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PubMed2006

Cardiac cine MR-imaging at 3T: FLASH vs SSFP.

The implications of an increase in field strength, from 1.5 T to 3 T, for routine functional cardiac examinations have been systematically investigated. Flip angle optimization was carried out for identical SSFP and FLASH cine imaging sequences at 1.5 T and 3 T, which supported the use of 20 degrees (FLASH 1.5 T and 3 T) and >60 degrees (SSFP 1.5 T and 3 T). The optimized sequences were applied in a study of cardiac function in a group of ten normal volunteers. Both SSFP and FLASH sequences showed significant SNR increases in the myocardium and blood at 3 T compared with 1.5 T, increases of 48% and 30% (myocardium and blood, respectively) for the SSFP sequence and 19% and 13% for the FLASH sequence. The SSFP sequence also showed a significant increase in CNR (22%). Image quality assessment revealed that the SSFP acquisitions were superior to FLASH at both field strengths. Although SSFP contained more artifacts at 3 T, they would not prevent its clinical use. We conclude that cardiac functional examinations at 3 T should use SSFP sequences.

Artifacts↗
PubMed1997

Actions of Ca2+ on an early stage in phototransduction revealed by the dynamic fall in Ca2+ concentration during the bright flash response.

To study the actions of Ca2+ on "early" stages of the transduction cascade, changes in cytoplasmic calcium concentration (Ca2+i) were opposed by manipulating Ca2+ fluxes across the rod outer segment membrane immediately following a bright flash. If the outer segment was exposed to 0 Ca2+/0 Na+ solution for a brief period immediately after the flash, then the period of response saturation was prolonged in comparison with that in Ringer solution. But if the exposure to 0 Ca2+/0Na+ solution instead came before or was delayed until 1 s after the flash then it had little effect. The degree of response prolongation increased with the duration of the exposure to 0 Ca2+/0 Na+ solution, revealing a time constant of 0.49 +/- 0.03 s. By the time the response begins to recover from saturation, Ca2+i seems likely to have fallen to a similar level in each case. Therefore the prolongation of the response when Ca2+i was prevented from changing immediately after the flash seems likely to reflect the abolition of actions of the usual dynamic fall in Ca2+i on an early stage in the transduction cascade at a site which is available for only a brief period after the flash. One possibility is that the observed time constant corresponds to the phosphorylation of photoisomerized rhodopsin.

Ambystoma↗
PubMed1963

THE DEPENDENCE OF THE PHOTOPUPIL RESPONSE ON FLASH DURATION AND INTENSITY.

The changes in pupil size were recorded by infrared pupillographic methods in response to light flashes of different durations and intensities for a 13 degree 34 minute centrally fixated circular field. For such stimuli, the threshold intensities for (rod) vision and for the pupil response were found to be about the same. The response amplitudes were related to the logarithm of the flash energy, the reciprocity law remaining valid up to about one-half second. The curve relating flash energy and pupil response was clearly divisible into two parts commensurate with the duplex character of the human retina. A similar dichotomy appears in curves relating response amplitude to response latency. Since the pupil response is determined by total flash energy, intense long flashes produce larger pupil responses than shorter (and perceptually brighter) ones of the same intensity.

Adaptation, Ocular↗
PubMed1981

Flash photolysis of rhodopsin in the cat retina.

The bleaching of rhodopsin by short-duration flashes of a xenon discharge lamp was studied in vivo in the cat retina with the aid of a rapid, spectral-scan fundus reflectometer. Difference spectra recorded over a broad range of intensities showed that the bleaching efficacy of high-intensity flashes was less than that of longer duration, steady lights delivering the same amount of energy. Both the empirical results and those derived from a theoretical analysis of flash photolysis indicate that, under the conditions of these experiments, the upper limit of the flash bleaching of rhodopsin in cat is approximately 90%. Although the fact that a full bleach could not be attained is attributable to photoreversal, i.e., the photic regeneration of rhodopsin from its light-sensitive intermediates, the 90% limit is considerably higher than the 50% (or lower) value obtained under other experimental circumstances. Thus, it appears that the duration (approximately 1 ms) and spectral composition of the flash, coupled with the kinetic parameters of the thermal and photic reactions in the cat retina, reduce the light-induced regeneration of rhodopsin to approximately 10%.

