alzheimer s disease ( ad ) is the most frequent neurodegenerative disease ( ferri et al . , 2005 ) and is characterized by a progressive dementia that occurs in middle or late life .
the neuropathological hallmark of ad is the presence of cortical intracellular neurofibrillary tangles ( nft ) and extracellular -amyloid ( a ) plaques ( braak and braak , 1991 ) , which leads to synapse dysfunction , neuronal cell loss and subsequent brain atrophy ( ballard et al . , 2011 ) .
in the past few decades the knowledge of the key pathogenic mechanisms of the disease has improved , but they are still not completely understood ( querfurth and laferla , 2010 ) . the natural history of ad could be divided in three different phases : the pre - clinical phase , where the pathogenic mechanisms of the disease have started but no symptoms can be identified ; the prodromal phase , where mild cognitive symptoms appear , but there are not severe enough to meet dementia criteria ; and the dementia phase ( dubois et al . , 2007 ) . on the other hand , mild cognitive impairment ( mci ) is a clinical construct created to capture patients with subtle cognitive symptoms at risk for ad ( petersen et al . , 2009 ) .
however , some subjects develop other types of dementias , do not progress , or even revert to normal cognition ( ganguli et al . , 2004 ; brooks and loewenstein , 2010 ) .
therefore , it is difficult to label mci patients as having prodromal ad . currently , the most frequently used clinical diagnostic criteria for ad are the diagnostic and statistical manual of mental disorders ( dsm - iv - tr ; apa , 2000 ) and the national institute of neurological disorders and stroke - alzheimer disease and related disorders ( nincds  adrda ; mckhann et al . , 1984 ) working group .
first , the dementia syndrome is established and then , criteria based on the clinical features of the ad type dementia are applied .
the dsm - iv - tr criteria require the presence of impairment in multiple cognitive domains of sufficient severity to interfere with the individuals activities of daily living to diagnose dementia .
hence , the clinical diagnosis of ad is not possible until a patient reach the dementia phase of the disease . on the other hand ,
these criteria have shown to have good sensitivity and specificity to distinguish between patients with ad type dementia and non - demented subjects , but are less accurate to differentiate ad dementia from other primary dementias ( knopman et al .
in fact , up to 20% of patients clinically diagnosed with ad do not have ad pathology at autopsy ( mayeux et al .
a more accurate and earlier diagnosis of ad ( e.g. , in mci patients that will progress to the ad dementia phase or ideally those subjects in the pre - clinical phase ) could enable the administration of potential disease - modifying drugs that would have a great impact on patients life and profound implications for public health ( brookmeyer et al .
, 1998 ) . in this context , there is a need of specific biological markers for ad diagnosis in the earliest stages .
a biomarker is defined as a measureable feature that can be used to diagnose a physiologic or pathologic condition .
the ideal biomarker of ad would be : ( 1 ) directed at the fundamental pathophysiology of the disease , ( 2 ) a marker of the presence of disease itself , ( 3 ) efficacious at prodromal , and even pre - clinical stages of ad , ( 4 ) an indicator of disease s severity , ( 5 ) a marker of treatment effectiveness , and ( 6 ) inexpensive and non - invasive ( klunk , 1998 ) . in the case of ad , different biomarkers have been described using diverse approaches .
the development of cerebrospinal fluid ( csf ) assays and neuroimaging techniques that can provide information about the presence of ad pathological changes has been a major step forward in the field .
consequently , biomarkers for ad are called to play a central role in the clinical characterization of the disease . more recently , the nincds  adrda criteria have been revised by the national institute on aging and the alzheimer s association ( nia  aa ) workgroup to include the experience in research over the past 25  years ( mckhann et al . , 2011 ) .
the nia  aa proposed a series of diagnostic classification from pre - clinical ad to mci to full - blown ad , and introduced csf and neuroimaging biomarkers as supportive features of the disease ( albert et al .
