Student Number : 8802466H - MSc research report - School of Physics - Faculty of Science / Medical ionising radiation sources give by far the largest contribution to the population dose from man-made sources. About 90% of this contribution is due to x-ray diagnostic procedures. Doses from diagnostic radiology procedures are nevertheless small and usually do not approach thresholds for deterministic effects. However, they must be accurately determined in order to maintain a reasonable balance between image quality and patient exposure. There is, thus, a need to establish quality assurance for diagnostic procedures that will provide the required clinical information in its optimal form and with minimum dose to the patient. In order to achieve this, dose measurements must be reproducible and the uncertainties associated with that measurement should be known. One of key factors for ensuring that appropriate levels of accuracy and long-term reproducibility of dose measurements are maintained is a calibration of the measuring equipment. The IEC (International Electrotechnical Commission) issued a standard IEC 61267 that deals with methods for generating radiation beams with radiation conditions which can be used under test conditions typically found in test laboratories for the determination of characteristics of medical diagnostic X-ray equipment. The document is currently being revised and publication of the new version is expected soon. Standard radiation qualities were established at a laboratory following the new IEC 61267 standard. Radiation qualities that characterize radiation beams emerging from the X-ray target (RQR qualities) were established. They were further filtered by Copper to obtain RQT beam qualities that simulate those used in Computed Tomography (CT). The spatial uniformity of a commercial CT dosimeter was then determined.

A necessidade de estimar a dose efetiva de radiação recebida pelos pacientes em exames de tomografia computadorizada (TC) é muito discutida atualmente devido ao fato de que este exame está entre aqueles que mais depositam dose em pacientes submetidos a exames radiológicos. O objetivo deste trabalho foi implementar um método de estimativa de dose efetiva em exames de TC baseado em evidências bem estabelecidas e em informações contidas nas imagens. Após dosimetria realizada no equipamento de TC, mapeamento dos protocolos e definição dos fatores de correção a serem utilizados, uma rotina de processamento dos dados foi criada para conversão da informação do cabeçalho DICOM da imagem radiológica em dose efetiva estimada, de modo que as informações levantadas poderão ser disponibilizadas, em registro no sistema de informação de radiologia do Hospital das Clínicas da Faculdade de Medicina de Ribeirão Preto da Universidade de São Paulo - HCFMRP/USP, aos médicos e profissionais de saúde envolvidos. / Computed tomography scan is the procedure that most deposits radiation dose in patients undergoing radiologic examinations. That is the reason why the need to record the dose received is currently discussed. The aim of this study was to implement an method to estimate effective dose received in CT scans based on well-established evidences and information contained in images. After performing dosimetry in CT equipment, mapped protocols and define correction factors, a data processing routine was created for conversion of DICOM header information in estimated effective dose, so that the information raised could be avalable in Hospital\'s radiology information system for doctors and health professionals involved.

A necessidade de estimar a dose efetiva de radiação recebida pelos pacientes em exames de tomografia computadorizada (TC) é muito discutida atualmente devido ao fato de que este exame está entre aqueles que mais depositam dose em pacientes submetidos a exames radiológicos. O objetivo deste trabalho foi implementar um método de estimativa de dose efetiva em exames de TC baseado em evidências bem estabelecidas e em informações contidas nas imagens. Após dosimetria realizada no equipamento de TC, mapeamento dos protocolos e definição dos fatores de correção a serem utilizados, uma rotina de processamento dos dados foi criada para conversão da informação do cabeçalho DICOM da imagem radiológica em dose efetiva estimada, de modo que as informações levantadas poderão ser disponibilizadas, em registro no sistema de informação de radiologia do Hospital das Clínicas da Faculdade de Medicina de Ribeirão Preto da Universidade de São Paulo - HCFMRP/USP, aos médicos e profissionais de saúde envolvidos. / Computed tomography scan is the procedure that most deposits radiation dose in patients undergoing radiologic examinations. That is the reason why the need to record the dose received is currently discussed. The aim of this study was to implement an method to estimate effective dose received in CT scans based on well-established evidences and information contained in images. After performing dosimetry in CT equipment, mapped protocols and define correction factors, a data processing routine was created for conversion of DICOM header information in estimated effective dose, so that the information raised could be avalable in Hospital\'s radiology information system for doctors and health professionals involved.

