Optimizing computer tomography examinations by using anthropomorphic phantoms and MOSFET dosimeters

The number of computed tomography (CT) examinations has increased in recent years due to developments in scanner technology and the increased diagnostic capabilities of CT. Nowadays, CT has become a major contributor to accumulated radiation doses from radiological examinations, accounting for approximately 60% of the overall medical radiation dose in Western countries. Ionizing radiation is generally considered harmful to health, and current knowledge suggests that the risk for stochastic effects increases linearly with radiation dose. Minimizing patient doses in CT requires effective optimization practices, including both technical and clinical approaches. CT optimization aims to reduce patients exposure to radiation without compromising image quality for diagnosis. The aim of this dissertation was to explore the feasibility of using anthropomorphic phantoms and metal-oxide-semiconductor field-effect transistors (MOSFETs) in CT optimization and patient dose measurements, and to study CT optimization in versatile clinical situations. Specifically, this thesis focused on studying the effects of patient centering on the CT scanner isocenter by determining changes in patient dose and image quality. Additionally, as a part of this thesis, we constructed and optimized ultralow-dose CT protocols for craniosynostosis imaging, and explored different optimization methods for reducing radiation exposure to eye lenses. Moreover, fetal radiation doses were assessed in the most typical CT examinations of a pregnant woman which also place the fetus at the highest risk for ionizing radiation-induced health detriments. Anthropomorphic phantoms and MOSFET dosimeters proved feasible in CT optimization even with the use of ultralow-dose levels. Patient vertical off-centering posed a common and serious problem in chest CT, as a majority of the scanned patients were positioned below the isocenter of the CT scanner, which significantly affected both radiation dose and image quality. This exposes the radiosensitive anterior surface tissues, including the breasts and thyroid gland, to greater risk. Special attention should focus on pediatric patients in particular, as they were typically miscentered lower than adults were. The use of constructed ultralow-dose CT protocols with model-based iterative reconstruction can enable craniosynostosis CT imaging with sufficient image quality for diagnosis with an effective dose of less than 20 μSv for the patient. This dose level was approximately 85% lower than the level used in routine CT protocols in the hospital for craniosynostosis, and was comparable to the radiation exposure of a plain-skull radiography examination. The most efficient method for reducing the dose to the eye lens proved to be gantry tilting, which leaves the eye lenses outside the primary radiation beam, thereby reducing the absorbed dose up to 75%. However, measurements with two different anthropomorphic head phantoms showed that patient geometry significantly affects dose-reduction capabilities. If lenses can only partially be cropped outside the primary beam, organ-based tube current modulation or bismuth shields may also be used for reducing the dose to the lenses. Based on the measured absorbed doses in this thesis, the radiation dose to the fetus poses no obstacle to an optimized CT examination with a medically necessary indication. The volumetric CT dose index (CTDIvol) provides a rough estimate of the fetal dose when the uterus is in the primary radiation beam, although the extent of the scan range has a substantial effect on the fetal dose. The results support the conception that when the fetus or uterus is not in the scan range, the fetal dose is affected mainly by the distance from the scan range.Tietokonetomografiatutkimusten (TT) määrä on kasvanut laitekehityksen sekä TT:n lisääntyneiden diagnostisten sovelluskohteiden ansiosta viime vuosien