion-metabolism mismatch has beenidentified as an unstable state associatedwith a high risk and poor outcome if nottreated immediately (69,70).

Other techniques have been intro-duced for prediction of functional recov-ery and were validated compared withPET metabolic imaging (71,72). Studiesof the contractile response to low-dosedobutamine by using echocardiographyor MR imaging were shown to be diag-nostically useful. Measurements of con-tractile reserve have a marginally higherspecificity but somewhat lower sensitiv-

ity for prediction of functional recoverywhen compared with FDG metabolicimaging (71). This has been explainedby the fact that FDG may be used todetect residual viable tissue in some ar-eas where extensive fibrosis already co-exists. These areas of advanced hiber-nation may show only slow functionalimprovement but may still be importantto revascularize because of an associa-tion with poor outcome (73).

Further, MR imaging has been usedto identify scar tissue by means of theretention of gadolinium-based contrastagent. Imaging of gadolinium-basedcontrast agent late enhancement allowsaccurate detection of the regional andtransmural extent of irreversibly dam-aged myocardium. It has been shownthat the transmural extent of gado-linium-based contrast agent enhance-ment is correlated with the likelihood ofregional functional recovery (74). Thediagnostic and prognostic accuracy ofthis test, however, needs to be definedin detail. Findings in other studies havesuggested that functional measures ofregional viability may be superior to thestatic measurement of the extent of ne-crotic tissue for prediction of recovery(75) and, thus, that biologic and molec-ular mechanisms other than scar devel-

Figure 3

Figure 3: Examples of patterns of myocardial viability from perfusion-metabolism PETimaging. Representative left ventricular (LV) short- and long-axis sections from two patientswith severe ventricular dysfunction are depicted in “hot metal” color scale (brighter color indi-cates higher radioactivity concentration). In top two rows, an anteroseptal perfusion defect ispresent in [13N]ONH3 perfusion images, with concomitant matched reduction of uptake ofthe metabolic tracer FDG. This pattern indicates the presence of scar tissue, which will notbenefit from revascularization. In bottom two rows, a perfusion defect is shown in the anteriorand apical wall. Enhanced FDG uptake is found in the metabolic study, indicating the presenceof ischemically compromised hibernating myoardium, which will benefit from revasculariza-tion. Note that relatively reduced uptake of FDG in normally perfused inferior wall is consistentwith use of fatty acids as substrate in this area of normally perfused myocardium. LA � leftatrium, RA � right atrium, RV � right ventricle.



opment are important in chronic ische-mic ventricular dysfunction.

Innovations in molecular imagingmay allow refinement of the character-ization of jeopardized myocardium and,thus, further improvement of diagnosticand prognostic assessment of myocar-dial viability in the future. The extent ofapoptotic cell death in hibernating myo-cardium may be identified by using trac-ers such as radiolabeled annexin V. Thismay allow one to identify the severity ofischemic damage and the transitionfrom programmed cell survival to pro-grammed cell death. Other biologic tar-gets for molecular imaging in chronicdysfunctional myocardium may be ex-tracellular matrix activation, collagendeposition, or inflammation. Finally,multimodality imaging may be of specialvalue in chronic left ventricular dysfunc-tion by providing a combination of mor-phologic, functional, and metabolic in-formation (Fig 4).

Heart Failure

In the United States, approximately 5million patients have HF, with an esti-mated 500 000 new cases diagnosedeach year (76). HF is the leading causeof morbidity, mortality, and hospitaliza-tion in patients older than 60 years andis the most common Medicare diagno-sis-related group (77). The direct andindirect cost of HF was estimated at$23.2 billion in 2002 (78). Clearly, thisdisease exerts major societal burdenand costs. Yet, despite recent advancesin medical therapy, nearly 300 000 pa-tients will die of HF as a primary orsecondary cause each year (79). There-fore, other treatment approaches, suchas implantable cardioverter defibrillator(ICD), cardiac resynchronization ther-apy, cardiac gene therapy, and cardiacstem cell transplantation, have been ad-vocated. The last two subjects will bediscussed in subsequent sections of thisarticle.

