Motion dependence of myocardial transverse relaxation time in magnetic resonance imaging

We discuss the effects of motion on the computation of the myocardial transverse relaxation time by use of magnetic resonance imaging. Equations describing its behavior are derived and illustrated graphically under different conditions. It is shown that the myocardial transverse relaxation time calculated from magnetic resonance images depends on the actual myocardial transverse relaxation time ex vivo (T2) as well as the phase of the cardiac cycle in which it is computed, heart rate, cardiac wall velocity, choice of spin-echoes used in the calculation, and the spin-echo times employed. In particular, the error in T2 decreases when both the first and third echoes are employed in the calculation, rather than only the first two echoes. However, the myocardial transverse relaxation time is more strongly dependent on heart rate in the former case rather than in the latter. Furthermore, the error in T2, when both the first and second spin echoes are used in the calculation, is seen to increase as the spin-echo time shortens. On the other hand, the error in T2 decreases for shorter spin-echo times when both the first and third spin echoes are used instead. The results are relevant to the noninvasive assessment of ischemia, cardiac transplantation rejection, and other myocardial disorders.     

Identification of patients with hereditary haemochromatosis by magnetic resonance imaging and spectroscopic relaxation time measurements

A total of 4302 healthy blood donors were screened for elevated serum ferritin and transferrin saturation. Fifteen had increased serum ferritin at a follow-up examination. Five relatives of these donors also entered the study. Eleven patients had elevated liver iron concentrations, while five had normal liver iron concentrations. The R₂ relaxation rate in the liver was first measured with a conventional multi-spin-echo imaging sequence, and then by a volume-selective spectroscopic multi-spin-echo sequence, in order to achieve a minimum echo time of 4 msec. No correlation was found between the relaxation rate R₂ and the liver iron concentration, when R₂ was calculated from the imaging data. Multi-exponential transverse relaxation could be resolved when the spectroscopic sequence was used. A strong correlation between the initial slope of the relaxation curve and the liver iron concentration was found (r = 0.90, p < 0.001). Signal intensity ratios between liver and muscle were calculated from the first three echoes in the multi-echo imaging sequence, and from a gradient echo sequence. A strong correlation between the logarithm of the signal intensity ratios and the liver iron concentration was found. Although both spectroscopic T₂ relaxation time measurements and signal intensity ratios could be used to quantify liver iron concentration, the gradient echo imaging seemed to be the best choice. Gradient echo imaging could be performed during a single breath hold, so motion artifacts could be avoided. The accuracy of liver iron concentration estimates from signal intensity ratios in the gradient echo images was about 35%.     

The dependence on magnetic field strength of correlated internal gradient relaxation time distributions in heterogeneous materials

Magnetic susceptibility differences in porous media produce local gradients within the pore space. At high magnetic fields, these inhomogeneities have the potential to greatly affect nuclear magnetic resonance measurements. We undertake a study using a new NMR technique to measure the internal gradients present in highly heterogeneous samples over a wide range of magnetic field strengths. Our results show that even at ultra-high fields there can exist signal at internal gradient strengths sufficiently small that techniques for suppressing unwanted side effects have the possibility to be used. Our findings encourage the use of these high and ultra-high field strengths for a broader range of samples. Our results also give experimental evidence to support the theory of internal gradient scaling as a function of field strength within pores.     

Transverse relaxation time constraints on resolution in one-dimensional,phase- and frequency-encoded nuclear magnetic resonance imaging

The theoretical dependence of the resolution on the relationship of sampling time to transverse relaxation time (T₂) for frequency-encoded, one-dimensional NMR imaging using constant field gradients has been investigated. A resolution function that is explicitly dependent on the sampling time is derived, and it is shown that the observed image of an object can be written as a convolution of the sample magnetization with this resolution function. This function is explicitly calculated for two cases of interest: (1) for sampling times much shorter than T₂, and (2) for sampling times much longer than T₂. These cases are illustrated for two examples: (1) a uniform magnetic bar, and (2) uniform periodic magnetic bars. When oscillating gradients are utilized, these results still hold in the limit of slow oscillation. The resolution in phase-encoded NMR imaging is not explicitly dependent on the sampling time.     

