lidge, U

article information

Article history:Received 20 November 2009Received in revised form22 April 2010Accepted 30 April 2010

1960s, NMR was used to evaluate a wide variety ofsubstances and tissues, but it was not until the 1970s thatthe medical applications of NMR became realized.4 The

the typical resolution of clinical MRI is still several orders ofmagnitude larger than the size of a cell, which in turn isseveral orders of magnitude larger than a biological mole-cule.6 However, in isolated tissue specimens it is possible toproduce isotropic MR images at a resolution of 20 mm,which is comparable to the size of a cell.6 This method, oftentermedMRmicroscopy, has not been translated into humanimaging and is limited by practical considerations, such asacquisition time.

* Guarantor and correspondent: F. A. Gallagher, Department of Radiology,University of Cambridge, Box 218 Level 5, Addenbrookes Hospital, Cam-bridge CB2 0QQ, UK. Tel.: 44 (0)1223 336890; fax: 44 (0)1223 330915.

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Clinical Radiology 65 (2010) 557e566E-mail address: fag1000@cam.ac.ukhuman imaging and some are used routinely in many radiology departments. 2010 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

Background

Molecular and functional imaging using MRI is based onthe established principles of nuclear magnetic resonance(NMR), which have been used to analyse molecular struc-tures since the 1940s.1e3 Throughout the 1950s and the

major breakthrough for medical imaging came in 1973when Paul Lauterbur demonstrated that the origin of theNMR signal could be distinguished in three-dimensionalspace.5

Although great advances have been made towardsincreasing the spatial resolution of the images produced,0009-9260/$ e see front matter 2010 The Royal Codoi:10.1016/j.crad.2010.04.006Magnetic resonance imaging (MRI) has been applied to many aspects of functional andmolecular imaging. Many of the parameters used to produce image contrast in MRI areinuenced by the local chemical environment around the atoms being imaged; theseparameters can be exploited to probe the molecular content of tissues and this has been shownto have many applications in radiology. Diffusion-weighted imaging is a well-establishedmethod for measuring small changes in the molecular movement of water that occursfollowing the onset of ischaemia and in the presence of tumours. Exogenous contrast agentscontaining gadolinium or iron oxide have been used to image tissue vascularity, cell migration,and specic biological processes, such as cell death. MR spectroscopy is a technique formeasuring the concentrations of tissue metabolites and this has been used to probe metabolicpathways in cancer, in cardiac tissue, and in the brain. Several groups are developing positron-emission tomography (PET)-MRI systems that combine the spatial resolution of MRI with themetabolic sensitivity of PET. However, the application of MRI to functional and molecularimaging is limited by its intrinsic low sensitivity. A number of techniques have been developedto overcome this which utilize a phenomenon termed hyperpolarization; these have been usedto image tissue pH, cellular necrosis, and to image the lungs. Although most of these appli-cations have been developed in animal models, they are increasingly being translated intoDepartment of Radiology, UniversityInstitute, Li Ka Shing Centre, Cambrmbridge, Addenbrookes Hospital, Cambridge, and Cancer Research UK Cambridge ResearchKof CaAn introduction to functionawith MRIF.A. Gallagher*llege of Radiologists. Published byand molecular imaging

le at ScienceDirect

diology

ierheal th.com/journals /cradElsevier Ltd. All rights reserved.

can be detected with imaging and report on fundamental

adiobiological states, such as the presence of disease, itsseverity, or response to therapy.12 Although most of theseapplications have been developed in animal models, theyare increasingly being translated into human imaging andsome are used routinely in many clinical centres.

Dynamic contrast-enhanced MRI (DCE-MRI)

DCE-MRI is performed following injection of intravenouscontrast medium and is used to assess tissue vascularity.Most MRI contrast agents use gadolinium, which shortensthe T1 relaxation time of the adjacent protons in water andproduces increased signal on T1-weighted imaging (the T2relaxation time will also be shortened).7 Free gadoliniumions are toxic and therefore they are combined witha chelate such as diethylene triamine penta-acetic acid(DTPA).7 DCE-MRI uses either low molecular weight agents(30,000 Da) thatdemonstrate prolonged intravascular retention.13 Theformer are in widespread clinical use and will be discussedhere. MRI has a number of advantages over other tech-niques in imaging vascularity including its high spatialresolution, absence of ionizing radiation, and high signal-to-noise and contrast-to-noise ratios.13

