Abstract
Acquiring CT images with thin slices can improve resolution and detectability, but cause an increase in the image noise. To compensate for the additional image noise, the kVp or mA can be increased, which carries a dose penalty to the patient. We investigate the image quality achieved in MPR images reformatted from different slice thicknesses 0.625 mm and 5 mm, to determine if a thicker slice could be resampled to smaller thickness with minimal loss of image information. Catphan?600 phantom was imaged using selected kVp/mA settings (80 kVp/250 mA, 100 kVp/ 150 mA and 120 kVp/200 mA) to generate slices with thicknesses of 0.625 mm and 5 mm using a GE Discovery HD750 64-slice CT scanner to investigate the impact of the acquisition slice thickness on the overall image quality in MPRs. Measurements of image noise, uniformity, contrast-to-noise ratio (CNR), low contrast detectability and limiting spatial resolution were performed on axial and coronal multiplanar reformatted images (MPRs). Increased noise, reduced contrast-to-noise ratio, and improved limiting spatial resolution and low contrast detection were observed in 2 mm coronal MPRs generated with 0.625 mm thin slices when compared to the MPRs from 5 mm thick slices. If the 2 mm coronal MPRs acquired with 5 mm slices are resampled to 0.6 mm slice thickness, the reductions in limiting resolution and low contrast detection are compensated, although with reduced uniformity and increased image noise. Thick slice image acquisitions yield better CNR and less noise in the images, whereas thin slices exhibited improved spatial resolution and low contrast detectability. Retrospectively resampling into thinner slices before obtaining the coronal MPRs provided a balance between image smoothness and identifying fine image detail. Which approach provides the optimal image quality may also depend on the imaging task, size and composition of the features of interest, and radiologist preference.
Highlights
Advances in CT technology have resulted in its increased use for a multitude of diagnostic tasks in both adult and paediatric patients [1]
We investigate the image quality achieved in multiplanar reformatted images (MPRs) images reformatted from different slice acquisition thicknesses (0.625 mm and 5 mm), to determine if a thicker slice could be resampled to smaller thickness with minimal loss of image information, as this may allow users to reduce kVp or mA and so achieve dosesavings to the patient
Image Noise: Images generated from 5 mm thick slice acquisitions (5 mm AX5 mm and 2 mm COR5 mm) were less noisy compared to images obtained with thin 0.625 mm slice thickness (5 mm AX0.6 mm and 2 mm COR0.6 mm ), as expected (Figure 1)
Summary
Advances in CT technology have resulted in its increased use for a multitude of diagnostic tasks in both adult and paediatric patients [1]. The scan time reduction diminished the need for sedation of paediatric patients that was previously necessary in order to decrease motion artifacts. These scanners enabled larger volume coverage and acquisition of thinner image slices with nearly isotropic voxels, enabling the production of high quality images in the coronal, sagittal and axial planes. Choosing the appropriate slice thickness for a certain diagnostic task is very important [3] since it influences the sensitivity of lesion detection, spatial resolution and image noise in addition to affecting radiation dose to the patient. A thicker beam width includes most of the detectors in z-direction, decreases the penumbra effect and enhances the dose efficiency. Thin slices provide images with higher resolution compared to images acquired with thicker slices
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