UNIVERSIDADE ESTADUAL DE CAMPINAS FACULDADE DE ODONTOLOGIA DE PIRACICABA
DANIELI MOURA BRASIL
OTIMIZAÇÃO DA QUALIDADE DE IMAGEM DE UM APARELHO
DE TOMOGRAFIA COMPUTADORIZADA ODONTOLÓGICA COM
DETECTOR VERTICAL ESTREITO
Piracicaba 2019
OTIMIZAÇÃO DA QUALIDADE DE IMAGEM DE UM APARELHO DE
TOMOGRAFIA COMPUTADORIZADA ODONTOLÓGICA COM
DETECTOR ESTREITO
Tese apresentada à Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do título de Doutora em Radiologia Odontológica, na Área de Radiologia Odontológica
Orientador: Prof. Dr. Francisco Haiter Neto
ESTE TRABALHO CORRESPONDE À VERSÃO FINAL DA TESE DEFENDIDA PELA ALUNA DANIELI MOURA BRASIL E ORIENTADA PELO PROF. DR. FRANCISCO HAITER NETO
Piracicaba 2019
Identificação e informações acadêmicas e profissionais da aluna:
Àqueles que são minha origem, meu porto seguro, meu aconchego e meus maiores incentivadores,
meus pais, Daniel e Eliane, e irmãos Daniel e Davyd.
nascer e pôr do sol de cada dia, pelas pessoas que me rodeiam e pelos desafios diários que me permitem crescer.
Aos meus pais, Daniel Martins Brasil e Eliane Amaro de Moura Brasil, por estarem sempre presentes, por terem me cuidado e me dado o exemplo mais verdadeiro de humanidade e compaixão, de respeito, e acima de tudo, por terem dado a nós três um lar cheio de amor, amor que não se acaba. Por compartilharem das minhas alegrias e das minhas lutas. Por estarem muito perto, mesmo estando longe. Por acreditarem nos meus sonhos, por me incentivarem a voar.
Aos meus irmãos, Daniel Moura Brasil e Davyd Moura Brasil, por saber que em vocês eu tenho o maior amor do mundo, pelo carinho e cuidado sempre presentes, por serem essenciais na minha vida. E a minha cunhada Marina Pessoa Bezerra Brasil pela cumplicidade, carinho e suporte, por ser família.
Às minhas avós, Rocilda Martins Brasil e Maria José Amaro de Moura, por serem fonte de vida e de amor, pela boa educação dada aos filhos e transmitida aos netos; e aos meus tios e
primos, levo vocês no meu coração sempre.
Às minhas tias Sandra, Cláudia e Célia por estarem sempre unidas, sendo família, sendo meus exemplos de mulheres fortes, guerreiras e amorosas.
AGRADEÇO TAMBÉM
O presente trabalho foi realizado com apoio da Coordenação de Aperfeiçoamento de
Pessoal de Nível Superior – Brasil (CAPES) – Código de Financiamento 001.
À Faculdade de Odontologia de Piracicaba, na pessoa do Prof. Dr. Francisco Haiter Neto e do Prof. Dr. Guilherme Elias Pessanha Henriques, pela oportunidade de estudar numa escola que oferece excelente suporte estrutural, profissionais altamente capacitados e oportunidade de realizar pesquisas de alto impacto científico.
Ao Prof. Dr. Frab Norberto Bóscolo, pelos ensinamentos repassados a nós com entusiasmo e destreza. Obrigada também pelo afeto e simpatia dedicados aos seus alunos.
À Profa. Dra. Deborah Queiroz de Freitas, obrigada pela dedicação para tornar este curso cada vez melhor, pelos ensinamentos, pelo sorriso largo e brilho no olhar, por ser sempre solícita e por me mostrar um exemplo de profissional que eu quero seguir.
Ao Prof. Dr. Matheus Lima de Oliveira, pelo exemplo que você é para mim, um excelente professor e também amigo, pelo zelo e dedicação que você aplica em tudo que faz, por partilhar conosco o que você tem de melhor e por querer também que a gente busque o nosso melhor.
Aos professores membros da banca de defesa, Prof. Dr. Francisco Haiter Neto, Profa.
Dra. Deborah Queiroz de Freitas, Prof. Dr. Christiano de Oliveira Santos, Profa. Dra. Alynne Vieira de Menezes e Profa. Dra. Maria Augusta Portella Guedes Visconti, e aos professores
suplentes Prof. Dr. Yuri Nejaim, Prof. Dr. Amaro Ilídio Vespasiano e Profa. Dra. Monikelly do
Carmo Chagas do Nascimento que prontamente aceitaram o convite e se dispuseram a participar
desse momento e a contribuir para o aperfeiçoar esse trabalho.
À Profa. Dra. Reinhilde Jacobs pela oportunidade de trabalhar no grupo OMFS-IMPATH (KU Leuven), viabilizando assim o desenvolvimento desse estudo, e pela atenção e incentivo.
Ao colaborador e amigo Dr. Ruben Pauwels pela solicitude, incentivo, suporte e por compartilhar as alegrias a cada pequena conquista que envolveu esse estudo.
Ao Dr. Wim Coucke pela execução da análise estatística e por todo auxílio prestado na interpretação e escrita dos resultados.
Agradeço também aos amigos Jardel Chaves, Patrícia Milagros e Danilo Schneider por terem sido meus grandes parceiros e por toda a experiência vivida em Leuven/Bélgica, no ano de 2017. E aos companheiros de laboratório Mariana Quirino, Rogério Caldas, Bennaree Awarun,
Daniel Vasconcelos, Joeri Meyns, Dorra Chaabouni, Myrthel Vranckx, Emad Albdour, Maria Ignacia, Irem Ayaz, Yan Huang, Laura Nicolielo, Jeroen Van Dessel e Gabriela Casteels pela
convivência fraterna. Com certeza esse foi um tempo de grande crescimento em minha vida profissional e pessoal.
Aos meus queridos professores Lucio Mitsuo Kurita e Alynne Vieira de Menezes, por, lá no começo, terem me mostrado possibilidades além das quais eu conseguia ver e por terem me incentivado e apoiado a seguir esse caminho que me trouxe até onde estou hoje. Tenho muito orgulho de ter sido aluna de vocês.
AGRADEÇO AINDA
Ao querido Gustavo Santaella, porque ter você ao meu lado me faz bem, e me faz feliz. Pela parceria, cumplicidade e cuidado sempre presentes. Por ser um dos grandes incentivadores dos meus planos, por compartilhar comigo seus sonhos e por topar ir junto nos meus também, seja perto ou seja longe.
À amiga e parceira Gina, você é uma irmã que a vida me deu. Obrigada por me colocar em desafios e assim me mostrar que posso ser e ir além. Por ser um exemplo de amiga e de profissional. Você está no meu coração.
Aos amigos Yuri Nejaim e Amaro Ilídio, pela parceria, pela compreensão, por encontrar em vocês carinho e proteção, porque juntos compartilhamos o que temos de melhor, por serem exemplos para mim, por serem meus irmãos.
quem conto sem hesitar, pois sei que está comigo.
Ao amigo Hugo Gaêta, pela forma simples e leve como você vê a vida e me inspira a ver também, pelo sorriso contagiante e pela lealdade que você dedica aos seus companheiros, pela parceria e pelo compromisso que você tem com o que se propõe a fazer.