Animals↗
PubMed1987

Intensity of the flash associated with laser-induced plasma in the eye.

A bright flash of light is emitted by the plasma formed at the focus of an ophthalmic Nd: YAG laser. The purpose of this investigation is to determine whether the plasma flash constitutes a hazard to the operator. The intensity of the flash at the slit lamp eyepiece was measured and the experimentally derived value was verified by independent calculation. For hazard assessment purposes, the concept of a spectrally weighted maximum permissible exposure (MPE) is described in the paper. Based on the MPE and our measurements and calculations our conclusion is that viewing the flash in a patient's eye through the microscope would not be considered hazardous to the operator. In contrast, during technical procedures, targets in air are used to produce a plasma and in this situation eye protection is recommended when viewing the flash through the microscope.

Cornea↗
PubMed2002

Sound-induced illusory flash perception: role of gamma band responses.

In the recently discovered sound-induced illusory flash phenomenon, a single flash accompanied with two auditory beeps is perceived as two flashes in a majority of trials. Here we asked what the neural substrates distinguishing illusion and no-illusion (i.e. perception of single flash) percepts are under identical stimulus configuration. Wavelet based method was used to analyze gamma band (> 30 Hz) responses in the event-related potential (ERP) signals recorded over visual cortical regions. We found: (i) significantly higher oscillatory and induced gamma band responses in illusion than in no-illusion trials, and (ii) significant supra-additive audio-visual interactions only in illusion trials. These results provide a clear neurophysiological correlate to the perception of illusion. Furthermore, the results suggest that auditory stimuli modulate cortical processing of visual stimuli, and the flash illusion (qualitative alteration of visual percept) only takes place when this modulation exceeds some critical threshold for the registration of conscious awareness.

Acoustic Stimulation↗
PubMed1992

Assessment of osteosarcoma response to preoperative chemotherapy using dynamic FLASH gadolinium-DTPA-enhanced magnetic resonance mapping.

RATIONALE AND OBJECTIVES: To improve the accuracy of magnetic resonance imaging (MRI) in evaluating the response of osteosarcomas to preoperative chemotherapy, the authors developed a technique of mapping tumor necrosis and viability by quantitating slope values of gadolinium-DTPA (Gd-DTPA) uptake on dynamic fast low-angle shot (FLASH) images. METHODS: Dynamic contrast-enhanced FLASH imaging of a single representative plane was performed on six osteosarcomas. Tumors were mapped by dividing resultant images into contiguous regions of interest and deriving slopes representing percentage increase in signal intensity (SI) per minute over the baseline for each region. The results were compared with estimations of viable tumor volume on subtracted Gd-DTPA-enhanced T1-weighted images and histologic maps of necrotic and viable tumor. RESULTS: Dynamic FLASH estimations of percent tumor necrosis using a critical slope value of 45% per minute correctly predicted histologic response to chemotherapy in all six patients. Comparison of dynamic FLASH and histologic maps showed a high degree of correlation. Static enhanced T1-weighted images overestimated the amount of residual viable tumor. CONCLUSIONS: Dynamic FLASH Gd-DTPA-enhanced mapping is a potentially useful noninvasive method of quantitating tumor response to chemotherapy.

Adolescent↗
PubMedDate not supplied

Single breath-holding scans of the abdomen using FISP and FLASH at 1.5 T.

Single breath-holding gradient echo techniques fast imaging with steady-state free precession (FISP) and fast low angle shot (FLASH) images were evaluated in the study of the abdomen in 16 patients (13 liver, two kidney, and one pancreas examinations). Gradient echo images were compared retrospectively with conventional spin echo images for image quality (depiction of pathology and representation of anatomic detail), and contrast characteristics were evaluated. All lesions were shown on gradient echo images, and in three of 16 cases gradient echo images were more diagnostic than spin echo images. On both FISP and FLASH images, most hepatic metastases were hyperintense relative to normal liver. The predicted flip angles for maximal contrast for the liver were modeled from signal intensity equations for FISP and FLASH and yielded predicted flip angles of approximately 40-55 degrees for FISP and 15-25 degrees for FLASH. Peak signal-to-noise ratio in liver of normal volunteers occurred at approximately 30 degrees for both FISP and FLASH. Single breath-holding gradient echo images are useful in the evaluation of abdominal structures and this study provides a framework for future work.