, 2011 ; mckhann et al . , 2011 ; sperling et al . , 2011 ) .
however , the authors cautioned about the use of biomarkers , and stated that their use should be limited to research and drug trials .
nevertheless , the core clinical criteria for ad remained relatively intact in the new criteria .
this review will focus on the most widely studied and currently accepted sources of biomarkers in ad : csf , magnetic resonance imaging ( mri ) , and nuclear medicine techniques , including amyloid imaging .
typically , the csf better reflects the brain neurochemistry , while biochemical values in plasma or serum may be affected by many non - neurological factors .
csf biomarker research has focused mainly on molecules related to the central neuropathological features of ad , such as a , the main component of the senile neuritic plaques ( snp ) , and tau proteins that are part of the nfts .
a is a 3643 amino acid peptide originated from proteolysis of the amyloid protein precursor ( querfurth and laferla , 2010 ) .
snps are mainly composed by a protein with 40 ( a40 ) or 42 ( a42 ) amino acids , and it seems that a deposition begins with a42 , and during plaque maturation , they acquire a40 ( iwatsubo et al . , 1994 ) .
by contrast , nfts are formed by tau proteins , a microtubule - associated stabilizing protein essential for axonal transport .
tau is hyperphosphorylated in ad , which tends to aggregate , leading to synaptic and neuronal dysfunction ( lee , 1996 ; blennow et al . , 2010 ;
this process of nfts formation starts in the entorhinal cortex , spreading to the hippocampus , and then to the rest of the brain .
these neuropathological changes begin early , even decades before the onset of clinical symptoms of dementia ( braak and braak , 1991 ) , but the relative contributions of altered tau and amyloid metabolism to neurodegeneration remain under investigation .
nevertheless , nfts are more robust associated with measures of synapse loss and cognitive impairment that snps ( gomez - isla et al . , 1997 ) .
typically , ad patients have lower levels of a42 in the csf compared to normal controls , a40 tends to remain constant , and tau ( t - tau ) and phosphorylated tau ( p - tau ) increase ( arai et al . , 1995 ; blennow et al . , 1995 ;
, 1995 ; galasko et al . , 1998 ; kurz et al . , 1998 ; shoji et al . , 1998 ; hulstaert et al . , 1999 ;
, 2001 ; sunderland et al . , 2003 ; lewczuk et al . , 2004 ;
the low csf a42 levels in this entity may represent the retention of the peptide into plaques , resulting in reduced availability to diffuse into the csf .
this is supported by the findings that a42 in the csf is inversely correlated with amyloid plaque load at autopsy ( strozyk et al . , 2003 ; tapiola et al . ,
2009 ) and as measured in vivo by amyloid imaging ( fagan et al . , 2006
the increase in t - tau and p - tau in the csf of ad patients may stem from release into csf during neurodegeneration ( blennow et al . , 1995 ) , although the exact mechanism is not known .
t - tau and p - tau concentrations in the csf have been associated with nft load at autopsy ( buerger et al .
2009 ) , as well as with cognitive decline and brain atrophy ( buerger et al . , 2002 ;
low csf a42 and high t - tau or p - tau proteins have shown high accuracy for ad diagnosis ( galasko et al . , 1998 ; andreasen et al . , 2001 ; schoonenboom et al . ,
2004 ) . studies that compared ad patients with normal control subjects have demonstrated that low a42 had 78100% sensitivity and 4781% specificity for ad diagnosis ; high csf t - tau levels had 70% sensitivity and 92% specificity and p - tau showed 77% sensitivity and 87% specificity .
however , p - tau appears to be better than t - tau in the diagnosis of ad , and it has shown a positive predictive value of 90% , especially p - tau phosphorylated at threonine 181 ( p - tau181 ; hampel et al .
figure 1 shows the t - tau levels in ad , frontotemporal dementia ( ftd ) , and normal autopsied subjects , and figure 2 shows the a42 levels in the same population ( schoonenboom et al . ,
ad , alzheimer s disease ; ftd , frontotemporal dementia . because some studies found that a42 and tau levels alone were not sufficient to differentiate ad from normal controls or other dementias , the a42/tau ratio is used to improve the diagnosis of ad .