À amiga Querida Amanda Candemil, pela amizade sincera, pela verdade presente no teu olhar, pelo compartilhar da cozinha, do alimento e do carinho, por tornar os meus dias mais leves e alegres. Por dividir comigo não só um espaço, mas também sentimentos, experiências, vinhos e sorrisos.
À amiga Anne Oenning, pelo exemplo que você sempre foi para mim, pelo carinho, por saber que em você tenho uma Amiga, por se fazer tão presente. Obrigada também por ser exemplo de coragem, responsabilidade e competência. Ter você por perto me inspira!
À amiga Karla Vasconcelos, por me fazer querer ser alguém melhor e por ser exemplo de humanidade e amor para todos nós.
À amiga Luciana Jacome, por todos os momentos que compartilhamos juntas, pelo apoio, força e incentivo que você sempre me deu e por cuidar de mim como uma irmã.
À amiga Eliana Dantas, por dividir comigo suas experiências e me fazer aprender mais sobre as várias nuances da vida e das pessoas que nos rodeiam, e pela dedicação e persistência com as quais você realiza seus trabalhos. Você me inspira a ser valente e ao mesmo tempo doce, pequena por fora, mas gigante por dentro. Você é uma amiga muito especial para mim.
Ao amigo Leonardo Peroni, pela simpatia e bom humor, por se importar com o bem-estar de todos e por estar sempre disposto a ajudar.
À amiga Mayra Cristina Yamasaki, por todo suporte que me deu desde que eu cheguei a Piracicaba, por dividir comigo um pouco da sua família e pelo respeito que sempre existiu entre nós. Serei sempre grata a vocês pela acolhida e pelo carinho que me deram.
Aos amigos Thiago de Oliveira Gamba, Débora Távora, Priscila Peyneau, Fran Verner,
Priscila Lopes, Liana Ferreira e Ana Caroline Brito porque vocês fazem parte da minha história
e são exemplos para mim.
Aos amigos de pós-graduação, Gustavo Nascimento, Luciano Cano, Neiandro Galvão,
Carlos Augusto, Victor de Aquino, Rocharles Fontenele, Nicolly Silva, Larissa Moreira, Larissa Lagos, Mariane Michels, Carolina Valadares, Wilson Cral, Amanda Farias, Daniela Madlum, Maria Clara Pinheiro, Fernanda Coelho, Lucas Lopes e Daniele Caldas, por fazem a diferença
Aos funcionários da Clínica de Radiologia Odontológica, Sarah, Wal e Fer, pelo bom convívio, pelo carinho, por serem sempre prestativos e pelo respeito com que tratam os demais. Obrigada também pelo zelo com que fazem seu trabalho, tornando o ambiente na clínica funcional, organizado e agradável.
À querida Luciane Sattolo, pela dedicação em tudo que se propõe a fazer, por ser um exemplo de profissional, pela simpatia e por estar sempre disposta a ajudar. Muito obrigada!
AGRADECIMENTOS ESPECIAIS
Ao meu orientador, Prof. Dr. Francisco Haiter Neto, agradeço pela confiança, pelo tempo dedicado a discussão e aprimoramento desse trabalho, por me mostrar os possíveis caminhos e me fazer sentir segura para tomar decisões e seguir a diante, pelo apoio e pelo direcionamento nos momentos de incertezas da vida, por encontrar em você mais que um orientador, mas também uma figura de pai e de amigo.
À Profa. Dra. Solange Maria de Almeida Bóscolo, por apoiar meus planos ousados, pelo carinho e pelas conversas, pela segurança transmitida e pelo zelo e atenção com os quais você sempre tratou minhas questões. Foi um prazer ser sua orientanda.
odontológica devem ser ajustados e otimizados de acordo com a finalidade clínica e com o paciente, mantendo a dose de radiação tão baixa quanto diagnosticamente aceitável. O objetivo deste estudo foi determinar a kV ideal para um aparelho de tomografia computadorizada com detector estreito, para pacientes adultos e infantis. Avaliações clínicas e quantitativas da qualidade da imagem foram realizadas usando um fantoma compatível com homem adulto e dois fantomas antropomórficos compatíveis com crianças de 5 e 10 anos de idade. As avaliações técnicas de qualidade de imagem foram realizadas com um fantoma de polimetilmetacrilato (SEDENTEXCT-IQ). As imagens foram obtidas utilizando o tomógrafo PaX-i3D Green Premium (Vatech, Hwaseong, Coreia do Sul), aplicando um campo de visão (FOV) grande, de 21x19 cm, e um médio de 12x9 cm. Protocolos de alta dose (HD – com kV variando de 85 a 110) e de baixa dose (LD – com kV variando de 75 a 95) foram usados para o fantoma correspondente a um adulto, enquanto apenas os protocolos com kV variando de 85 a 110 foram usados para os fantomas infantis. A dose de radiação dentro de cada grupo foi fixada adaptando a miliamperagem de acordo com uma estimativa de produto dose-área predeterminado para cada protocolo. Para avaliação clínica, as imagens foram analisadas considerando a qualidade geral da imagem, nitidez, contraste, artefatos e ruído, usando uma escala de 4 pontos. Para avaliação quantitativa foram calculados o desvio médio do valor cinza, a porcentagem do aumento do desvio padrão, o percentual de endurecimento do feixe e a relação contraste-ruído (CNR). Para avaliação técnica, foram mensurados a acurácia da segmentação, CNR e nitidez; e ainda, o desvio padrão normalizado de artefatos metálicos e a área de objetos metálicos, apenas para o fantoma correspondente a um adulto. Com base em gráficos biplot, os parâmetros mais representativos foram escolhidos, para cada avaliação, para determinar a kV ideal. No geral, para adultos, os valores de kV dentro do mesmo grupo mostraram qualidade semelhante (p> 0,05), exceto para 110 kV no protocolo 21x19 cm HD e para 85 kV no protocolo 12x9 cm HD, que apresentaram pior qualidade, considerando o aspecto clínico; além disso as kVs 85 e 90 no protocolo 21x19 cm HD, e kVs 75 e 80 no protocolo 21x19 cm LD foram piores (p < 0,05) no aspecto quantitativo. Para os fantomas pediátricos, clinicamente, os valores de kV apresentaram qualidade semelhante (p > 0,05), enquanto no aspecto quantitativo, o kV 85 apresentou qualidade pior (p < 0,05), exceto no protocolo 12x9 cm do fantoma de 5 anos. Tecnicamente, os valores de 85 e 110 kV no FOV 21x19 cm apresentaram pior qualidade. Conclui-se que, numa dose de
Palavras-chave: Odontologia pediátrica; Otimização; Qualidade de imagem; Tomografia computadorizada de feixe cônico
