Kinetics, energetics, and electronic coupling of the primary electron transfer reactions in mutated reaction centers of Blastochloris viridis.

Femtosecond spectroscopy in combination with site-directed mutagenesis has been used to study the dynamics of primary electron transfer in native and 12 mutated reaction centers of Blastochloris (B) (formerly called Rhodopseudomonas) viridis. The decay times of the first excited state P* vary at room temperature between of 0.6 and 50 ps, and at low temperatures between 0.25 and 90 ps. These changes in time constants are discussed within the scope of nonadiabatic electron transfer theory using different models: 1) If the mutation is assumed to predominantly influence the energetics of the primary electron transfer intermediates, the analysis of the room temperature data for the first electron transfer step to the intermediate P(+)B(A)(-) yields a reorganization energy lambda = 600 +/- 200 cm(-1) and a free energy gap Delta G ranging from -600 cm(-1) to 800 cm(-1). However, this analysis fails to describe the temperature dependence of the reaction rates. 2) A more realistic description of the temperature dependence of the primary electron transfer requires different values for the energetics and specific variations of the electronic coupling upon mutation. Apparently the mutations also lead to pronounced changes in the electronic coupling, which may even dominate the change in the reaction rate. One main message of the paper is that a simple relationship between mutation and a change in one reaction parameter cannot be given and that at the very least the electronic coupling is changed upon mutation.

Optimization and evaluation of the signal intensity change in multisection oxygen-enhanced MR lung imaging.

The behavior of the signal intensity in MRI of human lungs was investigated during inhalation of pure oxygen. Nine volunteers were examined, five using a breath-hold and four using a non-breath-hold technique. Four coronal slices were acquired in each volunteer using an inversion recovery turbo spin-echo sequence. The inversion time of the sequence was optimized for maximum contrast. Breathing of pure oxygen and room air was alternated in the volunteers. Breath-hold and non-breath-hold cases were compared. Breathing pure oxygen lead to a statistically significant signal intensity increase (up to 18%) compared to breathing room air. In addition, T(1) maps were acquired during breathing 100% oxygen and room air. Inhalation of pure oxygen reduced the mean T(1) time of the lungs from 1280 (+/-85) msec to 1224 (+/-139) msec without breath-hold and from 1219 (+/-176) to 1074 (+/-92) msec with breath-hold. Therefore, an optimized sequence and measurement protocol provided significant signal intensity changes utilizing 100% oxygen. Magn Reson Med 43:860-866, 2000.

[Myocardial perfusion at rest and during stress. MR signal characteristics of persistent and reversible myocardial ischemia]. / Myokardperfusion in Ruhe und unter Belastung. MR-Signalcharakteristika persistierender und reversibler Myokardischämien.

AIM: Performance of combined rest/stress MR perfusion studies and the analysis of qualitative signal intensity parameters in comparison with 99mTc-SestaMIBI SPECT in patients with known coronary artery disease (CAD). METHODS: Sixteen patients with CAD underwent MR myocardial perfusion assessment at rest and after dipyridamole-induced hyperemia. Qualitative parameters (SI increase, SI upslope) of the SI time-curves were evaluated and characteristics of normal, reversible and persistent hypoperfused myocardium as assessed by 99mTc-SestaMIBI SPECT were compared. RESULTS: Compared with the rest values, normal myocardium showed a significant increase of the SI upslope during hyperemia (P < 0.001), whereas persistent (P = 0.07) and reversible (P = 0.15) hypo-perfusions showed only minor changes. SI increase over baseline also showed a significant increase only in normal myocardium (P < 0.001). At rest, reversible ischemic areas showed no significant differences from normal myocardium, whereas during hyperemia SI increase was significantly lower (P = 0.02). CONCLUSIONS: Qualitative SI parameters of a combined rest/stress MR myocardial perfusion study allow to differentiate normal from reversibly or persistently hypoperfused myocardium.

Assessment of myocardial perfusion using multisection first-pass MRI and color-coded parameter maps: a comparison to 99mTc Sesta MIBI SPECT and systolic myocardial wall thickening analysis.

The most recently reported magnetic resonance first-pass myocardial perfusion studies were restricted to single slice imaging or a data analysis based on interactively placed regions of interest. This study was designed to investigate a new saturation recovery TurboFLASH sequence for multisection myocardial perfusion imaging and to develop a pixel-based software tool to calculate qualitative perfusion parameters. The findings of perfusion imaging were compared to percent systolic myocardial wall thickening analysis and 99mTc Sesta MIBI SPECT. Six healthy volunteers and twelve patients with proven coronary artery disease (CAD) or chronic myocardial infarction were examined. Diagnostic images were acquired for all volunteers and patients with the multisection saturation recovery TurboFLASH sequence. Perfusion defects could be visualized on parameter maps for signal intensity increase over baseline and signal intensity upslope. Sensitivity and specificity were 76.9% and 97.1% for first-pass perfusion MRI, and respectively 84.6% and 94.3% for CINE imaging. All perfusion defects determined with 99mTc Sesta MIBI SPECT were identified by the combined analysis of myocardial perfusion and wall thickening. The presented software demonstrated a pixel-based analysis of first-pass perfusion studies and simplified image interpretation in a clinical setting. The combination of perfusion and wall motion imaging provided complementary information for the treatment of patients suffering from CAD.