a meta - analysis showed that the a42/tau ratio had a sensitivity of 71% and specificity of 83% for ad ( hampel et al . , 2004b ) ; and a recent study showed that the  signature  of ad , based on the csf a42/p - tau181 ratio cutoffs , was present in 90% of the ad patients compared to 36% in the normal control group ( de meyer et al . , 2010 ) .
although , these csf biomarkers seem to be useful to diagnose ad , they are not sensitive enough to assess disease progression ( sunderland et al . , 1999 ; vemuri et al . , 2010b ) .
nevertheless , higher csf t - tau and lower a42 values have been associated with rapid cognitive decline , no response to cholinesterase inhibitor treatment and a higher mortality in patients with ad dementia ( wallin et al . , 2010 ) .
cerebrospinal fluid biomarkers might have a role in the differential diagnosis of ad and other dementias .
patients with dementia with lewy bodies ( dlb ) , like ad patients , have lower levels of a42 in the csf compared to normal controls , reflecting that most of these subjects also present ad pathology
. nevertheless , shorter amyloid peptides ( a37 , a38 , and a40 ) appear to be slightly more elevated in ad dementia ( bibl et al . , 2006 ) .
consequently , the a42/a37 and a42/a38 ratios seemed to better discriminate ad from dlb ( bibl et al .
t - tau and p - tau levels are normal or high in dlb , but most studies have shown that these markers levels in dlb are definitely lower than those found in ad .
other proposed biomarker for the differentiation of ad from dlb was monomeric soluble -synuclein , but the available studies in dlb patients showed conflicting results .
elevated , unchanged and decreased total -synuclein levels have been reported in these patients compared to ad and control subjects ( mukaetova - ladinska et al . , 2010 ) .
in addition , there are limited data about oligomeric -synuclein in csf , although it could be potential biomarker for synucleinopathies ( tokuda et al . , 2010 ) .
possibly , the combination of all these markers will help to distinguish between ad and dlb in the future . in ftd ,
csf a42 levels are lower than in control subjects but higher than in ad , while t - tau and p - tau levels are higher than in controls but lower that in ad patients ( hulstaert et al .
hence , t - tau / a42 and p - tau / a42 ratios could be a useful tool to distinguish ad from ftd ( bian et al .
creutzfeldt  jakob disease ( cjd ) is characterized by decreased csf a42 concentrations and very high t - tau levels , reflecting intense neuronal damage , while p - tau levels are normal ( kapaki et al . , 2001 ;
current data suggest that p - tau levels could be the most relevant marker to differentiate ad from other types of dementias ( hampel et al .
cerebrospinal fluid biomarkers may also be useful in predicting progression to ad in subjects with mci ( andreasen et al .
, 1999 ; hampel et al . , 2004c ; ewers et al . , 2007
low csf a42 levels predicted conversion with 6080% sensitivity and 65100% specificity , while t - tau levels had 8386% sensitivity and 5690% specificity , and p - tau had 7384% sensitivity and 4788% specificity ( hampel et al . , 2004b ) .
however , the conversion to ad dementia in mci subjects has shown significant variability among different csf studies .
( 2006 ) found high sensitivity ( 95% ) and specificity ( 87% ) for the conversion from mci to ad dementia using a combination of t - tau and a42/p - tau181 ratio cutoff values .
by contrast , a multicenter csf study found lower sensitivity ( 83% ) and specificity ( 72% ) for the conversion from mci to ad than single - site studies , with a considerable inter - site assay variability that highlights a need for standardization of analytical techniques and subject selection ( mattsson et al . , 2009 ) .
other large multicenter studies have confirmed the high predictive value of these csf biomarkers in the identification of cases with prodromal ad in the context of the mci syndrome ( shaw et al . , 2009 ; visser et al . , 2009 ) .
the csf biomarkers have been used to identify the pre - clinical stage of ad .
cross - sectional studies in normal control individuals carrying the apoe4 allele showed lower csf a42 and higher p - tau181 levels ( peskind et al .
interestingly , csf levels of a42 correlated with brain volume in this group of subjects , while in mci and ad dementia patients , whole - brain volume correlated with csf t - tau and p - tau181 ( fagan et al . , 2009 ) .
