Technological developments of computed tomography (CT) have led to a drastic increase of its clinical utilization,
creating concerns about patient exposure. To better control dose to patients, we propose a methodology to find an
objective compromise between dose and image quality by means of a visual discrimination model.
A GE LightSpeed-Ultra scanner was used to perform the acquisitions. A QRM 3D low contrast resolution phantom
(QRM - Germany) was scanned using CTDIvol values in the range of 1.7 to 103 mGy. Raw data obtained with the
highest CTDIvol were afterwards processed to simulate dose reductions by white noise addition. Noise realism of the
simulations was verified by comparing normalized noise power spectra aspect and amplitudes (NNPS) and standard
deviation measurements. Patient images were acquired using the Diagnostic Reference Levels (DRL) proposed in
Switzerland. Noise reduction was then simulated, as for the QRM phantom, to obtain five different CTDIvol levels, down
to 3.0 mGy.
Image quality of phantom images was assessed with the Sarnoff JNDmetrix visual discrimination model and compared
to an assessment made by means of the ROC methodology, taken as a reference. For patient images a similar approach
was taken but using as reference the Visual Grading Analysis (VGA) method.
A relationship between Sarnoff JNDmetrix and ROC results was established for low contrast detection in phantom
images, demonstrating that the Sarnoff JNDmetrix can be used for qualification of images with highly correlated noise.
Patient image qualification showed a threshold of conspicuity loss only for children over 35 kg.
Technological developments of computed tomography (CT) have led to an increase of its clinical utilization. To optimize patient dose and image quality, scanner manufacturers have introduced X-ray tube current modulation coupled to Automatic Exposure Control (AEC) devices. The purpose of this work was to assess the performance of the CT-AEC of three different MSCT manufacturers by means of two phantoms: a conical PMMA phantom to vary the thickness of the absorber in a monotonous way, and an anthropomorphic chest phantom to assess the response of the CT-AEC in more realistic conditions. Noise measurements were made by standard deviation assessments, and dose indicators (CTDIvol and DLP) were calculated. All scanners were able to compensate for thickness variation by an adaptation of tube current. Initial current adaptation lengths varied for all systems in the range of 1 to 5 cm. With the anthropomorphic phantom, noticeable differences appeared concerning the adaptation rapidity in a sudden X-ray attenuation change, and non-intuitive behavior of current evolution was noticed for some acquisitions. The xyz-modulation allowed to reduce the DLP of the acquisition by 18% compared to the z-modulation. It is also showed that a homogeneous test object is not sufficient to characterize CT-AEC devices.
This work compares the detector performances of the recent Kodak Min-R EV 190/Min-R EV and current Kodak Min-R 2190/Min-R 2000 mammography screen-film combinations with the Kodak CR 850M system using the new EHR-M and standard HR plates. Basic image quality parameters (MTF, NNPS and DQE) were evaluated according to ISO 9236-3 conditions (i.e. 28 kV; Mo/Mo; HVL = 0.64 mm eq. Al) at an entrance air kerma level of 60 μGy. Compared with the Min-R 2000, the Kodak Min-R EV screen-film system has a higher contrast and an intrinsically lower noise level, leading to a better DQE. Due to a lower noise level, the new EHR-M plate improves the DQE of the CR system, in comparison with the use of the standard HR plate (30 % improvement) in a mammography cassette. Compared with the CR plates, screen-film systems still permit to resolve finer details and have a significantly higher DQE for all spatial frequencies.