Open half-volume quadrature transverse electromagnetic coil for high-field magnetic resonance imaging

A half-volume quadrature head transverse electromagnetic (TEM) coil has been constructed for 4 T imaging applications. This coil produces a sufficiently large homogeneous B(1) field region for the use as a volume coil. It provides superior transmission efficiency, resulting in significantly lower power deposition, as well as greater sensitivity and improved patient comfort and accessibility compared with conventional full-volume coils. Additionally, this coil suppresses the RF penetration artifact that distorts the RF magnetic field profile and alters the intensity in high-field images recorded with linear surface and volume coils. These advantages make it possible to apply this device as an efficient transmit/receive coil for high-field imaging with a restricted field of view.

Quantitative functional imaging of the brain: towards mapping neuronal activity by BOLD fMRI

Quantitative magnetic resonance imaging (MRI) and spectroscopy (MRS) measurements of energy metabolism (i.e. cerebral metabolic rate of oxygen consumption, CMR(O2)), blood circulation (i.e. cerebral blood flow, CBF, and volume, CBV), and functional MRI (fMRI) signal over a wide range of neuronal activity and pharmacological treatments are used to interpret the neurophysiologic basis of blood oxygenation level dependent (BOLD) image-contrast at 7 T in glutamatergic neurons of rat cerebral cortex. Multi-modal MRI and MRS measurements of CMR(O2), CBF, CBV and BOLD signal (both gradient-echo and spin-echo) are used to interpret the neuroenergetic basis of BOLD image-contrast. Since each parameter that can influence the BOLD image-contrast is measured quantitatively and separately, multi-modal measurements of changes in CMR(O2), CBF, CBV, BOLD fMRI signal allow calibration and validation of the BOLD image-contrast. Good agreement between changes in CMR(O2) calculated from BOLD theory and measured by (13)C MRS, reveals that BOLD fMRI signal-changes at 7 T are closely linked with alterations in neuronal glucose oxidation, both for activation and deactivation paradigms. To determine the neurochemical basis of BOLD, pharmacological treatment with lamotrigine, which is a neuronal voltage-dependent Na(+) channel blocker and neurotransmitter glutamate release inhibitor, is used in a rat forepaw stimulation model. Attenuation of the functional changes in CBF and BOLD with lamotrigine reveals that the fMRI signal is associated with release of glutamate from neurons, which is consistent with a link between neurotransmitter cycling and energy metabolism. Comparisons of CMR(O2) and CBF over a wide dynamic range of neuronal activity provide insight into the regulation of energy metabolism and oxygen delivery in the cerebral cortex. The current results reveal the energetic and physiologic components of the BOLD fMRI signal and indicate the required steps towards mapping neuronal activity quantitatively by fMRI at steady-state. Consequences of these results from rat brain for similar calibrated BOLD fMRI studies in the human brain are discussed.

Changes in rat cerebral blood volume due to modulation of the 5-HT(1A) receptor measured with susceptibility enhanced contrast MRI

Brain blood volume changes in the rat in response to 5-HT(1A) agonist and antagonist administration were measured using susceptibility contrast enhanced magnetic resonance imaging (MRI). Administration of the 5-HT(1A) agonist 8-OH-DPAT resulted in decreases in fractional brain blood volumes. Administration of the 5-HT(1A) antagonist WAY-100635 following a dose of 8-OH-DPAT resulted in increases in fractional blood volumes greatest in hippocampus and cortex and smallest in thalamus and caudate-putamen. The magnitude of the regional increases in blood volumes paralleled the distribution of 5-HT(1A) receptors in the rat brain. Administration of WAY-100635 alone resulted in decreases in cortical blood volume and increases in cerebellar blood volume.

Dysprosium-bearing red cells as potential transverse relaxation agents for MRI

The cytosol of intact human red blood cells was loaded with 28.1 +/- 3.4 mM of dysprosium DTPA-BMA using a hypoosmotic technique. When loaded cells were diluted with saline and control cells to give an average dysprosium concentration of 3.3 +/- 0.5 mM, the transverse relaxation rate constants R(*)(2) and R(2) increased. R(*)(2) increased from 7.5 +/- 0.9 sec(-1) to 356 +/- 50 sec(-1), and R(2) increased from 7.4 +/- 0.7 sec(-1) to 148 +/- 40 sec(-1). After lysing, R(*)(2) was 6.0 +/- 0.6 sec(-1) in the control and 13.4 +/- 1.5 sec(-1) in the mixture; R(2) was 6.4 +/- 1.1 sec(-1) and 9.8 +/- 2.4 sec(-1), respectively. Thus, the relaxivity effects were enhanced by sequestration of the dysprosium within intact red cells, and this effect was lost after lysis. At a circulating whole-blood concentration of 0.81 +/- 0.15 mM in rats, the liver signal intensity dropped 29.9% +/- 3.7% and kidney signal intensity dropped 19.4% +/- 8.7%. Dysprosium-loaded cells might be useful in the study of perfusion and tissue blood volume.