(CT) according to the clinical purpose of the scan and the patient, keeping radiation dose as low as diagnostically acceptable. The aim of this study was to determine the optimised kV setting for a narrow detector computed tomography unit for adult and paediatric patients. Clinical and quantitative evaluations of image quality were performed using a consistent with adult and a 5- and 10-year-old anthropomorphic phantoms. Technical evaluations were performed with a polymethyl methacrylate (SEDENTEXCT-IQ) phantom. Images were obtained using the PaX-i3D Green Premium CT device, applying a large 21x19 cm and a medium 12x9 cm field of view (FOV). High-dose (HD - kV ranging from 85 to 110) and low-High-dose (LD - kV ranging from 75 to 95) protocols were used for adult patients, while protocols with kV ranging from 85 to 110 were used for paediatric patients. The radiation dose within each group was fixed by adapting the mA for each protocol according to a predetermined dose-area product estimate. For clinical evaluation, three observers assessed images based on overall image quality, sharpness, contrast, artefacts and noise, using a 4-point scale. For quantitative evaluation, mean grey value shift, percentage of increase of standard deviation, percentage of beam-hardening and contrast-to-noise ratio (CNR) were calculated. For technical evaluation, segmentation accuracy, CNR and sharpness were measured, besides normalized metal artefact standard deviation and metal object area for adults. Based on biplot graphs, representative parameters were chosen for each evaluation to determine the optimal kV. Overall, for adult patients, kV values within the same group showed similar quality (p > 0.05), except for 110 kV in 21x19 cm HD and 85 kV in 12x9 cm HD of clinical score which were worse; also 85, 90 kV in 21x19 cm HD and 75, 80 kV in 21x19 cm LD of quantitative score were worse (p < 0.05). While, for paediatric patients, clinically, kV values showed similar quality (p > 0.05), for the quantitative aspect, 85 kV showed worst quality (p < 0.05), except in 12x9 cm 5 y-o. Technically, 85 and 110 kV in the large FOV showed worst quality. In conclusion, at a constant radiation dose, for a narrow-detector computed tomography unit, low and high kV protocols can yield an acceptable image quality, while for paediatric indications 95 kV or higher are optimal. Key-words: Computed-assisted image analysis; Cone-beam computed tomography; Image Quality; Phantoms, Imaging; Optimization; Paediatric dentistry
1 INTRODUÇÃO ... 14
2 ARTIGOS ... 17
2.1. ARTIGO: IMAGE QUALITY OPTIMIZATION USING A NARROW VERTICAL DETECTOR DENTAL CONE-BEAM CT 17 2.2. ARTIGO: IMAGE QUALITY OPTIMIZATION OF A NARROW DETECTOR DENTAL COMPUTED TOMOGRAPHY FOR PAEDIATRIC PATIENTS ... 36
3 DISCUSSÃO ... 56
4 CONCLUSÃO ... 60
5 REFERÊNCIAS* ... 61
APÊNDICE 1 – METODOLOGIA ESTENDIDA ... 64
ANEXOS ... 79
ANEXO 1 – RELATÓRIO DE VERIFICAÇÃO DE ORIGINALIDADE E PREVENÇÃO DE PLÁGIO ... 79
ANEXO 2 - DOCUMENTO DE ACEITE PARA PUBLICAÇÃO DO ARTIGO ... 80
ANEXO 3 - DOCUMENTO DE SUBMISSÃO DO ARTIGO 2 ... 81
ANEXO 4 – NORMAS DA EDITORA REFERENTE A PERMIÇÃO PARA INCLUSÃO DO ARTIGO NA TESE ... 82
1 INTRODUÇÃO
O uso difundido da tomografia computadorizada de feixe cônico (TCFC) na Odontologia tem levado a uma crescente preocupação com relação à justificativa e otimização das exposições para aquisição de exames. O princípio de otimização em relação a proteção radiológica recomenda que a dose de radiação ao paciente seja mantida tão baixa quanto diagnosticamente aceitável (ICRP, 2007; Jaju and Jaju, 2015). Isso implica que os parâmetros de aquisição devem ser ajustados para realização do exame e otimizados de acordo com a indicação e com o paciente, sendo de grande importância a determinação adequada da região de interesse avaliada. Nesse contexto, a aquisição de exames que utilizam radiação ionizante em crianças é de particular preocupação (Pauwels et al., 2014a), pois esses pacientes possuem maior quantidade de células em estágio de mitose, sendo assim mais susceptíveis ao acúmulo de mutações genéticas induzidas por radiação ionizante. Os exames de TCFC para crianças são indicados principalmente para avaliação de impactação dentária, dentes supranumerários, trauma dentoalveolar, fenda palatina, anomalias dentárias, patologias ósseas, planejamento cirúrgico de autotransplante dentário e acompanhamento de síndromes (Oenning et al., 2018a).
A fim de otimizar a aquisição de exames reduzindo a dose de radiação, novos aparelhos tem apresentado melhorias em termos de detector, colimação do feixe e algoritmos de reconstrução, sendo capazes de adquirir exames de uma grande região de interesse utilizando protocolos de baixa dose (Pauwels, 2015; Scarfe et al., 2017). O aparelho PaX-i3D Green Premium (Vatech, Hwaseong, Coreia do Sul), introduzido no mercado nacional e internacional recentemente, trabalha com uma geometria de aquisição singular, usando um detector vertical estreito que adquire imagens da região de interesse por incremento por meio de múltiplas rotações rápidas e consecutivas. Além disso, possibilita seleção manual de quilovoltagem (kV - de 60 a 110) e miliamperagem (mA - de 4 a 10). No entanto, a qualidade de imagem deste sistema em termos de otimização em relação às indicações dentomaxilofaciais ainda não foi investigada.
A qualidade da imagem em TCFC está intrinsecamente relacionada aos parâmetros de exposição. A corrente do tubo afeta diretamente a qualidade da imagem, de forma que o uso de uma maior miliamperagem aumenta proporcionalmente o sinal que chega no detector e a dose, reduzindo, consequentemente, o ruído da imagem (Pauwels et al., 2015). Todavia, o efeito da kV
não é tão previsível, pois essa produz uma combinação de diferentes interações dependentes de energia, além de alterar o número de fótons no feixe primário de raios X. Diante da complexidade do efeito energético na qualidade da imagem, é sugerido que a condução do processo de otimização seja realizada inicialmente pela determinação da kV ideal, em uma dose fixa, seguida pela redução da miliamperagem até se alcançar a menor dose com qualidade de imagem aceitável para cada tarefa diagnóstica (Panmekiate et al., 2018).
A variabilidade das informações disponíveis sobre avaliação da qualidade de imagem em TCFC dificulta a comparação entre os estudos, e também o estabelecimento de conclusões gerais. Estudos anteriores avaliaram qualidade de imagem com foco na perspectiva clínica (Kwong et al., 2008; Lofthag-Hansen, 2010; Lofthag-Hansen et al., 2011), outras no aspecto técnico (Pauwels et al., 2011; Bamba et al., 2013; De Moura et al., 2016; EzEldeen et al., 2017). Porém, o uso de diferentes tipos de modelos para avaliação de estruturas anatômicas, como fantomas constituídos por materiais que mimetizam a atenuação de estruturas humanas, ou mesmo crânios e mandíbulas com diferentes tamanhos e simuladores de tecidos moles, limita a padronização dessa avaliação. No contexto pediátrico, fantomas infantis foram desenvolvidos e validados especificamente para realização de estudos de imagem (Oenning et al., 2018b). Do mesmo modo, entendendo-se a necessidade da produção de mensurações padronizadas e possíveis de serem comparadas, fantomas geométricos tem sido usados para controle de qualidade e comparação entre aparelhos e protocolos de aquisição (Pauwels et al., 2011; Bamba et al., 2013).
