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

Tissue characterization using R1rho dispersion imaging at low locking fields

Measurements of the variations of spin-locking relaxation rates (R_1ρ) with locking field amplitude allow the derivation of quantitative parameters that describe different dynamic processes, such as slow molecular motions, chemical exchange and diffusion. In some samples, changes in R_1ρ values between locking frequency 0 and 200 Hz may be dominated mainly by diffusion of water in intrinsic field gradients, while those at higher locking fields are due to exchange processes. The exchange and diffusion effects act independently of each other, as confirmed by simulation and experimentally. In tissues, the relevant intrinsic field gradients may arise from the magnetic inhomogeneities caused by microvascular blood so that R_1ρ dispersion over weak locking field amplitudes (≤ 200 Hz) is affected by changes in capillary density and geometry. Here we first review the theoretical and experimental background to the interpretation of R_1ρ dispersions caused by intrinsic magnetic susceptibility variations within the tissue. We then provide new empirical results of R_1ρ dispersion imaging of the human brain and skeletal muscle at low locking field amplitudes for the first time and identify potential applications of R_1ρ dispersion imaging in clinical studies.

Dispersion of relaxation rates in the rotating frame under the action of spin-locking pulses and diffusion in inhomogeneous magnetic fields

PURPOSE

A method is described for characterizing magnetically inhomogeneous media and the spatial scales of intrinsic susceptibility variations within samples. The rate of spin-lattice relaxation in the rotating frame, R1ρ , is affected by diffusion effects to a degree that depends on the magnitude of an applied spin-locking field. Appropriate analysis of the dispersion of R1ρ with locking field may be used to characterize susceptibility variations in inhomogeneous tissues.

THEORY AND METHODS

The contribution of diffusion to R1ρ is quantified by an analytic expression derived by analyzing of the effects of diffusion through periodic variations of magnetic susceptibility and is used to predict the effects of inhomogeneities in simple phantoms. The theory is further applied to imaging to derive parametric images that portray the dimensions of susceptibility inhomogeneities independent of their magnitude.

RESULTS

Significant dispersion of R1ρ with locking field was predicted and measured experimentally for suspensions of microspheres ranging from 1 to 90 μm in diameter. For scales of practical interest, these dispersion effects occur at much lower locking fields than the range in which chemical exchange effects cause similar dispersion.

CONCLUSION

There is good agreement between theory and experiment, and the method has potential for quantitative tissue characterization and functional imaging.

Surface-to-volume ratio with oscillating gradients

Restrictions to diffusion result in the dispersion of the bulk diffusion coefficient. We derive the exact universal high-frequency behavior of the diffusion coefficient in terms of the surface-to-volume ratio of the restrictions. This frequency dependence can be applied to quantify structure of complex samples with NMR using oscillating field gradients and static-gradient CPMG. We also demonstrate the inter-relations between different equivalent diffusion metrics, and describe how to calculate the effect of restrictions for arbitrary gradient waveforms.

Gadolinium-enhanced off-resonance contrast angiography

We describe a novel physical basis and methodology for gadolinium (Gd)-enhanced MRA, which we call "off-resonance contrast angiography" (ORCA). Unlike standard contrast-enhanced (CE) MR angiography (MRA), ORCA contrast depends not on T(1) but on Gd-induced shifts in intravascular resonance frequency due to the bulk magnetic susceptibility (BMS) effects of Gd. The method was tested at 3 Tesla in phantoms with a range of dilutions of Gd-DTPA and ultrasmall iron oxide contrast agent (CA). With the use of ORCA, complete background suppression was obtained without image subtraction. As a result, catheters filled with various Gd dilutions proved to be highly conspicuous in ORCA projection images. This feature may make ORCA particularly attractive for passive catheter tracking during MR-guided endovascular procedures. Gd-induced intravascular frequency shifts were measured in human subjects and found to be in the expected range. ORCA was used to create angiograms of forearm veins that were comparable in quality to standard CE-MRA. In addition, ORCA images of the extracranial carotid bifurcation were successfully acquired during intravenous contrast administration. However, significant technical restrictions also exist, including a dependence on vessel orientation with respect to B(0), and sensitivity to static field inhomogeneities. Further study is needed to determine the practicality and potential clinical utility of this method.

