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1.
Paper (German National Licenses)
Magnetic resonance imaging in obstetrics and gynecology: progress and limitations (1994)
Springer
ISSN: 1352-8661
Keywords infertility ; tumor ; uterus ; ovary ; fetus ; echo-planar imaging
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract The introduction of phase-array coils, fast spin echo, and certain other pulse sequences together with use of contrast agents has refined the application of magnetic resonance imaging (MRI) in pelvic disease. It makes management decisions in a number of benign conditions including uterine anomalies, adenomyosis, and leiomyomas of the uterus and endometriosis, especially in the context of infertility; it facilitates identification and characterisation of adnexal masses. In uterine malignancy, the multiplanar capability and excellent soft tissue contrast permit accurate assessment of depth of tumor invasion, tumor volume, and extension to adjacent structures. Its precise role in the management of primary and recurrent ovarian cancer remains to be decided. In pelvic malignancy, contrast facilitates identification of viable tumor but does not improve tissue specificity. In obstetrics, MRI is an attractive alternative to X-ray pelvimetry and assists in the evaluation of associated uterine and pelvic pathology. The use of echo-planar imaging eliminates movement artifact and has the potential to complement ultrasound in the assessment of fetal abnormalities and provide a method of identifying growth retardation from volume measurements of body organs.
Type of Medium: Electronic Resource
2.
Paper (German National Licenses)
ISSN: 1352-8661
Keywords Magnetic resonance spectroscopy ; tumors ; ion balance ; functional MRI ; blood flow ; pattern recognition
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Cancer was an obvious disease to study by magnetic resonance spectroscopy (MRS); it produces large lesions that give clearly abnormal spectra, all treatment methods leave much to be desired, and radiotherapy, in particular, is limited by tissue hypoxia, a process that can be investigated by MRS.31P MRS has shown that tumor cells are not acidic, as had been thought; instead, the pH gradient across the tumor cell membrane is the reverse of that in a normal cell. This change in hydrogen-ion gradient is accompanied by changes in gradients of many other ions. Tumor oxygenation can be monitored in animal tumor models using the techniques employed for functional magnetic resonance imaging (MRI) of the brain. Large changes in signal are observed when drugs that reduce tumor blood flow are administered. 1H NMR spectra of acid extracts of tumor or normal tissue biopsies contain sufficient information to permit classification (and thus, perhaps, diagnosis) if computer-based pattern recognition techniques are employed. Surprisingly, the same technique gives quite good classification of31P spectra takenin vivo. Can MRS beappliedin cancer therapy? Studies on tumor ion balance will help in the design of anticancer drugs and other therapies. Tumor blood flow studies using MRI could be applied to individual patients to predict the usefulness of radiotherapy and to assist in radiotherapy planning. Pattern recognition methods could automate the screening of biopsies and could also assist in interpretation of human spectra takenin vivo.
Type of Medium: Electronic Resource
3.
Paper (German National Licenses)
Clinical applications and techniques of echo-planar imaging (1994)
Springer
ISSN: 1352-8661
Keywords ultrafast MRI ; echo-planar imaging ; EPI
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract The ultrafast magnetic resonance imaging technique known as echo-planar imaging has undergone considerable technical improvements in recent years. It is currently being evaluated at only a few institutions worldwide. Although EPI, invented by P. Mansfield in 1977, is the oldest fast MRI technique, it is still not widely available on clinical scanners. Only 20–30 EPI scanners exist worldwide, compared to about 7000 conventional MRI scanners. The main reason why EPI has not emerged from the scientific prototype niche is its high demands on hardware and software. However, the time is now coming when EPI is entering the clinical stage. We describe the common EPI sequence types, show clinical results, and describe the contrast in the measured images.
Type of Medium: Electronic Resource
4.
Paper (German National Licenses)
Progress toward whole-body proton—electron double-resonance imaging of free radicals (1994)
Springer
ISSN: 1352-8661
Keywords free radicals ; imaging ; double resonance ; field cycling ; DNP
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Proton—electron double-resonance imaging (PEDRI) has been developed recently for imaging free radicals in biological samples and small animals. This article summarizes the techniques of PEDRI and the related field-cycled method, FC-PEDRI, and discusses the difficulties in scaling the techniques up to whole-body size. Imaging free radicals with broad EPR lines in humans would require excessive radiofrequency (RF) power, but the use of magnetic field cycling alleviates this problem and improves the signal-to-noise ratio. The results of computer simulations of field-cycled PEDRI are presented, which show that optimum EPR irradiation frequencies exist, depending on the free radical's electron relaxation times and on the applied RF power.
Type of Medium: Electronic Resource
5.
Paper (German National Licenses)
ISSN: 1352-8661
Keywords selective pulse ; inverse scattering theory ; SPINCALC ; soliton pulse ; refocusing pulse ; self-focusing pulse
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Selective pulse design for noninteracting spins is equivalent to inversion of the Bloch equations. Until recently, few analytical solutions to this problem were known. However, approaches based on inverse-scattering theory have now led to general solutions that offer ever higher precision in meeting target responses. The concept of soliton pulses (pulses that leave the spin system unaffected) turns out to be a particularly valuable one because half-solitons (both π/2 and π pulses) are inherently phase compensated. Such pulses are important for observation of shortT 2 species, where substantial signal loss could occur in any refocusing period. Multiply-selective pulses, suitable for simultaneous suppression of several “solvent” lines have been generated by inverse-scattering theory and have considerable potential in bothin vivo magnetic resonance spectroscopy and in routine high-resolution NMR. Although analytical solutions show great promise, it is likely that optimization methods will continue to be of value for the foreseeable future. The use of the SPINCALC scheme that operates in a switched stationary reference frame is illustrated through its use to design adiabatic refocusing pulses that do not lead to cumulative errors when used in multiple-echo trains.