Centric scan SPRITE magnetic resonance imaging: optimization of SNR, resolution, and relaxation time mapping

Two strategies for the optimization of centric scan SPRITE (single point ramped imaging with T1 enhancement) magnetic resonance imaging techniques are presented. Point spread functions (PSF) for the centric scan SPRITE methodologies are numerically simulated, and the blurring manifested in a centric scan SPRITE image through PSF convolution is characterized. Optimal choices of imaging parameters and k-space sampling scheme are predicted to obtain maximum signal-to-noise ratio (SNR) while maintaining acceptable image resolution. The point spread function simulation predictions are verified experimentally. The acquisition of multiple FID points following each RF excitation is described and the use of the Chirp z-Transform algorithm for the scaling of field of view (FOV) of the reconstructed images is illustrated. Effective recombination of the rescaled images for SNR improvement and T*2 mapping is demonstrated.     

Nondestructive detection of internal bruise and spraing disease symptoms in potatoes using magnetic resonance imaging

Magnetic resonance imaging (MRI) was applied to detect nonvisible internal bruise and spraing symptoms and to get insight on the chemical and anatomical causes of such defects. Cultivar Saturna with internal bruise and cultivar Estima with spraing symptoms were investigated by comparison of different MR images as proton density-, T(1)- and T(2)-weighted images and T(2) maps. In all these types of MR images, it was possible to identify internal bruise and spraing spots in the potatoes, where these phenomena were present. When combining the information in the MR images, the interior of the internal bruise was characterised as being very dry (low signal in the proton-weighted image) with a small amount of highly mobile water in the shell around the bruise (high signal in T(2)-weighted image and high relaxation time in T(2) map). The spraing spots were more diffuse; however, the dry interior and highly mobile water around the spraing dots were somewhat similar to the appearance of internal bruise but resembled more the appearance of human tumour tissue than bruise disorders in, for example, fruits. In conclusion, this study demonstrated that MRI can detect nonvisible internal bruise and spraing symptoms in potatoes, which has not been published before. MRI may, therefore, be an appropriate method for detecting and for studying developmental changes of such disorders and related disorders during postharvest storage in future experiments.     

11B magnetic resonance imaging and MAS spectroscopy of trimethylborate-treated radiata pine wood

Boron-11 nuclear magnetic resonance imaging and spectroscopy have been used to characterise the nature and distribution of boron compounds after preservative treatment of radiata pine wood with trimethylborate (TMB). One day after treatment, 11B magnetic resonance imaging microscopy showed significant differences in the morphological distribution of boron species, with there apparently being no treatment penetration of the latewood. 11B MAS NMR spectroscopy of freshly preservative-treated radiata pine wood, which had been separated into latewood and earlywood, showed the only boron species present in the latewood to be boric acid while in the earlywood both TMB and boric acid were initially present. Due to quadrupolar broadening and a short T2 value, the boric acid signal relaxed too quickly to be observable and this caused the apparent lack of 11B in the image when only boric acid was present. TMB undergoes hydrolysis to form boric acid and in radiata pine latewood, this hydrolysis reaction is rapid.     

Assessment of pulmonary artery stiffness using velocity-encoding magnetic resonance imaging: evaluation of techniques

The loss of pulmonary artery (PA) compliance has significant pathophysiological effect on the right ventricle. Noninvasive and reliable assessment of PA wall stiffness would be an essential determiner of right heart load and a clinically useful factor to assess cardiovascular risk. Two MRI techniques have been proposed for assessing PA stiffness by measuring pulse wave velocity (PWV): transit time (TT) and flow area (QA). However, no data are available that compares the two techniques and evaluates their performance, especially over a wide range of PWV values or at 3.0-T, which is the purpose of the present study. Thirty-three patients with different heart conditions were imaged using optimized high-temporal resolution and high-spatial resolution velocity-encoding MRI sequences. Statistical analysis was conducted to study intermethod, interobserver and intraobserver variabilities. The PWV measurements using TT and QA techniques showed good agreement (P>0.1). The Bland-Altman analysis showed negligible differences between the two methods (mean±S.D.=0.11±0.35 m/s, correlation coefficient r=0.94). The repeated measurements showed low interobserver and intraobserver variabilities, although the S.D. of the differences was larger in the QA technique. The mean±S.D. of the TT/QA measurement differences were −0.05±0.2/0.0±0.36 m/s and 0.02±0.26/0.02±0.39 m/s for the interobserver and intraobserver differences, respectively. In conclusion, each technique has its own advantages and disadvantages. The two techniques result in similar measurements, although the QA method is more subjective due to its dependency on operator intervention.     

