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PubMed1999

Signal enhancement using 45 degrees water flipback for 3D 15N-edited ROESY and NOESY HMQC and HSQC.

A novel implementation of the water flipback technique employing a 45 degrees flip-angle water-selective pulse is presented. The use of this water flipback technique is shown to significantly enhance signal in 3D 15N-edited ROESY in a 20 kDa complex of the vnd/NK-2 homeodomain bound to DNA. The enhancement is seen relative to the same experiment using weak water presaturation during the recovery delay. This enhancement is observed for the signals from both labile and nonlabile protons. ROESY and NOESY pulse sequences with 45 degrees water flipback are presented using both HMQC and HSQC for the 15N dimension. The 45 degrees flipback pulse is followed by a gradient, a water selective 180 degrees pulse, and another gradient to remove quadrature images and crosspeak phase distortion near the water frequency. Radiation damping of the water magnetization during the t1 and t2 evolution periods is suppressed using gradients. Water resonance planes from NOESY-HMQC and NOESY-HSQC spectra show that the HMQC version of the pulse sequences can provide stronger signal for very fast exchanging protons. The HSQC versions of the ROESY and NOESY pulse sequences are designed for the quantitative determination of protein-water crossrelaxation rates, with no water-selective pulses during the mixing time and with phase cycling and other measures for reducing axial artifacts in the water signal.

Artifacts↗
PubMed2001

Three-stage approach to ultrasound contrast detection.

A new method for detecting ultrasound contrast agents using a three-stage pulsing sequence is proposed. The method is based on observations showing that the scattering properties of contrast agents are modified by ultrasonic insonation at high power, but remain unchanged at low power. The objective of the first stage of the pulsing sequence is to use low power pulses to obtain a high resolution reference image without altering the agent. Higher power pulses in the second stage modify the contrast agent. The third stage detects the changes imposed to the contrast agent using low power pulses. A temporal filter is proposed to discriminate contrast response from clutter signal. The method is similar to power Doppler methods in that it uses several pulses to survey the target while destroying the agent. The new idea is to separate detection and destruction to circumvent a trade-off between sensitivity and resolution. Results from in vitro experiments with three different contrast agents are presented. The results are compared with harmonic power Doppler processed from the same data and show that an improvement in sensitivity is achievable by including the high power burst in the pulsing sequence. The results also show that the proposed filter reduces clutter artifacts from moving tissue.

Biomedical Engineering↗
PubMed1993

Evaluation of renal masses with contrast-enhanced rapid acquisition spin echo MR imaging.

To assess the use of dynamic contrast-enhanced rapid acquisition spin-echo (RASE) imaging for the detection and characterization of renal masses, we evaluated 18 patients with CT evidence of 67 renal masses. The masses included 58 simple cysts, 8 solid neoplasms, and 1 pseudotumor. Patients were examined with standard spin-echo (SE) pulse sequences including pre- and postcontrast T1-weighted images and noncontrast T2-weighted images as well as pre- and postcontrast RASE images. Each pulse sequence was reviewed individually and in a group with other pulse sequences by two blinded observers. The performance of contrast-enhanced RASE imaging either alone or in combination with a T2-weighted SE sequence resulted in improved diagnostic accuracy compared with unenhanced conventional SE and RASE sequences. The contrast-enhanced RASE sequence outperformed the contrast-enhanced T1-weighted SE sequence for one observer; similar diagnostic accuracy was achieved with the two examinations by the other observer. Combining the T2-weighted sequence with contrast-enhanced RASE images did not increase lesion detection but did increase the observers' confidence in making the diagnosis. Contrast-enhanced MR imaging resulted in significant improvement in the detection and characterization of renal lesions compared to unenhanced MR imaging.

Adult↗
PubMed1994

Utility of magnetization prepared GRE MRI for the detection of focal liver lesions.

