ticals, since correlation found between PET or microspheres and SPEC!' blood flow agents has demonstratedthat the relationship is curvilinear. Recently, several researchers suggested that the rising asymptotic plateau iscaused by washout from high-flow regions (3,4). The up

take, extraction fraction and retention of the agents usedvaiy among different regions of normal and abnormal brain(1) and the fixation can be altered by local or regionalparameters. The kinetic behavior and trapping mechanismspecific to each tracer must be well understood for the cmical application and interpretation of SPECT images. Technetium-99m-d,1HMPAO (exametazime) and @Fc-1,1ECD(bicisate) are both available and easy to label, however,HMPAO has poor in vitro stability (RP = 85% after 30 min)(5) and susceptibility to radiolysis, whereas ECD is muchmorestable(RP = 96%at6 hr)(6). These technetium-aminesdemonstrate in vivo instability following blood contact andhave a short arterial input of the bioavailable compound(7-9). The moiety of compoundschangesin circulatingblood, which results in relative'y poor extraction (E) by thebrain (HMPAO 77%, ECD 60%) (Table 1). The retention inthe brain parenchymais linked to the enzymatic reactionswith glutathione (GSH) for HMPAO (10,11) and with stereospecific deesterification to acid derivatives for ECD (12—15). These mechanisms have slow turnover and yield nondiffusible polar metabolites.

The first-passextractionrate is flow-dependentfor ECD,similar to HMPAO, and results in decreased uptake in thehigher flow of a normal range (16). Conversely, extractionis higherwhen the flow is reduced. Due to a low backfluxfrom brain (k2), the retention of ECD is higher thanHMPAO (Table 1). Backdiffusionof HMPAO is more pronounced in high flow regions and a higher extraction in lowflow regions results in poor contrast. Technetium-99m-ECD images hold a slightly better contrast between highand low flow regions. ECD has some washout from thebrain and negligible redistribution (17,18). HMPAO hasslow clearance and liver accumulation contrary to ECD,which has rapid blood clearance with renal excretion ofacid metabolites (EC) yielding a better signal-to-noiseratio.

IMP, which is not readily available in every country andis cumbersome to label, was the first iodo-amine (19) usedfor brain imagingwith a high brain extraction rate (96%)

J NuciMed1995;36:359-363

R adionucide imagingand semiquantitationof cerebralperfusion by SPEC!' are important issues in patients

with neurological and psychiatric diseases. The search foran ideal brain SPECT radiopharmaceuticalis complicatedand competes with other imaging modalities such as dynamic CT and MRI. Unfortunately, currently availableradiopharmaceuticals are far from ideal.

The ideal agent should be commercially available, easyto label, possess in vivo and in vitro stability, have shortarterialinput to facilitate intervention (stimulation)studiesand a high extraction fraction at the first pass. Such anagent should reach a steady state in the brain quickly andbe able to penetrate intact blood-brain barriers regardlessof the blood cell-plasma protein partition. They also musthave high passive retention in the brain independent of ametabolic or enzymatic system or a specific binding site.An ideal agent would also reflect regional distribution,which is linear to the cerebral blood flow over a wide rangeand remain fixed in the brain with a constant pattern topermit SPECT imaging with minimal extra brain activity.

The ideal radiopharmaceutical should be neutral, iipophilic (log p octanol/water between 1.0—2.5)and of smallsize (under 500 dalton) (1). All of the technetium-aminesandiodo-aminesdiscussed in this editorial[hexemethylpropyleneamine oxime (HMPAO), bicisate (1,1-ECD) or isopropyl iodoamphatamine (IMP)J meet these criteria atdifferent levels oflipophilicity, which determines biodistribution. At higher levels of lipophilicity, there is a trade-offbetween binding to the brain cells and plasma proteins infavor of the latter (2). A @‘Fc-labeledcompound thatbehaves like IMP would represent a major advance forroutinebrainperfusionimaging.However, a consensus hasnot yet been reached on the best agent.