De fato, a associação da avaliação clínica e técnica da qualidade de imagem é ideal, uma vez que permite conectar a percepção visual do profissional com a quantificação de aspectos técnicos, viabilizando assim o alcance de resultados mais sólidos e conclusivos. Entretanto, lidar com a grande quantidade de variáveis geradas pela aplicação desses diferentes métodos é um desafio. Uma alternativa para superar esse obstáculo é a utilização de gráficos biplot, o qual é uma representação gráfica que permite a análise exploratória de um conjunto de dados multivariados, de modo a reproduzir as principais características das variáveis visualmente, tornando-as comparáveis. Na construção do gráfico a dimensionalidade de grandes conjuntos de dados é reduzida, aumentado a interpretabilidade dos mesmos com mínima perda de informação (Jolliffe and Cadima, 2016). As variáveis são representadas por vetores e o ângulo formado entre elas
mostra a correlação entre uma e outra. Assim, torna-se possível a comparação de variáveis do mesmo aspecto entre si.
Estudos sobre o efeito da kV e mA em exames de TCFC para indicações odontológicas têm mostrado que a qualidade da imagem se mantém suficiente mesmo reduzindo os parâmetros de exposição consideravelmente, e consequentemente a dose, abaixo do padrão indicado pelo fabricante, sendo a maioria com foco no efeito da redução desses parâmetros na qualidade da imagem. Pauwels et al (Pauwels et al., 2014b) indicam o uso de valores de kV relativamente altos para alcançar uma diminuição da quantidade de radiação espalhada, sendo os melhores resultados encontrados utilizando 90 kV, a mais alta disponível no aparelho estudado por eles. Resta saber se uma kV ainda maior pode mostrar melhores resultados. No entanto, as pesquisas são limitadas pelas configurações de aparelhos disponíveis no mercado que muitas vezes fixam a kV e permitem apenas algumas opções de mA pré-definidos. Além disso, observou-se que para pacientes de tamanhos menores, é possível uma considerável diminuição na dose, mantendo-se a qualidade da imagem adequada, através da redução da mA e tempo em vez da kV (Pauwels et al., 2017).
Considerando a geometria de aquisição inovadora do aparelho de tomografia computadorizada odontológica PaX-i3D Green Premium, seu anúncio de aquisição utilizando baixas doses de radiação, a vasta possibilidade de ajuste de parâmetros de exposição e as recomendações internacionais de que esses parâmetros devem ser ajustáveis e otimizados na tomografia odontológica de acordo com a finalidade clínica da aquisição, o objetivo deste estudo foi iniciar o processo de otimização para o referido aparelho, determinando o ajuste de kV otimizado para aquisições infantis e em adultos. Além disso, propor o uso de um método consistente de avaliação de qualidade de imagem pela associação de parâmetros clínicos, quantitativos e técnicos.
2 ARTIGOS
2.1. Artigo: Image quality optimization using a narrow vertical detector dental cone-beam CT
Artigo publicado no periódico Dentomaxillofacial Radiology (doi: 10.1259/dmfr.20180357) – Esta corresponde a última versão do manuscrito com alterações prévias à publicação. A estruturação do manuscrito baseou-se nas instruções aos autores preconizadas pela editora do periódico. Em anexo, estão as normas da editora quanto a autorização para inclusão do material na tese (Anexo 4).
Running title: Image quality optimization of a narrow beam CT Type of manuscript: Research article
Danieli M Brasil
- DDS, MS, PhD student, Department of Oral Diagnosis, Division of Oral Radiology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil. E-mail: danielibrasil@hotmail.com
Ruben Pauwels
- MS, PhD, OMFS-IMPATH Research Group, Department of Imaging and Pathology, Faculty of Medicine, Catholic University of Leuven, and Oral & Maxillofacial Surgery, University Hospitals Leuven, Leuven, Belgium. Department of Mechanical Engineering, Catholic University of Leuven, Leuven, Belgium. Department of Radiology, Faculty of Dentistry, Chulalongkorn University, Bangkok, Thailand. E-mail: pauwelsruben@hotmail.com
Wim Coucke
- MS, PhD, Freelance statistician, Brugstraat 107, 3001 Heverlee, Leuven, Belgium. E-mail: wim.coucke@jewidaco.be
Francisco Haiter-Neto
- DDS, MS, PhD, Professor, Department of Oral Diagnosis, Division of Oral Radiology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil. E-mail: haiter@unicamp.br
Reinhilde Jacobs
- DDS, PhD, MSc, Dr hc, OMFS-IMPATH Research Group, Department of Imaging and Pathology, Faculty of Medicine, Catholic University of Leuven, and Oral & Maxillofacial Surgery, University Hospitals Leuven, Leuven, Belgium. Department of Dental Medicine, Karolinska Institutet, Stockholm, Sweden. E-mail: reinhilde.jacobs@uzleuven.be
Address correspondence and reprint requests to Dr Brasil:
Department of Oral Diagnosis, Piracicaba Dental School, University of Campinas, Av. Limeira, 901, Piracicaba, Sao Paulo 13414-903, Brazil; e-mail: danielibrasil@hotmail.com
ABSTRACT
Objectives: To determine the optimised kV setting for a narrow detector cone-beam CT (CBCT) unit.
Methods: Clinical (CL) and quantitative (QUANT) evaluations of image quality were performed using an anthropomorphic phantom. Technical (TECH) evaluation was performed with a polymethyl methacrylate phantom. Images were obtained using a PaX-i3D Green CBCT (Vatech, Hwaseong, Korea) device, with a large 21x19 and a medium 12x9 cm field of view, and high-dose (HD– ranging from 85 to 110 kV) and low-dose (LD– ranging from 75 to 95 kV) protocols, totalling four groups (21x19 cm HD, 21x19 cm LD, 12x9 cm HD, 12x9 cm LD). The radiation dose within each group was fixed by adapting the mA according to a predetermined dose-area product. For CL evaluation, three observers assessed images based on overall quality, sharpness, contrast, artefacts, and noise. For QUANT evaluation, mean grey value shift, % increase of standard deviation (SD), % of beam-hardening and contrast-to-noise ratio (CNR) were calculated. For TECH evaluation, segmentation accuracy, CNR, metal artefact SD, metal object area, and sharpness were measured. Representative parameters were chosen for CL, QUANT and TECH evaluations to determine the optimal kV based on biplot graphs. kV values of the same protocol were compared by bootstrapping approach. The ones that had statistical differences with the best kV were considered as worse quality.
Results: Overall, kV values within the same group showed similar quality (p > 0.05), except for 110 kV in 21x19 cm HD and 85 kV in 12x9 cm HD of CL score; also 85, 90 kV in 21x19 cm HD and 75, 80 kV in 21x19 cm LD of QUANT score which were worse (p < 0.05).
Conclusion: At a constant dose, low and high kV protocols yield an acceptable image quality for a narrow-detector CBCT unit.
Key-words: Computed-assisted image analysis; Cone-beam computed tomography; Image
INTRODUCTION
Cone-beam computed tomography (CBCT) has been increasingly used in many of dental specialties. Manufacturers have developed systems able to cover different indications, offering a large variety of fields of view and exposure parameters combinations which should provide good quality images as well as low radiation dose,1,2 in compliance with the ALARA (as low as
reasonably achievable) principle.3
Image quality assessment of CBCT devices is sometimes focused on clinical image quality aspects,4–6 and sometimes on technical aspects.7–10 The use of different skull and jaw models to
assess anatomical structures and landmarks from a clinical perspective limits the standardization of image quality evaluation. On the other hand, geometrical phantoms have been used allowing reproducible and objective measurements; these measurements can guide the evaluation of a scanner’s performance but are often very difficult to link to clinical acceptability of an image.11
Indeed, the use of clinical and technical image quality assessment is preferable to allow obtaining more conclusive and consistent data.
