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PubMed1998

[Recent developments and applications of MRI sequence technique. I: turbo spin echo, HASTE, turbo inversion recovery, turbo gradient echo, turbo gradient spin sequences].

Goal of this two-part report is to provide clinical MRI radiologists with a guide to the world of new and clinically available MRI pulse sequences. Discussed are the principles of rapid scan techniques like multiple spin-echo imaging, multiple gradient echo imaging, echo planar imaging, diffusion and perfusion imaging, and future perspectives (review article).

Echo-Planar Imaging↗
PubMed1999

Detection of homonuclear decoupled in vivo proton NMR spectra using constant time chemical shift encoding: CT-PRESS.

A new pulse sequence, termed CT-PRESS, is presented, which allows the detection of in vivo 1H NMR spectra with effective homonuclear decoupling. A PRESS sequence with a short echo-time TE, used for spatial localization, is supplemented by an additional 180 degrees pulse. The temporal position of this 180 degree pulse is shifted within a series of experiments, while the time interval between signal excitation and detection is kept constant. CT-PRESS is a two-dimensional (2D) spectroscopic experiment as far as data acquisition and processing are concerned, although only diagonal signals are generated in the 2D spectrum. However, since the principle of constant time chemical shift encoding is used in the t1 domain, effective homonuclear decoupling is obtained by projecting the 2D spectrum onto the corresponding f1 axis. Thus, good spectral resolution and high signal-to-noise ratio are obtained. The main advantage, as compared to localized 2D J-resolved MRS, is that optimized experiments can be performed for coupled resonances of interest by choosing the sequence parameters dependent on the type of multiplets, the J-coupling constants and T2. Major fields of application will be parametric studies on coupled resonances, (e.g., T1, diffusion behavior or magnetization transfer) and/or the detection of spatial and temporal changes of metabolites with coupled spin systes.

Animals↗
PubMed1999

Sources of variability in the response of coupled spins to the PRESS sequence and their potential impact on metabolite quantification.

Using a numerical method of solving the equation of motion of the density matrix, an evaluation is presented of the sources of the marked variability in the response to the point resolved spectroscopy (PRESS) pulse sequence of coupled proton spin systems. The consequences of an inappropriate 180 degrees pulse design and of the limitations on radiofrequency power are demonstrated for a weakly coupled example, lactate. The dominating role of strong coupling, which is present in most brain metabolites, is demonstrated for glutamate, in which 160 terms in the density operator were tracked to monitor the gross changes in lineshape and signal intensity as a function of the two echo times. The predictions of the numerical solutions were confirmed by experiments on phantoms of aqueous metabolite solutions.

Brain↗
PubMed1997

Stray-field imaging of quadrupolar nuclei of half integer spin in solids.

A report is presented on the observation of Hahn echoes from the following quadrupolar nuclei of half integer spin (I) in polycrystalline solids in the large static magnetic field gradient (37.5 T/m) which exists in the fringe field of a superconducting solenoid: 7Li, 23Na, 11B, 65Cu (I = 3/2); 27Al (I = 5/2); 51V, 59Co (I = 7/2); and 115In (I = 9/2). 23Na echo-trains from NaCl (with non-selective excitation) and from Na2SO4 (with selective excitation) are compared quantitatively for two different RF pulse sequences: 90x-(tau-90y-tau-echo-)n and 90x-(tau-90x-tau-echo-)n. The signals obtained from RF pulses corresponding to non-selective 90 degrees pulses were shown to be quantitative, whereas in the selective case smaller signals were obtained since only the central transition contributed. The loss of signal from this cause can be distinguished from small signals resulting from low density of nuclei by use of the second sequence. A 7Li image obtained from LiF in a cylindrical glass-vial is shown.

Calibration↗
PubMedDate not supplied

MR assessment of spinal metastases using the late 26 sequence.

Ninety-two patients with suspected spinal involvement by systemic cancer underwent MR examinations using both conventional and phase-contrast (LATE 26) spin-echo pulse sequences. MR imaging was considered positive for metastatic disease in 73 cases. Implants were extradural in 69 patients and intradural in 4 patients. Nineteen cases had no MRI evidence of metastases and the abnormalities suspected were shown to be due to either inflammatory or degenerative changes. MR appearances during and after chemo- and/or radiotherapy were studied in 5 patients. The impact of the LATE 26 sequence in the assessment of metastatic spinal disease is evaluated.

Adult↗
PubMed2006

R2 relaxometry with MRI for the quantification of tissue iron overload in beta-thalassemic patients.

