Image quality in CT: From physical measurements to model observers.

Evaluation of image quality (IQ) in Computed Tomography (CT) is important to ensure that diagnostic questions are correctly answered, whilst keeping radiation dose to the patient as low as is reasonably possible. The assessment of individual aspects of IQ is already a key component of routine quality control of medical x-ray devices. These values together with standard dose indicators can be used to give rise to 'figures of merit' (FOM) to characterise the dose efficiency of the CT scanners operating in certain modes. The demand for clinically relevant IQ characterisation has naturally increased with the development of CT technology (detectors efficiency, image reconstruction and processing), resulting in the adaptation and evolution of assessment methods. The purpose of this review is to present the spectrum of various methods that have been used to characterise image quality in CT: from objective measurements of physical parameters to clinically task-based approaches (i.e. model observer (MO) approach) including pure human observer approach. When combined together with a dose indicator, a generalised dose efficiency index can be explored in a framework of system and patient dose optimisation. We will focus on the IQ methodologies that are required for dealing with standard reconstruction, but also for iterative reconstruction algorithms. With this concept the previously used FOM will be presented with a proposal to update them in order to make them relevant and up to date with technological progress. The MO that objectively assesses IQ for clinically relevant tasks represents the most promising method in terms of radiologist sensitivity performance and therefore of most relevance in the clinical environment.

Optimization of Radiation Dose and Image Quality in Musculoskeletal CT: Emphasis on Iterative Reconstruction Techniques (Part 1).

Computed tomography (CT) is a modality of choice for the study of the musculoskeletal system for various indications including the study of bone, calcifications, internal derangements of joints (with CT arthrography), as well as periprosthetic complications. However, CT remains intrinsically limited by the fact that it exposes patients to ionizing radiation. Scanning protocols need to be optimized to achieve diagnostic image quality at the lowest radiation dose possible. In this optimization process, the radiologist needs to be familiar with the parameters used to quantify radiation dose and image quality. CT imaging of the musculoskeletal system has certain specificities including the focus on high-contrast objects (i.e., in CT of bone or CT arthrography). These characteristics need to be taken into account when defining a strategy to optimize dose and when choosing the best combination of scanning parameters. In the first part of this review, we present the parameters used for the evaluation and quantification of radiation dose and image quality. In the second part, we discuss different strategies to optimize radiation dose and image quality at CT, with a focus on the musculoskeletal system and the use of novel iterative reconstruction techniques.

Optimization of Radiation Dose and Image Quality in Musculoskeletal CT: Emphasis on Iterative Reconstruction Techniques (Part 2).

Computed tomography (CT) is a modality of choice for the study of the musculoskeletal system for various indications including the study of bone, calcifications, internal derangements of joints (with CT arthrography), as well as periprosthetic complications. However, CT remains intrinsically limited by the fact that it exposes patients to ionizing radiation. Scanning protocols need to be optimized to achieve diagnostic image quality at the lowest radiation dose possible. In this optimization process, the radiologist needs to be familiar with the parameters used to quantify radiation dose and image quality. CT imaging of the musculoskeletal system has certain specificities including the focus on high-contrast objects (i.e., in CT of bone or CT arthrography). These characteristics need to be taken into account when defining a strategy to optimize dose and when choosing the best combination of scanning parameters. In the first part of this review, we present the parameters used for the evaluation and quantification of radiation dose and image quality. In the second part, we discuss different strategies to optimize radiation dose and image quality of CT, with a focus on the musculoskeletal system and the use of novel iterative reconstruction techniques.

Evolution of mammographic image quality in the state of Rio de Janeiro

Objective:To evaluate the evolution of mammographic image quality in the state of Rio de Janeiro on the basis of parameters measured and analyzed during health surveillance inspections in the period from 2006 to 2011.Materials and Methods:Descriptive study analyzing parameters connected with imaging quality of 52 mammography apparatuses inspected at least twice with a one-year interval.Results:Amongst the 16 analyzed parameters, 7 presented more than 70% of conformity, namely: compression paddle pressure intensity (85.1%), films development (72.7%), film response (72.7%), low contrast fine detail (92.2%), tumor mass visualization (76.5%), absence of image artifacts (94.1%), mammography-specific developers availability (88.2%). On the other hand, relevant parameters were below 50% conformity, namely: monthly image quality control testing (28.8%) and high contrast details with respect to microcalcifications visualization (47.1%).Conclusion:The analysis revealed critical situations in terms of compliance with the health surveillance standards. Priority should be given to those mammography apparatuses that remained non-compliant at the second inspection performed within the one-year interval.