Abdominal Neoplasms↗
PubMedDate not supplied

Inversion recovery snapshot FLASH MR imaging.

Snapshot fast low angle shot (FLASH) magnetic resonance (MR) imaging techniques have been developed to enable real time imaging of MR parameters. The method is based on a 64 x 128 FLASH tomogram acquired within less than 200 ms. This work describes snapshot FLASH MR using a single 180 degrees pulse prior to the acquisition of a series of FLASH images. The experiment creates continuous dynamic inversion recovery (IR) T1 contrast in successive images. The total acquisition time of 16 images displaying the IR behavior is less than 4 s. Representative snapshot FLASH IR MR images of the abdomen of healthy rats and of an implanted hepatic tumor are illustrated.

Animals↗
PubMed1985

Laser flash effects on laser speckle shift visual evoked potential.

Steady-state visual evoked potentials (VEP's) were recorded from four cynomolgus monkeys in response to a sinusoidally oscillating 10 degrees helium-neon laser speckle field (632.8 nm), moving vertically 2.5 degrees at 8 shifts per second. A 5-pulse flash train at the maximum permissible exposure (MPE) dose from a collimated Q-switched frequency-doubled neodymium laser (532 nm) was superimposed on the foveal stimulus and the subsequent disruption and recovery of the VEP measured. Minimal disruption of the response signal magnitude was demonstrated (0.1 greater than p greater than 0.05) which recovered within 300 ms of the termination of the pulse train. A small but significant (p less than 0.01) disruption of phase entrainment was also noted that recovered within the same period. This is contrasted with a second experiment with three monkeys in which an argon (514 nm) laser served both as the speckle stimulus source and as the shuttered flash. Exposure to collimated MPE argon radiation for 250 ms immediately depressed the VEP (97%, p less than 0.01) and showed recovery to 70% of the pre-flash baseline only after 3 s. Phase lock was also severely degraded for several seconds. These results imply that visual processing of nonacuity-limited medium contrast stimuli with broad spatial frequency content will probably not be materially affected by ultra-short pulsed laser exposure at these energy levels and frequencies. However, even safe levels of collimated continuous laser light may have severe effects on vision that could parallel flash effects seen with Xenon discharge flash lamps.

Animals↗
PubMed1980

Flash Inactivation of Oxygen Evolution: IDENTIFICATION OF S(2) AS THE TARGET OF INACTIVATION BY TRIS.

Brief saturating light flashes were used to probe the mechanism of inactivation of O(2) evolution by Tris in chloroplasts. Maximum inactivation with a single flash and an oscillation with period of four on subsequent flashes was observed. Analyses of the oscillations suggested that only the charge-collecting O(2)-evolving catalyst of photosystem II (S(2)-state) was a target of inactivation by Tris. This conclusion was supported by the following observations: (a) hydroxylamine preequilibration caused a three-flash delay in the inactivation pattern; (b) the lifetimes of the Tris-inactivable and S(2)-states were similar; and (c) reagents accelerating S(2) deactivation decreased the lifetime of the inactivable state. Inactivation proved to be moderated by F, the precursor of Signal II(s), as shown by a one flash delay with chloroplasts having high abundance of F. Evidence was obtained for cooperativity effects in inactivation and NH(3) was shown to be a competitive inhibitor of the Tris-induced inactivation. S(2)-dependent inactivation was inhibited by glutaraldehyde fixation of chloroplasts, possibly suggesting that inactivation proceeds via conformational changes of the S(2)-state.

Journal Article↗
PubMed2002

Flash-cooling and annealing of protein crystals.