Most radiologists will be very familiar with the use ofdynamic contrast agents for both CT and MRI, althoughinterpretation is usually qualitative or semi-quantitative inthe clinical setting. Following injection, contrast medium istransiently conned within the vessels and can be used todemonstrate perfusion. Low molecular weight agents willDespite the limits of spatial resolution, MRI is particu-larly suited tomolecular imaging as many of the parametersthat are used to produce contrast, such as the spin-latticerelaxation (T1) and spinespin relaxation (T2) times, aredependent on the local chemical structure of the moleculesbeing imaged.7,8 Consequently, these parameters can beexploited to reect the molecular content of the tissuebeing imaged. Furthermore, this can be performed non-invasively in the absence of exogenous contrast media,unlike many other approaches for molecular imaging. Inparticular, these parameters can be used to image thespatial distribution of fat and water, which has manyapplications in medicine.9

More recent developments in the eld of MRI havefacilitated imaging of tissues, cells and molecules.10 Thisreview is a very brief introduction to these techniques:from perfusion and diffusion imaging, to methods thatprobe more specic molecular processes such as cellsurface molecules, enzyme activity, and intracellularevents. The biodistribution of molecular imaging probesshould be more specic in diagnosing and assessingdisease than the morphological information acquiredusing anatomical imaging alone.11 The term imagingbiomarkers is often used to describe objective anatomical,physiological, biochemical or molecular parameters that

F.A. Gallagher / Clinical R558then pass into the extravascular and extracellular space(EES) and the rate at which this occurs is dependent on thepermeability of the vessels, their surface area, and the rateat which blood ows through them.13,14 T1-weightedimages are acquired before injection of the contrast agentand continue to be acquired during uptake into the tissueand during washout.15 However, unlike CT, the relationshipbetween signal intensity and contrast medium concentra-tion using MRI is not linear.15 DCE-MRI derives quantitativedata using models which attempt to represent mathemat-ically the distribution of contrast medium as it ows intothe organ of interest, across the vessel wall and into theextracellular compartment.16 A simple approach is touse the initial area under the curve (IAUC), which describesthe shape of the graph of contrast agent concentration overtime; although this is frequently used in trials, it is difcultto interpret physiologically.17,18

An alternative is to use a simple pharmacokinetic modeldescribing physiological parameters such as: the trans-endothelial transport of the contrast agent by diffusion,termed the volume transfer constant (Ktrans); the volume ofthe EES or leakage space (ve); and the rate constant betweenEES and plasma (kep).15 These three are related by theequation:

kep Ktrans/veDCE-MRI exploits the fact that the onset of many

diseases is associated with an alteration in vascular density,vascular permeability, and blood ow. In particular,tumours develop a network of new vessels as they grow, butunlike normal vasculature, tumour angiogenesis is chaoticand inefcient with permeable vessels.19

In general, malignant tissues enhance early followingintravenous contrast medium with rapid and largeincreases in signal intensity on T1-weighted imagingcompared to benign lesions.20,21 Tumours that show anelevated Ktrans often have a poorer prognosis,22 but in somestudies the opposite has been shown to be true.23 There isstronger evidence that Ktrans can be used to demonstratewhich tumours are responding to therapy as a biomarker ofdrug activity, particularly in the context of antiangiogenicdrugs or vascular disrupting agents (Fig. 1).17,18,24 Changesin Ktrans have been shown to correlate both with theadministration of vascular endothelial growth factor (VEGF,a signalling molecule that stimulates the growth of newblood vessels), as well as the administration of therapeuticmonoclonal antibodies that block its effect.25 This demon-strates that DCE-MRI is an indirect tool for molecularimaging, as well as a functional imaging tool, and can beused as part of a multimodal platform to understand drugand tumour interaction.26

Another approach has been to use dynamic susceptibilitycontrast MRI or DSC-MRI. Paramagnetic contrast agentsproduce magnetic eld inhomogeneities that decrease thesignal intensity of surrounding tissues and this effect canbeobservedwith susceptibility T1-or T2*-weighted sequences;the latter provides greater sensitivityand is sometimes termedT2*-weighted DCE-MRI.13 The degree of signal loss is depen-dent on the concentration of the contrast agent as well as

logy 65 (2010) 557e566vessel size and density and, therefore, this can be used to

impd 3se in

adioestimate the relative blood volume (rBV) of the tissue underassessment.27 Using this technique, changes in relative cere-