Analysis of the meal-dependent intragastric performance of a gastric-retentive tablet assessed by magnetic resonance imaging

BACKGROUND

Modern medical imaging modalities can trace labelled oral drug dosage forms in the gastrointestinal tract, and thus represent important tools for the evaluation of their in vivo performance. The application of gastric-retentive drug delivery systems to improve bioavailability and to avoid unwanted plasma peak concentrations of orally administered drugs is of special interest in clinical and pharmaceutical research.

AIM

To determine the influence of meal composition and timing of tablet administration on the intragastric performance of a gastric-retentive floating tablet using magnetic resonance imaging in the sitting position.

METHODS

A tablet formulation was labelled with iron oxide particles as negative magnetic resonance contrast marker to allow the monitoring of the tablet position in the food-filled human stomach. Labelled tablet was administered, together with three different solid meals, to volunteers seated in a 0.5-T open-configuration magnetic resonance system. Volunteers were followed over a 4-h period.

RESULTS

Labelled tablet was detectable in all subjects throughout the entire study. The tablet showed persistent good intragastric floating performance independent of meal composition. Unfavourable timing of tablet administration had a minor effect on the intragastric tablet residence time and floating performance.

CONCLUSION

Magnetic resonance imaging can reliably monitor and analyse the in vivo performance of labelled gastric-retentive tablets in the human stomach.

Macroscopic susceptibility in ultra high field MRI. II: acquisition of spin echo images from the human head

The presence of magnetic susceptibility can lead to substantial geometric distortions when imaging the human head at 8 T. These are particularly significant in gradient echo images where susceptibility often results in a noticeable loss of MR signal in the temporal lobe, the frontal lobe, and the paranasal sinus regions. In this work, anterior coronal gradient and spin echo images were acquired from the frontal lobe and sinus regions. The spin echo was shown to significantly overcome the loss of signal observed in the corresponding gradient echo images, resulting in data of greatly increased quality. In conclusion, whereas susceptibility artifacts are significant in ultra high field MRI, they can be largely surmounted by using spin echo techniques, as had been previously demonstrated in studies at lower field strength.

Black blood magnetic resonance angiography with Dy-DTPA polymer: effect on arterial intraluminal signal intensity, lumen diameter, and wall thickness

Four rabbits in which atherosclerotic disease was induced by diet and balloon angioplasty underwent conventional angiography and MR angiography (MRA) using a black blood pulse sequence before and 10 minutes after the i.v. injection of a macromolecular contrast agent, NC 100283 (1.0 mmol/kg), a dysprosium diethylenetriaminepentaacetic acid hexamethylenediamine copolymer (Dy-DTPA polymer). Intraluminal signal intensity, apparent wall thickness, and lumen size measurements of the aorta and proximal common iliac arteries on precontrast MRA images were compared with postcontrast images. Aortic lumen diameter measurements on the precontrast and postcontrast MRA studies were compared with lumen diameters from conventional angiograms. Intraluminal signal intensity decreased on postcontrast MRA images compared with precontrast images, with an average loss of signal equal to 29% (P < .05). Apparent wall thickness decreased by 24% (P < .05). Lumen diameter and area were generally larger (average of 15% and 33%, respectively) on postcontrast MRA images than on precontrast images. Aortic lumen diameter measurements from postcontrast MRA agreed closely (95% confidence interval of the mean difference was -.2 to .3 mm), and precontrast MRA images tended to underestimate aortic lumen diameter (95% confidence interval of the mean difference was .3 to .8 mm) compared with conventional angiography. Postcontrast MRA with NC 100283, a macromolecular Dy-DTPA contrast agent, provides more accurate assessment of aortic lumen diameter than precontrast MRA, using conventional angiography as the standard reference.

Non-invasive in vivo mapping of tumour vascular and interstitial volume fractions

Non-invasive measurement of haemodynamic parameters and imaging of neovasculature architecture is of importance in determining tumour prognosis, in directing tissue sampling and in assessing treatment efficacy. In the current research we investigated a dual tracer nuclear magnetic resonance (NMR) technique to map the tumour vascular (VVF) and interstitial volume fraction (IVF) non-invasively in vivo. We hypothesised that a NMR signal emanating after intravenous administrations of a vascular paramagnetic probe (MPEG-PL-GdDTPA) can be maximised so that additional signal after administration of a second interstitial probe (GdDTPA) would only reflect the IVF but not the VVF. The method and its assumptions were verified and experimental conditions optimised both in phantoms and in C6 glioma bearing rats. Data derived from in vivo studies show tumoral VVF and IVF values that are consistent with histology data and literature values; the relative ranking order of values was tumour > muscle > brain. Image maps showed intratumoral and intertumoral heterogeneity of both parameters at submillimetre pixel resolution. The method is applicable to a wide variety of tumour models and can theoretically be performed repeatedly to study tumour growth or involution during therapy.