PURPOSE: To evaluate the usefulness of a time-efficient MRI method for the quantitative determination of tissue iron in the liver and heart of beta-thalassemic patients using spin-spin relaxation rate, R2, measurements. MATERIALS AND METHODS: Images were obtained at 1.5 T from aqueous Gd-DTPA solutions (0.106-8 mM) and from the liver and heart of 46 beta-thalassemic patients and 10 controls. The imaging sequence used was a respiratory-triggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spin-echo (SE) pulse sequence (TR = 2000 msec, TE(min) = 5 msec, echo spacing (ES) = 5 msec, matrix = 192 x 256, slice thickness = 10 mm). Liver iron concentration (LIC) measurements were obtained for 22 patients through biopsy specimens excised from the relevant liver segment. Biopsy specimens were also evaluated regarding iron grade and fibrosis. Serum ferritin (SF) measurements were obtained in all patients. RESULTS: A statistically significant difference was found between patients and healthy controls in mean liver (P < 0.004) and myocardium (P < 0.004) R2 values. The R2 values correlated well with Gd DTPA concentration (r = 0.996, P < 0.0001) and LIC (r = 0.874, P < 0.0001). A less significant relationship (r = 0.791, P < 0.0001) was found between LIC measurements and SF levels. R2 measurements appear to be significantly affected (P = 0.04) by different degrees of hepatic fibrosis. The patients' liver R2 values did not correlate with myocardial R2 values (r = 0.038, P < 0.21). CONCLUSION: Tissue iron deposition in beta-thalassemic patients may be adequately quantified using R2 measurements obtained with a 16-echo MRI sequence with short ES (5 msec), even in patients with a relatively increased iron burden.

Adolescent↗
PubMed2001

Sensitivity enhancement of a two-dimensional experiment for the measurement of heteronuclear long-range coupling constants, by a new scheme of coherence selection by gradients.

In this work we present a new pulse sequence for the measurement of long-range heteronuclear coupling constants in which the optimization of coherence selection by pulsed field gradients offers a net increase in sensitivity. This type of experiments is extremely valuable for conformational studies of molecules in natural abundance and in this context the use of gradients is essential for an efficient suppression of (12)C bound proton signals. A comparative analysis of the different gradient schemes available is presented with a conclusive elucidation of the relative sensitivities. Our gradient scheme could be advantageous as a building block for other related experiments.

Journal Article↗
PubMed1998

Comparison of sequences for depicting bone marrow alterations in osteomyelitis applied in a low field strength magnetic resonance imaging system.

OBJECTIVE/PATIENTS: To investigate the efficacy of standard sequences of a low field system for the detection of osteomyelitis, we tested T1wI pre and post i.v. contrast, T2w and fat suppressed IR sequences. DESIGN: On the basis of clinical and laboratory evidence, pathology reports, and three phase granulocyte scintigraphy, osteomyelitis was diagnosed in 18 of 21 patients with Charcot's joints. A consecutive low and high field magnetic resonance (MR) scan confirmed osteomyelitic bone marrow changes in the same osseous regions. These 18 diabetic patients were then studied on a 0.2 Tesla dedicated MR system (Esaote ArtoScan) using T1wI (SE: relaxation time (TR) 520/echo time (TE) 24: axial and coronal) before and after i.v. application of 0.1 mmol/l Gd-DTPA/kg BW, T2w imaging (TSE: TR 3500/TE 80 or TR 2000/TE 120: axial), and fat suppressed inversion recovery (IR) imaging (short tau inversion recovery (STIR): TR 3000/TE 30/TI 80 or inversion recovery gradient echo (IRGE)/fat suppressed IRGE (GEFS): TR 1000/TE 16m 80: coronal). RESULTS: The SE T1w sequence showed a significantly higher contrast-to-noise ratio (CNR) before administration of i.v. contrast. The TSE T2w pulse sequence demonstrated bone marrow changes superiorily utilizing a TE of 120 ms (CNR = 16.5+/-2.7 compared to 5.5+/-2.5 with TE = 80 ms). The IRGE showed a higher CNR than the standard STIR (CNR = 19.2+/-2.5 compared to 12.4+/-2.9). CONCLUSION: Fat suppressed IRGE imaging and longer TE in T2w TSE sequences result in a significantly better CNR in osteomyelitis. This way, using optimized sequences, low field systems are apt to depict bone marrow changes in the course of osteomyelitis.

Adolescent↗
PubMed1996

Conformation of surface exposed N-terminus part of bacteriorhodopsin studied by transferred NOE technique.

