Purpose Chemical exchange saturation transfer (CEST) imaging is a new MRI technology allowing the detection of low concentration endogenous cellular proteins and metabolites indirectly through their exchangeable protons. A new technique, variable delay multi-pulse CEST (VDMP-CEST), is proposed to eliminate the need for recording full Z-spectra and performing asymmetry analysis to obtain CEST contrast. Methods The VDMP-CEST scheme involves acquiring images with two (or more) delays between radiofrequency saturation pulses in pulsed CEST, producing a series of CEST images sensitive to the speed of saturation transfer. Subtracting two images or fitting a time series produces CEST and relayed-nuclear Overhauser enhancement CEST maps without effects of direct water saturation and, when using low radiofrequency power, minimal magnetization transfer contrast interference. Results When applied to several model systems (bovine serum albumin, crosslinked bovine serum albumin, l-glutamic acid) and in vivo on healthy rat brain, VDMP-CEST showed sensitivity to slow to intermediate range magnetization transfer processes (rate < 100–150 Hz), such as amide proton transfer and relayed nuclear Overhauser enhancement-CEST. Images for these contrasts could be acquired in short scan times by using a single radiofrequency frequency. Conclusions VDMP-CEST provides an approach to detect CEST effect by sensitizing saturation experiments to slower exchange processes without interference of direct water saturation and without need to acquire Z-spectra and perform asymmetry analysis.

Axonal degeneration is an important determinant of progressive neurological disability in multiple sclerosis (MS).

The local presence and concentration of metal ions in biological systems has been extensively studied ex vivo using fluorescent dyes. However, the detection of multiple metal ions in vivo remains a major challenge. We present a magnetic resonance imaging (MRI)-based method for noninvasive detection of specific ions that may be coexisting, using the tetrafluorinated derivative of the BAPTA (TF-BAPTA) chelate as a 19F chelate analogue of existing optical dyes. Taking advantage of the difference in the ion-specific 19F nuclear magnetic resonance (NMR) chemical shift offset (Δω) values between the ion-bound and free TF-BAPTA, we exploited the dynamic exchange between ion-bound and free TF-BAPTA to obtain MRI contrast with multi-ion chemical exchange saturation transfer (miCEST). We demonstrate that TF-BAPTA as a prototype single 19F probe can be used to separately visualize mixed Zn2+ and Fe2+ ions in a specific and simultaneous fashion, without interference from potential competitive ions.

Purpose Demonstrate the applicability of natural D-glucose as a T2 MRI contrast agent. Methods D-glucose solutions were prepared at multiple concentrations and variable pH. The relaxation rate (R2 = 1/T2) was measured at 3, 7, and 11.7T. Additional experiments were performed on blood at 11.7 T. Also, a mouse was infused with D-glucose (3.0 mmol/kg) and dynamic T2 weighted images of the abdomen acquired. Results The transverse relaxation rate depended strongly on glucose concentration and solution pH. A maximum change in R2 was observed around physiological pH (pH 6.8-7.8). The transverse relaxivities at 22°C (pH 7.3) were 0.021, 0.060, and 0.077 s-1mM-1 at 3.0, 7.0, and 11.7T, respectively. These values showed good agreement with expected values from the Swift-Connick equation. There was no significant dependence on glucose concentration or pH for T1 and the diffusion coefficient for these solutions. The transverse relaxivity in blood at 11.7 T was 0.09 s-1mM-1. The dynamic in vivo experiment showed a 10% drop in signal intensity after glucose infusion followed by recovery of the signal intensity after about 50-100 s. Conclusion Glucose can be used as a T2 contrast agent for MRI at concentrations that are already approved for human use.

Inorganic nanocrystals (NCs) have been extensively developed for a variety of uses. The ability to obtain high-resolution NMR signals from the core nuclei of NCs in solution could offer new opportunities in materials sciences and MR imaging. Herein, we demonstrate that small, water-soluble F-ionic NCs can average out homonuclear dipolar interactions, enabling one to obtain high-resolution F NMR signals in solution that reflect the MR properties of F in the crystal core. Decorating F-NC surfaces with a biocompatible poly(ethylene glycol) coating maintains colloidal stability in water while preserving the NC high-resolution F NMR properties, even after further functionalization. The high content and magnetic equivalence of the fluorides within the NCs enable their use as imaging tracers for in vivo F MRI by facilitating a "hot-spot" display of their distribution.

Chemical Exchange Saturation Transfer (CEST) is a new approach to generate magnetic resonance imaging (MRI) contrast that allows monitoring of protein properties in vivo. In this method, radiofrequency is used to saturate the magnetization of specific protons on a target molecule, where it is then transferred to water protons via chemical exchange and detected using MRI. One advantage of CEST imaging is that the magnetization of the 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 to detect 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, each with different expression levels of CDase, show 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.

Synthetic chemistry has revolutionized the understanding of many biological systems. Small compounds that act as agonists and antagonists of proteins, and occasionally as imaging probes, have contributed tremendously to the elucidation of many biological pathways. Nevertheless, the function of thousands of proteins is still elusive, and designing new imaging probes remains a challenge. Through screening and characterization we identified thymidine analog as probe for imaging the expression of the Herpes Simplex Virus type-1 thymidine kinase (HSV1-TK). To detect the probe, we used chemical exchange saturation transfer magnetic resonance imaging (CEST-MRI), in which a dynamic exchange process between an exchangeable proton and the surrounding water protons is used to amplify the desired contrast. Initially, five pyrimidine-based molecules were recognized as putative imaging agents, since their exchangeable imino protons resonate at 5–6ppm from the water proton frequency and their detection is therefore less affected by endogenous CEST contrast or confounded by direct water saturation. Increasing the pKa value of the imino proton by reduction of its 5,6-double bond results in a significant reduction of the exchange rate (kex) between this proton and the water protons. This reduced kex of the dihydropyrimidine nucleosides fulfills the “slow to intermediate regime” condition for generating high CEST-MRI contrast. Consequently, we identified 5-methyl-5,6-dihydrothymidine as the optimal probe and demonstrated its feasibility for in vivo imaging of the HSV1-TK. In light of these findings, this new approach can be generalized for designing specific probes for the in vivo imaging of a variety of proteins and enzymes.

Genetically engineered reporters have revolutionized the understanding of many biological processes. MRI-based reporter genes can dramatically improve our ability to monitor dynamic gene expression and allow coregistration of subcellular genetic information with high-resolution anatomical images. We have developed a biocompatible MRI reporter gene based on a human gene, the human protamine-1 (hPRM1). The arginine-rich hPRM1 (47% arginine residues) generates high MRI contrast based on the chemical exchange saturation transfer (CEST) contrast mechanism. The 51 amino acid-long hPRM1 protein was fully synthesized using microwave-assisted technology, and the CEST characteristics of this protein were compared to other CEST-based contrast agents. Both bacterial and human cells were engineered to express an optimized hPRM1 gene and showed higher CEST contrast compared to controls. Live cells expressing the hPRM1 reporter gene, and embedded in three-dimensional culture, also generated higher CEST contrast compared to wild-type live cells.