A clear understandingof the absence of a trueflow agentis crucial before comparingthe available radiopharmaceu

ReceivedOct 18,1994;acceptedOct.24,1995.Forcorrespondenceorreprintsonnt@Jean-LieMcmlii,MD,PhD,Med@ine

Nucieaire,Ho@ Avicenne,99009Bobigny,France.

CerebralPertu@onImagingTracesforSPECT•Morethetal. 359

Cerebral Perfusion Imaging Tracers for SPECT:Which One to Choose?

J.L. Moretti, M. Caglar and P. Weinmann

HopitalAvicenne, CHU Leonard de Vsni4 Bob@zy, France

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TABLE IComparisonofBrainSPECTImagingAgents

nism and metabolism of the tracer. This washin/washoutbalance is clearly demonstrated in the study performed byNishizawa et al. (24) who observed a temporal change ofradioactivity among normal brain structures. Likewise,IMP uptake is different between white and gray matter andfavors the latter. The first possible explanation for this isthe first come, first served principle. Since the penetratingcapillaries enter the gray matter, more tracer is available tobe extracted by gray matter before reaching the whitematter. The second explanation is the difference in theextraction coefficient of IMP between gray and white matter (21). Rapinet al. demonstratedby autoradiographythatthe activity in the brain redistributeswith time in favor ofthe white matter possibly due to backflux from gray towhite matter (25). For this reason, it is crucial to performearly imaging at 20 min. Afterwards, polar metabolitessuch as p-iodobenzoic acid (PIBA) will appear in theplasma in non-negligibleamounts with time (26), diffusingpassively through a damaged blood-brain barrier. Therefore the delayed images have to be acquired at 3 hr.

One of the most active and exciting fields of nuclearmedicine has been evaluatingcerebralperfusion in severalpathologies. There are certain conditions where differentlipophilic tracers give similar images such as drug addiction, dementia, interictal epilepsy, psychiatric disorders

+ and head trauma. Ictal, postictal epilepsy, stimulations and

stroke are not among these conditions.

ICTAL AND POSflCTAL EPILEPSY

Interictal perfusion SPECT was performed with iodoamines (27) andTc-amines (28)yielding a sensitivity of 60%to75%whichcorrelateswellwithEEGstereolocalization.Fluctuations in perfusion and metabolism in the interictalperiod do not allow the detection of more than 75% epileptic hypoactive foci with perfusion SPECF or FDG-PET(29). During the ictal phase, higher metabolic activity coupled with high perfusion results in increased uptake in theepileptic foci. True ictal studies are difficult to perform invitro with unstable HMPAO, rendering it impossible tostore in the neurology department. In this case, compounds of higher stability such as IMP or ECD are moreappropriate. In the early postictal phase, cerebral bloodflow remains elevated decoupling from the rapidly dccliiiing glucose metabolism (30). Blood flow remains elevatedlong enough to allow lipophilictracers to detect the hyperactive foci. Since ECD has a shorter arterial input, it appears to be more appropriate than IMP. Sensitivity increases by 10% during the immediate postictal phase (31).

S11MULA11ONIn normal cerebral autoregulation, cerebral blood flow

increases in response to neuronal metabolism; SPED.' hasdemonstrated sufficient sensitivity to detect changes in activity related to stimulation of metabolic activity (32).These activation tasks must generate detectable and reproducible changes in cerebral blood flow which are sustained

CharecteristlcIMPHMPAOECDLogP1.401.901.64Purity96%85%96%Stabllfty6hr30mm6hrHead

uptake (%lD)6.0 ±15.2 ±16.3 ±1.9BUI1137773E%907760ki0.430.26

0.27t0.290.25tk20.0140.60*

0.59t0.220.19tA

= k1ik@32.40.43*0.46t1.31*0.31tk30.930.58*

0.57@0.5rO.59@ak2ik3700.9r

0.9r2.6@3.l@R%98%50%@

49%t72%75%tE%88365k

37.7k43.2*45tK0.420.13*

O.l3@0.21*O.l9@RediStributiOn++100t(mln)201010Meanwashoutrateof1%2%10%steady

state(hr)Heterogeneouswashout + 0

*SeeReferences49,50and51. SeeReference48for IMPdatatSee References16 and 51.kl = unidirectIonalinfluxconstantcorrespordngtoflowx extraction

(firstpass)(Fx g1m@1;@=back-flwinh/rT@,k1/k2=pardtion@e1ficientinthecoitex@k3= transformationconstant(1/mm),a k3@2

= conversion rate, R = retention fr@ion = k3/k2 + k3, K = steady

state influxconstant= K1.@ + K@,/E = extr@ion at first pass,E =E*R Extractionat steadystate,BUI = bran u@akeindex@logP =dedmaiioga@thmofthepartitioncoefficientoctanolonwater,t = timetoreachthesteadystate,W = washoutrateaftersteadystate(hr1) frombrain.

