Impact of ROI Size on the Accuracy of Noise Measurement in CT on Computational and ACR Phantoms.

Background: The effect of region of interest (ROI) size variation on producing accurate noise levels is not yet studied. Objective: This study aimed to evaluate the influence of ROI sizes on the accuracy of noise measurement in computed tomography (CT) by using images of a computational and American College of Radiology (ACR) phantoms. Material and Methods: In this experimental study, two phantoms were used, including computational and ACR phantoms. A computational phantom was developed by using Matlab R215a software (Mathworks Inc., Natick, MA Natick, MA) with a homogeneously +100 Hounsfield Unit (HU) value and an added-Gaussian noise with various levels of 5, 10, 25, 50, 75, and 100 HU. The ACR phantom was scanned with a Philips MX-16 slice CT scanner in different slice thicknesses of 1.5, 3, 5, and 7 mm to obtain noise variation. Noise measurement was conducted at the center of the phantom images and four locations close to the edge of the phantom images using different ROI sizes from 3 × 3 to 41 × 41 pixels, with an increased size of 2 × 2 pixels. Results: The use of a minimum ROI size of 21 × 21 pixels shows noise in the range of ± 5% ground truth noise. The measured noise increases above the ± 5% range if the used ROI is smaller than 21 × 21 pixels. Conclusion: A minimum acceptable ROI size is required to maintain the accuracy of noise measurement with a size of 21 × 21 pixels.

Characteristics of laser induced breakdown investigated by a compact, nongated optical multichannel analyzer system and its potential application.

Laser induced breakdown is a highly temporally and spatially dynamic phenomenon, normally studied using a highly temporally resolved optical detector system. In this work, a compact, low cost optical multichannel analyzer (OMA) system without a built-in temporal gating device and thus operated under a free running mode was used to investigate the characteristics of laser induced plasma. A Nd-YAG laser beam was used as the excitation source from several samples, namely, copper, zinc, and aluminum plates. The characteristics of the plasma emission produced under various experimental parameters, including the pulse energy, surrounding gas pressure, and collection fiber position, were examined. It was found that the essential features of emission spectra can be investigated even using the ungated, compact OMA system even without a highly temporally resolved gating system. The plasma emission characteristics critically depend on the experimental parameters. A quality emission spectrum, featuring a high intensity with a low background, can be obtained using the ungated, compact OMA system under optimized conditions, namely, a pulse energy of approximately 8 mJ, a surrounding gas pressure of 10 Torr, and a collection fiber position of more than 5 mm above the surface of the sample. The features of the emission spectra detected under optimized conditions are only similar to those obtained using a sophisticated, gated OMA system. The characteristics of the emission spectra are in good agreement with the previous assumption of the shockwave role in plasma excitation. Having quality emission spectra under the optimized conditions, a preliminary practical laser induced breakdown spectroscopy (LIBS) analysis using the ungated, compact OMA system was performed on several samples, such as standard brass, commercial pure gold, and natural stone samples. The aluminum emission lines are strongly detected from the standard brass sample (C1118) containing aluminum at 2.8%. The LIBS system also unequivocally revealed a qualitatively abandoned impurity presence in the purportedly pure commercial gold sample. It also effectively confirmed qualitatively a Cu presence in the blinking spots of the natural stone collected from a traditional mining site in Aceh. This result implies the effectiveness of the LIBS using the ungated, compact OMA system for quick, practical analysis.

Trace metal analysis of element on material surface using pulse CO2 laser-induced breakdown spectroscopy applying vaporization technique.

Trace elemental analysis on a surface of material has been recently imperative to be carried out especially in material industries. In this study, sophisticated setup of laser-induced breakdown spectroscopy has been arranged and demonstrated by employing vaporization technique for the trace elemental analysis on a surface of material without ablating the material itself. Experimentally, a pulse transversely excited atmospheric CO2 laser was directed and defocused at +5 mm on a Si surface at inclining degree of approximately 25o to vaporize the trace metal element from the Si surface to the Pt mesh combined with Cu plate. The vaporized trace metal element then attached and deposited on the mesh surface. The trace metal attached-Pt mesh was then bombarded by focused laser beam to induce a luminous plasma and finally the trace element was identified. Results certified that sensitive trace elemental analysis of Cr deposited on the Si surface has been successfully carried out without any ablation of Si surface. Good linear calibration curve of Cr with an intercept zero was produced, which results in limit of detection of Cr of approximately 100 ppb.

Emission Spectrochemical Analysis of Soft Samples Including Raw Fish by Employing Laser-Induced Breakdown Spectroscopy with a Subtarget at Low-Pressure Helium Gas.

Laser-induced breakdown spectroscopy (LIBS) to detect the light elements such as lithium (Li) and boron (B) and heavy elements such as copper (Cu) and lead (Pb) in raw fish samples is reported in this work. This is made possible by understanding that the soft target absorbs recoil energy and as a result, the ablated atoms gushing from the soft target do not acquire sufficient speed to form a shock wave. In order to overcome this problem, we set a subtarget on the back of the soft target so as to produce the repulsion force by which the gushing speed of the ablated atoms is increased, yielding a sufficiently high plasma temperature or sufficiently large thermal energy needed for the excitation of the ablated atoms. Excellent spectral qualities of various soft samples such as margarine, butter, peanut butter, strawberry jam, raw tuna, raw gindara, and raw salmon are presented. Furthermore, a linear calibration curve with a zero intercept is also obtained for Li, Cu, and Pb. The detection limit of Li, Cu, and Pb is found to be around 0.1 mg/L. This modification of LIBS for soft samples by using a subtarget effect clearly promises a rapid and in situ soft sample analysis since there is practically no sample digestion in the analysis.

