Effective detector width

Implementation of the complete functionality of 64-detector capabilities in the 320-detector-row system enables use of all whole-body CT protocols and applications developed for 64-detector-row scanners. The wide volume-stitching mode is slightly faster than the helical technique is and likewise, requires no special training since the integrated software automatically joins the individual volumes and adjusts the effective detector width. 

The increase in effective detector width is associated with a proportionate increase in the effect of over ranging in the helical scan mode, and therefore, increased proportion of the overall radiation exposure from the first and last half rotation that does not contribute to image reconstruction. This effect is more significant when smaller volumes are imaged, e.g., when examining children, where this effect may account for up to 50% of the overall dose. The only effective means to reduce this excess radiation is the use of an active colh-mator in front of the X-ray tube, which is commercially available. 

The geometric slit width is associated with the effective mechanical widths (in mm or )um) at the entrance and exit slits for a given spectral bandpass. The entrance and exit slits, thus, control the portion of the radiation from the source that enters the monochromator and falls on the detector. By use of a wide entrance slit, large amounts of radiation energy reach the detector. In this case, the noise is small compared to the signal, and lower amplification can be employed. When the noise is low, the signal is stable and precise and low detection limits can be measured. The entrance and exit slits should have very similar mechanical dimensions. 

Lock-in amplifiers provide a very narrow frequency band pass and thus achieve an excellent signal-to-noise ratio. With suitable resistance-capacitance filters, an effective bandpass width of 1 Hz can be obtained. The lock-in amplifier uses a chopper to modulate the energy source. A reference source, usually a small flashlight bulb, is modulated at exactly the same frequency. The two signals are combined in a synchronous detector to produce sum and difference frequencies the sum frequency will be twice the chopping frequency and the difference frequency will be zero. 

A 16-slice scanner reduces the overall scan time to less than 15 s even when very thin slices are used. With most 16-slice CT scanners, the operator can only choose fi-om two primary slice thicknesses, depending on detector width. In contrast, the pitch factor is variable however, a pitch factor of 1 or slightly less than 1 provides optimal results on most CT scanners in terms of signal-to-noise ratio and slice sensitivity profile (SSP). Since effective scan time is not an issue on 16-slice scaimers, there is no need to use a higher pitch factor. 

At the conclusion of the northern hemisphere survey, the telescopes were refurbished and modified. The side lobe rejection was improved and the detector widths were increased by 50 percent. Also, the 4.2 pm band was replaced by one which had a 27.4 pm effective wavelength. The telescopes were twice successfully flown from Woomera, Australia These southern hemisphere flights mapped 36 percent of the celestial sphere, the majority of which lies at negative declinations. The 11.0 pm coverage is now 90 percent of the sky (37000 square degrees). 

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

Figure 2 Effect of slit-width on linearity— benzoic acid at 254 nm, 1.0 AUFS. (From Pfeiffer, C. D., Larson, J. R., and Ryder, J. F., Linearity testing of ultraviolet detectors in liquid chromatography, Anal. Chem., 54,1622,1983. Copyright American Chemical Society Publishers. With permission.)...

Thus, the region 2100-1830 cm 1 can be covered. This allows us to monitor CO(v,J) by resonance absorption and various M(CO)n [n = 3-6] as a result of near coincidences between the CO laser lines and the carbonyl stretching vibrations of these species. The temporal response of the detection system is ca. 100 ns and is limited by the risetime of the InSb detector. Detection limits are approximately 10 5 torr for CO and M(CO)n. The principal limitation of our instrumentation is associated with the use of a molecular, gas discharge laser as an infrared source. The CO laser is line tuneable laser lines have widths of ca. lO cm 1 and are spaced 3-4 cm 1 apart. Thus, spectra can only be recorded point-by-point, with an effective resolution of ca. 4 cm 1. As a result, band maxima (e.g. in the carbonyl stretching... 

This effective Q,t-range overlaps with that of DLS. DLS measures the dynamics of density or concentration fluctuations by autocorrelation of the scattered laser light intensity in time. The intensity fluctuations result from a change of the random interference pattern (speckle) from a small observation volume. The size of the observation volume and the width of the detector opening determine the contrast factor C of the fluctuations (coherence factor). The normalized intensity autocorrelation function g Q,t) relates to the field amplitude correlation function g (Q,t) in a simple way g t)=l+C g t) if Gaussian statistics holds [30]. g Q,t) represents the correlation function of the fluctuat-... 

The effect of the detector time constant on the apparent efficiency depends only on the time width of the bands. It has been shown by Sch-mauch 41) and by Me William and Bolton 42) that the profile recorded with a detector having a time constant r is wider than the actual profile by a factor (1 -f r/ert), where is the time standard deviation of the profile, provided this factor is less than about 1.2. Moreover, the peak heigh becomes smaller although the peak area remains unchanged. 1 he (list mu ment (retention time) of a peak increases by r and the retention time of the... 

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