<p>Cone beam computed tomography (CBCT) is a 3D x-ray imaging technique in which the x-ray beam is transmitted to an object with wide beam geometry producing a 2D image per projection. Due to its faster image acquisition time, wide coverage length per scan, and fewer motion artifacts, the CBCT system is rapidly replacing the conventional CT system and becoming popular in diagnostic and therapeutic radiology. However, there are few studies performed in CBCT dosimetry because of the absence of a standard dosimetric protocol for CBCT. Computed tomography dose index (CTDI), a standardized metric in conventional CT dosimetry, or direct organ dose measurements have been limitedly used in the CBCT dosimetry.</p> <p>This dissertation investigated the CBCT dosimetry from the CTDI method to the organ, effective dose, risk estimations with physical measurements and Monte Carlo (MC) simulations.</p> <p>An On-Board Imager (OBI, Varian Medical Systems, Palo Alto, CA) was used to perform old and new CBCT scan protocols. The new CBCT protocols introduced both partial and full angle scan modes while the old CBCT protocols only used the full angle mode. A metal-oxide-semiconductor-field-effect transistor (MOSFET) and an ion chamber were employed to measure the cone beam CTDI (CTDI_CB) in CT phantoms and organ dose in a 5-year-old pediatric anthropomorphic phantom. Radiochromic film was also employed to measure the axial dose profiles. A point dose method was used in the CTDI estimation.</p> <p>The BEAMnrc/EGSnrc MC system was used to simulate the CBCT scans; the MC model of the OBI x-ray tube was built into the system and validated by measurements characterizing the cone beam quality in the aspects of the x-ray spectrum, half value layer (HVL) and dose profiles for both full-fan and half-fan modes. Using the validated MC model, CTDI_CB, dose profile integral (DPI), cone beam dose length product (DLP_CB), and organ doses were calculated with voxelized MC CT phantoms or anthropomorphic phantoms. Effective dose and radiation risks were estimated from the organ dose results.</p> <p>The CTDI_CB of the old protocols were found to be 84 and 45 mGy for standard dose, head and body protocols. The CTDI_CB of the new protocols were found to be 6.0, 3.2, 29.0, 25.4, 23.8, and 7.7 mGy for the standard dose head, low dose head, high quality head, pelvis, pelvis spotlight, and low dose thorax protocols respectively. The new scan protocols were found to be advantageous in reducing the patient dose while offering acceptable image quality.</p> <p>The mean effective dose (ED) was found to be 37.8 ±0.7 mSv for the standard head and 8.1±0.2 mSv for the low dose head protocols (old) in the 5-year-old phantom. The lifetime attributable risk (LAR) of cancer incidence ranged from 23 to 144 cases per 100,000 exposed persons for the standard-dose mode and from five to 31 cases per 100,000 exposed persons for the low-dose mode. The relative risk (RR) of cancer incidence ranged from 1.003 to 1.054 for the standard-dose mode and from 1.001 to 1.012 for the low-dose mode.</p> <p>The MC method successfully estimated the CTDI_CB, organ and effective dose despite the heavy calculation time. The point dose method was found to be capable of estimating the CBCT dose with reasonable accuracy in the clinical environment.</p> / Dissertation

Introduction: Despite the relatively low dose (0.5 mSv – 1 mSv) generated by Attenuation Correction CT (ACCT) in PET examinations, the ALARA principle is still applicable. The currently used ACCT standard protocol at the Karolinska University Hospital in Solna uses 7.6 effective mAs (mAseff) and 120 kVp, but reducing mAseff and/or kVp would decrease patient dose as well as facilitate an increased number of research subjects. A CT reconstruction algorithm called Quantification Achieved Consistently (Q.AC.) (Lonn, 2012) has recently been developed to enable reduced doses from ACCT, while preserving quantitative PET data. The purposes of this study were to investigate possible limitations of the Q.AC. with respect to patient size, and to optimise protocols, aiming at minimising ACCT dose in terms of Volumetric Computer Tomography Dose Index CTDIvol. Methods: Measurements were performed with a GE PET/CT Discovery system, which offers Q.AC. reconstruction. The NEMA NU-2 protocol was followed to quantify PET quality, including evaluations of relative count error in the artificial lung in the phantom centre (lung), hot- and cold-sphere contrast (Q), and background variability (N). Two phantoms were used; the NEMA body phantom (elliptical cross section sized 30 cm laterally and 23 cm anterior-posterior (AP)), here representing paediatric patients and small-sized adults, and the same phantom with an additional (20 cm laterally and 4 cm AP) ellipsoid plastic (PMMA) extension ring, representing mid- and large-sized patients. ACCTs were acquired with 15 mAseff values, range [2.3 - 260], in combination with four kVp values [80, 100, 120, 140] and reconstructed with two algorithms (Q.AC. and a regular soft CT algorithm). Consequently, PET reconstructions were performed based on each mAseff, kVp and CT-reconstruction combination. Results: Quantitatively similar PET results to the standard protocol were achieved with the Q.AC. CT reconstruction algorithm using a CTDIvol = 0.06 mGy (2.3 mAseff and 80 kVp) for the NEMA body phantom, respectively a CTDIvol = 0.20 mGy (2.3 mAseff and 120 kVp) for the phantom with additional extension ring. Conclusions: This study indicates that the Q.AC. CT reconstruction algorithm enables accurate PET results at lower ACCT mAseff and kVp settings than the currently used clinical standard protocol. For paediatric patients and small-sized adults, a reduction of CTDIvol by approximately 90% may be achieved, while for mid- and large-sized patients, the CTDIvol can be reduced by approximately 70% without loss of quantitative PET data.

Diagnostic X-rays are the most important man-made source of radiation exposure for the general population, contributing about 14% of the total annual exposure worldwide from all sources. However, although diagnostic X-rays provide great benefits, it is generally accepted that their use involves some small risk of developing cancer. The risk to an individual is probably small because radiation doses are usually low, but the large number of people exposed annually means that even small individual risks could translate into a considerable number of cancer cases. The aim of this work was to reestimate this risk on the basis of the annual number of CT examinations undertaken in the Czech Republic and typical doses applied. But there wasn't possible to find necessary data for planned reckonings by return, because their archiving isn't obligatory. This study, therefore, only describes common incidence of ionising radiation with human organism and further, all inevitable mathematically-physical equations needed for realization of primary planned investigation, with all necessary datal inputs included. In findings of this work there are drafted and demonstrated procedures, how to obtain and cultivate needed data ,so it was possible to compare it with technical literature.