aikana huomattavasti. Siitä on nykyisellään tullut länsimaissa radiologisista menetelmistä eniten kollektiivista sädeannosta kerryttävä menetelmä noin 60 %:n osuudella kaikkien lääketieteellisten röntgentutkimusten aiheuttamasta yhteisestä kokonaisannoksesta. Ionisoivaa säteilyä pidetään yleisesti ottaen terveydelle haitallisena, ja nykytietämyksen mukaan säteilyn tilastollisten haittavaikutusten riski kasvaa lineaarisesti säteilyannoksen kasvaessa. Jotta potilaiden saamaa säteilyaltistusta voitaisiin TT:ssä vähentää, on tehokkaiden optimointimenetelmien, niin teknisten kuin myös kliinisten, käyttö tarpeen. TT-optimoinnin tarkoituksena on vähentää potilaiden saamia säteilyannoksia ilman että diagnostinen kuvanlaatu oleellisesti kärsii. Tämän työn tarkoituksena oli tutkia ihmisenkaltaisten potilasvasteiden (l. antropomorfisten fantomien) ja puolijohdetekniikkaan perustuvien MOSFET-dosimetrien soveltuvuutta TT-optimointiin sekä tutkia TT-optimointia useissa kliinisissä sovelluksissa. Työssä tutkittiin erityisesti potilaan vertikaalisuunnan keskittämisen vaikutuksia potilasannosten sekä kuvanlaadun osalta. Lisäksi tämän väitöskirjan osana luotiin kraniosynostoosipotilaiden kuvantamista varten erittäin matalaa annostasoa hyödyntävät TT-protokollat sekä tutkittiin erilaisten optimointimenetelmien käyttöä silmän linssien säteilyaltistuksen pienentämiseksi. Työssä määritettiin myös sikiön saamia säteilyannoksia yleisimmissä TT-tutkimuksissa, joita raskaana olevalle naiselle mahdollisesti joudutaan tekemään, ja jotka aiheuttavat sikiölle merkittävimmän ionisoivasta säteilystä peräisin olevan terveysriskin. Antropomorfiset fantomit ja MOSFET-dosimetrit osoittautuivat TT-tutkimusten optimointiin soveltuviksi jopa erittäin matalilla annostasoilla. Potilaan vertikaalinen keskitysvirhe havaittiin olevan vakava ja yleinen ongelma keuhkojen TT-tutkimuksissa, sillä suurin osa kliinisistä potilaista keskitettiin TT-laitteen isosentriin nähden liian alas, vaikuttaen huomattavasti sekä säteilyannoksiin että kuvanlaatuun. Tämä altistaa erityisesti säteilyherkät anterioriset pintakudokset, kuten rinnat ja kilpirauhasen, suuremmalle riskille. Erityisesti lasten kohdalla huolelliseen keskittämiseen tulisi kiinnittää huomiota, sillä keskitysvirhe oli lapsipotilailla aikuisia suurempi. Kraniosynostoosipotilaiden TT-tutkimus voitiin tehdä työssä kehitetyllä mallipohjaista iteratiivista rekonstruktiota hyödyntävällä erittäin matalan annostason omaavalla TT-protokollalla jopa alle 20 μSv efektiivisellä annoksella potilaalle ilman että diagnostiikkaan tarvittava kuvanlaatu oleellisesti kärsi. Tämä oli noin 85 % vähemmän kuin sairaalassa rutiinisti käytettävä TT-protokolla kraniosynostoosipotilaiden kuvaukseen tuottaa, vastaten samalla myös tavallisen kalloröntgenkuvan tuottamaa annostasoa. TT-gantryn kippaus siten, että silmän linssit jäävät primäärisäteilykeilan ulkopuolelle, osoittautui tehokkaimmaksi menetelmäksi pienennettäessä silmän linssien annostasoa tavallisissa pään TT-tutkimuksissa. Näin saavutettiin jopa 75 %:n annossäästö verrattuna protokollaan, jossa ei käytetty erillisiä optimointimenetelmiä. Mittaukset kahdella pääfantomilla kuitenkin osoittivat pään geometrian vaikuttavan huomattavasti annosoptimointiin. Kuvauksissa, joissa silmän linssit voidaan jättää vain osittain primäärikeilan ulkopuolelle, voidaan käyttää silmän linssien suojaamiseen myös joko elinkohtaista putkivirran modulaatiota tai vismuttisuojia. Sikiön saamat säteilyannokset eivät ole tässä työssä määritettyjen absorboituneiden annosten perusteella este optimoidulle TT-tutkimukselle lääketieteellisen indikaation niin vaatiessa. TT-annosten tilavuuskeskiarvoa (CTDIvol) voidaan pitää sikiöannokselle karkeana arviona kohdun ollessa primäärisäteilykeilassa, joskin kuvausalueen laajuudella on huomattava vaikutus sikiön saamaan säteilyannokseen. Saadut tulokset tukevat myös käsitystä, että sikiön tai kohdun ollessa kuvausalueen ulkopuolella, sikiöannos riippuu pääosin sikiön etäisyydestä kuvausalueelta