Development of a computational phantom for validation of automated noise measurement in CT images.

The purpose of this study was to develop a computational phantom for validation of automatic noise calculations applied to all parts of the body, to investigate kernel size in determining noise, and to validate the accuracy of automatic noise calculation for several noise levels. The phantom consisted of objects with a very wide range of HU values, from -1000 to +950. The incremental value for each object was 10 HU. Each object had a size of 15 × 15 pixels separated by a distance of 5 pixels. There was no dominant homogeneous part in the phantom. The image of the phantom was then degraded to mimic the real image quality of CT by convolving it with a point spread function (PSF) and by addition of Gaussian noise. The magnitude of the Gaussian noises was varied (5, 10, 25, 50, 75 and 100 HUs), and they were considered as the ground truth noise (NG). We also used a computational phantom with added actual noise from a CT scanner. The phantom was used to validate the automated noise measurement based on the average of the ten smallest standard deviations (SD) from the standard deviation map (SDM). Kernel sizes from 3 × 3 up to 27 × 27 pixels were examined in this study. A computational phantom for automated noise calculations validation has been successfully developed. It was found that the measured noise (NM) was influenced by the kernel size. For kernels of 15 × 15 pixels or smaller, the NMvalue was much smaller than the NG. For kernel sizes from 17 × 17 to 21 × 21 pixels, the NMvalue was about 90% of NG. And for kernel sizes of 23 × 23 pixels and above, NMis greater than NG. It was also found that even with small kernel sizes the relationship between NMand NGis linear with R2more than 0.995. Thus accurate noise levels can be automatically obtained even with small kernel sizes without any concern regarding the inhomogeneity of the object.

Assessment of patient dose and noise level of clinical CT images: automated measurements.

We investigated comparisons between patient dose and noise in pelvic, abdominal, thoracic and head CT images using an automatic method. 113 patient images (37 pelvis, 34 abdominal, 25 thoracic, and 17 head examinations) were retrospectively and automatically examined in this study. Water-equivalent diameter (Dw), size-specific dose estimates (SSDE) and noise were automatically calculated from the center slice for every patient image. The Dw was calculated based on auto-contouring of the patients' edges, and the SSDE was calculated as the product of the volume CT dose index (CTDIvol) extracted from the Digital Imaging and Communications in Medicine (DICOM) header and the size conversion factor based on the Dw obtained from AAPM 204. The noise was automatically measured as a minimum standard deviation in the map of standard deviations. A square region of interest of about 1 cm2 was used in the automated noise measurement. The SSDE values for the pelvis, abdomen, thorax, and head were 21.8 ± 7.3 mGy, 22.0 ± 4.5 mGy, 21.5 ± 4.7 mGy, and 65.1 ± 1.7 mGy, respectively. The SSDEs for the pelvis, abdomen, and thorax increased linearly with increasing Dw, and for the head with constant tube current, the SSDE decreased with increasing Dw. The noise in the pelvis, abdomen, thorax, and head were 5.9 ± 1.5 HU, 5.2 ± 1.4 HU, 4.9 ± 0.8 HU and 3.9 ± 0.2 HU, respectively. The noise levels for the pelvis, abdomen, and thorax of the patients were relatively constant with Dw because of tube current modulation. The noise in the head image was also relatively constant because Dw variations in the head are very small. The automated approach provides a convenient and objective tool for dose optimizations.

An algorithm for automated modulation transfer function measurement using an edge of a PMMA phantom: Impact of field of view on spatial resolution of CT images.

PURPOSE: The purpose of this study was to introduce a new algorithm for automated measurement of the modulation transfer function (MTF) using an edge of a readily available phantom and to evaluate the effect of reconstruction filter and field of view (FOV) on the spatial resolution in the CT images. METHODS: Our automated MTF measurement consisted of several steps. The center of the image was established and an appropriate region of interest (ROI) designated. The edge spread function (ESF) was determined, and a suitably interpolated ESF curve was differentiated to obtain the line spread function (LSF). The LSF was Fourier transformed to obtain the MTF. All these steps were accomplished automatically without user intervention. The results of the automated MTF from the edge phantom were validated by comparing them with a point image, and the results of the automated calculation were validated by the standard fitting method. The automated MTF calculation was then applied to the images of two polymethyl methacrylate (PMMA) phantoms and a wire phantom which had been scanned by a Toshiba Alexion 4-slice CT scanner and reconstructed with various filter types and FOVs. RESULTS: The difference in the 50% MTF values obtained from the edge and point phantoms were within ±4%. The values from the automated and fitted methods agreed to within ±2%, indicating that the automated MTF calculation was accurate. The automated MTF calculation was able to differentiate MTF curves for various filters. The spatial resolution values were 0.37 ± 0.00, 0.71 ± 0.01, and 0.78 ± 0.01 cycles/mm for FC13, FC30 and FC52 filters, respectively. The spatial resolution of the images decrease linearly (R2  > 0.98) with increasing FOVs. CONCLUSION: An automated MTF method was successfully developed using an edge phantom, the PMMA phantom. The method is easy to implement in a clinical environment and is not influenced by user experience.