The relationship of problems in biomedical MRI to the study of porous media

The NMR methods that are used to characterize inanimate porous media measure relaxation times and related phenomena and material transport, fluid displacement and flow. Biological tissues are comprised of multiple small, fluid-filled compartments, such as cells, that restrict the movement of the bulk solvent water and whose constituents influence water proton relaxation times via numerous interactions with macromolecular surfaces. Several of the methods and concepts that have been developed in one field of application are also of great value in the other, and it may be expected that technical developments that have been spurred by biomedical applications of MR imaging will be used in the continuing study of porous media. Some recent specific studies from our laboratory include the development of multiple quantum coherence methods for studies of ordered water in anisotropic macromolecular assemblies, studies of the degree of restriction of water diffusion in cellular systems, multiple selective inversion imaging to depict the ratios of proton pool sizes and rates of magnetization transfer between proton populations, and diffusion tensor imaging to depict tissue anisotropies. These illustrate how approaches to obtain structural information from biological media are also relevant to porous media. For example, the recent development of oscillating gradient spin echo techniques (OGSE), an approach that extends our ability to resolve apparent diffusion changes over different time scales in tissues, has also been used to compute surface to volume measurements in assemblies of pores. Each of the new methods can be adapted to provide spatially resolved quantitative measurements of properties of interest, and these can be efficiently acquired with good accuracy using fast imaging methods such as echo planar imaging. The community of NMR scientists focused on applications to porous media should remain in close communication with those who use MRI to study problems in biomedicine, to their mutual benefits.

A model for susceptibility artefacts from respiration in functional echo-planar magnetic resonance imaging

Respiration causes variations in the signals acquired during magnetic resonance imaging (MRI) and therefore is a significant source of noise in functional brain imaging. A primary component of respiratory noise may arise from variations of bulk susceptibility or air volume in the chest. Here we investigate the nature of the image artefacts that can be caused by such changes. We develop a simple model which attempts to mimic the effects of variations in susceptibility and volume during respiration. Theoretical calculations, computer simulations and imaging experiments with this model show that small variations in susceptibility within the thorax from alterations in the paramagnetism of cavity gas may lead to a shift of the image on the order of 0.1 pixels as well as a shading of the intensity by +/-1%. These effects are observed to be predominant in the phase-encoding direction. They may lead to the production of spurious activations in functional MRI and are likely to be of more importance at higher field strengths.

Intravascular susceptibility agent effects on tissue transverse relaxation rates in vivo

Since vascular architecture differs among tissues, it was hypothesized that the change in transverse relaxation rate produced by a given tissue concentration of susceptibility contrast agent also varies by tissue. This is relevant to strategies to map regional blood volume by MRI using indicator dilution techniques. R*(2) was measured in rat organs over a range of susceptibility agent concentrations at 1.5 T. Rat red blood cells loaded with dysprosium-DTPA-BMA served as an intravascular susceptibility agent. Tissue samples were frozen in vivo and dysprosium concentrations were independently measured using inductively coupled plasma atomic emission spectroscopy. The slope (k) of R*(2) vs. tissue dysprosium concentration in sec(-1) mM(-1) for myocardium was 97.1 (95% C.I. 77. 0-117.2), liver 122.6 (108.3-136.9), spleen 22.5 (8.8-36.3), kidney 68.1 (58.6-77.6), and skeletal muscle 77.9 (4.1-151.6); k was significantly different (P < 0.05) for all pairings except those with skeletal muscle. Therefore, relative values of tissue blood volume derived from dynamic images of first pass contrast effects may be in error because k is not constant for different conditions.

Assessment of heating effects in skin during continuous wave near infrared spectroscopy

Near infrared spectroscopy is an increasingly important tool for the investigation of human brain function, however, to date there have been few systematic evaluations of accompanying thermal effects due to absorption. We have measured the spatial distribution of temperature changes during near infrared irradiation (789 nm) as a function of laser power, in both excised tissue (chicken meat and skin) and in the forearm of an awake human volunteer. Light was applied using a 1 mm optical fiber which is characteristic of the topographic system. The temperature of excised chicken tissue increased linearly with power level as 0.097 and 0.042 degrees C/mW at depths of 0 and 1 mm, respectively. Human forearm studies yielded temperature changes of 0.101, 0.038, and 0.030 degrees C/mW at depths of 0.5, 1.0, and 1.5 mm, respectively. Due to direct irradiation of the thermocouple all measurements represent the maximum temperature increase from the laser. In all cases the estimated heating effects from continuous wave optical topography systems were small and well below levels which would endanger tissue cells. The close similarity between ex vivo and in vivo measurements suggests negligible contributions from blood flow in the skin which was further supported by measurements during cuff ischemia. Heating effects decreased sharply with both depth and lateral position; thus, for optode spacings greater than a few millimeters, fibers can be treated independently. Finite element analysis confirms that the experimental results are consistent with a simple heat conduction model.