The impact of vertical off-centering on image noise and breast dose in chest CT with organ-based tube current modulation : A phantom study

Publisher Copyright: © 2022 Associazione Italiana di Fisica Medica e SanitariaPurpose: To determine the effects of patient vertical off-centering when using organ-based tube current modulation (OBTCM) in chest computed tomography (CT) with focus on breast dose. Materials and methods: An anthropomorphic adult female phantom with two different breast attachment sizes was scanned on GE Revolution EVO and Siemens Definition Edge CT systems using clinical chest CT protocols and anterior-to-posterior scouts. Scans with and without OBTCM were performed at different table heights (GE: centered, ±6 cm, and ± 3 cm; Siemens: centered, −6 cm, and ± 3 cm). The dose effects were studied with metal-oxidesemiconductor field-effect transistor dosimeters with complementary Monte Carlo simulations to determine full dose maps. Changes in image noise were studied using standard deviations of subtraction images from repeated acquisitions without dosimeters. Results: Patient off-centering affected both the behavior of the normal tube current modulation as well as the extent of the OBTCM. Generally, both OBTCM techniques provided a substantial decrease in the breast doses (up to 30% local decrease). Lateral breast regions may, however, in some cases receive higher doses when OBTCM is enabled. This effect becomes more prominent when the patient is centered too low in the CT gantry. Changes in noise roughly followed the expected inverse of the change in dose. Conclusions: Patient off-centering was shown to affect the outcome of OBTCM in chest CT examination, and on some occasions, resulting in higher exposure. The use of modern dose optimization tools such as OBTCM emphasizes the importance of proper centering when preparing patients to CT scans.Peer reviewe

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

Metal artifacts in intraoperative O-arm CBCT scans

Background Cone-beam computed tomography (CBCT) has become an increasingly important medical imaging modality in orthopedic operating rooms. Metal implants and related image artifacts create challenges for image quality optimization in CBCT. The purpose of this study was to develop a robust and quantitative method for the comprehensive determination of metal artifacts in novel CBCT applications. Methods The image quality of an O-arm CBCT device was assessed with an anthropomorphic pelvis phantom in the presence of metal implants. Three different kilovoltage and two different exposure settings were used to scan the phantom both with and without the presence of metal rods. Results The amount of metal artifact was related to the applied CBCT imaging protocol parameters. The size of the artifact was moderate with all imaging settings. The highest applied kilovoltage and exposure level distinctly increased artifact severity. Conclusions The developed method offers a practical and robust way to quantify metal artifacts in CBCT. Changes in imaging parameters may have nonlinear effects on image quality which are not anticipated based on physics.Peer reviewe

Ejection fraction in myocardial perfusion imaging assessed with a dynamic phantom : comparison between IQ-SPECT and LEHR

BACKGROUND: Developments in single photon emission tomography instrumentation and reconstruction methods present a potential for decreasing acquisition times. One of such recent options for myocardial perfusion imaging (MPI) is IQ-SPECT. This study was motivated by the inconsistency in the reported ejection fraction (EF) and left ventricular (LV) volume results between IQ-SPECT and more conventional low-energy high-resolution (LEHR) collimation protocols. IQ-SPECT and LEHR quantitative results were compared while the equivalent number of iterations (EI) was varied. The end-diastolic (EDV) and end-systolic volumes (ESV) and the derived EF values were investigated. A dynamic heart phantom was used to produce repeatable ESVs, EDVs and EFs. Phantom performance was verified by comparing the set EF values to those measured from a gated multi-slice X-ray computed tomography (CT) scan (EF(True)). The phantom with an EF setting of 45, 55, 65 and 70% was imaged with both IQ-SPECT and LEHR protocols. The data were reconstructed with different EI, and two commonly used clinical myocardium delineation software were used to evaluate the LV volumes. RESULTS: The CT verification showed that the phantom EF settings were repeatable and accurate with the EF(True) being within 1% point from the manufacture’s nominal value. Depending on EI both MPI protocols can be made to produce correct EF estimates, but IQ-SPECT protocol produced on average 41 and 42% smaller EDV and ESV when compared to the phantom’s volumes, while LEHR protocol underestimated volumes by 24 and 21%, respectively. The volume results were largely similar between the delineation methods used. CONCLUSIONS: The reconstruction parameters can greatly affect the volume estimates obtained from perfusion studies. IQ-SPECT produces systematically smaller LV volumes than the conventional LEHR MPI protocol. The volume estimates are also software dependent.Peer reviewe