Relationship between acetabular labrum evaluation by using radial magnetic resonance imaging and progressive joint space narrowing in mild hip dysplasia

OBJECTIVE: The aim of this study was to investigate the relationship between abnormal acetabular labrum depicted by radial magnetic resonance and progressive joint space narrowing (JSN) of hip dysplasia. METHODS: Subjects were 23 joints [21 patients; mean age: 35.1 years (16-53 years)] that had acetabular dysplasia with lateral center-edge angle of Wiberg (CE angle) greater than 5 degrees and smaller than 25 degrees (mean, 16.4 degrees ), which did not show any arthrotic changes on plain radiograms and were followed up for 3 years or longer. Radial images of acetabular labrum were classified into three stages. RESULTS: Progression of JSN was not significantly related to CE angle but to progression of MRI stage (P=.006). In multivariate analysis, one rank progression of MRI stage was significantly associated with progression of JSN (adjusted OR=11.41, 95% CI: 1.51-86.24, P=.018). CONCLUSION: Our findings showed that in patients whose acetabular dysplasia has 5-25 degrees CE angle, MRI staging based on radial MRI is a better factor for prediction of progression of JSN than CE angle.     

Earlier detection of tumor treatment response using magnetic resonance diffusion imaging with oscillating gradients

An improved method for detecting early changes in tumors in response to treatment, based on a modification of diffusion-weighted magnetic resonance imaging, has been demonstrated in an animal model. Early detection of therapeutic response in tumors is important both clinically and in pre-clinical assessments of novel treatments. Noninvasive imaging methods that can detect and assess tumor response early in the course of treatment, and before frank changes in tumor morphology are evident, are of considerable interest as potential biomarkers of treatment efficacy. Diffusion-weighted magnetic resonance imaging is sensitive to changes in water diffusion rates in tissues that result from structural variations in the local cellular environment, but conventional methods mainly reflect changes in tissue cellularity and do not convey information specific to microstructural variations at sub-cellular scales. We implemented a modified imaging technique using oscillating gradients of the magnetic field for evaluating water diffusion rates over very short spatial scales that are more specific for detecting changes in intracellular structure that may precede changes in cellularity. Results from a study of orthotopic 9L gliomas in rat brains indicate that this method can detect changes as early as 24 h following treatment with 1,3-bis(2-chloroethyl)-1-nitrosourea, when conventional approaches do not find significant effects. These studies suggest that diffusion imaging using oscillating gradients may be used to obtain an earlier indication of treatment efficacy than previous magnetic resonance imaging methods.     

Rapid determination of the RF pulse flip angle and spin-lattice relaxation time for materials imaging

For samples with T1s longer than 10s, calibration of the RF probe and a measurement of T1 can be very time-consuming. A technique is proposed for use in imaging applications where one wishes to rapidly obtain information about the RF flip angle and sample T1 prior to imaging. The flip angle measurement time is less than 1s for a single scan. Prior knowledge of the RF flip angle is not required for the measurement of T1. The resulting time savings in measuring the values of flip angle and T1 are particularly significant in the case of samples with very long T1 and short T2*. An imaging extension of the technique provides RF flip angle mapping without the need for incrementing the pulse duration, i.e., RF mapping can be performed at fixed RF amplifier output.     

Dynamic magnetic resonance imaging of swallowing and laryngeal motion using parallel imaging at 3 T

Object

To evaluate the feasibility of an optimized MRI protocol based on high field imaging at 3 T in combination with accelerated data acquisition by parallel imaging for the analysis of oropharyngeal and laryngeal function.

Materials and Methods

Fast 2D gradient echo (GRE) MRI with different spatial resolutions (1.7×2.7 and 1.1×1.5 mm²) and image update rates (4 and 10 frames per second) was employed to assess pharyngeal movements and visualize swallowing via tracking of an oral contrast bolus (blueberry juice). In a study with 10 normal volunteers, image quality was semi-quantitatively graded by three independent observers with respect to the delineation of anatomical detail and depiction of oropharynx and larynx function. Additionally, the feasibility of the technique for the visualization of pathological pre- and post-surgical oropharynx and larynx function was evaluated in a patient with inspiratory stridor.