Effects of hypoglycemia on functional magnetic resonance imaging response to median nerve stimulation in the rat brain

The authors studied the effects of a standardized mild-moderate hypoglycemic stimulus (glucose clamp) on brain functional magnetic resonance imaging (fMRI) responses to median nerve stimulation in anesthetized rats. In the baseline period (plasma glucose 6.6 +/- 0.3 mmol/L), the MR signal changes induced by median nerve activation were determined within a fixed region of the somatosensory cortex from preinfusion activation maps. Subsequently, insulin and a variable glucose infusion were administered to decrease plasma glucose. The goal was to produce a stable hypoglycemic plateau (2.8 +/- 0.2 mmol/L) for 30 minutes. Thereafter, plasma glucose was restored to euglycemic levels (6.0 +/- 0.3 mmol/L). In the early phase of insulin infusion (15 to 30 minutes), before hypoglycemia was reached (4.7 +/- 0.3 mmol/L), the activation signal was unchanged. However, once the hypoglycemic plateau was achieved, the activation signal was significantly decreased to 57 +/- 6% of the preinfusion value. Control regions in the brain that were not activated showed no significant changes in MR signal intensity. Upon return to euglycemia, the activation signal change increased to within 10% of the original level. No significant activation changes were noted during euglycemic hyperinsulinemic clamp experiments. The authors concluded that fMRI can detect alterations in cerebral function because of insulin-induced hypoglycemia. The signal changes observed in fMRI activation experiments were sensitive to blood glucose levels and might reflect increases in brain metabolism that are limited by substrate deprivation during hypoglycemia.

High-resolution CMR(O2) mapping in rat cortex: a multiparametric approach to calibration of BOLD image contrast at 7 Tesla

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) method, which is sensitive to vascular paramagnetic deoxyhemoglobin, is dependent on regional values of cerebral metabolic rate of oxygen utilization (CMR(O2)), blood flow (CBF), and volume (CBV). Induced changes in deoxyhemoglobin function as an endogenous contrast agent, which in turn affects the transverse relaxation rates of tissue water that can be measured by gradient-echo and spin-echo sequences in BOLD fMRI. The purpose here was to define the quantitative relation between BOLD signal change and underlying physiologic parameters. To this end, magnetic resonance imaging and spectroscopy methods were used to measure CBF, CMR(O2), CBV, and relaxation rates (with gradient-echo and spin-echo sequences) at 7 Tesla in rat sensorimotor cortex, where cerebral activity was altered pharmacologically within the autoregulatory range. The changes in tissue transverse relaxation rates were negatively and linearly correlated with changes in CBF, CMR(O2), and CBV. The multiparametric measurements revealed that CBF and CMR(O2) are the dominant physiologic parameters that modulate the BOLD fMRI signal, where the ratios of (deltaCMR(O2)/CMR(O2)/(deltaCBF/ CBF) and (deltaCBV/CBV)/(deltaCBF/CBF) were 0.86 +/- 0.02 and 0.03 +/- 0.02, respectively. The calibrated BOLD signals (spatial resolution of 48 microL) from gradient-echo and spin-echo sequences were used to predict changes in CMR(O2) using measured changes in CBF, CBV, and transverse relaxation rates. The excellent agreement between measured and predicted values for changes in CMR(O2) provides experimental support of the current theory of the BOLD phenomenon. In gradient-echo sequences, BOLD contrast is affected by reversible processes such as static inhomogeneities and slow diffusion, whereas in spin-echo sequences these effects are refocused and are mainly altered by extravascular spin diffusion. This study provides steps by which multiparametric MRI measurements can be used to obtain high-spatial resolution CMR(O2) maps.

Quantitative multi-modal functional MRI with blood oxygenation level dependent exponential decays adjusted for flow attenuated inversion recovery (BOLDED AFFAIR)

A magnetic resonance imaging (MRI) method is described that allows interleaved measurements of transverse (R(2)(*) and R(2)) and longitudinal (R(1)) relaxation rates of tissue water in conjunction with spin labeling. The image-contrasts are intrinsically blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) weighted, but each contrast is made quantitative by two echo time (TE) and inversion recovery time (TIR) acquisitions with gradient echo (GE) and spin echo (SE) weighted echo-planar imaging (EPI). The EPI data were acquired at 7 Tesla with nominal spatial resolution of 430 x 430 x 1000 microm(3) in rat brain in vivo. The method is termed as blood oxygenation level dependent exponential decays adjusted for flow attenuated inversion recovery (BOLDED AFFAIR) and allows acquisition of R(2)(*), R(2), and CBF maps in an interleaved manner within approximately 12 minute. The basic theory of the method, associated experimental/systematic errors, and temporal restrictions are discussed. The method is validated by comparison of multi-modal maps obtained by BOLDED AFFAIR (i.e., two TE and TIR values with GE and SE sequences) and conventional approach (i.e., multiple TE and TIR values with GE and SE sequences) during varied levels of whole brain activity. Preliminary functional data from a rat forepaw stimulation model demonstrate the feasibility of this method for functional MRI (fMRI) studies. It is expected that with appropriate precautions this method in conjunction with contrast agent-based MRI has great potential for quantitative fMRI studies of mammalian cortex.

