A simple theorem relating noise and patient dose in computed tomography

Harpen, Michael D.

doi:10.1118/1.598778

Cited by 30 publications

(32 citation statements)

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“…For example, one can target uniform signal/noise at the center of the detector by specifying a fluence inversely proportional to the transmissivity (Kalra et al 2004). Gies et al (Gies et al 1999) and Harpen (Harpen 1999) directly minimized variance (from quantum noise) at a particular voxel in a FBP reconstruction through a TCM pattern that is proportional to the square root of the transmissivity.…”

Section: A Introductionmentioning

confidence: 99%

Task-driven optimization of CT tube current modulation and regularization in model-based iterative reconstruction

2017

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Tube current modulation (TCM) is routinely adopted on diagnostic CT scanners for dose reduction. Conventional TCM strategies are generally designed for filtered-backprojection (FBP) reconstruction to satisfy simple image quality requirements based on noise. This work investigates TCM designs for model-based iterative reconstruction (MBIR) to achieve optimal imaging performance as determined by a task-based image quality metric. Additionally, regularization is an important aspect of MBIR that is jointly optimized with TCM, and includes both the regularization strength that controls overall smoothness as well as directional weights that permits control of the isotropy/anisotropy of the local noise and resolution properties. Initial investigations focus on a known imaging task at a single location in the image volume. The framework adopts Fourier and analytical approximations for fast estimation of the local noise power spectrum (NPS) and modulation transfer function (MTF) - each carrying dependencies on TCM and regularization. For the single location optimization, the local detectability index (d′) of the specific task was directly adopted as the objective function. A covariance matrix adaptation evolution strategy (CMA-ES) algorithm was employed to identify the optimal combination of imaging parameters. Evaluations of both conventional and task-driven approaches were performed in an abdomen phantom for a mid-frequency discrimination task in the kidney. Among the conventional strategies, the TCM pattern optimal for FBP using a minimum variance criterion yielded worse task-based performance compared to an unmodulated strategy when applied to MBIR. Moreover, task-driven TCM designs for MBIR were found to have the opposite behavior from conventional designs for FBP, with greater fluence assigned to the less attenuating views of the abdomen and less fluence to the more attenuating lateral views. Such TCM patterns exaggerate the intrinsic anisotropy of the MTF and NPS as a result of the data weighting in MBIR. Directional penalty design was found to reinforce the same trend. The task-driven approaches outperform conventional approaches, with the maximum improvement in d′ of 13% given by the joint optimization of TCM and regularization. This work demonstrates that the TCM optimal for MBIR is distinct from conventional strategies proposed for FBP reconstruction and strategies optimal for FBP are suboptimal and may even reduce performance when applied to MBIR. The task-driven imaging framework offers a promising approach for optimizing acquisition and reconstruction for MBIR that can improve imaging performance and/or dose utilization beyond conventional imaging strategies.

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Section: A Introductionmentioning

confidence: 99%

Task-driven optimization of CT tube current modulation and regularization in model-based iterative reconstruction

2017

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“…These examples with uniform noise can represent scans of air in a system with no bowtie filter 28 or scans of a circularly symmetric object with a bowtie filter that perfectly compensate for the object attenuation.…”

Section: Iib Computer Simulationsmentioning

confidence: 99%

The noise power spectrum in CT with direct fan beam reconstruction

Baek

Pelc

2010

Medical Physics

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The noise power spectrum ͑NPS͒ is a useful metric for understanding the noise content in images. To examine some unique properties of the NPS of fan beam CT, the authors derived an analytical expression for the NPS of fan beam CT and validated it with computer simulations. The nonstationary noise behavior of fan beam CT was examined by analyzing local regions and the entire field-of-view ͑FOV͒. This was performed for cases with uniform as well as nonuniform noise across the detector cells and across views. The simulated NPS from the entire FOV and local regions showed good agreement with the analytically derived NPS. The analysis shows that whereas the NPS of a large FOV in parallel beam CT ͑using a ramp filter͒ is proportional to frequency, the NPS with direct fan beam FBP reconstruction shows a high frequency roll off. Even in small regions, the fan beam NPS can show a sharp transition ͑discontinuity͒ at high frequencies. These effects are due to the variable magnification and therefore are more pronounced as the fan angle increases. For cases with nonuniform noise, the NPS can show the directional dependence and additional effects.

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“…For CBCT, conventional bow tie filter is able to match a cylindrical phantom pretty well in all the projections over a circular trajectory, because the phantom's geometry is very simple and it is easy to align the phantom's symmetry axis with the system's rotation axis [8][9] . The breast, however, varies in size and shape among women.…”

Section: Design Of the Breast Shaped Compensation Filtermentioning

confidence: 99%

Evaluation of a new breast-shaped compensation filter for a newly built breast imaging system

et al. 2007

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A new breast-shaped compensation filter has been designed and fabricated for breast imaging using our newly built breast imaging (CBCTBI) system, which is able to scan an uncompressed breast with pendant geometry. The shape of this compensation filter is designed based on an average-sized breast phantom. Unlike conventional bow-tie compensation filters, its cross-sectional profile varies along the chest wall-to-nipple direction for better compensation for the shape of a breast. Breast phantoms of three different sizes are used to evaluate the performance of this compensation filter. The reconstruction image quality was studied and compared to that obtained without the compensation filter in place. The uniformity of linear attenuation coefficient and the uniformity of noise distribution are significantly improved, and the contrast-to-noise ratios (CNR) of small lesions near the chest wall are increased as well. Multi-normal image method is used in the reconstruction process to correct compensation flood field and to reduce ring artifacts.

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A simple theorem relating noise and patient dose in computed tomography

Cited by 30 publications

References 1 publication

Task-driven optimization of CT tube current modulation and regularization in model-based iterative reconstruction

Task-driven optimization of CT tube current modulation and regularization in model-based iterative reconstruction

The noise power spectrum in CT with direct fan beam reconstruction

Evaluation of a new breast-shaped compensation filter for a newly built breast imaging system

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