Measurements of system sharpness for two digital breast tomosynthesis systems

Area of Science:

• Medical Imaging Physics
• Radiological Quality Assurance
• Digital Breast Tomosynthesis (DBT)

Background:

• Accurate assessment of imaging system sharpness is crucial for effective quality assurance in digital breast tomosynthesis (DBT).
• Existing quality assurance protocols may not fully account for the dynamic factors affecting image sharpness, such as X-ray focus size and motion.
• Understanding these factors is essential for optimizing diagnostic performance in DBT.

Purpose of the Study:

• To propose system sharpness parameters for DBT systems that integrate the influence of focus size and focus motion.
• To establish parameters suitable for routine quality assurance protocols in DBT imaging.
• To evaluate the impact of varying object height above the detector on imaging performance.

Main Methods:

• Measured X-ray focus size using a multiple pinhole test object.
• Determined detector presampling modulation transfer function (MTF) using a steel edge.
• Measured in-plane and in-depth MTF/PSF using tungsten wires and an aluminum bead within PMMA phantoms at varying heights.

Main Results:

• Modulation transfer function (MTF) at 0.50 cycles/mm (MTF(0.50)) decreased with increasing height above the table for both Siemens Inspiration and Hologic Selenia Dimensions systems.
• Measured MTF(0.50) values showed a maximum deviation of 13% from theoretically calculated values.
• In-depth point spread function (PSF) full-width half-maximum (FWHM) was 3.0 mm for Inspiration and 5.9 mm for Dimensions; 3D MTF was independent of bead height.

Conclusions:

• Evaluating MTF as a function of height above the table in both projection images and reconstructed planes provides critical insights into DBT system performance.
• The proposed sharpness parameters, considering focus size and motion, are valuable for quality assurance protocols.
• A 25 µm tungsten wire in a PMMA plate is suitable for measuring in-plane MTF.