Flash-cooling and annealing of macromolecular crystals have been investigated using in situ X-ray imaging, diffraction-peak lineshape measurements and conventional crystallographic diffraction. The dominant mechanisms by which flash-cooling creates disorder are suggested and a fixed-temperature annealing protocol for reducing this disorder is demonstrated that should be more reliable and flexible than existing protocols. Flash-cooling tetragonal lysozyme crystals degrades diffraction resolution and broadens the distributions of lattice orientations (mosaicity) and lattice spacings. The diffraction resolution strongly correlates with the width of the lattice-spacing distribution. Annealing at fixed temperatures of 253 and 233 K consistently reduces the lattice-spacing spread and improves the resolution for annealing times up to approximately 30s. X-ray images show that this improvement arises from the formation of well ordered domains with characteristic sizes >10 microm and narrower mosaicities than the crystal as a whole. Flash-cooled triclinic crystals of lysozyme, which have a smaller water content than the tetragonal form, diffract to higher resolution with smaller mosaicities and exhibit pronounced ordered domain structure even before annealing. It is suggested that differential thermal expansion of the protein lattice and solvent may be the primary cause of flash-cooling-induced disorder. Mechanisms by which annealing at T << 273 K reduce this disorder are discussed.

Animals↗
PubMed1992

Flash and pattern-reversal visual evoked potential abnormalities in infants and children with cerebral blindness.

Visual evoked potentials (VEPs) were recorded of 60 infants and children with cerebral blindness, aged between six weeks and 10 years, and compared with age-matched normative data. Every patient had abnormal VEPs. 18 had absent flash and pattern VEPs and 13 had atypical or atypical and asymmetrical flash and pattern VEPs. Of the remaining 29, most had greater abnormality of pattern than of flash VEPs and greater abnormalities over parietal and temporal than occipital areas. Eight patients had normal occipital responses to flash and five others had delayed responses with normal morphology. One had normal occipital responses to pattern stimuli. All of these had abnormal late occipital responses or abnormal responses over the parietal and temporal areas. It is recommended that visual assessments using VEPs employ both flash and pattern stimuli, that pre-occipital as well as occipital recordings be made and that tracings be compared with age-specific normative data.

Blindness↗
PubMed1970

Responses of cat retinal ganglion cells to brief flashes of light.

1. The responses of cat retinal ganglion cells to brief flashes of light have been illustrated and described with a view to providing material for comparison with psychophysical experiments in the scotopic (rod-dominated) range of performance.2. There is a minimum response duration of 50-70 msec no matter how brief the flash is made. This duration is reached with stimuli lasting 32 msec or shorter.3. Reducing the background illumination obviously increases the latency of responses to stimuli at 4 times threshold intensity (about 10 msec increment per log. unit decrement) but has no obvious effect on the minimum response duration.4. The relation between intensity and duration of a flash for threshold responses closely resembles that in human psychophysical experiments. The Bunsen-Roscoe law is applicable for flash durations up to 64 msec.5. If equal amounts of energy are delivered in the form of a pair of flashes of varying separation rather than by rectangular pulses, the shape of the response changes more abruptly with the temporal factor.6. Non-linear performance is apparent for stimuli as weak as 4 times threshold.7. A method is developed for quantitative analysis of individual responses. It is based upon cross-correlation of the train of impulses with a Gaussian smoothing function and represents local impulse frequency as a smooth function of time. The method also improves the signal-to-noise ratio of post-stimulus time-histograms of the sum of many responses.8. The measurement of variability of individual responses is the main result of the method; its magnitude indicates that it is a significant new factor limiting temporal resolution with suprathreshold stimuli.

Animals↗
PubMed2004

Shorter latencies for motion trajectories than for flashes in population responses of cat primary visual cortex.

Psychophysical evidence in humans indicates that localization is different for stationary flashed and coherently moving objects. To address how the primary visual cortex represents object position we used a population approach that pools spiking activity of many neurones in cat area 17. In response to flashed stationary squares (0.4 deg) we obtained localized activity distributions in visual field coordinates, which we referred to as profiles across a 'population receptive field' (PRF). We here show how motion trajectories can be derived from activity across the PRF and how the representation of moving and flashed stimuli differs in position. We found that motion was represented by peaks of population activity that followed the stimulus with a speed-dependent lag. However, time-to-peak latencies were shorter by approximately 16 ms compared to the population responses to stationary flashes. In addition, motion representation showed a directional bias, as latencies were more reduced for peripheral-to-central motion compared to the opposite direction. We suggest that a moving stimulus provides 'preactivation' that allows more rapid processing than for a single flash event.

Action Potentials↗
PubMed2001

The b-wave of the dark adapted flash electroretinogram in patients with advanced asymmetrical glaucoma and normal subjects.