It is known that approximately 50%of all HF deaths are caused by ventricu-lar tachycardia and approximately 80%of patients with symptomatic systolicdysfunction have ventricular tachycar-dia (80). Results of the Multicenter Au-

tomatic Defibrillator Implantation Trial(also known as MADIT-II) showed thatroutine implantation of ICDs in patientswith a prior MI and an ejection fractionof less than 30% led to reduction ofmortality rates from 19.8% (conven-tional therapy group) to 14.2% (ICDgroup) (hazard ratio � 0.69, P � .16)(81). Findings in this study suggestedthat ICD implantation should be consid-ered routinely in all patients with re-duced ventricular ejection fraction afterMI (82). Besides risk of sudden cardiacdeath, patients with HF can have intra-ventricular dyssynchrony (or left bundlebranch block), which causes the twoventricles to beat in an asynchronousfashion, reduces systolic function, andincreases systolic volume.

Intraventricular dyssynchrony isseen in approximately 15%–30% of pa-tients with HF (83). Cardiac resynchro-nization therapy paces both the right

and left ventricle, which can synchro-nize the activation of the heart and,thus, improve left ventricular systolicfunction (84). This differs from the typ-ical pacemaker, which paces only theright ventricle. In the Multicenter In-Sync Randomized Clinical Evaluation(also called MIRACLE), results indi-cated that cardiac resynchronizationtherapy improved symptoms, exercisetolerance, and quality of life substan-tially (84). However, the estimated costof cardiac resynchronization therapyand/or ICD and hospitalization for im-plantation is between $40 000 and$50 000 per patient (78). Applying thisexpense to the estimated 400 000 to 1.5million patients with HF who may bene-fit from ICD poses an astronomic bur-den to health care costs. In addition, anestimated 20%–30% of patients do notrespond to cardiac resynchronizationtherapy (85). Thus, these issues empha-

Figure 4

Figure 4: Multimodality imaging of myocardial viability. Short-axis MR images show late enhancement ofgadopentetate dimeglumine (in gray scale) and PET images show glucose metabolism (using FDG) and per-fusion (using [13N]ONH3[NH3]). PET images are displayed in a hot metal color scale where brighter colorindicates higher radioactivity concentration. Nontransmural late enhancement, indicating subendocardialscar tissue, is shown on MR images. PET scans show reduced perfusion in the same area (bottom middle), butFDG uptake is less reduced and higher compared with perfusion (top middle). This perfusion-metabolismmismatch indicates residual viability in the subepicardial portion of the area with subendocardial scar. IRTrueFISP �Gd � inversion-recovery true fast imaging with steady-state precession and gadolinium-basedcontrast agent.



size the need for additional measuresthat can be used to selectively identifywhich patients will benefit the mostfrom ICD and cardiac resynchronizationtherapy.

Traditionally, the single most usefuldiagnostic test for the evaluation of pa-tients with HF is two-dimensional echo-cardiography coupled with a Dopplerflow study. Echocardiography can pro-vide global and regional function, de-gree of ventricular remodeling, contrac-tile reserve, ischemia, and viability, par-ticularly in patients after acute coronarysyndromes or patients with chronic HF.However, these measurements do notaddress the issue of intraventricularconduction delays. Newer echocardio-graphic measurements, which include

tissue Doppler imaging (86), strain rateimaging (87), and tissue tracking (88),have all been advocated. These parame-ters may provide optimal informationon intraventricular dyssynchrony andallow prospective identification of re-sponders to cardiac resynchronizationtherapy (89). A detailed discussion ofeach modality is beyond the scope of thisreview, but interested readers should re-fer to published articles (89,90).