Development and validation of a visual grading scale for assessing image quality of AP pelvis radiographic images.

OBJECTIVE: The aim of this article was to apply psychometric theory to develop and validate a visual grading scale for assessing the visual perception of digital image quality anteroposterior (AP) pelvis. METHODS: Psychometric theory was used to guide scale development. Seven phantom and seven cadaver images of visually and objectively predetermined quality were used to help assess scale reliability and validity. 151 volunteers scored phantom images, and 184 volunteers scored cadaver images. Factor analysis and Cronbach's alpha were used to assess scale validity and reliability. RESULTS: A 24-item scale was produced. Aggregated mean volunteer scores for each image correlated with the rank order of the visually and objectively predetermined image qualities. Scale items had good interitem correlation (≥0.2) and high factor loadings (≥0.3). Cronbach's alpha (reliability) revealed that the scale has acceptable levels of internal reliability for both phantom and cadaver images (α = 0.8 and 0.9, respectively). Factor analysis suggested that the scale is multidimensional (assessing multiple quality themes). CONCLUSION: This study represents the first full development and validation of a visual image quality scale using psychometric theory. It is likely that this scale will have clinical, training and research applications. ADVANCES IN KNOWLEDGE: This article presents data to create and validate visual grading scales for radiographic examinations. The visual grading scale, for AP pelvis examinations, can act as a validated tool for future research, teaching and clinical evaluations of image quality.

Effect of alternative photostimulable phosphor plates erasing times on subjective digital image quality

Objective: To evaluate the influence of alternative erasing times of DenOptix (R) (Dentsply/Gendex, Chicargo, IL) digital plates oil subjective image quality and the probability of double exposure image not Occurring.Methods: Human teeth were X-rayed with phosphor plates using tell different erasing times. Two observers evaluated the images for subjective Image quality (sharpness, brightness, contrast, enamel definition, dentin definition and dentin-enamal Junction definition) and for the presence or absence of double exposure image. Spearman's correlation analysis and ANOVA was performed to verify the existence ora linear association between the subjective image quality parameters and the alternative erasing times. A contingency table was constructed to evaluate the agreement among the observers, and a binominal logistic regression was performed to verify the correlation between the erasing time and the probability of double exposure image not occurring.Results: All 6 parameters or image quality were rated high by the examiners for the erasing times between 25 s and 130 s. The same erasing time range, from 25 to 130 s, was considered a safe erasing time interval, with no probability of a double exposure image Occurring.Conclusions: The alternative erasing times from 25 s to 130 s showed high quality and no probability of double image Occurrence. Thus, it is possible to reduce the operating time or the DenOptix (R) digital system Without jeopardizing the diagnostic task.Dentomaxillofacial Radiology (2010) 39, 23-27. doi: 10.1259/dmfr/49065239.

Implant image quality in dental radiographs recorded using a customized imaging guide or a standard film holder

Objective: To compare a customized imaging guide and a standard film holder for obtaining optimally projected intraoral radiographs of dental implants.Material and methods: Intraoral radiographs of four screw-type implants with different inclination placed in an upper or lower dental phantom model were recorded by 32 groups of examiners after a short instruction in the use of the RB-RB/LB-LB mnemonic rule. Half of the examiners recorded the images using a standard film holder and the other half used a customized imaging guide. Each radiograph was assessed under blinded conditions with regard to rendering of the implant threads and was assigned to one of four quality categories: (1) perfect, (2) not perfect, but clinically acceptable, (3) not acceptable, and (4) hopeless.Results: For the upper jaw, the same number of exposures per implant were made to achieve an acceptable image (P = 0.86) by the standard film holder method (median = 2) and the imaging guide method (median = 2). For the lower jaw, medians for the imaging guide method and the film holder method were 1 and 2, respectively (P = 0.004). For the imaging guide method, the first exposure was rated as perfect/acceptable in 62% of the cases and for the film holder method in 41% of the cases (P = 0.013). After <= 2 exposures, 78% (imaging guide method) and 69% (film holder method) of the implant images were perfect/acceptable (P=0.23). The implant inclination did not have a major influence on the outcomes.Conclusion: Perfect or acceptable images were achieved after two exposures with the same frequency either using a customized imaging guide method or a standard film holder method. However, the use of a customized imaging guide method was overall significantly superior to a standard film holder method in terms of obtaining perfect or acceptable images with only one exposure.