a longitudinal csf study in normal controls found increased t - tau levels and no change in a42 after a 1-year follow - up ( vemuri
a few longitudinal studies have assessed which csf biomarker or combination of biomarkers better predict ad dementia or cognitive decline .
fagan et al . ( 2007 ) demonstrated that csf tau / a42 ratios may predict future dementia in cognitively normal older subjects and two other studies showed that low levels of csf a42 predicted cognitive decline , as measured with the mini mental status examination ( mmse ; gustafson et al .
although there are studies that showed that csf a42 could predict prodromal ad , there was a wide sd in these subjects , which made difficult to determine individually who will progress to ad .
further studies are needed to assess changes in csf markers over time and to determine the best csf markers of change .
structural neuroimaging techniques are normally performed during the clinical assessment of patients with dementia , mainly to rule out structural lesions , such as brain tumors , normal pressure hydrocephalus , or vascular lesions .
the mri has shown high sensitivity to detect brain atrophy caused by the neurodegenerative process ( bobinski et al . , 2000 ) .
several methods are used to analyze mri data including visual inspection , volumetry , or region of interest ( roi ) analysis ( applying manual tracing , automated , or semi - automated techniques ) and voxel based morphometry ( vbm ; vemuri and jack , 2010 ) .
the visual rating of the images and roi analysis depend on the a priori choice of structures , while vbm is a whole - brain operator - independent analysis ( ashburner and friston , 2000 ) .
however , vbm analysis only allows comparing groups of patients , and it is not useful to classify individual cases .
visual assessment of mri scans is a fast method to assess brain atrophy but is not the most precise ( raji et al . , 2010 ) .
volumetry of the medial temporal lobe structures is the most common quantitative method used in ad .
the traditional manual tracing is time - consuming , and consequently , automated or semi - automated methods that require no significant manual intervention have been developed .
other methods to analyze sequential mris are boundary shift integral ( bsi ) and tensor - based morphometry ( tbm ) .
bsi has been developed to quantify the global percentage of change in the brain surfaces between two scans , while tbm provides a three - dimension profile of brain atrophy ( vemuri and jack , 2010 ) .
brain mri studies have shown that ad patients have a widespread cortical volume loss , including the frontal lobes , the temporoparietal regions , precuneus , and mesial temporal lobes , with relative sparing of the sensorimotor , visual , and cerebellar regions ( du et al . , 2001 ; good et al . , 2001
; thompson et al . , 2001 ; karas et al . , 2003 ; testa et al . , 2004 ; ishii et al . , 2005 ; dickerson et al . , 2008 ; burton et al . , 2009 )
longitudinal studies have suggested that atrophy stars in the medial temporal lobes and fusiform gyrus in the prodromal phase of the disease , then spreads out to the posterior temporal and parietal lobes , and by the time of the dementia phase , it involves the medial temporal lobes , temporoparietal cortex , and the frontal lobe ( whitwell et al . , 2007 ) .
this pattern of atrophy progression on mri appears to follow the distribution of nft in the different braak and braak ( 1991 ) pathological stages ( whitwell et al . , 2008 ) .
in ad , there is also a greater rate of brain atrophy over time compared with healthy elderly subjects ( 1.92.2% per year versus 0.50.7% per year ; obrien et al . , 2001 ; sluimer et al . ,
hippocampal and whole - brain atrophy in ad correlated well with cognitive performance and measures of disease severity ( jack et al . , 2008a ; ridha et al . , 2008
mri studies show a similar pattern of brain atrophy to that observed in ad ( jack et al . , 2000 ;
chetelat et al . , 2002 ; karas et al . , 2004 ; bell - mcginty et al . , 2005 ; whitwell et al . ,
2008 ) and rates of whole - brain atrophy are around 1% per year , between ad and control subjects ( jack et al . , 2004 )
. whole - brain atrophy rate in mci is associated with impaired cognitive performance ( evans et al . , 2010 ) and predicts progression to ad dementia ( spulber et al . , 2008 ) .
