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PubMed1987

Instant images of the human heart using a new, whole-body MR imaging system.

An extremely rapid MR imaging technique is described, and its use on a new 2.0-T high-speed MR system is demonstrated. This implementation permits complete filling of the two-dimensional spatial-frequency domain (k-space) within an acquisition window of 26 msec. With this acquisition window placed under the spin-echo signal envelope generated by a 90-180 degree pulse pair, the image contrast is the same as that of a conventional spin-echo pulse sequence. Resultant proton images have a motion-independent voxel resolution of 0.08 cm3 and a signal-to-noise ratio for cardiac muscle of approximately 30:1 (for TE = 30 msec) with no signal averaging. The pulse sequence yields images that are chemical shift-resolved. The total proton density distribution is optionally presented with lipid and water signals displayed in two different colors. Cardiac function is observed by displaying multiple images, acquired at different times in successive cardiac periods, in a cyclic movie format. Such motion pictures are obtained within a single period of suspended respiration, thereby assuring freedom from respiratory related motion artifacts. As preliminary examples, we present MR images of the normal adult human heart that have total acquisition times of only 40 msec/image and that show the major cardiac anatomy. Frames from movie loops show contraction of cardiac chambers and left ventricular wall thickening. The extremely rapid acquisition time of this technique suggests that it may hold promise for the routine and cost-effective evaluation of cardiac anatomy and function.

Adult↗

PubMed1996

Investigation of molecular order and dynamics in liquid crystals confined in porous media using the dipolar-correlation effect on the stimulated echo.

A new application of the stimulated echo pulse sequence is presented that permits the elucidation of molecular order and dynamics in a time scale between about 100 microseconds and the spin-lattice relaxation time. The technique exploits the influence of dipolar coupling on the quotient of the stimulated and primary echoes produced by the standard three 90 degrees-pulse sequence. Results obtained for a nematic liquid crystal in bulk and confined in porous glass (mean pore diameter 4 nm) are compared. In both cases the echo amplitude quotient oscillates as a function of the pulse spacing. In a bulk nematic crystal these oscillations originate from strong unaveraged dipolar interactions and directly reflect the molecular order in the material. In porous glass a real nematic order is absent. In this case, the oscillations can be attributed to spin exchange between inequivalent protons. Exchange rates are estimated.

Crystallography↗

PubMed1993

Generalized K-space analysis and correction of motion effects in MR imaging.

A new approach to understanding and reducing motion artifacts in magnetic resonance imaging (MRI) is introduced. This paper presents a novel technique for correcting generalized motion artifacts arising from translation, rotation, dilation, and compression, or any combination thereof. We also describe a new pulse sequence and a specialized postprocessing technique required to suppress these motion artifacts. The correction algorithm corrects for generalized motion. The theoretical basis of the correction scheme is founded upon the (k,t)-space formalism and the concept of pulse sequence contrast mapping functions. The proposed (k,t) formalism is based on the Fourier projection slice theorem and allows us to determine how motion artifacts arise. The correction technique currently suffers from some spatial resolution and signal-to-noise ratio limitations, and works better for small objects than large objects. These problems will be investigated in subsequent studies.

Artifacts↗

PubMed1997

Techniques for high-speed cardiac magnetic resonance imaging in rats and rabbits.

Progress in research on hypertension, heart failure, aging, post-infarct remodeling, and the molecular basis of cardiovascular diseases in general has been greatly facilitated in recent years by the development of specialized small-mammal models by selective breeding and/or genetic alteration. Routine noninvasive evaluation of cardiac function and perfusion in these animals models, however, is difficult using existing methods. In principle, MRI can be used for this purpose, but in practice this is difficult because of problems related to RF coils, cardiac gating, and imaging pulse sequences. In this article, solutions to these problems are described that have allowed us to use MRI to routinely image the hearts of rats and rabbits. Specifically described are four RF coils, cardiac gating schemes, and an imaging pulse sequence specially designed for cardiac imaging in these animals on a 4.7 T Omega chemical-shift imaging (CSI) spectrometer. These techniques can be used to obtain, within 2 min, eight double-oblique short-axis images of the rat at different cardiac phases with 200 x 400 microm in-plane resolution and a slice thickness of 2 mm. Moreover, myocardial tissue tagging can be performed with tag thicknesses and separations comparable to those used routinely in humans. The technical information is presented in sufficient detail to allow researchers at other sites to reproduce the results. This information should facilitate the use of MRI for the noninvasive examination of cardiac function and perfusion, which can be combined with other established techniques for the study of cardiovascular disease in specialized animal models.