Investigators in several studies havealso demonstrated that 123I-MIBG imag-ing can provide powerful diagnostic andprognostic information in patients withHF. Many radiotracers for scintigraphicimaging of cardiac neurotransmissionhave been developed with radiolabelingof the neurotransmitters or their struc-

tural analogs (Fig 5a). Of these, MIBGshares many cellular uptake and storageproperties with norepinephrine, and,thus, MIBG scans have been used toevaluate cardiac sympathetic nervoussystem distribution and function. In pa-tients with HF, MIBG scans typicallyshow a reduced heart-mediastinum up-take ratio, heterogeneous distributionwithin the myocardium, and increasedMIBG washout from the heart (91–93)(Fig 5b, 5c). For example, Arimoto et al(94) showed that patients with abnor-mally rapid washout levels had a signifi-cantly higher cardiac event rate than didthose with normal washout levels (57%vs 12%, P � .0001) during the follow-upperiod (6–30 months).

Kyuma et al (95) showed incremen-

Figure 5

Figure 5: 123I-MIBG imaging in patients with HF. (a) Schematic shows mostcommonly used radioligands for assessment of cardiac pre- and postsynaptic pro-cesses. (b) SPECT MIBG study in healthy volunteer. Short-axis tomograms andreconstructed polar maps show normal MIBG distribution and washout. (c) SPECTMIBG study in patient with dilated cardiomyopathy. Short-axis tomograms andreconstructed polar maps show decreased and heterogeneous myocardial MIBGactivity. ATP � adenosine triphosphate, DOPA � dihydroxyphenylalanine,cAMP � cyclic adenosine monophosphate, NE � norepinephrine. (Reprinted,with permission, from reference 91.)



tal prognostic levels when the plasmabrain natriuretic peptide level and theheart-mediastinum uptake ratio wereused together. More important, in a pi-lot study, Arora et al (96) showed thatpatients with ICD discharge had a sub-stantially lower MIBG heart-mediasti-num tracer uptake ratio, higher MIBGdefect scores, and more extensive sym-pathetic denervation. Clearly, largerstudies are needed in the future. If con-firmed, cardiac autonomic assessmentby using MIBG scans may help in theselection of patients who would benefitthe most from an ICD by means of iden-tification of those at increased risk forpotentially fatal arrhythmias, leading tomore cost-effective implementation ofthis life-saving device.

Cardiac Gene Therapy

Gene transfer has been heralded as themost promising therapy of molecularmedicine in the 21st century. It is usu-ally defined as the transfer and expres-sion of DNA to somatic cells in an indi-vidual, with a resulting therapeutic ef-fect. In cardiovascular diseases, genetherapy offers an exciting new approachto express the therapeutic factors lo-cally in the myocardium (97). In gen-eral, the successful application of genetherapy requires three essential ele-ments: (a) a vector for gene delivery,(b) delivery of the vector to the targettissue, and (c) a therapeutic gene to beexpressed in a particular patient popu-lation.

An ideal vector should enable effi-cient gene delivery to the target tissue,have minimal local or systemic toxicity,deliver enough concentration and dura-tion to induce a therapeutic effect, andcause no germ-line transmission to theoffspring. No single vector has all ofthese attributes, and, therefore, thetype of vector chosen will need to betailored to the specific clinical applica-tions. Vectors can be classified as viralvectors and nonviral vectors. Commonviral vectors include adenovirus, adeno-associated virus, gutless adenovirus,and lentivirus. Nonviral vectors includeplasmids, naked DNA, and liposomes

(98). The technique of vector deliveryto the heart also depends on the in-tended target area but in general can becategorized as the following: (a) directepicardial injection, (b) endocardialinjection, (c) intracoronary infusion,(d) retrograde coronary sinus infusion,and (e) pericardial injection (99).

Likewise, the choice of a therapeu-tic gene to be expressed is often drivenby the intended application. In animalstudies, successful gene therapy hasbeen demonstrated for the following:(a) the treatment of CAD by usingangiogenic factors such as vascular en-dothelial growth factor (VEGF) (100),fibroblast growth factor (101), andhypoxia-inducible factor 1� (102);(b) reduction of restenosis after angio-plasty through inhibition of smoothmuscle cell proliferation with suicidegene therapy by using thymidine kinase(103); (c) improvement of congestiveHF with gene transfer of a calcium aden-osine triphosphatase pump (SERCA2a)(104); (d) inhibition of atherosclerosiswith overexpression of an high-densitylipoprotein receptor (105); and (e) re-duction of hypoxia-induced apoptosis ofcardiomyocytes (106).

