Scattering in CT is a major process that may result in severe image artifacts.
In order to suppress this scattering, most CT scanners are equipped with post collimation anti-scatter grid along the fan beam direction. In the longitudinal direction (z-direction) no hardware solution has been implemented since the scattering is negligible in narrow coverage CT scanners. As the coverage becomes wider in recent MSCT scanners the scattering level in the z-direction increases significantly. This scattering increase in z-direction, results in image artifacts appearing as dark shadows along highly attenuating directions.
In the present work we measure the scattering level in the z-direction for a wide coverage scanner, using various phantoms.
Based on the results, a semi-empirical model for scattering correction in MSCT is presented and validated.
The proposed model is based on a subtraction of a low frequency offset. This offset is proportional to the scattering, corresponding to the detector that has the lowest signal at each rotation angle.
To validate the model, we first calculate this low frequency offset directly from the scatter measurements and apply it to the data acquired with the wide coverage. We then use a semi-empirical function to estimate the scattering fraction from the raw data, and use it to replace the directly measured scatter values in the correction scheme.
Applying the proposed semi-empirical scatter correction model to the data acquired with the wide coverage, the scattering signal is significantly decreased. The images reconstructed from the corrected data exhibit a clear reduction of the artifact level.
In multi-slice CT (MSCT), as the coverage becomes wider, the scattering contribution along the longitudinal direction (z) to the detectors' signal increases. The scattering results in image artifacts, appearing as dark shadows between highly attenuating objects.
In this work we measure the scattering level systematically, using phantoms of various sizes, shapes, and materials. We study the dependencies and their effect on the scattering amount.
We derive an empirical function for the scattering fraction, based on the maximal attenuation at each rotation angle. The function contains a single constant (SC). The variation of SC as a function of the different phantoms is analyzed, showing a clear dependence on the minimal water equivalent axis of each phantom.
The strong dependence of the scattering fraction on the maximal attenuation along each view is shown. This phenomenon can be correlated to a single scatter process along the z axis in the presence of an anti-scatter grid along the direction of the detectors.
The dependence of SC on the minimal axis indicates an additional significant scatter process.
The results validate that the scattering level estimation can be achieved using the derived function, with a minimal variation in the solution parameters. Hence, enabling the introduction of this scatter estimation into an MSCT scattering correction scheme.
One of the most important advantages of the novel multi slice CT (MSCT) with increased number of slices is the ability to reduce the scan time. However, does the increased number of slices in the MSCT enforce us to reduce the pitch, in order to avoid windmill artifacts, hence preventing us from decreasing the scan time?
In this work we address this issue along with other aspects of the windmill artifacts. We study the dependence of the windmill artifacts, their strength and frequency, on the number of slices and on the pitch.
The study demonstrates, that when retaining constant bed speed while increasing the number of slices, the intensity of the windmill artifacts is reduced significantly.
Images of scans performed with the same pitch, yet with various number of slices are compared. It is observed, that the intensity of the windmill artifacts is similar, independent of the number of slices. The frequency of the artifacts however, increases with the number of slices.
The study concludes that updating a clinical protocol performed with a low number of slices MSCT, to a similar protocol performed with high number of slices MSCT, the same pitch can be used attaining better IQ. Scanning with the same pitch using wider coverage enables an advantageous shorter scan time in novel MSCT.
The new multi slice CT scanners are characterized by a wide coverage (collimation) in the longitudinal (z) direction. This large collimation provides large and rapid coverage, it may however introduce artifacts caused by scatter, which are enlarged with collimation. The scattering in the z direction induces artifacts that appear as dark shadows between bones.We present a software correction that reduces the scattering artifacts.The correction is based on a subtraction of a low frequency offset, as foreseen in simulations of scatter effect, from the raw data. The study includes tests of the scattering correction of both phantoms and clinical scans.The corrected images are compared to un-corrected images scanned with the same z-coverage (collimation) and to images scanned with narrower z-coverage with the same scan parameters. The results demonstrate that the correction of the images, scanned with the wide collimation, reduces the scattering artifact significantly. The obtained image quality levels to that of the images scanned with narrower collimation.