The effect of vertical centering and scout direction on automatic tube voltage selection in chest CT : a preliminary phantom study on two different CT equipments

Purpose: To determine the effect of patient's vertical off-centering and scout direction on the function of automatic tube voltage selection (ATVS) and tube current modulation (TCM) in chest computed tomography (CT). Methods: Chest phantom was scanned with Siemens and GE CT systems using three clinical chest CT protocols exploiting ATVS and a fixed 120 kVp chest protocol. The scans were performed at five vertical positions of the phantom (- 6 to + 6 cm from the scanner isocenter). The effects of scout direction (posterior-to-anterior, anterior-to-posterior, and lateral) and vertical off-centering on the function of ATVS and TCM were studied by examining changes in selected voltage, radiation dose (volume CT dose index, CTDIvol), and image noise and contrast. Results: Both scout direction and vertical off-centering affected ATVS. The effect differed between the vendors for the studied geometry, demonstrating differences in technical approaches. The greatest observed increase in CTDI vot due to off-centering was 91%. Anterior-to-posterior scout produced highest doses at the uppermost table position, whereas posterior-to-anterior scout produced highest doses at the lowermost table position. Dose varied least using lateral scouts. Vertical off-centering impacted image noise and contrast due to the combined effect of ATVS, TCM, structural noise, and bowtie fillers. Conclusions: Patient vertical off-centering and scout direction affected substantially the CTDI vot and image quality in chest CT examinations. Vertical off-centering caused variation also in the selected tube voltage. The function of ATVS and TCM methods differ significantly between the CT vendors, resulting in differences in CTDIvol and image noise characteristics.Peer reviewe

CT dose optimization with model based iterative reconstruction

The aim of this thesis is to assess the feasibility of using model-based iterative reconstruction (MBIR) to develop new low-dose CT (computed tomography) protocols in the areas of neck, chest, and abdominal imaging without compromising diagnostic performance. Medical imaging has become the largest source of radiation exposure for humans other than natural background radiation. The availability of and improvements in diagnostic imaging have led to a sevenfold increase in the use of imaging over the past 30 years. This is especially true for CT, with a 7.8% annual increase in the use of CT from 1996 to 2010. The major concern associated with the widespread uptake of CT is the parallel increase in radiation exposure incurred by patients, and while the relationship of diagnostic radiation exposure to a quantifiable risk of cancer induction remains a controversial topic, physicians are beholden to keep radiation doses from diagnostic imaging as low as reasonably possible to limit the risk of radiation-induced cancer. We conducted preliminary phantom and cadaveric studies to examine the performance of MBIR at different radiation dose levels in the thorax and abdomen. Cadavers and phantoms provide a means of safely assessing new technologies and optimizing scan protocols prior to clinical validation. An anthropomorphic torso phantom and 5 human cadavers were scanned at varying radiation dose levels and images reconstructed using three different reconstruction techniques: filtered back projection, hybrid IR and MBIR. MBIR reduced image noise and improved image quality even in CT images acquired with a mean radiation dose reduction of 62%, compared with conventional dose studies reconstructed with hybrid IR, with lower levels of objective image noise, superior diagnostic acceptability and contrast resolution, and comparable subjective image noise and streak artifact scores. We subsequently performed clinical studies with the objectives of assessing MBIR as a tool for image quality improvement and radiation dose reduction in CT, and for the development of new low-dose carotid angiography, chest, and abdominopelvic CT protocols. We developed a low-dose carotid CTA protocol reconstructed with MBIR comparable to a conventional dose CTA protocol in terms of image quality and diagnostic accuracy while enabling a dose reduction of 49.6%. 20 patients were scanned using a split-dose technique with radiation dose divided into a low-dose acquisition reconstructed with MBIR and a conventional dose acquisition reconstructed with hybrid IR. Mean effective dose was significantly lower for the low-dose studies (1.84mSv versus 3.71mSv) and subjective image noise, contrast resolution, and spatial resolution were significantly higher for the low-dose studies. There was excellent agreement for stenosis grading accuracy between low- and conventional dose studies (Cohen κ = 0.806). A modified low-dose CT thorax protocol reconstructed with MBIR was also developed to monitor pulmonary disease progression in patients with cystic fibrosis with our low-dose protocol enabling the acquisition of a full-volume diagnostic quality chest CT at a dose equivalent to that of a chest radiograph (0.09±0.01mSv). Finally, we assessed the feasibility of low-dose abdominopelvic CT performed with MBIR as a radiation dose reduction strategy for imaging patients presenting with acute abdominal pain. A 74.7% mean radiation dose reduction was achieved with scans performed in the peri- and submillisievert range in patients of normal and low BMI, respectively, without compromising diagnostic performance. Dose reduction to the submillisievert range for patients with an elevated BMI was a challenge. The current era is extremely exciting in terms of radiation dose optimization in CT. This thesis is a demonstration of the potential for substantial reductions in radiation exposure, when the benefits of iterative reconstruction are combined with automated tube current modulation and other CT scanner technologies. The combination of all these hardware and software developments is now seeing major benefits for the patient and moving beyond the narrow aim of radiation exposure reduction to a complete change in practice, towards replacement of conventional radiography with low-dose CT, without any penalty for the patient in terms of radiation exposure