Interaction of the monoclonal antibody A5 raised against native bacteriorhodopsin (BR) with the synthetic peptide pGlu1-Ala-Gln-Ile-Thr-Gly-Arg7-NH2, corresponding to the amino acid sequence 1-7 was studied by transferred nuclear Overhauser effect (TRNOE) spectroscopy. The denaturing reagents and the specially designed pulse sequences which eliminate broad signals from the TRNOE spectra were used to favour evaluation of the TRNOE peaks. On the basis of the data obtained, the conformation of peptide bound with A5 was calculated. A model of the mutual arrangement of bacteriorhodopsin N-terminus and the first transmembrane alpha-helical segment 8-32 was proposed.

Amino Acid Sequence↗
PubMed1997

Frequency- and phase-modulated heteronuclear decoupling in rotating solids.

The mechanism of heteronuclear dipolar decoupling by the TPPM sequences, proposed by Bennett et al. (J. Chem. Phys. 103 (1995) 6951) is investigated by comparison with a modified pulse sequence, called TPFM, that uses frequency instead of phase modulation. By combining frequency and phase modulations, circularly modulated sequences are designed. The fact that only the left-handed modulation sequence FMPML leads to improved proton decoupling, while the right-handed modulation sequence FMPMR is ineffective, proves that the efficient decoupling is caused by a secondary resonance effect.

Chemical Phenomena↗
PubMed2004

Sensitivity-enhanced quantitative 13C NMR spectroscopy via cancellation of 1J(CH) dependence in DEPT polarization transfers.

A scheme has been developed to eliminate virtually all signal intensity dependence on 1JCH in polarization transfers between 1H and 13C nuclei, reducing differences in signal intensity to only 1.5% over the entire natural 1JCH range. The scheme relies on the summation of time-domain data acquired with four suitably selected Delta delays so that the J dependence is essentially canceled in the final, signal-averaged free-induction decay. These Delta delays have been incorporated into the DEPT pulse sequence to create sensitivity-enhanced experiments for collecting quantitative 13C{1H} spectra. Four experiments, each with unique read pulse angles, give quantitative spectra with 200-300% more sensitivity than conventional 13C spectra acquired with inverse-gated 1H decoupling. The experiments are ideal for recording spectra with improved quantitative information or for substantially reducing the long acquisition times indicative of quantitative 13C experiments. The ability of the experiments to provide quantitative spectra was demonstrated with a simple ethylbenzene solution, however, they can easily be adapted to various applications for analysis of complex mixtures.

Journal Article↗
PubMed2004

Correlation between magnetic resonance angiography (MRA) and quantitative coronary angiography (QCA) in ectatic coronary vessels.

Coronary artery ectasia (CAE) is defined as a dilatation of an arterial segment to a diameter at least 1.5 times that of the adjacent normal artery. The correct follow-up of ectatic vessels is hampered by the need for repeat angiograms. In this work we compared quantitative coronary angiography (QCA) measurements of the diameter of the proximal most ectatic part of coronary vessels, with corresponding measurements obtained by magnetic resonance angiography (MRA) using both gradient echo and turbo spin echo imaging sequences. Fifteen patients (14 male), aged 45-65 years, with known CAE were prospectively studied. Two electrocardiogram (ECG)-triggered pulse sequences were implemented for coronary magnetic resonance angiography. The first was a three-dimensional (3D), segmented, k-space gradient-echo sequence, employing a T2-weighted preparation prepulse and a frequency-selective, fat-saturation prepulse to enhance "white blood" (WB) contrast of the coronary arteries. The second sequence was an M2D dual Inversion Recovery (IR) Turbo Spin-Echo with a linear k-space acquisition scheme, providing "black-blood" (BB) contrast of the coronaries. All scans were carried out with the patient free breathing using a 2D, real-time Navigator beam, for respiratory motion tracking and gating. All patients underwent QCA, and the diameter of the proximal most ectatic part of each vessel was measured and compared with "white-blood" and "black-blood" MRA measurements. The average length of continuously visualized LM, LAD, LCx, and RCA by MRA was 2.5 +/- 0.3, 5.8 +/- 0.8, 3.9 +/- 1.0, and 7.2 +/- 1.2 cm, respectively. There were no statistically significant differences between diameter measurements of the proximal most ectatic part of each vessel, obtained with WB and BB sequences. There was a close correlation between MRA and QCA measurements (r = 0.87, p < 0.001). Bland-Altman analysis showed no systematic differences between the examined methods, over the whole range of vessel diameters measured. Coronary MRA is in close correlation with QCA for CAE detection. Magnetic resonance angiography, being noninvasive, may prove of significant value for the efficient follow-up of these patients.

Aged↗
PubMedDate not supplied

An algorithm for MR imaging of the short T2 fraction of sodium using the FID signal.