Moreover, CBCT image quality is intrinsically related to exposure parameters.11,12 A higher
tube current-exposure time product (mAs) reduces image noise, while the manifestation of beam hardening artefacts is most affected by the tube voltage (kV). However, compared to mAs, the effect of kV is not so predictable due to a combination of different energy-dependent X-ray interactions affecting both detector signal and contrast.13 Obtaining a suitable image for clinical
use involves the balancing between image quality and radiation dose parameters that are differently influenced by the exposure settings.
Recently, an impressive number of CBCT scanners have been introduced in the field of Dentomaxillofacial radiology trying to provide a balance between device cost, image quality and radiation dose to the patient.14 Pax-i3D Green is a recent CBCT device developed by Vatech
(Hwaseong, Korea) providing a new design in which the X-ray source and detector are inside a gantry (rather than a U-arm). The unique aspect of this unit involves the use of a narrow (36.4 mm width) vertical detector and several consecutive fast rotations around the isocenter with incremental detector off-set. Another particular and convenient aspect of this unit for our purpose is that it allows the tube voltage to be selected between 60 and 110 kV, allowing for an in-depth
investigation of the optimal kV for Dentomaxillofacial imaging, which is an issue that has only been partially resolved as of yet.13,15 No investigator has assessed image quality of this system.
Thus, the purpose of this study was to use an optimisation strategy for Dentomaxillofacial applications. In addition, this study proposed to use a consistent image quality assessment method by combining clinical, quantitative and technical parameters.
METHODS AND MATERIALS
Clinical (CL) and quantitative (QUANT) evaluations were done using an anthropomorphic phantom (SK150, The Phantom Laboratory, Salem, NY, USA) containing a natural human skull, upper cervical vertebrae and urethane as soft-tissue simulating material. Technical (TECH) evaluation was performed with a polymethyl methacrylate (SEDENTEXCT-IQ; Leeds Test Objects, Boroughbridge, UK) phantom filled with four inserts: 1-cm diameter cylinders of hydroxyapatite, air, aluminium and an insert containing 3 small (~5.2 mm) titanium cylinders positioned in the central region. Phantoms were scanned using the PaX-i3D Green CBCT device, using a large 21x19 cm (0.3 mm voxel size) and a medium 12x9 cm (0.2 mm voxel size) field of view (FOV) as well as high-dose (HD – ranging from 85 to 110 kV) and low-dose (LD - ranging from 75 to 95 kV) protocols, totalling four groups (21x19 cm HD, 21x19 cm LD, 12x9 cm HD, 12x9 cm LD). These FOV sizes were chosen for covering most of clinical applications. Technical specifications of the PaX-i3D Green are listed in Table 1.
Table 1. Technical specification of the PaX-i3D Green cone-beam CT.
Voltage 60 ~ 110 kV (1 kV increment ±10%) Tube current 4 ~ 10 mA (0.1 mA increment ± 10%) Exposure type Continuous
Focal spot 0.5 mm
Filtration 2.5 mm Al + 2.0 mm Al (added) Anode material Tungsten
Field of view size (cm) 8x8, 12x9, 17x11, 17x15 and 21x19 Detector type CMOS photodiode array
Detector size Active area: 310.4 mm x 36.4 mm Reconstruction algorithm FDK
The radiation dose within each group was fixed by adapting the mA for each protocol according to a predetermined dose-area product (DAP) value.Table 2 shows DAP range, kV and mA combinations for each protocol. All acquisitions for each group were registered automatically (through Maximization of Mutual Information) using elastix software, ensuring that regions used for evaluation were matched exactly.16
Table 2. DAP range, kV and mA combination for each protocol group.
FOV/
voxel size Dose level DAP range (mGy.cm2) kV/mA 21x19cm/
0.3mm HD LD 1233.5 - 1243.7 932.1 - 940 85/7 75/7 90/6.2 80/6.1 95/5.5 85/5.3 100/5.0 90/4.7 105/4.5 95/4.2 110/4.2 - 12x9cm/
0.2mm HD LD 520.1 - 524.4 393 - 396.4 85/7 75/7 90/6.2 80/6.1 95/5.5 85/5.3 100/5.0 90/4.7 105/4.5 95/4.2 110/4.2 -
FOV, field of view; HD, high-dose; LD, low-dose; DAP, dose-area product; kV, kilovoltage; mA, milliamperage.
CL evaluation
Three previously calibrated observers independently assessed images containing bone and dental structures. Corresponding images from the same group were shown simultaneously in a randomized order (Figure 1), in a darkened room on a 46’’ ED46D LCD display (Samsung, Seoul, South Korea).
Figure 1. Clinical evaluation screen (axial reconstructions) of upper jaw region (FOV of 21 x 19 cm, high-dose protocols). A. 85 kV/7 mA, B. 105 kV/4.5 mA, C. 90 kV/6.2 mA, D. 100 kV/5 mA, E. 95 kV/5.5 mA, F. 110 kV/4.2 mA.
CL evaluation was performed based on overall image quality, sharpness, contrast, artefacts, and noise, using 4-point scales. To perform the descriptive analysis, all parameters were adjusted to the same crescent image quality scale, as described below: overall image quality, sharpness and contrast ranging from 1 to 4 (1-poor, 2-moderate, 3-good, 4-excelent), and presence of artefact and noise ranging from 1 to 4 [1- high quantity (useless), 2- moderate quantity, 3- low quantity, 4- no artefact/noise]. Apart from these scores, the observers were asked which image was best and worst for overall diagnosis for set of images shown simultaneously. All the evaluations were performed twice with a 2-week interval to assess intraobserver agreement.
QUANT evaluation
To objectively estimate the effect of kV on image quality by means of measuring artefact and noise, mean grey value (MGV) and its standard deviation (SD) were measured joining three regions of interest (ROI) within air, buccal soft-tissue (bst) and central soft-tissue (cst). These ROIs were traced on images of the following anatomical regions: alveolar level of upper jaw, upper teeth level and lower jaw. Images of upper teeth level were selected due to show a higher number of artefacts in the anthropomorphic phantom. ROIs were measured over 5 consecutive slices. The ROIs were reproduced exactly for all scans of a given FOV using a ROI Manager tool (ImageJ software, National Institutes of Health, Bethesda, MD, USA), thereby eliminating operator variability.13
Based on MGV and SD measurements,13 MGV shift, percentual increase of standard
deviation (SD), contrast-to-noise ratio (CNR) and percentual beam hardening were calculated as the following formulas:
𝑀𝑀𝑀𝑀𝑀𝑀 𝑠𝑠ℎ𝑖𝑖𝑖𝑖𝑖𝑖 =100∗(𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀−𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀)(𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀−𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀)
% 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑠𝑠𝑖𝑖 𝑜𝑜𝑖𝑖 𝑆𝑆𝑆𝑆 = 100 ∗𝑆𝑆𝑆𝑆𝑀𝑀𝑀𝑀𝑀𝑀𝑆𝑆𝑆𝑆𝑀𝑀𝑀𝑀𝑀𝑀 − 1
TECH evaluation
For TECH evaluation, segmentation accuracy, CNR, normalized metal artefact SD, metal object area, and full width at half maximum (FWHM) of the point spread function were measured using a fully automated procedure developed in-house.9,17 Table 3 shows the definition of each
parameter used in each evaluation and its connotation in terms of image quality. Table 3. Parameters’ definition and connotation in image quality.