whole - brain techniques have also shown that higher rates of atrophy in specific areas such as the hippocampus , posterior cingulated gyrus , and superior parietal cortex were associated with incident ad in mci cases ( chetelat et al . , 2005 ) , and hippocampal and basal forebrain volumes in normal subjects predicted future development of ad ( hall et al . , 2008 ) .
interestingly , a recent study showed that normal subjects with the ad csf profile had increased rates of brain atrophy over time , suggesting they may be in the pre - clinical phase of the neurodegenerative process ( schott et al . , 2010 ) .
nevertheless , this observation requires a longer follow - up period to demonstrate that this particular group of subjects has a higher risk to develop ad dementia . because the most important clinical manifestation of ad is memory loss ,
the majority of the researchers have focused on the study of hippocampal atrophy as a marker of the disease .
different neuroimaging studies assessing the mesial temporal lobe structures have consistently shown smaller hippocampal volumes in ad patients compared to controls ( pennanen et al . , 2004 ) .
measures of hippocampal atrophy correlate well with the degree of cognitive impairment ( jack et al .
, 2000 ; mortimer et al . , 2004 ) and with ad pathology and hippocampal sclerosis in mri - post - mortem studies ( jagust et al . , 2008 ) .
both normal aging and ad have gradual volume loss overtime , but as with whole - brain volume is greater in ad ; the annual rate of hippocampal atrophy is 1.41% for healthy controls and
several studies have shown that hippocampal atrophy measured by mri predicts conversion from mci to ad ( jack et al . , 1999 ; visser et al . , 1999 , 2002 ; geroldi et al .
, 2006 ; devanand et al . , 2007 ; teipel et al . , 2007 ) .
a recent meta - analysis showed that this measure could identify mci converters with a sensitivity of 73% and a specificity of 81% ( yuan et al . , 2009 ) .
although decreased entorhinal cortical thickness has been proposed as a predictor of conversion from mci to ad , measures of decrease hippocampal volume appeared to be a more robust predictor .
there are several limitations for the use of whole - brain or hippocampal volume as a sole biomarker for ad .
one of the most important is that small brain volumes can also be seen in normal aging , and consequently , is difficult to separate cases with early ad from normal controls . on the other hand , the rate of change on whole - brain , entorhinal , and hippocampal volumes seemed to be good markers of neurodegeneration and they can be useful in determining the effects of ad therapies , but these techniques are labor intensive and limited to research studies .
brain functional neuroimaging techniques  positron emission tomography ( pet ) and single photon emission computed tomography ( spect )  allow a broad range of cerebral functions to be assessed in patients currently living with dementia .
actually , brain metabolism , brain perfusion , and different neurotransmitter systems can be measured with a great variety of pet and spect tracers .
however , brain metabolism with the glucose analog 2-[f]-fluoro-2-deoxy - d - glucose ( f - fdg ) and brain perfusion with technetium-99 m hexamethylpropylamine oxime ( 99mtc - hmpao ) are the two more widely employed techniques in the evaluation of patients with dementia , both in clinical practice and in research .
brain spect with 99mtc - hmpao studies provide information of the regional cerebral blood flow , and f - fdg pet estimates the regional brain rate of glucose consumption , therefore providing information about the pattern of neuronal loss or synapse dysfunction in patients with dementia .
the pattern of spect hypoperfusion and pet hypometabolism usually seen ad involves the anterior medial temporal lobes , the posterior cingulate and posterior temporoparietal cortex ( kogure et al . , 2000 ;
nevertheless , spect seems to be less accurate than pet for the diagnosis of ad .
one autopsy study found that this technique alone had a sensitivity of 63% and a specificity of 93% to diagnose ad ( jagust et al . , 2001 ) . on the other hand
the sensitivity of this procedure discriminating ad from ftd was 71.5% and the specificity was 78.2% , while its sensitivity and specificity to differentiate ad from vascular dementia were 71.3 and 75.9% , respectively ( dougall et al .
