Technical evaluation of a clinical, bi-planar, digital and upright X-ray imaging unit.

We describe the technical evaluation of the commercially available, clinical, bi-planar, low dose, digital X-ray system (EOS System, EOS imaging, France). The unit is used for upright, weight-bearing musculoskeletal pathologies, in particular, in the spine and lower limbs. The evaluation incorporated tests on the X-ray generator performance, radiation/imaging field alignment, dose area product accuracy and image quality. The assessment methodology was based on objective parameters and required equipment readily available for technical evaluation of other radiological equipment. Results demonstrated that the system performs well within acceptable performance criteria with regard to X-ray generator performance, radiation/imaging field alignment and dose area product accuracy. In addition, results from the image-quality assessment were aligned with previously published work. The work presented in this article can be used for the technical evaluation of the EOS System at other clinical sites.

Use of weight-based vs age-based groupings in the study of typical values of air kerma area product (PKA) for paediatric radiographs of chest and abdomen.

OBJECTIVE: To compare age groupings versus weight groupings in the calculation of typical air kerma area product (PKA) values in paediatric X-ray exams of chest and abdomen in our hospital. METHODS: Data were analysed from 687 abdominal and 1374 chest X-ray examinations. The PKA of exams was extracted with Radimetrics, and patient weights were collected from electronic records. Data were organised in different age groups and typical PKA values were estimated. The process was repeated by organising data in different weight groups. RESULTS: Typical PKA values for the four younger age groups (<1m, 1m - < 4y, 4y - < 10y and 10y - < 14y) were comparable to typical values for their equivalent weight groups (<5 kg, 5-15 kg, 15-30 kg and 30-50 kg, respectively). However, typical PKA values at the late adolescent age group (14y - < 18y) were much lower than its equivalent weight group (>50 kg). CONCLUSIONS: Age and weight groupings were found at our site to be interchangeable for the calculation of typical paediatric PKA values. The only exception was the late adolescent group, whose weight distribution can account for the difference in typical PKA results within its equivalent weight group. ADVANCES IN KNOWLEDGE: In calculating typical PKA values for radiological paediatric body examinations, departments must ascertain if using age groups, which is typical practice, is equivalent to using weight groups. Otherwise, results may misrepresent local practice.

Assessment of the limiting spatial resolution of an MRI scanner by direct analysis of the edge spread function.

Limiting spatial resolution is a key metric of the quality of magnetic resonance (MR) images, which can provide an indication of the smallest region that can effectively be imaged. In this paper a novel methodology for measuring the limiting spatial resolution of MR images is mathematically analyzed and successfully implemented on phantom images. The methodology presented in this paper is based on a direct fit of a mathematical expression of the edge spread function (ESF) profile to the ESF data acquired at the interface between different materials. The mathematical expression of ESF was determined by approximating the line spread function (LSF) of the system with a sinc function. The proposed methodology can be applied using signal data from magnitude MRI spin echo images and is not sensitive to noise amplification introduced by differentiating the ESF to produce the LSF, as performed in previous studies. In addition, the proposed methodology provides a quantitative, representative measurement of the limiting spatial resolution of MR images.

Wavelet-based de-noising algorithm for images acquired with parallel magnetic resonance imaging (MRI).

Wavelet-based de-noising has been shown to improve image signal-to-noise ratio in magnetic resonance imaging (MRI) while maintaining spatial resolution. Wavelet-based de-noising techniques typically implemented in MRI require that noise displays uniform spatial distribution. However, images acquired with parallel MRI have spatially varying noise levels. In this work, a new algorithm for filtering images with parallel MRI is presented. The proposed algorithm extracts the edges from the original image and then generates a noise map from the wavelet coefficients at finer scales. The noise map is zeroed at locations where edges have been detected and directional analysis is also used to calculate noise in regions of low-contrast edges that may not have been detected. The new methodology was applied on phantom and brain images and compared with other applicable de-noising techniques. The performance of the proposed algorithm was shown to be comparable with other techniques in central areas of the images, where noise levels are high. In addition, finer details and edges were maintained in peripheral areas, where noise levels are low. The proposed methodology is fully automated and can be applied on final reconstructed images without requiring sensitivity profiles or noise matrices of the receiver coils, therefore making it suitable for implementation in a clinical MRI setting.

Performance evaluation of contrast-detail in full field digital mammography systems using ideal (Hotelling) observer vs. conventional automated analysis of CDMAM images for quality control of contrast-detail characteristics.

