Quantifying exchange rates in chemical exchange saturation transfer agents using the saturation time and saturation power dependencies of the magnetization transfer effect on the magnetic resonance imaging signal (QUEST and QUESP): Ph calibration for poly-L-lysine and a starburst dendrimer

pCEST: Positive contrast using Chemical Exchange Saturation Transfer

Chemical Exchange Saturation Transfer (CEST) contrast utilizes selective pre-saturation of a small pool of exchanging protons and subsequent detection of the decrease in bulk water signal. The CEST contrast is negative and requires detection of small signal change in the presence of a strong background signal. Here we develop a Positive CEST (pCEST) detection scheme utilizing the analogous nature of the CEST and off-resonance T(1)(ρ) experiments and exploring increased apparent relaxation rates in the presence of the selective pre-saturation. pCEST leads to the positive contrast, i.e., increased signal intensity as the result of the presence of the agent and RF pre-saturation. Simultaneously substantial background suppression is achieved. The contrast can be switched "ON" and "OFF", similar to the original CEST.

Imaging in vivo extracellular pH with a single paramagnetic chemical exchange saturation transfer magnetic resonance imaging contrast agent

The measurement of extracellular pH (pHe) has potential utility for cancer diagnoses and for assessing the therapeutic effects of pH-dependent therapies. A single magnetic resonance imaging (MRI) contrast agent that is detected through paramagnetic chemical exchange saturation transfer (PARACEST) was designed to measure tumor pH(e) throughout the range of physiologic pH and with magnetic resonance saturation powers that are not harmful to a mouse model of cancer. The chemical characterization and modeling of the contrast agent Yb(3+)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid,10-o-aminoanilide (Yb-DO3A-oAA) suggested that the aryl amine of the agent forms an intramolecular hydrogen bond with a proximal carboxylate ligand, which was essential for generating a practical chemical exchange saturation transfer (CEST) effect from an amine. A ratio of CEST effects from the aryl amine and amide was linearly correlated with pH throughout the physiologic pH range. The pH calibration was used to produce a parametric pH map of a subcutaneous flank tumor on a mouse model of MCF-7 mammary carcinoma. Although refinements in the in vivo CEST MRI methodology may improve the accuracy of pHe measurements, this study demonstrated that the PARACEST contrast agent can be used to generate parametric pH maps of in vivo tumors with saturation power levels that are not harmful to a mouse model of cancer.

CEST phase mapping using a length and offset varied saturation (LOVARS) scheme

Chemical exchange saturation transfer MRI is a promising new technique for cellular and molecular imaging. This contrast allows the detection of tumors and ischemia without the use of gadolinium as well as the design of microenvironment-sensitive probes that can be discriminated based on their exchange contrast properties and saturation frequency. Current acquisition schemes to detect and analyze this contrast suffer from sensitivity to spatial B0 inhomogeneity and low contrast-to-noise-ratio, which is an obstacle to widespread adoption of the technology. A new method to detect chemical exchange saturation transfer contrast is proposed here, termed "length and offset varied saturation" which acquires a set of images with the saturation parameters varied so as to modulate the exchange contrast. Either fast fourier transform or the general linear model can be employed to decompose the modulation patterns into separate sources of water signal loss. After transformation, a length and offset varied saturation phase map is generated, which is insensitive to B0 inhomogeneity. When collected on live mice bearing 9L gliosarcomas, and compared to the conventional asymmetry in the magnetization transfer ratio map using offset increment correction, the results show that length and offset varied saturation phase mapping obtains about three to four times contrast-to-noise-ratio and exhibits less B0 artifacts.

MRI of CEST-based reporter gene

In recent years, several reporter genes have been designed for non-invasive magnetic resonance imaging (MRI). Here, we offer a brief summary of recent advances in MRI reporter gene technology, as well as elaborated protocols for cloning, expression, and imaging of reporter genes based on a chemical exchange saturation transfer (CEST) method. These protocols emphasize new developments in CEST-MRI data acquisition and processing.

Improved pH measurements with a single PARACEST MRI contrast agent

The measurement of extracellular pH has potential utility for assessing the therapeutic effects of pH-dependent and pH-altering therapies. A PARAmagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agent, Yb-DO3A-oAA, has two CEST effects that are dependent on pH. A ratio derived from these CEST effects was linearly correlated with pH throughout the physiological pH range. The pH can be measured with a precision of 0.21 pH units and an accuracy of 0.09 pH units. The pH measurement is independent of concentration and T₁ relaxation times, but is dependent on temperature. Although MR coalescence affects the CEST measurements, especially at high pH, the ratiometric analysis of the CEST effects can account for incomplete saturation of the agent's amide and amine that results from MR coalescence. Provided that an empirical calibration is determined with saturation conditions, magnetic field strength and temperature that can be used for subsequent studies, these results demonstrate that this single PARACEST MRI contrast agent can accurately measure pH.

Lanthanide(III) complexes that contain a self-immolative arm: potential enzyme responsive contrast agents for magnetic resonance imaging

Enzyme-responsive MRI-contrast agents containing a "self-immolative" benzylcarbamate moiety that links the MRI-reporter lanthanide complex to a specific enzyme substrate have been developed. The enzymatic cleavage initiates an electronic cascade reaction that leads to a structural change in the Ln(III) complex, with a concomitant response in its MRI-contrast-enhancing properties. We synthesized and investigated a series of Gd(3+) and Yb(3+) complexes, including those bearing a self-immolative arm and a sugar unit as selective substrates for β-galactosidase; we synthesized complex LnL(1), its NH(2) amine derivatives formed after enzymatic cleavage, LnL(2), and two model compounds, LnL(3) and LnL(4). All of the Gd(3+) complexes synthesized have a single inner-sphere water molecule. The relaxivity change upon enzymatic cleavage is limited (3.68 vs. 3.15 mM(-1) s(-1) for complexes GdL(1) and GdL(2), respectively; 37 °C, 60 MHz), which prevents application of this system as an enzyme-responsive T(1) relaxation agent. Variable-temperature (17)O NMR spectroscopy and (1)H NMRD (nuclear magnetic relaxation dispersion) analysis were used to assess the parameters that determine proton relaxivity for the Gd(3+) complexes, including the water-exchange rate (k(ex)(298), varies in the range 1.5-3.9×10(6) s(-1)). Following the enzymatic reaction, the chelates contain an exocyclic amine that is not protonated at physiological pH, as deduced from pH-potentiometric measurements (log K(H)=5.12(±0.01) and 5.99(±0.01) for GdL(2) and GdL(3), respectively). The Yb(3+) analogues show a PARACEST effect after enzymatic cleavage that can be exploited for the specific detection of enzymatic activity. The proton-exchange rates were determined at various pH values for the amine derivatives by using the dependency of the CEST effect on concentration, saturation time, and saturation power. A concentration-independent analysis of the saturation-power-dependency data was also applied. All these different methods showed that the exchange rate of the amine protons of the Yb(III) complexes decreases with increasing pH value (for YbL(3), k(ex)=1300 s(-1) at pH 8.4 vs. 6000 s(-1) at pH 6.4), thereby resulting in a diminution of the observed CEST effect.