The “Knowledgeable” CT Scanner: Optimization by technological advancements

Optimization of a computed tomography (CT) exam can be challenging, as there is a wide variety in patient characteristics, continuously emerging technologies and the inc

Application of Energy Dispersive X-Ray Diffraction (EDXRD) in the detection of fat nodules in liver disease and plaques in the blood vessels of the heart

Fat deposits in tissue or organs are a major health problem. Assessment of fat nodules in the liver (particularly prior to a liver transplant) or plaques in coronary arteries are important investigations that are undertaken in hospitals throughout the world. Many of the current techniques for investigation, e.g., computed tomography coronary angiography (CTCA), lead to high radiation doses or carry significant risks (catheter angiography) and hence new, quantitative tests are required. This thesis looks at using localised X-ray diffraction measurements as an alternative approach to quantifying the fat content of blood vessels or liver tissue. Energy dispersive X-ray diffraction (EDXRD) has been used to study the diffraction profiles of specially designed tissue phantoms. Phantoms have been built to represent a fatty-liver and a diseased heart where the size of the fatty inclusion can be varied. Combinations of tissue equivalent materials and real tissues have been used in each phantom. Diffraction profiles have been measured on these phantoms for a range of fat inclusions. All measurements were made on a fixed geometry diffractometer using a nominal scattering angle of 60 , a High Purity Germanium (HPGe) detector system and a tungsten target X-ray source. The diffraction data has been subjected to different analysis procedures. The results on the phantoms have shown that the minimum detectable levels of fat inclusions are clinically relevant. These results have then been used to scale up the measured diffraction data to patient-sized objects. Although these estimates demonstrate that doses need to be increased they also indicate that useful clinical data could be obtained at acceptable dose levels

Patient radiation dose issues resulting from the use of CT in the UK

In this report, COMARE presents a comprehensive review of the radiation dose issues associated with CT scans in the UK. The implications of the increase in the numbers of CT scans in the UK are considered in the report, with focus on the number of younger patients undergoing CT scans, who have greater sensitivity to x-rays. The report provides an update on the radiation protection aspects of justification (balancing risk and benefit) and optimisation (balancing the risk from the radiation dose with the quality of the image)

Radiation Dose And Image Quality From Pelvic Localisation Computed Tomography In Oncology

DissertationIntroduction: Computed tomography (CT) in radiation therapy plays an important role in the accurate identification of the position of the tumour and organs at risk, through high geometric fidelity of the CT image. It has been determined that the radiation dose from CT is amongst the highest from all medical imaging. There is concern over the increased radiation dose from pelvic CT localisation scans, due to the increased scan length and the necessity for high image quality used in radiation therapy planning. The necessity for high image quality, while lowering the CT dose and honouring the ALARA principles, is essential. Four article-style studies, which evaluate the CT dose and image quality produced for pelvic localisation scans and that are ultimately aimed at publication, are presented in this research paper. Purpose: The purpose of this research was to determine the CT dose and image quality produced for pelvic localisation scans in a department of oncology, Free State. The aim was to measure the dose received by patients during pelvic CT localisation scans and to determine whether the dose is justified in terms of imaging requirements for radiation therapy planning. The objectives of the research were to (i) determine baseline dose level for pelvic CT scan utilising an anthropomorphic phantom, (ii) measure patients’ CT pelvic localisation dose by using size-specific dose estimates, (iii) to verify whether the field of view (FOV) modified image quality for patients of different sizes, utilising water phantoms and (iv) objectively examine image quality using the contrast-to-noise ratio (CNR) for patients’ CT localisation scans. The significance of this research is reflected in filling the gap in existing literature, as most published studies were conducted on diagnostic CT dose and image quality. Methodology: The research was conducted as a prospective research study, performed between January and June 2019, after ethical approval was obtained. All CT scans were produced on a TSX-201A (Toshiba Aquilion © Large Bore) CT scanner. The CT baseline dose level was established utilising an anthropomorphic phantom. The patient dose for pelvic CT localisation was calculated by the size-specific dose estimate (SSDE) that determines the dose, based on individual patient dimensions. The participants were divided into three body mass index (BMI) categories; these were underweight, normal weight and overweight. The CT image quality was examined based on scans of different sized water phantoms utilising CT quality assurance tests. The patients’ CT image quality was derived from the contrast-to-noise ratio (CNR). Results: A total of 131 participants met the inclusion criteria of the research study and were grouped according to their BMI into categories, these being categorised as: overweight-, normal- and underweight BMI category. For the baseline dose level, the best kV and FOV combination was determined for the 120 kV setting with a large (L), large-large (LL) or extra-large (XL) FOV, which calculated at 14.0 cGy using SSDE. In terms of the BMI patient category the median dose was determined as: 12.3 cGy for an overweight BMI; 14.8 cGy for a normal BMI, which is in line with the baseline CT dose and 17.1 cGy for the underweight BMI category. The image quality determined as per phantom indicated that the 135 kV demonstrated the highest quality as well as the highest dose. In terms of FOV the small and medium sized phantom could be scanned with any size FOV. However, the large phantom excelled with the L, LL and XL FOV. The results for the patients in terms of image quality, based on CNR illustrated that the normal BMI category patients had the highest quality. It was furthermore concluded that the overweight BMI patient category reflected the lowest image quality. Conclusion: The research questions were addressed and the objectives of this research were indeed met. In addition, this research addressed the gap in relevant literature, by determining results based on oncological CT scans and protocols in terms of dose and image quality. The fact that only one anthropomorphic phantom was available for dose calculations and that no tissue types were present in the phantom to utilise for the image quality, limited the research. The researcher recommends that protocols for patients in different BMI categories be established for CT pelvic localisation scans whilst simultaneously adhering to the ALARA principles. Research demonstrates that there is an industrial drive to decrease dose while maintaining image quality. Numerous techniques have been introduced to assist with the reduction of dose. One of these techniques was illustrated by Irish researchers who established national diagnostic reference levels (DRL’s) for breast CT protocols in oncology. It is believed that by utilising knowledge from both diagnostic and oncology CT scan techniques, the reduction of CT dose - while maintaining image quality - is an achievable goal