AIMS: To evaluate whether the b-wave of the dark adapted flash electroretinogram (ERG) is affected by glaucomatous damage. METHODS: ERGs were recorded in 35 patients aged 33-65 years with advanced asymmetrical glaucomas (interocular difference of perimetric defects (mean deviation) >2 dB between the two fellow eyes of the glaucoma patients, primary and secondary open angle and low tension glaucomas) and 17 normal subjects matched for age and sex using white flashes of a xenon discharge tube in a Ganzfeld stimulator. After 30 minutes of dark adaptation luminance response functions were obtained using flashes of increasing scotopic luminance (highest 9.4 cd/s/m2, lowest 5.5 log units below it). The parameters Vmax, n, and K of the Naka-Rushton equation were computed from the measurement values based on the usual fitting procedure. These parameters, together with b-wave amplitudes and implicit times for all flash intensities, were compared interocularly and between the normal subjects and those with glaucoma. Correlations were computed between interocular differences of the mean deviation and interocular differences of Vmax, n, K, b-wave amplitudes, and implicit times between the two fellow eyes of the patients with asymmetrical glaucomatous damage. RESULTS: Implicit times were significantly longer (p<0.005) in the glaucoma patients than in the normal group for flash intensities of 9.4, 5.3, 1.7, 0.53, and 0.17 cd/s/m2. b-Wave amplitudes did not differ significantly between the two study groups. Comparing the two fellow eyes of each patient with glaucoma, Vmax was significantly higher in the less damaged eye than in the more damaged eye. The interocular differences in the mean deviation correlated significantly with the interocular differences in the b-wave amplitudes, implicit times, and Vmax. CONCLUSIONS: These results suggest that glaucomas can lead to electrophysiologically measurable damage of the inner nuclear layer.

Adaptation, Ocular↗
PubMed1976

A possible means of monitoring the progress of demyelination in multiple sclerosis: effect of body temperature on visual perception of double light flashes.

The ability to discriminate closely separated pairs of light flashes as being double is impaired in multiple sclerosis. The effects of altering body temperature on double flash resolution and on visual acuity were studied in four multiple sclerosis patients and in control subjects. At demyelinated sites heating impaired and cooling improved double flash resolution. Visual acuity behaved similarly. The double flash test was very sensitive, changing up to 75 ms in response to simple heating and cooling procedures that produced small variations in acuity. Apart from its diagnostic value, the double flash test furnishes a simple in vivo model to study the effect of temperature change (and potential symptomatic therapy) on conduction in partially demyelinated axons in the visual system.

Adult↗
PubMed2002

Breath-hold FLASH and FISP cardiovascular MR imaging: left ventricular volume differences and reproducibility.

PURPOSE: To compare fast imaging with steady-state precession (FISP) and fast low-angle shot (FLASH) magnetic resonance acquisitions to quantify left ventricular volumes, mass, and function and to determine if the two techniques are comparable. MATERIALS AND METHODS: Left ventricular volume studies were performed in 10 patients with heart failure and in 10 healthy subjects by using FISP and FLASH imaging. Identical section positions were used for section-by-section contour comparisons. Manual analysis was performed by two experienced observers. The study was repeated on a different day and interobserver and interstudy reproducibility assessed. RESULTS: With FISP, end-diastolic volume was larger (healthy subjects: +18 mL [13%], P <.001; patients: +6 mL [3%], not significant), end-systolic volume larger (healthy subjects: +9 mL [17%], P =.001; patients: +8 mL [6%], P =.001) and left ventricular mass smaller (healthy subjects: -25 g (19%), P <.001; patients: -21 g (11%), P <.001). There were no significant differences in ejection fraction. Both sequences had excellent interstudy and interobserver reproducibility, with statistically better reproducibility for interstudy healthy-subject ejection fraction on FISP images (P =.05). Section-by-section analysis determined that at FISP, endocardial contours were drawn larger and the epicardial contours smaller than on FLASH images. FISP enabled better delineation of epicardial fat from myocardium, of blood-myocardium interface in areas of trabeculation or papillary muscles, and of the atrioventricular ring. CONCLUSION: FISP produces small but significantly higher left ventricular volume measurements, as compared with FLASH imaging. FLASH imaging and FISP have similar reproducibility.

Adult↗