Chemical exchange saturation transfer magnetic resonance imaging of human knee cartilage at 3 T and 7 T

The sensitivity of chemical exchange saturation transfer (CEST) on glycosaminoglycans (GAGs) in human knee cartilage (gagCEST) in vivo was evaluated at 3 and 7 T field strengths. Calculated gagCEST values without accounting for B(0) inhomogeneity (~0.6 ppm) were >20%. After B(0) inhomogeneity correction, calculated gagCEST values were negligible at 3 T and ~6% at 7 T. These results suggest that accurate B(0) correction is a prerequisite for observing reliable gagCEST. Results obtained with varying saturation pulse durations and amplitudes as well as the consistency between numerical simulations and our experimental results indicate that the negligible gagCEST observed at 3 T is due to direct saturation effects and fast exchange rate. As GAG loss from cartilage is expected to result in a further reduction in gagCEST, gagCEST method is not expected to be clinically useful at 3 T. At high fields such as 7 T, this method holds promise as a viable clinical technique.

Multi-angle ratiometric approach to measure chemical exchange in amide proton transfer imaging

Amide proton transfer imaging, a specific form of chemical exchange saturation transfer imaging, has previously been applied to studies of acute ischemic acidosis, stroke, and cancer. However, interpreting the resulting contrast is complicated by its dependence on the exchange rate between amides and water, the amide concentration, amide and water relaxation, and macromolecular magnetization transfer. Hence, conventional chemical exchange saturation transfer contrast is not specific to changes such as reductions in pH due to tissue acidosis. In this article, a multi-angle ratiometric approach based on several pulsed-chemical exchange saturation transfer scans at different irradiation flip angles is proposed to specifically reflect exchange rates only. This separation of exchange effects in pulsed-chemical exchange saturation transfer experiments is based on isolating rotation vs. saturation contributions, and such methods form a new subclass of chemical exchange rotation transfer (CERT) experiments. Simulations and measurements of creatine/agar phantoms indicate that a newly proposed imaging metric isolates the effects of exchange rate changes, independent of other sample parameters.

In vivo three-dimensional whole-brain pulsed steady-state chemical exchange saturation transfer at 7 T

Chemical exchange saturation transfer (CEST) is a technique to indirectly detect pools of exchangeable protons through the water signal. To increase its applicability to human studies, it is needed to develop sensitive pulse sequences for rapidly acquiring whole-organ images while adhering to stringent amplifier duty cycle limitations and specific absorption rate restrictions. In addition, the interfering effects of direct water saturation and conventional magnetization transfer contrast complicate CEST quantification and need to be reduced as much as possible. It is shown that for protons exchanging with rates of less than 50-100 Hz, such as imaged in amide proton transfer experiments, these problems can be addressed by using a three-dimensional steady state pulsed acquisition of limited B(1) strength (≈ 1 μT). Such an approach exploits the fact that the direct water saturation width, magnetization transfer contrast magnitude, and specific absorption rate increase strongly with B(1) , while the size of the CEST effect for such protons depends minimally on B(1) . A short repetition time (65 ms) steady-state sequence consisting of a brief saturation pulse (25 ms) and a segmented echo-planar imaging train allowed acquisition of a three-dimensional whole-brain volume in approximately 11 s per saturation frequency, while remaining well within specific absorption rate and duty cycle limits. Magnetization transfer contrast was strongly reduced, but substantial saturation effects were found at frequencies upfield from water, which still confound the use of magnetization transfer asymmetry analysis. Fortunately, the limited width of the direct water saturation signal could be exploited to fit it with a Lorentzian function allowing CEST quantification. Amide proton transfer effects ranged between 1.5% and 2.5% in selected white and grey matter regions. This power and time-efficient 3D pulsed CEST acquisition scheme should aid endogenous CEST quantification at both high and low fields.

Magnetic resonance characterization of ischemic tissue metabolism

Magnetic resonance imaging (MRI) and spectroscopy (MRS) are versatile diagnostic techniques capable of characterizing the complex stroke pathophysiology, and hold great promise for guiding stroke treatment. Particularly, tissue viability and salvageability are closely associated with its metabolic status. Upon ischemia, ischemic tissue metabolism is disrupted including altered metabolism of glucose and oxygen, elevated lactate production/accumulation, tissue acidification and eventually, adenosine triphosphate (ATP) depletion and energy failure. Whereas metabolism impairment during ischemic stroke is complex, it may be monitored non-invasively with magnetic resonance (MR)-based techniques. Our current article provides a concise overview of stroke pathology, conventional and emerging imaging and spectroscopy techniques, and data analysis tools for characterizing ischemic tissue damage.

Monitoring enzyme activity using a diamagnetic chemical exchange saturation transfer magnetic resonance imaging contrast agent

Chemical exchange saturation transfer (CEST) is a new approach for generating magnetic resonance imaging (MRI) contrast that allows monitoring of protein properties in vivo. In this method, a radiofrequency pulse is used to saturate the magnetization of specific protons on a target molecule, which is then transferred to water protons via chemical exchange and detected using MRI. One advantage of CEST imaging is that the magnetizations of different protons can be specifically saturated at different resonance frequencies. This enables the detection of multiple targets simultaneously in living tissue. We present here a CEST MRI approach for detecting the activity of cytosine deaminase (CDase), an enzyme that catalyzes the deamination of cytosine to uracil. Our findings suggest that metabolism of two substrates of the enzyme, cytosine and 5-fluorocytosine (5FC), can be detected using saturation pulses targeted specifically to protons at +2 ppm and +2.4 ppm (with respect to water), respectively. Indeed, after deamination by recombinant CDase, the CEST contrast disappears. In addition, expression of the enzyme in three different cell lines exhibiting different expression levels of CDase shows good agreement with the CDase activity measured with CEST MRI. Consequently, CDase activity was imaged with high-resolution CEST MRI. These data demonstrate the ability to detect enzyme activity based on proton exchange. Consequently, CEST MRI has the potential to follow the kinetics of multiple enzymes in real time in living tissue.

Magnetic resonance imaging of the Amine-Proton EXchange (APEX) dependent contrast

Chemical exchange between water and labile protons from amino-acids, proteins and other molecules can be exploited to provide tissue contrast with magnetic resonance imaging (MRI) techniques. Using an off-resonance Spin-Locking (SL) scheme for signal preparation is advantageous because the image contrast can be tuned to specific exchange rates by adjusting SL pulse parameters. While the amide-proton transfer (APT) contrast is obtained optimally with steady-state preparation, using a low power and long irradiation pulse, image contrast from the faster amine-water proton exchange (APEX) is optimized in the transient state with a higher power and a shorter SL pulse. Our phantom experiments show that the APEX contrast is sensitive to protein and amino acid concentration, as well as pH. In vivo 9.4-T SL MRI data of rat brains with irradiation parameters optimized to slow exchange rates have a sharp peak at 3.5 ppm and also broad peak at -2 to -5 ppm, inducing negative contrast in APT-weighted images, while the APEX image has large positive signal resulting from a weighted summation of many different amine-groups. Brain ischemia induced by cardiac arrest decreases pure APT signal from ~1.7% to ~0%, and increases the APEX signal from ~8% to ~16%. In the middle cerebral artery occlusion (MCAO) model, the APEX signal shows different spatial and temporal patterns with large inter-animal variations compared to APT and water diffusion maps. Because of the similarity between the chemical exchange saturation transfer (CEST) and SL techniques, APEX contrast can also be obtained by a CEST approach using similar irradiation parameters. APEX may provide useful information for many diseases involving a change in levels of proteins, peptides, amino-acids, or pH, and may serve as a sensitive neuroimaging biomarker.