Evaluation Parameters Definition Connotation in image quality
CL
Overall quality Image quality in general, not considering a specific parameter Higher = better
Sharpness Ability to discriminate small structures in an image Higher = better
Contrast Ability to distinguish different densities in an image Higher = better
Artefact Any distortion or error (like dark bands or dark streaks) unrelated to the real object Higher = worse
Noise Graininess in appearance of a supposedly homogeneous structure Higher = worse
QUANT
MGV shift Mean of dark and light artefacts mainly from beam hardening but also from other phenomena Higher = worse
% of increase SD Percentage of mixed artefacts (light and dark) normalized to the SD of a non-artefact region Higher = worse
% of beam hardening Percentage of grey value shift between artefact and control region, divided by the difference between control region and
air Higher = worse
CNR Difference in grey value between average of soft-tissues and air, divided by root sum of squares of the SDs Higher = better
TECH
Segm accur Air Segmentation accuracy error of an air-filled region Higher = worse
Segm accur Al Segmentation accuracy error of an aluminium-filled region Higher = worse
CNR Air Contrast-to-noise ratio of air vs. PMMA Higher = better
CNR Al Contrast-to-noise ratio of aluminium vs. PMMA Higher = better
CNR hydrox Contrast-to-noise ratio of hydroxyapatite vs. PMMA Higher = better
Norm metal SD Normalized standard deviation of a homogeneous region containing metal artefacts Higher = worse
Metal object area Average of the segmented cross-sectional areas of three small titanium cylinders Higher = worse
FWHM Sharpness of an edge between air and PMMA Higher = worse
CL, clinical evaluation; QUANT, quantitative evaluation; TECH, technical evaluation; MGV, mean grey value; SD, standard deviation; CNR, contrast-to-noise ratio; Segm accur, segmentation accuracy; Al, aluminium; hydrox, hydroxyapatite; Norm metal, normalized metal; FWHM, full width at half maximum, PMMA, polymethyl methacrylate.
Statistical analysis
To assess the inter and intraobserver agreement, weighted kappa statistics was done using IBM SPSS Statistics for Windows, (Version 22.0. IBM Corp., Armonk, NY, USA). Descriptive statistics were used for each protocol group combining all observers together to show the most frequently chosen score for each parameter, and the percentage of best and worst kV chosen in the
CL evaluation. To determine the optimal kV, a set of non-correlated parameters were selected using biplots based on principal components analysis for CL, QUANT and TECH evaluations separately. Subsequently, for CL, QUANT and TECH evaluations separately, the squared difference between each value and the corresponding optimal value, set per parameter, was calculated and summed for the parameters that were chosen to be included. Differences between voltages were performed by comparing means. The standard error on the means was determined by a 100-fold bootstrap and a normal test using these means and their standard errors was applied to comparison between voltages. p ≤ 0.05 was considered statistically significant. The voltages that had no statistical differences with the best kV chosen from the score graphs were considered as having the same quality.
RESULTS
The weighted kappa was determined for the scores of each clinical parameter separately. In table 4, it can be seen that for the intraobserver comparisons, weighted kappa values were found into the slight to moderate agreement range. While, for the interobserver comparisons, weighted kappa values were found into slight to fair agreement range.18
Table 4. Observer’s agreement (weighted kappa).
Observer Observer 1 Observer 2 Observer 3
Overall quality Observer 1 Observer 2 0.52 0.13 0.05 0.34 0.13
Observer 3 0.37
Sharpness Observer 1 Observer 2 0.28 0.20 0.20 0.21 0.24
Observer 3 0.14
Contrast Observer 1 Observer 2 0.17 0.15 0.33 0.17 0.02
Observer 3 0.31
Artefact Observer 1 Observer 2 0.37 0.29 0.43 0.35 0.31
Observer 3 0.29
Noise Observer 1 Observer 2 0.40 0.14 0.39 0.25 0.13
Figure 2 shows the graphs of the most frequently chosen score during clinical image quality evaluation, as well as the percentage in which each image was chosen as best or worst for each parameter and each kV. Overall, all parameters showed a moderate to good score. In addition, for LD protocols, it is possible to see a tendency for higher kV protocols to be chosen as the best and lower kV protocols as the worst, while in HD protocols this choice is better distributed between kV. In the large FOV, HD and LD protocols exhibited similar image quality, being parameters scored between moderate and good in both. Meantime, in the medium FOV variations are more evident, being overall image quality and artefact greater scored in HD protocols, while in the LD protocols noise was perceived as high quantity in the lower kV.
Figure 2. Most frequently chosen scores for each parameter and percentage of choice of the best and worst image considering each kV from CL evaluation’s descriptive analysis. Color lines represent each clinical parameter [overall image quality, sharpness and contrast (score: 1-poor, 2-moderate, 3-good, 4-excelent), and presence of artefact and noise (score: 1- high quantity/useless, 2- moderate quantity, 3- low quantity, 4- no artefact/noise)]. Chosen scores, see vertical left axis. Columns show the percentage of best and worst choice for each kV, see vertical right axis.
The following parameters were chosen as representative to generate the scores: overall image quality, sharpness and artefacts for CL evaluation; % of beam hardening, % increase of SD and CNR for QUANT evaluation; segmentation accuracy of air and aluminium, normalized metal artefact SD for TECH evaluation (Figure 3).
Figure 3. Biplot graphs showing correlation between the parameters related to clinical (A), quantitative (B) and technical (C) evaluations. Parameters are represented by arrows. Components 1 and 2 mean the two directions that hold the most variability of the dataset. For every set of arrows that pointed into the same or opposite direction, one representative parameter was chosen to be used for the scores incorporating multiple parameters. Bolded parameters followed by * were chosen as representative for each evaluation (A, B and C). CL, clinical; QUANT, quantitative; TECH, technical.
Figure 4 shows the best kV for each score based on comparison between distances from ideal values. In the CL and QUANT scores, the best kV of HD protocols presented lower distance from ideal value, i.e. better image quality, than the best kV of LD protocols. However, for the TECH score the same occurred only for the medium FOV.
Figure 4.Relation between the distance from ideal values and kV for CL (A), QUANT (B) and TECH (C) evaluations. The closer to zero (on distance from ideal axis), the better image quality of that kV. Yellow points show the best kV for each group. Vertical axes represent the distance from ideal value. Horizontal axes represent the kV. Coloured lines represent each protocol group. CL, clinical; QUANT, quantitative; TECH, technical.
Based on the bootstrapping statistical approach and comparison between individual kV (Table 5), the ones that had no statistical differences with the best kV chosen from the score graphs were considered as having the same quality.
Table 5. Comparison between individual kV.
