Paramagnetic Chemical Exchange Saturation Transfer Agents Research Articles

Chemical exchange saturation transfer magnetic resonance imaging and its main and potential applications in pre-clinical and clinical studies.

Chemical exchange saturation transfer (CEST) imaging is a novel contrast mechanism, relying on the exchange between mobile protons in amide (-NH), amine (-NH2) and hydroxyl (-OH) groups and bulk water. Due to the targeted protons present in endogenous molecules or exogenous compounds applied externally, CEST imaging can respectively, generate endogenous or exogenous contrast. Nowadays, CEST imaging for endogenous contrast has been explored in pre-clinical and clinical studies. Amide CEST, also called amide proton transfer weighted (APT) imaging, generates CEST effect at 3.5 ppm away from the water signal and has been widely investigated. Given the sensitivity to amide proton concentration and pH level, APT imaging has shown robust performance in the assessment of ischemia, brain tumors, breast and prostate cancer as well as neurodegenerative diseases. With advanced methods proposed, pure APT and Nuclear Overhauser Effect (NOE) mediated CEST effects were separately fitted from original APT signal. Using both effects, early but promising results were obtained for glioma patients in the evaluation of tumor response to therapy and patient survival. Compared to amide CEST, amine CEST is also mobile proton concentration and pH dependent, but has a faster exchange rate between amine protons and water. The resultant CEST effect is usually introduced at 1.8-3 ppm. Glutamate and creatine, as two main metabolites with amine groups for CEST imaging, have been applied to quantitatively assess diseases in the central nervous system and muscle system, respectively. Glycosaminoglycan (Gag) as a representative metabolite with hydroxyl groups has also been measured to evaluate the cartilage of knee or intervertebral discs in CEST MRI. Due to limited frequency difference between hydroxyl protons and water, 7T for better spectral separation is preferred over 3T for GagCEST measurement. The applications of CEST MRI with exogenous contrast agents are still quite limited in clinic. While certain diamagnetic CEST agents, such as dynamic-glucose, have been tried in human for brain tumor or neck cancer assessment, most exogenous agents, i.e., paramagnetic CEST agents, are still tested in the pre-clinical stage, mainly due to potential toxicity. Engineered tissues for tissue regeneration and drug delivery have also shown a great potential in CEST imaging, as many of them, such as hydrogel and polyamide materials, contain mobile protons or can be incorporated with CEST specific chemical compounds. These engineered tissues can thus generate CEST effect in vivo, allowing a possibility to understand the fate of them in vivo longitudinally. Although the CEST MRI with engineered tissues has only been established in early stage, the obtained first evidence is crucial for further optimizing these biomaterials and finally accomplishing the translation into clinical use.

A Cu(II)2 Paramagnetic Chemical Exchange Saturation Transfer Contrast Agent Enabled by Magnetic Exchange Coupling.

The ability of magnetic exchange coupling to enable observation of paramagnetic chemical exchange saturation transfer (PARACEST) in transition metal ions with long electronic relaxation times (τs) is demonstrated. Metalation of the dinucleating, tetra(carboxamide) ligand HL with Cu(2+) in the presence of pyrophosphate (P2O7)(4-) affords the complex [LCu(II)2(P2O7)](-). Solution-phase variable-temperature magnetic susceptibility data reveal weak ferromagnetic superexchange coupling between the two S = 1/2 Cu(II) centers, with a coupling constant of J = +2.69(5) cm(-1), to give an S = 1 ground state. This coupling results in a sharpened NMR line width relative to a GaCu analogue, indicative of a shortening of τs. Presaturation of the amide protons in the Cu2 complex at 37 °C leads to a 14% intensity decrease in the bulk water (1)H NMR signal through the CEST effect. Conversely, no CEST effect is observed in the GaCu complex. These results provide the first example of a Cu-based PARACEST magnetic resonance contrast agent and demonstrate the potential to expand the metal ion toolbox for PARACEST agents through introduction of magnetic exchange coupling.