Figure 1 Demonstration of DCE-MRI. (a) False-colour Ktrans map supera patient with a known hepatic metastasis. (b) The Ktrans was reduceblocks VEGF. The colour scale for Ktrans is shown. (c) Percentage decreaManchester.F.A. Gallagher / Clinical Rbral blood volume (rCBV) maps have been correlated withgliomagrade28andthis approachcanbeused totargetabiopsyto areas of potential dedifferentiation within the tumour. Themethod has also been used to distinguish radiation necrosisfrom recurrent disease, to determine response to therapy, andas a prognostic marker.29,30

Drawing denitive conclusions about the role of DCE-MRI in clinical practice has been hampered by the relativelysmall number of patients in many trials, the use of varyingimaging parameters and acquisition techniques, as well asthe use of diverse disease endpoints. Current attempts tostandardize DCE-MRIwill help to address these issues in thefuture. Although DCE-MRI has shown much promise, it hasyet to be incorporated into routine clinical practice.

Diffusion-weighted imaging (DWI)

DWI is based upon the thermal movement of molecules,which is random and often referred to as Brownianmotion.8

If the diffusion constant is high, then the molecules willdiffuse further in a xed time interval compared to mole-cules with a lower diffusion constant.7 Conversely, bymeasuring how far a molecule moves in a xed timeinterval, the diffusion constant can be calculated. DWIusually employs magnetic eld gradients, which sensitizea spin-echo sequence to these small molecular move-ments.8 The b-value (in s/mm2) is a composite term, whichis dependent on a number of parameters that dene thesegradients.7 In the case of freely mobile water, signal will belost due to the random phase shifts acquired by the water

osed over an axial, T2-weighted image through the upper abdomen ofdays after treatment with bevacizumab, a monoclonal antibody thatKtrans over time. Images courtesy of Dr James OConnor, University of

logy 65 (2010) 557e566 559protons. These distances are very small: water, if unre-stricted, will diffuse 20e30 mm in 50e100 ms, which is thetypical timescale for DWI.8,31 Although DWI has a spatialresolution in the millimetre range, it is a totally non-inva-sive method for reporting on a molecular movement, whichis several orders of magnitude smaller.

Watermovement in the extracellular space is restricted bythe surrounding cells, and therefore the measured diffusionconstant is an apparent (rather than a real) value; this istermed the apparent diffusion co-efcient (ADC) and iscalculated from the slope of relative signal intensity (ona logarithmic scale) against a series of b-values. There isincreasing evidence to suggest that DWI and the calculatedADC correlatewith tissue cellularity: water-containing tissueswith few cells have mobile molecules, which give lower DWIsignal intensity thanmore solid tissueswith high cellularity.32

One early discovery was that DWI could be used to detectbrain ischaemia before any changes could be detected withconventional MRI methods;33 the increase in DWI signalintensity that is seen may reect cell swelling with a shift ofwater from an unrestricted diffusion environment into therestricted intracellular space.8 Recent technical advanceshave allowed DWI to be applied towhole-body imaging andit is now increasingly used in oncological imaging (Fig. 2).32

Many cancers demonstrate restricted diffusion (or highsignal intensity on DWI), which has classically been attrib-uted to the increased cellular density seen within tumours,but other factors are likely to play a role such as the tor-tuousity of the extracellular space, extracellular brosis, and

adioF.A. Gallagher / Clinical R560the shapeand size of the intercellular spaces.34DWIhasbeenused to differentiate benign frommalignant lesions (Fig. 2b),to detect tumour recurrence, and to monitor response tochemotherapy; successful treatment is generally reectedby an increase in the ADC value, although transient earlydecreases can occur following treatment.32,34,35

The non-invasive nature of DWI makes it an attractiveclinical technique but its full clinical potential has yet to beexplored and further work is required to understand thecomplicated interplay between ADC and the biophysicaland histological environment.