Simplified quantification of labile proton concentration-weighted chemical exchange rate (k(ws) ) with RF saturation time dependent ratiometric analysis (QUESTRA): normalization of relaxation and RF irradiation spillover effects for improved quantitative chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer MRI is an emerging imaging technique capable of detecting dilute proteins/peptides and microenvironmental properties, with promising in vivo applications. However, chemical exchange saturation transfer MRI contrast is complex, varying not only with the labile proton concentration and exchange rate, but also with experimental conditions such as field strength and radiofrequency (RF) irradiation scheme. Furthermore, the optimal RF irradiation power depends on the exchange rate, which must be estimated in order to optimize the chemical exchange saturation transfer MRI experiments. Although methods including numerical fitting with modified Bloch-McConnell equations, quantification of exchange rate with RF saturation time and power (QUEST and QUESP), have been proposed to address this relationship, they require multiple-parameter non-linear fitting and accurate relaxation measurement. Our work extended the QUEST algorithm with ratiometric analysis (QUESTRA) that normalizes the magnetization transfer ratio at labile and reference frequencies, which effectively eliminates the confounding relaxation and RF spillover effects. Specifically, the QUESTRA contrast approaches its steady state mono-exponentially at a rate determined by the reverse exchange rate (k(ws) ), with little dependence on bulk water T(1) , T(2) , RF power and chemical shift. The proposed algorithm was confirmed numerically, and validated experimentally using a tissue-like phantom of serially titrated pH compartments.

Using the water signal to detect invisible exchanging protons in the catalytic triad of a serine protease

Chemical Exchange Saturation Transfer (CEST) is an MRI approach that can indirectly detect exchange broadened protons that are invisible in traditional NMR spectra. We modified the CEST pulse sequence for use on high-resolution spectrometers and developed a quantitative approach for measuring exchange rates based upon CEST spectra. This new methodology was applied to the rapidly exchanging Hδ1 and Hε2 protons of His57 in the catalytic triad of bovine chymotrypsinogen-A (bCT-A). CEST enabled observation of Hε2 at neutral pH values, and also allowed measurement of solvent exchange rates for His57-Hδ1 and His57-Hε2 across a wide pH range (3-10). Hδ1 exchange was only dependent upon the charge state of the His57 (k (ex,Im+) = 470 s(-1), k (ex,Im) = 50 s(-1)), while Hε2 exchange was found to be catalyzed by hydroxide ion and phosphate base (k(OH)⁻ = 1.7 × 10(10) M(-1) s(-1), K(HPO)²⁻₄ = 1.7 × 10(6) M(-1) s(-1)), reflecting its greater exposure to solute catalysts. Concomitant with the disappearance of the Hε2 signal as the pH was increased above its pK (a), was the appearance of a novel signal (δ = 12 ppm), which we assigned to Hγ of the nearby Ser195 nucleophile, that is hydrogen bonded to Nε2 of neutral His57. The chemical shift of Hγ is about 7 ppm downfield from a typical hydroxyl proton, suggesting a highly polarized O-Hγ bond. The significant alkoxide character of Oγ indicates that Ser195 is preactivated for nucleophilic attack before substrate binding. CEST should be generally useful for mechanistic investigations of many enzymes with labile protons involved in active site chemistry.

Quantitative separation of CEST effect from magnetization transfer and spillover effects by Lorentzian-line-fit analysis of z-spectra

Chemical exchange saturation transfer (CEST) processes in aqueous systems are quantified by evaluation of z-spectra, which are obtained by acquisition of the water proton signal after selective RF presaturation at different frequencies. When saturation experiments are performed in vivo, three effects are contributing: CEST, direct water saturation (spillover), and magnetization transfer (MT) mediated by protons bound to macromolecules and bulk water molecules. To analyze the combined saturation a new analytical model is introduced which is based on the weak-saturation-pulse (WSP) approximation. The model combines three single WSP approaches to a general model function. Simulations demonstrated the benefits and constraints of the model, in particular the capability of the model to reproduce the ideal proton transfer rate (PTR) and the conventional MT rate for moderate spillover effects (up to 50% direct saturation at CEST-resonant irradiation). The method offers access to PTR from z-spectra data without further knowledge of the system, but requires precise measurements with dense saturation frequency sampling of z-spectra. PTR is related to physical parameters such as concentration, transfer rates and thereby pH or temperature of tissue, using either exogenous contrast agents (PARACEST, DIACEST) or endogenous agents such as amide protons and -OH protons of small metabolites.

On-bead combinatorial synthesis and imaging of chemical exchange saturation transfer magnetic resonance imaging agents to identify factors that influence water exchange

The sensitivity of magnetic resonance imaging (MRI) contrast agents is highly dependent on the rate of water exchange between the inner sphere of a paramagnetic ion and bulk water. Normally, identifying a paramagnetic complex that has optimal water exchange kinetics is done by synthesizing and testing one compound at a time. We report here a rapid, economical on-bead combinatorial synthesis of a library of imaging agents. Eighty different 1,4,7,10-tetraazacyclododecan-1,4,7,10-tetraacetic acid (DOTA)-tetraamide peptoid derivatives were prepared on beads using a variety of charged, uncharged but polar, hydrophobic, and variably sized primary amines. A single chemical exchange saturation transfer image of the on-bead library easily distinguished those compounds having the most favorable water exchange kinetics. This combinatorial approach will allow rapid screening of libraries of imaging agents to identify the chemical characteristics of a ligand that yield the most sensitive imaging agents. This technique could be automated and readily adapted to other types of MRI or magnetic resonance/positron emission tomography agents as well.

Multimodal imaging of sustained drug release from 3-D poly(propylene fumarate) (PPF) scaffolds

The potential of poly(propylene fumarate) (PPF) scaffolds as drug carriers was investigated and the kinetics of the drug release quantified using magnetic resonance imaging (MRI) and optical imaging. Three different MR contrast agents were used for coating PPF scaffolds. Initially, iron oxide (IONP) or manganese oxide nanoparticles (MONP) carrying the anti-cancer drug doxorubicin were absorbed or mixed with the scaffold and their release into solution at physiological conditions was measured with MRI and optical imaging. A slow (hours to days) and functional release of the drug molecules into the surrounding solution was observed. In order to examine the release properties of proteins and polypeptides, protamine sulfate, a chemical exchange saturation transfer (CEST) MR contrast agent, was attached to the scaffold. Protamine sulfate showed a steady release rate for the first 24h. Due to its biocompatibility, versatile drug-loading capability and constant release rate, the porous PPF scaffold has potential in various biomedical applications, including MR-guided implantation of drug-dispensing materials, development of drug carrying vehicles, and drug delivery for tumor treatment.

