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22.03.2014
Hard X-rays can penetrate solid objects, and their largest use is to take images of the inside of objects in diagnostic radiography and crystallography. The roentgen (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create one electrostatic unit of charge of each polarity in one cubic centimeter of dry air.
The rad is the (obsolete) corresponding traditional unit, equal to 10 millijoules of energy deposited per kilogram. The sievert (Sv) is the SI unit of equivalent dose, which for X-rays is numerically equal to the gray (Gy). X-rays are generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. In crystallography, a copper target is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem.
X-ray fluorescence: If the electron has enough energy it can knock an orbital electron out of the inner electron shell of a metal atom, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. So the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the dose.
To generate an image of the cardiovascular system, including the arteries and veins (angiography) an initial image is taken of the anatomical region of interest. A specialized source of X-rays which is becoming widely used in research is synchrotron radiation, which is generated by particle accelerators.
The most commonly known methods are photographic plates, photographic film in cassettes, and rare earth screens. Before the advent of the digital computer and before invention of digital imaging, photographic plates were used to produce most radiographic images. Since photographic plates are sensitive to X-rays, they provide a means of recording the image, but they also required much X-ray exposure (to the patient), hence intensifying screens were devised. Areas where the X-rays strike darken when developed, causing bones to appear lighter than the surrounding soft tissue. Contrast compounds containing barium or iodine, which are radiopaque, can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels.
An increasingly common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s.
The PSP plate can be reused, and existing X-ray equipment requires no modification to use them. For many applications, counters are not sealed but are constantly fed with purified gas, thus reducing problems of contamination or gas aging. Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier.
Turning Safe Search OFF may display content intended for mature audiences.You must be at least 18 years old to continue. Please enter at least one email addressYou are trying to send out more invites than you have remaining. We will be provided with an authorization token (please note: passwords are not shared with us) and will sync your accounts for you. While changes in cochlear frequency tuning are thought to play an important role in the perceptual difficulties of people with sensorineural hearing loss (SNHL), the possible role of temporal processing deficits remains less clear. In addition to diminished spectral resolution, perceptual difficulties in people with SNHL might also reflect changes in auditory sensitivity to the temporal structure of sound. In the present study, we extend this previous physiological work by studying temporal envelope coding using more complex, Gaussian noise stimuli with broad frequency bandwidth and a dynamic temporal envelope.
All procedures were performed in chinchillas and approved by the Purdue Animal Care and Use Committee.
Single fibers were isolated by listening for spikes on a monitor speaker while advancing the electrode through the auditory nerve during periodic stimulation with broadband noise. We quantified the temporal precision of envelope coding using spectral and temporal analysis methods.
Finally, we used the amplitude envelope of h2 to quantify the amplitude and latency of temporal envelope coding (Figure 1E, black cross). Acoustic overexposure was associated with increases in both the threshold and 10-dB bandwidth of auditory-nerve fiber tuning curves (Figure 3).
Patterns of hearing loss were similar between the 2 kHz narrowband and 500 Hz octave band overexposure paradigms (Figure 3, red triangles and blue circles, respectively). TMTFs generated from Wiener-kernel analyses of auditory-nerve fiber responses to broadband Gaussian noise invariably exhibited a roll-off in response amplitude with increasing temporal modulation frequency (Figure 1F). Cut-off modulation frequencies were generally greater in noise-overexposed fibers than in normal-hearing control fibers (Figure 6). This process produces an emission spectrum of X-ray frequencies, sometimes referred to as the spectral lines. The intensity of the X-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the X-ray tube.


A second image is then taken of the same region after iodinated contrast material has been injected into the blood vessels within this area. The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen. In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate. This means that you will not need to remember your user name and password in the future and you will be able to login with the account you choose to sync, with the click of a button. This page doesn't support Internet Explorer 6, 7 and 8.Please upgrade your browser or activate Google Chrome Frame to improve your experience.
Our knowledge of temporal envelope coding in the impaired cochlea is limited to two studies that examined auditory-nerve fiber responses to narrowband amplitude modulated stimuli. Research conducted over the last several decades has uncovered changes in cochlear frequency tuning with SNHL that most likely contribute to speech perception problems in these listeners.