Dental cone beam CT : An updated review

Cone beam computed tomography (CBCT) is a diverse 3D x-ray imaging technique that has gained significant popularity in dental radiology in the last two decades. CBCT overcomes the limitations of traditional twodimensional dental imaging and enables accurate depiction of multiplanar details of maxillofacial bony structures and surrounding soft tissues. In this review article, we provide an updated status on dental CBCT imaging and summarise the technical features of currently used CBCT scanner models, extending to recent developments in scanner technology, clinical aspects, and regulatory perspectives on dose optimisation, dosimetry, and diagnostic reference levels. We also consider the outlook of potential techniques along with issues that should be resolved in providing clinically more effective CBCT examinations that are optimised for the benefit of the patient.Peer reviewe

Myocardial tissue characterization in patients with hereditary gelsolin (AGel) amyloidosis using novel cardiovascular magnetic resonance techniques

Gelsolin (AGel) amyloidosis is a hereditary condition with common neurological effects. Myocardial involvement, especially strain, T1, or extracellular volume (ECV), in this disease has not been investigated before. Local myocardial effects and possible amyloid accumulation were the targets of interest in this study. Fifty patients with AGel amyloidosis were enrolled in the study. All patients underwent cardiovascular magnetic resonance imaging, including cine imaging, T1 mapping, tagging, and late gadolinium enhancement (LGE) imaging at 1.5 T. Results for volumetry, myocardial feature-tracking strain, rotation, torsion, native T1, ECV, and LGE were investigated. The population mean native T1 values in different segments of the left ventricle (LV) varied between 1003 and 1080 ms. Myocardial mean T1 was 1031 ± 37 ms. T1 was highest in the basal plane of the LV (1055 ± 40 ms), similarly to ECV (30.0% ± 4.4%). ECV correlated with native T1 in all LV segments (p Peer reviewe

Automated daily quality control analysis for mammography in a multi-unit imaging center

Background: The high requirements for mammography image quality necessitate a systematic quality assurance process. Digital imaging allows automation of the image quality analysis, which can potentially improve repeatability and objectivity compared to a visual evaluation made by the users. Purpose: To develop an automatic image quality analysis software for daily mammography quality control in a multi-unit imaging center. Material and Methods: An automated image quality analysis software using the discrete wavelet transform and multi-resolution analysis was developed for the American College of Radiology accreditation phantom. The software was validated by analyzing 60 randomly selected phantom images from six mammography systems and 20 phantom images with different dose levels from one mammography system. The results were compared to a visual analysis made by four reviewers. Additionally, long-term image quality trends of a full-field digital mammography system and a computed radiography mammography system were investigated. Results: The automated software produced feature detection levels comparable to visual analysis. The agreement was good in the case of fibers, while the software detected somewhat more microcalcifications and characteristic masses. Long-term follow-up via a quality assurance web portal demonstrated the feasibility of using the software for monitoring the performance of mammography systems in a multi-unit imaging center. Conclusion: Automated image quality analysis enables monitoring the performance of digital mammography systems in an efficient, centralized manner.Peer reviewe

Radiation Doses to Staff in a Hybrid Operating Room : An Anthropomorphic Phantom Study with Active Electronic Dosimeters

