Double Spin Echo Research Articles

ENDOR and ESEEM studies of cytochrome c oxidase: evidence for exchangeable protons at the CuA site.

Electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies were used to study whether protons in the immediate protein environment around CuA in cytochrome c oxidase are susceptible to solvent exchange. The enzyme was incubated in buffered D2O under resting or turnover conditions for 90 min and then frozen to quench the hydrogen/deuterium-exchange process. ENDOR spectra of the deuterated sample were essentially identical to those of control samples. The ESEEM spectra, however, provided a clear indication of the introduction of deuterium into the CuA environment following incubation in buffered D2O. The extent of deuterium incorporation was not affected by enzyme turnover. An analysis of the ESEEM data indicated that water is in reasonably close proximity to the CuA site, but not in the immediate coordination sphere of the metal(s). We estimate a minimum distance of 5.4 A between the CuA center and the protein/water interface. This relatively short surface separation distance is consistent with the role of CuA as the immediate oxidant of cytochrome c in the cytochrome oxidase (Hill, B. C. (1991) J. Biol. Chem. 266, 2219-2226).

Spezielle MR-Methoden bei primären Knochentumoren: II. Volumenselektive1H-Spektroskopie

The method of localised 1H MR spectroscopy was used in 19 examinations in 9 patients with primary bone tumours in order to obtain spectral signals and to determine whether these are specific for tumour classification. The spectra were obtained by a double spin echo method and the tumour tissue showed very high water content and only minor lipid and proton signals which showed J-coupling characteristics. The latter are not found in normal bone marrow and are probably due to products of tumour metabolism. The resolution of the spectra within a 13 mm or 20 mm tissue cube depends largely on the structural homogeneity of the tumour. The signal patterns in the spectra were not specific to histological tumour types.

ESR, ENDOR and ESEEM studies of Lewis acid site interactions with tetrachloro-1,2-benzoquinone in aluminosilicate catalysts

The interactions of o-chloranyl (tetrachloro-1,2-benzoquinone) with Lewis acid sites in HY, H-mordenite and H-ZSM-5 zeolites and in aluminum oxide have been studied using electron spin resonance, electron nuclear double resonance and electron spin echo envelope modulation spectroscopies. The results are compared to those obtained from paramagnetic complexes generated in a solution of o-quinone and AlCl 3 which serves as a model system. The detected 27Al hyperfine interactions lead to the conclusion that adsorption of o-chloranyl on activated catalysts yields a paramagnetic complex with Al 3+ Lewis acid sites. Hence, o-chloranyl can be used as an indicator for the detection and quantification of such sites.

Ionic radicals on silica surfaces — an EPR, ENDOR and ESE study of benzene radical cations adsorbed on HY and silica gel

Electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and electron spin echo, (ESE) spectroscopy have been used to characterise radical cations of benzene (benzene- d 6 and benzene- d 1), generated by ionising radiation, and stabilised on silica gel and HY molecular sieve surfaces. The electronic structure and dynamic features of the benzene cation were found to be different from those in the case of stabilisation in a Freon matrix (CFCl 3). The monomeric cation was found to undergo pseudorotation at 3.5 K with hyperfine structure (h.f.s.) constants due to six averaged hydrogen nuclei. Simulations of the time-domain deuterium electron spin echo envelope modulation (ESEM) (C 6D + 6 cation) gave the following h.f.s. constants: the perpendicular component of the axially symmetric tensor T ⊥ = −0.8 MHz, and the isotropic component a = 2.17 MHz, consistent with ENDOR results at 105 K. It was concluded that the monodeuterated benzene cation ‘slowed down’ the rotation at 3.5 K, although not enough to allow an analysis in the rigid limit. ENDOR spectra of the protonated and deuterated monomeric and dimeric benzene radical cations on silica gel and HY molecular sieve surfaces are presented. On HY molecular sieve the ENDOR results revealed two types of dimer, one stabilised at low temperature (below 110 K) characterised by h.f.s. constants of 4.8 and 8.9 MHz, and the other stabilised at higher temperatures with an h.f.s. constant of 6.5 MHz. Only the latter type of dimer could be detected on silica gel. In addition to hyperfine splittings from ring protons, a smaller hyperfine splitting attributed to hydroxyl protons situated on the surface was determined from the ENDOR results.