We compared the efficacy of rapidly acquired magnetization prepared gradient-echo (MP-GRE) sequences with CT and standard MRI pulse sequences for the detection of focal liver lesions. Fourteen patients with 28 focal liver lesions were scanned. TI times of 300, 450, and 600 ms were used. MP-GRE lesion conspicuity was compared to corresponding CT, T1, T2, T2-post-superparamagnetic-iron-oxide (SPIO), and STIR images. It was found that the differences between MP-GRE and CT and MP-GRE and T1 MRI were not significant. However, overall anatomic detail was better with CT and T1 MRI than MP-GRE. Lesion conspicuity was significantly worse with the MP-GRE than with the T2, T2-post-SPIO, and STIR sequences (all p values = 0.00). Maximal liver signal nulling occurred at TI = 300 ms in 13 out of 14 patients. However, the T1 for optimal focal liver lesion conspicuity varied widely and could not be predicted before scanning. No new lesions were seen on the MP-GRE sequence that could not be seen on the CT or standard MRI sequences. As currently implemented, MP-GRE imaging offers no advantage in the detection of focal liver lesions over CT and standard MRI pulse sequences.

Adult↗
PubMed1996

Quantitative magnetic resonance methods for in vivo investigation of the human liver and spleen. Technical aspects and preliminary clinical results.

This project was initiated with the introduction of magnetic resonance (MR) in Denmark in order to evaluate the possibilities of this technique as a diagnostic aid in non-focal liver and splenic diseases. The signal intensities in the MR image are sensitive to the longitudinal relaxation (T1), the transverse relaxation (T2), flow and chemical shift. All these parameters may be quantified by developing specific pulse sequences sensitive to the parameter in question. Previous studies had indicated that relaxation time measurements might be of value in the diagnosis of liver cirrhosis and haemochromatosis. Measuring relaxation times in these 2 groups of patients posed different challenges. In patients with liver cirrhosis a method had to be developed for simultaneous T1 and T2 relaxation time measurements, which was robust to the respiratory motion of the liver. A combination of multi-echo pulse sequences with different repetition times was chosen, because motion effects were partly refocused. Multi-acquisition was used to improve the signal-to-noise ratio in the heavily saturated experiments with short repetition times, to further reduce the sensitivity to motion. To test the quality of this pulse sequence, phantom experiments were performed, and sensitivity to motion was tested by measuring with and without respiratory synchronization. Respiratory synchronization gave a marked improvement in focal liver diseases, whereas no difference was found in non-focal diseases. Standard imaging sequences with a minimum echo time of 30 ms could not be used to measure the short T2 relaxation times found in patients with increased liver iron. A volume-selective multi-echo spectroscopic pulse was developed with a minimum echo time of 4 ms. Biexponential signal decay could be shown in patients with increased liver iron by using this sequence. Patients with liver cirrhosis, as a group, had increased T1 relaxation times compared to normal volunteers, but an overlap in T1 values was found. No correlation between the degree of fibrosis and the T1 relaxation time was found. Liver iron concentration could be quantified either by using the fast component of the T2 signal decay or by using the decreased signal in spin-echo and gradient echo images. Patients with leukemias and myeloproliferative disorders had prolonged T1 relaxation times in the spleen, but a considerable overlap was found between this group and a group of patients with benign hyperplasia and patients with splenomegaly secondary to portal hypertension. Volume-selective proton spectroscopy was developed and used to quantify the liver fat concentration. The accuracy of the method was about 3 g/100 g. With the implementation of a second generation scanner system it became possible to develop a pulse sequence, using the phase information in the MR signal, to measure portal vein flow during breath-holding. This method made it possible to estimate the portal vein flow during fasting, and the flow increase after eating. Quantitative MR methods may contribute to the diagnosis of non-focal liver diseases by estimation of liver fat and liver iron and by assessment of portal vein blood flow. Increased T1 relaxation time is a sign of a disease process in the liver rather than specific for any liver disease.