Magnetic resonance spectroscopy (MRS)

Certain nuclei will resonate in a magnetic eld ata frequency that is determined by the local chemical envi-ronment around them. For example, the two hydrogen

Figure 2 Demonstration of DWI in oncology. (a) An axial, T2-weightedintensity within the right peripheral zone of the prostate in a patient wi(TR) 3711 ms]. (b) The equivalent diffusion-weighted image shows signthis corresponded to an area of low signal on ADC (not shown). These feacourtesy of Dr Evis Sala, University of Cambridge. (c) Whole-body diffusiowoman with metastatic breast cancer. This has been shown as an invertareas representing areas of greater cell density, e.g., normal spleen, spMetastatic disease is seen in the liver with scattered metastases in the rImage courtesy of Dr Anwar Padhani, Paul Strickland Scanner Centre, Mology 65 (2010) 557e566nuclei (1H or protons) in each molecule of water havea different resonant frequency from the hydrogen nuclei infat and this can be exploited in fat-suppressed imaging. Thisprinciple allows MRS to probe many metabolites simulta-neously; the relative size of eachpeak in a spectrumacquiredfrom a volume of tissue is proportional to the concentrationof the metabolite it represents. Common metabolites thatcan be identied are: choline-containing molecules (Cho),creatine (Cr), phosphocreatine (PCr), and N-acetylaspartate(NAA).8 NAA is present predominately in neurons and theloss of NAA is associated with neuronal damage as occurs instroke, or neuronal loss as in the presence of an intracranialtumour (Fig. 3).36e38 Choline is part of the lipid biosynthesispathway and because tumours contain a higher proportionof lipids than normal tissue, the choline-containing metab-olite peak is a dominant feature of most tumour spectra.8

Furthermore, a decrease may be seen following successfultreatment with a chemotherapeutic agent.39

image through the pelvis showing an area of ill-dened low signalth a known prostate cancer [echo time (TE) 86 ms, repetition timeicant high signal intensity in this area (b-value of 1400 s/mm2) andtures are consistent with restricted diffusion within a tumour. Imagesn study at a b-value of 800 s/mm2 in a separate patient, a 70-year-olded greyscale maximum intensity projection (MIP) image with darkerinal cord, lymph nodes in the neck, axillae, groin, and spinal cord.ibs and lumbar spine; these were difcult to identify with CT alone.unt Vernon Hospital, London.

tero(TEin

n C

adioMRS can be applied to molecules that contain nucleiother than protons, such as phosphorus-31 (31P), and uo-rine-19 (19F). 31P is particularly useful in probing cellularenergetics as it can detect the basic energy unit within thecell, adenosine triphosphate (ATP). 31P-MRS can be used todetect phosphomonoesters (PME), which are commonlyelevated in tumours; this has been interpreted as an alter-ation in membrane metabolism.40 The PME peak is oftenused as an indicator of tumour aggressiveness and itdecreases in response to therapy.41 31P-MRS can also beused to measure intracellular pH using the pH-sensitiveinorganic phosphate (Pi) peak; this has been used to show

Figure 3 Investigation of a grade 2 astrocytoma using MRS. (a) The heand the six highlighted voxels on the T2-weighted image are displayedThe ratio of NAA:Cr declines and Cho:Cr increases from normal brainMcLean, Cancer Research UK Cambridge Research Institute and Dr Justifrom McLean and Cross.80F.A. Gallagher / Clinical RpH changes in a number of conditions including somemuscle disorders.42e44 Finally, 19F-MRS has been utilized tomonitor the metabolism of uorinated drugs, such as 5-uorouracil, and may be able to identify patients who arelikely to respond to the drug.40

MRS is attractive as a non-invasive method formeasuring metabolism but due to its low sensitivity, thespectra are acquired at relatively low spatial resolution. It isnot widely used yet as a routine clinical tool.

Targeted contrast agents and CEST agents

Water-soluble contrast media are used in DCE-MRI,which consequently reects the movement of free water.An alternative approach is to link a gadolinium chelateto a probe that will target a specic molecule of bio-logical interest. A relatively high concentration ofcontrast agent (0.01e0.1 mM) is necessary to producea local alteration in the water signal intensity and,therefore, amplication strategies are required to accu-mulate a large number of gadolinium ions at the site ofinterest; these carriers include micelles, liposomes, orbiological structures, such as apoferritin andlipoproteins.45 Apoferritin (ferritin that is not combinedwith iron) is particularly efciently at trapping gadoli-nium chelates, and this can be incorporated into hepa-tocytes via the ferritin receptors.46,47