Screening CEST contrast agents using ultrafast CEST imaging

A chemical exchange saturation transfer (CEST) experiment can be performed in an ultrafast fashion if a gradient field is applied simultaneously with the saturation pulse. This approach has been demonstrated for studying dia- and para-magnetic CEST agents, hyperpolarized Xe gas and in vivo spectroscopy. In this study we present a simple method for the simultaneous screening of multiple samples. Furthermore, by interleaving a number of saturation and readout periods within the TR, a series of images with different saturation times can be acquired, allowing for the quantification of exchange rates using the variable saturation time (QUEST) approach in a much accelerated fashion, thus enabling high throughput screening of CEST contrast agents.

MRI CEST at 1T with large µeff Ln(3+) complexes T m(3+)-HPDO3A: An efficient MRI pH reporter.

Chemical exchange saturation transfer (CEST) sensitivity relies on the prototropic exchange rate kex between the agent and the "bulk" water protons. To exploit large kex, a large frequency separation (Δω) between the pools of exchanging protons is necessary. For this reason, high magnetic fields are preferred. Herein it is shown that the use of paramagnetic CEST agents based on lanthanide (III) ions with large effective magnetic moments allows the carrying out of CEST experiments at the relatively low field strength of 1 tesla (T). Measurements were performed on a 1T MR-scanner using continuous wave (cw)-presaturation with a spin echo sequence. ParaCEST complexes have been synthetized by mixing the ligand and Ln(III)Cl3 in a stoichiometric ratio at room temperature and pH 7. Different lanthanide chelates were investigated (Tm-, Dy-, Yb-, Eu-HPDO3A, and Eu-DOTAMGly). Ratiometric (Tm-HPDO3A) and selective detection (Eu-DOTAMGly and Tm-HPDO3A) experiments have been proven feasible in vivo. In vitro experiments demonstrated the feasibility of the CEST methodology at 1T for nearly every paraCEST candidate under investigation, except for Eu-HPDO3A. Among the studied compounds, Tm-HPDO3A proved suitable for the application of a ratiometric method for assessing pH both in vitro and in vivo.

Modulation of CEST Images in Vivo by T1 Relaxation: A New Approach in the Design of Responsive PARACEST Agents

A novel approach for the design of responsive paramagnetic chemical exchange saturation transfer (PARACEST) magnetic resonance imaging (MRI) agents has been developed where the signal is "turned on" by altering the longitudinal relaxation time (T1) of bulk water protons. To demonstrate this approach, a model Eu(DOTA-tetraamide) complex (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) containing two nitroxide free radical units was synthesized. The nitroxide groups substantially shortened the T1 of the bulk water protons which, in turn, resulted in quenching of the CEST signal. Reduction of paramagnetic nitroxide moieties to a diamagnetic species resulted in the appearance of CEST. The modulation of CEST by T1 relaxation provides a new platform for designing biologically responsive MRI agents.

Nanoparticle‐based chemical exchange saturation transfer (CEST) agents

The frequency-encoding property of chemical exchange saturation transfer (CEST) agents places them in a unique position among the MRI contrast agents, as it allows the visualization of more agents in the same MR image, as well as making it possible to set up innovative MRI-responsive agents. The sensitivity issue shown by molecular CEST agents (either diamagnetic or paramagnetic) has been tackled with the design of nanoparticle-based CEST agents endowed with a large number of exchangeable protons that ensure large saturation transfer levels. Several systems have been considered, namely supramolecular adducts, dendrimers, micelles and liposomes loaded with CEST agents (in most cases, paramagnetic CEST agents). A particularly sensitive system is represented by lipoCEST agents, consisting of liposomes whose inner water resonance is shifted by the co-presence of paramagnetic lanthanide complexes. The exchangeable pool of protons is represented by all the water molecules contained in the inner liposome cavity (10(6) -10(8) ). Although in vitro work has provided excellent results, in vivo translation appears to be hampered to some extent by the peculiar behavior shown by these particles on administration to living animals.