These encouraging results led to theinitiation of several clinical trials begin-ning in the 1990s. Of the 509 ongoinggene therapy trials in the United States,46 are related to cardiovascular dis-eases (107). The majority of these stud-ies are aimed at testing the safety andefficacy of therapeutic angiogenesisand, to a lesser extent, examining reste-nosis. In the late 1990s, results of sev-eral phase 1 open-labeled trials that in-volved small numbers of patients withmyocardial ischemia and peripheralvascular disease were uniformly posi-tive (108–110). However, phase 2 ran-domized, double-blind, placebo-con-trolled trials have yielded conflicting, ifnot disappointing, results. In the Vascu-lar Endothelial Growth Factor in Ische-mia for Vascular Angiogenesis (alsoknown as VIVA) (111), FGF InitiatingRevascularization Trial (also calledFIRST) (112), Adenovirus FibroblastGrowth Factor Angiogenic Gene Ther-apy (also known as AGENT) (113,114),and the Kuopio Angiogenesis Trial (also

called KAT) (115), gene therapy wastested by using either vascular endothe-lial growth factor (VEGF) or fibroblastgrowth factor. Unfortunately, these tri-als failed to show any consistent im-provement in various parameters, suchas symptoms, ejection fraction, wallmotion scores, myocardial perfusion,and restenosis rate.

Nonetheless, important lessons canbe learned from these trials. Theyshowed that angiogenesis is a complexprocess regulated by the interaction ofvarious growth factors and may be dif-ficult to stimulate by using a singleprotein or gene injection. The idealinjection method, delivery vector, andpatient population remain to be deter-mined. The pharmacokinetics and phar-macodynamics of therapeutic gene ex-pression will need to be defined firstbefore gene therapy can proceed fur-ther to widespread clinical use, and thisprocess is similar to that for the re-search and development of experimen-tal drugs. Finally, since there is no avail-able method of assessing gene expres-sion in vivo, investigators are unable todetermine whether the lack of symp-tomatic improvement is due to poor in-jection technique, insufficient gene ex-pression, the host inflammatory re-sponse, or an inappropriate therapeuticgene (116).

Most molecular imaging studieshave focused on cancer biology (117,118). Imaging of cardiac transgene ex-pression was established in severalproof-of-principle studies that involvedthe injection of various reporter genesinto the myocardium and in which thekinetics of transgene expression wasfollowed over time by using opticalbioluminescence, micro-PET, and clin-ical PET imaging (119–123). The con-cept of imaging reporter gene expres-sion (Fig 6) involves a reporter genefirst being introduced into target tis-sues by various methods, which in-clude viral and nonviral vectors. Theupstream promoter that regulates thetranscription of the reporter gene canbe constitutive (always on), inducible(can be turned on or off), or tissuespecific (expressed only in the tissuesof interest).



Transcription of the reporter geneand translation of the messenger RNAlead to a reporter protein product thatcan interact with the reporter probe.This interaction may be enzyme based(eg, phosphorylation of a reporter

probe with intracellular retention ofmetabolites) or receptor based (eg,binding of a radiolabeled ligand to cellsurface receptors) (98). Clearly, muchflexibility exists within these systems.By altering various components, the re-

porter gene can provide informationabout the efficiency of vector transfec-tion into cells, regulation of DNA byupstream promoters, and the fate of in-tracellular protein trafficking. In addi-tion, the reporter probe itself does not