Gadolinium-based agents have been produced thatrespond to changes in pH, calcium levels, anion concen-trations, enzyme activity, and protein-binding.48 Somespecic examples of agents that are targeted to biologicalmolecules in animal studies are listed below. Phosphati-dylserine is a phospholipid that is normally found on theinner leaet of the plasma membrane; as a cell undergoesapoptosis or programmed cell death, it becomes expressed

geneity of metabolite distribution was studied using a 1616 matrix 109 ms, TR 4630 ms). (b) Spectra from these six voxels are shown.voxel 7 to tumour in voxels 11 and 12. Images courtesy of Dr Maryross, Addenbrookes Hospital Cambridge; reproduced with permission

logy 65 (2010) 557e566 561on the outer leaet of the membrane. MRI contrast agentshave been developed that will bind to this externalizedphosphatidylserine providing a tool to image cell death(Fig. 4).49,50 Another example of a targeted agent is thespecic binding of labelled nanoparticles to brin whichhas been used to detect thrombus within vulnerableatherosclerotic plaque.51 Anti-carcinoembryonic antigen(CEA) antibodies coupled to a gadolinium chelate havebeen developed which potentially could be used fortumour detection.52 Finally, the formation of new bloodvessels has been targeted using an MRI-specic agent thatbinds to an endothelial marker known as aVb3 integrin andthis has been shown to correlate with tumour grade.53

These are just a few examples of targeted MR agents indevelopment.

Chemical exchange saturation transfer (CEST) agentsrepresent a new class of MRI probes that can be switchedon and off on demand.48 A large sensitivity enhance-ment is obtained by exploiting a contrast agent (or anendogenous molecule) that undergoes rapid protonexchange with the high concentration of water. A selectivelow-power radiofrequency pulse is applied to the CESTagent which eliminates, or saturates, the NMR signal.However, given that this agent is at a low concentration in

axiaareene ont unenan

adioa biological system, the signal intensity will be difcult todetect and therefore an alternative method is required tovisualize it. The protons in the surrounding water are easilydetectable and as they are exchanging with the CEST agent,the saturation will also be transferred to this bulk water

Figure 4 Demonstration of a targeted MRI contrast agent. Images ofsuperimposed upon the greyscale proton MR images and the tumoursa high concentration of contrast medium as shown by the yellow/greintravenous injection of a contrast agent targeted to phosphatidylserin(a) and (b) have been treated with chemotherapy; (c) and (d) represenform of the contrast agent, whereas those in (b) and (d) have been givthe treated tumour (a) indicating cell death. Images courtesy of Dr Anet al.50F.A. Gallagher / Clinical R562producing a visible reduction in signal intensity (Fig. 5).45

Therefore, changes in the intensity of the water signal areindirectly reporting on the molecule of interest and theseCEST agents are particularly promising as sensors ofthe biological environment.

These various approaches could be used in the future todetect specic cellular processes or to demonstrateresponse to therapy at a very early stage. However, fora targeted MRI agent to be used in patients, it has to beeffectively cleared from the body and, therefore, few ofthese methods have been used in humans to date.

Cell labelling and gene expression with MRI

Contrast agents incorporating superparamagnetic ironoxide (SPIO) nanoparticles have shown promise as a meansto image labelled cells using MRI.54 They are usuallyinjected as carbohydrate-coated particles measuringapproximately 50e100 nm in diameter that can be trans-ported across cell membranes.55 Even at very lowconcentration, SPIOs create magnetic eld inhomogeneity;this dephases the protons which reduces the signalintensity seen on T2*- or T2-weighted images.7 Cells can belabelled by incubating them in contrast medium outsidethe body56 or alternatively, macrophages and other similarcells can be imaged following the intravenous injection ofcontrast medium.57Cell labelling has allowed the fate of transplanted cells tobe imaged in living organisms. This has been applied toneurological disorders,56 cardiac disease,58 the study ofpancreatic tissue59 as well as to imaging the immunesystem and stem cells.60 SPIO-labelled cells have been used

l sections through mice with lymphoma. Colour T1 maps have beenoutlined in white. A low T1 value corresponds to the accumulation ofcolour within the tumour. Images have been acquired following thethe cell surface, which is expressed during cell death. The animals intreated controls. Mice in (a) and (c) have been administered an activean inactive form. The active contrast agent selectively accumulates int Krishnan, University of Cambridge. For further details see Krishnanlogy 65 (2010) 557e566in humans to track dendritic cells61 and pancreatic isletgrafts.62