Amide proton transfer imaging with improved robustness to magnetic field inhomogeneity and magnetization transfer asymmetry using saturation with frequency alternating RF irradiation

Amide proton transfer (APT) imaging has shown promise as an indicator of tissue pH and as a marker for brain tumors. Sources of error in APT measurements include direct water saturation, and magnetization transfer (MT) from membranes and macromolecules. These are typically suppressed by postprocessing asymmetry analysis. However, this approach is strongly dependent on B(0) homogeneity and can introduce additional errors due to intrinsic MT asymmetry, aliphatic proton features opposite the amide peak and radiation damping-induced asymmetry. Although several methods exist to correct for B(0) inhomogeneity, they tremendously increase scan times and do not address errors induced by asymmetry of the z-spectrum. In this article, a novel saturation scheme-saturation with frequency alternating RF irradiation (SAFARI)-is proposed in combination with a new magnetization transfer ratio (MTR) parameter designed to generate APT images insensitive to direct water saturation and MT, even in the presence of B(0) inhomogeneity. The feasibility of the SAFARI technique is demonstrated in phantoms and in the human brain. Experimental results show that SAFARI successfully removes direct water saturation and MT contamination from APT images. It is insensitive to B(0) offsets up to 180 Hz without using additional B(0) correction, thereby dramatically reducing scanning time.

Iopamidol as a responsive MRI-chemical exchange saturation transfer contrast agent for pH mapping of kidneys: In vivo studies in mice at 7 T

Iopamidol (Isovue®-Bracco Diagnostic Inc.) is a clinically approved X-Ray contrast agent used in the last 30 years for a wide variety of diagnostic applications with a very good clinical acceptance. Iopamidol contains two types of amide functionalities that can be exploited for the generation of chemical exchange saturation transfer effect. The exchange rate of the two amide proton pools is markedly pH-dependent. Thus, a ratiometric method for pH assessment has been set-up based on the comparison of the saturation transfer effects induced by selective irradiation of the two resonances. This ratiometric approach allows to rule out the concentration effect of the contrast agent and provides accurate pH measurements in the 5.5-7.4 range. Upon injection of Iopamidol into healthy mice, it has been possible to acquire pH maps of kidney regions. Furthermore, it has been also shown that the proposed method is able to report about pH-changes induced in control mice fed with acidified or basified water for a period of a week before image acquisition.

Chemical exchange saturation transfer (CEST): what is in a name and what isn't?

Chemical exchange saturation transfer (CEST) imaging is a relatively new magnetic resonance imaging contrast approach in which exogenous or endogenous compounds containing either exchangeable protons or exchangeable molecules are selectively saturated and after transfer of this saturation, detected indirectly through the water signal with enhanced sensitivity. The focus of this review is on basic magnetic resonance principles underlying CEST and similarities to and differences with conventional magnetization transfer contrast. In CEST magnetic resonance imaging, transfer of magnetization is studied in mobile compounds instead of semisolids. Similar to magnetization transfer contrast, CEST has contributions of both chemical exchange and dipolar cross-relaxation, but the latter can often be neglected if exchange is fast. Contrary to magnetization transfer contrast, CEST imaging requires sufficiently slow exchange on the magnetic resonance time scale to allow selective irradiation of the protons of interest. As a consequence, magnetic labeling is not limited to radio-frequency saturation but can be expanded with slower frequency-selective approaches such as inversion, gradient dephasing and frequency labeling. The basic theory, design criteria, and experimental issues for exchange transfer imaging are discussed. A new classification for CEST agents based on exchange type is proposed. The potential of this young field is discussed, especially with respect to in vivo application and translation to humans.

Simulation and optimization of pulsed radio frequency irradiation scheme for chemical exchange saturation transfer (CEST) MRI-demonstration of pH-weighted pulsed-amide proton CEST MRI in an animal model of acute cerebral ischemia

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) is capable of measuring dilute labile protons and microenvironmental properties. However, the CEST contrast is dependent upon experimental conditions-particularly, the radiofrequency (RF) irradiation scheme. Although continuous-wave RF irradiation has been used conventionally, the limited RF pulse duration or duty cycle of most clinical systems requires the use of pulsed RF irradiation. Here, the conventional numerical simulation is extended to describe pulsed-CEST MRI contrast as a function of RF pulse parameters (i.e., RF pulse duration and flip angle) and labile proton properties (i.e., exchange rate and chemical shift). For diamagnetic CEST agents undergoing slow or intermediate chemical exchange, simulation shows a linear regression relationship between the optimal mean RF power of pulsed-CEST MRI and continuous-wave-CEST MRI. The optimized pulsed-CEST contrast is approximately equal to that of continuous-wave-CEST MRI for exchange rates less than 50 s(-1), as confirmed experimentally using a multicompartment pH phantom. In the acute stroke animals, we showed that pulsed- and continuous-wave-amide proton CEST MRI demonstrated similar contrast. In summary, our study elucidated the RF irradiation dependence of pulsed-CEST MRI contrast, providing useful insights to guide its experimental optimization and quantification.

Saturation power dependence of amide proton transfer image contrasts in human brain tumors and strokes at 3 T

Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides in tumor and monitoring pH effects in stroke, through the saturation transfer between irradiated amide protons and water protons. In this work, four healthy subjects, eight brain tumor patients (four with high-grade glioma, one with lung cancer metastasis, and three with meningioma), and four stroke patients (average 4.3 ± 2.5 days after the onset of the stroke) were scanned at 3 T, using different radiofrequency saturation powers. The APT effect was quantified using the magnetization transfer ratio (MTR) asymmetry at 3.5 ppm with respect to the water resonance. At a saturation power of 2 μT, the measured APT-MRI signal of the normal brain tissue was almost zero, due to the contamination of the negative conventional magnetization transfer ratio asymmetry. This irradiation power caused an optimal hyperintense APT-MRI signal in the tumor and an optimal hypointense signal in the stroke, compared to the normal brain tissue. The results suggest that the saturation power of 2 μT is ideal for APT imaging of these two pathologies at 3 T with the existing clinical hardware.

High-throughput screening of chemical exchange saturation transfer MR contrast agents

A new high-throughput MRI method for screening chemical exchange saturation transfer (CEST) agents is demonstrated, allowing simultaneous testing of multiple samples with minimal attention to sample configuration and shimming of the main magnetic field (B(0)). This approach, which is applicable to diamagnetic, paramagnetic and liposome CEST agents, employs a set of inexpensive glass or plastic capillary tubes containing CEST agents put together in a cheap plastic tube holder, without the need for liquid between the tubes to reduce magnetic susceptibility effects. In this setup, a reference image of direct water saturation spectra is acquired in order to map the absolute water frequency for each volume element (voxel) in the sample image, followed by an image of saturation transfer spectra to determine the CEST properties. Even though the field over the total sample is very inhomogeneous due to air-tube interfaces, the shape of the direct saturation spectra is not affected, allowing removal of susceptibility shift effects from the CEST data by using the absolute water frequencies from the reference map. As a result, quantitative information such as the mean CEST intensity for each sample can be extracted for multiple CEST agents at once. As an initial application, we demonstrate rapid screening of a library of 16 polypeptides for their CEST properties, but in principle the number of tubes is limited only by the available signal-noise-ratio, field of view and gradient strength for imaging.