Figure 6

Figure 6: Four strategies of imaging reporter gene and reporter probe. A, Enzyme-based bioluminescence imaging. Expression of the firefly luciferase reporter geneleads to the firefly luciferase reporter enzyme, which catalyzes the reporter probe (D-luciferin) that results in a photochemical reaction. This yields low levels of photonsthat can be detected and quantified by a charge-coupled device camera. B, Enzyme-based PET imaging. Expression of the herpes simplex virus type 1 thymidine kinase(HSV1-tk) reporter gene leads to the thymidine kinase reporter enzyme, HSV1-TK, which phosphorylates and traps the PET reporter probe 9-(4-[18F]fluoro-3-hy-droxymethylbutyl)guanine (FHBG) intracellularly. Radioactive decay of 18F isotopes can be detected with PET. C, Receptor-based PET imaging. 3-(2-[18F]fluoroethyl)-spiperone (18FESP) is a reporter probe that interacts with the dopamine 2 receptor (D2R) to result in probe trapping on or in cells expressing the D2R gene. D, Receptor-based MR imaging. Overexpression of engineered transferrin receptor (TfR) results in increased cell uptake of the transferrin-monocrystalline iron oxide nanoparticles.These changes result in a detectable contrast change on MR image. FPCV � 8-[18F]fluoropenciclovir, holo-Tf � holo-transferrin. (Reprinted, with permission, fromreference 98.)



have to be changed if one wishes tostudy a new biological process, whichsaves valuable time needed to synthe-size, test, and validate new radiotraceragents.

The feasibility of linking a PET re-porter gene to a therapeutic gene wasdemonstrated in two separate studies(124,125). In this case, an adenoviruscontaining two constitutive cytomegalo-virus (CMV) promoters driving aVEGF121 therapeutic gene and anHSV1-sr39tk PET reporter gene sepa-rated by poly-adenine sequences wasconstructed (Ad-CMV-VEGF121-CMV-HSV1-sr39tk) (Fig 7a). Wu et al in-jected the construct into the myocar-dium of a rat in an MI model. Reportergene expression, which indirectly re-flects the VEGF121 therapeutic gene ex-pression, persisted for only approxi-mately 2 weeks because of host immuneresponse against the adenovirus (124).At 2 months, there was no substantialimprovement in myocardial contractil-ity, perfusion, or metabolism as mea-sured by using echocardiography, 13Nammonia ([13N]ONH3) perfusion, andFDG imaging between study and controlgroups (Fig 7b). Thus, this study high-lights the importance of monitoring thepharmacokinetics of gene expression. Italso demonstrates the proof of principlethat any other cardiac therapeutic genesof interest (eg, hypoxia-inducible factor1�, sarcoplasmic endoplasmic reticu-lum Ca2�, heat shock protein, or en-dothelial nitric oxide synthase) canlikewise be coupled to a PET reportergene for subsequent noninvasive mon-itoring.

In the future, it will also be useful tohave the following arsenals for cardiacgene therapy: (a) less immunogenic vec-tors such as adenoassociated virus thatcan prolong transgene expression in themyocardium (126); (b) cardiac tissue-specific promoter (eg, myosin lightchain kinase or troponin) that can di-minish unwanted extracardiac activity(82); (c) codelivery of proangiogenicgenes with stem cells to enhance revas-cularization (127); and (d) multimodal-ity molecular imaging approaches thatcan monitor the location, magnitude,and duration of transgene expressions,

as well as their downstream functionaleffects (98,116).

Stem Cell Transplantation

In an acute MI, tissue loss leads to he-modynamic stress followed by compen-satory left ventricular hypertrophy anddilatation. Many patients continue todevelop refractory HF symptoms andrequire more aggressive approaches,such as a left ventricular assist deviceand orthotopic heart transplantation.

However, the limitations of these ap-proaches (eg, infection and bleeding forleft ventricular assist device and organshortage and graft rejection for ortho-topic heart transplantation) justify thesearch for alternative therapeutic op-tions.

