Image Modulation Transfer Function Research Articles

SU‐FF‐I‐17: Methodology for Image Quality Characterization of Cone‐Beam CT Systems Using a Novel Adaptation of Fundamental Imaging Metrics

Purpose: A methodology to characterize reconstructed image quality in cone‐beam computed tomography (CBCT) is presented in which, imaging performance is characterized using fundamental imaging metrics: modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Traditionally, imaging MTF, NPS and DQE have only been used to describe spatial resolution, noise and contrast in 2D imaging. In a novel adaptation to 3D imaging, the MTF and NPS are measured using a conventional CatPhan phantom. An effective DQE parameter is then evaluated by normalization to incident beam flunece characteristics using a dose index. Method and Materials: The modulations from CatPhan line‐pairs are used to calculate the MTF using a technique that we recently validated for megavoltage imaging. Blank/uniform slices are used to calculate the NPS, while the DQE is obtained from MTF and NPS measurements normalized by a quantum fluence term. While the fluence normalization is relatively straightforward with unscattered beams in 2D imaging, its evaluation in CBCT imaging is complicated by the effects of collimators, compensators, and scatter. In our calculations, we obtain an effective incident fluence term for each beam combination based on its weighted CT dose index (CTDIw) and by using Monte Carlo simulations to generate a transmitted fluence to CTDI dose ratio. The methodology was implemented for clinical CBCT systems to observe the influence of image acquisition parameters. Results: CatPhan based MTF, NPS and DQE measurements of clinical CBCT systems were found to be useful in evaluating the effects of various image acquisition and reconstruction parameters. The technique was extended to near automated metric analysis with minimal hindrances from scatter and associated CBCT artifacts. Conclusions: The use of fundamental imaging metrics to characterize CBCT image quality can be effective in optimizing image acquisition parameters and ensure optimal imaging performance at the lowest possible dose.

Effect of nonlinear three-dimensional optimized reconstruction algorithm filter on image quality and radiation dose: Validation on phantoms

A new technique called the nonlinear three-dimensional optimized reconstruction algorithm filter (3D ORA filter) is currently used to improve CT image quality and reduce radiation dose. This technical note describes the comparison of image noise, slice sensitivity profile (SSP), contrast-to-noise ratio, and modulation transfer function (MTF) on phantom images processed with and without the 3D ORA filter, and the effect of the 3D ORA filter on CT images at a reduced dose. For CT head scans the noise reduction was up to 54% with typical bone reconstruction algorithms (H70) and a 0.6 mm slice thickness; for liver CT scans the noise reduction was up to 30% with typical high-resolution reconstruction algorithms (B70) and a 0.6 mm slice thickness. MTF and SSP did not change significantly with the application of 3D ORA filtering (P > 0.05), whereas noise was reduced (P < 0.05). The low contrast detectability and MTF of images obtained at a reduced dose and filtered by the 3D ORA were equivalent to those of standard dose CT images; there was no significant difference in image noise of scans taken at a reduced dose, filtered using 3D ORA and standard dose CT (P > 0.05). The 3D ORA filter shows good potential for reducing image noise without affecting image quality attributes such as sharpness. By applying this approach, the same image quality can be achieved whilst gaining a marked dose reduction.

１６ＤＡＳ‐ＭＤＣＴ使用のＭＰＲ画像における角度変化に伴う解像度評価

In this study, we evaluated the resolving power of three-dimensional (3D) multiplanar reformation (MPR) images with various angles by using 16 data acquisition system multi detector row computed tomography (16DAS-MDCT) . We reconstructed the MPR images using data with a 0.75 mm slice thickness of the axial image in this examination. To evaluate resolving power, we used an original new phantom (RC phantom) that can be positioned at any slice angle in MPR images. We measured the modulation transfer function (MTF) by using the methods of measuring pre-sampling MTF, and used Fourier transform of image data of the square wave chart. The scan condition and image reconstruction condition that were adopted in this study correspond to the condition that we use for three-dimensional computed tomographic angiography (3D-CTA) examination of the head in our hospital. The MTF of MPR images showed minimum values at slice angles in parallel with the axial slice, and showed maximum values at the sagittal slice and coronal slice angles that are parallel to the Z-axis. With an oblique MPR image, MTF did not change with angle changes in the oblique sagittal slice plane, but in the oblique coronal slice plane, MTF increased as the tilt angle increased from the axial plane to the Z plane. As a result, we could evaluate the resolving power of a head 3D image by measuring the MTF of the axial image and sagittal image or the coronal image.

IKONOS spatial resolution and image interpretability characterization

Five individual projects characterized the spatial performance of the IKONOS commercial imaging sensor. The result was determination of the spatial image quality of IKONOS data products in terms of the National Imagery Interpretability Rating Scale (NIIRS), the system Modulation Transfer Function (MTF), the system stability over its first year of operation, the characteristics of the Space Imaging MTF Compensation (MTFC) procedure, and the application-specific capabilities of IKONOS imagery. Both panchromatic and multispectral imagery were evaluated. Major conclusions of this work are that the system was stable in imaging performance during the first year of operation, that its MTF meets the specification for the NASA Scientific Data Purchase program, that the initial MTFC processing appears to be transposed in the in-track and the cross-track directions, that the MTFC results in a noise amplification of 2× to 4× in addition to sharpening the imagery, and that IKONOS panchromatic imagery achieves an average NIIRS rating of 4.5.

Loss of Resolution in Water as a Result of Multiple Small-Angle Scattering

Multiple scattering of light by particles suspended in liquid can be a cause of degraded optical resolution. This is especially significant when the particles are relatively large and numerous, but most of the scattered power is deflected through very small angles, owing to a near match between the refractive indices of the particles and the medium. Formulas which convert the volume scattering function to an image modulation transfer function are derived for this case. This conversion allows calculation of image degradation, given the scattering properties of the medium. The inverse transform is also derived so that the scattering properties can be calculated, given the loss of resolution. The latter is best measured by imaging a set of paralled bar patterns having a wide range of spatial frequencies. The conversion from scattering function to optical transfer function is interpreted in general and evaluated for one published scattering function measured in sea water. The results are compared to published image measurements in fresh water.