Dependence of oxygen delivery on blood flow in rat brain: a 7 tesla nuclear magnetic resonance study

Magnetic resonance imaging (MRI) and spectroscopy (MRS) were used at a magnetic field strength of 7 T to measure CBF and CMRO2 in the sensorimotor cortex of mature rats at different levels of cortical activity. In rats maintained on morphine anesthesia, transitions to lower activity and higher activity states were produced by administration of pentobarbital and nicotine, respectively. Under basal conditions of morphine sulfate anesthesia, CBF was 0.75 +/- 0.09 mL x g(-1) x min(-1) and CMRO2 was 3.15 +/- 0.18 micromol x g(-1) x min(-1). Administration of sodium pentobarbital reduced CBF and CMRO2 by 66% +/- 16% and 61% +/- 6%, respectively (i.e., "deactivation"). In contrast, administration of nicotine hydrogen tartrate increased CBF and CMRO2 by 41% +/- 5% and 30% +/- 3%, respectively (i.e., "activation"). The resting values of CBF and CMRO2 for alpha-chloralose anesthetized rats were 0.40 +/- 0.09 mL x g(-1) x min(-1) and 1.51 +/- 0.06 micromol x g(-1) x min(-1), respectively. Upon forepaw stimulation, CBF and CMRO2 were focally increased by 34% +/- 10% and 26% +/- 12%, respectively, above the resting nonanesthetized values (i.e., "activation"). Incremental changes in CBF and CMRO2, when expressed as a percentage change for "deactivation" and "activation" from the respective control conditions, were linear (R2 = 0.997) over the entire range examined with the global and local perturbations. This tight correlation for cerebral oxygen delivery in vivo is supported by a recent model where the consequence of a changing effective diffusivity of the capillary bed for oxygen, D, has been hypothetically shown to be linked to alterations in CMRO2 and CBF. This assumed functional characteristic of the capillary bed can be theoretically assessed by the ratio of fractional changes in D with respect to changes in CBF, signified by omega. A value 0.81 +/- 0.23 was calculated for omega with the in vivo data presented here, which in turn corresponds to a supposition that the effective oxygen diffusivity of the capillary bed is not constant but presumably varies to meet local requirements in oxygen demand in a similar manner with both "deactivation" and "activation."

Density matrix simulations of the effects of J coupling in spin echo and fast spin echo imaging

A computer simulation has been used to calculate the effects of J coupling on the amplitudes of echoes produced by CPMG sequences. The program computes the evolution of the density matrix for different pulse intervals and can predict the signals obtainable from spin systems of any size and complexity. Results from the simulation confirm the prediction that a decrease in the effects of J coupling is largely responsible for the bright fat signal seen in fast spin echo imaging at high pulse rates. The effects of J coupling on CPMG echotrains are examined for A3B2 and A3B2C2 spin systems over a wide range of J coupling and chemical shift values and pulse spacings. The effects of J coupling on the point spread function obtained with fast spin echo imaging are also discussed.

Quantitative imaging of magnetization transfer using multiple selective pulses

New spectroscopic and imaging methods have been developed for quantitatively measuring magnetization transfer (MT). These methods use trains of radiofrequency (rf) pulses with pulse separations much longer than 1/k(mf) and pulse durations much shorter than 1/k(mf), where k(mf) is the rate of MT from the immobile (macromolecular) protons to the mobile (free water) protons. Signal sensitivity to MT occurs when these pulses affect the mobile and immobile proton pools to different degrees. The signal from water may be quantitatively related to the macromolecular content of the sample using theory. The method has been used to make quantitative measurements of macromolecular content in cross-linked bovine serum albumin and employed in conjunction with echoplanar imaging to produce maps of the spatial distribution of the macromolecular content.

Analysis of J coupling-induced fat suppression in DIET imaging

The DIET (or dual interval echo train) sequence, a modification of the fast spin echo (FSE) sequence that selectively reduces signal from fat in MR images, has been investigated. The DIET sequence uses an initial echo spacing longer than that of a conventional FSE sequence, thus allowing J coupling-induced dephasing to take effect. The sequence is evaluated theoretically, and its effectiveness on a hydrocarbon (1-pentene) is demonstrated numerically using density matrix calculations. The sequence is also evaluated experimentally using in vitro solutions and in vivo imaging. The efficacy of the sequence is compared for different lipid chemical structures, field strengths, and pulse sequence parameters.

Physiological basis for BOLD MR signal changes due to neuronal stimulation: separation of blood volume and magnetic susceptibility effects

An NMR method is applied for separating blood volume and magnetic susceptibility effects in response to neuronal stimulation in a rat model. The method uses high susceptibility contrast agents to enhance blood volume induced signal changes. In the absence of exogenous agent, the dominant source of signal change on neuronal activation is associated with the signal increase from the blood oxygen level dependent (BOLD) effect. The relative negative contribution of blood volume changes to BOLD changes is maximally estimated to be 34%. The blood volume changes associated with median nerve stimulation (7 Hz) in the motor cortex are 26+/-7% and the corresponding blood susceptibility changes are 0.021+/-0.006 ppm. These methods can be applied to enhance the sensitivity of fMRI signal response and provide accurate quantitative measures of blood volume response to stimulation.