Low-molecular weight lanthanide contrast agents: evaluation of susceptibility and dipolar effects in red blood cell suspensions

Red blood cell (RBC) suspensions, containing low-molecular weight (LMW) dysprosium (Dy) and gadolinium (Gd) chelates, were selected as a two-compartment system for the evaluation of the magnetic dipolar and susceptibility contributions to the transverse (T2) relaxation of solvent water protons. The influence of RBC geometry and degree of metal chelate compartmentalization on T2 was investigated by variation of the osmolality and hematocrit (HC), respectively. The T2-relaxation ability of Dy-chelates was markedly improved in RBC suspensions, in comparison to aqueous solutions, due to the presence of susceptibility effects that more than compensated for the low dipolar relaxation efficacy. Despite a smaller susceptibility effect, the Gd-chelates were still the most efficacious in shortening T2 due to their comparatively larger dipolar relaxation contribution. The results obtained with the Dy-chelates allowed the evaluation of the relative contributions of susceptibility and dipolar mediated relaxation for the Gd-chelates. The RBC geometry and degree of compartmentalization influenced strongly the T2 relaxation efficacy of Dy-chelates, as opposed to the Gd-chelates. Hemolysis eliminated the susceptibility effect, essentially removing the T2 relaxation ability of Dy-chelates. The T2 relaxation efficacy of Gd-chelates was improved by hemolysis due to enhancement of the dipolar relaxation. As a conclusion, RBC suspensions have clearly been shown to be a suitable ex vivo model with which to distinguish the different contrast mechanisms of LMW Dy- and Gd-based MRI contrast agents.

The relaxivity of Gd-EOB-DTPA and Gd-DTPA in liver and kidney of the Wistar rat

The NMR relaxivities of Gd-EOB-DTPA and Gd-DTPA were determined in the kidney and liver of intact male Wistar rats immediately following sacrifice and in vitro in solutions and gels, at 1.5 T using a clinical MR scanner, T1 and T2 values of tissue samples were derived from spin-echo image sequences. Tissue gadolinium concentrations were determined by radioassay of Gd153, Gd-EOB-DTPA T1 and T2 relaxivities, R1 and R2 (s-1 mmole-1 kg), were found to be 10.7 +/- 0.5 and 22.5 +/- 3.2 respectively, for liver, 2.4 +/- 0.2 and 12.1 +/- 1.7 for kidney cortex, 2.7 +/- 0.2 and 14.5 +/- 1.9 for kidney outer medulla, 2.0 +/- 0.2 and 11.4 +/- 2.1 for kidney inner medulla. Gd-DTPA R1 and R2 were found to be 4.8 +/- 0.4 and 14.5 +/- 3.7 for liver, 1.2 +/- 0.1 and 7.9 +/- 0.8 for kidney cortex, 1.6 +/- 0.1 and 10.2 +/- 1.4 for kidney outer medulla, 1.3 +/- 0.1 and 10.2 +/- 1.2 for kidney inner medulla. Gd-EOB-DTPA and Gd-DTPA R1 was increased in liver compared to agarose gets at 38 degrees C (4.49 +/- 0.03 and 3.47 +/- 0.06), but reduced in kidney tissues. All R2 were elevated compared to agarose gels at 38 degrees C (5.72 +/- 0.12 and 4.12 +/- 0.03). Elevated R2 and R1 (expressed in terms of the concentration of gadolinium per kg of tissue) can be accounted for in part by the lower water content of tissues compared with gels or solutions increased microviscosity and binding to macromolecules. In addition, susceptibility effects may give rise to further increases in R2. By contrast, the reduced R1 observed in kidney may be the result of compartmentalization of the magnetopharmaceuticals. Statistically improved fits were obtained for T1 recovery curves for liver in the presence of Gd-EOB-DTPA when a dual exponential model was used. Assuming in vitro values for the relaxivities of these artificial contrast agents will lead to inaccuracies when relating observed signal enhancement factors to tissue gadolinium concentration.