Anothermajor goal of molecular imaging is to allow geneexpression to be imaged non-invasively, and in particular, toimage genes that have been articially introduced into cellsto report on a specic molecular event. Measuring geneexpression has traditionally been carried out by extractingribonucleic acid (RNA) or protein, but clearly this is invasiveand cannot easily provide temporal or spatial information.There have been a number of approaches to image geneexpression using MRI:63 (1) expression of proteins that altermagnetic susceptibility, such as ferritin, or expression ofproteins that shuttle superparamagnetic nanoparticles intocells, such as the transferrin receptor;54 (2) expression ofsurface receptors that enable binding of a specic contrastagent, for instance, overexpression of the inammatoryadhesion molecule ICAM-1 can be detected with antibodiesconjugated to iron oxide;64 (3) enzymatic cleavage ofgroups that alter the water exchange of an MR contrastagent.65 However, imaging of gene expression using MRI isstill some way from being translated into routine use forpatients.

PET-MRI

Recently, there has been an interest in combining thehigh sensitivity of metabolic imaging using PET with the

prior to PET acquisition69 or can be split to incorporate thePMTs between the two halves (Fig. 6).70 A nal approach isto use new semiconductor-based PMTs, which are lesssensitive to the magnetic eld.66 However, all of theseapproaches are expensive and lack CT for attenuation-correction calculations. A recent study of the human brainhas been performed using PET detectors integrated witha clinical MR system.71 PET-MRI is likely to remaina research tool in specialist centres for the foreseeable

adiology 65 (2010) 557e566 563F.A. Gallagher / Clinical Rhigh spatial and contrast resolution of MRI. PET-CT is nowan important clinical tool but PET-MRI would havea number of advantages, including a reduction in radiationdose and improved soft-tissue resolution.66,67 The majorproblem for PET-MRI is that conventional photomultipliertubes (PMTs) are very sensitive to even weak magneticelds.67 To overcome this, optical bres have been used totransmit the scintillation light outside the magnetic eld tothe PMTs, although this may degrade sensitivity.67,68

Alternatively, the magnet can be designed to shut down

future.

Hyperpolarization techniques

Although MRI has many advantages for molecularimaging, its major disadvantage is low sensitivity todetection. To put this into context, when protons areplaced into a magnetic eld they enter one of two energylevels; approximately half will enter the lower energylevel and approximately half will enter the higher energylevel. There is a small difference between these two pools,which is typically of the order of a few parts per million,and it is this small number of protons that is used toproduce MR images. Therefore, MRI is fundamentallyinsensitive when compared with other techniques such asPET.

Figure 5 Schematic to explain CEST and how it can be switched onand off. (a) Two proton pools are shown which represent the lowconcentration of the CEST agent and the high concentration of bulkwater; protons are constantly exchanging between the two pools. (b)A selective radiofrequency pulse saturates, or eliminates, the CESTsignal. (c) As the protons are exchanging with bulk water, the satu-ration is also transferred to the water; the resulting drop in the watersignal can be used as an indirect marker of the presence of theoriginal molecule.Figure 6 PET-MRI system. This novel 1 T magnet is split in two andthe photomultiplier tubes are seen outside the magnet with bre-optics bundles connecting them to the scintillating crystal blocks inthe centre. Image courtesy of Dr Rob Hawkes and Prof. Adrian

Carpenter, University of Cambridge.

adioF.A. Gallagher / Clinical R564If the population difference between the two energylevels described above could be enhanced, then the sensi-tivity of MRI could be signicantly improved. A number oftechniques have been described to do this and the processinvolved is termed hyperpolarization; for instance, thenoble gases helium and xenon can be hyperpolarized usinglasers and hyperpolarized helium has been used to inves-tigate pulmonary structure and function.72