Stem cell transplantation holds po-tential promise for treatment of ische-mic heart disease. Orlic et al (128) re-ported transdifferentiation of bone mar-row–derived hematopoietic stem cellsinto cardiomyocytes after myocardial

Figure 7

Figure 7: Molecular imaging of cardiac perfusion, metabolism, and gene expression. (a) Schematic ofAd-CMV-VEGF121-CMV-HSV1-sr39tk mediated gene expression. The translated product of VEGF121 is solu-ble and excreted extracellularly, whereas the translated product of HSV1-sr39tk (HSV1-sr39TK) traps FHBGintracellularly by phosphorylation. PCMV �CMV promoter. (b) At day 2, representative images showing normalperfusion ([13N]ONH3) and metabolism (FDG) in a sham rat, anterolateral infarction in a control rat, andanterolateral infarction in a study rat (Ad-CMV-VEGF121-CMV-HSV1-sr39tk) in short, vertical, and horizontalaxis (gray scale). The color scale is expressed as percentage injected dose per gram (%ID/g) for FHBG uptake.Only the study rat showed robust HSV1-sr39tk reporter gene activity near the site of injection. (Reprinted, withpermission, from reference 124.)



injury in mice. However, researchers insubsequent studies have questioned theconcept of differentiation across tissuelineage boundaries (129,130). Regard-less, several phase 1 clinical trials in-volving transplantation of bone marrowstem cells, skeletal myoblasts, and en-dothelial progenitor cells have been ini-tiated since then (131–138). Assmus etal (132) showed that transplantation ofprogenitor cells was associated with asubstantial increase in global left ven-tricular ejection fraction, improved re-gional wall motion in the infarct zone,and reduced end-systolic left ventricularvolumes at 4-month follow-up (Fig 8).

Wollert et al (138) showed that themean global left ventricular ejectionfraction, as determined with cardiacMR imaging, increased by 6.7% in pa-tients after MI who received autologousbone marrow stem cells (n � 30) com-pared with 0.7% in the control group(n � 30) (P � .0026). The mechanismsof improvement may be related to im-proved angiogenesis or vasculogenesis,better survival of hibernating myocar-dium, paracrine effects of injured cells,or modulation of the wound tissue. In-terestingly, the 18-month follow-up datafrom the same study showed that thedifference in left ventricular ejectionfraction was no longer significant (P �.27) (139). The authors suggested thatbone marrow stem cell transplantationmay not lead to substantial formation ofnew myocardial tissues and that phar-macologic or genetic strategies may bewarranted to enhance myocardial en-graftment and increase its therapeuticefficacy.

One of the main limitations of car-diac stem cell transplantation is the lackof available methods to assess stem cellsurvival. In clinical studies, changes inmyocardial perfusion, viability, and per-fusion are assessed with echocardiogra-phy, nuclear SPECT and PET imaging,or MR imaging (Table 2). These pa-rameters can be used to measure thetherapeutic effects but not to actuallydetect the presence or absence of trans-planted stem cells. This is an importantdistinction because most patients alsoundergo concurrent coronary artery by-pass graft or percutaneous coronary in-

Figure 8

Figure 8: Functional imaging studies in a patient after MI who underwent intracoronary infusion of circu-lating blood– derived progenitor cells. A, Left ventricular angiograms before circulating blood– derived pro-genitor cell therapy and, B, at 4-month follow-up. C, Pretherapeutic and, D, posttherapeutic correspondingFDG PET bull’s-eye views of the left ventricle of the patient. LAD � left anterior descending artery, LCX � leftcircumflex artery, RCA � right coronary artery. (Reprinted, with permission, from reference 132.)



tervention, and it is not clear whetherthe improvement is due to these proce-dures or to the transplanted stem cells.Likewise, in animal studies, analysis ofstem cell survival is based on postmor-tem histologic examination, which is in-vasive and precludes longitudinal moni-toring (128–130,140). Thus, the abilityto study stem cells in the context of theintact whole-body system rather thanwith indirect (clinical trials) or invasive(animal studies) means will give betterinsight into the underlying biology andphysiology of stem cells. Several investi-gators have started addressing this is-sue through different approaches, includ-ing radionuclide labeling, ferromagneticlabeling, and reporter gene labeling (Ta-ble 3).