Animals↗

PubMed1986

Influence of physiologic motion on the appearance of tissue in MR images.

Studies were performed to determine the possible influence of physiologic motion on the parenchymal intensity of organs in magnetic resonance (MR) images. It is known that periodic motion associated with respiration and cardiac function causes characteristic artifacts in spin-warp images. The present study shows that bulk motion can also cause striking intensity changes at velocities equivalent to the craniocaudal respiratory excursion of organs in the upper abdomen. The magnitude of the effect depends on the velocity and direction of motion with respect to the three orthogonal axes of the imager and on the technical details of the imager and pulse sequence. Large systematic errors in calculated tissue relaxation times are possible due to this phenomenon. The findings have important implications for clinical imaging because motion can cause artifactual changes in the gray-scale relationships among tissues. Some pulse sequences are much less sensitive to these effects. These results provide guidance for selecting MR techniques that reduce the detrimental effect of respiratory and other physiologic motion on examinations of the upper abdomen and thorax.

Abdomen↗

PubMed1990

[Diffusion imaging using three orthogonal diffusion encoding gradients].

A new pulse sequence for in vivo diffusion measurements by magnetic resonance imaging is introduced. The proposed sequence employed gradient sensitization in three orthogonal directions to accentuate the effects of diffusion. The amplitudes and durations of the diffusion encoding gradients were carefully selected to achieve optimal signal-to-noise ratio in diffusion imaging. The pulse sequence was implemented on a superconducting whole body imager operating at 1.5 T. The self-diffusion coefficient of water in a bottle was measured to be 2.17 +/- 0.089 X 10(-9) m2/sec at 23 degrees C consistent with other previous measurements. A preclinical study with a human volunteer was also performed and results were presented.

Brain↗

PubMed2006

Order parameters based on (13)C(1)H, (13)C(1)H(2) and (13)C(1)H(3) heteronuclear dipolar powder patterns: a comparison of MAS-based solid-state NMR sequences.

Order parameters describing conformational exchange processes on the nanosecond to microsecond timescale can be obtained from powder patterns in solid-state NMR (SSNMR) experiments. Extensions of these experiments to magic-angle spinning (MAS) based high-resolution experiments have been demonstrated, which show a great promise for site-specific probes of biopolymers. In this study, we present a detailed comparison of two pulse sequences, transverse Manfield-Rhim-Elleman-Vaughn (T-MREV) and Lee-Goldburg cross-polarization (LGCP), using experimental and simulation tools to explore their utility in the study of order parameters. We discuss systematic errors due to passively coupled (13)C or (1)H nuclei, as well as due to B(1) inhomogeneity. Both pulse sequences can provide quantitative measurements of the order parameter, but the LGCP experiment is capable of greater accuracy provided that the B(1) field is highly homogeneous. The T-MREV experiment is far better compensated for B(1) inhomogeneity, and it also performs better in situations with limited signal.

Carbon Isotopes↗

PubMed2002

Time-resolved, undersampled projection reconstruction imaging for high-resolution CE-MRA of the distal runoff vessels.