Comparison of the diagnostic information in relative cerebral blood volume, maximum concentration, and subtraction signal intensity maps based on magnetic resonance imaging of gliomas.

RATIONALE AND OBJECTIVES: The authors investigate the validity of regional relative cerebral blood volume (rCBV) versus maximum concentration and subtraction signal intensity (SI) maps using simple reconstruction modes in patients with gliomas. METHODS: Twenty-five patients were studied using a 1.5 T magnetic resonance imaging scanner. To calculate the rCBV map, the magnetic resonance susceptibility effect SI/time curves were first transformed into concentration/time curves; then a gamma-variate function was fitted and the area under the curve was integrated. From the concentration/time data, the maximum concentration (MAX) maps were calculated pixel per pixel as the maximum peak amplitude of the concentration/time curve. Subtraction (SUB) maps are a result of simple image subtraction of pixelwise baseline SI minus the highest peak of susceptibility change pixel per pixel. Region of interest means SI measurement of the different maps was compared using statistical t test correlation. RESULTS: Normal gray to white matter contrast did not show a significant difference among the rCBV, MAX, and SUB maps. Based on statistical evaluation, the low-grade lesions did not differ significantly in the rCBV, MAX, and SUB maps. The group with high-grade lesions (12 patients) showed no significant difference in standardized rCBV, MAX, and SUB maps. CONCLUSION: Compared to rCBV maps, the simple MAX and SUB maps demonstrated good correlation in both high-grade and low-grade gliomas. This simpler approach could establish first-pass reconstruction in clinical settings because it reduces the need for time-consuming postprocessing.

Multislice MR perfusion imaging and regional myocardial function analysis: complimentary findings in chronic myocardial ischemia.

PURPOSE: The purpose of this study is to assess the reliability of multislice MR perfusion imaging in comparison to regional wall function and nuclear medicine and to test different qualitative and quantitative parameters for perfusion assessment. MATERIAL AND METHODS: 15 patients with chronic myocardial ischemia underwent CINE and first-pass perfusion MR imaging. Functional myocardial imaging was performed using a segmented CINE FLASH sequence and systolic myocardial wall thickening was assessed after semiautomated segmentation. MR first-pass perfusion studies were performed using a multislice saturation recovery TurboFLASH sequence. Different parameters were calculated for assessment of hypoperfused segments and results of MR imaging compared to 99mTc-SestaMIBI SPECT. RESULTS: MR perfusion imaging showed a sensitivity of 72% and a specificity of 98%. In combination with MR CINE imaging and wall thickening analysis we calculated a sensitivity of 100% and a specificity of 93%. Qualitative and quantitative perfusion parameter analysis showed significant differences between normal and hypoperfused segments for the signal intensity increase (p < 0.001), the signal intensity upslope (p < 0.001) as well as for the myocardial mean transit time (p < 0.001). CONCLUSION: The combination of systolic wall thickening analysis and myocardial perfusion can markedly improve the sensitivity of MRI in depiction of LV myocardial perfusion abnormalities. For assessment of hypoperfusion, different quantitative and qualitative parameters can be calculated showing significant differences between normal state and hypoperfusion.

MRI assessment of cerebral blood volume in patients with brain infarcts.

MRI perfusion studies have focussed mainly on acute ischaemia and characterisation in ischaemia. Our purpose was to analyse regional brain haemodynamic information in acute, subacute, and chronic ischaemia. We performed 16 examinations of 11 patients on a 1.5 T MR images. Conventional and dynamic contrast-enhanced imaging were employed in all examinations. For the dynamic susceptibility sequences, a bolus (0.2 mmol/kg) of gadopentetate dimeglumine was injected. Reconstructed regional relative cerebral blood volume (rCBV) maps, bolus maps, and conventional images were analysed by consensus reading. In all examinations decreases in rCBV were observed in the lesions. The distribution of regional rCBV in lesions was heterogeneous. The rCBV of the periphery of the lesions was higher than that at their center. There was a correlation between the time since onset and abnormalities on the rCBV map and T2-weighted images (T2WI). In the early stage of acute stroke, the abnormalities tended to be larger on the rCBV than on T2WI. Many patterns of bolus passage were observed in ischaemic regions. rCBV maps provide additional haemodynamic information in patients with brain infarcts.

Patency of coronary bypass grafts: assessment with breath-hold contrast-enhanced MR angiography--value of a non-electrocardiographically triggered technique.