Establishing an evidence-base for erect pelvis radiography : positioning, radiation dose and image quality

Purpose: Pelvic radiography using X-ray imaging has traditionally been used for the identification of hip joint changes, including the identification of pathologies such as osteoarthritis. For patients suffering from hip pain, the supine pelvis X-ray examination is one of the initial diagnostic steps. Despite this, many recent studies have recommended that the position should now be undertaken erect and not supine to reflect the functional appearances of the hip joint. This thesis aims to establish an evidence base for erect pelvis radiography, and it will include assessing radiographic positioning, radiation dose and image quality. Methods: The experimental work described in this thesis was conducted in three phases. Each phase has its own methods with the purpose of achieving a specific set of aims. Phase One was the evaluation of the postural effects of different erect (standing) positions in order to recommend an optimal one for erect pelvic radiography. Eight different erect positions were investigated. A sample group of 67 healthy people participated, and a range of spinal and pelvis measurements were acquired using a 3D video rasterography system (Diers) and an inclinometer.Phase Two was a phantom study evaluating the potential changes to radiation dose and image quality when moving between supine and erect imaging. Phase two was undertaken using three experiments (experiment #1, experiment #2 and experiment #3). Experiment #1 evaluated the impact of increased patient size on the radiation dose and image quality. In this experiment, animal fat was positioned anteriorly on a pelvic anthropomorphic phantom and the thickness increased incrementally in 1cm steps from 1 to 15cm. Image quality was evaluated physically and visually. The effective dose was calculated using Monte Carlo simulation software (PCXMC). During experiment #2, the anterior thicknesses for 109 patients, with a range of BMIs, who were referred for pelvis radiography, was measured in the erect and supine position. Experiment #3 evaluated the potential differences between the positions (supine and erect) in terms of image quality and radiation dose by modelling patient thickness changes between positions using the data obtained in experiment #2. An anthropomorphic phantom was used and modified (by adding additional fat) to simulate tissue changes for both erect and supine X-ray positions. Visual grading analysis was used (VGA) to evaluate image quality. The effective dose and absorbed dose were calculated using PCXMC.During Phase Three, 60 patients were imaged in erect and supine positions. The paired pelvis X-ray images were then compared, taking into account radiation dose and image quality.Results: Phase One demonstrated no statistical differences between the eight-different standing positions for pelvic and spine metrics (P>0.05). Results also demonstrated no significant postural differences between BMIs across all eight standing positions (P>0.05). Also, no differences (P>0.05) were identified in the pelvis and spinal metrics when comparing between males and females .Standing relaxed with feet internally rotated by 20°and the upper arms supported was a recommendation derived from this phase. Results from Phase Two showed an increase in effective dose (E) as the fat thickness increased. Also, all physical and visual image quality metrics decreased as fat thickness increased. Physical and visual image quality measures also decreased for erect images when compared to supine images, and the E also increased. 90kVp, 130/145 SID, using both outer chambers, were the recommended exposure parameters settings for obtaining erect pelvis X-ray images. Results from Phase Three showed that anterior patient thickness was 17% (P<0.001) higher in an erect position .The DAP and absorbed dose were 46% and 45% (P<0.001) greater in the erect position. Also, the effective dose was 67% (P<0.001) higher in the erect position when compared with supine. In regard to the image quality (IQ), that of the erect position decreased by 10% when compared with supine (P<0.001).Conclusion: The eight proposed standing positions could theoretically be suitable for erect pelvis imaging. People in a relaxed standing position, with their feet internally rotated by 20°and their upper arms supported would be recommended. In terms of IQ and radiation dose for erect positions, this position decreases image quality (both physical and visual) and increased radiation dose. Changes were largely due to the effect of gravity on the anterior soft tissue distribution. These issues should be considered and optimised more fully when deciding if to move from supine to erect pelvis imaging