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Quantifying image quality in diagnostic radiology using simulation of the imaging system and model observers
Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Linköping University, Faculty of Health Sciences.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Accurate measures of both clinical image quality and patient radiation risk are needed for successful optimisation of medical imaging with ionising radiation. Optimisation in diagnostic radiology means finding the image acquisition technique that maximises the perceived information content and minimises the radiation risk or keeps it at a reasonably low level. The assessment of image quality depends on the diagnostic task and may in addition to system and quantum noise also be hampered by overlying projected anatomy.

The main objective of this thesis is to develop methods for assessment of image quality in simulations of projection radiography. In this thesis, image quality is quantified by modelling the whole x‐ray imaging system including the x‐ray tube, patient, anti‐scatter device, image detector and the observer. This is accomplished by using Monte Carlo (MC) simulation methods that allow simultaneous estimates of measures of image quality and patient dose. Measures of image quality include the signal‐to‐noise‐ratio, SNR, of pathologic lesions and radiation risk is estimated by using organ doses to calculate the effective dose. Based on high‐resolution anthropomorphic phantoms, synthetic radiographs were calculated and used for assessing image quality with model‐observers (Laguerre‐Gauss (LG) Hotelling observer) that mimic real, human observers. Breast and particularly chest imaging were selected as study cases as these are particularly challenging for the radiologists.

In chest imaging the optimal tube voltage in detecting lung lesions was investigated in terms of their SNR and the contrast of the lesions relative to the ribs. It was found that the choice of tube voltage depends on whether SNR of the lesion or the interfering projected anatomy (i.e. the ribs) is most important for detection. The Laguerre‐Gauss (LG) Hotelling observer is influenced by the projected anatomical background and includes this into its figure‐of‐merit, SNRhot,LG. The LG‐observer was found to be a better model of the radiologist than the ideal observer that only includes the quantum noise in its analysis. The measures of image quality derived from our model are found to correlate relatively well with the radiologist’s assessment of image quality. Therefore MC simulations can be a valuable and an efficient tool in the search for dose‐efficient imaging systems and image acquisition schemes.

Place, publisher, year, edition, pages
Institutionen för medicin och hälsa , 2008.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1050
Keyword [en]
radiology, radiation physics, image quality, optimisation, effective dose, chest radiography
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:liu:diva-11662ISBN: 978‐91‐7393‐952‐2 OAI: oai:DiVA.org:liu-11662DiVA: diva2:18076
Public defence
2008-05-09, Eken, Campus US, Linköpings universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2015-03-20
List of papers
1. A search for optimal x‐ray spectra in iodine contrast media mammography
Open this publication in new window or tab >>A search for optimal x‐ray spectra in iodine contrast media mammography
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2005 (English)In: Physics in medicine and biology, ISSN 0031-9155, Vol. 50, no 13, 3143-3152 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this work was to search for the optimal x-ray tube voltage and anode–filter combination in digital iodine contrast media mammography. In the optimization, two entities were of interest: the average glandular dose, AGD, and the signal-to-noise ratio, SNR, for detection of diluted iodine contrast medium. The optimum is defined as the technique maximizing the figure of merit, SNR2/AGD. A Monte Carlo computer program was used which simulates the transport of photons from the x-ray tube through the compression plate, breast, breast support plate, anti-scatter grid and image detector. It computes the AGD and the SNR of an iodine detail inside the compressed breast. The breast thickness was varied between 2 and 8 cm with 10–90% glandularity. The tube voltage was varied between 20 and 55 kV for each anode material (Rh, Mo and W) in combination with either 25 µm Rh or 0.05–0.5 mm Cu added filtration. The x-ray spectra were calculated with MCNP4C (Monte Carlo N-Particle Transport Code System, version 4C). A CsI scintillator was used as the image detector. The results for Rh/0.3mmCu, Mo/0.3mmCu and W/0.3mmCu were similar. For all breast thicknesses, a maximum in the figure of merit was found at approximately 45 kV for the Rh/Cu, Mo/Cu and W/Cu combinations. The corresponding results for the Rh/Rh combination gave a figure of merit that was typically lower and more slowly varying with tube voltage. For a 4 cm breast at 45 kV, the SNR2/AGD was 3.5 times higher for the Rh/0.3mmCu combination compared with the Rh/Rh combination. The difference is even larger for thicker breasts. The SNR2/AGD increases slowly with increasing Cu-filter thickness. We conclude that tube voltages between 41 and 55 kV and added Cu-filtration will result in significant dose advantage in digital iodine contrast media mammography compared to using the Rh/Rh anode/filter combination at 25–32 kV.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13217 (URN)10.1088/0031-9155/50/13/012 (DOI)
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2015-03-20
2. Distributions of scatter to primary ratios and signal to noise ratios per pixel in digital chest imaging
Open this publication in new window or tab >>Distributions of scatter to primary ratios and signal to noise ratios per pixel in digital chest imaging
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2005 (English)In: Radiation protection dosimetry, ISSN 0144-8420, Vol. 114, no 1-3, 355-358 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this work was to calculate distributions of scatter-to-primary ratios (s/p) and signal-to-noise ratios per pixel (SNRp) in chest images. Such distributions may provide useful information on how physical image quality (contrast, SNR) is distributed over the posterior/anterior (PA) chest image. A Monte Carlo computer program was used for the calculations, including a model of both the patient (voxel phantom) and the imaging system (X-ray tube, anti-scatter grid and image detector). The calculations were performed for three PA thicknesses 20, 24 and 28 cm. For a 24 cm patient, the s/p varies between 0.5 in the lung to 2.5 behind the spine and heart. The corresponding variation of the SNRp is a factor of 3, with the highest values in the lung. Increasing the patient thickness from 20 to 28 cm increases the s/p by a factor of 2.2 behind the spine and heart.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13218 (URN)10.1093/rpd/nch530 (DOI)
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2015-03-20
3. Towards optimization in digital chest radiography using Monte Carlo modelling
Open this publication in new window or tab >>Towards optimization in digital chest radiography using Monte Carlo modelling
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2006 (English)In: Physics in medicine and biology, ISSN 0031-9155, Vol. 51, no 11, 2729-2743 p.Article in journal (Refereed) Published
Abstract [en]