Advantages of paramagnetic chemical exchange saturation transfer (CEST) complexes having slow to intermediate water exchange properties as responsive MRI agents

Paramagnetic chemical exchange saturation transfer (PARACEST) complexes are exogenous contrast agents that have great potential to further extend the functional and molecular imaging capabilities of magnetic resonance. As a result of the presence of a central paramagnetic lanthanide ion (Ln(3+) ≠ La(3+) , Gd(3+) , Lu(3+) ) within the chelate, the resonance frequencies of exchangeable protons bound to the PARACEST agent are shifted far away from the bulk water frequency. This large chemical shift, combined with an extreme sensitivity to the chemical exchange rate, make PARACEST agents ideally suited for the reporting of significant biological metrics, such as temperature, pH and the presence of metabolites. In addition, the ability to turn PARACEST agents 'off' and 'on' using a frequency-selective saturation pulse gives them a distinct advantage over Gd(3+) -based contrast agents. A current challenge for PARACEST research is the translation of the promising in vitro results into in vivo systems. This short review article first describes the basic theory behind PARACEST contrast agents, their benefits over other contrast agents and their applications to MRI. It then describes some of the recent PARACEST research results: specifically, pH measurements using water molecule exchange rate modulation, T2 exchange contrast caused by water molecule exchange, the use of ultrashort TEs (TE < 10 µs) to overcome T2 exchange line broadening and the potential application of T2 exchange as a new contrast mechanism for MRI.

Magnetic resonance chemical exchange saturation transfer imaging and nanotechnology

Chemical exchange saturation transfer (CEST) agents and paramagnetic CEST (PARACEST) agents display bound water signals that exchange protons with the bulk water. CEST magnetic resonance imaging (MRI) relies on exchangeable protons that resonate at a chemical shift that is distinguishable from the bulk water signal. In some cases, paramagnetic chelates are utilized to shift the bound water frequency further away from the bulk water. Radiofrequency prepulses applied at the appropriate frequency can saturate the exchangeable protons, which transfer into the bulk water pool and lead to reduced equilibrium magnetization. Therefore, CEST and PARACEST agents allow the image contrast to be switched 'on' and 'off' by simply changing the pulse sequence parameters. One of the main limitations with this approach is the inherent insensitivity of MRI to CEST and PARACEST agents. Nanoscale carriers have been developed to improve the limit of detection for these agents, demonstrating the feasibility of in vivo molecular or cellular MRI based on CEST or PARACEST contrast. These carriers have been based on a number of different nanoparticle constructs, such as liposomes, dendrimers, polymers, adenovirus particles, and perfluorocarbon nanoparticles. The unique MRI properties of CEST and PARACEST nanoparticle systems have spawned research into an array of potential medical applications.

Chemical Exchange Saturation Transfer Is Unaffected by Modest Changes in Pressure

ParaCEST (paramagnetic Chemical Exchange Saturation Transfer) agents offer an unparalleled opportunity to perform quantitative molecular imaging by MRI. Agents that can alter the image contrast they generate in response to changes in local environmental parameters such as pH, glucose concentration or lactate concentration can be used ratiometrically to quantitatively describe the local tissue environment. However, when performing such quantitative measurements it is important that the results are not confounded by changes in a second environmental parameter. In vivo pressure varies quite considerably, both through the respiratory cycle and from tissue to tissue (tumors in particular have high interstitial pressures). Since paraCEST agents have positive activation volumes, their exchange kinetics and therefore the CEST effect that they generate are necessarily related to pressure. The purpose of this investigation was to examine whether the relatively small changes in pressure exhibited in vivo could affect CEST sufficiently to confound attempts to quantify other local environmental parameters. The CEST properties of a rigid EuDOTA-tetraamide was examined at temperatures ranging from 288 to 319 K, at applied pressures ranging from 0 to 414 kPa and pre-saturation (B1) powers ranging from 524 to 935 Hz. At no point was pressure found to affect the CEST generated by this chelate, indicating that changes in in vivo pressure is unlikely to confound the quantitative measurement of physiologically relevant parameters by paraCEST MRI.