For radionuclide labeling, Aicher etal (141) injected endothelial progenitorcells with indium 111 (111In) oxine intonude rats with infarcted myocardium.At 24 to 96 hours, images obtained witha double-headed gamma camera showedthat the ratio of specific radioactivity ofthe heart was approximately twice theactivity of peripheral skeletal muscle tis-sue. The main limitation of this ap-proach is that 111In-oxine has a physicalhalf-life of 2.8 days, so cell distributioncan be studied only for 5–7 days. Forferromagnetic labeling, Kraitchman etal (142) injected swine mesenchymalstem cells with ferumoxides injectablesolution (Feridex; Berlex, Montville,NJ), with 25 �g of iron per milliliter,into pig hearts. Images were obtainedby using a 1.5-T MR imager (CV/i; GEMedical Systems, Milwaukee, Wis). Af-ter 24 hours, the injected sites appearedas ovoid hypointense lesions with sharpborders. At 1–3 weeks after cell injec-tion, the delineation of the borders wasless clear because of degradation of theferumoxides particles. The main limita-tion of MR imaging for this application isits lack of capability for use in a serialfashion. Because ferumoxides injectablesolution will still register an MR signaleven when the injected cells have un-dergone apoptosis or cell death, it maybe difficult to relate the MR signal to thenumber of viable cells.

For reporter gene labeling, the cellsare transfected with reporter genes

Tabl

e2

Phas

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Clin

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Tria

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tal(

131)

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nary

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3m

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nem

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and

cont

ract

ility

20

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est

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echo

card

iogr

aphy

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tven

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lar

angi

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phy

Assm

uset

al(1

32),

2002

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entio

n4

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prog

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and

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FDG

PET,

dobu

tam

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stre

ssec

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entri

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ran

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raph

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Bone

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20

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(138

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0430

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Note

.—20

1 Tl�

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lium

201.



prior to implantation into the myocar-dium. If the cells are alive, the reportergene will be expressed. If the cells aredead, the reporter gene will not be ex-pressed. By using this approach, Wu etal (143) injected embryonic cardiomyo-blasts expressing either firefly luciferaseor HSV1-sr39tk reporter genes thatwere tracked noninvasively by using op-tical bioluminescence or micro-PET im-aging, respectively (Fig 9). Drastic re-ductions of signal intensity within thefirst 1–4 days were noted and werelikely due to acute donor cell death frominflammation, adenoviral toxicity, ische-mia, and apoptosis. The pattern of celldeath was consistent with data in otherreports in which invasive assays, suchas serial histologic staining (144), termi-nal transferase uridyl nick end labelingapoptosis assays (145), and TaqManreal-time polymerase chain reactionassays, were used (146).

In contrast, transplantation of mouseembryonic stem cells in the myocar-dium led to intracardiac and extracar-diac teratoma formation, as determinedby using molecular imaging techniques,and raised serious doubts about the useof undifferentiated embryonic stem cells

for myocardial regeneration (147). In-terestingly, the PET reporter gene (her-pes simplex virus type 1 truncated thy-midine kinase [HSV1-ttk]) can alsoserve as a suicide gene by treating theanimals with ganciclovir and allows for abackup safety measure against stem cellmisbehavior. Finally, the safety of re-porter gene expression in stem cells wasrecently evaluated by comparing thetranscriptional profile of normal mouseembryonic stem cells versus embryonicstem cells stably expressing fluores-cence (monomeric red fluorescenceprotein), bioluminescence (firefly lucif-erase), and PET (HSV1-ttk) fusion re-porter genes (148). Of 20 371 genes an-alyzed, there were only 296 genes, rep-resenting approximately 1.4% of totalgenes, that had more than twofold up-or downregulation on the basis of geneontology annotation. More important,expression of these reporter genes hadno major adverse effects on embryonicstem cell viability, proliferation, or dif-ferentiation.