Humans↗
PubMed2002

Fully refocused gradient recalled echo (FRGRE): factors affecting flow and motion sensitivity in cardiac MRI.

The Fully Refocused Gradient Recalled Echo (FRGRE) magnetic resonance pulse sequence, previously reported as True FISP, was implemented and reported on a GE Signa CV/i 1.5T scanner. The purpose of this research was to optimize the pulse sequence design and scanning parameters to improve image quality for cardiac applications. A 2-D, multi-slice, multi-phase, breathold, segmented k-space, prospectively gated FRGRE sequence was implemented with TE and TR values as short as 0.9 and 3.0 msec, respectively. Pulse sequence design changes were investigated theoretically in terms of moment calculations and by assessing the quality of long- and short-axis cardiac images. The most influential scanning parameter for the improvement of image quality was both a short TR and TE. Placement of the z gradient rephaser immediately before the slice select gradient reduced the accumulated moments and improved image quality. Increasing the slice thickness from 3 to 8-10 mm significantly reduced flow artifact. The number of views per segment was doubled, without decreasing image quality, cutting breathold time in half compared to cardiac scanning with conventional sequences with longer TRs. The optimal flip angle was approximately 60 degrees. The use of a full or fractional echo had no noticeable effect on image quality. Surprisingly, the addition of flow compensation pulses significantly decreased image quality. In summary, optimization of sequence design and scanning parameters significantly reduced flow artifacts seen in FRGRE images.

Algorithms↗
PubMed2000

Microscopic displacement imaging with pulsed field gradient turbo spin-echo NMR.

We present a pulse sequence that enables the accurate and spatially resolved measurements of the displacements of spins in a variety of (biological) systems. The pulse sequence combines pulsed field gradient (PFG) NMR with turbo spin-echo (TSE) imaging. It is shown here that by ensuring that the phase of the echoes within a normal spin-echo train is constant, displacement propagators can be generated on a pixel-by-pixel basis. These propagators accurately describe the distribution of displacements, while imaging time is decreased by using separate phase encoding for every echo in a TSE train. Measurements at 0.47 T on two phantoms and the stem of an intact tomato plant demonstrate the capability of the sequence to measure complete and accurate propagators, encoded with 16 PFG steps, for each pixel in a 128 x 128 image (resolution 117 x 117 x 3,000 microm) within 17 min. Dynamic displacement studies on a physiologically relevant time resolution for plants are now within reach.

Solanum lycopersicum↗
PubMed1996

Possible Systematic Errors in Single-Shot Measurements of the Trace of the Diffusion Tensor

There have been recent proposals of diffusion-weighting pulse sequences that purport to measure the trace of the diffusion tensor, or to weight an image by that rotationally invariant quantity. Here the effect of a time-dependent diffusion tensor on such sequences is investigated theoretically. It is found that the time dependence of the diffusion tensor both breaks the rotational invariance of the pulse sequence and affects the overall magnitude of the diffusion weighting. These deviations are calculated for two pulse sequences, with parameters relevant to experiments on brain tissue. The departure from rotational invariance is found to be surprisingly small: <1%, for reasonable parameters and approximately5% for the most extreme choice of parameters. The overall magnitude of the diffusion weighting, however, may be significantly altered; 10% is estimated for reasonable parameters. It is proposed that this effect should prove a sensitive probe of time dependence in the diffusion tensor.

Journal Article↗
PubMed1994

Fast spin-echo MR in the detection of vertebral metastases: comparison of three sequences.