Imaging of the blood vessels below the knee using contrast-enhanced (CE) MRI is challenging due to the need to coordinate image acquisition and arrival of the contrast in the targeted vessels. Time-resolved acquisitions have been successful in consistently capturing images of the arterial phase of the bolus of contrast agent in the distal extremities. Although time-resolved exams are robust in this respect, higher spatial resolution for the depiction of tight stenoses and the small vessels in the lower leg is desirable. A modification to a high-spatial-resolution T(1)-weighted pulse sequence (projection reconstruction-time resolved imaging of contrast kinetics (PR-TRICKS)) that improves the through-plane spatial resolution by a factor of 2 and maintains a high frame rate is presented. The undersampled PR-TRICKS pulse sequence has been modified to double the spatial resolution in the slice direction by acquiring high-spatial-frequency slice data only after first pass of the bolus of contrast agent. The acquisition reported in the present work (PR-hyperTRICKS) has been used to image healthy volunteers and patients with known vascular disease. The temporal resolution was found to be beneficial in capturing arterial phase images in the presence of asymmetric filling of vessels.

Contrast Media↗

PubMedDate not supplied

Comparison of helical CT and MR imaging in detecting and staging small pancreatic adenocarcinoma.

BACKGROUND: To compare the value of helical computed tomography (CT) and various pulse sequences of magnetic resonance (MR) imaging in the detection and staging of small pancreatic adenocarcinoma. METHODS: Small pancreatic adenocarcinomas (< or = 2 cm in diameter) in eight patients were evaluated with both helical CT and MR imaging. Five MR imaging pulse sequences that included fat-suppressed T1-weighted images and dynamic study using fast multiplanar spoiled gradient-recalled technique were compared for the tumor detectability. To evaluate the tumor vascularity, angiographic findings were also investigated. RESULTS: Helical CT delineated the tumor in five cases, and MR imaging depicted the tumor in seven cases. MR imaging could detect the tumor of 0.8 cm in diameter clearly. Although helical CT and dynamic MR imaging missed the tumor of 2 cm with relative hypervascularity, fat-suppressed T1-weighted MR imaging demonstrated it precisely. As for the tumor staging, MR imaging was equal or slightly superior to helical CT. CONCLUSION: MR imaging is the first modality of choice to evaluate small pancreatic adenocarcinoma, and fat-suppressed T1-weighted images and dynamic study must be performed.

Adenocarcinoma↗

PubMed1988

Practical aspects of 2D NMR for assigning the non-exchangeable protons in DNA-RNA fragments.

Using the branched trimer A2'-5'A3'-5'A as an example, different 2D NMR experiments such as homonuclear correlations via single-quantum coherence, double-quantum filtration via double-quantum coherence, isotropic mixing, relayed connectivities, cross relaxation and 31P/1H correlations are presented together with the corresponding spectra. The discussion pointed out the advantages and the difficulties of 14 pulse sequences, used for assigning non-exchangeable protons in DNA or RNA fragments. The emphasis has been put on the methodological aspects of 2D NMR as a simple technique, allowing an easier assimilation of the new coming 2D NMR pulse sequences.

Adenine Nucleotides↗

PubMed1992

Magnetic resonance imaging of the uterus at an ultra low (0.02 T) magnetic field.

In vivo pelvic imaging of 39 women and in vitro relaxation time measurements of four uterine specimens were performed using an ultra low field (0.02 T) MRI unit. Average T1 times measured in vitro at 37 degrees C for the myometrium and endometrium were 206 ms (SD 47 ms) and 389 ms (SD 21 ms), respectively. Corresponding T2 times were 95 ms (SD 20 ms) and 167 ms (SD 13 ms). The proton relaxation of almost all myometrial specimens proved to be biexponential, but of all endometrial specimens was monoexponential. Contrast measurements between endometrium versus myometrium and myometrium versus the junctional zone were performed after imaging 18 volunteer women using different pulse sequence parameters. Normal uterine structures were optimally demonstrated by SE 700/70. Relatively short repetition times could be used, because spin-lattice relaxation times were short at the low magnetic field. Consequently, the short repetition times allowed averaging of four excitations to create adequate images within an acceptable scanning time. In addition to T2-weighted images a T1-weighted inversion recovery sequence with a short inversion time of 50 ms (IR 1000/50/40) adequately differentiated the three uterine zones. Although pathologic lesions of the uterus including leiomyomas, anomalies and carcinomas were well demonstrated, especially with the T2-weighted spin echo pulse sequence, further investigations are needed to evaluate the optimal technique for ultra low field MR imaging of uterine tumors.