Quantification of intravascular and extravascular contributions to BOLD effects induced by alteration in oxygenation or intravascular contrast agents

A simple model is presented that allows quantitative separation of the contributions of signals from water in blood and extravascular parenchyma due to changes in blood oxygenation, induced either by brain activation or by alteration of inspired oxygen. The separation is based on the progressive attenuation of the signals in the vasculature of different levels when bipolar field gradient pulses are applied. Diffusion-weighted spin-echo echo-planar imaging sequences were used to measure signal changes under various conditions in both animals and human volunteers. Normoxic-hyperoxic episodes were induced in rats before and after injection of a superparamagnetic iron oxide contrast agent. Signal changes produced by visual stimulation were measured in human volunteers, and in volunteers subject to alternating normoxic-hyperoxic episodes, and with administration of Gd-DTPA. Analysis of the results with our simple model suggests that the apparent diffusion coefficient increases and R2 (= 1/T2) decreases upon brain activation, with a large component from extravascular water related to the decrease in the blood deoxyhemoglobin concentration. Furthermore, this study suggests that apparent diffusion coefficient of the extravascular component alone may provide localization of neuronal activation.

Isometric and dynamic exercise studied with echo planar magnetic resonance imaging (MRI)

PURPOSE

The effect of different types of exercise upon echo planar (EP) magnetic resonance (MR) images was examined during and after both dynamic and isometric dorsi-flexion exercises at matched workloads and durations.

METHODS

Healthy untrained subjects performed either dynamic exercise through a full range of motion and against a constant resistance or isometric exercise at the center of the range of motion and against a constant resistance at 25 or 70% their measured maximum voluntary contraction (MVC). EP MR images were acquired at 1.5 T every 4 s before (4 images), during (27 images), and after (29-65 images) exercise. A spin echo EP sequence was employed with TE = 30 ms, TR = 4000 ms, FOV = 20 x 40 cm, 64 x 128 matrix. The changes in proton transverse relaxation rate (deltaR2, [s(-1)]) relative to values obtained before exercise were calculated from individual images at different times during and after exercise.

RESULTS

At both 70 and 25% of MVC, the maximum deltaR2 after dynamic exercise (-8.38+/-0.32 s(-1) (70%), -6.47+/-1.23s(-1) (25%)) was significantly greater (P < or = 0.05) than after isometric exercise (-5.91+/-0.67 s(-1) (70%), -3.80+/-0.87s(-1) (25%)). Throughout the period that recovery was monitored, the recovery patterns of deltaR2 following isometric and dynamic exercise at both workloads remained parallel.

CONCLUSIONS

We conclude that exercise-induced changes in MR images are influenced not only by workload and exercise duration but also by the type of exercise, and we postulate that these differences result from the different physiological responses elicited by the different types of exercise.

Asymmetric spin-echo imaging of magnetically inhomogeneous systems: theory, experiment, and numerical studies

The ability of the asymmetric spin-echo (ASE) pulse sequence to provide different degrees of spin-echo (SE)-type and gradient-echo (GE)-type contrast when imaging media containing magnetic inhomogeneities is investigated. The dependence of the ASE signal on the size of magnetic field perturbers is examined using theory, computer simulations, and experiment. A theoretical prediction of the ASE signal is obtained using the Anderson-Weiss mean field theory, the results of which are qualitatively supported by computer simulations and experimental studies. It is shown that the ASE sequence can be used to tune the range of perturber sizes that provide the largest contributions to susceptibility contrast effects.

Gadolinium-bearing red cells as blood pool MRI contrast agents

Human and rat red blood cells (RBCs) were loaded with gadolinium DTPA dimeglumine using an osmotic pulse technique to create a blood pool contrast agent for MRI. The resulting packed red cells contained 30.9 +/- 3.3 (1 SD) mmol Gd/liter for humans and 24.7 +/- 3.5 (1 SD) mmol Gd/liter for rats. Longitudinal relaxation rate constant of human RBCs increased from 2.0 +/- 0.1 to 145.6 +/- 36.2 s(-1); the transverse relaxation rate constant increased from 6.8 +/- 1.2 to 562 +/- 410 s(-1). For rat RBCs, R1 increased from 1.45 +/- 0.15 to 84.8 +/- 23.9 s(-1); R2 increased from 7.1 +/- 0.64 to 247 +/- 158 s(-1). Affinity for oxygen was slightly reduced (control P50 = 22.3 +/- 2.3 versus experimental P50 = 27.3 +/- 1.3, P < 0.01), as was mechanical deformability. No drop in relaxivities was seen after 5 days of storage. The apparent volume of distribution was 0.0164 +/- 0.003 liter/kg, biologic half-life 4.38 +/- 0.34 h, and total plasma clearance 0.003 +/- 0.0006 liter/kg/h. Compared with Gd-DTPA "free" in the plasma, tissue enhancement from RBCs was initially lower but was much prolonged. Preparation is simple enough to be reproduced by most laboratories.