CL QUANT TECH
Protocol group kV P-value kV P-value kV P-value
21x19cm HD 85-90 0.96 85-90 0.001 85-90 0.99 21x19cm HD 85-95 0.45 85-95 0.001 85-95 0.99 21x19cm HD 85-100 0.73 85-100 0.001 85-100 0.98 21x19cm HD 85-105 0.68 85-105 0.001 85-105 0.92 21x19cm HD 85-110 0.27 85-110 0.001* 85-110 0.93 21x19cm HD 90-95 0.46 90-95 0.46 90-95 0.99 21x19cm HD 90-100 0.68 90-100 0.15 90-100 0.98 21x19cm HD 90-105 0.70 90-105 0.20 90-105 0.92 21x19cm HD 90-110 0.22 90-110 0.02* 90-110 0.93 21x19cm HD 95-100 0.24 95-100 0.50 95-100 0.98 21x19cm HD 95-105 0.73 95-105 0.60 95-105 0.92 21x19cm HD 95-110 0.03* 95-110 0.14 95-110 0.92 21x19cm HD 100-105 0.43 100-105 0.87 100-105 0.94 21x19cm HD 100-110 0.44 100-110 0.44 100-110 0.95 21x19cm HD 105-110 0.10 105-110 0.35 105-110 0.99 21x19cm LD 75-80 0.92 75-80 0.41 75-80 0.98 21x19cm LD 75-85 0.43 75-85 0.02 75-85 0.95 21x19cm LD 75-90 0.70 75-90 0.01 75-90 0.97 21x19cm LD 75-95 0.98 75-95 0.001* 75-95 0.84 21x19cm LD 80-85 0.49 80-85 0.11 80-85 0.98 21x19cm LD 80-90 0.63 80-90 0.11 80-90 0.95 21x19cm LD 80-95 0.95 80-95 0.01* 80-95 0.81 21x19cm LD 85-90 0.24 85-90 0.81 85-90 0.92 21x19cm LD 85-95 0.47 85-95 0.23 85-95 0.79 21x19cm LD 90-95 0.69 90-95 0.14 90-95 0.86 12x9cm HD 85-90 0.64 85-90 0.88 85-90 0.97 12x9cm HD 85-95 0.12 85-95 0.45 85-95 0.96 12x9cm HD 85-100 0.04* 85-100 0.06 85-100 0.92 12x9cm HD 85-105 0.25 85-105 0.06 85-105 0.98 12x9cm HD 85-110 0.18 85-110 0.05 85-110 0.96 12x9cm HD 90-95 0.27 90-95 0.51 90-95 0.99 12x9cm HD 90-100 0.12 90-100 0.06 90-100 0.95 12x9cm HD 90-105 0.52 90-105 0.06 90-105 0.99 12x9cm HD 90-110 0.40 90-110 0.05 90-110 0.98 12x9cm HD 95-100 0.72 95-100 0.30 95-100 0.96 12x9cm HD 95-105 0.56 95-105 0.32 95-105 0.97 12x9cm HD 95-110 0.67 95-110 0.28 95-110 0.99 12x9cm HD 100-105 0.29 100-105 0.94 100-105 0.94 12x9cm HD 100-110 0.36 100-110 0.98 100-110 0.96 12x9cm HD 105-110 0.85 105-110 0.91 105-110 0.97 12x9cm LD 75-80 0.62 75-80 0.93 75-80 0.95 12x9cm LD 75-85 0.88 75-85 0.54 75-85 0.99 12x9cm LD 75-90 0.50 75-90 0.42 75-90 0.94 12x9cm LD 75-95 0.74 75-95 0.33 75-95 0.90 12x9cm LD 80-85 0.69 80-85 0.49 80-85 0.95 12x9cm LD 80-90 0.82 80-90 0.39 80-90 0.99 12x9cm LD 80-95 0.87 80-95 0.29 80-95 0.95 12x9cm LD 85-90 0.54 85-90 0.75 85-90 0.94 12x9cm LD 85-95 0.83 85-95 0.64 85-95 0.90 12x9cm LD 90-95 0.71 90-95 0.92 90-95 0.96
Bootstrapping statistical approach. Bolded numbers are comparisons between the best kV from score graphs and other kV values at the same protocol. (*) means statistical difference, p < 0.05.
In general, kV values within the same group showed similar quality (p > 0.05), except for 110 kV in 21x19 cm HD and 85 kV in 12x9 cm HD of CL score; also 85, 90 kV in 21x19 cm HD and 75, 80 kV in 12x9 cm LD of QUANT score which were worse (p < 0.05). Also, the choice of the best kV values presented in all the three evaluations revealed that, in the largest FOV, the range of best kV was reduced to the highest ones, while in the smaller FOVs, almost all kV values remained present (Figure 5).
Figure 5. Schematic representation of the material and methods resulting in the best kV values found for each protocol. Parameters followed by * were chosen to be used in the scores. Absent kV had lower image quality. Overall result shows the best kV values present in all the three evaluations for each protocol group. CL, clinical; QUANT, quantitative; TECH, technical; MGV, mean grey value; SD, standard deviation; CNR, contrast-to-noise ratio; Segm accur, segmentation accuracy; Al, aluminium; hydrox, hydroxyapatite; Norm metal, normalized metal; FWHM, full width at half maximum.
DISCUSSION
Confronted with the challenge of obtaining images with diagnostic quality acquired achieving the lowest radiation dose possible for the patient, it is of great importance to perform image quality assessments on CBCT machines, through the evaluation of image quality considering FOV, kV, mA and subsequently radiation dose. In the present study, a commercially available CBCT scanner with a unique exposure geometry using a narrow vertical detector was assessed for clinical, quantitative and technical image quality. From this combination of perspectives, we found that, at a fixed dose per scanning protocol, both low and high kV protocols can yield an acceptable image quality from a statistical point of view, whereas distinct differences between protocols were found for individual image quality parameters.
Combining clinical and technical image quality assessment, using both anthropomorphic and geometric phantoms, enable for connecting the professional clinical view (i.e. direct application in diagnostic practice) with a quantification of technical image aspects, allowing for a degree of standardization,8 and achieving more conclusive and consistent data. Yet, dealing with
so many variables and data generated by this set of assessments is a challenge. A biplot is a graphical representation of a multivariate data which reduces the dimensionality of large datasets, increasing interpretability and with minimal information loss.19 In our study, based on biplot
graphs, correlation between several parameters from CL, QUANT and TECH evaluations were found. The most representative parameters were thus selected to obtain scores considering the different image quality aspects.
Clinical image quality is essentially a descriptor of the subjective visual interpretation of an image that should convey enough information to allow a clinical decision to be made with an acceptable degree of certainty. Subjective evaluation is being considered as a factor of greater weight to assess the image quality in a task-based approach; however, the possibility of standardizing this method is limited.6,20 We found in CL score a slight difference in image quality
between different kV settings at the same group. Because the dose within each group was fixed, and the available range for kV within the groups was limited, consistent and significant trends between kV and clinical image quality were not identified, partly due to the high subjectivity of this type of evaluation.6 This also can explain the slightly lower intra and interobserver agreement
showed at the same screen during the observation section had very subtle differences, making it hard for observers to score each one. On the other hand, the tendency for higher kV protocols to be chosen as the best and lower kV protocols as the worst for low-dose protocols can be explained by the fact that, since acquired images in low-dose protocols have lower overall image quality, the increasing of the beam energy in higher kV can provide a visually impactful noise reduction and improvement of overall image quality.13 In high-dose protocols, the selection of the best and worst
protocol was more evenly distributed between protocols with different kV, seeing that images acquired using high-dose protocol already had good overall quality.
