Study On Isotropic Imageing At A Multi-Slice Spiral Ctsytem

Objective To derive a formula between the scanning parameters and full width at half maximum (FWHM) of spiral slice sensitivity profile (SSP) which is also described as z direction resolution, and search for the optimal scanning parameters that agree with isotropic imaging by objective evaluation.Methods Gauss SSP was analyzed and as a result a formula was derived which can be used to calculate slice width. Additionally, reconstruction increment and FOV was analyzed with theory of signal processing. Using Philips Mx8000 4-slice spiral CT system and the same CT technique and inner ear protocol as those used for clinical CT, Phantom Catphan(?)500,which is generally used for quality control on CT system, was scanned with Scanning parameters as follows:120Kv,145mAs,180mm FOV,512*512 Matrix,a filter of D,0.6mm nominal slice width,0.1mm reconstruction increment,180°linear interpolation(180°LI), and different pitch=0.5,0.625,0.875,1.0,1.25,1.5,1.75. Then relative FWHM of point spread function (PSF) was measured respectively in x, y, and z direction. Furthermore, images were reconstructed with the varieties of other parameters using the raw data and relative FWHM was got.Results (1)The measured FWHM in z direction increases overall, but not in a monotonous way as the pitch increases, while that in x and y direction had no variety; (2) The measured FWHM in 3 dimension is equivalent at pitch=0.875 and effective slice width arrives at the minimum at pitch=0.625 and the maximum at pitch=1(for 0.5≤pitch≤1.75) respectively; (3) In z direction, the measured FWHM is equivalent with reconstruction increment(RI) of 0.1mm and 0.3mm respectively, but both are less than that with RI of 0.7mm;(4)The measured FWHM with 360°LI is greater than that with 180°LI;(5)The measured FWHM in z direction does not change with the variety of filter parameter, while that in x and y direction changes obviously; (6)For different FOV, The measured FWHM in z direction does not change, while that in x and y direction rises with the increase of FOV.Conclusions (1)The identity between theoretic analysis and the experimental results proved that the derived formula is correct; (2)Isotropic imaging with sub-millimeter resolution would come true with right scanning parameters on MSCT; (3)Collimation of 0.5mm, reconstruction increment of 0.3mm, pitch 0.875,and 180°LI are the optimal scanning parameters for high resolution isotropic imaging; (4)FOV and filter have only effect on the in-plane spatial resolution. On the contrary, pitch, RI, and interpolation have effect on the spatial resolution in z direction only.