PURPOSE: To determine the value of non-electrocardiographically triggered contrast material-enhanced magnetic resonance (MR) angiography in assessing the patency of venous and internal thoracic artery (ITA) grafts after coronary bypass surgery. MATERIALS AND METHODS: Twenty-seven patients with 76 coronary bypass grafts (48 venous, 28 ITA) were examined 26.5 months +/- 5.8 after surgery with MR angiography and conventional angiography. MR angiography was performed with a three-dimensional gradient-echo sequence after automated injection of contrast material; contrast agent administration was based on measurement of the individual transit time of the agent. Results of MR angiography were interpreted by two independent observers and compared with results of conventional angiography. RESULTS: The independent interpretations of the MR angiograms agreed with the results of conventional angiography in 96% and 91% of the grafts. After a final consensus reading, sensitivity was 95% for all grafts, 94% for venous grafts, and 96% for ITA grafts. Specificity was 85% for venous grafts and 67% for ITA grafts. Positive predictive value was 95% for all grafts, 94% for venous grafts, and 96% for ITA grafts. CONCLUSION: Non-electrocardiographically triggered contrast-enhanced MR angiography allows reliable assessment of the patency of venous and arterial coronary bypass grafts.

[Calculation of absolute cerebral blood volume and cerebral blood flow by means of electron-beam computed tomography (EBT) in acute ischemia]. / Berechnungen des absoluten zerebralen Blutvolumens und des zerebralen Blutflusses mit der Elektronenstrahltomographie (EBT) bei akuten Ischämien.

The utility of electron beam computed tomography (EBT) to estimate cerebral blood volume (CBV) and cerebral blood flow (CBF) was evaluated. Eleven patients with suspected acute cerebral ischemia were investigated. The EBT was performed with an acquisition time of 50 ms per slice at eight parallel levels. To compare signal/noise and contrast/noise ratios the data from the EBT investigation were compared to a similar examination on a spiral CT. The signal/noise ratio with EBT was about 30%, the contrast/noise ratio 25% of that with spiral CT. The absolute values of CBV were 4.9 +/- 1.2 ml/100 g (EBT); CBF was 50.5 +/- 7.0 ml/100 g/min in normal contralateral brain tissue. In four patients with proven infarcts on follow-up, the ischemic areas had a CBV ranging from 1.7 to 3.8 ml/100 g, while CBF ranged from 9.4 to 24.5 ml/100 g/min. Using a bolus injection of contrast material, calculation of absolute CBV and CBF is feasible using EBT. Advantages of EBT are the absolute measurements possible and it's multislice capability. Disadvantages, however, are caused by the high image noise, limiting the demarcation of ischemic tissue.

[Qualitative and quantitative determination of regional myocardial perfusion with magnetic resonance tomography]. / Qualitative und quantitative Bestimmung der regionalen Myokardperfusion mittels Magnetresonanztomographie.

PURPOSE: Purpose of this study was to test a triple slice saturation recovery turbo FLASH sequence for myocardial perfusion imaging. In addition data-evaluation-tools for qualitative and quantitative perfusion parameters are presented and preliminary tested. MATERIAL AND METHODS: We examined 8 healthy volunteers and 4 patients with myocardial infarction. Parameters of the saturation recovery turbo FLASH sequence were as follows: TR = 2.5 msec, TE = 1.2 msec, alpha = 8 degrees, 3 slices, thickness 10 mm. For data analysis signal-intensity time curves were calculated pixel by pixel and evaluated for signal-intensity-increase over baseline and signal-intensity-upslope. Images were displayed color-coded. For quantitative data analysis we used the indicator dilution theory and developed a deconvolution algorithm which takes the arterial input function into account to calculate the myocardial mean transit time (MTT). RESULTS: The color-coded parametermaps showed uniform conditions in normal myocardium of volunteers, but reduced signal-intensity-increase over baseline and signal-intensity-upslope for infarcted areas in patients. The MTTs calculated using our algorithm were significantly shorter than those assessed with previous methods and matched better with values derived from literature. Infarcted areas show prolonged MTTs in comparison to normal myocardium. CONCLUSION: A triple slice saturation recovery turbo FLASH sequence is suitable for myocardial perfusion imaging. Color-coded parametermaps can visualize hypoperfused areas. For calculating myocardial MTTs using indicator dilution therapy a deconvolution algorithm is necessary.

[Contrast media enhanced magnetic resonance angiography for determining patency of a coronary bypass. A comparison with coronary angiography]. / Kontrastmittelverstärkte Magnetresonanzangiographie zur Uberprüfung der Durchgängigkeit koronarer Bypasses. Vergleich zur Koronarangiographie.

AIM: Assessment of graft patency with current non-invasive MRA techniques is particularly difficult for evaluating internal mammary artery grafts. Our aim is to determine the accuracy of a contrast enhanced MRA technique is assessing graft patency. METHODS: We examined 19 patients with a total of 53 grafts (32 venous/21 arterial), using an ultrafast contrast enhanced 3D gradient-echo technique and compared this with the results of selective angiography. RESULTS: Sensitivity of the contrast enhanced method was 95.2% for venous grafts, 94.4% for IMA grafts and 94.8% overall. Specificity was 85.7% overall, 90.9% for venous and 66.7% for IMA grafts. Positive predictive value was 94.4%. CONCLUSION: Compared with previous studies, visualisation of IMA grafts was improved by using contrast enhanced MRA. In this preliminary study, contrast enhanced MRA proved promising for the assessment of graft patency.