Consistent with enhancement of envelope coding, these studies showed that noise-induced SNHL amplifies phase locking to the temporal envelope of sinusoidally amplitude modulated (SAM) tones and single-formant stimuli in auditory-nerve fibers of anesthetized chinchillas (Kale and Heinz, 2010).
Here, we quantify the effects of SNHL on the temporal precision, amplitude, and latency of envelope coding at the level of the auditory nerve in anesthetized chinchillas. When a fiber was encountered, a tuning curve was recorded (Figure 1D, black line) using an automated procedure that tracked, as a function of stimulus frequency, the minimum SPL of a 50-ms tone required to evoke at least 1 more spike than a subsequent 50-ms silent period (Chintanpalli and Heinz, 2007).
In its 2-dimensional Fourier transform, the envelope component of h2 falls into the 2nd and 4th quadrants while the fine structure component falls into quadrants 1 and 3 (Recio-Spinoso et al., 2005).
For the spectral analysis, we computed the STRF as the 1-dimensional Fourier transform of h2 (Figure 1B; Lewis and van Dijk, 2004). The amplitude envelope of this eigenvector (Figure 1E, gray line), determined using the Hilbert transform, represents the temporal window to which the fiber responds to acoustic stimulation with an increase in spike rate. To facilitate comparison of data across fibers with varying driven rates and response threshold levels, we normalized h2 by dividing by N0A. Dependent variables were log10-transformed in all cases except for the analysis of temporal window latency.
The spectral lines generated depend on the target (anode) element used and thus are called characteristic lines. These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels. Photographic film largely replaced these plates, and it was used in X-ray laboratories to produce medical images. In the pursuit of a non-toxic contrast material, many types of high atomic number elements were evaluated. After the plate is X-rayed, excited electrons in the phosphor material remain "trapped" in "colour centres" in the crystal lattice until stimulated by a laser beam passed over the plate surface. Electrons accelerate toward the anode, in the process causing further ionization along their trajectory.
In the present study, we used Wiener-kernel analyses of auditory-nerve fiber responses to broadband Gaussian noise in anesthetized chinchillas to quantify changes in temporal envelope coding with noise-induced SNHL. Amplified envelope coding was observed over a wide range of modulation frequencies and did not appear to alter the shape of temporal modulation transfer functions (TMTFs) plotting response amplitude as a function of modulation frequency (Kale and Heinz, 2012). The results show that at equal stimulus sensation level, noise-induced SNHL increases the temporal precision of envelope coding by 20–30%. CF (Figure 1D, cross) was identified as the frequency of best sensitivity or, in noise-overexposed fibers, as the frequency of the breakpoint in the high frequency slope of the tuning curve because this value provides a robust estimate of CF prior to cochlear damage (Liberman, 1984). Histograms showing the distribution of noise stimulus levels presented to normal-hearing and noise-overexposed fibers. Because our primary focus was envelope coding, we removed the fine structure component from h2 by discarding the contents of the 1st and 3rd quadrants. So normalized, the amplitude values relate to a modulation factor of the mean firing at the average SPL of the noise stimulus. Temporal response patterns were also similar between the overexposed groups, and are therefore described together. Notably, the roll-off rate was also slower in noise-overexposed fibers than in normal-hearing control fibers.
The radiologist or surgeon then compares the image obtained to normal anatomical images to determine if there is any damage or blockage of the vessel. In more recent years, computerized and digital radiography has been replacing photographic film in medical and dental applications, though film technology remains in widespread use in industrial radiography processes (e.g. For example, the first time the forefathers used contrast it was chalk, and was used on a cadaver's vessels.
This process, known as a Townsend avalanche, is detected as a sudden current, called a "count" or "event". Temporal modulation transfer functions (TMTFs) and temporal windows of sensitivity to acoustic stimulation were computed from 2nd-order Wiener kernels and analyzed to estimate the temporal precision, amplitude, and latency of envelope coding.
Furthermore, SNHL causes downward shifts in the best frequency of tuning, particularly in the base of the cochlea, which are also likely to contribute to perceptual impairment (Liberman, 1984; Henry and Heinz, 2013).