Ytterbium-based PARACEST Agent: Feasibility of CEST Imaging on a Clinical MR Scanner

We investigated the feasibility of performing chemical exchange saturation transfer (CEST) imaging using ytterbium-based paramagnetic CEST (PARACEST) agents on a clinical magnetic resonance (MR) scanner. We prepared solutions of 3 different ytterbium-based PARACEST agents at concentrations of 5, 10, 20, and 50 mM at a pH of 7.4 and at a concentration of 50 mM at pHs of 3.0, 5.0, 7.4, and 9.5. We acquired images with a turbo spin echo technique using a quadrature head coil and a clinical 3.0-tesla MR system in accordance with the safety limits of the specific absorption rate (SAR). We acquired CEST images with presaturation offset frequencies from -5,000 Hz (-39.1 ppm) to 5,000 Hz (39.1 ppm) with an interval of 500 Hz (3.9 ppm) for each condition. We repeated each scan 3 times and then calculated the mean and standard deviations of the magnitude of the CEST effect at different concentrations and pH values for each agent. We used one-way analysis of variance and Tukey's honestly significant difference post hoc test to compare mean values of the magnitude of the CEST effect obtained at different concentrations and pH values. P < 0.05 was considered significant. PARACEST agents showed a strong CEST effect at their specific presaturation offset frequencies. For each agent, the CEST effect showed significant concentration dependency (P < 0.05), increasing with agent concentration, and significant pH dependency (P < 0.05), with strong effect near physiological pH. CEST imaging using ytterbium-based PARACEST agents might be feasible on a clinical MR scanner with further modifications, such as adjustments of the presaturation radiofrequency pulse and imaging protocols.

SWIFT‐CEST: A new MRI method to overcome T2 shortening caused by PARACEST contrast agents

The exchange of water molecules between the inner sphere of a paramagnetic chemical exchange saturation transfer (PARACEST) contrast agent and bulk water can shorten the bulk water T(2) through the T(2) -exchange (T(2ex) ) mechanism. The line-broadening T(2ex) effect is proportional to the agent concentration, the chemical shift of the exchanging water molecule, and is highly dependent on the water molecule exchange rate. A significant T(2ex) contribution to the bulk water linewidth can make the regions of agent uptake appear dark when imaging with conventional sequences like gradient-echo and fast spin-echo. The minimum echo times for these sequences (1-10 ms) are not fast enough to capture signal from the regions of shortened T(2) . This makes "Off" (saturation at -Δω) minus "On" (saturation at +Δω) imaging of PARACEST agents difficult, because the regions of uptake are dark in both images. It is shown here that the loss of bulk water signal due to T(2ex) can be reclaimed using the ultrashort echo times (<10 μs) achieved with the sweep imaging with Fourier transform pulse sequence. Modification of the sweep imaging with Fourier transform sequence for PARACEST imaging is first discussed, followed by parameter optimization using in vitro experiments. In vivo PARACEST studies comparing fast spin-echo to sweep imaging with Fourier transform were performed using EuDOTA-(gly) 4- uptake in healthy mouse kidneys. The results show that the negative contrast caused by T(2ex) can be overcome using the ultrashort echo time achieved with sweep imaging with Fourier transform, thereby enabling fast and sensitive in vivo PARACEST imaging.