The TECH scores showed no significant differences between kV within groups with identical FOV and dose, whereas differences were found between the four groups due to varying FOV size and dose level. Earlier studies have found no changes in image uniformity23 and
segmentation accuracy24 under different exposure parameters using geometrical phantoms, even
without fixing radiation dose. This might be explained by the fact that the material from which the SEDENTEXCT-IQ phantom is made (polymethyl methacrylate) and its spatial distribution is homogeneous compared to bone and soft-tissues inside an anthropomorphic phantom. Therefore, the SEDENTEXCT-IQ phantom may not be able to represent subtle differences in clinical image quality due to varying amounts of X-ray scatter and beam hardening. In addition, parameters such as noise and sharpness are not expected to vary considerably when the dose, FOV and voxel size are fixed. Using a polymethyl methacrylate, Loubele et al.24 found a small correlation between
segmentation accuracy on a skull phantom and in the contrast phantom, which point out that information about diagnostic quality cannot be predicted by a physical phantom. For research involving variation of beam energy, the association with clinical evaluation using an anthropomorphic model may thus be preferred.25
In contrast, in the QUANT evaluation, measurements were performed on scans obtained using an accurate head model. It was found that image quality may be improved by increasing the kV, since this higher kV may result in an increased contrast-to-noise ratio as well as reduced artefacts,13 just as Elkhateeb et al.26 also found less noise and more image uniformity when they
used higher kV. In addition, these results show that for indications that require a large FOV the choice of higher kV values produce images with better quality, thus contributing to a good start for the study of the optimization of this device.
Although CBCT is widely used in a variety of dental specialties, still there is a scientific debate about the level of risk associated with this imaging modality.3 Hence, it is essential to apply
the optimization principle every possible as an effort to reduce patient dose to the minimum level. X-ray tube current and exposure time are directly proportional to dose when other factors remain constant. The effect of kV on dose and image quality is more complex. kV is only altering beam energy, yet it may also vary the number of photons determining the resulting image and dose.13 In
order to verify the influence of kV on image quality, it is necessary to fix the dose. As DAP has been relates reasonably well with effective dose,3 in the present study, based on DAP, a dose range
with a small as possible variation between dose of each kV was stablished for each scanning protocol, to maintain a minimal, also unavoidable, dose difference between kV within the same group. It was not possible to compare the full range of kV values at the same dose because of the limited mA range, so we split kV values into HD and LD groups, but with enough overlap between groups. Nevertheless, in clinical practice, the choice of using HD or LD protocols depends on the diagnostic task to be solved.
The current results are based on a novel CBCT unit which uses a unique acquisition geometry involving a narrow vertical detector and several consecutive rotations with incremental detector off-set. A benefit of such approach is that the primary-to-scattered ratio at the detector is increased, also it is possible to increase the width of the FOV while using a smaller area detector, thereby maybe reducing manufacturing costs.1,27,28 Furthermore, future work should investigate
whether applying image processing tools (e.g. high-sharpness filters vs low-noise filters) in the reconstruction process are needed to truly optimize the CBCT exposure. Considering that X-ray scatter magnitude as well as its angular distribution depends on beam energy, other CBCT units using a large-area flat panel detector may yield different results. Moreover, we introduced a novel method to combine clinical, quantitative and technical evaluations, which was used for the first time for this narrow-detector CT optimization process. While the technical aspect may be more standardised, the clinical visual perception does not always correspond to the technical results, but it is more relevant and connected to the clinical radiodiagnostics. So, when striving for clinical optimization, it is necessary to start from a combined assessment of clinical and technical parameters, apart from considering dose aspects.
CONCLUSION
From a clinical and technical perspective, at a fixed dose per scanning protocol for the CBCT device studied, both low and high kV protocols yield an acceptable image quality for a narrow-detector CBCT unit. Nonetheless, higher kV values (and correspondingly low mA values) should be preferred, as they lead to reduced noise and artefacts.
ACKNOWLEDGEMENTS
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.
We also thank the observers Jardel Mazzi-Chaves, Danilo Schneider, Daniel Vasconcelos for making possible to obtain the clinical results of this study.
CONFLICTS OF INTEREST
None. The authors have no conflict of interest, also disclose any financial and personal relationships with other people or organizations that could inappropriately influence their work. There is no financial support received from a company.
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2.2. Artigo: Image quality optimization of a narrow detector dental computed tomography for paediatric patients
Artigo aceito para publicação no periódico Dentomaxillofacial Radiology – Esta corresponde a última versão do manuscrito com alterações prévias à publicação. A estruturação do manuscrito baseou-se nas instruções aos autores preconizadas pela editora do periódico. Em anexo, estão as normas da editora quanto a autorização para inclusão do material na tese (Anexo 4).
Danieli Moura Brasil
- DDS, MS, PhD student, Department of Oral Diagnosis, Division of Oral Radiology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil. E-mail: danielibrasil@hotmail.com
Ruben Pauwels
- MS, PhD, OMFS-IMPATH Research Group, Department of Imaging and Pathology, Faculty of Medicine, Catholic University of Leuven, and Oral & Maxillofacial Surgery, University Hospitals Leuven, Leuven, Belgium. Department of Mechanical Engineering, Catholic University of Leuven, Leuven, Belgium. Department of Radiology, Faculty of Dentistry, Chulalongkorn University, Bangkok, Thailand. E-mail: pauwelsruben@hotmail.com
Wim Coucke
- MS, PhD, Freelance statistician, Brugstraat 107, 3001 Heverlee, Leuven, Belgium. E-mail: wim.coucke@jewidaco.be
Francisco Haiter-Neto
- DDS, MS, PhD, Professor, Department of Oral Diagnosis, Division of Oral Radiology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil. E-mail: haiter@unicamp.br
Reinhilde Jacobs
- DDS, PhD, MSc, Dr hc, OMFS-IMPATH Research Group, Department of Imaging and Pathology, Faculty of Medicine, Catholic University of Leuven, and Oral & Maxillofacial Surgery, University Hospitals Leuven, Leuven, Belgium. Department of Dental Medicine, Karolinska Institutet, Stockholm, Sweden. E-mail: reinhilde.jacobs@uzleuven.be
Address correspondence and reprint requests to Dr Brasil:
Department of Oral Diagnosis, Piracicaba Dental School, University of Campinas, Av. Limeira, 901, Piracicaba, Sao Paulo 13414-903, Brazil; e-mail: danielibrasil@hotmail.com
ABSTRACT
Objective: Dental Cone-beam computed tomography (CBCT) exposure parameters should be optimized according to patient-specific indications, mainly for children that are most vulnerable to harmful effects of ionizing radiation. The aim of this study was to determine optimized kV settings for pediatric acquisitions for a dental CBCT device.
Methods: Clinical and quantitative evaluations of image quality were performed using 5 and 10 years old (y/o) anthropomorphic phantoms. Technical evaluation was performed with the SEDENTEXCT-IQ phantom. Images were obtained using a PaX-i3D Green CBCT (Vatech, Korea) device, combining tube voltages ranging from 85 kV to 110 kV and 2 fields of view (FOV: 21x19 and 12x9 cm), while maintaining the radiation dose fixed by adjusting the mA accordingly. Clinically, observers assessed images based on overall quality, sharpness, contrast, artifacts, and noise. For quantitative evaluation, mean grey value shift, % increase standard deviation, % beam-hardening and contrast-to-noise ratio (CNR) were calculated. For technical evaluation, segmentation accuracy, CNR and full width at half maximum were measured. Biplot graphs were used to choose representative parameters, from which the best kV was selected for each protocol and evaluation. kV values that had no statistical differences (p>0.05) with the best kV chosen were considered as having the same quality.
Results: Clinically, 95 kV was found as a cut-off value. From the quantitative aspect, 85 kV (p<0.05) showed the worst quality, except in 12x9 cm 5y/o. Technically, 85 and 110 kV in the large FOV showed significantly worse quality for the large FOV.
Conclusion: For pediatric indications, 95 kV or higher (and correspondingly low mA values) was found as optimal.
Key-words: Pediatric dentistry; Cone-beam computed tomography; Optimization; Image Quality; Phantoms, Imaging