A magnetization-driven gradient echo pulse sequence for the study of myocardial perfusion

A T1-weighted imaging pulse sequence for contrast-based studies of myocardial perfusion is presented and evaluated in phantoms and in vivo. The sequence is similar to spoiled gradient-recalled echo sequences except that nonselective preparatory RF pulses drive magnetization to steady state prior to image acquisition. Steady state is thus obtained in both tissue and blood resulting in a stable, homogeneous, and dark pre-contrast baseline. Tip angles and timings are chosen so that pixel intensity approximates a linear relation to 1/T1. The dynamic range of signal response to contrast agent concentration is greater than that of an inversion-recovery fast low angle shot sequence. The sequence proposed should be useful for myocardial perfusion studies.

Comparison of T2 relaxation in blood, brain, and ferritin

T2 was measured in samples of human blood and monkey brain over a field range of 0.02-1.5 Tesla, with variable interecho times, and was compared with previous data on ferritin solutions (taken with the same apparatus). 1/T2 in deoxygenated blood increased quadratically with field strength, as noted previously, but in brain gray matter the increase was linear, as also was the case in ferritin solution. In both deoxygenated blood and gray matter, 1/T2 increased with interecho time, but appeared to level off at times around 50 msec, as expected from the theory of diffusion through magnetic gradients. Diffusion times estimated by using the chemical exchange approximation were 3.4 msec for deoxygenated blood and 5.7 msec for the globus pallidus. The quadratic field dependence in blood is consistent with this same theory, but the linear dependence in brain tissue and in ferritin solutions remains unexplained.

Assessment of myocardial perfusion using contrast-enhanced MR imaging: current status and future developments

Excellent inherent tissue contrast is one of the great promises of clinical magnetic resonance (MR) imaging, but functional information is relatively limited. However, MR imaging complemented by the administration of contrast agents can provide such functional assessment. The perfusion status of the myocardium is one of the most important functional information in cardiovascular imaging. Because the clinical acceptance of a contrast agent is measured by its ability to improve patient outcome and to guide therapy, it is unlikely that detection of myocardial infarction, the final stage of ischemic heart disease, should be the target for contrast media development. It would obviously be better if occult regional myocardial perfusion deficits could be reliably detected. The current article was prepared to help the clinical radiologist to keep pace with new strategies for myocardial enhancement and their potential clinical applicability for detection of early perfusion deficits. Several techniques for noninvasive measurement of myocardial perfusion are currently evolving which have the potential to be introduced into routine MR imaging. Most investigators favor a first-pass analysis of the contrast agent bolus through the myocardium using ultrafast sequences. However, such a technique may require clinical introduction of a blood pool agent. There are good reasons to favor T1-weighted sequences over susceptibility imaging in such first-pass studies. In the future, assessment of myocardial perfusion status using contrast-enhanced MR imaging may be done producing perfusion maps with high spatial resolution (e.g., 256 x 128), with sequences available on most scanners without special hardware requirements (e.g., IR-Turboflash, keyhole imaging), and requiring only a short period of time for examination (approximately 3 min).

Effects of myocardial water exchange on T1 enhancement during bolus administration of MR contrast agents

Interpretation of first-pass myocardial perfusion studies employing bolus administration of T1 magnetic resonance (MR) contrast agents requires an understanding of the relationship between contrast concentration and image pixel intensity. The potential effects of myocardial water exchange rates among the intravascular, interstitial, and cellular compartments on this relationship are controversial. We directly studied these issues in isolated, nonbeating canine interventricular septa. Myocardial T1 was measured three times/s during bolus transit of intravascular (albumin-Gd-DTPA and polylysine-Gd-DTPA) and extracellular (gadoteridol) contrast agents. For polylysine-Gd-DTPA, the peak changes in myocardial 1/T1 (delta R1) scaled nonlinearly with perfusate contrast concentration whereas a linear relationship would be expected for fast water exchange among the vascular, interstitial, and cellular compartments. For all agents, the peak delta R1 were much smaller than the values expected on the basis of fast myocardial water exchange. The data demonstrate that in isolated myocardial tissue, myocardial T1 enhancement during bolus administration of contrast can be strongly affected by myocardial water exchange for both intravascular and extracellular MR contrast agents.