Quantification of water exchange kinetics for targeted PARACEST perfluorocarbon nanoparticles

PARACEST (PARAmagnetic Chemical Exchange Saturation Transfer) agents offer the ability to generate "contrast on demand", negating the need to image before contrast agent injection. Perfluorocarbon (PFC) nanoparticles can deliver very large payloads of PARACEST agents, lowering the effective detection limit for molecular imaging of sparse biomarkers. Also, the PFC core provides a quantitative (19)F signal for measuring particle binding with high signal intensity and no background signal. (19)F quantization coupled with mathematical modeling of the PARACEST signal showed that incorporating PARACEST chelates onto the nanoparticle surface reduces the bound water lifetime and diminishes the available contrast to noise ratio compared to the parent small molecule PARACEST chelate. PARACEST nanoparticles were targeted to fibrin, an early biomarker for atherosclerotic plaque rupture, and bound to the surface of in vitro clots, yielding a detection limit of 2.30 nM at 11.7T. When the particles bind to a target surface, the image contrast is higher than predicted from phantom experiments, perhaps due to improved water exchange kinetics. We demonstrated that PARACEST PFC nanoparticles can provide two unique signatures, (19)F and PARACEST, for quantitative targeted molecular imaging of fibrin.

Europium(III) DOTA-Derivatives Having Ketone Donor Pendant Arms Display Dramatically Slower Water Exchange

A series of new 1,4,7,10-tetraazacyclododecane-derivatives having a combination of amide and ketone donor groups as side-arms were prepared, and their complexes with europium(III) studied in detail by high resolution NMR spectroscopy. The chemical shift of the Eu(3+)-bound water resonance, the chemical exchange saturation transfer (CEST) characteristics of the complexes, and the bound water residence lifetimes (τ(m)) were found to vary dramatically with the chemical structure of the side-arms. Substitution of ketone oxygen donor atoms for amide oxygen donor atoms resulted in an increase in residence water lifetimes (τ(m)) and a decrease in chemical shift of the Eu(3+)-bound water molecule (Δω). These experimental results along with density functional theory (DFT) calculations demonstrate that introduction of weakly donating oxygen atoms in these complexes results in a much weaker ligand field, more positive charge on the Eu(3+) ion, and an increased water residence lifetime as expected for a dissociative mechanism. These results provide new insights into the design of paramagnetic CEST agents with even slower water exchange kinetics that will make them more efficient for in vivo imaging applications.

In vivo detection of MRI‐PARACEST agents in mouse brain tumors at 9.4 T

Paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents are under development for biological target identification by magnetic resonance imaging. Image contrast associated with PARACEST agents can be generated by radiofrequency irradiation of the chemically shifted protons bound to a PARACEST contrast agent molecule or by direct irradiation of the on-resonance bulk water protons. The observed signal change in a magnetic resonance image after the administration of a PARACEST contrast agent is due to both altered relaxation time constants and the CEST effect. Despite high sensitivity in vitro, PARACEST agents have had limited success in vivo where sensitivity is reduced by the magnetization transfer effect from endogenous macromolecules. The purpose of this study was to demonstrate the in vivo detection of a PARACEST contrast agent using the on-resonance paramagnetic chemical exchange effect (OPARACHEE) in a mouse glioblastoma multiforme tumor model and to isolate the OPARACHEE effect from the changes in relaxation induced by the PARACEST agent. Three mice with tumors were imaged on a 9.4 T MRI scanner following tail vein injection of 150 μL 50 mM Tm(3+)-DOTAM-glycine-lysine. A fast low angle shot pulse sequence with a low power radiofrequency pulse train (WALTZ-16) as the preparation pulse was used to generate OPARACHEE contrast. To study the dynamics of agent uptake, reference images (without the preparation pulse) and OPARACHEE images were acquired continuously in an alternating fashion before, during and after agent injection. Signal intensity decreased by more than 10% in tumor in the control images after agent administration. Despite these changes, a clear OPARACHEE contrast of 1-5% was also observed in brain tumors after contrast agent injection and maintained in the hour following injection. This result is the first in vivo observation of OPARACHEE contrast in brain tumors with correction of T(1) and T(2) relaxation effects.