The role of specific side groups and pH in magnetization transfer in polymers

The nature of water-macromolecule interactions in aqueous model polymers has been investigated using quantitative measurements of magnetization transfer. Cross-linked polymer gels composed of 94% water, 3% N,N'-methylene-bis-acrylamide, and 3% functional monomer (acrylamide, methacrylamide, acrylic acid, methacrylic acid, 2-hydroxyethyl-acrylate, or 2-hydroxyethyl-methacrylate) were studied. Water-macromolecule interactions were modified by varying the pH and specific functional group on the monomer. The magnitudes of the interactions were quantified by measuring the rate of proton nuclear spin magnetization exchange between the polymer matrix and the water. This rate was highly sensitive to the presence of carboxyl side groups on the macromolecule. However, the dependence of the rate on pH was not consistent with simple acid/base-catalyzed chemical exchange, and instead, the data suggest that multiequilibria proton exchange, a wide distribution in surface group pK values, and/or a macromolecular structural dependence on pH may play a significant role in magnetization transfer in polymer systems. These model polymer gels afford useful insights into the relevance of chemical composition and chemical dynamics on relaxation in tissues.

Quantitative studies of magnetization transfer by selective excitation and T1 recovery

Water proton longitudinal relaxation has been measured in agar and cross-linked bovine serum albumin (BSA) using modified selective excitation (Goldman-Shen and Edzes-Samulski) pulse sequences. The resulting recovery curves are fit to biexponentials. The fast recovery rate gives magnetization transfer (MT) information, which is complementary to that given by steady-state saturation methods. This rate provides an estimate of the strength of the coupling of the immobile proton pool to the mobile proton pool. Near their optimal pulse power values, the Goldman-Shen and Edzes-Samulski sequences give fast recovery rates that agree with each other. However, these measured fast recovery rates are dependent on the pulse power, an effect not predicted by the coupled two-pool model. For 8% agar and 17% BSA, both methods (at optimal pulse powers) give rates in the neighborhoods of 210 and 64 Hz, respectively. The Goldman-Shen and Edzes-Samulski pulse sequences have several advantages over those techniques based on steady state saturation: no long saturating pulses, shorter measurement time, and reduced necessity for making lineshape or fitting technique assumptions. The principle disadvantages are smaller effects on the NMR signal, less complete characterization of the MT system, and, in the case of the Goldman-Shen sequence, greater pulse power.

Physiologic basis for BOLD MR signal changes due to hypoxia/hyperoxia: separation of blood volume and magnetic susceptibility effects

An NMR method is presented for separating blood volume and magnetic susceptibility effects in response to respiratory challenges such as hypoxia and hyperoxia. The technique employs high susceptibility contrast agents to enhance blood volume induced signal changes. The results show that for a rat model the dominant source of signal variation upon changing breathing gas from 100% oxygen to 10% oxygen/90% nitrogen is the change in blood magnetic susceptibility associated with the BOLD effect. The results imply that signal changes associated with respiratory challenges can be regarded as indicators of local blood oxygenation in vivo.

Functional magnetic resonance imaging of median nerve stimulation in rats at 2.0 T

An animal model of sensory activation using fMRI at 2.0 T has been developed, demonstrating that fMRI studies on animals need not be limited to high field magnets. These methods produced reliable image intensity changes of 2% using median nerve stimulation in rats at 3 Hz and propofol as the anesthetic agent. At 6 Hz the activation was slightly but not statistically significantly greater. The feasibility of fMRI studies in animals using propofol suggests that it may be a useful anesthetic for fMRI studies in agitated adult patients or in children.

Magnetic resonance evidence of hypoxia in a homozygous alpha-knockout of a transgenic mouse model for sickle cell disease

All transgenic mouse models for sickle cell disease express residual levels of mouse globins which complicate the interpretation of experimental results. We now report on a mouse expressing high levels of human betaS and 100% human alpha-globin. These mice were created by breeding the alpha-knockout and the mouse beta(major)-deletion to homozygosity in mice expressing human alpha- and betaS-transgenes. These betaS-alpha-knockout mice have accelerated red cell destruction, altered hematological indices, ongoing organ damage, and pathology under ambient conditions which are comparable with those found in alphaH betaS-Ant[betaMDD] mice without introduction of additional mutations which convert betaS into a "super-betaS" such as the doubly mutated betaS-Antilles. This is of particular importance for testing strategies for gene therapy of sickle cell disease. Spin echo magnetic resonance imaging at room air and 100% oxygen demonstrated the presence of blood hypoxia (high levels of deoxygenated hemoglobin) in the liver and kidneys that was absent in control mice. We demonstrate here that transgenic mice can be useful to test new noninvasive diagnostic procedures, since the magnetic resonance imaging technique described here potentially can be applied to patients with sickle cell disease.

