Slice sensitivity profile

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. 

Fig. 1.8. Effective slice width in spiral/helical CT the collimated slice profile, which is a trapezoidal in general, is indicated in red. The slice sensitivity profiles (SSP) after spiral/ helical interpolation are bell-shaped see the green curves for the most commonly used single-slice approach (180-LI) at dif-...

Hein I, Taguchi K, Silver M D, Kazarna M, Mori I (2003) Feld-kamp-based cone-beam reconstruction for gantry-tilted helical multislice CT. Med Phys 30 3233-3242 Hsieh J (2001) Investigation of the slice sensitivity profile for step-and-shoot mode multi-slice computed tomography. Med Phys 28 491-500... 

The profiling method requires the sensitive slice to be shifted through the object. Figure 2.4.2 shows the mechanical lift used to move the sensor with respect to the sample. The object under study, for instance the lower surface of the arm in the picture, is positioned on top of a flat holder (A) and the NMR sensor is placed under it on a movable plate (B). The mechanical construction allows one to move the sensor up and down with a precision of 10 pm. The distance between the rf coil and the sensitive slice defines the maximum penetration depth into the sample (maximum field of view of the ID image). Depending on the application, the position of the rf coil with respect to the sensitive slice can be changed to maximize the sensitivity. 

Fig. 12. Sequences for volume selective single voxel spectroscopy. Both techniques work with three slice-selective RF-pulses. (a) The Point RESolved Spectroscopy (PRESS) sequence generates a volume selective double spin-echo. The entire time delay between the initial 90° excitation and the echo is sensitive to transverse relaxation, (b) The Stimulated Echo Acquisition Mode (STEAM) sequence generates a stimulated echo. Maximal signal intensity (without relaxation effects) is only half the signal intensity of PRESS under comparable conditions, but slice profiles are often better (only 90° pulses instead of 180° pulses) and the TM interval is not susceptible to transverse relaxation, (c) The recorded echo signal is only generated in a volume corresponding to the intersection of all three slices.

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