Type of Medium: Electronic Resource
6.
Paper (German National Licenses)
ISSN: 1352-8661
Keywords hyperfast MRI ; 3D MRI ; echo-volumar imaging
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Echo-volumar imaging is a hyperfast technique capable of producing volumetric magnetic resonance images in times of the order of 100 ms. By increasing the gradient strengths and introducing real-time processing and display, we have been able to produce the first 16×16×16 voxel snapshot head images on volunteers.
Type of Medium: Electronic Resource
7.
Paper (German National Licenses)
Short-echo-time magnetic resonance phase-shift velocity mapping for assessment of heart valve and great vessel stenoses: three years' experience (1994)
Springer
ISSN: 1352-8661
Keywords MRI ; velocity mapping ; stenosis ; hemodynamics ; coarctation ; valve disease
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Shortening the echo times of magnetic resonance (MR) sequences used for phase-shift velocity mapping to 3.6 ms has extended use of the technique to measurement of velocities in turbulent, poststenotic jet flows. We used a 0.5-T MR machine and field even-echo rephasing (FEER) sequences with 3.6 ms echo times for jet velocity mapping.In vitro trials used continuous flow through a phantom with a 6-mm stenosis. Fifteen patients with mitral and/or aortic valve stenosis and 20 patients with repaired aortic coarctation were studied prospectively, with Doppler ultrasonic measurement of peak jet velocity performed independently on the same day. The clinical contribution of MR jet velocity mapping, used during a 3-year period in 306 patients with congenital and acquired disease of heart valves, great vessels, and conduits, was assessed retrospectively. The 3.6-ms sequence allowed accurate measurement of jet velocities up to 6 m s−1 in vitro (r=0.996). Prospective studies in patients showed good agreement between MR and Doppler measurements of peak velocity:n=38; range, 1.2–6.1 m s−1; mean, 2.7 m s−1; mean of differences (Doppler-MR), 0.22 ms−1; standard deviation of differences, ±0.38 m s−1 (±14%). MR jet velocity mapping proved particularly valuable for assessment and localization of stenoses at sites where ultrasonic access was limited. The technique represents a diagnostic advance which can obviate the need for catheterization in selected cases.
Type of Medium: Electronic Resource
8.
Paper (German National Licenses)
ISSN: 1352-8661
Keywords functional magnetic resonance imaging ; fMRI ; functional magnetic resonance angiography ; fMRA ; motor systems ; vessels
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Functional MR angiography (fMRA) images have been generated from the brain of healthy volunteers, in response to a hand motor task. Two sets of 3D phase-contrast MR images were, therefore, acquired, one during a resting and one during a task activation period. The MR images measured during rest were subtracted from those measured during task performance. The fMRA images were eventually obtained by calculating maximum intensity projections from the set of subtraction images. The results confirm earlier observations that there is a significant functional response from pial veins to motor activity.
Type of Medium: Electronic Resource
9.
Paper (German National Licenses)
ISSN: 1352-8661
Keywords functional imaging ; EPI ; brain transfer function
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Functional magnetic resonance imaging (fMRI) is becoming an important tool in the mapping of brain activation. However there are two main concerns that need to be answered before functional imaging can be considered truly useful as a neurophysiological tool. The first is that the detected activation may be derived from large veins and, thus, be spatially separate from the underlying brain activity. The second is the incomplete understanding of the brain transfer function and its relation to brain activity, blood flow, and metabolism. This work contains initial results that will help address these points. Models of the brain vasculature predict that signal changes on SE (spin-echo) images are expected to be much smaller in magnitude but very accurate in localizing true areas of activation than on GE (gradient-echo) images which are susceptable to large veins. By comparing activation from SE and GE EPI at 3 T, we have shown that the regions of activation are spatially very similar, suggesting that GE activation is closely linked to the underlying brain activity. We have identified an experimental impulse response of the brain following 8-s visual stimulation. This impulse response can be used to successfully predict the frequency response obtained experimentally and its shape suggests a resonance phenomenon. This suggests the brain transfer function can be modeled from linear response theory corresponding to the inherent feedback control mechanisms of the brain homeostasis. Continuation of this early work will help to identify the links between fMRI signal change and underlying brain physiology.
Type of Medium: Electronic Resource
10.
Paper (German National Licenses)
Quantitative proton MRS of brain tumors reveals increased cholineT 2 in meningiomas (1994)
Springer
ISSN: 1352-8661
Keywords proton MRS ; quantitation ; choline ; T 2 relaxation ; meningioma
Source: Springer Online Journal Archives 1860-2000
Kategorien: Medicine , Physics
Notes: Abstract Quantitative proton spectroscopy was performed on 26 volunteers and 9 patients using STEAM. Voxels (8 ml) were localized within white matter or meningioma and water-suppressed spectra acquired with TR=2 s at three echo times. Concentrations were calculated using individual relaxation parameter values with tissue water as an internal reference. Compared to white matter, meningiomas were characterized by an increased choline/creatine ratio, the absence ofN-acetylasparate, and the presence of alanine. Further, theT 2 of choline and its concentration were both significantly greater in meningiomas than white matter (p<0.01). Thus, the high choline/creatine ratio seen in meningioma spectra is the consequence of a greater concentration and a longerT 2. The longerT 2 may reflect differences in the relative proportions of choline-containing compounds. Our data demonstrate that individual measurements of relaxation parameters are important for long echo spectra and may reveal important metabolic information.
Type of Medium: Electronic Resource