In the future, it will be importantto use these molecular imaging ap-proaches to assess the in vivo biology ofdifferent stem cell types. Because of its

noninvasive nature, molecular imaginghas the potential to help accelerate re-search progress through evaluation ofvariables such as the optimal cell type,cell dosage, or delivery route. Perhapseven more important is the investiga-tion of the means aimed at preventingacute donor cell death, which may bethe most critical variable for determin-ing the efficacy of stem cell therapy(149–151). Ultimately, the goal is to de-velop and standardize stem cell trans-plantation protocols that are safe, quan-tifiable, and reproducible in the future,thereby avoiding the controversies thathave plagued cardiac gene therapy trials(152).

Conclusion

In summary, molecular imaging is anexciting field. It combines the disciplinesof molecular biology, radiochemistry,pharmacology, instrumentation, andclinical medicine into a new imagingparadigm. As discussed in this review,molecular imaging can be used to studycardiac neurotransmission, plaque vul-nerability, inflammation, myocardial vi-ability, as well as newer research sub-

Table 3

Imaging Strategies in Cardiovascular Diseases

Target Past Strategy Present Strategy Future Strategy

Atherosclerosis Invasive angiography Invasive angiography and intravascular US,CT angiography

Invasive angiography and optical coherence tomography;noninvasive angiography with CT and MR imaging;morphologic and biologic plaque characterization withCT, MR imaging, and nuclear imaging

Ischemia Perfusion SPECT, stressechocardiography

SPECT, PET, echocardiography, and MRimaging of perfusion and function

One-stop shop MR imaging of morphology, function,perfusion, and viability; hybrid SPECT/CT and PET/CTimaging; molecular imaging with markers of ischemicmemory, remodeling, and myocardial adaptivemechanisms

Viability FDG PET, 201Tl SPECT FDG PET, dobutamine echocardiography,and gadolinium-enhanced MR imaging

One-stop shop MR imaging of morphology, function,perfusion, and viability; multimodality imaging ofbiology, contractile reserve, and geometry

HF Two-dimensional and Dopplerechocardiography

Two-dimensional and Dopplerechocardiography

Echocardiography with tissue Doppler imaging, strain rateimaging, and tissue tracking to help select candidatesfor cardiac resynchronization therapy; nuclear imagingwith 123I-MIBG to help select candidates for ICDimplantation

Gene therapy None Functional imaging with SPECT, PET, andechocardiography

Functional imaging and molecular imaging of cardiactransgene expression

Stem cell therapy None Functional imaging with SPECT, PET,echocardiography, and MR imaging

Functional imaging and molecular imaging of stem cellsurvival and proliferation



jects, such as gene transfer and stemcell survival, that have generated muchattention over recent years. Future di-rections include understanding the mo-lecular mechanisms of individual dis-ease processes, developing newer andmore physiologic tracers that targetspecific disease sites, and implementingmultimodality imaging approaches thatproduce fast throughput analysis inanimal studies (optical), high-resolu-tion images (echocardiography, MRimaging), and functional information(SPECT, PET). All of these measureswill be essential as the field moves for-ward from translational animal studiesto the clinical arena.

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Figure 9

Figure 9: Optical bioluminescence and PET imaging of cell transplantation in rat myocardium. A, Study animal transplanted with embryonic H9c2 cardiomyoblastsemits significant cardiac bioluminescence activity at days 1, 2, 4, 8, 12, and 16 (P � .05 vs control). Control rat shows background signal only. B, The location, magni-tude, and duration of cell survival are determined by longitudinal imaging of FHBG activity (gray scale) within the same rat. C, Tomographic views of cardiac micro-PETimages shown in short, vertical, and horizontal axes. At day 2, study animal transplanted with cardiomyoblasts expressing HSV1-sr39tk shows significant FHBG uptake(color scale) superimposed on [13N]ONH3 images (gray scale). Control animal shows homogeneous [13N]ONH3 perfusion but background FHBG uptake. D, Autora-diography in the same study animal at day 2 confirms trapping of 18F by transplanted cells at the lateral wall at finer spatial resolution (approximately 50 �m). %ID/g �percentage infective dose per gram, p/sec/cm2/sr � photons per second per square centimeter per steradian. (Reprinted, with permission, from reference 143.)



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