, spect has shown that can predict conversion to ad with a test accuracy of 82% ( huang et al . , 2007 ) .
positron emission tomography studies in early ad have shown a pattern of reduced metabolism in the posterior cingulate ( minoshima et al . , 1997 ) and mesial temporal cortices .
as the disease progresses , there is a greater involvement of the parietotemporal and frontal cortices ( ishii et al . , 2001 ;
this progression of the areas of hypometabolism correlated with cognitive deterioration over time as measured by neuropsychological tests ( engler et al .
1984 ; minoshima et al . , 1995 ; silverman et al . , 2001 ; mosconi et al . , 2005 ) .
a meta - analysis showed that pooled f - fdg pet sensitivity and specificity to differentiate ad patients from normal subjects were 86% in both cases , but the variability was important between the studies ( sensitivity from 61 to 100% and specificity from 54 to 100% ; patwardhan et al . , 2004 ) .
pet also seems to be a good tool to differentiate ad from other forms of dementia . in an autopsy confirmed study , the presence of occipital hypometabolism was able to distinguish dlb from ad with 90% sensitivity and 80% specificity ( minoshima et al .
this technique also has a high specificity and sensibility to differentiate ad and ftd ( koeppe et al . , 2005 ;
f - fdg pet appears to be a promising instrument to discriminate those patients with mci who will progress to ad dementia .
these patients show a medial temporal and posterior cingulate cortices hypometabolism compared with healthy controls ( chetelat et al .
has also shown that f - fdg pet performs slightly better than spect and structural mri in identifying the prodromal phase of ad in patients with a clinical diagnosis of mci ( pooled sensitivity of 89% and specificity of 85% ; yuan et al . , 2009 ) .
interestingly , similar metabolic reductions in the same regions as those found in mci and ad patients have been seen in asymptomatic subjects at risk of ad , i.e. , in persons with subjective memory complains ( caselli et al . , 2008 ) , carriers of an apolipoprotein e 4 ( apoe4 ) allele ( reiman et al . , 1996 , 2001 , 2004 , 2005 ) , with abnormalities in the csf markers ( petrie et al . , 2009 ) , or with a maternal history of ad ( mosconi et al
, 2007 , 2009 ) . furthermore , hippocampal hypometabolism during normal aging predicted cognitive decline years in advance of the clinical diagnosis ( de leon et al . , 2001 ; jagust et al .
these data indicate that f - fdg pet has high sensitivity to distinguish ad from controls and from other dementing entities , and also to identify subjects at high ad risk .
consequently , the pet ad pattern has been incorporated into the new clinical criteria for prodromal ad ( dubois et al . , 2007 ) , pre - clinical ad ( sperling et al . , 2011 ) , and mci due to ad ( albert et al . ,
the development of pet radiotracers that bind to brain amyloid has revolutionized the use of neuroimaging techniques in patients with dementia .
these molecules allow the localization of ad pathology in vivo , and have helped to further our understanding of the underlying biology of ad .
the most actively tested tracers are n - methyl-[c]2-(4-methylaminophenyl)-6-hydroxybenzothiazole ( c - pib ; klunk et al . ,
2003 ) , ( e)-4-(2-(6-(2-(2-(2-(18)f - fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-n - methyl benzenamine ( f - av-45 ; choi et al . , 2009 ) and 2-(1 - 96-(2-f - fluoroethyl)(methyl)amino)-2-naphthyl)ethyldene ) malono nitrile ( f - fddnp ; small et al . , 2006 ; shin et al . , 2008 ) , but there are multiple c and f compounds under investigation such as trans-4-(n - methyl - amino)-4-{2-[2-(2-[18f]fluoro - ethoxy)-ethoxy]-ethoxy}-stilbene ( f - bay94 - 9172 ; rowe et al . , 2008 ) , 4-n-[c - methyl]amino-4-hydroxystilbene ( c - sb-13 ; verhoeff et al . , 2004 ) and 2-(2-[2-dimethylaminothiazol-5-yl]ethenyl)-6-(2-[fluoro]ethoxy)benzoxazole ( c - bf-227 ; kudo et al . , 2007 ) .
while c have better affinity for fibrillar amyloid than f compounds , the former have a very short life , which limits its use to centers with an available cyclotron that can produce them . among these tracers ,
this thioflavin - t derived binds fibrillar a with high affinity and its brain retention correlates well with levels of amyloid in ad brain tissue ( mathis et al . , 2002 , 2003 ;
positron emission tomography studies with c - pib have demonstrated an increased retention of this tracer in frontal and parietotemporal cortices , as well as in the striatum of almost all ad patients compared with controls ( klunk et al .