The effect of sacrifice on image signal, T1, T2, and T*2 in liver, kidney, and brain of the Wistar rat

Signal intensity in SE240/25 and GE240/25 images, T*2, T2, and T1 were measured prior to and after sacrifice at 1.5 T in liver, kidney, and brain of Wistar rats. Presacrifice, animals were maintained on 2-3% halothane, ensuring a high saturation of venous haemoglobin with oxygen, which emphasised blood oxygenation level dependent (BOLD) effects upon deoxygenation of haemoglobin postsacrifice. T*2 values (in ms) changed on sacrifice from 27.8 +/- 4.1 to 15.0 +/- 1.1 in liver (p << .001), from 57.4 +/- 6.5 to 52.8 +/- 5.7 in brain (p < .02), and from 52.3 +/- 10.3 to 37.6 +/- 11.9 in kidney cortex (p < .01). T*2 was also reduced in kidney medulla and papilla. T2 reduced on sacrifice (by a lesser amount than T*2), except in brain, which showed a small increase. T1 (ms) changed only in liver, showing a significant increase from 670 +/- 31 to 834 +/- 69 (p << .001). The ratio of signal postsacrifice to presacrifice was lowest in the SE240/25 and GE240/25 images of liver (0.73 +/- 0.03 and 0.52 +/- 0.05, respectively), but increased T1 may account for some of these changes. Expected signal ratios at sacrifice calculated from measured relaxation times agreed well with those observed experimentally. Relaxation changes observed on sacrifice were consistent with deoxygenation of haemoglobin. These results give an estimate of the maximum effects which changes of blood oxygen level might produce at 1.5 T and suggest that contrast due to variation of endogenous deoxyhaemoglobin may be exploitable in the liver.

Oxygen tension in a murine tumor: a combined EPR and MRI study

The effects of fusinite, a new agent for the measurement of the concentration of oxygen in vivo by EPR, on MRI images have been studied. There was little effect on spin-echo T1-weighted images, but the fusinite resulted in large effects on T2-weighted images. Especially large effects could be observed when using spoiled gradient echo sequences (SPGR). The observed measurements of oxygen by EPR corresponded to the relative vascularity at the site of the fusinite both histologically and by MRI studies of vascularity using Gd-DTPA as a contrast agent. We conclude that by using the effects of fusinite on magnetic susceptibility, it can be located accurately and noninvasively with MRI and thereby the value of the use of fusinite to measure concentration of oxygen in vivo is enhanced.

Susceptibility changes following bolus injections

The general mechanism of bulk magnetic susceptibility (BMS) induced MRI contrast following a bolus injection is elaborated. Combining radiolabeled tracer data for the first pass of a bolus injection through the human brain with the application of Wiedemann's law allows us to calculate the lower limit for the time course of the vascular BMS following the injection of any contrast agent. Superparamagnetic iron oxide particles produce a much larger effect than any mononuclear Ln(III) chelate. We also calculate the BMS changes occurring after a dilution bolus injection (of isosmolal physiological saline) subsequent to a prior slow infusion of an intravascular contrast agent. This technique bears some resemblance to the increasingly important approach that exploits changes in only the level of blood oxygenation. The calculation indicates that contrast changes after the dilution bolus injection are smaller than those following Ln(III) agent injections but larger than those due to changes in blood oxygenation and suggests a way to possibly enhance the latter. We present an in vivo study demonstrating the dilution bolus injection technique in the mouse brain, and that features its rapid repeatability. Extrapolation of these results to the human, however, indicates that the saline volumes required for venous injections, except possibly for cardiac studies, would be prohibitively large. Smaller, catheter-delivered arterial bolus injections are feasible. We also suggest a method for using an agent bolus injection to measure the parenchymal BMS, and thus the iron content, of pathologically iron-loaded tissue.

Proton relaxation enhancement

Paramagnetic and superparamagnetic substances are used as contrast agents to enhance proton relaxation in magnetic resonance imaging. This review summarizes the physics of contrast agents, specifically the mechanisms by which contrast agents enhance T1 and T2 relaxation. The purpose is to provide a background for understanding the behavior of existing contrast agents in basic experimental and clinical studies. Terms such as magnetic dipole, dipole moment, magnetic susceptibility, diamagnetism, paramagnetism, superparamagnetism, and ferromagnetism are introduced. Two important interactions between the magnetic dipole moments of paramagnetic substances and the dipole moments associated with protons are described. The Solomon-Bloembergen-Morgan equations and other basic relaxation theory that has been confirmed experimentally are introduced to account for the dependence of relaxation on such parameters as the Larmor frequency, magnetic moment, accessibility of water molecules to the core of a contrast agent, and frequency of molecular motions.