(Table 1). Its uptake linearly follows a wide range of flowassessed by microspheres (20). The gradual increase in thebrain uptake of IMP (50%at 5 miii and 90% at 20 min) islinked to its clearance from the lung, which functions as abuffer releasing the tracer into the circulation (21). Thestereoselective retention mechanism of IMP in brain tissueinvolves a possible conversion to hydrophilic metabolites(22), and an affinity to high capacity and relatively nonspecific binding sites. Intracellular pH can play a role in manyinstances: (1) rate of exchange through the BBB; (2)strength of binding to the nonspecific sites of the brain; and(3)metabolicconversionrate of aminesthrough the enzymatic system linked with binding sites. When blood pH isdecreased due to local lactate discharge, the uptake of IMPis notably decreased (23).

Brainretention of IMPreflects the balance of the washinandwashout ofthe tracerin individualstructures.Washoutfrom brain is influenced by blood flow, retention mecha

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during the period after injection, while the tracer reachessteady state. In the activation studies, the tracer distribution has to reflect high flow rates, like IMP, in order toquantify the stimulation accurately. The trapping processof the tracer should not be saturable in test/retest studiesusing split dose technique within a short interval. In thetest/retest studies, assumption is that the original distribution pattern is still the same during the second SPECFimage acquiredafteractivationon the same day. Due to theheterogeneous washout pattern, a test/retest protocolwould introduce significant errors, therefore a two dayprotocol is preferable (33).

As the radioactivity of d,1 HMPAO changes with timeafterpreparation(in vitro and in vivo), even if equal dosesof radioactivity are injected, one cannot be sure of thesame bioavailabiity for brain. Using HMPAO in a test/retest activation study requires a long stimulation of thesame intensity. ECD has the same uptake and retentioncharacteristics of HMPAO, but it is more stable and moreconvenient to handle. Like HMPAO, however it underestimates high flow areas (5).

CEREBROVASCULARDISEASE

Tomographic images of cerebral perfusion with 11-pophilic radiotracers are extremely useful in early detection of acute and chronic stroke. SPECF is superiorto Cl'in detecting ischemia since it reflects alterations in perfusion before structuralchanges occur. This is an advantagefor determining the appropriate therapy, which must beinitiated as early as possible in acute stroke.

In stroke, two differentiatedregions are observed in thebrain: one the central infarct core known as the structuralabnormality, which can be imaged by CF and two a peripheralarea that reflects pen-infarct ischemia, known asthe penumbral zone (34). SPECT perfusion imaging candemonstrateboth of these regions dependingon the tracerused. There is a correlation between the penumbralzoneand the severity of the neurological deficit which may berelated to outcome (35).

Because of the high extraction rate in low-flow regionsand the high level of perfusion in ischemic regions due tovasodilation, HMPAO cannot be reliable to detect periinfarct ischemia in the subacute phase (4—15days). If images are acquired at 4 hr, clearance of the plasma activityand leakage of the tracer from the cells allows imagingofhypoactive peri-infarct ischemia (36). On the contrary,early IMP images demonstrate both structural abnormalities (40% decrease) and the penumbral zone (15% decrease). With time, even if the central area remains unchanged, redistribution occurs in the peripheral zoneresulting in a partiallyfilled delayed images. The degree ofredistribution is correlated with the local cerebral bloodflow and regional consumption of oxygen but not with theregional oxygen extraction fraction which reflects viability(37). IMP redistribution indicates a tissue with low CBFand a remainingmetabolism (38). This implies that blood

flow is inadequaterelative to the energy/metabolicdemandfor oxygen and substrate (misery perfusion). In these instances, slower metabolism and washout of IMP wouldcause retentionin the pen-infarctzone. The only definitiveconclusions made on redistribution IMP SPEC!' images iswhether non-necrotic cells are present or not. The redistribution of IMP was formerly related to a good clinicaloutcome (39).