Multiecho magnetic resonance imaging of 23Na in the human body could provide valuable information regarding the distribution of the sodium in the intracellular and extracellular spaces in living tissues. Since it is known that more than half the intracellular sodium has a T2 value of less than 3 ms, the conventional spin-echo imaging technique is not appropriate for the quantitative study of sodium in living tissues. A pulse sequence using the three-dimensional planar-integral projection reconstruction algorithm was developed to provide images from the free induction decay (FID) signal and from the spin-echo. Using this imaging sequence, we are able to produce human head images with the FID signal immediately following the 90 degrees radiofrequency pulse thus preserving the signal from the very short T2 components. This technique with a multiple echo imaging scheme may open the possibility of discriminating between intracellular and extracellular sodium compartments in the human brain based on the T2 relaxation properties.

Algorithms↗
PubMed2004

Observation of enhanced field-free molecular alignment by two laser pulses.

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of 1:3 and by sending the second pulse near the time where the alignment created by the first pulse peaks.

Journal Article↗
PubMed2005

Carrier envelope phase noise in stabilized amplifier systems.

At present most laser systems for generating phase-stabilized high-energy pulses are chirped pulse amplifier systems that involve the selection and subsequent amplification of pulses from a phase-stabilized seed oscillator. We investigate the effect of the picking process on the carrier envelope phase stability and how the phase noise of the picked pulse sequence can be estimated from the phase noise properties of the seed oscillator. All noise components from the original pulse train above the picking frequency are aliased into the picked pulse train and therefore cannot be neglected.

Journal Article↗
PubMed2001

Characteristics of spongiform leukoencephalopathy induced by heroin: MRI detection.

OBJECTIVE: To elucidate the imaging characteristics of spongiform leukoencephalopathy induced by heroin and the diagnostic value of MRI examination. METHODS: Eleven cases of heroin addiction were examined by MRI. The sequences included T1-weighted imaging (T1WI), T2-weighted imaging (T2WI) and fast fluid-attenuated inversion recovery pulse sequence (Flair). RESULTS: Bilateral symmetrical lesions were located in the white matter of the cerebrum and cerebellum, the internal capsule in all cases, and the callosum in 6 cases. Elliptical lesions in the mesencephalon were found in two cases. These lesions were shown as hypointense signals on T1WI and hyperintense signals on T2WI and Flair's imaging. Cerebellar white matter suffered more severely, but the dentate nuclei were not invaded. CONCLUSION: Heroin-induced spongiform leukoencephalopathy has characteristic patterns in MRI, which are very specific to this form of the disease.

Adult↗
PubMed1989

Physical principles of magnetic resonance imaging.

When a nucleus contains either unpaired protons, neutrons, or both, it has angular momentum. This property provides the basis for magnetic resonance imaging (MRI). Because of its abundance in the human body, hydrogen is used for clinical MRI. Within the MR magnet, the atoms are aligned with the magnetic field. The MR signal is generated by subjecting the atoms to radiofrequency (RF) pulsations, and the pulse sequences selected will determine the appearance of the image. The repitition time (TR) is the time between the RF pulses, and the echo time (TE) is the time between the RF pulse and the recording of the signal. T1-weighted images (short TR/short TE) provide the best anatomic detail, but T2-weighted images (long TR/long TE) are often needed to demonstrate pathology. Many unfamiliar artifacts may be encountered on MR images. These can be due to several factors, including inhomogeneities in the magnetic field (for example, those resulting from metallic orthopedic applicances or prior surgery), extraneous RF interference, and motion. Surface coils are commonly used in musculoskeletal MR imaging to improve the quality of the examination by increasing the signal-to-noise ratio over the area of interest.

Humans↗
PubMed1995

MR angiography of the coronary arteries.

Magnetic resonance angiography (MRA) of the coronary arteries has recently become possible due to the development of a new group of ultrafast imaging sequences. Coronary arteries are small, tortuous vessels subjected to significant physiological motions (respiration and cardiac contractions). Most coronary MRA pulse sequences rely on a combination of segmental acquisition of data (in k-space) to minimize cardiac motion artifacts and patient breath-holding to minimize respiratory motion artifacts. MR is noninvasive, does not require any contrast agents or ionizing radiation, and thus offers the potential of becoming a very important cardiac screening tool. Coronary MRA can routinely depict the proximal and middle portion of most coronary arteries and some coronary artery branches. However, its clinical utility for detecting lesions has not yet been proven. As coronary MRA techniques improve they may become an integral part of the evaluation of patients with ischemic heart disease. Existing coronary MRA techniques can already play a significant clinical role in the evaluation of coronary artery anomalies, in the follow-up of proximal lesions after angioplasty, and in the noninvasive determination of patency of bypass grafts and coronary stents.

Artifacts↗