More recently, a technique termed dynamic nuclearpolarization (DNP) has been described that allowscarbon-13 (13C) containing molecules to be hyper-polarized and this can produce an increase in sensitivityof >10,000-fold.73 This large increase in signal allows thespatial distribution of an injected molecule to be imaged,as well as the metabolites formed from it (as they reso-nate at different frequencies). This technique has beenapplied to several endogenous molecules and has beenused to image a number of basic biological phenomena,such as pH, tumour response to chemotherapy, cellproliferation, and necrosis (Fig. 7).74e77 The major limi-tation of all forms of hyperpolarization is the short half-life of the hyperpolarized state, which is often less thana minute, and therefore requires rapid image acquisition.Hyperpolarized carbon-13 imaging is currently under-going clinical trial.78

Figure 7 Demonstration of how hyperpolarized MRI can be used to imagelabelled sodium bicarbonate. Axial sections through a mouse with an impGreyscale proton MR image. (b) The pH map is superimposed over the prohas been derived from the spatial distribution of bicarbonate (c) and cacourtesy of Dr Mikko Kettunen and Prof Kevin Brindle and reproduced wlogy 65 (2010) 557e566Conclusion

The basic principles of NMR have allowed MRI to beexploited for molecular and functional imaging. MRI isa versatile technique that can be used to image tissueanatomy, probe vascularity and water diffusion, as well asto image specic molecular targets on or within cells. Ingeneral, functional and molecular techniques probeprocesses that require amplication for detection. There-fore, they represent a compromise between spatial resolu-tion and sensitivity; instead of traditional high spatialresolution MRI, lower resolution parameter maps or spectraare produced. Consequently, if these techniques are to nda place in routine clinical practice it is likely that functionalandmolecular MR images will be combinedwith traditionalanatomical imaging in much the same way that PET and CTare co-registered in PET-CT.

Many of these techniques, such as DCE-MRI and DWI,are already used for clinical imaging and some of the morenovel methods are undergoing clinical assessment.However, translating an interesting pre-clinical tool intoa routine clinical technique is not a trivial process and maytake many years of development. Some of the techniquesdescribed here require extra hardware, which may rangefrom a simple coil to purchasing an additional magnet.

tumour pH following the intravenous injection of hyperpolarized 13C-lanted subcutaneous lymphoma tumour, which is outlined in red. (a)ton image as a false-colour map; the tumour is acidic (green). The pHrbon dioxide (d) using the HendersoneHasselbalch equation. Imageith permission from Gallagher et al.75

as DWI) often require dedicated software, sequence

agents, the pharmaceutical industry, as well as govern-

F.A. Gallagher / Clinical Radiology 65 (2010) 557e566 565mental and non-governmental funding bodies. A jointapproach will greatly accelerate the translation of newideas from the bench to the bedside, particularly in thecase of relatively expensive imaging technologies such asPET and MRI. Furthermore, it seems likely that one of therst uses of these techniques will be in early drug devel-opment where they can provide a means to assess drugfunction in a cost-effective way.

Although this translation of functional and molecularimaging to routine use in radiology departments facesmanychallenges, it promises to offer powerful tools toaid diagnosis, identify disease heterogeneity, predictoutcome, target biopsies, and determine treatment responsenon-invasively. The latter may be particularly important asmany novel cancer therapies do not produce signicantchanges in tumour size and conventional response criteriaare not always appropriate to assess disease response.79

When will these techniques become a standard clinicaltool for most radiologists? The answer to this question isdifcult to predict but history suggests that it may be rapid.Paul Lauterbur published his seminal paper demonstratingthe feasibility of MRI in 1973 and within 30 years, MRIprogressed from an experimental tool to widespread clin-ical use.5 If these new techniques can be shown to havea signicant benet for patient diagnosis and management,then they soon could be appearing in the daily requests thatarrive in a radiology department.

Acknowledgements

The author holds funding from Cancer Research UK, theRoyal College of Radiologists UK, the National Institute forHealth Research Cambridge Biomedical Research Centre,and the School of Clinical Medicine at the University ofCambridge. The author is grateful to Dr James OConnor ofthe University of Manchester, for providing helpfulcomments on the manuscript.

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An introduction to functional and molecular imaging with MRIBackgroundDynamic contrast-enhanced MRI (DCE-MRI)Diffusion-weighted imaging (DWI)Magnetic resonance spectroscopy (MRS)Targeted contrast agents and CEST agentsCell labelling and gene expression with MRIPET-MRIHyperpolarization techniquesConclusionAcknowledgementsReferences