![]() A single 1-mm slice was acquired with a field of view 5 × 5 mm and a matrix size of 32 × 32 pixels with an in plane resolution of 156 μm. CEST imaging was performed with the parameters TR/TE = 9000/6.35 ms, saturation power = 0.5 μT, saturation time = 4000 ms, and Δω = ± 3.76 ppm from the water 1H frequency (to coincide with the PLL amide protons at +3.76 ppm and for a control measurement at −3.76 ppm). For T2 measurements, a spin-echo sequence was used (TR = 3000 ms, TE = 9.2, 20, 30, 40, 50, 60, 70, 80, and 90 ms), with a single scan being acquired for each echo time. Louis, MO) and different concentrations of SPIO (Feridex, 0–50 μg Fe/mL) were imaged at 11.7 Tesla (T) using a Bruker MR spectrometer (using the software Paravision 3.0.2). MATERIALS AND METHODSĪ phantom composed of glass NMR tube, 5 mm in diameter, containing five inner capillaries tubes, each with or without 250 μM PLL, (MW 22 kDa, Sigma, St. ![]() The results show a range of concentrations in which the two contrast agents can be used concurrently and distinguished from each other. To assess the feasibility of using CEST and iron oxide-induced contrast simultaneously, we conducted an in vitro study using both agents. A different source of cellular contrast (T2) may be obtained by using superparamagnetic agents, either by labeling with iron oxide (SPIO) particles ( 10) or by overexpression of ferritin as an MRI reporter gene ( 11- 13). Our initial CEST reporter gene is designed based on these properties of PLL ( 9) and can produce a relatively constant intracellular concentration in cells independent of cell division. Before such studies are undertaken, however, the interactions between these two different contrast mechanisms should be examined carefully.Īmong the previously reported diamagnetic CEST contrast agents, Poly-L-Lysine (PLL) has a high exchange rate as well as multiple exchangeable protons resulting in amplification of contrast by several orders of magnitude ( 3). The switchable “on–off” property raises the possibility for CEST agents to co-exist with T1 and T2 contrast agents, potentially enabling simultaneously imaging and tracking of two different targets such as two cell populations at the same location. Recently, its application has been demonstrated for imaging of cells using an artificial lysine-rich protein (LRP) as CEST agent ( 9). A variety of organic ( 1- 4) and organometallic ( 5- 8) compounds have a sufficient number of protons to be detected at low concentrations and with suitable chemical exchange rates and chemical shifts to be selectively labeled.ĬEST contrast agents have a major advantage in that they are switchable, that is, the contrast is detectable only when a saturation pulse is applied at the specific frequency where the agent's exchangeable protons resonate. As a result, these agents are being referred to as chemical exchange saturation transfer (CEST) contrast agents ( 1). Recently, a new type of MRI contrast has been developed that relies on selective labeling of protons on contrast agents using a radiofrequency (RF) saturation pulse followed by transfer of saturation by means of exchange to bulk water, producing a fractional reduction in the water signal. These initial findings are a first step toward using dual CEST/T2 contrast imaging for studying multiple cellular or molecular targets simultaneously in vivo. At higher concentrations, the iron-based agent can be used to “shut off” the signal arising from the CEST agent. We found that at concentrations lower than 5 μg(Fe)/mL both contrast agents can be imaged simultaneously. Color-coded overlay maps demonstrated the feasibility of concurrent dual contrast enhancement. T2 maps, CEST maps, and frequency-dependent saturation spectra were then measured. Various amounts of superparamagnetic iron oxide (SPIO) nanoparticles were mixed with a fixed concentration (250 μM) of the CEST agent poly-L-lysine. ![]() 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. A major challenge for cellular and molecular MRI is to study interactions between two different cell populations or biological processes. ![]()
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