With ECD in the acute phase of stroke, when the esterase enzyme system is still functioning, a higher extractionand retention of the tracer results in low blood flow areas(40). In the subacute phase, hypoxia may alter the functionof esterase, resulting in minimalretention, even if the extraction is preserved. The hypoactive area will be largerand deeper on delayed images, demonstrating increasedclearance from the ischemic areas (41,42). Diminished activity of ECD in subacute stroke may be due to the lack of

oxygen and enzyme activity, which causes an accumulation of unmetabolizedtracercandidate to leak out from theperipheral ischemic zone. By contrast, at low blood flow,since the GSH system is more resistant to hypoxia andcontinues to function, cells retain HMPAO in the ischemicarea.

During the period of luxury perfusion after recanalization, there is transientdecouplingof metabolic demandandcerebral blood flow in the subacute phase of ischemic regions. Increased glucose consumption is observed, indicatinganaerobic glycolysis and production of lactate accompaniedby localacidosisallofwhichdecreasetheuptakeof IMP.Inthis condition, the behavior of the perfusion tracers is different. Most of the time HMPAOshows a focal areaof hyperactivity, whereas IMP and ECD demonstrate hypoactivezones and IMPis relatedto low extractionin acidicpH andECD caused by low retention, which is linked to alteredesterase function in hypoxia (43,44).

In reversible ischemic stroke, structuralimaginghas litlie to offer and perfusion imaginghas low sensitivity (60%within the first 24 hr (45), declining further with time). Toincrease sensitivity, it is necessary to assess vasodilatoryreserve by an acetazolamide stimulation test as demonstrated by us with IMP (46,47). The weak contrast obtamed with HMPAO and ECD in very mild ischemia is adrawback incorrectly delineating lesions. Because activityof these @“@Tc-labeledamines is not linear at high flowrates, the response to acetazolamide is blunted as is lesioncontrast.

CONCLUSION

Although IMP because of its linearity with flow andcapacity to depict mild ischemic regions seems to remainthe reference in assessing cerebrovascular diseases, ECD,due to its availability and improved stability, will likelybecome widely used even though it is far from being anideal chemical microsphere agent. Technetium-99m-ECDis a powerful diagnostic tool to facilitate critical management decisions in neurologic and psychiatric diseases. It is

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necessary to be cautious, however, in using this tracer inextreme conditions oflow and high brain blood flow and intest/retest examinations. Iodune-123-IMPhas not been replaced by Tc-labeled amines. The search continues for anoptimal brain tracer of perfusion and viability as a reliablefirst step for functional and receptor imaging. Although thetask to find the ideal agent is difficult, the challenge isworthwhile. With early diagnosis of cerebrovascular disease, there is an opportunity for improved outcome.

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developmentof ‘231-MIBG,‘‘In-pentetreotideor otherreceptor-specificradiolabeledligands. Idid not tell them ofthe men and women who work in the middle ofthe night to produceanddeliver those magic bullets, or ofthose who work day after day in hospitals and officesobtaining images and data. Nor did I describe the wonder ofa gamma camera or my delightwhen watchingthree-dimensionalimage acquisition,volume reconstructionand fusion withMRIor CT images.Thereis neverenoughtimeto relatethe fascinationofwatchingthebrainthink,theheartpump,or themarvelsof otherorganfunction.

Nuclear medicine is more than a medical specialty. It is a wonder to behold.

Stanley J. Goldsmith, MD, Editor-in-ChiefTheJournalofNuclearMedicine

—March1995

363CerebralPerfusionImagingTracesfor SPECT•Morethet al.

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1995;36:359-363.J Nucl Med.   J.L. Moretti, M. Caglar and P. Weinmann  Cerebral Perfusion Imaging Tracers for SPECT: Which One to Choose?

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