The effects of cross-link density and chemical exchange on magnetization transfer in polyacrylamide gels

The effects of polymer structure and water-macromolecule interactions on proton relaxation in an aqueous model polymer have been investigated using quantitative measurements of magnetization transfer. Polyacrylamide gels composed of 95% water, 5% comonomers acrylamide and N,N'-methylene-bis-acrylamide were studied. The structure and rigidity were varied by changing the cross-linking density of the polymer. The polymer showed a biphasic change in transverse relaxation with increasing cross-linking density which was accompanied by a sudden increase in magnetization transfer above 40% cross linking. This change may be attributed to the formation of rigid domains in the polymer which exhibit solid-like behavior with a short T2 (11 microseconds) and a Gaussian lineshape. Water-macromolecule interactions were controlled by varying the pH of the gel. At high pH (> 8), there was an increase in magnetization transfer and transverse relaxivity consistent with a chemical-exchange-mediated interaction between water protons and the polymer. By analyzing the system as two proton reservoirs coupled by magnetization exchange, the proton populations, intrinsic relaxation rates, and exchange rates were estimated, for different degrees of cross linking and pH. This model affords useful insights into the relevance of both supramolecular structure and chemical exchange on relaxation in tissues.

Dynamic echo planar imaging of exercised muscle

Echo planar imaging was used to record dynamic changes in tissue transverse relaxation rates (delta R2) in the anterior tibialis muscle during dorsi-flexion exercise and recovery. Using a single spin-echo technique to calculate the change in relaxation rate produced by the exercise a time resolution of 4 s was achieved for each measurement of delta R2. For a fixed workload of 70% of maximum voluntary contraction (MVC), the duration of dorsi-flexion exercise was varied and measurements of delta R2 were obtained throughout exercise and for at lease 5 min of recovery. Comparisons were made between the single echo results and those obtained using multiple echo measurements of T2 with much lower time resolution, to verify that the two techniques gave the same results. We found on average that delta R2 decreased by an average of 8.7 s-1 within the tibialis with an average rate of decrease during exercise of delta R2/ delta t(ex) = -0.061 s-2. For the high time resolution studies we consistently observed that there was a continued decrease in the measured value of delta R2 after the exercise, reaching a minimum value about a minute after the exercise ceased. This average rate of undershoot during the postexercise period was given by delta R2/ delta t(us) = -0.035 s-2. This effect has not been noted previously in MR imaging studies and may be attributed to increased flow within the tissue as contracting muscle fibers relax following exercise. The results can be interpreted using simple fast exchange or slow exchange models for tissue water relaxation. For the case of rapid exchange the changes in delta R2 may be indicative of an increase in the net water volume within the muscles, whereas in the case of slow exchange delta R2 is primarily a measure of intracellular volume increases.

Functional NMR imaging using fast spin echo at 1.5 T

Functional NMR imaging of the brains response to a simple visual task has been performed using a fast spin echo (FSE) imaging sequence at 1.5 T. The FSE method refocuses dephasing effects induced by large-scale susceptibility variations, and permits imaging in regions where macroscopic field gradients produce artifacts in gradient echo sequences. At 1.5 T, gradient echo (GRE) sequences are sensitive to the effects of brain activation, but relatively large effects may arise from large vessels and veins, and these may dominate the effects produced by smaller capillaries. Spin echo (SE) sequences with short echo times are relatively immune to large vessel effects and emphasize the susceptibility induced losses from small capillaries, but the imaging time for these sequences is prohibitive for most functional brain studies. We demonstrate that multislice functional brain imaging may be performed in reasonable imaging times at 1.5 T using an FSE imaging sequence. The FSE sequence with short echo spacing but long effective TE is sensitive to susceptibility induced effects at the capillary level. It is not sensitive to larger scale inhomogeneities such as those found in veins and can be used in regions near tissue/air boundaries. Results are shown comparing conventional GRE and FSE images in activation of the visual cortex and these are supported by theoretical calculations and phantom experiments.

A general model of microcirculatory blood flow effects in gradient sensitized MRI

A general expression is derived for the NMR signal from a fluid undergoing random directional flow such as encountered within the microcirculation. The dependence of the echo amplitude on flow velocity, sample morphology, and experimental parameters are described in terms of a temporal velocity autocorrelation function. The width of the correlation function determines whether the flow can properly be described as diffusive. Comparison is made between the velocity autocorrelation method outlined here and the IVIM model for tissue perfusion. Conditions for the validity of the latter approach for extracting physiologic information from apparent diffusion measurements are discussed. The approach outlined leads to a more robust measure of microcirculatory blood velocity from NMR measurements.