Amide proton transfer imaging of the human brain

Amide proton transfer (APT) imaging is a new MRI technique that detects endogenous mobile proteins and peptides in tissue via saturation of the amide protons in the peptide bonds. Initial studies have shown promise in detecting tumor and stroke, but this technique was hampered by magnetic field inhomogeneity and a low signal-to-noise ratio. Several important prerequisites for performing APT imaging experiments include designing an effective APT imaging pulse sequence based on the hardware capability, optimizing the experimental protocol for the best clinical imaging quality, and developing data-processing approaches for effective image assessment. In this chapter, technical issues, such as pulse sequence design and optimization, magnetic field inhomogeneity correction, specific absorption rate minimization, and scan duration, are addressed.

CEST MRI reporter genes

In recent years, several reporter genes have been developed that can serve as a beacon for non-invasive magnetic resonance imaging (MRI). Here, we provide a brief summary of recent advances in MRI reporter gene technology, as well as detailed "hands-on" protocols for cloning, expression, and imaging of reporter genes based on chemical exchange saturation transfer (CEST).

Polymeric PARACEST MRI contrast agents as potential reporters for gene therapy

Gene therapy is a potentially powerful treatment approach that targets molecular remedies for disease. Among other challenges it remains difficult to monitor gene delivery and its downstream metabolic consequences. Approaches to MRI gene reporters have been reported but few have the potential for translation beyond isolated cell systems. Herein, we report a polycationic polymer MRI contrast agent that binds to DNA in a ratio of one monomer unit per phosphate group of DNA. Significantly, this binding event diminishes the MR contrast signal from the agent itself potentially providing a platform for imaging delivery and release of a gene into cells and tissues. Importantly, we demonstrate here the proof of concept that a positively charged polymeric contrast agent can also act as a transfection agent, delivering the gene for encoding green fluorescent protein into cells. These observations provide support for the radical, new idea of creating a combined transfection/imaging agent for monitoring gene delivery in real time by MRI.

A self-calibrating PARACEST MRI contrast agent that detects esterase enzyme activity

The CEST effect of many PARACEST MRI contrast agents changes in response to a molecular biomarker. However, other molecular biomarkers or environmental factors can influence CEST, so that a change in CEST is not conclusive proof for detecting the biomarker. To overcome this problem, a second control CEST effect may be included in the same PARACEST agent, which is responsive to all factors that alter the first CEST effect except for the biomarker to be measured. To investigate this approach, a PARACEST MRI contrast agent was developed with one CEST effect that is responsive to esterase enzyme activity and a second control CEST effect. The ratio of the two CEST effects was independent of concentration and T(1) relaxation, so that this agent was self-calibrating with respect to these factors. This ratiometric method was dependent on temperature and was influenced by MR coalescence as the chemical exchange rates approached the chemical shifts of the exchangable protons as temperature was increased. The two CEST effects also showed evidence of having different pH dependencies, so that this agent was not self-calibrating with respect to pH. Therefore, a self-calibrating PARACEST MRI contrast agent can more accurately detect a molecular biomarker such as esterase enzyme activity, as long as temperature and pH are within an acceptable physiological range and remain constant.

Spin-locking versus chemical exchange saturation transfer MRI for investigating chemical exchange process between water and labile metabolite protons

Chemical exchange saturation transfer (CEST) and spin-locking (SL) experiments were both able to probe the exchange process between protons of nonequivalent chemical environments. To compare the characteristics of the CEST and SL approaches in the study of chemical exchange effects, we performed CEST and SL experiments at varied pH and concentrated metabolite phantoms with exchangeable amide, amine, and hydroxyl protons at 9.4 T. Our results show that: (i) on-resonance SL is most sensitive to chemical exchanges in the intermediate-exchange regime and is able to detect hydroxyl and amine protons on a millimolar concentration scale. Off-resonance SL and CEST approaches are sensitive to slow-exchanging protons when an optimal SL or saturation pulse power matches the exchanging rate, respectively. (ii) Offset frequency-dependent SL and CEST spectra are very similar and can be explained well with an SL model recently developed by Trott and Palmer (J Magn Reson 2002;154:157-160). (iii) The exchange rate and population of metabolite protons can be determined from offset-dependent SL or CEST spectra or from on-resonance SL relaxation dispersion measurements. (iv) The asymmetry of the magnetization transfer ratio (MTR(asym)) is highly dependent on the choice of saturation pulse power. In the intermediate-exchange regime, MTR(asym) becomes complicated and should be interpreted with care.

In vivo mapping of brain myo-inositol

Myo-Inositol (MI) is one of the most abundant metabolites in the human brain located mainly in glial cells and functions as an osmolyte. The concentration of MI is altered in many brain disorders including Alzheimer's disease and brain tumors. Currently available magnetic resonance spectroscopy (MRS) methods for measuring MI are limited to low spatial resolution. Here, we demonstrate that the hydroxyl protons on MI exhibit chemical exchange with bulk water and saturation of these protons leads to reduction in bulk water signal through a mechanism known as chemical exchange saturation transfer (CEST). The hydroxyl proton exchange rate (k=600 s(-1)) is determined to be in the slow to intermediate exchange regime on the NMR time scale (chemical shift (∆ω)>k), suggesting that the CEST effect of MI (MICEST) can be imaged at high fields such as 7 T (∆ω=1.2×10(3)rad/s) and 9.4 T (∆ω=1.6×10(3) rad/s). Using optimized imaging parameters, concentration dependent broad CEST asymmetry between ~0.2 and 1.5 ppm with a peak at ~0.6 ppm from bulk water was observed. Further, it is demonstrated that MICEST detection is feasible in the human brain at ultra high fields (7 T) without exceeding the allowed limits on radiofrequency specific absorption rate. Results from healthy human volunteers (N=5) showed significantly higher (p=0.03) MICEST effect from white matter (5.2±0.5%) compared to gray matter (4.3±0.5%). The mean coefficient of variations for intra-subject MICEST contrast in WM and GM were 0.49 and 0.58 respectively. Potential overlap of CEST signals from other brain metabolites with the observed MICEST map is discussed. This noninvasive approach potentially opens the way to image MI in vivo and to monitor its alteration in many disease conditions.

CEST-FISP: a novel technique for rapid chemical exchange saturation transfer MRI at 7 T

Chemical exchange saturation transfer (CEST) and magnetization transfer techniques provide unique and potentially quantitative contrast mechanisms in multiple MRI applications. However, the in vivo implementation of these techniques has been limited by the relatively slow MRI acquisition techniques, especially on high-field MRI scanners. A new, rapid CEST-fast imaging with steady-state free precession technique was developed to provide sensitive CEST contrast in ∼20 sec. In this study at 7 T with in vitro bovine glycogen samples and initial in vivo results in a rat liver, the CEST-fast imaging with steady-state free precession technique was shown to provide equivalent CEST sensitivity in comparison to a conventional CEST-spin echo acquisition with a 50-fold reduction in acquisition time. The sensitivity of the CEST-fast imaging with steady-state free precession technique was also shown to be dependent on k-space encoding with centric k-space encoding providing a 30-40% increase in CEST sensitivity relative to linear encoding for 256 or more k-space lines. Overall, the CEST-fast imaging with steady-state free precession acquisition technique provides a rapid and sensitive imaging platform with the potential to provide quantitative CEST and magnetization transfer imaging data.