1H and 2D electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) investigations on γ-irradiated betaine arsenate

Electron spin resonance, electron nuclear double resonance and electron spin echo envelope modulation measurements were performed on γ-irradiated betaine arsenate single crystals. The investigations revealed that two radicals, Ṅ(CH 3) 3 and (CH 3) 3N +ĊHCOO −, are formed by the γ-irradiation at room temperature. The concentration of both radicals decays in time with a time constant of a few days. In deuterated betaine arsenate the protons of the CH 2 group are partially replaced by deuterons. The 14N hyperfine coupling tensor of the Ṅ(CH 3) 3 radical as well as the 1H, 2D hyperfine coupling tensor and the 2D quadrupole tensor of the (CH 3) 3N +ĊHCOO − ((CH 3) 3N +ĊDCOO −) radicals were determined.

Localized flip angle based editing of lactate proton NMR spectra using PRESS

Spectral editing of localized proton spectra was performed using a modified version of the double spin echo PRESS sequence. Subtraction of signals obtained with two different double spin echo sequences results in a signal the spectrum of which is characterized by suppressed signals for uncoupled spins and enhanced coupled signals. This flip angle based localized spectral editing process is shown to work with a gain in S/N for lactate. Experiments were carried out on a Siemens Magnetom 1.5T whole-body imager.

Proton relaxation times in human red bone marrow by volume selective magnetic resonance spectroscopy

In hematological diseases the composition of red bone marrow shows alterations. The relaxation timesT1 andT2 of water and lipids in human hemopoietic bone marrow of 14 normal volunteers and 10 patients with acute leukemia and bone marrow carcinosis are determined using a double spin echo spectroscopy sequencein vivo. The volumes of interest (VOI) of (13 mm)3 in the center of vertebral bodies are examined using different measurement parameters. ForT1 measurements an inversion-recovery method is used.T2 is evaluated from spectra with differentTE. T1 (water) is found in a range between 1000 and 1700 ms,T1 (lipids) in a range between 260 and 320 ms in healthy volunteers.T2 (water) is determined between 32 and 65 ms. In some cases phase distortions of the water signals occur in the spectra. Water flow within the VOI may be a possible reason.T2 (lipids) is evaluated between 73 and 91 ms. The patients with acute leukemia exhibit clearly reduced lipid signals in their spectra. Lipid relaxation times could not be determined in these cases.T2 (water) is prolonged in acute leukemia to 51–98 ms.T1 (water) was not significantly different from values of healthy volunteers in our measurements. Results are discussed in comparison to relaxometric data from imaging and STEAM spectroscopic methods of other authors.

Localised in vivo 1H spectroscopy of human bone and soft tissue tumours

Localised 1H in vivo magnetic resonance spectroscopy was applied to fibrous and bone tumours before and during cytostatic treatment and radiotheraphy. The results of 24 studies in 18 patients with malignant tumours of the leg or pelvis are presented including cases of sarcoma, malignant fibrous histiocytoma, multiple myeloma, malignant lymphoma and metastasis. A double spin echo localisation method with water suppression was implemented on a 1.5 Tesla whole body unit. Voxel size was (13 mm)3 or (20 mm)3. The most common resonances besides lipids (16/18) were those of choline (10/18) and creatine (5/18). Creatine was always decreased in comparison to choline and often absent from tumour spectra. Additional resonances with phase distorsions from J-coupling (chemical shift region 1.8–2.5 and 2.2–4.0 ppm) were recorded. In the presence of lipids, lactate remained undetectable because special editing techniques were not available. Significant spectral differences between different tumour types were not evident. In about 30% of the investigations the spectra contained only water and lipid signals. Follow-up studies in three patients during radio- and chemotheraphy showed a decrease in metabolites (choline, creatine, unassigned signals between 1.0 and 2.5 ppm) after weeks and months. The decrease in choline was most pronounced paralleled by an increase in lipid/choline ratios.