, 2004 ; edison et al . , 2006 ; rowe et al . , 2007 ) . however , it is important to note that a positive c - pib pet scan can also be seen in other entities , often misdiagnosed as ad , such as dlb and cerebral amyloid angiopathy ( edison et al . , 2008 ; gomperts et al . , 2008 ; dierksen et al . ,
c - pib retention in ad correlates with rates of whole - brain atrophy ( archer et al . , 2006 ) , parietotemporal hypometabolism on f - fdg pet ( klunk et al .
, 2004 ; edison et al . , 2006 ; engler et al . , 2006 ) , and decreased csf a42 levels ( fagan et al . , 2006 ; grimmer et al .
there are few longitudinal studies using amyloid ligands , and the observation time has been relatively short , because these techniques have only been available for research since 2004 .
recent longitudinal studies in ad patients assessing amyloid load and progression of neurodegeneration using structural mri or f - fdg pet found that c - pib retention was relatively stable over time , while neurodegenerative markers worsen in parallel with cognitive decline ( engler et al . , 2006 ; jack et al . , 2009 ; scheinin et al . , 2009 ; kadir et al . ,
these findings are consistent with the hypothesis that there is dissociation between the amyloid deposition and the neurodegenerative process , where amyloid accumulation precedes the clinical symptoms and reaches its maximum detectable level before the cognitive deficits are evident ( jack et al . , 2010a ) .
however , another recent study has shown increases of c - pib retention in mild to moderate ad dementia , questioning this hypothesis and suggesting that a deposition might slow down in the later stages of the disease , but it is still present ( villemagne et al . , 2011 ) . in mci patients ,
c - pib pet shows a bimodal distribution with some subjects having a pattern of retention similar to ad patients and others similar to control subjects ( pike et al . , 2007 ; rowe et al .
in these patients , the a burden is related to episodic memory impairment but not to other cognitive tasks ( pike et al . , 2007 ) .
the pib - positive mci subjects are more likely to carry an apoe4 allele and to progress to ad ( pike et al . , 2007 ;
factors that seem to influence a shorter time to progression in pib - positive subjects is the presence of hippocampal atrophy , higher c - pib retention values and having an apoe4 allele ( okello et al . , 2009 ;
( 2009 ) rescanned mci subjects after a single year , but only a minimal change has seen in this group of persons
2011 ) have shown small but significant increases in c - pib cortical retention in mci patients from the baseline studies to scans to those performed from 20  months to 3  years later ; other studies have reported similar results ( kadir et al . , 2010 ; koivunen et al . ,
, approximately 2030% of elderly cognitively normal subjects show some degree of c - pib retention , mainly in the prefrontal cortex , posterior cingulate and precuneus regions ( mintun et al . , 2006 ; pike et al .
, 2007 ; rowe et al . , 2007 ; aizenstein et al . , 2008 ) .
in fact , the proportion of c - pib - positive healthy control subjects increases with age ( morris et al . , 2010 ) . in these subjects ,
some studies have reported a strong relationship between impaired episodic memory performance and c - pib binding that could be modified by cognitive reserve ( pike et al .
2010 ) , but some others have not found this association ( mintun et al . , 2006 ; jack et al . , 2008b ; stornadt et al . , 2009 ) .
studies directly relating structural mri data to a burden in control subjects has yield contradictory results .
2009 ) , a cortical thinning pattern consistent with early ad ( dickerson et al . , 2008 ;
becker et al . , 2010 ) and an increased rate of brain atrophy ( scheinin et al . , 2009 ) , while others have found brain atrophy only in c - pib - positive subjects or in normal individuals with subjective cognitive impairment ( chetelat et al . ,
there are only two longitudinal studies reporting progression of cognitively normal subjects scanned with c - pib pet to symptomatic ad . in one study ,
2009 ) reported that the relative risk of conversion from cognitive normal to ad was increased almost five - fold with a c - pib - positive pet scan .