Fast multislice pH-weighted chemical exchange saturation transfer (CEST) MRI with Unevenly segmented RF irradiation

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging technique for measuring microenvironment properties via dilute CEST labile groups. Conventionally, CEST MRI is implemented with a long radiofrequency irradiation module, followed by fast image acquisition to obtain the steady state CEST contrast. Nevertheless, the sensitivity, scan time, and spatial coverage of the conventional CEST MRI method may not be optimal. Our study proposed a segmented radiofrequency labeling scheme that includes a long primary radiofrequency irradiation module to generate the steady state CEST contrast and repetitive short secondary radiofrequency irradiation module immediately after the image acquisition so as to maintain the steady state CEST contrast for multislice acquisition and signal averaging. The proposed CEST MRI method was validated experimentally with a tissue-like pH phantom and optimized for the maximal contrast-to-noise ratio. In addition, the proposed sequence was evaluated for imaging ischemic acidosis via pH-weighted endogenous amide proton transfer MRI, which showed similar contrast as conventional amide proton transfer MRI. In sum, a fast multislice relaxation self-compensated CEST MRI sequence was developed, with significantly improved sensitivity and suitable for in vivo applications.

Lanthanide complexes of triethylenetetramine tetra-, penta-, and hexaacetamide ligands as paramagnetic chemical exchange-dependent saturation transfer contrast agents for magnetic resonance imaging: nona- versus decadentate coordination

The solid state and solution structure of a series of lanthanide complexes of the decadentate ligand triethylenetetramine-N,N,N',N'',N''',N'''-hexaacetamide, (ttham), its two decadentate derivatives di-tert-butyl N,N,N''',N'''-tetra(carbamoylmethyl)-triethylenetetramine-N',N''-diacetate (Bu(2)ttha-tm) and N,N,N''',N'''-tetra(carbamoylmethyl)-triethylenetetramine-N',N''-diacetic acid (H(2)ttha-tm), and its two nonadentate derivatives N-benzyl-triethylenetetramine-N,N',N'',N''',N'''-pentaacetamide (1bttpam) and N'-benzyl-triethylenetetramine-N,N,N'',N''',N'''-pentaacetamide (4bttpam) have been investigated by infrared and Raman spectroscopy, X-ray crystallography, cyclovoltammetry, and NMR spectroscopy. In these mononuclear lanthanide complexes, the first coordination sphere is generally saturated by four amine nitrogens of the triethylenetetramine ligand backbone and five or six carbonyl oxygen atoms of the pendent amide or acetate donor groups. In the [Ln(ttham)](3+) complex series, a switch from a decadentate to a nonadentate coordination occurs between [Er(ttham)](3+) and [Tm(ttham)](3+). This switch in coordination mode, which is caused by decreasing metal ion radii going from lanthanum to lutetium (lanthanide contraction), has no significant effect on the T(1)-relaxivity of these complexes. It is shown that the T(1)-relaxivity is dominated by second coordination sphere interactions, with an ascendant contribution of the classical dipolar relaxation mechanism for the earlier (Ce-Sm) and very late (Tm, Yb) lanthanides, and a prevailing Curie relaxation mechanism for most of the remaining paramagnetic lanthanide ions. In chemical exchange-dependent saturation transfer (CEST) (1)H NMR experiments, most of the above complexes exhibit multiple strong CEST peaks of the paramagnetically shifted amide protons spread over a >100 ppm chemical shift range. The effective CEST effect of the studied thulium complexes strongly depends on temperature and pH. The pH at which the CEST effect maximizes (generally between pH 7 and 8) is determined by the overall charge of the complex. Depending on the used saturation frequency offset, the temperature-dependence varies between the extremes of strongly linearly dependent and fully independent in the case of [Tm(ttham)](3+). In combination with the strong pH-dependence of the CEST effect at the latter frequency offset, this complex is highly suitable for simultaneous temperature and pH mapping using magnetic resonance imaging.

Simplified and scalable numerical solution for describing multi-pool chemical exchange saturation transfer (CEST) MRI contrast

Chemical exchange saturation transfer (CEST) imaging is sensitive to dilute labile proton and microenvironment properties such as pH and temperature, and provides vital information complementary to the conventional MRI methods. Whereas the Bloch equations coupled by exchange terms (i.e., Bloch-McConnell equations) have been utilized to quantify 2-pool CEST contrast, it is tedious to extend the Bloch-McConnell equations to describe CEST contrast beyond four saturation transfer sites. Hence, it is necessary to develop a scalable yet reasonably accurate numerical solution to describe the complex multi-pool CEST contrast. It is postulated here that the multi-pool CEST contrast can be quantified by modifying the classic 2-pool model. Although the direct exchange among labile proton groups is often negligible, labile protons may be coupled indirectly through their interaction with bulk water protons, which has to be quantified. The coupling term was solved empirically, and the proposed simplified solution was shown in good agreement with the conventional simulation. Moreover, the proposed solution is scalable, and can be easily extended to describe multi-pool CEST contrast. In sum, our study established a simplified and scalable, yet reasonably accurate numerical solution, suitable for quantitatively describing multi-pool CEST contrast.

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.

A concentration-independent method to measure exchange rates in PARACEST agents

The efficiency of chemical exchange dependent saturation transfer (CEST) agents is largely determined by their water or proton exchange kinetics, yet methods to measure such exchange rates are variable and many are not applicable to in vivo measurements. In this work, the water exchange kinetics of two prototype paramagnetic agents (PARACEST) are compared by using data from classic NMR line-width measurements, by fitting CEST spectra to the Bloch equations modified for chemical exchange, and by a method where CEST intensity is measured as a function of applied amplitude of radiofrequency field. A relationship is derived that provides the water exchange rate from the X-intercept of a plot of steady-state CEST intensity divided by reduction in signal caused by CEST irradiation versus 1/omega(1)(2), referred to here as an omega plot. Furthermore, it is shown that this relationship is independent of agent concentration. Exchange rates derived from omega plots using either high-resolution CEST NMR data or CEST data obtained by imaging agree favorably with exchange rates measured by the more commonly used Bloch fitting and line-width methods. Thus, this new method potentially allows access to a direct measure of exchange rates in vivo, where the agent concentration is typically unknown.

Simultaneous determination of labile proton concentration and exchange rate utilizing optimal RF power: Radio frequency power (RFP) dependence of chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer (CEST) MRI is increasingly used to probe mobile proteins and microenvironment properties, and shows great promise for tumor and stroke diagnosis. However, CEST MRI contrast mechanism is complex, depending not only on the CEST agent concentration, exchange and relaxation properties, but also varying with experimental conditions such as magnetic field strength and RF power. Hence, it remains somewhat difficult to quantify apparent CEST MRI contrast for properties such as pH, temperature and protein content. In particular, CEST MRI is susceptible to RF spillover effects in that RF irradiation may directly saturate the bulk water MR signal, leading to an optimal RF power at which the CEST contrast is maximal. Whereas RF spillover is generally considered an adverse effect, it is noted here that the optimal RF power strongly varies with exchange rate, although with negligible dependence on labile proton concentration. An empirical solution suggested that optimal RF power may serve as a sensitive parameter for simultaneously determining the labile proton content and exchange rate, hence, allowing improved characterization of the CEST system. The empirical solution was confirmed by numerical simulation, and experimental validation is needed to further evaluate the proposed technique.