Comparison of the methodologies for measuring the electronic relaxation times in order-disorder lattices: SeO 43- in KH 2PO 4

Electron nuclear double resonance (ENDOR) and electron spin echo (ESE) techniques were utilized for measuring the spin-lattice relaxation time ( T 1) and spin-phase memory time ( T M ) of the SeO 4 3- center in KH 2PO 4. Comparison of the ESE/ENDOR data with that obtained via the classical CW power saturation techniques showed that the power saturation methods yield T 1 and T 2 values differing by at least an order of magnitude from the ESE/ENDOR results. Such large errors can mask or artefactually invert the T 1/ T 2 anomalies that generally occur in the phase transition region.

Localized in vivo 1H spectroscopy and chemical shift imaging of the bone marrow in leukemic patients

Six healthy volunteers, ten patients with acute leukemia, one patient with hypersplenia and two with bone marrow carcinoris were studied. Nine patients with leukemia were restudied during chemotheraphy. A double spin echo localization method, implemented on a 1.5 T whole body unit was used for 1H magnetic resonance spectroscopy (MRS). A cubic (13 mm)3 voxel was chosen in a midlumbar vertebra. For chemical shift imaging (CSI) the SENEX sequence was used. We recorded fat and water images in a representative midsagittal plane. Patients with acute leukemia and hypercellular bone marrow a severe reduction or loss bone marrow fat signal and an increased water signal. Water T1 increaed during therapy in three patients. The bone marrow fat reappeared in the spectra and chemical shift images within 2 or 3 weeks in responders and remained unchanged or reappeared later in non-responders. A normal fat signal could be detected in leukemic patients without hypercellular bone marrow. Specificity was missing for 1H MRS and CSI; marrow carcinosis and benign stimulation (hypersplenia) could not be seperated from leukemia. In clinical routine, CSI may have advantages over 1H MRS, because a large anatomic field can be examined. Inhomogenous fat signal distrbutions can be detected and were seen in sveral cases during therapy. 1H MRS and CSI allow non-invasive therapy monitoring of leukemic patients adn might be of prognostic value.

Proton spectroscopic imaging of human brain

Signals from water and fat can cause artifacts in proton spectroscopic imaging in the human brain. The major problem is variation of the B 0 field over a range of several ppm within the sensitive volume of the standard whole-head coil. Here, the coherence-pathway formalism is used to describe and evaluate the origin of artifacts in a double spin-echo (PRESS) sequence. The attenuation of unwanted coherences using pulsed field gradients is described for homogeneous and inhomogeneous B 0 fields. The effect of the following parameters on the quality of the spectroscopic images is analyzed: (a) directional order of plane selection, (b) positioning of phase-encode gradients in the sequence, (c) postprocessing spatial windowing, and (d) motion. It is shown that, for a typical echo time of 272 ms, it is not necessary to first select a region of interest within the brain borders when sufficient phase-encode steps are used. Examples of 2D proton spectroscopic images with a nominal voxel volume of 0.85 ml are given for a healthy volunteer and a patient with a low-grade glioma.

Time-reduced T2-weighted celebral MRI with optimized flip angles

This study was set up to see whether lowering the flip angle in proton density- and T2-weighted double-spin echo sequences allows for shortening of repetition time (TR) and imaging time without significant change of image quality. Ten patients with celebral white matter lesions were investigated with an 1.5 T MR scanner using a conventional long- TR double-spin echo sequence (TR = 2500 ms, TE = 15 and 70 ms) and reduced-TR double-spin echo sequences (TR = 1900 ms, TE = 15 and 70 ms) at flip angles of 90°, 80°, 70°, 60°, and 50°. Lowering the flip angle resulted in less T1-contrast and a relative increase of T2-contrast. At a flip angle of 70°, contrast-to noise ratios (NNRs) between lesions and brain, as well as image artifacts of the reduced-TR sequence (CNR: 22.4) were similar to the conventional long-TR sequence (CNR:21.1), while imaging time was shortened by about 25%.

