The amide proton transfer (APT) effect has emerged as a unique endogenous molecular imaging contrast mechanism with great clinical potentials. However, in vivo quantitative mapping of APT using the conventional asymmetry analysis is difficult due to the confounding Nuclear Overhauser Effect (NOE) and the asymmetry of the magnetization transfer (MT) effect. Here we showed that the asymmetry of MT contrast from immobile macromolecules is highly significant, and the wide spectral separation associated with a high magnetic field of 9.4 T delineates APT and NOE peaks in a Z-spectrum. Therefore, high resolution apparent APT and NOE maps can be obtained from measurements at three offsets. The apparent APT value was greater in gray matter compared to white matter in normal rat brain, and was sensitive to tissue acidosis and correlated well with ADC in the rat focal ischemic brain. In contrast, no ischemia-induced contrast was observed in the apparent NOE map. The concentration-dependence and the pH insensitivity of NOE were confirmed in phantom experiments. Our results demonstrate that in vivo apparent APT and NOE maps can be easily obtained at high magnetic fields, and the pH-insensitive NOE may be a useful indicator of mobile macromolecular contents.

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. Recently, there has been an increasing number of in vivo studies that have used the chemical exchange (CE) effect to probe the tissue microenvironment and provide novel imaging contrasts that are not available from conventional MRI techniques. Most of these studies adopted either a chemical exchange saturation transfer (CEST) or a spinlocking (SL) approach to detect contrast in tissue pH or the population of labile protons, which have a Larmor frequency different from water. Ideally, a CE-sensitive imaging contrast should have good sensitivity and vary monotonically with pH and linearly with labile proton concentration. The CE contrast is determined by many parameters, such as the exchange rate between water and labile protons (k ex ), the difference in their Larmor frequencies (d), the populations of the exchangeable protons, water T 1 , and the magnetic field strength (B 0 ). The CE effect in MRI is also highly sensitive to a ratio of k ex to d. k ex /d, which indicates the CE kinetics, is usually divided into three regimes: slow (k ex /d ( 1), intermediate (k ex /d $ 1), and fast exchange (k ex /d ) 1). CEST techniques are mostly applied at the slow-or slow-to intermediate-exchange regime (1,2), whilereas the CE is often assumed to occur at the fast-exchange regime for SL applications (3,4).In CEST studies that are based upon endogenous contrast, selective off-resonance irradiation of labile protons of protein or peptide side chains attenuates the water signal via exchange between these labile protons and bulk water. The signal intensity as a function of irradiation frequency, often referred to as the Z-spectrum, can be expressed by the magnetization transfer ratio (MTR):where V is the frequency offset with respect to water. In practice, the conventional non-CE magnetization transfer effect and direct water saturation (or the so-called spillover effect) also affect the Z-spectrum, and these effects are assumed to be symmetrical around the water resonance frequency. To minimize these non-CE contributions, CEST contrast in MRI is usually extracted from two images-one acquired with off-resonance irradiation on the targeted labile proton and the other as a control with opposite offset frequency from the water (5). The norma...

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.

The spatiotemporal characteristics of cerebral blood volume (CBV) and flow (CBF) responses are important for understanding neurovascular coupling mechanisms and blood oxygenation leveldependent (BOLD) signals. For this, cortical layer-dependent BOLD, CBV and CBF responses were measured at the cat visual cortex using fMRI. Major findings are: (i) The time-dependent fMRI cortical profile is dependent on imaging modality. Overall, the peak across the cortex occurs at the cortical surface for BOLD, but at the middle cortical layer for CBV and CBF. Compared to an initial stimulation period (4-10 s), the spatial specificity of CBV to the middle cortical layer increases significantly at a later time, while the specificity of BOLD and CBF slightly changes. (ii) The CBV response at the upper cortical area containing large pial vessels has a faster onset time and time to peak than the BOLD response at the same area, and a faster time to peak than CBV at the middle cortical area with microvessels. This suggests that the dilation of microvessels at the middle cortical area follows arterial volume increase at the surface of the cortex. (iii) For all three modalities, the post-stimulus undershoot was observed with the 60-s stimulation paradigm, indicating that the poststimulus BOLD undershoot cannot be explained by the delayed venous CBV recovery theory under our experimental conditions. (iv) The relationship between CBV and CBF responses is both spatially and temporally dependent. Thus, a single power-law scaling constant (gamma value) may not be applicable for high-resolution study.

