Standardization of NPS measurement: interim report of AAPM TG16

Image guided radiation therapy solutions based on megavoltage computed tomography (MVCT) involve the extension of electronic portal imaging devices (EPIDs) from their traditional role of weekly localization imaging and planar dose mapping to volumetric imaging for 3D setup and dose verification. To sustain the potential advantages of MVCT, EPIDs are required to provide improved levels of portal image quality. Therefore, it is vital that the performance of EPIDs in clinical use is maintained at an optimal level through regular and rigorous quality assurance (QA). Traditionally, portal imaging QA has been carried out by imaging calibrated line-pair and contrast resolution phantoms and obtaining arbitrarily defined QA indices that are usually dependent on imaging conditions and merely indicate relative trends in imaging performance. They are not adequately sensitive to all aspects of image quality unlike fundamental imaging metrics such as the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) that are widely used to characterize detector performance in radiographic imaging and would be ideal for QA purposes. However, due to the difficulty of performing conventional MTF measurements, they have not been used for routine clinical QA. The authors present a simple and quick QA methodology based on obtaining the MTF, NPS, and DQE of a megavoltage imager by imaging standard open fields and a bar-pattern QA phantom containing 2 mm thick tungsten line-pair bar resolution targets. Our bar-pattern based MTF measurement features a novel zero-frequency normalization scheme that eliminates normalization errors typically associated with traditional bar-pattern measurements at megavoltage x-ray energies. The bar-pattern QA phantom and open-field images are used in conjunction with an automated image analysis algorithm that quickly computes the MTF, NPS, and DQE of an EPID system. Our approach combines the fundamental advantages of linear systems metrics such as robustness, sensitivity across the full spatial frequency range of interest, and normalization to imaging conditions (magnification, system gain settings, and exposure), with the simplicity, ease, and speed of traditional phantom imaging. The algorithm was analyzed for accuracy and sensitivity by comparing with a commercial portal imaging QA method (PIPSPRO, Standard Imaging, Middleton, WI) on both first-generation lens-coupled and modern a-Si flat-panel based clinical EPID systems. The bar-pattern based QA measurements were found to be far more sensitive to even small levels of degradation in spatial resolution and noise. The bar-pattern based QA methodology offers a comprehensive image quality assessment tool suitable for both commissioning and routine EPID QA.

Section: Iib Noise Power Spectrummentioning

confidence: 99%

Use of a line‐pair resolution phantom for comprehensive quality assurance of electronic portal imaging devices based on fundamental imaging metrics

Gopal

Samant

2009

“…Many studies [2][3][4][5][6][7][8][9][10][11][12] have reported largely standardized methods to measure the physical performance of imaging systems based on the International Electrotechnical Commission IEC 62220-1 standard. 4 These studies were based on the metric-the detective quantum efficiency (DQE)-which reflects detector efficiency when forming an image using a limited number of x-ray photons, providing an input signalto-noise ratio (SNR) at a specific exposure level.…”

Section: Introductionmentioning

confidence: 99%

A comparison of digital radiography systems in terms of effective detective quantum efficiency

Bertolini¹,

Nitrosi²,

Rivetti

et al. 2012

The authors confirm the robustness and reproducibility of the eDQE method. As expected, the DR systems performed better than the CR systems due to their superior signal-to-noise transfer characteristics. The results of this study suggest the eDQE method may provide an opportunity to more accurately assess the clinical performance of digital radiographic imaging systems by accounting for factors such as the presence of scatter, use of an antiscatter grid, and magnification and focal spot blurring effects, which are not reflected in conventional DQE measures.

“…17,18,33,34 Dob-bins et al 17 described the importance of measuring the twodimensional NPS for complete noise analysis. 35 For our measurements of image quality, we use an edge method for the presampling MTF and a two-dimensional Fourier analysis method for the NPS.…”

Section: Introductionmentioning

confidence: 99%

Investigation of physical image quality indices of a bone densitometry system

Wilkie¹,

Giger²,

Chinander³

et al. 2004

Self Cite

Osteoporosis is a disease characterized by a loss of bone mass and a deterioration of bone structure. Bone mineral density (BMD) measures bone mass and is currently the method used to diagnose osteoporosis, while computerized radiographic texture analysis (RTA) is being investigated as a measure of bone structure. The GE/Lunar PIXI peripheral bone densitometer (PD) system, which uses dual-energy subtraction to measure BMD, also provides a digital image of the heel or forearm. The goal of our current research was to evaluate the physical imaging properties of the PIXI system (pixel size of 0.2 mm) compared to a Fuji computed radiography (CR) system (pixel size of 0.1 mm) to determine its suitability for texture analysis from image data. Contrast was measured using a series of uniform images covering the useful clinical exposure range. Spatial resolution was characterized by the presampling modulation transfer function (MTF) determined by an edge method. Noise power spectra (NPS) for different exposures were calculated using a two-dimensional Fourier analysis method. The expectation modulation transfer function was measured and combined with the NPS data to calculate the noise-equivalent number of quanta. The slope of the characteristic curve of the peripheral densitometer (PD) system was found to be position dependent across the image, although this dependence was substantially reduced by use of the system's clinical-settings corrections. An MTF value of 0.5 was found at 0.5 cycles/mm for the densitometry system compared to the same value at 1.6 cycles/mm for the CR system. Unlike the CR system, the NPS of the densitometry system was found not to be directionally dependent and did not drop off at higher spatial frequencies.