Results

Image grading demonstrated the feasibility of dynamic MRI for the assessment of normal oropharynx and larynx anatomy and function. Superior image quality (P<.05) was found for data acquisition with four frames per second and higher spatial resolution. In the patient, dynamic MRI detected pathological hypermobility of the epiglottis resulting in airway obstruction. Additional post-surgical MRI for one clinical case revealed morphological changes of the epiglottis and improved function, i.e., absence of airway obstruction and normal swallowing.

Conclusion

Results of the volunteer study demonstrated the feasibility of dynamic MRI at 3 T for the visualization of the oropharynx and larynx function during breathing, movements of the tongue and swallowing. Future studies are necessary to evaluate its clinical value compared to existing modalities based on endoscopy or radiographic techniques.     

Four-angle method for practical ultra-high-resolution magnetic resonance mapping of brain longitudinal relaxation time and apparent proton density

Changes in longitudinal relaxation time (T₁) and proton density (PD) are sensitive indicators of microstructural alterations associated with various central nervous system diseases as well as brain maturation and aging. In this work, we introduce a new approach for rapid and accurate high-resolution (HR) or ultra HR (UHR) mapping of T₁ and apparent PD (APD) of the brain with correction of radiofrequency field, B₁, inhomogeneities. The four-angle method (FAM) uses four spoiled-gradient recalled-echo (SPGR) images acquired at different flip angles (FA) and short repetition times (TRs). The first two SPGR images are acquired at low-spatial resolution and used to accurately map the active B₁⁺ field with the recently introduced steady-state double angle method (SS-DAM). The estimated B₁⁺ map is used in conjunction with the two other SPGR images, acquired at HR or UHR, to map T₁ and APD. The method is evaluated with numerical, phantom, and in-vivo imaging measurements. Furthermore, we investigated imaging acceleration methods to further shorten the acquisition time. Our results indicate that FAM provides an accurate method for simultaneous HR or UHR mapping of T₁ and APD in human brain in clinical high-field MRI. Derived parameter maps without B₁⁺correction suffer from large inaccuracies, but this issue is well-corrected through use of the SS-DAM. Furthermore, the use of SPGR imaging with short TR and phased-array coil acquisition permits substantial imaging acceleration and enables robust HR or UHR T₁ and APD mapping in a clinically acceptable time frame, with whole brain coverage obtained in less than 2 min or 5 min, respectively. The method exhibits high reproducibility and benefits from the use of the conventional SPGR sequence, available in all preclinical and clinical MRI machines, and very simple modeling to address a critical outstanding issue in neuroimaging.     

Overall and internal motions of substituted benzenes studied by means of selective proton magnetic longitudinal relaxation time measurements

Selective T ₁ measurements at different temperatures on seven substituted benzenes in CDCl₃ solution show that for these molecules the rotational diffusion model applies. Anisotropic reorientation is important if the molecules contain two polar substituents para to each other. Anomalies in the temperature dependence of the T ₁ values in a 2-methyl substituted phenol are explained by intermolecular hydrogen bonds, which strongly influence the molecular motion. In a 2,6-dimethyl substituted phenol this effect is absent. The correlation times and the energy barriers for methyl rotation of methyl and methoxy groups are determined. The reorientation of methoxy groups around the aryl oxygen bond is slower than the molecular motion. The T ₁ values of ring protons and substituents can in some cases be used for spectral assignment. Expressions are given for the T ₁ value of a ring proton relaxed by an ortho methyl or methoxy group and for the T ₁ value of a reorienting methoxy group in the case of dipolar relaxation and axially symmetric behaviour of the molecule.     

Labeling of macrophages using bacterial magnetosomes and their characterization by magnetic resonance imaging

This work investigated macrophages labeled with magnetosomes for the possible detection of inflammations by MR molecular imaging. Pure magnetosomes and macrophages containing magnetosomes were analyzed using a clinical 1.5 T MR-scanner. Relaxivities of magnetosomes and relaxation rates of cells containing magnetosomes were determined. Peritonitis was induced in two mice. T₁, T₂ and T₂* weighted images were acquired following injection of the probes. Pure magnetosomes and labeled cells showed slight effects on T₁, but strong effects on T₂ and T₂* images. Labeled macrophages were located with magnetic resonance imaging (MRI) in the colon area, thus demonstrating the feasibility of the proposed approach.