Objective: To quantify the effects of different imaging settings on radiation exposure to the operator and surgical team in a hybrid operating room (OR). Methods: Measurements to determine scatter radiation in different imaging and geometry settings using an anthropomorphic phantom were performed in a hybrid OR equipped with a robotic C arm interventional angiography system (Artis Zeego; Siemens Healthcare, Erlangen, Germany). The radiation dose (RD) was measured with seven calibrated Philips DoseAware active electronic dosimeters and a Raysafe Xi survey detector, which were placed at different locations in the hybrid OR. The evaluated set ups included low dose, medium dose, and high dose fluoroscopy for abdomen; fluoroscopy fade; roadmap; and digital subtraction angiography (DSA), all using 20 s exposures. The effect of magnification, tube angulation, field size, source to skin distance, and RADPAD protection shields were assessed. Finally RD during cone beam computed tomography (CBCT) was obtained. Results: In the operator position the initial settings with low dose fluoroscopy caused a RD of 1.03 mu Gy. The use of fluorofade did not increase the radiation dose (1.02 mu Gy), whereas the roadmap increased it threefold (2.84 mu Gy). The RD with "normal fluoro" was 4.13 mu Gy and increased to 6.44 mu Gy when high dose fluoroscopy mode was used. Magnification or field size varying from 42 cm to 11 cm led the RD to change from 0.86 mu Gy to 2.10 mu Gy. Decreasing the field of view to 25% of the initial size halved the RD (0.48 mu Gy). The RDs for the left anterior oblique 30 degrees and right anterior oblique 30 degrees were 3.26 mu Gy and 1.63 mu Gy, respectively. DSA increased the cumulative dose 33 fold but the RADPAD shield decreased the DSA RD to 4.92 mu Gy. The RD for CBCT was 47.2 mu Gy. Conclusion: Radiation exposure to operator and personnel can be significantly reduced during hybrid procedures with proper radiation protection and dose optimisation. A set of six behavioural rules were established.Peer reviewe

Medieval Wood Sculpture of an Unknown Saint from Nousiainen : from Materials to Meaning

This article takes up from an interdisciplinary perspective the issue of placing relics inside wooden sculptures during the Middle Ages in Finland. In the focus of the study is a late thirteenth-century sculpture depicting a standing saint from the memorial church of Saint Henrik in Nousiainen (Nousis), South-Western Finland, now in the collection of the National Museum of Finland. The identity and even the gender of the “Unknown Saint’ has puzzled art historians, most recently in the 1960s. On the basis of a new visual and technical examination as well as iconographical and stylistic analysis of the sculpture, alternative interpretations of the identity of the depicted saint as well as of the origin and function of the sculpture are suggested. The current article also includes a detailed description of the dual-energy and ultrahigh-resolution CT-scanning of the sculpture at the HUS Medical Imaging Center at the Helsinki University Hospital. This is the first time that CT-scanning has been used to research medieval wooden sculpture in Finland. Discoveries made during the inspection were interpreted as possible relics. Consequently, the function of the sculpture is further discussed in the context of the cult of relics in Finland.This article takes up from an interdisciplinary perspective the issue of placing relics inside wooden sculptures during the Middle Ages in Finland. In the focus of the study is a late thirteenth-century sculpture depicting a standing saint from the memorial church of Saint Henrik in Nousiainen (Nousis), South-Western Finland, now in the collection of the National Museum of Finland. The identity and even the gender of the “Unknown Saint” has puzzled art historians, most recently in the 1960s. On the basis of a new visual and technical examination as well as iconographical and stylistic analysis of the sculpture, alternative interpretations of the identity of the depicted saint as well as of the origin and function of the sculpture are suggested. The current article also includes a detailed description of the dual-energy and ultrahigh-resolution CT-scanning of the sculpture at the HUS Medical Imaging Center at the Helsinki University Hospital. This is the first time that CT-scanning has been used to research medieval wooden sculpture in Finland. Discoveries made during the inspection were interpreted as possible relics. Consequently, the function of the sculpture is further discussed in the context of the cult of relics in Finland.Peer reviewe