Image quality evaluation of chest for computed radiography

The daily-to-day of medical practice is marked by a constant search for an accurate diagnosis and therapeutic assessment. For this purpose the doctor serves up a wide variety of imaging techniques, however, the methods using ionizing radiation still the most widely used because it is considered cheaper and above all very efficient when used with control and quality. The optimization of the risk-benefit ratio is considered a major breakthrough in relation to conventional radiology, though this is not the reality of computing and digital radiology, where Brazil has not established standards and protocols for this purpose. This work aims to optimize computational chest radiographs (anterior-posterior projection-AP). To achieve this objective were used a homogeneous phantoms that simulate the characteristics of absorption and scattering of radiation close to the chest of a patient standard. Another factor studied was the subjective evaluation of image quality, carried out by visual grading assessment (VGA) by specialists in radiology, using an anthropomorphic phantom to identify the best image for a particular pathology (fracture or pneumonia). Quantifying the corresponding images indicated by the radiologist was performed from the quantification of physical parameters (Detective Quantum Efficiency - DQE, Modulation Transfer Function - MTF and Noise Power Spectrum - NPS) using the software MatLab®. © 2013 Springer-Verlag.

A brief discussion about image quality and SEM methods for quantitative fractography of polymer composites

The methodology for fracture analysis of polymeric composites with scanning electron microscopes (SEM) is still under discussion. Many authors prefer to use sputter coating with a conductive material instead of applying low-voltage (LV) or variable-pressure (VP) methods, which preserves the original surfaces. The present work examines the effects of sputter coating with 25 nm of gold on the topography of carbon-epoxy composites fracture surfaces, using an atomic force microscope. Also, the influence of SEM imaging parameters on fractal measurements is evaluated for the VP-SEM and LV-SEM methods. It was observed that topographic measurements were not significantly affected by the gold coating at tested scale. Moreover, changes on SEM setup leads to nonlinear outcome on texture parameters, such as fractal dimension and entropy values. For VP-SEM or LV-SEM, fractal dimension and entropy values did not present any evident relation with image quality parameters, but the resolution must be optimized with imaging setup, accompanied by charge neutralization. © Wiley Periodicals, Inc.

Effect of digitization parameters on periapical radiographic image quality with regard to anatomic landmarks

The aim of this study was to evaluate the influence of digitization parameters on periapical radiographic image quality, with regard to anatomic landmarks. Digitized images (n = 160) were obtained using a flatbed scanner with resolutions of 300, 600 and 2400 dpi. The radiographs of 2400 dpi were decreased to 300 and 600 dpi before storage. Digitizations were performed with and without black masking using 8-bit and 16-bit grayscale and saved in TIFF format. Four anatomic landmarks were classified by two observers (very good, good, moderate, regular, poor), in two random sessions. Intraobserver and interobserver agreements were evaluated by Kappa statistics. Inter and intraobserver agreements ranged according to the anatomic landmarks and resolution used. The results obtained demonstrated that the cement enamel junction was the anatomic landmark that presented the poorest concordance. The use of black masking provided better results in the digitized image. The use of a mask to cover radiographs during digitization is necessary. Therefore, the concordance ranged from regular to moderate for the intraobserver evaluation and concordance ranged from regular to poor for interobserver evaluation.

An in vitro comparison of subjective image quality of panoramic views acquired via 2D or 3D imaging

Objectives: The objective of this study is to compare subjective image quality and diagnostic validity of cone-beam CT (CBCT) panoramic reformatting with digital panoramic radiographs. Materials and methods: Four dry human skulls and two formalin-fixed human heads were scanned using nine different CBCTs, one multi-slice CT (MSCT) and one standard digital panoramic device. Panoramic views were generated from CBCTs in four slice thicknesses. Seven observers scored image quality and visibility of 14 anatomical structures. Four observers repeated the observation after 4 weeks. Results: Digital panoramic radiographs showed significantly better visualization of anatomical structures except for the condyle. Statistical analysis of image quality showed that the 3D imaging modalities (CBCTs and MSCT) were 7.3 times more likely to receive poor scores than the 2D modality. Yet, image quality from NewTom VGi® and 3D Accuitomo 170® was almost equivalent to that of digital panoramic radiographs with respective odds ratio estimates of 1.2 and 1.6 at 95% Wald confidence limits. A substantial overall agreement amongst observers was found. Intra-observer agreement was moderate to substantial. Conclusions: While 2D-panoramic images are significantly better for subjective diagnosis, 2/3 of the 3D-reformatted panoramic images are moderate or good for diagnostic purposes. Clinical relevance: Panoramic reformattings from particular CBCTs are comparable to digital panoramic images concerning the overall image quality and visualization of anatomical structures. This clinically implies that a 3D-derived panoramic view can be generated for diagnosis with a recommended 20-mm slice thickness, if CBCT data is a priori available for other purposes.