Time axes (A,B,E) indicate time relative to the occurrence of a spike, and are plotted from a lower limit of 1 ms rather than 0 ms to more clearly show the structure of the kernels.
The bandwidth of the tuning curve 10 dB above threshold was also quantified (Figure 1D, gray line). The noise floor of the temporal window was calculated as the mean amplitude plus 3 standard deviations during the first 2 ms and last 3 ms of the waveform. In general, mixed model analyses were conducted with two continuous independent variables, log10(CF) and [log10(CF)]2. Data from subpopulations of fibers with low and high spontaneous rate are shown in panels (B) and (C).
When the film is developed, the parts of the image corresponding to higher X-ray exposure are dark, leaving a white shadow of bones on the film.
Noise overexposure was associated with slower (less negative) TMTF roll-off with increasing modulation frequency and reduced temporal window duration.
Physiological data from non-human animals have the potential to clarify changes in temporal envelope sensitivity with SNHL. The skin and muscles overlying the skull were transected to expose the ear canals and bullae, and both ear canals were dissected to allow insertion of hollow ear bars.
Next, a sequence of 9 broadband Gaussian noise stimuli were presented repeatedly for up to 10 mins at 10–15 dB above the threshold for the noise stimulus until approximately 20,000 driven spikes were recorded. Portions of the temporal window falling below the noise floor were excluded as not statistically significant. At CFs above 2.5 kHz, noise overexposure was associated with an increase in driven rate and decrease in the proportion of low-SR fibers. Photographic plates are mostly things of history, and their replacement, the "intensifying screen", is also fading into history. The results show that at equal stimulus sensation level, SNHL increases the temporal precision of envelope coding by 20–30%. Due to noise-induced threshold elevation, the SPL of noise stimuli was on average ~20 dB higher in noise-overexposed fibers than in unexposed controls (Figure 2). We quantified the duration of the temporal window at amplitude values corresponding to 20, 30, 40, and 50% of peak amplitude (Figure 1D, red dotted lines).
Roll-off rate was 30% slower (less negative) in noise-overexposed fibers than in normal-hearing controls. The metal silver (formerly necessary to the radiographic & photographic industries) is a non-renewable resource.
A craniotomy was opened in the posterior fossa and the cerebellum partially aspirated and retracted medially to expose the trunk of the auditory nerve bundle. Regions of the TMTF rising less than 3 dB above the noise floor were excluded as not statistically significant.
In general, shorter durations at any given amplitude value reflect greater temporal precision. Thus it is beneficial that this is now being replaced by digital (DR) and computed (CR) technology. While a previous study of envelope coding demonstrated a similar increase in response amplitude, the present study is the first to show enhanced temporal precision. Acoustic stimuli were presented through the right ear bar with a dynamic loudspeaker (Beyerdynamic DT48) and calibrated using a probe microphone placed within a few mm of the tympanum (Etymotic ER7C).
In general, slower (less negative) roll-off rates and greater cut-off modulation frequencies correspond to greater temporal precision of envelope coding. Statistical inferences were drawn based on F-tests and T-tests comparing least squares means.
Where photographic films required wet processing facilities, these new technologies do not. This new finding may relate to the use of a more complex stimulus with broad frequency bandwidth and a dynamic temporal envelope. Exaggerated neural coding of fast envelope modulations may contribute to perceptual difficulties in people with SNHL by acting as a distraction from more relevant acoustic cues, especially in fluctuating background noise. Temporal windows were used to calculate the amplitude and latency of envelope coding (black cross) and duration of temporal sensitivity at 50, 40, 30, and 20% of peak amplitude (red dotted lines).
Recordings were amplified (Dagan 2400A) and band-pass filtered from 0.03 to 6 kHz (Krohn-Hite 3550). Finally, the results underscore the value of studying sensory systems with more natural, real-world stimuli. Spikes were identified using a time-amplitude window discriminator (BAK Electronics) and timed with 10-μs resolution. TMTFs were characterized based on roll-off rate (green line) and cut-off modulation frequencies measured 3, 4, 5, and 6 dB down from peak amplitude (red dotted lines).



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Comments Low frequency nerve hearing loss jobs

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