PURPOSE: To examine the relative capabilities for the detection of vertebral metastases of three available fast spin-echo sequences: T1-weighted fast spin-echo, short tau inversion recovery (STIR) fast spin-echo, and T2-weighted fast spin-echo sequences with chemical shift selective saturation pulse fat suppression. METHODS: Fourteen patients were evaluated prospectively over a 2-month period with T1-weighted fast spin-echo (four echo train, four acquisitions, 1 min 59 sec-2 min 37 sec). STIR fast spin-echo (16 echo train, four acquisitions, 2 min 30 sec-3 min 19 sec), and T2-weighted fast spin-echo (16 echo train, 4 acquisitions, 2 min 27 sec-3 min 16 sec). For all three pulse sequences, measurements were obtained of the signal intensities of normal marrow, abnormal marrow, fat, and noise posterior to the spine. Contrast-to-noise ratios were calculated for metastases in each case. Lesions were evaluated by three observers and rated for size, location, and conspicuity. RESULTS: Signal intensities of fat, normal marrow, and noise were highest for T1-weighted fast spin-echo sequences. STIR fast spin-echo and fat-suppressed T2-weighted fast spin-echo had approximately similar fat-suppression capabilities. Though contrast-to-noise ratios were highest overall for STIR fast spin-echo, the finding was not statistically significant and lesion conspicuity was deemed better with fat-suppressed T2-weighted fast spin-echo and T1-weighted fast spin-echo images. Discrete lesions were well identified on all three pulse sequences. CONCLUSION: Fast spin-echo sequences appear promising for the detection of vertebral metastases. Further work should be directed toward comparison with conventional spin-echo to determine whether fast spin-echo may replace conventional spin-echo sequences for evaluation of vertebral metastases.

Adult↗
PubMed2006

Selective averaging for high-resolution solid-state NMR spectroscopy of aligned samples.

Solid-state NMR experiments benefit from being performed at high fields, and this is essential in order to obtain spectra with the resolution and sensitivity required for applications to protein structure determination in aligned samples. Since the amount of rf power that can be applied is limited, especially for aqueous protein samples, the most important pulse sequences suffer from bandwidth limitations resulting from the same spread in chemical shift frequencies that aids resolution. SAMPI4 is a pulse sequence that addresses these limitations. It yields separated local field spectra with narrower and more uniform linewidths over the entire spectrum than the currently used PISEMA and SAMMY experiments. In addition, it is much easier to set up on commercial spectrometers and can be incorporated as a building block into other multidimensional pulse sequences. This is illustrated with a two-dimensional HETCOR experiment, where it is crucial to transfer polarization from the amide protons to their directly bonded nitrogens over a wide range of chemical shift frequencies. A quantum-mechanical treatment of the spin Hamiltonians under high-power rf pulses is presented which gives the scaling factor for SAMPI4 as well as the durations of the rf pulses to achieve optimal decoupling.

Algorithms↗
PubMed1989

[Magnetic resonance imaging of the metastatic vertebral tumor].

Fourty two patients underwent MR studies for a variety of lesions in the vertebral body. A 0.15-T MR system was employed. Twenty five patients were found to have malignant metastatic lesions (group 1); 16 had non-neoplastic lesions (group 2). The ability to discriminate between group 1 and group 2 with MR imaging was evaluated. All malignant metastatic lesions appeared as low intensity areas on both T1-weighted spin echo image and inversion recovery image, but 44 to 53% of the non-neoplastic lesions appeared as low intensity areas, respectively. The diagnostic ability with signal intensity of the vertebral column was evaluated on various pulse sequences; sensitivity of inversion recovery and T1-weighted spin echo image was 100%, in contrast specificity of these pulse sequences was 47 to 56%, overall accuracy was the highest on T1-weighted spin echo image (86%). The signal intensity of intervertebral disk was also evaluated in both groups. The intervertebral disks adjacent to the all malignant metastatic lesions showed normal intensity on both T1-weighted spin echo image and inversion recovery image, but non-neoplastic lesions showed variable intensities on images with all pulse sequences. The diagnostic ability with the signal intensities of the vertebral column and intervertebral disk was higher than that of the vertebral columns alone. Consequently accuracy was the highest in that case of both intervertebral disk and bone marrow which were imaged on T1-weighted spin echo (93%). We concluded that this diagnostic method was useful in distinguishing malignant metastatic from non-neoplastic lesions.