Intravascular susceptibility contrast mechanisms in tissues

The factors affecting the rate of loss of transverse magnetization in gradient echo and spin-echo pulse sequences have been quantified using computer modeling for media containing arrays of susceptibility variations. The results are particularly relevant for describing the signal losses that occur in tissues containing capillaries of altered intrinsic susceptibility from the administration of exogenous contrast agents or arising from changes in blood oxygenation. The precise magnitudes and relationship of gradient echo and spin-echo decay rates depend on geometrical factors such as the sizes and spacings of the inhomogeneities, the rate of water diffusion, field strength, and echo times. The conventional separation of contributions to transverse decay rates arising from so-called static field effects and diffusion is shown to be inappropriate for many situations of practical interest because diffusion introduces a motional averaging of the static field even in gradient echo sequences. The result of diffusion in some regimes is to reduce the decay rate from field inhomogeneities in gradient echo sequences, so that T2* is longer in media such as tissue where water diffuses reasonably rapidly, than would be the case for stationary nuclei. The effects of different types of contrast agent and the implications for functional imaging based on the effects of deoxyhemoglobin in brain tissue are considered.

Imaging oxygen tension in liver and spleen by 19F NMR

19F NMR imaging of the perfluorocarbon emulsion Fluosol has been used to study regional variations in oxygen tension in rat liver and spleen. We have used the linear dependence of spin lattice relaxation rate (1/T1) on the partial oxygen pressure (pO2) of Fluosol to determine the oxygen tension in the reticuloendothelial system (RES) of the liver and spleen of male Sprague-Dawley rats which have received serial infusions of Fluosol. Oxygen tension maps have been computed from 19F NMR images using a calibration obtained for Fluosol in vitro at 37.5 degrees C. The spatial resolution of the pO2 maps computed using this technique is 1.2 x 1.2 mm in 3-mm thick slices. Calculations from in vivo pO2 maps indicate an average change in the median pO2 of the RES from 118 to 80 mmHg for (n = 7) rats breathing 95% O2 and 5% CO2 (carbogen) and air, respectively.

NMR relaxation enhancement in gels polymerized and cross-linked by ionizing radiation: a new approach to 3D dosimetry by MRI

A new type of tissue-equivalent medium for magnetic resonance imaging of the dose distributions produced by ionizing radiation has been developed. Agarose gel is infused with acrylamide and N,N'-methylene-bis-acrylamide (Bis) comonomers, which are readily polymerized by free radical initiators in de-aerated aqueous solutions. Polymerization and cross-linking induced locally by free radical products of water radiolysis increase the rate of water proton spin relaxation gradually up to doses of about 15 Gy. The slopes of the dose-response curves at 64 MHz are 0.015 and 0.28 s-1 Gy-1 for R1 and R2, respectively. The agarose matrix as well as the high (50% by weight) relative concentration of the cross-linker (Bis) per total comonomer limit the spread of polymerization so that the spatial distribution of the radiation dose is faithfully represented in the resultant spatial distribution of relaxation rates. The gel can be imaged with conventional magnetic resonance imaging devices with high spatial resolution and accuracy. In addition, due to the well established effect of the precipitation of insoluble agglomerates of highly cross-linked acrylamide, the optical turbidity of the gel increases gradually with the absorbed dose. This may provide an additional means of visualizing the dose distribution in three dimensions. The major advantage of the acrylamide-Bis-agarose gels over those that depend on ionic chemical dosimeters, for example, Fricke-infused gels, lies in the lack of diffusion of radiation-induced chemical changes subsequent to or concurrent with irradiation.

Solubility of inert gases in PFC blood substitute, blood plasma, and mixtures

Measurements are reported of the solubility of nonreactive gases, e.g., hydrogen and xenon, in the following liquids: (a) Oxypherol (FC-43 emulsion) blood substitute, (b) blood plasma, (c) mixtures of Oxypherol and blood plasma, and (d) perfluorotributylamine. Typical results for Ostwald solubility at 25 degrees C for Xe gas in various liquids are 0.118 in H2O, 0.12 in blood plasma, and 1.51 in N(C4F9)3. Observed solubilities for the mixtures can be calculated from the relation: L(mixture) = L(emulsion)xv(emulsion) + L (plasma)xv(plasma), in which the v's are the volume fractions in the mixture. This linear relation implies that the gas dissolves independently in each liquid in the mixture. The effect of the emulsifier (Pluronic F-68, 2.6%), on gas solubility in the mixture, is small. Results for the temperature dependence of Ostwald solubility, L(T), in the range 10-37 degrees C are reported.

On the relative importance of paramagnetic relaxation and diffusion-mediated susceptibility losses in tissues

Susceptibility agents such as dysprosium may reduce the apparent T2 of a tissue by inducing magnetic field gradients so that diffusion of water molecules causes dephasing of the transverse magnetization. Gadolinium has a susceptibility that is about 30% lower than dysprosium, so that diffusion losses are expected to be only half as big, but it also may produce paramagnetic relaxation by dipolar interactions. The relative importance of these two processes is dependent on several parameters, including the metal concentration, pulse sequence timing, field strength, and the permeability of tissue interfaces to water exchange. The conditions under which exchange-mediated dipolar interactions are less important than diffusion losses have been derived for capillary borne contrast agents in realistic situations.