Responsive MRI agents for sensing metabolism in vivo

Magnetic resonance imaging (MRI) has inherent advantages in safety, three-dimensional output, and clinical relevance when compared with optical and radiotracer imaging methods. However, MRI contrast agents are inherently less sensitive than agents used in other imaging modalities primarily because MRI agents are detected indirectly by changes in either the water proton relaxation rates (T(1), T(2), and T(*)(2)) or water proton intensities (chemical exchange saturation transfer and paramagnetic chemical exchange saturation transfer, CEST and PARACEST). Consequently, the detection limit of an MRI agent is determined by the characteristics of the background water signal; by contrast, optical and radiotracer-based methods permit direct detection of the agent itself. By virtue of responding to background water (which reflects bulk cell properties), however, MRI contrast agents have considerable advantages in "metabolic" imaging, that is, spatially resolving tissue variations in pH, redox state, oxygenation, or metabolite levels. In this Account, we begin by examining sensitivity limits in targeted contrast agents and then address contrast agents that respond to a physiological change; these responsive agents are effective metabolic imaging sensors. The sensitivity requirements for a metabolic imaging agent are quite different from those for a targeted Gd(3+)-based T(1) agent (for example, sensing cell receptors). Targeted Gd(3+) agents must have either an extraordinarily high water proton relaxivity (r(1)) or multiple Gd(3+) complexes clustered together at the target site on a polymer platform or nanoparticle assembly. Metabolic MRI agents differ in that the high relaxivity requirement, although helpful, is eased because these agents respond to bulk properties of tissues rather than low concentrations of a specific biological target. For optimal sensing, metabolic imaging agents should display a large change in relaxivity (deltar(1)) in response to the physiological or metabolic parameter of interest. Metabolic imaging agents have only recently begun to appear in the literature and only a few have been demonstrated in vivo. MRI maps of absolute tissue pH have been obtained with Gd(3+)-based T(1) sensors. The requirement of an independent measure of agent concentration in tissues complicates these experiments, but if qualitative changes in tissue pH are acceptable, then these agents can be quite useful. In this review, we describe examples of imaging extracellular pH in brain tumors, ischemic hearts, and pancreatic islets with Gd(3+)-based pH sensors and discuss the potential of CEST and PARACEST agents as metabolic imaging sensors.

Feasibility of concurrent dual contrast enhancement using CEST contrast agents and superparamagnetic iron oxide particles

A major challenge for cellular and molecular MRI is to study interactions between two different cell populations or biological processes. We studied the possibility to simultaneously image contrast agents based on two different MRI contrast mechanisms: chemical exchange saturation transfer (CEST) and enhancement of T2 relaxation. Various amounts of superparamagnetic iron oxide (SPIO) nanoparticles were mixed with a fixed concentration (250 microM) of the CEST agent poly-L-lysine. T2 maps, CEST maps, and frequency-dependent saturation spectra were then measured. Color-coded overlay maps demonstrated the feasibility of concurrent dual contrast enhancement. We found that at concentrations lower than 5 microg(Fe)/mL both contrast agents can be imaged simultaneously. At higher concentrations, the iron-based agent can be used to "shut off" the signal arising from the CEST agent. These initial findings are a first step toward using dual CEST/T2 contrast imaging for studying multiple cellular or molecular targets simultaneously in vivo.

Transverse relaxation and magnetization transfer in skeletal muscle: effect of pH

Exercise increases the intracellular T(2) (T(2,i)) of contracting muscles. The mechanism(s) for the T(2,i) increase have not been fully described, and may include increased intracellular free water and acidification. These changes may alter chemical exchange processes between intracellular free water and proteins. In this study, the hypotheses were tested that (a) pH changes T(2,i) by affecting the rate of magnetization transfer (MT) between free intracellular water and intracellular proteins, and (b) the magnitude of the T(2,i) effect depends on acquisition mode (localized or nonlocalized) and echo spacing. Frog gastrocnemius muscles were excised and their intracellular pH was either kept at physiological pH (7.0) or modified to model exercising muscle (pH 6.5). The intracellular transverse relaxation rate (R(2,i) = 1/T(2,i)) always decreased in the acidic muscles, but the changes were greater when measured using more rapid refocusing rates. The MT rate from the macromolecular proton pool to the free water proton pool, its reverse rate, and the spin-lattice relaxation rate of water decreased in acidic muscles. It is concluded that intracellular acidification alters the R(2,i) of muscle water in a refocusing rate-dependent manner, and that the R(2,i) changes are correlated with changes in the MT rate between macromolecules and free intracellular water.

Imaging the tissue distribution of glucose in livers using a PARACEST sensor

Noninvasive imaging of glucose in tissues could provide important insights about glucose gradients in tissue, the origins of gluconeogenesis, or perhaps differences in tissue glucose utilization in vivo. Direct spectral detection of glucose in vivo by (1)H NMR is complicated by interfering signals from other metabolites and the much larger water signal. One potential way to overcome these problems is to use an exogenous glucose sensor that reports glucose concentrations indirectly through the water signal by chemical exchange saturation transfer (CEST). Such a method is demonstrated here in mouse liver perfused with a Eu(3+)-based glucose sensor containing two phenylboronate moieties as the recognition site. Activation of the sensor by applying a frequency-selective presaturation pulse at 42 ppm resulted in a 17% decrease in water signal in livers perfused with 10 mM sensor and 10 mM glucose compared with livers with the same amount of sensor but without glucose. It was shown that livers perfused with 5 mM sensor but no glucose can detect glucose exported from hepatocytes after hormonal stimulation of glycogenolysis. CEST images of livers perfused in the magnet responded to changes in glucose concentrations demonstrating that the method has potential for imaging the tissue distribution of glucose in vivo.

PARACEST MRI with improved temporal resolution

PARAmagnetic Chemical Exchange Saturation Transfer (PARACEST) is a novel contrast mechanism for MRI. A PARACEST MRI methodology with high temporal resolution is highly desired for in vivo MRI applications of molecular imaging. To address this need, a strategy has been developed that includes a long selective saturation period before each repetition of a Rapid Acquisition with Relaxation Enhancement (RARE) pulse sequence. This strategy is suitable for the application of PARACEST contrast agents to environments with long T1 relaxation times. An alternative strategy uses short selective saturation periods before the acquisition of each k-space trajectory to maintain steady state conditions, which can be implemented with a Fast Low Angle Shot (FLASH) pulse sequence. These short saturation periods lengthen the total scan time as compared to the first approach but compensate for the loss in PARACEST contrast related to T1 relaxation. Both approaches have been demonstrated in vitro and in vivo with significantly improved temporal resolutions as compared to a conventional gradient-echo PARACEST method without sacrificing CNR efficiency. These demonstrations also adopted a strategy for measuring the PARACEST effect that only requires selective saturation at a single MR frequency, which further improves temporal resolution for PARACEST detection.