Adenocarcinoma↗
PubMed1990

Proton and phosphorus magnetic resonance spectroscopy of human astrocytomas in vivo. Preliminary observations on tumor grading.

We have obtained localized, water-suppressed proton magnetic resonance spectra from eleven astrocytomas in vivo. Localized phosphorus spectra were also obtained from three of these tumors. All tumors were examined prior to surgery, radiotherapy or chemotherapy. Examinations were performed with a commercially available 1.5 Tesla combined imaging and spectroscopy system using a stimulated echo pulse sequence for protons and an ISIS pulse sequence for phosphorus. A relatively high lactate resonance intensity correlated with a more malignant histological tumor grade and more aggressive behaviour. The resonance intensity of N-acetylaspartate/creatine was decreased and choline/creatine was increased, but these did not reliably discriminate between tumor grades. Other unidentified resonances not present in spectra of normal brain were sometimes seen. Proton magnetic resonance spectroscopy provides a new method for determining the metabolic behaviour of astrocytomas that may be useful in the clinical assessment of patients with these tumors.

Astrocytoma↗
PubMed2000

MR findings of necrotic lesions and the extralesional area of osteonecrosis of the femoral head.

OBJECTIVE: To investigate the MR findings of necrotic lesions and the extralesional area of osteonecrosis of the femoral head (ONFH) for each of the radiological stages. DESIGN AND PATIENTS: Forty-nine hips in 29 patients (15 female, 14 male; mean age 38 years, range 17-59 years) were imaged using a 1.0-T superconducting magnet. T2-weighted spin echo pulse sequences (T2WI), spoiled gradient recalled echo pulse sequences (SPGR) and fat suppression SPGR (FS-SPGR), followed by Gd-DTPA enhanced fat suppression SPGR (Gd-FS-SPGR), were all obtained with the aid of a TORSO surface coil. RESULTS AND CONCLUSIONS: While a normal fat intensity area with a low-intensity band on SPGR (band pattern) was seen in 16 of 16 stage 1 (100%), nine of 11 stage 2 (82%), four of 17 stage 3 (24%), and none of five stage 4 hips, all hips showed peripheral rim enhancement on Gd-FS-SPGR (100%). This enhancement band on Gd-FS-SPGR corresponded to histological findings of necrotic trabecular bone, repaired marrow, and fibrous reparative tissue. Bone marrow edema was also clearly demonstrated as a diffuse, high-intensity area outside this enhancement band on Gd-FS-SPGR in two stage 2 (18%), 12 stage 3 (71%), and one stage 4 hip (20%). In cases at stage 2 or more advanced stages with homogeneous or inhomogeneous low intensity on nonenhanced MRI, the reparative process both inside and outside the necrotic lesion, including bone marrow edema, was detected clearly on contrast-enhanced MRI.

Adolescent↗
PubMed1986

A multinuclear magnetic resonance imaging technique--simultaneous proton and sodium imaging.

Simultaneous imaging of proton and sodium is achieved using a new two coil system and a time-multiplexing technique with a 1.5 T NMR imaging system. Distinctly different NMR parameters of protons and sodium, such as resonant frequencies and T1 relaxation rates, are incorporated in the imaging with a new dual coil arrangement. Since the T1 relaxation time of sodium is substantially shorter than that of protons, a set of fast repeating sodium pulse sequences is inserted between the proton pulse repetition intervals. The different resonant frequencies of the nuclei provide good isolation between the two RF coils. In this paper, the system configuration and the pulse sequence employed for the proposed simultaneous multinuclear imaging of protons and sodium are presented along with some preliminary results.

Humans↗
PubMed2001

Identification of amino acids in wines by one- and two-dimensional nuclear magnetic resonance spectroscopy.