Localized double spin echo spectroscopy of weakly homonuclear coupled spin systems: Influence of chemical shift artifacts

In localized double spin echo proton NMR spectroscopy of weakly homonuclear coupled spin systems unusual echo time dependent signal intensity modulations can be observed. Recently a model for a theoretical description assuming rectangular slice profiles and exact pulse angles was published. Here we use the product operator formalism to calculate the intensity modulations caused by the chemical shift between weakly coupled spins. We point out that the effects of this chemical shift artifact should be dependent on the size of the selected volume. A quantification of the effect is given, and experiments with different voxel sizes are performed with a solution of calcium lactate using a 1.5 T SIEMENS whole-body imager. The experimental results do not show the theoretically expected effects. It can be stated that the proposed model very well enables understanding of some dynamical properties of coupled spin systems in volume selective spectroscopy, but it does not solve the problem of sufficient theoretical description of the double spin echo spectroscopy.

Schnellere MRT mit T2-gewichteten Spinecho-Sequenzen durch Variation des Anregungswinkels

In the present study the influence of the flip angle on image contrast in spin echo imaging was investigated. It was evaluated, whether variation of the flip angle allows for shorter repetition and imaging times in T2-weighted spin echo sequences. 10 patients with cerebral white matter lesions were investigated with an 1.5 Tesla whole body tomograph using a conventional double-spin echo sequence (TR = 2500 ms, TE = 15 and 70 ms) and time-optimized double-spin echo sequences (TR = 1900 ms, TE 15 and 70 ms) at flip angles of 90, 80, 70, 60 and 50 degrees. A reduction of the flip angles resulted in a decrease of T1-weighting and a relative increase of T2-weighting of the images. Despite the reduced repetition time at a flip angle of 70 degrees visually and quantitatively assessed contrast between lesions and brain as well as image artifacts of the time-optimized sequence were comparable to the conventional spin echo sequence; however, imaging time was shortened about 25%.

In vivo localized1H-spectroscopy with spin echos and stimulated echos: A quantitative comparison

Stimulated echos (STE) are frequently used for localization inin vivo spectroscopy. However, in theory the signal is reduced by a factor of 2 compared to spin echos. We have compared the STEAM technique and a method utilizing a double spin echo (DSE) for localization. Projections and spectra of phantoms and the human brain have been measured with a 4 T whole body system. Both methods provide a reliable localization in one scan. The spectra obtained with both techniques are similar, with one significant exception: the DSE signal strength is stronger than that of STEAM by a factor of about 1.9. The advantages and disadvantages of both methods with respect to RF- power, gradient strength, and spectral editing will be discussed.

Double Spin Echo Volume Selective NMR Spectroscopy with a 1.5 T Whole Body Imager

Abstract For volume selective nuclear magnetic resonance spectroscopy with protons a modified version of the double spin echo method has been implemented on a 1.5 T whole body imager. The high performance of the localization sequence has been verified with phantoms and tissue. A spectral resolution of about 2 · 10-8 was obtained. The volume elements are well shaped and positioned, the minimal size was 2.2 cm3. Spectra of porcine leg muscle tissue with different echo times are presented.

Klinische Bedeutung einer bewegungskompensierten Doppelechosequenz bei der Kernspintomographie (Magnetic Resonance Imaging of the Brain)

This study is a clinical evaluation on 5 volunteers and 20 patients of an improved sequence using rephasing gradients for motion compensation, applied to MR imaging of the brain at 1.5 T. The sequence is a double spin echo sequence with a normal first echo at TE = 15 ms and a second echo at TE = 90 ms with first and second order motion compensation in the readout gradient direction; the band width of the second echo is halved for improved S/N. Results demonstrate a dramatic improvement in image quality. Vascular and CSF flow are rephased, ghosting is reduced and signal voids no longer exist. Most remarkable is the substantial suppression of artifacts from voluntary movements such as from swallowing or rotation of the eyes. ECG triggering does not further improve image quality and is thus no longer needed.