A Monte Carlo based computer model of the x-ray imaging system was used to investigate how various image quality parameters of interest in chest PA radiography and the effective dose E vary with tube voltage (90–150 kV), additional copper filtration (0–0.5 mm), anti-scatter method (grid ratios 8–16 and air gap lengths 20–40 cm) and patient thickness (20–28 cm) in a computed radiography (CR) system. Calculated quantities were normalized to a fixed value of air kerma (5.0 µGy) at the automatic exposure control chambers. Soft-tissue nodules were positioned at different locations in the anatomy and calcifications in the apical region. The signal-to-noise ratio, SNR, of the nodules and the nodule contrast relative to the contrast of bone (C/CB) as well as relative to the dynamic range in the image (Crel) were used as image quality measures. In all anatomical regions, except in the densest regions in the thickest patients, the air gap technique provides higher SNR and contrast ratios than the grid technique and at a lower effective dose E. Choice of tube voltage depends on whether quantum noise (SNR) or the contrast ratios are most relevant for the diagnostic task. SNR increases with decreasing tube voltage while C/CB increases with increasing tube voltage.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13219 (URN)10.1088/0031-9155/51/11/003 (DOI)
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2015-03-20
4. Comparison of clinical and physical measures of image quality in chest and pelvis computed radiography at different tube voltages
Open this publication in new window or tab >>Comparison of clinical and physical measures of image quality in chest and pelvis computed radiography at different tube voltages
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2006 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 11, 4169-4175 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this work was to study the dependence of image quality in digital chest and pelvis radiography on tube voltage, and to explore correlations between clinical and physical measures of image quality. The effect on image quality of tube voltage in these two examinations was assessed using two methods. The first method relies on radiologists' observations of images of an anthropomorphic phantom, and the second method was based on computer modeling of the imaging system using an anthropomorphic voxel phantom. The tube voltage was varied within a broad range (50–150  kV), including those values typically used with screen-film radiography. The tube charge was altered so that the same effective dose was achieved for each projection. Two x-ray units were employed using a computed radiography (CR) image detector with standard tube filtration and antiscatter device. Clinical image quality was assessed by a group of radiologists using a visual grading analysis (VGA) technique based on the revised CEC image criteria. Physical image quality was derived from a Monte Carlo computer model in terms of the signal-to-noise ratio, SNR, of anatomical structures corresponding to the image criteria. Both the VGAS (visual grading analysis score) and SNR decrease with increasing tube voltage in both chest PA and pelvis AP examinations, indicating superior performance if lower tube voltages are employed. Hence, a positive correlation between clinical and physical measures of image quality was found. The pros and cons of using lower tube voltages with CR digital radiography than typically used in analog screen-film radiography are discussed, as well as the relevance of using VGAS and quantum-noise SNR as measures of image quality in pelvis and chest radiography.

Keyword
diagnostic radiography, Monte Carlo methods, image denoising, dosimetry, phantoms, biomedical equipment
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13220 (URN)10.1118/1.2362871 (DOI)
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2015-03-20
5. Calculation of images from an anthropomorphic chest phantom using Monte Carlo methods
Open this publication in new window or tab >>Calculation of images from an anthropomorphic chest phantom using Monte Carlo methods
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2006 (English)In: Proceedings of SPIE, 2006, Vol. 6142Conference paper, Published paper (Refereed)
Abstract [en]

Monte Carlo (MC) computer simulation of chest x-ray imaging systems has hitherto been performed using anthropomorphic phantoms with too large (3 mm) voxel sizes. The aim for this work was to develop and use a Monte Carlo computer program to compute projection x-ray images of a high-resolution anthropomorphic voxel phantom for visual clinical image quality evaluation and dose-optimization. An Alderson anthropomorphic chest phantom was imaged in a CT-scanner and reconstructed with isotropic voxels of 0.7 mm. The phantom was segmented and included in a Monte Carlo computer program using the collision density estimator to derive the energies imparted to the detector per unit area of each pixel by scattered photons. The image due to primary photons was calculated analytically including a pre-calculated detector response function. Attenuation and scatter of x-rays in the phantom, grid and image detector was considered. Imaging conditions (tube voltage, anti-scatter device) were varied and the images compared to a real computed radiography (Fuji FCR 9501) image. Four imaging systems were simulated (two tube voltages 81 kV and 141 kV using either a grid with ratio 10 or a 30 cm air gap). The effect of scattered radiation on the visibility of thoracic vertebrae against the heart and lungs is demonstrated. The simplicity in changing the imaging conditions will allow us not only to produce images of existing imaging systems, but also of hypothetical, future imaging systems. We conclude that the calculated images of the high-resolution voxel phantom are suitable for human detection experiments of low-contrast lesions.

Series
, 6142
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13221 (URN)10.1117/12.644119 (DOI)
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2015-03-20
6. Development of a Monte Carlo based model for optimization using the Laguerre‐Gauss Hotelling observer
Open this publication in new window or tab >>Development of a Monte Carlo based model for optimization using the Laguerre‐Gauss Hotelling observer
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2008 (English)In: Medical physics, ISSN 0094-2405Article in journal (Refereed) Submitted
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13222 (URN)
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2017-01-11

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