Amino acids are minor compounds in wines, but they have a profound influence on wine quality, and amino acids composition can be used to differentiate wines according to the vine variety, geographical origin, and year of production. The NMR signals of amino acids in NMR spectra are overlapped by the signals of other compounds present and especially by the signals of dominant compounds such as water, ethanol, and glycerol. In this work we used 1D (1)H and (13)C, 2D homonuclear COSY, TOCSY, and 2D heteronuclear HSQC and HMQC pulse sequences, also with an incorporated WET pulse sequence element that allows the simultaneous suppression of several frequencies. Complete (1)H and (13)C NMR assignments for 17 amino acids commonly present in wine and of gamma-aminobutyric acid at pH 3 have been achieved in wine sample of Sauvignon from the Coastal wine-growing region of Slovenia, vintage 1994.

Amino Acids↗
PubMed1989

Liver metastases: detection with MR imaging at 0.5 and 1.5 T.

Nineteen patients with known liver metastases were examined with two magnetic resonance (MR) pulse sequences at 0.5 T and three pulse sequences at 1.5 T. In addition, the patients were studied with computed tomography (CT) enhanced with ethiodized oil emulsion-13 (EOE-13). At 0.5 T a spin-echo (SE) 300/22 (repetition time/echo time, msec) sequence prospectively demonstrated 92.4% of the detectable liver metastases, while an SE 2,000/80 sequence showed 52.1% of the lesions. At 1.5 T, an SE 300/25 sequence depicted 68.3% and an SE 2,000/80 sequence 71.6%, while the more T1-weighted inversion recovery (IR) 2,000/600 (repetition time/inversion time, msec) sequence demonstrated 89.5% of the lesions. EOE-13 CT scans depicted 93.3%. These findings suggest that T1-weighted imaging is as successful at demonstrating liver metastases at 1.5 T with an IR 2,000/600 sequence as at 0.5 T with an SE 300/22 sequence. At both field strengths, MR imaging of the liver is comparable to state-of-the-art CT.

Humans↗
PubMed1995

Diaphragmatic motion: fast gradient-recalled-echo MR imaging in healthy subjects.

PURPOSE: To investigate the feasibility of imaging diaphragmatic motion with a fast gradient-recalled-echo (GRE) magnetic resonance (MR) pulse sequence. MATERIALS AND METHODS: Fast GRE pulse sequences in sagittal and coronal planes were used to acquire repeated, single-level, 1.2-second scans in 10 healthy volunteers during deliberately slowed, approximate-vital-capacity breathing. Motion was analyzed subjectively by viewing the image sequences as cine loops and quantitatively by measuring the displacement of different points on the diaphragm at a workstation. RESULTS: Temporal and spatial resolutions were adequate in all subjects. Absolute excursion of the domes was 4.4 cm on the right and 4.2 cm on the left. Analysis of diaphragmatic displacement at different locations revealed a gradient of excursion that increased from anterior to middle to posterior (P < .05-.001; paired t test). Excursion of the lateral aspects was greater than that of the medial aspect (P < .001). CONCLUSION: Fast GRE MR imaging can be reliably used to demonstrate diaphragmatic motion and may prove useful in the investigation of normal and abnormal respiratory mechanics.

Adult↗
PubMed1990

Practical aspects of vascular imaging using MRI.

MRI is an accurate means of noninvasively assessing vascular abnormalities. However, multiple potential pitfalls exist. In order to minimize error, a knowledge of the flow phenomena seen on various pulse sequences, along with an understanding of pulse sequence modifications that alter these flow phenomena, is necessary. With such an understanding, the current utility of MRI for solving commonly encountered clinical problems of the vascular tree is addressed. Emphasis is placed on a "how-I-do-it" approach, realizing that the reader will make modifications where necessary due to differences in hardware, software, and experience. As appropriate, references to the literature are made in an attempt to justify the use of MRI for the particular problems being discussed.

Blood Vessels↗