Calcium-responsive paramagnetic CEST agents

The assessment of changes in the extracellular calcium concentration by magnetic resonance imaging would be a valuable biomedical research tool to monitor brain neuronal activity. In this perspective, we report here the synthesis of novel ligands consisting of tetraamide and bisamide derivatives of cyclen, L(1) and L(2), respectively, each bearing imino(diacetate) moieties for Ca(2+) binding. Yb(3+) and Eu(3+) complexes are investigated as chemical exchange saturation transfer (CEST) agents that respond to the presence of Ca(2+). A CEST effect is observed for both YbL(1) and EuL(1) complexes (B=11.7T), originating from the slow exchange of the amide protons and those of the coordinated water, respectively, whilst no CEST is detected for complexes of L(2). Upon calcium binding, the CEST effect decreases considerably (from 60% to 20% for YbL(1) and from 35% to 10% for EuL(1)). A similar variation is observed in the presence of Mg(2+). The affinity constants between the lanthanide complexes and the alkaline earth metal ions have been estimated from the variation of the CEST effect to be K(YbL(1)-Ca)(aff) = 8 ± 2M(-1), K(YbL(1)-Mg)(aff) = 23 ± 3M(-1) and K(EuL(1)-Ca)(aff) = 10 ± 3M(-1). These low values imply the coordination of the alkaline earth ions to a single iminodiacetate arm. Ca(2+)/Mg(2+) binding to the lanthanide complexes slows down the exchange of the amide protons on YbL(1) which is responsible for the diminished CEST effect. This has been evidenced by assessing the proton exchange rates from the dependency of the CEST effect on the saturation time and the saturation power, in the absence and in the presence of Ca(2+) and Mg(2+). The applicability of the PARACEST MRI agents for Ca(2+) detection has been evaluated on a 16T MRI scanner.

In vivo detection of PARACEST agents with relaxation correction

Several pulse sequences have been used to detect paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents in animals to quantify the uptake over time following a bolus injection. The observed signal change is a combination of relaxation effects and PARACEST contrast. The purpose of the current study was to isolate the PARACEST effect from the changes in bulk water relaxation induced by the PARACEST agent in vivo for the fast low-angle shot pulse sequence. A fast low-angle shot–based pulse sequence was used to acquire continuous images on a 9.4-T MRI of phantoms and the kidneys of mice following PARACEST agent (Tm3+-DOTAM-Gly-Lys) injection. A WALTZ-16 pulse was applied before every second image to generate on-resonance paramagnetic chemical exchange effects. Signal intensity changes of up to 50% were observed in the mouse kidney in the control images (without a WALTZ-16 preparation pulse) due to altered bulk water relaxation induced by the PARACEST agent. Despite these changes, a clear on-resonance paramagnetic chemical exchange effect of 4-7% was also observed. A four-pool exchange model was used to describe image signal intensity. This study demonstrates that in vivo on-resonance paramagnetic chemical exchange effect contrast can be isolated from tissue relaxation time constant changes induced by a PARACEST agent that dominate the signal change. Magn Reson Med 63:1184–1192, 2010. © 2010 Wiley-Liss, Inc.

PARACEST Properties of a Dinuclear Neodymium(III) Complex Bound to DNA or Carbonate

A dinuclear Nd(III) macrocyclic complex of 1 (1,4-bis[1-(4,7,10-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane]-p-xylene) and mononuclear complexes of 1,4,7-tris-1,4,7,10-tetraazacyclododecane, 2, and 1,4,7-tris[(N-N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane, 3, are prepared. Complexes of 1 and 2 give rise to a PARACEST (paramagnetic chemical exchange saturation transfer) peak from exchangeable amide protons that resonate approximately 12 ppm downfield from the bulk water proton resonance. The dinuclear Nd(III) complex is promising as a PARACEST contrast agent for MRI applications, because it has an optimal pH of 7.5 and the rate constant for amide proton exchange (2700 s(-1)) is nearly as large as it can be within slow exchange conditions with bulk water. Dinuclear Ln(2)(1) complexes (Ln(III) = Nd(III), Eu(III)) bind tightly to anionic ligands including carbonate, diethyl phosphate, and DNA. The CEST amide peak of Nd(2)(1) is enhanced by certain DNA sequences that contain hairpin loops, but decreases in the presence of diethyl phosphate or carbonate. Direct excitation luminescence studies of Eu(2)(1) show that double-stranded and hairpin-loop DNA sequences displace one water ligand on each Eu(III) center. DNA displaces carbonate ion despite the low dissociation constant for the Eu(2)(1) carbonate complex (K(d) = 15 microM). Enhancement of the CEST effect of a lanthanide complex by binding to DNA is a promising step toward the preparation of PARACEST agents containing DNA scaffolds.

