Página 209 - irpa

209 ST 3.2 – Protección radiológica en radiodiagnóstico
(orientada a la tomografía computada)
COMPARISON BETWEEN FILM-SCREEN AND COMPUTED
RADIOGRAPHY SYSTEMS IN BRAZILIAN MAMMOGRAPHY
de A. Vieira, Leandro1*; Nogueira, Maria do Socorro2 ;
V. Barbosa, Nayra1 ; Oliveira, Jeyselaine Rocha1
1 Superintendência de Vigilância Sanitária. Secretaria de Estado da Saúde de Minas Gerais –
SES/MG – Belo Horizonte, MG. Brasil.
2 Centro de Desenvolvimento de Tecnologia Nuclear. Brasil.
* Responsible author, email: leandro.vieira@saude.mg.gov.br
Since 2004 the Public Health Office of the State of Minas Gerais in Brazil has established the
Image Quality Control Program in Mammography. It evaluates the image quality based on an
accredited phantom of the Brazilian College of Radiology (CBR). This phantom follows
international standards such as masses, specks, fibers, contrast details and spatial resolution.
The contrast index (CI) is accessed through optical density (OD) measurements. Although OD
is defined under film-screen (FS) scope, among all accessible mammographic systems under
the health office surveillance, almost 80% are computed radiography (CR) based. A necessity to
adapt the protocol has emerged to consider OD as a conformity parameter. Objective: To verify
the OD accessibility under CR´s printed out films and the feasibility to calculate contrast index,
in comparison with FS´s. Results: A total of 56 images were evaluated with three different CBR
phantom-models. They were equally divided into FS and CR systems and a densitometer was
used to read out their OD values. The correlation between their contrast-to-noise ratio (CNR)
was found to be in the order of 0,98 (σ of 0,17). The samples were not significantly different
(inside 5% incertitude) for every phantom. The CNR correlation coefficient was 0,871. For OD,
correlation coefficient was 0,989 and a log-fit function has shown good agreement with detector
response. The OD-normalized standard eviation difference between CR and FS was 36.6%,
2.8% and 20.2%. Conclusions: Different phantoms models were successfully tested in both CR
and FS to evaluate the feasibility in use contrast index as a conformity parameter since their
correlations are strictly related to calibration curve, as provided by phantom manufacturer. The
relative CR-FS OD σ-difference provides a spreading indicator, where the first and last
phantoms are considerably out of expectation. Such differences are probably due to their batch
production.