Gantry rotation

Helical pitch <1 ( high-speed helical techniques produce unacceptable reconstruction artifacts in the posterior fossa) Slow table speed (8-11 mm/s) Rapid gantry rotation (0.5-0.8 rotations/s) Smallest possible focal spot setting ... 

Certain user-defined imaging parameters are unique to helical CT scanning. Pitch refers to the ratio between table increment per gantry rotation and X-ray beam collimation [28]. 

Current MDCT scanners have sub-second gantry rotation times. Reducing the scan rotation time from 1.0-0.5 s will halve the time taken to perform... 

Beam collimation and table movement together (i.e. pitch) determine the volume of tissue irradiated per gantry rotation, whereas section collimation determines the minimal slice (also known as section) thickness that can be reconstructed from an examination s raw data. That is, the narrowest selected reconstructed slice thickness available will equal the selected size of the detector elements (Dalrymple et al. 2005). Section collimation may also be referred to as (effective) detector (row) thickness. The term detector configuration merely couples together the value selected for the section collimation with the number of detector elements. For example 4 x 1.25 refers to a four- channel scanner acquiring four channels of data 1.25 mm thick. Similarly, 16 x 0.625 refers to 16 channels of data 0.625 mm thick obtained on a 16-channel scanner. From this information and the equation for pitch, it can be seen that detector configuration is numerically equal to the beam collimation (Rydberg et al. 2003 Saini 2004). 

Now in 2006, 64 row CT scanners are being installed i diich allow 64 x 0.6 mm slices to be obtained in a single gantry rotation with gantry rotation times decreasing to 0.33 s. The progression of data acquisition can be depicted by displaying the... 

The CT parameters will also impact the quality of the 2D, multiplanar and 3D reconstructed images. The details of scanner type, collimation, pitch, detector array for volume scanner, gantry rotation... 

With this approach, constant temporal resolution equivalent to a quarter of the gantry rotation time is achieved in a centered region of the scan field of view. For trot= 0.33 s, the temporal resolution is t t/4 = 83 ms, independent of the patients heart rate. 

Fig. 1.5a,b. Schematic illustration of a dual-source CT (DSCT) system using two tubes and two corresponding detectors offset by 90°. A scanner of this type provides temporal resolution equivalent to a qiuirter of the gantry rotation time, independent of the patient s heart rate. In a technical realization, one detector (a) covers the entire scan field of view with a diameter of 50 cm, while the other detector (b) is restricted to a smaller, central field of view... 

Spiral/hehcal scanning is characterized by continuous gantry rotation and continuous data acquisition while the patient table is moving at constant speed see Fig. 1.7. 

SORGE 2001), which provide a temporal resolution up to half the gantry rotation time per image in a sufficiently centered region of interest. The 16-slice and 64-sHce CT systems offer gantry rotation times as short as 0.4 s, 0.37 s or even 0.33 s. In this case, temporal resolution can be as good as 200 ms, 185 ms or 165 ms. 

The major improvements of 4-sHce to 64-slice scanners include improved temporal resolution due to shorter gantry rotation times, better spatial resolution owing to sub-millimeter coUimation and considerably reduced examination times (Flohr and Ohnesorge 2001 Flohr et al. 2003b) see Fig. 1.16. 

A substantial increase in patient dose results from helical acquisition in cardiac CT. Regardless of the number of X-ray tubes, the table feed must be sufficiently slow such that projection data of 180° gantry rotation are available for image reconstruction at all points in time and space even in patients with variable heart rate. Though radiation exposure can be markedly reduced... 

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