Image quality and dose evaluation of filtered back projection versus iterative reconstruction algorithm in multislice computed tomography.

Il presente lavoro di tesi è stato svolto presso il servizio di Fisica Sanitaria del Policlinico Sant'Orsola-Malpighi di Bologna. Lo studio si è concentrato sul confronto tra le tecniche di ricostruzione standard (Filtered Back Projection, FBP) e quelle iterative in Tomografia Computerizzata. Il lavoro è stato diviso in due parti: nella prima è stata analizzata la qualità delle immagini acquisite con una CT multislice (iCT 128, sistema Philips) utilizzando sia l'algoritmo FBP sia quello iterativo (nel nostro caso iDose4). Per valutare la qualità delle immagini sono stati analizzati i seguenti parametri: il Noise Power Spectrum (NPS), la Modulation Transfer Function (MTF) e il rapporto contrasto-rumore (CNR). Le prime due grandezze sono state studiate effettuando misure su un fantoccio fornito dalla ditta costruttrice, che simulava la parte body e la parte head, con due cilindri di 32 e 20 cm rispettivamente. Le misure confermano la riduzione del rumore ma in maniera differente per i diversi filtri di convoluzione utilizzati. Lo studio dell'MTF invece ha rivelato che l'utilizzo delle tecniche standard e iterative non cambia la risoluzione spaziale; infatti gli andamenti ottenuti sono perfettamente identici (a parte le differenze intrinseche nei filtri di convoluzione), a differenza di quanto dichiarato dalla ditta. Per l'analisi del CNR sono stati utilizzati due fantocci; il primo, chiamato Catphan 600 è il fantoccio utilizzato per caratterizzare i sistemi CT. Il secondo, chiamato Cirs 061 ha al suo interno degli inserti che simulano la presenza di lesioni con densità tipiche del distretto addominale. Lo studio effettuato ha evidenziato che, per entrambi i fantocci, il rapporto contrasto-rumore aumenta se si utilizza la tecnica di ricostruzione iterativa. La seconda parte del lavoro di tesi è stata quella di effettuare una valutazione della riduzione della dose prendendo in considerazione diversi protocolli utilizzati nella pratica clinica, si sono analizzati un alto numero di esami e si sono calcolati i valori medi di CTDI e DLP su un campione di esame con FBP e con iDose4. I risultati mostrano che i valori ricavati con l'utilizzo dell'algoritmo iterativo sono al di sotto dei valori DLR nazionali di riferimento e di quelli che non usano i sistemi iterativi.

Iterative Reconstruction Algorithm for Abdominal Multidetector CT at Different Tube Voltages: Assessment of Diagnostic Accuracy, Image Quality, and Radiation Dose in a Phantom Study

To assess the diagnostic accuracy, image quality, and radiation dose of an iterative reconstruction algorithm compared with a filtered back projection (FBP) algorithm for abdominal computed tomography (CT) at different tube voltages.

Detection of pancreatic tumors, image quality, and radiation dose during the pancreatic parenchymal phase: effect of a low-tube-voltage, high-tube-current CT technique--preliminary results

To intraindividually compare a low-tube-voltage (80 kVp), high-tube-current (675 mA) computed tomographic (CT) technique with a high-tube-voltage (140 kVp) CT protocol for the detection of pancreatic tumors, image quality, and radiation dose during the pancreatic parenchymal phase.

Low-tube-voltage, high-tube-current multidetector abdominal CT: improved image quality and decreased radiation dose with adaptive statistical iterative reconstruction algorithm--initial clinical experience

To investigate whether an adaptive statistical iterative reconstruction (ASIR) algorithm improves the image quality at low-tube-voltage (80-kVp), high-tube-current (675-mA) multidetector abdominal computed tomography (CT) during the late hepatic arterial phase.