A method for quantification of noise non-uniformity in computed tomography images: A computational study

In computed tomography (CT), the noise is sometimes non-uniform, i.e. the noise magnitude may vary with the gradient level within the image. The purpose of this study was to quantify the noise non-uniformity in CT images using appropriate 1D and 2D computational phantoms, and to validate the effectiveness of the proposed concept in images filtered by the bilateral filter (BF), as an example of a non-linear filter. We first developed 1D and 2D computational phantoms, and Gaussian noises with several noise levels were then added to the phantoms. In addition, to simulate the real form of noise from images obtained in a real CT scanner, a homogeneous water phantom image was used. These noise levels were referred to as ground truth noise (σG). The phantoms were then filtered by the bilateral filter with various pixel value spreads (σ) to produce non-uniform noise. The original gradient phantoms (G) were subtracted from both the noisy phantoms (IN) and the filtered noisy phantoms (IBF), and the magnitudes of the resulting noise for each gradient were computed. The noise-gradient dependency (NGD) curve was used to display the dependency of noise magnitude on image gradient in the non-uniform noise. It is found that for uniform noise, the magnitude of noise was constant for all gradients. However, for non-uniform noise, the measured noise was dependent on the gradient levels and on the strength of the BF for every ground truth noise (σG). It was found that the noise magnitude was large for the large gradients and decreased with the magnitude of the image gradient

Impact of Noise Level on the Accuracy of Automated Measurement of CT Number Linearity on ACR CT and Computational Phantoms

Background: Methods for segmentation, i.e., Full-segmentation (FS) and Segmentation-rotation (SR), are proposed for maintaining Computed Tomography (CT) number linearity. However, their effectiveness has not yet been tested against noise.Objective: This study aimed to evaluate the influence of noise on the accuracy of CT number linearity of the FS and SR methods on American College of Radiology (ACR) CT and computational phantoms.Material and Methods: This experimental study utilized two phantoms, ACR CT and computational phantoms. An ACR CT phantom was scanned by a 128-slice CT scanner with various tube currents from 80 to 200 mA to acquire various noises, with other constant parameters. The computational phantom was added by different Gaussian noises between 20 and 120 Hounsfield Units (HU). The CT number linearity was measured by the FS and SR methods, and the accuracy of CT number linearity was computed on two phantoms.Results: The two methods successfully segmented both phantoms at low noise, i.e., less than 60 HU. However, segmentation and measurement of CT number linearity are not accurate on a computational phantom using the FS method for more than 60-HU noise. The SR method is still accurate up to 120 HU of noise. Conclusion: The SR method outperformed the FS method to measure the CT number linearity due to its endurance in extreme noise

Scatter index measurement using a CT dose profiler

The CT dose index (CTDI) is usually measured using a pencil chamber with a length of 100 mm on a CTDI phantom with a length of 150 mm. The scattering radiation dose beyond 100 mm is usually still significant despite using a small beam width (below 10 mm). This study aims to measure the scattering index of CT dose for several variations of input parameters. The scatter index measurements were performed on a multi-slice CT (MSCT) Alexion™ using a CT dose profiler detector connected to a Black Piranha electrometer (RTI Electronic, Sweden). The measurements used the helical mode and a beam width of 2 x 4 mm, and resulted in 150 mm dose profiles. Values of CTDI150, CTDI130 and CTDI100 were calculated and used to obtain values of the scatter indices (SI130 and SI150). We varied input parameters, such as tube voltage, tube current, and pitch, and used two types of CTDI phantoms, i.e. body and head. In the tube voltage variation (from 80 to 135 kV), we found SI130 and SI150 values of 1.13 ± 0.01 and 1.19 ± 0.01 for the body CTDI phantom; and  SI130 and SI150 values of 1.08 ± 0.01 and 1.11 ± 0.01 for the head CTDI phantom. For tube current variations from 25 to 120 mA, and pitch variations from 0.75 to 1.5, SI130 and SI150 values were 1.14 ± 0.00 and 1.20 ± 0.00 for the body CTDI phantom; and 1.08 ± 0.00 and 1.11 ± 0.00 respectively for the head CTDI phantom. We showed that the more frequently used CTDI100 value is too small because it ignores scattering beyond the 100 mm boundary, even for beam widths less than 10 mm. The scatter index values were strongly influenced by the size of the CTDI phantom, and were slightly affected by the tube voltage. Variations in tube currents and pitch did not affect the value of the scatter index. The scatter index values of SI130 and SI150 were significantly different, and suggests that the use of SI150 is even more appropriate for describing the scattering dose

Measurement of the secondary neutron dose distribution from the LET spectrum of recoils using the CR-39 plastic nuclear track detector in 10 MV X-ray medical radiation fields

We measured the recoil charged particles from secondary neutrons produced by the photonuclearreaction in a water phantom from a 10-MV photon beam from medical linacs. The absorbed dose andthe dose equivalent were evaluated from the linear energy transfer (LET) spectrum of recoils using theCR-39 plastic nuclear track detector (PNTD) based on well-established methods in the field of spaceradiation dosimetry. The contributions and spatial distributions of these in the phantom on nominal photonexposures were verified as the secondary neutron dose and neutron dose equivalent. The neutrondose equivalent normalized to the photon-absorbed dose was 0.261 mSv/100 MU at source to chamberdistance 90 cm. The dose equivalent at the surface gave the highest value, and was attenuated to less than10% at 5 cm from the surface. The dose contribution of the high LET component of P100 keV/lmincreased with the depth in water, resulting in an increase of the quality factor. The CR-39 PNTD is apowerful tool that can be used to systematically measure secondary neutron dose distributions in a waterphantom from an in-field to out-of-field high-intensity photon beam