Uptake of administered D-glucose (Glc) or 2-deoxy-D-glucose (2DG) has been indirectly mapped through the chemical exchange (CE) between glucose hydroxyl and water protons using CE-dependent saturation transfer (glucoCEST) magnetic resonance imaging (MRI). We propose an alternative technique—on-resonance CE-sensitive spin-lock (CESL) MRI—to enhance responses to glucose changes. Phantom data and simulations suggest higher sensitivity for this ‘glucoCESL' technique (versus glucoCEST) in the intermediate CE regime relevant to glucose. Simulations of CESL signals also show insensitivity to B0-fluctuations. Several findings are apparent from in vivo glucoCESL studies of rat brain at 9.4 Tesla with intravenous injections. First, dose-dependent responses are nearly linearly for 0.25-, 0.5-, and 1-g/kg Glc administration (obtained with 12-second temporal resolution), with changes robustly detected for all doses. Second, responses at a matched dose of 1 g/kg are much larger and persist for a longer duration for 2DG versus Glc administration, and are minimal for mannitol as an osmolality control. And third, with similar increases in steady-state blood glucose levels, glucoCESL responses are ∼2.2 times higher for 2DG versus Glc, consistent with their different metabolic properties. Overall, we show that glucoCESL MRI could be a highly sensitive and quantifiable tool for glucose transport and metabolism studies.

Amide-proton-transfer weighted (APTw) MRI has emerged as a non-invasive pH-weighted imaging technique for studies of several diseases such as ischemic stroke. However, its pH-sensitivity is relatively low, limiting its capability to detect small pH changes. In this work, computer simulations, protamine phantom experiments, and in vivo gas challenge and experimental stroke in rats showed that, with judicious selection of the saturation pulse power, the amide-CEST at 3.6 ppm and guanidyl-CEST signals at 2.0 ppm changed in opposite directions with decreased pH. Thus, the difference between amide-CEST and guanidyl-CEST can enhance the pH measurement sensitivity, and is dubbed as pHenh. Acidification induced a negative contrast in APTw, but a positive contrast in pHenh. In vivo experiments showed that pHenh can detect hypercapnia-induced acidosis with about 3-times higher sensitivity than APTw. Also, pHenh reduced gray and white matter contrast compared to APTw. In stroke animals, the CEST contrast between the ipsilateral ischemic core and contralateral normal tissue was −1.85 ± 0.42% for APTw and 3.04 ± 0.61% (n = 5) for pHenh, and the contrast to noise was 2.9 times higher for pHenh than APTw. Our results suggest that pHenh can be a useful tool for non-invasive pH-weighted imaging.

Purpose Amide proton transfer (APT) and amine-water proton exchange (APEX) can be viable to map pH-decreasing ischemic regions. However, their exact contributions are unclear. Methods We measured APEX- and APT-weighted magnetization transfer ratio asymmetry (denoted as APEXw and APTw), ADC, T2 and T1 images, and localized proton spectra in rats with permanent middle cerebral artery occlusion at 9.4 T. Phantoms and theoretical studies were also performed. Results Within one hour post-occlusion, APEXw and APTw maps showed hyperintensity (3.1% of M0) and hypointensity (−1.8%), respectively, in regions with decreased ADC. Ischemia increased lactate and gamma aminobutyric acid (GABA) concentrations, but decreased glutamate and taurine concentrations. Over time, the APEXw contrast decreased with glutamate, taurine and creatine, while the APTw contrast and lactate level were similar. Phantom and theoretical studies suggest that the source of APEXw signal is mainly from proteins at normal pH, while at decreased pH, GABA and glutamate contributions increase, inducing the positive APEXw contrast in ischemic regions. The APTw contrast is sensitive to lactate concentration and pH, but contaminated from contributions of the faster amine-water proton exchange processes. Conclusion Positive APEXw contrast is more sensitive to ischemia than negative APTw contrast. They may provide complementary tissue metabolic information.

Purpose We aimed to detect, map and quantify a novel nuclear Overhauser enhancement (NOE)-mediated magnetization transfer (MT) with water at around −1.6 ppm (NOE(−1.6)) in rat brain using MRI. Methods Continuous wave MT sequences with a variety of RF irradiation powers were optimized to achieve the maximum contrast of this NOE(−1.6) effect at 9.4 T. The distribution of effect magnitudes, resonance frequency offsets, and line widths in healthy rat brains and the differences of the effect between tumors and contralateral normal brains were imaged and quantified using a multi-Lorentzian fitting method. MR measurements on reconstituted model phospholipids as well as two cell lines (HEK293 and 9L) were also performed to investigate the possible molecular origin of this NOE. Results Our results demonstrate that the NOE(−1.6) effect can be reliably detected in rat brain. Pixel-wise fittings demonstrate the regional variations of the effect. Measurements on rodent tumor model show that the amplitude of NOE(−1.6) in brain tumor is significantly diminished compared with that in normal brain tissue. Measurements on reconstituted phospholipids suggest that this effect may originate from choline phospholipids. Conclusion The NOE(−1.6) could be used as a new biomarker for the detection of brain tumor.