Adenocarcinoma↗

PubMedDate not supplied

MR imaging of normal extrahepatic bile ducts.

Abdominal scans of 48 patients without evidence of biliary tree or pancreatic head pathology were retrospectively evaluated to determine how often the normal common bile duct (CBD) could be visualized, as well as to determine which projections and pulse sequences optimized its visualization. Axial sequences with long repetition time (TR) and for echo time (TE) intervals (2,000-2,500 and 60-80 ms, respectively) were the most useful, identifying the normal CBD in 15 of 30 diagnostic studies. Axial sequences using short TR/TE intervals (300-800 and 20-25 ms, respectively) were less informative, identifying the CBD in only eight of 30 studies. Coronal studies were not useful, identifying the CBD in only one of 10 long TR/TE pulse sequences.

Bile Ducts↗

PubMed1987

Magnetite albumin suspension: a superparamagnetic oral MR contrast agent.

Suspensions of magnetite albumin microspheres (MAM), a new biodegradable particulate iron superparamagnetic MR contrast material, were evaluated in vitro and in vivo as an oral contrast agent. MAM is stable over a broad range of pH and tolerates proteolytic enzyme exposure over 24 hr in vitro. MAM possesses a much larger magnetic moment than do paramagnetic contrast agents. The transverse relaxation rate (R2) of MAM can be as much as 40 times the longitudinal relaxation rate (R1). In vitro spectroscopy studies confirm the potency of MAM in promoting T2 relaxation at concentrations of 10-1000 mg/l. Preliminary studies in rabbits and dogs show that in contrast to oral gadolinium-DTPA, which causes increased signal in bowel, MAM causes marked signal loss in the stomach and small bowel on both T1- and T2-weighted pulse sequences. Radionuclide labeling studies of MAM suspension with 99mTc show no evidence of absorption of MAM suspension from the gastrointestinal tract in small animals. Superparamagnetic suspensions such as MAM that reduce bowel signal on T1- and T2-weighted pulse sequences offer the unique benefit of reducing motion artifacts throughout the gastrointestinal tract, which should allow for improved evaluation of intra- and retroperitoneal diseases, particularly with high-field strength and gradient-echo "fast-scan techniques." Unlike paramagnetic material, MAM appears effective as a small-bowel contrast material.

Administration, Oral↗

PubMed2001

Optimization of field-cycled PEDRI for in vivo imaging of free radicals.

A numerical model of the behaviour of the magnetization in a field-cycled dynamic nuclear polarization (DNP) experiment is presented, with the aim of optimizing pulse sequence parameters in field-cycled proton-electron double-resonance free radical imaging. The model is used to predict the observed enhancement of the NMR signal as a function of the magnetic field strength, EPR irradiation frequency and pulse sequence timing, as well as the properties of the sample including the NMR and EPR relaxation times. The model allowed optimization of parameters in the field-cycled DNP experiment, in particular the EPR irradiation frequency, to find the value which would give the largest difference between NMR signals recorded with and without EPR irradiation. Experiments to verify the model were carried out using aqueous solutions of TEMPOL, which exhibits three hyperfine lines in its EPR spectrum and triarylmethyl (TAM), which has a single, narrow line. It was found that the model predicted very well the variation in DNP enhancement with EPR irradiation power for both samples. The behaviour of the NMR signal with EPR irradiation frequency in studies using TEMPOL was also accurately modelled, with the optimum frequency lying between 60 and 80 MHz, depending on the EPR irradiation power. The optimum frequency obtained from the model also agreed with the experimental data obtained using the TAM free radical, but with this sample the theoretical curves tended to deviate from the experimental data at irradiation frequencies below 70 MHz.

Electron Spin Resonance Spectroscopy↗

PubMed2000

The cardiovascular magnetic resonance machine: hardware and software requirements.