( 2011 ) performed serial c - pib pet scans in a group of normal subjects and found that 25% of individuals with a positive scan developed mci or ad dementia by 3  years .
in addition , the authors also reported an increase in c - pib brain retention in those control subjects with a positive baseline scan , suggesting a slow process of a accumulation in the brain over time ( villemagne et al .
these data are consistent with the hypothesis that amyloid imaging can detect a accumulation in advance of the onset of dementia ( jack et al . , 2010a ) , although more longitudinal studies are required to determinate the sequence of pathological events in the process from normalcy to ad .
figure 3 shows the spectrum of amyloid deposition in normal controls , mci and ad cases , as measured with c - pib pet ( mathis et al . , 2007 ) .
transaxial and sagittal planes of c - pib pet scans in a normal control ( nc ) , a c - pib - positive nc ( nc+ ) , c - pib - negative mci subject ( mci ) , a c - pib - positive mci subject ( mci+ ) , a highly c - pib - positive mci subject ( mci++ ) , and a c - pib - positive ad patient ( ad ) . reprinted from mathis et al .
although amyloid imaging is a relatively new technique , we can state that it accurately detects a accumulation in the brain .
however , it has been reported the case of a patient clinically diagnosed of mild ad with a negative c - pib pet scan , where low levels of amyloid pathology at autopsy were found ( cairns et al . , 2009 ) .
on the other hand , there are not reported cases of subjects with a positive amyloid pet without presence of a at autopsy .
these data suggests that c - pib pet is a technique with high specificity and positive predictive value for early ad diagnosis .
nevertheless , having an c - pib - positive scan does not mean to be at a pre - dementia stage of ad , and the prognostic value of increased a load on pet has to be established with more extensive longitudinal studies .
in this scenario , adding techniques that assess functional brain changes suggesting early ad could be of special interest .
definitely , the information obtained with current amyloid tracers will bring to light important issues in the biology of amyloid deposition in the transition from normal to ad .
current data suggest that diagnosis of ad can be enhanced by use of these promising biomarkers to increase accuracy and identify early stages of the disease .
each of these biomarkers seems to indicate a specific process in ad , so that amyloid imaging and decreased csf a42 are indicators of brain amyloid burden , while csf tau , brain atrophy and brain metabolism are biomarkers of the neurodegenerative process .
some authors have proposed a model of disease in which there is timing for the use of different biomarkers ( jack et al . , 2010a ) .
the ad pathologic cascade appears to have a sequential two stage process initiated by amyloid accumulation and followed by neuronal pathology . to validate this hypothesis and the right use of biomarkers in the clinical practice ,
it is necessary to determine the dynamics of the amyloid process with other biomarkers at the different stages of the disease .
an early ad diagnosis could enable to administer treatments with potential disease - modifying effect before neurodegeneration becomes severe .
the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 alzheimer s disease ( ad ) is the most common form of dementia in the elderly , and it is characterized by progressive impairment in multiple cognitive domains of sufficient severity to interfere with individuals daily living activities . 
 historically , the diagnosis of ad has been based on the identification of a clinical syndrome , and accuracy studies of the current clinical criteria conducted in referral clinics have shown high sensitivity for ad . however , the identification of the disease is still not perfect , and there is growing evidence that the use of biomarkers will increase our ability to better indentify the underlying biology of ad , especially in its early stages . 
 these biomarkers will improve the detection of the patients suitable for research studies and drug trials , and they will contribute to a better management of the disease in the clinical practice . in this review , we discuss the most studied biomarkers in ad : cerebrospinal fluid proteins , structural magnetic resonance imaging , functional neuroimaging techniques , and amyloid imaging . 
Introduction
Biomarkers in CSF
Structural Neuroimaging Markers
Functional Neuroimaging Markers
Molecular Neuroimaging Markers
Conclusion
Conflict of Interest Statement