Targeted PARACEST nanoparticle contrast agent for the detection of fibrin

A lipid-encapsulated perfluorocarbon nanoparticle molecular imaging contrast agent that utilizes a paramagnetic chemical exchange saturation transfer (PARACEST) chelate is presented. PARACEST agents are ideally suited for molecular imaging applications because one can switch the contrast on and off at will simply by adjusting the pulse sequence parameters. This obviates the need for pre- and postinjection images to define contrast agent binding. Spectroscopy (4.7T) of PARACEST nanoparticles revealed a bound water peak at 52 ppm, in agreement with results from the water-soluble chelate. Imaging of control nanoparticles showed no appreciable contrast, while PARACEST nanoparticles produced >10% signal enhancement. PARACEST nanoparticles were targeted to clots via antifibrin antibodies and produced a contrast-to-noise ratio (CNR) of 10 at the clot surface.

MRI thermometry based on PARACEST agents.

A novel magnetic resonance imaging (MRI) thermometry technique is demonstrated in vitro based upon the use of a PARACEST (PARAmagnetic Chemical Exchange Saturation Transfer) agent. This new method takes advantage of the high concentration of bulk water (the readout signal for imaging) and the hyperfine frequency shift properties of PARACEST agents. For two prototypes, Dy(1)3+ and Eu(2)-, the chemical shifts (delta, in ppm) of the Ln3+-bound water molecules are linearly dependent on temperature (T, in degrees C) over the range of 20-50 degrees C (delta = 6.9 x T - 944.7 and delta = -0.4 x T + 64.6, respectively). This offers the exciting possibility of improving the temperature dependencies approximately 690- and approximately 40-fold over the most widely used water PRF thermometry (Proton Resonance Frequency: -0.01 ppm/ degrees C).

Numerical solution of the Bloch equations provides insights into the optimum design of PARACEST agents for MRI

Paramagnetic lanthanide complexes that display unusually slow water exchange between an inner sphere coordination site and bulk water may serve as a new class of MRI contrast agents with the use of chemical exchange saturation transfer (CEST) techniques. To aid in the design of paramagnetic CEST agents for reporting important biological indices in MRI measurements, we formulated a theoretical framework based on the modified Bloch equations that relates the chemical properties of a CEST agent (e.g., water exchange rates and bound water chemical shifts) and various NMR parameters (e.g., relaxation rates and applied B(1) field) to the measured CEST effect. Numerical solutions of this formulation for complex exchanging systems were readily obtained without algebraic manipulation or simplification. For paramagnetic CEST agents of the type used here, the CEST effect is relatively insensitive to the bound proton relaxation times, but requires a sufficiently large applied B(1) field to highly saturate the Ln(3+)-bound water protons. This in turn requires paramagnetic complexes with large Ln(3+)-bound water chemical shifts to avoid direct excitation of the exchanging bulk water protons. Although increasing the exchange rate of the bound protons enhances the CEST effect, this also causes exchange broadening and increases the B(1) required for saturation. For a given B(1), there is an optimal exchange rate that results in a maximal CEST effect. This numerical approach, which was formulated for a three-pool case, was incorporated into a MATLAB nonlinear least-square optimization routine, and the results were in excellent agreement with experimental Z-spectra obtained with an aqueous solution of a paramagnetic CEST agent containing two different types of bound protons (bound water and amide protons).