SNR

The detectability of critical defects with CT depends on the final image quality and the skill of the operator, see figure 2. The basic concepts of image quality are resolution, contrast, and noise. Image quality are generally described by the signal-to-noise ratio SNR), the modulation transfer function (MTF) and the noise power spectrum (NFS). SNR is the quotient of a signal and its variance, MTF describes the contrast as a function of spatial frequency and NFS in turn describes the noise power at various spatial frequencies [1, 3]. 

To verify the modelling of the data eolleetion process, calculations of SAT 4, in the entrance window of the XRII was compared to measurements of RNR p oj in stored data as function of tube potential. The images object was a steel cylinder 5-mm) with a glass rod 1-mm) as defect. X-ray spectra were filtered with 0.6-mm copper. Tube current and exposure time were varied so that the signal beside the object. So, was kept constant for all tube potentials. Figure 8 shows measured and simulated SNR oproj, where both point out 100 kV as the tube potential that gives a maximum. Due to overestimation of the noise in calculations the maximum in the simulated values are normalised to the maximum in the measured values. Once the model was verified it was used to calculate optimal choice of filter materials and tube potentials, see figure 9. 

Sandborg, M. and G. Alm-Carlsson, Influence of x-ray energy spectrum, contrasting detail and detector on the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) in projection radiography. Phys. Med. Biol., 1992. 37(6) p. 1245-1263. 

To find now the optimum excitation frequency, we calculate the first derivative of Equ. (3.1) to find the maximum value of the response field of cracks in different depths (Fig. 3,1). For example a crack (20 x 0.6 x 0,2 mm) in a depth of 9 mm in an aluminium sample (a = 20 MS/m) could be found with highest SNR when using a frequency of 260 Hz Here a double-D... 

By choosing the proper correlation algorithm, it is possible to realise sensitive filters for other types of defects (e.g. corrosion). Fig. 5.2 shows an example for the suppression of signals which do not exhibit the expected defect stmcture (Two parallel white lines near upper central rim portion of Fig. 5.2). The largest improvement in SNR is obtained here by using the expression (ai ai+x /ai+yj), since for a gradiometric excitation, one expects the crack response to show two maxima (a, aj+x) with a minimum (a m) in the centre (see Fig. 5.3). 

In fig. 2 an ideal profile across a pipe is simulated. The unsharpness of the exposure rounds the edges. To detect these edges normally a differentiation is used. Edges are extrema in the second derivative. But a twofold numerical differentiation reduces the signal to noise ratio (SNR) of experimental data considerably. To avoid this a special filter procedure is used as known from Computerised Tomography (CT) /4/. This filter based on Fast Fourier transforms (1 dimensional FFT s) calculates a function like a second derivative based on the first derivative of the profile P (r) ... 

The filter according equation (1) allows a practical application of a second derivative, because it has only the noise amplification like a first derivative. This is shown in fig. 3 on a experimental data set. The SNR of the true second derivative is too low for correct edge detection, whereas the CT filter gives reliable results. 

Prompted by the success, TOFD measurements were conducted on a fatigue crack in a stainless steel compact tension specimen. Test and system parameters were optimised following the same procedure used for carbon steel specimens. A clear diffracted signal was observed with relatively good SNR and its depth as measured from the time-of-flight measurements matched exactly with the actual depth. 

Materials for MO Media. The materials classes best suited for MO media are RE-TM alloys, Co/Pt multilayers, and ferrites. The quality of disks is mainly characterised by the signal-to-noise ratio (SNR). 

Each of the systems has an optimum distance for which the SNR has a maximum. For Tb—FeCo this is at about 220 K (50),... 

Fig. 13. Expected signal and noise levels for RE-TM alloys and Pt/Co multilayers (schematic). The total noise entering the SNR is the sum of the system noise, disk noise, and write noise. The system noise is electronic noise and photon shot noise and is comparable for disks with the same reflectivity.

If the microstmcture becomes ever finer by improved deposition technology, the domain irregularities should diminish. The SNR is limited by the shot noise of the laser source and is equal to i . In this region a high 9 is of great value. 

When used for superresolution, the laser beam is incident on b, which hides the domains in s. During read-out, b is heated and the domains in s are copied to b. The optical system sees only the overlap area between the laser spot and the temperature profile which is lagging behind, so that the effective resolution is increased. Experimentally it is possible to double the linear read-out resolution, so that a four times higher area density of the domains can be achieved when the higher resolution is also exploited across the tracks. At a domain distance of 0.6 pm, corresponding to twice the optical cutoff frequency, a SNR of 42 dB has been reached (82). 

Fig. 16. Maximum achievable signal-to-noise ratio (SNR) on read-out of different writable optical data storage systems as a function of the writing energy (laser power) (121). SQS = Organic dye system (WORM) PC = phase change system (TeSeSb) MO = magnetooptical system (GbTbFe). See text.

TOCSY data are acquired in tbe pbase-sensitive mode using quadrature detection, and aU. tbe data phases are positive. Tbis increases the SNR for the matrix, and the time required for the experiment is short because very Htde, if any, phase cycling is necessary. In some cases a single scan per FID suffices, and the data can be acquired in approximately 10 min,... 

Heteronuclear chemical shift-correlated spectroscopy, commonly called H-X COSY or HETCOR has, as the name implies, different and F frequencies. The experiment uses polarization transfer from the nuclei to the C or X nuclei which increases the SNR. Additionally, the repetition rate can be set to 1—3 of the rather than the longer C. Using the standard C COSY, the ampHtude of the C signals are modulated by the... 

Time constraints ate an important factor in selecting nmr experiments. There are four parameters that affect the amount of instmment time requited for an experiment, A preparation delay of 1—3 times should be used. Too short a delay results in artifacts showing up in the 2-D spectmm whereas too long a delay wastes instmment time. The number of evolution times can be adjusted. This affects the F resolution. The acquisition time or number of data points in can be adjusted. This affects resolution in F. EinaHy, the number of scans per EID can be altered. This determines the SNR for the 2-D matrix. In general, a lower SNR is acceptable for 2-D than for 1-D studies. 

The resolution in an ultrasound image is, among other things, related to the duration of the ultrasound pulse, ie, the shorter the pulse the better the resolution. Imaging may not be performed when the pulse duration is longer than the time to receive an echo. The shorter the ultrasound pulse the more difficult it is to discern it from noise, and the poorer the SNR of the image. As the pulse duration is decreased, the power of the ultrasound pulse is typically increased to compensate for the poorer SNR. 

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