Chemical Shift Selective Magnetic Resonance Imaging of the Optic Nerve in Patients with Acute Optic Neuritis

Optic neuritis is often the first manifestation of multiple sclerosis (MS). Sixteen patients with acute optic neuritis and one patient with benign intracranial hypertension (BIH) were investigated by magnetic resonance imaging, using a chemical shift selective double spin echo sequence. In 3 of the 16 patients, abnormalities were seen. In one patient with bilateral symptoms, signal hyperintensity and swelling of the right side of the chiasm were found. In another patient the optic nerve was found diffusely enlarged with only a marginally increased signal in the second echo. In the third patient an area of signal hyperintensity and swelling was seen in the left optic nerve. In the patient with BIH the subarachnoid space which surrounds the optic nerves was enlarged. Even using this refined pulse sequence, avoiding the major artefact in imaging the optic nerve, the chemical shift artefact, lesions were only shown in 3/16 (19%) of the patients with optic neuritis. Nevertheless, the presented chemical shift selective double spin echo sequence may be of great value for detection of retrobulbar lesions.

MR imaging of pulmonary arterial hypertension and pulmonary emboli.

Intraluminal signal in the pulmonary arteries on spin-echo, ECG-gated MR images is limited to the diastolic phase of the cardiac cycle in normal subjects. Initial experience has indicated that signal persisting during systole may be characteristic of slow blood flow associated with pulmonary arterial hypertension (PAH) or of thrombotic material secondary to pulmonary embolism. This study analyzes our cumulative experience (31 patients) with multiphasic, double spin-echo MR for assessing PAH and/or suspected pulmonary embolism. In PAH, the abnormal systolic signal showed an intensity increase from first to second echo. This pattern was observed in 92% of PAH patients, including 100% of patients with pulmonary systolic pressures greater than or equal to 80 mm Hg and 60% of patients with pressures less than 80 mm Hg. At any focus in the pulmonary arteries, such signal disappeared at some phase of the cardiac cycle. In patients with pulmonary embolism, signal from thrombus was fixed throughout the cardiac cycle and showed little or no increase in relative intensity change from first- to second-echo image. Using this guideline, MR made six confirmed positive and four confirmed negative diagnoses of proximal pulmonary embolism, while it failed to identify thrombus in the one patient with a peripheral pulmonary embolism. Intraluminal signal in the pulmonary arteries caused by PAH or pulmonary embolism can be differentiated in most instances using multiphasic, double spin-echo, ECG-gated MR. However, at its current stage of development, the procedure does not appear to be useful for the evaluation of peripheral pulmonary embolism.

Evidence for N coordination to Fe in the [2Fe-2S] center in yeast mitochondrial complex III Comparison with similar findings for analogous bacterial [2Fe-2S] proteins

Yeast mitochondrial complex III contains a subunit with a [2Fe-2S] cluster (the Rieske center) that has unusual physical and chemical properties. For apparently similar centers isolated from bacteria, it has been shown by electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) measurements that these [2Fe-2S] centers are coordinated by at least one and probably two nitrogen ligands. This work describes similar ENDOR and ESEEM studies on the intact mitochondrial complex. We find that this [2Fe-2S] cluster exhibits ESEEM and ENDOR properties that appear to be indistinguishable from those observed with the isolated bacterial systems. Furthermore, changes in EPR lineshape that occur as complex III is progressively reduced are not accompanied by any changes in the nitrogen coupling parameters. This spectroscopic evidence for nitrogen coordination is supported by published sequence data on four Rieske iron-sulfur subunits. It seems likely that this is a general characteristic of such [2Fe-2S] redox active centers.