The flexibility of cardiac magnetic resonance imaging (MRI) includes faster imaging for applications such as stress tests, ventricular function, myocardial perfusion and coronary artery imaging. Faster imaging makes greater demands on the hardware and software. Although some cardiac imaging can be performed at 0.5 T, some of the faster techniques demand the higher signal-to-noise ratio of higher main field, and fat suppression in cardiac images is more easily achieved at higher field. Main field inhomogeneity affects rapid imaging and performance in open-access magnets. High gradient performance, low eddy currents and surface receiver coils are essential for fast cardiac imaging and the hardware of these systems including interventional imaging is discussed. The use of ECG signals for prospective and retrospective cardiac synchronization of MRI is examined. Techniques for reducing the major problem of respiratory motion in MRI are surveyed. Flexibility in the computer architecture of the scanner and the electronics generating the pulse sequence and controlling data acquisition is vital in cardiac imaging, for retrospective cardiac gating, respiratory navigator-controlled imaging and "real-time interactive" imaging in a similar manner to ultrasound imaging. Automated measurements from MR images remain under development. The pulse sequences and image display functions a cardiovascular MRI system should support for basic cardiac imaging applications and current clinical research areas are summarized.

Cardiovascular System↗

PubMed1994

Understanding chemical shift induced boundary artefacts as a function of field strength: influence of imaging parameters (bandwidth, field-of-view, and matrix size).

OBJECTIVE: To study the importance of chemical shift induced boundary artefact (CSA) at different field strengths and the implications for pulse sequence design with respect to receiver bandwidth (BW), field-of-view (FOV) and matrix size. MATERIALS AND METHODS: A fat-water phantom was examined in MR systems of different field strength (1.5 T, 1.0 T and 0.2 T), using pulse sequences with different receiver BW, FOV, and matrix size. The chemical shift was quantified by measuring the width of the bright and dark misregistration rims seen at the planar fat-water interface. The measured chemical shift was compared with the theoretically calculated chemical shift. RESULTS: Excellent correlations were found between predicted chemical shift and measurement results in our experiments. The width of the CSA (in millimetres) is directly proportional to field strength, inversely proportional to receiver BW and hence to the strength of the readout gradient, directly proportional to FOV, and inversely proportional to matrix size. CONCLUSION: CSA occurs at all magnetic field strengths, but given a certain BW it is more pronounced at higher fields. Although the CSA in Hz is directly proportional to field strength, the visible CSA at low-field was slightly higher than theoretically expected. The relative lack of CSA in low-field strength images permits the application of narrow receiver BW sequences, resulting in increased signal to noise ratio.

Artifacts↗

PubMed1994

Measurement of 13C chemical shift tensor principal values with a magic-angle turning experiment.

The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of 13C chemical shift tensors in powdered solids. A large-volume MAT probe with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that measurements of the principal values of chemical shift tensors in complex compounds can be enhanced by using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the 13C principal values in 2,3-dimethylnaphthalene and Pocahontas coal are reported as typical examples.

Anisoles↗

PubMed1984

Magnetic resonance of the brain: the optimal screening technique.

Seventy consecutive patients were examined with magnetic resonance (MR) and computed tomography (CT) of the brain. Each study was independently reviewed. Focal abnormalities were detected by one or both modalities in 51 patients. Neoplastic, infectious, vascular, demyelinating, metabolic, and congenital disorders of the brain were included. The MR pulse sequence that best detected these abnormalities was a spin-echo multisection technique that used a long interval between RF excitations (TR = 1500 or 2000 msec). Forty-eight of 51 patients showed focal lesions with this technique. A supplementary MR pulse sequence with a short TR (500 msec) was useful in helping to characterize certain lesions with a long T1 relaxation component, but in 10 of 26 positive cases in which this sequence was added it would have missed the abnormality had it been the sole sequence used. MR missed focal lesions in 3 of 51 patients. These were lesions that required thin-section (1.5 mm) CT techniques. Two were intrasellar, and one was an intracanalicular neurinoma. In 17 of 48 patients, CT missed the focal lesion seen with MR. Based on this experience, it is concluded that the long TR multisection spin-echo sequence is the optimal MR screening technique for detection of most brain abnormalities, and is more sensitive than CT. Currently, CT remains the screening modality of choice when high-resolution, thin-section studies in the pituitary, inner ear, and orbital regions are indicated.

Brain Diseases↗