PURPOSE: The purpose of this work was to evaluate the contrast-detail performance of full field digital mammography (FFDM) systems using ideal (Hotelling) observer Signal-to-Noise Ratio (SNR) methodology and ascertain whether it can be considered an alternative to the conventional, automated analysis of CDMAM phantom images. METHODS: Five FFDM units currently used in the national breast screening programme were evaluated, which differed with respect to age, detector, Automatic Exposure Control (AEC) and target/filter combination. Contrast-detail performance was analysed using CDMAM and ideal observer SNR methodology. The ideal observer SNR was calculated for input signal originating from gold discs of varying thicknesses and diameters, and then used to estimate the threshold gold thickness for each diameter as per CDMAM analysis. The variability of both methods and the dependence of CDMAM analysis on phantom manufacturing discrepancies also investigated. RESULTS: Results from both CDMAM and ideal observer methodologies were informative differentiators of FFDM systems' contrast-detail performance, displaying comparable patterns with respect to the FFDM systems' type and age. CDMAM results suggested higher threshold gold thickness values compared with the ideal observer methodology, especially for small-diameter details, which can be attributed to the behaviour of the CDMAM phantom used in this study. In addition, ideal observer methodology results showed lower variability than CDMAM results. CONCLUSION: The Ideal observer SNR methodology can provide a useful metric of the FFDM systems' contrast detail characteristics and could be considered a surrogate for conventional, automated analysis of CDMAM images.

Developing a quality control protocol for diffusion imaging on a clinical MRI system.

This work describes the development of a quality control protocol, which can be implemented to assess the accuracy, precision and reproducibility of the apparent diffusion coefficient (ADC) measurement on a clinical magnetic resonance imaging (MRI) system. The precision and accuracy of the ADC measurement are analysed with regard to MRI system noise, signal reproducibility and differences between nominal and effective b values. Two aqueous test-solutions of CuSO4 and sucrose are prepared for the quality control protocol. ADC measurement with the CuSO4 solution is more sensitive to differences between nominal and effective b values, on account of the solution's high ADC. ADC measurement with the sucrose solution is more sensitive to signal reproducibility due to the solution's low baseline signal intensity. The ADC of the test-solutions is measured on an MRI system at our centre with a sequence used for clinical studies using diffusion imaging. Two parameters, Q and R, are defined for the analysis of the quality control ADC values. The Q parameter is the ratio of the standard deviation of the quality control mean ADC values over time to the optimal standard deviation, as derived from the effect of thermal noise on the ADC measurement uncertainty. Analysis with the Q parameter indicates that signal reproducibility errors contribute to ADC variations on our MRI system when imaging with high b values (b > 500 mm s(-2)), whereas differences between nominal and effective b values have a greater impact on the ADC measurement when imaging with low b values (b < 500 mm s(-2)). The R parameter is defined as the ratio of the directional variation of the ADC quality control values to the uncertainty of the ADC measurement. Analysis with the R parameter shows that the effect of directional variation of the ADC measurement on our MRI system is more pronounced when imaging with low b values. The quality control protocol identified a systematic error, which introduced a small system-induced anisotropy in the ADC measurement. This error is currently taken into account in the analysis of clinical studies employing the diffusion imaging sequence used in this quality control protocol.

[Safety aspects in interventional MRI]. / Sicherheitsaspekte bei der interventionellen MRT.

Because of its high soft-tissue contrast, Magnetic Resonance Imaging (MRI) is used increasingly for guidance and control of minimal invasive and neurological surgical procedures. Besides common precautions during an MRI investigation, special attention has to be paid to the consequences of MR compatibility, accuracy of localisation of interventional tools and geometrical distortions. As a new application of interventional MR intravascular procedures are developing that involve the introduction of guidewires, catheters and miniaturized coils with their leads into the blood vessels. Resonant currents and high electric fields can develop at the conductor ends, possibly causing burning lesions.

MRI receiver frequency response as a contributor to Nyquist ghosting in echo planar imaging.

PURPOSE: To study the frequency response characteristic of the MRI signal receiver system as a contributing factor to the formation of Nyquist ghosting in echo-planar imaging (EPI). MATERIALS AND METHODS: Experimental work was undertaken on a 1.5 T system. A cylindrical test object filled with water was imaged axially with EPI in the center of the quadrature, transmit-receive head coil. In the first set of experiments, the water conductivity was increased progressively with the addition of salt between EPI acquisitions. In the second set of experiments, the conductivity of the water in the test object was kept constant and EPI images were acquired at several different bandwidths. A computer simulation was also implemented to demonstrate the impact of changes in the frequency response characteristic of the signal receiver system on EPI Nyquist ghosting. RESULTS: Experimental and simulation results showed that Nyquist ghosting increased with the variation of the frequency response characteristic within the effective frequency range determined by the image bandwidth. One can increase the variation in the frequency response characteristic by increasing its steepness over the image's bandwidth window when coil loading is decreased, or by increasing the effective frequency range when image bandwidth is increased. CONCLUSIONS: The results of this research may help reduce Nyquist ghosting in EPI studies when the imaging coil is not sufficiently loaded, such as in pediatric and phantom studies.