New "multicolor" polypeptide diamagnetic chemical exchange saturation transfer (DIACEST) contrast agents for MRI

An array of 33 prototype polypeptides was examined as putative contrast agents that can be distinguished from each other based on the chemical exchange saturation transfer (CEST) mechanism. These peptides were chosen based on predictions of the chemical exchange rates of exchangeable amide, amine, and hydroxyl protons that produce this contrast, and tested at 11.7T for their CEST suitability. Artificial colors were assigned to particular amino acid units (lysine, arginine, threonine, and serine) based on the separate resonance frequencies of these exchangeable protons. The magnitude of the CEST effect could be fine-tuned by altering the amino acid sequence, and these three exchangeable groups could be distinguished in an MR phantom based on their different chemical shifts ("colors"). These new diamagnetic CEST (DIACEST) agents possess a wide range of electrostatic charges, compositions, and protein stabilities in vivo, making them potentially suitable for a variety of biological applications such as designing MR reporter genes for imaging cells and distinguishing multiple targets within the same MR image.

Practical data acquisition method for human brain tumor amide proton transfer (APT) imaging

Amide proton transfer (APT) imaging is a type of chemical exchange-dependent saturation transfer (CEST) magnetic resonance imaging (MRI) in which amide protons of endogenous mobile proteins and peptides in tissue are detected. Initial studies have shown promising results for distinguishing tumor from surrounding brain in patients, but these data were hampered by magnetic field inhomogeneity and a low signal-to-noise ratio (SNR). Here a practical six-offset APT data acquisition scheme is presented that, together with a separately acquired CEST spectrum, can provide B(0)-inhomogeneity corrected human brain APT images of sufficient SNR within a clinically relevant time frame. Data from nine brain tumor patients at 3T shows that APT intensities were significantly higher in the tumor core, as assigned by gadolinium-enhancement, than in contralateral normal-appearing white matter (CNAWM) in patients with high-grade tumors. Conversely, APT intensities in tumor were indistinguishable from CNAWM in patients with low-grade tumors. In high-grade tumors, regions of increased APT extended outside of the core into peripheral zones, indicating the potential of this technique for more accurate delineation of the heterogeneous areas of brain cancers.

Molecular and functional imaging of breast cancer

Despite several major advances in breast cancer diagnosis and treatment, the American Cancer Society has estimated that in the US alone 43300 women and 400 men will die from breast cancer in 2007. Breast cancer typically is a multi-focal, multi-faceted disease, with the major cause of mortality being complications due to metastasis. Whereas a decade ago genetic alterations were the primary focus in cancer research, it is now apparent that the physiological tumor microenvironment, interactions between cancer cells and stromal cells such as endothelial cells, fibroblasts and macrophages, the extracellular matrix, and a multitude of secreted factors and cytokines influence progression, aggressiveness, and response of the disease to treatment. Prevention, early diagnosis, and treatment are the three broad challenges for MR molecular and functional imaging in reducing mortality from this disease. Multi-parametric molecular and functional MRI provides unprecedented opportunities for identifying novel targets for imaging and therapy at the bench, as well as for accurate diagnosis and monitoring response to therapy at the bedside. Here we provide an overview of the current status of molecular and functional MRI of breast cancer, outlining some key developments, as well as identifying some of the important challenges facing this field in the future.

Imaging pH using the chemical exchange saturation transfer (CEST) MRI: Correction of concomitant RF irradiation effects to quantify CEST MRI for chemical exchange rate and pH

Chemical exchange saturation transfer (CEST) MRI has been shown capable of detecting dilute labile protons and abnormal tissue glucose/oxygen metabolism, and thus, may serve as a complementary imaging technique to the conventional MRI methods. CEST imaging, however, is also dependent on experimental parameters such as the power, duration, and waveform of the irradiation RF pulse. As a result, its sensitivity and specificity for microenvironment properties such as pH is not optimal. In this study, the dependence of CEST contrast on experimental parameters was solved and an iterative compensation algorithm was proposed that corrects the experimentally measured CEST contrast from the concomitant RF irradiation effects. The proposed algorithm was verified with both numerical simulation and experimental measurements from a tissue-like pH phantom, and showed that pH derived from the compensated CEST imaging agrees reasonably well with pH-electrode measurements within 0.1 pH unit. In sum, our study validates the use of a correction algorithm to compensate CEST imaging from concomitant RF irradiation effects for accurate calibration of the chemical exchange rate, and demonstrates the feasibility of pH imaging with CEST MRI.

Investigation of optimizing and translating pH-sensitive pulsed-chemical exchange saturation transfer (CEST) imaging to a 3T clinical scanner

Chemical exchange saturation transfer (CEST) MRI provides a sensitive detection mechanism that allows characterization of dilute labile protons usually undetectable by conventional MRI. Particularly, amide proton transfer (APT) imaging, a variant of CEST MRI, has been shown capable of detecting ischemic acidosis, and may serve as a surrogate metabolic imaging marker. For preclinical CEST imaging, continuous-wave (CW) radiofrequency (RF) irradiation is often applied so that the steady state CEST contrast can be reached. On clinical scanners, however, specific absorption rate (SAR) limit and hardware preclude the use of CW irradiation, and instead require an irradiation scheme of repetitive RF pulses (pulsed-CEST imaging). In this work, CW- and pulsed-CEST MRI were systematically compared using a tissue-like pH phantom on an imager capable of both CW and pulsed RF irradiation schemes. The results showed that the maximally obtainable pulsed-CEST contrast is approximately 95% of CW-CEST contrast, and their optimal RF irradiation powers are equal. Moreover, the pulsed-CEST sequence was translated to a 3 Tesla clinical scanner and detected pH contrast from the labile creatine amine groups (1.9 ppm). Furthermore, pilot endogenous APT imaging of normal human volunteers was demonstrated, warranting future APT MRI of stroke patients to elucidate its diagnostic value.

Amide proton transfer imaging of 9L gliosarcoma and human glioblastoma xenografts

Amide proton transfer (APT) imaging is a variant of magnetization transfer (MT) imaging, in which the contrast is determined by a change in water intensity due to chemical exchange with saturated amide protons of endogenous mobile proteins and peptides. In this study, eight Fisher 344 rats implanted with 9L gliosarcoma cells and six nude rats implanted with human glioblastoma cells were imaged at 4.7 T. There were increased signal intensities in tumors in the APT-weighted images. The contrast of APT imaging between the tumor and contralateral brain tissue was about 3.9% in water intensity (1.49 +/- 0.66% vs -2.36 +/- 0.19%) for the more uniformly hypercellular 9L brain tumors, and it was reduced to 1.6% (-1.18 +/- 0.60% vs -2.77 +/- 0.42%) for the human glioblastoma xenografts that contained hypocellular zones of necrosis. The preliminary results show that the APT technique at the protein level may provide a unique MRI contrast for the characterization of brain tumors.